A comparison of the iddingsite alteration products in two terrestrial basalts and the Allan Hills 77005 martian meteorite using Raman spectroscopy and electron microprobe analyses



[1] We document the secondary mineral assemblages in two occurrences of terrestrial iddingsite, Lunar Crater, Nevada (LC) and Mauna Kea, Hawaii (MK), and compare these with the iddingsite in Allan Hills (ALHA) 77005. Short Raman spectroscopic traverses across olivine alteration fronts provide information about changes in mineralogy with alteration. Data from the Raman traverses are combined with electron microprobe (EMP) traverses at the same locations which provide information regarding element mobility and confirm mineral identifications made by Raman spectroscopy. This information is used with petrographic observations to argue for the martian origin of the iddingsite and jarosite, infer the sequence of alteration, and deliberate on the conditions and settings of alteration. Raman spectra indicate the presence of different iron oxides/oxyhydroxides in each sample (goethite in LC, maghemite in MK, and akaganéite in ALHA), and the terrestrial samples show different element mobility trends (loss of MgO and SiO2, retention of FeO) than ALHA (loss of MgO and FeO, influx of SiO2), whose trends reflect the deposition of jarosite. Altered olivine occur throughout the LC samples but only in the exteriors of the MK samples. The LC and MK alteration products formed by surface alteration, but ALHA 77005 is a lherzolite, and the olivine hosting the iddingsite are enclosed by orthopyroxene (appear to be restricted to the light lithology), suggesting that it formed at depth during magma consolidation. The ALHA iddingsite is an example of “deuteric alteration” (reaction with fluids that separated from the magma as crystallization progressed towards completion).

1 Introduction and Purpose of Research

[2] Recent surface investigations of Mars by the Mars Exploration Rovers (MER) have focused on the acquisition of bulk rock and soil spectra and their chemical compositions from which mineral speciation is inferred. Particular interest is placed on the identification of primary minerals in association with their secondary alteration products because these help constrain reaction pathways and the physical conditions of past aqueous activity [Wang et al., 1999, 2006, 2009]. Secondary assemblages in the martian meteorites provide constraints for interpreting remote data, but their use is complicated by the need to distinguish martian alteration products from potential terrestrial contaminants [Smith and Steele, 1983, 1984; Gooding, 1992; Bridges et al., 2001]. Many of the secondary materials in the martian meteorites are hosted by olivine so the alteration products of this mineral are of special interest [Delvigne et al., 1979; Burns, 1989; Delvigne, 1998; Tosca et al., 2004; Stopar et al., 2007]. Olivine is a common basaltic mineral that occurs in martian outcrops and soils and readily alters in aqueous environments because it consists of isolated SiO4 tetrahedra bonded by cations that are easily oxidized (Fe2+) or mobilized in solution (Mg2+) [Delvigne et al., 1979]. Instead of using bulk chemistry to infer secondary mineral speciation, we use Raman spectroscopy to directly detect secondary phases and study the reaction process.

[3] A critical assessment of the climatic history of Mars depends on our ability to merge the evidence of the martian hydrosphere present in the secondary assemblages of the martian meteorites with that of regional (MER, Pathfinder, and Viking) and global data sets, such as those from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM); the Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activité (OMEGA); and the Mars Global Surveyor Thermal Emission Spectrometer (MGS-TES). The suites of known martian meteorites are mafic, and most are relatively unaltered, but Allan Hills (ALHA) 77005, Miller Range (MIL) 03346, and Yamato (Y) 793605 contain jarosite; Elephant Moraine (EETA) 79001 and Allan Hills 84001 contain carbonates; and phyllosilicates occur in the Nakhlites. A summary of the secondary minerals previously reported to occur in Antarctic martian meteorites and their presently accepted origin (terrestrial or martian) is provided in Table 1a of Kuebler et al. [2007], and Meyer [2003] reviews their occurrence on a sample by sample basis.

[4] Most research on Mars' secondary mineralogy is presently focused on sulfates and phyllosilicates. Jarosite [KFe33+(SO4)2(OH)6] peaks are observed in the Mössbauer spectra of outcrops on Mars [Klingelhöfer et al., 2004; Morris et al., 2006] and the presence of kieserite [MgSO4·H2O] inferred from chemical data (Alpha Particle X-Ray Spectrometer, APXS [Rieder et al., 2004; Wang et al., 2006, 2006]). The identification of jarosite and other sulfate minerals at the MER landing sites has renewed an interest in the potential martian origin of the sulfates in the Shergottite-Nakhlite-Chassignite (SNC) meteorites [Fries et al., 2006; Papike et al., 2006, 2006; Treiman, 2006]. Jarosite is reported to occur in the lherzolitic shergottites and MIL 03346 [Smith and Steele, 1983, 1984; Ikeda, 1997; Mittlefehldt et al., 1997; Fries et al., 2006; Kuebler et al., 2007, 2008], but its martian origin (as opposed to an Antarctic origin [Gooding, 1984; Wentworth and Gooding, 1993; Wentworth et al., 2005]) has yet to be firmly established. We present petrographic evidence in support of a martian origin for the jarosite in ALHA. Hydrous phyllosilicates have been identified in regional (MER, Pathfinder, and Viking) and global data sets (OMEGA, CRISM, and MGS-TES) of Mars and are of interest because these occur in the nakhlites and may yield clues to the martian hydrosphere [Treiman et al., 1993, 2004; Bridges et al., 2001; Wang et al., 2002]. Mg-Fe smectites are suggested to occur in the Nili/Syrtis area [Mustard et al., 2008; Ehlmann et al., 2009] and Al-rich species in Mawrth Vallis (montmorillonite or chlorite), and kaolinite is suggested to occur in a few locations [Poulet et al., 2005; Bibring et al., 2005, 2007].

2 Analytical Techniques and Methods

[5] Laser Raman spectroscopy (LRS) and electron microprobe (EMP) imaging and quantitative analyses were made on polished thin or thick sections. All Raman spectra were collected with a HoloLab 5000® spectrometer (Kaiser Optical Systems, Inc.) using a 532.3 nm line of a frequency-doubled Nd:YAG laser, partially polarized at the sample, with a 6 µm condensed beam under a 20X long-working distance objective (0.4 NA). A HoloPlex filter determines the 0 to 4370 cm−1 spectral region, and a super-notch filter is used to reject the reflected laser wavelength. The motorized stage attached to the microscope is capable of taking steps <1 µm and is used to make “manual” traverses on the sample. Manual traverses permit the analyst to focus on the sample as frequently as needed (whereas automated traverses only permit the user to focus on the sample once). The absolute wavelength scale of the CCD camera (1024 × 256 pixels) was calibrated using the standard emission lines of a Ne arc lamp and a cubic spline routine used to smooth the spectrum and fit it with a third order polynomial. The resulting calibration curve has a standard deviation of 0.003 nm/pixel. The absolute frequency of the Nd:YAG laser showed slight daily variations, ±1.6 cm−1, over the time frame of data collection but was stable for several months at a time so the olivine peak positions were corrected according to the Si wafer's deviation from a standard value of 520.5 cm−1. Spectral resolution (peak separation) of the CCD camera is 3 pixels or 6.2 cm−1.

[6] Most spectra were acquired using a low laser power (≤6.0 mW) because of the potential for oxidizing components of the alteration phases [De Faria et al., 1997; Wang et al., 1998, 2004]. Some samples were analyzed under a dry N2 purge to provide additional protection from oxidation. Spectral accumulation times varied from 50 to 100 s. Olivine doublets and spectral alteration features were curve-fit using a least-squares curve-fitting routine in the Grams AI software package with a mixed Gaussian-Lorentzian peak shape and linear baseline. The constraint-free iteration option was used for fitting all parameters until convergence or a minimum was attained. Table 1a lists the samples and parameters used to collect each Raman traverse.

Table 1a. Samples Used in Study, Raman Data Collection Parameters
Sample, thin section usedSection used, olivine labelRaman traverseaObjective usedDate collectedLaser power (mW)Under N2 purgeRaman accumulation time# of spectra in traverseStep size (µm)
  1. a

    Repeated some Raman traverses with different parameters or at different orientations relative to laser polarization.

  2. b

    Lunar Crater thin and thick sections made from the same parent samples used by Israel [1996].

(1) Lunar CraterQBV1, ol phenocryst120X22 January 20032.0no5s × 10942
terrestrialQBV1, ol phenocryst220X22 January 20032.0no5s × 10942
alkali basaltQBV1, ol phenocryst320X27 January 20032.1no5s × 10722
QBV1QBV1, ol phenocryst4100X25 and 26 June 20080.8yes10s × 52011
and QBV2bQBV1, 1st gmass ol520X26 June 20032.1yes10s × 5731
 QBV1, 2nd gmass ol620X30 June 20082.2yes10s × 5761
 QBV1, 3rd gmass ol720X30 June 20032.2yes10s × 5661
 QBV1, ol phenocryst820X10 July 20032.1yes10s × 52001.4
 QBV1, ol phenocryst920X07 March 20044.7yes10s × 5701
 QBV1, ol phenocryst1020X13 March 20044.8yes10s × 51132
 QBV1, ol phenocryst1120X15 April 20044.9yes10s × 5771
 QBV1, ol phenocryst1220X16 April 20045.0yes10s × 5871
 QBV1, ol phenocryst1320X19 April 20045.0yes10s × 5621
 QBV1, ol phenocryst1420X24 April 20045.5yes10s × 5741
 QBV1, ol phenocryst1520X25 April 20045.2yes10s × 5781
 QBV1, ol phenocryst1620X29 April 20045.2yes10s × 5781
 QBV1, ol phenocryst1720X13 and 14 June 200411.6yes10s × 52871
(2) Mauna Kea16G1-30120X08 August 20032.0yes10s × 5741
post-shield16G1-30220X09 August 20032.0part of traverse10s × 5811
flows16G1-30320X11 August 20032.0no10s × 51361
hawaiite16G1-30420X12 August 20032.0no10s × 5491
and mugearite16J-30520X8 and 9 September 20032.0yes10s × 51822
16J-3016J-30620X10 September 20032.0no?10s × 5592
and 16G1-3016J-30720X10 September 20032.0no?10s × 51510
(3) ALHA 77005,51 on olivine 10120X26 May 20084.8no10s × 101401
Lherzolitic,51 on olivine 15220X4 and 5 June 20085.3no8 s × 102431.5
Shergottite,51 on olivine 14320X6 and 7 June 20085.5no8 s × 102261
,51 and, 34,51 on olivine 11420X8 and 9 June 20085.5no8 s × 102441
 ,51 on olivine 15520X10 June 20085.5no8 s × 10881

[7] Chemical compositions of the altered terrestrial samples were measured with a JEOL 733 electron microprobe at Washington University. EMP analyses on thin sections, 51 and, 34 of ALHA 77005 were collected using the Cameca SX100 at the Johnson Space Center (JSC). The accelerating voltage, beam current, and spot size used for the terrestrial samples were 15 kV, ~30 nA, and 1 µm, respectively. The same accelerating voltage was used for ALHA 77005 but with a lower beam current and larger spot size, 10–20 nA and 5 µm, to limit the amount of beam damage experienced by the martian alteration products which may contain volatiles. All quantitative analyses used a combination of silicate, metal, and oxide standards plus anhydrite or troilite for S, apatite for P, tugtupite for Cl, and fluorite for F. More elements (Na, K, Ni, P, S, F, and Cl) were analyzed in ALHA 77005 in order to accommodate the presence of jarosite [Smith and Steele, 1983, 1984]. X-ray matrix effects were corrected using ZAF methods incorporated into the JEOL Probe-for-Windows control software on the JEOL microprobe and a PAP correction routine applied on the Cameca SX100 [Armstrong, 1988; Pouchou and Pichior, 1991]. Table 1b summarizes the parameters used during the collection of each microprobe traverse.

Table 1b. Samples Used in Study, Microprobe Data Collection Parameters, and Instruments Used for Microprobe Data Collection
Sample, thin section usedRaman traverseCorresponding EMP traverseaEMP instrumentbDate collectedAccelerating voltage (keV)Beam current (nA)Spot size (µm)Same location as
  1. a

    Note that the probe traverse collection parameters are repeated for repeated Raman traverses (see comments).

  2. b

    JEOL 733 located at Washington University, St. Louis, MO; Cameca SX100 at Johnson Space Center (JSC) in Houston, TX.

(1) Lunar Crater1unk 32JEOL 73326 August 200315301automated traverse 1
terrestrial2unk 7JEOL 73325 August 200315301automated traverse 2
alkali basalt3noneJEOL 733noneautomated traverse 3
QBV14unk 7JEOL 73325 August 200315301manual traverse 2
and QBV25unk 15JEOL 73325 August 200315301 
 6unk 30JEOL 73326 August 200315301 
 7unk 31JEOL 73326 August 200315301 
 8unk 33JEOL 73326 August 200315301automated traverse 5
 9unk 7JEOL 73325 August 200315301manual traverse 2, thick section rotated ~90°
 10unk 32JEOL 73326 August 200315301manual traverse 1, thick section rotated 90°
 11noneJEOL 733nonemanual traverse 3, thick section rotated 90°
 12noneJEOL 733noneautomated traverse 7, thick section rotated 90°
 13noneJEOL 733noneautomated traverse 8, thick section rotated 90°
 14unk 15JEOL 73325 August 200315301manual traverse 5, thick section rotated 90°
 15unk 30JEOL 73326 August 200315301manual traverse 6, thick section rotated 90°
 16unk 31JEOL 73326 August 200315301manual traverse 7, thick section rotated 90°
 17unk 33JEOL 73326 August 200315301manual traverse 8, thick section rotated 130°
(2) Mauna Kea1unk 8JEOL 73317 December 200315301automated traverse 2
post-shield2unk 9JEOL 73317 December 200315301automated traverse 3
flows3noneJEOL 733none 
hawaiite4noneJEOL 733none 
and mugearite5unk 12JEOL 73311 December 200315301automated traverse 6
16J-306unk 13JEOL 73311 December 200315301automated traverse 5, did not cross entire grain
and 16G1-307unk 9JEOL 73312 December 200315301 
(3) ALHA 770051aCameca SX10013 June 200615105 
shergottite3fCameca SX10027 June 200615105 
,51 and, 344cCameca SX10013 June 200815105 
 5hCameca SX10027 June 200615105 

3 Studying Weathering Reactions With Laser Raman Spectroscopy and Electron Microprobe Traverses

[8] Raman spectral patterns provide information regarding crystal structure, crystallinity (regularity of the long-range crystal structure), and composition. Raman peaks are produced by molecular vibrations specific to the crystal structure and bonding environment so any change in the olivine structure due to alteration will be apparent in the spectra. By studying the degradation of the olivine spectral pattern in conjunction with the appearance and occurrence of Raman peaks from the alteration phases, we hope to constrain the conditions present during alteration (e.g., T, P, fO2, fluid composition, water/rock ratio). In addition, the Raman spectra of hydrated minerals have peaks at high Raman shifts (between 2500 and 4000 cm−1) due to OH and H2O, so the presence of waters of hydration are directly detected, not inferred [Wang et al., 1999].

[9] Electron microprobe analyses provide information on composition, which permit the evaluation of element mobility across the alteration fronts and place constraints on solution chemistry. The depth of penetration of the microprobe electron beam is shallower than that of the Raman laser so the difference in the sample volumes being analyzed by the two techniques results from the diameter of the microprobe beam and the depth of penetration of the Raman laser. The depth to which the Raman laser penetrates the sample varies with the opacity of the material being analyzed and is generally deeper than that of the focused EMP electron beam. A larger (5 µm) spot size was used during the EMP analysis of the ALHA alteration products so the volumes analyzed by the two techniques are deemed to be most alike in this sample. The alteration products in all three samples are fine-grained intergrowths so the microprobe analyses in the alteration represent a mixture of phases. Variations in the major element compositions are used to make inferences regarding the phases present and confirm secondary mineral phase identifications made by Raman spectroscopy where necessary.

[10] We used 1–2 µm steps in relatively short Raman traverses (total lengths of tens to hundreds of micrometers) to study the reactions of olivine alteration in all three samples, maintaining an optimal focus on the upper surface of the section throughout the traverses. The effective Raman sample volume of the alteration phases is generally small in comparison to the coexisting olivine, because these phases are not optically transparent. The tight spacing of data points and small sample volumes provide good spatial control on the apparent reaction inside the alteration, but the secondary phases are poorly crystalline and have weak Raman signals. Standard thin sections have discrete thicknesses, ~30 µm, and should not be thought of as being infinitely thin. Reflected and transmitted light images demonstrate differences in the materials visible at the surface of the thin sections and at depth. Unaltered olivine are transparent so the spectra outside the alteration fronts may sample material at depth in the thin section, including epoxy, so most of the Raman analyses were made on thick sections rather than thin sections. The olivine are more crystalline and have inherently larger Raman cross-sections than the alteration products so the spectra acquired outside of the alteration fronts are stronger (have better signal-to-noise ratios).

[11] Throughout this paper, changes in the Raman spectra are described as the traverses pass from unaltered olivine into patches of alteration. Spectral changes are related to the chemical trends established by electron microprobe analyses (EMPA) at the same locations. Cross-cutting (petrographic) relationships constrain the timing and sequence of alteration events (i.e., multiple episodes of alteration, preterrestrial from potentially Antarctic alteration).

4 The Raman Spectrum of Olivine and the Forsterite-Fayalite (Fo-Fa) Olivine Calibration

[12] Olivine is an orthosilicate of the Pbnm space group with a complete solid solution between the Fe and Mg end-members [Papike, 1987; Khisina et al., 1995]. We use the same spectral regions as Kuebler et al. [2006] to describe changes in the olivine spectra (see Figure 1): <400 cm−1, 400–700 cm−1, and 700–1100 cm−1. The dominant feature of the olivine spectrum is a doublet in the 700–1100 cm−1 region with peaks near 820 and 850 cm−1 (referred to as DB1 and DB2 by Kuebler et al. [2006]) that result from the coupled symmetric (ν1) and asymmetric (ν3) stretching vibrations of SiO4 tetrahedra [Pâques-Ledent and Tarte, 1973; Piriou and McMillan, 1983; Chopelas, 1991]. Peaks in the 400–700 cm−1 region are mainly from SiO4 internal bending vibrational modes, and peaks below 400 cm−1 arise from lattice modes: rotations and translations of SiO4 units and translations of octahedral cations in the crystal lattice [Chopelas, 1991]. Peak positions of the olivine doublet vary with composition; the curve-fit peak positions of the fayalite doublet occur near 815 cm−1 and 838 cm−1, and those of forsterite near 825 cm−1 and 857 cm−1, as reported by Kuebler et al. [2006]. The increase in peak position with increasing Fo content is linked to the change in cation mass between the end-members (Mg versus Fe), the change in the polyhedral volume of the octahedral sites, and the degree of coupling between the SiO4 symmetric and asymmetric stretching vibrational modes [Chopelas, 1991; Kuebler et al., 2006].

Figure 1.

Raman spectra of the olivine end-members forsterite and fayalite (same as Figure 1 of Kuebler et al. [2006]).

[13] Kuebler et al. [2006] present a calibration for estimating olivine compositions from the peak positions of their Raman doublets and discuss its accuracy with low signal-to-noise data. Alteration degrades the olivine crystal structure, influences the spectral quality of the olivine doublet (peak resolution and peak height/width ratios), and may also hinder our ability to determine olivine composition. Observations of olivine peak degradation through a sequence of spectra in conjunction with the appearance of plausible secondary phases will provide a useful indication of alteration in remotely acquired data sets (e.g., from an instrument mounted on the arm of a lander or rover).

[14] The laser in this study is only partially polarized after passing through the optics of the Raman system so we cannot distinguish which has the greater influence on changes in the peak positions of the olivine doublets: crystal orientation or alteration. Data from the Lunar Crater sample demonstrate that alteration preferentially influences the 820 cm−1 peak in this sample because this peak degrades (weakens and broadens) faster than the 850 cm−1 peak regardless of which peak dominates the doublets ahead of the alteration front. The magnitude of the changes in the doublet peak positions, however, varies with crystal orientation and may be due to differences in the relative peak heights of the doublet (which change with crystal orientation). Data from the Mauna Kea sample do not indicate a preferential degradation of the 820 cm−1 peak, but the crystal structure of the olivine we focused on appears to be less affected by alteration. The 820 cm−1 peak appears to have a stronger M1 cation contribution [Kuebler et al., 2006], suggesting that polarized Raman spectra on oriented crystals would be useful for studying the cation site-specific effects of alteration on the olivine structure.

5 Olivine Alteration Products—Background Information

[15] Olivine is a common primary mineral occurring in mafic and ultramafic igneous rocks whose alteration assemblages may be indicative of the environment of alteration, be it deuteric, hydrothermal, surface, or metamorphic. Iddingsite has been narrowly defined as the product of deuteric alteration but has also been used to describe the products of surface alteration (weathering). According to Delvigne et al. [1979], surface alteration products include iron hydroxide and phyllosilicate (smectite) intergrowths where drainage is good and the water/rock ratio high and infer nontronite or other phyllosilicate clays to form where drainage is poor and the water/rock ratio low. Serpentine and talc commonly occur with magnetite as the products of metamorphic reactions. Multiple generations of alteration are commonly present and may complicate their interpretation. A more complete discussion of olivine alteration is given in the review paper of Delvigne et al. [1979] and illustrated in his Atlas of Micromorphology of Mineral Alteration and Weathering [Delvigne, 1998]. We compare the alteration products in our samples to select images from the Atlas. Figure 2 shows reference spectra of each of the phases identified in this study or suggested to be present by previous studies (quenstedtite). Spectra of lepidocrocite and hematite are included for reference to their polymorphs. Wang et al. [1998] compiled the reference Raman spectra (using a 633 nm laser) of several Fe oxide and Fe oxyhydroxide samples from D. Morris (NASA-JSC).

Figure 2.

Standard Raman spectra of the secondary phases identified in this study plus hematite, lepidocrocite, and quenstedtite. Sources of the mineral standards shown here are (from top to bottom): (1) natrojarosite from D. Morris and D. Ming, (2) quenstedtite belonging to A. Wang, (3) a synthetic hematite (HMS3) from D. Morris, (4) akaganéite from Johnson Space Center (JSC) lunar rusty rock 66055,155, (5) goethite from Arizona State University (ASU) collection (WAR 3674), (6) and (7) lepidocrocite (LPS2) and synthetic maghemite (MHS3) also from D. Morris. The lepidocrocite spectrum is shown for reference to its polymorphs goethite and akaganéite and the hematite spectrum for comparison to its polymorph maghemite (hematite spectrum was acquired at 633 nm).

[16] Olivine alteration products commonly occur as fine-grained intergrowths whose components are frequently only poorly constrained and ill defined. In particular, the term “iddingsite” has been used as a catchall phrase for the reddish-brown alteration products of olivine. All three of the samples discussed in this paper contain reddish-brown alteration products that appear similar in plane-polarized light (PPL) but differ in mode of occurrence [Kuebler, 2009]. As mentioned above, some authors ascribe a more specific mode of formation, that of deuteric alteration, which occurs in situ as the magma crystallizes—that the concentration of volatiles (H2O, HCl, and H2SO4) increases in the residual melt as they are excluded from the growing crystals. When the melt becomes acidic enough, the volatiles begin to alter the primary minerals (McSween, personal communication; possible reference to Edwards [1938]). The ALHA 77005 iddingsite fits this definition, whereas the Lunar Crater and Mauna Kea iddingsite suggest surface alteration (see petrographic descriptions below for justification). Typically, the alteration products of both processes are poorly crystalline intergrowths of iron oxides (goethite, hematite) and phyllosilicates (smectites/saponite) that may also contain amorphous components (presence of goethite initially established by X-ray diffraction (XRD) [Sun, 1957; see also Gay and LeMaitre, 1961; Delvigne et al., 1979; Eggleton, 1984; Eggleton et al., 1987; Smith et al., 1987]). The amount of smectite produced is variable, abundant in some samples but lacking in others [Delvigne et al., 1979].

[17] Jarosite-group minerals [KFe3(SO4)2(OH)6] are hydrous secondary minerals whose compositions are highly variable; K is commonly replaced by Na but can also be replaced by H3O (hydronium substitution is more common in jarosite than in alunite, the Al3+ end-member, and is usually inferred where chemical analyses suggest a deficiency in cations and an excess of H2O [Ripmeester et al., 1986]). Jarosite forms most commonly as a supergene product of Fe sulfide oxidation in acidic settings, is less common in hydrothermal settings, and only rarely occurs as a hypogene mineral, even in S-rich volcanic settings [Stöffregen et al., 2000; Dutrizac and Jambor, 2000]. Jarosite formation requires a lower pH and more oxidizing conditions than alunite and is therefore less common (e.g., terrestrial occurrences: Rio Tinto, Spain; Goldfield, NV, USA; and Queensland, Australia). Supergene or hypogene alunite formed at or below the water table may be replaced by jarosite if the groundwater becomes more acidic or oxidizing (e.g., when a drop in the water table exposes sulfides to weathering [Burns, 1988; Stöffregen et al., 2000; Scott, 1987]).

[18] In the following section, we describe the petrographic differences of the iddingsite in these samples, particularly in the mode of occurrence, and provide as much geologic context as possible for the alteration setting of the terrestrial samples.

6 Geologic Setting and Sample Descriptions of Alteration

6.1 Lunar Crater, NV

[19] This sample is a vesicular and porphyritic basalt cobble collected from a pavement in the Lunar Crater Volcanic Field (LCVF, see Figure 3), Nevada; a 10 × 100 km alkali basalt field located in the center of the Great Basin, an internally drained region that includes most of Nevada and portions of Utah (see Figure 1 of Israel [1996], for the sample location 250 km north of Las Vegas [Israel et al., 1997; Bergman, 1984; Frazier and Schwimmer, 1987]). The LCVF is relatively isolated in comparison to other Quaternary volcanic fields and contains some of the youngest alkali basalts in the Great Basin. The region is underlain by Paleozoic sediments and intermediate to felsic Miocene to Oligocene volcanics. The first basalt flows formed during the late Pliocene (5.7 Ma), and volcanic activity migrated north through time, with the most recent flows erupted during the Holocene (10–15 ka [Bergman, 1984; Scott and Trask, 1971; Shepard, 1994; Shepard et al., 1995]). The region became increasingly arid during the Pliocene with the uplift of the adjacent Sierra Nevada Range, marked by the onset of desert conditions. At present, the mean annual temperature (MAT) is ~11 °C, and the mean annual precipitation (MAP) is ~12 cm. During the Pleistocene, the MAT is suggested to have been several degrees colder and the MAP not much greater than at present [Dohrenwend et al., 1987].

Figure 3.

Photos of the Lunar Crater South, NV lava flow (top, foreground) and the summit of Mauna Kea, HI (bottom) indicate the respective environments of alteration of these samples, although the Lunar Crater flow has experienced several pluvial episodes since eruption. The Mauna Kea photo is from http://www.ucolick.org/~bolte/AY4_00/week1/telescopes.html (accessed 8/2012).

[20] Shepard [1994] notes the presence of altered olivine in the Lunar Crater flow (unofficial designation, the flow adjacent to the Lunar Crater cinder cone; ~1.25 km in diameter) at his sample location but indicates that olivine in the nearby Black Rock flow are unaltered. Our sample was collected from a presumably related flow 8.5 miles south of Lunar Crater. None of our samples are from the interior of the flows, but similarly altered samples were observed on top of the cone and in the alluvium surrounding the Lunar Crater South flow. Per their K-Ar ages, the Lunar Crater flow is about 600 ± 30 ka; the Black Rock flow is younger and fresher, 350 ± 50 ka (Kargel [1986], in Shepard [1994]; Shepard et al. [1995]). The Lunar Crater flow is mantled by aeolian sediments at both sites (at least 1 m thick, one of Shepard's samples was collected from a depth of 1 m) and topped by a pavement, while the Black Rock flow retains more of its initial surface roughness [Wells et al., 1985; Shepard, 1994; Arvidson et al., 1993]. Shepard [1994] suggests that excess Ar may be present in the Black Rock flow and reports the morphology of the Black Rock flow to be more consistent with its much younger 36Cl and 10Be exposure ages, 35 and 40 ka [Kargel, 1986; Foland and Bergman, 1992; Shepard, 1994; Arvidson et al., 1993].

[21] The schematic map of Shepard's [1994] field site indicates the presence of a playa three miles east of his sample site (Lunar Lake). The K-Ar age of the Lunar Crater, Nevada (LC) flow indicates that it erupted during the Aftonian I interglacial episode. Lakes Lahontan and Bonneville formed during the pluvial period associated with the Wisconsinan Glacial Episode (10,000–70,000 years ago, in NW Nevada and NW Utah, respectively) of the Pleistocene, and Figures 9–84 of Frazier and Schwimmer [1987] indicate the presence of a lake (present-day Butterfield Marsh, 20 miles to the NE) in the vicinity of the LCVF at the same time. Saline lakes and more abundant groundwater may have accompanied earlier glacial episodes as well (core samples acquired near the shores of the Great Salt Lake, UT suggest that Lake Bonneville had two precursors [Flint, 1971]). The younger 36Cl and 10Be exposure ages for the Black Rock flow may explain why its olivine are not as altered. The 36Cl exposure ages of the LC flow range from 133 ± 19 ka to 1372 ± 1047 ka suggesting that these were affected by sediments from the salt lake and are unreliable.

[22] Olivine throughout the Lunar Crater sample are altered along their edges and fractures; these reddish-brown alteration products are optically homogeneous (Figures 4a and 4b, intergrowth too fine to be resolved petrographically, materials exhibit uniform extinction under cross-polarized light) and have irregular boundaries. In addition, the extinction angle of these materials parallels that of the host olivine, implying that the oxygen network of the goethite is inherited from the olivine [Brown and Stephen, 1959; Delvigne et al., 1979]. The alteration products are uniformly bright in reflected light (brightness does not vary with respect to the degree of alteration, suggesting that an iron oxide or oxyhydroxide is present throughout the alteration) and are isovolumetric with the host olivine. Green alteration products are also present in the more ferroan groundmass olivine and are presumably the unoxidized equivalent of the iddingsite [Baker and Haggerty, 1967; Delvigne et al., 1979]. We focus on the red alteration products in one large phenocryst (~5 mm in its longest dimension); smaller grains that contain both colors of alteration were analyzed but not studied in any great detail. The composition of this large phenocryst ranges from Fo51 to Fo94 (by EMPA), but analyses were focused in the vicinity of the alteration fronts (where compositions average ~Fo64) and not the core of the olivine. The appearance of the Lunar Crater iddingsite is similar to that featured in plates 102–109 of Delvigne [1998].

Figure 4.

Transmitted light images of altered olivine in the Lunar Crater, Mauna Kea, and ALH 77005 thin sections. (a) and (b) 10X PPL images of iddingsitized olivine from the Lunar Crater samples; altered olivine occur throughout these samples. (c) and (d) 10X PPL photos of altered olivine in the oxidized exteriors of the Mauna Kea samples (both olivine occur within 1 cm of the exterior of the sample). The location of traverse 5 is shown in Figure 4c, and the location of traverse 6 is shown in Figure 4d. (e) and (f) 10X PPL images of brown olivine (beige in this thin section) with patchy alteration (darker red-brown materials) and cross-cutting jarosite veins (yellow-brown) in thin section, 51 of ALHA 77005. The locations of traverses 1 and 4 are shown in Figure 4e, and traverses 2 and 3 are shown in Figure 4f. (g) and (h) 20X PPL images of altered chromites in ALHA 77005,51. (i) and (j) X-ray maps of the altered olivine shown in Figure 4e and altered chromite shown in Figure 4g. The X-ray maps in Figure 4i demonstrate the removal of Mg and influx of S and Si with alteration. The X-ray maps shown in Figure 4j indicate that the jarosite rimming this chromite is reacting with the clear feldspathic glass that surrounds it and implies that the jarosite is martian (clear glass predates fusion crust; if the jarosite were terrestrial, the glass would be altered to jarosite also).

[23] Cobbles within a pavement generally derive from the underlying flow, which is most permeable and susceptible to alteration during the first 50–200 ka after eruption (50 ka = estimated time of mantle formation on Lunar Crater flow [Shepard, 1994]; 200 ka = phase of initial flow degradation and accumulation of aeolian mantles in the three-stage models of Dohrenwend et al. [1987]; Arvidson et al. [1993]). Coarse-grained calcite and disordered carbon (identified by Raman spectroscopy) occur in vesicles, but their occurrences are not spatially correlated with the iddingsite and were probably deposited later (presumably derived from the aeolian mantle surrounding the flow). Clay may also be present, but the calcite is fluorescent and inhibits the detection of other phases.

6.2 Mauna Kea, HI

[24] The Hawaiian samples were collected from the Laupahoehoe flows on the southern flank of Mauna Kea. These are 4–65 ka post-shield volcanic flows. Sample 16G-30 is from the younger Laupahoehoe flow emanating from Puu Keonehehee, and 16J-30 is from the older flow; both flows were sampled from Observatory Road. The 16J collection site is ~100 m east of the 16G site (see Table 1a of [Kristall, 2000] for GPS coordinates). The summit of the volcano is cold and dry; the average low temperature is −2.0 °C and the average high 7.8 °C, with about 18.7 cm of precipitation, usually in the form of snow (Western Regional Climate Center Table on Wikipedia, 2012, http://en.wikipedia.org/wiki/Mauna_Kea, hereinafter referred to as Western Regional Climate Center Table on Wikipedia, 2012). Eruption of these flows was separated by the Makanaka glacial episode 13 ka [Wolfe et al., 1997]. Both are highly vesicular basalts, but 16G1-30 is more altered than 16J-30. 16G1-30 contains fine-grained calcite, clay (montmorillonite), and hydrous (opaline) silica in its vesicles (compositions reported in Table 7 of Kristall [2000]), while the groundmass of 16J-30 contains clasts of unaltered brown volcanic glass and abundant magnetite grains.

[25] Altered olivine only occur in the red, oxidized exteriors of these two samples (within 1 cm of the edge), probably the undersides that sat in and were exposed to moisture in the Mauna Kea soil. The degree of olivine alteration is highly variable from grain to grain, ranging from thin rims to almost complete replacement. In transmitted light, the Mauna Kea alteration appears similar to that of Lunar Crater in color, extinction, and morphology (Figures 4c and 4d). Alteration occurs along edges and fractures in olivine and has irregular boundaries, and the extinction angle of the alteration parallels that of the olivine. However, in the grains that exhibit well-developed alteration, the alteration is not uniformly bright in reflected light and is only bright in localized areas. Hematite or maghemite is present as grain coatings in the altered exterior of the sample as seen in reflected light and may be the locally reflective material in the olivine alteration products of this sample. Small olivine grains with various degrees of alteration in the oxidized rims were analyzed as well as an unaltered olivine in the middle of each thin section. These occurrences are similar to plates 201 and 202 in Delvigne [1998].

6.3 ALHA 77005

[26] ALHA 77005 is a lherzolitic shergottite consisting of two lithologies: light and dark. The olivine in both lithologies are light beige to brown in plane-polarized light, although not necessarily uniform in color (Figures 4e and 4f [Treiman et al., 2007]), and are annealed colorless near impact melt veins, indicating that the brown alteration products are martian [Smith and Steele, 1983]. The brown coloration is attributed to the presence of Fe3+ in the olivine crystal structure, but Raman and Mössbauer spectra indicate that these are not equivalent to terrestrial ferri-olivine [Ostertag et al., 1984; Burns, 1989; Treiman et al., 2007]. The brown olivine are not spatially associated with fusion crust, grain boundaries, or cross-cutting fractures and may be poikilitically enclosed by orthopyroxene, so it is likely that the brown color formed just before or during the solidification of the magma.

[27] Four thin sections of ALHA 77005 were examined petrographically while at JSC (,34; ,46; ,51; and, 52). Only two of these (,34 and, 51) contained significant alteration features beyond the presence of the brown olivine (patchy iddingsite alteration and later jarosite) and were selected for further study. Patchy alteration products are uncommon in the meteorite and appear to occur in olivine poikilitically enclosed by orthopyroxene (i.e., in the light lithology, confirmation of whether these materials are restricted to the light lithology requires examination of more thin sections). Similarly altered olivine occur in Y-793605 [Ikeda, 1997]. Likewise, the jarosite that is relatively abundant in these two thin sections is uncommon in the rest of the meteorite and appears to be rare among the martian meteorites (or under-reported; identified in five shergottites, see Table 1a of Kuebler et al. [2007]). The later jarosite crosscuts and overprints most of the phases in these two thin sections precluding a reflected light comparison with the iddingsite of the two terrestrial samples. We do not know of any instances of iddingsite in ALHA (or Y-793605) without jarosite. The chromite in these thin sections are also altered to jarosite. Both, 51 and, 34 were cut from potted butt, 13, which Smith and Steele [1984] indicate is from the interior of the meteorite. We have one thick section (,202) with fusion crust, but it contains no iddingsite or jarosite. The brown olivine are similar to plates 246 and 247 in the Delvigne [1998] and the iddingsite comparable to plates 102–109.

[28] The secondary K-Fe sulfates in thin sections, 51 and, 34 were reported to be potassium jarosite [KFe3+3(SO4)2(OH)6] and quenstedtite [Fe3+2(SO4)3*10(H2O)] by Smith and Steele [1983]. These phases were initially suggested to be preterrestrial because this meteorite contains little H2O and because its trace-element composition appears to be unaffected by weathering [Marvin, 1980; Biswas et al., 1980]. The martian status of these phases was later called into question after sulfates were reported to occur in the fusion crusts of other Allan Hills meteorites and because the sulfates clearly postdate the other alteration products [Gooding, 1981] (the MAT of the Allan Hills is −30 °C and the MAP too low to measure accurately [Buchwald and Clarke, 1989]). Similar materials coat fracture surfaces of the meteorite so a terrestrial origin was deemed likely [Smith and Steele, 1984]. Ikeda [1997] reports three varieties of alteration in another lherzolitic shergottite, Y-793605: Fe-K-S-P, Si-Fe-Mg, and Si-Al-Fe. The first two correspond to jarosite and iddingsite, and the third variety is reported to occur in or with maskelynite and glass and is probably amorphous silica.

[29] Discussion regarding the martian versus terrestrial provenance of the jarosite has re-opened as a result of the following: (1) the discovery of jarosite on Mars by the MER Rovers and (2) the suggestion that the jarosite in the fusion crust and lining fractures in the meteorite may have been remobilized from elsewhere in the meteorite during its passage through the terrestrial atmosphere [Treiman et al., 1993]. We observed one chromite with an altered rim of jarosite enclosed in and reacting with a clear, unaltered feldspathic glass in thin section, 51 that suggests the jarosite is martian (see Figure 4g; i.e., if the jarosite was an Antarctic alteration feature, the glass would be altered too). The iddingsite predates the jarosite and is also considered to be martian. The lack of K-bearing minerals within the primary assemblage may stipulate a martian origin, assuming that little K and S are available in the brines of the encapsulating Antarctic ice. Secondary free silica, goethite, a low-Al phyllosilicate, and Mg-Fe phosphates are also reported to occur in this meteorite [Wentworth and Gooding, 1993; Treiman, 2006].

7 Sample Analysis

[30] In this manuscript, we describe the effects of alteration along traverses as they pass from unaltered olivine across the alteration fronts and into the alteration products (locations inferred from reflected light microscopy), paying attention to where each alteration phase appears and the distance over which the reaction appears to take place. These data are used to constrain the physical processes of alteration. Raman spectra obtained within a given patch of iddingsite are generally similar throughout, but the peak positions of the poorly crystallized materials can be highly variable. Because of this inherent variability, less effort is devoted to describing changes in the spectral patterns inside the alteration. Raman peaks of the alteration phases are usually less noisy where the alteration is better developed relative to those acquired at the alteration front.

[31] For each sample, we present the results of the Raman analysis first, beginning with a general description of the alteration phases occurring in, or associated with, the olivine and then describe the spectral changes across the alteration front of one representative traverse. The Raman results are followed by a presentation of the corresponding electron microprobe analyses with a discussion of element mobility and an assessment of the mineral identifications where the Raman results are tentative or ambiguous.

7.1 Lunar Crater, NV

7.1.1 Raman Analysis

[32] A sequence of spectra from traverse 2 across an olivine alteration front in the Lunar Crater phenocryst is shown in Figure 5. Three groups of peaks are distinguished: (1) the olivine doublet or a “remnant” olivine feature (we refer to the phase as a remnant when the two olivine peaks can no longer be resolved), (2) four peaks attributed to goethite [FeO(OH)], and (3) a “polymerized” silicate phase. Goethite peaks occur at 245, 302, 390, and 478 cm−1, the strongest of which is the ~390 cm−1 peak. The polymerized silicate phase has two multicomponent spectral features in the 400–700 cm−1 and 700–1100 cm−1 regions that cannot be attributed to olivine, goethite, saponite, or any other smectite. The polymerized silicate phase in Lunar Crater has a strong, broad feature in the 400–700 cm−1 region where Si-Ob-Si peaks occur (Ob = bridging oxygen). It is centered at 630 cm−1 but has a shoulder near 690 cm−1. A weaker feature is located in the 700–1100 cm−1 region where Si-Onb peaks occur (Onb = nonbridging oxygen), centered at 935 cm−1, usually with a shoulder near 1020 cm−1. This phase is categorized as a polymerized silicate phase (or mixture of phases) according to the correlation of Wang et al. [1994], who relate the relative intensities of the Si-Ob-Si and Si-Onb Raman bands to the ratio of Onb to Ob bonds (see Figure 6). Silicates with Onb = 2 have relatively strong Raman Si-Ob-Si peaks in comparison to their Si-Onb peaks, and the peak positions of the Si-Onb bands increase as the number of Onb bonds decreases [Wang et al., 1994]. The stronger 630 cm−1 band suggests a degree of polymerization higher than that of pyroxene, whereas the peak position of the Si-Onb band suggests an incipient layered structure. These two constraints imply a structure whose degree of polymerization is between that of a double-chain and sheet silicate [Kuebler et al., 2003, 2004]. The presence of multiple peak components and large peak shifts suggest that the degree of polymerization is variable and that a mixture of phases may be present.

Figure 5.

Stack of spectra showing spectral changes across the alteration front of traverse 2 on the Lunar Crater phenocryst. Distances are reported relative to the starting point of each traverse (here, 2 µm steps between spectra). The alteration products contain goethite and two groups of peaks belonging to a “polymerized” silicate phase (see text for explanation); the group of peaks near 635 cm−1 represents Si-Ob-Si bonds, and the group of peaks near 935 cm−1 represents Si-Onb peaks. The olivine doublet degrades as the traverse approaches the alteration front (spectrum 01223144) and becomes a “remnant,” while the peaks belonging to the goethite and polymerized phase appear and strengthen as the traverse proceeds into the alteration.

Figure 6.

There is a direct correlation between the relative intensities of the Si-Ob-Si and Si-Onb Raman peaks and the ratio of Onb/Ob bonds in a crystal lattice. Raman Si-Ob-Si peak intensities are much stronger than those of the Si-Onb peaks in silicates with Onb < 2. The peak positions of the Si-Onb bands also increase as the number of Onb bonds decreases. The relative peak intensities and peak positions of the “polymerized” silicate phase in the Lunar Crater and Mauna Kea samples imply a structure whose degree of polymerization lies between that of double-chain and sheet silicate. The ~935 cm−1 feature is weaker than the ~630 cm−1 feature but stronger than those of phyllosilicates. The breadth of the peaks implies that the structure is poorly crystalline.

[33] Alteration begins as the olivine doublet degrades (broadens and weakens) into a variably developed remnant ~850 cm−1 in proximity to, but outside of, the alteration front. The olivine remnant disappears within 10 µm of the inside of the alteration front (which occurs at spectrum 01223144; see Figure 5). Goethite and polymerized silicate peaks appear in the olivine spectra ahead of the alteration front in the yellowish-brown fringes (with a possible contribution from alteration at depth in the thick section), but their peaks are weaker than the olivine doublet. Peaks of the goethite and polymerized phases only dominate the spectra acquired inside of the alteration. The polymerized phase is the strongest phase just inside the alteration front, but the spectra acquired here are usually noisy with a low signal-to-noise ratio. Peaks of both the goethite and polymerized phases strengthen and are better resolved deeper into the alteration, but the strengthening of the goethite peaks outpaces that of the polymerized phase so that goethite becomes the dominant phase inside the alteration. A broad, weak band in the OH stretching mode region (3400–3800 cm−1) appears in some spectra, but its peak positions are too low for goethite. Some spectra acquired just inside the alteration front have high backgrounds that may indicate the presence of a phyllosilicate clay (halloysite was reported to occur in the iddingsite fringes of olivine from the Baynton basalt of Australia by Eggleton [1984] and Eggleton et al. [1987], and we have found that saponite can fluoresce under a green laser), but this is speculation. A search for smectite was made at higher magnification (100X), but none was detected. Hematite peaks occur in some spectra within the alteration but are rare and may represent goethite that has dehydrated. This description of the alteration reaction holds for all of the Raman traverses acquired on the large phenocryst in the Lunar Crater sample.

[34] A couple of spectral features combine to indicate the alteration of olivine and provide information about the alteration process. When the ~820 and ~850 cm−1 peak positions are plotted against each other on the olivine calibration curve of Kuebler et al. [2006], the data points are found to scatter away from the curve (see Figure 7a; dark gray data points represent strong doublets whose peak height/width ratios are >10). This scattering of data points could be a potentially useful indicator of alteration during in situ measurements on Mars if the focal quality of the spectra is otherwise known to be good. Degradation of the olivine doublet concurrent with the appearance of goethite reflects the oxidation of Fe2+ to Fe3+, whereas the multicomponent features near ~630 and ~935 cm−1 indicate the presence of an additional phase (or phases).

Figure 7.

Data points from Lunar Crater traverses (a) 1–3 and (b) 9–11 plotted on the calibration curve of Kuebler et al. [2006]. Dark gray data points in both Figures 7a and 7b represent the curve-fit peak positions of strong olivine doublets—those whose 820 cm−1 peaks have peak height/width ratios >10 and whose 850 cm−1 peaks have height/width ratios >15. Black data points represent weaker doublets with peaks whose height/width ratios are <10 and <15, respectively, but also include doublets whose 820 cm−1 peak has a height/width ratio >10 but whose 850 cm−1 peak has a height/width ratio <15. Light gray data points in Figure 7a represent doublets acquired in unaltered olivine ahead of the alteration front. Olivine doublets disappear ~10 µm behind the alteration front in all traverses so all of the data points represent spectra acquired in the vicinity of the alteration front. The manner in which the data points deviate from the olivine calibration curve in Figure 7a is due to changes in the SiO4 bonding environment with alteration and to changes in the curve-fit peak positions with the diminishing strength of the doublets. Traverses 9–11 were acquired in approximately the same locations as traverses 1–3 but with the thick section turned 90° relative to the laser beam; data demonstrate that crystal orientation influences the pattern of scattering; alteration appears to have a preferential effect on the 820 cm−1 peak positions (see Figure 11).

7.1.2 Microprobe Data

[35] The microprobe data across the alteration fronts in the Lunar Crater phenocryst record a loss of MgO and SiO2 with progressive alteration and compositions that deviate from the Fo-Fa lines (along which stoichiometric olivine compositions plot) and trend toward the theoretical composition of goethite as shown in Figure 8 (~80 wt % FeO at 0 wt % SiO2). EMP analyses that correlate with the traverse 2 spectra shown in Figure 5 are provided in Table 2. Olivine analyses (those having oxide totals >97 wt %) were reduced using four oxygen atoms per formula unit and indicate the phenocryst to have compositions between Fo51 and Fo94. Iddingsite analyses (oxide totals <97 wt %) were reduced using three oxygen atoms per formula unit. Iron in the olivine is assumed to be in the Fe2+ state and in the Fe3+ state in the iddingsite. Magnesium numbers, Mg # = Mg/(Mg + Fe + Mn), were calculated for all of the analyses and demonstrate the removal of Mg relative to Fe. The microprobe data record little influx of other cations; the oxides of the other cations present in the alteration products (Cr2O3, TiO2, MnO, V2O3, Al2O3, CaO, ZnO) sum to less than ~2.2 wt % on average, rarely exceed 4.0 wt %, and are less than 7.2 wt % in all analyses. All microprobe analyses acquired within the alteration contain less SiO2 than the host olivine and do not support the presence of a smectite component. If present, the smectite is a relatively minor component, volumetrically, and does not contribute much to the measured compositions. Spot analyses taken in the olivine adjacent to the alteration tend to have low totals (between 99.0 and 99.5 wt %), and plots of the major oxides support the inference that material is being mobilized out of the olivine (Figure 8).

Figure 8.

(a) Plot of the EMP wt % FeO and MgO versus SiO2 data from the Lunar Crater phenocryst (EMP traverses corresponding to Raman traverses 1, 2, and 5–8). Unaltered olivine (stoichiometric) compositions fall along the dashed lines, data points taken in altered areas fall away from these trends. The FeO data points from altered olivine extrapolate to a value of ~81 wt % FeO at 0 wt % SiO2—consistent with the stoichiometric composition of goethite, while the MgO data points indicate the MgO content of the iddingsite (goethite + polymerized phase). (b) Plot of the minor oxide contents of the Lunar Crater analyses indicating a low influx of cations into the olivine structure during alteration (<6 wt % Al2O3, <2 wt % CaO or MnO). Al2O3 may be due to the removal of an earlier carbon coat with alumina grit.

Table 2. EMP Analyses Acquired along Raman Traverse 2 of the Lunar Crater Phenocryst
Line #55566162636465
  1. a

    “Distance along traverse” reported according to the spacing of points during acquisition of EMP data = 5 µm.

  2. b

    Corresponding spectra shown in Figure 5, Raman traverse step size = 2 µm.

  3. c

    All Fe in olivine assumed to be in 2+ state, all Fe in alteration 3+ state.

  4. Note: Olivine data reduced using four oxygen atoms per formula unit, alteration data reduced using three oxygen atoms per formula unit in goethite. Mn included in calculation of Mg# = Mg/(Mg + Fe2+ +Mn); n.d. = not detected.

Distance (µm)a50 µm45 µm20 µm15 µm10 µm5 µm0 µm
Notesalterationalterationalterationalterationalterationalteration frontolivine
Corresponding spectrumb~01223119~01223122~01223138=01223139~01223140=01223144~01223145
Corrected total94.6794.1994.7594.7395.9398.88100.37
sum octahedral1.331.341.351.351.311.992.00
sum tetrahedral0.650.630.610.600.681.001.00
Total cations1.971.971.961.951.992.993.00
Mg# of ol0.980.970.970.960.980.580.60

7.2 Mauna Kea

7.2.1 Raman Analysis

[36] Several olivine grains of various degrees of alteration were studied in the two Mauna Kea samples, from unaltered to almost completely altered. Both the moderately and highly altered grains have alteration whose brightness is variable in reflected light. A sequence of Raman spectra across an alteration front in the moderately altered olivine (traverse 5) of 16J-30 is shown in Figure 9 and briefly described here. In general, the spectral patterns of the alteration products in the Mauna Kea samples are similar to those of Lunar Crater except that the peak positions of the polymerized silicate occur at higher Raman shifts, reflecting the more magnesian olivine compositions, and the goethite peaks are lacking (Figure 10). The Si-Ob-Si bands of the polymerized silicate phase in the Mauna Kea samples (in the 400–700 cm−1 region) are centered near 640 cm−1 with shoulders near 710 cm−1. The weaker, overlapping Si-Onb features (in the 700–1100 cm−1 region) are centered at ~945 cm−1 and ~1035 cm−1. Instead of goethite, many spectra have two broad features below 500 cm−1 with variable peak positions (Figure 10). These could be the secondary peaks of an iron oxide (ilmenite) whose primary peak (~670 cm−1) is obscured by the polymerized silicate, but ilmenite has a strong peak near 225 cm−1, which is missing from these spectra. A broad peak whose center varies from 1300 to 1360 cm−1 is common and suggests hematite or maghemite; the absence of good 220 and 290 cm−1 peaks (hematite) suggest maghemite. As noted above, hematite (or maghemite) is present as grain coatings in the alteration rinds of the Mauna Kea samples, but the quality of the peaks (weak, broad, noisy) imply that it is poorly crystalline.

Figure 9.

Stack of spectra showing spectral changes across the alteration front of traverse 5 on Mauna Kea sample 16J-30 (made across the olivine shown in Figure 4 C); 2 µm step size, alteration front located at spectrum 09080457. Secondary phases include a polymerized silicate and traces of an iron oxide, inferred to be maghemite from the microprobe data.

Figure 10.

Comparison of iddingsite spectra from the Lunar Crater and Mauna Kea samples. The Lunar Crater iddingsite contains distinctive goethite peaks, but the Mauna Kea spectra only have weak, broad features in this area, which may belong to maghemite. The “polymerized” phase of the Mauna Kea sample has higher peak positions reflecting more magnesian olivine compositions. The 853 cm−1 peak in the top spectrum represents an olivine “remnant” (our phase identification when the olivine doublet can no longer be resolved); weak 824 and 848 cm−1 peaks in the bottom spectrum indicate that olivine is still present at this location.

[37] Degradation of the olivine doublet in the Mauna Kea samples differs from that in Lunar Crater such that the olivine doublet is observed further into the alteration products, up to 30 µm, and only becomes a “remnant” and disappears in proximity to alteration products that are bright in reflected light (Figure 9, alteration front occurs at 09080457). The doublets may even appear sharp where the alteration appears to be well developed (more opaque) in transmitted light (as at the end of traverse 5). When the doublet does degrade into a remnant near the reflective material, it does so over a short distance, within 5–10 µm. As in the Lunar Crater sample, the peaks of the alteration phases appear in the olivine ahead of the alteration front, and spectra acquired at, or just inside, the alteration front sometimes have high backgrounds.

[38] Data from the entire length of traverse 5 are shown in Figure 11a. The curve-fit peak positions of the Mauna Kea olivine largely fall along the olivine calibration curve between Fo60 and Fo90; some scatter is observed to either side of the curve, but the data do not spread away from the curve systematically as alteration progresses (unlike the Lunar Crater data points shown in Figure 7a). This finding supports the interpretation that alteration has had less of an effect on the olivine crystal structure in these two samples. The same 820 cm−1 peak height/width cutoff ratio (=10) was used to define “good” data points (dark gray in Figures 7 and 11) in the Mauna Kea samples, although some of the data points defined as good were acquired behind the alteration front, particularly in traverse 5. The observed spread of the good data points along the calibration curve in Figure 11 can be explained by compositional zoning in the host olivine. The scattering of points away from the calibration curve in the Mauna Kea samples may also be due in part to zoning (see the discussion in Kuebler et al. [2006] regarding the degree of zoning and its influence on the breadth of the olivine doublet peaks and their peak positions) or to a decrease in the strength of the peaks with alteration.

Figure 11.

Data points from (a) traverse 5 on Mauna Kea and (b) traverse 1 on ALHA 77005,51 on olivine calibration plot. Again, dark gray data points represent the curve-fit peak positions of olivine doublets with strong peak height/width ratios, and the black data points represent doublets with weaker peak height/width ratios (same definitions as in Figure 7). Traverse 5 crossed the olivine phenocryst shown in Figure 4c; most of the data points fall along the median curve of the calibration plot and indicate zoning in the host olivine. Only a few data points scatter more than 1–2 cm−1 away from the calibration curve so we interpret the structure of this olivine to be less affected by alteration. ALHA 77005 data points from traverse 1 on thin section, 51 fall away from the calibration curve in a different manner than that observed for the Lunar Crater phenocryst (see Figure 7), probably because the c-axis of the olivine is at a different orientation relative to the direction of laser polarization.

[39] The ~640 and ~710 cm−1 peaks of the polymerized phase emerge over a short distance (<10 µm) across the alteration front in traverse 5 (which appears sharp in both transmitted and reflected light) and imply that the reaction occurs abruptly over a short distance. This is probably true of most reaction fronts, but the alteration is sometimes detected at depth ahead of the alteration front (as observed in reflected light). Therefore, some traverses give the impression that the reactions are more drawn out than others (e.g., the reaction at the end of traverse 1 or across the alteration front in traverse 6).

7.2.2 Microprobe Data

[40] Microprobe analyses that correlate with the traverse 5 spectra shown in Figure 9 are provided in Table 3. Without concrete phase identifications from the Mauna Kea alteration and because the Raman data suggest that the olivine structure persists further into the alteration, all EMP analyses were reduced as olivine using four oxygen atoms per formula unit, and the Fe2+/Fe3+ ratios varied to bring the total cation sum to 3.0. Even though the totals are low, the stoichiometric calculations produced octahedral cation sums >1.98 for all olivine analyses and >1.80 for most iddingsite analyses.

Table 3. EMP Analyses Acquired Across Two Alteration Fronts in Raman Traverse 1 on ALHA 77005,51
Line #131132133136138
  1. a

    “Distance along traverse” reported according to the spacing of points set during acquisition of EMP data = 5 µm.

  2. b

    Corresponding spectra shown in Figure 9, Raman traverse step size = 1 µm.

  3. c

    Fe2+/Fe3+ ratio varied to satisfy stoichiometry (sum oct = 2.0), all Fe in unaltered olivine should be in Fe2+ state. Note: Mn included in calculation of Mg#.

Distancea0 µm5 µm10 µm25 µm35 µm
Corresponding spectrumb0908045509080459090804630908047509080483
Corrected total98.394.894.996.993.5
sum octahedral2.
sum tetrahedral1.000.940.970.990.99
Total cations3.
Mg# of ol0.790.710.690.700.56

[41] The Mauna Kea alteration products also demonstrate a loss of MgO and SiO2 and an increase in FeO (see Figure 12). These olivine are more magnesian (Fo63–88) than those analyzed in the Lunar Crater sample, so the data trend toward an alteration product containing less FeO than goethite, ~70 wt % FeO at 0 wt % SiO2, consistent with the presence of either hematite or maghemite. As with the Lunar Crater alteration, the Mauna Kea samples show little influx of other elements; the sum of cations other than FeO and MgO is ~2.2 wt % (on average) and never exceeds 4.0 wt %. Again, EMPA of the Mauna Kea alteration products indicate less SiO2 than the host olivine; if smectite is present, it is volumetrically minor.

Figure 12.

(a) Plot of wt % FeO and MgO versus SiO2 from the EMP traverses parallel to Raman traverses 5–7 on both altered and unaltered olivine in the Mauna Kea 16J-30 sample. Unaltered olivine compositions fall along the dashed lines and indicate more magnesian olivine compositions than in the rim of the Lunar Crater phenocryst. The FeO data points extrapolate to a value near ~70 wt % FeO at 0 wt % SiO2 (suggesting the presence of hematite or maghemite), and the MgO data points indicate that the Mauna Kea iddingsite, like the host olivine, is more magnesian than that in LC. (b) Plot of the minor oxide components in the Mauna Kea alteration, indicates an even lower influx of cations into the olivine of this sample than into the Lunar Crater phenocryst (all minor oxides <2 wt %).

7.3 ALHA 77005

7.3.1 Raman Analysis

[42] The ALHA 77005 thin sections indicate multiple generations of alteration; early alteration products include the brown color of the olivine (predating the injection of impact melt [Smith and Steele, 1983, 1984]) and “patchy” olivine alteration that resembles the iddingsite of the LC and Mauna Kea, Hawaii (MK) samples. A short traverse was made in a brown olivine that is annealed colorless near a pocket of impact melt in ALHA, 202 to identify the source of the brown coloration. No major spectral changes were observed along this traverse, but the brown olivine have an extra (minor) peak near 660 cm−1. These peaks are too small to confidently attribute to any single phase but could indicate the presence of magnetite (unfortunately, the epoxy used to make many thin sections has peaks at 640, 670, and 735 cm−1 that may appear as small peaks in many olivine spectra). No goethite or hematite was detected in the patchy alteration; instead, two broad peaks at 300 and 390 cm−1 and another near 720 cm−1 (frequently with a shoulder ~685 cm−1) suggest the presence of its polymorph, akaganéite (compare spectrum 05260817 in Figure 13 to the standard spectrum shown in Figure 2). Later veins of jarosite cut across the thin section, altering most primary phases and over-printing the patchy alteration. No quenstedtite (suggested to be present by Smith and Steele [1983]) or amorphous silica was detected by Raman spectroscopy, but EMPA indicate the presence of amorphous silica (see below).

Figure 13.

Stack of spectra showing spectral changes across the alteration front and into the patchy alteration near the beginning of traverse 1 on olivine 10 of ALHA 77005,51 (upper left olivine shown in Figure 4e; 1 µm steps between spectra, alteration front located at spectrum 05260822). Spectra indicate the presence of secondary akaganéite and jarosite.

[43] Despite the low laser power (<5.5 mW), the Raman laser left a trace in the jarosite-rich portions of some traverses, suggesting that oxidation occurred and that a lower laser power (or N2 purge) should have been used. Akaganéite (a polymorph of goethite) was detected instead of hematite and, like jarosite, is a nominally Fe3+-bearing mineral. Ferrous iron was required to reduce the microprobe analyses of the olivine-hosted jarosite and produce the proper cation sums (see discussion below); we therefore infer that Fe2+ is present in the jarosite and that the phase being oxidized is jarosite.

[44] Spectral changes across the alteration fronts in ALHA 77005 are complex because multiple alteration phases are present and because the jarosite spectrum contains several peaks. As shown in Figure 13, the olivine doublets diminish over a distance of about 15 µm and are replaced by a minor jarosite peak at 870 cm−1 inside the patchy alteration. The akaganéite is most easily detected near the fringes of the iddingsite before the jarosite peaks come to dominate the spectra. Weak jarosite peaks appear several spectra ahead of the alteration front (spectrum 05260822) but strengthen and sharpen inside the alteration. Jarosite has a relatively large Raman cross-section so this phase dominates most of the spectra acquired inside the alteration. Instead of a polymerized silicate, ALHA contains variants of the akaganéite spectrum having broad 640 and 670 cm−1 peaks (or a single ~660 cm−1 peak) in the Si-Ob-Si region but lacking a Si-Onb feature. The 310 and 390 cm−1 peaks of the akaganéite are sometimes masked by jarosite. Raman spectra of the olivine- and chromite-hosted jarosite of ALHA 77005 are shown with three standard jarosite spectra in Figure 14 (a Chinese jarosite from A. Wang, an Iconofile synthetic jarosite pigment, and natrojarosite from Mauna Kea HWMK515—both from D. Morris and D. Ming; see Ming et al. [1996] for an XRD pattern of HWMK515 and Chio et al. [2005] for a Raman spectrum of natrojarosite).

Figure 14.

Comparison of jarosite spectra from ALHA 77005 (a) olivine and (b) chromite to three standard jarosite spectra. Most peak positions are similar and vary <10 cm−1 between samples. Only the 430 cm−1 and OH peak positions vary significantly. Variations in the 430 cm−1 peak positions are probably due to the presence of a second peak or shoulder near 450 cm−1. The OH peak positions vary ~40 cm−1 between the five spectra; the high OH peak positions in the chromite-hosted alteration products may be due to the presence of elevated MgO or Cr2O3 concentrations. Additional peaks near 471 and 720 cm−1 in the ALHA 77005 spectra belong to akaganéite (compare to reference spectrum in Figure 2). The Chinese jarosite sample #AW 82E-68 (c) belongs to A. Wang, and the Mauna Kea natrojarosite HWMK515 (d) and Iconofile jarosite pigment (e) are from D. Morris and D. Ming (see Ming et al. [1996] for XRD pattern of HWMK and Chio et al. [2005] for a discussion on the Raman spectra of Fe sulfates).

7.3.2 Microprobe Data

[45] Electron microprobe analyses of locations correlating with the traverse 1 spectra shown in Figure 13 are provided in Table 4. EMP analyses of the olivine (analyses having oxide totals >97% and typically 36–38 wt % SiO2) were reduced using four oxygen atoms per formula unit, the iddingsite alteration (analyses having totals <97 wt %, more SiO2 than the host olivine, and usually containing some SO3) with three oxygen atoms per formula unit, and the jarosite alteration (analyses having >12.5 wt % SO3 but frequently containing SiO2) with 11 oxygen atoms per formula unit. All Fe2+/Fe3+ ratios were varied to bring the sum of the octahedral cations to 2.0 for olivine, the total cation sum of the iddingsite as close to 2.0 as possible, and the sum of the B cations to 3.0 for jarosite. Our jarosite cation assignments are based on the cation assignments of Scott [1987], who states that the A sites contain large mono- or divalent cations in 12-fold coordination [Na, K, Ag, NH4, H3O, Ca, Pb, Ba, Sr, Ce, and other rare earth elements (REE)], that the B sites are occupied by cations in octahedral coordination (Al, Fe3+, Cu, or Zn), and that the X sites may contain PO4, AsO4, CO3, SbO4, CrO4, or SiO4 in addition to SO4. The ALHA 77005 jarosite does not have an end-member composition so we included Fe2+, Mn, Mg, Ca, Na, and Ni in the A cation sums, Al and Ti in the B sums, and SiO2, Cr2O3, and P2O5 in the X sums. Although Scott [1987] does not specifically state that Fe2+ may substitute into the A cation sites, there was not enough Na2O or CaO to make up the difference so we found it necessary to include some Fe2+ with the A cations in order to produce the proper B cation sums. If all of the Fe was assumed to be in the Fe3+ state, the B cation sums jumped into the hundreds. However, adjusting the Fe2+/Fe3+ ratio to satisfy the B cation sums produced high A and X cation sums (A sums should = 1 but ranged from 1.5 to 2.9, and X sums should = 2 but ranged from 3.4 to 4.4). Although Scott [1987] suggests that SiO2 may substitute into jarosite, some of the SiO2 in the jarosite analyses may be due to the broad microprobe beam (overlap onto amorphous silica), and Figure 15 indicates that SiO2 is negatively correlated with K2O (peaks in the K2O profile coincide with dips in the SiO2 profile). We left the SiO2 in the X cation sums but assume that jarosite occurs as a mixture with amorphous silica (see below). Other cation assignments were based on valence, but these elements (Mg, Mn, Ni, and Ti) are not present in great abundance and were included mainly so that all of the elements analyzed in the routine would be accounted for. We acknowledge that the calculated Fe2+/Fe3+ ratios are not unique and may be in error because (1) a broad beam was used for the EMP analyses, (2) the alteration phases are hydrated (totals are low), and (3) there may be unanalyzed components (e.g., REE—although REE are not implied by the Raman spectra).

Table 4. EMP Analyses Acquired Along Raman Traverse 1 on ALHA 77005,51 Going from Olivine into Alteration
Line #pt 54pt 56pt 57pt 59pt 60pt 61pt 63pt 66pt 70pt 77
  1. a

    “Distance along traverse” reported according to the spacing of points during acquisition of probe data = 1.16 µm.

  2. b

    Corresponding spectra shown in Figure 13, Raman traverse step size = 1 µm.

  3. c

    Analyzed as SO2 and converted to SO3 by multiplying by (80.0642/64.0648 = 1.24973776551242).

  4. d

    Fe2+/Fe3+ ratios varied to satisfy stoichiometry; to bring sum oct to 2.0 for olivine and sum B as close to 3.0 as possible for sulfate.

  5. Notes: Mn included in calculation of Mg# = Mg/(Mg + Fe2+ +Mn). Olivine data reduced using four oxygen atoms per formula unit, jarosite using 11 oxygen atoms, and iddingsite three oxygen atoms. Cation sums for olivine: sum oct = (Al, Ti, V, Cr, Fe2+, Fe3+, Mn, Mg, Zn, Ca, Na, K, Ni), sum tet. = (Si, P, S), Cation sums for sulfate: A = (Fe2+, Mn, Mg, Ca, Na, K, Ni); B = (Al, Ti, Fe3+); X = (Si, P, S, Cr). Cation sum for iddingsite alteration include all cations. Alteration front located at spectrum 05260822 (pt. 63).

Distancea61.48 µm63.8 µm64.96 µm67.28 µm68.44 µm69.6 µm71.92 µm75.4 µm80.04 µm88.16 µm
Phase identificationiddingsiteiddingsiteolivineolivineolivineolivineiddingsiteiddingsitejarositeSi-rich alt
Corresponding spectrumb052608120525081405250815052608170526081805260820052608220526082405260838
-O = F0.
-O = Cl0.
New Total91.7989.7897.4797.5096.3797.4290.9587.5286.3288.97
Corrected total91.889.897.597.596.497.491.087.586.489.0
For olivine         
sum octahedral    
sum tetrahedral    
Total cations    
For sulfate         
A cations        1.86 
B cations        3.00 
X cations        4.16 
Total cations        9.03 
For iddingsite        
Total cations2.012.01    2.032.03 2.01
Figure 15.

This figure was created using the quantitative analyses from microprobe traverse a on ALHA 77005,51, which corresponds to Raman traverse 1. Data for the major olivine elements (MgO, FeO, and SiO2) are at the top. The Mg profile indicates its removal by alteration. FeO also appears to have been mobilized from the leading edge of the alteration front where it is negatively correlated with SiO2 (negative peaks left of center) but is enriched at the trailing edge of the alteration where it is positively correlated with SiO2 (three peak pattern to the right of center). The K2O and SO3 profiles correlate well with the middle and trailing edge of the alteration in the FeO profile; the same three peaks are seen in the Na2O and P2O5 profiles suggesting that a phosphate supplied some of the cations to the jarosite. A few Al2O3 peaks also correlate with jarosite; the last Al2O3 peak correlates with the single CaO peak.

[46] Figure 15 demonstrates that MgO has largely been removed from the altered area. FeO is negatively correlated with K2O and SO3 at the leading edge of patchy alteration but is correlated with them in the middle and end of the traverse (jarosite); this suggests an oxidizing reaction (Fe2+ → Fe3+). FeO is negatively correlated with SiO2 throughout the traverse. The pattern of three positive peaks at the trailing edge of the patch of alteration (right side of Figure 15) indicates a good correlation between FeO, K2O, Na2O, SO3, and P2O5, suggesting that a phosphate supplied some of the cations for the jarosite. The Na2O and P2O5 profiles are similar in the middle of the traverse, suggesting some Na substitution into the jarosite. Most Al2O3 peaks correlate with the jarosite but not all; the last Al2O3 peak correlates with the single CaO peak (addition of cations from anorthite? Si profile does not correlate). Al2O3 correlates with SiO2 at the beginning of the traverse but not at the end (where the plot indicates an influx of Al2O3 and loss of SiO2). The CaO content is <1 wt % throughout the most of the traverse, but there is a narrow peak to 2.8 wt % near the trailing edge of the patch of alteration (see Figure 15). This CaO peak correlates with dips in K2O, SO3, Na2O, and P2O5 pattern, suggesting that it belongs to another phase; we do not know what this might be (there is no evidence for Ca carbonate or Ca sulfate in the Raman spectra, and the presence of jarosite suggests an acidic environment, possible smectite?).

[47] The brown host olivine in ALHA 77005 have compositions similar to the Mauna Kea olivine but are less strongly zoned, Fo68–74. Unlike the Lunar Crater and Mauna Kea samples, plots of the EMP data through the patchy alteration in ALHA 77005 indicate the presence of both silica-rich and silica-poor alteration products (trends reflect later jarosite deposition, see Figures 15 and 16). The chemical trends indicate that the silica-rich alteration phase has SiO2 contents ranging up to ~70 wt % (the highest silica contents occur in microprobe analyses with ~5 wt % SO3 and oxide totals ~88 wt %). This is greater than the silica content of smectite (which has 36–38 wt % SiO2), suggesting the presence of amorphous silica. Amorphous silica was not detected in the Raman spectra—presumably because amorphous silica has a relatively small Raman cross-section versus jarosite. A broad microprobe beam (5 µm) was used to analyze the ALHA 77005 alteration (multiple phases analyzed simultaneously) so the SiO2 content of the pure phase is probably higher than 70 wt %. The trend of the FeO data from the unaltered olivine into the silica-poor secondary materials extrapolates to an FeO content appropriate for jarosite (~40 wt % FeO at 0 wt % SiO2; see Figure 16) and do not indicate the presence of akaganéite. The linear trend of the FeO data points in Figure 16 suggests that this is a mixing line between a low-SiO2 jarosite and amorphous silica. The trend of the MgO data points indicates that the silica and jarosite both have <6.5 wt % MgO.

Figure 16.

(a) Plot of wt % FeO and MgO versus SiO2 data from the EMP traverse parallel to Raman traverse 1 on ALHA 77005,51 olivine 10. Multiple alteration phases are present; data points with SiO2 contents approaching 75 wt % indicate the presence of an amorphous silica phase. The FeO data points extrapolate to a value near 40 wt % FeO at 0 wt % SiO2, consistent with the presence of jarosite. The trend of the MgO data points indicates that the jarosite and amorphous silica contain <6.5 wt % MgO. The microprobe data give no indication of the akaganéite because of the later jarosite. (b) Plot of the minor oxide contents of the ALHA 77005,51 analyses indicate that the influx of cations into the martian olivine was also minor (all minor oxides <3 wt %).

7.3.3 Comparison of Jarosite in Olivine and Chromite (Raman Spectra and EMP Data)

[48] Like the alteration associated with the olivine, the chromite-hosted alteration products in ALHA 77005 are highly variable in composition. Representative analyses of a few ALHA 77005 chromite and their associated alteration products are shown in Table 5a for comparison with the analyses of the olivine-hosted alteration shown in Table 4. Table 5b shows EMP analyses of the feldspathic glass (Or1.5-2.7An41–47Ab50–58, labradorite) that surrounds and appears to have reacted with the jarosite-altered chromite mentioned above (see Figures 4g and 4j). The analyses in Table 5a demonstrate the range of compositions observed in the chromite alteration products. Chromite analyses were reduced using four oxygen atoms per formula unit and varying the Fe2+/Fe3+ ratios until the total number of cations equaled 3.0. As with the olivine-hosted jarosite, the chromite-hosted jarosite was reduced using 11 oxygen atoms per formula unit, and the Fe2+/Fe3+ ratios varied until the number of B cations equaled 3.0. We used the same formulas for calculating the cation sums of the jarosite residing in the chromite as for that in the olivine but did not encounter the same issue with the Fe2+/Fe3+ ratios and B cation sums.

Table 5a. EMP Analyses of Two Chromites in ALHA 77005,51 with Their Alteration Products
Chromite #Chromite 9Chromite 42
  1. a

    Analyzed as SO2 and converted to SO3 by multiplying by (80.0642/64.0648 = 1.24973776551242).

  2. b

    Fe2+/Fe3+ of chromite was varied to bring cation total to 3.0000 and B cation sum of alteration as close to 3.0 as possible.

  3. c

    Chromite data reduced assuming four oxygen atoms per formula unit and the sulfate data using 11 oxygen atoms per formula unit (assuming jarosite). Sulfate sums: A = Fe2+ + Mn + Mg + Ca + Na + K + Ni, B = Al + Ti + V + Zn + Fe3+, X = Si + Cr + S + P. Chromite sums: A = Fe2+ + Mn + Mg + Zn + Ca + Na + K + Ni; B = Si + Al + Ti + V + Cr + Fe3+.

Phase idchrchraltaltaltaltaltaltaltaltaltchrchr
Forlocation seepic30pic30pic30pic30pic30pic47pic47pic47pic47pic47pic 47pic47pic47
-O = Cl0.
New total98.1598.0181.1680.4771.6067.4171.7384.2974.7775.6277.0096.7895.63
Corrected total98.398.383.682.875.470.274.586.477.578.
For sulfatec             
Sum A  1.030.371.190.560.481.030.660.730.83  
Sum B  1.991.503.002.973.002.402.932.713.00  
Sum X  3.213.872.152.442.603.022.582.952.22  
Total cations  6.235.746.355.986.086.456.176.396.05  
For chromite             
sum A1.111.12         1.051.20
sum B1.891.88         1.951.80
Total cations3.003.00           
Opaque end-members (in %)           
(Al + Ti + Cr + Fe3+ + Mg)1.01.0         1.01.0
% mgt2.33.2         2.54.7
% chr56.551.3         76.257.7
% ulvo10.211.2         4.618.4
% spinel31.034.3         16.719.2
Table 5b. EMP Analyses of Glass Surrounding Chromite 42
Line #50/151/152/153/154/1
  1. a

    Fe2+/Fe3+ adjusted to bring cation total as close to 5.0 as possible.

  2. b

    Reduced the glass analyses surrounding chr 42 as feldspar, points 1−3 have the composition of labradorite, but the last two glass analyses are mixed with the chromite, the jarosite around the chromite, and adjacent pyroxene.

Notesglass aroundglass aroundglass aroundglass aroundglass around
 chr 42, pt 1chr 42, pt 2chr 42, pt 3chr 42, pt 4chr 42, pt 5
Phase idfeld glassfeld glassfeld glassglassglass
For location seepic 47pic 47pic 47pic 47pic 47
-O = Cl0.
New Total96.6795.5996.0495.4696.08
corrected sum96.7395.5996.1196.0896.68
Fe +
Fe +
Sum tet.b3.834.
Sum oct1.160.981.011.741.71
Total cations4.994.985.014.994.99

[49] The chromite-hosted alteration products are relatively enriched in Cr2O3 (ranging from 2.0 to 37 wt %), P2O5 (0–12.7 wt %), and Al2O3 (up to 8.6 wt %) versus that in the olivine (Cr2O3, P2O5, and Al2O3 all <3.0 wt %). CaO and Na2O contents are low (<1 wt %) in both. Silica contents range up to 31 wt % in the chromite-hosted alteration and up to 72 wt % in the olivine-hosted alteration products (again, implying silica-saturated fluids and the formation of amorphous silica with the jarosite). The sulfate content of the alteration in both cases is highly variable, ranging from 11 to 20 wt % SO3 in the chromite alteration and from 1.5 to 20 wt % SO3 in the olivine alteration, but both have less than the theoretical SO3 content of jarosite (32 wt % SO3).

[50] Raman spectra of the olivine and chromite-hosted jarosite are similar despite the chemical variability indicated in Tables 4, 5a, and 5b (see Figure 14). Most of the variation is in the peak position of the peak near 430 cm−1 and the OH peaks. Variation in the 430 cm−1 peak position is mostly due to the presence of a second peak near 450 cm−1 (where the two are better resolved) and the variable peak heights of these two peaks. Some Raman spectra of the chromite-hosted jarosite have additional peaks near 471 and 720 cm−1, which probably belong to akaganéite. Variation in the OH peak positions probably reflects the elevated MgO or Cr2O3 contents of the jarosite. The chromite-hosted jarosite spectrum shown in Figure 14 is from chromite 9, which contains 5.2 wt % MgO and 6.9 wt % Cr2O3 (Table 5a) and has an OH peak position of 3427 cm−1, whereas the olivine-hosted jarosite spectrum is from olivine 11 in traverse c with 0.4 wt % MgO and 1.3 wt % Cr2O3 and has an OH peak position of ~3412 cm−1. A soil associated with the impact melt glass in fractures of thick section ALHA 77005,202 was found to contain goethite and could be the source of Fe for the jarosite.

8 Inferences Regarding Alteration

8.1 Inferences Regarding the Setting of Alteration

[51] We chose to group the patchy alteration products of ALHA together with the Lunar Crater and Mauna Kea alteration products on the basis of their similarity in appearance. However, ALHA 77005 is a lherzolite (a deep-seated igneous lithology), not a basalt, and the iddingsite appears to be restricted to the light lithology so the patchy alteration is inferred to have formed at depth, fitting the definition of deuteric alteration given by McSween (personal communication; possible reference to Edwards [1938])—reaction of olivine with acidic volatiles (H2O, HCl, H2SO4) that became concentrated around the crystal during the final stages of crystallization. More thin sections will have to be studied to confirm that the ALHA iddingsite is restricted to the light lithology.

[52] Altered olivine only occur in the oxidized surface rinds (outer 1 cm) of the Mauna Kea samples and are <65 ka surface weathering products formed atop of Mauna Kea sometime after extrusion. The Lunar Crater iddingsite is also attributed to surface alteration (speculation that the flow was more susceptible to alteration during the pluvial episodes of the Pleistocene before the accumulation of its aeolian mantle or simply to a longer duration of exposure, ~600 ka). The ALHA, LC, and MK alteration products are optically continuous with their host olivine and presumed to have formed by topotactic growth of the secondary phases. Raman spectra of the Lunar Crater and Mauna Kea alteration products demonstrate their iddingsite to be fine-scale intergrowths of a polymerized silicate with either goethite or maghemite. The Lunar Crater alteration products are uniformly bright in reflected light, but the Mauna Kea alteration are not, indicating a greater proportion of FeO(OH) in LC. The ALHA iddingsite is also a fine-scale intergrowth containing akaganéite overprinted by a later generation of jarosite (causing it to be dark in reflected light so its reflected light character cannot be compared with LC and MK).

[53] An unresolved observation regarding the Lunar Crater iddingsite is the fact that olivine throughout the samples are altered (their occurrence is not restricted to the outer rind as in the MK samples). Olivine throughout the sample Shepard collected from a meter's depth in the mantle of the LC flow are similarly altered. The Lunar Crater flows are positive features (>100 feet thick) that lie above the local modern groundwater table as indicated by Lunar Lake and Hot Creek (the elevation of Lunar Lake is about the same as the bottom of the Lunar Crater cinder cone, and the elevation of the base of the Lunar Crater South flow is about 240 feet above Hot Creek). The distribution of the altered olivine is not spatially associated with drainage patterns on the flows, and there is no evidence for hydromagmatic volcanism. No sulfates are present in the thick sections to suggest evaporite processes and only minor carbonate (inferred to derive from the aeolian mantle). At present, Nevada is dry with a lower MAP than Mauna Kea, and Dohrenwend et al. [1987] indicates that the MAT and MAP of the Basin and Range during the Pleistocene was similar to that of modern-day Mauna Kea, yet the abundance of the altered olivine and the greater hydration state of the goethite in the LC samples versus the maghemite in the MK samples imply that more water was available during alteration.

8.2 Alteration Reactions, Water/Rock Ratios

[54] The alteration products of the terrestrial samples include iron oxides and oxyhydroxides with a poorly crystallized polymerized silicate. Coincidence of the iron oxides and hydroxides with the degradation of the olivine doublet reflects the concurrent oxidation of iron with alteration. The chemical trends of the Lunar Crater and Mauna Kea samples demonstrate the removal of Mg and Si and indicate the formation of goethite and maghemite (or hematite), respectively. The ALHA 77005 alteration products indicate the removal of Mg and Fe and the formation of akaganéite but indicate the addition of Si, S, and K associated with the jarosite and amorphous silica. The chromite alteration products in ALHA also contain both jarosite and amorphous silica so the silica-saturated fluids were those associated with the later deposition of jarosite and not those responsible for the formation of iddingsite.

[55] Balanced chemical equations can be written for the alteration reactions observed in each sample using a Fo75 olivine composition, assuming pure phases, and using the component oxides (MgO, SiO2, and H2) to represent the polymerized phase. Note that each of the reactants and products are neutral species so the positive and negative charges do not need to be balanced. In each of the samples, oxidation of Fe from Fe2+ → Fe3+ is balanced by the reduction of hydrogen from H+ in H2O to H0 in H2 or OH.

[56] Lunar Crater:

display math(1)

[57] Mauna Kea:

display math(2)

[58] ALHA 77005:

[59] (includes the akaganéite and jarosite in a single reaction, assumes all Fe is Fe3+)

display math(3)
display math

[60] These chemical reactions suggest that more water was required to drive alteration in the LC sample than the MK samples and that even more water was required for the ALHA 77005 reaction (and more H2 evolved by the ALHA reaction, consistent with the inferred acidic environment needed to produce jarosite). These reactions are consistent with the petrographic observation that all of the olivine in the LC samples exhibit some degree of alteration. The ternary diagrams shown in Figure 17 suggest that alteration in MK was minor (points fall along the Fo-Fa tie line; trend reflects zoning, not alteration), but the LC ternary reflects the oxidation of Fe and the addition of OH (data trend from the Fo-Fa tie line towards the FeO + Fe2O3 apex). The chemical reaction provided for ALHA 77005 suggests that even more water was added to produce the jarosite, but the patchy alteration products and jarosite are only reported in a few thin sections of this meteorite. The data points in the ALHA ternary diagram scatter towards the SiO2 apex and reflect the addition of silica-rich materials.

Figure 17.

Ternary plots of normalized microprobe data (wt % FeO + wt % Fe2O3 versus wt % MgO versus wt % SiO2) from altered and unaltered olivine in all three samples. The altered Mauna Kea data points follow the Fo-Fa tie line suggesting that alteration in Mauna Kea was minor, whereas the altered Lunar Crater data points deviate from the tie line toward the FeO + Fe2O3 apex suggesting more intense oxidation. The ALHA 77005 data points scatter away from the Fo-Fa tie line indicating a loss of MgO and the addition of SiO2 with the jarosite. There is a break in Mg# between the altered and unaltered Lunar Crater data as indicated by the dividing line and arrows, but the Mg# of the altered and unaltered Mauna Kea data form a continuous trend.

[61] The lack of a primary K-bearing mineral within the ALHA meteorite, coupled with the presence of K2O and SiO2 in the alteration, suggests that the aqueous solution was potassic and silica-rich. Some of the SiO2 in the olivine-hosted jarosite may have been mobilized from the host olivine, but the chromite alteration also has high SiO2 supporting the inference that the jarosite-altering fluids had a high dissolved silica content. The K2O content of either lithology, and that of the bulk meteorite, is ~0.03 wt %, and the reported maskelynite compositions range from An24-56 (see Table VIII-1a in Meyer [2003] for bulk compositions; Treiman et al. [1994] for maskelynite compositions) so it is doubtful that enough K2O could be leached from the primary phases of the host lithology to produce the jarosite in these two thin sections. The disparate compositions of the olivine and chromite-hosted sulfates suggest limited Cr mobility; the elevated P in the chromite-hosted jarosite (ave. ~2.8 wt % P2O5) suggests that some of the cations were supplied by a phosphate.

8.3 Inferences Regarding the Setting of Jarosite Alteration

[62] Burns [1988] states that the jarosite-forming reactions related to gossan deposits (the oxidized caps topping massive sulfide ores) occur in the zone of oxidation at or above the water table and are facilitated by warm (40 °C) and humid to semi-arid climates. Fe oxyhydroxides frequently coexist with jarosite in these settings but are inferred to form together, whereas we interpret the iddingsite in ALHA to predate the jarosite. We could infer the presence of a massive sulfide deposit associated with bimodal volcanics (coexisting felsic and mafic volcanics in rift-related settings, as are found on Earth) as a potential source for the sulfur and alkalis in the jarosite of ALHA, but this sample is a lherzolite with a complex shock history, which does not contain much sulfide (1–2 vol. %, Meyer [2003]), and the rift systems of Mars are fundamentally unlike those on Earth (no subduction, plate tectonics). The jarosite surrounding the altered chromite pictured in Figures 4g and 4j is enclosed in and reacting with the surrounding vein of clear, feldspathic melt glass. This vein of melt glass is cross-cut by a crushed zone, and PPL images of ALHA, 46 suggest that the feldspathic glass is older than the dark impact melt glass that is also present in this meteorite. These observations imply that the jarosite (and iddingsite) predate the shock events that produced these glasses, while the presence of altered chromite and the width of some veins suggest that the jarosite formed at elevated temperatures and pressures (high enough to produce veins that cross-cut mineral grains). An alternative scenario for the formation of jarosite involves the oxidation of the sulfides in the surface fines of Mars in response to a fluctuating water table akin to the acid-sulfate weathering of pyrite-bearing, black shales on Earth [Keller, 1996].

8.4 Temperature of Alteration

[63] The abundance of altered olivine in the Lunar Crater samples along with the low T and P parageneses of other forms of goethite (limonite, ocher, and gossan deposits, which form at temperatures below a few hundred degrees) prompt our surface alteration hypothesis. Further investigation of the green alteration products in the Lunar Crater olivine seems warranted, as they might provide more evidence regarding the conditions of alteration. Calcite is present but is not spatially correlated with the iddingsite, suggesting that the calcite was precipitated later from the surrounding aeolian mantle.

[64] The identification of akaganéite in the iddingsite of ALHA 77005 was initially perplexing because previous citations of this mineral's occurrence suggested formation at low temperatures. This polymorph of goethite is reported to be the oxyhydration product of lawrencite (FeCl2) and Fe-Ni metal in Antarctic meteorites [Taylor et al., 1974; Bland et al., 1997] and is also reported to occur in samples from every station of the Apollo 16 landing site. Whether the lunar akaganéite formed on the Moon or by exposure to moisture upon return to the Earth is unclear [Taylor et al., 1974]. The oxyhydration reaction of lawrencite to akaganéite given by Taylor et al. [1974] is as follows: FeCl2 + Fe + 2H2O + O2 → 2 FeO(OH) + 2H+ + 2Cl and is described as being very rapid at 25 °C and 40% relative humidity. Akaganéite can be synthesized at 40 °C and is the FeO(OH) polymorph deposited by chloride hydrothermal fluids at 60 °C (Schwertmann and Cornell [2000] in Glotch and Kraft [2008]) but converts to hematite upon heating. Glotch and Kraft [2008] indicate that conversion may be incomplete at temperatures ~150 °C but that conversion is complete at T > 300 °C. Because of the lawrencite association, akaganéite frequently contains chlorine, but chlorine is not mandatory for its formation [Mackay, 1962]. Table 4 indicates that some chlorine (≤0.2 wt % Cl) is present in the ALHA iddingsite, but this is less than the 1–6 wt % Cl reported to occur in lunar rust [Taylor et al., 1981], and the lunar akaganéite pictured in Taylor et al. [1973] does not resemble the patchy alteration in ALHA. The jarosite postdates the iddingsite and was deposited by silica-saturated fluids. X-ray and PPL images demonstrate the alteration of chromite (and pyroxene), suggesting that the jarosite formed at high temperature.

[65] Altered olivine are only present in the oxidized surface rinds of the two Hawaiian samples and are clear examples of surface alteration; the younger 16G1-30 is more altered than 16J-30 (we presume that the younger sample has spent more time exposed to the elements). Conditions at the summit of Mauna Kea are cold and dry (average low −2.0 °C, average high 7.8 °C) with an average of 18.7 cm of precipitation (mostly snow; Western Regional Climate Center Table on Wikipedia, 2012). Morris et al. [1998] indicate that maghemite is the dehydroxylation product of lepidocrocite and suggests that lepidocrocite is the initial (favored) weathering product of the martian soil. The laboratory experiments of Morris et al. [1998] describe the formation of polycrystalline pseudomorphs by the heating a synthetic lepidocrocite powder (LPS2) to various temperatures between 55 and 525 °C for 3 or 300 h. The transformation of the lepidocrocite powder to maghemite (then maghemite + hematite and finally to hematite) occurs faster in samples heated to higher temperatures. Maghemite appeared at 223 °C in samples heated for 3 h but at lower temperatures in samples heated for longer periods (300 h). Although Morris's experiments produced maghemite by heating, we assume that the maghemite in our sample formed under ambient Mauna Kea conditions, promoted by the dry climate.

9 Conclusions

[66] We analyzed the iddingsite mineral assemblages in two terrestrial basalts (from Lunar Crater, NV and Mauna Kea, HI) and compared these with the iddingsite in the Allan Hills (ALHA) 77005 shergottite. Correlated Raman spectroscopic and EMP traverses (tens to hundreds of micrometers long) were made across olivine alteration fronts in each of the samples to analyze the changes in mineralogy and chemical composition attending alteration. Petrographic observations were used to make inferences about the origin of the alteration products, the timing and setting of alteration, and the results used to discuss the environmental conditions and modes of alteration.

[67] Virtually all of the olivine in the Lunar Crater sample are altered to iddingsite, suggesting a plentiful source of water. Shifting climate patterns during the glacial episodes of the Pleistocene led to the formation of large salty lakes in the Basin and Range Province (notably Lakes Lahontan and Bonneville during the Wisconsinan Glacial Stage, 10–70 ka; Frazier and Schwimmer [1987]); the LC flows were presumably altered during these pluvial episodes, but field work suggests that they were not inundated. In contrast, altered olivine only occur in the exterior of the Mauna Kea samples, having formed by surface alteration/oxidation in the presence of only minor quantities of fluid. ALHA 77005 is a lherzolite (an ultramafic intrusive lithology) and the olivine hosting the iddingsite enclosed by orthopyroxene, suggesting that the iddingsite formed by the reaction of the olivine with volatiles exsolved during the final stages of magma consolidation (H2O, HCl, H2SO4, deuteric alteration). Veins of jarosite cross-cut and postdate all of the primary phases in these thin sections, altering the olivine and chromite and over-printing the iddingsite. The jarosite in ALHA is suggested to have formed at depth and at elevated temperatures (implied by the width of the jarosite veins, the presence of altered chromite, and because the jarosite predates both generations of melt glass).

[68] All three samples yielded different spectra of their alteration products. The Lunar Crater iddingsite has definitive goethite Raman peaks and two spectral features attributed to a poorly crystallized “polymerized” silicate. The polymerized silicate presumably formed by the linkage of the SiO4 tetrahedra remaining in the olivine crystal structure, whereas the amorphous silica in ALHA 77005 is a precipitate. The Mauna Kea iddingsite also contains a polymerized silicate phase (with higher peak positions) but contains no goethite—only broad, weak features occur in its place. Microprobe data of the MK iddingsite suggest that the Fe oxide is hematite or maghemite; the lack of clear hematite Raman peaks suggests that the Fe oxide is maghemite. The oxyhydroxide in the ALHA iddingsite is akaganéite (β-FeO(OH)), a polymorph of goethite (α-FeO(OH)); it is most apparent near the alteration fronts where the jarosite peaks are weak. Weak 640 and 670 cm−1 peaks (or one peak ~660 cm−1 where unresolved) occur in the Si-Ob-Si region of many ALHA spectra. There is no corresponding Si-Onb feature so we interpret these to be incipient akaganéite. Jarosite is present in both the altered olivine and chromite of ALHA, but its Raman spectra do not vary much and do not reflect the chemical variability observed in the microprobe data. A chromite grain (pictured in Figure 4g and the X-ray maps shown in Figure 4j) with an altered rim of jarosite was found to be enclosed by and reacting with a clear, unaltered feldspathic glass in thin section, 51 demonstrating that the jarosite is martian, not Antarctic.

[69] The Lunar Crater and Mauna Kea samples demonstrate similar element mobility trends (loss of Mg and Si, retention and oxidation of Fe) during alteration, but the second generation of jarosite + amorphous silica alteration in ALHA produces distinct trends in this sample (loss of Mg and Fe; influx of Si, K, and S). The ternary diagrams in Figure 17 suggest only minor alteration of the MK olivine (trend reflects zoning rather than alteration), whereas the LC olivine are more oxidized (data trend away from the Fo-Fa tie line towards the FeO + Fe2O3 apex). The chemical reactions provided in the discussion suggest a higher water/rock ratio for the LC samples than the MK samples; this is consistent with our petrographic observations. The ALHA reaction suggests that even more water was needed to produce the jarosite + silica assemblage, but these phases have only been reported in a few thin sections of this meteorite (possibly under-reported).

[70] The Lunar Crater iddingsite presumably formed at the ambient temperatures of the Pleistocene. Further investigation of the green alteration products is warranted as these may provide further constraints on the conditions of alteration. Although some publications infer a low T paragenesis for akaganéite (i.e., samples collected from sensibly cold environments such as the Moon, Antarctica), the lawrencite to akaganéite oxyhydration reaction reported by Taylor et al. [1974] occurs rapidly at 25 °C and 40% relative humidity, and Schwertmann and Cornell [2000] report akaganéite to be deposited by chloride hydrothermal fluids at 60 °C. Likewise, Glotch and Kraft [2008] indicate that the (complete) conversion of akaganéite to hematite requires T > 300 °C so that alteration of the ALHA olivine by late-stage magmatic fluids (at T between 60 and 150 °C) may not be unreasonable. The complex shock history of ALHA suggests that the jarosite formed at depth under elevated temperatures.

[71] The Hawaiian samples are clear examples of surface alteration; the older sample (16J) is less altered than the younger sample (16G), suggesting that the younger sample has been exposed to the elements for a longer period of time. Morris et al. [1998] indicate that lepidocrocite is the favored weathering product under martian conditions and report maghemite to be its dehydroxylation product. Morris et al. [1998] produced polycrystalline maghemite + hematite pseudomorphs by heating LPS2 to various temperatures between 55 and 525 °C for 3 or 300 h; they found that the temperature at which the lepidocrocite powder transforms to maghemite (then hematite) is lower in samples heated for longer periods of time (appeared at 223 °C in samples heated for 3 h). We presume that lepidocrocite can transform into maghemite at the cold, dry conditions atop Mauna Kea.


[72] I gratefully acknowledge the LPI and Allan Treiman for supporting my research during the summer of 2006 and the Johnson Space Center for the use of their electron microprobe facilities and for access to the Antarctic Meteorite Collection. I would like to thank Alian Wang for her assistance and advice and for granting me access to the Raman spectrometer at Washington University. Drs. Randy Korotev, Brad Jolliff, Alian Wang, Bob Dymek, and Jill Pasteris at Washington University in St. Louis all provided constructive reviews. The reviewers from JGR, Mark Fries and Melanie Kaliwoda, were also helpful. I would also like to extend my appreciation to everyone in the Earth & Planetary Sciences Department at WashU and especially the Planetary Surface Materials Research Group for their guidance and support. This research was supported in part by grant NNX07AQ34G (Mars Fundamental Research Program) to Dr. Brad Jolliff.