The weathering products present in igneous terrestrial Antarctic samples were analyzed, and compared with those found in the four Miller Range nakhlite Martian meteorites. The aim of these comparisons was to determine which of the alteration phases in the Miller Range nakhlites are produced by terrestrial weathering, and what effect rock composition has on these phases. Antarctic terrestrial samples MIL 05031 and EET 96400, along with the Miller Range nakhlites MIL 03346 and 090032, were found to contain secondary alteration assemblages at their externally exposed surfaces. Despite the difference in primary mineralogy, the assemblages of these rocks consist mostly of sulfates (jarosite in MIL 05031, jarosite and gypsum in EET 96400) and iddingsite-like Fe-clay. As neither of the terrestrial samples contains sulfur-bearing primary minerals, and these minerals are rare in the Miller Range nakhlites, it appears that SO42−, possibly along with some of the Na+, K+, and Ca+ in these phases, was sourced from wind-blown sea spray and biogenic emissions from the southern ocean. Cl enrichment in the terrestrially derived “iddingsite” of MIL 05031 and MIL 03346, and the presence of halite at the exterior edge of MIL 090032, can also be explained by this process. However, jarosite within and around the olivine-bound melt inclusions of MIL 090136 is present in the interior of the meteorite and, therefore, is probably the product of preterrestrial weathering on Mars.
The presence of sulfate alteration minerals has been reported in a number of Antarctic Martian meteorite finds (e.g., Losiak and Velbel 2011 and references therein). For example, jarosite (KFe3+3(OH)6(SO4)2) has been detected in the Yamato (Y)-000593/749 paired nakhlites (Treiman and Steele 2008; Noguchi et al. 2009; Changela and Bridges 2011), the Miller Range (MIL) nakhlite 03346 (Vicenzi et al. 2007; McCubbin et al. 2009; Hallis and Taylor 2011), and the two shergottites Roberts Massif (RBT)-04262 (Greenwood et al. 2009) and Queen Alexandra Range (QUE)-94201 (Ross et al. 2010). In addition, it has previously been reported as an alteration mineral on the surface of micrometeorites from the Yamato ice field region (Osawa et al. 2003). Gypsum (CaSO4·2H2O) has also been reported in the paired nakhlites MIL 03346 and 090030 (Hallis and Taylor 2011), as well as in the shergottite Elephant Moraine (EET) A79001 and various other Antarctic meteorite finds (Gooding 1986, 1992). The petrologic and isotopic data of numerous studies suggest that these sulfates, along with other evaporate minerals, have a terrestrial origin in Antarctic meteorites (Marvin 1980; Gooding et al. 1988; Jull et al. 1988; Grady et al. 1989; Gounelle and Zolensky 2001; Changela and Bridges 2011; Hallis and Taylor 2011). However, the data of Vicenzi et al. (2007) and McCubbin et al. (2009) raise the possibility that some jarosite in the Martian meteorites may be preterrestrial. This possibility is supported by the detection of sulfates (including jarosite) both from orbit and on the surface of Mars in recent years (e.g., Squyres et al. 2004; Bibring et al. 2005; Clark et al. 2005; McLennan et al. 2005; Poulet et al. 2005; Ming et al. 2006; Dohm et al. 2009). In addition, early calculations predicted the production of jarosite via the oxidation of primary igneous sulfides on Mars (Burns 1986, 1987; Burns and Fisher 1990).
Sulfates have also been reported in Antarctic terrestrial samples. For example, Berkley (1982) reported anhydrite in basanite flows and tuffs from Ross Island, Antarctica, as well as zeolite secondary alteration phases, K-feldspar, calcite, and phyllosilicates in hawaiites from the same area. Allen and Conca (1991) reported the presence of a small proportion of sulfates within secondary alteration assemblages in basaltic rocks from Victoria Land, Antarctica. These assemblages mostly consisted of illite and poorly crystalline quartz (95%), thought to be converted from silica gel, with minor (5%) sulfate salts, hematite, and leucoxene particles (a poorly defined, microcrystalline form of TiO2) in varying proportions. Wentworth et al. (2005) noted the similarities between Ca-sulfate in samples from Wright Valley, Antarctica with known occurrences in the Martian meteorites. In addition to Ca-sulfates, Wentworth et al. (2005) reported the presence of bloedite (Na2Mg(SO4)2·4H2O) and thenardite (Na2SO4) sulfates, as well as halite and the zeolite chabazite ((Na2,Ca)6(Al12Si24O72)·40H2O) (see also Gibson et al. 1983). However, not all secondary alteration assemblages produced in the Antarctic contain sulfates. Chevrier et al. (2006) studied the Ferrar dolerite of Antarctica as an analogue for Martian weathering, and reported that, while the presence of sulfates is probable as a product of pyrrhotite weathering, none were actually observed. (Oxy)hydroxides were the most common form of weathering observed in this lithology.
The aim of this study was to analyze the weathering products present in igneous terrestrial Antarctic samples, and compare these products with those found in the four Miller Range nakhlite Martian meteorites. The two terrestrial samples, Elephant Moraine (EET) 96400 and Miller Range (MIL) 05031, were mis-identified as meteorites and inadvertently collected by the Antarctic Search for Meteorites program (ANSMET). As these samples have been curated in the same way as the ANSMET Martian meteorite finds (including the Miller Range nakhlites), their weathering products can be accurately compared. Comparisons of this kind will help determine which of the alteration minerals in the Miller Range nakhlites are produced by terrestrial weathering. These results could then be extended to the Antarctic Martian meteorite group as a whole. In addition, detailed analysis of the alteration minerals present in different types of rock will enhance our understanding of the role rock composition plays in the formation of Antarctic weathering products.
MIL 03346 was collected during the 2003 collaborative expedition between ANSMET and the Mars Exploration Program. Petrological and geochemical analyses of MIL 03346 indicate that it originated from the same augitic cumulate layer(s) as the nakhlite group of Martian meteorites (e.g., Trieman 2005). However, in contrast to the other nakhlites, the presence of a vitrophyric intercumulus matrix in MIL 03346 and the absence of albitic feldspar laths suggest that rapid cooling occurred toward the end of crystallization (Day et al. 2006). Estimates of oxygen fugacity (Dyar et al. 2005; Hammer and Rutherford 2005; Szymanski et al. 2010) indicate that MIL 03346 may be the most oxidized SNC meteorite recognized to date. MIL 090030, 090032, and 090136 were collected six years later during the 2009 ANSMET field season, and are reported to be paired with MIL 03346 (Corrigan et al. 2011; Hallis and Taylor 2011; Udry et al. 2012).
Secondary alteration in the form of olivine-bound “iddingsite” alteration veins has previously been reported in MIL 03346 and its paired meteorites (Anand et al. 2005; Day et al. 2006; Stopar 2007; Hallis and Taylor 2011). Generally, on Earth, “iddingsite” is a mixture of smectite clays and Fe-oxides, but the Martian use of the word relates to the mixture of phases within the olivine alteration veins, and this can include other phases such as gypsum, siderite, and amorphous silicate gel (e.g., Changela and Bridges 2011; Velbel 2011). Abundant SiO2 and Fe2O3, moderate MgO, and minor Al2O3 and SO3, in the “iddingsite” veins suggest that this assemblage is composed of a heterogeneous mixture of Fe-rich phyllosilicates and fine-grained Fe-oxides (Hallis and Taylor 2011). Low analytical totals—even when all compositional iron is assumed to be Fe2O3—indicate the presence of water. Therefore, Fe-oxides within this “iddingsite” phase are likely to be at least partially hydrous—probably a mixture of goethite and hematite. The high SiO2 content of the Miller Range olivine alteration veins, along with variability in the position of the H2O Raman peaks, and the heterogeneous density of individual veins detected during Raman analyses, may indicate the presence of an additional amorphous silica gel component (Hallis and Taylor 2011). It is mostly the dominance of this amorphous component that separates Martian “iddingsite” from its terrestrial equivalent (see Velbel 2011 and references therein for further discussion). The similarity of these “iddingsite” veins in the meteorites of the nakhlite group (e.g., Bridges et al. 2001; Changela and Bridges 2011), in addition to H isotopic data (Hallis et al. 2012), indicates a preterrestrial origin. The Miller Range nakhlites also contain secondary Fe-oxide alteration veins, which, chemical and Raman spectral data suggest, are composed of mostly magnetite (Day et al. 2006; Hallis and Taylor 2011). The association of these veins with the “iddingsite” alteration veins suggests that they too have a preterrestrial origin. As mentioned above, jarosite and gypsum are present in interstitial areas in MIL 03346. The strong association of these phases with exterior edges indicates that they are terrestrially derived in this meteorite (Hallis and Taylor 2011).
The sample set included two thin sections of the terrestrial sample EET 96400, classified as a dolerite at the NASA Johnson Space Center. These thin sections were prepared at the University of Hawai‘i (UH) from parent chips 32 and 33, taken from the external and internal parts of the sample, respectively. Two thin sections of the unclassified terrestrial sample MIL 05031 were also prepared at UH from parent chips 4 (external) and 5 (internal). I was allocated four MIL 03346 thin sections (126, 128, 173 and 174), and two thin sections of MIL 090030(23 and 25), MIL 090032(24 and 25), and MIL 090136(21 and 25), from the NASA Johnson Space Center.
The JEOL JSM-5900LV scanning electron microscope at UH was utilized to produce high-resolution backscattered electron images and elemental X-ray images to locate areas of interest in each thin section. These X-ray images were used to determine the mineralogical mode of each sample, via the production of false color composite images and subsequent pixel counting using Adobe Photoshop software. As each whole thin section X-ray image was produced at a magnification of ×250, the pixel size was kept constant for each (approximately 20 μm). Specific areas of interest were subsequently X-ray imaged at higher resolutions, with various pixel sizes (approximately 1–5 μm).
The JEOL JXA-8500F electron microprobe at UH was also used to produce backscattered and X-ray images, as well as analyze the major- and minor- element chemistry of mineralogical phases within each sample. Analytical conditions included a 10 nA primary beam current, 15 keV accelerating voltage, and 5 μm spot size. The count time for beam-sensitive elements (e.g., Na and K) was set at 15 s, while that for less sensitive elements was set at 30 s. Beam sensitive elements were measured first. Matrix effects were corrected using PAP procedures. The elemental detection limits are 0.03 wt% for SiO2, Al2O3 and MgO; 0.04 wt% for TiO2, CaO, and K2O; 0.06 wt% for Na2O and Cr2O3; 0.07 wt% for MnO; 0.08 wt% for FeO. San Carlos olivine (NMNH 111312), Rockport fayalite (USNM 85276), and Makaopuhi Lake basaltic glass (USNM 113498/1) were utilized as standards, along with USNM 104021 apatite and NMNH 117075 chromite. UH standardized orthoclase Or-1 (5–168), Kakanui augite, Lake Co labradorite, and Staunton meteorite troilite were also used as standards.
Secondary alteration phases were defined by Raman spectroscopy at UH. Spectra were acquired with a confocal Raman microscope (alpha-300, WITec), using the 532 nm excitation wavelength (green) of a frequency-doubled solid-state Nd:YAG laser. The maximum power of the laser was approximately 23–24 mW, but a filter with an optical density of 200 was inserted between the laser and the sample to reduce the laser power accordingly. The laser beam was focused on the samples by a 50× objective, leading to an approximately 2 μm diameter spot. Each spectrum was acquired in a spectral range of approximately 3700 cm−1 (shift relative to the laser wavelength). The calibration of the spectrometer was checked daily for any shift in spectral peak position, via analysis of a pure silicon wafer with known major Raman peak at 524 cm−1. The acquisition time was fixed at 5 s and each spectrum is an average of five acquisitions.
Igneous Petrology and Mineral Chemistry
Terrestrial sample EET 96400 was classified at the Johnson Space Center as a dolerite, and this study's modal mineralogical and mineral chemical data support this classification (Table 1; Fig. 1). The sample consists of euhedral to subhedral clinopyroxene and feldspar phenocrysts (<2 and <1 mm, respectively), with minor ilmenite (<100 μm) and silica (<200 μm), in a Si- and K-rich mesostasis (Fig. 2). Clinopyroxene is mostly augite to ferroaugite and subcalcic ferroaugite, with some intermediate pigeonite. Feldspar varies from bytownite to labradorite, with some K-feldspar.
Table 1. Modal mineralogy of terrestrial and martian samples
Terrestrial sample MIL 05031 is more complex. It contains subhedral to anhedral feldspar and silica phenocrysts (all <500 μm), with rare ilmenite (<200 μm) and phosphate (<50 μm). Plagioclase composition varies from andesine to oligoclase, with rare anorthite. K-feldspar phenocrysts are present in approximately the same proportion as plagioclase (Fig. 3). The composition of the phenocrysts suggests that this sample is a quartz monzodiorite, but the very Fe-rich composition of the mesostasis negates this classification. The K, Na, and Si contents of the mesostasis indicate that MIL 05031 is a picritic basalt, or even a foidite. The mismatch may be the result of mixing between two melts, one basic and one acidic. This process usually results in the development of embayments on sodic plagioclase phenocrysts, or sieve texture, followed by calcic plagioclase overgrowths (Shelley 1993). In MIL 05031, a number of feldspar grains appear partially to almost completely dissolved, displaying embayments and sieve texture. Both the sodic and the K-feldspar phenocrysts are affected, with a number of K-feldspars exhibiting sodic feldspar overgrowths—possibly as a result of the low Ca content of this rock. In addition, both silica and feldspar phenocrysts commonly show a shattered texture, and a number of silica phenocrysts exhibit undulose extinction. These features indicate that MIL 05031 probably formed as a result of an explosive volcanic event, where a crystallized/partially crystallized acidic melt was mixed with another, much more basic and uncrystallized melt.
MIL 05031 has a very different mineralogical composition to EET 96400, which is very similar to the Miller Range nakhlites (Table 1; Fig. 1). These nakhlites are clinopyroxenites, containing cumulus euhedral to subhedral augites (<2.5 mm in length), that commonly show polysynthetic twinning and distinct Fe-rich rims (10–70 μm wide). Alongside the clinopyroxene grains are rarer subhedral to anhedral olivine phenocrysts (<1.5 mm in diameter), small anhedral grains of orthopyroxene (10–50 μm), and minor skeletal Ti-magnetite (<500 μm). A fine-grained felsic mesostasis surrounds the phenocrysts, punctuated with fayalite (<25 μm), pyrrhotite (<20 μm), cristobalite (as reported by Imae and Ikeda 2007), and acicular apatite (<15 μm long). Skeletal Ti-magnetite contains Ti-rich lamellae in all four meteorites, although no pure ilmenite was observed.
Thin sections from the internal rock chips of both EET 96400 (33a) and MIL 05031 (5a) did not contain any sulfate or secondary alteration veins large enough to be detected during initial SEM analysis. However, both external rock chip thin sections (EET 96400,32a and MIL 05031,4a) did show signs of alteration at their terrestrially exposed edges.
Terrestrial Sample EET 96400
EET 96400,32a contains narrow Ca-sulfate veining along the terrestrially exposed edge. One area along this edge shows Fe-rich alteration alongside a Ca-sulfate vein approximately 1 μm wide and approximately 100 μm long (Table 2; Fig. 2). The narrowness of this vein results in mixed chemical analyses, with the inclusion of some Si (15–20 wt% SiO2) from the surrounding mesostasis. Therefore, despite characteristically low stoichiometric totals, it is not possible to determine from these data whether the Ca-sulfate in this vein is composed of gypsum or anhydrite. Raman spectral data show four small peaks at 414, 462, 499, and 637 cm−1, and sharp peaks at 1015 and 1130 cm−1 (Fig. 4). Gypsum has characteristic peaks at 413, 493, 618, 1008, and 1140 wavenumbers and anhydrite at 465 (low intensity), 499, 608, 675, 1026, and 1130. Bassanite (2CaSO4·H2O) has its main peak at 1014 (Prasad et al. 2001). Pure anhydrite can be excluded on the basis that the Raman spectra show fluorescence under the green (523 nm) laser—a characteristic of gypsum (e.g., Berenblut et al. 1971; Krishnamurthy and Soots 1971). Therefore, the sulfate vein is probably a mixture of anhydrite and gypsum, possibly with a bassanite component. The Fe-rich altered area appears to be a heavily altered ilmenite grain, which becomes more S-rich toward the exposed edge (Fig. 2). Its composition grades from Fe-, Al, and Mg-rich, to K-, S-, and slightly more Na-rich toward the edge, with a corresponding decrease in Mg and Al. Fe content stays approximately the same throughout, but Fe, K, S, and Na highs correspond to Si lows. Raman spectra indicate that this area is hydrated, showing spectral peaks indicative of hematite and goethite (Fig. 5). Based on the chemical composition, hydrated Raman spectra, and altered appearance under an optical microscope, the silicate component in this alteration area is probably Fe-rich clay with minor Al and Mg (Table 2; Fig. 5). The increase in K, S, and Na toward the edge of the sample indicates a jarosite component. Raman spectral data support the presence of jarosite (Fig. 5), which appears to become dominant <50 μm from the terrestrially exposed edge (Fig. 2).
Table 2. Major-element mineral chemistry of secondary alteration phases
Fe-clay2 denotes data from the terrestrially derived iddingsite-like material surrounding gypsum and jarosite—as shown in Fig. 6h. Fe-clay1 denotes data from the olivine bound iddingsite-like veins reported to be of preterrestrial origin.
Terrestrial Sample MIL 05031
MIL 05031,4a shows a weathering rind of altered mesostasis approximately 500 μm thick along the terrestrially exposed edge of the rock (Fig. 3). This rind is depleted in Fe, and enriched in K relative to the unaltered mesostasis. The outside of the rind (within approximately 100 μm of the terrestrially exposed edge) is S-rich in certain areas, and Cl-rich in other areas, relative to the unaltered mesostasis. Chemical (Table 2) and Raman spectral analyses (Fig. 4) show that the S-rich areas are composed of Na,K-jarosite. Raman spectral data from the Cl-rich areas show characteristic hematite peaks, with fluorescence at high wavenumbers indicating hydration. This hydration could indicate the presence of göethite, but chemical analysis indicates a silicate component. The characteristic Raman peaks of hematite and goethite are similar to those shown by preterrestrial “iddingsite” veins in the Miller Range nakhlites (Fig. 5), which consist of a mixture of smectite clays, Fe-oxides, amorphous silica, and evaporites. Therefore, the Cl-rich alteration areas in MIL 05031,4a are probably composed of a mixture of phases similar to these preterrestrial “iddingsite” veins, but with a greater Fe-oxide component (Table 2; Fig. 5). In addition to being Fe-rich, the MIL 05031,4a “iddingsite” is Al-, Na-, K, and S-rich, and Mg- and Si-poor, relative to the preterrestrial “iddingsite” of MIL 03346 and its paired meteorites (Table 2). The same is also true when the “iddingsite” of MIL 05031 is compared with the terrestrially derived “iddingsite” in MIL 03346 (Fig. 6), except that the latter contains even more Si and less Fe than its preterrestrial counterpart. It also contains more S due to its proximity to areas of jarosite and gypsum (Table 2). Both “iddingsite” and jarosite are present in areas of altered mesostasis in the weathered rind, and as veins running through cracks in feldspar and silica phenocrysts (Fig. 3). The composition of each does not appear to be dependent on the surrounding material (i.e., “iddingsite” within a silica phenocryst crack is not correspondingly Si-rich).
The Miller Range Nakhlites
The sulfate composition of MIL 03346, along with those of the internal thin sections of MIL 090030, 090032, and 090136, was documented in Hallis and Taylor (2011). No jarosite and little gypsum were found in the internal thin sections, but jarosite and gypsum are associated with the terrestrially exposed edges of external thin section MIL 03346,174. External thin sections of MIL 090030(25), 090032(25), and 090136(25) were examined for this study.
MIL 090032(25) was found to contain a Ca-sulfate grain approximately 1 mm in diameter, with sulfate veins up to 300 μm wide extending from this area in all directions for up to 3 mm (Fig. 6). Most of the thickest and longest of these veins extend toward the edge of the thin section, which is approximately 1 mm from the main sulfate area. In addition, one edge of the Ca-sulfate grain runs along a large crack, which opens out to the edge of the sample. The presence of Antarctic varnish—a windblown-layered deposit with one layer rich in Si with minor Al, Fe, and Mg, and another layer rich in S and Fe, with minor Al, K, and P (Giorgetti and Baroni 2007)—shows this to be part of the external edge of the meteorite, and so the Ca-sulfate is associated with terrestrially exposed areas. Jarosite is associated with the edge of the Ca-sulfate grain and in places penetrates into the grain. Compared with that of MIL 05031, this jarosite is K-rich and Na-poor.
Iddingsite-like material also appears to be associated with this sulfate area (Table 2; Fig. 6c). This association indicates that this “iddingsite” has a terrestrial origin, and is of a separate generation to the olivine-bound “iddingsite” veins in the Miller Range nakhlites. The preterrestrial origins of the olivine-bound veins is confirmed in thin section MIL 090136,25, where two of these veins are visibly vesiculated and warped by the fusion crust (Fig. 7). Terrestrial “iddingsite” in MIL 090032,25 is present in too small an amount for accurate chemical comparisons with the terrestrial “iddingsite” of MIL 03346. However, the mineralogical assemblage of gypsum surrounded by jarosite surrounded by “iddingsite” in an area of mesostasis is the same in both meteorites (Fig. 6). One way in which the terrestrially derived “iddingsite” in the Miller Range nakhlites is different from the “iddingsite” in MIL 05031 is Cl-content—“iddingsite” in the latter is more Cl-rich (Table 2). However, in MIL 03346, one area of this terrestrial “iddingsite” contains a Cl-rich grain (approximately 5 μm diameter), which X-ray imaging indicates is O-poor, Ca-, Al- and K-rich, and Fe- and Si-neutral relative to the surrounding “iddingsite.” It is possible that this grain is a calcium chloride, although the presence of Fe and Si suggest a clay mixture.
Chemical differences between the “iddingsite” of MIL 05031 and MIL 03346 reflect the differing mineralogical composition of these two rocks—MIL 05031,4a “iddingsite” is Fe-, Al-, Na-, and K-rich, and Mg- and Si-poor, relative to the preterrestrial and terrestrial “iddingsite” of MIL 03346 and its paired meteorites (Table 2). The Fe-rich mesostasis of MIL 05031 ensures a clay form, which is Fe-rich and contains a large Fe-oxide component. The Al-, K-, and Na-rich nature of the feldspar phenocrysts ensures that these elements are well represented in the clay. Si is present in lower levels than in the clay of MIL 03346 because of the relatively higher abundances of Fe-oxides and jarosite, and the low-Mg content reflects the absence of Mg-bearing minerals such as olivine or pyroxene. The small percentage of K and S in the MIL 05031,4a “iddingsite” areas represents a jarosite component.
Another, less extensive, area of sulfate veins (again both Ca-sulfate and jarosite) is present farther along the same edge of MIL 090032,25, approximately 2 mm inside the meteorite. These veins center on a approximately 500 μm area of K-jarosite, bordering an angular hole in the section (approximately 1.5 mm across). Small halite grains are also present in this area, both within voids in the jarosite and on the surface of nearby clinopyroxene phenocrysts. It is difficult to determine whether this hole was present before sample preparation, but a pore space close to the terrestrially exposed meteorite surface would explain the presence of sulfates at this particular place, and any major plucking of grains during sample preparation probably would have disrupted them.
No sulfate was found in MIL 090030,25 or 090136,25. However, investigation of MIL 090136,21 olivine melt inclusion composition revealed that four large inclusions contain jarosite. This jarosite commonly exists as narrow veins (<1 μm wide), both within the melt inclusions and within cracks in the surrounding olivine. One of the four inclusions (MI3, see Fig. 8) contains a central jarosite-rich zone, surrounded by Fe-oxide (Table 3). The low analytical totals of this Fe-oxide suggest that it is hydrated, in agreement with previous data indicating that jarosite is associated with goethite and hematite in MIL 03346 clinopyroxene-bound melt inclusions (McCubbin et al. 2009). As these inclusions are within an internal thin section of MIL 090136, the sulfates are not associated with terrestrially exposed areas of the meteorite. In total, six olivine-bound melt inclusions were found within MIL 090136,21. In addition to jarosite and Fe-oxide, these inclusions were found to contain dendritic clinopyroxene within Al-rich glass, and in some cases, amphibole, phlogopite, and Ca-phosphate (Table 3; Fig. 8). Some areas of amphibole are Cl-rich, similar to the potasic-chlorohastingsite reported by McCubbin et al. (2009), except with less Fe and more Mg and Ti. In other cases, the amphibole is Cl-poor and comparatively enriched in Si and Ca, making it nonstoichiometric. Sulfur is also slightly enriched in the Cl-poor amphiboles, indicating that alteration has occurred. Melt inclusions (MI) 1, 2, and 3 are penetrated by iddingsite alteration veins, and MI 1, 3, 4, and 5 contain a chlorite-like phase (Table 3)—chlorite is commonly formed via the alteration of amphibole and phlogopite in the presence of water. The chlorite-like phase in these inclusions has a similar composition to amphibole, excepting its lack of Ca and increased S content. It is nonstoichiometric when compared with classic terrestrial chlorite, as it contains an excess of Si and Fe, and a small amount of S. These excesses indicate that chlorite may be mixed with penetrating “iddingsite” in these melt inclusions, as may be the case for the Cl-poor, nonstoichiometric amphiboles.
Table 3. Major-element mineral chemistry of melt inclusion phases
MI1, MI2, etc., indicate the melt inclusion number.
Of the two terrestrial Antarctic samples analyzed in this study, dolerite EET 96400 has the most comparable mineralogical composition to the Miller Range nakhlites. The major differences between the two rock types are the absence of olivine and sulfides in EET 96400. MIL 05031 has a completely different composition, with the phenocrysts of an acidic rock (silica and feldspars), but the Fe-rich mesostasis of a basic rock. No olivine, pyroxene, or sulfides were observed in MIL 05031. However, despite the differences in primary mineralogical composition between the samples, their secondary alteration minerals are strikingly similar, consisting mostly of jarosite and Ca-sulfates alongside Fe-rich clay/“iddingsite” deposits. Production of the latter could be explained by weathering of the Fe-rich mesostasis and feldspar in MIL 05031, and of ilmenite, feldspar, and mesostasis in EET 96400 (Figs. 2c, 2d, and 3b). Weathering of feldspars and the mesostasis could, at least to some extent, explain the dominance of Ca, K, and Na in the sulfates of these two rocks (see Velbel 2011 for detailed discussion), although it does not explain where the S originates, as both rocks contain no S-rich primary minerals. Nor can this mechanism of weathering account for the presence of Cl in the Fe-clays of MIL 05031, or the halite observed associated with jarosite at the exposed edge of MIL 090032. The Miller Range nakhlites do contain minor sulfide, and this is commonly observed to be weathered. However, the amount of primary sulfide in these meteorites totals <0.1 vol% of the whole rock, whereas MIL 03346 contains 0.4 vol% sulfates. Therefore, it is probable that an additional source of sulfur is required to explain the dominance of sulfate secondary minerals in these Antarctic rock and meteorite samples.
The presence of all interstitial sulfate areas so close to the terrestrially exposed edge of MIL 090032, as well as their absence in the remainder of this thin section and the internally cut thin section, strongly suggests a terrestrial origin for sulfate in this meteorite.
Sulfates make up approximately 7.7% of the total salt in the Earth's oceans and recent research suggests that these salts may be blown far into the interior of Antarctica. Udisti et al. (2012) recorded sea spray deposition 1100 km from the Antarctic coastline, at the Italian-French Concordia Station base on the East Antarctic Plateau. Detailed analysis suggests that these deposits are composed of winter layers, consisting of sea spray-rich aerosols of submicrometric particles with trajectories over the Antarctic Plateau, and summer layers, consisting of mostly 1–2 μm sized SO42−-rich particles originating from the closer Indian-Pacific sector. The difference in SO42− content between the seasons is thought to be caused by an increase in nonsea salt sulfate (nssSO42−) and methanesulfonate acid (MSA) during the summer. These are both produced by the biological activities of certain phytoplankton groups, which bloom in the southern ocean's summer (Kaufmann et al. 2010 and references therein). In Antarctica, the deposition of nssSO42− from other sources, such as volcanic SO42− in the stratosphere, continental dust, and recent anthropogenic SO2 contamination in the lower troposphere, is reported to be minimal compared with these biogenic deposits (e.g., Prospero et al. 1991; Legrand 1995; Minikin et al. 1998; Alexander et al. 2003; Kaufmann et al. 2010). The Miller Range and Elephant Moraine regions of the Transantarctic Mountains are much closer to the nearest source of open water than Concordia Station: McMurdo Sound is approximately 500 km north of Miller Range and approximately 150 km east of Elephant Moraine. Therefore, it may be supposed that SO42− from sea spray and phytoplankton blooms is deposited at a higher rate in these regions. In addition, other salts should also be deposited. The Dry Valleys of Antarctica are located approximately 120 km southeast of Elephant Moraine, and are known to contain soils with exceptionally high salt contents, including sulfates (Claridge and Campbell 1977; Key and Williams 1981; Bockheim 1997; Wentworth et al. 2005 and references therein). Oxygen isotope analyses indicate that the vast majority of the sulfate in these soils is produced by biogenic emissions from the ocean (e.g., Bao et al. 2000).
This wealth of evidence points toward a wind-blown origin for the sulfate in jarosite and Ca-sulfates found at the terrestrially exposed edges of Antarctic meteorites and terrestrial rocks. It may also indicate that at least some of the K, Na, and Ca ions in these sulfate minerals are wind-blown. However, this implication is in contrast to the findings of Velbel et al. (1991). These authors determined that Na, K, and Ca cations in the terrestrially derived Mg-carbonate of the H5 chondrite Lewis Cliff (LEW) 85320 occur at abundances within an order of magnitude of their chondritic ratios with Mg. Therefore, Na, K, and Ca in LEW 85320 appear to be derived from the meteorite itself. Therefore, this aspect of weathering in the Antarctic meteorites deserves further study.
The presence of sea spray aerosols in these regions can explain the elevated Cl content of “iddingsite” in the weathered rind of MIL 05031 (Table 2), the possible calcium chloride grain in the terrestrially derived “iddingsite” of MIL 03346, and the presence of halite in MIL 090032 (Fig. 6), As noted above, halite has previously been reported in the dry valley soils (e.g., Wentworth et al. 2005). Preterrestrial “iddingsite” veins in the Miller Range nakhlites also contain elevated Cl abundances (Table 2). However, the uniformity of “iddingsite” vein Cl-content throughout, and between, each of the four meteorites indicates that this Cl is of a preterrestrial origin. The same is true of the S in these veins. This suggests that even though “iddingsite” is a structurally weak hydrous clay and Fe-oxide mixture, its bulk chemistry has not been significantly altered by terrestrial weathering where it is not directly exposed to the atmosphere in these meteorites (i.e., at an external edge). Where “iddingsite” is observed at a terrestrially exposed surface, it is commonly covered with Antarctic varnish, but no obvious leaching or change in the composition of the “iddingsite” beneath is observed. These observations all indicate that weathering processes in the Antarctic are limited to the very surface of each rock or meteorite.
The absence of interstitial sulfates in both the external and internal thin sections of MIL 090030 and MIL 090136 suggest that specific conditions are required for them to develop in the Antarctic. It may be that MIL 090030 and MIL 090136 were more sheltered from the elements than the other sulfate-bearing nakhlites after they fell to Earth. They may have been trapped in glacial ice for a longer period of time, thus minimizing the effects of freeze-thaw weathering. Alternatively, and/or additionally, they may have been more sheltered from prevailing winds.
Formation via sea-spray deposition probably does not apply to the jarosite within the olivine-bound melt inclusions of MIL 090136,21. This is the second observed occurrence of sulfate not associated with a terrestrially exposed edge in the Miller Range nakhlites. McCubbin et al. (2009) analyzed an apparently unbreached, amphibole-bearing, augite-bound melt inclusion, which these authors reported as containing a separate generation of preterrestrial hydrothermal jarosite. While the unbreached nature of this melt inclusion cannot be guaranteed (due to the cleavable structure of augite and the lack of 3-D information in a rock thin section), McCubbin et al. (2009) argued that the presence of Cl-rich amphibole strongly suggests that this inclusion was sealed, at least during cooling. The presence of Cl-rich amphibole in MIL 090136,21 MI2—which is clearly breached by an “iddingsite” vein (Fig. 8)—indicates that this primary mineral can survive melt inclusion breach by Martian secondary alteration. Therefore, the augite-bound melt inclusion of McCubbin et al. (2009) may have been similarly, if imperceptibly, breached after cooling—the absence of tell-tale “iddingsite” veins in augite makes this difficult to determine. However, it can be stated that the presence of chlorite in MI1, 3, 4, and 5 and its association with “iddingsite” veins suggest that amphibole and phlogopite in these inclusions were altered by the “iddingsite”—forming Martian fluids. Significantly, the most jarosite-rich melt inclusion (MI3) is also richest in the chlorite-like phase. This chlorite-like phase, which appears to be a mixture of chlorite and “iddingsite,” completely surrounds the central jarosite and Fe-oxide area, indicating that this too was a product of Martian alteration.
The fact that jarosite is only present at the external edges of the Miller Range nakhlites when it is not associated with melt inclusions points toward the presence of at least two separate populations of sulfate, produced by preterrestrial and terrestrial alteration, respectively. In the case of MIL 090136 olivine-bound melt inclusions, it seems that jarosite, Fe-oxide, and the chlorite-like phase are Martian alteration phases, overprinting the original clinopyroxene, amphibole, phlogopite, and glass of the melt inclusions. This is supported by the fact that the nearby mesostasis, olivine, and sulfides do not show signs of terrestrial weathering. In contrast, the external surface sulfates appear to be produced by Antarctic wind-blown sulfates.
Antarctic terrestrial samples MIL 05031 and EET 96400, along with the Miller Range nakhlites MIL 03346 and 090032, contain secondary alteration assemblages at their externally exposed surfaces. Despite the difference in primary mineralogy, the assemblages of these rocks consist mostly of sulfates (jarosite in the former, jarosite and gypsum in the latter) and iddingsite-like Fe-clay. “Iddingsite” is present in the Fe-rich mesostasis of MIL 05031, and within at least one heavily weathered ilmenite in EET 96400; these areas are strongly associated and intergrown with sulfates. As neither of these rocks contains any sulfur-bearing primary minerals, it appears that the SO42− that formed sulfates, possibly along with some of the Na+, K+, and Ca+ in these phases, was sourced from wind-blown sea spray and biogenic emissions from the southern ocean. The enrichment in Cl− in the terrestrially derived “iddingsite” of MIL 05031 and MIL 03346, and the presence of halite at the exterior edge of MIL 090032 can also be explained by this process. However, jarosite within and around the olivine-bound melt inclusions of MIL 090136 cannot be produced by wind-blown deposits, as it is present in the interior of the meteorite in terrestrially unweathered areas. This jarosite, along with hydrated Fe-oxide and chlorite within the melt inclusions, appears to be the product of preterrestrial weathering on Mars.
This material is based upon work supported by the National Aeronautics and Space Administration, through the NASA Astrobiology Institute under Cooperative Agreement No. NNA09DA77A, issued through the Office of Space Science. Thanks to the NASA Johnson Space Center for allocation of the Miller Range nakhlite thin sections, and rock chips of MIL 05031 and EET 96400. I would also like to thank JoAnn Sinton for thin-section production and Eric Hellebrand for his assistance with EMP analyses. Prof. Jeff Taylor is thanked for his helpful comments and corrections, as are Francis McCubbin, Michael Velbel, and Gretchen Benedix.