4.1 The Terrestrial or Martian Provenance of Nakhlite Secondary Minerals
 The Martian origin of the phyllosilicates is demonstrated by the presence of veins truncated by fusion crust, offsets produced by micro-faulting during impact events, and Ar-Ar ages that predate their arrival on Earth [Gooding et al., 1991; Treiman et al., 1993; Shih et al., 1998; Swindle and Olson, 2002]. Jarosite has been identified in Mössbauer spectra of rocks and soils at both MER landing sites [Klingelhöfer et al., 2004; Morris et al., 2006] and is present in the shergottite ALHA 77005 [Smith and Steele, 1983, 1984; Kuebler, 2009], and its presence previously noted in MIL per the Raman spectral maps of Fries et al. . Jarosite is present in X-ray maps of the groundmass of all three MIL thin sections (see S map in Figure 10); we infer the jarosite to be Martian because it predates the Ca-sulfates, which are inferred to be Martian.
 The carbonate and sulfate components of the nakhlites are inferred to be Martian because they occur in both falls and finds from around the globe [Bridges and Grady, 2000; Bridges et al., 2001; Gooding et al., 1991; Gooding, 1992; Dreibus et al., 2006], but similar phases occur in Antarctic soils and terrestrial contaminants may be present [Wentworth et al., 2005]. Treiman et al.  observed occurrences of Ca-sulfates in both open and closed (i.e., not exposed to the atmosphere) vugs of the Lafayette fusion crust and inferred these sulfates to have been mobilized during passage through the Earth's atmosphere. Photo-documentation of MIL 03346 demonstrates that no efflorescent salts (calcium sulfates) were present on its surface at the time of collection but were present after an accidental thawing [Satterwhite and Righter, 2006], presumably having migrated to the surface from the interior of the meteorite. Occurrences of gypsum and bassanite in the X-ray maps of ,168 and ,169 (exterior samples) are obvious late-stage deposits, appearing in a fracture crosscutting a stilpnomelane-bearing vein of one olivine and with impact melt glass in another (Figures 11a and 11f). Calcium sulfates were not detected in the X-ray maps of the olivine in ,177 (cut from a more interior piece of the meteorite), but weak 1007 cm−1 peaks are observed in multiple spectra from the Raman point count and indicate trace amounts of gypsum and jarosite. No siderite or other carbonate was detected in any of the thin sections by EMP or Raman spectroscopy.
Figure 11. (a) BSE image of Ca-sulfate in a fracture crosscutting a vein of stilpnomelane in olivine 2 of thin section ,168. (b–f) Sulfur X-ray maps of the Ca-sulfate in thin sections ,168 and ,169; maps indicate that low levels of jarosite (gray) occur throughout the mesostasis but that Ca-sulfates (white, ~41 wt % CaO) only occur along late-stage fractures and cracks.
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4.2 Inferences Regarding Conditions, Sequence of Alteration
 Most references report laihunite to be an oxidized variety of fayalite with the approximate composition of Fe+2Fe+32(SiO4)2, but the terms ferrifayalite and oxidized olivine are also found in the literature [Kitamura et al., 1984; Schaefer, 1985; Kan and Coey, 1985; Kondoh et al., 1985; Khisina et al., 1995; Dyar et al., 1998]. Dyar et al.  report the CNMMN to have approved usage of the term laihunite to describe distorted olivine having Fe3+ in M2 sites, Fe2+ in M1 sites, and vacancies in alternate rows parallel to (001). The WU Raman standard is from the laihunite type locality, a metamorphic magnetite mine near the village of Lai-He in Liaoning Province, China, and was originally identified by XRD [A. Wang, personal communication, 2003]. Laihunite has been identified in a variety of geologic settings—iron ores, eulysite (an Fe-Mn rich metamorphic rock), banded iron formations (BIF), as well as volcanic tuffs [Ferrifayalite Research Group, 1976; Laihunite Research Group, 1976, 1982 in Schaefer, 1985; references in Kitamura et al., 1984]—but the literature is not clear regarding the mechanism of formation at these locations (oxidation, alteration, metasomatism).
 Laihunite has reportedly been synthesized by the oxidation of fayalite at elevated temperatures with or without an aqueous fluid (400–800°C [Kondoh et al., 1985]; 100 and 300°C [Banfield et al., 1990; Iishi et al., 1997]). That laihunite was only detected in the olivine overgrowths of ,168, ,169, and ,177 (does not form continuous rims) and not observed adjacent to any of the traversed fractures inside the olivine of ,177 suggests that it formed by the oxidation of fayalite as the magma cooled (a late primary phase or early secondary phase) and not as a reaction product of the fluids that produced the stilpnomelane or jarosite. Crosscutting relationships demonstrates the stilpnomelane to have formed after the laihunite (stilpnomelane crosscuts the laihunite at the edge of the olivine in Figure 1e and crosscuts primary zoning in the Mg X-ray map in Figure 1i), as do the young geochronologic ages reported for the vein-filling materials in Lafayette. We found stilpnomelane veins crosscut by jarosite (see Figure 1d) and one stilpnomelane vein offset by a planar fracture that is crosscut by a continuous zone of jarosite alteration. We therefore infer the jarosite to represent a third generation of alteration. Late-stage fractures hosting Ca-sulfates crosscut both the stilpnomelane and jarosite and are considered to be late third or fourth generation alteration products.
 Stilpnomelane occurs in a variety of geologic environments (greenschists, iron formations, in xenoliths within Skaergaard gabbros and granophyres, and as a surface weathering product of sulfide deposits [Deer et al., 1992; formula from Eggleton, 1972; Klein, 2005] and is usually indicative of low to medium-grade metamorphism (late diagenesis to garnet-grade metamorphism) [Klein, 1983]. The Nassau stilpnomelane Raman standard is a fragment of schist (banded quartz and stilpnomelane) whose metamorphic history does not extrapolate to the MIL alteration environment. Stilpnomelane is common in very low grade metamorphic mafic rocks and metasediments of appropriate composition (i.e., glauconite bearing); it has been synthesized from gels and from magnesian chlorite + albite at 630°C and 2 kbar, perhaps requiring pressures as low as 1 kbar to form [Deer et al., 1992; Winkler, 1979]. French  reviews the criteria for evaluating whether a secondary phase formed under the conditions of diagenesis (1–2 kbars, 150–200°C) or low-grade metamorphism (2–3 kbars, 200–350°C) and indicates that the stability range of stilpnomelane may be large enough to encompass both diagenetic and low-grade metamorphic conditions. The vitric texture of the MIL mesostasis suggests rapid crystallization; MIL is presumed to have formed in a relatively shallow environment, so we infer the stilpnomelane in MIL to have crystallized at the lower end of its range of stability.
 The alteration products in the analog image of the Delvigne Atlas [1998; plates 130 and 131] more closely resemble the alteration products in Lafayette than those in MIL and are indicated to be a mixture of chlorite and smectite. Chlorite and saponite are both olivine alteration products but may also form from other minerals and may occur together or separately. Saponite is a microcrystalline to lamellar clay, whereas chlorite has a micaceous habit [Delvigne et al., 1979]. Smectites occur as hydrothermal alteration products near hot springs and geysers or in soils produced by the alteration of basic rocks [Deer et al., 1992]. Chlorite is a common hydrothermal alteration product in igneous rocks but may also occur in argillaceous sediments and low to moderate grade metamorphic rocks. On Earth, the increased temperatures and pressures associated with diagenesis are thought to facilitate the conversion of trioctahedral smectites into chlorite [Deer et al., 1992].
 Cristobalite, in particular, indicates that the mesostasis of MIL crystallized rapidly at elevated temperatures (β-cristobalite is the stable silica polymorph between 1470 and 1713°C) [Deer et al., 1992]. The coexistence of fayalite, magnetite, and cristobalite in the groundmass suggests that fO2 reached QFM during the late stages of crystallization, and the presence of laihunite and hematitic stains in the groundmass suggests that conditions may have become even more oxidizing during the final stages of crystallization (or post consolidation). The presence of an oxidizing environment is consistent with results from Viking experiments (surface materials inferred to contain oxidants—peroxides or perchlorates formed by exposure to short wavelength UV light) [Oyama and Berdahl, 1977; Klein, 1998; Lodders and Fegley, 1998; Hecht et al., 2009]; these atmospheric oxidants when combined with volcanogenic SO3 and HCl, plus H2O in the form of frost or dew, form acids capable of dissolving basalts [Haskin et al., 2005].
 Jarosite is present as thin films in cracks and veins of MIL olivine and is present in the mesostasis where it postdates the stilpnomelane. The sulfur X-ray map in Figure 10 gives a visual estimate of the volume of solution present during alteration. These films are narrower than the jarosite veins found in ALHA 77005,34 and 51, but jarosite has been identified in more thin sections of MIL than ALHA. The MIL pattern of jarosite deposition suggests infiltration. The presence of jarosite suggests an acidic environment, but gypsum and bassanite (and other previously reported secondaries: halite, siderite, anhydrite, and epsomite) suggest deposition from more neutral chloride solutions. The sulfates and salts may later precipitate from the same fluids that deposited the jarosite or represent a separate generation of alteration (compare occurrences of jarosite and Ca-sulfate in Figures 10 and 11).
4.3 Inferences Regarding the Setting of Nakhlite Olivine Alteration
 Raman spectra of the Lunar Crater and Mauna Kea alteration products demonstrate their iddingsite to be fine-scale intergrowths of a volumetrically minor polymerized silicate with either goethite or maghemite [Kuebler et al., 2013]. Altered olivine only occur in the oxidized surface rinds (outer 1 cm) of the Mauna Kea samples and are surface weathering products formed atop of Mauna Kea sometime after extrusion. The Lunar Crater iddingsite is attributed to surface alteration during the pluvial episodes of the Pleistocene (the Lunar Crater flow is inferred to have been more permeable before the accumulation of its aeolian mantle). ALHA 77005 iddingsite is also a fine-scale intergrowth containing akaganéite but is overprinted by a later generation of jarosite alteration; the iddingsite appears to be restricted to the light lithology (olivine poikilitically is enclosed by orthopyroxene and predates many of the shock features) and is therefore interpreted to have formed at depth as a product of deuteric alteration. 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 (the secondary phases inherit part of the olivine structure).
 In contrast, the secondary alteration veins in the nakhlites are dominated by phyllosilicates that are not optically continuous with their host olivine, occur in veins that are in sharp contact with the host olivine (suggesting complete disruption of the olivine crystal structure), indicate different element mobility trends, and are significantly younger than their host lithology. Lafayette and the deeper-formed nakhlites host smectite with masses of goethite or ferrihydrite [see images in Treiman et al., 1993; Treiman, 2005], but analysis of the MIL alteration products indicates none of these (except for the potential ferrihydrite/jarosite laser oxidation features in vein 2).
 The nakhlite analog sample pictured in the Delvigne Atlas  resembles the saw-tooth vein-filling materials in Lafayette and is likely to be from Volcans National Park which lies a few miles north of Kivu Lake and straddles the boundary between the Democratic Republic of the Congo and Rwanda (the Atlas gives the sample location as Northern Kivu, DRC, and states that the sample was collected from an outcrop). According to Google Earth Satellite imagery, this is a modern basalt complex hosting several calderas (e.g., Nyiragongo and Nyamulagira) and is associated with the western limb of the East African Rift. Proximity to Lake Kivu implies alteration at high water/rock ratios but SO2 fumaroles and dry CO2, CH4, and hydrocarbon gas vents (“mazuku” or “evil winds”) are also present [Smets et al., 2010; Tedesco et al., 2007]. This is a modern, equatorial volcanic complex implying that alteration occurred in a warm, humid climate shortly after extrusion (in contrast to the discrepancy between the reported nakhlite crystallization age and the age of their alteration products). Unfortunately, no Kivu samples were available for analysis so we cannot review the secondary phase identifications and element mobility trends of MIL with respect to a terrestrial analog as we did for ALHA 77005.
 In reference to the analog site (but without specific knowledge of where in the park the sample pictured in the Atlas was collected; i.e., its proximity to a groundwater source or fumarole), the MIL phyllosilicate and sulfate alteration products are inferred to have formed in discrete episodes; that the phyllosilicates formed in a warm, humid climate in the presence of groundwater and the sulfates deposited later by gas vents or fumaroles (or related seepage; as per the sulfur X-ray map in Figure 10). Phyllosilicate alteration of the Kivu basalts occurred in a shallow environment shortly after extrusion. However, the MIL alteration veins contain stilpnomelane, suggesting conditions akin to diagenesis, and are significantly younger (650–680 Ma) [Shih et al., 1998; Swindle and Olson, 2002] than their host clinopyroxenite (1.3 Ga) indicating a significant span of time between accumulation and alteration. It is unclear at present how to reconcile this age discrepancy relative to the terrestrial analog. Volcanism on Mars is longer lived than on Earth, and again, there is a difference in the host lithology. MIL is inferred to be a shallow-formed nakhlite but is still a cumulate lithology. We anticipate differences in the temperature, composition, and possibly the volume of the fluids effecting alteration in the Kivu basalt and MIL; confirmation of the conditions of alteration will require field work.
 MIL secondaries indicate the oxidation of olivine to form laihunite, the addition of SiO2 with the formation of stilpnomelane, and the loss of FeO and MgO with an influx of CaO associated with sulfate deposition. No mixing is indicated between the olivine and stilpnomelane which form discrete clusters of data points in Figures 8c and 8d (instead of the linear trends observed for the iddingsite samples), reflecting the sharp contacts between these phases. Laihunite is inferred to have formed early at an elevated temperature. An oxidation reaction for the conversion of fayalite to laihunite can be written as
The presence of magnetite and silica as reaction products is consistent with the inference that laihunite formed during the final stages of crystallization (magnetite and cristobalite occur in the mesostasis). We can write an approximate chemical reaction for the formation of stilpnomelane in MIL assuming a Fo50 olivine composition and the stilpnomelane formula of Eggleton  (see footnote in Table 2). The cations supplied to form the stilpnomelane are assumed to have been mobilized from the surrounding augite and mesostasis. The formula used in the reaction below requires the influx of alkalis (Na, Ca, K, and Al), but our microprobe analyses indicate the stilpnomelane in MIL to be alkali poor (sum of CaO, Na2O, and K2O < 0.1 wt %; Al2O3 < 0.5 wt %):
This reaction assumes that the glass in the mesostasis has the composition of K-feldspar, assumes stoichiometric alteration, and that fluid was present to aid cation mobility.
 A few of the data points in the MIL ternary fall along the Fo–Fa tie line (olivine zoning; see Figure 12) but others scatter above or below this toward the SiO2—(FeO + Fe2O3) join. Data that trend toward the FeO + Fe2O3 apex represent jarosite and those that fall above the Fo–Fa tie line represent stilpnomelane. The reaction for MIL implies a water/rock ratio similar to the ALHA reaction [Kuebler et al., 2013]. Both jarosite and stilpnomelane are hydrated and require an influx of water (more so than hematite or goethite), but their distribution within MIL is irregular; portions of the MIL groundmass contain jarosite and are visibly altered, whereas other areas retain their glassy texture.
Figure 12. MIL ternary diagram; data points that fall along the Fo–Fa tie line illustrate the range of zoned olivine compositions. Data trending toward the FeO + Fe2O3 apex represent jarositic alteration, while those that fall above the Fo–Fa tie line represent stilpnomelane.
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 Deeper-formed nakhlites are reported to contain more abundant alteration products suggesting that alteration may have been longer lived at depth with higher water/rock ratios (e.g., Nakhla, Lafayette). Siderite was not observed in MIL so no direct inferences can be made regarding the relative timing of its formation versus that of the stilpnomelane. Bridges and Grady  review the textural and geochemical evidence pertaining to the formation of the siderite-anhydrite-halite-chlorapatite assemblage of Nakhla, citing textural evidence in favor of a high temperature, hydrothermal origin (i.e., vein deposits) but report geochemical data that support a low temperature, low pressure origin. This conflicting evidence led Bridges and Grady  to conclude that the siderite, apatite, and halite in Nakhla most likely formed during the final cooling stages but Bridges and Grady  and Bridges et al.  to favor deposition from a brine. Kivu Lake consists of mildly basic (pH ~9.5) and CO2 saturated meteoric waters [see Tassi et al., 2009, Table 1], for concentrations of dissolved constituents in depth profiles of the five main basins, temperatures not reported); we do not know how the local meteoric groundwaters compare to the lake water but expect the Bridges and Grady  hypothesis is more accurate. The jarosite films in MIL may represent fluids associated with the venting of volcanic gases. Halite's solubility suggests that dry conditions persisted after its deposition and implies that the stilpnomelane formed prior to the siderite assemblage. While Bridges and Grady  report these salts to be roughly coeval with the alteration veins in Nakhla, the gypsum and bassanite in MIL clearly postdate all other secondary phases. It may be that the Ca-sulfate efflorescents in MIL were remobilized from anhydrite.