Aqueous alteration of the Nakhla meteorite

Abstract— Interior samples of three different Nakhla specimens contain an iron-rich silicate “rust” (which includes a tentatively identified smectite), Ca-carbonate (probably calcite), Ca-sulfate (possibly gypsum or bassanite), Mg-sulfate (possibly epsomite or kieserite), and NaCl (halite); the total abundance of these phases is estimated as <0.01 weight percent of the bulk meteorite. Rust veins are truncated and decrepitated by fusion crust and are preserved as faulted segments in partially healed olivine crystals, indicating that the rust is pre-terrestrial in origin. Because Ca-carbonate and Ca-sulfate are intergrown with the rust, they are also indicated to be of pre-terrestrial origin. Similar textural evidence regarding origins of the NaCl and Mg-sulfate is lacking. Impure and poorly crystallized sulfates and halides on the fusion crust of the meteorite suggest leaching of interior (pre-terrestrial) salts from the interior after Nakhla arrived on Earth but coincidental addition of these same salts by terrestrial contamination cannot be excluded. At least the clay-like silicate “rust,” Ca-carbonate, and Ca-sulfate were formed by precipitation from water-based solutions on the Nakhla parent planet although temperature and pressure conditions of aqueous precipitation are unconstrained by currently available data. It is possible that aqueous alteration on the parent body was responsible for the previously observed disturbance of the Rb-Sr geochronometer in Nakhla at or near 1.3 Ga.     

Mars regolith versus SNC meteorites: Possible evidence for abundant crustal carbonates

Viking XRF analyses of the Martian regolith are compared with typical igneous rocks of the Earth, the Moon, the eucrite parent asteroid, and especially the shergottites, nakhlites, and Chassigny (SNC) meteorites, which are suspected to be basalts and mafic cumulates from Mars. A striking feature of the Martian regolith, compared to igneous rocks with similar molar (Mg + Fe)/Si ratios, is its extraordinarily low Ca/Si ratio. The regolith's low Ca/Si ratio is probably not a result of simple mixing (isochemical weathering) of SNC-like rocks with other igneous rocks, unless the regolith contains a large component of rock with an improbable combination of extremely low Ca/Si and (Mg + Fe)/Si, and yet low K₂O and Zr. Several other models might conceivably account for the low Ca/Si ratio, but I suggest that most of the “missing” Ca was removed from the regolith as Ca-carbonate. Formation of a mass of carbonate equivalent to a global shell 20 m thick would suffice to remove 1000 mbar of CO₂ from the Martian atmosphere. Thus, the peculiar Ca/Si ratio of the Martian regolith tends to support the hypothesis that the climate of Mars was once far warmer and wetter than it is today.     

40Ar‐39Ar and cosmic‐ray exposure ages of nakhlites—Nakhla,Lafayette, Governador Valadares—and Chassigny

Abstract– We present ⁴⁰Ar‐³⁹Ar dating results of handpicked mineral separates and whole‐rock samples of Nakhla, Lafayette, and Chassigny. Our data on Nakhla and Lafayette and recently reported ages for some nakhlites and Chassigny ( Misawa et al. 2006 ; Park et al. 2009 ) point to formation ages of approximately 1.4 Ga rather than 1.3 Ga that is consistent with previous suggestions of close‐in‐time formation of nakhlites and Chassigny. In Lafayette mesostasis, we detected a secondary degassing event at approximately 1.1 Ga, which is not related to iddingsite formation. It may have been caused by a medium‐grade thermal event resetting the mesostasis age but not influencing the K‐Ar system of magmatic inclusions and the original igneous texture of this rock. Cosmic‐ray exposure ages for these meteorites and for Governador Valadares were calculated from bulk rock concentrations of cosmogenic nuclides ³He, ²¹Ne, and ³⁸Ar. Individual results are similar to literature data. The considerable scatter of T₃, T₂₁, and T₃₈ ages is due to systematic uncertainties related to bulk rock and target element chemistry, production rates, and shielding effects. This hampers efforts to better constrain the hypothesis of a single ejection event for all nakhlites and Chassigny from a confined Martian surface terrain ( Eugster 2003 ; Garrison and Bogard 2005 ). Cosmic‐ray exposure ages from stepwise release age spectra using ³⁸Ar and neutron induced ³⁷Ar from Ca in irradiated samples can eliminate errors induced by bulk chemistry on production rates, although not from shielding conditions.     

Alteration assemblages in the Miller Range and Elephant Moraine regions of Antarctica: Comparisons between terrestrial igneous rocks and Martian meteorites

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 SO₄^2?, 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.     

Noble gases in the Martian meteorite Northwest Africa 2737: A new chassignite signature

Abstract— We report noble gas data for the second chassignite, Northwest Africa (NWA) 2737, which was recently found in the Moroccan desert. The cosmic ray exposure (CRE) age based on cosmogenic ³He, ²¹Ne, and ³⁸Ar around 10–11 Ma is comparable to the CRE ages of Chassigny and the nakhlites and indicates ejection of meteorites belonging to these two families during a discrete event, or a suite of discrete events having occurred in a restricted interval of time. In contrast, U‐Th/He and K/Ar ages <0.5 Ga are in the range of radiometric ages of shergottites, despite a Sm‐Nd signature comparable to that of Chassigny and the nakhlites (Misawa et al. 2005). Overall, the noble gas signature of NWA 2737 resembles that of shergottites rather than that of Chassigny and the nakhlites: NWA 2737 does not contain, in detectable amount, the solar‐like xenon found in Chassigny and thought to characterize the Martian mantle nor apparently fission xenon from ²⁴⁴Pu, which is abundant in Chassigny and some of the nakhlites. In contrast, NWA 2737 contains Martian atmospheric noble gases trapped in amounts comparable to those found in shergottite impact glasses. The loss of Martian mantle noble gases, together with the trapping of Martian atmospheric gases, could have occurred during assimilation of Martian surface components, or more likely during shock metamorphism, which is recorded in the petrology of this meteorite.     

Glass‐bearing inclusions in olivine of the Chassigny achondrite: Heterogeneous trapping at sub‐igneous temperatures

Abstract— Three types of glass‐bearing inclusions are present in olivine and chromite of the Chassigny achondrite: pure glass, monocrystal (glass plus a single mineral grain), and multiphase (glass plus a variety of minerals) inclusions. The occurrence, texture, and mineralogy of these inclusions and the chemical composition of the glass suggest an origin by heterogeneous trapping of these phases. The glass is rich in SiO₂, Al₂O₃, Na₂O, K₂O; and poor in MgO, FeO, and CaO; and contains appreciable amounts of Cl. The compositional variability of the glass is independent of the mineral content of the inclusions. Heating experiments with final temperatures of 900, 1000, and 1200 °C were performed with Chassigny inclusions for the first time. The glass of the heated inclusions has a chemical composition similar to that of unheated inclusions. This situation suggests that the glass cannot be a residual melt but rather is an independent component that was trapped with or without mineral phases. The extreme heterogeneity in alkali contents, and in particular Rb and Sr contents, also suggests precipitation and mixing of solid precursors. The most Rb‐rich glasses have near‐chondritic Rb/Sr ratios, possibly indicating a chondritic source for their precursor(s). None of the inclusions contain bubbles like those of typical melt inclusions in terrestrial igneous minerals. Furthermore, many inclusions are at the center of radial cracks in the host olivine, which indicates development of an overpressure within the inclusions at some time. A volume increase of the inclusions could have been achieved by differential thermal expansion of the content of the inclusion during a heating event. That mechanism requires bubble‐free and solid preheating inclusion contents. These features are incompatible with an origin of the inclusions by trapping of a silicate melt and point toward heterogeneous trapping of solid phases. The first N analyses performed in Chassigny glass‐bearing inclusions by nuclear reaction analysis (NRA) revealed high and variable N contents of the glass, which suggests trapping of a solid precursor (presumably at relatively low temperatures) from a fluid rather than a melt. In conclusion, the glass‐bearing inclusions in Chassigny olivine are not residuals after a closed‐system evolution of a trapped melt, but rather heterogeneously trapped precipitates of a fluid that existed during formation of Chassigny constituents. Consequently, it is very unlikely that the host olivine has an igneous origin.     

ALH84001, a cumulate orthopyroxenite member of the martian meteorite clan

Abstract— ALH84001, originally classified as a diogenite, is a coarse-grained, cataclastic, orthopyroxenite meteorite related to the martian (SNC) meteorites. The orthopyroxene is relatively uniform in composition, with a mean composition of Wo_3.3En_69.4Fs_27.3. Minor phases are euhedral to subhedral chromite and interstitial maskelynite, An_31.1Ab_63.2Or_5.7, with accessory augite, Wo_42.2En_45.1Fs_12.7, apatite, pyrite and carbonates, Cc_11.5Mg_58.0Sd_29.4Rd_1.1. The pyroxenes and chromites in ALH84001 are similar in composition to these phases in EETA79001 lithology A megacrysts but are more homogeneous. Maskelynite is similar in composition to feldspars in the nakhlites and Chassigny. Two generations of carbonates are present, early (pre-shock) strongly zoned carbonates and late (post-shock) carbonates. The high Ca content of both types of carbonates indicates that they were formed at moderately high temperature, possibly ～700 °C. ALH84001 has a slightly LREE-depleted pattern with La 0.67x and Lu 1.85x CI abundances and with a negative Eu anomaly (Eu/Sm 0.56x CI). The uniform pyroxene composition is unusual for martian meteorites, and suggests that ALH84001 cooled more slowly than did the shergottites, nakhlites or Chassigny. The nearly monomineralic composition, coarse-grain size, homogenous orthopyroxene and chromite compositions, the interstitial maskelynite and apatite, and the REE pattern suggest that ALH84001 is a cumulate orthopyroxenite containing minor trapped, intercumulus material.     

Igneous and shock processes affecting chassignite amphibole evaluated using chlorine/water partitioning and hydrogen isotopes

Amphibole in chassignite melt inclusions provides valuable information about the volatile content of the original interstitial magma, but also shock and postshock processes. We have analyzed amphibole and other phases from NWA 2737 melt inclusions, and we evaluate these data along with published values to constrain the crystallization Cl and H₂O content of phases in chassignite melt inclusions and the effects of shock on these amphibole grains. Using a model for the Cl/OH exchange between amphibole and melt, we estimate primary crystallization OH contents of chassignite amphiboles. SIMS analysis shows that amphibole from NWA 2737 currently has 0.15 wt% H₂O. It has lost ~0.6 wt% H₂O from an initial 0.7–0.8 wt% H₂O due to intense shock. Chassigny amphibole had on average 0.3–0.4 wt% H₂O and suffered little net loss of H₂O due to shock. NWA 2737 amphibole has δD ≈ +3700‰; it absorbed Martian atmosphere‐derived heavy H in the aftermath of shock. Chassigny amphibole, with δD ≤ +1900‰, incorporated less heavy H. Low H₂O/Cl ratios are inferred for the primitive chassignite magma, which had significant effects on melting and crystallization. Volatiles released by the degassing of Martian magma were more Cl‐rich than on Earth, resulting in the high Cl content of Martian surface materials.     

Alteration assemblages in the nakhlites: Variation with depth on Mars

Secondary mineral assemblages in the nakhlite meteorites, Lafayette, Governador Valadares (GV), Nakhla, Yamato (Y)‐000593/Y‐000749 have been studied using scanning electron microscopy, transmission electron microscopy, and electron probe micro analysis. The different nakhlites have distinctive secondary assemblages in their olivine grains and mesostases, showing compositional fractionation correlated with their relative depths below the Martian surface. Fracture‐filled veins in Lafayette at the bottom of the pile consist of a siderite‐phyllosilicate‐Fe oxide‐hydrated silicate gel assemblage. Corresponding veins in Nakhla and GV further up the pile are predominantly a siderite‐gel assemblage, with additional evaporites including gypsum. Y‐000593/Y‐000749 veins are dominated by gel. The gel’s Mg/(Mg + Fe) ratio decreases from Lafayette (0.37) to GV (0.32), Nakhla (0.24), and Y‐000593 (0.15). We suggest that hydrothermal fluid flowed up this depth profile, initiated by melting of buried H₂O–CO₂ ice. Our results show a complex mix of Fe‐rich phyllosilicate within the veins and mesostasis of Lafayette with d‐spacings of 0.7–1.1 nm suggesting a mixture of smectite and serpentine. The phyllosilicate formed at close to neutral pH, ≤150 °C. We also suggest that water rock ratios (W/R) of 1–10 occurred in Lafayette with smaller values for the other nakhlites. This is reflected in the volume of alteration minerals: 10% of olivine in Lafayette to 3% in Nakhla. Textural evidence of rapid cooling, together with the W/R and likely fluid velocities, suggest that the secondary assemblages formed quickly, e.g., within months. A model is proposed in which the secondary assemblages formed in an impact‐induced hydrothermal system terminated by precipitation of the gel and evaporation of soluble salts.     

Density,porosity, and magnetic susceptibility of achondritic meteorites

As part of a large‐scale survey of meteorite bulk and grain densities, porosities, and magnetic susceptibilities, we measured these properties for 174 stones from 106 achondritic meteorites. These include four lunar meteorites, 15 stones from 10 shergottites, nakhlites, and chassignites (SNCs), 96 stones from 56 howardites, eucrites, and diogenites (HEDs), 17 stones from nine aubrites, two angrites, and 16 stones from 10 ureilites, four stones of three acapulcoites, as well as four stones of three lodranites, and 15 stones from eight primitive achondrites. Those meteorites derived from basalts and crustal material of differentiated parent bodies have lower densities and magnetic susceptibilities, on an average, than the more primitive achondrites, which have a higher percentage metal. A notable exception is the one chassignite in the study (Chassigny), which has a high grain density of 3.73 ± 0.04 g cm^?3. Ureilites have magnetic susceptibilities consistent with primitive achondrites, but lower grain densities. Porosities do not vary considerably between most of the groups, with most stones 5–14% porous, although on an average, ureilites and brachinites have lower porosities, with most stones less than 7% porous. For primitive achondrites, the higher metal content causes finds to exhibit weathering effects similar to what is observed in ordinary chondrites, with a reduction in grain density, magnetic susceptibility, and porosity as compared with unweathered falls. For lunites, SNCs, and HEDs, no such effect is observed. We also observe that grain density and magnetic susceptibility used in conjunction distinguish shergottites, nakhlites, and chassignites from each other. Shergottites and nakhlites have low grain densities (averaging 3.31 and 3.41 g cm^?3, respectively) whereas Chassigny is 3.7 g cm^?3. In magnetic susceptibility, shergottities and chassignites are similar (averaging 2.85 and 2.98 in log units of 10^?9 m³ kg^?1, respectively) with nakhlites averaging higher at 3.42.     

Comparisons of the four Miller Range nakhlites,MIL 03346, 090030, 090032 and 090136: Textural and compositional observations of primary and secondary mineral assemblages

Abstract– Petrological and geochemical analyses of Miller Range (MIL) 03346 indicate that this meteorite originated from the same augitic cumulate layer(s) as the nakhlite Martian meteorites, but underwent rapid cooling prior to complete crystallization. As with the other nakhlites, MIL 03346 contains a secondary alteration assemblage, in this case consisting of iddingsite‐like alteration veins in olivine phenocrysts, Fe‐oxide alteration veins associated with the mesostasis, and Ca‐ and K,Fe‐sulfate veins. We compared the textural and mineralogical compositions of MIL 090030, 090032, and 090136 with MIL 03346, focusing on the composition and Raman spectra of the alteration assemblages. These observations indicate that the meteorites are paired, and that the preterrestrial olivine‐bound alteration assemblages were produced by weakly acidic brine. Although these alteration assemblages resemble similar assemblages in Nakhla, the absence of siderite and halite in the Miller Range nakhlites indicates that the parental alteration brine was comparatively HCO₃^? depleted, and less concentrated, than that which altered Nakhla. This indicates that the Miller Range nakhlite alteration brine experienced a separate evolutionary pathway to that which altered Nakhla, and therefore represents a separate branch of the Lafayette‐Nakhla evaporation sequence. Thin‐sections cut from the internal portions of these meteorites (away from any fusion crust or terrestrially exposed edge), contain little Ca‐sulfate (identified as gypsum), and no jarosite, whereas thin‐sections with terrestrially exposed edges have much higher sulfate abundances. These observations suggest that at least the majority of sulfate within the Miller Range nakhlites is terrestrially derived.     

Search for sulfates on the surface of Ceres

The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile‐rich protoplanets like Ceres. Sulfates, present on water‐bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres’ UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres’ cryogenic, low‐pressure environment and the consequent spectral effects, using UV–Vis–IR reflectance spectroscopy. H₂O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO₄·6H₂O by X‐ray powder diffraction. Long‐term (>40 h), continuous exposure of MgSO₄·nH₂O (n = 0, 6, 7) to low pressure (10^?3–10^?6 Torr) causes material decomposition and strong UV absorption below 0.5 μm. Our measurements suggest that MgSO₄·6H₂O grains (45–83 μm) dehydrate to 2% of the original 1.9 μm water band area over ~0.3 Ma at 200 K on Ceres and after ~42 Ma for 147 K. These rates, inferred from an Avrami dehydration model, preclude MgSO₄·6H₂O as a component of Ceres’ surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5–17 μm) suggests sulfates cannot be excluded from Ceres’ mineralogy.     

Epitaxial fluorapatite vein in Northwest Africa 998 host apatite: Clues on the geochemistry of late hydrothermal fluids on Mars

Secondary minerals in martian nakhlites provide a powerful tool for investigating the nature, composition, and duration of aqueous activity in the martian crust. Northwest Africa (NWA) 998 crystallized early from the nakhlite magmatic source and has evidence of minimal signatures of the late hydrothermal alteration event that altered the nakhlites. Using FIB-TEM techniques to study a cumulus apatite grain in NWA 998, we report the first evidence of a submicron-scale vein consisting of fluorapatite and an SiO₂-rich phase. Fluorapatite grew epitaxially on the walls of an opened cleavage plane of host F-bearing chlorapatite and the SiO₂-rich phase filled the center of the vein. The presence of nanoporosity and nanometer-scale amorphous material and the sharp interface between the vein and the host apatite indicate the vein represents a coupled dissolution–reprecipitation process that generated apatite of a different composition that was more stable with the fluid. Using experimental data and diffusion coefficients of Cl in apatite from the literature, we conclude that the vein was caused by a low temperature (~300°C), slightly acidic, F-, Si-rich, aqueous fluid that acted as a closed system. Based on the characteristics of the vein (formation by rapid injection of fluid) and the fluid (composition, temperature, pH), and the lack of terrestrial weathering products in our SEM and TEM images, we infer that the vein is pre-terrestrial in origin. Our observations support the hypothesis that the heat source triggering a hydrothermal system was a low-shock velocity impact and rule out a magmatic origin. Finally, the vein could have formed from a late-stage fluid different from that reported in other nakhlites, but formation during the same magmatic event by, for example, a less evolved fluid might also be plausible.     

Sulfide petrology of four nakhlites: Northwest Africa 817, Northwest Africa 998, Nakhla,and Governador Valadares

Abstract– The nakhlites contain small proportions of Cu‐Fe‐Ni sulfide minerals; we have studied these sulfides in Northwest Africa (NWA) 998, Nakhla, Governador Valadares, and NWA 817 with optical microscopy, scanning electron microscope, and electron microprobe. Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are uniformly low (0.02 to 0.05 ± 0.03 vol%), i.e., a factor 5 lower than in shergottites. Sulfides occur within the glassy mesostasis, as composite two‐phase Fe‐Ti oxide‐sulfide grains, intimately associated with interstitial grains or locally enclosed in postcumulus melt inclusions (e.g., Governador Valadares) in olivine. They exhibit a uniform low‐Ni monoclinic pyrrhotite composition ± chalcopyrite. There is a gradation of sulfide grain sizes and textures across the nakhlites flow(s): droplets in NWA 817; resorbed blebs in Governador Valadares; more massive, true intercumulus blebs in Nakhla and NWA 998. These nakhlites also show evidence for sulfide weathering. Hot desert finds (e.g., NWA 998 and NWA 817) show a few percent fracture‐filling iron (oxy) hydroxides of likely terrestrial origin. Original sulfides are 50% altered in our NWA 998 section, with iron (oxy) hydroxides at grain boundaries and as complete pseudomorphs. The compositions of unaltered pyrrhotites are homogeneous, close to that of the monoclinic endmember Fe₇S₈, and are too sulfur‐rich to have been in chemical equilibrium with the late magmatic redox state fixed by the fayalite‐magnetite‐quartz buffer. Therefore, the compositions of the pyrrhotites must have been altered during the later stages of magmatic crystallization, by assimilation of S‐rich regolith and hydrothermal circulation.     

Petrology of the Miller Range 03346 nakhlite in comparison with the Yamato‐000593 nakhlite

Abstract— We petrologically examined the Miller Range (MIL) 03346 nakhlite. The main‐phase modal abundances are 67.7 vol% augite, 0.8 vol% olivine, and 31.5 vol% mesostasis. Among all known nakhlites, MIL 03346's modal abundance of olivine is the smallest and of mesostasis is the largest. Augite occurs as cumulus phenocrysts having a homogeneous core composition (En_36–38Fs_24–22Wo₄₀), which is identical with other nakhlites. They accompany thin ferroan rims divided into inner and outer rims with a compositional gap at the boundary between the two rims. Olivine grains have magnesian cores (Fa ≥ 55) and show normal zoning toward ferroan rims (Fa ≤ 84). Mesostasis consists mostly of glass (26.0 vol%) with minor skeletal fayalites, skeletal titanomagnetites, acicular phosphate, massive cristobalite, and sulfides. We conclude that MIL 03346 is the most rapidly cooled nakhlite among all known nakhlites based on the petrography. We obtain the intercumulus melt composition for MIL 03346 from the mass balance calculation using the modal abundances and discuss the crystallization sequence of MIL 03346 in comparison with that of Yamato (Y‐) 000593. Although magnesian olivines of Y‐000593 are phenocrystic, magnesian olivine grains of MIL 03346 seem to have texturally crystallized from the intercumulus melt. After the MIL 03346 magma intruded upward to the Martian surficial zone, the magnesian olivine crystallized, and then the ferroan inner rim formed on phenocrystic core augite. The outer rim of phenocrystic augites formed after the crystallization of skeletal fayalites and skeletal titanomagnetites, resulting in a compositional gap between the inner and outer rims. Finally, glassy mesostasis formed from the residual melt. This crystallization sequence of MIL 03346 is different from those of other nakhlites, including Y‐000593.     

Estimate of the magnetic field of Mars based on the magnetic characteristics of the Yamato 000593 nakhlite

Abstract— Yamato 000593, a nakhlite, was analyzed in terms of its magnetic record and magnetomineralogy. The natural remanent magnetization (NRM: 3.55–6.07 times 10^?5 Am²/kg) was thermally demagnetized at ～320 °C, and it was unstable against alternating field demagnetization. Based on analyses of thermomagnetic curves, the temperature dependence of hysteresis parameters, and microscopic observations, the magnetic minerals mainly consist of magnetite (0.68 wt% of the sample, including ～5% Fe₂TiO₄) of less than 100 μm in size, associated with minor amounts of monoclinic pyrrhotite (<0.069 wt% of the sample) and goethite. Thermal demagnetization of NRM at ～330 °C is explained due to an offset of magnetization of antipodal NRM components of magnetite, whereas it is not due to a pyrrhotite Curie point. Large magnetite grains show exsolution texture with ilmenite laths, and are cut by silicate (including goethite) veins that formed along cracks. Numerous single‐domain (SD) and pseudo‐single‐domain (PSD) magnetite grains are scattered in the mesostasis and adjacent olivine grains. Moderate coercive forces of H_C = 6.8 mT and H_RC = 31.1 mT suggest that Yamato 000593 is fundamentally able to carry a stable NRM; however, NRM was found to be unstable. Accordingly, the meteorite was possibly crystallized at 1.3 Ga under an extremely weak or absent magnetic field, or was demagnetized by impact shock at 12 Ma (ejection age) on Mars. This finding differs from the results of previous paleomagnetic studies of SNC (shergottites, nakhlites, chassignites, and orthopyroxenite) Martian meteorites. The significant dipole magnetic field resulting from the molten metallic core was probably absent during the Amazonian Epoch (after 1.8 Ga) on Mars.     

Noble gases and chemical composition of Shergotty mineral fractions,Chassigny, and Yamato 793605: The trapped argon-40/argon-36 ratio and ejection times of Martian meteorites

Abstract— We report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene, maskelynite/mesostasis glass, and bulk material of Shergotty and of bulk samples from Chassigny and Yamato 793605. The ⁴⁰K-⁴⁰Ar isochron for the Shergotty minerals yields a gas retention age of 196 Ma, which is, within errors, in agreement with previously determined Rb-Sr internal isochron ages. Argon that was trapped at this time has a ⁴⁰Ar/³⁶Ar ratio of 1100. For Chassigny and Y-793605, we obtain trapped ⁴⁰Ar/³⁶Ar ratios of 1380 and 950, respectively. Using these results and literature data, we show that the three shergottites, Shergotty, Zagami, and QUE 94001; the lherzolites ALH 77005, LEW 88516, and Y-793605; as well as Chassigny and ALH 84001 contain a mixture of Martian mantle and atmospheric Ar; whereas, the trapped ⁴⁰Ar/³⁶Ar ratio of the nakhlites, Nakhla, Lafayette, and Governador Valadares cannot be determined with the present data. We show that Martian atmospheric trapped Ar in Martian meteorites is correlated with the shock pressure that they experienced. Hence, we conclude that the Martian atmospheric gases were introduced by shock into the meteoritic material. For the Shergotty minerals, we obtain ³He-, ²¹Ne-, and ³⁸Ar-based cosmic-ray exposure ages of 3.0 Ma, and for the lherzolite Y-793605, 4.0 Ma, which confirms our earlier conclusion that the lherzolites were ejected from Mars ～1 Ma before the shergottites. Chassigny yields the previously known ejection age of 11.6 Ma.     

Formation of a martian pyroxenite: A comparative study of the nakhlite meteorites and Theo's Flow

Abstract— The unusual composition of the nakhlites, a group of pyroxenitic martian meteorites with young ages, presents an opportunity to learn about nonbasaltic magmatic activity on another planet. However, the limited number of these meteorites makes unraveling their history difficult. A promising terrestrial analog for the formation of the nakhlites is Theo's Flow in Ontario, Canada. This atypical, 120 m-thick flow differentiated in place, forming distinct layered lithologies of peridotite, pyroxenite, and gabbro. Theo's pyroxenite and the nakhlites share strikingly similar petrographies, with concentrated euhedral to subhedral augite grains set in a plagioclase-rich matrix. These two suites of rocks also share specific petrologic features, mineral and whole-rock compositional features, and size and spatial distributions of cumulus grains. The numerous similarities suggest that the nakhlites formed by a similar mechanism in a surface lava flow or shallow intrusion. Their formation could have involved settling of crystals in a phenocryst-laden flow or in situ nucleation and growth of pyroxenes in an ultramafic lava flow. The latter case is more likely and requires steady-state nucleation and growth of clusters of pyroxene grains (and olivine in the nakhlites), circulating in a strongly convecting melt pool, followed by settling and continued growth in a thickening cumulate pile. Trapped pockets of intercumulus liquid in the pile gradually evolved, finally growing Fe-enriched rims on cumulus grains. With sufficient evolution, the melt reached plagioclase supersaturation, causing rapid growth of plagioclase sprays and late-stage mesostasis growth.     

A petrogenetic model for the comagmatic origin of chassignites and nakhlites: Inferences from chlorine‐rich minerals,petrology, and geochemistry

Twelve samples belonging to the chassignite and nakhlite subgroups of Martian meteorites were investigated using a variety of micro‐beam analytical techniques to gain insight into the petrogenesis of these two meteorite classes. There are a striking number of geochemical similarities between the chassignites and nakhlites, including mineralogy and petrology, crystallization age, cosmic‐ray exposure age, and radiogenic isotopic compositions. However, there are also geochemical differences, namely in trace element systematics of pyroxenes, that have led some authors to conclude that the nakhlites are comagmatic with each other, but not comagmatic with the chassignites. On the basis of data presented here, we propose a model in which these differences can be reconciled by the addition of an exogenous Cl‐rich fluid to the chassignite‐nakhlite magma body shortly after the formation of the cumulate horizon that was sampled by the Chassigny meteorite. This model is supported by the textural and chemical associations of the volatile‐bearing minerals apatite, amphibole, and biotite, which record a history starting with the addition of a Cl‐ and LREE‐enriched fluid to the magma body. As the magma continued to crystallize, it eventually reached chloride saturation and degassed a Cl‐rich fluid phase. Depending on the provenance of the Cl‐rich fluid, this model could explain how the chassignites and nakhlites originated from an LREE‐depleted source, yet all exhibit LREE‐enriched bulk‐rock patterns. Additionally, the model explains the range in oxygen fugacity that is recorded by the chassignites and nakhlites because eventual exsolution and loss of Cl‐rich fluid phases near the end of crystallization of the nakhlite sequence leads to auto‐oxidation of the magma body due to the preferential partitioning of Fe²⁺ into the fluid phase.     

Investigating the igneous petrogenesis of Martian volcanic rocks using augite quantitative textural analysis of the Yamato nakhlites

To better understand volcanism on planetary bodies other than the Earth, the quantification of physical processes is needed. Here, the petrogenesis of the achondrite Martian Yamato (Y) nakhlites (Y 000593, Y 000749, and Y 000802) is reinvestigated via quantitative analysis of augite (high-Ca clinopyroxene) phenocrysts: crystal size distribution (CSD), spatial distribution patterns (SDP), and electron backscatter diffraction (EBSD). Results from CSD and EBSD quantitative data sets show augite to have continuous uninterrupted growth resulting in calculated minimum magma chamber residence times of either 88–117 ± 6 yr or 9–12 yr. All samples exhibit low-intensity S-LS type crystallographic preferred orientation. Directional strain is observed across all samples with intracrystalline misorientation patterns indicative of (100)[001]:(001)[100] (Y 000593 and Y 000802) and {110}<001>or {110}¹/₂<110> (Y 000749) slip systems. SDP results indicate phenocryst-bearing crystal-clustered rock signatures. Combined findings from this work show that the Yamato nakhlites formed on Mars as individual low-viscosity lava flows or sills. This study shows that through combining these different quantitative techniques over multiple samples, one can more effectively compare and interpret resulting data to gain a more robust, geologically contextualized petrogenetic understanding of the rock suite being studied. The techniques used in this study should be equally applicable to igneous achondrites from other parent bodies.