Experimental shock decomposition of siderite and the origin of magnetite in Martian meteorite ALH 84001

Abstract Shock recovery experiments to determine whether magnetite could be produced by the decomposition of iron‐carbonate were initiated. Naturally occurring siderite was first characterized by a variety of techniques to be sure that the starting material did not contain detectable magnetite. Samples were shocked in tungsten‐alloy holders (W = 90%, Ni = 6%, Cu = 4%) to further ensure that any iron phases in the shock products were contributed by the siderite rather than the sample holder. Each sample was shocked to a specific pressure between 30 to 49 GPa. Transformation of siderite to magnetite as characterized by TEM was found in the 49 GPa shock experiment. Compositions of most magnetites are >50% Fe⁺² in the octahedral site of the inverse spinel structure. Magnetites produced in shock experiments display the same range of sizes (?50–100 nm), compositions (100% magnetite to 80% magnetite‐20% magnesioferrite), and morphologies (equant, elongated, euhedral to subhedral) as magnetites synthesized by Golden et al. (2001) and as the magnetites in Martian meteorite Allan Hills (ALH) 84001. Fritz et al. (2005) previously concluded that ALH 84001 experienced ?32 GPa pressure and a resultant thermal pulse of ?100–110°C. However, ALH 84001 contains evidence of local temperature excursions high enough to melt feldspar, pyroxene, and a silica‐rich phase. This 49 GPa experiment demonstrates that magnetite can be produced by the shock decomposition of siderite as a result of local heating to > 470°C. Therefore, magnetite in the rims of carbonates in Martian meteorite ALH 84001 could be a product of shock devolatilization of siderite as well.     

Petrogenesis of Allan Hills 84001: Constraints from impact‐melted feldspathic and silica glasses

Abstract— Compositional and textural relationships of shock‐melted glasses in the Allan Hills (ALH) 84001 meteorite have been examined by optical microscopy, electron microprobe analysis, and compositional mapping. The feldspathic and silica glasses exhibit features which constrain the relative timing of shock events and carbonate deposition in ALH 84001. The feldspathic glasses are stoichiometric and have compositions plausibly described as forming from igneous plagioclase (An_27–39Ab_58–68Or_3–7) or sanidine (Or₅₁Ab₄₆An₃), or from a mixture of these phases (mixed‐feldspar glasses). These observations argue against prior interpretations of feldspathic glasses as unflowed maskelynite, hydrothermal precipitates or alteration products, or shock melts that have undergone alkali volatilization. Carbonate was deposited around previously formed mixed‐feldspar glass clasts, suggesting that carbonate deposition occurred after the shock event that formed the granular bands (crushed zones) in this meteorite. SiO₂‐rich glasses appear to be silica remobilized during shock, with little addition of other material. A petrogenetic history of ALH 84001 consistent with the observations of feldspathic and silica glasses is (1) igneous crystallization and cumulate formation; (2) a pre‐carbonate shock event that formed the granular bands (crushed zones) and sheared chromites, and melted igneous plagioclase and sanidine to form mixed‐feldspar glasses; (3) carbonate and silica deposition in the granular bands (veining of plagioclase glasses by SiO₂ and deposition of carbonate around mixed‐feldspar and plagioclase glass clasts); (4) a post‐carbonate shock event that resulted in invasion of carbonate by feldspathic melts, shock faulting and decarbonation of carbonate, high‐temperature mobilization of silica melts, and minor dissolution of orthopyroxene by silica melts.     

Multi‐generational carbonate assemblages in martian meteorite Allan Hills 84001: Implications for nucleation,growth, and alteration

Abstract— The carbonate mineralogy of several complex carbonate‐rich regions in Allan Hills (ALH) 84001 has been examined. These regions contain familiar forms of carbonate, as well as textural forms previously unreported including carbonate rosettes, planiform “slab” carbonates, distinct “post‐slab” magnesites, and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite‐rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite‐siderite layers on the exterior surfaces of the carbonate. Post‐slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts.     

The history of Allan Hills 84001 revised: Multiple shock events

Abstract— The geologic history of Martian meteorite Allan Hills (ALH) 84001 is more complex than previously recognized, with evidence for four or five crater-forming impacts onto Mars. This history of repeated deformation and shock metamorphism appears to weaken some arguments that have been offered for and against the hypothesis of ancient Martian life in ALH 84001. Allan Hills 84001 formed originally from basaltic magma. Its first impact event (I1) is inferred from the deformation (D1) that produced the granular-textured bands (“crush zones”) that transect the original igneous fabric. Deformation D1 is characterized by intense shear and may represent excavation or rebound flow of rock beneath a large impact crater. An intense thermal metamorphism followed D1 and may be related to it. The next impact (I2) produced fractures, (Fr2) in which carbonate “pancakes” were deposited and produced feldspathic glass from some of the igneous feldspars and silica. After I2, carbonate pancakes and globules were deposited in Fr2 fractures and replaced feldspathic glass and possibly crystalline silicates. Next, feldspars, feldspathic glass, and possibly some carbonates were mobilized and melted in the third impact (I3). Microfaulting, intense fracturing, and shear are also associated with I3. In the fourth impact (I4), the rock was fractured and deformed without significant heating, which permitted remnant magnetization directions to vary across fracture surfaces. Finally, ALH 84001 was ejected from Mars in event I5, which could be identical to I4. This history of multiple impacts is consistent with the photogeology of the Martian highlands and may help resolve some apparent contradictions among recent results on ALH 84001. For example, the submicron rounded magnetite grains in the carbonate globules could be contemporaneous with carbonate deposition, whereas the elongate magnetite grains, epitaxial on carbonates, could be ascribed to vapor-phase deposition during I3.     

Epitaxial growth of nanophase magnetite in Martian meteorite Allan Hills 84001: Implications for biogenic mineralization

Abstract— Crystallographic relationships between magnetite, sulfides, and carbonate rosettes in fracture zones of the Allan Hills (ALH) 84001 Martian meteorite have been studied using analytical electron microscopy. We have focused on those magnetite grains whose growth mechanisms can be rigorously established from their crystallographic properties. Individual magnetite nanocrystals on the surfaces of carbonates are epitaxially intergrown with one another in “stacks” of single-domain crystals. Other magnetite nanocrystals are epitaxially intergrown with the surfaces of the carbonate substrates. The observed magnetite/carbonate (hkl) Miller indices orientation relationships are (?, ?, 3)_m ‖ (1, ?, 0)_c and (1, ?, 1)_m ‖ (0,0, 3)_c with lattice mismatches of ～13% and ～11%, respectively. Epitaxy is a common mode of vapor-phase growth of refractory oxides like magnetite, as is the spiral growth about axial screw dislocations previously observed in other magnetite nanocrystals in ALH 84001. Epitaxy rules out intracellular precipitation of these magnetites by (Martian) organisms, provides further evidence of the high-temperature (>120 °C) inorganic origins of magnetite in ALH 84001, and indicates that the carbonates also have been exposed to elevated temperatures.     

Alteration minerals,fluids, and gases on early Mars: Predictions from 1‐D flow geochemical modeling of mineral assemblages in meteorite ALH 84001

Clay minerals, although ubiquitous on the ancient terrains of Mars, have not been observed in Martian meteorite Allan Hills (ALH) 84001, which is an orthopyroxenite sample of the early Martian crust with a secondary carbonate assemblage. We used a low‐temperature (20 °C) one‐dimensional (1‐D) transport thermochemical model to investigate the possible aqueous alteration processes that produced the carbonate assemblage of ALH 84001 while avoiding the coprecipitation of clay minerals. We found that the carbonate in ALH 84001 could have been produced in a process, whereby a low‐temperature (~20 °C) fluid, initially equilibrated with the early Martian atmosphere, moved through surficial clay mineral and silica‐rich layers, percolated through the parent rock of the meteorite, and precipitated carbonates (thereby decreasing the partial pressure of CO₂) as it evaporated. This finding requires that before encountering the unweathered orthopyroxenite host of ALH 84001, the fluid permeated rock that became weathered during the process. We were able to predict the composition of the clay minerals formed during weathering, which included the dioctahedral smectite nontronite, kaolinite, and chlorite, all of which have been previously detected on Mars. We also calculated host rock replacement in local equilibrium conditions by the hydrated silicate talc, which is typically considered to be a higher temperature hydrothermal phase on Earth, but may have been a common constituent in the formation of Martian soils through pervasive aqueous alteration. Finally, goethite and magnetite were also found to precipitate in the secondary alteration assemblage, the latter associated with the generation of H₂. Apparently, despite the limited water–rock interaction that must have led to the formation of the carbonates ~ 3.9 Ga ago, in the vicinity of the ALH 84001 source rocks, clay formation would have been widespread.     

The igneous crystallization history of an ancient Martian meteorite from rare earth element microdistributions

Abstract— Rare earth element (REE) and other selected trace and minor element concentrations were measured in individual grains of orthopyroxene, feldspathic glass (of plagioclase composition) and merrillite of the ALH 84001 Martian meteorite. Unlike in other Martian meteorites, phosphate is not the main REE carrier in ALH 84001. The REE pattern of ALH 84001 bulk rock is dependent on the modal abundances of three REE-bearing phases, namely, orthopyroxene, which contains most of the heavy rare earth elements (HREEs); feldspathic glass, which dominates the Eu abundances; and merrillite, which contains the majority of the light rare earth elements (LREEs). Variations in the REE abundances previously observed in different splits of ALH 84001 can easily be explained in terms of small variations in the modal abundances of these three minerals without the need to invoke extensive redistribution of LREEs. At least some orthopyroxenes (i.e., those away from contacts with feldspathic glass) in ALH 84001 appear to have preserved their original REE zonation from igneous fractionation. An estimate of the ALH 84001 parent magma composition from that of the unaltered orthopyroxene “core” (i.e., zoned orthopyroxene with the lowest REE abundances) indicates that it is LREE depleted. This implies that the Martian mantle was already partly depleted within ～100 Ma of solar system formation, which is consistent with rapid accretion and differentiation of Mars. Although equilibration and exchange of REEs between phases (in particular, transport of LREEs into the interstitial phases, feldspathic glass and merrillite) cannot be ruled out, our data suggest that the LREE enrichment in melts “in equilibrium” with these interstitial phases is most likely the result of late-stage infiltration of the cumulate pile by a LREE-enriched melt.     

An experimental study on kinetically‐driven precipitation of calcium‐magnesium‐iron carbonates from solution: Implications for the low‐temperature formation of carbonates in martian meteorite Allan Hills 84001

Abstract— Spherical carbonate globules of similar composition, size, and radial Ca‐, Mg‐, and Fe‐zonation to those in martian meteorite Allan Hills (ALH) 84001 were precipitated from Mg‐rich, supersaturated solutions of Ca‐Mg‐Fe‐CO₂‐H₂O at 150 °C. The supersaturated solutions (pH ? 6–7) were prepared at room temperature and contained in Teflon^TM‐lined stainless steel vessels, which were sealed and heated to 150 °C for 24 h. Experiments were also conducted at 25 °C and no globules comparable to those of ALH 84001 were precipitated. Instead, amorphous Fe‐rich carbonates were formed after 24 h and Mg‐Fe calcites formed after 96 h. These experiments suggest a possible low‐temperature inorganic origin for the carbonates in martian meteorite ALH 84001.     

Olivine in Martian meteorite Allan Hills 84001: Evidence for a high-temperature origin and implications for signs of life

Abstract— Olivine from Martian meteorite Allan Hills (ALH) 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock glass. The inclusions are irregular in shape and range in size from ～40 μm to submicrometer. Some of the inclusions are elongate and boudinage-like. The olivine grains are in sharp contact with the enclosing orthopyroxene and often contain small inclusions of chromite. The olivine exhibits a very limited range of composition from Fo₆₅ to Fo₆₆ (n = 25). The δ¹⁸O values of the olivine and orthopyroxene analyzed by ion microprobe range from +4.3 to +5.3‰ and are indistinguishable from each other within analytical uncertainty. The mineral chemistries, O-isotopic data, and textural relationships indicate that the olivine inclusions were produced at a temperature >800 °C. It is unlikely that the olivines formed during the same event that gave rise to the carbonates in ALH 84001, which have more elevated and variable δ¹⁸O values, and were probably formed from fluids that were not in isotopic equilibrium with the orthopyroxene or olivine. The reactions most likely instrumental in the formation of olivine could be either the dehydration of hydrous silicates that formed during carbonate precipitation or the reduction of orthopyroxene and spinel. If the olivine was formed by either reaction during a postcarbonate heating event, the implications are profound with regards to the interpretations of McKay et al. (1996). Due to the low diffusion rates in carbonates, this rapid, high-temperature event would have resulted in the preservation of the fine-scale carbonate zoning, while partially devolatilizing select carbonate compositions on a submicrometer scale (Brearley, 1998a). This may have resulted in the formation of the minute magnetite grains that McKay et al. (1996) attributed to biogenic activity.     

Transmission electron microscopy of minerals in the martian meteorite Allan Hills 84001

Abstract— We have studied carbonate and associated oxides and glasses in a demountable section of Allan Hills 84001 (ALH 84001) using optical, scanning, and transmission electron microscopy (TEM) to elucidate their origins and the shock history of the rock. Massive, fracture‐zone, and fracture‐filling carbonates in typical locations were characterized by TEM, X‐ray microanalysis, and electron diffraction in a comprehensive study that preserved textural and spatial relationships. Orthopyroxene is highly deformed, fractured, partially comminuted, and essentially unrecovered. Lamellae of diaplectic glass and other features indicate shock pressures >30 GPa. Bridging acicular crystals and foamy glass at contacts of orthopyroxene fragments indicate localized melting and vaporization of orthopyroxene. Carbonate crystals are >5 mm in size, untwinned, and very largely exhibit the R3c calcite structure. Evidence of plastic deformation is generally found mildly only in fracture‐zone and fracture‐filling carbonates, even adjacent to highly deformed orthopyroxene, and appears to have been caused by low‐stress effects including differential shrinkage. High dislocation densities like those observed in moderately shocked calcite are absent. Carbonate contains impactderived glasses of plagioclase, silica, and orthopyroxene composition indicating brief localized impact heating. Stringers and lenses of orthopyroxene glass in fracture‐filling carbonate imply flow of carbonates and crystallization during an impact. Periclase (MgO) occurs in magnesite as 30–50 nm crystals adjacent to voids and negative crystals and as ?1 μm patches of 3 nm crystals showing weak preferred orientation consistent with (111)_MgO//(0001)_carb, as observed in the thermal decomposition of CaCO₃ to CaO. Magnetite crystals that are epitaxially oriented at voids, negative crystals, and microfractures clearly formed in situ. Fully embedded, faceted magnetites are topotactically oriented, in general with (111)_mag//(0001)_carb, so that their oxygen layers are aligned. In optically opaque rims, magnetites are more irregularly shaped and, except for the smallest crystals, poorly aligned. All magnetite and periclase crystals probably formed by exsolution from slightly non‐stoichiometric, CO₂‐poor carbonate following impact‐induced thermal decomposition. Any magnetites that existed in the rock before shock heating could not have preserved evidence for biogenic activity.     

Thermal history of ALH 84001 meteorite by Fe2+‐Mg ordering in orthopyroxene

Abstract— A single orthopyroxene crystal from the Martian meteorite Allan Hills (ALH) 84001 was studied by X‐ray diffraction (XRD) and electron microprobe analysis (EMPA) to retrieve information about its thermal history. Both sets of data were used to measure the Fe²⁺‐Mg order degree between the M1 and M2 sites expressed by the distribution coefficient k_D. The 529 ± 30°C closure temperature (T_c) of the Fe²⁺‐Mg ordering process of ALH 84001 orthopyroxene (Fs₂₈) was calculated using Stimpfl (2005a, 2005b) ln k_D versus 1/T equation obtained for intermediate iron sample. At this T_c, the orthopyroxene cooling rate, calculated by Ganguly's (1982) numerical method, was 0.1 °C/day. This study puts new constraints on the last high‐temperature thermal episode recorded by orthopyroxene. With reference to the geological history (Treiman 1998), we ascribe this episode to the I3 event, and we interpret the T_c of 529 °C as a lower limit for this impact heating. Our data confirm that experimentally defined physical conditions for the formation of magnetite from decomposition of carbonates took place on the Martian surface during event I3.     

Hosts of hydrogen in Allan Hills 84001: Evidence for hydrous martian salts in the oldest martian meteorite?

Abstract— The martian meteorite, Allan Hills (ALH) 84001, contains D‐rich hydrogen of plausible martian origin (Leshin et al., 1996). The phase identity of the host(s) of this hydrogen are not well known and could include organic matter (McKay et al., 1996), phlogopite (Brearley, 2000), glass (Mittlefehldt, 1994) and/or other unidentified components of this rock. Previous ion microprobe studies indicate that much of the hydrogen in ALH 84001 as texturally associated with concretions of nominally anhydrous carbonates, glass and oxides (Boctor et al., 1998; Sugiura and Hoshino, 2000). We examined the physical and chemical properties of the host(s) of this hydrogen by stepped pyrolysis of variously pre‐treated subsamples. A continuous‐flow method of water reduction and mass spectrometry (Eiler and Kitchen, 2001) was used to permit detailed study of the small amounts of this hydrogen‐poor sample available for study. We find that the host(s) of D‐rich hydrogen released from ALH 84001 at relatively low temperatures (?500 °C) is soluble in orthophosphoric and dilute hydrochloric acids and undergoes near‐complete isotopic exchange with water within hours at temperatures of 200 to 300 °C. These characteristics are most consistent with the carrier phase(s) being a hydrous salt (e.g., carbonate, sulfate or halide); the thermal stability of this material is inconsistent with many examples of such minerals (e.g., gypsum) and instead suggests one or more relatively refractory hydrous carbonates (e.g., hydromagnesite). Hydrous salts (particularly hydrous carbonates) are common on the Earth only in evaporite, sabkha, and hydrocryogenic‐weathering environments; we suggest that much (if not all) of the “martian” hydrogen in ALH 84001 was introduced in analogous environments on or near the martian surface rather than through biological activity or hydrothermal alteration of silicates in the crust.     

A petrographic history of martian meteorite ALH84001: Two shocks and an ancient age

Abstract— ALH84001 is an igneous meteorite, an orthopyroxenite of martian origin. It contains petrographic evidence of two shock metamorphic events, separated by thermal and chemical events. The evidence for two shock events suggests that ALH84001 is ancient and perhaps a sample of the martian highlands. From petrography and mineral chemistry, the history of ALH84001 must include: crystallization from magma, a first shock (impact) metamorphism, thermal metamorphism, low-temperature chemical alteration, and a second shock (impact) metamorphism. Originally, ALH84001 was igneous, an orthopyroxene-chromite cumulate. In the first shock event, the igneous rock was cut by melt-breccia or cataclastic veinlets, now bands of equigranular fine-grained pyroxene and other minerals (crush zones). Intact fragments of the cumulate were fractured and strained (now converted to polygonized zones). The subsequent thermal metamorphism (possibly related to the first shock) annealed the melt-breccia or cataclastic veinlets to their present granoblastic texture and permitted chemical homogenization of all mineral species present. The temperature of metamorphism was at least 875 °C, based on mineral thermometers. Next, Mg-Fe-Ca carbonates and pyrite replaced plagioclase in both clasts and granular bands, producing ellipsoidal carbonate globules with sub-micron scale compositional stratigraphy, repeated identically in all globules. The second shock event produced microfault offsets of carbonate stratigraphy and other mineral contacts, radial fractures around chromite and maskelynite, and strain birefringence in pyroxene. Maskelynite could not have been preserved from the first shock event, because it would have crystallized back to plagioclase. The martian source area for ALH84001 must permit this complex, multiple impact history. Very few craters on young igneous surfaces are on or near earlier impact features. It is more likely that ALH84001 was ejected from an old igneous unit (Hesperian or Noachian age), pocked by numerous impact craters over its long exposure at the martian surface.     

Comprehensive imaging and Raman spectroscopy of carbonate globules from Martian meteorite ALH 84001 and a terrestrial analogue from Svalbard

Abstract— We report a comprehensive imaging study including confocal microRaman spectroscopy, scanning electron microscopy (SEM), and 3‐D extended focal imaging light microscopy of carbonate globules throughout a depth profile of the Martian meteorite Allan Hills (ALH) 84001 and similar objects in mantle peridotite xenoliths from the Bockfjorden volcanic complex (BVC), Svalbard. Carbonate and iron oxide zoning in ALH 84001 is similar to that seen in BVC globules. Hematite appears to be present in all ALH 84001 carbonate‐bearing assemblages except within a magnesite outer rim found in some globules. Macromolecular carbon (MMC) was found in intimate association with magnetite in both ALH 84001 and BVC carbonates. The MMC synthesis mechanism appears similar to established reactions within the Fe‐C‐O system. By inference to a terrestrial analogue of mantle origin (BVC), these results appear to represent the first measurements of the products of an abiotic MMC synthesis mechanism in Martian samples. Furthermore, the ubiquitous but heterogeneous distribution of hematite throughout carbonate globules in ALH 84001 may be partly responsible for some of the wide range in measured oxygen isotopes reported in previous studies. Using BVC carbonates as a suitable analogue, we postulate that a low temperature hydrothermal model of ALH 84001 globule formation is most likely, although alteration (decarbonation) of a subset of globules possibly occurred during a later impact event.     

Bulk and stable isotopic compositions of carbonate minerals in Martian meteorite Allan Hills 84001: No proof of high formation temperature

Abstract— Understanding the origin of carbonate minerals in the Martian meteorite Allan Hills (ALH) 84001 is crucial to evaluating the hypothesis that they contain traces of ancient Martian life. Using arguments based on chemical equilibria among carbonates and fluids, an origin at >650 °C (inimical to life) has been proposed. However, the bulk and stable isotopic compositions of the carbonate minerals are open to multiple interpretations and so lend no particular support to a high-temperature origin. Other methods (possibly less direct) will have to be used to determine the formation temperature of the carbonates in ALH 84001.     

Experimental hydrothermal alteration of a martian analog basalt: Implications for martian meteorites

Abstract— A number of martian meteorite samples contain secondary alteration minerals such as Ca‐Mg‐Fe carbonates, Fe oxides, and clay minerals. These mineral assemblages hint at hydrothermal processes occurring in the martian crust, but the alteration conditions are poorly constrained. This study presents the results of experiments that examined the alteration of a high‐Fe basalt by CO₂‐saturated aqueous fluids at 23 and 75 °C and by mixed H₂O‐CO₂ vapors at 200 and 400 °C and water‐rock ratios of 1:1 and 1:10. Results indicate that observable alteration of the basalt takes place after runs of only seven days. This alteration includes mobilization of silica into phases such as opal‐CT and quartz, as well as the formation of carbonates, oxides, and at some conditions, zeolites and hydrous silicates. The degree of alteration increases with run temperature and, in high‐temperature vapor experiments, with increasing water content of the vapor. The degree of alteration and the mineralogy observed in the martian meteorites suggests that none of these samples were exposed to aqueous fluids for long periods of time. Nakhla and Lafayette probably interacted with water for relatively brief periods of time; if so, silica may have been leached from the parent rocks by the altering fluids. Allan Hills 84001 shows possible evidence for very limited interaction with an aqueous fluid, but the overall slight degree of alteration described for this meteorite strongly suggests that it never interacted extensively or at high temperature with any water‐bearing fluid. Elephant Moraine A79001 may not have been altered by aqueous fluids at all. The results of this study best support models wherein the meteorite parent rocks were wetted intermittently or for brief periods of time rather than models that invoke long‐term reaction with large volumes of water. Our experiments studied alteration of a high‐Fe basalt by dilute, CO₂‐saturated, aqueous solutions at 23 and 75 °C and by mixed H₂O‐CO₂ vapors at 200 and 400 °C. The results suggest that alteration of the parent rock takes place even after very short reaction times of seven days. All experiments produced carbonate minerals, including calcite, and in some cases, magnesite, siderite, and ankerite. A free silica phase, either opal, quartz, or hydrated silica, formed in most experiments. More altered experiments also contained minerals such as zeolites and hydrous phyllosilicates. Clay minerals were not observed to form in any experiments. In aqueous fluids, higher temperature corresponded with a higher degree of alteration, whereas changing fluid composition had no observable effect. In high‐temperature vapors, the degree of alteration was controlled by temperature and the proportion of H₂O to CO₂, with water‐rock ratio also playing a role in transport of silica. Application of these results to martian meteorites that contain secondary alteration minerals suggests that none of the martian rocks underwent extensive interaction with aqueous fluids. Nakhla and Lafayette contain clay minerals, which suggests that they interacted with water to some extent, possibly at elevated temperatures. Although ALH84001 shows possible evidence of very limited interaction with aqueous fluids, EETA79001 does not. These results support models for the alteration of these meteorites that do not invoke long‐term interaction with water or reaction with large volumes of water. Except for some models for alteration of ALH84001, this conclusion agrees with most of the literature on alteration of martian meteorites.     

An abiotic origin for hydrocarbons in the Allan Hills 84001 martian meteorite through cooling of magmatic and impact‐generated gases

Abstract— Thermodynamic calculations of metastable equilibria were used to evaluate the potential for abiotic synthesis of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite Allan Hills (ALH) 84001. The calculations show that PAHs and normal alkanes could form metastably from CO, CO², and H² below approximately 250–300°C during rapid cooling of trapped magmatic or impact‐generated gases. Depending on temperature, bulk composition, and oxidation‐reduction conditions, PAHs and normal alkanes can form simultaneously or separately. Moreover, PAHs can form at lower H/C ratios, higher CO/CO² ratios, and higher temperatures than normal alkanes. Dry conditions with H/C ratios less than approximately 0.01–0.001 together with high CO/CO² ratios also favor the formation of unalkylated PAHs. The observed abundance of PAHs, their low alkylation, and a variable but high aromatic to aliphatic ratio in ALH 84001 all correspond to low H/C and high CO/CO² ratios in magmatic and impact gases and can be used to deduce spatial variations of these ratios. Some hydrocarbons could have been formed from trapped magmatic gases, especially if the cooling was fast enough to prevent reequilibration. We propose that subsequent impact heating(s) in ALH 84001 could have led to dissociation of ferrous carbonates to yield fine‐grain magnetite, formation of a CO‐rich local gas phase, reduction of water vapor to H², reequilibration of the trapped magmatic gases, aromatization of hydrocarbons formed previously, and overprinting of the synthesis from magmatic gases, if any. Rapid cooling and high‐temperature quenching of CO‐, H²‐rich impact gases could have led to magnetite‐catalyzed hydrocarbon synthesis     

Hydrogen‐isotopic compositions in Allan Hills 84001 and the evolution of the martian atmosphere

Abstract— Hydrogen‐isotopic compositions of carbonate and maskelynite in Allan Hills (ALH) 84001 were measured by secondary ion mass spectrometry (SIMS). the δd values of both minerals show considerable deviation. The deviation seems to be caused by addition of varying amounts of terrestrial water in the case of carbonate. In the case of maskelynite, H is heterogeneously distributed and the deviation in δD values seems to be due to mixing of this indigenous heavy H with isotopically normal H present in the SIMS chamber. The indigenous δD value in ALH 84001 seems to be ～2000%‰. Carbonate rather than maskelynite seems to be the main carrier of H in ALH 84001. Because ALH 84001 is ～4 Ga old, the H‐isotopic composition suggests that a large fraction of the initial martian atmosphere had already escaped by 4 Ga.     

A nitrogen and argon stable isotope study of Allan Hills 84001: Implications for the evolution of the Martian atmosphere

Abstract— The abundances and isotopic compositions of N and Ar have been measured by stepped combustion of the Allan Hills 84001 (ALH 84001) Martian orthopyroxenite. Material described as shocked is N-poor ([N] ～ 0.34 ppm; δ¹⁵N ～ +23%); although during stepped combustion, ¹⁵N-enriched N (δ¹⁵N ～ +143%) is released in a narrow temperature interval between 700 °C and 800 °C (along with ¹³C-enriched C (δ¹³C ～ +19%) and ⁴⁰Ar). Cosmogenic species are found to be negligible at this temperature; thus, the iso-topically heavy component is identified, in part, as Martian atmospheric gas trapped relatively recently in the history of ALH 84001. The N and Ar data show that ALH 84001 contains species from the Martian lithosphere, a component interpreted as ancient trapped atmosphere (in addition to the modern atmospheric species), and excess ⁴⁰Ar from K decay. Deconvolution of radiogenic ⁴⁰Ar from other Ar components, on the basis of end-member ³⁶Ar/¹⁴N and ⁴⁰Ar/³⁶Ar ratios, has enabled calculation of a K-Ar age for ALH 84001 as 3.5–4.6 Ga, depending on assumed K abundance. If the component believed to be Martian palaeoatmos-phere was introduced to ALH 84001 at the time the K-Ar age was set, then the composition of the atmosphere at this time is constrained to: δ¹⁵N ≥ +200%, ⁴⁰Ar/³⁶Ar ≤ 300 and ³⁶Ar/¹⁴N ≥ 17 × 10^?5. In terms of the petrogenetic history of the meteorite, ALH 84001 crystallised soon after differentiation of the planet, may have been shocked and thermally metamorphosed in an early period of bombardment, and then subjected to a second event. This later process did not reset the K-Ar system but perhaps was responsible for introducing (recent) atmospheric gases into ALH 84001; and it might mark the time at which ALH 84001 suffered fluid alteration resulting in the formation of the plagioclase and carbonate mineral assemblages.     

Regolith breccia Northwest Africa 7533: Mineralogy and petrology with implications for early Mars

Northwest Africa 7533, a polymict Martian breccia, consists of fine‐grained clast‐laden melt particles and microcrystalline matrix. While both melt and matrix contain medium‐grained noritic‐monzonitic material and crystal clasts, the matrix also contains lithic clasts with zoned pigeonite and augite plus two feldspars, microbasaltic clasts, vitrophyric and microcrystalline spherules, and shards. The clast‐laden melt rocks contain clump‐like aggregates of orthopyroxene surrounded by aureoles of plagioclase. Some shards of vesicular melt rocks resemble the pyroxene‐plagioclase clump‐aureole structures. Submicron size matrix grains show some triple junctions, but most are irregular with high intergranular porosity. The noritic‐monzonitic rocks contain exsolved pyroxenes and perthitic intergrowths, and cooled more slowly than rocks with zoned‐pyroxene or fine grain size. Noritic material contains orthopyroxene or inverted pigeonite, augite, calcic to intermediate plagioclase, and chromite to Cr‐bearing magnetite; monzonitic clasts contain augite, sodic plagioclase, K feldspar, Ti‐bearing magnetite, ilmenite, chlorapatite, and zircon. These feldspathic rocks show similarities to some rocks at Gale Crater like Black Trout, Mara, and Jake M. The most magnesian orthopyroxene clasts are close to ALH 84001 orthopyroxene in composition. All these materials are enriched in siderophile elements, indicating impact melting and incorporation of a projectile component, except for Ni‐poor pyroxene clasts which are from pristine rocks. Clast‐laden melt rocks, spherules, shards, and siderophile element contents indicate formation of NWA 7533 as a regolith breccia. The zircons, mainly derived from monzonitic (melt) rocks, crystallized at 4.43 ± 0.03 Ga (Humayun et al. 2013 ) and a ¹⁴⁷Sm‐¹⁴³Nd isochron for NWA 7034 yielding 4.42 ± 0.07 Ga (Nyquist et al. 2016 ) defines the crystallization age of all its igneous portions. The zircon from the monzonitic rocks has a higher Δ¹⁷O than other Martian meteorites explained in part by assimilation of regolith materials enriched during surface alteration (Nemchin et al. 2014 ). This record of protolith interaction with atmosphere‐hydrosphere during regolith formation before melting demonstrates a thin atmosphere, a wet early surface environment on Mars, and an evolved crust likely to have contaminated younger extrusive rocks. The latest events recorded when the breccia was on Mars are resetting of apatite, much feldspar and some zircons at 1.35–1.4 Ga (Bellucci et al. 2015 ), and formation of Ni‐bearing pyrite veins during or shortly after this disturbance (Lorand et al. 2015 ).