Petrographic and geochemical evidence for multiphase formation of carbonates in the Martian orthopyroxenite Allan Hills 84001

Martian meteorites can provide valuable information about past environmental conditions on Mars. Allan Hills 84001 formed more than 4 Gyr ago, and owing to its age and long exposure to the Martian environment, and this meteorite has features that may record early processes. These features include a highly fractured texture, gases trapped during one or more impact events or during formation of the rock, and spherical Fe‐Mg‐Ca carbonates. In this study, we have concentrated on providing new insights into the context of these carbonates using a range of techniques to explore whether they record multiple precipitation and shock events. The petrographic features and compositional properties of these carbonates indicate that at least two pulses of Mg‐ and Fe‐rich solutions saturated the rock. Those two generations of carbonates can be distinguished by a very sharp change in compositions, from being rich in Mg and poor in Fe and Mn, to being poor in Mg and rich in Fe and Mn. Between these two generations of carbonate is evidence for fracturing and local corrosion.     

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.     

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     

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.     

Cation diffusion in calcite: Determining closure temperatures and the thermal history for the Allan Hills 84001 meteorite

Abstract— The presence of zoned Fe, Mg, Ca, and Mn in the carbonate phases associated with the cracks and inclusions of the Allan Hills (ALH) 84001 meteorite provides evidence for constraining the thermal history of the meteorite. Using self- and tracer-diffusion coefficients obtained from laboratory experiments on natural calcite, cooling rates are calculated for various temperatures and diffusion distances to assist in the evaluation of the compositional zoning associated with the carbonate phases in ALH 84001. The closure temperature model provides the average temperature below which compositional zoning will be preserved for a given cooling rate, that is, the temperature at which diffusion will be ineffective in homogenizing the phase. The validity of various theories for the formation of the carbonate globules may be examined, therefore, in view of the diffusion-limited kinetic constraints. Experiments using a thin film-mineral diffusion couple and ion microprobe for depth profiling analysis were performed for the temperature range of 550–800 °C to determine self- and tracer-diffusion coefficients for Ca and Mg and in calcite. The resulting activation energies for Ca (E_a(Ca) = 271 ± 80 kJ/mol) and for Mg (E_a(Mg) = 284 ± 74 kJ/mol) were used then to calculate a series of cooling rate, grain size, and closure temperature curves. The data indicate, for example, that by the diffusion of Mg in calcite, a 10 μm compositional zone would be completely homogenized at a temperature of 300 °C for cooling rates <100 K/Ma. These data provide no constraint on formation models that propose a low-temperature fluid precipitation mechanism; however, they indicate that the carbonate globules were not exposed to a high-temperature environment for long time scales following formation.     

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.     

Bhawad LL6 chondrite: Chemistry,petrology, noble gases,nuclear tracks,and cosmogenic radionuclides

Abstract— Chemical and mineral analysis of the Bhawad chondrite, which fell in Rajasthan in 2002, suggest that this stone belongs to LL6 group of chondrites. Based on helium, neon, and argon isotopes, it has a cosmic ray exposure age of 16.3 Ma. The track density in the olivines shows a narrow range of 1.7–6.8 times 10⁶/cm². The ²²Na/²⁶Al ratio of 1.13 is about 25% lower than the solar cycle average value of about 1.5, but is consistent with irradiation of the meteoroid to modulated galactic cosmic ray fluxes as expected for a fall around the solar maximum. The cosmogenic records indicate a pre‐atmospheric radius of about 7.5 cm. Based on U/Th‐⁴He and K‐⁴⁰Ar, the gas retention ages are low (about 1.1 Ga), indicating a major thermal event or shock event that lead to the complete loss of radiogenic ⁴He and ⁴⁰Ar and the partial loss of radiogenic ¹²⁹Xe and fission Xe from ²⁴⁴Pu.     

Argon-39-argon-40 “ages” and trapped argon in Martian shergottites,Chassigny, and Allan Hills 84001

Abstract— Argon-isotopic abundances were measured in neutron-irradiated samples of Martian meteorites Chassigny, Allan Hills (ALH) 84001, ALH 77005, Elephant Moraine (EET) 79001, Yamato (Y) 793605, Shergotty, Zagami, and Queen Alexandra Range (QUE) 94201, and in unirradiated samples of ALH 77005. Chassigny gives a ³⁹Ar-⁴⁰Ar age of 1.32 ± 0.07 Ga, which is similar to radiometric ages of the nakhlites. Argon-39-Argon-40 data for ALH 84001 indicate ages between 3.9 and 4.3 Ga. A more precise definition of this age requires detailed characterization of the multiple trapped Ar components in ALH 84001 and of ³⁹Ar recoil distribution. All six shergottite samples show apparent ³⁹Ar-⁴⁰Ar ages substantially older than the ～165–200 Ma range in ages given by other isotope dating techniques. Shergottites appear to contain ubiquitous Ar components acquired from the Martian atmosphere, the Martian mantle, and commonly terrestrial atmospheric contamination. Zagami feldspar also suggests inherited radiogenic ⁴⁰Ar. These data analyses indicate that the recent Martian atmospheric component trapped in shergottites has a ⁴⁰Ar/³⁶Ar ratio possibly as low as ～1750 and no greater than ～1900. These ratios are less than the value of 3000 ± 500 reported by Viking. The ⁴⁰Ar/³⁶Ar ratio for the Martian mantle component is probably <500 but is poorly constrained. The correlation between trapped ⁴⁰Ar/³⁶Ar and ¹²⁹Xe/¹³²Xe ratios in shergottite impact glasses and unirradiated samples of ALH 77005 shows considerable scatter and suggests that the ³⁶Ar/¹³²Xe ratio in the Martian components may vary. Resolution of Martian atmospheric ⁴⁰Ar/³⁶Ar ratio at different time periods (i.e., at ～4.0 and 0.2 Ga) is also difficult without an understanding of the composition of various trapped components.     

Cosmogenic and trapped noble gases in individual chondrules: Clues to chondrule formation

Abstract— We studied the elemental and isotopic abundances of noble gases (He, Ne, Ar in most cases, and Kr, Xe also in some cases) in individual chondrules separated from six ordinary, two enstatite, and two carbonaceous chondrites. Most chondrules show detectable amounts of trapped ²⁰Ne and ³⁶Ar, and the ratio (³⁶Ar/²⁰Ne)_t (from ordinary and carbonaceous chondrites) suggests that HL and Q are the two major trapped components. A different trend between (³⁶Ar/²⁰Ne)_t and trapped ³⁶Ar is observed for chondrules in enstatite chondrites indicating a different environment and/or mechanism for their formation compared to chondrules in ordinary and carbonaceous chondrites. We found that a chondrule from Dhajala chondrite (DH‐11) shows the presence of solar‐type noble gases, as suggested by the (³⁶Ar/²⁰Ne)_t ratio, Ne‐isotopic composition, and excess of ⁴He. Cosmic‐ray exposure (CRE) ages of most chondrules are similar to their host chondrites. A few chondrules show higher CRE age compared to their host, suggesting that some chondrules and/or precursors of chondrules have received cosmic ray irradiation before accreting to their parent body. Among these chondrules, DH‐11 (with solar trapped gases) and a chondrule from Murray chondrite (MRY‐1) also have lower values of (²¹Ne/²²Ne)_c, indicative of SCR contribution. However, such evidences are sporadic and indicate that chondrule formation event may have erased such excess irradiation records by solar wind and SCR in most chondrules. These results support the nebular environment for chondrule formation.     

Light noble gases in 11 achondrites: Cosmic ray exposure ages,gas retention ages,and preatmospheric sizes

We report light noble gas (He, Ne, and Ar) concentrations and isotopic ratios in 11 achondrites, Tafassasset (unclassified primitive achondrite), Northwest Africa (NWA) 12934 (angrite), NWA 12573 (brachinite), Jiddat al Harasis (JaH) 809 (ureilite), NWA 11562 (ungrouped achondrite), four lodranites (NWA 11901, NWA 7474, NWA 6685, and NWA 6484), NWA 2871 (acapulcoite), and Sahara 02029 (winonaite), most of which have not been previously studied for noble gases. We discuss their noble gas isotopic composition, determine their cosmogenic nuclide content, and systematically calculate their cosmic ray exposure (CRE) and gas retention ages. In addition, we estimate their preatmospheric radii and preatmospheric masses based on the shielding parameter (²²Ne/²¹Ne)_cos. None of the studied meteorites shows evidence of contribution from solar cosmic rays (SCRs). JaH 809 and NWA 12934 show evidence of ³He diffusive losses of >90% and 40%, respectively. The winonaite Sahara 02029 has lost most of its noble gases, either during or before analysis. The average CRE age of Tafassasset of ~49 Ma is lower than that reported by Patzer et al. (2003), but is consistent with it within the uncertainties; this confirms that Tafassasset and CR chondrites are not source paired, CR chondrites having CRE ages from 1 to 25 Ma (Herzog & Caffee, 2014). The ureilite JaH 809 has a CRE age of ~5.4 Ma, which falls into the typical range of exposure ages for ureilites; the angrite NWA 12934 has a CRE age of ~49 Ma, which is within the main range of exposure ages reported for angrites (0.2–56 Ma). We calculate a CRE age of ~2.4 Ma for the brachinite NWA 12573, which falls into a possible “cluster” in the brachinite CRE age histogram around ~3 Ma. Three lodranites (NWA 11901, NWA 7474, and NWA 6685) have CRE ages higher than the average CRE ages of lodranites measured so far, NWA 11901 and NWA 6685 having CRE ages far higher than the CRE age already reported by Li et al. (2019) on NWA 8118. The measured ⁴⁰K-⁴⁰Ar gas retention ages fit well into established systematics. The gas retention age of Tafassasset is consistent, within respective uncertainties, with that previously calculated by Patzer et al. (2003). Our study indicates that Tafassasset originates from a meteoroid with a preatmospheric radius of ~20 cm, however discordant with the radius of ~85 cm inferred in a previous study (Patzer et al., 2003).     

Light noble gases and cosmogenic radionuclides in Estherville,Budulan, and other mesosiderites: Implications for exposure histories and production rates

Abstract— We report measurements of ²⁶AI, ¹⁰Be, ⁴¹Ca, and ³⁶Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station. Average production rate ratios (atom/atom) for metal phase samples from Estherville and Budulan are ²⁶Al/¹⁰Be = 0.77 ± 0.02; ³⁶Cl/¹⁰Be = 5.3 ± 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find ²⁶Al/¹⁰Be = 0.72 ± 0.01 and ³⁶Cl/¹⁰Be = 4.5 ± 0.2. The average ⁴¹Ca/³⁶Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The ⁴¹Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from <400 to >1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of ²⁶Al and ¹⁰Be (dpm/kg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content. Unless shielding effects are larger than suggested by the ³⁶Cl and ⁴¹Ca activities of the metal phases, matrix effects are unimportant for ¹⁰Be and minor for ²⁶Al. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of ³⁶Cl/³⁶Ar exposure ages for the metal phases agree well with published values. Neon‐21 production rates for mesosiderite silicates calculated from these ages and from measured ²¹Ne contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites. We suggest that the elevation of the ²¹Ne production rate in mesosiderite silicates reflects a “matrix effect,” that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low‐energy secondary particles and hence the ²¹Ne production from Mg. As ¹⁰Be production is relatively insensitive to this matrix effect, ¹⁰Be/²¹Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard ²²Ne/²¹Ne based “shielding” corrections give fairly reliable ²¹Ne production rates in the mesosiderite silicates.     

Petrography,classification, oxygen isotopes,noble gases,and cosmogenic records of Kamargaon (L6) meteorite: The latest fall in India

A single piece of meteorite fell on Kamargaon village in the state of Assam in India on November 13, 2015. Based on mineralogical, chemical, and oxygen isotope data, Kamargaon is classified as an L‐chondrite. Homogeneous olivine (Fa: 25 ± 0.7) and low‐Ca pyroxene (Fs: 21 ± 0.4) compositions with percent mean deviation of <2, further suggest that Kamargaon is a coarsely equilibrated, petrologic type 6 chondrite. Kamargaon is thermally metamorphosed with an estimated peak metamorphic temperature of ~800 °C as determined by two‐pyroxene thermometry. Shock metamorphism studies suggest that this meteorite include portions of different shock stages, e.g., S3 and S4 (Stöffler et al. 1991 ); however, local presence of quenched metal‐sulfide melt within shock veins/pockets suggest disequilibrium melting and relatively higher shock stage of up to S5 (Bennett and McSween 1996 ). Based on noble gas isotopes, the cosmic‐ray exposure age is estimated as 7.03 ± 1.60 Ma and nitrogen isotope composition (δ¹⁵N = 18‰) also correspond well with the L‐chondrite group. The He‐U, Th, and K‐Ar yield younger ages (170 ± 25 Ma 684 ± 93, respectively) and are discordant. A loss of He during the resetting event is implied by the lower He‐U and Th age. Elemental ratios of trapped Ar, Kr, and Xe can be explained through the presence of a normal Q noble gas component. Relatively low activity of ²⁶Al (39 dpm/kg) and the absence of ⁶⁰Co activity suggest a likely low shielding depth and envisage a small preatmospheric size of the meteoroid (<10 cm in radius). The Kr isotopic ratios (⁸²Kr/⁸⁴Kr) further argue that the meteorite was derived from a shallow depth.     

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.     

Comparison of cosmic‐ray exposure ages and trapped noble gases in chondrule and matrix samples of ordinary,enstatite, and carbonaceous chondrites

Abstract— We performed a comprehensive study of the He, Ne, and Ar isotopic abundances and of the chemical composition of bulk material and components of the H chondrites Dhajala, Bath, Cullison, Grove Mountains 98004, Nadiabondi, Ogi, and Zag, of the L chondrites Grassland, Northwest Africa 055, Pavlograd, and Ladder Creek, of the E chondrite Indarch, and of the C chondrites Hammadah al Hamra 288, Acfer 059, and Allende. We discuss a procedure and necessary assumptions for the partitioning of measured data into cosmogenic, radiogenic, implanted, and indigenous noble gas components. For stone meteorites, we derive a cosmogenic ratio ²⁰Ne/²²Ne of 0.80 ± 0.03 and a trapped solar ⁴He/³He ratio of 3310 ± 130 using our own and literature data. Chondrules and matrix from nine meteorites were analyzed. Data from Dhajala chondrules suggest that some of these may have experienced precompaction irradiation by cosmic rays. The other chondrules and matrix samples yield consistent cosmic‐ray exposure (CRE) ages within experimental errors. Some CRE ages of some of the investigated meteorites fall into clusters typically observed for the respective meteorite groups. Only Bath's CRE age falls on the 7 Ma double‐peak of H chondrites, while Ogi's fits the 22 Ma peak. The studied chondrules contain trapped ²⁰Ne and ³⁶Ar concentrations in the range of 10^?6–10^?9 cm³ STP/g. In most chondrules, trapped Ar is of type Q (ordinary chondritic Ar), which suggests that this component is indigenous to the chondrule precursor material. The history of the Cullison chondrite is special in several respects: large fractions of both CR‐produced ³He and of radiogenic ⁴He were lost during or after parent body breakup, in the latter case possibly by solar heating at small perihelion distances. Furthermore, one of the matrix samples contains constituents with a regolith history on the parent body before compaction. It also contains trapped Ne with a ²⁰Ne/²²Ne ratio of 15.5 ± 0.5, apparently fractionated solar Ne.     

Noble gases and cosmogenic radionuclides in the Gold Basin L4 chondrite shower: Thermal history,exposure history,and pre‐atmospheric size

Abstract— We measured the concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in the stone and metal fractions of 15 fragments of the Gold Basin L4 chondrite shower, as well as noble gases in 18 Gold Basin fragments. A comparison of ¹⁰Be, ²⁶Al, and ⁴¹Ca concentrations with calculated production rates from two different models indicates that the Gold Basin samples came from depths of about 10 cm to more than 150 cm in an object with a radius of 3–5 m. As was predicted by recent model calculations, the noble gases show a reversal of the ²²Ne/²¹Ne ratio at very high shielding. The ²¹Ne/¹⁰Be and ²¹Ne/²⁶Al ratios in most samples are constant and correspond to a 4π exposure age of 18 ± 2 Myr. However, three Gold Basin samples show a 30–120% excess of ²¹Ne implying that they were previously exposed close to the surface of the parent body, whereas the other samples were buried several meters deeper. Concentrations of neutron‐capture ³⁶Ar in most samples are consistent with measured concentrations of neutron‐capture ³⁶Cl and an exposure age of 18 Myr. Large excesses of neutron‐capture ³⁶Ar were found in those samples with an excess of ²¹Ne, providing additional evidence of a first‐stage exposure on the parent body. The excess of spallation‐produced ²¹Ne and neutron‐capture‐produced ³⁶Ar in these samples indicate a first‐stage exposure of 35–150 Myr on the parent body. The radiogenic ⁴He and ⁴⁰Ar concentrations indicate a major impact on the parent body between 300 and 400 Myr ago, which must have preceded the impacts that brought the Gold Basin meteoroid to the surface of the parent body and then expelled it from the parent body 18 Myr ago.     

Iron‐rich aureoles in the CM carbonaceous chondrites Murray,Murchison, and Allan Hills 81002: Evidence for in situ aqueous alteration

Abstract— Iron‐rich aureoles in CM carbonaceous chondrites are previously unidentified domains of aqueously altered matrix material, whose FeO content may exceed that of the surrounding matrix by up to more than 15 wt%. We describe the petrography and mineralogy of these objects in the CM chondrites Murray, Murchison, and Allan Hills (ALH) 81002. The size of Fe‐rich aureoles ranges from a few hundred microns to several millimeters in diameter and appears to be a function of the degree of alteration of the host chondrite. The origin of Fe‐rich aureoles is related to the alteration of large metal grains that has resulted in the formation of characteristic PCP‐rich reaction products that are frequently observed at the centers of the aureoles. This suggests that Fe‐rich aureoles in CM chondrites are the result of the mobilization of Fe from altering metal grains into the matrix. The fact that Fe‐rich aureoles enclose numerous chondritic components such as chondrules, calcium‐aluminum‐rich inclusions (CAIs), and mineral fragments, as well as their radial symmetric appearance, are strong evidence that they formed in situ and that significant directional fluid flow was not involved in the alteration process. This and additional constraints, such as the distribution of S and other elements, as well as the inferred alteration conditions, are consistent with in situ parent‐body alteration. The observations are, however, entirely incompatible with preaccretionary alteration models in which the individual CM chondrite components have experienced diverse alteration histories. The presence of numerous intact aureoles in the brecciated CM chondrites Murray and Murchison further suggests that the alteration occurred largely after brecciation affected these meteorites. Therefore, the progressive aqueous alteration of CM chondrites may not be necessarily coupled to brecciation as has been previously proposed.     

Noble gases in angrites Northwest Africa 1296, 2999/4931, 4590, and 4801: Evolution history inferred from noble gas signatures

Noble gases in the five angrites Northwest Africa (NWA) 1296, 2999, 4590, 4801, and 4931 were analyzed with total melting and stepwise heating methods. The noble gases consist of in situ components: spallogenic, radiogenic, nucleogenic, and fission. Cosmic-ray exposure ages of the angrites (including literature data) spread uniformly from <0.2 to 56 Ma, and coarse-grained angrites have longer exposure ages than fine-grained angrites. It is implied that the parent bodies from which the two subgroups of angrites were ejected are different and have distinct orbital elements. The ²⁴⁴Pu-¹³⁶Xe relative ages of the angrites obtained by using ²⁴⁴Pu/¹⁵⁰Nd ratios are as old as that of Angra dos Reis, reflecting their early formation. On the other hand, another method to obtain ²⁴⁴Pu-¹³⁶Xe relative ages, using fission ¹³⁶Xe, spallogenic ¹²⁶Xe, and Ba/REE ratios, yields systematically older ²⁴⁴Pu-¹³⁶Xe ages than those obtained by using ²⁴⁴Pu/¹⁵⁰Nd ratios, which is explained by apparently high Ba/REE ratios caused by Ba contamination during terrestrial weathering. The ²⁴⁴Pu/²³⁸U ratio at 4.56 Ga of angrites is estimated as 0.0061 ± 0.0028, which is consistent with those for chondrules, chondrites, achondrites, and a terrestrial zircon. It is suggested that initial ²⁴⁴Pu/²³⁸U ratio has been spatially homogeneous at least in the inner part of the early solar system.     

Noble gases in chondrules and associated metal‐sulfide‐rich samples: Clues on chondrule formation and the behavior of noble gas carrier phases

Abstract— Chondrules are generally believed to have lost most or all of their trapped noble gases during their formation. We tested this assumption by measuring He, Ne, and Ar in chondrules of the carbonaceous chondrites Allende (CV3), Leoville (CV3), Renazzo (CR2), and the ordinary chondrites Semarkona (LL3.0), Bishunpur (LL3.1), and Krymka (LL3.1). Additionally, metalsulfide‐rich chondrule coatings were measured that probably formed from chondrule metal. Low primordial ²⁰Ne concentrations are present in some chondrules, while even most of them contain small amounts of primordial ³⁶Ar. Our preferred interpretation is that‐in contrast to CAIs‐the heating of the chondrule precursor during chondrule formation was not intense enough to expel primordial noble gases quantitatively. Those chondrules containing both primordial ²⁰Ne and ³⁶Ar show low presolar‐diamond‐like ³⁶Ar/²⁰Ne ratios. In contrast, the metal‐sulfide‐rich coatings generally show higher gas concentrations and Q‐like ³⁶Ar/²⁰Ne ratios. We propose that during metalsilicate fractionation in the course of chondrule formation, the Ar‐carrying phase Q became enriched in the metal‐sulfide‐rich chondrule coatings. In the silicate chondrule interior, only the most stable Ne‐carrying presolar diamonds survived the melting event leading to the low observed ³⁶Ar/²⁰Ne ratios. The chondrules studied here do not show evidence for substantial amounts of fractionated solar‐type noble gases from a strong solar wind irradiation of the chondrule precursor material as postulated by others for the chondrules of an enstatite chondrite.     

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.     

Noble gases and mineralogy of meteorites from China and the Grove Mountains,Antarctica: A 0.05 Ma cosmic ray exposure age of GRV 98004

Abstract— We determined the mineralogical and chemical characteristics and the He, Ne, and Ar isotopic abundances of 2 meteorites that fell in China and of 2 meteorites that were recovered by the 15th Chinese Antarctic Research Expedition. Guangmingshan (H5), Zhuanghe (H5), and Grove Mountain (GRV) 98002 (L5) yield cosmic ray exposure (CRE) ages of 68.7 ± 10.0 Ma, 3.8 ± 0.6 Ma, and 17.0 ± 2.5 Ma, respectively. These ages are within the range typically observed for the respective meteorite types. GRV 98004 (H5) had an extremely short parent body‐Earth transfer time of 0.052 ± 0.008 Ma. Its petrography and mineral chemistry are indistinguishable from other typical H5 chondrites. Only 3 other meteorites exist with similarly low CRE ages: Farmington (L5), Galim (LL6), and ALH 82100 (CM2). We show that several asteroids in Earth‐crossing orbits, or in the main asteroid belt with orbits close to an ejection resonance, are spectrally matching candidates and may represent immediate precursor bodies of meteorites with CRE ages ≤0.1 Ma.