H and Cl isotope systematics of apatite in brecciated lunar meteorites Northwest Africa 4472, Northwest Africa 773, Sayh al Uhaymir 169, and Kalahari 009

We have investigated the H and Cl systematics in apatite from four brecciated lunar meteorites. In Northwest Africa (NWA) 4472, most of the apatites contain ～2000–6000 ppm H₂O with δD between ?200 and 0‰, except for one grain isolated in the matrix, which contains ～6000 ppm H₂O with δD of ～500–900‰. This low‐δD apatite contains ～2500–7500 ppm Cl associated with δ³⁷Cl of ～15–20‰, while the high‐δD grain contains ～2500 ppm Cl with δ³⁷Cl of ～7–15‰. In NWA 773, apatites in a first group contain ～700–2500 ppm H₂O with δD values averaging around ～0 ± 100‰, while apatites in a second group contain ～5500–16500 ppm H₂O with δD ～250 ± 50‰. In Sayh al Uhaymir (SaU) 169 and Kalahari (Kal) 009, apatites are similar in terms of their H₂O contents (～600–3000 ppm) and δD values (?100 to 200‰). In SaU 169, apatites contain ～6000–10,000 ppm Cl, characterized by δ³⁷Cl of ～5–12‰. Overall, most of the analyzed apatite grains have δD within the range reported for carbonaceous chondrites, similar to apatite analyzed in ancient (>3.9 Ga) lunar magmatic. One grain in NWA 4472 has H and Cl isotope compositions similar to apatite from mare basalts. With an age of 4.35 Ga, this grain could be a representative of the oldest known lunar volcanic activity. Finally, since numerous evolved clasts in NWA 773 formed through silicate liquid immiscibility, the apatite grains with extremely high H₂O contents, reaching pure hydroxylapatite composition, could provide insights into the effects of such process on the evolution of volatiles in lunar magmas.     

Rb‐Sr and Sm‐Nd isotopic and REE studies of igneous components in the bulk matrix domain of Martian breccia Northwest Africa 7034

The bulk matrix domain of the Martian breccia NWA 7034 was examined petrographically and isotopically to better understand the provenance and age of the source material that make up the breccia. Both ¹⁴⁷Sm‐¹⁴³Nd and ¹⁴⁶Sm‐¹⁴²Nd age results for mineral separates from the bulk matrix portion of breccia NWA 7034 suggest that various lithological components in the breccia probably formed contemporaneously ~4.44 Ga ago. This old age is in excellent agreement with the upper intersection ages (4.35–4.45 Ga) for U‐Pb discordia and also concordia defined by zircon and baddeleyite grains in matrix and igneous‐textured clasts. Consequently, we confirm an ancient age for the igneous components that make up the NWA 7034 breccia. Substantial disturbance in the Rb‐Sr system was detected, and no age significance could be gleaned from our Rb‐Sr data. The disturbance to the Rb‐Sr system may be due to a thermal event recorded by bulk‐rock K‐Ar ages of 1.56 Ga and U‐Pb ages of phosphates at about 1.35–1.5 Ga, which suggest partial resetting from an unknown thermal event(s), possibly accompanying breccia formation. The NWA 7034 bulk rock is LREE enriched and similar to KREEP‐rich lunar rocks, which indicates that the earliest Martian crust was geochemically enriched. This enrichment supports the idea that the crust is one of the enriched geochemical reservoirs on Mars that have been detected in studies of other Martian meteorites.     

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 ).     

The chlorine isotopic composition of Martian meteorites 1: Chlorine isotope composition of Martian mantle and crustal reservoirs and their interactions

The Martian meteorites record a wide diversity of environments, processes, and ages. Much work has been done to decipher potential mantle sources for Martian magmas and their interactions with crustal and surface environments. Chlorine isotopes provide a unique opportunity to assess interactions between Martian mantle‐derived magmas and the crust. We have measured the Cl‐isotopic composition of 17 samples that span the range of known ages, Martian environments, and mantle reservoirs. The ³⁷Cl of the Martian mantle, as represented by the olivine‐phyric shergottites, NWA 2737 (chassignite), and Shergotty (basaltic shergottite), has a low value of approximately ?3.8‰. This value is lower than that of all other planetary bodies measured thus far. The Martian crust, as represented by regolith breccia NWA 7034, is variably enriched in the heavy isotope of Cl. This enrichment is reflective of preferential loss of ³⁵Cl to space. Most basaltic shergottites (less Shergotty), nakhlites, Chassigny, and Allan Hills 84001 lie on a continuum between the Martian mantle and crust. This intermediate range is explained by mechanical mixing through impact, fluid interaction, and assimilation‐fractional crystallization.     

The noble gas concentrations of the Martian meteorites GRV 99027 and paired NWA 7906/NWA 7907

Here we present the isotopic concentrations of He, Ne, Ar, Kr, and Xe for the three Martian meteorites, namely Grove Mountains 99027 (GRV 99027), Northwest Africa 7906 (NWA 7906), and Northwest Africa 7907 (NWA 7907). The cosmic ray exposure (CRE) age for GRV 99027 of 5.7 ± 0.4 Ma (1σ) is consistent with CRE ages for other poikilitic basaltic shergottites and suggests that all were ejected in a single event ~5.6 Ma ago. After correcting for an estimated variable sodium concentration, the CRE ages for NWA 7906 and NWA 7907 of 5.4 ± 0.4 and 4.9 ± 0.4 Ma (1σ), respectively, are in good agreement with the CRE age of ~5 Ma favored by Cartwright et al. ( 2014 ) for NWA 7034. The data, therefore, support the conclusion that all three basaltic regolith breccias are paired. The ⁴⁰Ar gas retention age for NWA 7907 of ~1.3 Ga is in accord with Cartwright et al. ( 2014 ). For NWA 7906, we were unable to determine a ⁴⁰Ar gas retention age. The ⁴He gas retention ages for NWA 7906 and 7907 are in the range of 200 Ma and are much shorter than the ⁴⁰Ar gas retention age of NWA 7907, indicating that about 86–88% of the radiogenic ⁴He has been lost. The Kr and Xe isotopic concentrations in GRV 99027 are composed almost exclusively of Martian interior (MI) gases, while for NWA 7906 and NWA 7907, they indicate gases from the MI, elementally fractionated air, and possibly Martian atmosphere.     

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.     

The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars

We determined the chlorine isotope composition of 16 Martian meteorites using gas source mass spectrometry on bulk samples and in situ secondary ion microprobe analysis on apatite grains. Measured δ³⁷Cl values range from ?3.8 to +8.6‰. The olivine‐phyric shergottites are the isotopically lightest samples, with δ³⁷Cl mostly ranging from ?4 to ?2‰. Samples with evidence for a crustal component have positive δ³⁷Cl values, with an extreme value of 8.6‰. Most of the basaltic shergottites have intermediate δ³⁷Cl values of ?1 to 0‰, except for Shergotty, which is similar to the olivine‐phyric shergottites. We interpret these data as due to mixing of a two‐component system. The first component is the mantle value of ?4 to ?3‰. This most likely represents the original bulk Martian Cl isotope value. The other endmember is a ³⁷Cl‐enriched crustal component. We speculate that preferential loss of ³⁵Cl to space has resulted in a high δ³⁷Cl value for the Martian surface, similar to what is seen in other volatile systems. The basaltic shergottites are a mixture of the other two endmembers. The low δ³⁷Cl value of primitive Mars is different from Earth and most chondrites, both of which are close to 0‰. We are not aware of any parent‐body process that could lower the δ³⁷Cl value of the Martian mantle to ?4 to ?3‰. Instead, we propose that this low δ³⁷Cl value represents the primordial bulk composition of Mars inherited during accretion. The higher δ³⁷Cl values seen in many chondrites are explained by later incorporation of ³⁷Cl‐enriched HCl‐hydrate.     

Postcrystallization metasomatism in shergottites: Evidence from the paired meteorites LAR 06319 and LAR 12011

Apatite is the major volatile‐bearing phase in Martian meteorites, containing structurally bound fluorine, chlorine, and hydroxyl ions. In apatite, F is more compatible than Cl, which in turn is more compatible than OH. During degassing, Cl strongly partitions into the exsolved phase, whereas F remains in the melt. For these reasons, the volatile concentrations within apatite are predictable during magmatic differentiation and degassing. Here, we present compositional data for apatite and merrillite in the paired enriched, olivine‐phyric shergottites LAR 12011 and LAR 06319. In addition, we calculate the relative volatile fugacities of the parental melts at the time of apatite formation. The apatites are dominantly OH‐rich (calculated by stoichiometry) with variable yet high Cl contents. Although several other studies have found evidence for degassing in the late‐stage mineral assemblage of LAR 06319, the apatite evolutionary trends cannot be reconciled with this interpretation. The variable Cl contents and high OH contents measured in apatites are not consistent with fractionation either. Volatile fugacity calculations indicate that water and fluorine activities remain relatively constant, whereas there is a large variation in the chlorine activity. The Martian crust is Cl‐rich indicating that changes in Cl contents in the apatites may be related to an external crustal source. We suggest that the high and variable Cl contents and high OH contents of the apatite are the results of postcrystallization interaction with Cl‐rich, and possibly water‐rich, crustal fluids circulating in the Martian crust.     

Annealing of radiation damage in zircons from Apollo 14 impact breccia 14311: Implications for the thermal history of the breccia

Impact breccia 14311, was collected from the Apollo 14 landing site as a potential sample of the underlying Fra Mauro Formation. Published zircon U‐Pb ages of >4000 Ma date the source material of the breccia and the apatite U‐Pb age of ~3940 Ma is interpreted as dating thermal resetting of the apatite U‐Pb systems. In this contribution we present new age information on the late stage thermal history of the breccia based on the annealing of radiation damage in the zircons. From Raman spectroscopic determination of the radiation damage within SIMS analytical spots on the zircons and the U and Th concentrations determined on these spots, we demonstrate that the radiation damage in the zircons has been annealed and we estimate the age of annealing at 3410 ± 80 Ma. This age is interpreted as a cooling age following heating of the breccia to above the annealing temperature of ~230 °C for stage 1 radiation damage in zircon, but below the temperature needed to reset the U‐Pb system of apatite (~500 °C). It is proposed that this thermal event was associated with the prolonged period of Mare volcanism, from 3150 to 3750 Ma, that generated massive basalt flows in the vicinity of the sample location.     

U‐Pb dating of zircons from an impact melt of the Nördlinger Ries crater

In situ U‐Pb measurements on zircons of the Ries impact crater are presented for three samples from the quarry at Polsingen. The U‐Pb data of most zircons plot along a discordia line, leading to an upper intercept of Carboniferous age (331 ± 32 Ma [2σ]). Four zircons define a concordia age of 313.2 ± 4.4 Ma (2σ). This age most probably represents the age of a granite from the basement target rocks. From granular textured zircon grains (including baddeleyite and anatase/Fe‐rich phases, first identified in the Ries crater), most probably recrystallized after impact (13 analyses, 4 grains), a concordia age of 14.89 ± 0.34 Ma (2σ) and an error weighted mean ²⁰⁶Pb*/²³⁸U age of Ma 14.63 ± 0.43 (2σ) is derived. Including the youngest concordant ages of five porous textured zircon grains (24 spot analyses), a concordia age of 14.75 ± 0.22 Ma (2σ) and a mean ²⁰⁶Pb*/²³⁸U age of 14.71 ± 0.26 Ma (2σ) can be calculated. These results are consistent with previously published ⁴⁰Ar/³⁹Ar ages of impact glasses and feldspar. Our results demonstrate that even for relatively young impact craters, reliable U‐Pb ages can be obtained using in situ zircon dating by SIMS. Frequently the texture of impact shocked zircon grains is explained by decomposition at high temperatures and recrystallization to a granular texture. This is most probably the case for the observed granular zircon grains having baddeleyite/anatase/Fe‐rich phases. We also observe non‐baddeleyite/anatase/Fe‐rich phase bearing zircons. For these domains, reset to crater age is more frequently for high U,Th contents. We tentatively explain the higher susceptibility to impact resetting of high U,Th domains by enhanced Pb loss and mobilization due to higher diffusivity within former metamict domains that were impact metamorphosed more easily into porous as well as granular textures during decomposition and recrystallization, possibly supported by Pb loss during postimpact cooling and/or hydrothermal activity.     

Determination of volatile concentrations in fluorapatite of Martian shergottite NWA 2975 by combining synchrotron FTIR,Raman spectroscopy,EMPA, and TEM,and inferences on the volatile budget of the apatite host‐magma

We combined the focused ion beam sample preparation technique with polarized synchrotron‐based FTIR (Fourier transform infrared) spectroscopy, laser‐Raman spectroscopy, electron microprobe analysis (EMPA), and transmission electron microscope (TEM) analysis to identify and quantify structurally bound OH, F, Cl, and CO₃ groups in fluorapatite from the Northwest Africa 2975 (NWA 2975) shergottite. In this study, the first FTIR spectra of the OH‐stretching region from a Martian apatite are presented that show characteristic OH‐bands of a F‐rich, hydroxyl‐bearing apatite. Depending on the method of apatite‐formula calculation and whether charge balance is assumed or not, the FTIR‐based quantification of the incorporated OH, expressed as wt% H₂O, is in variably good agreement with the H₂O concentration calculated from electron microprobe data. EMP analyses yielded between 0.35 and 0.54 wt% H₂O, and IR data yielded an average H₂O content of 0.31 ± 0.03 wt%, consistent with the lower range determined from EMP analyses. The TEM observations implied that the volatiles budget of fluorapatite is magmatic. The water content and the relative volatile ratios calculated for the NWA 2975 magma are similar to those established for other enriched or intermediate shergottites. It is difficult to define the source of enrichment: either Martian wet mantle or crustal assimilation. Comparing the environment of parental magma generation for NWA 2975 with the terrestrial mantle in terms of water content, it displays a composition intermediate between enriched and depleted MORB.     

The chlorine isotope composition of iron meteorites: Evidence for the Cl isotope composition of the solar nebula and implications for extensive devolatilization during planet formation

The bulk chlorine concentrations and isotopic compositions of a suite of non‐carbonaceous (NC) and carbonaceous (CC) iron meteorites were measured using gas source mass spectrometry. The δ³⁷Cl values of magmatic irons range from ?7.2 to 18.0‰ versus standard mean ocean chloride and are unrelated to their chlorine concentrations, which range from 0.3 to 161 ppm. Nonmagmatic IAB irons are comparatively Cl‐rich containing >161 ppm with δ³⁷Cl values ranging from ?6.1 to ?3.2‰. The anomalously high and low δ³⁷Cl values are inconsistent with a terrestrial source, and as Cl contents in magmatic irons are largely consistent with derivation from a chondrite‐like silicate complement, we suggest that Cl is indigenous to iron meteorites. Two NC irons, Cape York and Gibeon, have high cooling rates with anomalously high δ³⁷Cl values of 13.4 and 18.0‰. We interpret these high isotopic compositions to result from Cl degassing during the disruption of their parent bodies, consistent with their low volatile contents (Ga, Ge, Ag). As no relevant mechanisms in iron meteorite parent bodies are expected to decrease δ³⁷Cl values, whereas volatilization is known to increase δ³⁷Cl values by the preferential loss of light isotopes, we interpret the low isotope values of <?5‰ and down to ?7.2‰ to most closely represent the primordial isotopic composition of Cl in the solar nebula. Similar conclusions have been derived from low δ³⁷Cl values down to ?6, and ?3.8‰ measured in Martian and Vestan meteorites, respectively. These low δ³⁷Cl values are in contrast to those of chondrites which average around 0‰ previously explained by the incorporation of isotopically heavy HCl clathrate into chondrite parent bodies. The poor retention of low δ³⁷Cl values in many differentiated planetary materials suggest that extensive devolatilization occurred during planet formation, which can explain Earth's high δ³⁷Cl value by the loss of approximately 60% of the initial Cl content.     

The Northwest Africa 1500 meteorite: Not a ureilite,maybe a brachinite

Abstract– The Northwest Africa (NWA) 1500 meteorite is an olivine‐rich achondrite containing approximately 2–3 vol% augite, 1–2 vol% plagioclase, 1 vol% chromite, and minor orthopyroxene, Cl‐apatite, metal and sulfide. It was originally classified as a ureilite, but is currently ungrouped. We re‐examined the oxygen three‐isotope composition of NWA 1500. Results of ultra‐high precision (～0.03‰ for Δ¹⁷O) laser fluorination analyses of two bulk chips, and high precision (～0.3‰) secondary ion mass spectrometry (SIMS) analyses of olivine and plagioclase in a thin section, show that the oxygen isotope composition of NWA 1500 (Δ¹⁷O = ?0.22‰ from bulk samples and ?0.18 ± 0.06‰ from 16 mineral analyses) is within the range of brachinites. We compare petrologic and geochemical characteristics of NWA 1500 with those of brachinites and other olivine‐rich primitive achondrites, including new petrographic, mineral compositional and bulk compositional data for brachinites Hughes 026, Reid 013, NWA 5191, NWA 595, and Brachina. Modal mineral abundances, texture, olivine and pyroxene major and minor element compositions, plagioclase major element compositions, rare earth element abundances, and siderophile element abundances of NWA 1500 are within the range of those in brachinites and, in most cases, well distinguished from those of winonaites/IAB silicates, acapulcoites/lodranites, ureilites, and Divnoe. NWA 1500 shows evidence of internal reduction, in the form of reversely zoned olivine (Fo ～65–73 core to rim) and fine‐grained intergrowths of orthopyroxene + metal along olivine grain margins. The latter also occur in Reid 013, Hughes 026, NWA 5191, and NWA 595. We argue that reduction (olivine→enstatite + Fe⁰ + O₂) is the best hypothesis for their origin in these samples as well. We suggest that NWA 1500 should be classified as a brachinite, which has implications for the petrogenesis of brachinites. Fe‐Mn‐Mg compositions of brachinite olivine provide evidence of redox processes among bulk samples. NWA 1500 provides evidence for redox processes on a smaller scale as well, which supports the interpretation that these processes occurred in a parent body setting. SIMS data for ²⁶Al‐²⁶Mg isotopes in plagioclase in NWA 1500 show no ²⁶Mg excesses beyond analytical uncertainties (1–2‰). The calculated upper limit for the initial ²⁶Al/²⁷Al ratio of the plagioclase corresponds to an age younger than 7 Ma after CAI. Compared to ⁵³Mn‐⁵³Cr data for Brachina ( Wadhwa et al. 1998b ), this implies either a much younger formation age or a more protracted cooling history. However, Brachina is atypical and this comparison may not extend to other brachinites.     

Caleta el Cobre 022 Martian meteorite: Increasing nakhlite diversity

Caleta el Cobre (CeC) 022 is a Martian meteorite of the nakhlite group, showing an unbrecciated cumulate texture, composed mainly of clinopyroxene and olivine. Augite shows irregular core zoning, euhedral rims, and thin overgrowths enriched in Fe relative to the core. Low‐Ca pyroxene is found adjacent to olivine. Phenocrysts of Fe‐Ti oxides are titanomagnetite with exsolutions of ilmenite/ulvöspinel. Intercumulus material consists of both coarse plagioclase and fine‐grained mesostasis, comprising K‐feldspars, pyroxene, apatite, ilmenite, Fe‐Ti oxides, and silica. CeC 022 shows a high proportion of Martian aqueous alteration products (iddingsite) in olivine (45.1 vol% of olivine) and mesostasis. This meteorite is the youngest nakhlite with a distinct Sm/Nd crystallization age of 1.215 ± 0.067 Ga. Its ejection age of 11.8 ± 1.8 Ma is similar to other nakhlites. CeC 022 reveals contrasted cooling rates with similarities with faster cooled nakhlites, such as Northwest Africa (NWA) 817, NWA 5790, or Miller Range 03346 nakhlites: augite irregular cores, Fe‐rich overgrowths, fine‐grained K‐feldspars, quenched oxides, and high rare earth element content. CeC 022 also shares similarities with slower cooled nakhlites, including Nakhla and NWA 10153: pyroxene modal abundance, pyroxenes crystal size distribution, average pyroxene size, phenocryst mineral compositions, unzoned olivine, and abundant coarse plagioclase. Moreover, CeC 022 is the most magnetic nakhlite and represents an analog source lithology for the strong magnetization of the Martian crust. With its particular features, CeC 022 must originate from a previously unsampled sill or flow in the same volcanic system as the other nakhlites, increasing Martian sample diversity and our knowledge of nakhlites.     

U‐Pb SIMS ages of Apollo 14 zircon: Identifying distinct magmatic episodes

U‐Pb ages of zircon in four different Apollo 14 breccias (14305, 14306, 14314, and 14321) were obtained by secondary ion mass spectrometry. Some of the analyzed grains occur as cogenetic, poikilitic zircon grains in lithic clasts, revealing magmatic events at ~4286 Ma, ~4200–4220 Ma, and ~4150 Ma. The age distribution of the crystal clasts in the breccias exhibits a minor peak at ~4210 Ma, which can be attributed to a magmatic event, as recorded in zircon grains located in noritic clasts. An age peak at ~4335 Ma is present in all four breccias, as well as zircon grains from different Apollo landing sites, enhancing the confidence that these grains recorded a global zircon‐forming event. The overall age distribution among the four breccias exhibits minor differences between the breccias collected farther away from the Cone Crater and the ones collected within the continuous ejecta blanket of the Cone Crater. A granular zircon grain yielded a ²⁰⁷Pb/²⁰⁶Pb age of 3936 ± 8 Ma, which is interpreted as an impact event. A similar age of 3941 ± 5 Ma (n = 17, MSWD = 0.89, P = 0.58) was obtained for a large zircon grain (~430 × 340 μm in size). This grain might have crystallized in the same impact melt sheet which formed the granular zircon or the age is representative of the final extrusion of KREEP magma. The majority of zircon grains, however, occur as isolated crystal clasts within the matrix and their ages cannot be correlated with any real events (impact or magmatic) nor can the possibility be excluded that these ages represent partial resetting of the U‐Pb system.     

Petrography,mineral chemistry,and crystallization history of olivine‐phyric shergottite NWA 6234: A new melt composition

Knowledge of Martian igneous and mantle compositions is crucial for understanding Mars' mantle evolution, including early differentiation, mantle convection, and the chemical alteration at the surface. Primitive magmas provide the most direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The new Martian meteorite Northwest Africa (NWA) 6234 is an olivine‐phyric shergottite. Its most magnesian olivine cores (Fo₇₈) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that it represents a melt composition. Thermochemical calculations show that NWA 6234 not only represents a melt composition but is a primitive melt derived from an approximately Fo₈₀ mantle. Thus, NWA 6234 is similar to NWA 5789 and Y 980459 in the sense that all three are olivine‐phyric shergottites and represent primitive magma compositions. However, NWA 6234 is of special significance because it represents the first olivine‐phyric shergottite from a primitive ferroan magma. On the basis of Al/Ti ratio of pyroxenes in NWA 6234, the minor components in olivine and merrillite, and phosphorus zoning of olivine, we infer that the rock crystallized completely at pressures consistent with conditions in Mars' upper crust. The textural intergrowths of the two phosphates (merrillite and apatite) indicate that at a very last stage of crystallization, merrillite reacted with an OH‐Cl‐F‐rich melt to form apatite. As this meteorite crystallized completely at depth and never erupted, it is likely that its apatite compositions represent snapshots of the volatile ratios of the source region without being affected by degassing processes, which contain high OH‐F content.     

A new type of isotopic anomaly in shergottite sulfides

The isotopic composition and abundance of sulfur in extraterrestrial materials are of interest for constraining models of both planetary and solar system evolution. A previous study that included phase‐specific extraction of sulfur from 27 shergottites found the sulfur isotopic composition of the Martian mantle to be similar to that of terrestrial mid‐ocean ridge basalts, the Moon, and nonmagmatic iron meteorites. However, the presence of positive Δ³³S anomalies in igneous sulfides from several shergottites, indicating incorporation of atmospherically processed sulfur into the subsurface, complicated this interpretation. The current study expands upon the previous work through analyses of 20 additional shergottites, enabling tighter constraints on the isotopic composition of juvenile Martian sulfur. The updated composition (δ³⁴S = ?0.24 ± 0.05‰, Δ³³S = 0.0015 ± 0.0016‰, and Δ³⁶S = 0.039 ± 0.054‰, 2 s.e.m.), representing the weighted mean for all shergottites within the combined population of 47 without significant Δ³³S anomalies, strengthens our earlier result. The presence of sulfur isotopic anomalies in igneous sulfides of some meteorites suggests that their parent magmas may have assimilated crustal material. We observed small negative Δ³³S anomalies in sulfides from two meteorites, NWA 7635 and NWA 11300. Although negative Δ³³S anomalies have been observed in nakhlites and ALH 84001, previous anomalies in shergottites have all shown positive values of Δ³³S. Because NWA 7635 has formation age of 2.4 Ga and is much more ancient than shergottites analyzed previously, this finding expands our perspective on the continuity of Martian atmospheric sulfur photochemistry over geologic time.     

Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages,trapped gases,and a re‐evaluation of the evidence for solar cosmic ray‐produced neon in shergottites and other achondrites

We present noble gas data for 16 shergottites, 2 nakhlites (NWA 5790, NWA 10153), and 1 angrite (NWA 7812). Noble gas exposure ages of the shergottites fall in the 1–6 Ma range found in previous studies. Three depleted olivine‐phyric shergottites (Tissint, NWA 6162, NWA 7635) have exposure ages of ~1 Ma, in agreement with published data for similar specimens. The exposure age of NWA 10153 (~12.2 Ma) falls in the range of 9–13 Ma reported for other nakhlites. Our preferred age of ~7.3 Ma for NWA 5790 is lower than this range, and it is possible that NWA 5790 represents a distinct ejection event. A Tissint glass sample contains Xe from the Martian atmosphere. Several samples show a remarkably low (²¹Ne/²²Ne)_cos ratio < 0.80, as previously observed in a many shergottites and in various other rare achondrites. This was explained by solar cosmic ray‐produced Ne (SCR Ne) in addition to the commonly found galactic cosmic ray‐produced Ne, implying very low preatmospheric shielding and ablation loss. We revisit this by comparing measured (²¹Ne/²²Ne)_cos ratios with predictions by cosmogenic nuclide production models. Indeed, several shergottites, acalpulcoites/lodranites, angrites (including NWA 7812), and the Brachina‐like meteorite LEW 88763 likely contain SCR Ne, as previously postulated for many of them. The SCR contribution may influence the calculation of exposure ages. One likely reason that SCR nuclides are predominantly detected in meteorites from rare classes is because they usually are analyzed for cosmogenic nuclides even if they had a very small (preatmospheric) mass and hence low ablation loss.     

Petrography and composition of Martian regolith breccia meteorite Northwest Africa 7475

The Northwest Africa (NWA) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars ≤4.4 Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7–1.4 Ga, and a third that launched NWA 7475 from Mars ~5 Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15 GPa. The mild postshock‐heating that resulted from these shock pressures would have been insufficient to sterilize this water‐bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium‐carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma‐ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of ~5 Ma and an apparent diameter of 6.9 km, marks one possible launch site of NWA 7475.     

Petrology and oxygen isotopic compositions of clasts in HED polymict breccia NWA 5232

Northwest Africa (NWA) 5232, an 18.5 kg polymict eucrite, comprises eucritic and exogenic CM carbonaceous chondrite clasts within a clastic matrix. Basaltic clasts are the most abundant eucritic clast type and show a range of textures and grain size, from subophitic to granoblastic. Other eucritic clast types present include cumulate (high‐En pyroxene), pyroxene‐lath, olivine rich with symplectite intergrowths as a break‐down product of a quickly cooled Fe‐rich metastable pyroxferroite, and breccia (fragments of a previously consolidated breccia) clasts. A variable cooling rate and degree of thermal metamorphism, followed by a complex brecciation history, can be inferred for the clasts based on clast rounding, crystallization (and recrystallization) textures, pyroxene major and minor element compositions, and pyroxene exsolution. The range in δ¹⁸O of clasts and matrix of NWA 5232 reflects its origin as a breccia of mixed clasts dominated by eucritic lithologies. The oxygen isotopic compositions of the carbonaceous chondrite clasts identify them as belonging to CM group and indicate that these clasts experienced a low degree of aqueous alteration while part of their parent body. The complex evolutionary history of NWA 5232 implies that large‐scale impact excavation and mixing was an active process on the surface of the HED parent body, likely 4 Vesta.