Dar Al Gani Research Articles

Olivine microstructure and thermometry in olivine‐phyric shergottites Sayh al Uhaymir 005 and Dar al Gani 476

AbstractOlivine‐phyric shergottites are relatively young Martian meteorites that resemble primitive mantle‐derived melts, so offer insight into the causes of recent magmatism on Mars. The Al‐in‐olivine geothermometer offers the potential to examine (near‐)liquidus melt temperatures. However, the ubiquitous shock features in most Martian meteorites, caused by high‐energy impacts, can change the structure and composition of olivine crystals, making the applicability of mineral geothermometry methods uncertain. This study examines microstructure and mineral chemistry in two shocked primitive, depleted olivine‐phyric shergottites, Sayh al Uhaymir (SaU) 005 and Dar al Gani (DaG) 476. DaG 476 is unsuitable for Al‐in‐olivine thermometry because of the presence of difficult‐to‐observe but pervasive networks of undulating veins in olivine down to sub‐micron sizes, caused by melting and providing pathways for cation diffusion. In contrast, SaU 005 can be used for Al‐in‐olivine thermometry despite the presence of conjugate shear and fracture sets and micron‐scale cpx‐spinel exsolution. The average crystallization temperature of Fo>70 olivine in SaU 005, 1380°C, is near‐identical to the average temperature of new and published Fo>70 data from all olivine‐phyric shergottites. When corrected for equilibrium with mantle olivine (Fo80) this corresponds to a mantle temperature of approximately 1500°C, 130°C hotter than ambient Martian mantle when shergottites formed. Shergottites were generated by melting within a moderately hot mantle plume or thermal anomaly, in support of other evidence that the Martian mantle is actively convecting. However, it does not support the extremely high potential temperatures estimated for the shergottite source by a whole‐rock petrological method.

Petrogenesis of the Dar al Gani (DaG) 1.1 Ma ejection‐paired olivine‐phyric shergottites and implications for ~470 Ma Martian volcanism

AbstractThe Dar al Gani (DaG) olivine‐phyric shergottites share mineralogical and geochemical characteristics, which confirm that these meteorites are derived from a single source. Bulk trace elements (La/Yb—0.12), in situ maskelynite 87Sr/86Sr (~0.7014) and redox estimates (FMQ ~ −2) indicate derivation from a depleted, reduced mantle reservoir; identical to all ~470 Ma shergottites ejected at 1.1 Ma. The DaG shergottites have been variably affected by terrestrial alteration, which precipitated carbonate along fractures and modified bulk‐rock fluid mobile (e.g., Ba) elements. Nonetheless, sufficient data are available to construct a multi‐stage formation model for the DaG shergottites and other 1.1 Ma ejection‐paired shergottites that erupted at ~470 Ma. First, partial melting of a depleted mantle source occurred at 1540 ± 20°C and 1.2 ± 0.1 GPa, equivalent to > ~100 km depth. Then, initial crystallization in a staging chamber at ~85 km depth at the crust–mantle boundary took place, followed by magma evolution and variable incorporation of antecrystic olivine ± orthopyroxene. Subsequently, crystallization of olivine phenocrysts and re‐equilibration of olivine antecrysts occurred within an ascending magma. Finally, magmas with variable crystal loads erupted at the surface, where varied cooling rates produced a range of groundmass textures. This model is similar to picritic flood basalt magmas erupted on Earth.

Sm and Gd Isotopic Shifts in Eucrites and Implications for Their Cosmic-Ray Exposure History

The isotopic compositions of Sm and Gd in eight eucrites—five from a desert, Dar al Gani (DaG) 380, DaG 391, DaG 411, DaG 443, and DaG 480, and three from nondesert areas, Juvinas, Millibillillie, and Stannern—were determined to understand the cosmic-ray exposure (CRE) history for each meteorite from the isotopic shifts of 149Sm–150Sm and 157Gd–158Gd caused by the neutron capture reactions induced by cosmic-ray irradiation. Seven of the eight samples, excepting DaG 443, show readily detectable isotopic shifts of Sm and Gd corresponding to neutron fluences in the range of (0.28–2.38) × 1015 neutrons cm−2. The degrees of Sm isotopic shifts for six of these seven eucrites can be consistently explained by the CRE age histogram of eucrites obtained in previous studies. Exceptionally, DaG 480 shows larger isotopic shifts of Sm than those expected from the CRE age histogram, suggesting a multiple-irradiation history, including irradiation on the parent body. However, there is no clear difference in the CRE conditions between DaG 480 and other eucrites from the parameter ε Sm/ε Gd to identify the difference in the thermalization degree of neutrons in association with the CRE conditions.

Constraints on martian depleted shergottite volcanism from the petrogenesis of olivine-phyric shergottites NWA 2046 and NWA 4925

The 1.1Ma ejection-paired depleted shergottites are sourced from a long-lived volcanic centre, likely located on the Tharsis plateau. Here we present bulk-rock geochemistry, mineral major- and trace-element concentrations, and in-situ maskelynite and pyroxene 87Sr/86Sr data for two depleted olivine-phyric shergottites, Northwest Africa (NWA) 2046 and NWA 4925, from this 1.1 Ma ejection event. The earliest formed olivine megacryst cores in NWA 2046 (Fo83) and NWA 4925 (Fo81) are among the most Mg-rich recorded in martian meteorites and are also in Mg–Fe equilibrium with their respective bulk-rock compositions. We infer the olivine megacrysts in NWA 2046 and NWA 4925 are phenocrysts and suggest that both samples represent melt compositions. CI chondrite-normalised rare earth element patterns for melts calculated to be in equilibrium with pyroxene in NWA 2046 and NWA 4925 are parallel to those of melts in equilibrium with maskelynite and merrillite, which suggests that both samples crystallised from continuously fractionating melts unaffected by crustal contamination. Oxygen fugacity estimates for NWA 2046 (QFM -2.7) and NWA 4925 (QFM -2.9), derived from the V-in-olivine oxybarometer, are comparable to other depleted olivine-phyric shergottites, including Tissint, Larkman Nunatak (LAR) 12095/12240, Dar Al Gani (DaG) 476 and Dhofar (Dho) 019.Effects of terrestrial alteration are observed in bulk-rock and mineral compositions from NWA 2046 and NWA 4925. The degree of terrestrial alteration is most pronounced in NWA 4925, consistent with an older reported exposure age relative to NWA 2046. The low Ni-contents of bulk-rock and olivine megacrysts in NWA 2046 and NWA 4925 suggests that both samples are not primary mantle-derived melt, but rather have undergone 8–10% and 10–13% fractional crystallisation, respectively, in a staging chamber at depth. Pyroxenes Ti/Al for NWA 2046 and NWA 4925, suggest initial crystallisation at ∼ 10 kbar, consistent with other ejection-paired olivine-phyric shergottites, indicating either a common or multiple staging chambers near the base of the martian crust. The 87Sr/86Sr compositions for unaltered maskelynite grains in NWA 2046 (0.70130 ± 7) and NWA 4925 (0.70128 ± 10) suggest that both samples originate from a similar mantle source to other depleted olivine-phyric shergottites, particularly DaG 476, and Sayh al Uhaymir (SaU) 005 and Yamato 980459 (Y-980459).

Mineralogy, petrology, and oxygen isotopic compositions of aluminum-rich chondrules from unequilibrated ordinary and the Dar al Gani 083 (CO3.1) chondrite

Understanding the genetic relationship between different chondritic components will help to decipher their origin and dynamical evolution within the protoplanetary disk. Here, we obtain insight into these processes by acquiring O-isotope data from 17 Al-rich chondrules from unequilibrated ordinary chondrites (OCs, petrologic type ≤ 3.2) and four Al-rich chondrules from the CO3.1 carbonaceous chondrite Dar al Gani (DaG) 083. These particular kinds of chondrules are of special interest, as it is suggested that their precursors may have contained refractory material related to Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs).The four investigated Al-rich chondrules from the CO3.1 chondrite Dar al Gani 083 consist of olivine, low-Ca pyroxene, Ca pyroxene, and spinel phenocrysts embedded in mostly Na-rich glassy mesostasis. Two chondrules have a homogeneous O-isotopic composition and two are heterogeneous in their O-isotopic composition. One chondrule contains relict spinel grains with a Δ17O value of −24.3 ± 1.3‰, indicative of 16O-rich precursor refractory material, similar to constituents of CAIs and AOAs. The presence of CAI-like precursors for the Al-rich chondrules from CO chondrites is consistent with their previously reported presence of 50Ti excesses (Ebert et al., 2018).The Al-rich chondrules in the ordinary chondrites studied consist of olivine, low-Ca pyroxene, Ca pyroxene, and, occasionally, spinel phenocrysts embedded in mostly Na-rich glassy mesostasis. Hibonite is present in one Al-rich chondrule. The vast majority of these chondrules have heterogeneous O-isotopic compositions: Chondrule glasses are 16O-depleted compared to chondrule phenocrysts; the Δ17O values of the former approach those of aqueously formed fayalite and magnetite grains in type 3 OCs, ∼ +5‰. We infer that the chondrule glasses experienced O-isotope exchange with an aqueous fluid on the OC parent asteroids.Chondrule phenocrysts, like spinel, olivine, low-Ca pyroxene, and Ca pyroxene, were not affected by this isotope exchange and preserved their initial O-isotope compositions. The phenocrysts within individual chondrules have similar Δ17O, whereas the inter-chondrule Δ17O values range from −4.5 to +1.4‰, i.e., they are in general 16O enriched relative to the majority of ferromagnesian type I and type II porphyritic chondrules in OCs having Δ17O of ∼ +1‰. Because no relict grains were identified in the Al-rich chondrules from ordinary chondrites, the original O-isotopic composition of the refractory precursor material remains unknown.Additional detailed Na measurements within olivine grains show no major changes in the Na content of the chondrule melt during their crystallization. This implies either that the Na was part of the precursor material or that the Na was enriched in the chondrule melt/glass after crystallization of the olivines.

Geochemically heterogeneous Martian mantle inferred from Pb isotope systematics of depleted shergottites

Radiogenic isotopic compositions of shergottite meteorites suggest that early planetary differentiation processes, which are related to the crystallization of the Martian Magma Ocean (MMO), resulted in the geochemically heterogeneous Martian mantle. In order to understand the early geochemical evolution of Mars, we investigated the Pb isotope systematics in the depleted Martian mantle on the basis of the analyses of two geochemically depleted shergottites, Dar al Gani (DaG) 476 and Yamato 980459 (Y-980459). Their initial Pb isotopic compositions were estimated from geochemical analyses of highly leached acid residues and age-correction calculations using reference crystallization ages. This yielded μ-values (238U/204Pb) for the DaG 476 and Y-980459 source reservoirs of 2.33 ± 0.07 and 2.32 ± 0.06, respectively. These μ-values are distinct from those of other depleted shergottite source reservoirs (e.g., 1.4 ± 0.1 for the Tissint meteorite) and show a negative correlation with corresponding 147Sm/144Nd, 176Lu/177Hf, ɛ182W, and ε142Nd compositions. Such correlations between long- and short-lived isotopic signatures suggest that a geochemically heterogeneous depleted shergottite source mantle was formed on the early Mars. This geochemical heterogeneity would have been formed by variable mixing of depleted and enriched end-member components that originally formed by fractional crystallization in the MMO. Local remelting in the geochemically depleted Martian mantle after the crystallization of the MMO is another possible explanation for the formation of a geochemically heterogeneous depleted shergottite source mantle.

Insights into the formation of silica‐rich achondrites from impact melts in Rumuruti‐type chondrites

AbstractAncient, SiO2‐rich achondrites have previously been proposed to have formed by disequilibrium partial melting of chondrites. Here, we test the alternative hypothesis that these achondrites formed by fractional crystallization of impact melts of Rumuruti (R) chondrites. We identified two new melt clasts in R chondrites, one in Pecora Escarpment (PCA) 91241 and one in LaPaz Icefield (LAP) 031275. We analyzed major, minor, and trace element concentrations, as well as oxygen isotopes, of these two clasts and a third one that had been previously recognized (Bischoff et al. 2011) as an impact melt in Dar al Gani (DaG) 013. The melt clast in PCA 91241 is an R chondrite impact melt closely resembling the one previously recognized in DaG 013. The melt clast in LAP 031275 has an L chondrite provenance. We show that SiO2‐rich melts could form from the mesostases of R chondrite impact melts. However, their CI‐normalized rare earth element patterns are flat, whereas those of ancient SiO2‐rich achondrites (Day et al. 2012; Srinivasan et al. 2018) and those of disequilibrium partial melts of chondrites (Feldstein et al. 2001) have positive Eu anomalies from preferential melting of plagioclase. Thus, we conclude that ancient SiO2‐rich achondrites were probably formed by disequilibrium partial melting (due to an internal heat source on their parent bodies), rather than from impact melts.

Petrology and geochemistry of olivine-phyric shergottites LAR 12095 and LAR 12240: Implications for their petrogenetic history on Mars.

Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine-phyric shergottite lnds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine-phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low- and high-Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low-Ca pyroxene, which is one of the earliest formed REE-bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last-formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed-system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.

Formation of phosphorus-rich olivine in Dar al Gani 978 carbonaceous chondrite through fluid-assisted metamorphism

Phosphorus-rich olivine (P 2 O 5 > 1 wt%) is a mineral that has been reported only in a few terrestrial and extraterrestrial occurrences. Previous investigations suggest that P-rich olivine mainly forms through rapid crystallization from high-temperature P-rich melts. Here, we report a new occurrence of P-rich olivine in an ungrouped carbonaceous chondrite Dar al Gani (DaG) 978. The P-rich olivine in DaG 978 occurs as lath-shaped grains surrounding low-Ca pyroxene and olivine grains. The lath-shaped olivine shows a large variation in P 2 O 5 (0–5.5 wt%). The P-rich olivine grains occur in a chondrule fragment and is closely associated with chlorapatite, merrillite, FeNi metal, and troilite. Tiny Cr-rich hercynite is present as inclusions within the P-rich olivine. The lath-shaped texture and the association with Cr-rich hercynite indicates that the P-rich olivine in DaG 978 formed by replacing low-Ca pyroxene precursor by a P-rich fluid during a thermal event, rather than by crystallization from a high-temperature melt. The large variation of P 2 O 5 within olivine grains on micrometer-scale indicates a disequilibrium formation process of the P-rich olivine. The occurrence of P-rich olivine in DaG 978 reveals a new formation mechanism of P-rich olivine.

Young asteroidal fluid activity revealed by absolute age from apatite in carbonaceous chondrite.

Chondritic meteorites, consisting of the materials that have formed in the early solar system (ESS), have been affected by late thermal events and fluid activity to various degrees. Determining the timing of fluid activity in ESS is of fundamental importance for understanding the nature, formation, evolution and significance of fluid activity in ESS. Previous investigations have determined the relative ages of fluid activity with short-lived isotope systematics. Here we report an absolute 207Pb/206Pb isochron age (4,450±50 Ma) of apatite from Dar al Gani (DaG) 978, a type ∼3.5, ungrouped carbonaceous chondrite. The petrographic, mineralogical and geochemical features suggest that the apatite in DaG 978 should have formed during metamorphism in the presence of a fluid. Therefore, the apatite age represents an absolute age for fluid activity in an asteroidal setting. An impact event could have provided the heat to activate this young fluid activity in ESS.

Boron abundances and isotopic ratios of olivine grains on Itokawa returned by the Hayabusa spacecraft

AbstractWe report the B abundances and isotopic ratios of two olivine grains from the S‐type asteroid Itokawa sampled by the Hayabusa spacecraft. Olivine grains from the Dar al Gani (DaG) 989 LL6 chondrite were used as a reference. Since we analyzed polished thin sections in both cases, we expect the contribution from the solar wind B (rich in 10B) to be minimal because the solar wind was implanted only within very thin layers of the grain surface. The Itokawa and DaG 989 olivine grains have homogeneous B abundances (~400 ppb) and 11B/10B ratios compatible with the terrestrial standard and bulk chondrites. The observed homogeneous B abundances and isotopic ratios of the Itokawa olivine grains are likely the result of thermal metamorphism which occurred in the parent asteroid of Itokawa, which had a similar composition as LL chondrites. The chondritic B isotopic ratios of the Itokawa samples suggest that they contain little cosmogenic B (from cosmic‐ray spallation reactions) rich in 10B. This observation is consistent with the short cosmic‐ray exposure ages of Itokawa samples inferred from the small concentrations of cosmogenic 21Ne. If other Itokawa samples have little cosmogenic B as well, the enrichment in 10B found previously on the surface of another Itokawa particle (as opposed to the bulk grain study here) may be attributed to implanted solar wind B.

Origins of Al-rich chondrules: Clues from a compound Al-rich chondrule in the Dar al Gani 978 carbonaceous chondrite

Aluminum-rich chondrules are one of the most interesting components of primitive chondrites, because they have characteristics that are similar to both Ca, Al-rich inclusions (CAIs) and ferromagnesian chondrules. However, their precursor and formation history remain poorly constrained, especially with respect to their oxygen isotopic distributions. In this study, we report on the petrography, mineralogy, oxygen isotope ratios, and rare-earth-element compositions of a sapphirine-bearing Al-rich chondrule (SARC) in the ungrouped chondrite Dar al Gani (DaG) 978. The SARC has a complex core-mantle-rim texture; while both the core and the mantle are mainly composed of Al-rich enstatite and anorthite with minor amounts of mesostasis, these regions are distinguished by the presence of Fe-rich spinel and sapphirine in the core and their absence in the mantle. The rim of the SARC consists mainly of Fe-rich olivine, enstatite, and Fe–Ni metal. Spinel and some olivine grains in the SARC are 16O-rich, with Δ17O values down to −20‰ and −23‰, respectively. Enstatite, sapphirine, and most olivine grains have similar Δ17O values (∼ −7‰), which are lower than those of anorthite and the mesostasis (including augite therein) (Δ17O: ∼ −3‰). Mesostasis from both the core and mantle have Group II rare-earth-element (REE) patterns; however, the core mesostasis has higher REE concentrations than the mantle mesostasis. These observations provide a strong indication that the SARC formed by the melting and crystallization of a mixture of materials from Group II CAIs and ferromagnesian chondrules. Both spinel and olivine with 16O-rich features could be of relict origin. The 16O-poor isotopic compositions of most components in Al-rich chondrules can be explained by oxygen isotopic exchange between the melt and 16O-poor nebular gas (Δ17O: ∼ −7‰) during melting in chondrule-forming regions; whereas the anorthite and mesostasis could have experienced further oxygen isotopic exchange with a relatively 16O-poor reservoir (Δ17O: ∼ −3‰) on the parent body, likely during fluid-assisted thermal metamorphism. During the same thermal metamorphism event, spinel, olivine, some enstatite, and the mesostasis experienced Mg–Fe exchange to various extents.

SERS investigation of possible extraterrestrial life traces: Experimental adsorption of adenine on a Martian meteorite

Abstract– The identification of adenine by surface enhanced Raman scattering (SERS) on different mineral phases of a Martian meteorite Dar al Gani (DaG) 670 has been adopted as a test to verify the capability of this technique to detect trace amounts of organic or biological substances deposited over, or contained in, extraterrestrial materials. Raman spectra of different phases of meteorite (olivine, pyroxene, and ilmenite), representative of Martian basaltic rocks, have been measured by three laser sources with wavelengths at 785, 632.8, and 514.5 nm, coupled to a confocal micro‐Raman apparatus. Adenine deposited on the Martian meteorite cannot be observed in the normal Raman spectra; when, instead, meteorite is treated with silver colloidal nanoparticles, the SERS bands of adenine are strongly enhanced, allowing an easy and simple identification of this nucleobase at subpicogram level.

Osmium isotope and highly siderophile element constraints on ages and nature of meteoritic components in ancient lunar impact rocks

The concentrations of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au) and 187Os/188Os isotope compositions have been determined for 67 subsamples of six lunar impact rocks from the Apollo 14, 16 and 17 landing sites, and the lunar meteorite Dar al Gani (DaG) 400 using inductively coupled plasma mass spectrometry (ICP-MS) and negative thermal ionization mass spectrometry (N-TIMS).We report the first Re–Os isochron age on a lunar impact melt rock. 187Re–187Os isotope systematics for Apollo 16 sample 67935 define an isochron age of 4.21±0.13Ga (MSWD=1.5), which is interpreted to reflect localized partitioning processes between solid metal–liquid metal as this rock melted. The new age adds further constraints on the significance of pre-4.0Ga basin forming impacts on the Moon and possible mixing of ancient impactor compositions in lunar impact rocks.Linear correlations displayed by subsamples of a given impact rock in plots of HSE versus Ir concentrations are explained by dilution processes through essentially HSE-free anorthositic lunar crustal target rocks or binary mixing between a high HSE meteoritic end-member and a low HSE end-member composition. Slope-derived HSE ratios and 187Os/188Os of the meteoritic component in granulitic impactites 67915, 67955 and 79215 are similar to slightly volatile element depleted carbonaceous chondrites. Suprachondritic ratios of Ru/Ir, Pt/Ir, Rh/Ir, and Pd/Ir for Apollo 14 impact melt rock 14310 are similar to ratios observed for other Apollo 14 samples and Apollo 17 poikilitic impact melt rocks. Apollo 16 poikilitic and subophitic impact melt rocks 60315 and 67935 show slightly subchondritic Os/Ir and suprachondritic ratios of 187Os/188Os, Ru/Ir, Pt/Ir, Rh/Ir, Pd/Ir and Au/Ir. Their strongly fractionated HSE compositions are similar to some members of the IVA iron meteorite group and provide further evidence for an iron meteorite impactor component in Apollo 16 impact melt rocks.The range of chondritic to suprachondritic ratios of 187Os/188Os, Ru/Ir, Pt/Ir, Pd/Ir, and chondritic to slightly subchondritic Os/Ir of Apollo 17 and 14 impact melt rocks may be explained by variable mixing of a carbonaceous chondrite-like HSE composition (as in granulitic impactites) with a suprachondritic iron meteorite-like impactor composition as inferred from 67935 and 60315, and subsequent homogenization by younger impact events. The inferred dispersal of these two components across the lunar nearside and their antiquity (4.2Ga) suggest at least two basin-forming impacts that predate the 3.9–3.8Ga basin forming era. The HSE composition of the silicate Earth can be matched by a late veneer comprised of ∼80% volatile depleted carbonaceous chondrite-like material and ∼20% differentiated meteoritic metal, similar to what has been identified in ancient lunar impact rocks.

Terrestrial and Martian weathering signatures of xenon components in shergottite mineral separates

Abstract– Xenon‐isotopic ratios, step‐heating release patterns, and gas concentrations of mineral separates from Martian shergottites Roberts Massif (RBT) 04262, Dar al Gani (DaG) 489, Shergotty, and Elephant Moraine (EET) 79001 lithology B are reported. Concentrations of Martian atmospheric xenon are similar in mineral separates from all meteorites, but more weathered samples contain more terrestrial atmospheric xenon. The distributions of xenon from the Martian and terrestrial atmospheres among minerals in any one sample are similar, suggesting similarities in the processes by which they were acquired. However, in opaque and maskelynite fractions, Martian atmospheric xenon is released at higher temperatures than terrestrial atmospheric xenon. It is suggested that both Martian and terrestrial atmospheric xenon were initially introduced by weathering (low temperature alteration processes). However, the Martian component was redistributed by shock, accounting for its current residence in more retentive sites. The presence or absence of detectable 129Xe from the Martian atmosphere in mafic minerals may correspond to the extent of crustal contamination of the rock’s parent melt. Variable contents of excess 129Xe contrast with previously reported consistent concentrations of excess 40Ar, suggesting distinct sources contributed these gases to the parent magma.

Lunar meteorite regolith breccias: An in situ study of impact melt composition using LA-ICP-MS with implications for the composition of the lunar crust

Dar al Gani (DaG) 400, Meteorite Hills (MET) 01210, Pecora Escarpment (PCA) 02007, and MacAlpine Hills (MAC) 88104/88105 are lunar regolith breccia meteorites that provide sampling of the lunar surface from regions of the Moon that were not visited by the US Apollo or Soviet Luna sample return missions. They contain a heterogeneous clast population from a range of typical lunar lithologies. DaG 400, PCA 02007, and MAC 88104/88105 are primarily feldspathic in nature, and MET 01210 is composed of mare basalt material mixed with a lesser amount of feldspathic material. Here we present a compositional study of the impact melt and impact melt breccia clast population (i.e., clasts that were generated in impact cratering melting processes) within these meteorites using in situ electron microprobe and LA-ICP-MS techniques. Results show that all of the meteorites are dominated by impact lithologies that are relatively ferroan (Mg# 10), and have low incompatible trace element (ITE) concentrations (i.e., typically 10 ppm Sm), High Magnesium Suite (typically >70 Mg#) or High Alkali Suite (high ITEs, Sc/Sm ratios <2) target rocks. Instead the meteorite mafic melts are more ferroan, KREEP-poor and Sc-rich, and represent mixing between feldspathic lithologies and low-Ti or very low-Ti (VLT) basalts. As PCA 02007 and MAC 88104/05 were likely sourced from the Outer-Feldspathic Highlands Terrane our findings suggest that these predominantly feldspathic regions commonly contain a VLT to low-Ti basalt contribution.

Compositions and taxonomy of 15 unusual carbonaceous chondrites

Abstract– We used instrumental neutron activation analysis and petrography to determine bulk and phase compositions and textural characteristics of 15 carbonaceous chondrites of uncertain classification: Acfer 094 (type 3.0, ungrouped CM-related); Belgica-7904 (mildly metamorphosed, anomalous, CM-like chondrite, possibly a member of a new grouplet that includes Wisconsin Range (WIS) 91600, Dhofar 225, and Yamato-86720); Dar al Gani (DaG) 055 and its paired specimen DaG 056 (anomalous, reduced CV3-like); DaG 978 (type 3 ungrouped); Dominion Range 03238 (anomalous, magnetite-rich CO3.1); Elephant Moraine 90043 (anomalous, magnetite-bearing CO3); Graves Nunataks 98025 (type 2 or type 3 ungrouped); Grosvenor Mountains (GRO) 95566 (anomalous CM2 with a low degree of aqueous alteration); Hammadah al Hamra (HaH) 073 (type 4 ungrouped, possibly related to the Coolidge-Loongana [C-L] 001 grouplet); Lewis Cliff (LEW) 85311 (anomalous CM2 with a low degree of aqueous alteration); Northwest Africa 1152 (anomalous CV3); Pecora Escarpment (PCA) 91008 (anomalous, metamorphosed CM); Queen Alexandra Range 99038 (type 2 ungrouped); Sahara 00182 (type 3 ungrouped, possibly related to HaH 073 and/or to C-L 001); and WIS 91600 (mildly metamorphosed, anomalous, CM-like chondrite, possibly a member of a new grouplet that includes Belgica-7904, Dhofar 225, and Y-86720). Many of these meteorites show fractionated abundance patterns, especially among the volatile elements. Impact volatilization and dehydration as well as elemental transport caused by terrestrial weathering are probably responsible for most of these compositional anomalies. The metamorphosed CM chondrites comprise two distinct clusters on the basis of their Δ17O values: approximately −4‰ for PCA 91008, GRO 95566, DaG 978, and LEW 85311, and approximately 0‰ for Belgica-7904 and WIS 91600. These six meteorites must have been derived from different asteroidal regions.

Petrography, mineralogy, and geochemistry of lunar meteorite Sayh al Uhaymir 300

Abstract— We report here the petrography, mineralogy, and geochemistry of lunar meteorite Sayh al Uhaymir 300 (SaU 300). SaU 300 is dominated by a fine‐grained crystalline matrix surrounding mineral fragments (plagioclase, pyroxene, olivine, and ilmenite) and lithic clasts (mainly feldspathic to noritic). Mare basalt and KREEPy rocks are absent. Glass melt veins and impact melts are present, indicating that the rock has been subjected to a second impact event. FeNi metal and troilite grains were observed in the matrix. Major element concentrations of SaU 300 (Al2O3 21.6 wt% and FeO 8.16 wt%) are very similar to those of two basalt‐bearing feldspathic regolith breccias: Calcalong Creek and Yamato (Y‐) 983885. However, the rare earth element (REE) abundances and pattern of SaU 300 resemble the patterns of feldspathic highlands meteorites (e.g., Queen Alexandra Range (QUE) 93069 and Dar al Gani (DaG) 400), and the average lunar highlands crust. It has a relatively LREE‐enriched (7 to 10 x CI) pattern with a positive Eu anomaly (˜11 x CI). Values of Fe/Mn ratios of olivine, pyroxene, and the bulk sample are essentially consistent with a lunar origin. SaU 300 also contains high siderophile abundances with a chondritic Ni/Ir ratio. SaU 300 has experienced moderate terrestrial weathering as its bulk Sr concentration is elevated compared to other lunar meteorites and Apollo and Luna samples. Mineral chemistry and trace element abundances of SaU 300 fall within the ranges of lunar feldspathic meteorites and FAN rocks. SaU 300 is a feldspathic impact‐melt breccia predominantly composed of feldspathic highlands rocks with a small amount of mafic component. With a bulk Mg# of 0.67, it is the most mafic of the feldspathic meteorites and represents a lunar surface composition distinct from any other known lunar meteorites. On the basis of its low Th concentration (0.46 ppm) and its lack of KREEPy and mare basaltic components, the source region of SaU 300 could have been within a highland terrain, a great distance from the Imbrium impact basin, probably on the far side of the Moon.

Shock implantation of Martian atmospheric argon in four basaltic shergottites: A laser probe 40Ar/ 39Ar investigation

Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of two Martian basalts (Los Angeles and Zagami) and two Martian olivine-phyric basalts (Dar al Gani (DaG) 476 and North West Africa (NWA) 1068). With a ∼50 μm diameter focused infrared laser beam, it has been possible to distinguish between argon isotopic signatures from host rock (matrix) minerals and localized shock melt products (pockets and veins). The concentrations of argon in analyzed phases from all four meteorites have been quantified using the measured J values, 40Ar/ 39Ar ratios and K 2O wt% in each phase. Melt pockets contain, on average, 10 times more gas (7–24 ppb 40Ar) than shock veins and matrix minerals (0.3–3 ppb 40Ar). The 40Ar/ 36Ar ratio of the Martian atmosphere, estimated from melt pocket argon extractions corrected for cosmogenic 36Ar, is: Los Angeles (∼1852), Zagami (∼1744) and NWA 1068 (∼1403). In addition, Los Angeles shows evidence for variable mixing of two distinct trapped noble gas reservoirs: (1) Martian atmosphere in melt pockets, and (2) a trapped component, possibly Martian interior ( 40Ar/ 36Ar: 480–490) in matrix minerals. Average apparent 40Ar/ 39Ar ages determined for matrix minerals in the four analyzed meteorites are 1290 Ma (Los Angeles), 692 Ma (Zagami), 515 Ma (NWA 1068) and 1427 Ma (DaG 476). These 40Ar/ 39Ar apparent ages are substantially older than the ∼170–474 Ma radiometric ages given by other isotope dating techniques and reveal the presence of trapped 40Ar. Cosmic ray exposure (CRE) ages were measured using spallogenic 36Ar and 38Ar production. Los Angeles (3.1 ± 0.2 Ma), Zagami (2.9 ± 0.4 Ma) and NWA 1068 (2.0 ± 0.5 Ma) yielded ages within the range of previous determinations. DaG 476, however, yielded a young CRE age (0.7 ± 0.25 Ma), attributed to terrestrial alteration. The high spatial variation of argon indicates that the incorporation of Martian atmospheric argon into near-surface rocks is controlled by localized glass-bearing melts produced by shock processes. In particular, the larger (mm-size) melt pockets contain near end-member Martian atmospheric argon. Based on petrography, composition and argon isotopic data we conclude that the investigated melt pockets formed by localized in situ shock melting associated with ejection. Three processes may have led to atmosphere incorporation: (1) argon implantation due to atmospheric shock front collision with the Martian surface, (2) transformation of an atmosphere-filled cavity into a localized melt zone, and (3) shock implantation of atmosphere trapped in cracks, pores and fissures.

Remote laser‐induced breakdown spectroscopy analyses of Dar al Gani 476 and Zagami Martian meteorites

Laser‐induced breakdown spectroscopy (LIBS) was recently selected to provide active remote sensing compositional information on the Mars Science Laboratory (MSL) rover. The ability of LIBS to remotely determine differences between basaltic rock types on Mars is investigated by analyzing two Martian basaltic shergottite meteorites whose compositions differ slightly and which also differ slightly in texture. Using 14 analysis spots of ~400 µm diameter on Dar al Gani (DaG) 476 and 9 analysis spots on Zagami, the LIBS technique at a 5.4 m standoff distance clearly distinguished the olivine‐phyric (DaG 476) from the basaltic (Zagami) shergottite on the basis of MgO and CaO. Mean elemental abundances agreed with literature values for these meteorites to within ~12% for most of the major elements. The ability of LIBS to remotely distinguish between basaltic and andesitic compositions in a Martian environment was also investigated by analyzing a known andesite standard and comparing its spectra to the Martian basaltic shergottite spectra, yielding agreement with the literature andesite composition within 9%. The data presented here demonstrate the ability of LIBS to provide reasonable estimates of whole rock composition as well as determine relatively subtle differences in rock types from a distance.