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.     

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

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

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

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

Northwest Africa 5790: A previously unsampled portion of the upper part of the nakhlite pile

We present a geochemical study of recently discovered Martian meteorite Northwest Africa (NWA) 5790 and use our results to constrain its origin and relationship with the other nakhlites. This nakhlite is a clinopyroxene cumulate composed of phenocrysts of augite, olivine, and rare oxides surrounded by a mesostasis composed of vitrophyric glass, feldspars, oxides, phosphates, and fine‐grained olivines and augite. Petrography, and major and trace element compositions of the phases present are consistent with derivation of NWA 5790 from a parental magma common to all the nakhlites. Olivine cores grew from a distinct, incompatible‐element enriched magma and are surrounded by rims containing augite inclusions that grew from the nakhlite parental liquid, supporting previous arguments for xenocrystic olivine cores in nakhlites. Rare earth element microdistributions suggest derivation of NWA 5790 augites from an evolved, relatively oxidized magma, produced by augite fractionation from the common nakhlite parental liquid. Augite grain shapes and CSD patterns are consistent with rapid cooling and derivation near the top of the nakhlite cumulate pile, but patterns are distinct from other nakhlites thought to have formed near the stratigraphic top. The high mesostasis abundance (~44 vol%) indicates solidification near the top of the nakhlite pile close to locations suggested for nakhlites NWA 817 and Miller Range (MIL) 03346. However, the geochemical and petrographic characteristics of these three samples do not permit their placement in a simple stratigraphic order as would occur in a single lava flow. This lack of simple ordering suggests that the nakhlite lava flow split into multiple sections as would occur during breakouts from a single lava flow. Finally we note that NWA 5790 is unique among currently available nakhlites in having phenocryst abundances low enough to allow it to flow.     

Petrology and chemistry of MIL 03346 and its significance in understanding the petrogenesis of nakhlites on Mars

Abstract— Antarctic meteorite Miller Range (MIL) 03346 is a nakhlite composed of 79% clinopyroxene, ?1% olivine, and 20% vitrophyric intercumulus material. We have performed a petrological and geochemical study of MIL 03346, demonstrating a petrogenetic history similar to previously discovered nakhlites. Quantitative textural study of MIL 03346 indicates long (>1 × 10¹ yr) residence times for the cumulus augite, whereas the skeletal Fe‐Ti oxide, fayalite, and sulfide in the vitrophyric intercumulus matrix suggest rapid cooling, probably as a lava flow. From the relatively high forsterite contents of olivine (up to Fo₄₃) compared with other nakhlites and compositions of augite cores (Wo_38–42En_35–40Fs_22–28) and their hedenbergite rims, we suggest that MIL 03346 is part of the same or a similar Martian cumulate‐rich lava flow as other nakhlites. However, MIL 03346 has experienced less equilibration and faster cooling than other nakhlites discovered to date. Calculated trace element concentrations based upon modal abundances of MIL 03346 and its constituent minerals are identical to whole rock trace element abundances. Parental melts for augite have REE patterns that are approximately parallel with whole rock and intercumulus melt using experimentally defined partition coefficients. This parallelism reflects closed‐system crystallization for MIL 03346, where the only significant petrogenetic process between formation of augite and eruption and emplacement of the nakhlite flow has been fractional crystallization. A model for the petrogenesis of MIL 03346 and the nakhlites (Nakhla, Governador Valadares, Lafayette, Yamato‐000593, Northwest Africa (NWA) 817, NWA 998) would include: 1) partial melting and ascent of melt generated from a long‐term LREE depleted mantle source, 2) crystallization of cumulus augite (± olivine, ± magnetite) in a shallow‐level Martian magma chamber, 3) eruption of the crystal‐laden nakhlite magma onto the surface of Mars, 4) cooling, crystal settling, overgrowth, and partial equilibration to different extents within the flow, 5) secondary alteration through hydrothermal processes, possibly immediately succeeding or during emplacement of the flow. This model might apply to single—or multiple—flow models for the nakhlites. Ultimately, MIL 03346 and the other nakhlites preserve a record of magmatic processes in volcanic rocks on Mars with analogous petrogenetic histories to pyroxene‐rich terrestrial lava flows and to komatiites.     

Primitive olivine‐phyric shergottite NWA 5789: Petrography,mineral chemistry,and cooling history imply a magma similar to Yamato‐980459

Knowledge of Martian igneous basaltic compositions is crucial for constraining mantle evolution, including early differentiation and mantle convection. Primitive magmas provide direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The recently discovered Martian meteorite Northwest Africa (NWA) 5789 is an olivine‐phyric shergottite. NWA 5789 has special significance among the Martian meteorites because it appears to represent one of the most magnesian Martian magmas known, other than Yamato (Y) 980459. Its most magnesian olivine cores (Fo₈₅) are in Mg‐Fe equilibrium with a magma of the bulk rock composition, suggesting that the bulk represents a magma composition. Based on the Al/Ti ratio of its pyroxenes, we infer that the rock began to crystallize at a high pressure consistent with conditions in Mars’ lower crust/upper mantle. It continued and completed its crystallization closer to the surface, where cooling was rapid and produced a mesostasis of radiating sprays of plagioclase and pyroxene. The mineralogy, petrology, mineral chemistry, and bulk rock composition of NWA 5789 are very similar to those of Y‐980459. The similarities between the two meteorites suggest that NWA 5789 (like Y‐980459) represents a primitive, mantle‐derived magma composition. They also suggest the possibility that NWA 5789 and Y‐980459 formed in the same lava flow. However, based on the mineralogy and texture of its mesostasis, NWA 5789 must have cooled more slowly than Y‐980459. NWA 5789 will help elucidate the igneous geology and geochemistry of Mars.     

Analysis of ordinary chondrites using powder X‐ray diffraction: 1. Modal mineral abundances

Abstract– Powder X‐ray diffraction (XRD) is used to quantify the modal abundances (in wt%) of 18 H, 17 L, and 13 LL unbrecciated ordinary chondrite falls, which represents the complete petrologic range of equilibrated ordinary chondrites (types 4–6). The XRD technique presents an effective alternative to traditional methods for determining modal abundances, such as optical point counting and electron microprobe phase (EMP) mapping. The majority of chondrite powders in this study were previously prepared for chemical characterization from 8 to 20 g of material, which is consistent with the suggested mass (10 g) necessary to provide representative sampling of ordinary chondrites. Olivine and low‐Ca pyroxene are the most abundant phases present, comprising one‐half to three‐fourths of total abundances, while plagioclase, high‐Ca pyroxene, troilite, and metal comprise the remaining XRD‐measured mineralogy. Pigeonite may also be present in some samples, but it is fitted using a high‐Ca pyroxene standard, so exact abundances cannot be measured directly using XRD. Comparison of XRD‐measured abundances with calculated Cross, Iddings, Pirsson, Washington (CIPW) normative abundances indicates that systematic discrepancies exist between these two data sets, particularly in olivine and high‐Ca pyroxene. This discrepancy is attributed to the absence of pigeonite as a possible phase in the CIPW normative mineralogy. Oxides associated with pigeonite are improperly allocated, resulting in overestimated normative olivine abundances and underestimated normative high‐Ca pyroxene abundances. This suggests that the CIPW norm is poorly suited for determining mineralogical modal abundances of ordinary chondrites.     

Estimating the modal mineralogy of eucrite and diogenite meteorites using visible–near infrared reflectance spectroscopy

Reliable quantitative mapping of minerals exposed on Vesta's surface is crucial for understanding the crustal composition, petrologic evolution, and surface modification of the howardite, eucrite, and diogenite (HED) parent body. However, mineral abundance estimates derived from visible–near infrared (VIS–NIR) reflectance spectra are complicated by multiple scattering, particle size, and nonlinear mixing effects. Radiative transfer models can be employed to accommodate these issues, and here we assess the utility of such models to accurately and efficiently determine modal mineralogy for a suite of eucrite and olivine‐bearing (harzburgitic) diogenite meteorites. Hapke and Shkuratov radiative transfer models were implemented to simultaneously estimate mineral abundances and particle size from VIS–NIR reflectance spectra of these samples. The models were tested and compared for laboratory‐made binary (pyroxene–plagioclase) and ternary mixtures (pyroxene–olivine–plagioclase) as well as eucrite and diogenite meteorite samples. Results for both models show that the derived mineral abundances are commonly within 5–10% of modal values and the estimated particle sizes are within the expected ranges. Results for the Hapke model suggest a lower detection limit for olivine in HEDs when compared with the Shkuratov model (5% versus 15%). Our current implementation yields lower uncertainties in mineral abundance (commonly <5%) for the Hapke model, though both models have an advantage over typically used parameters such as band depth, position, and shape in that they provide quantitative information on mineral abundance and particle size. These results indicate that both the Hapke and Shkuratov models may be applied to Dawn VIR data in a computationally efficient manner to quantify the spatial distribution of pyroxene, plagioclase, and olivine on the surface of Vesta.     

ELEMENTAL COMPOSITIONS OF MAJOR SILICIC PHASES IN CHONDRULES OF UNEQUILIBRATED CHONDRITIC METEORITES

Elemental compositions of olivine, low-Ca pyroxene and mesostasis in chondrules from type-3 ordinary chondrites (OC), CV3, CO3, CM2 and EH3 chondrites were compiled in a search for mineral compositional differences among chondrules of different chondrite groups. Such differences are demonstrated. A few elements occur in silicic phases in amounts proportional to their bulk chondrule concentrations: e.g., Mn in OC chondrules, Ti in CV chondrules, Cr in EH chondrules. However, OC chondrules have higher bulk Cr than CM-CO chondrules, higher Cr in mesostasis, but lower Cr in olivine and low-Ca pyroxene. The higher oxidation state of OC chondrules implies that Cr is more likely to be trivalent, and thus, less likely to enter the olivine crystal structure and more likely to concentrate in pyroxene and mesostasis. CV and OC chondrules have similar high bulk Fe and mesostasis Fe, but OC chondrules have much more FeO in olivine and low-Ca pyroxene. The remaining Fe in CV chondrules is reduced and occurs as metal blebs in the mesostasis. Relative to OC chondrules, EH chondrules have lower bulk Ca, lower Ca in pyroxene and mesostasis, but higher (by a factor of 2) Ca in olivine. EH chondrules may have been incompletely melted, preserving relict refractory lithophile-rich olivine nuclei. OC chondrules are richer than EH chondrules in FeO; they have a lower melting temperature and may have been more completely melted during chondrule formation.     

Trace elements in olivine and the petrogenesis of the intermediate,olivine‐phyric shergottite NWA 10170

Olivine‐phyric shergottites represent primitive basaltic to picritic rocks, spanning a large range of Mg# and olivine abundances. As primitive olivine‐bearing magmas are commonly representative of their mantle source on Earth, understanding the petrology and evolution of olivine‐phyric shergottites is critical in our understanding of Martian mantle compositions. We present data for the olivine‐phyric shergottite Northwest Africa (NWA) 10170 to constrain the petrology with specific implications for magma plumbing‐system dynamics. The calculated oxygen fugacity and bulk‐rock REE concentrations (based on modal abundance) are consistent with a geochemically intermediate classification for NWA 10170, and overall similarity with NWA 6234. In addition, we present trace element data using laser ablation ICP‐MS for coarse‐grained olivine cores, and compare these data with terrestrial and Martian data sets. The olivines in NWA 10170 contain cores with compositions of Fo₇₇ that evolve to rims with composition of Fo₅₈, and are characterized by cores with low Ni contents (400–600 ppm). Nickel is compatible in olivine and such low Ni content for olivine cores in NWA 10170 suggests either early‐stage fractionation and loss of olivine from the magma in a staging chamber at depth, or that Martian magmas have lower Ni than terrestrial magmas. We suggest that both are true in this case. Therefore, the magma does not represent a primary mantle melt, but rather has undergone 10–15% fractionation in a staging chamber prior to extrusion/intrusion at the surface of Mars. This further implies that careful evaluation of not only the Mg# but also the trace element concentrations of olivine needs to be conducted to evaluate pristine mantle melts versus those that have fractionated olivine (±pyroxene and oxide minerals) in staging chambers.     

Paired nakhlites MIL 090030, 090032, 090136, and 03346: Insights into the Miller Range parent meteorite

Abstract– Miller Range (MIL) 03346 is the most oxidized and least equilibrated nakhlite known and displays the highest amount of intercumulus phase. The discovery of three new nakhlites, MIL 090030, MIL 090032, and MIL 090136, in the Miller Range, Antarctica, geographically close to the location of MIL 03346, suggests that they may come from the same parent meteorite. In this study, we investigate the mineralogy and texture of cumulus and intercumulus phases, in situ major and trace element compositions for the cumulus phases, as well as pyroxene crystal size distribution patterns and spatial distribution patterns of MIL 090030, 090032, and 090136. Using these combined results, we conclude that the three nakhlites studied here are paired with MIL 03346. However, modal mineral abundances of MIL 090030, 090032, 090136, and 03346 exhibit variations indicating that a single sample of a Miller Range nakhlite is not modally representative of the parent meteorite and that analyses of multiple samples for a single nakhlite may be necessary to obtain representative modal data for placement in the cumulate pile. Our calculated parental melt composition based on all the paired samples confirms a previous study suggesting that the nakhlite parent melt crystallized as a closed system.     

Analysis of ordinary chondrites using powder X‐ray diffraction: 2. Applications to ordinary chondrite parent‐body processes

Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X‐ray diffraction (XRD)‐measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD‐derived olivine and low‐Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H₂O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.     

KREEPy lunar meteorite Dhofar 287A: A new lunar mare basalt

Abstract— Dhofar 287 (Dho 287) is a new lunar meteorite, found in Oman on January 14, 2001. The main portion of this meteorite (Dho 287A) consists of a mare basalt, while a smaller portion of breccia (Dho 287B) is attached on the side. Dho 287A is only the fourth crystalline mare basalt meteorite found on Earth to date and is the subject of the present study. The basalt consists mainly of phenocrysts of olivine and pyroxene set in a finer‐grained matrix, which is composed of elongated pyroxene and plagioclase crystals radiating from a common nucleii. The majority of olivine and pyroxene grains are zoned, from core to rim, in terms of Fe and Mg. Accessory minerals include ilmenite, chromite, ulvöspinel, troilite, and FeNi metal. Chromite is invariably mantled by ulvöspinel. This rock is unusually rich in late‐stage mesostasis, composed largely of fayalite, Si‐K‐Ba‐rich glass, fluorapatite, and whitlockite. In texture and mineralogy, Dho 287A is a low‐Ti mare basalt, with similarities to Apollo 12 (A‐12) and Apollo 15 (A‐15) basalts. However, all plagioclase is now present as maskelynite, and its composition is atypical for known low‐Ti mare basalts. The Fe to Mn ratios of olivine and pyroxene, the presence of FeNi metal, and the bulk‐rock oxygen isotopic ratios, along with several other petrological features, are evidence for the lunar origin for this meteorite. Whole‐rock composition further confirms the similarity of Dho 287A with A‐12 and A‐15 samples but requires possible KREEP assimilation to account for its rare‐earth‐element (REE) contents. Cooling‐rate estimates, based on Fo zonation in olivine, yield values of 0.2–0.8°C/hr for the lava, typical for the center of a 10–20 m thick flow. The recalculated major‐element concentrations, after removing 10–15% modal olivine, are comparable to typical A‐15 mare basalts. Crystallization modeling of the recalculated Dho 287A bulk‐composition yields a reasonable fit between predicted and observed mineral abundances and compositions.     

Northwest Africa 1950: Mineralogy and comparison with Antarctic lherzolitic shergottites

Abstract— NWA 1950 is a new lherzolitic shergottite recently recovered from Morocco and is the first sample of this group found outside Antarctica. Major constituent phases of NWA 1950 are olivine, pyroxenes, and plagioclase glass (“maskelynite”) and the rock shows a two distinct textures: poikilitic and non‐poikilitic typical of lherzolitic shergottites. In poikilitic areas, several‐millimeter‐sized pyroxene oikocrysts enclose cumulus olivine and chromite. In contrast, pyroxenes are much smaller in non‐poikilitic areas, and olivine and plagioclase glass are more abundant. Olivine in non‐poikilitic areas is more Fe‐rich (Fa_29–31) and shows a narrower distribution than that in poikilitic areas (Fa_23–29). Pyroxenes in non‐poikilitic areas are also more Fe‐rich than those in poikilitic areas that show continuous chemical zoning suggesting fractional crystallization under a closed system. These observations indicate that pyroxene in non‐poikilitic areas crystallized from evolved interstitial melts and olivine was re‐equilibrated with such melts. NWA 1950 shows similar mineralogy and petrology to previously known lherzolitic shergottites (ALH 77005, LEW 88516, Y‐793605 and GRV 99027) that are considered to have originated from the same igneous body on Mars. Olivine composition of NWA 1950 is intermediate between those of ALH 77005‐GRV 99027 and those of LEW 88516‐Y‐793605, but is rather similar to ALH 77005 and GRV 99027. The subtle difference of mineral chemistry (especially, olivine composition) can be explained by different degrees of re‐equilibration compared to other lherzolitic shergottites, perhaps due to different location in the same igneous body. Thus, NWA 1950 experienced a high degree of re‐equilibration, similar to ALH 77005 and GRV 99027.     

Characterization of mesostasis regions in lunar basalts: Understanding late‐stage melt evolution and its influence on apatite formation

Recent studies geared toward understanding the volatile abundances of the lunar interior have focused on the volatile‐bearing accessory mineral apatite. Translating measurements of volatile abundances in lunar apatite into the volatile inventory of the silicate melts from which they crystallized, and ultimately of the mantle source regions of lunar magmas, however, has proved more difficult than initially thought. In this contribution, we report a detailed characterization of mesostasis regions in four Apollo mare basalts (10044, 12064, 15058, and 70035) in order to ascertain the compositions of the melts from which apatite crystallized. The texture, modal mineralogy, and reconstructed bulk composition of these mesostasis regions vary greatly within and between samples. There is no clear relationship between bulk‐rock basaltic composition and that of bulk‐mesostasis regions, indicating that bulk‐rock composition may have little influence on mesostasis compositions. The development of individual melt pockets, combined with the occurrence of silicate liquid immiscibility, exerts greater control on the composition and texture of mesostasis regions. In general, the reconstructed late‐stage lunar melts have roughly andesitic to dacitic compositions with low alkali contents, displaying much higher SiO₂ abundances than the bulk compositions of their host magmatic rocks. Relevant partition coefficients for apatite‐melt volatile partitioning under lunar conditions should, therefore, be derived from experiments conducted using intermediate compositions instead of compositions representing mare basalts.     

Sulfide mineralogy and redox conditions in some shergottites

Abstract— Magmatic sulfide mineralogy has been studied in 2 olivine‐phyric shergottites (DaG 476 and SaU 005) and 4 basaltic shergottites (Zagami, Shergotty, Los Angeles, and NWA 480). Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are high (0.16 to 0.53 area percent) and correlate positively with abundances of Fe‐Ti oxides. Sulfides are mesostasis minerals, being mostly interstitial grains or locally enclosed in post‐cumulus melt inclusions (e.g., SaU 005) in olivine. Sulfides in shergottites are composed of major pyrrhotite containing pentlandite exsolutions associated with minor amounts of Cu sulfides (chalcopyrite and/or cubanite). Hot desert finds (e.g., DaG 476) show abundant fracture‐filling iron (oxy)hydroxides of probable terrestrial origin. Unaltered sulfides show metal‐rich hexagonal pyrrhotite compositions with metal/sulfur (M/S) ratio ranging between 0.936 ± 0.005 and 0.962 ± 0.01. This compositional range corresponds to the two‐phase structural domain 2C + nC of the Fe‐S system; however, the high‐temperature disordered hexagonal 1C pyrrhotite structure would be in better agreement with magnetic properties of shergottites. Ni contents in pyrrhotite increase from Los Angeles (<0.05 at%) to Shergotty, Zagami, and NWA 480 (0.2–0.5 at%), and DaG 476 and SaU 005 (up to 3 at%). The higher Ni values of pyrrhotite in olivine‐phyric shergottites correlate with the abundance of pentlandite exsolutions, both reflecting more primitive Ni‐rich sulfide liquids where abundant olivine crystallized. This result and the strong correlation between sulfide abundances and Fe‐Ti oxides argue for a primary magmatic origin of these sulfides. Although they reproduce the trend of magmatic oxygen fugacity conditions determined from Fe‐Ti oxide pairs, observed pyrrhotite compositions are systematically more metal‐deficient compared to those calculated from the Fe‐S‐O system. This suggests post‐magmatic oxidation during cooling on Mars, followed by terrestrial weathering for hot desert finds.     

Petrogenesis of the new lherzolitic shergottite Grove Mountains 99027: Constraints of petrography,mineral chemistry,and rare earth elements

Abstract— We report petrography, mineral chemistry, and microdistribution of rare earth elements (REE) in a new lherzolitic shergottite, Grove Mountains (GRV) 99027. The textural relationship and REE patterns of minerals suggest precipitation of cumulus olivine and chromite, followed by equilibrium crystallization of a closed system with a bulk composition of the inferred intercumulus melt. Subsolidus equilibrium temperatures of pyroxenes and olivine range from 1100 to 1210 °C, based on a two‐pyroxene thermometry and Ca partitioning between augite and olivine. Oxygen fugacity of the parent magma is 1.5–2.5 (av. 2.0 ± 0.4) log units below the quartz‐fayalite‐magnetite (QFM) buffer at 960–1360 °C, according to the olivine‐orthopyroxene‐chromite barometer. The ilmenite‐chromite barometer and thermometer show much wider ranges of oxygen fugacity (1.0–7.0 log unit below QFM) and temperature (1130–480 °C), suggesting subsolidus equilibration of the oxides at low temperatures, probably due to deep burial of GRV 99027 on Mars. The low oxygen fugacity and LREE depletion of the parent magma of GRV 99027 suggest low contamination by martian crust. Characteristics of GRV 99027 demonstrate similarity of lherzolitic shergottites, suggesting a high possibility of launch pairing or a homogeneous upper mantle of Mars if they were ejected by individual impact events. However, GRV 99027 probably experienced severe post‐shock thermal metamorphism in comparison with other lherzolitic shergottites, based on the re‐crystallization of maskelynite, the homogeneity of minerals, and the low subsolidus equilibrium temperatures between chromite and ilmenite.     

Crystallization kinetics of olivine‐phyric shergottites

Crystal size distribution (CSD) and spatial distribution pattern (SDP) analyses are applied to the early crystallizing phases, olivine and pyroxene, in olivine‐phyric shergottites (Elephant moraine [EET] 79001A, Dar al Gani [DaG] 476, and dhofar [Dho] 019) from each sampling locality inferred from Mars ejection ages. Trace element zonation patterns (P and Cr) in olivine are also used to characterize the crystallization history of these Martian basalts. Previously reported 2‐D CSDs for these meteorites are re‐evaluated using a newer stereographically corrected methodology. Kinks in the olivine CSD plots suggest several populations that crystallized under different conditions. CSDs for pyroxene in DaG 476 and EET 79001A reveal single populations that grew under steady‐state conditions; pyroxenes in Dho 019 were too intergrown for CSD analysis. Magma chamber residence times of several days for small grains to several months for olivine megacrysts are calculated using the CSD slopes and growth rates inferred from previous experimental data. Phosphorus imaging in olivines in DaG 476 and Dho 019 indicate rapid growth of skeletal, sector‐zoned, or patchy cores, probably in response to delayed nucleation, followed by slow growth, and finally rapid dendritic growth with back‐filling to form oscillatory zoning in rims. SPD analyses indicate that olivine and pyroxene crystals grew or accumulated in clusters rather than as randomly distributed grains. These data reveal complex solidification histories for Martian basalts, and are generally consistent with the formation at depth of olivine megacryst cores, which were entrained in ascending magmas that crystallized pyroxenes, small olivines, and oscillatory rims on megacrysts.     

A critical evaluation of oxidation versus reduction during metamorphism of L and LL group chondrites,and implications for asteroid spectroscopy

Abstract— Modal mineralogies of individual, equilibrated (petrologic type 4–6 L and LL chondrites have been measured using an electron microprobe mapping technique, and the chemical compositions of coexisting silicate minerals have been analyzed. Progressive changes in the relative abundances and in the molar Fe/Mn and Fe/Mg ratios of olivine, low‐Ca pyroxene, and diopside occur with increasing metamorphic grade. Variations in olivine/low‐Ca pyroxene ratios (Ol/Px) and in metal abundances and compositions with petrologic type support the hypothesis that oxidation of metallic iron accompanied thermal metamorphism in ordinary chondrites. Modal Ol/Px ratios are systematically lower than normative Ol/Px ratios for the same meteorites, suggesting that the commonly used C.I.P.W. norm calculation procedure may not adequately estimate silicate mineral abundances in reduced chondrites. Ol/Px ratios calculated from visible and near‐infrared (VISNIR) reflectance spectra of the same meteorites are not in agreement with other Ol/Px determinations, possibly because of spectral complexities arising from other minerals in chondrites. Characteristic features in VISNIR spectra are sensitive to the proportions and compositions of olivine and pyroxenes, the minerals most affected by oxidative metamorphism. This work may allow spectral calibration for the determination of mineralogy and petrologic type, and thus may be useful for spectroscopic studies of asteroids.     

Assessing the limits of the Modified Gaussian Model for remote spectroscopic studies of pyroxenes on Mars

We investigate the ability to refine pyroxene composition and modal abundance from laboratory and remotely acquired spectra. Laboratory data including the martian meteorites, Shergotty, Zagami, MIL03346, and ALH84001 as well as additional pyroxene-rich spectra obtained from the OMEGA (Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité) spectrometer for Mars are characterized using the Modified Gaussian Model (MGM), a spectral deconvolution method developed by Sunshine et al. [Sunshine, J.M., Pieters, C.M., Pratt, S., 1990. J. Geophys. Res. 95, 6955-6966]. We develop two sensitivity tests to assess the extent to which the MGM can consistently predict (1) pyroxene composition and (2) modal abundance for a compositionally diverse suite of pyroxene spectra. Results of the sensitivity tests indicate that the MGM can be appropriately applied to remote spectroscopic measurements of extraterrestrial surfaces and can estimate pyroxene composition and relative abundance within a derived uncertainty. Deconvolved band positions for laboratory spectra of the meteorites Shergotty and Zagami are determined within ±17 nm while remotely acquired OMEGA spectra are defined within ±50 nm. These results suggest that absolute compositions can be uniquely derived from laboratory pyroxene-rich spectra and non-uniquely derived from the remote measurements of OMEGA at this time. While relative pyroxene chemistries are not assessed from OMEGA measurements at this time, relative pyroxene abundances are estimated using a normalized band strength ratio between the low-calcium (LCP) and high-calcium (HCP) endmember components and are constrained to ±10%. The fraction of LCP in a two-pyroxene mixture is the derived value from the normalized band strength ratio, LCP/(LCP + HCP). This calculation for relative abundance is robust in the presence of up to 10-15% olivine. Deconvolution results from the OMEGA spectra indicate that the ancient terrain in the Syrtis Major region is uniquely enriched in LCP (59±10% LCP) relative to HCP while the volcanics of Syrtis Major are uniquely enriched in HCP (39±10% LCP).