Shock and annealing in aubrites: Implications for parent‐body history

Several aubrites (e.g., LAP 03719, Bishopville, Khor Temiki, ALH 83015) contain orthopyroxene grains that exhibit more‐pronounced shock effects than associated olivine grains. The orthopyroxene grains in these samples have clinoenstatite lamellae on (100) and exhibit weak mosaic extinction, characteristic of shock stage S4; the olivine grains exhibit either sharp optical extinction, characteristic of shock stage S1 (as in LAP 03719), or undulose extinction (shock stage S2), as in Bishopville and ALH 83015. The Khor Temiki regolith breccia contains S1 and S2 olivine grains. Because literature data show that diffusion is much slower in orthopyroxene than in olivine, it seems likely that aubrites experienced postshock, impact‐induced annealing. After differentiation, the aubrite parent asteroid suffered major collisions that caused extensive brecciation of near‐surface materials and damaged orthopyroxene and olivine crystal lattices. As a result of these impact events, some aubrites were shocked and buried within warm ejecta blankets or beneath fallback debris under the crater floor. Entombed olivine crystal lattices healed (and became unstrained, reaching shock stage S1), but orthopyroxene lattices retained their S4‐level shock‐damaged features. Aubrites with S4 orthopyroxene and S2 olivine were probably very weakly shocked again after olivine was annealed to S1.     

STUDIES OF KAMACITE,PERRYITE AND SCHREIBERSITE IN E-CHONDRITES AND AUBRITES

The bulk composition of metal (kamacite plus perryite) was determined in eleven E-chondrites and eight aubrites. The data are compatible with the subdivision of the E-chondrites into two groups (Yavnel, 1963; Anders, 1964), St Mark's and St Sauveur belonging to type I (Easton, 1985). The Ni content of kamacite plus perryite in Kota Kota (5.49%) is within the range covered by the remaining E-chondrites. Normative perryite, (Fe, Ni)_x(Si,P)_y constitutes 2.1% of Kota Kota and 2.7% of South Oman. The Ni content in the bulk metal of Aubres, Bishopville, Norton County and Peña Blanca Spring is about half the average Ni content in the metal of E-chondrites or the remaining aubrites (Bustee, Khor Temiki, Mayo Belwa and Shallowater). High Ga/Ni and Ge/Ni ratios distinguish the metal in E-chondrites and aubrites from that in ordinary chondrites. In a large metal grain from Aubres perryite formed on reheating, whereas in one from Khor Temiki there is evidence of shock and displacement of fragmented schreibersite (rhabdite). Thirty-eight metal grains (< 1.5 mm diameter) from Khor Temiki have a wide compositional range like that in Mayo Belwa (Graham, 1978). In Shallowater the distribution of Ni in the metal is bimodal (5.2 and 11.6%) and there is evidence of rapid cooling. The composition of both bulk metal and individual grains in aubrites makes it unlikely that they represent residual metal trapped during magmatic differentiation and/or fractional crystallization of E6 material. Compositional differences between metal grains strongly indicate that the aubrites are polymict breccias.     

Trace elements in mineral separates of the Peña Blanca Spring aubrite: Implications for the evolution of the aubrite parent body

Abstract— The origin of the aubrite parent body (APB) and its relation to the enstatite chondrites is still unclear. Therefore we began a detailed chemical study of the aubrite Peña Blanca Spring. Bulk samples and mineral separates (oldhamite, troilite, alabandite, pyroxene) of Peña Blanca Spring were analyzed for major and trace elements by instrumental neutron activation analysis (INAA). In addition, a leaching experiment was performed on a powdered bulk sample to study the distribution of trace elements in aubrite minerals. The elemental abundances in Peña Blanca Spring are compared to abundances in EH-chondrites and EL-chondrites in an attempt to distinguish volatility related fractionations (evaporation, condensation) from planetary differentiation (melting and core formation). Low abundances of siderophile (e.g., Ir) and chalcophile (e.g., V) elements in bulk samples indicate that 25% (by mass) metal and about 6% (by mass) sulfide separated from an enstatite chondrite like-parent body to form a core and a residual mantle with aubrite composition. We argue that the high observed rare earth element (REE) abundances in oldhamite (>100 × EH-chondrite normalized) reflect REE incorporation into oldhamite during nebular condensation. Thus, oldhamite in aubrites is, at least in part, a relict phase as originally proposed by Lodders and Palme (1990). Some re-equilibration of CaS with silicates has, however, occurred, leading to partial redistribution of REE, as exemplified by the uptake of Eu by plagioclase. The distribution of the REE among aubritic minerals cannot be the result of fractional crystallization, which would occur if high degrees of partial melting took place on the APB. Instead, the REE distributions indicate incomplete equilibrium of oldhamite and other phases. Therefore, a short non-equlibrium melting episode led to segregation of metal and sulfides.     

Enstatite meteorites and their parent bodies*

Abstract— Enstatite meteorites are highly reduced rocks that consist of major, nearly FeO-free enstatite, variable amounts of metallic Fe, Ni and troilite, and a host of rare minerals formed under highly-reducing conditions. They are comprised of the EH and EL chondrites and the aubrites. Here I discuss some of their properties and the nature and number of their parent bodies. Conclusions: 1. EH and EL chondrites show bulk compositional differences in non-volatile major elements that were established by nebular, not planetary processes. Occurrence of abundant breccias among them but lack of clasts of EL in EH chondrites (and vice versa) suggests that EH and EL chondrites represent two separate parent bodies. 2. Aubrites were not derived from known enstatite chondrites on the same parent bodies. Aubrites represent samples from a third enstatite meteorite parent body. 3. The aubrite parent body may have experienced collisional break-up and gravitational reassembly of the debris into a rubble-pile object. 4. The aubrite source material (parent body) was probably enstatite chondrite-like in composition, but had a higher troilite/metallic Fe, Ni ratio, higher contents of titanium and diopside, and possibly less plagioclase than known enstatite chondrites. 5. Shallowater, the only non-brecciated aubrite, does not appear to have formed on the EH, EL, or aubrite parent bodies by either internal (igneous) or external (impact) melting processes. Instead, Shallowater may be a sample from yet a fourth enstatite meteorite parent body. 6. Shallowater experienced a complex three-stage cooling history, requiring an equally complex mode of origin: collisional break-up of a molten or partly molten body by impact with a solid body, followed by gravitational reassembly. 7. It is unknown why some enstatite meteorite parent bodies melted (the aubrite and Shallowater bodies), and others did not (the EH and EL bodies). If unipolar dynamo induction by a primordial T Tauri sun was the dominant heat source that heated asteroidal-sized bodies in the early Solar System, then the aubrite and Shallowater parent bodies may have melted because they were of intermediate sizes, whereas the EH and EL bodies did not melt because they were either much smaller or much larger.     

Raman spectroscopy of shocked enstatite-rich meteorites

We present Raman patterns of enstatite in different classes of enstatite-rich chondrites and achondrites of various shock levels as previously reported from petrographic observations and X-ray diffraction analyses. Thin sections or mineral separates of four enstatite chondrites (LaPaz Icefield [LAP] 02225, MacAlpine Hills [MAC] 02837, Pecora Escarpment [PCA] 91020, and Itqiy), three aubrites (Larkman Nunatak [LAR] 04316, Khor Temiki, and Allan Hills [ALH] 84008), and a ureilite (Sayh al Uhaymir [SaU] 559) were examined by laser Raman spectroscopy. We find that the frequencies of fundamental Raman peaks of enstatites from the chondrites and aubrites deviate by ≤2 cm⁻¹ from the values for unshocked enstatite. This small difference implies a negligible effect of shock metamorphism on peak positions. Significant differences (<6 cm⁻¹) for peak positions are found for the pyroxenes of SaU 559 and may be attributed to minor substitution of Fe and Ca for Mg. Linear regressions of peak widths of enstatite chondrites against their established shock stages show a strong positive correlation for each mode (r² > 0.94). From this linear relationship, the 343 and 1014 cm⁻¹ peaks of the aubrites coincide with S4 determined from petrography. For Itqiy, we find S4–5, while the shock levels of SaU 559 exceed the petrologic scheme (S1–6), suggesting that the ureilite might have sustained multiple shock events or have been deformed in a high-pressure environment. Alternatively, for Itqiy (peak 343 cm⁻¹) and SaU 559 (all peaks) enstatites, minor substitutions of Fe and Ca for Mg may have further broadened their peak widths.     

Igneous History of the Aubrite Parent Asteroid: Evidence from the Norton County Enstatite Achondrite

Abstract— We studied numerous specimens of the Norton County enstatite achondrite (aubrite) by optical microscopy, electron microprobe, and neutron-activation analysis. Our main conclusions are the following: 1. Norton County is a fragmental impact breccia, consisting of a clastic matrix made mostly of crushed enstatite, into which are embedded a variety of mineral and lithic clasts of both igneous and impact melt origin. 2. The Norton County precursor materials were igneous rocks, mostly plutonic orthopyroxenites, not grains formed by condensation from the solar nebula. 3. The Mg-silicate-rich aubrite parent body experienced extensive melting and igneous differentiation, causing formation of diverse lithologies, some of which have not been described previously. These lithologies include dunites (represented by forsterite crystals), plutonic orthopyroxenites (represented by most enstatite crystals in the matrix), plutonic pyroxenites (the pyroxenitic clasts), and plagioclase-silica rocks (like the feldspathic clasts). Presence of impact melt breccias (the microporphyritic clasts and the diopside-plagioclase-silica clast) of still different compositions further attest to the lithologic diversity of the aubrite parent body.     

Geochemical and petrographic studies of oldhamite,diopside, and roedderite in enstatite meteorites

Abstract— In Qingzhen (EH3), oldhamite contains numerous types of inclusions and intergrows with other phases; but in equilibrated enstatite chondrites and aubrites, it usually occurs as individual grains. I suggest that oldhamite in unequilibrated enstatite chondrites (UECs) crystallized from a melt, probably during chondrule formation. Subsequent thermal metamorphism on the parent bodies further modified the oldhamite occurrences in enstatite chondrites. This suggestion is consistent with the results of melting experiments on UECs and aubrites and with the volatile element enrichments in this mineral. I analyzed minor and trace element abundances in diopside from two aubrites. These data and petrographic observations suggest that diopside formed by igneous crystallization. I report the first known occurrence of roedderite in an aubrite and its major, minor, and trace element concentrations. This mineral is rich in alkalis but is depleted in siderophile and refractory lithophile elements. A negative Sm anomaly was noted in albite from equilibrated enstatite chondrites and aubrites.     

Microdistributions and petrogenetic implications of rare earth elements in polymict ureilites

Abstract— Polymict ureilites contain various mineral and lithic clasts not observed in monomict ureilites, including plagioclase, enstatite, feldspathic melt clasts and dark inclusions. This paper investigates the microdistributions and petrogenetic implications of rare earth elements (REEs) in three polymict ureilites (Elephant Moraine (EET) 83309, EET 87720 and North Haig), focusing particularly on the mineral and lithic clasts not found in monomict ureilites. As in monomict ureilites, olivine and pyroxene are the major heavy (H)REE carriers in polymict ureilites. They have light (L)REE‐depleted patterns with little variation in REE abundances, despite large differences in major element compositions. The textural and REE characteristics of feldspathic melt clasts in the three polymict ureilites indicate that they are most likely shocked melt that sampled the basaltic components associated with ureilites on their parent body. Simple REE modeling shows that the most common melt clasts in polymict ureilites can be produced by 20–30% partial melting of chondritic material, leaving behind a ureilitic residue. The plagioclase clasts, as well as some of the high‐Ca pyroxene grains, probably represent plagioclase‐pyroxene rock types on the ureilite parent body. However, the variety of REE patterns in both plagioclase and melt clasts cannot be the result of a single igneous differentiation event. Multiple processes, probably including shock melting and different sources, are required to account for all the REE characteristics observed in lithic and mineral clasts. The C‐rich matrix in polymict ureilites is LREE‐enriched, like that in monomict ureilites. The occurrence of Ce anomalies in C‐rich matrix, dark inclusions and the presence of the hydration product, iddingsite, imply significant terrestrial weathering. A search for ²⁶Mg excesses, from the radioactive decay of ²⁶Al, in the polymict ureilite EET 83309 was negative.     

An experimental and theoretical study of rare-earth-element partitioning between sulfides (FeS,CaS) and silicate and applications to enstatite achondrites

Abstract— Partition coefficients of the rare-earth-elements (REE) between sulfides (FeS or CaS) and silicate melt were determined experimentally at 1200–1300 °C. The REE sulfide/silicate partition coefficients (D) are ≤1 under the experimental O and S fugacities, which demonstrates that the REE are mainly located in the silicate phase. Rare-earth-element partition coefficients in the FeS/silicate system decrease from light to heavy REE, while the opposite behavior is found for the CaS/silicate system, where partition coefficients increase from light to heavy REE. In both sulfide systems, Eu is preferentially incorporated into the sulfide phases, as also expected from thermodynamic calculations. The Eu sulfide/silicate partition coefficient is about a factor of ten higher than that of neighboring Sm and Gd, in accordance with thermodynamic predictions of REE sulfide/silicate partition coefficients. The low sulfide/silicate partition coefficients indicate that CaS (oldhamite) in enstatite achondrites (aubrites) cannot have gained its high REE concentrations during igneous differentiation processes. The high REE concentrations and the REE patterns in aubritic oldhamite are more plausibly explained by REE condensation into refractory CaS. The refractory nature of CaS prevented major exchange reactions of the oldhamite with other aubritic minerals during the short differentiation and metamorphism period on the aubrite parent body. Thus, oldhamite in aubrites may be relict condensates altered to different degrees during short heating events, as originally suggested by Lodders and Palme (1990).     

Testing an integrated chronology: I‐Xe analysis of enstatite meteorites and a eucrite

Abstract— We have determined initial ¹²⁹I/¹²⁷I ratios for mineral concentrates of four enstatite meteorites and a eucrite. In the case of the enstatite meteorites the inferred ages are associated with the pyroxene‐rich separates giving pyroxene closure ages relative to the Shallowater standard of Indarch (EH4, 0.04 ± 0.67 Ma), Khairpur (EL6, ?4.22 ± 0.67 Ma), Khor Temiki (aubrite, ?0.06 Ma), and Itqiy (enstatite achondrite, ?2.6 ± 2.6 Ma), negative ages indicate closure after Shallowater. No separate from the cumulate eucrite Asuka (A?) 881394 yielded a consistent ratio, though excess ¹²⁹Xe was observed in a feldspar separate, suggesting disturbance by thermal metamorphism within 25 Ma of closure in Shallowater. Iodine‐129 ages are mapped to the absolute Pb‐Pb time scale using the calibration proposed by Gilmour et al. (2006) who place the closure age of Shallowater at 4563.3 ± 0.4 Ma. Comparison of the combined ¹²⁹I‐Pb data with associated ⁵³Mn ages, for objects that have been dated by both systems, indicates that all three chronometers evolved concordantly in the early solar system. The enstatite chondrites are offset from the linear array described by asteroid‐belt objects when ⁵³Mn ages are plotted against combined ¹²⁹I‐Pb data, supporting the suggestion that ⁵³Mn was radially heterogeneous in the early solar system.     

Alkali elements in the Earth's core: Evidence from enstatite meteorites

Abstract— The abundances of alkali elements in the Earth's core are predicted by assuming that accretion of the Earth started from material similar in composition to enstatite chondrites and that enstatite achondrites (aubrites) provide a natural laboratory to study core-mantle differentiation under extremely reducing conditions. If core formation on the aubrite parent body is comparable with core formation on the early Earth, it is found that 2600 (±1000) ppm Na, 550 (±260) ppm K, 3.4 (±2.1) ppm Rb, and 0.31 (±0.24) ppm Cs can reside in the Earth's core. The alkali-element abundances are consistent with those predicted by independent estimates based on nebula condensation calculations and heat flow data.     

Oxide‐bearing and FeO‐rich clasts in aubrites

Abstract— We report the occurrence of an oxide‐bearing clast and an FeO‐rich clast from aubrites. The FeO‐rich clast in Pesyanoe is dominated by olivine and pyroxene phenocrysts with mineral compositions slightly less FeO‐rich than is typical for H chondrites. In Allan Hills (ALH) 84008, the oxide‐bearing clast consists of a single forsterite grain rimmed by an array of sulfides, oxides, and phosphides. We consider a number of possible origins. We can exclude formation by melting of oxide‐bearing chondrules and CAIs formed in enstatite chondrites. The Pesyanoe clast may have formed in a more oxidized region of the aubrite parent body or, more likely, is a foreign clast from a more oxidized parent body. The ALH 84008 clast likely formed by reaction between sulfides and silicates as a result of cooling, oxidation, or de‐sulfidization. This clast appears to be the first oxide‐bearing clast from an aubritic breccia that formed on the aubrite parent body. Identification of additional oxide‐bearing clasts in aubrites could shed light on whether this was a widespread phenomenon and the origin of these enigmatic objects.     

Impact melting in the Cumberland Falls and Mayo Belwa aubrites

Abstract– Six chondritic clasts in the Cumberland Falls polymict breccia were examined: four texturally resemble ordinary chondrites (OCs) and two are impact melt breccias containing shocked OC clasts adjacent to a melt matrix. The six chondritic clasts are probably remnants of a single OC projectile that was heterogeneously shocked when it collided with the Cumberland Falls host. Mayo Belwa is the first known aubrite impact melt breccia. It contains coarse enstatite grains exhibiting mosaic extinction; the enstatite grains are surrounded by a melt matrix composed of 3–16 μm‐size euhedral and subhedral enstatite grains embedded in sodic plagioclase. Numerous vugs, ranging from a few micrometers to a few millimeters in size, constitute ～5 vol% of the meteorite. They occur nearly exclusively within the Mayo Belwa matrix; literature data show that some vugs are lined with bundles of acicular grains of the amphibole fluor‐richterite. This phase has been reported previously in only two other enstatite meteorites (Abee and St. Sauveur), both of which are EH‐chondrite impact melt breccias. It seems likely that in Mayo Belwa, volatiles were vaporized during an impact event and formed bubbles in the melt. As the melt solidified, the bubbles became cavities; plagioclase and fluor‐richterite crystallized at the margins of these cavities via reaction of the melt with the vapor.     

Experimental rare-earth-element partitioning in oldhamite: Implications for the igneous origin of aubritic oldhamite

Abstract— Aubritic oldhamite (CaS) has been the subject of intense study recently because it is the major rare-earth-element (REE) carrier in aubrites, has a variety of REE patterns comparable to those in unequilibrated enstatite chondrites and has an extraordinarily high melting point as a pure substance (2525 °C). These latter two facts have caused some authors to assert that much of the aubritic oldhamite is an unmelted nebular relict, rather than of igneous origin. We have conducted REE partitioning experiments between oldhamite and silicate melt using an aubritic bulk composition at 1200 °C and 1300 °C and subsolidus annealing experiments. All experiments produced crystalline oldhamite, with a range of compositions, glass and Fe metal, as well as enstatite, SiO₂, diopside and troilite in some charges. Rare-earth-element partitioning is strongly dependent on oldhamite composition and temperature. Subsolidus annealing results in larger partition coefficients for some oldhamite grains, particularly those in contact with troilite. All experimental oldhamite/silicate melt partition coefficients are <20 and the vast majority are <5, which is similar to those reported in the literature and is two orders of magnitude less than those inferred for natural aubritic oldhamite. These partition coefficients preclude a simple igneous model, since REE abundances in aubritic oldhamite are greater than would be predicted on the basis of the experimental partition coefficients. Our experimental partition coefficients are consistent with a relict nebular origin for aubritic oldhamite, although experimental evidence that suggests melting of oldhamite at temperatures lower than that reached on the aubrite parent body are clearly inconsistent with the nebular model. Our experiments are consistent also with a complex igneous history. Oldhamite REE patterns may reflect a complex process of partial melting, melt removal, fractional crystallization and subsolidus annealing and exsolution. These mechanisms (primarily fractional crystallization and subsolidus annealing) can produce a wide range of REE patterns in aubritic oldhamite, as well as elevated (100–1000 × CI) REE abundances observed in aubritic oldhamite.     

On the significance of diopside and oldhamite in enstatite chondrites and aubrites

Abstract— Enstatite is the primary silicate phase of equilibrated enstatite chondrites (EECs). The CaO contents of these enstatites lie close to or on the enstatite-diopside phase boundary, yet, curiously, diopside has always been absent from EEC assemblages. In contrast, aubrites contain abundant diopside even though they are thought to be derived from an E chondrite-like protolith. A phase equilibrium analysis of the Ca-Mg-Fe-Mn-Si-O-S system under reducing conditions solves this enigma and shows that diopside-bearing EECs should commonly be found. When S fugacity is sufficiently high (e.g., Fe-FeS buffer), low O fugacity limits the stability of diopside in favor of oldhamite. Under such conditions, the relative stability of diopside and oldhamite is described by the reaction: CaMgSi₂O₆ + MgS = CaS + Mg₂Si₂O₆ A large bulk compositional field exists where diopside and oldhamite are simultaneously stable. The existence of oldhamite does not preclude the stability of diopside. Phase diagram topology demonstrates that bulk compositions lying in the enstatite-oldhamite field and enstatite-oldhamite-alabandite field have enstatite CaO contents nearly identical to that of enstatite in equilibrium with diopside alone. This explains the high enstatite CaO contents of all EECs that do not contain diopside. This study also reports the discovery of the first EEC to contain metamorphic diopside, the Antarctic meteorite EET 90102. Elephant Moraine 90102 has a typical EL6 texture and contains the assemblage: enstatite, diopside, albite, kamacite, troilite, sinoite, and graphite. Trace quantities of alabandite, oldhamite and daubreelite are also present. Diopside is stable in EET 90102 because its bulk composition lies within either the enstatite-diopside-oldhamite-alabandite or diopside-alabandite-enstatite stability fields. In contrast, all other EECs analyzed to date have bulk compositions lying in the enstatite-oldhamite-alabandite stability field. The discovery of diopside in EET 90102 helps confirm the predictions of the phase equilibrium analysis. Elephant Moraine 90102 experienced a high-temperature metamorphic equilibration from which it was quenched. The enstatite-diopside, CaS in alabandite and Fe in alabandite, geothermometers yield temperatures of last equilibration of ～900 °C. The absence of daubreelite and schreibersite along with high troilite Cr contents and high kamacite P contents confirm a high-temperature metamorphic quench. The EET 90102 chondrite experienced a somewhat different cooling history and has a slightly different bulk composition than all other EECs studied to date; however, the close mineralogic, petrologic and textural similarities between EET 90102 and nominal EL6 chondrites signify that it should be classified as a diopside- and sinoite-bearing EL6 chondrite. Assuming that the aubrites formed from an E chondrite-like protolith, a source rock similar to that of a diopside-bearing EEC offers a clear advantage for aubrite formation. Melting of a diopside-saturated EEC protolith would not require conversion of CaS to achieve diopside-saturation upon cooling.     

Potassium isotopic compositions of enstatite meteorites

Enstatite chondrites and aubrites are meteorites that show the closest similarities to the Earth in many isotope systems that undergo mass‐independent and mass‐dependent isotopic fractionations. Due to the analytical challenges to obtain high‐precision K isotopic compositions in the past, potential differences in K isotopic compositions between enstatite meteorites and the Earth remained uncertain. We report the first high‐precision K isotopic compositions of eight enstatite chondrites and four aubrites and find that there is a significant variation of K isotopic compositions among enstatite meteorites (from ?2.34‰ to ?0.18‰). However, K isotopic compositions of nearly all enstatite meteorites scatter around the bulk silicate earth (BSE) value. The average K isotopic composition of the eight enstatite chondrites (?0.47 ± 0.57‰) is indistinguishable from the BSE value (?0.48 ± 0.03‰), thus further corroborating the isotopic similarity between Earth's building blocks and enstatite meteorite precursors. We found no correlation of K isotopic compositions with the chemical groups, petrological types, shock degrees, and terrestrial weathering conditions; however, the variation of K isotopes among enstatite meteorite can be attributed to the parent‐body processing. Our sample of the main‐group aubrite MIL 13004 is exceptional and has an extremely light K isotopic composition (δ⁴¹K = ?2.34 ± 0.12‰). We attribute this unique K isotopic feature to the presence of abundant djerfisherite inclusions in our sample because this K‐bearing sulfide mineral is predicted to be enriched in ³⁹K during equilibrium exchange with silicates.     

Accessory phases in aubrites: spectral properties and implications for asteroid 44 Nysa

The reflectance spectra of meteoritic metal, meteoritic troilite and the CR carbonaceous chondrite EET87770 have been measured in order to investigate the causes of the spectral differences between the surface of the E-class asteroid 44 Nysa and the opaque free fraction of the Happy Canyon aubrite meteorite. The data indicate that the spectral differences require the presence on Nysa's surface of a small amount of a spectrally red sloped material, of which metal and troilite are the most reasonable candidates, and a material possessing absorption bands near 0.9m and 1.8m. A material similar to the carbonaceous chondrite inclusions found in some aubrites can provide a match to the 0.9m feature and perhaps the 1.8m feature. The required abundances of these components depends on whether they are areally distributed or intimately mixed with an enstatite rich material. Based on the petrologic associations seen in aubrites and a series of simulated mineral mixtures, an intimate mixture of 69–92% enstatite and 1–11% metal + troilite and an areal component of 7–20% carbonaceous chondrite type material can provide a reasonable match to the 0.3–2.6m spectrum of Nysa.     

Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body

Abstract– Aubrites exhibit a wide range of highly siderophile element (HSE—Re, Os, Ir, Ru, Rh, Pt, Pd, Au) concentrations and ¹⁸⁷Os/¹⁸⁸Os compositions. Their HSE concentrations are one to three orders of magnitude less than chondrites, with the exception of the Shallowater and Mt. Egerton samples. While most aubrites show chondritic HSE abundance ratios, significant enrichments of Pd and Re relative to Os, Ir, and Ru are observed in 12 of 16 samples. Present‐day ¹⁸⁷Os/¹⁸⁸Os ratios range from subchondritic values of 0.1174 to superchondritic values of up to 0.2263. Half of the samples have ¹⁸⁷Os/¹⁸⁸Os ratios of 0.127 to 0.130, which is in the range of enstatite chondrites. Along with the brecciated nature of aubrites, the HSE and Re‐Os isotope systematics support a history of extensive postaccretion processing, including core formation, late addition of chondritic material and/or core material and potential breakup and reassembly. Highly siderophile element signatures for some aubrites are consistent with a mixing of HSE‐rich chondritic fragments with a HSE‐free aubrite matrix. The enrichments in incompatible HSE such as Pd and Re observed in some aubrites, reminiscent of terrestrial basalts, suggest an extensive magmatic and impact history, which is supported by both the ¹⁸⁷Re‐¹⁸⁷Os isotope system and silicate‐hosted isotope systems (Rb‐Sr, K‐Ar) yielding young formation ages of 1.3–3.9 Ga for a subset of samples. Compared with other differentiated achondrites derived from small planetary bodies, aubrites show a wide range in HSE concentrations and ¹⁸⁷Os/¹⁸⁸Os, most similar to angrites. While similarities exist between the diverse groups of achondrites formed early in solar system history, the aubrite parent body(ies) clearly underwent a distinct evolution, different from angrites, brachinites, ureilites, howardites, eucrites, and diogenites.     

Partial melting of the Indarch (EH4) meteorite: A textural,chemical, and phase relations view of melting and melt migration

Abstract— To test whether aubrites can be formed by melting of enstatite chondrites and to understand igneous processes at very low O fugacities, we have conducted partial melting experiments on the Indarch (EH4) chondrite at 1000–1500 °C. Silicate melting begins at 1000 °C, and Indarch is completely melted by 1500 °C. The metal-sulfide component melts completely at 1000 °C. Substantial melt migration occurs at 1300–1400 °C, and metal migrates out of the silicate charge at 1450 °C and ～50% silicate partial melting. As a group, our experiments contain three immiscible metallic melts (Si-, P-, and C-rich), two immiscible sulfide melts (Fe- and FeMgMnCa-rich), and silicate melt. Our partial melting experiments on the Indarch (EH4) enstatite chondrite suggest that igneous processes at low fO₂ exhibit several unique features. The complete melting of sulfides at 1000 °C suggests that aubritic sulfides are not relics. Aubritic oldhamite may have crystallized from Ca and S complexed in the silicate melt. Significant metal-sulfide melt migration might occur at relatively low degrees of silicate partial melting. Substantial elemental exchange occurred between different melts (e.g., S between sulfide and silicate, Si between silicate and metal), a feature not observed during experiments at higher fO₂. This exchange may help explain the formation of aubrites from known enstatite chondrites.     

Characterization of multiple lithologies within the lunar feldspathic regolith breccia meteorite Northeast Africa 001

Abstract– Lunar meteorite Northeast Africa (NEA) 001 is a feldspathic regolith breccia. This study presents the results of electron microprobe and LA‐ICP‐MS analyses of a section of NEA 001. We identify a range of lunar lithologies including feldspathic impact melt, ferroan noritic anorthosite and magnesian feldspathic clasts, and several very‐low titanium (VLT) basalt clasts. The largest of these basalt clasts has a rare earth element (REE) pattern with light‐REE (LREE) depletion and a positive Euanomaly. This clast also exhibits low incompatible trace element (ITE) concentrations (e.g., <0.1 ppm Th, <0.5 ppm Sm), indicating that it has originated from a parent melt that did not assimilate KREEP material. Positive Eu‐anomalies and such low‐ITE concentrations are uncharacteristic of most basalts returned by the Apollo and Luna missions, and basaltic lunar meteorite samples. We suggest that these features are consistent with the VLT clasts crystallizing from a parent melt which was derived from early mantle cumulates that formed prior to the separation of plagioclase in the lunar magma ocean, as has previously been proposed for some other lunar VLT basalts. Feldspathic impact melts within the sample are found to be more mafic than estimations for the composition of the upper feldspathic lunar crust, suggesting that they may have melted and incorporated material from the lower lunar crust (possibly in large basin‐forming events). The generally feldspathic nature of the impact melt clasts, lack of a KREEP component, and the compositions of the basaltic clasts, leads us to suggest that the meteorite has been sourced from the Outer‐Feldspathic Highlands Terrane (FHT‐O), probably on the lunar farside and within about 1000 km of sources of both Low‐Ti and VLT basalts, the latter possibly existing as cryptomaria deposits.