Ungrouped Achondrite Research Articles

NWA 11562: A Unique Ureilite with Extreme Mg-rich Constituents

A comprehensive study of an ungrouped achondrite meteorite, North West Africa (NWA) 11562, was conducted, involving petrology, mineralogy, and mass-independent chromium isotopic composition. NWA 11562 comprises 34.9 vol% olivine, 56.1 vol% pyroxenes, 6.7 vol% Fe–Ni metal and oxides, 2.2 vol% carbon, and 0.2 vol% anhydrite. The oxygen isotopic composition (δ 18O = 6.24‰ ± 0.13‰ and Δ17O = −1.81‰ ± 0.03‰; Meteoritical Bulletin Database) and chromium isotopic compositions (ε 54Cr = −0.82 ± 0.10, 2SE) are consistent with NWA 11562 being a ureilite, and petrographic observations show features similar to those of the common ureilite group meteorites. Olivine (core: Fo99.0 ± 0.1; rim: Fo98.9 ± 0.1) and pyroxene (orthopyroxene: Mg# 99.0 ± 0.2; clinopyroxene: Mg# 99.1 ± 0.7) core compositions are more magnesian than in any previously known ureilite and lack the characteristic reduction rims of ureilites. Rounded small olivine grains within NWA 11562 indicate that the meteorite experienced impact and associated melting. Combined with the characteristic Fe/Mn ratio (3.84 ± 0.16) and Mg# (99.0 ± 0.1) of olivine cores, we suggest that NWA 11562 represents a more Mg-rich ureilite than any previously reported. NWA 11562 has a high 55Mn/52Cr ratio, and when combined with literature data, it plots on a well-defined 53Mn–53Cr isochron, providing a more accurate age, i.e., 4566.7 ± 0.8 Ma, overlapping previous work. This age may represent the result of early partial melting of the ureilite parent body.

Igneous intrusion origin for the petrofabric of Erg Chech 002

AbstractAchondrites provide an opportunity to examine the igneous processes of differentiated bodies in our solar system. The recent discovery of several silica‐rich achondrites suggests that andesitic crusts were more common among planetesimals than previously thought, though the processes behind their emplacement are not well understood. Here, electron backscatter diffraction (EBSD) is used to investigate the igneous emplacement conditions of Erg Chech 002 (EC 002), a recently discovered ungrouped achondrite representing andesitic magmatism ~2 Myr after the formation of calcium–aluminum‐rich inclusions (CAIs). EBSD analyses of crystallographic preferred orientations (CPOs) for augite and plagioclase feldspar phenocrysts indicate that EC 002 exhibits a weak foliation CPO. Augite misorientation inverse pole figures (mIPF) indicate preferential slip along the (100)[001] system with a distinct shift toward the {0kl}[u0w] system in plastically deformed grains. Our findings support the hypothesis that EC 002 was likely emplaced in the lower regions of a magmatic intrusion. Augite slip signatures suggest that EC 002 crystallization and emplacement were restricted to high temperatures (>800°C) and experienced at least two strain regimes. The distinct shift from a dominant (100)[001] slip system, which corresponds to high temperatures (800–1050°C), to a [0kl][u0w] slip system indicates an increased strain rate due to shock deformation (1–5 GPa) attributed to ejection by hypervelocity impact.

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

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

Al Huwaysah 010: The most reduced brachinite, so far

AbstractAl Huwaysah 010 is an ungrouped achondrite meteorite, recently referred to as a brachinite‐like meteorite. This meteorite, showing a fine‐grained assemblage of low‐Ca pyroxene and opaque phases, is strongly reduced in comparison to other reduced brachinites. The occurrence of some tiny plates of graphite and oldhamite in this meteorite suggests that a partial melt residue has experienced a further reduction process. Olivine, the most abundant phase, is compositionally homogeneous (Fo83.3) as well as the clinopyroxene (En45.5Fs10.8Wo43.7) and the plagioclase (Ab69.5). Orthopyroxene (En85.4Fs13.9Wo0.7) also occurs but only in a fine intergrowth. Other accessory phases are Fe metal grains (Ni‐free or Cr‐bearing Fe‐Ni alloy), troilite, chlorapatite, pentlandite (as inclusions in chromite). The sample shows two different closure temperatures: the highest (≈900°C) is determined via the olivine–chromite intercrystalline geothermometer and the lowest temperature (≈520°C) is determined via the pyroxene‐based intracrystalline geothermometer. These temperatures may represent, respectively, the closure temperature associated with the formation and a subsequent impact event excavating the sample from the parental body. The visible to near‐infrared (VNIR) reflectance spectra of Al Huwaysah 010 exhibit low reflectance, consistent with the presence of darkening components, and weak absorptions indicative of olivine and pyroxene. Comparing the spectral parameters of Al Huwaysah 010 to potential parent bodies characterized by olivine–pyroxene mineralogy, we find that it falls within the field previously attributed to the SIII type asteroids. These results lead us to classify the Al Huwaysah 010 meteorite as the most reduced brachinite, whose VNIR spectral features show strong affinities with those of SIII asteroids.

Mass-dependent nickel isotopic variations in achondrites and lunar rocks

We present high-precision mass-dependent nickel isotopic data for a comprehensive suite of achondrites and lunar rocks, providing key insights into the early planetary differentiation and Earth-Moon system formation. The primitive achondrites display high Ni contents and invariant Ni isotopic compositions. Incomplete core-mantle differentiation in primitive achondrite parent bodies resulted in the retention of metal in the mantle, which dominated the Ni budget and accounted for the bulk chondritic Ni isotopic values. The highly reduced differentiated achondrites, aubrites and an ungrouped achondrite (NWA 8409), have variable, and extremely light Ni isotopic compositions. Acid leaching experiments demonstrate that the sulfides are a significant host of light Ni isotopes in aubrites. The most extreme Ni isotope values of aubrites may be due to large Ni isotope fractionation accompanied by silicate-sulfide-metal separation during differentiation of the parent bodies, and subsequent global disruptive collision and reassembly with variably high proportions of sulfides enriched in the mantle. The howardite-eucrite-diogenite (HED) meteorites show Ni isotopic variations that are positively correlated with Ni/Co ratios, a feature that cannot be produced by igneous differentiation. Late accretion of high-Ni and high-Ni/Co chondritic materials after core formation of their likely parent body, Vesta, could have accounted for this correlation. Thus, the primitive silicate mantle of Vesta may have sub-chondritic Ni isotopic compositions, implying possible Ni isotope fractionation during core-mantle differentiation of small planetary bodies. The lunar breccia meteorites have homogenously chondritic Ni isotope values, together with their high Ni/Co of bulk rock and metals therein, suggesting impact contamination. Lunar basalt meteorites have low Ni/Co ratios and are systematically isotopically lighter than the breccias, displaying a positive correlation between Ni isotope value and Ni/Co ratio, as that seen in the HEDs. Therefore, the Ni isotopic systematics in lunar rocks also indicates the effect of late accretion, with the primitive lunar mantle having sub-chondritic Ni isotope values. This implies that the Moon-forming impactor, Theia, was likely an aubrite-like differentiated planetary body whose mantle was enriched in light Ni isotopes. We suggest that there was significant Ni isotope fractionation between core and mantle during differentiation of early formed small planetary bodies, but this signature can be obscured by late accretion in the bulk achondrite records.

Tissemouminites: A new group of primitive achondrites spanning the transition between acapulcoites and winonaites

AbstractThe Northwest Africa (NWA) 090 meteorite, initially classified as an acapulcoite, presents petrological, chemical, and isotopic characteristics comparable to a group of seven primitive winonaites: Dhofar 1222, NWA 725, NWA 1052, NWA 1054, NWA 1058, NWA 1463, and NWA 8614. Five of these samples were previously classified as acapulcoites or ungrouped achondrites before being reclassified as winonaites based on their oxygen isotopic compositions. These misclassifications are indicative of the particular compositional nature of these primitive achondrites. All contain relict chondrules and a lower closure temperature of metamorphism of 820 ± 20 °C compared to other typical winonaites, as well as mineral elemental compositions similar to those of acapulcoites. The oxygen isotopic signature of these samples, δ17O of 1.18 ± 0.17‰, δ18O of 3.18 ± 0.30‰, and Δ17O of −0.47 ± 0.02, is in fact resolvable from both acapulcoites and winonaites. We investigate the relationship between these eight primitive achondrites, typical winonaites, and acapulcoites, to redefine petrological, mineralogical, and geochemical criteria of primitive achondrite classification. Distinguishing between winonaites, acapulcoites, and this group of eight primitive achondrites can be unambiguously done using a combination of several mineralogical and chemical criteria. A combination of olivine fayalite content and FeO/MnO ratio, as well as plagioclase potassium content allow us to separate these three groups without the absolute necessity of oxygen isotope analyses. NWA 090 as well as the other seven primitive achondrites, although related to winonaites, are most likely derived from a parent body distinct from winonaites and acapulcoites–lodranites, and define a new group of primitive achondrites that can be referred to as tissemouminites.

Evaluating the O‐Cr‐Mo isotope signatures in various meteorites representing core–mantle–crust fragments: Implications for partially differentiated planetesimal(s) accreted in the early outer solar system

AbstractThe Allende, Eagle Station pallasites (ESP), some ungrouped achondrites (UA), and ungrouped irons (UI) represent different types of meteorites; these are traditionally distinguished as chondrule‐bearing chondrite, chondrule‐free stony irons, stones, and irons, respectively. Oxygen isotopic compositions of meteorites have long been used as a robust tool to identify the parent bodies of these extraterrestrial materials. We revisited the δ18O‐∆17O, along with ε54Cr and ε92Mo, of these meteorites and established a genetic link that possibly suggests that undifferentiated carbonaceous chondrites and their differentiated counterparts belong to a common reservoir. We observed that the differentiated counterparts of Allende CV3 including ESP, ungrouped achondrites, and ungrouped irons collectively provide the oxygen isotope trends comparable to those of the Y&R and the PCM lines, and are likely to represent the primitive isotope reservoirs in the solar nebula. ε54Cr and ε92Mo data of these meteorite types support their oxygen isotope trends. The idea of a common partially differentiated parent body also lends support from natural remnant magnetization in the CV chondrites and Eagle Station olivine. Furthermore, ∆17O‐ε54Cr data suggest that these meteorites originated from carbonaceous planetesimal(s) accreted at different heliocentric distances in the solar nebula in the outer solar system. As proposed earlier, these bodies remained separated from the inner solar system due to formation of Jupiter. Taken together, we propose that the ungrouped irons, ESP, and ungrouped achondrites could possibly represent the differentiated sections, such as core, core–mantle, and mantle of a planetesimal(s), respectively, with the Allende CV3 representing an undifferentiated chondritic crust.

Sulfide‐dominated partial melting pathways in brachinites

AbstractPetrogenetic relationships among members of the brachinite family were established by analyzing major and trace element concentrations of minerals for 9 representative specimens: Al Huwaysah 010, Eagles Nest, Northwest Africa (NWA) 4882, NWA 5363, NWA 7297, NWA 7299, NWA 11756, Ramlat as Sahmah (RaS) 309, and Reid 013. The brachinite family, which includes brachinites and ungrouped achondrites with compositional and isotopic similarities to brachinites, comprises FeO‐rich, olivine‐dominated achondrites whose compositional and mineralogic variability is correlated with oxidation state. Most classical brachinites are derived from precursors that were more oxidized and sulfur‐rich than those of ungrouped “brachinite‐like” achondrites. This is manifest in the distinct Fe‐Ni‐S systems among brachinite family precursors, which were sulfide‐dominated for the most oxidized brachinites and metal‐dominated for the least oxidized brachinite‐like achondrites. Consequently, highly siderophile element behavior was controlled through melting and removal of their dominant host phase in the precursor, which was likely pentlandite in sulfide‐dominated systems and kamacite/taenite in metal‐dominated systems. Anomalous Ir/Os and Pt/Os ratios of oxidized brachinites may be attributed to selective complexing during melting of As‐rich pentlandite, consistent with our observations of impact‐melted sulfides in R chondrite NWA 11304, although further experimental work is needed to model this process. The apparent redox trend among the brachinite family is consistent with silicate FeO content and Fe/Mn ratios, which may be used as a proxy for determining the relative oxidation state of brachinite family members. Based on our analyses, we make several recommendations for reclassification of samples into a continuum of oxidized to reduced endmembers for the brachinite family. Along with a common range of Δ17O, this evidence is consistent with either formation on a common heterogeneous parent body, or at least from the same nebular reservoir, with variable O and S fugacities, resulting in mineralogically distinct igneous products for oxidized and reduced endmembers. Sulfur‐bearing, oxidized differentiation may extend to other bodies that formed at or beyond the snow line in the early solar system, and should be considered when interpreting observational data for asteroids in upcoming missions.

Early sulfur‐rich magmatism on the ungrouped achondrite Northwest Africa 7325 differentiated parent body

AbstractThe ungrouped achondrite Northwest Africa (NWA) 7325 parent body underwent a severe impact after primary crystallization, which completely melted plagioclase and partially melted pyroxene, followed by Mg diffusion into the adjacent plagioclase‐melt. The 26Al‐26Mg system was therefore modified, forming a pseudoisochron with an initial δ26Mg* of 0.094 ± 0.005‰ and an age of 4563.12 ± 0.33 Ma between the primary crystallization and subsequent impact event(s). The positive initial δ26Mg* can be interpreted by a model age of ~1.77 Ma after CAIs when a chondritic composition differentiated into a magma/rock with the Al/Mg ratio equivalent to that of NWA 7325 (~1.52). The LREE enrichments and a positive Eu anomaly suggest that the NWA 7325 parent magma formed by the melting of a plagioclase‐rich crustal lithology, which crystallized from a magma ocean. Differentiation of the magma ocean was prior to 1.77 Ma after CAIs. NWA 7325 is also unique by containing many rounded voids (5–6 vol%) interstitial to or enclosed in silicates, suggested to have formed by the leaching/vaporization of pre‐existing Fe‐Ca‐Mg‐Mn sulfides. This is supported by the similar morphology between voids and Cr‐bearing troilites, the discovery of relict oldhamite, and the highly reducing conditions of NWA 7325. The loss of pre‐existing sulfides could explain the unusual subchondritic Mn/Mg ratio of the bulk sample. Furthermore, the enrichment of moderately volatile elements (K/Th ratio ~2600–10,000) in the NWA 7325 parent body may result from the bonding with S2‐ in silicate melts under highly reducing conditions. NWA 7325 therefore provides evidence of sulfur‐rich magmatism in the early solar system.

Evidence for anorthositic crust formed on an inner solar system planetesimal

doi: 10.7185/geochemlet.1921 During the first million years of solar system history, planetesimals experienced extensive melting powered by the radioactive decay of 26 Al (Lee et al., 1977). To date, the only known anorthositic crust on a solar system body is that of the Moon, formed by plagioclase flotation on top of the magma ocean (Wood et al., 1970). Here we show evidence from the ungrouped achondrite meteorite Northwest Africa (NWA) 8486 that an anorthositic crust formed on a planetes-imal very early in solar system history (<1.7 Ma). NWA 8486 displays the highest anomalies in Eu and Sr found in achondrites so far and, for the first time, this characteristic is also identified in clinopyroxene. Elemental modelling , together with calculated timescales for crystal settling, show that only the melting of an anorthosite can produce NWA 8486 within the first 5 million years of solar system history. Our results indicate that such a differentiation scenario was achievable over short timescales within the inner solar system, and must have contributed to the making and elemental budget of the terrestrial planets.

Evidence for early impact on a hot differentiated planetesimal from Al-rich micro-inclusions in ungrouped achondrite Northwest Africa 7325

Abstract The ungrouped achondrite NWA 7325 is a cumulate olivine gabbro (Irving et al., 2013). It contains abundant and unique micro-inclusions of Ca-pyroxene (Bischoff et al., 2013) and spinel-like Al-Mg oxide (Goodrich et al., 2017) in plagioclase, indicating a remelting event induced either by impact (Goodrich et al., 2017) or by magma intrusion (Bischoff et al., 2013; Weber et al., 2016). In this work, a combined FIB-TEM study has been conducted on these micro-inclusions to address their petrogenesis and the related history of NWA 7325. TEM study revealed that micro-inclusions in the interiors of large plagioclase grains are Al-enriched spinel (Mg/Al atomic ratio: 0.03–0.4) with minor needle-like α-corundum, whereas those in the margins are predominantly Al-rich diopside (En44.5-46.6Fs1.2-1.5Wo31.2-36.7CaTs17.6-22.4) with minor forsterite (Fo94.6-94.7). The Mg/Al atomic ratios of the spinel micro-inclusions are negatively correlated with the distance away from the interface of plagioclase-pyroxene. Large plagioclase grains also exhibit a decrease in the Mg/Al atomic ratio from the rims to the cores. Based on the reaction texture at the interfaces of plagioclase-pyroxene, we infer that the Mg concentration gradient in large plagioclase grains could have resulted from Mg diffusion from the remelted rims of pyroxene into plagioclase. In addition, TEM observations showed that large plagioclase grains are not single crystals, but assemblages of submicron to micron-sized crystals. The preservation of Mg concentration gradients, submicron-sized polycrystalline plagioclases, and the consistent presence of micro-inclusions within large plagioclase grains likely indicate complete remelting of plagioclase and partial remelting of pyroxene (only rims of pyroxene with plagioclase) followed by fast cooling. We propose that micro-inclusions of diopside, forsterite, Al-rich spinel and corundum crystallized from the melt, which developed a Mg concentration gradient during the remelting of NWA 7325. The heating temperatures of pyroxene and plagioclase in the remelting event were estimated to be 1274–1327 °C and ≥1530 °C, respectively. A subsequent cooling rate of ∼500–650 °C/h at 1300 °C was found by fitting the measured Mg concentration gradient in large plagioclase grains with a Fick’s second law model that incorporated the diffusion coefficients of Mg in plagioclase-melt. These results are better explained by a shock event; a magmatic intrusion process is ruled out. To achieve the coexistence of shock-induced high temperature (≥1274 °C) in-situ melting and only undulatory extinction of forsterite grains, an ambient temperature of 1000–1100 °C in the surrounding, parent rock of NWA 7325 was required prior to impact. This work suggests a very early shock event when NWA 7325 was hot and buried in the crust of its parental planetesimal, which is a scenario consistent with its magma crystallization age (∼4.3 Ma after CAIs, e.g., Koefoed et al., 2016). This work also implies that impacts are a potential heat source for melting hot planetesimals in the early Solar System.

Petrological, petrofabric, and oxygen isotopic study of five ungrouped meteorites related to brachinites

AbstractNorthwest Africa (NWA) 6112, Miller Range (MIL) 090206 (plus its pairs: MIL 090340 and MIL 090405), and Divnoe are olivine‐rich ungrouped achondrites. We investigated and compared their petrography, mineralogy, and olivine fabrics. We additionally measured the oxygen isotopic compositions of NWA 6112. They show similar petrography, mineralogy, and oxygen isotopic compositions and we concluded that these five meteorites are brachinite clan meteorites. We found that NWA 6112 and Divnoe had a c axis concentration pattern of olivine fabrics using electron backscattered diffraction (EBSD). NWA 6112 and Divnoe are suggested to have been exposed to magmatic melt flows during their crystallization on their parent body. On the other hand, the three MIL meteorites have b axis concentration patterns of olivine fabrics. This indicates that the three MIL meteorites may be cumulates where compaction of olivine grains was dominant. Alternatively, they formed as residues and were exposed to olivine compaction. The presence of two different olivine fabric patterns implies that the parent body(s) of brachinite clan meteorites experienced diverse igneous processes.

U-Pb, Rb-Sr and Ar-Ar systematics of the ungrouped achondrites Northwest Africa 6704 and Northwest Africa 6693

We report U-Pb, 87Rb-87Sr, 40Ar-39Ar, and 238U/235U isotopic data for paired ungrouped achondrites NWA 6704 and NWA 6693 that were derived from a highly oxidised parent body with broadly chondritic composition (Warren et al., 2013; Hibiya et al., 2019). Pb-isotopic ages derived from isochrons for multiple acid-leached pyroxene fractions are 4562.76 + 0.22/−0.30 Ma for NWA 6704 and 4562.63 + 0.29/−0.21 Ma for NWA 6693, calculated using 238U/235U ratio of 137.7784 ± 0.0097 measured in NWA 6704. The Rb-Sr mineral isochron age of 4543 ± 46 Ma (initial 87Sr/86Sr = 0.699013 ± 0.000055) is consistent with the Pb-isotopic age. Together with 187Re-187Os isochron age of 4576 ± 250 Ma for NWA 6704 (Hibiya et al., 2019), and 26Al-26Mg and 53Mn-53Cr ages calculated using the rapidly crystallized angrite D’Orbigny as a time anchor are also consistent with the Pb-isotopic age (Sanborn et al., 2019), these data indicate that the parent rocks of NWA 6693 and NWA 6704 remained closed to migration of both lithophile and siderophile elements since crystallisation and initial cooling. The whole rock 40Ar-39Ar age of 4199 ± 32 Ma suggests a complete resetting of the K-Ar system approximately 360 Ma after crystallisation. A later event at ≤2.12 Ga partially reset the K-Ar system as shown by the low temperature heating steps. Both meteorites have high 87Rb/86Sr ratios (up to 7.0 in NWA 6693 pyroxene) and very radiogenic 87Sr/86Sr up to 1.15. Together with the absence of secondary disturbance in the Rb-Sr and U-Pb systems, this makes them suitable for cross-calibration of the isotopic chronometers. These meteorites are also promising candidates to serve as age reference samples for the early Solar System chronology, as an alternative or complement to angrites of the early generation (D’Orbigny, Sahara 99555) that are currently used for this purpose. Plagioclase in NWA 6704 has a sufficiently low Rb/Sr ratio to define precise initial 87Sr/86Sr of 0.698997 ± 0.000027, which corresponds to the time of separation of the parent body precursor material from the solar nebula of 1.5 ± 2.1 Ma. This value suggests that the parent asteroid accreted within 3.6 Ma after CAI formation, or before 4563.7 Ma using the CAI age of 4567.3 Ma (Connelly et al., 2012).

Shock history of the fossil ungrouped achondrite Österplana 065: Raman spectroscopy and TEM of relict chrome‐spinel grains

AbstractChrome‐spinel grains from the fossil ungrouped achondrite Österplana 065 (Öst 065) recovered from Middle Ordovician limestone in Sweden were studied using Raman spectroscopy and TEM. All the studied chrome‐spinel grains have a high density of planar fractures and planar features, not seen in chromites from the other L chondritic Ordovician fossil meteorites. Raman spectra of the host chrome‐spinel grain and its planar features are similar and no signatures of high‐pressure phases of chromite were found. The planar features occur along planar fractures, are enriched in ZnO, and are most probably produced due to enhanced leaching during terrestrial weathering in the marine sediment. Dislocation densities within two FIB sections prepared from two chrome‐spinel grains from Öst 065 are similar to the dislocation densities found within chromite grains from the matrix of Tenham L6 chondrite. Using this observation and taking into account the presence of significant fracturing in all the grains, we conclude that the Öst 065 chrome‐spinel grains were subjected to moderate to very strong shock corresponding to shock stages of S4–S6. This makes Öst 065 fossil achondrite the highest shocked fossil meteorite studied so far. This is consistent with the hypothesis that Öst 065 is a piece of the impactor that led to the L chondrite parent body breakup.

Meteorite falls in Bulgaria: Reappraisal of mineralogy, chemistry, and classification

AbstractWe present a summary of the mineralogy, mineral chemistry, and magnetic characteristics of all the five Bulgarian meteorite falls. We report the first mineralogical descriptions, chemical analyses, and magnetic measurements of the Konevo (1931) and Silistra (1917) meteorites. We classify Konevo as LL5, and Silistra as an ungrouped achondrite with HED affinities. Pavel (1966; previously classified as an H5) is reclassified as H3‐anomalous. We also provide precise mineralogy and mineral chemistry of the Virba meteorite (1873, L6), and more details on the mineral chemistry of Gumoschnik (1904, H5).

The Meteoritical Bulletin, No. 103

AbstractMeteoritical Bulletin 103 contains 2582 meteorites including 10 falls (Ardón, Demsa, Jinju, Križevci, Kuresoi, Novato, Tinajdad, Tirhert, Vicência, Wolcott), with 2174 ordinary chondrites, 130 HED achondrites, 113 carbonaceous chondrites, 41 ureilites, 27 lunar meteorites, 24 enstatite chondrites, 21 iron meteorites, 15 primitive achondrites, 11 mesosiderites, 10 Martian meteorites, 6 Rumuruti chondrites, 5 ungrouped achondrites, 2 enstatite achondrites, 1 relict meteorite, 1 pallasite, and 1 angrite, and with 1511 from Antarctica, 588 from Africa, 361 from Asia, 86 from South America, 28 from North America, and 6 from Europe. Note: 1 meteorite from Russia was counted as European. The complete contents of this bulletin (244 pages) are available on line. Information about approved meteorites can be obtained from the Meteoritical Bulletin Database (MBD) available on line at http://www.lpi.usra.edu/meteor/.

In search of the Earth‐forming reservoir: Mineralogical, chemical, and isotopic characterizations of the ungrouped achondrite NWA 5363/NWA 5400 and selected chondrites

AbstractHigh‐precision isotope data of meteorites show that the long‐standing notion of a “chondritic uniform reservoir” is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this “isotopic crisis” and to better understand the genetic relations of meteorites and the Earth‐forming reservoir, we performed a comprehensive petrographic, elemental, and multi‐isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent‐body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium‐tungsten chronology yields a model age of 2.2 ± 0.8 Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent‐body is not the “missing link” that could explain the composition of the Earth by the mixing of known meteorites. Until this “missing link” or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth.

Petrogenesis and provenance of ungrouped achondrite Northwest Africa 7325 from petrology, trace elements, oxygen, chromium and titanium isotopes, and mid-IR spectroscopy

Northwest Africa (NWA) 7325 is an ungrouped achondrite that has recently been recognized as a sample of ancient differentiated crust from either Mercury or a previously unknown asteroid. In this work we augment data from previous investigations on petrography and mineral compositions, mid-IR spectroscopy, and oxygen isotope compositions of NWA 7325, and add constraints from Cr and Ti isotope compositions on the provenance of its parent body. In addition, we identify and discuss notable similarities between NWA 7325 and clasts of a rare xenolithic lithology found in polymict ureilites.NWA 7325 has a medium grained, protogranular to poikilitic texture, and consists of 10–15vol.% Mg-rich olivine (Fo 98), 25–30vol.% diopside (Wo 45, Mg# 98), 55–60vol.% Ca-rich plagioclase (An 90), and trace Cr-rich sulfide and Fe,Ni metal. We interpret this meteorite to be a cumulate that crystallized at ⩾1200°C and very low oxygen fugacity (similar to the most reduced ureilites) from a refractory, incompatible element-depleted melt. Modeling of trace elements in plagioclase suggests that this melt formed by fractional melting or multi-stage igneous evolution. A subsequent event (likely impact) resulted in plagioclase being substantially remelted, reacting with a small amount of pyroxene, and recrystallizing with a distinctive texture.The bulk oxygen isotope composition of NWA 7325 plots in the range of ureilites on the CCAM line, and also on a mass-dependent fractionation line extended from acapulcoites. The ε54Cr and ε50Ti values of NWA 7325 exhibit deficits relative to terrestrial composition, as do ordinary chondrites and most achondrites. Its ε54Cr value is distinct from that of any analyzed ureilite, but is not resolved from that of acapulcoites (as represented by Acapulco).In terms of all these properties, NWA 7325 is unlike any known achondrite. However, a rare population of clasts found in polymict ureilites (“the magnesian anorthitic lithology”) are strikingly similar to NWA 7325 in mineralogy and mineral compositions, oxygen isotope compositions, and internal textures in plagioclase. These clasts are probably xenolithic in polymict ureilites, and could be pieces of NWA 7325-like meteorites.Using constraints from chromium, titanium and oxygen isotopes, we discuss two possible models for the provenance of the NWA 7325 parent body: (1) accretion in the inner solar system from a reservoir similar to that of acapulcoites in Δ17O, ε54Cr and ε50Ti; or (2) early (<1Ma after CAI formation) accretion in the outer solar system (beyond the snow line), before 54Cr and 50Ti anomalies were introduced to this region of the solar system. The mid-IR emission spectrum of NWA 7325 obtained in this work matches its modal mineralogy, and so can be compared with spectra of new meteorites or asteroids/planets to help identify similar materials and/or the parent body of NWA 7325.

U–Pb and Al–Mg systematics of the ungrouped achondrite Northwest Africa 7325

Northwest Africa (NWA) 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with a possible link to the planet Mercury. In order to understand the origin of this meteorite and the nature of its parent body, we have determined its crystallisation age using the long-lived U–Pb and short-lived Al–Mg chronometers. An internal Pb–Pb isochron defined by six acid leached pyroxene fractions yields an age of 4563.4±2.6Ma, assuming that the 238U/235U ratio for NWA 7325 is identical to the bulk Earth and Solar System value of 137.794. The Al–Mg isotope analyses of seven fractions (four plagioclase, one pyroxene, one olivine and one whole rock) define a regression line corresponding to 26Al/27Al0=(3.03±0.14)×10−7 and an initial δ26Mg∗ of 0.093±0.004‰. When anchored to the D’Orbigny angrite, this initial 26Al/27Al yields an age of 4563.09±0.26Ma. The Pb–Pb age of 4563.4±2.6Ma and Al–Mg age of 4563.09±0.26Ma are in complete agreement, but the low U concentrations of NWA 7325 resulted in a relatively low precision Pb–Pb age. The observed excess in initial δ26Mg∗ can be explained by 27Al/24Mg fractionation and subsequent Mg isotopic evolution after planetary differentiation. Furthermore, the parental magma of NWA 7325 most likely formed within 1.72Ma after calcium-aluminium rich inclusion (CAI) formation. NWA 7325 formed near simultaneously with quenched angrites and a number of ungrouped achondrites at ∼4563Ma, suggesting that a multitude of planetary bodies had formed and differentiated by ∼4–5Myr after CAI formation. This ancient age may be interpreted as an argument against NWA 7325 originating from Mercury, however it does not completely rule it out.

The initial abundance and distribution of 92Nb in the Solar System

Niobium-92 is an extinct proton-rich nuclide, which decays to 92Zr with a half-life of 37 Ma. This radionuclide potentially offers a unique opportunity to determine the timescales of early Solar System processes and the site(s) of nucleosynthesis for p-nuclei, once its initial abundance and distribution in the Solar System are well established. Here we present internal Nb–Zr isochrons for three basaltic achondrites with known U–Pb ages: the angrite NWA 4590, the eucrite Agoult, and the ungrouped achondrite Ibitira. Our results show that the relative Nb–Zr isochron ages of the three meteorites are consistent with the time intervals obtained from the Pb–Pb chronometer for pyroxene and plagioclase, indicating that 92Nb was homogeneously distributed among their source regions. The Nb–Zr and Pb–Pb data for NWA 4590 yield the most reliable and precise reference point for anchoring the Nb–Zr chronometer to the absolute timescale: an initial 92Nb/93Nb ratio of (1.4±0.5)×10−5 at 4557.93±0.36 Ma, which corresponds to a 92Nb/93Nb ratio of (1.7±0.6)×10−5 at the time of the Solar System formation. On the basis of this new initial ratio, we demonstrate the capability of the Nb–Zr chronometer to date early Solar System objects including troilite and rutile, such as iron and stony-iron meteorites. Furthermore, we estimate a nucleosynthetic production ratio of 92Nb to the p-nucleus 92Mo between 0.0015 and 0.035. This production ratio, together with the solar abundances of other p-nuclei with similar masses, can be best explained if these light p-nuclei were primarily synthesized by photodisintegration reactions in Type Ia supernovae.