Angrite Parent Body Research Articles

Potential Chronological Disturbance of the D’Orbigny Angrite Inferred from Discordant 26Al Ages

Angrites originate from the early-formed differentiated angrite parent body. The pristine volcanic angrite D’Orbigny is devoid of brecciation, shock effects, or any evidence of secondary processes and is thus key for studying the early stages of planetary accretion and differentiation. However, chronometers used to establish the formation chronology of angrites (including D’Orbigny) yield discordant ages, either (i) suggesting that secondary processes could have disturbed the apparent formation ages or (ii) being taken as evidence of heterogeneous distribution of 26Al in the early solar system. Yet spinel is minimally susceptible to secondary parent body processes and therefore a reliable target for establishing precise 26Al–26Mg ages. Here, we present the first in situ 26Al–26Mg analyses of spinel and plagioclase in D’Orbigny. Individual mineral assemblages provide distinct ages: olivine–spinel shows a well-defined isochron with an initial Al ratio ([26Al/27Al]i) of (5.39 ± 0.85) × 10−6, indicating formation at 2.35 +0.25−0.22 Myr after the formation of calcium–aluminum-rich inclusions (CAIs), whereas plagioclase–olivine defines an isochron with [26Al/27Al]i = (7.46 ± 1.87) × 10−7, implying formation at 4.40 +0.44−0.38 Myr after CAIs, consistent with previous MC-ICP-MS studies. This temporal gap can be attributed to secondary processes such as metamorphic or impact-generated diffusion. Thus, D’Orbigny and other angrites do not represent an immaculate anchor for chronometric comparison. This complexity should be considered in future studies, especially when using D’Orbigny as an anchor to discuss the chronology of the early solar system.

Rubidium isotopic compositions of angrites controlled by extensive evaporation and partial recondensation.

The planetesimals in the solar system exhibit varying degrees of moderately volatile elements (MVEs) depletion compared to the protosolar composition. Revealing the relevant mechanisms is crucial for exploring early solar system evolution. Most volatile-depleted materials in the solar system exhibit enrichments in the heavier isotopes of MVEs, which have traditionally been attributed to the loss of volatiles through partial evaporation. Angrites are so far an exception as they are enriched in the lighter isotopes of K. This has been interpreted as reflecting condensation processes. Here, we present Rb isotopic data of angrites and find that they have lighter Rb isotopic compositions than Vesta, Mars, and the Moon. The δ87Rb value of the angrite parent body (APB) is estimated to range between -1.19‰ and -0.67‰. The extremely light Rb isotopic composition of the APB is likely a result of the kinetic recondensation of Rb after near-complete evaporation during the magma ocean stage. This finding provides further support for the partial recondensation model to explain the light Rb and K isotopic compositions of the APB. In addition, the APB, alongside other terrestrial planetary bodies (e.g., Earth, Mars, Moon, and Vesta), exhibit a strong correlation between their Rb and K isotopic compositions. This coupling of Rb and K isotopes is indicative of a volatility-driven isotopic fractionation rather than nucleosynthetic anomalies. The extremely light Rb-K isotopic signatures of the APB suggest that beyond evaporation, condensation plays an equally significant role in shaping the planetary-scale distributions of volatile elements.

Partitioning of nickel and cobalt between metal and silicate melts: Expanding the oxy-barometer to reducing conditions

Moderately siderophile elements (MSEs) are potential tracers of the thermodynamic conditions prevailing during planetary core formation because their metal–silicate partition coefficients (Dmet/sil) vary as a function of P, T, and oxygen fugacity (fO2). Those properties result in the production of planetary mantles with unique MSE depletion signatures. Among the MSEs, Ni and Co are reliable barometers in magma oceans because their Dmet/sil values are strongly correlated with pressure, decreasing by almost 3 orders of magnitude between 1 bar and 100 GPa. Current pressure-dependent expressions of Dmet/sil were calibrated based on experiments performed under relatively oxidizing conditions, mostly at fO2 slightly below the iron–wüstite Fe–FeO buffer (IW), which is relevant to the mantles of Earth and Mars. However, planets and asteroids formed under a wide range of redox conditions, from Mercury, the most reduced (∼IW − 5.5), to the most oxidized angrite parent body (IW − 1.5 to IW + 1). In this study, we performed and analyzed 38 metal–silicate partitioning experiments over a wide range of pressures (1 bar to 26 GPa) and oxygen fugacities (IW − 6.4 to IW − 1.9) to expand the available Ni and Co Dmet/sil values to reducing conditions. We then parameterized 255 Ni and 194 Co Dmet/sil values as a function of T (1573–5700 K), P (1 bar to 100 GPa), and fO2 (IW − 6.4 to IW + 0.2). We also modeled the evolution of Ni and Co Dmet/sil values along the liquidus of a chondritic mantle at various P and fO2 conditions to investigate the thermodynamic conditions of various planetary bodies’ magma oceans. The P and fO2 conditions we obtained for Earth, Mars, the Moon, and Vesta are consistent with previous studies using similar methods, and the pressure during core formation is strongly correlated to planetary size. Finally, we also applied our model to several achondrite parent bodies; our results indicate a wide variety of objects, from the asteroid-sized, oxidized angrite parent body to the planet-sized, highly reduced aubrite parent body.

The impact history and prolonged magmatism of the angrite parent body

AbstractAs some of the oldest differentiated materials in our solar system, angrite meteorites can provide unique insights into the earliest stages of planetary evolution. However, the timing of planetary mixing, as evidenced by oxygen isotope variations in the quenched angrites, and the extent of magmatism on the angrite parent body (APB) remain poorly understood. Here, we report on microstructurally guided in situ geochemical and Pb–Pb isotopic measurements on angrites aimed at better understanding of the timing and nature of magmatic processes, as well as impact events, on the APB. The quenched angrite Northwest Africa (NWA) 12320 yielded a Pb–Pb date of 4571.2 ± 9.4 Ma, which we interpret as corresponding to the timing of planetary mixing. The only known shocked quenched angrite, NWA 7203, also yielded an ancient Pb–Pb date of 4562.9 ± 9.3 Ma, which is identical to the Pb–Pb date of 4563.6 ± 7.9 Ma obtained for the texturally intermediate angrite NWA 10463. Pb–Pb analyses in phosphates in the dunitic angrite NWA 8535 yielded a much younger date of 4514 ± 30 Ma, representing the youngest Pb–Pb date ever recorded for an angrite. Based on the evidence from the lack of shock deformation, olivine major and trace element compositions, and no apparent contamination in the oxygen isotope composition of NWA 8535, our findings are consistent with prolonged magmatism on the APB. This finding is consistent with a large size for the APB.

Mineralogical anatomy of the Cr-rich quenched angrite Northwest Africa 12774: Implication to mantle heterogeneity

Angrites are a small group of basaltic meteorites and their origin is currently disputed. Among these, Northwest Africa (NWA) 12774 is a quenched angrite that was reported having an anomalously high bulk Cr2O3 content (∼0.45 wt%). However, the reason behind this anomaly, which is critical for understanding the evolution of the angrite parent body, remains unknown. Here, we performed a detailed petrographic, mineralogical, and bulk oxygen and chromium isotopic composition study on this meteorite. NWA 12774 consists of porous olivine macrocrysts, phenocrysts of olivine and Al-Ti-rich augite, and spinel micro-phenocrysts with fine-grained groundmass. The olivine macrocrysts and the magnesian cores of olivine phenocrysts show compositional correlations distinctly different from typical olivine phenocrysts. The olivine macrocrysts contain small chromite/chrome-spinel inclusions which have the highest Cr2O3 content (53.2 wt%) for chromite/spinel in angrites to date. Based on these textural and chemical features, the olivine macrocrysts and the magnesian cores of olivine phenocrysts are identified as xenocrysts. Some pyroxene phenocrysts contain regions with complexly zoned microtextures, which have much larger chemical variations compared with those with simply zoned microtextures. The regions with complexly zoned microtextures are likely to be of xenocrystic origin. The bulk Cr2O3 content in NWA 12774 was estimated through two approaches, both of which show the bulk Cr2O3 content to be around 0.3 wt% or possibly up to ∼ 0.45 wt%, which is consistent with previously measured values. The high Cr2O3 content in NWA 12774 could be attributed to both the high abundance of spinel micro-phenocrysts and their high Cr2O3 contents, rather than the presence of Cr-rich xenocrysts. The calculated melt rare earth element concentration in NWA 12774 equilibrated with the most Mg-rich augite is essentially identical to those in LEW 87051 and Asuka 881371, which however have Cr2O3 contents much lower than NWA 12774. We suggest that the mantle source of NWA 12774 may not be as depleted in Cr and probably other volatile elements as other angrite sources. The various Cr2O3 contents in different quenched angrites probably reflect that their mantle sources have not been homogenized.

Volatile-depletion processing of the building blocks of Earth and Mars as recorded by potassium isotopes

The stable potassium isotopic ratios (41K/39K) of Earth and Mars have been interpreted to reflect either nucleosynthetic isotope anomalies or volatility-driven K depletion. Chondrites comprise primordial materials from which planetary bodies are assembled, and thus are critical samples for this discussion. Here, we present high-precision K isotopic analyses (reported as δ41K) of 33 chondrites and two achondrites, which reveal unprecedented variation from −1.08 to 4.68‰. In addition, there is considerable overlap in δ41K values between carbonaceous and non-carbonaceous meteorites despite their contrasting nucleosynthetic isotope anomalies. These findings are inconsistent with the nucleosynthetic origin of 41K variations in meteorites. Instead, the δ41K values of chondrites correlate positively with the isotopic compositions of other moderately volatile elements (e.g., Rb, Cu, Zn, Sn, Ga, and Te). These correlations suggest that volatility-controlled fractionation is a common mechanism for mass-dependent isotopic variations in the Solar System. In particular, carbonaceous chondrites and the angrite parent body exhibit a trend of concomitant decreases in K and its heavier isotope due to incomplete K condensation. Earth and Mars also follow this trend, suggesting that their K depletion may reflect similar volatile-depleting processes that occurred with their respective precursors. That Mars is isotopically heavier than Earth is consistent with it having less K-depleted precursors, in addition to the previous suggestion of a later-stage K loss from proto-Mars during accretionary collisions.

Appraising the late-stage magmatic fO2 of diabasic angrites with a novel phase equilibrium approach. Implications for new and existing models of angrite petrogenesis

One of the enduring problems in angrite petrogenesis is how to reconcile the extraordinary Fe-rich compositions of many angritic liquids – which seemingly require modestly oxidized fO2 conditions that fall outside the stability field of Fe-Ni alloys during primordial melting – with the geochemical and paleomagnetic evidence that the angrite parent body contains a small metallic Fe-Ni-S-C core. One of the major impediments to resolving these contradictions stems from a distinct lack of rigorous fO2 data for the angrite meteorite clan. To begin addressing this issue, we have developed a new approach for calculating magmatic fO2 values directly from the late-stage olivine-ulvöspinel- silicate liquid assemblages that are common in the volcanic or hypabyssal angrites. The adaptation of the olivine-spinel-aSiO2liq oxybarometer for use in angrites requires a careful assessment of aSiO2liq of angritic liquids. We apply the results of metal saturated angrite crystallization experiments and an analysis of the Ca-Tschermak’s-anorthite-aSiO2liq equilibrium to obtain estimates for the aSiO2liq values of angritic magmas. We then use the aSiO2liq estimates derived from the experiments and thermodynamic analysis in conjunction with the olivine-spinel-aSiO2liq oxybarometer to calculate the magmatic fO2 of Sahara 99555, D’Orbigny, and Northwest Africa 12004. With this approach we estimate oxygen fugacity values that range from ΔIW + 0 to ΔIW + 0.25. The relatively reducing fO2 values obtained from our analysis are inconsistent with redox conditions required by the oxidized melting hypothesis. Some caution is warranted in extrapolating these late-stage magmatic fO2 values to higher temperatures; however, we stress that fractionated silicate liquids typically become more oxidized with increasing crystallization via auto-oxidation (i.e., the accumulation of incompatible ferric iron in the liquid). Therefore, we suggest that late stage magmatic fO2 values from our calculations may represent the most oxidized conditions along the angrite liquid line of descent. If this interpretation is correct, a large-scale oxidation event on the APB need not be invoked to reconcile mildly reducing conditions (ΔIW−1.35) thought to have prevailed during core formation with the magmatic fO2 record preserved in angritic meteorites. Future redox studies of compositionally primitive angrites (e.g., Northwest Africa 12774) may shed new light on the redox relationships among primitive angrite magmas, evolved angrite magmas, and core forming alloys.

Potassium isotope heterogeneity in the early Solar System controlled by extensive evaporation and partial recondensation

Volatiles are vital ingredients for a habitable planet. Angrite meteorites sample the most volatile-depleted planetesimal in the Solar System, particularly for the alkali elements. They are prime targets for investigating the formation of volatile-poor rocky planets, yet their exceptionally low volatile content presents a major analytical challenge. Here, we leverage improved sensitivity and precision of K isotopic analysis to constrain the mechanism of extreme K depletion (>99.8%) in angrites. In contrast with the isotopically heavy Moon and Vesta, we find that angrites are strikingly depleted in the heavier K isotopes, which is best explained by partial recondensation of vaporized K following extensive evaporation on the angrite parent body (APB) during magma-ocean stage. Therefore, the APB may provide a rare example of isotope fractionation controlled by condensation, rather than evaporation, at a planetary scale. Furthermore, nebula-wide K isotopic variations primarily reflect volatility-driven fractionations instead of presolar nucleosynthetic heterogeneity proposed previously.

The H2O content of the ureilite parent body

The fate of highly volatile elements (H, C, F, Cl and S) during planetary accretion and differentiation is debated. Recent analyses of water in non-carbonaceous chondrites (RC, OC, EC) and achondrites (angrites, eucrites) have been used to argue that inner solar system parent bodies accreted and retained their highly volatile element budgets from their primary feedstock without substantial loss during accretion, metamorphism and differentiation. An alternative model posits that differentiated inner solar system parent bodies (e.g., the angrite parent body, 4 Vesta, Earth) derived the majority of their water from a carbonaceous chondrite-like source, delivered during the final stages of accretion.In order to add new constraints to this debate, we have measured water in nominally anhydrous minerals, melt inclusions, and interstitial glass in ureilites, the largest group of primitive achondrites in the terrestrial meteorite collection. Primitive achondrites did not experience global melting and homogenization. Therefore, these meteorites capture part of the transition from chondritic to achondritic parent bodies, allowing us to constrain the fate of water during the earliest stages of differentiation. Our nano-scale secondary ion mass spectrometry (nanoSIMS) analyses allow us to assess the viability of ureilite-like material as a potential source of terrestrial water. Analyses of pigeonite in main group ureilites yield a range of 2.0 – 6.0 µg/g H2O, and analyses of high-Ca pyroxene and glass (glassy melt inclusions and interstitial glass) in the Almahata Sitta ureilitic trachyandesite yield ranges of 13 – 19 µg/g H2O and 44 – 216 µg/g H2O, respectively. Mass balance, incremental melting, and batch melting calculations yield a preferred ureilite parent body H2O content of 2 – 20 µg/g, similar to previous estimates of water in the eucrite parent body (4 Vesta), but lower than estimates of Earth’s water budget. With these data, we demonstrate that 1) the ureilite parent body is H2O-depleted relative to the Earth; 2) ureilite-like material is unlikely to be a primary source of H2O to the Earth; 3) C and H are not necessarily coupled elements during planetary accretion and thermal processing; and 4) accretion, heating, partial melting, and degassing of rocky planetesimals likely results in significant depletion of H2O.

The case for the angrite parent body as the archetypal first-generation planetesimal: Large, reduced and Mg-enriched

Angrites are silica-undersaturated achondrites formed very early in the history of the Solar System, and the most volatile-depleted known meteorites. As such, the study of angrites can provide critical insights into the early stages of planetary formation, melting and differentiation. Yet, understanding the origins of angrites and the nature of their parent body has long been hindered by the initially small number of specimens available. Here, we leverage (i) the rapidly growing number of known angrites, and (ii) equilibrium crystallization experiments at various pressure, temperature and oxygen fugacity conditions (P-T-fO2), to revisit the petrogenesis of angrites and constrain key features of the angrite parent body (APB), such as its composition and size.We observe that quenched (i.e., volcanic) angrites define two compositional groups, which we show are readily related by fractional crystallization. This crystallization trend converges on an olivine-clinopyroxene-plagioclase (Ol + Cpx + Plag) multiple saturation boundary, whose composition is sampled by D’Orbigny, Sahara 99555 and NWA 1296. Using the observation that some quenched specimens represent primitive angritic melts, we derive a self-consistent bulk composition for the APB. We find that this composition matches the proposed Mg/Si ratio of 1.3 derived from the angrite δ30Si values, and yields a core size (18 ± 6 wt%) in agreement with the siderophile elements depletion in the APB mantle. Our results support a primary control of nebular fractionation (i.e., partial condensation) on the composition of the APB. To establish the liquid phase equilibria of angrites, a series of 1 atmosphere and high-pressure crystallization experiments (piston cylinder and internally heated pressure vessel) were performed on a synthetic powder of D’Orbigny. The results suggest that the APB was a large (possibly Moon-sized) body, formed from materials condensed at relatively high-temperature (∼1300–1400 K), and whose fO2 changed from mildly reducing (∼IW-1.5) to relatively oxidizing (∼IW+1 ± 1) in the ∼ 3 Myr between its core formation and the crystallization of D’Orbigny-like (Group 2) angrites. Based on its timing of accretion and differentiation, its composition, redox, and size, we argue that the APB represents the archetype of the first-generation of refractory-enriched planetesimals and embryos formed in the innermost part of the inner Solar System (<1 AU), and which accreted in the telluric planets.

Unique igneous textures and shock metamorphism of the Northwest Africa 7203 angrite: Implications for crystallization processes and the evolutionary history of the angrite parent body

AbstractNorthwest Africa (NWA) 7203 is a quenched angrite, showing mineralogical features typically not present in other quenched angrites. NWA 7203 exhibits textures whose grain size varies from fine grains (&lt;10 μm) to coarse grains (~3 mm), while other quenched angrites show only single‐sized textures. Fine‐grained and coarse‐grained lithologies have nearly the same bulk compositions. Cooling rates were estimated to be ~80 °C h−1 for fine‐grained lithologies and ~1 °C h−1 for coarse‐grained lithologies. Mg‐rich olivines (~Fo64) were found only in fine‐grained lithologies. Crystallization of NWA 7203 started in the fine‐grained lithologies with Mg‐rich olivine grains acting as seeds for crystallization. Coarse‐grained lithologies were subsequently formed under conditions of slower cooling. NWA 7203 shows clear shock metamorphic textures unlike other quenched angrites except for NWA 1670. We confirm that the oxygen isotopic ratios of NWA 7203 plot on the angrite fractionation line within uncertainty. However, the obtained Pb‐Pb age of NWA 7203 is 4543 ± 19 Ma, younger than the ages of other quenched angrites, which might be a result of disturbance by shock metamorphism. The finding of shock metamorphism of NWA 7203 suggests that some angrites might be derived from asteroids that remained large (&gt;10 km in diameter) during the late heavy bombardment.

Origin and timing of volatile delivery (N, H) to the angrite parent body: Constraints from in situ analyses of melt inclusions

Angrites are derived from the earliest generation of differentiated planetesimals that accreted sunward of Jupiter’s orbit, and are, thus, key to constraining the timing and source(s) of volatile delivery to planetary bodies in the inner solar system. Here we investigate the nitrogen and hydrogen isotopic signatures of angrite melts by in situ secondary ion mass spectrometry (SIMS) analyses of mineral-hosted melt inclusions and interstitial glass in two of the oldest volcanic angrites: D’Orbigny and Sahara 99555. The most primitive melt trapped in Mg-rich olivines in D'Orbigny is characterized by δ15N values ranging from 0 ± 25 to +56 ± 29‰ and δD values between −348 ± 53 and −118 ± 31‰. This shows that the angrite mantle source sampled by D'Orbigny has a N-H isotopic composition that is similar to that of CM carbonaceous chondrites, whose parent bodies are thought to have accreted in the outer solar system. The low nitrogen and water contents measured in Sahara 99555 possibly indicate that its parental melt underwent a higher degree of degassing compared to D’Orbigny or, alternatively, that the two angrites do not sample the same volatile reservoir within the angrite parent body. Given the very old crystallisation age of D’Orbigny, our findings imply that nitrogen- and water-rich objects, presumably formed beyond the orbit of Jupiter, must have been present in the terrestrial planet-forming region within the first ~4 Ma after the formation of Ca-Al-rich inclusions (CAIs, the oldest materials in the solar system).

Trace element geochemistry of coarse-grained angrites from Northwest Africa: Implications for their petrogenesis on the angrite parent body.

The angrites are a class of achondrites that encompass a wide range of igneous textures from quenched, volcanic, and subvolcanic stones to slowly cooled, plutonic rocks. The compositions of the various geochemical reservoirs generating this variety of rocks have not been investigated fully because historically the numbers and masses of angrites available for study have been quite small. However, the rapid increase in meteorites from Northwest Africa (NWA) has enabled a renewed opportunity for such an investigation. In particular, three samples, NWA 2999, 4590, and 4801, have provided a new window into our understanding of the origin and petrogenesis of the coarse-grained (plutonic) angrites. We report here the trace element abundances in individual mineral grains of pyroxene, plagioclase, olivine/kirschsteinite, and phosphate and in the whole-rock samples. We utilize these data to constrain the petrogenetic history of each of these samples on the angrite parent body (e.g., parental melt compositions and oxygen fugacity conditions) and assess genetic relationships to previously investigated angrites. The trace element abundances in each of the three coarse-grained angrites studied here indicate a unique history for each. The observed trace element abundances and patterns in NWA 2999 show similarities with previously studied fine-grained, volcanic angrites and potentially indicate a common geochemical source reservoir, even though NWA 2999 is temporally distinct from the volcanic angrites. In contrast, NWA 4590 has trace element characteristics analogous to other coarse-grained angrites (e.g., Lewis Cliff [LEW] 86010), suggesting that these samples originated from geochemically similar source reservoirs. The third angrite, NWA 4801, exhibits geochemical characteristics most similar to the plutonic, coarse-grained angrites, but also appears to have some affinities in its trace element characteristics to the volcanic, fine-grained angrites. This suggests that NWA 4801 may represent a petrogenetic link between two distinct geochemical reservoirs on the angrite parent body. In aggregate, the trace element distributions in these three plutonic angrites suggest that while they may have originated up to several million years after the fine-grained angrites, they sampled a range of source reservoirs on the angrite parent body. Some of these source reservoirs were likely similar to those of the fine-grained angrites, but others had distinct geochemical characteristics.

The speciation of carbon, nitrogen, and water in magma oceans and its effect on volatile partitioning between major reservoirs of the Solar System rocky bodies

The composition of atmospheres and the resulting potential for planetary habitability in the rocky bodies of our Solar System and beyond is strongly controlled by the volatile exchange between their silicate reservoirs and exospheres. The initial budget and speciation of major volatiles, like carbon (C), nitrogen (N) and water (H2O), in the silicate reservoirs and atmospheres was set during the formation stages of rocky bodies. However, the speciation of these major volatiles in reduced silicate melts prevalent during the differentiation stages of rocky bodies and its effect on the partitioning of volatiles between major rocky body reservoirs is poorly known. Here we present SIMS and vibrational spectroscopy (FTIR and Raman) data, determining C solubility, H content, and speciation of mixed C-O-N-H volatiles in graphite saturated silicate glasses from high P (1–7 GPa)-T (1500–2200 °C) experiments reported in Grewal et al. (2019b, 2019a). The experiments recorded oxygen fugacity (log fO2) between IW–4.3 and IW–0.8. C-O-N-H speciation varied systematically as function of fO2 at any given P-T. We find out that C-N−, CO32-, N2, and OH− are the dominant species in the oxidized range (>IW–1.5), along with some contributions from C-H, N-H, and C-O bearing species. Between IW–3.0 and IW–1.5, C is bonded as C-O either in the form of isolated C-O molecules or Fe-carbonyl complexes, or as C-H in hydrocarbons, or as combination of both in esters, while almost all of the H is bonded with the dominant N species, i.e., NH2− or NH2-. At the most reduced conditions (<IW–3.0), C is present mostly in the form of C-H bearing species, while anhydrous N3− followed by N-H bearing molecules are the dominant N bearing species. Magma oceans (MOs) in highly reduced bodies like Mercury would contain most of their C as graphite if MO is carbon saturated and the dissolved C and N would be chemically bonded with the silicate network either in the form of anhydrous C4− and N3−, or hydrogenated C-H and N-H bearing species depending on H content of the silicate melts. MOs relevant for Mars, the Moon, Vesta, and angrite parent body would contain C and N mostly in the form of C-O and N-H bearing species, respectively. If the composition of Earth’s accreting material evolved from reduced to oxidized, then initially a significant amount of the C and N budget would be locked in the silicate reservoirs, which would subsequently be released to the proto-atmosphere(s) at later stages. The retention of proto-atmosphere(s) formed by MO degassing on Earth could have provided important precursors for prebiotic chemistry which possibly led to the eventual habitability of our planet. Additionally, based on the dominant speciation of N versus C in silicate melt as a function of fO2, we also predict that DNalloy/silicate is unaffected by fH2 under highly reduced conditions (<IW–3), while DCalloy/silicate is affected. Therefore, caution must be taken during the application of experimentally determined DNalloy/silicate and DCalloy/silicate to nominally anhydrous MOs.

Formation of primitive achondrites by partial melting of alkali-undepleted planetesimals in the inner solar system

Acapulcoites-lodranites, ureilites, brachinites, brachinite-like achondrites and winonaites are the main groups of primitive achondrites. They are variably depleted in incompatible lithophile elements (Al, Na, K and rare earth elements) and siderophile/chalcophile elements relative to chondrites and are interpreted as the residual mantle of planetesimals from which silicate melts and sulfide/metal melts were extracted. We use a series of melting experiments conducted with various chondritic compositions (CV, CM, CI, H and LL) to constrain the oxygen fugacity (fO2), the temperature, extent of melting and the initial bulk composition of the parent bodies of primitive achondrites. They melted at different and variable fO2: ΔIW −0.5/−1.0 for brachinites, ΔIW −1.3/−2.5 for ureilites, ΔIW −1.6/−2.7 for acapulcoites/lodranites and ΔIW −2.5/−3.0 for winonaites (with ΔIW = log fO2 – (log fO2)IW; IW being the iron-wustite buffer). Those main groups of primitive achondrites, which have nucleosynthetic anomalies characteristic of the “non-carbonaceous” reservoir and the inner solar system, were not initially depleted in Na2O and K2O relative to the sun’s photosphere. This suggests, in accordance with the enrichment in the heavy isotopes of Zn, Rb and K in eucrites, that the depletion of moderately volatile elements in planetesimals that melted to a larger extent (e.g. Vesta, the angrite parent body) resulted from evaporative losses during partial melting. The depletion of moderately volatile elements in terrestrial planets is likely inherited from partial melting and differentiation of small planetary bodies rather than from the incomplete condensation of the solar nebula.

Unique angrite-like fragments in a CH3 chondrite reveal a new basaltic planetesimal

Most meteorites are believed to be chips from planetesimals and can provide clues to constrain the chemical evolution and dynamic history of the early Solar System. In this study, we report two unique fragments (ALF-1 and ALF-2) enclosed in the CH3 carbonaceous chondrite Sayh al Uhaymir 290. These two fragments are dominated by Ca,Fe-rich olivine with various amounts of Al,Ti-rich augite, anorthite, oxide minerals, Ca-phosphate mineral, FeNi metal, enstatite, and less Al,Ti-rich augite. The Ca-Mg-Fe systematics and Fe/Mn ratios of olivine grains in the two fragments are similar to that of the volcanic angrites. These similar features imply that the parent body of the two fragments might have bulk chemistry, oxygen fugacity, and differentiation resembling the angrite parent body. However, high-precision SIMS measurements reveal oxygen isotope compositions Δ17O = 0.91 ± 0.18‰ (2σ) of olivine in the two fragments are distinctly different from that of known angrite meteorites, possibly representing a new type of basaltic planetesimal.The two fragments also have a few mineralogical features distinct from angrite meteorites. They include: (1) the lack of a typical igneous texture; (2) the coexistence of two spatially associated Al,Ti-rich augites with different contents of CaO, Al2O3, MgO, and P2O5; (3) the presence of an enstatite-dominant rim in ALF-1; (4) the presence of a Cr,Mn-rich margin in ALF-1; and (5) complex microscale heterogeneity in oxide minerals. We argue that these features could be due to complex thermal histories in their parent body and/or after ejection from their parent body. Among these features, the Mn,Cr-rich margin and the enstatite-dominant rim in ALF-1 can be best explained with thermal events in nebular settings. This implies that the differentiation of the parent body of the two fragments might have taken place prior to the dissipation of the nebular gas.

An experimental assessment of the potential of sulfide saturation of the source regions of eucrites and angrites: Implications for asteroidal models of core formation, late accretion and volatile element depletions

The geochemistry of asteroidal magmas provides fundamental clues to the processes involved in the origin and early evolution of planetary bodies. Although sulfides are important reservoirs for a diverse suite of major and trace elements, it is currently unclear whether the interiors of asteroid Vesta and the Angrite Parent Body were sulfide liquid saturated during petrogenesis of non-cumulate eucrites and volcanic angrites. To assess the potential of sulfide liquid saturation in the interiors of these bodies, high pressure (P) – temperature (T) experiments were used to quantify the sulfur concentrations at sulfide saturation (SCSS) for volcanic angrites and non-cumulate eucrites. The sulfide-silicate partitioning behavior of various trace elements was simultaneously quantified to study their geochemical behavior at sulfide liquid saturation.It was found that the measured SCSS values agree well with the SCSS values predicted from a previous thermodynamic model for high-FeO* melts. To assess the possibility of sulfide liquid saturation of the source regions of non-cumulate eucrites and angrites, their S abundances were compared with the calculated SCSS values for their source regions. Results show that if eucritic and angritic source regions were saturated with FeS liquid, significant degassing (>50–80%) of S must have occurred during or following their magmatic emplacement. Such loss is inconsistent with the S, Cl, Zn and Rb isotopic compositions of non-cumulate eucrites. Sulfide liquid saturation of eucrite and angrite source regions is also excluded from the strongly incompatible behavior of Cu and HSE in non-cumulate eucrites and angrites (Riches et al., 2012).Additional calculations were performed to further explore the timing and extent of S loss during crystallization of the Vestan magma ocean. The assumption of chondritic bulk S abundances of bulk Vesta would correspond with extremely high S contents of the eucrite source region(s), even after consideration of depletion of S due to core formation. In light of the S, Cl, Zn and Rb stable isotopic compositions of eucrites, the S abundances in eucrites are most consistent with the hypothesis that the Vestan mantle was already strongly depleted in S (>70–80%) by the time of Vestan magma ocean crystallization, resulting in more realistic S contents of the eucrite source region(s). The depletion of S could have been established during initial accretion of Vesta or it could simply reflect accretion of volatile depleted components that experienced incomplete condensation (Wu et al., 2018). Modeling of the new experimentally determined sulfide-silicate partition coefficients and previously reported Vestan mantle depletions of the various chalcophile and siderophile elements suggests that sulfide liquid segregation during early Vestan magma ocean crystallization is also unlikely. The lack of sulfide liquid saturation in the source regions of non-cumulate eucrites and angrites, as well as during early Vestan magma ocean solidification, shows that current geochemical models of core formation and late accretion remain valid for these bodies.

Combined U-corrected Pb-Pb dating and 26Al-26Mg systematics of individual chondrules – Evidence for a reduced initial abundance of 26Al amongst inner Solar System chondrules

Abstract Chondrites are fragments of asteroids that avoided melting and, thus, provide a record of the material that accreted to form protoplanets. The dominant constituent of chondrites are millimeter-sized chondrules formed by transient heating events in the protoplanetary disk. Some chondritic components, including chondrules, contain evidence of the extinct short-lived radionuclide 26Al (half-life of 0.73 Myr). The decay of 26Al is postulated to have been an important heat source promoting asteroidal melting and differentiation. Thus, understanding the 26Al inventory in the accretion regions of differentiated asteroids is critical to constrain the accretion timescales of protoplanets. The current paradigm asserts that the canonical 26Al/27Al ratio of ∼ 5 × 10−5 recorded by the oldest dated solids, calcium-aluminium refractory inclusions, represents that of the bulk Solar System. We report, for the first time, the 26Al-26Mg systematics of chondrules from the North West Africa (NWA) 5697 L 3.10 ordinary chondrite and Allende CV3OxA (Vigarano type) carbonaceous chondrite that have been previously dated by U-corrected Pb-Pb dating. Eight chondrules, which record absolute ages ranging from 4567.57 ± 0.56 to 4565.84 ± 0.72 Ma, define statistically-significant internal isochron relationships corresponding to initial (26Al/27Al) ([26Al/27Al]0) ratios in their precursors at the time of CAI formation at 4567.3 ± 0.16 Ma ranging from ( 3.92 + 4.53 - 2.95 ) × 10−6 to ( 2.74 + 1.30 - 1.09 ) × 10−5. These initial ratios are much lower than those predicted by the Pb-Pb ages, corresponding to age mismatches between the Pb-Pb and 26Al-26Mg systems ranging from 0.69 + 0.54 - 0.44 to 2.71 + 0.66 - 0.59 Myr. All chondrules record 54Cr/52Cr compositions indicating an origin from inner Solar System precursor material and, as such, we interpret the age mismatch to reflect a reduced initial abundance of 26Al in the chondrule precursors, similar to that proposed for the angrite parent body. In particular, the range of [26Al/27Al]0 ratios essentially defines two groups, which are apparently correlated with the absolute ages of the chondrules. A first group, charactertized by chondrules with absolute Pb-Pb ages identical to CAIs, defines a mean [26Al/27Al]0 value of ( 4.75 + 1.99 - 1.21 ) × 10−6, whereas a second group, with absolute ages ∼1 Myr younger than CAIs, record a mean mean [26Al/27Al]0 of ( 1.82 + 0.57 - 0.40 ) × 10−5. We interpret this systematic variability in [26Al/27Al]0 values as reflecting progressive inward transport and admixing of dust of solar composition and 26Al content from the outer disk during chondrule recycling and remelting. Finally, a reduced [26Al/27Al]0 ratio in chondrule precursors impacts our understanding of the accretion timescales of differentiated planetesimals if chondrules are indeed representative of inner disk material. Using the average [26Al/27Al]0 ratio of (1.36 ± 0.72) × 10−5 defined by the eight chondrules, thermal modelling constrains the accretion of differentiated planetesimals formed with this 26Al inventory from ∼0.1 to ∼0.9 Myr after Solar System formation to ensure melting by 26Al decay.

Significance of high field strength and rare earth element distributions in deciphering the evolution of the inner solar system

Major processes affecting high field strength (HFSE) and rare earth (REE) element ratios in planetary basalts are degree of melting, separation of metal-sulfide melt fractions, addition and loss of silicate melt, ilmenite fractionation, and subduction. Fractional crystallization of planetary magma oceans has left a surviving imprint on only three bodies for which we have data: the Moon, Vesta, and the angrite parent body. Thorium mobilization in aqueous fluids may account for decoupling of Th and Nb in planetary systems, and this is especially notable on Earth but also possible on Mars, the Moon and some asteroids. On Earth, HFSE and REE ratios in young basalts characterize hydrated (HM), enriched (EM) and depleted (DM) mantle sources, associated with, respectively, subduction, mantle plumes and ocean ridges. Terrestrial hydrated and depleted mantle were in existence by at least 4 Ga and possibly they may have been produced in a stagnant lid tectonic regime before 3 Ga. Also, removal of Nb in metal-sulfide melts can force the composition of silicate restitic material into the hydrated mantle field on HFSE-REE graphs, thus not requiring hydration. Such an origin is probable for “hydrated” mantle in primitive achondrites and plutonic angrites. The record of all three types of mantle in basalts from other bodies in the Solar System indicates the three mantle reservoirs are not diagnostic of plate tectonics, but can be produced in stagnant lid settings.Enriched mantle is thus far recognized only in Earth and possibly Mars. There are at least two enriched mantle reservoirs in Earth: a primordial (>4 Ga) reservoir, perhaps hidden in the D” layer above the core and rarely sampled by basalts, and a recycled plate reservoir (<3 Ga), perhaps located in the two LLSVPs commonly sampled by oceanic island basalts. Between 3 and 2 Ga, the recycled enriched mantle reservoir became established in Earth, possibly in response to the widespread propagation of subduction. On Mars enriched mantle shows depleted radiogenic isotopic signatures and requires a multistage process to decouple trace element and isotopic systems.Although there are several processes by which Nb can be fractionated from Ta in planetary bodies, the low Nb/Ta (<15) characteristic of some planetary and asteroid basalts may reflect separation of a metal-sulfide melt enriched in Nb, which may or may not produce a core. This fractionation must occur early during a relatively reduced stage of planetary evolution (IW-3 to IW-5) such that Nb behaves as a chalcophile or siderophile element. If the average Nb/Ta ratio of both primitive and depleted mantle is equal to 15, production of basaltic magma in the terrestrial mantle through time has not fractionated Nb from Ta. On the other hand, if the Nb/Ta in primitive mantle equals 17, Nb must be fractionated from Ta before 4 Ga, perhaps by partitioning into the core during or soon after planetary accretion when reducing conditions may have existed.

Timing and Origin of the Angrite Parent Body Inferred from Cr Isotopes

Abstract Angrite meteorites are some of the oldest materials in the solar system. They provide important information on the earliest evolution of the solar system and accretion timescales of protoplanets. Here, we show that the 54Cr/52Cr ratio is homogeneously distributed among angrite meteorites within 13 parts per million, indicating that precursor materials must have experienced a global-scale melting such as a magma ocean. The 53Cr/52Cr and Mn/Cr ratios are correlated, which is evidence for an initial 53Mn/55Mn ratio of (3.16 ± 0.11) × 10−6. When anchored to the U-corrected Pb–Pb age for the D’Orbigny angrite, this initial 53Mn/55Mn corresponds to an absolute age of 4563.2 ± 0.3 Ma, i.e., 4.1 ± 0.3 Ma after Ca–Al-rich inclusion-formation. This age is distinct from that of the volatile depletion events dated by the 87Sr/86Sr initial ratio and therefore must correspond to the age of crystallization of the magma ocean and crust formation of the angrite parent body (APB), which can also constrain a slightly bigger size of APB than that of Vesta. Furthermore, this age is similar to those obtained from internal isochrons of the oldest volcanic angrites that cooled rapidly at the surface of the parent body (with ages of 4564 ∼ 4563 Ma), while older than those obtained from plutonic angrites (4561 ∼ 4556 Ma) that cooled down slowly, located deeper within the parent body. This implies that cooling of the APB took at least ∼8 Myr after its differentiation.