39Ar‐40Ar ages of eucrites and thermal history of asteroid 4 Vesta

Abstract— Eucrite meteorites are igneous rocks that derived from a large asteroid, probably 4 Vesta. Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ≥800°C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new ³⁹Ar‐⁴⁰Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic‐brecciated eucrites. Precise Ar‐Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ± 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar‐Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb‐Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4–4.515 Gyr, and ¹⁴⁷Sm‐¹⁴³Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41–4.55 Gyr. Similar Ar‐Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K‐Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar‐Ar ages at ?4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ?4.48 Gyr ago, which quickly cooled the samples and started the K‐Ar chronometer. A large (?460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb‐Pb and Sm‐Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar‐Ar age of 4.48 Gyr, and the few older Pb‐Pb and Sm‐Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar‐Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar‐Ar ages of 3.4–3.7 Gyr; and 2 unbrecciated eucrites give Ar‐Ar ages of ?3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar‐Ar impact‐reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5–4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct ²⁶Al, was strongly impact heated ?3.5 Gyr ago. When these data are combined with eucrite Ar‐Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ?4.1–3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta.     

Mineralogical records of early planetary processes on the howardite,eucrite, diogenite parent body with reference to Vesta

Abstract— Mineralogical information recovered from the howardite, eucrite, diogenite (HED) meteorites was employed to reconstruct the history of the parent body and relate it to 4 Vesta. These interpreted crustal evolution processes were then compared to the expected geological features on the surface of a likely proto-planet, 4 Vesta. The original crustal materials of the HED parent body were preserved as mineral grains and lithic clasts, but in many eucrites, Fe/Mg ratios in pyroxenes were homogenized by diffusion after crystallization. The crystallization trend of the protocrust has been deciphered by (1) examining monomict and crystalline samples and using their mineralogical and chemical information to formulate a sequence of crystallization and cooling trends; and by (2) reconstructing the original crust prior to cratering events from lithic clasts and mineral fragments in polymict breccias such as howardites and polymict eucrites. Mineral components are identical, both in the individual HED and in polymict breccias, and no remnants of primitive materials were preserved in the polymict breccias. A layered crust model reconstructed from such breccias consists of an upper crust with extrusive lava-like eucrites that have been brecciated and metamorphosed, diogenite mantle, and cumulate eucrites of varying thickness between them. This model can be used to explain the surface geological features of Vesta observed from the Hubble space telescope. A large crater with diogenitic orthopyroxene at the crater floor is consistent with the deepest diogenitic layer of the layered crust model; and an underlying olivine layer is expected from early crystallized olivine in the crystal fractionation model. The old terrain of eucritic surface materials of Vesta can be howardites, polymict eucrites, or regolith-like eucrites produced from eucrites extruded and impacted on the surface. Partial melting models of eucrites seem to be favored by the rare-earth element (REE) chemistry and experimental studies. Unfortunately, partial melting models have not demonstrated how the HED parent body is converted to a layered crust without producing any metamorphosed primitive material in the layered crust. The origin of cumulate eucrites with systematic variation of textures and chemistries of pyroxene can be explained by the layered crust model with excavation and mixing of trapped liquid. Discovery of basaltic materials with Na-rich plagioclase and augite in iron meteorites, which are the products of partial melting, suggests that eucrites may be unique to a body that underwent large-scale differentiation and metamorphism.     

Pristinity and petrogenesis of eucrites

New petrography, mineral chemistry, and whole rock major, minor, and trace element abundance data are reported for 29 dominantly unbrecciated basaltic (noncumulate) eucrites and one cumulate eucrite. Among unbrecciated samples, several exhibit shock darkening and impact melt veins, with incomplete preservation of primary textures. There is extensive thermal metamorphism of some eucrites, consistent with prior work. A “pristinity filter” of textural information, siderophile element abundances, and Ni/Co ratios of bulk rocks is used to address whether eucrite samples preserve endogenous refractory geochemical signatures of their asteroid parent body (i.e., Vesta), or could have experienced exogenous impact contamination. Based on these criteria, Cumulus Hills 04049, Elephant Moraine 90020, Grosvenor Range 95533, Pecora Escarpment 91245, and possibly Queen Alexander Range 97053 and Northwest Africa 1923 are pristine eucrites. Eucrite major element compositions and refractory incompatible trace element abundances are minimally affected by metamorphism or impact contamination. Eucrite petrogenesis examined through the lens of these elements is consistent with partial melting of a silicate mantle that experienced prior metal–silicate equilibrium, rather than as melts associated with cumulate diogenites. In the absence of the requirement of a large-scale magma ocean to explain eucrite petrogenesis, the interior structure of Vesta could be more heterogeneous than for larger planetary bodies.     

Shocked meteorites: Argon-40-argon-39 evidence for multiple impacts

Abstract— To contribute to the understanding of the impact history of asteroids, we performed a high-resolution ⁴⁰Ar-³⁹Ar study of ten moderately to highly shocked chondrites, which we selected according to the shock classification given by Stöffler et al. (1991). Two recent shocked chondrite falls and two highly shocked eucrites completed our sample suite. When possible, we separated impact melt from host rock for separate analysis. In total, we studied 28 samples from 14 meteorites. In some cases, atmospheric Ar that we associate with terrestrial weathering was identified and corrected for. The ages we obtained range between ～100 Ma and ～4.1 Ga and are clearly distinct from primordial ages that correspond to solar system formation. We reproduced the previously reported cluster of L-chondrite ages, ～500 Ma. The most prominent result of our study is that, in the case of chondrites, melts generally are older than host rocks or melt-embedded unmolten rocks. To solve this apparent paradox, we propose that the melt-forming event, which was the most severe shock episode in the history of these meteorites, has not been the only occasion affecting their K-Ar systems. At least one later impact metamorphism must have occured. The response of the K-Ar clock to this second event was more severe in the host rock than in the previously (in the first event) generated melt veins and pockets because of different Ar retention rates. Hence, impact metamorphism on meteorite parent bodies indeed was a multistage process extending in time over billions of years.     

L‐chondrite asteroid breakup tied to Ordovician meteorite shower by multiple isochron 40Ar‐39 Ar dating

Abstract— Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ～480 Myr ago. An association with the L‐chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ± 30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple ⁴⁰Ar‐³⁹Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L‐chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress.     

REE patterns versus the origin of the basaltic achondrites

The spectral uniqueness of asteroid 4 Vesta has led to suggestions that it is the eucrite parent body. However, there exist other basaltic achondrite types besides eucrites; either they also came from Vesta or else there exist other achondrite parent bodies. Howardites appear to be mixtures of eucrites and diogenites, and mesosiderites mixtures of eucrites or howardites and iron; thus one may infer that all four classes come from the same parent body. The REE patterns of eucrites and diogenites are modeled in order to test this hypothesis; eucrites can be made easily, but the patterns of diogenites are more difficult to match. The other basaltic achondrites are so rare that one cannot argue from statistics of abundances against a disrupted parent body for their origin. Pallasites and most irons likely had an origin separate from eucrites, again in parent bodies since disrupted.     

Young asteroid mixing revealed in ordinary chondrites: The case of NWA 5764, a polymict LL breccia with L clasts

Polymict chondritic breccias—rocks composed of fragments originating from different chondritic parent bodies—are of particular interest because they give insights into the mixing of asteroids in the main asteroid belt (occurrence, encounter velocity, transfer time). We describe Northwest Africa (NWA) 5764, a brecciated LL6 chondrite that contains a >16 cm³ L4 clast. The L clast was incorporated in the breccia through a nondestructive, low‐velocity impact. Identical cosmic‐ray exposure ages of the L clast and the LL host (36.6 ± 5.8 Myr), suggest a short transfer time of the L meteoroid to the LL parent body of 0.1 ± 8.1 Myr, if that meteoroid was no larger than a few meters. NWA 5764 (together with St. Mesmin, Dimmitt, and Glanerbrug) shows that effective mixing is possible between ordinary chondrite parent bodies. In NWA 5764 this mixing occurred after the peak of thermal metamorphism on the LL parent body, i.e., at least several tens of Myr after the formation of the solar system. The U,Th‐He ages of the L clast and LL host, identical at about 2.9 Ga, might date the final assembly of the breccia, indicating relatively young mixing in the main asteroid belt as previously evidenced in St. Mesmin.     

Ibitira: A basaltic achondrite from a distinct parent asteroid and implications for the Dawn mission

Abstract— I have done a detailed petrologic study of Ibitira, a meteorite that has been classified as a basaltic eucrite since 1957. The mean Fe/Mn ratio of pyroxenes in Ibitira with <10 mole% wollastonite component is 36.4 ± 0.4; this value is well resolved from those of similar pyroxenes in five basaltic eucrites studied for comparison, which range from 31.2 to 32.2. Data for the latter five eucrites completely overlap. Ibitira pyroxenes have lower Fe/Mg than the basaltic eucrite pyroxenes; thus, the higher Fe/Mn ratio does not reflect a simple difference in oxidation state. Ibitira also has an oxygen isotopic composition, alkali element contents, and a Ti/Hf ratio that distinguish it from basaltic eucrites. These differences support derivation from a distinct parent asteroid. Thus, Ibitira is the first recognized representative of the fifth known asteroidal basaltic crust, the others being the HED, mesosiderite, angrite, and NWA 011 parent asteroids. 4 Vesta is generally assumed to be the HED parent asteroid. The Dawn mission will orbit 4 Vesta and will perform detailed mapping and mineralogical, compositional, and geophysical studies of the asteroid. Ibitira is only subtly different from eucritic basalts. A challenge for the Dawn mission will be to distinguish different basalt types on the surface and to attempt to determine whether 4 Vesta is indeed the HED parent asteroid.     

Presidential Address: Presented 2000 August 28, Chicago,Illinois, USA The eucrite/Vesta story

Abstract— Many lines of evidence indicate that meteorites are derived from the asteroid belt but, in general, identifying any meteorite class with a particular asteroid has been problematical. One exception is asteroid 4 Vesta, where a strong case can be made that it is the ultimate source of the howardite‐eucrite‐diogenite (HED) family of basaltic achondrites. Visible and near‐infrared reflectance spectra first suggested a connection between Vesta and the basaltic achondrites. Experimental petrology demonstrated that the eucrites (the relatively unaltered and unmixed basaltic achondrites) were the product of approximately a 10% melt. Studies of siderophile element partitioning suggested that this melt was the residue of an asteroidal‐scale magma ocean. Mass balance considerations point to a parent body that had its surface excavated, but remains intact. Modern telescopic spectroscopy has identified kilometer‐scale “Vestoids” between Vesta and the 3:1 orbit‐orbit resonance with Jupiter. Dynamical simulations of impact into Vesta demonstrate the plausibility of ejecting relatively unshocked material at velocities consistent with these astronomical observations. Hubble Space Telescope images show a 460 km diameter impact basin at the south pole of Vesta. It seems that nature has provided multiple free sample return missions to a unique asteroid. Major challenges are to establish the geologic context of the HED meteorites on the surface of Vesta and to connect the remaining meteorites to specific asteroids.     

A survey of Rb-Sr systematics of eucrites

Abstract— This paper surveys the experimental material on the Rb-Sr systematics of eucrites accumulated in the literature for the last two decades. Among the meteorites studied, those representatives have been chosen for which the absence of disturbances of isotope systems seems reliably fixed (according to evidence from Rb-Sr, Sm-Nd, and U-Pb isotope data). These data have been subjected to joint statistical treatment. For ordinary eucrites the age has been estimated as T = 4.548 ± 0.058 Ga, and initial Sr isotope ratio, I_ord = 0.698925 ± 14. For cumulate eucrites, the initial Sr isotope ratio has been estimated as I_cum = 0.698872 ± 14. The substantial difference in initial Sr isotope composition suggests that those two eucrite groups originate from distinct parent bodies. Joint treatment of the available Rb-Sr data for angrites has been carried out for comparison, and the angrites initial Sr ratio has been estimated. On the basis of the estimates obtained formation intervals for the corresponding parent bodies have been calculated.     

Basaltic volcanism on the angrite parent body: Comparison with 4 Vesta

Carbon and nitrogen data from stepped combustion analysis of eight angrites, seven eucrites, and two diogenites, alongside literature data from a further nine eucrites and two diogenites, have been used to assess carbon and nitrogen incorporation and isotope fractionation processes on the angrite parent body (APB), for comparison with volatile behavior on the HED parent body (4 Vesta). A subset of the angrite data has been reported previously (Abernethy et al. 2013 ). Two separate families of volatile components were observed. They were (1) moderately volatile material (MVM), mostly combusting between ~500 and 750 °C and indistinguishable from terrestrial contamination and (2) refractory material (RM), mainly released above 750 °C and thought to be carbon (as ) and nitrogen (as N₂ or ) dissolved within the silicate lattice, fitting with the slightly oxidized (~IW to IW+2) angrite fO₂ conditions. Isotopic fractionation trends for carbon and nitrogen within the plutonic and basaltic (quenched) angrites suggest that the behavior of the two volatile elements is loosely coupled, but that the fractionation process differs between the two angrite subgroups. Comparison with results from eucrites and diogenites implies similarities between speciation of carbon and nitrogen on 4 Vesta and the APB, with the latter being more enriched in volatiles than the former.     

147Sm‐143Nd and 176Lu‐176Hf systematics of eucrite and angrite meteorites

Comparative planetary geochemistry provides insight into the origin and evolutionary paths of planetary bodies in the inner solar system. The eucrite and angrite achondrite groups are particularly interesting because they show evidence of early planetary differentiation. We present ¹⁴⁷Sm‐¹⁴³Nd and ¹⁷⁶Lu‐¹⁷⁶Hf analyses of eight noncumulate (basaltic) eucrites, two cumulate eucrites, and three angrites, which together place new constraints on the evolution and differentiation histories of the crusts of the eucrite and angrite parent bodies and their mantle mineralogies. The chemical compositions of both eucrites and angrites indicate similar evolutionary paths and petrogenetic models with formation and isolation of differentiated crustal reservoirs associated with segregation of ilmenite. We report a ¹⁴⁷Sm‐¹⁴³Nd mineral isochron age for the Moama cumulate eucrite of 4519 ± 34 Ma (MSWD = 1.3). This age indicates protracted magmatism within deep crustal layers of the eucrite parent body lasting up to about 50 Ma after the formation of the solar system. We further demonstrate that the isotopic compositions of constituent minerals are compromised by secondary processes hindering precise determination of mineral isochron ages of basaltic eucrites and angrites. We interpret the changes in geochemistry and, consequently, the erroneous ¹⁴⁷Sm‐¹⁴³Nd and ¹⁷⁶Lu‐¹⁷⁶Hf internal mineral isochron ages of basaltic eucrites and angrites as the result of metamorphic events such as impacts (effects from pressure, temperature, and peak shock duration) on the surfaces of the eucrite and angrite parent bodies.     

Populating the asteroid belt from two parent source regions due to the migration of giant planets—“The Grand Tack”

Abstract– The asteroid belt is found today in a dramatically different state than that immediately following its formation. It is estimated that it has been depleted in total mass by a factor of at least 1000 since its formation, and that the asteroids’ orbits evolved from having near‐zero eccentricity and inclination to the complex distributions we find today. The asteroid belt also hosts a wide range of compositions, with the inner regions dominated by S‐type and other water‐poor asteroids and the outer regions dominated by C‐type and other primitive asteroids. We discuss a model of early inner solar system evolution whereby the gas‐driven migration of Jupiter and Saturn brings them inwards to 1.5 AU, truncating the disk of planetesimals in the terrestrial planet region, before migrating outwards toward their current locations. This model, informally titled “The Grand Tack,” examines the planetary dynamics of the solar system bodies during the final million years of the gaseous solar nebula lifetime—a few million years (Myr) after the formation of the first solids, but 20–80 Myr before the final accretion of Earth, and approximately 400–600 Myr before the Late Heavy Bombardment of the inner solar system. The Grand Tack attempts to solve some outstanding problems for terrestrial planet formation, by reproducing the size of Mars, but also has important implications for the asteroid population. The migration of Jupiter causes a very early depletion of the asteroid belt region, and this region is then repopulated from two distinct source regions, one inside the formation region of Jupiter and one between and beyond the giant planets. The scattered material reforms the asteroid belt, producing a population the appropriate mass, orbits, and with overlapping distributions of material from each parent source region.     

Composition and petrology of HED polymict breccias: The regolith of (4) Vesta

We have done petrologic and compositional studies on a suite of polymict eucrites and howardites to better understand regolith processes on their parent asteroid, which we accept is (4) Vesta. Taking into account noble gas results from companion studies, we interpret five howardites to represent breccias assembled from the true regolith: Elephant Moraine (EET) 87513, Grosvenor Mountains (GRO) 95535, GRO 95602, Lewis Cliff (LEW) 85313, and Meteorite Hills (MET) 00423. We suggest that EET 87503 is paired with EET 87513, and thus is also regolithic. Pecora Escarpment (PCA) 02066 is dominated by melt‐matrix clasts, which may have been formed from true regolith by impact melting. These meteorites display a range in eucrite:diogenite mixing ratio from 55:45 to 76:24. There is no correlation between degree of regolith character and Ni content. The Ni contents of howardite, eucrite, and diogenites (HEDs) are mostly controlled by the distribution of coarse chondritic clasts and metal grains, which in some cases resulted from individual, low‐velocity accretion events, rather than extensive regolith gardening. Trace element compositions indicate that the mafic component of HED polymict breccias is mostly basalt similar to main‐group eucrites; Stannern‐trend basaltic debris is less common. Pyroxene compositions show that some trace element‐rich howardites contain abundant debris from evolved basalts, and that cumulate gabbro debris is present in some breccias. The scale of heterogeneity varies considerably; regolithic howardite EET 87513 is more homogeneous than fragmental howardite Queen Alexandra Range (QUE) 97001. Individual samples of a given howardite can have different compositions even at roughly 5 g masses, indicating that obtaining representative meteorite compositions requires multiple or large samples.     

Petrologic insights from the spectra of the unbrecciated eucrites: Implications for Vesta and basaltic asteroids

Abstract– We investigate the relationship between the petrology and visible–near infrared spectra of the unbrecciated eucrites and synthetic pyroxene–plagioclase mixtures to determine how spectra obtained by the Dawn mission could distinguish between several models that have been suggested for the petrogenesis of Vesta’s crust (e.g., partial melting and magma ocean). Here, we study the spectra of petrologically characterized unbrecciated eucrites to establish spectral observables, which can be used to yield mineral abundances and compositions consistent with petrologic observations. No information about plagioclase could be extracted from the eucrite spectra. In contrast, pyroxene dominates the spectra of the eucrites and absorption band modeling provides a good estimate of the relative proportions of low‐ and high‐Ca pyroxene present. Cr is a compatible element in eucrite pyroxene and is enriched in samples from primitive melts. An absorption at 0.6 μm resulting from Cr³⁺ in the pyroxene structure can be used to distinguish these primitive eucrites. The spectral differences present among the eucrites may allow Dawn to distinguish between the two main competing models proposed for the petrogenesis of Vesta (magma ocean and partial melting). These models predict different crustal structures and scales of heterogeneity, which can be observed spectrally. The formation of eucrite Allan Hills (ALH) A81001, which is primitive (Cr‐rich) and relatively unmetamorphosed, is hard to explain in the magma ocean model. It could only have been formed as a quench crust. If the magma ocean model is correct, then ALHA81001‐like material should be abundant on the surface of Vesta and the Vestoids.     

On the origin of shocked and unshocked CM clasts in H‐chondrite regolith breccias

Abstract— CM chondrite clasts that have experienced different degrees of aqueous alteration occur in H‐chondrite and HED meteorite breccias. Many clasts are fragments of essentially unshocked CM projectiles that accreted at low relative velocities to the regoliths of these parent bodies. A few clasts were heated and dehydrated upon impact; these objects most likely accreted at higher relative velocities. We examined three clasts and explored alternative scenarios for their formation. In the first scenario, we assumed that the H and HED parent bodies had diameters of a few hundred kilometers, so that their high escape velocities would effectively prevent soft landings of small CM projectiles. This would imply that weakly shocked CM clasts formed on asteroidal fragments (family members) associated with the H and HED parent bodies. In the second scenario, we assumed that weakly shocked CM clasts were spall products ejected at low velocities from larger CM projectiles when they slammed into the H and HED parent bodies. In both cases, if most CM clasts turn out to have ancient ages (e.g., ?3.4‐4.1 Ga), a plausible source for their progenitors would be outer main belt objects, some which may have been dynamically implanted 3.9 Ga ago by the events described in the so‐called “Nice model.” On the other hand, if most CM clasts have recent ages (<200 Ma), a plausible source location for their parent body would be the inner main belt between 2.1–2.2 AU. In that case, the possible source of the CM‐clasts' progenitors' parent fragments would be the breakup ?160 Ma ago of the parent body 170 km in diameter of the Baptistina asteroid family (BAF).     

Vesta as the howardite,eucrite and diogenite parent body: Implications for the size of a core and for large-scale differentiation

Abstract— If Vesta is the parent body of the howardite, eucrite, and diogenite (HED) meteorites, then geo-chemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of Vesta to (1) determine the size of a possible metal core in Vesta and (2) model the igneous differentiation and internal structure of Vesta. The density of Vesta and petrologic models for HED meteorites together suggest that the amount of metal in the parent body is <25 mass%, with a best estimate of ～5%, assuming no porosity. For a porosity of up to 5% in the silicate fraction of the asteroid, the permissible metal content is <30%. These results suggest that any metal core in the HED parent body and Vesta is not unusually large. A variety of geochemical and other data for HED meteorites are consistent with the idea that they originated in a magma ocean. It appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most noncumulate eucrites (excepting such eucrites as Bouvante and Statinem) formed by subsequent crystallization of the residual melts. Modelling results suggest that the HED parent body is enriched in rare earth elements by a factor of ～2.5–3.5 relative to CI-chondrites and that it has approximately chondritic Mg/Si and Al/Sc ratios. Stokes settling calculations for a Vesta-wide, nonturbulent magma ocean suggest that early-crystallizing magnesian olivine, orthopyroxene, and pigeonite would have settled relatively quickly, permitting fractional crystallization to occur, but that later-crystallizing phases would have settled (or floated) an order of magnitude more slowly, allowing, instead, a closer approach to equilibrium crystallization for the more evolved (eucritic) melts. This would have inhibited the formation of a plagioclase-flotation crust on Vesta. Plausible models for the interior of Vesta, which are consistent with the data for HED meteorites and Vesta, include a metal core (<130 km radius), an olivine-rich mantle (～65–220 km thick), a lower crustal unit (～12–43 km thick) composed of pyroxenite, from which diogenites were derived, and an upper crustal unit (～23–42 km thick), from which eucrites originated. The present shape of Vesta (with ～60 km difference in the maximum and minimum radius) suggests that all of the crustal materials, and possibly some of the underlying olivine from the mantle, could have been locally excavated or exposed by impact cratering.     

Diogenites as polymict breccias composed of orthopyroxenite and harzburgite

Abstract– A few relatively unbrecciated olivine‐rich diogenites consist of an equilibrium assemblage of olivine and magnesian orthopyroxene (harzburgite). More common diogenites with smaller amounts of olivine are breccias containing two distinct orthopyroxenes—one magnesian and one ferroan. These diogenites are mixtures of a harzburgite lithology that is more magnesian, with the “normal” orthopyroxenite lithology that is ferroan and may contain small amounts of plagioclase. Both lithologies likely formed by fractional crystallization in multiple plutons emplaced within the crust of asteroid 4 Vesta. Minor element trends in orthopyroxenes indicate that these plutons exhibited a range of compositions. We propose a revised taxonomy for the HED (howardites, eucrites, and diogenites) suite where all ultramafic samples are referred to as diogenites. Within this group, the prefixes dunitic, harzburgitic, and orthopyroxenitic are used to distinguish diogenites consisting of more than or equal to 90% olivine, olivine + orthopyroxene, and more than or equal to 90% orthopyroxene, respectively. The prefix polymict is used to describe brecciated mixtures of any of these rock types. The recognition that olivine is a significant phase in some diogenites is consistent with spectral interpretations of olivine in a deeply excavated crater on Vesta, and has important implications for the bulk composition and petrogenesis of that body.     

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.     

Origin of polymict breccias on asteroids deduced from their pyroxene fragments

Abstract— Cumulate eucrite, noncumulate eucrite, and diogenite meteorites are considered to have come from the crust of one (or similar) parent asteroid. Howardites are regarded as regolith breccias of eucrites and diogenites, and polymict eucrites are regarded as polymict breccias of eucrites. These polymict breccias show many textural and chemical features. In order to gain a better understanding of the origin of polymict breccias and the origin of their components, we investigated four polymict breccias, Yamato (Y)-791439, Y-791192, Y-82009, and Y-82049 with a scanning electron microscope (SEM) equipped with a chemical mapping system, and by electron probe microanalysis (EPMA). We analyzed all pyroxene grains with chemical maps, classified them by chemical composition, and observed their chemistry and mineralogy in detail. The characteristics of pyroxenes suggest that the polymict breccias were generated by gathering locally ordinary eucrites and cumulate eucrites. The chemical-evolutionary features of the pyroxenes (such as homogenization, chemical zoning, and exsolution lamellae) suggest that there were at least two long annealing events and one short (or low-temperature) annealing event, separated by mixing events. Local heterogeneity on the asteroidal crust is also suggested.