Asteroid 4-Vesta Research Articles

The origin of 4-Vesta's volatile depletion revealed by the zinc isotopic composition of diogenites.

Volatile element abundances vary substantially among terrestrial planetary bodies like Earth, Mars, Moon, and differentiated asteroids, leading to intense debate about the governing processes. howardites-eucrites-diogenites (HED) meteorites, most likely from asteroid 4-Vesta, represent highly volatile-depleted Solar System samples, offering critical insights into these processes. Zinc is a moderately volatile element and its isotopic composition reveals sources of volatiles in planetary bodies. Our study finds Zn isotopic anomalies in diogenites overlapping with noncarbonaceous reservoirs, indicating negligible contributions of outer solar system materials to 4-Vesta's volatiles. Besides, zinc isotopic composition of 4-Vesta is lighter than that of chondrites, contrary to the expected signature of evaporation-based volatile depletion. This suggests that after 4-Vesta lost all its volatiles through evaporation during the magma ocean stage, partial kinetic recondensation occurred that produced the observed isotopically light composition. These insights, combined with previous data, underscore the process of global evaporation followed by partial condensation as a key factor influencing the final volatile budget of planetary bodies.

ID-TIMS zircon U–Pb geochronology of the Camel Donga eucrite

Eucrite meteorites are considered to originate from the upper crust of asteroid 4-Vesta. These differentiated basaltic meteorites commonly contain accessory zircons with grain sizes smaller than 20–30 μm in diameter. U–Pb zircon dating using secondary ion mass spectrometry (SIMS) revealed that initial magmatism and global thermal metamorphism occurred in the Vestan crust ~4550–4555 million year ago (Ma). To improve the temporal resolution in order to distinguish zircons grown during different thermal events in the early stage of crustal evolution of Vesta, we have applied a chemical abrasion-based method for extracting small zircon crystals from bulk meteorite samples and obtained high-precision isotope dilution-thermal ionization mass spectrometry (ID-TIMS) U–Pb dates for zircons from the Camel Donga eucrite. Some of the Camel Donga zircons occur in contact with ilmenite, implying that they have formed by exsolution of ilmenite followed by reaction with silica during high temperature metamorphism. Raman measurements indicate that the zircons are moderately radiation damaged and that high temperature annealing (900 °C for 48 h) is likely to improve their crystallinities. Zircons separated from acid residues after dissolving other minerals in a mixture of hydrofluoric and nitric acid experienced U–Pb and Pb isotope fractionation during the acid treatment. High temperature annealing prior to acid extraction efficiently suppresses Pb isotope fractionation resulting in accurate and precise 207Pb–206Pb dates. The weighted mean 207Pb–206Pb date of the annealed zircons is 4552.56 ± 0.69 Ma (2σ), which is significantly older than that of Camel Donga zircons previously obtained by SIMS (4531 ± 10 Ma) but is consistent with 207Pb–206Pb dates of zircons in the other eucrites. Thus, the Camel Donga eucrite zircons formed during initial magmatism or global thermal metamorphism in the early stage of Vestan crustal formation.

Chondritic mercury isotopic composition of Earth and evidence for evaporative equilibrium degassing during the formation of eucrites

Variations in the abundances of moderately volatile elements (MVE) are one of the most fundamental geochemical differences between the terrestrial planets. Whether these variations are the consequence of nebular processes, planetary volatilization, differentiation or late accretion is still unresolved. The element mercury is the most volatile of the MVE and is a strongly chalcophile element. It is one of the few elements that exhibit large mass-dependent (MDF) and mass-independent (MIF) isotopic fractionations for both odd (odd-MIF, Δ199Hg and Δ201Hg) and even (even-MIF, Δ200Hg) Hg isotopes in nature, which is traditionally used to trace Hg biogeochemical cycling in surface environments. However, the Hg isotopic composition of Earth and meteorites is not well constrained. Here, we present Hg isotopic data for terrestrial basaltic, trachytic and granitic igneous samples. These rocks are isotopically lighter (δ202Hg = −3.3 ± 0.9‰; 1 standard deviation) than sedimentary rocks that have previously been considered to represent the terrestrial Hg isotope composition (δ202Hg=−0.7±0.5‰; 1 standard deviation). We show degassing during magma emplacement induces MIF that are consistent with kinetic fractionation in these samples. Also presented is a more complete dataset for chondritic (carbonaceous, ordinary and enstatite) meteorites, which are consistent with previous work for carbonaceous chondrites (positive odd-MIF) and ordinary chondrites (no MIF), and demonstrate that some enstatite chondrites exhibit positive odd-MIF, similar to carbonaceous chondrites. The terrestrial igneous rocks fall within the range of chondritic compositions for both MIF and MDF. Given the fact that planetary differentiation (core formation, evaporation) would contribute to Hg loss from the silicate portion of Earth and would likely fractionate Hg isotopes from chondritic compositions, we suggest that the budget of the mantle Hg is dominated by late accretion of chondritic materials to Earth, as also suggested for other volatile chalcophile elements (S, Se, Te). Considering the Hg isotopic signatures, materials with compositions similar to CO chondrites or ordinary chondrites are the most likely late accretion source candidates. Finally, eucrite meteorites, which are highly depleted in volatile elements, are isotopically heavier than chondrites and exhibit negative odd-MIF. The origin of volatile depletion in eucrites has been vigorously debated. We show that Δ199Hg versus Δ201Hg relationships point toward an equilibrium nuclear field shift effect, suggesting that volatile loss occurred during a magma ocean phase at the surface of the eucrite parent body, likely the asteroid 4-Vesta.

Blind deconvolution in astronomy with adaptive optics: the parametric marginal approach

ABSTRACT One of the major limitations of using adaptive optics (AO) to correct image post-processing is the lack of knowledge about the system’s point spread function (PSF). The PSF is not always available as direct imaging on isolated point-like objects, such as stars. The use of AO telemetry to predict the PSF also suffers from serious limitations and requires complex and yet not fully operational algorithms. A very attractive solution is to estimate the PSF directly from the scientific images themselves, using blind or myopic post-processing approaches. We demonstrate that such approaches suffer from severe limitations when a joint restitution of object and PSF parameters is performed. As an alternative, here we propose a marginalized PSF identification that overcomes this limitation. In this case, the PSF is used for image post-processing. Here we focus on deconvolution, a post-processing technique to restore the object, given the image and the PSF. We show that the PSF estimated by marginalization provides good-quality deconvolution. The full process of marginalized PSF estimation and deconvolution constitutes a successful blind deconvolution technique. It is tested on simulated data to measure its performance. It is also tested on experimental AO images of the asteroid 4-Vesta taken by the Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE)/Zurich Imaging Polarimeter (Zimpol) on the Very Large Telescope to demonstrate application to on-sky data.

Potassium isotopic compositions of howardite-eucrite-diogenite meteorites

We report new high-precision stable K isotope data for three martian meteorites, one lunar meteorite, one ordinary chondrite, four terrestrial igneous United States Geological Survey (USGS) reference materials, and twenty howardite–eucrite–diogenite [HED] meteorites. The three martian meteorites define a relatively narrow δ41K range with an average of −0.36 ± 0.12‰ (2 SD) that is slightly heavier than the Bulk Silicate Earth (BSE) K isotopic composition (−0.48 ± 0.03‰). Except for the four Northwest Africa samples which were terrestrially contaminated, all HED meteorites reveal substantial 41K enrichment compared to BSE, lunar samples, martian meteorites, and chondrites. We propose that the average δ41K (+0.36 ± 0.16‰) obtained from HED meteorites is representative of Bulk Silicate 4-Vesta. The coupled volatile depletion and heavy K isotope enrichment in 4-Vesta could be attributed to both nebula-scale processes and parent-body events. The asteroid 4-Vesta is likely to have accreted from planetary feedstocks that have been significantly volatile-depleted prior to the major phases of planetary accretion in the early Solar System, with secondary effects of K loss during accretionary growth and magma ocean degassing.

Differentiation and magmatic activity in Vesta evidenced by 26Al-26Mg dating in eucrites and diogenites

The 26Al-26Mg short-lived chronometer has been widely used for dating ancient objects in studying the early Solar System. Here, we use this chronometer to investigate and refine the geological history of the asteroid 4-Vesta. Ten meteorites widely believed to come from Vesta (4 basaltic eucrites, 3 cumulate eucrites and 3 diogenites) and the unique achondrite Asuka 881394 were selected for this study. All samples were analyzed for their δ26Mg∗ and 27Al/24Mg ratios, in order to construct both whole rock and model whole rock isochrons. Mineral separation was performed on 8 of the HED’s in order to obtain internal isochrons. While whole rock Al-Mg analyses of HED’s plot on a regression that could be interpreted as a vestan planetary isochron, internal mineral isochrons indicate a more complex history. Crystallization ages obtained from internal 26Al-26Mg systematic in basaltic eucrites show that Vesta’s upper crust was formed during a short period of magmatic activity at 2.66-0.58+1.39 million years (Ma) after Calcium-Aluminum inclusions (after CAI). We also suggest that impact metamorphism and subsequent age resetting could have taken place at the surface of Vesta while 26Al was still extant. Cumulate eucrites crystallized progressively from 5.48-0.60+1.56 to >7.25Ma after CAI. Model ages obtained for both basaltic and cumulate eucrites are similar and suggest that the timing of differentiation of a common eucrite source from a chondritic body can be modeled at 2.88-0.12+0.14 Ma after CAI, i.e. contemporaneously from the onset of the basaltic eucritic crust. Based on their cumulate texture, we suggest cumulate eucrites were likely formed deeper in the crust of Vesta. Diogenites have a more complicated history and their 26Al-26Mg systematics show that they likely formed after the complete decay of 26Al and thus are younger than eucrites. This refined chronology for eucrites and diogenites is consistent with a short magma ocean stage on 4-Vesta from which the basaltic eucrites rapidly crystallized. In order to explain the younger age and the complex history of diogenites, we postulate that a second episode of magmatism was possibly triggered by a mantle overturn. We bring a refined chronology of the geological history of Vesta that shows that the asteroid has known a more-complex differentiation than previously thought.

Chlorine and hydrogen degassing in Vesta's magma ocean

The hydrophilic nature of halogens makes these elements ideal for probing potential hydrous geologic processes. Generally, in magmatic settings the stable isotopes of Cl may fractionate when H is in low concentrations and little fractionation occurs when the H concentration is high. We determined the Cl isotope composition and halogen content (F, Cl, Br, and I) of apatite and merrillite in seven basaltic eucrites, which are meteorites linked to the asteroid 4-Vesta, by using secondary ion mass spectrometry. We compare our halogen results with H isotope data, existing bulk rock concentrations, and petrologic models. The inferred Cl isotope composition of eucrites from this study, expressed in standard δ37Cl notation, which ranges from −3.8 to 7.7‰, correlates with the bulk major- and trace-element content, e.g., the Cl isotope composition positively correlates with Mg and Sc, while Cl isotope composition negatively correlates with K, V, and Cr. Here we suggest that eucrites preserve evidence of a degassing magma ocean as evidenced by the decreasing bulk rock K content with increasing δ37Cl. If the eucrite parent body, 4-Vesta, accreted with a negative δ37Cl of −3.8±1.1‰, at least some parts of the solar nebula would have been isotopically light compared to most estimates of the Earth, which on average is close to 0‰.

Hiding in the howardites: Unequilibrated eucrite clasts as a guide to the formation of Vesta's crust

Abstract204 howardites in the National Meteorite Collection at the Smithsonian were examined for the presence of fine‐grained eucrite clasts, with the goal of better understanding the formation of the uppermost crust of asteroid 4Vesta. Eight clasts were identified and characterized in terms of their textures and mineral chemistry, and their degree of thermal metamorphism was assessed. The paucity of fine‐grained eucrites, both within the unbrecciated eucrites and as clasts within the howardites, suggests that they originate from small‐scale units on the surface of Vesta, most likely derived from partial melting. Six of the eight clasts described were found to be unequilibrated, meaning that they preserve their original crystallization trends. The vast majority of eucrites are at least partially equilibrated, making these samples quite rare and important for deciphering the petrogenesis of the vestan crust. Biomodal grain populations suggest that eucrite melts often began crystallizing pyroxene and plagioclase during their ascent to the surface, where they were subject to more rapid cooling, crystallization, and later metasomatism. Pyroxene compositions from this study and prior work indicate that the products of both primitive and evolved melts were present at the vestan surface after its formation. Two howardite thin sections contained multiple eucrite composition clasts with different crystallization and thermal histories; this mm‐scale diversity reflects the complexity of the current day vestan surface that has been observed by Dawn.

Constraints on core formation in Vesta from metal–silicate partitioning of siderophile elements

It is now widely accepted that the asteroid 4-Vesta has an Fe-rich metallic core, but the composition of the core and the conditions prevailing during core–mantle differentiation are poorly constrained. In light of new constraints on Vesta’s geophysical and geochemical properties obtained by the DAWN mission, we have re-examined the conditions at which core–mantle differentiation in Vesta may have occurred by linking the estimated mantle depletions of siderophile elements P, Co, Ni, Cu, Ga, Ge, Mo and W in the vestan mantle to newly derived predictive equations for core–mantle partitioning of these elements. We extend the number of elements previously considered in geochemical modeling of vestan core formation and use published metal–silicate partitioning data obtained at low pressures to characterize the dependence of metal/silicate partition coefficients (D) on pressure, temperature, oxygen fugacity and composition of the silicate and metallic melt. In our modeling we implement newly derived mantle depletions of P, Co, Ni and Ga through analysis of published HED meteorite analyses and assess two contrasting bulk compositional models for Vesta.Modeling results using Monte Carlo simulations constrain vestan core formation to have occurred at mildly reducing conditions of approximately 2log units below the iron–wüstite (IW) buffer (ΔIW=−2.05±0.20) if the two most likely bulk compositions (binary mixtures of H+CM or H+CV chondritic meteorites) are considered, assuming a temperature range between 1725 and 1850K and a sulfur-free pure Fe core. If the core is assumed to be sulfur-rich (15wt.% S) as predicted by the latter bulk compositional models, observed depletions for all eight siderophile elements can be simultaneously satisfied at ΔIW=−2.35±0.10 and 1725–1850K for the H+CV bulk composition and ΔIW=−2.30±0.15 and 1725–1850K for the H+CM bulk composition. More reducing conditions are not consistent with the observed siderophile element depletions in the vestan mantle, independent of the sulfur content of the vestan core and of the bulk compositional model chosen.Our analysis shows that a previously proposed shallow magma ocean on Vesta during core formation is not consistent with the observed mantle depletion of Ga for the two considered bulk compositions irrespective of core composition. Instead, our results are consistent with the existence of a deep magma ocean during core formation on Vesta, requiring >50–100% mantle melting.

High-pressure investigations on Piplia Kalan eucrite meteorite using in-situ X-ray diffraction and 57Fe Mössbauer spectroscopic technique up to 16 GPa

We report here high-pressure investigations on Piplia Kalan eucrite–a member of HED (Howardite–Eucrite–Diogenite) family from asteroid 4-Vesta based on synchrotron X-ray diffraction (up to 16 GPa) and 57Fe Mössbauer spectroscopy (up to 8 GPa). Dominant with anorthite-rich plagioclase, pigeonite-rich pyroxene and clino-ferrosilite, the sample displayed various phase transitions attaining amorphous character at 16 GPa. These phase transitions of individual components could be explained simultaneously through variations in high-pressure XRD patterns and the Mössbauer parameters. Most prominent P21/c to C2/c transition of pigeonite and ferrosilite was exhibited both as sudden variation in Mössbauer parameters and population inversion of Fe2+ in M1 and M2 sites between 2.9 and 3.8 GPa and variation in intensity profile in XRD patterns at 3.56 GPa. Anorthite seemed to respond more to such impact than other components in the sample. Complete amorphization in anorthite which occurred at lower pressure of ∼12 GPa implied residual stress experienced due to shock impact. The presence of high pressure (monoclinic) phase of pigeonite and ferrosilite at ambient condition in this eucrite sample confirmed earlier suggestions of an early shock event. This report is an attempt to emphasize the role of anorthite in the determination of the residual stress due to impact process in the parent body thus to understand the behavioral differences amongst HED members.

A deep crust-mantle boundary in the asteroid 4Vesta.

The asteroid 4Vesta was recently found to have two large impact craters near its south pole, exposing subsurface material. Modelling suggested that surface material in the northern hemisphere of Vesta came from a depth of about 20kilometres, whereas the exposed southern material comes from a depth of 60 to 100kilometres. Large amounts of olivine from the mantle were not seen, suggesting that the outer 100kilometres or so is mainly igneous crust. Here we analyse the data on Vesta and conclude that the crust-mantle boundary (or Moho) is deeper than 80kilometres.

Redox state during core formation on asteroid 4-Vesta

Core formation is the main differentiation event in the history of a planet. However, the chemical composition of planetary cores and the physicochemical conditions prevailing during core formation remain poorly understood. The asteroid 4-Vesta is the smallest extant planetary body known to have differentiated a metallic core. Howardite, Eucrite, Diogenite (HED) meteorites, which are thought to sample 4-Vesta, provide us with an opportunity to study core formation in planetary embryos.Partitioning of elements between the core and mantle of a planet fractionates their isotopes according to formation conditions. One such element, silicon, shows large isotopic fractionation between metal and silicate, and its partitioning into a metallic core is only possible under very distinctive conditions of pressure, oxygen fugacity and temperature. Therefore, the silicon isotope system is a powerful tracer with which to study core formation in planetary bodies. Here we show through high-precision measurement of Si stable isotopes that HED meteorites are significantly enriched in the heavier isotopes compared to chondrites. This is consistent with the core of 4-Vesta containing at least 1wt% of Si, which in turn suggests that 4-Vesta's differentiation occurred under more reducing conditions (ΔIW∼−4) than those previously suggested from analysis of the distribution of moderately siderophile elements in HEDs.

Direct measurement of hydroxyl in the lunar regolith and the origin of lunar surface water

Remote sensing discoveries of hydroxyl and water on the lunar surface have reshaped our view of the distribution of water and related compounds on airless bodies such as the Moon. The origin of this surface water is unclear, but it has been suggested that hydroxyl in the lunar regolith can result from the implantation of hydrogen ions by the solar wind. Here we present Fourier transform infrared spectroscopy and secondary ion mass spectrometry analyses of Apollo samples that reveal the presence of significant amounts of hydroxyl in glasses formed in the lunar regolith by micrometeorite impacts. Hydrogen isotope compositions of these glasses suggest that some of the observed hydroxyl is derived from solar wind sources. Our findings imply that ice in polar cold traps could contain hydrogen atoms ultimately derived from the solar wind, as predicted by early theoretical models of water stability on the lunar surface. We suggest that a similar mechanism may contribute to hydroxyl on the surfaces of other airless terrestrial bodies where the solar wind directly interacts with the surface, such as Mercury and the asteroid 4-Vesta.

The Australian Desert Fireball Network: a new era for planetary science

Through an international collaboration between Imperial College London, the Ondřejov Observatory in the Czech Republic and the Western Australian Museum, the installation of the Australian Desert Fireball Network in the Nullarbor Region of Western Australia was completed in 2007. Currently, the Network, which is the first to be established in the southern hemisphere, comprises four all-sky autonomous observatories providing precise triangulation of fireball records to constrain pre-atmospheric orbits and fall positions of meteorites over an area of approximately 200 000 km2. To date, the Network has led to the successful recovery of two observed meteorite falls. The first recovery was three fragments (174, 150 and 14.9 g) of the same meteorite fall recorded on 20 July 2007 at 19 h 13 m 53.2 s±0.1 s UT that were found within 100 m of the predicted fall line. Named Bunburra Rockhole, the meteorite is a basaltic achondrite with an oxygen isotopic composition (Δ17O = −0.112 ‰) distinguishing it from basaltic meteorites belonging to the Howardite–Eucrite–Diogenite clan thought to be derived from asteroid 4Vesta, and therefore must have come from another differentiated asteroid in the terrestrial planet region. Bunburra Rockhole was delivered to Earth from an Aten-like orbit that was almost entirely contained within the Earth's orbit. The second recovered fall was detected by the Network on 13 April 2010 and led to the recovery of a 24.54 g meteorite fragment that is yet to be fully described. To date, the Network has recorded ∼550 fireballs. Records from which precise orbits and trajectories can be determined number ∼150. In addition to the two recovered falls twelve fireballs are considered to have resulted in meteorite falls. Of these, four are probable falls (10's–100 g), and five are certain falls (>100 g). Having proved the potential of the Network, ultimately a large dataset of meteorites with orbits will provide the spatial context for the interpretation of meteorite composition that is currently lacking in planetary science.

Zinc isotopes in HEDs: Clues to the formation of 4-Vesta, and the unique composition of Pecora Escarpment 82502

The δ66Zn (permil deviation of the 66Zn/64Zn ratio from a terrestrial standard) values for a suite of 20 non-Antarctic HED (howardite–eucrite–diogenite) meteorites and one mesosiderite, and for eight Antarctic eucrites and diogenites, were measured in order to determine the role of volatization in the formation of their presumed parent body, the asteroid 4-Vesta. The 20 non-Antarctic HEDs had δ66Zn values that ranged from −2.0‰ to +1.67‰, with a mean value of −0.01±0.39‰ (2se); this range likely represents a small-scale heterogeneity due to brecciation induced by multiple impacts. The non-Antarctic eucrites (δ66Zn=+0.00±0.58‰ (2se), n=12) were isotopically the same as the diogenites (δ66Zn=−0.31±0.80‰ (2se), n=4), and the howardites (δ66Zn=+0.26±0.37‰ (2se), n=4). On average, non-Antarctic eucrite falls were isotopically heavier (+0.50‰) than non-Antarctic finds (−1.00‰). The Antarctic finds studied were all unbrecciated samples, and they were significantly heavier than the non-Antarctic samples with a δ66Zn range of +1.63‰ to +6.22‰ for four eucrites (mean, +4.32‰) and +0.94‰ to +1.60‰ for three diogenites (mean+1.23‰), excluding one anomalous sample, while their Zn concentration is significantly lower than the brecciated samples. These data suggest that the unbrecciated eucrites probably represent the eucritic crust shortly after differentiation and cooling of the parent asteroid, at which time volatization of lighter zinc isotopes led to an isotopically heavy crust. Early impact events caused the ejection of these unbrecciated meteorites, which were subsequently spared from brecciation caused by multiple additional impacts on the much larger Vesta. The range of δ66Zn values and Zn concentration for the brecciated HEDs in this study supports a major contribution to the Vestan surface by chondritic impactors (−1.30<δ66Zn<+0.76‰ for ordinary and carbonaceous chondrites). The anomalous eucrite PCA 82502 (δ66Zn=−7.75‰) is significantly isotopically lighter than the other HEDs and is the natural sample with the lightest Zn isotopic composition reported in the solar system to date. This meteorite most likely originated from a distinct parent body.

Possible fluid–rock interactions on differentiated asteroids recorded in eucritic meteorites

The eucritic meteorites are basaltic rocks that originate from the upper part of the crust of some small bodies as exemplified possibly by asteroid 4-Vesta. A few eucrites appear to have been modified by different degrees of a late stage alteration process that caused significant variations in mineralogy. Three distinct alteration stages are identified: (1) Fe-enrichment along the cracks that cross cut the pyroxene crystals (“Fe-metasomatism”); secondary olivine and minute amounts of troilite are found only occasionally in cracks at this stage; (2) deposits of Fe-rich olivine (Fa 64-86) and minor amounts of troilite are frequent inside the cracks; sporadic secondary Ca-rich plagioclase (An 97-98) is associated with the fayalitic olivine; (3) at this stage, the Fe-enrichment of the pyroxene is accompanied by a marked Al-depletion; moreover, secondary Ca-rich plagioclase is more frequent and partly fills some cracks or rims of the primary plagioclase crystals. The composition of the secondary phases on one hand, the lack of incompatible trace element enrichment in the metasomatized pyroxenes on the other hand, rule out a silicate melt as the metasomatic agent. Although no hydrous phase has been yet identified in the studied samples, aqueous fluids are plausible candidates for explaining the deposits of ferroan olivine and anorthitic plagioclase inside the fractures of the studied unequilibrated eucrites.

Relative chronology of crust formation on asteroid Vesta: Insights from the geochemistry of diogenites

The eucrites and diogenites are meteorites that probably originate from asteroid 4-Vesta. The upper part of the crust of this body is certainly composed of eucrites which are basaltic or gabbroic rocks. Diogenites are ultramafic cumulates whose relationships with eucritic lithologies are unknown. Here, we show that the orthopyroxenes of some diogenites display very deep negative Eu anomalies (Eu/Eu∗ close to 0.1 or lower). The contamination of the parental magmas of diogenites by melts derived by partial melting of the eucritic crust can satisfactorily explain the range of the Eu anomalies displayed by diogenites. Thus, these anomalies are the first firm indication that parental melts of diogenites have intruded the eucritic crust, and consequently are younger than eucrites.

Response to the comment by G. Dreibus and H. Wänke on “Comparative geochemistry of basalts from the Moon, Earth, HED asteroid, and Mars: Implications for the origin of the Moon” (2001) Geochim. Cosmochim. Acta 65, 979–997

Response to the comment by G. Dreibus and H. Wänke on “Comparative geochemistry of basalts from the Moon, Earth, HED asteroid, and Mars: Implications for the origin of the Moon” (2001) Geochim. Cosmochim. Acta 65, 979–997

Post-crystallization reheating and partial melting of eucrite EET90020 by impact into the hot crust of asteroid 4Vesta ∼4.50 Ga ago

We performed petrologic, radiometric (Ar-Ar, Sm-Nd, and Mn-Cr ages), and ion microprobe studies of the basaltic eucrite, EET90020. This is one of the few rare basaltic eucrites whose 39Ar-40Ar age has not been reset during impact bombardment on the HED parent body ≤4 Ga ago and, thus, should provide a unique opportunity to study the nature of the early thermal events on its parent body (presumably asteroid 4Vesta). Hand specimen inspection shows that the rock consists of a fine-grained and a coarse-grained lithology. Microscopy indicates that the fine-grained lithology has a granulitic texture, with a coarser-grained area and a large opaque assemblage embedded in the granulitic matrix. The coarse-grained lithology has an igneous, subophitic texture. The rock has pyroxenes similar to those in type 5 eucrites (type 5 pyroxene) and experienced prolonged thermal metamorphism after rapid crystallization from a near-surface melt. However, minor mineral assemblages are unusual and suggest a complex thermal history. Tridymite occurs as large laths, irregular crystals (<1.5 mm), and in a large assemblage of tridymite-plagioclase-pyroxene about 4 mm in size. One large opaque assemblage (∼1.4 mm in size) is a decomposition product of Cr-ulvöspinel and consists of zoned Ti-chromite, ilmenite and minor Fe-metal. The opaques are often rimmed by Fe-rich olivine and pigeonite with some chemical variations, indicating disequilibrium with the surrounding type 5 pyroxenes. Ca-phosphate with low REE abundances, and the LREE signatures observed in plagioclase suggest that melt was present during metamorphism. This indicates that EET90020 was reheated above the subsolidus temperature of eucrites of ∼1060°C, causing partial melting. Tridymite, Cr-ulvöspinel, and some pyroxene and plagioclase crystallized from the melt. The presence of unequilibrated phases related to opaques suggests cooling rates greater than several °C/day. The reheating event was too short to destroy the exsolution textures of the type 5 pyroxenes. The temperature of the rock just before the reheating could have been ∼870°C, based on the two-pyroxene temperature of the type 5 pyroxenes. The absence of shock effects in plagioclases suggests that EET90020 did not experience shock events >1–5 GPa after the reheating event. EET90020 seems to have experienced the following thermal history; (1) crystallization during rapid cooling near the surface; (2) some brecciation by impact; (3) thermal metamorphism that produced type 5 pyroxene; and (4) short reheating that caused partial melting and rapid cooling. 39Ar-40Ar measurements show a relatively flat pattern and an age of 4.49±0.01 Ga, which is consistent with rapid cooling from high temperature (event 4). Resetting of the Sm-Nd ages at 4.51 ± 0.04 Ga appears to be closely related to the remelting of Ca-phosphates. Rb-Sr data suggest Rb-loss from tridymite during partial melting. The resetting of the Mn-Cr age may have been related to the formation of Cr-ulvöspinels (event 4). We suggest that all these ages were reset by partial melting (event 4). We further suggest that the partial melting event (event 4) that reset the ages ∼4.50 Ga ago was caused by an impact into EET90020 which was part of the hot crust of 4Vesta and resulted in an increase in the temperature from the ambient temperature of ∼ 870°C to above the subsolidus temperature of eucrites of ∼1060°C.

Comparative geochemistry of basalts from the moon, earth, HED asteroid, and Mars: implications for the origin of the moon

Most hypotheses for the origin of the Moon (rotational fission, co-accretion, and collisional ejection from the Earth, including “giant impact”) call for the formation of the Moon in a geocentric environment. However, key geochemical data for basaltic rocks from the Moon, Earth, the howardite-eucrite-diogenite (HED) meteorite parent body (probably asteroid 4-Vesta), and the shergottite-nakhlite-chassignite (SNC) meteorite parent body (likely Mars), provide no evidence that the Moon was derived from the Earth, and suggest that some objects with lunar-like compositions were produced without involvement of the Earth. The source region compositions of basalts produced in the Moon (mare basalts) were similar to those produced in the HED asteroid (eucrites) with regard to volatile-lithophile elements (Na, K, Rb, Cs, and Tl), siderophile elements (Ni, Co, Ga, Ge, Re, and Ir), and ferromagnesian elements (Mg, Fe, Cr, and V), and less similar to those in the Earth or Mars. Mare and eucrite basalts differ in their Mn abundances, Fe/Mn values, and isotopic composition, suggesting that the Moon and HED asteroid formed in different nebular locations. However, previous claims that the Moon and HED parent body differ significantly in the abundances of some elements, such as Ni, Co, Cr, and V, are not supported by the data. Instead, Cr-Mg-Fe-Ni-Co abundance systematics suggest a close similarity between the source region compositions and conditions involved in producing mare and eucrite basalts, and a significant difference from those of terrestrial basalts. The data imply that the Moon and HED asteroid experienced similar volatile-element depletion and similar fractionation of metallic and mafic phases. Among hypotheses of lunar origin, rotational fission, and small-impact collisional ejection seem less tenable than co-accretion, capture, or a variant of giant-impact collisional ejection in which the Moon inherits the composition of the impactor. Both the Moon and HED asteroid may have been derived from a class of objects that were common in the early solar system. “The most plausible model for the origin of the Moon in line with geochemical and cosmochemical constraints is an impact-induced “fission” of the proto-Earth.” — Wänke and Dreibus (1986) “Clearly, the Moon and eucrite parent body resemble each other to a high degree. Nature produced such a composition not once but at least twice. This calls into question an entire class of models that invoke ad hoc processes to explain the Moon by a unique chance event.” — Anders (1977) “The similarity between eucrites and lunar mare basalts are remarkable. Were it not for the differences in age and oxygen-isotope signature, it might be difficult to distinguish them on petrological or geochemical grounds.” — Taylor (1986)