Composition and evolution of the eucrite parent body: evidence from rare earth elements

Eucrites are extraterrestrial plagioclase-pigeonite basalts. Experimental studies suggest that they were produced by partial melting of an olivine (Fo₆₅)-pigeonite (Wo₅En₆₅)-plagioclase (An₉₄)-spinel-metal source region. Quantitative modeling of the evolution of REE abundances in the eucrites indicates that the main group of eucrites (e.g. Juvinas) may be produced by approximately 10% equilibrium partial melting of a source region with initial REE abundances which were chondritic relative and absolute. Other eucrites appear to represent greater (e.g. Sioux County—15%) or smaller (e.g. Stannern—4%) degrees of melting. Moore County and Serra de Magé appear to be cumulates of pyroxene and plagioclase produced by fractional crystallization of a Juvinas-like melt. Nuevo Laredo may represent a residual liquid after such fractional crystallization. Our calculations are consistent with the conclusion that the eucrites were derived from a single type of source region. The close correspondence of the age of the eucrites (? 4.6 AE) to the age of the solar system appears to preclude the possibility of extensive chemical differentiation of the eucrite parent body prior to the event which produced the eucritic melts. Thus our calculations have yielded not only the mode of the source region but, assuming homogeneous accretion, the mode and hence the bulk composition of the eucrite parent body as well. We are unable to estimate quantitatively the ratio of metal to olivine in the parent body. If no metal is present, the bulk composition (in oxide wt%) is Na₂O—0.04, MgO—29.7, Al₂O₃—1.8, SiO₂—39.0, CaO—1.2, FeO—28.3. If, in contrast, the parent body contained 30% metal, the bulk composition of the silicate portion of the eucrite parent body is Na₂O—0.06, MgO—28.0, Al₂O₃—2.6, SiO₂—41.3, CaO—1.9, FeO—26.3. Relative abundances of the meteorites suggest that the eucrite parent body is still intact. The solar system object most closely resembling the eucrites is asteroid 4 Vesta. Because Vesta is unique among the asteroids, we have license to conclude that it is the source of the eucrites and its bulk composition is close to the analyses given above.     

Cosmic ray exposure age and time of formation of a new meteorite,Dhofar 007 achondrite from Oman: A rock fragment from the asteroid Vesta

The isotopic composition of noble gases was investigated in the Dhofar 007 meteorite. Petrographic and mineralogical observations suggested that it is a brecciated cumulate eucrite with high contents of siderophile elements. The concentrations of noble gases in Dhofar 007 are identical to those of other eucrites. Its cosmic ray exposure age was estimated as 11.8 ± 0.8 Ma, which coincides with a maximum on the histogram of comic ray exposure ages of eucrite meteorites. It can be supposed that, similar to other eucrites, Dhofar 007 was ejected from the surface of their parent body (presumably, asteroid Vesta) about 12.0 Ma ago. The crystallization age of the Dhofar 007 eucrite was estimated from the ratio of plutonogenic Xe to Nd as 4476 ± 22 Ma. The potassium-argon age is much younger, 3.7–4.1 Ga, which indicates partial loss of radiogenic argon during the history of the meteorite, most likely related to impact metamorphic events.     

A “chondritic” eucrite parent body: inference from trace elements

A total of 33 elements (Ag, Al, Au, Bi, Br, Cd, Ce, Co, Cr, Cs. Eu, Fe, Ge, Hf, Ir, Lu, Na, Ni, Os, Pd, Rb, Re, Sb, Se, Se, Si, Sm, Tb, Te, Tl, U, Yb and Zn) were analyzed by radiochemical and instrumental neutron activation in four eucrites: Juvinas (brecciated), Ibitira (vesicular, unbrecciated) and Moore County and Serra de Magé (cumulate, un brecciated).When arranged in order of volatility. Cl—normalized abundance patterns allow nebular and planetary effects to be distinguished. The stepped lithophile pattern reveals the dominance of nebular processes; in Ibitira, refractory elements (Hf, Lu, Tb, Ce, Sm, Yb, U, Eu) are (13.1 ± 0.7) × Cl chondrites; volatile elements (Rb. Cs, Br, Bi) are (6.0 + 1.5) × 10^?2 Cl. The depletion of Tl seems inherent to the eucrite parent body and is mirrored in the chalcophile elements by the marked deficit of Te relative to Se; apparently volatiles were accreted as a fractionated C3-like component. Consistent but subtle Cl-normalized abundance differences between eucrites (Serra de Magé < Moore County < Juvinas < Ibitira) result from crystal/liquid differentiation; Ibitira approximates the composition of an undifferentiated eucrite magma. The siderophile pattern retains little sign of nebular processes, but reflects planetary metal-silicate partition.The bulk composition of the eucrite parent body closely resembles that of H-chondrites, except for two features: moderately volatile elements (e.g. Na, K. Rb) are very much lower, apparently due to the accretion of more chondrule-like material; the metallic Fe-Ni content is only ~13%, even though total iron is very similar.     

Asteroid (4) Vesta: I. The howardite-eucrite-diogenite (HED) clan of meteorites

The howardite, eucrite and diogenite (HED) clan of meteorites are ultramafic and mafic igneous rocks and impact-engendered fragmental debris derived from a thoroughly differentiated asteroid. Earth-based telescopic observation and data returned from vestan orbit by the Dawn spacecraft make a compelling case that the asteroid (4) Vesta is the parent asteroid of HEDs, although this is not universally accepted. Diogenites are petrologically diverse and include dunitic, harzburgitic and noritic lithologic types in addition to the traditional orthopyroxenites. Diogenites form the lower crust of Vesta. Cumulate eucrites are gabbroic rocks formed by accumulation of pigeonite and plagioclase from a mafic magma at depth within the crust, while basaltic eucrites are melt compositions that likely represent shallow-level dikes and sills, and flows. Some basaltic eucrites are richer in incompatible trace elements compared to most eucrites, and these may represent mixed melts contaminated by partial melts of the mafic crust. Differentiation occurred within a few Myr of formation of the earliest solids in the Solar System. Evidence from oxygen isotope compositions and siderophile element contents favor a model of extensive melting of Vesta forming a global magma ocean that rapidly (period of a few Myr) segregated and crystallized to yield a metallic core, olivine-rich mantle, orthopyroxene-rich lower crust and basaltic upper crust. The igneous lithologies were subjected to post-crystallization thermal processing, and most eucrites show textural and mineral-compositional evidence for metamorphism. The cause of this common metamorphism is unclear, but may have resulted from rapid burial of early basalts by later flows caused by high effusion rates on Vesta. The observed surface of Vesta is covered by fragmental debris resulting from impacts, and most HEDs are brecciated. Many eucrites and diogenites are monomict breccias indicating a lack of mixing. However, many HEDs are polymict breccias. Howardites are the most thoroughly mixed polymict breccias, yet only some of them contain evidence for residence in the true regolith. Based on the numbers of meteorites, compositions of howardites, and models of magma ocean solidification, cumulate eucrites and their residual ferroan mafic melts are minor components of the vestan crust.     

Foreign meteoritic material of howardites and polymict eucrites

Howardites and polymict eucrites are fragments of regolith breccias ejected from the surface of a differentiated (eucritic) parent body, perhaps, of the asteroid Vesta. The first data are presented demonstrating that howardites contain, along with foreign fragments of carbonaceous chondrites, also fragments of ordinary chondrites, enstatite meteorites, ureilites, and mesosiderites. The proportions of these types of foreign meteoritic fragments in howardites and polymict eucrites are the same as in the population of cosmic dust particles obtained from Antarctic and Greenland ice. The concentrations of siderophile elements in howardites and polymict eucrites are not correlated with the contents of foreign meteoritic particles. It is reasonable to believe that cosmogenic siderophile elements are concentrated in howardites and polymict eucrites mostly in submicrometer-sized particles that cannot be examined mineralogically. The analysis of the crater population of the asteroid Vesta indicates that the flux of chondritic material to the surface of this asteroid should have been three orders of magnitude higher than the modern meteoritic flux and have been comparable with the flux to the moon’s surface during its intense meteoritic bombardment. This provides support for the earlier idea about a higher meteoritic activity in the solar system as a whole at approximately 4 Ga. The lithification of the regolith (into regolith breccia) of the asteroid Vesta occurred then under the effect of thermal metamorphism in the blanket of crater ejecta. Thus, meteorite fragments included in howardites provide record of the qualitative composition of the ancient meteorite flux, which was analogous to that of the modern flux at the Earth surface.     

U-Pb and Pb-Pb ages of zircons from basaltic eucrites: Implications for early basaltic volcanism on the eucrite parent body

We have undertaken petrologic and SHRIMP U-Th-Pb isotopic studies on zircons from basaltic eucrites (Yamato [Y]-75011, Y-792510, Asuka [A]-881388, A-881467 and Padvarninkai) with different thermal and shock histories. Eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites. Some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene, and typically have rounded to subrounded shapes due to recrystallization. Superchondritic Zr/Hf ratios of eucritic zircons indicate that they crystallized from incompatible element-rich melts after crystallization of ilmenite. Concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite Y-75011 are higher than those in metamorphosed eucrites.The U-Pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance. Radiogenic lead-loss up to 48% from zircons is observed in the shock-melted eucrite Padvarninkai. The ²⁰⁷Pb-²⁰⁶Pb ages of zircon in Y-75011 (4550 ± 9 Ma, n = 5) are nearly identical, within analytical uncertainty, to the ages of zircons from the metamorphosed eucrite Y-792510 (4545 ± 15 Ma, n = 13), the highly metamorphosed eucrites A-881388 (4555 ± 54 Ma, n = 5) and A-881467 (4558 ± 13 Ma, n = 8), and the shock-melted eucrite Padvarninkai (4555 ± 13 Ma, n = 18). The averaged ²⁰⁷Pb-²⁰⁶Pb age of zircon from five eucrites analyzed in this study is 4554 ± 7 Ma (95% confidence limits, n = 49), indistinguishable from the averaged U-Pb age (4552 ± 9 Ma) of the same samples. Because of the high closure temperature of lead in zircon (T_closure = ∼1050°C with a cooling rate of 0.2°C/yr), the ²⁰⁷Pb-²⁰⁶Pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas.This fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the Solar System history, 7-20 Ma after CAI formation (4567.2 ± 0.6 Ma), thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body. The U-Pb ages of eucritic zircons are older than the U-Pb, Sm-Nd and Rb-Sr ages of some basaltic eucrites, which is consistent with differences in closure temperatures of each isotopic system, and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene, plagioclase and phosphates.     

Petrology of the unbrecciated eucrites

Twenty-nine unbrecciated eucrites have been thoroughly characterized in terms of the petrologic factors that affect their spectra, such as mineral chemistry, modal adundances, grain sizes, and textures. We have conducted a combined petrologic and spectral study designed to provide insight into the petrogenesis of the basaltic crust of Vesta and the variety of rock-types that exist within it, as well as aid in the petrologic interpretation of spectra to be collected by the Dawn orbiting spacecraft. This paper details the petrology part of the study. Unbrecciated eucrite samples were selected to avoid the complications of lithologic mixing in the accompanying spectral study. A wide variety of textural types are seen within the basaltic eucrites, encompassing quenched, coarse-grained, and granoblastic samples. Zoned pyroxenes in eucrites and those that preserve a history of initial rapid cooling are rare. Nearly all eucrite samples have been thermally metamorphosed and would commonly be classified as equilibrated; however, this term reflects only the quadrilateral (Mg, Fe, and Ca) compositions of pyroxenes, and considerable variations are seen within the minor elements (Al, Ti, and Cr) in pyroxenes as well as plagioclase compositions. Determination of both pyroxene and plagioclase compositions together with pyroxene geothermometry provides a better estimate for the relative degree of thermal metamorphism a eucrite has experienced. The petrologic differences observed here might allow different eucrites to be distinguished spectrally. This is especially true for the varying pyroxene compositions as the spectra of eucrites are dominated by absorption features attributed to pyroxene.     

Experimental petrology of eucritic meteorites

Low pressure melting experiments on eucritic meteorites demonstrate that the compositions of most eucrites can be generated by low pressure fractionation of pigeonite and plagioclase from liquids similar in composition to the Sioux County and Juvinas eucrites. It is unlikely that the liquids with compositions similar to Sioux County and Juvinas were themselves residual liquids produced by extensive fractionation of more magnesian parental liquids. The compositions of Stannern and Ibitira cannot be produced by fractionation of liquids with compositions similar to other known eucrites. Liquid compositions similar to Stannern, Ibitira, and Sioux County could have been generated by increasing degrees of low pressure partial melting of source regions composed of olivine (~Fo65), pigeonite (~Wo5En65), plagioclase (~An94), Cr-rich spinel, and metal. These source assemblages may have been primitive, undifferentiated material of the basaltic achondrite parent body and the eucrites may represent melts produced in early stages of its melting and differentiation. Further melting in these source regions, after exhaustion of plagioclase, may have produced magnesian liquids from which the magnesian pyroxenes and olivines in howardites, diogenites, and mesosiderites crystallized in closed-system plutonic environments. Most of the cumulate eucrites (e.g. Moama, Moore County, Serra de Magé) could not have equilibrated with liquids similar in composition to known eucrites. These cumulates may have accumulated from liquids produced by extensive fractionation of advanced partial melts of the source regions of eucritic liquids. A depletion in Na, K, and Rb in Ibitira is noted.     

Differentiation and magmatic history of Vesta: Constraints from HED meteorites and Dawn spacecraft data

Quantifying the amounts of various igneous lithologies in Vesta’s crust allows the estimation of petrologic ratios that describe the asteroid’s global differentiation and subsequent magmatic history. The eucrite:diogenite (Euc:Diog) ratio measures the relative proportions of mafic and ultramafic components. The intrusive:extrusive (I:E) ratio assesses the effectiveness of magma ascent and eruption. We estimate these ratios by counting numbers and masses of eucrites, cumulate eucrites, and diogenites in the world’s meteorite collections, and by calculating their proportions as components of crustal polymict breccias (howardites) using chemical mixing diagrams and petrologic mapping of multiple thin sections. The latter two methods yield a Euc:Diog ratio of ∼2:1, although meteorite numbers and masses give slightly higher ratios. Surface lithologic maps compiled from spectra of Dawn spacecraft instruments (VIR and GRaND) yield Euc:Diog ratios that bracket estimates of Euc:Diog from the meteorites. The I:E ratios from HEDs lie between 0.5–2.1:1, due to uncertainties in identifying cumulate eucrite. Gravity mapping of Vesta by the Dawn spacecraft supports the existence of diogenite plutons in the crust. Quantifying the proportion of high-density diogenitic crust in the gravity map yields I:E ratios of 0.8-1:2:1, values which are bracketed by calculations based on HEDs. The I:E ratio for Vesta is lower than for Earth and Mars, consistent with physical modeling of asteroid-size bodies. Nevertheless, it indicates a significant role for pluton emplacement during the formation of Vesta’s crust. These results are inconsistent with simple differentiation models that produce the crust by crystallization of a global magma ocean, unless residual melts are extracted into crustal magma chambers.     

Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

A few eucrites have anomalous oxygen isotopic compositions. To help understand their origin and identify additional samples, we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites. Except for five eucrites, these meteorites have Δ¹⁷O values that lie within 2σ of their mean value viz., −0.242 ± 0.016‰, consistent with igneous isotopic homogenization of Vesta. The five exceptional eucrites—NWA 1240, Pasamonte (both clast and matrix samples), PCA 91007, A-881394, and Ibitira—have Δ¹⁷O values that lie, respectively, 4σ, 5σ, 5σ, 15σ, and 21σ away from this mean value. NWA 1240 has a δ¹⁸O value that is 5σ below the mean eucrite value. Four of the five outliers are unbrecciated and unshocked basaltic eucrites, like NWA 011, the first eucrite found to have an anomalous oxygen isotopic composition. The fifth outlier, Pasamonte, is composed almost entirely of unequilibrated basaltic clasts. Published chemical data for the six eucrites with anomalous oxygen isotopic compositions (including NWA 011) exclude contamination by chondritic projectiles as a source of the oxygen anomalies. Only NWA 011 has an anomalous Fe/Mn ratio, but several anomalous eucrites have exceptional Na, Ti, or Cr concentrations. We infer that the six anomalous eucrites are probably derived from five distinct Vesta-like parent bodies (Pasamonte and PCA 91007 could come from one body). These anomalous eucrites, like the isotopically normal, unbrecciated eucrites with 4.48 Gyr Ar-Ar ages, are probably deficient in brecciation and shock effects because they were sequestered in small asteroids (10 km diameter) during the Late Heavy Bombardment following ejection from Vesta-like bodies. The preservation of Vesta’s crust and the lack of deeply buried samples from the hypothesized Vesta-like bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets, rather than by collisional grinding.     

Stony-iron meteorites: History of the metal phase according to tungsten isotopes

The global composition of the early solar system is thought to be roughly chondritic in terms of refractory components, and this means that metal and silicate should be present together in early planetesimals. To fully understand the metal-silicate differentiation process within the eucrite parent body (EPB), it is important to try and identify the metal reservoir that is complementary to the silicate part. The isotope 182 of tungsten (W), a siderophile element, is partly formed from the decay of ¹⁸²Hf, and W isotopes are useful for examining metal-silicate segregation. The W isotopic composition expected for the metal that is complementary to eucrites falls in the range of iron meteorites. However, mesosiderites seem to be genetically linked to eucrites based on petrologic and oxygen isotopic similarities. Therefore, we undertook the analysis of the metal phase of these stony-irons. Here we present tungsten isotopic data for mesosiderite and pallasite metal to characterize their parent body (bodies) and to assess possible relationships with eucrites.All stony-iron metals are depleted in radiogenic tungsten by −1.3 to −4.2 ε units, relative to the terrestrial standard, while chondrites, for comparison, are depleted by −1.9 ε units. In addition to W isotopic heterogeneity from one stony-iron to another, there is also W isotopic heterogeneity within individual meteorites. A formation model is tentatively proposed, where we show that mesosiderites, pallasites, and eucrites could possibly come from the same parent body. Several hypotheses are discussed to explain the isotopic heterogeneity: the production of cosmogenic tungsten, the in situ decay of hafnium present in inclusions, and tungsten diffusion processes after metal-silicate mixing during the cooling of the meteorites. The two latter hypotheses provide the best explanation of our data.     

A Tiny Piece of Basalt Probably from Asteroid 4 Vesta

Grove Mountains (GRV) 99018 is a new eucrite (0.23 g), consisting mainly of pyroxene (50.5 vol%) and plagioclase (37.2 vol%) with minor silica minerals (7.0 Vol%) and opaque minerals (5.2 vol%). It was intensely shocked, leading to partial melting, formation of abundant tiny inclusions in pyroxenes and plagioclase, and heavy brecciation. Exsolution of most pyroxenes (1-3μm in width of the lamellae), recrystallization of the shpck-induced melt pockets and veins (5-20μm in size), and homogeneous compositions of pyroxenes of     

Challenges of identifying putative planetary-origin meteorites of non-igneous material

This paper summarizes the challenges of identifying planetary-origin meteorites of non-igneous composition-particularly those of sedimentary origin.Evidence for putative sedimentary-origin(sedtype)meteorites and their potential parent bodies is reviewed,suggesting that the list of candidate parent bodies for sed-type meteorites includes,but is not limited to,Mars,Enceladus,Ganymede,Europa,Ceres,Vesta,and other hypothetical planets that existed between the orbits of Mars and Jupiter in the past.The extraterrestrial origin and probable parent body for sed-type meteorites should be assessed based on multiple lines of evidence,and not solely limited to tests of oxygen and noble gas isotopes,whose signatures may undergo terrestrial contamination and which may exhibit significant heterogeneity within both the Solar System and parent cosmic bodies.The observed fall of a cosmic body,evidence of hypervelocity fall,signs of impact,presence of fusion crust,melting,and/or shock deformation features in impactor fragments should be considered as priority signs of meteoritic origin.     

The Stannern trend eucrites: Contamination of main group eucritic magmas by crustal partial melts

We report on the petrology of a new eucrite belonging to the Stannern trend and discuss the origin of this trend. The eucrite Northwest Africa 4523 (NWA 4523) is an equilibrated eucrite consisting of dark clasts embedded in a fine-grained crystallized matrix. Two types of clasts have been observed: medium-grained ophitic/subophitic clasts, and very fine-grained clasts. Despite textural differences, the clasts display the same mineralogy, in particular the same kind of pyroxenes with pigeonitic cores containing sparse exsolution lamellae, and augitic rims, zoned plagioclases and the occurrence of K-feldspar. The major and trace element abundances of a large medium-grained clast are very similar to Stannern or Bouvante.The Stannern trend eucrites are characterized by high incompatible trace element abundances. Their trace element patterns normalized to a representative Main Group eucrite, exhibit significant Eu, Sr and Be negative anomalies. In this paper, we show that contamination of Main Group eucritic magmas by melts derived by partial melting of the asteroid’s crust can successfully explain both the high incompatible trace elements concentrations and the distinctive Eu, Sr, Be anomalies shown by the Stannern trend eucrites. This model is in agreement with the view that Stannern and some Main Group-Nuevo Laredo trend eucrites have been contemporaneously erupted, and with the probable assumption that Stannern trend eucrites formed rather late in the history of the 4-Vesta’s crust.     

Trace element geochemistry of K-rich impact spherules from howardites

The howardite–eucrite–diogenite (HED) achondrites are a group of meteorites that probably originate from the asteroid Vesta. Howardites are complex polymict breccias that sometimes contain, in addition to various rock debris, impact melt glasses which show an impressive range of compositions. In this paper we report on the geochemistry and O isotopes of a series of 6 Saharan polymict breccias (4 howardites and 2 polymict eucrites), and on the trace element abundances of high-K impact spherules found in two of them, Northwest Africa (NWA) 1664 and 1769, which are likely paired.The high-K impact spherules found in the howardites NWA 1664 and NWA 1769 display remarkable trace element patterns. Compared to eucrites or howardites, they all show prominent enrichments in Cs, Rb, K, Li and Ba, strong depletion in Na, while the REE and other refractory elements are unfractionated. These features could not have been generated during impact melting of their host howardites, nor other normal HED target materials. The involvement of Na-poor rocks, and possibly rocks of granitic composition, appears likely. Although these lithologies cannot be well constrained at present, our results demonstrate that the surface of Vesta is certainly more diverse than previously thought. Indeed, despite the large number of available HED meteorites (about 1000 different meteorites), the latter are probably not sufficient to describe the whole surface of their parent body.     

Geochemistry of eucrites: genesis of basaltic eucrites, and Hf and Ta as petrogenetic indicators for altered antarctic eucrites

We performed instrumental neutron activation analysis on a large suite of antarctic and nonantarctic eucrites, including unbrecciated, brecciated, and polymict eucrites and cumulate and noncumulate eucrites. We evaluate the use of Hf and Ta, two highly incompatible elements, as sensitive indicators of partial melting or fractional crystallization processes. Comparison with rare earth element (REE) data from nonantarctic and antarctic eucrites shows that Hf and Ta are unaffected by the terrestrial alteration that has modified the REE contents and patterns of some antarctic eucrites. The major host phases for Hf and Ta—zircon, baddeleyite, ilmenite, and titanite—are much less susceptible to terrestrial alteration than the phosphate hosts of REEs. The host phases for Hf and Ta are minor or trace phases, so sample heterogeneity is a serious concern for obtaining representative compositions. The trace lithophile and siderophile element contents of noncumulate eucrites do not allow for a single, simple model for the petrogenesis of the howardite-eucrite-diogenite suite. Fractional crystallization models cannot reproduce the compositional relationship between eucrites of the main group-Nuevo Laredo trend and those of the Stannern trend. Equilibrium crystallization models cannot explain the trace element diversity observed among diogenites. Partial melting models cannot explain the W variations among eucrites, unless source regions had different metal contents. We suggest that slight variations in oxygen fugacity of eucrite source regions during partial melting can explain the W variations without requiring different metal contents. This hypothesis may fail to account for eucrite Co contents, however.     

Asteroid (4) Vesta II: Exploring a geologically and geochemically complex world with the Dawn Mission

More than 200 years after its discovery, asteroid (4) Vesta is thought to be the parent body for the howardite, eucrite and diogenite (HED) meteorites. The Dawn spacecraft spent ∼14 months in orbit around this largest, intact differentiated asteroid to study its internal structure, geology, mineralogy and chemistry. Carrying a suite of instruments that included two framing cameras, a visible-near infrared spectrometer, and a gamma-ray and neutron detector, coupled with radio tracking for gravity, Dawn revealed a geologically and geochemically complex world. A constrained core size of ∼110–130 km radius is consistent with predictions based on differentiation models for the HED meteorite parent body. Hubble Space Telescope observations had already shown that Vesta is scarred by a south polar basin comparable in diameter to that of the asteroid itself. Dawn showed that the south polar Rheasilvia basin dominates the asteroid, with a central uplift that rivals the large shield volcanoes of the Solar System in height. An older basin, Veneneia, partially underlies Rheasilvia. A series of graben-like equatorial and northern troughs were created during these massive impact events 1–2 Ga ago. These events also resurfaced much of the southern hemisphere and exposed deeper-seated diogenitic lithologies. Although the mineralogy and geochemistry vary across the surface for rock-forming elements and minerals, the range is small, suggesting that impact processes have efficiently homogenized the surface of Vesta at scales observed by the instruments on the Dawn spacecraft. The distribution of hydrogen is correlated with surface age, which likely results from the admixture of exogenic carbonaceous chondrites with Vesta's basaltic surface. Clasts of such material are observed within the surficial howardite meteorites in our collections. Dawn significantly strengthened the link between (4) Vesta and the HED meteorites, but the pervasive mixing, lack of a convincing and widespread detection of olivine, and poorly-constrained lateral and vertical extents of units leaves unanswered the central question of whether Vesta once had a magma ocean. Dawn is continuing its mission to the presumed ice-rich asteroid (1) Ceres.     

Impact melting of the Cachari eucrite 3.0 Gy ago

Both the host phase and glass veins of the Cachari eucrite have been analyzed by microprobe and neutron activation analysis for their chemical compositions and by mass spectrometry for their ³⁹Ar-⁴⁰Ar gas retention ages. Cachari is chemically similar to other non-cumulate eucrites. The vesicular glass veins vary from pure glass, to devitrified glass, to areas that are substantially crystalline. The glassy areas have nearly the same concentrations of major and trace elements as the unmelted portions of Cachari, but some differences, probably due to preferential dissolution, occur along melt contacts. The glass formed by shock melting of Cachari host or of rock identical to it. ³⁹Ar-⁴⁰Ar data for the host and glass suggest distinctly different ages of 3.04 ±.07 Gy and 3.47 ±.04 Gy, respectively. The time of glass formation, which may also be the time of brecciation, is most likely given by the 3.0 Gy age of the host. The higher age for the glass is interpreted to represent incomplete Ar degassing during the 3.0 Gy event due to the greater resistance to Ar diffusion shown by the glass compared to the host. Event ages significantly younger than 4.5 Gy have now been determined for several eucrites and howardites and suggest a long dynamic regolith history for the parent body.     

Siderophile and other geochemical constraints on mixing relationships among HED-meteoritic breccias

We have used neutron activation and electron-probe fused-bead techniques to analyze the bulk major and trace-element compositions of 104 named HED meteorites (about 100–102 distinct meteorites, depending upon pairings), including 32 polymict eucrites, 30 howardites and six diogenites. Most were not previously analyzed for siderophile trace elements; many not even for major elements. Our typical sample was 350 mg, and in some cases two separate chips were analyzed as a test of meteorite heterogeneity. Meteorites with extraordinary compositions include Bluewing 001, an unequilibrated eucrite that is rich in Ti, Sm and other incompatible elements; Y-791192, a cumulate-dominated polymict eucrite; and LEW 87002, an oddly Sm-rich howardite dominated by a ferroan variety of diogenite. The eucrite:diogenite mixing ratio is the single most important factor determining the compositions of polymict HEDs, but wide ranges in eucrite incompatible element contents, in diogenite Cr and V contents, and in Sc contents of both eucrites and diogenites, make for diversity among the polymict HEDs.As our new siderophile data help to show, the common practice of describing the entire class of howardites as regolith breccias is erroneous. Most howardites are fragmental breccias showing no sign of origin from true (in the lunar sense, i.e., soil-like) near-surface regolith. Howardites are highly diverse in Ni content, often remarkably Ni-poor, compared to lunar regolith breccias. However, the few (8) howardites with between 300 and 1200 μg/g Ni consistently show some combination of other traits suggestive of regolith origin. Most importantly, all four cases (or five if we include Malvern, which appears to have been altered by annealing) of howardites known to have enrichments in solar-wind noble gases belong to the >300 μg/g Ni group. In many cases, an abundance of glasses, particularly in spheroidal or turbid-brown form, provides additional evidence for regolith origin. We propose that the important subset of howardites that are regolith breccias be formally distinguished by the designation regolithic howardite.Apart from high siderophile levels, the regolithic howardites are compositionally distinctive in having Al₂O₃ consistently near 8–9 wt%; corresponding to a eucrite:diogenite mixing ratio of precisely 2:1. Assuming the HEDs are reasonably representative of the ancient (i.e., pre-vestoid-launch) surface of Vesta, this clustering of regolith composition is difficult to explain unless most of the ancient diogenite component was brought to the surface in a single early episode (i.e., probably a single great impact), after which smaller-scale cratering (with no further major excavations of diogenite until the vestoid-forming event), efficiently homogenized the surface. Such a single-excavation model may also help to explain why diogenites, in marked contrast with eucrites, are seldom polymict; and why Al₂O₃-poor (diogenite-dominated) howardites consistently lack major siderophile enrichments. The low siderophile contents of polymict eucrites are most enigmatic. Possibly in the HED-asteroidal context (low collision velocities, etc.), only materials blended by multiple impacts consistently acquire major enrichments in siderophile elements.