Mesosiderites and howardites: igneous formation and possible genetic relationships

We report neutron activation data for major, minor and trace elements determined in whole rock howardites and silicates from mesosiderites. Compositions of howardites and mesosiderites are similar, and intermediate between those of eucrites and diogenites. Relative to howardites mean mesosiderite abundances are slightly nearer the diogenites. Literature data indicate that mesosiderites have a higher normative silica component than howardites. It appears that this partly results from a higher content of a highly evolved igneous component, and partly from in situ reduction of FeO to Fe followed by magnetic separation of metal prior to analysis. Removal of a portion of the FeO in this manner yields a higher normative SiO₂ component for the nonmagnetic fraction. Petrographic observations demonstrate the formation of SiO₂ which may have resulted from a combination of various factors including accretion of a reducing agent together with the Fe-Ni metal, extensive reaction during the long cooling period, and catalysis by the finely divided metal.In the mesosiderites Mincy, Lowicz and Veramin the light rare earth elements (REE) are enriched. The resulting REE pattern is qualitatively similar to that in terrestrial basalts thought to have been formed by small degrees of partial melting. Of several partial melting models tested, the best match to the REE patterns is provided by one involving ~2–4% partial melting of a source containing low REE abundances. It appears that the light REE enrichment is not associated with the hypothetical silica enriched igneous phase.Since numerous properties separate mesosiderite silicates from howardites, it is clear that they are not composed of precisely the same material. Whether or not they originated on the same parent body is unresolved. If parent body regoliths were mixed vertically and horizontally on a planet-wide basis, then separate bodies would be required.     

Differentiation history of the mesosiderite parent body: constraints from trace elements and manganese-chromium isotope systematics in Vaca Muerta silicate clasts

We report here the results of a study of trace element microdistributions and ⁵³Mn-⁵³Cr systematics in several basaltic and orthopyroxenitic clasts from the Vaca Muerta mesosiderite. Ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that, following igneous crystallization, these clasts underwent extensive metamorphic equilibration that resulted in intra- and inter-grain redistribution of elements. There is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents (such as phosphorus, derived from the mesosiderite metal) at the time of metal-silicate mixing. Furthermore, our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal-silicate mixing event. After taking into account the effects of these postmagmatic secondary processes, comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites, respectively, suggests that the primary igneous petrogenesis, including parent magma and source compositions, of Vaca Muerta silicates were similar to those of achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association. Internal ⁵³Mn-⁵³Cr isochrons obtained for two basaltic (pebble 16 and 4679) and two orthopyroxenitic (4659 and 4670) clasts show that chromium isotopes are equilibrated within each clast. Nevertheless, just as for noncumulate eucrites and diogenites, ⁵³Cr excesses in whole-rock samples of the basaltic clasts (∼1.01 ε in pebble 16; ∼1.07 ε in 4679) are significantly higher than in the orthopyroxene-rich clasts (∼0.62 ε in 4659; ∼0.53 ε in 4670). As in the case of the HED parent body, this suggests that Mn/Cr fractionation in the parent body of the Vaca Muerta silicate clasts occurred very early in the history of the solar system, when ⁵³Mn was still extant. However, the slope of the ⁵³Mn-⁵³Cr isochron defined by the whole-rock samples of Vaca Muerta clasts (corresponding to a ⁵³Mn/⁵⁵Mn ratio of 3.3 ± 0.6 × 10⁻⁶) is distinctly lower than that defined by the HED whole-rock samples (corresponding to a ⁵³Mn/⁵⁵Mn ratio of 4.7 ± 0.5 × 10⁻⁶), indicating that the global Mn/Cr fractionation event that established mantle source reservoirs on the parent body of the Vaca Muerta silicate clasts occurred ∼2 Ma after a similar event on the HED parent body.     

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.     

The Antarctic achondrite ALHA 76005: a polymict eucrite

ALHA 76005 is a basaltic achondrite containing few. if any, orthopyroxenes. Its bulk major and trace element composition is like that of a non-cumulate eucrite, and unlike that of a howardite. It contains a variety of igneous clasts which differ in their textures, pyroxene/plagioclase ratios and pyroxene and plagioclase compositions. One clast, No. 4, was found to have the REE pattern of a cumulate eucrite and an oxygen isotopic composition different from that of the bulk meteorite. Both the chemical and oxygen isotopic composition of clast No. 4 suggest that it was derived from a source different from its host. These observations lead to the conclusion that ALHA 76005 is a polymict eucrite.     

Origin of ureilites inferred from a SIMS oxygen isotopic and trace element study of clasts in the Dar al Gani 319 polymict ureilite

Secondary ion mass spectrometer (SIMS) oxygen isotope analyses were performed on 24 clasts, representing 9 clast types, in the Dar al Gani (DaG) 319 polymict ureilite with precisions better than 1‰. Olivine-rich clasts with typical ureilitic textures and mineral compositions have oxygen isotopic compositions that are identical to those of the monomict ureilites and plot along the CCAM (Carbonaceous Chondrite Anhydrous Mineral) line. Other igneous clasts, including plagioclase-bearing clasts, also plot along the CCAM line, indicating that they were derived from the ureilite parent body (UPB). Thus, we suggest that some of the plagioclase-bearing clasts in the polymict ureilites represent the “missing basaltic component” produced by partial melting on the UPB.Trace element concentrations (Mg, K, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rb, Sr and Ba) in ureilitic plagioclase and glass from 13 clasts were obtained by using the SIMS high mass resolution method. The trace element contents of the plagioclase generally show monotonic variations with anorthite content (mol%) that are consistent with partial melting and fractional crystallization. Incompatible trace element concentrations (K, Ti, and Ba) are low and variable for plagioclase with An > 40, indicating that the parental magmas for some clasts were derived from a depleted source. We performed partial melt modeling for CI and CM precursor compositions and compared the results to the observed trace element (K, Ba, and Sr) abundances in the plagioclase. Our results indicate that (1) the UPB evolved from a alkali-rich carbonaceous chondritic precursor, (2) parent melts of porphyritic clasts could have formed by 5-20% equilibrium partial melting and subsequent fractional crystallization, and (3) parent melts of the incompatible trace element-depleted clasts could be derived from fractional melting, where low degree (<10%) partial melts were repeatedly extracted from their solid sources.Thus, both the oxygen isotopic and trace element compositions of the plagioclase bearing clasts in DaG-319 suggest that the UPB underwent localized low degree-partial melting events. The partial melts could have been repeatedly extracted from the precursor, resulting in the formation of the olivine-pigeonite monomict ureilites as the final residue.     

Mineralogy,petrology, and trace element geochemistry of the Johnstown meteorite: a brecciated orthopyroxenite with siderophile and REE-rich components

The Johnstown meteorite is a brecciated orthopyroxenite (diogenite) containing coarsegrained centimeter-sized clasts of cumulate origin that have undergone subsolidus recrystallization. The brecciated portion is dominated by subangular fragments of orthopyroxene (Wo_2–3En_72–74Fs_23–25) in a variably comminuted matrix of the same material. Minor and accessory phases include plagioclase (An_82–90Ab_10–18Or_0–1), diopside (Wo_44–45En_46–47Fs_9–10), olivine (Fo_71–72), tridymite, troilite, metallic Ni-Fe (~3% Ni), and chromite (Cm_71–80Hc_1–8Sp_11–19Mt_2–4Uv_1–3).The clastic component is parental to the brecciated matrix which contains no foreign lithic or mineralogic components. Siderophile trace element studies, however, reveal the presence of meteoritic (chondritic) contamination in the brecciated portion using unbrecciated clasts for indigenous values. Rare earth element abundances show a wide range of values for the light REE in different samples, although all samples exhibit a strong negative Eu anomaly, indicative of earlier plagioclase fractionation. Two pairs of adjacent brecciated and unbrecciated samples from different portions of the meteorite show, respectively, the most enriched and the most depleted light REE patterns. The variability in La content is over a factor of 100. However, in each case the REE pattern for the brecciated portion is very similar to that of the unbrecciated portion. These differences are attributed to sampling of variable amounts of residual, REE-enriched, trapped liquid. The most representative REE pattern for the bulk meteorite has an intermediate composition and was obtained from the largest sample. The data presented here indicate that Johnstown is a monomict breccia, in contrast to several other diogenites which may be considered to be polymict on the basis of their mineral compositions and/or clast populations.     

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.     

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.     

The variety of lithologies in the Yamato-86032 lunar meteorite: Implications for formation processes of the lunar crust

We performed a petrologic, mineralogical, geochemical, and isotopic study of several lithologies in the Y-86032 feldspathic breccia. This study leads us to conclude that Y-86032 likely originated on the lunar farside. Y-86032 is composed of several types of feldspathic clasts, granulitic breccias, and minor basaltic clasts set in a clastic matrix. We identify an “An97 anorthosite” that has An contents similar to those of nearside FANs. Mg′ (= molar Mg/(Mg + Fe) × 100) values vary significantly from ∼45 to ∼80 covering the ranges of both nearside FANs and the Mg′ gap between FANs and the Mg-suite. A light-gray feldspathic (LG) breccia making up ∼20% of the investigated slab (5.2 × 3.6 cm²) mainly consists of fragments of anorthosites (“An93 anorthosite”) more sodic than nearside FANs. LG also contains an augite-plagioclase clast which either could be genetically related to the An93 anorthosite or to slowly-cooled basaltic magma intruded into the precursor rock. The Na-rich nature of both An93 anorthosite and this clast indicates that the LG breccia was derived from a relatively Na-rich but incompatible-element-poor source. The Mg′ variation indicates that the “An97 anorthosite” is a genomict breccia of several types of primary anorthosites. Granulitic breccias in Y-86032 have relatively high Mg′ in mafic minerals. The highest Mg′ values in mafic minerals for the “An97 anorthosite” and granulitic breccias are similar to those of Mg-rich lithologies recently described in Dhofar 489. Basaltic clasts in the dark-gray matrix are aluminous, and the zoning trends of pyroxene are similar to those of VLT or LT basalts. The crystallization of these basaltic clasts pre-date the lithification age of the clastic matrix at ∼3.8 Ga. The low K contents of plagioclase in both the anorthositic and basaltic clasts and generally low incompatible element abundances in all the lithologies in Y-86032 indicate that KREEP was not involved during the formation of the precursor lithologies. This observation further suggests that urKREEP did not exist in the source regions of these igneous lithologies. All these facts support the idea that Y-86032 was derived from a region far distant from the PKT and that the lithic clasts and fragments are indigenous to that region. An An97 anorthositic clast studied here has distinct Sm-Nd isotopic systematics from those previously found for another An97 anorthositic clast and “An93 anorthosite”, and suggests either that An97 anorthosites come from isotopically diverse sources, or that the Sm-Nd isotopic systematics of this clast were reset ∼4.3 Ga ago. These lines of geochemical, isotopic, and petrologic evidence suggest that the lunar crust is geochemically more heterogeneous than previously thought.     

Intrinsic oxygen fugacities of diogenites and mesosiderite clasts

Oxygen fugacities of diogenite and mesosiderite clast material were measured with the double ZrO₂ cell technique between 800° and 1150°C. The samples were taken from large clasts in the diogenites Johnstown (En₇₃) and Tatahouine (En₇₅), and the mesosiderites Estherville (En₈₁), West Point (Fo₈₈) and Emery (En₆₈). Fugacity values for all except Emery plot near the wüstite-iron buffer curve and are interpreted as indicating similar source regions and environments of crystallization for the two suites. Emery orthopyroxene records a lower fugacity, close to the fayalite-quartz-iron buffer curve, probably as a result of equilibration with the mesosiderite matrix assemblage. The similarity of redox conditions experienced by mesosiderite orthopyroxenite and diogenites is not sufficient to require a single parent body and, if the common achondrites were derived from Vesta, mesosiderites probably came from a different 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.     

Olivines and olivine coronas in mesosiderites

Olivines and their surrounding coronas in mesosiderites have been studied texturally and compositionally by optical and microprobe methods. Most olivine is compositionally homogeneous but some is irregularly zoned. It ranges from Fo_58–92 and shows no consistent pattern of distribution within and between mesosiderites. Olivine occurs as large single crystals or as partially recrystallized mineral clasts, except for two lithic clasts. One is in Emery, the other in Vaca Muerta, and they are both shock-modified olivine orthopyroxenites. $\text{FeO}\text{MnO}$ ratios in olivine exhibit a variety of differing trends and range from 22–46, most commonly 35–40. These values are lower than those in olivine from diogenites sensu stricto (45–50) and have therefore experienced a different history. Some of the olivine clasts could have coexisted with some of the large orthopyroxene clasts as equilibrium assemblages, but some could not. Much of the olivine may be derived from mesosiderite olivine orthopyroxenites, which differ from diogenites sensu stricto. More magnesian olivine may be a residue from one or more source rocks, with varying degrees of melting. These events probably occurred in a highly evolved and differentiated parent body.Fine-grained coronas surround olivine, except for those in impact-melt group mesosiderites (Simondium, Hainholz, Pinnaroo) and those without tridymite in their matrices (Bondoc, Veramin). Coronas consist largely of orthopyroxene, plagioclase, clinopyroxene, chromite, merrillite and ilmenite and are similar to the matrix, but lack metal and tridymite. Coronas contain abundant orthopyroxene but are unusually rich in chromite (up to 7%) and merrillite (up to 20%). The outer parts of the corona grade into the matrix, but have little or no metal and tridymite. Texturally the innermost part of the corona can be divided into three stages of development: I Radiating acicular; II Intermediate; III Granular. Stage I is the result of the greatest disequilibrium between olivine and matrix orthopyroxene and Stage III has the least disequilibrium. Coronas are the result of the reaction olivine + tridymite = orthopyroxene, probably because FeO (and MgO) diffuse from olivine to tridymite in the matrix. Absence of metal and concentration of chromite in the corona are probably the result of an FeO potential gradient away from the olivine. Merrillite concentrations are a result of P₂O₅ migration into the corona but are controlled by the availability of calcic pyroxene, or possibly plagioclase. Although the coronas are texturally similar to terrestrial and lunar counterparts, they are unique and represent different kinds of reactions marked by a large degree of intra-corona diffusion under dry conditions. Opaque oxide-silicate-metal buffer assemblages yield apparent equilibration conditions of about 840°C and fO₂ near 10^?20. Poikiloblastic pyroxene textures in some coronas suggest a closing of reaction systems between 900 and 1000°C and such systems may record a higher temperature stage of development.     

Meteoritic zircon – Occurrence and chemical characteristics

In common with the remarkable variation in the bulk rock Zr content of distinct meteorite groups, ranging from <1 ppm to >800 ppm, the occurrence and abundance of accessory zircon is also highly diverse and limited to certain meteorite classes. A detailed literature study on the occurrence of meteoritic zircon, along with other Zr-bearing phases reveals that lunar rocks, eucrites and mesosiderites are the prime sources of meteoritic zircon. Rare zircon grains occur in chondrites, silicate-bearing iron meteorites and Martian meteorites, with grain sizes of >5 μm allowing chemical and chronological studies at high spatial resolution using secondary ion mass spectrometry (SIMS) technique. Grain sizes, crystal habits, structural and chemical characteristics of zircon grains derived from various meteorite types, including their REE abundances, minor element concentrations, and Zr/Hf values is diverse. Superchondritic Zr/Hf values (47 ± 8; s.d. with n = 97), i.e., typical for zircon in eucrites and mesosiderites, indicate crystallization from a fractionated, incompatible-element-rich (residual) melt. Differences in REE abundances, occurrence or absence of Ce- and Eu-anomalies, and overall REE patterns that are often fractionated with a depletion in LREE, might be primarily controlled by variable formation conditions of individual grains and/or differences in the residual melt compositions on a small, local scale within single samples. Subsequent fractionation/modification of the chemical fingerprint of meteoritic zircon can involve high-temperature annealing processes during thermal metamorphic reactions and/or impact events along with mixing of lithic fragments since many samples are breccias.     

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.     

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.     

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.     

Uranium and thorium in achondrites

The abundances of U and Th in 19 achondrites and two pallasite olivines have been measured by radiochemical neutron activation analysis. Brecciated eucrites are enriched relative to chondrites in both elements by factors between 10 and 20, perhaps as a result of a magmatic differentiation process. Two unbrecciated eucrites are far less enriched, possibly due to their origin as igneous cumulates. The diogenites Johnstown and Shalka contain approximately chondritic levels of U and Th, but Ellemeet is 10 times lower. The abundances in three howardites are in good agreement with those expected from major element data for a mixing model with eucrite and diogenite end members. The high O¹⁸ basaltic achondrites Nakhla, Shergotty and Angra dos Reis have a range of U and Th abundances similar to the brecciated eucrites and howardites, but have systematically higher Th/U ratios. The Bishopville aubrite has U and Th abundances and Th/U ratios similar to those of several enstatite chondrites, suggesting a genetic relationship. The Norton County aubrite has a low Th/U, similar to that observed in recrystallized and metamorphosed terrestrial ultrabasic rocks, indicating a more complex history. Pallasite olivines have low U and Th contents (0.5.4 ppb and 1.4.3 ppb, respectively) similar to those in terrestrial dunites. The Goalpara ureilite has very low U (<0–6 ppb) and Th (2.7 ppb) abundance consistent with an origin from carbonaceous chondrites by partial melting.     

Enstatite achondrite meteorites (aubrites) and the histories of their asteroidal parent bodies

The aubrites are nearly monomineralic enstatite pyroxenites, consisting mostly of nearly FeO-free enstatite, with minor albitic plagioclase, nearly FeO-free diopside and forsterite, metallic Fe,Ni, troilite, and a host of rare accessory minerals, many unknown from Earth, that formed under highly reducing conditions. As a result, many of the normally lithophile elements such as Ti, Cr, Mn, Na, etc. behave partly as chalcophiles (i.e., occur in sulfides), and Si is partly siderophile and occurs in metallic Fe,Ni. Aubrites must therefore have formed in a very unique part of the solar nebula, possibly within 1 AU of the Sun. While of the 27 aubrites, 15 are fragmental breccias, 6 are regolith breccias, and 6 are described as non-brecciated, their ingredients are clearly of igneous origin and formed by melting and fractional crystallization, possibly of a magma ocean. This is indicated by the occurrence of a variety of lithic clasts of igneous origin, and by the REE and other trace element distributions. Their highly reduced nature and their oxygen isotopic compositions suggest close kinship to the enstatite chondrites. However, they did not form from known EH or EL chondrites on their parent bodies. Rather, they formed from enstatite chondrite-like material on at least two separate parent bodies, the Shallowater parent body and, for all other aubrites, on the aubrite parent body. Visible and near-infrared reflectance spetra of asteroids suggest that the aubrite parent bodies may be asteroids of the E-type and perhaps the E(II) sub-class, such as 3103 Eger and 2867 Steins (the target of the Rosetta Mission). If aubrites formed by the melting and fractional crystallization of enstatite chondrite-like parent lithologies, which should have contained ～10 vol% plagioclase, then meteorites of enstatite-plagioclase basaltic composition should exist, which is not the case. These early basaltic melts may have been removed from the aubrite parent body by explosive pyroclastic volcanism, and these small pyroclasts would have been destroyed in space long ago. Age dates suggest that the aubrites formed very early in the history of the solar system, within a few Ma of CAI formation, and that the heat sources for heating and melting of their parent bodies were, most likely, short-lived radionuclides such as ²⁶Al and, perhaps, ⁶⁰Fe. Finally, attention has been drawn to the surface composition of Mercury of low bulk FeO and of nearly FeO-free enstatite, perhaps with plagioclase, diopside and sulfide. While known aubrites clearly did not originate from Mercury, recent calculations suggest that several percent of high-speed ejecta from Mercury reach Earth. This is only factors of 2–3 less than typical launches from Mars and, since there are now 53 Martian meteorites in our collections, meteoriticists should be alert to the potential discovery of a genuine meteorite from Mercury which, superficially, should resemble aubrites. However, recent results from the Neutron Spectrometer of the Messenger Flyby of Mercury have been interpreted to suggest that the planet’s surface may, in fact, contain abundant Fe–Ti-oxides and, if true, a meteorite from Mercury should not resemble any currently known meteorite type.     

Argon 40-argon 39 chronology of lithic clasts from the Kapoeta howardite

⁴⁰Ar-³⁹Ar age spectra have been measured on plagioclase separates from three basaltic clasts (A, B, C), a pyroxene separate from clast B, and a total sample of a fourth basaltic clast (ρ) from the Kapoeta achondritic meteorite. The Ar data show that three of the four clasts crystallized ≥4.5 AE ago. Xe measurements indicate all four formed within a 0.1 AE period (Huneke, et al., 1977, Lunar Science VIII, pp. 484–486). Three clasts have suffered various degrees of ⁴⁰Ar loss since that time. The times of ⁴⁰Ar degassing do not cluster about a single time analogous to the lunar cataclysm. The survival of ≥4.5 AE ages contrasts with the general absence of ages ≥4.0 AE on the moon.The Ar retention age of clast B of ≥4.57 AE is atypically older than the Rb-Sr age of 3.6 AE (Papanastassiouet al., 1974, Lunar Science V, p. 583). The 3.5 AE Ar age of clast A is distinctly younger than the Rb-Sr age of 3.9 AE (Papanastassiou et al., 1974). The K-Ar and Rb-Sr systems are clearly not equivalent dating techniques in these instances.The combined evidence of Ar, Xe and Rb-Sr studies suggests the period of volcanism on the Kapoeta parent planet was restricted to the first ~0.2 AE of solar system history. The subsequent thermal metamorphic histories recorded in each of the four clasts after formation are distinctly different. The clasts must have existed as independent fragments at least as recently as 3.5 AE ago. The cosmic ray exposure ages of all the four clasts are similar (~ 3 Myr), and are not significantly different from that of the bulk meteorite. The clasts spent essentially all of the time prior to the formation of Kapoeta at depths greater than a few meters.