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1.
Abstract— An evaluation of trapped melts effects during crystallization and subsolidus equilibration of cumulates is necessary to constrain the composition of their parental magmas. In this paper, a simple mass balance approach is described. It allows estimation of trace element abundances in these parental melts from phase compositions. It is used to discuss the genesis of cumulate eucrites and diogenites. The REE behavior is in full agreement with the view that cumulate eucrites formed from melts similar to noncumulate eucrites. Trapped melt fractions ranging from <10 wt% for Moama to ?30 wt% for Moore County were involved. The origin of diogenites is more complex. The assumption that eucrites and diogenites shared the same parental melts cannot satisfactorily explain the diversity of incompatible trace element ratios (e.g., Dy/Yb) observed in diogenitic orthopyroxenes, even if interstitial melt effects are taken into account. Moreover, some diogenites unambiguously crystallized from magmas displaying significant HREE (heavy rare earth elements) enrichments. More likely, diogenites formed from distinct batches of parental magmas, as previously proposed by Mittlefehldt (1994), Fowler et al. (1995), and Shearer (1997).  相似文献   

2.
Mandler and Elkins‐Tanton ( 2013 ) recently proposed an upgraded magma ocean model for the differentiation history of the giant asteroid 4 Vesta. They show that a combination of both equilibrium crystallization and fractional crystallization processes can reproduce the major element compositions of eucritic melts and broadly the range of mineral compositions observed in diogenites. They assert that their model accounts for all the howardites, eucrites, and diogenites (HEDs), and use it to predict the crustal thickness and the proportions of the various lithologies. Here, we show that their model fails to explain the trace element diversity of the diogenites, contrary to their claim. The diversity of the heavy REE enrichment exhibited by the orthopyroxenes in diogenites is inconsistent with crystallization of these cumulates in either shallow magma chambers replenished by melts from a magma ocean or in a magma ocean. Thus, proportions of the various HED lithologies and the crustal thickness predicted from this model are not necessarily valid.  相似文献   

3.
Abstract— According to a currently popular model for petrogenesis on the howardite, eucrite, and diogenite (HED) parent asteroid, the diogenites are not comagmatic with most eucrites but instead formed in separate orthopyroxenite-dominated plutons. This model can be tested for consistency with mass balance for MgO and FeO, assuming the overall diogenite/(diogenite + eucrite) ratio, d, of the parent asteroid is at least comparable to the average d for the eucrite + diogenite dominated howardite regolith breccias. Average mg# (=MgO/[MgO + FeO]) is much lower for eucrites, especially noncumulate eucrites, than for diogenites. Unless the diogenite parent magmas eventually produced a large proportion of low-mg# residual basalt and gabbro (RBG), the implied initial magma's mg# is vastly higher than that of any noncumulate eucrite. Starting from a source previously depleted by putative primary eucrite genesis, melt mg# can be estimated as a function of the exchange reaction KD and degree of melting. Using several very conservative assumptions (e.g., assuming that the total [MgO + FeO] concentration is nearly the same in the nascent melt as in the residual solids), the degree of melting required to yield a melt with mg# high enough to satisfy mass balance, without implying an RBG component that accounts for >50% of all eucrites, is an implausibly high 60–80 wt%. The separate orthopyroxenitic plutons (SOP) model also seems inconsistent with the uniform density of melts across the diogenite-eucrite compositional spectrum (2.77–2.82 g/cm3), which implies that diogenitic magmas should have been as capable as eucrites of extruding to form lavas. This difficulty cannot be reduced by simply assuming that later-formed magmas were systematically both more plutonic and more MgO-rich than earlier ones, because the plutonic cumulate eucrites equilibrated with melts systematically lower in mg# than noncumulate eucrites. Conceivably, the bulk mg# of the asteroid's silicate system was increased between primary-melt eucrite genesis and SOP diogenite genesis by graphite-fueled reduction of FeO. However, the graphite oxidation process generates a huge proportion of gas, which would have enhanced the buoyancy of the nascent diogenite-parent magmas, thus exacerbating the difficulty of achieving the implied high degrees of partial melting. To avoid these difficulties but still form most eucrites as rapidly cooled extrusives, I propose the NERD (noncumulate eucrites as extruded residua of diogenites) model. In this model, the diogenites form as early cumulates from a large magma system (probably a global “magma ocean”) that yields a large proportion of eucritic melt as residuum. This residual melt zone undergoes relatively little crystallization during a period when it is episodically tapped to produce extrusions, dikes and sills of rapidly cooled noncumulate eucrites. Slight (～5–10%) porosity in the nascent eucritic crust keeps it marginally buoyant over the residual melt zone. The common thermal metamorphism of noncumulate eucrites results from baking by superjacent flows during the episodic venting of the melt zone. The NERD model's greatest advantage is that it does not require implausibly high degrees of localized melting in the mature stages of igneous evolution of the HED asteroid.  相似文献   

4.
Abstract— Available evidence strongly suggests that the HED (howardite, eucrite, diogenite) meteorites are samples of asteroid 4 Vesta. Abundances of the moderately siderophile elements (Ni, Co, Mo, W and P) in the HED mantle indicate that the parent body may have been completely molten during its early history. During cooling of a chondritic composition magma ocean, equilibrium crystallization is fostered by the suspension of crystals in a convecting magma ocean until the crystal fraction reaches a critical value near 0.80, when the convective system freezes and melts segregate from crystals by gravitational forces. The extruded liquids are similar in composition to Main Group and Stannern trend eucrites, and the last pyroxenes to precipitate out of this ocean (before convective lockup) span the compositional range of the diogenites. Subsequent fractional crystallization of a Main Group eucrite liquid, which has been isolated as a body of magma, produces the Nuevo Laredo trend and the cumulate eucrites. The predicted cumulate mineral compositions are in close agreement with phase compositions analyzed in the cumulate eucrites. Thus, eucrites and diogenites are shown to have formed as part of a simple and continuous crystallization sequence starting with a magma ocean environment on an asteroidal size parent body that is consistent with Vesta.  相似文献   

5.
The asteroid 4 Vesta is one of the very few heavenly bodies to have been linked to samples on Earth: the howardite‐eucrite‐diogenite (HED) meteorite suite. This large and diverse suite of meteorites provides a detailed picture of Vesta's igneous and postigneous history. We have used the range of igneous rock types and compositions in the HED suite to test a series of chemical models for solidification processes following peak melting (magma ocean) conditions on Vesta. Fractional crystallization cannot have been a dominant early process in the magma ocean because it leads to excessive Fe‐enrichment in the melt. Models that are dominated by equilibrium crystallization cannot produce orthopyroxene cumulates (diogenites). Our best models invoke 60–70% equilibrium crystallization of a magma ocean followed by continuous extraction of the residual melt into shallow magma chambers. Fractional crystallization in these magma chambers combined with continuous or periodic addition of more melt from the slowly compacting crystal mush (magmatic recharge) can produce all of the igneous HED lithologies (noncumulate and cumulate eucrites, diogenites, dunites, harzburgites, and olivine diogenites). Magmatic recharge can also explain the narrow range in eucrite compositions and the variability of incompatible trace element concentrations in diogenites. We predict an internal structure for Vesta that permits excavation of the HEDs during the formation of the Rheasilvia basin, while remaining consistent with observations from the Dawn mission and most impact models.  相似文献   

6.
Dawn has recently revealed that the surface of Vesta is heterogeneously covered by polymictic regoliths represented by mixtures of howardite, eucrite, and diogenite (HED) meteorites. Mixing relations of the HED suite are examined here using a new computational statistical approach of independent component analysis (ICA). We performed eight‐component ICA (Si, Ti, Al, Cr, Fe, Mn, Mg, and Ca) for 209 HED bulk‐rock compositions. The ICA results indicate that the HED bulk‐rock compositions can be reduced into three independent components (IC) and these IC vectors can reasonably explain compositional variation, petrographic observations, and the mixing relations of the HED suite. The IC‐1 vector represents a eucrite variation that extends from cumulate eucrite toward main‐group (MG) and incompatible‐element enriched eucrites. The IC‐2 vector represents a compositional variation of howardites that extends from diogenites to MG‐eucrites, indicating the well‐known two‐component mixing trend of diogenite and eucrite. The IC‐3 vector represents a compositional variation defined by diogenites and olivine‐bearing diogenites, suggesting mixing of olivine and orthopyroxene. Among the three ICs, the diogenite‐eucrite mixing trend IC‐2 is most statistically robust and dominates the compositional variations of the HED suite. Our ICA study further indicates that the combination of only three elements (Mg, Si, and Fe) approximates the eight‐component ICA model, and that the limited number of resolvable γ‐ray spectra obtained by the Dawn mission possibly discriminates olivine lithologies from the olivine‐free regolith breccias on the surface of Vesta.  相似文献   

7.
New petrography, mineral chemistry, and whole rock major, minor, and trace element abundance data are reported for 29 dominantly unbrecciated basaltic (noncumulate) eucrites and one cumulate eucrite. Among unbrecciated samples, several exhibit shock darkening and impact melt veins, with incomplete preservation of primary textures. There is extensive thermal metamorphism of some eucrites, consistent with prior work. A “pristinity filter” of textural information, siderophile element abundances, and Ni/Co ratios of bulk rocks is used to address whether eucrite samples preserve endogenous refractory geochemical signatures of their asteroid parent body (i.e., Vesta), or could have experienced exogenous impact contamination. Based on these criteria, Cumulus Hills 04049, Elephant Moraine 90020, Grosvenor Range 95533, Pecora Escarpment 91245, and possibly Queen Alexander Range 97053 and Northwest Africa 1923 are pristine eucrites. Eucrite major element compositions and refractory incompatible trace element abundances are minimally affected by metamorphism or impact contamination. Eucrite petrogenesis examined through the lens of these elements is consistent with partial melting of a silicate mantle that experienced prior metal–silicate equilibrium, rather than as melts associated with cumulate diogenites. In the absence of the requirement of a large-scale magma ocean to explain eucrite petrogenesis, the interior structure of Vesta could be more heterogeneous than for larger planetary bodies.  相似文献   

8.
Abstract— Diogenites are recognized as a major constituent of the howardite, eucrite and diogenite (HED) meteorite group. Recently, several papers (Mittlefehldt, 1994; Fowler et al, 1994, 1995) have identified trace-element systematics in diogenites that appeared to mimic simple magmatic processes that involved large degrees of crystallization (up to 95% orthopyroxene) of basalt with extremely high normative hypersthene. Such a crystallization scenario linking all the diogenites is highly unlikely. The purpose of this study is to explore other possible models relating the diogenites. Computational major-element melting models of a variety of different potential bulk compositions for the eucrite parent body (EPB) mantle indicate that these compositions show a similar sequence in residuum mineral assemblage with increasing degrees of partial melting. Numerous bulk compositions would produce melts with Mg# appropriate for diogenitic parent magmas at low to moderate degrees of partial melting (15% to 30%). These calculations also show that melts with similar Mg# and variable incompatible element concentrations may be produced during small to moderate degrees of EPB mantle melting. The trace-element characteristic of the orthopyroxene in diogenites does not support a model for large amounts of fractional crystallization of a single “hypersthene normative” basaltic magma following either small-scale or large-scale EPB mantle melting. Small degrees of fractional crystallization of a series of distinct basaltic magmas are much more likely. Only two melting models that we considered hold any promise for producing different batches of “diogenitic magmas.” The first model involves the fractional melting of a homogeneous source that produces parental magmas to diogenites with an extensive range of incompatible elements and limited variations in Mg#. There are several requirements for this model to work. The first requirement of this model is that the Dorthopyroxene/melt must change during melting or crystallization to compress the range of incompatible elements in the calculated diogenitic magmas. The second prerequisite is that either some of the calculated diogenitic magmas are parental to eucrites or the Mg# in diogenitic magmas are influenced by slight changes in oxygen fugacity during partial melting. The second model involves batch melting of a source that reflects accretional heterogeneities capable of generating diogenitic magmas with the calculated Mg# and incompatible element contents. Both of these models require small to moderate degrees of partial melting that may limit the efficiency of core separation.  相似文献   

9.
The incompatible trace element-enriched Stannern-trend eucrites have long been recognized as requiring a distinct petrogenesis from the Main Group-Nuevo Laredo (MGNL) eucrites. Barrat et al. ( 2007 ) proposed that Stannern-trend eucrites formed via assimilation of crustal partial melts by a MGNL-trend magma. Previous experimental studies of low-degree partial melting of eucrites did not produce sufficiently large melt pools for both major and trace element analyses. Low-degree partial melts produced near the solidus are potentially the best analog to the assimilated crustal melts. We partially melted the unbrecciated, unequilibrated MGNL-trend eucrite NWA 8562 in a 1 atm gas-mixing furnace, at IW-0.5, and at temperatures between 1050 and 1200 °C. We found that low-degree partial melts formed at 1050 °C are incompatible trace element enriched, although the experimental melts did not reach equilibrium at all temperatures. Using our experimental melt compositions and binary mixing modeling, the FeO/MgO trend of the resultant magmas coincides with the range of known Stannern-trend eucrites when a primary magma is contaminated by crustal partial melts. When experimental major element compositions for eucritic crustal partial melts are combined with trace element concentrations determined by previous modeling (Barrat et al. 2007 ), the Stannern-trend can be replicated with respect to both major, minor, and trace element concentrations.  相似文献   

10.
The large collection of howardite‐eucrite‐diogenite (HED) meteorites allows us to study the initial magmatic differentiation of a planetesimal. We report Pb‐Pb ages of the unequilibrated North West Africa (NWA) 4215 and Dhofar 700 diogenite meteorites and their mass‐independent 26Mg isotope compositions (26Mg*) to better understand the timing of differentiation and crystallization of their source reservoir(s). NWA 4215 defines a Pb‐Pb age of 4484.5 ± 7.9 Myr and has a 26Mg* excess of +2.3 ± 1.6 ppm whereas Dhofar 700 has a Pb‐Pb age of 4546.4 ± 4.7 Myr and a 26Mg* excess of +25.5 ± 1.9 ppm. We interpret the young age of NWA 4215 as a thermal overprint, but the age of Dhofar 700 is interpreted to represent a primary crystallization age. Combining our new data with published Mg isotope and trace element data suggests that approximately half of the diogenites for which such data are available crystallized within the first 1–2 Myr of our solar system, consistent with a short‐lived, early‐formed magma ocean undergoing convective cooling. The other half of the diogenites, including both NWA 4215 and Dhofar 700, are best explained by their crystallization in slowly cooled isolated magma chambers lasting over at least ~20 Myr.  相似文献   

11.
Abstract— If Vesta is the parent body of the howardite, eucrite, and diogenite (HED) meteorites, then geo-chemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of Vesta to (1) determine the size of a possible metal core in Vesta and (2) model the igneous differentiation and internal structure of Vesta. The density of Vesta and petrologic models for HED meteorites together suggest that the amount of metal in the parent body is <25 mass%, with a best estimate of ～5%, assuming no porosity. For a porosity of up to 5% in the silicate fraction of the asteroid, the permissible metal content is <30%. These results suggest that any metal core in the HED parent body and Vesta is not unusually large. A variety of geochemical and other data for HED meteorites are consistent with the idea that they originated in a magma ocean. It appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most noncumulate eucrites (excepting such eucrites as Bouvante and Statinem) formed by subsequent crystallization of the residual melts. Modelling results suggest that the HED parent body is enriched in rare earth elements by a factor of ～2.5–3.5 relative to CI-chondrites and that it has approximately chondritic Mg/Si and Al/Sc ratios. Stokes settling calculations for a Vesta-wide, nonturbulent magma ocean suggest that early-crystallizing magnesian olivine, orthopyroxene, and pigeonite would have settled relatively quickly, permitting fractional crystallization to occur, but that later-crystallizing phases would have settled (or floated) an order of magnitude more slowly, allowing, instead, a closer approach to equilibrium crystallization for the more evolved (eucritic) melts. This would have inhibited the formation of a plagioclase-flotation crust on Vesta. Plausible models for the interior of Vesta, which are consistent with the data for HED meteorites and Vesta, include a metal core (<130 km radius), an olivine-rich mantle (～65–220 km thick), a lower crustal unit (～12–43 km thick) composed of pyroxenite, from which diogenites were derived, and an upper crustal unit (～23–42 km thick), from which eucrites originated. The present shape of Vesta (with ～60 km difference in the maximum and minimum radius) suggests that all of the crustal materials, and possibly some of the underlying olivine from the mantle, could have been locally excavated or exposed by impact cratering.  相似文献   

12.
Abstract— Nine howardites and two diogenites were recovered from the Pecora Escarpment Icefield (PCA) in 2002. Cosmogenic radionuclide abundances indicate that the samples are paired and that they constituted an approximately 1 m (diameter) meteoroid prior to atmospheric entry. At about 1 m in diameter, the PCA 02 HED group represents one of the largest single pre‐atmospheric pieces of the Vestan surface yet described. Mineral and textural variations were measured in six of the PCA 02 howardites to investigate meter‐scale diversity of the Vestan surface. Mineral compositions span the range of known eucrite and diogenite compositions. Additional non‐diogenitic groups of Mg‐ and Fe‐rich olivine are observed, and are interpreted to have been formed by exogenic contamination and impact melting, respectively. These howardites contain olivine‐rich impact melts that likely formed from dunite‐ and harzburgite‐rich target rocks. Containing the first recognized olivine‐rich HED impact melts, these samples provide meteoritic evidence that olivine‐rich lithologies have been exposed on the surface of Vesta. Finally, we present a new method for mapping distributions of lithologies in howardites using 8 elemental X‐ray maps. Proportions of diogenite and eucrite vary considerably among the PCA 02 howardites, suggesting they originated from a heterogeneous portion of the Vestan surface. While whole sample modes are dominated by diogenite, the finer grain size fractions are consistently more eucritic. This discrepancy has implications for near‐infrared spectral observations of portions of Vesta’s surface that are similar to the PCA 02 howardites, as the finer grained eucritic material will disproportionately dominate the spectra.  相似文献   

13.
Abstract– We investigate the relationship between the petrology and visible–near infrared spectra of the unbrecciated eucrites and synthetic pyroxene–plagioclase mixtures to determine how spectra obtained by the Dawn mission could distinguish between several models that have been suggested for the petrogenesis of Vesta’s crust (e.g., partial melting and magma ocean). Here, we study the spectra of petrologically characterized unbrecciated eucrites to establish spectral observables, which can be used to yield mineral abundances and compositions consistent with petrologic observations. No information about plagioclase could be extracted from the eucrite spectra. In contrast, pyroxene dominates the spectra of the eucrites and absorption band modeling provides a good estimate of the relative proportions of low‐ and high‐Ca pyroxene present. Cr is a compatible element in eucrite pyroxene and is enriched in samples from primitive melts. An absorption at 0.6 μm resulting from Cr3+ in the pyroxene structure can be used to distinguish these primitive eucrites. The spectral differences present among the eucrites may allow Dawn to distinguish between the two main competing models proposed for the petrogenesis of Vesta (magma ocean and partial melting). These models predict different crustal structures and scales of heterogeneity, which can be observed spectrally. The formation of eucrite Allan Hills (ALH) A81001, which is primitive (Cr‐rich) and relatively unmetamorphosed, is hard to explain in the magma ocean model. It could only have been formed as a quench crust. If the magma ocean model is correct, then ALHA81001‐like material should be abundant on the surface of Vesta and the Vestoids.  相似文献   

14.
Abstract– We have done bulk rock compositional analyses (INAA, ICP‐MS) and petrologic study of a suite of diogenite meteorites. Most contain orthopyroxenes with mg#s of 70.6–79.0. Meteorite Hills (MET) 00425 is magnesian (mg# of 83.9). Lewis Cliff (LEW) 88011 contains orthopyroxene grains of varying mg# (76.3–68.6). Queen Alexandra Range (QUE) 93009 (orthopyroxene mg# 70.6) contains coarse‐grained noritic clasts (plagioclase An84.7–88.3), and is rich in incompatible trace elements. It has Eu/Eu* < 1, indicating that cumulate norites do not dominate its trace element inventory. Queen Alexandra Range 93009 may be transitional between diogenites and magnesian cumulate eucrites. Lewis Cliff 88679, a dimict breccia of harzburgite and orthopyroxenite, has anomalously low concentrations of highly incompatible elements (e.g., Nb, La, Ta, U) compared to other diogenites, but is similar to them in less highly incompatible elements (e.g., Y, Zr, Yb, Hf). It is unlikely that this characteristic reflects a low proportion of a trapped melt component. The highly incompatible elements were likely mobilized after impact mixing of the two parent lithologies. Graves Nunataks 98108 shows an extreme range in Eu/Eu* attributable to the heterogeneous distribution of plagioclase; one sample has the lowest Eu/Eu* among diogenites. We find no compelling evidence to support the hypothesis that diogenite parent magmas were contaminated by partial melts of the eucritic crust. We posit that subsolidus equilibration between orthopyroxene and minor/trace phases (including phosphates) resulted in preferential redistribution of Eu2+ relative to Eu3+ and other rare earth elements, and results in anomalously low Eu/Eu* in samples leached in acids that dissolve phosphates.  相似文献   

15.
Abstract— The major element, trace element, and isotopic compositional ranges of the martian basaltic meteorite source regions have been modeled assuming that planetary differentiation resulted from crystallization of a magma ocean. The models are based on low to high pressure phase relationships estimated from experimental runs and estimates of the composition of silicate Mars from the literature. These models attempt to constrain the mechanisms by which the martian meteorites obtained their superchondritic CaO/Al2O3 ratios and their source regions obtained their parent/daughter (87Rb/86Sr, 147Sm/144Nd, and 176Lu/177Hf) ratios calculated from the initial Sr, Nd, and Hf isotopic compositions of the meteorites. High pressure experiments suggest that majoritic garnet is the liquidus phase for Mars relevant compositions at or above 12 GPa. Early crystallization of this phase from a martian magma ocean yields a liquid characterized by an elevated CaO/Al2O3 ratio and a high Mg#. Olivine‐pyroxene‐garnet‐dominated cumulates that crystallize subsequently will also be characterized by superchondritic CaO/Al2O3 ratios. Melting of these cumulates yields liquids with major element compositions that are similar to calculated parental melts of the martian meteorites. Furthermore, crystallization models demonstrate that some of these cumulates have parent/daughter ratios that are similar to those calculated for the most incompatible‐element‐depleted source region (i.e., that of the meteorite Queen Alexandra [QUE] 94201). The incompatible‐element abundances of the most depleted (QUE 94201‐like) source region have also been calculated and provide an estimate of the composition of depleted martian mantle. The incompatible‐element pattern of depleted martian mantle calculated here is very similar to the pattern estimated for depleted Earth's mantle. Melting the depleted martian mantle composition reproduces the abundances of many incompatible elements in the parental melt of QUE 94201 (e.g., Ba, Th, K, P, Hf, Zr, and heavy rare earth elements) fairly well but does not reproduce the abundances of Rb, U, Ta and light rare earth elements. The source regions for meteorites such as Shergotty are successfully modeled as mixtures of depleted martian mantle and a late stage liquid trapped in the magma ocean cumulate pile. Melting of this hybrid source yields liquids with major element abundances and incompatible‐element patterns that are very similar to the Shergotty bulk rock.  相似文献   

16.
Abstract— The 1.2 μm band in near‐infrared spectra of pyroxenes results from Fe2+in the M1 crystallographic site. The distribution of Fe and Mg between the M1 and M2 sites is in part a function of the cooling rate and thermal history of a pyroxene. Combining near‐infrared and Mössbauer spectra for a series of compositionally controlled synthetic Mg, Fe, Ca pyroxenes, we quantify the strength of the 1.2 μm band as a function of Fe2+in the M1 site. Near‐infrared spectra are deconvolved into component absorptions that can be assigned to the M1 and M2 sites using the modified Gaussian model. The relative strength of the 1.2 μm band is shown to be directly related to the amount of Fe2+in the M1 site measured by Mössbauer spectroscopy. The strength of the 1.2 μm band relative to the combined strengths of the 1.2 and 2 μm bands, or the M1 intensity ratio, is calculated for 51 howardite, eucrite, and diogenite (HED) meteorites. Diogenites and cumulate eucrites exhibit the lowest M1 intensity ratios, consistent with their formation as slowly cooled cumulates. Basaltic eucrites exhibit a large range of M1 intensity ratios, all of which are consistently higher than the diogenites and cumulate eucrites. This example illustrates how the M1 intensity ratio can be a used as a tool for characterizing the cooling history of remotely detected pyroxene‐dominated rocks.  相似文献   

17.
Abstract— We have analyzed the modal abundances of 23 of the known 24 diogenites in 31 thin sections using an energy dispersive spectrometer (EDS) and automated phase distribution analysis software. Orthopyroxene is predictably the most abundant phase, ranging from 27.7 vol% to 99.8 vol% in these samples. The grand average mode of all the analyzed diogenites includes the “olivine diogenites” but not ALH 85015, a probable howardite, and ALHA81208, a sample with an abundant silica phase. The grand average of these 21 diogenites is: orthopyroxene 92.2 vol%, olivine 4.2 vol%, clinopyroxene 1.2 vol%, chromite 0.9 vol%, plagioclase 0.4 vol%, FeNi metal 0.1 vol%, troilite 0.6 vol%, and silica phase 0.4 vol%. Plagioclase feldspar is extremely depleted in all samples, with modal abundance from none detected to 4.6 vol% in range. Such a low volume of plagioclase may indicate that the diogenite parental melts originated in a source region depleted in Al (Warren, 1985; Stolper, 1975), which is consistent with crystallization from a melt derived from material that had previously experienced extraction of a eucrite-type melt.  相似文献   

18.
Abstract— This study explores the controls of oxygen fugacity and temperature on the solubilities of Fe, Ni, Co, Mo, and W in natural eucritic liquids to better constrain the formation of eucritic melts. The solubilities of all five elements in molten silicate in equilibrium with FeNiCo‐, FeMo‐, and FeW‐ alloys increase with increasingly oxidizing conditions and decrease with decreasing temperatures. In applying these data to formation scenarios of the eucrite parent body, we find that the siderophile element abundances in eucrites (meteoritic basalts) cannot be explained by a single‐step partialmelting process from a chondritic, metal‐containing source. The Ni content of the partial melt is too high, and the W and Mo contents are too low compared to the abundances in eucritic meteorites. But Fe, Ni, and Co concentrations in eucrites can be modeled by metal‐silicate equilibrium during more or less complete melting of the eucrite parent body with subsequent fractional crystallization of olivine and orthopyroxene. However, the computed values of Mo are still too low and those of W too high when compared with Mo and W abundances in eucritic meteorites. One possibility is that the Mo and W partition coefficients strongly depend on pressure, although the howardite‐eucrite‐diogenite (HED) parent body only had a minimal pressure gradient (maximum interior pressure = 0.1 GPa). Alternatively, sulfides may have played some role in establishing Mo abundances.  相似文献   

19.
We present global lithological maps of the Vestan surface based on Dawn mission's Visible InfraRed (VIR) Spectrometer acquisitions with a spatial sampling of 200 m. The maps confirm the results obtained with the data set acquired by VIR with a spatial sampling of 700 m, that the reflectance spectra of Vesta's surface are dominated by pyroxene absorptions that can be interpreted within the context of the distribution of howardites, eucrites, and diogenites (HEDs). The maps also partially agree with the ground and Hubble Space Telescope observations: they confirm the background surface being an assemblage of howardite or polymict eucrite, as well as the location of a diogenitic‐rich spot; however, there is no evidence of extended olivine‐rich regions in the equatorial latitudes. Diogenite is revealed on the Rheasilvia basin floor, indicating that material of the lower crust/mantle was exposed. VIR also detected diogenites along the scarp of Matronalia Rupes, and the rims of Severina and a nearby, unnamed crater, and as ejecta of Antonia crater. The diogenite distribution is fully consistent with petrological constraints; although the mapped distribution does not provide unambiguous constraints, it favors the hypothesis of a magma ocean.  相似文献   

20.
Identifying and mapping olivine on asteroid 4 Vesta are important components to understanding differentiation on that body, which is one of the objectives of the Dawn mission. Harzburgitic diogenites are the main olivine‐bearing lithology in the howardite‐eucrite‐diogenite (HED) meteorites, a group of samples thought to originate from Vesta. Here, we examine all the Antarctic harzburgites and estimate that, on scales resolvable by Dawn, olivine abundances in putative harzburgite exposures on the surface of Vesta are likely at best in the 10–30% range, but probably lower due to impact mixing. We examine the visible/near‐infrared spectra of two harzburgitic diogenites representative of the 10–30% olivine range and demonstrate that they are spectrally indistinguishable from orthopyroxenitic diogenites, the dominant diogenitic lithology in the HED group. This suggests that the visible/near‐infrared spectrometer onboard Dawn (VIR) will be unable to resolve harzburgites from orthopyroxenites on the surface of Vesta, which may explain the current lack of identification of harzburgitic diogenite on Vesta.  相似文献   

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