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1.
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   

2.
Melting of Martian mantle, formation, and evolution of primary magma from the depleted mantle were previously modeled from experimental petrology and geochemical studies of Martian meteorites. Based on in situ major and trace element study of a range of olivine‐hosted melt inclusions in various stages of crystallization of Tissint, a depleted olivine–phyric shergottite, we further constrain different stages of depletion and enrichment in the depleted mantle source of the shergottite suite. Two types of melt inclusions were petrographically recognized. Type I melt inclusions occur in the megacrystic olivine core (Fo76‐70), while type II melt inclusions are hosted by the outer mantle of the olivine (Fo66‐55). REE‐plot indicates type I melt inclusions, which are unique because they represent the most depleted trace element data from the parent magmas of all the depleted shergottites, are an order of magnitude depleted compared to the type II melt inclusions. The absolute REE content of type II displays parallel trend but somewhat lower value than the Tissint whole‐rock. Model calculations indicate two‐stage mantle melting events followed by enrichment through mixing with a hypothetical residual melt from solidifying magma ocean. This resulted in ~10 times enrichment of incompatible trace elements from parent magma stage to the remaining melt after 45% crystallization, simulating the whole‐rock of Tissint. We rule out any assimilation due to crustal recycling into the upper mantle, as proposed by a recent study. Rather, we propose the presence of Al, Ca, Na, P, and REE‐rich layer at the shallower upper mantle above the depleted mantle source region during the geologic evolution of Mars.  相似文献   

3.
The crustal dichotomy and the Tharsis rise are the most prominent topographic features on Mars. The dichotomy is largely an expression of different crustal thicknesses in the northern and southern hemispheres, while Tharsis is centered near the equator at the dichotomy boundary. However, the cause for the orientation of the dichotomy and the equatorial location of Tharsis remains poorly understood. Here we show that the crustal thickness variations associated with the dichotomy may have driven true polar wander, establishing the north-south orientation of the dichotomy very early in martian history. Such a reorientation that placed the dichotomy boundary near the equator would also have constrained the Tharsis region on the dichotomy boundary to have originated near the equator. We present a scenario for the early generation and subsequent reorientation of the hemispheric dichotomy, although the reorientation is independent of the formation mechanism. Our results also have implications for the sharply different remanent magnetizations between the two hemispheres.  相似文献   

4.
A Melt-through Model for Chaos Formation on Europa   总被引：1，自引：0，他引：1  
The character of chaotic terrain on Europa is consistent with its formation by the melting of a thin conducting ice shell from below. Tidal dissipation can provide adequate energy for such a process. For example, only a few percent of Europa's predicted tidal heat, spread over a region 200 km in diameter, can lead to large melt regions within a few tens of thousands of years. Stronger, more localized concentrations result in melt-through in significantly shorter times (i.e., a few hundred years). The time scale for melt-through is shorter than the time scale for the solid-state viscous inflow of ice by several orders of magnitude. In general, modest concentrations of tidal heat can melt ice away faster than viscous inflow, leading to melt-through. A mechanism to transmit these heat concentrations through the ocean is required for this model. Such heat transport could be the result of convective plumes in the ocean driven by seafloor volcanism or by the destabilization of a stratified ocean.  相似文献   

5.
Petrological analysis of the Martian meteorites suggests that rheologically significant amounts of water are present in the Martian mantle. A bulk mantle water content of at least a few tens of ppm is thus expected to be present despite the potentially efficient degassing during accretion, magma ocean solidification, and subsequent volcanism. We examine the dynamical consequences of different thermochemical evolution scenarios testing whether they can lead to the formation and preservation of mantle reservoirs, and compare model predictions with available data. First, the simplest scenario of a homogenous mantle that emerges when ignoring density changes caused by the extraction of partial melt is found to be inconsistent with the isotopic evidence for distinct reservoirs provided by the analysis of the Martian meteorites. In a second scenario, reservoirs can form as a result of partial melting that induces a density change in the depleted mantle with respect to its primordial composition. However, efficient mantle mixing prevents these reservoirs from being preserved until present unless they are located in the stagnant lid. Finally, reservoirs could be formed during fractional crystallization of a magma ocean. In this case, however, the mantle would likely end up being stably stratified as a result of the global overturn expected to accompany the fractional crystallization. Depending on the assumed density contrast, little secondary crust would be produced and the lithosphere would be extremely cool and dry, in contrast to observations. In summary, it is very challenging to obtain a self‐consistent evolution scenario that satisfies all available constraints.  相似文献   

6.
We report results of systematic experimental simulation of fractional crystallization of a lunar magma ocean (LMO) with the Lunar Primitive Upper Mantle bulk composition. These results complement prior work that simulated equilibrium crystallization. In contrast to previous numerical models for investigating magma ocean solidification processes and implications, our combined program simulates these processes directly using petrologic experimentation. Our experiments mimic LMO crystallization that is fractional throughout the process, rather than switching from initially equilibrium to fractional crystallization partway through. To do this, we adopted an iterative approach in which the starting material for each run is synthesized using the composition of the melt phase from the prior run. We compare our results to those from long-standing numerical models of LMO crystallization and show that although some features of those models are broadly reproduced, there are key differences in liquid lines of descent and the cumulate lithologies generated. Our results can be used to estimate the possible thickness of a primordial lunar crust formed from flotation of plagioclase during magma ocean solidification. Our estimate is greater than that from the recent Gravity Recovery and Interior Laboratory (GRAIL) mission, but consistent with the criteria on which the starting bulk composition was originally calculated. It assumes perfectly efficient separation of all plagioclase formed from the crystallizing magma ocean, which is likely not the case. We also demonstrate that a non-chondritic bulk composition, with respect to trace elements, is not required in order to generate a KREEP (potassium, rare earth elements, and phosphorus) signature from magma ocean crystallization.  相似文献   

7.
The early thermal evolution of Moon has been numerically simulated to understand the magnitude of the impact-induced heating and the initially stored thermal energy of the accreting moonlets. The main objective of the present study was to understand the nature of processes leading to core–mantle differentiation and the production and cooling of the initial convective magma ocean. The accretion of Moon was commenced over a time scale of 100 yr after the giant impact event around 30–100 million years in the early solar system. We studied the dependence of the planetary processes on the impact scenarios, the initial average temperature of the accreting moonlets, and the size of the protomoon that accreted rapidly beyond the Roche limit within the initial 1 yr after the giant impact. The simulations indicate that the accreting moonlets should have a minimum initial averaged temperature around 1600 K. The impacts would provide additional thermal energy. The initial thermal state of the moonlets depends upon the environment prevailing within the Roche limit that experienced episodes of extensive vaporization and recondensation of silicates. The initial convective magma ocean of depth more than 1000 km is produced in the majority of simulations along with the global core–mantle differentiation in case the melt percolation of the molten metal through porous flow from bulk silicates was not the major mode of core–mantle differentiation. The possibility of shallow magma oceans cannot be ruled out in the presence of the porous flow. Our simulations indicate the core–mantle differentiation within the initial 102 to 103 yr of the Moon accretion. The majority of the convective magma ocean cooled down for crystallization within the initial 103 to 104 yr.  相似文献   

8.
Tidally forced viscous heating in a partially molten Io   总被引：1，自引：0，他引：1  
M.N. Ross  G. Schubert 《Icarus》1985,64(3):391-400
We investigate tidal dissipative heating in two different models of Io. The partially molten asthenosphere model consists of a rigid inner core and a thin (less than 40 km thick) partially molten “decoupling” layer (asthenosphere) surrounded by an elastic lithosphere. In the partially molten interior model the interior beneath the lithosphere is partially molten throughout. The partially molten region in each model assumed to possess negligible shear strength and to be characterized by a Newtonian viscosity. Tidal deformation and dissipation in the core of the thin asthenosphere model are assumed negligible. Fluid in the viscous layers is forced to circulate by the tidal distortion of the outer shell, modeled here as a sinusoidal variation with time of the distortion amplitude. As a result, heat is generated in the fluid by viscous dissipation. There are two heating mechanisms in our models: “elastic” dissipation in the lithosphere ∞ 1/Q and viscous dissipation in the partially molten region. Numerical calculatons are carried out for a 90-km-thick lithosphere with Q = 100. This thickness maximizes dissipation in a decoupled lithosphere; other reasonable values of lithosphere thickness do not alter our conclusions. Under the constraint that total dissipation equals the observed radiated heat loss we derived the iscosity of the partially molten region in each model. We a posteriori evaluate the assumption that the lithosphere is decoupled from the interior by calculating the distortion of an elastic shell due to the viscous stresses on the lower surface of the outr shell. If the interior viscosity is such that the total dissipation is equal to the observed heat flux from Io, viscous stresses produce negligible distortion of a 90-km-thick shell. This validates the assumption of a decoupled shell. The derived viscosity for both models is characteristic of a partially molten rock. In the thin asthenosphere model the derived viscosity is so low that a very high degree of partial melt is necessary, about 40% crystal fraction in a 400-km-thick asthenosphere and about 0% in a 1-km-thick asthenosphere. In the partially molten interior model the derived viscosity corresponds to a magma with about 60% crystals. Consideration of convective efficiencies demonstrates the plausibility of a stable thermal steady state for both models. A significant portion (75% for Q = 100) of Io's tidal heating can be the result of viscous dissipation in a partially molten region that decouples the outer shell from the interior. The partially molten layer can be considered a “global magma ocean”.  相似文献   

9.
This study uses experimentally determined plagioclase‐melt D values to estimate the trace element concentrations of Sr, Hf, Ga, W, Mo, Ru, Pd, Au, Ni, and Co in a crystallizing lunar magma ocean at the point of plagioclase flotation. Similarly, experimentally determined metal‐silicate partition experiments combined with a composition model for the Moon are used to constrain the concentrations of W, Mo, Ru, Pd, Au, Ni, and Co in the lunar magma ocean at the time of core formation. The metal‐silicate derived lunar mantle estimates are generally consistent with previous estimates for the concentration of these elements in the lunar mantle. Plagioclase‐melt derived concentrations for Sr, Ga, Ru, Pd, Au, Ni, and Co are also consistent with prior estimates. Estimates for Hf, W, and Mo, however, are higher. These elements may be concentrated in the residual liquid during fractional crystallization due to their incompatibility. Alternatively, the apparent enrichment could reflect the inappropriate use of bulk anorthosite data, rather than data for plagioclase separates.  相似文献   

10.
The origin of the ancient martian crustal dichotomy and the massive magmatic province of Tharsis remains an open problem. Here, we explore numerically a hypothesis for the origin of these two features involving both exogenic and endogenic processes. We propose a giant impact event during the late stage of planetary formation as the source of the southern highland crust. In a second stage, the extraction of excess heat by vigorous mantle convection on the impacted hemisphere leads to massive magmatism, forming a distinct Tharsis-like volcanic region. By coupling short-term and long-term numerical simulations, we are able to investigate both the early formation as well as the 4.5 Gyr evolution of the martian crust. We demonstrate numerically that this exogenic-endogenic hypothesis is in agreement with observational data from Mars.  相似文献   

11.
Abstract— Crystallization of a magma ocean on a large terrestrial planet that is significantly melted by the energy of accretion may lead to an unstable cumulate density stratification, which may overturn to a stable configuration. Overturn of the initially unstable stratification may produce an early basaltic crust and differentiated mantle reservoirs. Such a stable compositional stratification can have important implications for the planet's subsequent evolution by delaying or suppressing thermal convection and by influencing the distribution of radiogenic heat sources. We use simple models for fractional crystallization of a martian magma ocean, and calculate the densities of the resulting cumulates. While the simple models presented do not include all relevant physical processes, they are able to describe to first order a number of aspects of martian evolution. The models describe the creation of magma source regions that differentiated early in the history of Mars, and present the possibility of an early, brief magnetic field initiated by cold overturned cumulates falling to the coremantle boundary. In a model that includes the density inversion at about 7.5 GPa, where olivine and pyroxene float in the remaining magma ocean liquids while garnet sinks, cumulate overturn sequesters alumina in the deep martian interior. The ages and compositions of source regions are consistent with SNC meteorite data.  相似文献   

12.
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   

13.
The identification of hydrogen in a range of lunar samples and the similarity of its abundance and isotopic composition with terrestrial values suggest that water could have been present in the Moon since its formation. To quantify the effect of water on early lunar differentiation, we present new analyses of a high‐pressure, high‐temperature experimental study designed to model the mineralogical and geochemical evolution of the solidification material equivalent to 700 km deep lunar magma oceans first reported in Lin et al. (2017a). We also performed additional experiments to better quantify water contents in the run products. Water contents in the melt phases in hydrous run products spanning a range of crystallization steps were quantified directly using a secondary ion mass spectrometry (SIMS). Results suggest that a significant but constant proportion (68 ± 5%) of the hydrogen originally added to the experiments was lost from the starting material independent of run conditions and run duration. The volume of plagioclase formed during our crystallization experiments can be combined with the measured water contents and the observed crustal thickness on the Moon to provide an updated lunar interior hygrometer. Our data suggest that at least 45–354 ppm H2O equivalent was present in the Moon at the time of crust formation. These estimates confirm the inference of Lin et al. (2017a) that the Moon was wet during its magma ocean stage, with corrected absolute water contents now comparable to estimates derived from the water content in a range of lunar samples.  相似文献   

14.
Stress models for Tharsis formation, Mars   总被引：1，自引：0，他引：1  
A critical survey is presented of most stress models proposed for the formation of the tectonic structures in the Tharsis volcano-tectonic province on Mars and provides new constraints for further models. First papers, in the 1970s, attempted to relate the Tharsis formation to asthenospheric movements and lithosphere loading by magma bodies. These processes were then quantified in terms of stress trajectory and magnitude models in elastic lithosphere (e.g. Banerdt et al., J. Geophys. Res. 87(B12), 9723–9733, 1982). Stresses generated by dynamic lithosphere uplift were rapidly dismissed because of the poor agreement between the stress trajectories provided by the elastic models and the structural observations. The preferred stress models involved lithosphere loading, inducing isostatic compensation, and then lithosphere flexure. Some incomsistency with structural interpretation of Viking imagery has been found. In the early 1990s, an attempt to solve this problem resulted in a model involving the existence of a Tharsis-centred brittle crustal cap, deteched from the strong mantle by a weak crustal layer (Tanaka et al., J. Geophys. Res. 96(E1), 15617–15633, 1991). Such a configuration should produce loading stresses akin to those predicted by some combination of the two loading modes. This model has not been quantified yet, however it is expected to reconcile stress trajectories and most structural patterns. Nevertheless, some inconsistencies with observed structures are also expected to remain. Parallel to this approach focused on loading mechanisms, the idea that volcanism and tectonic structures could be related to mantle circulation began to be considered again through numerical convection experiments, whose results have however not been clearly correlated with surface observations. Structural clues to early Tharsis dynamic uplift are reported. These structures have little to do with those predicted by elastic stress modelling of dynamic lithosphere uplift. They denote the existence of unsteady stress trajectories responsible for surface deformations that cannot be readily predicted by elastic models. These structures illustrate that improving current stress models for Tharsis formation shall come from deeper consideration of rock failure criterion and load growth in the lithosphere (e.g. Schultz and Zuber, J. Geophys. Res. 99(E7), 14691–14702, 1994). Improvements should also arise from better understanding rheological layering in the lithosphere and its evolution with time, and from consideration of stress associated to magma emplacement in the crust, which may have produced many tectonic structures before loading stress resulting from magma freezing became significant (Mège and Masson, Planet. Space Sci. 44, 1499–1546, 1996a).  相似文献   

15.
Melt inclusions in igneous minerals can provide constraints on magma compositions, especially for planetary samples where mass is severely limited. Small inclusions (<15 μm diameter) are more abundant than large ones, but have been used little from concern that they did not entrap average magma, but are rich in melt of a diffusional layer against the host mineral. We compared bulk compositions and calculated original compositions of small and large melt inclusions in the Martian basalt meteorite (shergottite) Tissint. Small and large melt inclusions are consistent with the same line of igneous differentiation, have the same abundance ratios for incompatible elements (P, Ti, Al, K, Na), and are consistent with derivation from the bulk composition of Tissint (inferred to represent its parent melt composition). For Tissint, then, small melt inclusions show no evidence of entrapping diffusional boundary layers, and appear to have entrapped bulk magma. Thus, its small inclusions can be as useful as larger ones; this may be so for other planetary samples, and thus provides an additional tool for investigating planetary magmas.  相似文献   

16.
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   

17.
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   

18.
The crustal dichotomy of Mars describes the topographic division between the young plains in the northern hemisphere and the old terrain in the southern hemisphere. The highland-lowland boundary separates the younger plains from the older, high-standing terrain and consists of three geologically-distinct regions: the Tharsis Province, the chaotic terrain, and the fretted terrain (which includes gradational boundary types)-all are characterised by tensional tectonics. This paper presents new geological evidence that shows the topographic division at the fretted terrain formed in the late Noachian-early Hesperian time period: the same time period in which the Tharsis Province and chaotic terrain formed, and fracturing of a southern-hemisphere-type surface beneath the northern plains occurred. These are inherent features of the crustal dichotomy, indicating it must have also formed during the late Noachian-early Hesperian time period. An analogy is made between the northern lowlands and sedimentary basins on Earth: both are basin like and are surrounded by provinces that have been subjected to pronounced tensional tectonics. This paper uses the White and McKenzie model (1989a) to propose that a lithospheric-stretching event on Mars, in the late Noachian-early Hesperian time period, produced the crustal dichotomy; the Tharsis Province formed by uplift (over a sub-surface hotspot) and gave rise to lithospheric stretching, and the northern lowlands formed by subsidence (over normal asthenospheric temperatures). Detachment faults, operating from the Tharsis Province and around northern lowlands, allowed structural equilibrium and large lithospheric extensions to be attained during this period: they also defined the geometry of the lowlands. The proposal is supported with calculations used to estimate the amount of subsidence that can be achieved in this way.  相似文献   

19.
The global martian volcanic evolutionary history   总被引：1，自引：0，他引：1  
Viking mission image data revealed the total spatial extent of preserved volcanic surface on Mars. One of the dominating surface expressions is Olympus Mons and the surrounding volcanic province Tharsis. Earlier studies of the global volcanic sequence of events based on stratigraphic relationships and crater count statistics were limited to the image resolution of the Viking orbiter camera. Here, a global investigation based on high-resolution image data gathered by the High-Resolution Stereo Camera (HRSC) during the first years of Mars Express orbiting around Mars is presented. Additionally, Mars Orbiter Camera (MOC) and Thermal Emission Imaging System (THEMIS) images were used for more detailed and complementary information. The results reveal global volcanism during the Noachian period (>3.7 Ga) followed by more focused vent volcanism in three (Tharsis, Elysium, and Circum-Hellas) and later two (Tharsis and Elysium) volcanic provinces. Finally, the volcanic activity became localized to the Tharsis region (about 1.6 Ga ago), where volcanism was active until very recently (200-100 Ma). These age results were expected from radiometric dating of martian meteorites but now verified for extended geological units, mainly found in the Tharsis Montes surroundings, showing prolonged volcanism for more than 3.5 billions years. The volcanic activity on Mars appears episodic, but decaying in intensity and localizing in space. The spatial and temporal extent of martian volcanism based on crater count statistics now provides a much better database for modelling the thermodynamic evolution of Mars.  相似文献   

20.
Abstract— We report on major and trace element analyses of 17 eucrites, including three cumulate eucrites (Binda, Moore County, and Serra de Magé), determined by, respectively, inductively‐coupled plasma atomic emission spectrometry and inductively‐coupled plasma mass spectrometry. The results obtained for Binda and Moore County are consistent with the model of Treiman (1997) for the formation of cumulate eucrites, which holds that these meteorites were produced from a eucritic melt. Our sample of Serra de Magé contains unusually large amounts of pyroxene and probably an accessory phase rich in heavy rare earth elements and is therefore not representative of this eucrite as known from literature data. Our results for the noncumulate eucrites Bereba, Bouvante, Cachari, Caldera, Camel Donga, Ibitira, Jonzac, Juvinas, Lakangaon, Millbillillie, Padvarninkai, Pasamonte, Sioux County, and Stannern are in good agreement with literature data. The observed decoupling between major and trace elements for noncumulate eucrites can be explained by in situ crystallization during the differentiation of an asteroidal magma ocean. This model can further account for both the Nuevo Laredo and the Stannern trends but has as a consequence that none of the analyzed eucrites represents a primary melt.  相似文献   

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