An investigation of the behavior of Cu and Cr during iron meteorite crystallization

Abstract— The measured Cu and Cr contents in magmatic iron meteorites appear to contradict the behavior predicted by experimental fractional crystallization studies currently available. To investigate the origin of Cu and Cr concentrations observed in these meteorites, a thorough set of solid metal/liquid metal experiments were conducted in the Fe‐Ni‐S system. In addition to Cu and Cr, partitioning values were also determined for As, Au, Bi, Co, Mo, Ni, Pb, Rh, Ru, Sb, Sn, V, and Zn from the experiments. Experimental results for Cu and Cr showed similar chalcophile partitioning behavior, whereas these elements have differently sloped trends within magmatic iron meteorite groups. Thus, fractional crystallization alone cannot control both the Cu and Cr concentrations in these iron meteorite groups. A simple fractional crystallization model based on our experimental Cu partitioning results was able to match the Cu versus Au trend observed in the S‐poor IVB iron meteorite group but not the decreasing Cu versus Au trends in the IIAB and IIIAB groups or the unique S‐shaped Cu versus Au trend in the IVA group. However, the crystallization model calculations were found to be very sensitive to the specific choice for the mathematical expression of D(Cu), suggesting that any future refinement of the parameterization of D(Cu) should include a reassessment of the Cu fractional crystallization trends. The Cr versus Au trends in magmatic iron meteorite groups are steeper than those of Cu and not explained by fractional crystallization. Other influences, such as the removal of chromite from the crystallizing system or sampling biases during iron meteorite compositional analyses, are likely responsible for the Cr trends in magmatic iron meteorite groups.     

Crystallization of magmatic iron meteorites: The effects of phosphorus and liquid immiscibility

Abstract— Magmatic iron meteorites are commonly thought to have formed by fractional crystallization of the metallic cores of asteroid‐sized bodies. As fractional crystallization proceeds, light elements such as P and S become enriched in the molten portion of the core. The light element content of the metallic liquid influences the partitioning behavior of trace elements and may cause liquid immiscibility to occur. The elemental trends observed in magmatic iron meteorites may have been affected by both of these processes. We have examined experimentally the effect of P on the solid‐metal‐liquid‐metal partitioning behavior of Ag and Pd, Re and Os, two element pairs used to date iron meteorite processes. Phosphorus has no effect on the partition coefficient of either Ag or Pd, which are incompatible and identical within experimental error. Compatible Re and Os also have identical partitioning behavior, within experimental error, and show increasing compatibility in the solid metal with increasing P content of the metallic liquid. Including the effects of both S and P on the partitioning behavior of Re and Os, simple fractional crystallization calculations can reproduce the large variation of Re and Os concentrations observed in four magmatic iron meteorite groups but have difficulty matching the later crystallizing portions of the trends. We have also conducted experiments with three phases—solid metal and two immiscible metallic liquids—to determine the location of the liquid immiscibility field near conditions thought to be relevant to magmatic iron meteorites. Our results show a significantly smaller liquid immiscibility field as compared to the previously published Fe‐P‐S phase diagram. Our revised phase diagram suggests that liquid immiscibility was encountered during the crystallization of asteroidal cores, but much later during the crystallization process than predicted by the previously published diagram.     

Comment on “Fractional crystallization of iron meteorites: Constant versus changing partition coefficients” by J. H. Jones

Abstract— Jones (1994) demonstrated that fractional crystallization of IIIAB iron meteorites may result in a log Au vs. log Ni plot of constant slope even though k_Au and k_Ni change. Jones' example is a special case, however, and does not necessarily describe behavior by other elements or in other metallic magmas.     

The phosphates of IIIAB iron meteorites

Abstract— Thirteen phosphate minerals are found in IIIAB iron meteorites. Four of these (sarcopside, graftonite, johnsomervilleite, and galileiite) constitute the majority of occurrences. The IIIB iron meteorites are confined to occurrences of only these four phosphates. The IIIA iron meteorites may contain one or more of these four phases; they may also contain other rarer phosphates, and silica (in two instances) and a silicate rock (in one instance). Thus, the IIIA lithophile chemistry is more varied than that of the IIIB meteorites. Based on petrographic relations, sarcopside appears to be the first phosphate to form. Graftonite is probably formed by recrystallization of sarcopside. Johnsomervilleite and galileiite exsolved as enclaves in sarcopside or graftonite at lower temperatures, although some of these also nucleated as separate crystals. The IIIAB phosphates are carriers of a group of incompatible lithophile elements: Fe, Mn, Na, Ca, and K, and, rarely, Mg as well as Pb. These elements (and O) were concentrated in a residual, S-rich liquid during igneous fractional crystallization of the IIIAB core mass. The phosphates formed by oxidation of P as the core solidified and excluded O, which increased its partial pressure in the residual liquid. The trace siderophile trends in bulk IIIAB metal are paralleled by a mineralogical trend of the phosphate minerals that formed. For IIIAB meteorites with low-Ir contents in the metal, the phosphates are mainly Fe-Mn phases; at intermediate Ir values, more Na-bearing phosphates appear; at the highest Ir values, the rarer Na-, K-, Mg-, Cr-, and Pb-bearing phosphates appear. The absence of significant amounts of Mg, Si, Al, and Ti suggest depletion of these elements in the core by the overlying mantle.     

Formation of the Treysa quintet and the main‐group pallasites by impact‐generated processes in the IIIAB asteroid

Treysa and Delegate have compositions closely similar to those of IIIAB irons but plot above the IIIAB field on Ir‐Au diagrams; for this reason they are designated anomalous members of IIIAB. All refractory siderophiles share this anomaly. Wasson ( 1999 ) interpreted the large spread on IIIAB Ir‐Au diagrams to result from melt‐trapping and generated solid and liquid fractional crystallization tracks; almost all IIIAB irons fall between the tracks. In contrast, Treysa, Delegate, and three other irons (the Treysa quintet) plot beyond the liquid track. Ideal fractional crystallization cannot account for compositions that plot outside the region between the tracks. Possible explanations for the anomalous compositions of the Treysa quintet are that (1) these meteorites did not form in the IIIAB magma or (2) they formed by the mixing of early crystallized solids with a late liquid. The weight of the evidence including cosmic‐ray ages favor the second explanation. Although this explanation can account for positions plotting above the liquid track, it requires special circumstances. The infalling blocks must be assimilated and the resulting melt must crystallize quickly into pockets small enough (<1 m) to allow igneous gradients to be leveled by subsequent diffusion. The Treysa quintet shares the region beyond the liquid track with most main‐group pallasites (PMG), which may have also originated in the IIIAB body. It appears that Treysa, its relatives, and the PMG were formed in one or more impact events that mixed olivine and solid metal formed near the core‐mantle boundary with nearby magma. It is then necessary to cool the melt rapidly; the best way to achieve rapid cooling is by heat exchange with cooler solids. That the Treysa quintet and the PMG can be explained by the same processes operating on late IIIAB magma supports the conclusion that PMG formed on the IIIAB parent asteroid.     

Effects of liquid immiscibility on trace element fractionation in magmatic iron meteorites: A case study of group IIIAB

Abstract— Magmatic iron meteorites are generally agreed to represent metal that crystallized in asteroidal cores from a large pool of liquid. Estimates suggest that the metallic liquid contained significant amounts of S and P, both of which are incompatible and exert a strong effect on trace element partitioning. In tandem, S and P are also prone to cause immiscibility between sulfide liquid and P-rich metal liquid. The liquid immiscibility field occupies ～70% of the portion of the Fe-Ni-S-P system in which Fe is the first phase to crystallize. In spite of this, previous fractional crystallization models have taken into account only one liquid phase and have encountered significant discrepancies between the meteorite data and model values for the key elements Ni, Ir, Ga, Ge and Au at even moderate degrees of fractionation. For the first time, a model for trace element partitioning between immiscible liquids in the Fe-Ni-S-P system is presented in order to assess the effects on fractionation in magmatic iron meteorite groups. The onset of liquid immiscibility causes a significant change in the enrichment patterns of S and P in both liquids; so elements with contrasting partitioning behavior will show trends deviating clearly from one-liquid trends. A trend recorded in the solid metal will either be a smooth curve as long as equilibrium is maintained between the two liquids or the trend may diverge into a field limited by two extreme curves depending on the degree of disequilibrium. Bulk initial liquids for most magmatic groups have S/P (wt%) ratios well below 25. In these cases and due to the constitution of the Fe-Ni-S-P system, most of the metal will crystallize from the rapidly decreasing volume of metal liquid and only a subordinate amount from the sulfide liquid. Because of the strong extraction of P into the metal liquid, P will have a much larger influence on trace element partitioning than a low initial P content might suggest. My model calculations suggest that liquid immiscibility played a significant role during the solidification of the IIIAB parent body's core. The two-liquid model reproduces the IIIAB trends more closely than previous one-liquid models and can account for: (a) the general widening of the IIIAB trend with increasing Ni and decreasing Ir contents, (b) the occurrence of high-Ni members that are not strongly depleted in Ir, Ga and Ge; and (c) an upper limit at ～11 wt% Ni where the metal liquid was almost consumed.     

Compositional Study of a Suite of Samples from the 28-t Armanty (Xinjiang) Iron Meteorite

Abstract— Neutron activation analysis of 15 samples from widely separated locations on the surface of the 28-t Armanty (Xinjiang) group-IIIE iron meteorite showed no compositional variations attributable to magmatic fractionation processes such as fractional crystallization. The homogeneity contrasts with that observed in the Cape York IIIAB iron, in keeping with the idea that the latter is an exceptional case. From the maximum compositional gradients we estimate that the radius of the HIE core was > 1 km. One sample taken from a surface formed by fracture associated with its fall to Earth is slightly anomalous in composition; we suspect that this region is near the border between two parental γ crystals and has a high content of minor phases.     

Metal in mesosiderites: Evidence for redox and fractional crystallization

Mesosiderites are an amalgamation of crustal silicates and molten metal, and their formational history is not well understood. It is widely believed that redox reactions occurred in the mesosiderites during metal–silicate mixing. Previous studies evaluated redox reactions by studying the silicates within mesosiderites, but little attention has been given to the metal for complementary evidence of such processes. Here, the evidence for redox within the metal portion of five mesosiderites is documented, most notably lower P content in the matrix metal relative to clast metal (nodule). These observations, together with the noted FeO reduction in silicates, provide further support for redox reactions occurring during metal–silicate mixing. Samples with differing Ir concentrations, such as Chaunskij and RKP A70015, have been previously classified as anomalous. However, the marked variation in highly siderophile element concentrations in all of these mesosiderites is consistent with fractional crystallization. These compositional trends could be explained by isolated metallic masses that underwent fractional crystallization before mixing or by hit-and-run collisions that produced metallic masses that ranged in size.     

Anhydrous liquid line of descent of Yamato‐980459 and evolution of Martian parental magmas

We report the results of nominally anhydrous equilibrium and fractional crystallization experiments on a synthetic Yamato‐980459 (Y98) bulk composition at 0.5 GPa. These experiments allow us to test a suggested fractional crystallization model, calculated using MELTS by Symes et al. ( 2008 ), in which a Y98‐like initial liquid yielded a magma closely resembling the bulk composition of QUE 94201. Although the two meteorites cannot be cogenetic owing to their age difference, they are thought to represent bona fide magmatic liquids rather than products of crystal accumulation, as are most Martian basaltic meteorites. Hence, understanding possible petrogenetic links between these types of liquids could be revealing about processes of melting and crystallization that formed the range of Martian basalts. We find that Y98 can, in fact, generate a residual liquid closely resembling QUE, but only after a very different crystallization process, and different degree of crystallization, than that modeled using MELTS. In addition, both the identity and sequence of crystallizing phases are very different between model and experiments. Our fractional crystallization experiments do not produce a QUE‐like liquid, and the crystallizing phases are an even poorer match to the MELTS‐calculated compositions than in the equilibrium runs. However, residual liquids from our experiments define a liquid line of descent that encompasses bulk compositions of parental melts calculated for several Martian basaltic meteorites, suggesting that the known Martian basaltic meteorites had their ultimate origin from the same or very similar source lithologies. These are, in turn, similar to source rocks modeled by previous studies as products of extensive crystallization of an initial Martian magma ocean.     

Properties and mixing of soil components in Apollo 17 double-drive tube 79001/2

Abstract— Previous studies of Apollo 17 double-drive tube 79001/2 showed that portions of this lunar regolith segment have some unusual properties, such as very high I_s/FeO values (Monis et al., 1989) and N contents (Stone and Clayton, 1989). To understand the geologic significance of these features in this core, we determined the grain-size distribution and modal abundance of the petrographic constituents for samples from 12 different depths of the core. Also, we measured the elemental and isotopic compositions of noble gases in the coarse-grained (150–250 μm) and fine-grained (<20 μm) sample fractions from four depths of this core. The agglutinate abundance and ³⁶Ar contents show depth-related variations similar to those observed for I_s/FeO and N in this core. Samples from the top (～0.5 cm depth) and the bottom (～45 cm depth) of the drive tube are related to Apollo 17 submature soils with about 250–300 Ma galactic cosmic-ray (GCR) exposure age. But the soil at the top of the drive tube received additional surface irradiation for ～2 Ma after deposition at Van Serg. The samples at intermediate depths (i.e., ～7 cm (upper zone) and ～20 cm (lower zone) of the 79001/2 core) show features characteristic of mixtures of Apollo 17 mature soils and finely comminuted regolith breccias having about 600–800 Ma GCR exposure age. The mixing ratios between the coarse and fine fractions of the intermediate-depth samples are similar to each other. Though the mixing ratios for the samples from the top and the bottom of the core are also similar to each other, they differ significantly from the ratios at intermediate depths. The results presented here are consistent with the two-component Van Serg core model proposed by Stone and Clayton (1989) and McKay et al. (1988).     

Estimation of trace element concentrations in the lunar magma ocean using mineral‐ and metal‐silicate melt partition coefficients

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.     

Martian basalt (shergottite) Queen Alexandra Range 94201 and lunar basalt 15555: A tale of two pyroxenes

Abstract— We present here ion microprobe analyses of rare earth and other selected trace and minor elements in pyroxenes of shergottite Queen Alexandra Range 94201 and lunar basalt 15555. Pyroxene zonation patterns record the crystallization histories of these two basaltic samples from Mars and the Moon, respectively, and allow a comparison of mafic melt evolution on these two planetary bodies. Elemental abundances and trends in pyroxenes of these two rocks indicate that their minerals formed by continuous, closed system fractional crystallization of their respective parent melts. This further supports the idea that QUE 94201 closely represents the composition of a true Martian basaltic melt (McSween et al., 1996). The main differences in pyroxene elemental zonation patterns in these two objects are attributed to earlier crystallization of whitlockite in QUE 94201 (i.e., before the Fe-rich pyroxenes) than in 15555 (after the Fe-rich pyroxenes). The size of Eu anomalies in pyroxenes of QUE 94201 is intermediate between that in pyroxenes of 15555 and the other shergottites and may imply that fO₂ conditions during crystallization of this Martian basalt were significantly more reducing than for other shergottites, although not quite as reducing as for lunar basalts. Cerium anomalies appear to be less prevalent in pyroxenes of QUE 94201 than other Antarctic shergottites and could be indicative of lesser degree of weathering in the Antarctic.     

Mn‐Cr chronology of five IIIAB iron meteorites

Abstract— Mn‐Cr systematics in phosphates (sarcopside, graftonite, beusite, galileiite, and johnsomervilleite) in IIIAB iron meteorites were investigated by secondary ion mass spectrometry (SIMS). In most cases, excesses in ⁵³Cr are found and δ⁵³Cr is well correlated with Mn/Cr ratios, suggesting that ⁵³Mn was alive at the time of IIIAB iron formation. The inferred Mn‐Cr “ages” are different for different phosphate minerals. This is presumably due to a combined effect of the slow cooling rates of IIIAB iron meteorites and the difference in the diffusion properties of Cr and Mn in the phosphates. The ages of sarcopside are the same for the IIIAB iron meteorites. Johnsomervilleite shows apparent old ages, probably because of a gain of Cr enriched in ⁵³Cr during the closure process. Apparently, old Mn‐Cr ages reported in previous studies can also be explained in a similar way. Therefore, the IIIAB iron meteorites probably experienced identical thermal histories and thus derived from the core of a parent body. Thermal histories of the parent body of IIIAB iron meteorites that satisfy the Mn‐Cr chronology and metallographic cooling rates were constructed by computer simulation. The thermal history at an early stage (<10 Ma after CAI formation) is well determined, though later history may be more model‐dependent. It is suggested that relative timing of various events in the IIIAB parent body may be estimated with the aid of the thermal history. There is a systematic difference in Mn and Cr concentrations in various minerals (phosphates, sulfide, etc.) among the IIIAB iron meteorites, which seems to be mainly controlled by redox conditions.     

In situ formation of palisade bodies in calcium,aluminum-rich refractory inclusions

Abstract— It has been suggested that palisade bodies—shells of spinel found within some calcium, aluminum-rich inclusions (CAIs) and the phases the shells enclose—are intact mini-CAIs that predate and were captured by their current hosts while the latter were still molten. We present new data and observations that indicate that most palisade bodies formed instead in situ while their host inclusions were crystallizing. The evidence includes observations of spinel-lined cavities and glass-filled, circular structures outlined by spinel in experimental run products crystallized from melts; a partially formed palisade body in an inclusion; a fassaite crystal that is optically continuous across a palisade wall; and similarity of unusual mineral compositions in some palisade bodies and their hosts. Our observations can be used to refute arguments for exotic origin and are most consistent with a model for in situ formation involving: (1) formation of vesicles in a largely molten inclusion; (2) nucleation of spinel upon and/or adherence to vapor-melt interfaces, forming spinel shells around vesicles; (3)leakage of vesicles and filling with melt while spinel shells remain largely intact; and (4) crystallization of melt inside shells. This model is similar to one proposed for formation of segregation vesicles, which are partially- to completely-filled vesicles found in some terrestrial basalts. In addition, we interpret framboids (i.e., dense clusters of spinel with little material between grains, found in most inclusions that contain palisade bodies) as polar or near-polar sections through palisade bodies and therefore do not make a genetic distinction between the two features.     

Petrogenetic models for magmatism on the eucrite parent body: Evidence from orthopyroxene in diogenites

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 D^{orthopyroxene/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.     

A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites

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.     

Magmatic inclusions and felsic clasts in the Dar al Gani 319 polymict ureilite

Abstract— Magmatic inclusions occur in type II ureilite clasts (olivine‐orthopyroxene‐augite assemblages with essentially no carbon) and in a large isolated plagioclase clast in the Dar al Gani (DaG) 319 polymict ureilite. Type I ureilite clasts (olivine‐pigeonite assemblages with carbon), as well as other lithic and mineral clasts in this meteorite, are described in Ikeda et al.(2000). The magmatic inclusions in the type II ureilite clasts consist mainly of magnesian augite and glass. They metastably crystallized euhedral pyroxenes, resulting in feldspar component‐enriched glass. On the other hand, the magmatic inclusions in the large plagioclase clast consist mainly of pyroxene and plagioclase, with a mesostasis. They crystallized with a composition along the cotectic line between the pyroxene and plagioclase liquidus fields. DaG 319 also contains felsic lithic clasts that represent various types of igneous lithologies. These are the rare components not found in the common monomict ureilites. Porphyritic felsic clasts, the main type, contain phenocrysts of plagioclase and pyroxene, and their groundmass consists mainly of plagioclase, pyroxene, and minor phosphate, ilmenite, chromite, and/or glass. Crystallization of these porphyritic clasts took place along the cotectic line between the pyroxene and plagioclase fields. Pilotaxitic felsic clasts crystallized plagioclase laths and minor interstitial pyroxene under metastable conditions, and the mesostasis is extremely enriched in plagioclase component in spite of the ubiquitous crystallization of plagioclase laths in the clasts. We suggest that there are two crystallization trends, pyroxene‐metal and pyroxene‐plagioclase trends, for the magmatic inclusions and felsic lithic clasts in DaG 319. The pyroxene‐metal crystallization trend corresponds to the magmatic inclusions in the type II ureilite clasts and the pilotaxitic felsic clasts, where crystallization took place under reducing and metastable conditions, suppressing precipitation of plagioclase. The pyroxene‐plagioclase crystallization trend corresponds to the magmatic inclusions in the isolated plagioclase clast and the porphyritic felsic clasts. This trend developed under oxidizing conditions in magma chambers within the ureilite parent body. The felsic clasts may have formed mainly from albite component‐rich silicate melts produced by fractional partial melting of chondritic precursors. The common monomict ureilites, type I ureilites, may have formed by the fractional partial melting of alkali‐bearing chondritic precursors. However, type II ureilites may have formed as cumulates from a basaltic melt.     

On the internal structure and magnetic fields of

First, a sequence of four-zone models for the interior of Venus is constructed under the assumption of hydrostatic equilibrium. While the equation of state for each zone is taken to be the Bullen's relation with its coefficients consistent with the PREM Earth model (Dziewonski and Anderson, 1981), the position of core-mantle boundary is determined by matching solutions of the Emden's equation in different regions. The results of hydrostatic models indicate the presence of a reasonably large molten iron core in Venus, broadly similar to the Earth. It is also found that the position of the core-mantle interface is nearly model-independent. Second, we focus on the question why Venus does not possess a significant global magnetic field and on what we can learn from this fact. Solutions of magnetohydrodynamic equations appropriate for the molten core of Venus are discussed. It is argued that, because the Elsasser number measuring the relative importance of Coriolis and Lorentz forces satisfies 1, equations for the problem of thermal convection in the Venusian fluid core must be nearly uncoupled with the dynamo equation. The existence of a global magnetic field, though small, then suggests that the size of the magnetic Reynolds numberR _m must beR _m =O(10), sustaining a dynamo action near its marginal state but not an active dynamo in the Venusian molten core. On the basis of asymptotic relations for finite amplitude convection, a useful constraint on important physical parameters for the liquid core of Venus is derived and discussed.     

Comment on “The origin of eucrites,diogenites, and olivine diogenites: Magma ocean crystallization and shallow magma processes on Vesta” by B. E. Mandler and L. T. Elkins‐Tanton

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.