Refractory Element Ratios Research Articles

The dynamic formation process of the CB chondrite Gujba

The many unique characteristics of CB chondrites have resulted in the impact hypothesis becoming the favoured model for their formation. Here, we further investigate the formation mechanisms of CB chondrites by analysing the elemental and K isotope compositions of chondrules and bulk fractions from the CBa chondrite Gujba. Similar to previous work, the refractory element ratios in the Gujba chondrules show evidence of a differentiated precursor, with the Nb/Ta, Zr/Hf, Sc/Th and Zr/Th ratios showing fractionation relative to other chondrites. In addition, the bulk fractions, and to a lesser extent the chondrules with attached matrix and metals, display significantly more refractory element fractionation and a large enrichment in light REEs. Based on EDS elemental mapping and comparisons with previous studies, the most likely source of this highly fractionated material appears to be the small amount of heterogeneously distributed interstitial fine-grained material within Gujba. These large refractory element fractionations (i.e., Nb/Ta, Zr/Hf, Sc/Th Zr/Th, and LREE/HREE) are best explained by a significant partial melting process such as crustal formation. Nevertheless, the mechanism of patrial melting cannot be conclusively determined with the data available here. The K isotopic compositions of the Gujba chondrules analyzed here range from −2.24‰ to −0.41‰ in δ41K, whereas the bulk analyses show δ41K values of −0.81‰ to −0.72‰. This range of chondrule K isotope compositions is significantly larger, and extends to much lighter compositions, compared to all other chondrites measured so far by bulk ICP-MS. In addition, the Gujba chondrules display a clear negative correlation of K isotopic composition with K concentration, with the chondrules showing the lightest K isotope compositions having the highest K concentrations. This distinctive correlation indicates that evaporation was likely the dominant process affecting the K isotopic variation observed in the Gujba chondrules. Nevertheless, the extremely light δ41K values seen in the most K-rich chondrules (which are lighter than any other early solar system material so far measured) indicate that incomplete condensation likely took place before evaporation. As such, we propose a two-stage model to explain the formation of chondrules in Gujba, with Stage 1 characterized by incomplete condensation of vaporized material with an average isotopic fractionation factor (α) of 0.9984 (when using the most K enriched chondrule to constrain the model), and Stage 2 representing partial evaporation in a vapor plume with an average α range of 0.9976 to 0.9990. Using these α values we calculate an approximate vapor saturation index value of 0.935 for condensation and between 0.903 and 0.960 for evaporation. This formation process requiring both condensation and evaporation for CB chondrules is consistent with an impact generated vapor plume and further expands our understanding of CB chondrite formation.

Forming Planets Around Stars With Non-Solar Elemental Composition

Context. Stars in the solar neighbourhood have refractory element ratios slightly different from that of the Sun. It is unclear how much the condensation of solids and thus the composition of planets forming around these stars is affected. Aims. We aim to understand the impact of changing the ratios of the refractory elements Mg, Si, and Fe within the range observed in solar-type stars within 150 pc of the Sun on the composition of planets forming around them. Methods. We use the GGchem code to simulate the condensation of solids in protoplanetary disks with a minimum mass solar nebula around main sequence G-type stars in the solar neighbourhood. We extract the stellar elemental composition from the Hypatia Database. Results. We find that a lower Mg/Si ratio shifts the condensation sequence from forsterite (Mg2SiO4) and SiO to enstatite (MgSiO3) and quartz (SiO2); a lower Fe/S ratio leads to the formation of FeS and FeS2 and few or no Fe-bearing silicates. Ratios of refractory elements translate directly from the gas phase to the condensed phase for T < 1000 K. However, ratios with respect to volatile elements (e.g. oxygen and sulphur) in the condensates – the building blocks of planets – differ from the original stellar composition. Conclusions. Our study shows that the composition of planets crucially depends on the abundances of the stellar system under investigation. Our results can have important implications for planet interiors, which depend strongly on the degree of oxidation and the sulphur abundance.

Magmatic evolution of the host magma of plutonic rocks in the Procellarum KREEP Terrane

The origin of the KREEP (K, Rare Earth Element and P)-rich component of the Mg-suite rocks of the Procellarum KREEP Terrane (PKT), one of three major lunar crustal terranes, is unclear. In an attempt to determine its origin, we estimated the composition of host magmas of plutonic rocks, including Mg-suite rocks and evolved rocks, from the PKT (PKT-host magmas) by using secondary ion mass spectrometry analyses of plagioclase. Calculated partition coefficients for Sr, Ba and Ti between plagioclase and melts, taking into account the anorthite content of plagioclase, temperature and bulk rock major-element compositions, were applied to determine parental magma compositions. The PKT-host magmas contained 160–360 ppm Sr, 520–7600 ppm Ba and 1.1–7.0 wt.% TiO2; most of them had higher Ti and Ba concentrations than KREEP basalts and high-K KREEP. We used phase relations based on the Rhyolite-MELTS algorithm to explore the evolution of the PKT-host magmas and KREEP basalts from two bulk silicate moon (BSM) starting compositions, a BSM with chondritic ratios of refractory elements, and a crustal-component-enriched BSM with non-chondritic ratios of refractory elements. We propose a three-stage evolution model. Stage-1: polybaric multi-step fractionation from a BSM magma to form ferroan anorthosite (FAN) crust and an evolved magma as the first KREEP-rich component (M0). Stage-2: assimilation and fractional crystallization (AFC) of M0, early cumulate and FAN associated with deep mantle overturn and impact events to form the PKT-host magmas, Mg-suite rocks and an evolved magma as the second KREEP-rich component (K0). Stage-3: further AFC cycles of K0, Mg-suite rocks or FAN associated with shallow mantle overturn and impact events to form KREEP basalts. This three-stage model for a crustal-component-enriched BSM with non-chondritic ratios of refractory elements (e.g., a sub-chondritic Ti/Ba ratio) reproduced the compositions of both the host magmas of FAN and the PKT-host magmas that fractionated to form the Mg-suite rocks and KREEP basalts of the PKT region. In particular, the model reproduced the high Ti and Ba contents of the PKT-host magmas we estimated from plagioclase composition. Variations of TiO2 and Ba contents (and hence Ti/Ba ratios) of the magmas were critical controls on their evolution.

Accretion and differentiation of early planetary bodies as recorded in the composition of the silicate Earth

The abundances of the chemical elements and radiogenic isotopes in the silicate Earth provide key information on the composition of planetary building blocks, the accretion process, including its timing, and the early planetary-wide chemical differentiation. The abundances of the lithophile and highly siderophile elements in the bulk silicate Earth can be modeled as a mixture of three distinct components. Component A (proto-Earth) consisted of volatile element-depleted and strongly reduced material to which a highly oxidized component B (impactor, Theia) was added with chondritic element abundances for the refractory elements to slightly depleted in the volatile elements. Finally, a late veneer (component C) added more material with a composition similar to carbonaceous chondrites. These components make up ~85%, ~15% and ~0.4% of the mass of the silicate Earth, respectively. The sequence of their accretion led to a first core formation that produced a metallic core and depleted the silicate portion in siderophile elements including most of the Fe. Addition of the oxidized and volatile richer component B was followed by a second core formation event with removal of a sulfide melt and depletion of the mantle in chalcophile and siderophile elements. The final addition of a late chondritic veneer established a near CI-chondritic abundance among the highly siderophile elements, but also among S, Se and Te. The significant chemical differences between the two first and major components imply that they formed in different regions of the solar system and from isotopically distinct material. The homogeneity of the isotope ratios of refractory elements in the Earth-Moon system then requires a giant impact that was energetic enough to homogenize the material from the two bodies. The combination of the two major components that formed the Earth is contemporaneous with the formation of the Moon. The initial Sr-isotope composition of the Moon indicates that this impact occurred at 4.507 (15) Ga. The most-likely major source for the highly volatile elements, including water on Earth, is the Moon-forming impactor. Thus, the habitability of Earth and its ability to develop plate tectonic processes is the result of the chance collision of proto-Earth with a planetary body that had formed dominantly from material originating beyond the orbit where Earth formed and therefore had accreted a higher amount of volatile elements.

Carbonaceous Chondrites and the Condensation of Elements from the Solar Nebula

Abstract Common features of all carbonaceous chondrite groups are invariant refractory element ratios, depletions of moderately volatile elements as a function of their condensation temperature (T C), and strongly depleted highly volatile element concentrations independent of T C. The depletion of volatile elements with respect to the bulk solar system composition requires a separation of gas from solids in the solar nebula. Several models have been proposed to explain the decoupling of gas and solids, but not all are compatible with astrophysical, chemical, and petrologic constraints. Here existing physical models are integrated with measured element concentrations, measured and modeled physical properties of protoplanetary disks, and planetary-scale nucleosynthetic and stable isotope variations to establish a conceptual model for the condensation and accretion of elements into planetesimals. In this model, the chemical composition of chondrites is established by element condensation in a cooling solar nebula that changed its surface density as a function of time and temperature. The model predicts peak temperatures at the condensation sites of about 1400 K that consequently decreased due to a diminishing heat source originating from viscous heating and radiation, accompanied by continuous removal of gas from the nebula surface by photoevaporation. The coupled evolution of condensing solids from a nebula of diminishing surface density resulted in a pattern of decreasing moderately volatile abundances with decreasing T C. The reduction of nebula opacity due to the chondrule-forming process significantly increased nebula cooling rates and led to the near-chondritic relative abundances of highly volatile elements observed in carbonaceous chondrites.

Impact of monsoon, vegetation, and landscape on pedogenesis: A case study using organic and inorganic tracers from the Himalayan foreland sediments

The physical and chemical properties of paleosols are widely used to explain their genesis. However, the physical attributes of paleosols subjected to burial cannot be used for paleoenvironmental reconstructions. In contrast, selective pedogenic tracers such as rare earth (REE) and trace elements, and organic proxies such as pristane/phytane (Pr/Ph) ratios, and D/H and 13C/12C ratios of n-alkanes that are resistant to post-depositional processes can provide insight into pedogenic conditions. Although the upper Miocene Siwalik paleosols from the Himalayan Foreland Basin (HFB) have been used for past vegetation and climate reconstructions, little emphasis has been given to understand their formation. In this regard, the Jabbarkhad Siwaliks of the Kangra sub-basin, identified as distal alluvial fan deposits with records of temporal variability in rainfall and vegetation composition, have been used to investigate the combined impact of depositional environment, topography, climate and vegetation composition on paleosol formation.Prominent negative Eu-anomalies in CI-normalised REE patterns along with uniform ratios of refractory elements suggest a dominant and consistent source of sediments from K-rich granitic upper continental crust. A wide variation in the abundance of REE indicates the modification of sediments by pedogenetic processes. In the Jabbarkhad region, oxic (higher Pr/Ph) and well-drained soils favoured the proliferation of grasses. The high root density and organic turnover of grasses facilitated the development of a mature, strongly leached soil (high Ba/Sr) under high rainfall and humid conditions (lower δ2Η values). Less mature (lower Ba/Sr) paleosols were developed in low-lying anoxic (Pr/Ph <1) floodplain areas dominated by C3 trees near the active channel. Comparison of soil maturity from the Gangetic floodplain, a modern analogue of the Siwalik sediments, suggests that the spatial variability of paleosol maturity was controlled by the landscape mosaic of the Siwalik floodplain. Since pedogenesis and vegetation distribution are controlled by floodplain morphology, sampling of paleosols with similar pedogenic and geomorphic characteristics is essential for evaluation of vegetation distribution in temporally separated paleosols.

Composition, petrology, and chondrule‐matrix complementarity of the recently discovered Jbilet Winselwan CM2 chondrite

AbstractJbilet Winselwan (JW) is a recently found CM2 chondrite breccia containing two lithologies. A study of 508 chondrules provides the first statistically reliable size distribution for CM chondrite chondrules, which is about log normal with a mean chondrule size of 149 μm (lithology I) and 141 μm (lithology II). Chondrules are surrounded by fine‐grained rims. Apparent chondrule diameters and their apparent rim thicknesses are positively correlated with slopes of 0.12 (lithology II) and 0.18 (lithology I), the latter typical of CM chondrites. The CM chondrites are generally primitive and parts of JW experienced only mild aqueous alteration. Bulk JW element ratios are solar (=CI chondritic), e.g., Si/Mg (1.12), Fe/Mg (1.80–1.83), Ti/Al (0.053), and about solar for Ca/Al. The 26 chemically studied chondrules have subchondritic Si/Mg (0.88) and Fe/Mg ratios (0.21). Matrix and the fine‐grained chondrule rims on the other hand have superchondritic Si/Mg ratios with means of 1.34 and 1.41, respectively. The Fe/Mg ratios are also superchondritic, with means of 2.41 (matrix) and 2.61 (fine‐grained rims). The refractory element ratios in chondrules are superchondritic (Ti/Al: 0.106; Ca/Al: 1.64), and subchondritic in the JW matrix (Ti/Al: 0.031; Ca/Al: 0.71) and in the fine‐grained rims (Ti/Al: 0.023; Ca/Al: 0.68). These complementary element ratios require formation of chondrules and matrix/rims from the same reservoir in order to obtain a chondritic bulk composition. Most chondrules are mineralogically zoned, with olivine in the core and low‐Ca pyroxene in the rim; hence, CM chondrules were open systems, exchanging material with the surrounding gas.

Complementary element relationships between chondrules and matrix in Rumuruti chondrites

The complimentary chemical composition of chondrules and matrix has so far only been studied in carbonaceous chondrites. We have extended these studies to the matrix-rich Rumurutis We have determined the chemical composition of 27 bulk chondrules and 100 matrix spots in unequilibrated fragments of three different Rumuruti (R) chondrites (NWA 2446, NWA 753, Hughes 030). Also, the bulk chemical composition of NWA 753 was determined. Bulk R chondrites have about CI chondritic (= solar) ratios of Fe/Mg (1.89), Al/Ti (18.62), and Al/Ca (0.94), while chondrules and matrix are complementary to each other. Mean Fe/Mg ratios in chondrules are 0.43 (NWA 2446), 0.36 (NWA 753), and 0.34 (Hughes). Chondrules are depleted in Fe, while matrices are enriched in Fe with respective values of 2.59, 2.45, and 2.39. Bulk Fe/Mg ratio of NWA 753, the only sample which is unaffected by terrestrial weathering, is reproduced by careful mass balance calculations using compositions and abundances of chondrules, matrix and sulphides from the same thin section. Refractory element ratios are also complementary: Al/Ti and Al/Ca are sub-chondritic in chondrules (Al/Ti: 10.43 in NWA 753, 12.24 in NWA 2446 and 11.47 in Hughes 030; Al/Ca: 0.66 in NWA 2446, 0.53 in NWA 753, and 0.46 in Hughes 030), while matrices generally have super-chondritic Al/Ca and Al/Ti ratios (Al/Ti: 28.00 in NWA 2446, 23.20 in NWA 753 and 19.00 in Hughes 030; Al/Ca: 2.30 in NWA 2446, 1.90 in NWA 753, and 0.41 in Hughes 030). Calcium is enriched in the Hughes 030 matrix due to terrestrial weathering. These complementary element ratios and the CI chondritic bulk strongly suggest a common reservoir from which all R chondrite components formed, similar to carbonaceous chondrites. Further, super-chondritic Si/Mg, and CI chondritic Fe/Mg ratios in bulk R chondrites require addition of Si to their reservoir, most probably before chondrule formation.

An experimental study of partial melting and fractional crystallization on the HED parent body

AbstractWe have performed an experimental and modeling study of the partial melting behavior of the HED parent body and of the fractional crystallization of liquids derived from its mantle. We estimated the mantle composition by assuming chondritic ratios of refractory lithophile elements, adjusting the Mg# and core size to match the density and moment of inertia of Vesta, and the compositions of Mg‐rich olivines found in diogenites. The liquidus of a mantle with Mg# (=100*[Mg/(Mg+Fe)]) 80 is ~1625 °C and, under equilibrium conditions, the melt crystallizes olivine alone until it is joined by orthopyroxene at 1350 °C. We synthesized the melt from our 1350 °C experiment and simulated its fractional crystallization path. Orthopyroxene crystallizes until it is replaced by pigeonite at 1200 °C. Liquids become eucritic and crystal assemblages resemble diogenites below 1250 °C. MELTS correctly predicts the olivine liquidus but overestimates the orthopyroxene liquidus by ~70 °C. Predicted melt compositions are in reasonable agreement with those generated experimentally. We used MELTS to determine that the range of mantle compositions that can produce eucritic liquids and diogenitic solids in a magma ocean model is Mg# 75–80 (with chondritic ratios of refractory elements). A mantle with Mg# ~ 70 can produce eucrites and diogenites through sequential partial melting.

EH3 matrix mineralogy with major and trace element composition compared to chondrules

AbstractWe investigated the matrix mineralogy in primitive EH3 chondrites Sahara 97072, ALH 84170, and LAR 06252 with transmission electron microscopy; measured the trace and major element compositions of Sahara 97072 matrix and ferromagnesian chondrules with laser‐ablation, inductively coupled, plasma mass spectrometry (LA‐ICPMS); and analyzed the bulk composition of Sahara 97072 with LA‐ICPMS, solution ICPMS, and inductively coupled plasma atomic emission spectroscopy. The fine‐grained matrix of EH3 chondrites is unlike that in other chondrite groups, consisting primarily of enstatite, cristobalite, troilite, and kamacite with a notable absence of olivine. Matrix and pyroxene‐rich chondrule compositions differ from one another and are distinct from the bulk meteorite. Refractory lithophile elements are enriched by a factor of 1.5–3 in chondrules relative to matrix, whereas the matrix is enriched in moderately volatile elements. The compositional relation between the chondrules and matrix is reminiscent of the difference between EH3 pyroxene‐rich chondrules and EH3 Si‐rich, highly sulfidized chondrules. Similar refractory element ratios between the matrix and the pyroxene‐rich chondrules suggest the fine‐grained material primarily consists of the shattered, sulfidized remains of the formerly pyroxene‐rich chondrules with the minor addition of metal clasts. The matrix, chondrule, and metal‐sulfide nodule compositions are probably complementary, suggesting all the components of the EH3 chondrites came from the same nebular reservoir.

Evidence against a chondritic Earth

The (142)Nd/(144)Nd ratio of the Earth is greater than the solar ratio as inferred from chondritic meteorites, which challenges a fundamental assumption of modern geochemistry--that the composition of the silicate Earth is 'chondritic', meaning that it has refractory element ratios identical to those found in chondrites. The popular explanation for this and other paradoxes of mantle geochemistry, a hidden layer deep in the mantle enriched in incompatible elements, is inconsistent with the heat flux carried by mantle plumes. Either the matter from which the Earth formed was not chondritic, or the Earth has lost matter by collisional erosion in the later stages of planet formation.

Accretion and core formation: constraints from metal–silicate partitioning

Experimental metal-silicate partitioning data for Ni, Co, V, Cr, Nb, Mn, Si and W were used to investigate the geochemical consequences of a range of models for accretion and core formation on Earth. The starting assumptions were chondritic ratios of refractory elements in the Earth and the segregation of metal at the bottom of a magma ocean, which deepened as the planet grew and which had, at its base, a temperature close to the liquidus of the silicate. The models examined were as follows. (i) Continuous segregation from a mantle which is chemically homogeneous and which has a fixed oxidation state, corresponding to 6.26 per cent oxidized Fe. Although Ni, Co and W partitioning is consistent with chondritic ratios, the current V content of the silicate Earth cannot be reconciled with core segregation under these conditions of fixed oxidation state. (ii) Continuous segregation from a mantle which is chemically homogeneous but in which the Earth became more oxidized as it grew. In this case, the Ni, Co, W, V, Cr and Nb contents of core and mantle are easily matched to those calculated from the chondritic ratios of refractory elements. The magma ocean is calculated to maintain a thickness approximately 35 per cent of the depth to the core-mantle boundary in the accreting Earth, yielding a maximum pressure of 44GPa. This model yields a Si content of the core of 5.7 per cent, in good agreement with cosmochemical estimates and with recent isotopic data. (iii) Continuous segregation from a mantle which is not homogeneous and in which the core equilibrates with a restricted volume of mantle at the base of the magma ocean. This is found to increase depth of the magma ocean by approximately 50 per cent. All of the other elements (except Mn) have partitioning consistent with chondritic abundances in the Earth, provided the Earth became, as before, progressively oxidized during accretion. (iv) Continuous segregation of metal from a crystal-melt mush. In this case, pressures decrease to a maximum of 31GPa and it is extremely difficult to match the calculated mantle contents of the highly incompatible elements Nb and W to those observed. Progressive oxidation is required to fit the observed mantle contents of vanadium. All of the scenarios discussed above point to progressive oxidation having occurred as the Earth grew. The Earth appears to be depleted in Mn relative to the chondritic reference.

HE 0515–4414: an unusual sub-damped Ly αsystem revisited

Aims. We examine the ionization, abundances, and differential dust depletion of metals, the kinematic structure, and the physical conditions in the molecular hydrogen-bearing sub-damped Ly α system toward HE 0515−4414. Methods. We used the STIS and VLT UVES spectrographs to obtain high-resolution recordings of the damped Ly α profile and numerous associated metal lines. Observed element abundances were corrected with respect to dust depletion effects. Results. The sub-damped Ly α absorber at redshift z = 1.15 is unusual in several aspects. The velocity interval of associated metal lines extends for 700 km s −1 . In addition, saturated H i absorption is detected in the blue damping wing of the NHi = 8 × 10 19 cm −2 main component. The column density ratios of associated Al ii ,A liii ,a nd Feii lines indicate that the absorbing material is ionized. Nineteen of a total of 31 detected metal line components are formed within peripheral H ii regions, while only 12 components are associated with the predominantly neutral main absorber. The bimodal velocity distribution of metal line components suggests two interacting absorbers. For the main absorber, the observed abundance ratios of refractory elements to Zn range from a Galactic warm disk [Si/Zn]g = −0.40 ± 0.06, [Fe/Zn]g = −1.10 ± 0.05 to halo-like and essentially undepleted patterns. The dust-corrected metal abundances indicate a nucleosynthetic odd-even effect and might imply an anomalous depletion of Si relative to Fe for two components, but otherwise they do correspond to solar ratios. The intrinsic average metallicity is almost solar [Fe/H]m = −0.08 ± 0.19, whereas the uncorrected average is [Zn/H]g = −0.38 ± 0.04. The ion abundances in the periphery conform to solar element composition. Conclusions. The detection of H ii, as well as the large variation in dust depletion for this sight line, raises the question whether in future studies of damped Ly α systems ionization and depletion effects have to be considered in more detail. Ionization effects, for instance, may mimic an enrichment of α elements. An empirical recipe for detecting H ii regions is provided.

Impact glasses from the Apollo 14 landing site and implications for regional geology

Lunar impact glasses possess the unmodified refractory element ratios of the original fused target materials at the sites of impacts. These target materials are regolith developed on different terrains and local rock types. Almost 800 glasses from the Apollo 14 landing site have been analyzed by electron microprobe in this study. These glasses show significant variation and hint at the existence of multiple terrains of differing compositions near the landing site. To test this idea, we have used Clementine color image data to construct iron and titanium maps from which a petrologic map in an elemental system analogous to the sample database was made. This map shows the regional provenance of Apollo 14 samples and suggests that the highlands in the Fra Mauro region of the Moon consist of a potassium, rare Earth element, and phosphorus (KREEP)‐rich, basaltic debris layer that overlies a more feldspathic terrain in some areas. This mapping effort demonstrates the efficacy of using Clementine image data to place lunar sample information into a regional, and ultimately global, context. Lunar impact glasses and orbital data can provide geochemical constraints on the local and regional geology of the Moon.

Refractory forsterite in primitive meteorites: Condensates from the solar nebula?

Abstract— All groups of chondritic meteorites contain discrete grains of forsteritic olivine with FeO contents below 1 wt% and high concentrations of refractory elements such as Ca, Al, and Ti. Ten such grains (52 to 754 μg) with minor amounts of adhering matrix were separated from the Allende meteorite. After bulk chemical analysis by instrumental neutron activation analysis (INAA), some samples were analyzed with an electron microprobe and some with an ion microprobe.Matrix that accreted to the forsterite grains has a well‐defined unique composition, different from average Allende matrix in having higher Cr and lower Ni and Co contents, which implies limited mixing of Allende matrix.All samples have approximately chondritic relative abundances of refractory elements Ca, Al, Sc, and rare‐earth elements (REE), although some of these elements, such as Al, do not quantitatively reside in forsterite; whereas others (e.g., Ca) are intrinsic to forsterite. The chondritic refractory element ratios in bulk samples, the generally high abundance level of refractory elements, and the presence of Ca‐Al‐Ti‐rich glass inclusions suggest a genetic relationship of refractory condensates with forsteritic olivine. The Ca‐Al‐Ti‐rich glasses may have acted as nuclei for forsterite condensation.Arguments are presented that exclude an origin of refractory forsterite by crystallization from melts with compositions characteristic of Allende chondrules: (a) All forsterite grains have CaO contents between 0.5 and 0.7 wt% with no apparent zoning, requiring voluminous parental melts with 18 to 20 wt% CaO, far above the average CaO content of Allende chondrules. Similar arguments apply to Al contents. (b) The low FeO content of refractory forsterite of 0.2‐0.4 wt% imposes an upper limit of ∼1 wt% of FeO on the parental melt, too low for ordinary and carbonaceous chondrule melts, (c) The Mn contents of refractory forsterites are between 30 to 40 ppm. This is at least one order of magnitude below the Mn content of chondrule olivines in all classes of meteorites. The observed Mn contents of refractory forsterite are much too low for equilibrium between olivine and melts of chondrule composition, (d) As shown earlier, refractory forsterites have O‐isotopic compositions different from chondrules (Weinbruch et al., 1993a).Refractory olivines in carbonaceous chondrites are found in matrix and in chondrules. The compositional similarity of both types was taken to indicate that all refractory forsterites formed inside chondrules (e.g., Jones, 1992). As refractory forsterite cannot have formed by crystallization from chondrule melts, we conclude that refractory forsterite from chondrules are relic grains that survived chondrule melting and probably formed in the same way as refractory forsterite enclosed in matrix. We favor an origin of refractory forsterite by condensation from an oxidized nebular gas.

Mantle plumes, mantle stirring and hotspot chemistry

Recent advances in understanding plume dynamics allow the role of mantle plumes, the presumed cause of volcanic hotspots and source of the hotspots' oceanic island basalts (OIBs), to be specified in more detail than hitherto. Their role in sampling a mantle with single-layer convection and an increase in viscosity with depth by two or three orders of magnitude is considered here. Plumes would sample thin tabular regions at the bottom of the mantle. This region would be expected in the assumed mantle model to have properties appropriate to OIB sources, including greater heterogeneity, greater age and less depletion of incompatible elements than the shallow mantle sampled by ocean ridges. All proposed sources of recycled material, including oceanic crust, oceanic sediments and continental lithosphere, can be accommodated in this model. However it is noted that clear evidence for a source with primitive refractory element ratios is still lacking, as is undisputed evidence for differences between MORB source and plume source 40Ar/ 36Ar and 129Xe/ 130Xe ratios. 40Ar/ 36Ar ratios do not exclude nearly complete early degassing of the mantle. High 3He/ 4He ratios in some plumes might come from some remaining less degassed mantle or from the core.

The composition of the mantle of the eucrite parent body and the origin of eucrites

Previous estimates of the composition of the EPB have tacitly assumed chondritic abundances of refractory LIL elements. A model is presented here which utilizes refractory element ratios instead. Consideration of these ratios and of known olivine-liquid partition coefficients for Sc, Mg and Si imply that the EPB mantle may be accurately modeled as a simple mixture of 25% eucrite + 75% olivine. The model also implies that eucrites such as Juvinas and Sioux County were derived by large degrees of partial melting (20–30%). Comparison of the model EPB mantle to known meteorite classes indicates a similarity between the element abundances of the EPB and C3 chondrites.

Netschaëvo: A New Class of Chondritic Meteorite

The ratios of refractory elements to silicon and of zinc to silicon indicate that the silicate portion of the Netschaëvo meteorite is an ordinary chondrite. The scarcity of chondrules, the large dimensions (about 100 micrometers) of plagioclase grains, and the low indium content (0.09 nanogram per gram) indicate that Netschaëvo belongs to petrologic type 6. On a diagram of reduced iron versus oxidized iron, Netschaëvo lies along an extrapolation of the LL-L-H ordinary chondrite fractionation trend. The abundances of siderophile elements (nickel, germanium, iridium, and gold) are about 1.6 to 2.0 times greater than in H-group chondrites, and siderophile/nickel ratios are, with one exception, those expected from LL-L-H trends. This evidence indicates that Netschaëvo is an extremely iron-rich member of the ordinary chondrite sequence, and that plausible models to account for the ordinary chondrite sequence must produce materials having iron/silicon ratios 25 percent greater than those in CI carbonaceous chondrites. The existence of Netschaëvo emphasizes that the chondritic meteorites in terrestrial collections are a biased and incomplete selection of primitive solar system materials.