Early solar system processes recorded in the matrices of two highly pristine CR3 carbonaceous chondrites, MET 00426 and QUE 99177

The mineralogy and bulk compositions of the matrices of the CR chondrites MET 00426 and QUE 99177 have been studied using a combination of SEM, EPMA, and TEM techniques. The matrices of these two chondrites are texturally, chemically, and mineralogically similar and are characterized by significant FeO-enrichments with respect to other CR chondrite matrices, nearly flat refractory lithophile patterns, variable volatile element patterns, and a simple mineral assemblage dominated by amorphous silicate material and Fe,Ni sulfides. Fine-grained, crystalline silicate phases such as olivine and pyroxene appear to be extremely rare in the matrices of both meteorites. Instead, the mineralogy of matrices and fine-grained rims of both meteorites consists of abundant amorphous FeO-rich silicate material, containing nanoparticles of Fe,Ni sulfides (troilite, pyrrhotite, and pentlandite). Secondary alteration minerals that are characteristic of other CR chondrites (e.g., Renazzo and Al Rais), such as phyllosilicates, magnetite, and calcite are also rare. The texture and mineralogy of the matrices of MET 00426 and QUE 99177 share many features with matrices in the primitive carbonaceous chondrites ALH A77307 (CO3.0) and Acfer 094 (unique). These observations show that MET 00426 and QUE 99177 are very low petrologic type 3 chondrites that have escaped the effects of aqueous alteration, unlike other CR chondrites, which are typically classified as petrologic type 2. We suggest that these meteorites represent additional samples of highly primitive, but extremely rare carbonaceous chondrites of petrologic type 3.00, according to the classification scheme of Grossman and Brearley (2005). The highly pristine nature of MET 00426 and QUE 99177 provides important additional insights into the origins of fine-grained materials in carbonaceous chondrites. Based on our new observations, we infer that the amorphous silicate material and nanosulfide particles that dominate the matrices of these meteorites formed in the solar nebula by rapid condensation of material following high-temperature events, such as those that formed chondrules.     

Petrographic and oxygen-isotopic study of refractory forsterites from R-chondrite Dar al Gani 013 (R3.5-6), unequilibrated ordinary and carbonaceous chondrites

We have conducted petrographic, chemical and in-situ oxygen isotopic studies of refractory forsterites from unequilibrated ordinary and carbonaceous chondrites as well as an unequilibrated R-chondrite. Refractory forsterites occur in all types of unequilibrated chondrites and all have very similar chemical composition with low FeO and high refractory lithophile element (RLE) contents. Refractory forsterites are typically enriched in ¹⁶O relative to ‘normal’ olivine independent of the bulk O-isotope ratios of the parent meteorites. Analyses of refractory forsterites spread along a Δ¹⁷O mixing line with Δ¹⁷O ranging from +2 to −10‰. Due to similarities in oxygen isotopes and chemical compositions, we conclude that refractory forsterites of various types of chondrites come from a single common reservoir. Implications of this hypothesis for the chemical and O-isotope evolution of silicates in the early solar nebular are discussed.     

Matrix material in type 3 chondrites—occurrence,heterogeneity and relationship with chondrules

Matrix material in type 3 chondrites forms rims on chondrules, metal-sulfide aggregates, Ca,Al-rich inclusions and chondritic clasts; it also forms lumps up to a millimeter in size, which may contain coarser silicates. Chondrules of all types were found with internal matrix lumps that appear to have entered the chondrules before the latter had crystallized. Mean concentrations of Mg, Na, Al and Ca in matrix occurrences show up to fivefold variations in a single chondrite. Variations between mean matrix compositions of individual type 3 ordinary chondrites are almost as large and partly reflect systematic differences between H, L and LL matrices. Such variations are probably a result of nebular separation of feldspathic material and ferromagnesian silicates.Compositions of chondrules and their matrix rims are normally unrelated, although rim compositions are correlated with those of matrix lumps inside chondrules. A single chondrule was found with a composition nearly identical to that of its internal matrix lump, suggesting that some chondrules may have formed from matrix material. Matrix lumps are as heterogeneous as chondrules, but mean chondrule and matrix compositions differ, even allowing for possible loss of metallic Fe,Ni during chondrule formation. Since bulk compositions of matrix lumps and rims have probably not changed significantly since their formation except for Fe-Mg exchange, our matrix samples cannot represent typical chondrule precursor materials.     

The composition of the Earth

Compositional models of the Earth are critically dependent on three main sources of information: the seismic profile of the Earth and its interpretation, comparisons between primitive meteorites and the solar nebula composition, and chemical and petrological models of peridotite-basalt melting relationships. Whereas a family of compositional models for the Earth are permissible based on these methods, the model that is most consistent with the seismological and geodynamic structure of the Earth comprises an upper and lower mantle of similar composition, an Fe---Ni core having between 5% and 15% of a low-atomic-weight element, and a mantle which, when compared to CI carbonaceous chondrites, is depleted in Mg and Si relative to the refractory lithophile elements.The absolute and relative abundances of the refractory elements in carbonaceous, ordinary, and enstatite chondritic meteorites are compared. The bulk composition of an average CI carbonaceous chondrite is defined from previous compilations and from the refractory element compositions of different groups of chondrites. The absolute uncertainties in their refractory element compositions are evaluated by comparing ratios of these elements. These data are then used to evaluate existing models of the composition of the Silicate Earth.The systematic behavior of major and trace elements during differentiation of the mantle is used to constrain the Silicate Earth composition. Seemingly fertile peridotites have experienced a previous melting event that must be accounted for when developing these models. The approach taken here avoids unnecessary assumptions inherent in several existing models, and results in an internally consistent Silicate Earth composition having chondritic proportions of the refractory lithophile elements at 2.75 times that in CI carbonaceous chondrites. Element ratios in peridotites, komatiites, basalts and various crustal rocks are used to assess the abundances of both non-lithophile and non-refractory elements in the Silicate Earth. These data provide insights into the accretion processes of the Earth, the chemical evolution of the Earth's mantle, the effect of core formation, and indicate negligible exchange between the core and mantle throughout the geologic record (the last 3.5 Ga).The composition of the Earth's core is poorly constrained beyond its major constituents (i.e. an Fe---Ni alloy). Density contrasts between the inner and outer core boundary are used to suggest the presence ( 10 ± 5%) of a light element or a combination of elements (e.g., O, S, Si) in the outer core. The core is the dominant repository of siderophile elements in the Earth. The limits of our understanding of the core's composition (including the light-element component) depend on models of core formation and the class of chondritic meteorites we have chosen when constructing models of the bulk Earth's composition.The Earth has a bulk Fe/Al of 20 ± 2, established by assuming that the Earth's budget of Al is stored entirely within the Silicate Earth and Fe is partitioned between the Silicate Earth ( 14%) and the core ( 86%). Chondritic meteorites display a range of Fe/Al ratios, with many having a value close to 20. A comparison of the bulk composition of the Earth and chondritic meteorites reveals both similarities and differences, with the Earth being more strongly depleted in the more volatile elements. There is no group of meteorites that has a bulk composition matching that of the Earth's.     

Presolar diamond, silicon carbide, and graphite in carbonaceous chondrites: implications for thermal processing in the solar nebula

We have determined abundances of presolar diamond, silicon carbide, graphite, and Xe-P1 (Q-Xe) in eight carbonaceous chondrites by measuring the abundances of noble gas tracers in acid residues. The meteorites studied were Murchison (CM2), Murray (CM2), Renazzo (CR2), ALHA77307 (CO3.0), Colony (CO3.0), Mokoia (CV3_ox), Axtell (CV3_ox), and Acfer 214 (CH). These data and data obtained previously by Huss and Lewis (1995) provide the first reasonably comprehensive database of presolar-grain abundances in carbonaceous chondrites. Evidence is presented for a currently unrecognized Ne-E(H) carrier in CI and CM2 chondrites.After accounting for parent-body metamorphism, abundances and characteristics of presolar components still show large variations across the classes of carbonaceous chondrites. These variations correlate with the bulk compositions of the host meteorites and imply that the same thermal processing that was responsible for generating the compositional differences between the various chondrite groups also modified the initial presolar-grain assemblages. The CI chondrites and CM2 matrix have the least fractionated bulk compositions relative to the sun and the highest abundances of most types of presolar material, particularly the most fragile types, and thus are probably most representative of the material inherited from the sun's parent molecular cloud. The other classes can be understood as the products of various degrees of heating of bulk molecular cloud material in the solar nebula, removing the volatile elements and destroying the most fragile presolar components, followed by chondrule formation, metal-silicate fractionation in some cases, further nebula processing in some cases, accretion, and parent body processing. If the bulk compositions and the characteristics of the presolar-grain assemblages in various chondrite classes reflect the same processes, as seems likely, then differential condensation from a nebula of solar composition is ruled out as the mechanism for producing the chondrite classes. Presolar grains would have been destroyed if the nebula had been completely vaporized. Our analysis shows that carbonaceous chondrites reflect all stages of nebular processing and thus are no more closely related to one another than they are to ordinary and enstatite chondrites.     

太阳星云凝聚过程的岩石学模型：（1）球粒陨石的凝聚成因

建立类地行星区太阳星云凝聚过程的岩石学模型，对于合理解释陨石、地球和类地行星的成因关系，探讨地球起源和估算地球的整体成分都有着重要意义。本文中根据天体化学和太阳系演化学说关于太阳星云物理化学条件的基本分析，以及实验凝聚岩石学的研究结果，推断在太阳星云盘的类地行星区中可能有星云的气－固和气－液－固两种凝聚作用发生。通过对球粒陨石中球粒和基质矿物成分及结构构造特征的对比，论证了绝大多数球粒的气－液－固凝聚成因和基质的气－固凝聚成因，并讨论了球粒陨石各化学群的凝聚成因模式。     

Heavily fractionated noble gases in an acid residue from the Klein Glacier 98300 EH3 chondrite

Noble gases were measured both in bulk samples (stepped pyrolysis and total extraction) and in a HF/HCl residue (stepped pyrolysis and combustion) from the Klein Glacier (KLE) 98300 EH3 chondrite. Like the bulk meteorite and as seen in previous studies of bulk type 3 E chondrites (“sub-Q”), the acid residue contains elementally fractionated primordial noble gases. As we show here, isotopically these are like those in phase-Q of primitive meteorites, but elementally they are heavily fractionated relative to these. The observed noble gases are different from “normal” Q noble gases also with respect to release patterns, which are similar to those of Ar-rich noble gases in anhydrous carbonaceous chondrites and unequilibrated ordinary chondrites (with also similar isotopic compositions). While we cannot completely rule out a role for parent body processes such as thermal and shock metamorphism (including a later thermal event) in creating the fractionated elemental compositions, parent body processes in general seem not be able to account for the distinct release patterns from those of normal Q noble gases. The fractionated gases may have originated from ion implantation from a nebular plasma as has been suggested for other types of primordial noble gases, including Q, Ar-rich, and ureilite noble gases. With solar starting composition, the corresponding effective electron temperature is about 5000 K. This is lower than inferred for other primordial noble gases (10,000-6000 K). Thus, if ion implantation from a solar composition reservoir was a common process for the acquisition of primordial gas, electron temperatures in the early solar system must have varied spatially or temporally between 10,000 and 5000 K.Neon and xenon isotopic ratios of the residue suggest the presence of presolar silicon carbide and diamond in abundances lower than in the Qingzhen EH3 and Indarch EH4 chondrites. Parent body processes including thermal and shock metamorphism and a late thermal event also cannot be responsible for the low abundances of presolar grains. KLE 98300 may have started out with smaller amounts of presolar grains than Qingzhen and Indarch.     

Aqueous alteration in carbonaceous chondrites: Mass balance constraints on matrix mineralogy

Bulk chemical compositions of matrix material in Antarctic CM chondrites and other non-Antarctic CM and CI chondrites have been determined using microprobe defocused beam techniques. These are used, along with the results of previously published mineralogical studies, to provide mass balance constraints on the relative proportions of intergrown and intermixed phyllosilicate phases in carbonaceous chondrite matrices. Results of these calculations indicate differing amounts of PCP (a mixture of approximately 25% tochilinite and 75% cronstedtite) and serpentines (Mg-rich and Fe-rich varieties in varying proportions or intermediate compositional varieties). Additional sulfide phases are also probably necessary to account for excess Ni and S. Fe/Si ratios for matrices of individual meteorites range from 1.21 to 2.77, corresponding to PCP/(PCP + SERF) ratios of 0.16 to 0.58. Progressive aqueous alteration of matrix appears to have occurred by formation of tochilinite, then cronstedtite and Mg-rich serpentine, and finally Fe-rich serpentine and sulfides. CM matrix clearly did not behave as an isolated system during alteration. CI chondrite matrices appear to contain little if any PCP; this may be a natural consequence of the absence of chondrule-associated metal, from which PCP forms, in the unaltered precursor material. These data provide a more quantitative picture of low-temperature aqueous alteration processes in carbonaceous chondrite parent bodies than has heretofore been possible from TEM studies alone.     

Alteration in CM carbonaceous chondrites inferred from modal and chemical variations in matrix

The modal abundance of matrix in CM chondrites appears to vary from 57–85 vol%. The concentrations of volatile elements that should occur in matrix remain approximately constant despite differences in the proportions of matrix, suggesting that the differing matrix contents may not be real primary variations but are optical effects due to aqueous alteration processes that make other petrologic components unrecognizable. Apparent matrix content can be used as a qualitative measure of the degree of alteration experienced by each CM chondrite. Fe/Si ratios in matrices decrease progressively with increasing alteration due to the formation of new phyllosilicate phases with higher Mg/Fe ratios and optically recognizable opaque minerals that are not counted as matrix. The aqueous alteration process in CM chondrites appears to have been largely isochemical if the bulk meteorites are considered as the reacting systems, although depletion patterns and isotopic anomalies indicate open-system behavior for a few highly mobile components.     

Chemical and physical studies of type 3 chondrites—III. Chondrules from the Dhajala H3.8 chondrite

Fifty-eight chondrules were separated from the Dhajala H3.8 chondrite and their thermoluminescence properties were measured. Chips from 30 of the chondrules were examined petrographically and with electron-microprobe techniques; the bulk compositions of 30 chondrules were determined by the fused bead technique. Porphyritic chondrules, especially 5 which have particularly high contents of mesostasis, tend to have higher TL (mass-normalized) than non-porphyritic chondrules. Significant correlations between log(TL) and the bulk CaO, Al₂O₃ and MnO content of the chondrules, and between log(TL) and the CaO, Al₂O₃, SiO₂ and normative anorthite content of the chondrule glass, indicate an association between TL and the abundance and composition of mesostasis. Unequilibrated chondrules ( i.e. those whose olivine is compositionally heterogeneous and high in Ca) have low TL, whereas equilibrated chondrules have a wide range of TL, depending on their chemical and petrographic properties.We suggest that the TL level in a given chondrule is governed by its bulk composition (which largely determined the abundance and composition of constituent glass) and by metamorphism (which devitrfied the glass in those chondrules with high Ca glass to produce the TL phosphor). We also suggest that one reason why certain chondrules in type 3 ordinary chondrites are unequilibrated, while others are equilibrated, is that the mesostasis of the unequilibrated chondrules resisted the devitrification. This devitrification is necessary for the diffusive communication between chondrule grains and matrix that enables equilibration.     

Highly siderophile element evidence for early solar system processes in components from ordinary chondrites

Separated magnetic and nonmagnetic components from the ordinary chondrites Dhajala (H3.8) and Ochansk (H4) were analyzed for their Re-Os isotopic compositions, as well as for the abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt and Pd. The Re-Os isotopic systematics of these components are used to constrain the timing of HSE fractionations, and assess the level of open-system behavior of these elements in each of the different components. The high precision, isotope dilution mass spectrometric analyses of the HSE are used to constrain the origins of, and possible relations between some of the diverse components present in these chondrites. The relative and absolute abundances of the HSE differ considerably among the components. Metal fractions have Re/Os that are factors of ∼2 (Dhajala) to ∼3 (Ochansk) higher than those of their nonmagnetic fractions. The isotopic data for both meteorites are consistent with the largest Re-Os fractionations occurring between metal and nonmagnetic components early in solar system history, although minor to moderate late stage, open-system behavior, and limited variations in Re/Os preclude a precise determination of the age for that fractionation. Open-system behavior is generally absent to minor in the metal fractions, and highly variable in nonmagnetic fractions. Re/Os ratios of nonmagnetic fractions deviate as much as 40% from a primordial isochron. Although some deviations are large for isochron applications, nearly all are negligible with respect to consideration of fractionation processes controlling the HSE.Metal from both meteorites contains about 90% of the total budget of HSE. Metal in Ochansk has ∼2 to 10 times the abundances of the bulk meteorite, while metal from the matrix of Dhajala has ∼2 to 4 times the abundances of the bulk. Fine metal in both meteorites has higher abundances than coarse metal, as has been previously observed. Nonmagnetic components, consisting of chondrules and matrix from which metal was removed in the laboratory, have highly fractionated HSE, characterized by much lower Re/Os than the bulk meteorites, as well as large relative depletions in Pd. The abundances of Re, Os, Ir, Ru and Pt in the nonmagnetic fractions are 14-120 ng/g, much higher than would be expected if they had equilibrated with the metal phases present (150-16,000 ng/g). Collectively, the data are consistent with the HSE budget in ordinary chondrites being dominated by two HSE-bearing carrier phases with distinct compositions. These phases formed separately, and never subsequently equilibrated. Metal components incorporated a HSE carrier that formed at high through moderate temperatures and relatively high pressures, such that the relatively volatile Pd behaved coherently with the more refractory HSE. Nonmagnetic fractions from both chondrules and matrix have HSE compositions that likely require at least two processes that fractionated the HSE. Depletions in Pd are consistent with the presence of HSE carriers that formed as either highly refractory condensates, or residues of high degrees of metal melting. Depletions in Re may implicate a period of relatively high f_O2 during which a volatile form of Re was separated from the other HSE.     

陨石氧同位素组成及其地学意义

介绍了各类陨石氧同位素组成的特点,对陨石氧同位素组成的主要成因观点进行了评述,结合地球的原始物质组成,讨论了陨石氧同位素组成的地球科学意义。     

An Fe isotope study of ordinary chondrites

The Fe isotope composition of ordinary chondrites and their constituent chondrules, metal and sulphide grains have been systematically investigated. Bulk chondrites fall within a restricted isotopic range of <0.2‰ δ⁵⁶Fe, and chondrules define a larger range of >1‰ (−0.84‰ to 0.21‰ relative to the IRMM-14 Fe standard). Fe isotope compositions do not vary systematically with the very large differences in total Fe concentration, or oxidation state, of the H, L, and LL chondrite classes. Similarly, the Fe isotope compositions of chondrules do not appear to be determined by the H, L or LL classification of their host chondrite. This may support an origin of the three ordinary chondrite groups from variable accretion of identical Fe-bearing precursors.A close relationship between isotopic composition and redistribution of Fe during metamorphism on ordinary chondrite parent bodies was identified; the largest variations in chondrule compositions were found in chondrites of the lowest petrologic types. The clear link between element redistribution and isotopic composition has implications for many other non-traditional isotope systems (e.g. Mg, Si, Ca, Cr). Isotopic compositions of chondrules may also be determined by their melting history; porphyritic chondrules exhibit a wide range in isotope compositions whereas barred olivine and radial pyroxene chondrules are generally isotopically heavier than the ordinary chondrite mean. Very large chondrules preserve the greatest heterogeneity of Fe isotopes.The mean Fe isotope composition of bulk ordinary chondrites was found to be −0.06‰ (±0.12‰ 2 SD); this is isotopically lighter than the terrestrial mean composition and all other published non-chondritic meteorite suites e.g. lunar and Martian samples, eucrites, pallasites, and irons. Ordinary chondrites, though the most common meteorites found on Earth today, were not the sole building blocks of the terrestrial planets.     

Absolute isotopic abundances of Ti in meteorites

The absolute isotope abundance of Ti has been determined in Ca-Al-rich inclusions from the Allende and Leoville meteorites and in samples of whole meteorites. The absolute Ti isotope abundances differ by a significant mass dependent isotope fractionation transformation from the previously reported abundances, which were normalized for fractionation using ${}^{46}\text{Ti}{}^{48}\text{Ti}$. Therefore, the absolute compositions define distinct nucleosynthetic components from those previously identified or reflect the existence of significant mass dependent isotope fractionation in nature. We provide a general formalism for determining the possible isotope compositions of the exotic Ti from the measured composition, for different values of isotope fractionation in nature and for different mixing ratios of the exotic and normal components. The absolute Ti and Ca isotopic compositions still support the correlation of ⁵⁰Ti and ⁴⁸Ca effects in the FUN inclusions and imply contributions from neutron-rich equilibrium or quasi-equilibrium nucleosynthesis. The present identification of endemic effects at ⁴⁶Ti, for the absolute composition, implies a shortfall of an explosive-oxygen component or reflects significant isotope fractionation. Additional nucleosynthetic components are required by ⁴⁷Ti and ⁴⁹Ti effects. Components are also defined in which ⁴⁸Ti is enhanced.Bulk samples of carbonaceous meteorites (C2 and C3 types) show distinct excesses at ⁵⁰Ti but no nonlinear effects at the other Ti isotopes. Other chondrites, including Orgueil (Cl), show no nonlinear effects. Relative to terrestrial Ti, a small isotope fractionation is found for only an enstatite chondrite. The Ti absolute compositions in Ca-Al-rich inclusions show significant isotope fractionation effects corresponding to an enhancement in the heavier isotopes relative to the lighter isotopes as compared to Ti in a TiO₂ standard and in chondrites. The absence of a correlation of Ti isotope fractionation effects with those for Ca and Mg is indicative of multiple processes of condensation, volatilization and recondensation; however, the mechanisms causing the isotope fractionation are not well understood.     

The Confirmation of a New Type of Chondrites and Their Cosmochemical Significance

The data available show that some Antarctic carbonaceous chondrites are similar to Cl meteorites.Tehy contain a lot of phyllosilicate aggregates and the oxygen isotopic composition of the whole-rock samples is approximate to that of C1 chondrites,so they are named after quasi-C1(Q-C1)chondrites Unlike Cl metcorites,the Q-Cl chondites possess chondrule structrue,and the compositions of hih temperature condensates(chondrule fragments,mineral grains or aggregates)show that the oxygen fugacity varied within a wide range in the surroundings where they were formed,similar to the variation range from E.H.L,LL to C group chondrites.It is inferred that the Q-C1 chondrites could be formed at the edges far from the equator in the whole asteroid region of the solar nebular disk.where the nebula was lower in density and the condensates were lower in accretion velocity,so that the hydration of chon drules and matrix occurred during the late stage of nebular condensation.The discovery of the Q-Cl chondrites and the fact that the earth and other terrestrial planets contain water indicate that at the edges far from the equator in the terrestrial reigion of the solar nebular disk,a large amount of water was incorporated into the lattice of minerals in the condensates as a result of hydration during nebular condensation,and then found its way into the interior parts of the Earth and other terrestrial planets due to accretion.     

Electron microprobe study of a ‘mysterite’-bearing inclusion from the Krymka LL-chondrite

The black inclusion from the Krymka LL3 chondrite previously found to contain ‘mysterite’ by Lewiset al. (1979) belongs to a hitherto unknown class of carbonaceous chondrites. Its olivine and pyroxene compositions. Fo 97–99 and En 96, respectively, are characteristic of carbonaceous chondrites and its plagioclase composition. An 100, is characteristic of C3's. It contains a peculiar group of Co-, Cr-rich metal grains whose compositions are similar, but not identical, to those in C2 chondrites and which also bear some similarities to those in Renazzo. Its weight ratios of total $\text{Fe}\text{SiO}{}_2$ and $\text{solSiO}{}_2\text{MgO}$ are 0.74 and 1.43, respectively, and its atomic ratio of $\text{Si}\text{Al}$ is 10.7, exactly the same as in carbonaceous chondrites. Its bulk chemical composition is very close to that of the Murchison C2 chondrite. The association of mysterite with a special type of carbonaceous chondrite material suggests that mysterite formed by low-temperature condensation in a different region of the nebula from other carbonaceous chondrites.     

Glasses in the D'Orbigny angrite

The angrites are a small and heterogeneous group of achondritic meteorites with highly unusual chemical and mineralogical features. The abundant presence of glasses in D'Orbigny makes this rock a unique member of the angrite group. Glasses fill open spaces, form pockets, and occur as inclusions in olivines. Their physical settings exclude an incorporation from an external source. Major and trace element (rare earth elements [REE], Li, B, Be, transition elements, N and C) contents of these glasses and host olivines were measured combining laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), Nuclear Reaction Analysis (NRA), and EMP techniques. Based on the major element composition, glasses filling voids could represent either a melt formed by melting an angritic rock or a melt from which angrites could have crystallized. Trace element contents of these glasses strongly indicate a direct link to the D'Orbigny bulk meteorite. They are incompatible with the formation of the glasses by partial melting of a chondritic source rock or by shock melting. The refractory elements (e.g., Al, Ti, Ca) have about 10 × CI abundances with CaO/TiO₂ and FeO/MnO ratios being approximately chondritic. Trace element abundances in the glasses appear to be governed by volatility and suggest that the refractory elements in the source had chondritic relative abundances. Although the glasses (and the whole rock) lack volatile elements such as Na and K, they are rich in some moderately volatile elements such as B, V, Mn, Fe (all with close to CI abundances), and Li (about 3-5 × CI). These elements likely were added to the glass in a sub-solidus metasomatic elemental exchange event. We have identified a novel mechanism for alteration of glass and rock compositions based on an exchange of Al and Sc for Fe and other moderately volatile elements in addition to the well-known metasomatic exchange reactions (e.g., Ca-Na and Mg-Fe).Because glass inclusions in olivine were partly shielded from the metasomatic events by the host crystal, their chemical composition is believed to be closer to the original composition than that of any other glasses. The relative trace element abundances in glasses of glass inclusions in olivine and glass pockets are also unfractionated and at the 10 to 20 × CI level. These glasses are chemically similar to the common void-filling glasses but show a much wider compositional variation. Inclusion glasses demonstrate that at least olivine grew with the help of a liquid. In analogy to olivines in carbonaceous chondrites, initial formation could also have been a vapor-liquid-solid condensation process. At that time, the glass had a purely refractory composition. This composition, however, was severely altered by the metasomatic addition of large amounts of FeO and other moderately volatile elements. The presence of volatile elements such as carbon and nitrogen in glasses of glass inclusions is another feature that appears to give these glasses a link with those hosted by olivines of carbonaceous chondrites. All these features point to an origin from a vapor with relative abundances of condensable elements similar to those in the solar nebula.     

Trace element composition of silicate minerals in the porphyritic and nonporphyritic chondrules of Elenovka (L5) and Knyahinya (L/Ll5) meteorites

The results of SIMS and EPMA studies on the silicate minerals and bulk compositions (SEM-EDS) of porphyritic and nonporphyritic chondrules from Elenovka and Knyahinya meteorites are reported. The trace element composition of silicate minerals (olivine, low-Са pyroxene) in equilibrated ordinary chondrites (EOC) has not been affected considerably by thermal metamorphism on the chondritic parent bodies. Therefore, equilibrated chondrites can be used for chondrule-forming processes studies. Low-Са pyroxene in nonporphyritic chondrules contains higher REE, Ba, Sr concentrations than that in porphyritic chondrules at similar trace element concentrations in the olivine of chondrules. The data obtained indicate that the formation of non-porphyritic chondrules was triggered by an increase in the cooling rate of chondrules upon the formation of pyroxene, rather than a difference in the initial conditions of chondrule formation. Higher refractory incompatible element (Nb, LREE) concentrations in the olivine of chondrules than those in the olivine of the matrix and contrasting trace element (Zr, Sr, Cr, REE) concentrations in the low-Са pyroxene of the chondrules and the matrix suggest that the matrix and chondrules of the meteorites formed in one reservoir under different physico-chemical conditions (density, redox state, rotation speed, homogeneity, temperature, shocks, electrical discharge, etc.).     

Deuterium enrichments in chondritic macromolecular material—Implications for the origin and evolution of organics, water and asteroids

Here we report the elemental and isotopic compositions of the insoluble organic material (IOM) isolated from several previously unanalyzed meteorites, as well as the reanalyses of H isotopic compositions of some previously measured samples (Alexander et al., 2007). The IOM in ordinary chondrites (OCs) has very large D enrichments that increase with increasing metamorphism and decreasing H/C, the most extreme δD value measured being almost 12,000‰. We propose that such large isotopic fractionations could be produced in the OC parent bodies through the loss of isotopically very light H₂ generated when Fe was oxidized by water at low temperatures (<200 °C). We suggest that similar isotopic fractionations were not generated in the IOM of CV and CO chondrites with similar metamorphic grades and IOM H/C ratios because proportionately less water was consumed during metamorphism, and the remaining water buffered the H isotopic composition of the IOM even a H was being lost from it.Hydrogen would also have been generated during the alteration of CI, CM and CR carbonaceous chondrites. The IOM in these meteorites exhibit a considerable range in isotopic compositions, but all are enriched in D, as well as ¹⁵N, relative to terrestrial values. We explore whether these enrichments could also have been produced by the loss of H₂, but conclude that the most isotopically anomalous IOM compositions in meteorites from these groups are probably closest to their primordial values. The less isotopically anomalous IOM has probably been modified by parent body processes. The response of IOM to these processes was complex and varied, presumably reflecting differences in conditions within and between parent bodies.The D enrichments associated with H₂ generation, along with exchange between D-rich IOM and water in the parent bodies, means that it is unlikely that any chondrites retain the primordial H isotopic composition of the water ice that they accreted. The H isotopic compositions of the most water-rich chondrites, the CMs and CIs, are probably the least modified and their compositions (δD ? −25‰) suggest that their water did not form at large radial distances from the Sun where ice is predicted to be very D-rich. Yet models to explain the O isotopic composition of inner Solar System bodies require that large amounts of ice were transported from the outer to the inner Solar System.     

Hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites

We present new hydrogen isotope data for separated matrix, hydrated chondrules, and other hydrated coarse silicate fragments from nine carbonaceous chondrites. These data were generated using a micro-analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids. We also re-evaluate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites.Hydrogen isotope compositions of matrix and whole-rock samples of CM chondrites are correlated with oxygen isotope indices, major and minor-element abundances, and abundance and isotope ratios of other highly volatile elements. These correlations include a monotonic decrease in δD with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements (including C, N and Ar), between extremes of ∼0‰ (least altered, most volatile rich) and −200‰ (most altered, least volatile rich). In plots involving only abundances and/or isotope ratios of highly volatile elements, CI chondrites fall on the high-δD, volatile rich end of the trends defined by CM chondrites; i.e., CI chondrites resemble the least altered CM chondrites in these respects. These trends suggest the protoliths of the CM chondrites (i.e., before aqueous alteration) contained an assemblage of volatiles having many things in common with those in the CI chondrites. If so, then the volatile-element inventory of the CI chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized CI meteorites. Differences in volatile-element chemistry between the CI and average CM chondrites can be attributed to aqueous alteration of the latter.Previous models of carbonaceous chondrite aqueous alteration have suggested: (1) the protoliths of the CM chondrites are volatile poor objects like the CO or CV chondrites; and (2) the CI chondrites are more altered products of the same process producing the CM chondrites. Both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile-element geochemistry of these rocks. We present a model for aqueous alteration of the CM chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaceous chondrites. Our model requires, among other things, that the water infiltrating CM chondrites had a δD value of ∼−158‰, consistent with initial accretion of CM parent bodies at ∼4 AU.