Lithium,sodium and potassium abundances in carbonaceous chondrites

Concentrations of lithium, sodium, and potassium in 18 carbonaceous chondrites were determined in the same sample solution by atomic absorption. Mean abundances in carbonaceous Type I chondrites are, in atoms 10⁶ Si: Li = 60.1, Na = 5800, K = 3700. Relative to Type I carbonaceous chondrites, abundances in Type II's are: Li = 0.87, Na = 0.61, K = 0.58; and in Type III's Li = 0.82, Na = 0.49, K = 0.36. Evidently there is a differential depletion of potassium relative to sodium in Type III's, suggesting a fractionation after accretion.     

Carbon,hydrogen and nitrogen isotopes in solvent-extractable organic matter from carbonaceous chondrites

Solvent extractions were done on the carbonaceous chondrites Murray, Murchison, Orgueil and Renazzo, using CCl₄ and CH₃OH. Between 2 and 10% of the total carbon in these meteorites is extractable by ordinary techniques, most of it in CH₃OH. After demineralization with HF, perhaps as much as 30% of the total carbon in Murray may be extractable with CH₃OH. The extracts from Renazzo have isotopic ratios which suggest that they are mainly terrestrial organic matter, with lesser contributions from indigenous organics. The CH₃OH-soluble organic matter from Murchison and both untreated and HF-treated Murray has δ¹³C values of about +5 to + 10%. and δ¹⁵N values of about +90 to +100%., both of which are significantly higher than the bulk meteorite values. The Orgueil CH₃OH-extract also has a δ¹⁵N value well above the value in residual organic matter. Values for δD of +300 to +500%. are found for the CH₃OH-soluble organic matter. The combined data for C, H and N isotopes makes it highly unlikely that the CH₃OH-soluble components are derivable from, or simply related to, the insoluble organic polymer found in the same meteorites. A relationship is suggested between the event that formed hydrous minerals in CI1 and CM2 meteorites and the introduction of water-soluble (methanol-soluble) organic compounds. Organic matter soluble in CCl₄ has essentially no nitrogen, and δ³C and δD values are lower than for CH₃OH-soluble phases. Either there are large isotopic fractionations for carbon and hydrogen between different soluble organic phases, or the less polar components are partially of terrestrial origin.     

Elemental composition of individual chondrules from carbonaceous chondrites,including Allende

Major and trace element analyses of over 180 individual chondrules from 12 carbonaceous chondrites are reported, including individual analyses of 60 chondrules from Pueblito de Allende. Siderophile elements in most chondrules are depleted, compared to the whole chondrite. Correlations of Al-Ir and Ir-Sc among chondrules high in Ca and Al were observed. A Cu-Mn correlation was also found for chondrules from some meteorites. No correlation was observed between Au and other siderophile elements (Fe, Ni, Co and Ir). It is suggested that these elemental associations were present in the material from which the chondrules formed. Compositionally, chondrules appear to be a multicomponent mixture of remelted dust. One component displaying an Al-Ir correlation is identified as Allende-type white aggregates. The other components are a material chemically similar to the present matrix and sulfides-plus-metal material. Abundances of the REE (rare earth elements) were measured in ‘ordinary’ Allende chondrules and were 50% higher than REE abundances in Mokoia chondrules; REE abundances in Ca-Al rich chondrules were similar to REE abundances in Ca-rich white aggregates.     

Mineralogy,petrography, and oxygen isotopic compositions of ultrarefractory inclusions from carbonaceous chondrites

We report on the mineralogy, petrography, and in situ oxygen isotopic composition of twenty-five ultrarefractory calcium-aluminum-rich inclusions (UR CAIs) in CM2, CR2, CH3.0, CV3.1–3.6, CO3.0–3.6, MAC 88107 (CO3.1-like), and Acfer 094 (C3.0 ungrouped) carbonaceous chondrites. The UR CAIs studied are typically small, < 100 μm in size, and contain, sometimes dominated by, Zr-, Sc-, and Y-rich minerals, including allendeite (Sc₄Zr₃O₁₂), and an unnamed ((Ti,Mg,Sc,Al)₃O₅) mineral, davisite (CaScAlSiO₆), eringaite (Ca₃(Sc,Y,Ti)₂Si₃O₁₂), kangite ((Sc,Ti,Al,Zr,Mg,Ca,□)₂O₃), lakargiite (CaZrO₃), warkite (Ca₂Sc₆Al₆O₂₀), panguite ((Ti,Al,Sc,Mg,Zr,Ca)_1.8O₃), Y-rich perovskite ((Ca,Y)TiO₃), tazheranite ((Zr,Ti,Ca)O_2−x), thortveitite (Sc₂Si₂O₇), zirconolite (orthorhombic CaZrTi₂O₇), and zirkelite (cubic CaZrTi₂O₇). These minerals are often associated with 50–200 nm-sized nuggets of platinum group elements. The UR CAIs occur as: (i) individual irregularly-shaped, nodular-like inclusions; (ii) constituents of unmelted refractory inclusions – amoeboid olivine aggregates (AOAs) and Fluffy Type A CAIs; (iii) relict inclusions in coarse-grained igneous CAIs (forsterite-bearing Type Bs and compact Type As); and (iv) relict inclusions in chondrules. Most UR CAIs, except for relict inclusions, are surrounded by single or multilayered Wark-Lovering rims composed of Sc-rich clinopyroxene, ±eringaite, Al-diopside, and ±forsterite. Most of UR CAIs in carbonaceous chondrites of petrologic types 2–3.0 are uniformly ¹⁶O-rich (Δ¹⁷O ∼ −23‰), except for one CH UR CAI, which is uniformly ¹⁶O-depleted (Δ ¹⁷O ∼ −5‰). Two UR CAIs in Murchison have heterogeneous Δ¹⁷O. These include: an intergrowth of corundum (∼ ‒24‰) and (Ti,Mg,Sc,Al)₃O₅ (∼ 0‰), and a thortveitite-bearing CAI (∼ −20 to ∼ ‒5‰); the latter apparently experienced incomplete melting during chondrule formation. In contrast, most UR CAIs in metamorphosed chondrites are isotopically heterogeneous (Δ¹⁷O ranges from ∼ −23‰ to ∼ −2‰), with Zr- and Sc-rich oxides and silicates, melilite and perovskite being ¹⁶O-depleted to various degrees relative to uniformly ¹⁶O-rich (Δ¹⁷O ∼ −23‰) hibonite, spinel, Al-diopside, and forsterite. We conclude that UR CAIs formed by evaporation/condensation, aggregation and, in some cases, melting processes in a ¹⁶O-rich gas of approximately solar composition in the CAI-forming region(s), most likely near the protoSun, and were subsequently dispersed throughout the protoplanetary disk. One of the CH UR CAIs formed in an ¹⁶O-depleted gaseous reservoir providing an evidence for large variations in Δ¹⁷O of the nebular gas in the CH CAIs-forming region. Subsequently some UR CAIs experienced oxygen isotopic exchange during melting in ¹⁶O-depleted regions of the disk, most likely during the epoch of chondrule formation. In addition, UR CAIs in metamorphosed CO and CV chondrites, and, possibly, the corundum-(Ti,Mg,Sc,Al)₃O₅ intergrowth in Murchison experienced O-isotope exchange with aqueous fluids on the CO, CV, and CM chondrite parent asteroids. Thus, both nebular and planetary exchange with ¹⁶O-depleted reservoirs occurred.     

Presolar spinel grains from the Murray and Murchison carbonaceous chondrites

With a new type of ion microprobe, the NanoSIMS, we determined the oxygen isotopic compositions of small (<1μm) oxide grains in chemical separates from two CM2 carbonaceous meteorites, Murray and Murchison. Among 628 grains from Murray separate CF (mean diameter 0.15 μm) we discovered 15 presolar spinel and 3 presolar corundum grains, among 753 grains from Murray separate CG (mean diameter 0.45 μm) 9 presolar spinel grains, and among 473 grains from Murchison separate KIE (mean diameter 0.5 μm) 2 presolar spinel and 4 presolar corundum grains. The abundance of presolar spinel is highest (2.4%) in the smallest size fraction. The total abundance in the whole meteorite is at least 1 ppm, which makes spinel the third-most abundant presolar grain species after nanodiamonds (if indeed a significant fraction of them are presolar) and silicon carbide. The O-isotopic distribution of the spinel grains is very similar to that of presolar corundum, the only statistically significant difference being that there is a larger fraction of corundum grains with large ¹⁷O excesses (¹⁷O/¹⁶O > 1.5 × 10⁻³), which indicates parent stars with masses between 1.8 and 4.5 M_⊙.     

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.     

Oxygen- and magnesium-isotope compositions of calcium-aluminum-rich inclusions from CR2 carbonaceous chondrites

We report both oxygen- and magnesium-isotope compositions measured in situ using a Cameca ims-1280 ion microprobe in 20 of 166 CAIs identified in 47 polished sections of 15 CR2 (Renazzo-type) carbonaceous chondrites. Two additional CAIs were measured for oxygen isotopes only. Most CR2 CAIs are mineralogically pristine; only few contain secondary phyllosilicates, sodalite, and carbonates - most likely products of aqueous alteration on the CR2 chondrite parent asteroid. Spinel, hibonite, grossite, anorthite, and melilite in 18 CAIs have ¹⁶O-rich (Δ¹⁷O = −23.3 ± 1.9‰, 2σ error) compositions and show no evidence for postcrystallization isotopic exchange commonly observed in CAIs from metamorphosed CV carbonaceous chondrites. The inferred initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, in 15 of 16 ¹⁶O-rich CAIs measured are consistent with the canonical value of (4.5-5) × 10⁻⁵ and a short duration (<0.5 My) of CAI formation. These data do not support the “supra-canonical” values of (²⁶Al/²⁷Al)₀ [(5.85-7) × 10⁻⁵] inferred from whole-rock and mineral isochrons of the CV CAIs. A hibonite-grossite-rich CAI El Djouf 001 MK #5 has uniformly ¹⁶O-rich (Δ¹⁷O = −23.0 ± 1.7‰) composition, but shows a deficit of ²⁶Mg and no evidence for ²⁶Al. Because this inclusion is ¹⁶O-rich, like CAIs with the canonical (²⁶Al/²⁷Al)₀, we infer that it probably formed early, like typical CAIs, but from precursors with slightly nonsolar magnesium and lower-than-canonical ²⁶Al abundance. Another ¹⁶O-enriched (Δ¹⁷O = −20.3 ± 1.2‰) inclusion, a spinel-melilite CAI fragment Gao-Guenie (b) #3, has highly-fractionated oxygen- and magnesium-isotope compositions (∼11 and 23‰/amu, respectively), a deficit of ²⁶Mg, and a relatively low (²⁶Al/²⁷Al)₀ = (2.0 ± 1.7) × 10⁻⁵. This could be the first FUN (Fractionation and Unidentified Nuclear effects) CAI found in CR2 chondrites. Because this inclusion is slightly ¹⁶O-depleted compared to most CR2 CAIs and has lower than the canonical (²⁶Al/²⁷Al)₀, it may have experienced multistage formation from precursors with nonsolar magnesium-isotope composition and recorded evolution of oxygen-isotope composition in the early solar nebula over  My. Eight of the 166 CR2 CAIs identified are associated with chondrule materials, indicating that they experienced late-stage, incomplete melting during chondrule formation. Three of these CAIs show large variations in oxygen-isotope compositions (Δ¹⁷O ranges from −23.5‰ to −1.7‰), suggesting dilution by ¹⁶O-depleted chondrule material and possibly exchange with an ¹⁶O-poor (Δ¹⁷O > −5‰) nebular gas. The low inferred (²⁶Al/²⁷Al)₀ ratios of these CAIs (<0.7 × 10⁻⁵) indicate melting >2 My after crystallization of CAIs with the canonical (²⁶Al/²⁷Al)₀ and suggest evolution of the oxygen-isotope composition of the inner solar nebula on a similar or a shorter timescale. Because CAIs in CR2 and CV chondrites appear to have originated in a similarly ¹⁶O-rich reservoir and only a small number of CR2 and CV CAIs were affected by chondrule melting events in an ¹⁶O-poor gaseous reservoir, the commonly observed oxygen-isotope heterogeneity in CAIs from metamorphosed CV chondrites is most likely due to fluid-solid isotope exchange on the CV asteroidal body rather than gas-melt exchange. This conclusion does not preclude that some CV CAIs experienced oxygen-isotope exchange during remelting, instead it implies that such remelting is unlikely to be the dominant process responsible for oxygen-isotope heterogeneity in CV CAIs. The mineralogy, oxygen and magnesium-isotope compositions of CAIs in CR2 chondrites are different from those in the metal-rich, CH and CB carbonaceous chondrites, providing no justification for grouping CR, CH and CB chondrites into the CR clan.     

Uranium content and radiogenic ages of hypersthene,bronzite, amphoterite and carbonaceous chondrites

U was measured by fission track analysis in 115 samples of hypersthene, bronzite, amphoterite and carbonaceous chondrites. On a weight basis the average values for the Cl carbonaceous and bronzite chondrites are similar to the “classic” value of 11 ppb, but the hypersthenes and amphoterites are ~50 per cent higher. Each class shows a well-determined peak in the U abundance distribution, allowing the calculation of radiogenic ages and comparison with other elements of interest.     

Early core formation in asteroids and late accretion of chondrite parent bodies: Evidence from Hf-W in CAIs, metal-rich chondrites, and iron meteorites

The ¹⁸²Hf-¹⁸²W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the ¹⁸²Hf/¹⁸⁰Hf and ε_W at the time of CAI formation to (1.07 ± 0.10) × 10⁻⁴ and −3.47 ± 0.20, respectively. All magmatic iron meteorites examined here have initial ε_W values that are similar to or slightly lower than the initial value of CAIs. These low ε_W values may in part reflect ¹⁸²W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ∼0.3 ε units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ∼1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.Chondrule formation ∼2-5 Myr after CAIs, as constrained by published Pb-Pb and Al-Mg ages, postdates core formation in planetesimals, and indicates that chondrites do not represent the precursor material from which asteroids accreted and then differentiated. Chondrites instead derive from asteroids that accreted late, either farther from the Sun than the parent bodies of magmatic iron meteorites or by reaccretion of debris produced during collisional disruption of older asteroids. Alternatively, chondrites may represent material from the outermost layers of differentiated asteroids. The early thermal and chemical evolution of asteroids appears to be controlled by the decay of ²⁶Al, which was sufficiently abundant (initial ²⁶Al/²⁷Al >1.4 × 10⁻⁵) to rapidly melt early-formed planetesimals but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. The preservation of the primitive appearance of chondrites thus at least partially reflects their late formation rather than their early and primitive origin.     

Investigating the variations in carbon and nitrogen isotopes in carbonaceous chondrites

The carbonaceous chondrites contain significant amounts of carbon- and nitrogen-bearing components, the most abundant of which is organic matter. Stepped combustion data of whole rock and HF/HCl residues of carbonaceous chondrites reveal that the organic material can be subdivided operationally into three components: (1) free organic matter (FOM), which is readily extractable from whole-rock meteorites and is enriched in ¹³C and ¹⁵N; (2) labile organic matter (LOM), which has a macromolecular structure but is liberated by hydrous pyrolysis; LOM is the parent structure for some FOM and is also enriched in ¹³C and ¹⁵N; and (3) refractory organic matter (ROM), which is also macromolecular but is virtually unaffected by hydrous pyrolysis and is relatively depleted in ¹³C and ¹⁵N. The macromolecular entities (LOM and ROM) are by far the most abundant organic components present, and as such, the relative abundances of the ¹³C- and ¹⁵N-enriched LOM and the ¹³C- and ¹⁵N-depleted ROM will have a major influence on the overall isotopic composition of the whole-rock meteorite. Laboratory experiments designed to simulate the effects of parent body aqueous alteration indicate that this form of processing removes LOM from the macromolecular material, allowing ROM to exert a stronger influence on the overall isotopic compositions. Hence, aqueous alteration of macromolecular materials on the meteorite parent body may have a significant control on the stable isotopic compositions of whole-rock carbonaceous chondrites. The enstatite chondrites are also carbon rich but have been subjected to high levels of thermal metamorphism on their parent body. Stepped combustion data of HF/HCl residues of enstatite chondrites indicate, that if they and carbonaceous chondrites inherited a common organic progenitor, metamorphism under reducing conditions appears to incorporate and preserve some of the ¹³C enrichments in LOM during graphitisation. However, when metamorphism is at its most extreme, the ¹⁵N enrichments in LOM are lost.     

南极碳质球粒陨石中两个富尖晶石球粒状难熔包体的岩石学和矿物化学特征

富尖晶石球粒状CAI(富Ca-Al难熔包体)是球粒陨石中一种特殊类型的CAI,在南极格罗夫山碳质球粒陨石GRV020025和GRV021579中共发现两个富尖晶石球粒状CAI———GRV020025-3RI8和GRV021579-3RI5。GRV020025-3RI8具有占统治地位的尖晶石,在球粒的最外边存在严重蚀变的不规则边,钙钛矿主要分布在靠近边的位置。与GRV020025-3RI8比较,GRV021579-3RI5的尖晶石中的钙钛矿消失,深绿辉石出现,薄薄的蚀变层位于尖晶石核和富钙辉石边之间。两个富尖晶石球粒状CAI的尖晶石均具有低含量FeO和ZnO的特征,而且GRV021579-3RI5具有较GRV020025-3RI8更高的TiO2含量。岩石学和矿物化学特征表明,GRV020025-3RI8和GRV021579-3RI5都经历过熔融结晶过程,它们的蚀变均发生在非氧化的含水或无水的环境中。     

Planetesimal sulfate and aqueous alteration in CM and CI carbonaceous chondrites

Water-soluble sulfate salts extracted from six CM chondrites have oxygen isotope compositions that are consistent with an extraterrestrial origin. The Δ¹⁷O of sulfate are correlated with previously reported whole rock δ¹⁸O and with an index of meteorite alteration, and may display a correlation with the date of the fall. The enrichments and depletions for Δ¹⁷O of water-soluble sulfate from the CM chondrites relative to the terrestrial mass dependent fractionation line are consistent with sulfate formation in a rock dominated asteroidal environment, and from aqueous fluids that had undergone relatively low amounts of oxygen isotope exchange and little reaction with anhydrous components of the meteorites. It is unresolved how the oxidation of sulfide to sulfate can be reconciled with the inferred low oxidation state during the extraterrestrial alteration process. Oxygen isotope data for two CI chondrites, Orgueil and Ivuna, as well as the ungrouped C2 chondrite Essebi are indistinguishable from sulfate of terrestrial origin and may be terrestrial weathering products, consistent with previous assertions. Our oxygen isotope data, however, can not rule out a preterrestrial origin either.     

Kainsaz(CO3)陨石中两个富Al球粒的氧同位素组成特征与形成演化

富Al球粒是原始球粒陨石中一种矿物岩石学特征介于富钙铝包体(CAIs)和镁铁质硅酸盐球粒之间的特殊集合体,所以常常认为富Al球粒在认识CAIs和镁铁质硅酸盐球粒形成演化过程中的相互联系具有特殊意义。然而,对富Al球粒的初始物质组成以及形成演化过程一直存在较多争议,而氧同位素组成研究能够对球粒演化和早期星云环境等提供重要的信息。在本文中我们报导了来自Kainsaz(1937年降落于俄罗斯,CO3型)碳质球粒陨石中的2个富Al球粒(编号K1-CH1和K2-CH2)的矿物岩石学和氧同位素组成特征。K1-CH1的矿物组成主要为橄榄石、低钙辉石和富钙长石,K2-CH2为橄榄石和富钙长石。2个球粒中的矿物均具有贫~(16)O同位素组成特征。K1-CH1中矿物的△~(17)O组成基本上位于2个区间:-11.1‰～-8.7‰和-3.9‰～0.4‰;而K2-CH2的△~(17)O介于-6.6‰～-0.6‰之间,且具有从中部至边部升高的趋势。矿物岩石学和氧同位素特征表明,这2个富Al球粒的初始物质组成为富CAIs和镁铁质硅酸盐。在球粒熔融结晶过程中,与贫~(16)O同位素组成(△~(17)O:-8.7‰～-7.8‰)的星云发生了氧同位素交换。球粒形成后,发生迁移进入陨石母体,在相对更贫~(16)O同位素组成(△~(17)O:-0.6‰～0.4‰)的母体中(流体参与)发生变质作用,并再次发生了氧同位素交换。     

Determination of REE,Ba, Fe,Mg, Na and K in carbonaceous and ordinary chondrites

Precise determination of REE and Ba abundances in three carbonaceous (Orgueil Cl, Murchison C2 and Allende C3) and seven olivine-bronzite chondrites were carried out by mass spectrometric isotope dilution technique. Replicate analyses of standard rock and the three carbonaceous chondrites demonstrated the high quality of the analyses (accuracies for REE are ±1–2 per cent). Certain carbonaceous chondrite specimens showed small positive irregularities in Yb abundance. The Yb ‘anomaly’ (approximately + 5 per cent relative to the average of 10 ordinary chondrites) in Orgueil may relate to high temperature components. The REE pattern of Guareña (H6) exhibits comparatively extensive fractionation (about factor 2) with a negative anomaly for Eu (17 ± 1 percent) compared to the average H chondrite. This could be interpreted in terms of extensive thermal metamorphism leading to melting.Apart from absolute abundance differences, there appears to be small but recognizable fractionation among the average relative REE abundances of Cl, E, H and L chondrites. However, individual chondrites within these groups showed more or less fractionated REE patterns relative to each other. The distinction between H and L chondrites was well demonstrated in Eu-Sm correlation curves and absolute abundance differences of REE and major elements.Si-normalized atomic ratios of the REE abundances in different kinds of chondrites to those in Orgueil (Cl) chondrite were 0.58 (E), 0.75 (H), 0.81 (L), 1.07 (C2) and 1.32 (C3).     

Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites

TEM, HRTEM, HVEM and SEM methods, coupled with energy dispersive X-ray analysis, have been used to study the microstructure and the phases comprising the matrix of carbonaceous chondrites Murchison, Cold Bokkeveld, Nawapali and Cochabamba. A wide variety of phyllosilicate morphologies occurs in each. Very small crystals and clasts of olivine, pyroxene and other unhydrated minerals are mixed intimately with the phyllosilicates. Intergrowths of carbonates and Sulfides within the phyllosilicates also occur, as well as a ubiquitous spongey material which is difficult to characterize, but contains elementary phyllosilicate units and embryo crystals. The identifiable large crystalline phyllosilicates are principally Fe-rich serpentine-group minerals and intermediate more Mg-rich chrysotilelike group members, with characteristic ~ 7.0–7.4 Å basal layer spacings. Complex interlayered and intergrown hydrous minerals also occur associated with the spongey material, and other poorly crystalline silicates and finely divided Fe-Ni sulphides. Fe/Si and Mg/Si ratios vary on a sub-micron scale, and the morphologies of the larger phyllosilicate crystals correlate broadly with these variations. Small crystals of sodium chloride and potassium chloride have been identified, occluded within a predominantly organic mass.The matrix minerals have a multistage history of formation in which the effects of aqueous alteration are dominant. Few, if any, of matrix minerals can be unmodified nebular condensates, although some clasts and inclusions have escaped alteration and predate the alteration process.     

Search for nucleosynthetic and radiogenic tellurium isotope anomalies in carbonaceous chondrites

Tellurium isotope data acquired by multiple-collector inductively coupled plasma-mass spectrometry (MC-ICPMS) are presented for sequential acid leachates of the carbonaceous chondrites Orgueil, Murchison, and Allende. Tellurium isotopes are produced by a broad range of nucleosynthetic pathways and they are therefore of particular interest given the isotopic anomalies previously identified for other elements in these meteorites. In addition, the data provide new constraints on the initial solar system abundance of the r-process nuclide ¹²⁶Sn, which decays to ¹²⁶Te with a half-life of 234,500 years. The ¹²⁶Te/¹²⁸Te ratios of all leachates were found to be identical, within uncertainty, despite variations in ¹²⁴Sn/¹²⁸Te of between about 0.002 and 1.4. The data define a ¹²⁶Sn/¹²⁴Sn ratio of <7.7 × 10⁻⁵ at the time of last isotopic closure, consistent with the value of <18 × 10⁻⁵ previously reported for bulk carbonaceous chondrites. How close this is to the initial ¹²⁶Sn/¹²⁴Sn ratio of the solar system depends on when the investigated samples last experienced redistribution of Sn and Te. No clear evidence is found for nucleosynthetic anomalies in the abundances of p-, s-, and r-process nuclides. The largest effect detected in this study is a small excess of the r-process nuclide ¹³⁰Te in a nitric acid leachate of Murchison. This fraction displays an anomalous ε¹³⁰Te of +3.5 ± 2.5. Although barely resolvable given the analytical uncertainties, this is consistent with the presence of a small excess r-process component or an s-process deficit. The general absence of anomalies contrasts with previous results obtained for K, Cr, Zr, Mo, and Ba isotopes in similar leachates, which display nucleosynthetic anomalies of up to 3.8%. The reason for this discrepancy is unclear but it may reflect volatility and more efficient mixing of Te in the solar nebula.     

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.     

Chemical and petrographic constraints on the origin of chondrules and inclusions in carbonaceous chondrites

Bulk chemical compositions of the various petrographie types of chondrules and inclusions in Type 3 carbonaceous chondrites (excluding those affected by metamorphism) have been determined by microprobe defocused beam analysis. Inclusion compositions follow approximately the theoretical compositional trajectory for equilibrium condensation. Analyses of chondrules occurring in the same meteorites have higher silica contents and show only slight overlap with inclusion compositions. Dust fusion is apparently an inadequate mechanism for producing the wide chemical variations observed among chondrules. Impact melting models require sampling of complex target rocks which are unknown as components of meteorites; this mechanism also demands efficient mechanical processing of chondrules before accretion. A genetic relationship between chondrules and inclusions in carbonaceous chondrites is suggested by the compositional continuum between these objects. A condensation sequence which dips into the liquid stability field at lower temperatures is advocated for the production of both inclusions and chondrules. Textural relationships between intergrown chondrules and inclusions support such a sequence. This model suggests that the assembled components (inclusions and chondrules) of carbonaceous chondrites are related by a common process.     

Mass spectrometric isotope dilution analyses of palladium,silver, cadmium and tellurium in carbonaceous chondrites

The mass spectrometric isotope dilution technique was used to measure the elemental abundances of Pd, Ag, Cd and Te in Orgueil (C1), Ivuna (C1), Murray (C2) and Allende (C3) chondrites. The Pd abundance of 554 ppb for the C1 chondrites is almost identical to the recommended value of Anders and Ebihara (1982); that for Cd (712 ppb) is approximately 5% higher, whereas that for Ag (198 ppb) is approximately 10% lower than the recommended values. A smooth distribution for the abundances of the odd-A nuclides between65 ≦ A ≦ 209 have been observed except for small irregularities in the Pd-Ag-Cd and the Sm-Eu mass regions (ANDERS and Ebihara, 1982). The results from the present work have the effect of smoothing out the dip in the Pd-Ag-Cd region and indicate that there is no systematic fractionation of cosmochemical element groups in this mass region.A Te abundance of 2.25 ppm has been determined for the C1 chondrites Orgueil and Ivuna in agreement 2+with the value of Smith et al. (1977). This value is some 30% lower than the value of Krähenbühl et al. (1973) but is in good agreement with the more recent measurements from Chicago. The Krähenbühl et al. value causes ¹²⁸Te and ¹³⁰Te to lie approximately 30% above the r-process peak at A = 130 (Käppeler el al., 1982), whereas the new value fits smoothly into the general trend.