Xenon isotopes in size separated nanodiamonds from Efremovka: Xe*, Xe-P3, and Xe-P6

Xenon isotopic data were acquired by high resolution step pyrolysis and combined step pyrolysis/combustion of aliquots of size separated nanodiamonds. ¹²⁹Xe excess (¹²⁹Xe*) from in situ decay of ¹²⁹I is preferentially associated with the larger grain size separates. This observation rules out trapping by recoil from surrounding material. The releases of Xe-P3 and ¹²⁹Xe occur in the same low temperature pyrolysis steps and exhibit similar distributions among the size separates. These observations imply a common site for the components and, in consequence, suggest a common incorporation event.Whether one component or two, our observations require that ¹²⁹Xe* and Xe-P3 were incorporated into a subpopulation of nanodiamonds before nanodiamonds were mixed and incorporated into parent bodies. Their susceptibilities to loss during heating in the laboratory are similar, but the ratio of ¹²⁹Xe* to Xe-P3 varies among nanodiamond separates from different meteorites (literature data). We conclude that the ¹²⁹Xe* we observe today was present as ¹²⁹I during parent body processing. Furthermore, the range of ¹²⁹Xe*/¹³²Xe_P3 ratios across all the separates requires that even nanodiamonds from CI chondrites were at least 5-10× more rich in Xe-P3 during ¹²⁹I decay than they are today.We present a simple model involving one degassing event per parent body between incorporation of nanodiamonds and final decay of ¹²⁹I. The observed variations among parent bodies require degassing events separated by several ¹²⁹I half lives (∼50Ma), consistent with low-temperature processing on parent bodies but longer than expected for nebular processing. In this model, nanodiamonds from ALHA77307 degassed at an unusually early stage, suggesting they alone may retain the signature of processing in the nebula in their P3 and ¹²⁹Xe* abundances.The isotopic signature associated with Xe-P6 is also found only in the larger size separates. Concentration of Xe-HL increases with increasing grain size, but its relative abundance with respect to Xe-P3 and P6 is higher in smaller grain-size fractions. We argue that Xe-P6 is best seen as a variant of Xe-HL, and that they are both mixtures of a “normal” component akin to solar xenon and a slightly variable exotic component. We show that both current models of Xe-H formation can account for the observed variability, and propose a scenario according to which Xe-HL and P6 were implanted into separate diamond populations before incorporation of Xe-P3 and ¹²⁹I.     

Hydrogen Isotopes of Glassy and Phyllosilicate Spherules in AI Rais （CR） and Orgueil （CI） Chondrites

The hydroxyl in phyllosilicate minerals is the most common occurrence of water in primitive meteorites.Direct hydrogen isotopic analysis of this water component using an ion microprobe has been made in some glassy or phyllosilicate spherules from the A1 Rais (CR) and Orgueil (CI) chondrites. The spherules from A1 Rais show largedeuterium excesses (δD= 200- 800‰) relative to terrestrial standards, whereas deuterium-enrichments in the spherules from Orgueil are much smaller (δD= 40- 130‰). The phyllosilicate spherules are products of aqueous alteration of glassy precursors. In A1 Rais the phyllosilicate spherules have relatively higher δD values than the glassy ones, indicating that water introduced during aqueous alteration was deuterium-enriched. The deuterium-enrichments in the phyllosilicate spherules from Orgueil could result from isotopic exchange under thermodynamic conditions within the solar nebula. The much larger δD excesses of the A1 Rais spherules, however, cannot be attributed to the similar process;instead, an interstellar origin needs to be invoked.     

Hydrogen Isotopes of Glassy and Phyllosilicate Spherules in Al Rais (CR) and Orgueil (CI) Chondrites

The hydroxyl in phyllosilicate minerals is the most common occurrence of water in primitive meteorites. Direct hydrogen isotopic analysis of this water component using an ion microprobe has been made in some glassy or phyllosilicate spherules from the Al Rais (CR) and Orgueil (CI) chondrites. The spherules from Al Rais show large deuterium excesses (δD = +200 -+800‰) relative to terrestrial standards, whereas deuterium-enrichments in the spherules from Orgueil are much smaller (δD = +40 - +130‰). The phyllosilicate spherules are products of     

Heterogeneous distribution of paramagnetic radicals in insoluble organic matter from the Orgueil and Murchison meteorites

An electron paramagnetic resonance (EPR) investigation was performed on the insoluble organic matter from the Orgueil and Murchison meteorites and on terrestrial coals with similar elemental composition. A complementary electron nuclear double resonance (ENDOR) study was also carried out. The measured g-factors of the observed paramagnetic radicals in the meteoritic organic matter exhibit a similar correlation with the chemical composition as for the type III (i.e., hydrogen-poor) terrestrial coals. The main result, obtained from EPR saturation and ¹H ENDOR enhancement measurements, showed that the effective local concentration in radicals of about 3 to 4 × 10¹⁹ spin.g⁻¹ in the meteoritic organic matter is much higher than the average concentration, hence the occurrence of radical-rich regions accounting for 5% and 20% of the total volume for Murchison and Orgueil, respectively. This heterogeneity of concentration seems to be unique among natural organic macromolecules. It is proposed that these radical-rich regions correspond to pristine parts of the organic matter synthesized in conditions close to those prevailing in the interstellar medium, and which have survived the hydrothermal process on the parent body.     

Trapped Xe and I-Xe ages in aqueously altered CV3 meteorites

Twenty-two dark inclusions (DIs) from Allende (18), Leoville (2), Vigarano (1) and Efremovka (1) were studied by the I-Xe method. All except two of these DIs (Vigarano 2226 and Leoville LV2) produce well-defined isochrons, and precise I-Xe ages. The Allende DIs formed a tight group about 1.6 Ma older than Shallowater (4.566 ± 0.002 Ga), about 5 Ma older than four previously studied Allende CAIs. Most of the dark inclusions require trapped Xe with less ¹²⁹Xe (or more ¹²⁸Xe) than conventional planetary Xe (well restricted in composition by Q-Xe or OC-Xe). Studies of an irradiated/unirradiated DI pair from Allende demonstrate that the ¹²⁸Xe/¹³²Xe ratio in trapped is normal planetary, so that a ¹²⁹Xe/¹³²Xe ratio below planetary seems to be required. Yet, this is not possible given constraints on ¹²⁹Xe evolution in the early solar system. Trends among all of the Allende DIs suggest that an intimate mixture of partially decayed iodine and Xe formed a pseudo trapped Xe component enriched in both ¹²⁹Xe and ¹²⁷I, and subsequently in ¹²⁸Xe after n-capture during reactor irradiation. Enrichment in radiogenic ¹²⁹Xe, but with a ¹²⁹Xe/¹²⁷I ratio less than that observed in the iodine host phase, places closure of this trapped mixture ≥13 Ma after precipitation of the major iodine-bearing phase. Because the I-Xe isochron is a mixing line between iodine-derived and trapped Xe (pseudo or not), I-Xe ages, given by the slope of this mixing line, are not compromised by the presence of pseudo trapped Xe, and the precision of the I-Xe ages is given by the statistics of the line fit.     

Oxygen isotopic evolution of amoeboid olivine aggregates in the reduced CV3 chondrites Efremovka, Vigarano, and Leoville

Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Vigarano, and Leoville consist of forsteritic olivine, FeNi-metal and a refractory component composed of spinel, Al-diopside, ±anorthite. Secondary ferrous olivine and alkali-rich minerals (nepheline and sodalite), commonly observed in the oxidized CVs, are rare. Mineralogy and chemical compositions of AOAs are similar to those predicted by equilibrium thermodynamic condensation models, suggesting that AOAs formed primarily by gas-solid condensation over a narrow temperature range, slightly below the temperatures over which most Ca-Al-rich inclusions (CAIs) formed. AOAs in the reduced CVs preserve a 1^st-generation ¹⁶O-rich signal (δ^17,18O ∼ −40‰) similar to that observed in many CAIs, suggesting that these refractory objects originated from a common source in the solar nebula. In fact AOAs and many fine-grained CAIs may have formed by the same processes, but at slightly different temperatures, and can be considered a single class of refractory objects.Alteration of the AOAs is manifested by differing extents of ¹⁶O-depletion in original AOA minerals, FeO-enrichment in olivine, and formation of interstitial very fine grained Na-bearing phases. From the six AOAs and one fine-grained, melilite-pyroxene-rich CAI examined in this study, five distinct patterns of alteration were identified. (1) One unaltered AOA from Vigarano is characterized by ¹⁶O-rich forsterite without FeO-rich rims and interstitial Na-bearing phases. (2) Weak alteration in the melilite-pyroxene-rich CAI is characterized by incomplete ¹⁶O-depletion in some melilite and precipitation of Na-bearing phases near the CAI rim. (3) Oxygen isotopic composition and mineralogy are correlated in two AOAs from Leoville with ¹⁶O-rich olivine, ¹⁶O-poor anorthite and a range of intermediate compositions in Al-diopside. This pattern is consistent with model diffusion between original grains and a ¹⁶O-poor reservoir during a relatively short-term (<60 yr), high-temperature (900-1100°C) event. (4) Original forsterite has been enriched in FeO, but remained ¹⁶O-rich in one AOA from Vigarano. This result is consistent with the slower rate of diffusion of O than Fe and Mg in olivine. At least some interstitial phases are ¹⁶O-rich, and Na-bearing phases are abundant in this AOA. (5) In contrast, oxygen isotopic composition and Fo-content are correlated in two AOAs from Efremovka. The olivine in these AOAs tends to have forsteritic ¹⁶O-rich cores and FeO-rich ¹⁶O-depleted rims. The general correlation between oxygen isotopic composition and Fo-content is difficult to model by diffusion, and may have formed instead by aqueous dissolution and precipitation along the margins of preexisting olivine grains.Independent evidence for aqueous alteration of the Efremovka AOAs is provided by OH-rich signals detected during ion beam sputtering of some of the ¹⁶O-poor olivine. Elevated ¹⁶OH-count rates and order of magnitude increases in ¹⁶OH detected during single analyses reflect trapping of an aqueous phase in ¹⁶O-depleted olivine. An elevated ¹⁶OH signal was also detected in one analysis of relatively ¹⁶O-poor melilite in the melilite-pyroxene CAI from Vigarano, suggesting that this object also was altered by aqueous fluid.     

Tungsten and hafnium distribution in calcium-aluminum inclusions (CAIs) from Allende and Efremovka

Recent ¹⁸²Hf-¹⁸²W age determinations on Allende Ca-, Al-rich refractory inclusions (CAIs) and on iron meteorites indicate that CAIs have initial ε¹⁸²W (−3.47 ± 0.20, 2σ) identical to that of magmatic iron meteorites after correction of cosmogenic ¹⁸²W burn-out (−3.47 ± 0.35, 2σ). Either the Allende CAIs were isotopically disturbed or the differentiation of magmatic irons (groups IIAB, IID, IIIAB, and IVB) all occurred <1 m.y. after CAI formation. To assess the extent of isotopic disturbance, we have analyzed the elemental distribution of Hf and W in two CAIs, Ef2 from Efremovka (CV3 reduced), and Golfball from Allende (CV3 oxidized). Fassaite is the sole host of Hf (10-25 ppm) and, therefore, of radiogenic W in CAIs, with ¹⁸⁰Hf/¹⁸⁴W > 10³, which is lowered by the ubiquitous presence of metal inclusions to ¹⁸⁰Hf/¹⁸⁴W > 10 in bulk fassaite. Metal alloy (Ni ∼ 50%) is the sole host of W (∼500 ppm) in Ef2, while opaque assemblages (OAs) and secondary veins are the hosts of W in Golfball. A large metal alloy grain from Ef2, EM2, has ¹⁸⁰Hf/¹⁸⁴W < 0.006. Melilite has both Hf and W below detection limits (<0.01 ppm), but the presence of numerous metallic inclusions or OAs makes melilite a carrier for W, with ¹⁸⁰Hf/¹⁸⁴W < 1 in bulk melilite. Secondary processes had little impact on the ¹⁸²Hf-¹⁸²W systematics of Ef2, but a vein cross-cutting fassaite in Golfball has >100 ppm W with no detectable Pt or S. This vein provides evidence for transport of oxidized W in the CAI. Because of the ubiquitous distribution of OAs, interpretations of the ¹⁸²Hf-¹⁸²W isochron reported for Allende CAIs include: (i) all W in the OAs was derived by alteration of CAI metal, or (ii) at least some of the W in OAs may have been equilibrated with radiogenic W during metamorphism of Allende. Since (ii) cannot be ruled out, new ¹⁸²Hf-¹⁸²W determinations on CAIs from reduced CV3 chondrites are needed to firmly establish the initial W isotopic composition of the solar system.     

New insight on aliphatic linkages in the macromolecular organic fraction of Orgueil and Murchison meteorites through ruthenium tetroxide oxidation

Ruthenium tetroxide oxidation was used to examine the macromolecular insoluble organic matter (IOM) from the Orgueil and Murchison meteorites and especially to characterize the aliphatic linkages. Already applied to various terrestrial samples, ruthenium tetroxide is a selective oxidant which destroys aromatic units, converting them into CO₂, and yields aliphatic and aromatic acids. In our experiment on chondritic IOM, it produces mainly short aliphatic diacids and polycarboxylic aromatic acids. Some short hydroxyacids are also detected.Aliphatic diacids are interpreted as aliphatic bridges between aromatic units in the chemical structure, and polycarboxylic aromatic acids are the result of the fusion of polyaromatic units. The product distribution shows that aliphatic links are short with numerous substitutions. No indigenous monocarboxylic acid was detected, showing that free aliphatic chains must be very short (less than three carbon atoms). The hydroxyacids are related to the occurrence of ester and ether functional groups within the aliphatic bridges between the aromatic units. This technique thus allows us to characterize in detail the aliphatic linkages of the IOMs, and the derived conclusions are in agreement with spectroscopic, pyrolytic, and degradative results previously reported.Compared to terrestrial samples, the aliphatic part of chondritic IOM is shorter and highly substituted. Aromatic units are smaller and more cross-linked than in coals, as already proposed from NMR data. Orgueil and Murchison IOM exhibit some tiny differences, especially in the length of aliphatic chains.     

Kinetics of C, N, and Xe release during the quasi-isothermal pyrolysis and subsequent oxidation of nanodiamond from the Orgueil CI meteorite

Analysis of the C, N, and Xe release kinetics of intermediate-sized nanodiamond fraction from the Orgueil CI meteorite during isothermal pyrolysis conducted for the first time and subsequent oxidation indicates that (a) the rate of C, N, and Xe release at pyrolysis at a constant temperature decreases with time; (b) the relative amount of released Xe, which mostly has a normal isotopic composition (Xe-P3) at various pyrolysis time up to 800°C, is controlled, first of all, by the heating temperature, whereas the amount of N is controlled by both the temperature and heating time; and (c) prolonged pyrolysis notably modifies the distribution of nitrogen of normal (δ¹⁵N = 0) and anomalous (δ¹⁵N= ?350‰) isotopic composition in diamond grains. The identified features of the C and N release kinetics are explained by differences in the binding energy of chemically adsorbed O with C atoms and the accommodation of the main amounts of N in extended defects of the crystal structure of nanodiamond. The major factors of the decrease in the Xe-P3 release rate during the isothermal pyrolysis of nanodiamond are either the differences between the Xe desorption parameters of the traps in graphite-like phases containing Xe-P3 or the differences between the radiation-induced defectiveness of grains of the population containing implanted Xe-P3. Our results led us to conclude that (1) meteoritic nanodiamond contains relatively low amounts of a phases carrying the P3 component of noble gases, regardless of the nature of this component, and (2) the population of nanodiamond grains containing most of isotopically anomalous nitrogen was produced at a high rate to preserve this nitrogen, first of all, at extended defects in the diamond crystal structure.     

南极三块（L3）陨石球粒结构类型及其矿物学物征

我国第16次南极考察队回收到6块稀少种类陨石-13型,GRV99001,GRV99019,GRV99020,GRV99021,GRV99022,GRV99026。本文对其中3块陨石进行研究,研究它们的球粒结构和矿物化学成分．它们虽然部属于非平衡普通球粒陨石（L3）,但它们的亚类不同,GRV99001为13．4,GRV99026为L3．5,GRV99019为L3．6．它们的球粒结构和球粒内的矿物晶体完整性和矿物组合变化比较大．橄榄石和辉石以高镁为特征．这三块陨石的球粒结构种类比较多．有班状的、炉条状的、扇形的和隐晶质的等．在GRV99001陨石中班状结构的球粒内能见到一个或两个以上完整的单晶橄榄石构成的球粒,也能见到多个细小的或是破碎橄榄石,被包襄在辉石晶体内．而在GRV99019和GRV99026陨石中只能见到多个细小单晶体或是破碎的橄榄石晶体．GRV99001陨石的炉条状结构,好象是由一条带状长石矿物,穿插在单个橄榄石晶体中构成．扇形和伞形结构的球粒,以一个点为中心,向外放射呈扇形．如GRV99019陨石中扇形结构球粒,它们是以辉石为主,陨硫铁充填在低钙辉石缝隙中,形成扇形．另一种是以多个点为中心,如GRV99001陨石,它们是由橄榄石、低钙辉石和长石质的玻璃．构成多个小伞形,形状类似三维立体的球,裂缝中也充填有金属矿物．隐晶质的球粒在GRV99026陨石中有两种,一种是在一厘米等于100un时呈现隐晶质矿物,而放大到一厘米等于5un时,就可以清楚看到两种低钙辉石矿物,在低钙辉石中还有金属矿物．另一种隐晶质结构球粒由极细小破碎的橄榄石和辉石矿物构成．这三块陨石中的橄榄石和辉石都以高镁为特征．班状结构球粒,在GRV99001陨石中橄榄石的MgO～33．37～51．21,辉石为35．9～36．61;GRV99019陨石中橄榄石23．33～56．58．辉石21.38-33.07,GRV99026陨石中榄橄榄石45.91-52.63,辉石34.48-37.35,扇形结构球粒,在GRV99001陨石中橄榄石29.94-46.22,辉石28.17-30.36;GRV99019陨石中橄榄石29.17-34.38,辉石23.11-27.79,炉条状结构的球粒,在GRV99001陨石中橄榄石51.84-56.03,隐晶质结构球粒,在GRV99026陨石中辉石20.17-21.54,橄榄石30.84-32.66,由此看出矿物晶体完整性越好镁的含量越高。     

Modal mineralogy of CV3 chondrites by X-ray diffraction (PSD-XRD)

Using position sensitive detector X-ray diffraction (PSD-XRD) we determine a complete modal mineralogy for all phases present in abundances greater than 1 wt% in Vigarano, Efremovka, Mokoia, Grosnaja, Kaba and Allende. Reduced CV3 samples are comprised of (vol%): olivine (83-85%); enstatite (6.5-8.1%); anorthite (1.1-1.2%); magnetite (1.4-1.8%); sulphide (2.4-5.1%); Fe, Ni metal (2-2.2%). The oxidized samples are comprised of: olivine (76.3-83.9%); enstatite (4.8-7.8%); anorthite (1.1-1.7%); magnetite (0.3-6.1%); sulphide (2.9-8.1%); Fe, Ni metal (0.2-1.1%); Fe-oxide (0-2.7%) and phyllosilicate (1.9-4.2%). When our modal data is used to calculate a bulk chemistry that is compared to literature data a near 1:1 correlation is observed. PSD-XRD data indicates that olivine compositions may span almost the entire Fe-Mg solid solution series in all CV samples and that these contain a component (4-13%) of fine-grained olivine that is more Fe-rich (>Fa₆₀) than is typically reported. Modal mineralogy shows that there are mineralogic differences between CV3 samples classified as oxidized and reduced but that these sub-classes are most clearly distinguished by the relative abundance of metal and Ni content of sulphide, rather than abundance of magnetite. The most significant difference in modal mineralogy observed is the relative absence of phyllosilicate in reduced CV that essentially escaped aqueous alteration.Fayalite, ferrous olivine and magnetite are typically considered secondary alteration products. The abundances of these minerals overlap in oxidized and reduced samples and correlate positively supporting common conditions of formation in a relatively oxidizing environment. The abundances of fayalite, ferrous olivine and magnetite show no relationship to petrographic type and if these abundances were used as a proxy for alteration, Allende would be the least altered CV - contrary to all previous data. The implication is that thermal metamorphism on the parent body was de-coupled from formation of Fe-rich secondary minerals. Low temperature fluid-assisted metamorphism can also not easily explain the origin of fayalite, ferrous olivine and magnetite, since the reduced CVs appear to be largely unaffected by this process. Parent body models require an anhydrous low-temperature mechanism of secondary alteration. The alternative is that these phases formed prior to accretion of the final CV parent body.     

Infrared spectra of the Orgueil (C-1) chondrite and serpentine minerals

The infrared spectrum of Orgueil is discordant with spectra of chrysotile, antigorite, and lizardite polymorphs of serpentine. The spectrum of chamosite is in good, but not exact, agreement with that of Orgueil.     

Recently fallen Bukhara (CV3) and Kilabo (LL6) chondrites: A parallel study of luminescence, tracks, and cosmogenic radionuclides

Thermoluminescence, tracks of VH nuclei, and cosmogenic radionuclides with various half-lives were studied of the recently fallen Bukhara CV3 and Kilabo LL6 chondrites. The obtained experimental information and theoretical modeling were utilized to examine the thermal impact and exposure histories of these chondrites, estimate the sizes and masses they had before entering the atmosphere, extent of their ablation, the circumferences of the orbits, and to evaluate the distribution and variations in cosmic radiation in the heliosphere in the maximum phase of the 23rd solar cycle. The Kilabo LL6 chondrite and the Bensour LL6 (which fall six months earlier) were determined to be genetically related to the asteroid 3628 Boznemcova.     

26Al, 244Pu, 50Ti,REE, and trace element abundances in hibonite grains from CM and CV meteorites

The ion microprobe was used to measure Ti and Mg isotopes as well as rare earth and other trace elements in ten hibonites from the CM carbonaceous chondrites Murchison, Murray, and Cold Bokkeveld and in two hibonites and Ti-rich pyroxene from the CV chondrite Allende. In hibonites from Murchison and Murray fission track densities were also measured, as were Th and U concentrations. Eight of the hibonites, from all four meteorites, exhibit large Ti isotopic anomalies, particularly in ⁵⁰Ti. Two grains from Murray have ⁵⁰Ti excesses of ~ 10%. At least four nucleosynthetic components are required to account for all the Ti isotopic data. Neutron-rich nuclear statistical equilibrium nucleosynthesis is the most likely process to account for a ⁵⁰Ti-rich component (with ${}^{50}\text{Ti}{}^{49}\text{Ti}\text{\$}\text{̆}\text{20}$). The ion probe Ti isotopic measurements confirm that the solar nebula was isotopically heterogeneous on a small spatial scale and argue for a chemical memory origin of the Ti isotopic anomalies which were probably carried into the solar system in the form of refractory dust grains. However, there is no experimental evidence that such interstellar grains survived the formation of the hibonites. The REE and trace element patterns of the hibonites are similar to those seen in CAIs and can be interpreted in terms of fractionation effects during condensation from a gas of solar composition, thus arguing for a solar system origin of the hibonites. Additional evidence for such an origin is provided by the $\text{Pu}\text{Th}$ ratios, which are ~ 10⁻⁴, and by the Mg isotopic compositions which are normal except for ²⁶Mg1 due to ²⁶Al.Only three out of ten hibonites exhibit ²⁶Mg1, consistent with previous studies which demonstrated the paucity of ²⁶Mg1 in hibonites. Because of the refractory nature of hibonite and the presence of large Ti isotopic effects, we conclude that a heterogeneous distribution of ²⁶Al in the early solar system is the most likely reason. In particular, our observations of δ⁵⁰Ti = 15%. and of an isochron with (${}^{26}\text{Al}{}^{27}\text{Al}\text{)}{}_0\text{= 5 × 10}{}^{\mathrm{-8}}$ in the FUN inclusion HAL are evidence against both late formation and Mg redistribution to explain the lack of ²⁶Al in hibonites.There are no obvious correlations between the Ti isotopic compositions, the presence of ²⁶Mg1, the presence of ²⁴⁴Pu, and the REE and trace element patterns in individual hibonites. This indicates that the anomalous ⁵⁰Ti, as well as ²⁶A1 and ²⁴⁴Pu, were not co-produced in a single astrophysical source, and/or that these nuclides were introduced into the solar nebula by different carriers before being incorporated into the hibonites.     

A computer simulation study of (OH) defects in olivine

Recent experiments (Miller et al. 1987; Bell and Rossman 1992; Bai and Kohlstedt 1992) have shown that olivine, the dominant mineral in the Earth's upper mantle, can contain substantial amounts of water in the form of OH. There is uncertainty, however, as to the mechanisms by which water dissolves into the mineral structure and as to the site it occupies in the host lattice. We have therefore used atomistic computer simulation techniques based on the Born model of solids, to investigate the structures and energies of OH defects in olivine. Our calculations suggest that the most favourable route for incorporation of OH into olivine involves reactions with water accompanied by the reduction of ferric iron for which we obtain a solution energy of 0.46 eV. We propose therefore that the OH content will be largely controlled by the concentration of Fe³⁺.     

Petrologic study of SJ101, a new forsterite-bearing CAI from the Allende CV3 chondrite

The forsterite-bearing Type B (FoB) CAI SJ101 consists of three major structural units: (1) light patches of sector-zoned, poikilitic Al-rich clinopyroxene (Cpx) with numerous inclusions of small spinel grains and aggregates and subordinate amounts of Mg-rich melilite (Mel) and anorthite (An) (Sp-Cpx lithology), (2) dark sinuous bands of Al-rich clinopyroxene with large (up to ∼300 × 60 μm) poikilitically enclosed euhedral forsterite (Fo) crystals (Fo-Cpx lithology), and (3) the external Cpx-Sp-An rim overlying the entire inclusion. The two major lithologies are always separated by a transition zone of clinopyroxene poikilitically enclosing both forsterite and spinel. The patches of the Sp-Cpx lithology exhibit significant textural and mineralogical variability that is size-dependent. Small patches typically consist of Cpx and spinel with minor remnants of melilite and/or its alteration products. Large patches contain Mel-An-rich cores with either equigranular-ophitic-subophitic or ‘lacy’ textures reminiscent of those in Types B or C CAIs, respectively. All silicates poikilitically enclose numerous spinel grains of identical habit. Both melilite and anorthite gradually disappear toward the boundary with the Fo-Cpx lithology. Neither the evaporation mantle of Al-rich melilite typical of other FoBs nor the Wark-Lovering rim is present. Secondary minerals include grossular, monticellite, magnetite, and a few grains of wollastonite, andradite, and nepheline.Being a rather typical FoB mineralogically and chemically, texturally SJ101 differs from other FoBs in displaying the nearly complete segregation of forsterite from spinel which occur only in the Fo-Cpx and Sp-Cpx lithologies, respectively. The complex, convoluted internal structure of SJ101 suggests that the coarse-grained Sp-An-Mel-Cpx cores and Fo-Cpx lithology represent the precursor materials of FoBs, proto-CAIs and Fo-rich accretionary rims. While the inferred chemistry and mineralogy of the Fo-rich rims are fairly typical, the high Åk content in SJ101 melilite (78.7-82.3 mol.%) implies that the SJ101 proto-CAIs represent a new type of CAIs that has not been sampled before. This type of CAIs might have formed by remelting of spinel-rich condensates.The Group II REE pattern, slightly negative δ²⁹Si and δ²⁵Mg values, and nearly solar ratios of the major elements in the bulk SJ101 suggest that its precursors, proto-CAIs and Fo-rich rims, could have formed by a non-equilibrium condensation in a closed system of solar composition somewhat depleted in a super-refractory evaporation residue. The proposed formation scenario of SJ101 invokes a non-steady cooling and condensation of the nebular gas interrupted by at least two distinct melting episodes required to account for the igneous textures of the Mel-An-Cpx-rich cores (proto-CAIs) and the Fo-Cpx lithology.     

Isotopic anomalies of noble gases in meteorites and their origins—IV. C3 (Ornans) carbonaceous chondrites

The C3O chondrites Kainsaz, Lancé and Ornans were studied by an acid dissolution technique, to characterize the noble-gas components in 3 mineral fractions: HF, HCl-solubles (99% of the meteorite), chromite and carbon (0.3–0.9%), and ‘phase Q’, a poorly characterized trace mineral (0.05–0.4%) containing most of the Ar, Kr, Xe. For all fractions, gas contents decline in the order Kainsaz > Lancé > Ornans; this trend parallels volatile contents but not heterogeneity of olivine composition or degree of metamorphism and seems to reflect progressively higher condensation temperatures from the solar nebula.Solubles contain nearly unfractionated Xe, and show ${}^{136}\text{Ar}{}^{132}\text{Xe}$ ratios up to 850. Hence the high $\text{Ar}\text{Xe}$ ratios (200–400) of bulk C3O chondrites must be due to an HF-soluble mineral (possibly magnetite). Phase Q contains ordinary planetary gases and a Ne component of ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{= 10.3 ± 0.4.}$Chromite and carbon contain Ne of ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{= 8.6 ± 0.1}$ and ‘CCF’ xenon (a peculiar component of possibly fissiogenic origin, enriched in the heavy isotopes but accompanied by a component enriched in the light isotopes).In all primitive chondrites, both the amount and the chemical separability of CCFXe parallel the abundance of promordial noble gases and other volatiles, such as C, N, Tl, Bi and In. The close correlation of CCFXe with various properties of undoubtedly local origin (volatile content, petrologic type, presence of ferrichromite and carbon, etc.) is more consistent with a local than with an extrasolar origin of this component. A volatile superheavy element seems to be the most plausible source, but the evidence is not conclusive.     

Mercury abundances and isotopic compositions in the Murchison (CM) and Allende (CV) carbonaceous chondrites

The abundance and isotopic composition of Hg was determined in bulk samples of both the Murchison (CM) and Allende (CV) carbonaceous chondrites using single- and multi-collector inductively coupled plasma mass spectrometry (ICP-MS). The bulk abundances of Hg are 294 ± 15 ng/g in Murchison and 30.0 ± 1.5 ng/g in Allende. These values are within the range of previous measurements of bulk Hg abundances by neutron activation analysis (NAA). Prior studies suggested that both meteorites contain isotopically anomalous Hg, with δ^196/202Hg values for the anomalous, thermal-release components from bulk samples ranging from −260 ‰ to +440 ‰ in Murchison and from −620 ‰ to +540 ‰ in Allende Jovanovic and Reed 1976a, Jovanovic and Reed 1976b, Kumar and Goel 1992. Our multi-collector ICP-MS measurements suggest that the relative abundances of all seven stable Hg isotopes in both meteorites are identical to terrestrial values within 0.2 to 0.5 ‰.On-line thermal-release experiments were performed by coupling a programmable oven with the single-collector ICP-MS. Powdered aliquots of each meteorite were linearly heated from room temperature to 900°C over twenty-five minutes under an Ar atmosphere to measure the isotopic composition of Hg released from the meteorites as a function of temperature. In separate experiments, the release profiles of S and Se were determined simultaneously with Hg to constrain the Hg distribution within the meteorites and to evaluate the possibility of Se interferences in previous NAA studies. The Hg-release patterns differ between Allende and Murchison. The Hg-release profile for Allende contains two distinct peaks, at 225° and 343°C, whereas the profile for Murchison has only one peak, at 344°C. No isotopically anomalous Hg was detected in the thermal-release experiments at a precision level of 5 to 30 ‰, depending on the isotope ratio. In both meteorites the Hg peak at ∼340°C correlates with a peak in the S-release profile. This correlation suggests that Hg is associated with S-bearing phases and, thus, that HgS is a major Hg-bearing phase in both meteorites. The Hg peak at 225°C for Allende is similar to release patterns of physically adsorbed Hg on silicate and metal grains. Prior studies suggested that the isotopic anomalies reported from NAA resulted from interference between ²⁰³Hg and ⁷⁵Se. However, the amount of Se released from both meteorites, relative to Hg, is insufficient to produce all of the observed anomalies.     

Occurence of abundant diradicaloid moieties in the insoluble organic matter from the Orgueil and Murchison meteorites: a fingerprint of its extraterrestrial origin?

The highly aromatic structure of the macromolecular organic matter (OM) of the Murchison and Orgueil meteorites was recently shown to contain free organic radicals which are concentrated in micro-regions in contrast with terrestrial samples which always show an homogeneous distribution of radicals. An additional signature is revealed, in the present study, by the evolution of the radical concentration with temperature. Whereas in terrestrial samples, this concentration is independent of temperature (Curie magnetism), a significant increase is observed above 150 K in the two meteorites. Based on the electronic structure of organic radicals, calculated by Extended Hückel and Density Functional methods, this behavior was assigned to the occurrence of diradicaloid moieties hosted by aromatic structures of 10 to 15 rings and having a quinoidal structure. They represent 40 and 25% of the total radicals in Orgueil and Murchison, respectively. The search for the cosmochemical interpretation of this unique observation should open a new field of experimental investigations.