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11.
Ghislaine Crozaz 《Geochimica et cosmochimica acta》1979,43(1):127-136
Uranium distributions have been determined in seventeen meteorites using fission track techniques. In seven cases, Th was also determined by a new method using fast neutrons. The actinides are generally concentrated in phosphates, usually whitlockite and/or chlorapatite. Wherever whitlockite and chlorapatite coexist, chlorapatite is richer in uranium. U concentrations in a given phosphate phase are highly variable from meteorite to meteorite and sometimes also show large variations in the same meteorite. A clinopyroxene phase enriched in U (0.2–0.3 ppm) is usually found in Ca-rich achondrites. The ratios of phosphates differ considerably from whole rock values indicating that these elements were fractionated during the meteorite formation. 相似文献
12.
Thomas Rigaudier Véronique Gardien François Martineau Ghislaine Reverdy Christophe Lécuyer 《Geostandards and Geoanalytical Research》2012,36(1):51-59
Fluid inclusions represent a unique opportunity for a straightforward determination of the chemical and isotopic composition of fluids that composed the hydrosphere and atmosphere over Earth’s history. The production of reference materials in the laboratory is needed to monitor and to validate the determination of hydrogen and oxygen isotope compositions of water inclusions. We propose a protocol leading to the experimental synthesis of halite crystals that contain water inclusions whose δD and δ18O values can be related to those of surrounding evaporating waters where the crystals grew. Corrections to isotopic measurements were performed by applying an orthogonal projection of the raw data to the water evaporation trajectory line whose slope can be predicted by taking into account the parameters developed in the linear resistance model of Craig and Gordon (1965). Several hundreds of grams of halite reference material can be produced rapidly (within 2 d) at a low cost and can be stored within a vacuum desiccator at ambient temperature over several months or years. The described method is especially useful for the analysis of anhydrous salts and the interpretation of isotopic fractionations that operate within the surficial water cycle. 相似文献
13.
Abstract— Electron and ion microprobe measurements of major, minor, and trace element concentrations were made in individual grains of pyroxene, plagioclase, and Ca phosphates in Pomozdino and Peramiho, two eucrites previously classified as anomalous. Although Pomozdino pyroxene is highly magnesian, minor and trace element concentrations in both pyroxene and plagioclase of this meteorite are similar to those in other noncumulate eucrites. High incompatible element concentrations (similar to those in Stannern) coupled with mg# typical of cumulate eucrites confirm the anomalous character of this meteorite but do not allow us to distinguish unequivocally between different possible modes of origin. Peramiho has mg# and trace element concentrations similar to main group eucrites, indicating that this meteorite most probably belongs to this group. A previously reported low incompatible element concentration for Peramiho may be due to a sampling problem. 相似文献
14.
Abstract— The Ca isotopic compositions of 32 oldhamite (CaS) grains from the Qingzhen (EH3), MAC88136 (EL3), and Indarch (EH4) enstatite chondrites were determined by ion microprobe mass spectrometry. Also measured were the S isotopic compositions of eight oldhamite, two niningerite (MgS), and seven troilite (FeS) grains. The S isotopic compositions of all minerals are normal, but oldhamite grains of the first two meteorites exhibit apparent small 48Ca excesses and deficits that are correlated with isotopic mass fractionation as determined from the 40Ca-44Ca pair. The interpretation of these results is complicated by the fact that none of the established mass fractionation laws can account for the data in the Norton County oldhamite standard. The method of analysis is carefully scrutinized for experimental artifacts. Neither interferences nor any known mass fractionation effect can satisfactorily explain the observed small deviations from normal isotopic composition. If these are truly isotopic anomalies, they are much smaller than those observed in hibonite. The nucleosynthetic origin of Ca isotopes is discussed. 相似文献
15.
Christine Floss Ghislaine Crozaz Takashi Mikouchi 《Geochimica et cosmochimica acta》2003,67(24):4775-4789
The recent discovery of two new angrites, Sahara 99555 and D'Orbigny, has revived interest in this small group of achondrites. We measured trace element abundances in the individual minerals of these two angrites and compared them with the three Antarctic angrites, LEW 86010, LEW 87051 and Asuka 881371. Trace element variations in four of these meteorites (LEW 87051, Asuka 881371, Sahara 99555 and D'Orbigny) indicate rapid crystallization under near closed system conditions, consistent with their mineralogical and textural features. All four appear to be closely related and crystallized from very similar magmas. Discrepancies between their bulk REE compositions and melts calculated to be in equilibrium with the major phases may be due in part to kinetic effects of rapid crystallization. Prior crystallization of olivine and/or plagioclase may also account for the elevated parent melt composition of clinopyroxene in some of the angrites such as Asuka 881371.LEW 86010 also crystallized from a melt and represents a liquid composition, but trace element trends in clinopyroxene and olivine differ from those of the other angrites. This meteorite seems to have crystallized from a different source magma. 相似文献
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17.
R. Bibron R. Chesselet G. Crozaz G. Leger J.P. Mennessier E. Picciotto 《Earth and Planetary Science Letters》1974,21(2):109-116
The reasons why53Mn (a cosmogenic radionuclide with a half-life of 3.7 × 106 y) appears as one of the best indicators of the presence of interplanetary dust are summarized. This paper reports the detection of53Mn in pre-1952 snow samples collected on the Eastern Antarctic Plateau in the vicinity of Plateau Station. The measurements were carried out by neutron activation and X-ray spectrometry on three samples weighing a few hundred kg and covering each the time interval 1935–1950. The specific activity of53Mn was found to be (0.82 ± 0.17) disint.min?1/103 tons of snow, corresponding to a deposition rate at Plateau Station of (2.2 ± 0.5) × 10?5 disint. min?1 m?2 y?1. The mean global deposition rate would be three times higher if53Mn were assumed to behave in the same way as stratospheric90Sr. By comparing this figure with existing data on the meteorite flux reaching the earth and with the galactic and solar production rates of53Mn, it is concluded that the bulk of the53Mn found at Plateau Station is associated with interplanetary dust in which it had been produced by the action of solar protons on iron. The deposition rate of extra-terrestrial dust-borne iron must be between 1.3 × 10?5 and 1.3 × 10?4 g m?2 y?1 at Plateau Station. These results support jointly with other studies the concept of an interplanetary zodiacal cloud of dust with a chemical composition and density not essentially different from chondritic meteorites, with a relatively ‘flat’ grain size distribution and a mass influx to the earth of the order of 105 tons/y. 相似文献
18.
Track densities in 85 feldspar crystals from L-2009 range from 2.5 × 106/cm2 to > 109/cm2. This track distribution represents an intermediate case between what have been previously defined as lightly and heavily irradiated soils and suggests that the Luna 20 sample consists of a mixture of a mature, heavily irradiated component with another, lightly irradiated component. Using a two component mixing model, the age of the lightly irradiated component is ~270 × 106 yr. It is possible, but by no means certain, that this is associated with the formation of the crater Apollonius C. At ~200°C the ratio of natural TL to that induced by a standard irradiation is similar to that in Apollo 12 and 14 cores below ~7 cm. This confirms that most of the Luna 20 sample represents sub-surface material. 相似文献
19.
Abstract— This paper explores the possible origin of the light rare earth element (LREE) enrichments observed in some ureilites, a question that has both petrogenetic and chronologic implications for this group of achondritic meteorites. Rare earth element and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites. The spatial microdistributions of REEs in C‐rich matrix areas of the three ureilites with the most striking V‐shaped whole‐rock REE patterns (Kenna, Goalpara, and Novo Urei) were investigated using the ion imaging capability of the ion microprobe. All olivines and clinopyroxenes measured have LREE‐depleted patterns with little variation in REE abundances, despite large differences in their major element compositions from ureilite to ureilite. Furthermore, we searched for but did not find any minor mineral phases that carry LREEs. The only exception is one Ti‐rich area (～20μm) in Lewis Cliff (LEW) 85400 with a major element composition similar to that of titanite; REE abundances in this area are high, ranging from La ? 400 × CI to Lu ? 40 × CI. In contrast, all ion microprobe analyses of C‐rich matrix in Kenna, Goalpara, and Novo Urei revealed large LREE enrichments. In addition, C‐rich matrix areas in the three polymict ureilites, Elephant Moraine (EET) 83309, EET 87720, and North Haig, which have less pronounced V‐shaped whole‐rock REE patterns, show smaller but distinct LREE‐enrichments. The C‐rich matrix in Antarctic ureilites tends to have much lower LREE concentrations than the matrix in non‐Antarctic ureilites. There is no obvious association of the LREEs with other major or minor elements in the C‐rich areas. Ion images further show that the LREE enrichments are homogeneously distributed on a microscale in most C‐rich matrix areas of Kenna, Goalpara, and Novo Urei. These observations suggest that the LREEs in ureilites most probably are absorbed on the surface of fine‐grained amorphous graphite in the C‐rich matrix. It is unlikely that the LREE enrichments are due to shock melts or are the products of metasomatism on the ureilite parent body. We favor LREE introduction by terrestrial contamination. 相似文献
20.
Meenakshi WADHWA Ernst K. ZINNER Ghislaine CROZAZ 《Meteoritics & planetary science》1997,32(2):281-292
Abstract— We measured with a secondary ion mass spectrometer Mn/Cr ratios and Cr isotopes in individual grains of Mn-bearing sulfides (i.e., sphalerites, ZnS; alabandites, MnS; and niningerites, MgS) in nine unequilibrated enstatite chondrites (UECs). The goals were to determine whether live 53Mn (half-life ～3.7 Ma) was incorporated in these objects at the time of their isotopic closure and to establish whether Mn-Cr systematics in sulfides in UECs can be used as a high-resolution chronometer to constrain formation time differences between these meteorites. Sulfide grains analysed in four of these UECs, MAC 88136 (EL3), MAC 88184 (EL3), MAC 88180 (EL3), and Indarch (EH4), have clear 53Cr excesses. These 53Cr excesses can be very large (δ53Cr/52Cr ranges up to ～18,400%, the largest 53Cr excess measured so far) and, in some grains, are well correlated with the Mn/Cr ratios. Thus, they were most likely produced by the in situ decay of 53Mn in the meteorite samples. In the remaining five meteorites, no detectable excesses of 53Cr were found, and only upper limits on the initial 53Mn/55Mn ratios could be established. The four meteorites with 53Cr excesses show variations in the inferred 53Mn/55Mn ratios in various sulfide grains of the same meteorite. The Mn-Cr systematics in these sulfides were disturbed (during and/or after the decay of 53Mn) by varying degrees of reequilibration. Provided 53Mn was homogeneously distributed in the region of the early solar system where these objects formed, the data suggest that the time of the last isotopic equilibration of sulfides in EL chondrites occurred at least 3 Ma after a similar episode in EH chondrites. 相似文献