In situ analysis of oxygen isotopes and Fe/Mg ratios in olivine using SIMS: Preliminary results for an Allende chondrule

A secondary ion mass spectrometer (SIMS; CAMECA ims-6f) was used for in situ spot analyses of chemical and oxygen isotopic compositions in olivine. San Carlos olivine and Burma spinel were used for oxygen isotope standards. Repeated analyses of oxygen isotopes were performed using these terrestrial standards by changing several factors which might control instrumental mass fractionation (such as entrance slit position, field aperture position, etc.) and conditions required for reproducible oxygen isotope analysis were investigated. For an optimum condition, the results of &acd;40 min analyses were reproducible within ±3‰ (1σ) for ^O/^O and ±2‰ (1σ) for ^O/^O. For chemical composition (e.g., Fe/Mg ratio) analyses, calibration was made using various terrestrial olivine grains with EPMA data. The oxygen isotope and Fe/Mg ratio analyses were performed for olivine grains in an Allende chondrule, which seems to be a compound chondrule consisting of two parts, (I) and (II). The results show that most of the analyzed points (14 out of 17 points) have relatively ^O-rich compositions (δ^O=-14 to -4‰ and δ^O=-10 to -5‰), while three points near the edge of the chondrule (two points belong to (I) and one belongs to (II)) have relatively ^O-poor compositions (δ^O=-6 to -1‰ and δ^O=&acd;0‰). The latter three points are more FeO-rich (the Fe/(Fe+Mg) atomic ratios of 4.5-8.2%) than the others (1.6-4.4%; mostly <3.2%). The present results, though rather preliminary, suggest that there is heterogeneity in oxygen isotopic composition even within a single chondrule

General characterization of Antarctic micrometeorites collected by the 39th Japanese Antarctic Research Expedition: Consortium studies of JARE AMMs (III)

From November 1998 to January 1999,the 39th Japanese Antarctic Research Expedition (JARE-39) undertook Japanese first large-scale collection of Antarctic micrometeorites (AMMs), with sizes larger than 10μm, at the Meteorite Ice Field around the Yamato Mountains in Antarctica (at three different locations, for a total of 24 collection sites). The number of collected AMMs larger than 40μm is estimated to be about 5000. Here we present the general characterization (i.e., micro-morphology and surface chemical composition using SEM/EDS) of &acd;810 AMMs chosen from 5 of the 24 sites. Additionally, the mineral composition of 61 out of 810 AMMs was determined by Synchrotron X-ray radiation. Preliminary results on mineralogical and chemical compositions show similarities with that of previous studies, even though a pronounced alteration of some AMMs is noticed. A correlation is found between the Mg/Si ratio at the sample\u27s surfaces of unmelted AMMs and the age of snow/ice in which the AMMs are embedded

Comprehensive study of carbon and oxygen isotopic compositions, trace element abundances, and cathodoluminescence intensities of calcite in the Murchison CM chondrite

We have performed in situ analyses of C and O isotopic compositions, trace element concentrations, and cathodoluminescence (CL) intensities on calcite in Murchison, a weakly altered CM chondrite. We found that the trace element (Mg, Mn, and Fe) concentrations are heterogeneous within single calcite grains. Grain to grain heterogeneity is even more pronounced. The analyzed calcite grains can be separated into two distinct types with respect to their C isotopic ratios, trace element concentrations, and CL characteristics: Calcite grains with higher δ13CPDB values (∼75‰) have low trace element concentrations and uniformly dark CL, while grains with lower δ13C values (∼35‰) have higher trace element concentrations and CL zoning. In contrast to the C isotopic ratios, O isotopic ratios are similar for both types of calcites (δ18OSMOW ∼ 34‰).The O isotopic ratios, trace element concentrations, and CL characteristics provide no evidence for C-isotope evolution in fluids from a single C reservoir by Rayleigh-type isotope fractionation (i.e., removal of C-bearing gaseous species). Also, it seems difficult to explain the O and C isotopic compositions of the two types of calcites by their formation at different temperatures from a single fluid. Instead, the δ13C variation suggests the presence of at least two C reservoirs with different isotopic ratios in the aqueous fluids from which the calcites precipitated. The C reservoirs with lower δ13C values are likely to be organic matter. The same holds for the C reservoirs with higher δ13C values which might have significant contributions from the 13C-enriched grains identified in meteoritic insoluble organic matter. Thermodynamic calculations show that calcite with lower Fe concentrations formed under more reduced conditions than calcite with higher Fe concentrations. If this is the case, the 13C-rich organic grains may have been destroyed and dissolved in the fluids under more reduced conditions than other organic components. The fact that the two types of calcites were found in different domains in the same thin section suggests that microenvironments with diverse physicochemical conditions such as redox states were present at scales of 100’s μm