PLATINUM GROUP ELEMENT MICROANALYSIS OF ORDINARY CHONDRITE METAL GRAINS BY LASER ABLATION ICP-MS. A. J. Campbell and M. Humayun, Dept. of the Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 (acampbel@midway.uchicago.edu).

Introduction: Platinum group element abundances in metal grains in ordinary chondrites have been measured using laser ablation ICP-MS. An effort was made to determine the microdistribution of Re and Os in ordinary chondrite metal, knowledge of which is critical to the interpretation of ¹⁸⁷Re-¹⁸⁷Os ages and of chondrite genesis [1]. Relative abundances of Re, Os, Ir, and Pt in metallic grains in the Weston H4 and Soko-Banja LL4 chondrites are reported here.

Experimental: Specimens were polished and examined by SEM to select grains for microanalysis by laser ablation ICP-MS. Inclusion-free, single-phase regions of metallic grains were chosen. Six grains of Weston metal, seven in Soko-Banja, and three in Allegan H5 have been analyzed to date.

A CETAC LSX-200 laser ablation peripheral was used for solid sample introduction into a magnetic sector ICP mass spectrometer, the Finnigan MAT Element [2]. Each point on the sample was analyzed by three successive 15-pulse laser bursts; the laser-ablated pit produced was approximately 50 µm in diameter and 25 µm deep. The isotopes monitored were ⁵⁷Fe, ⁵⁹Co, ⁶⁰Ni, ¹⁰²Ru, ¹⁰³Rh, ¹⁰⁵Pd, ¹⁸⁷Re, ¹⁹²Os, ¹⁹³Ir, ¹⁹⁵Pt, and ¹⁹⁷Au. Instrumental sensitivity factors for each isotope were determined by measuring signal intensity from the ataxite Hoba, which has known concentrations of the elements of interest [3]. In the analyses discussed below a smaller mass range, ¹⁸⁷Re to ¹⁹⁵Pt, was analyzed to enhance the precision (typically 3.5%) with which relative abundances of the HPGEs were determined. In this mode of analysis the absolute concentrations were determined using external calibration of the signal intensities, which was accurate only to within a factor of 1.5 or better.

Results and Discussion: Weston matrix kamacite grains were found to contain siderophile elements in approximately chondritic relative proportions, within a factor of 2. Soko-Banja matrix metal was observed to have a wider range of PGE concentrations; for example, Os concentrations ranged from about 0.08x chondritic to 16x chondritic. In Allegan, chondrule metal and matrix metal showed similar PGE concentrations.

The relative HPGE abundances are presented in Figure 1, which shows CI-normalized Re/Os, Ir/Os, and Pt/Os ratios for different matrix metal grains in Soko-Banja and Weston. Errors are smaller than the symbols in this figure for most data, except ~20% errors on the three Soko-Banja grains that show high Re/Os ratios, and that had low PGE concentrations. The normalized Ir/Os (0.98-1.15) and Pt/Os (1.19-1.46) ratios in Weston show a limited range of values among the six analyses in Figure 1. The normalized Re/Os ratios in Weston, in contrast, have a broader range of 1.33-2.39. All of the Re/Os values in Weston kamacite are well above the CI value; this is consistent with the observation by Chen et al. [1] that Re is enriched with respect to Os in the metal fraction of ordinary chondrites, and suggests the presence of a low-Re/Os phase not yet identified.

Four of the Soko-Banja metal analyses in Figure 1 exhibit patterns similar to those observed in Weston. The Ir/Os and Pt/Os ratios cluster around CI values, and the Re/Os values are systematically superchondritic and show a greater range. The other three grains in Soko-Banja had higher Re/Os, Ir/Os, and Pt/Os ratios, and low PGE concentrations. This PGE signature is understandable for the sulfide grain, but it is unusual for kamacite and taenite to exhibit low PGE concentrations and PGE patterns similar to those observed in sulfide (Figure 1). It is conceivable that these metal grains were produced by decomposition of sulfides, and that they retained the original sulfide PGE signatures. An analogous dephosphidization of a metallic grain has been documented in CAI metal [4]. However, no further evidence of such a history for the Soko-Banja grains was obtained from petrographic study of the sample.

References: [1] Chen J. H. et al. (1998) GCA, 62, 3379-3392. [2] Campbell A. J. and Humayun M. (1999) Anal. Chem., 71, 939-946. [3] Campbell A. J. and Humayun M. (1999) LPSC XXX, #1974. [4] Campbell A. J. et al. (1999) LPSC XXX, #1609.