AN in-situ TRACE ELEMENT STUDY OF REFRACTORY FORSTERITES FROM DIFFERENT TYPES OF UNEQUILIBRATED CHONDRITES

Introduction: Refactory forsterites are widespread components of all types of unequilibated chondrites. They occur in concentrations up to 0.3 vol.% and are extremly poor in Fe, Ni, Co and Mn and are rich in refractory lithophile elements (Ca, Al, Ti, Sc, V, REEs) [1,2]. A further characteristic of refractory forsterites ist that they are often significantly enriched in O [3]. In order to trace the origin of refractory forsterites, we have conducted an in-situ LA-ICP-MS trace element study of refractory forsterites from carbonaceous chondrites (Murchison, Allende, Vigarano) and, for the first time, from unequilibated ordinary chondrites (Dar al Gani 369, Dar al Gani 378 and Chainpur) and an R-chondrite (Dar al Gani 013). Preliminary results of these analyses are presented here. Analytical technique: Prior to LA-ICPMS analysis, major and minor elements were analyzed by EPMA. Trace element analyses were performed using the ANU RSES LA-ICPMS apparatus, consisting of a 193 nm ArF-excimer laser interfaced to an Agilent 7500 ICPMS. The size of the ablation spot was varied between 24 and 40 μm Calibration as well as drift correction was performed by measuring NIST 612 and BCR2G glass standards after each 5–10 sample spots. A single analysis (38 isotopes) took about two minutes. For analyses of refractory forsterites, intensities were normalized relative to Si. RLEs: All analyzed refractory forsterites show fractionation among the rare earth elements with LREE/HREE ≈ 10 (Fig. 1). Concentrations of Lu are between 0.5 and 1 x C1. The concentrations of LREEs were close to or below the detection limit of the LA-ICPMS setup used. Therefore no statement can be made about possible anomalies in Eu and Ce. There is no apparent difference in the REE content of refractory forsterites from carbonaceous. chondrites, ordinary chondrites or the R-chondrite Dar al Gani 013 (Fig. 1). The amount of REEs in the refractory forsterites correlates weakly with the concentrations of Ca, Al and Ti. The correlation between Ca and the other refractory lithophile elements is less well developed than the correlation between Al and the other RLEs. Correlations between Al2O3 and Ti, Zr and Y, which is indicative of the amount of REEs, are well developed (Fig. 2). This is in agreement with earlier observations that other refractory elements, such as Ca, Al and Ti also have similar abundances in forsterites of different groups of chondritic meteorites. As bulk carbonaceous chondrites and in particular CV-chondrites are enriched in refractory elements compared to ordinary chondrites, we conclude that the abundances of refractory elements in forsterites are independent of the bulk refractory element content of the host meteorite, suggesting independent formation of the forsterites. The patterns of RLEs in forsterites reflect forsterite/melt partition coefficients, suggesting that the forsterites could have crystallized from a melt with chondritic relative abundances and a ten to twenty fold enrichment in RLEs [1].