A single asteroidal source for extraterrestrial Ordovician chromite grains from Sweden and China: High-precision oxygen three-isotope SIMS analysis

Abstract

We examined oxygen three-isotope ratios of 48 extraterrestrial chromite (EC) grains extracted from mid-Ordovician sediments from two different locations in Sweden, and one location in south-central China. The ages of the sediments (similar to 470 Ma) coincide with the breakup event of the L chondrite parent asteroid. Elemental compositions of the chromite grains are generally consistent with their origin from L or LL chondrite parent bodies. The average Delta O-17 (parts per thousand-deviation from the terrestrial mass-fractionation line, measured in situ from 15 mu m spots by secondary ion mass spectrometry; SIMS) of EC grains extracted from fossil meteorites from Thorsberg and Brunflo are 1.17 +/- 0.09 parts per thousand (2 sigma) and 1.25 +/- 0.16 parts per thousand, respectively, and those of fossil micrometeorites from Thorsberg and Puxi River are 1.10 +/- 0.09 parts per thousand, and 1.11 +/- 0.12 parts per thousand, respectively. Within uncertainty these values are all the same and consistent with the L chondrite group average Delta O-17 = 1.07 +/- 0.18 parts per thousand, but also with the LL chondrite group average Delta O-17 = 1.26 +/- 0.24 parts per thousand. (Clayton et al., 1991). We conclude that the studied EC grains from correlated sediments from Sweden and China are related, and most likely originated in the same event, the L chondrite parent body breakup. We also analyzed chromites of modern H, L and LL chondrites and show that their Delta O-17 values coincide with averages of Delta O-17 of bulk analyses of H, L and LL chondrites. This study demonstrates that in situ oxygen isotope data measured by SIMS are accurate and precise if carefully standardized, and can be used to classify individual extraterrestrial chromite grains found in sediments. (c) 2009 Elsevier Ltd. All rights reserved. (Less)