DISRUPTION OF THE L CHONDRITE PARENT BODY: NEW OXYGEN ISOTOPE EVIDENCE FROM ORDOVICIAN RELICT CHROMITE GRAINS

Abstract

Mid-Ordovician fossil meteorites found in the Thorsberg quarry, southern Sweden, are believed to have been deposited during a period of enhanced meteorite flux following the fragmentation of the L chondrite parent body. During diagenesis, the fossil meteorites were largely replaced by a secondary mineral assemblage. However, primary chromite grains have been preserved. High-precision oxygen isotope analysis by laser-assisted fluorination has been undertaken in order to confirm the chemical group (H, L or LL) to which the fossil meteorites belong. To test our methodology, chromites extracted from recent ordinary chondrite falls (Holbrook L6, Appley Bridge LL6 and Kernouve H6) have been analyzed and these show that ordinary chondrites can be classified into their respective groups (H, L, or LL) using the oxygen isotopic composition of chromite alone. Results from the Golvsten 001 meteorite demonstrate that this sample is an equilibrated L chondrite. The uniform major and minor element composition of chromites throughout the southern Swedish fossil meteorite section means that it is highly probable that all are L chondrites. High-precision oxygen isotope analysis of relict chromites thus further strengthens the link between the fossil meteorites and the disruption of the L chondrite parent body. The evidence presented here demonstrates that relict chromite grains survive diagenesis and can be used to classify ancient meteoritic material. Analysis of such fossil grains may prove to be a powerful tool, not only in the case of the mid-Ordovician event, but also in examining changes in the relative distribution of meteorite groups throughout geological time. © 2007 Elsevier B.V. All rights reserved.