CARBON IN OLIVINES OF CHONDRITIC METEORITES

Abstract

Carbon concentrations have been measured, using the 12C(d,p)13C nuclear reaction, in olivines from different chondritic meteorites: CV3 (Allende and Ningqiang), CH (Acfer214), CM (Mighei and Murchison), LL3.1 (Bishunpur) and from a dark inclusion of the Allende CV3 meteorite. Their chemical compositions vary from Mg-rich (primitive olivines grown under reducing conditions) to FeO-rich (olivines that have been affected by elemental exchange reactions in a more oxygen-rich environment), with variable contents of MnO and Cr2O3. The ways these olivines occur in the chondrites (e.g., as isolated olivines or olivines in aggregates or in a barred chondrule) have also been considered as they can indicate different processes involved in their genesis. Nevertheless, no correlation can be established between the chemical composition of olivines, their possible genesis and their carbon contents. Contrary to previous measurements of carbon contents in chondrules, Nuclear Reaction Analyses (NRA) show that olivines have carbon contents of less than 120 ppm. Notwithstanding the different physico-chemical conditions under which olivines could have been formed or been altered in meteorites (from highly reducing to oxidising conditions) carbon does not enter the structure of olivine. This widespread feature appears as an intrinsic characteristic of olivines regardless of their origin, either on Earth or in the Solar Nebula.