Content and Isotopic Composition of Carbon in the Light and Dark Portions of Gas-Rich Chondrites

Abstract

The total carbon content and its isotopic composition has been determined in the light and dark portions of the gas-rich bronzite chondrites Breitscheid and Pantar. In accordance with previous measurements by Wlotzka the dark portions were found to contain more carbon than the light ones. In both meteorites the isotope ratio ¹³C/¹²C is approximately 5 permil higher in the dark portions than in the light ones. For Pantar, however, this holds only for Pantar I, i.e. those samples where the dark portions contain large amounts of light primordial gases. In case of Breitscheid the ratios do not fall into two distinct sets but rather seem to form a sequence with a positive correlation between carbon content and ¹³C/¹²C ratio.

There appear to be two possibilities to account for the differences:

• (1) The carbon in the dark portions consists of a mixture from two different sources with different isotope ratios. The admixed fraction could be solar wind proper or carbonaceous chondritic matter.

• (2) Both portions contained originally the same amount of carbon with identical isotopic composition and the change in the isotopic composition came about during the subsequent depletion of carbon at some time during the history of the meteorite.