The origin of magnetite in carbonaceous chondrites

Abstract

The content and distribution of magnetite in carbonaceous chondrites is of particular interest since it is a substantial component in these, the most primitive materials surviving from solar system formation. Previous attempts to determine the abundance of magnetite in the Orgueil carbonaceous chondrite resulted in values ranging from 6% to 50%. We developed two new non-destructive techniques for the accurate and precise measurement of magnetite in these rare meteorites. One utilized a Faraday balance, the other a vibrating-sample magnetometer. Agreement between the two methods is excellent. Over one thousand measurements were taken of the magnetic contents of these meteorites. The CI chondrites ranged from 8.5% to 11.1% magnetite. Unexpectedly, magnetite was quite uniform within a given meteorite, the variation less than or equal to 10%. Magnetite distribution in the three other groups of carbonaceous chondrites shows a wider range, varying from < 0.4% to 16%.<p> An examination of our data, coupled with previous discussions in the literature, lead us to propose that much of the magnetite in carbonaceous chondrites has resulted from the oxidation of iron sulfide species. It is not now possible to determine unambiguously whether this oxidation occurred in the solar nebula or during aqueous alteration on the surface of a planetessimal. However, strong evidence suggests the latter as the more likely site.