Links between petrographic, chemical and isotopic properties of chondrites

Seminar Date: 
24 Oct 2013 - 15:00
Brigitte Zanda
Muséum National d’Histoire Naturelle

Chondrites, which come from asteroids that escaped the melting event(s) leading to differentiation, are a window on processes that were taking place over 4.5 Byr ago in the protoplanetary disk. They share first order common properties: all are an assemblage of high temperature components (refractory inclusions, chondrules, metal grains, and also presolar grains) embedded in a fine grained low temperature matrix, and their chemical composition mimics that of the Sun (except for H and the most volatile elements).
        Yet chondrites are incredibly varied: (1) Petrography: chondrule sizes are in most cases sorted within a given group, commonly ranging from a few tens of microns (in CH chondrites) up to a millimeter (in CVs), abundance of matrix varies from no more than a few vol% (in ECs) up to above 95 vol% (in CIs), metal abundance ranges from 0% (in CIs) up to several tens of %. (in CHs). (2) Chemistry: redox state, total Fe abundance and volatile element budget vary tremendously from group to group. (3) Oxygen (Cr and other element) isotopic compositions also exhibit a large range of variation, much larger than that of differentiated bodies known so far.
        Some of these properties are likely to have been acquired or modified as a result of a complex history involving parent body processes (e.g. mineralogy, redox state), while others must derive from preaccretionary processes (e.g. chondrule sizes, component proportions). In this talk, we will discuss some of the connections between property variations in chondrites. For example, we will suggest that matrix abundance not only controls the volatile element budget of carbonaceous chondrites, but also contributes to their oxygen isotopic composition. We will also show that matrix-chondrule complementarity is more likely to result from parent-body alteration overprint rather than from preaccretionary processes.