Organic Constituents in Carbonaceous Chondrites; Evidence for Preservation of Pristine Particles of Mixed Origins

Introduction: Carbonaceous chondrites are primitive meteorites that have sampled the oldest conden-sates of the solar system. They have accreted components of various origins: CAIs (also known as refrac-tory inclusions), chondrules, ice, noble gases and or-ganics. Radioactive decay of short lived radionucleides quickly heated these parent bodies and drove thermal metamorphism and aqueous alteration of their various constituents. Despites this post-accretionary modification , at least some components of the organic matter in the carbonaceous chondrites have retained distinctive isotopic and molecular properties that plausibly relate to their pre-accretionary origins in the early solar neb-ula or in the molecular cloud that gave birth to it [1, 2]. The processes that gave rise to early solar-system or-ganics and the extent to which these compounds were modified by parent body processes are still a matter of debate [3], and this work provides new insight into those questions. One way to address these questions is to study the relationships between the organic constituents of the carbonaceous chondrites and their surrounding inorganic matrix, in situ. Our goal is to assess how the organic matter was accreted on the parent body of the carbonaceous chondrites and what alterations it has undergone during the 4.5 billion years evolution of the parent bodies. Moreover we want to shed light on the potential of the minerals that are associated with the organics to induce catalytic processes for organic synthesis or isotopic exchange which could alter the original isotopic signatures. Preliminary work on the most hydrated carbona-ceous chondrites (CI) confirmed previous observations that organic matter occurs mainly as individual particles randomly distributed in the matrix within clay minerals [4]. No specific association with any inorganic component was observed, ruling out the hypothesis that the main fraction of the organic matter could be synthesized by catalytic reactions on grain surfaces. The present contribution reports new observations performed on CM, CV and CR chondrites along with the study of the isotopic processes that occurred on the parent body of these meteorites. Experimental: We have used the Cameca Nano-SIMS 50L installed at Caltech to image matrix fragments of Orgueil (CI), Tagish Lake (CI), Murchison (CM), Cold Bokkeveld (CM), Allende (CV) and Renazzo (CR) carbonaceous chondrites. A focused Cs