Early aqueous activity on primitive meteorite parent bodies
Magnus Endress*, Ernst Zinner†‡ & Adolf Bischoff*
* Institute of Planetology, University of Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany
† McDonnell Center for the Space Sciences and Physics Department, Washington University, St Louis, Missouri 63130, USA
‡ To whom correspondence should be addressed.
THE interstellar material from which the Solar System formed has been modified by many processes1: evaporation and condensation in the solar nebula, accretion into protoplanetary bodies and post-accretion processes within these bodies. Meteorites provide a record of these events and their chronology2. Carbonaceous CI chondrites are among the most primitive, undifferen-tiated meteorites3–6, but nevertheless show evidence of post-accretion alteration7; they contain carbonates that are believed to have formed by
reactions between anhydrous CI precursor materials and circulating fluids in the meteorite parent body (or bodies), yet little is known about the nature of these reactions or the timescale on which they occurred. Here we report measurements of excess 53Cr—formed by the decay of short-lived 53Mn— in five carbonate fragments from the CI chondrites Orgueil and Ivuna. Our results show that aqueous alteration on small protoplanetary bodies must have begun less than 20 Myr after the time of formation of the oldest known solar-nebula
condensates2 (Allende refractory inclusions). This upper limit is much shorter than that of 50 Myr inferred from previous studies8, and clearly establishes aqueous alteration as one of the earliest processes in the chemical evolution of the Solar System.
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