EARLY AQUEOUS ACTIVITY ON PRIMITIVE METEORITE PARENT BODIES

THE interstellar material from which the Solar System formed has been modified by many processes(1): evaporation and condensation in the sol ar nebula, accretion into protoplanetary bodies and post-accretion pro cesses within these bodies, Meteorites provide a record of these event s and their chronology(2), Carbonaceous CI chondrites are among the mo st primitive, undifferentiated meteorites(3-6), but nevertheless show evidence of post-accretion alteration(7); they contain carbonates that are believed to have formed by reactions between anhydrous CI precurs or materials and circulating fluids in the meteorite parent body (or b odies), yet little is known about the nature of these reactions or the timescale on which they occurred, Here we report measurements of exce ss Cr-53-formed by the decay of short-lived Mn-53-in five carbonate fr agments from the CI chondrites Orgueil and Ivuna. Our results show tha t aqueous alteration on small protoplanetary bodies must have begun le ss than 20 Myr after the time of formation of the oldest known solar-n ebula condensates(2) (Allende refractory inclusions). This upper limit is much shorter than that of 50 Myr inferred from previous studies(8) , and clearly establishes aqueous alteration as one of the earliest pr ocesses in the chemical evolution of the Solar System.