Early Solar System aqueous activity: Sr isotope evidence from the Orgueil CI meteorite.

The CI meteorites are rare but important objects because they may represent our best sample of chemically unfractionated Solar System material. Despite the fact the these meteorites apparently retain their original chemical composition, they clearly contain secondary mineral phases, some at least believed to have been produced through the action of liquid water on the parent body. The timing of this event, however, was unknown. In an attempt to solve this problem, we have measured the Sr isotopic composition and 87Rb/86Sr in carbonates and sulphate separated from the Orgueil meteorite. Both of these phases probably precipitated from aqueous solution. Our first results, reported here, show that carbonate deposition occurred contemporaneously with parent body formation or shortly after it probably within 100 Myr. On the other hand, at least some of the calcium sulphate seems to have been formed recently.