Radiogenic 53Cr* in Orgueil Carbonates: Chronology of Aqueous Activity on the CI Parent Body

Details Radiogenic 53Cr* in Orgueil Carbonates: Chronology of Aqueous Activity on the CI Parent Body Radiogenic 53Cr* in Orgueil Carbonates: Chronology of Aqueous Activity on the CI Parent Body

Mar 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996lpi....27..577h&link_type=abstract

Lunar and Planetary Science, volume 27, page 577

Mathematics

Logic

7

Alteration: Aqueous, Chondrites: Ci, Isotope Anomalies, Orgueil

Scientific paper

The petrographic and mineralogical features characteristic of CI carbonaceous chondrites, including carbonate- and sulfate-filled veins, magnetite morphologies and the ubiquitous presence of carbonates provide strong evidence for the action of aqueous fluids (water) on the CI parent planetesimal. Although commonly described as "early", the time scale of aqueous alteration is not well constrained. Dolomitic (Ca-Mg) carbonates in Orgueil are characterized by very low 87Sr/86Sr ratios, consistent with the lowest values reported for Allende, and must have been deposited within ~50 Ma of Allende refractory inclusions, while 53Cr* excesses in some, but not all, carbonates in two CI chondrites suggest an even earlier onset of aqueous activity. We present here a new study of the Mn-Cr isotope system in Mn-rich carbonates (breunnerite) in Orgueil, providing unambiguous evidence of large excesses of radiogenic 53Cr* from the in situ decay of short-lived 53Mn (t1/2 ~ 3.7 Ma). The inferred initial abundance of 53Mn, 53Mn/55Mn ~ 2 x 10-6, implies early formation of these Orgueil carbonates, within ~16 Ma of Allende Ca-, Al-rich inclusions.

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