MANGANESE-CHROMIUM SYSTEMATICS IN SULFIDES OF UNEQUILIBRATED ENSTATITE CHONDRITES

We measured with a secondary ion mass spectrometer Mn/Cr ratios and Cr isotopes in individual grains of Mn-bearing sulfides (i.e., sphalerit es, ZnS; alabandites, MnS; and niningerites, MgS) in nine unequilibrat ed enstatite chondrites (UECs). The goals were to determine whether li ve Mn-53 (half-life similar to 3.7 Ma) was incorporated in these objec ts at the time of their isotopic closure and to establish whether Mn-C r systematics in sulfides in UECs can be used as a high-resolution chr onometer to constrain formation time differences between these meteori tes. Sulfide grains analysed in four of these UECs, MAC 88136 (EL3), M AC 88184 (EL3), MAC 88180 (EL3), and Indarch (EH4), have clear Cr-53 e xcesses. These Cr-53 excesses can be very large (delta(53)Cr/Cr-52 ran ges up to similar to 18,400 parts per thousand, the largest Cr-53 exce ss measured so far) and, in some grains, are well correlated with the Mn/Cr ratios. Thus, they were most likely produced by the in situ deca y of Mn-53 in the meteorite samples. In the remaining five meteorites, no detectable excesses of Cr-53 were found, and only upper limits on the initial Mn-53/Mn-55 ratios could be established. The four meteorit es with Cr-53 excesses show variations in the inferred Mn-53/Mn-55 rat ios in various sulfide grains of the same meteorite. The Mn-Cr systema tics in these sulfides were disturbed (during and/or after the decay o f Mn-53) by varying degrees of: reequilibration. Provided Mn-53 was ho mogeneously distributed in the region of the early solar system where these objects formed, the data suggest that the time of the last isoto pic equilibration of sulfides in EL chondrites occurred at least 3 Ma after a similar episode in EH chondrites.