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., sphalerites, ZnS; alabandites, MnS; and niningerites, MgS) in nine unequilibrated enstatite chondrites (UECs). The goals were to determine whether live Mn-53 (half-life similar to 3.7 Ma) was incorporated in these objects at the time of their isotopic closure and to establish whether Mn-Cr systematics in sulfides in UECs can be used as a high-resolution chronometer to constrain formation time differences between these meteorites. Sulfide grains analysed in four of these UECs, MAC 88136 (EL3), MAC 88184 (EL3), MAC 88180 (EL3), and Indarch (EH4), have clear Cr-53 excesses. These Cr-53 excesses can be very large (delta(53)Cr/Cr-52 ranges up to similar to 18,400 parts per thousand, the largest Cr-53 excess 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 decay 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 meteorites with Cr-53 excesses show variations in the inferred Mn-53/Mn-55 ratios in various sulfide grains of the same meteorite. The Mn-Cr systematics in these sulfides were disturbed (during and/or after the decay of Mn-53) by varying degrees of: reequilibration. Provided Mn-53 was homogeneously distributed in the region of the early solar system where these objects formed, the data suggest that the time of the last isotopic equilibration of sulfides in EL chondrites occurred at least 3 Ma after a similar episode in EH chondrites.