Mn-Cr 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 manganese-bearing sulfides, i.e., sphalerites (ZnS), alabandites (MnS), and niningerites (MgS), in nine unequilibrated enstatite chondrites (UECs). The goals were to determine whether live 53Mn (half life ~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, MAC88136 (EL3), MAC88184 (EL3), MAC88180 (EL3), and Indarch (EH4), have clear 53Cr excesses. These 53Cr excesses can be very large (d53Cr/52Cr ranges up to ~18,400 , the largest 53Cr 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 53Mn in the meteorite samples. In the remaining five meteorites, no detectable excesses of 53Cr were found, and only upper limits on the initial 53Mn/55Mn ratios could be established. The four meteorites with 53Cr excesses show variations in the inferred 53Mn/55Mn 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 53Mn) by varying degrees of reequilibration. Provided 53Mn was homogeneously distributed in the region of the early solar system where these objects formed, the data suggest that the time of last isotopic equilibration of sulfides in EL chondrites occurred at least 3 Ma after a similar episode in EH chondrites.