53Mn-53Cr systematics of sphalerite in enstatite chondrites

We investigated the Mn-53-Cr-53 isotopic composition of a suite of enstatite chondrites by in situ analyses of various mineral phases with the Cameca IMS 1280-HR ion probe at Heidelberg, Germany, and a Cameca NanoSIMS at Paris, France. Only in sphalerite we found anomalies in Cr-53/Cr-52-ratios correlating with Mn-55/Cr-52 which are due to decay of short-lived 53 Mn (t(1/2) = 3.7 Ma). A sphalerite in the EH-impact melt LAP 02225 showed the largest excess of Cr-53, with Cr-53/Cr-52-ratios ranging up to 2.2. The calculated initial Mn-53/Mn-55-ratios are within the range of previous Mn-Cr studies. We observed a spatial variation within a large sphalerite in LAP 02225, translating in distinct initial Mn-53/Mn-55 values. In case of the EL3 chondrite MAC 88136 the initial Mn-53/Mn-55 derived for one sphalerite is at the lower end of previously reported values and may reflect a variable influence of alteration-induced exchange of common Cr. This is supported by the total reset of the Cr-53/Cr-52-ratio in another sphalerite in contact with an alteration vein irrespective of high Mn/Cr-ratios. Our observed initial Mn-53/Mn-55-ratios of Sahara 97158, Indarch and EET 96135 correspond to the I-Xe systematics and hence, show that sphalerites can preserve reasonable age information. For Indarch however, if compared with other initial Mn-53/Mn-55 values from literature, a considerable scatter is obvious. This clearly demonstrates that the Mn-Cr system in sphalerite can be disturbed by various, still poorly investigated, processes (e.g., by thermal events, weathering, diffusion-controlled remobilization). Future application of Mn-Cr dating to sphalerite in enstatite chondrites thus requires a better understanding of how such processes influence the Mn-Cr systematics and demands for tools to identify the undoubtly present sphalerites carrying a true chronologic information. (C) 2021 Elsevier Ltd. All rights reserved.