Cation distribution in orthopyroxenes from São João Nepomuceno iron meteorite inferred from 57Fe Mössbauer spectroscopy: Implications for thermal history and origin of IVA parent body

Abstract

São João Nepomuceno (SJN) is an IVA iron meteorite, found in Minas Gerais state (Brazil) in 1960, that consists of Fe-Ni metal matrix and coarse-grained silicate inclusions. In spite of the extensive work performed on the IVA irons, there is still no consensus about their origin and thermal history. Their particular chemistry and range in metallographic cooling rates are difficult to explain using conventional models. Furthermore, metal?silicate mixing of the IVA group remains a complex issue. In this work, the 57Fe Mössbauer spectroscopy was applied for measuring the intracrystalline Fe-Mg distribution in orthopyroxenes extracted from SJN. The Mössbauer data associated with Ganguly´s cooling rate numerical model were used to investigate the thermal history of SJN meteorite. The results are the background for discussions about the IVA formation models, aiming to improve the understanding of the origin of IVA iron meteorite group.