¹Edivaldo Dos Santos,²Rosa B. Scorzelli,³Maria E. Varela
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13121]
¹Instituto de Ciência e Tecnologia—ICT/UFVJM, Minas Gerais, Brazil
Centro Brasileiro de Pesquisas Físicas—CBPF, Rio de Janeiro, Brazil
²Instituto de Ciencias Astronómicas, de la Tierra y del
³Espacio—ICATE/CONICET, San Juan, Argentina
Published by arrangement with John Wiley & Sons

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.

^1,2,3Alan E. Rubin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13128]
¹Department of Earth, Planetary and Space Sciences, University of CaliforniaLos Angeles, California, USA
²Institute of Geophysics and Planetary Physics, University of CaliforniaLos Angeles, California, USA
³Maine Mineral & Gem Museum, Bethel, Maine, USA
Published by arrangement with John Wiley

Iron‐meteorite groups that appear from published isotopic data to have been derived from melted carbonaceous‐chondrite‐like precursors (CC irons) (IIC, IID, IIF, IIIF, IVB) tend to have higher median refractory siderophile element (RSE) contents, higher median Ni contents, and higher median Ir/Ni and Ir/Au ratios than magmatic noncarbonaceous (NC) iron‐meteorite groups (IC, IIAB, IIIAB, IIIE, IVA). (Group IIG is also NC.) One potential source of RSEs in magmatic CC irons is the set of refractory metal nuggets from inherited CAIs. Magmatic CC‐iron groups tend to have longer cosmic‐ray exposure (CRE) ages than magmatic NC‐iron groups, indicating long residence times as small bodies in interplanetary space. The lower membership of CC‐iron groups is probably mainly due to the high oxidation state of their precursors. Such oxidation would have produced lesser amounts of free metal; parent body differentiation of such bodies would have produced smaller cores, resulting in fewer samples available to make CC‐iron meteorites in the first place. (Ungrouped magmatic irons, most of which can be considered groups with only one member, also tend to be carbonaceous.) It is possible that a subset of the chondrule‐poor dark inclusions in many carbonaceous chondrites represent unmelted materials related to the precursors of the CC irons. The Eagle Station pallasites (also CC‐related) are analogous to CC irons in being more oxidized, richer in Ni and RSEs, and fewer in number than main‐group pallasites (PMG). However, Eagle Station has a shorter CRE age than most PMG.