Combined chemical-oxygen isotope study of large igneous inclusions in ordinary chondrites

The Open University's repository of research publications and other research outputs Combined chemical-oxygen isotope study of large igneous inclusions in ordinary chondrites Conference Item (2016). Combined chemical-oxygen isotope study of large igneous inclusions in ordinary chondrites. For guidance on citations see FAQs. c [not recorded] Version: Version of Record Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyright owners. For more information on Open Research Online's data policy on reuse of materials please consult the policies page. Introduction: Large igneous-textured inclusions poor in metal and sulfide occur in ~4% of ordinary chondrites [1] but are otherwise diverse, suggesting various formation mechanisms [2,3]. Recent work on the petrology of 29 inclusions suggested that they can be subdivided into different bulk chemical groups, with no evidence that any were produced by igneous differentiation [4]. This is in contrast to other inclusions that could have formed by differentiation [5]. Here we expand the geochemical database to 41 inclusions and report on the oxygen isotopic compositions of 12. Our results have important implications for the origins of such inclusions. Chemical groups: Bulk chemical compositions of inclusions reconstructed from modal and phase composition data (SEM + EMPA) confirm earlier work [4] that inclusions comprise a few basic chemical groups, none of which correspond to that expected for igneous differentiation. The groups include: vapor-fractionated (Vfr, n=18 examples), unfractionated (Unfr, n=13), K-enriched but otherwise unfractionated (Unfr+K, n=4), feldspar-rich (FldR, n=4), and unique (n=2). Examples of each group can be found in both less and more metamorphosed chondrites, although Unfr is more prevalent in type 5 and 6 (9 of 13 examples) and Vfr in type 3 and 4 chondrites (17 of 18). The unfractionated (Unfr) group has lithophile element abundances similar to ordinary chondrites and depletions in Ni, Fe and S (Fig. 1a). This composition is best explained by the melting of chondrite accompanied by metal and sulfide loss. Five such inclusions are located near coarse metal-sulfide nodules that could have separated from the inclusions during in situ melting. Unfr inclusions sometimes show evidence for brecciation while partly molten, consisting of obviously brecciated olivine microphenocrysts or brecci-ated regions of olivine + mesostasis embedded in inclusion mesostasis that is not brecciated. Some or all Unfr inclusions could have formed by shock melting of chondrites. The Unfr+K group (inclusions 869-I1, 8231-I1, MET-I2, Dim-I1) is chemically and texturally similar to …