Sulfide‐dominated partial melting pathways in brachinites. (9th September 2020)

• Record Type: Journal Article
• Authors: Crossley, Samuel D.; Ash, Richard D.; Sunshine, Jessica M.; Corrigan, Catherine M.; M Coy, Timothy J.; Mittlefehldt, David W.; Puchtel, Igor S.
• Is Part Of: Meteoritics & planetary science. Volume 55:Number 9(2020)
• Page Start: 2021
• Page End: 2043
• Publication Date: 2020-09-09
• Subjects: Meteorites -- Periodicals; Planetology -- Periodicals; 523.4
• DOI: 10.1111/maps.13558
• Languages: English
• ISSNs: 1086-9379

Abstract

Abstract: Petrogenetic relationships among members of the brachinite family were established by analyzing major and trace element concentrations of minerals for 9 representative specimens: Al Huwaysah 010, Eagles Nest, Northwest Africa (NWA) 4882, NWA 5363, NWA 7297, NWA 7299, NWA 11756, Ramlat as Sahmah (RaS) 309, and Reid 013. The brachinite family, which includes brachinites and ungrouped achondrites with compositional and isotopic similarities to brachinites, comprises FeO‐rich, olivine‐dominated achondrites whose compositional and mineralogic variability is correlated with oxidation state. Most classical brachinites are derived from precursors that were more oxidized and sulfur‐rich than those of ungrouped "brachinite‐like" achondrites. This is manifest in the distinct Fe‐Ni‐S systems among brachinite family precursors, which were sulfide‐dominated for the most oxidized brachinites and metal‐dominated for the least oxidized brachinite‐like achondrites. Consequently, highly siderophile element behavior was controlled through melting and removal of their dominant host phase in the precursor, which was likely pentlandite in sulfide‐dominated systems and kamacite/taenite in metal‐dominated systems. Anomalous Ir/Os and Pt/Os ratios of oxidized brachinites may be attributed to selective complexing during melting of As‐rich pentlandite, consistent with our observations of impact‐melted sulfides in R chondrite NWA 11304, although further experimental work is needed to model this process. The apparent redox trend among the brachinite family is consistent with silicate FeO content and Fe/Mn ratios, which may be used as a proxy for determining the relative oxidation state of brachinite family members. Based on our analyses, we make several recommendations for reclassification of samples into a continuum of oxidized to reduced endmembers for the brachinite family. Along with a common range of Δ 17 O, this evidence is consistent with either formation on a common heterogeneous parent body, or at least from the same nebular reservoir, with variable O and S fugacities, resulting in mineralogically distinct igneous products for oxidized and reduced endmembers. Sulfur‐bearing, oxidized differentiation may extend to other bodies that formed at or beyond the snow line in the early solar system, and should be considered when interpreting observational data for asteroids in upcoming missions.