Sulfide–oxide assemblages in Acfer 094—Clues to nebular metal–gas interactions. (16th October 2017)

• Record Type: Journal Article
• Authors: Barth, M. I. F.; Harries, D.; Langenhorst, F.; Hoppe, P.
• Is Part Of: Meteoritics & planetary science. Volume 53:Number 2(2018)
• Page Start: 187
• Page End: 203
• Publication Date: 2017-10-16
• Subjects: Meteorites -- Periodicals; Planetology -- Periodicals; 523.4
• DOI: 10.1111/maps.12992
• Languages: English
• ISSNs: 1086-9379

Abstract

Abstract: The ungrouped carbonaceous chondrite Acfer 094 is among the least altered samples of the early solar system. We have studied concentric sulfide–oxide aggregates from this meteorite by transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS). The main minerals present are magnetite, pentlandite, and pyrrhotite/troilite. The outer parts of the aggregates include μm‐sized olivine and pyroxenes with variable Mg/Fe ratios. One aggregate contains taenite (56.7 wt% Ni) within its central part that is surrounded by pentlandite and magnetite. We conclude that both phases have formed by oxidation and sulfidization of metal and, based on the metal and sulfide Fe/Ni ratio, a (sulfide)‐formation temperature of 400–550 °C can be constrained. This temperature is higher than any temperature that could be expected to have occurred on the Acfer 094 parent body, and also textural evidence indicates that the aggregates formed before parent‐body accretion. We therefore conclude that the formation of the sulfide–oxide aggregates occurred most likely in the solar nebular at highly variable H2 O and H2 S fugacities. Oxygen‐isotopic compositions of magnetite within these assemblages show that they are indistinguishable from the meteorite's matrix (δ 17 OSMOW ≈ 4 ± 8‰, δ 18 OSMOW ≈ 10 ± 6‰, and ∆ 17 OSMOW ≈ −1 ± 5‰). An enrichment of 17, 18 O relative to chondrules of Acfer 094 suggests a link between the formation of the sulfide–oxide aggregates and the preaccretionary processing of matrix grains in a volatile‐enriched nebular environment.