Speciation of sulfur in the insoluble organic matter from carbonaceous chondrites by XANES spectroscopy

Sulfur speciation in a comprehensive set of insoluble organic matter (IOM) samples extracted from 3 CI (Orgueil, Alais, Ivuna), 5 CM (QUE97990, Murchison, Murray, QUE99355, Cold Bokkeveld), 1 CR (Renazzo) and ungrouped C2 Tagish Lake chondrites has been studied by K-edge XANES micro-spectroscopy. Five main sulfur groups were identified: (1) sulfides, (2) aliphatic sulfur, (3) heterocyclic organic sulfur, (4) oxidized organic sulfur and (5) sulfates. The IOM of the 3 CIs and the extensively altered Cold Bokkeveld CM exhibit a higher abundance of heterocyclic versus aliphatic organic sulfur compared to the other CMs and compared to the Renazzo and Tagish Lake chondrites. This suggests greater thermal heating on the parent body of CIs, consistent with the higher temperatures experienced by these chondrites (~ 70 °C for the most altered clasts of Cold Bokkeveld; 100–150 °C for CIs). Alais may have experienced more heating than Ivuna and Orgueil. The IOM of CIs contains oxidized organic sulfur, suggesting the presence of a mild-temperature oxidation process on the parent body. Among type 2 chondrites other than Cold Bokkeveld, no significant variation of the sulfur speciation was detected. This suggests tenuous oxidation processes and a low-temperature aqueous alteration (< ~ 35 °C) on the parent body. The global chemical variations between the CR and CM groups (e.g. H/C elemental ratio, alkyl groups concentration) reported in earlier studies appear to be more the result of chemical variations among the accreted precursors than of post-accretional processes.

Research Highlights
► 5 main sulfur groups have been identified.
► CI and C. Bokkeveld IOM underwent chemical transformation by mild thermal heating.
► CI and CM show differences consistent with accretional heterogeneity.
► Oxidation of sulfur in the CM group is independent of the alteration index.