[S-MP24] Supercontinents and Crustal Evolution

[SMP24-P04] Sulfur isotope study of metacarbonate rocks from Highland Complex, Sri Lanka

*吉岡 葵¹、Madhusoodhan Satish-Kumar¹ (1.新潟大学理学部地質学科)

キーワード：硫黄同位体、変成作用、微生物硫酸塩還元、スリランカハイランド複合岩体、変成炭酸塩岩

Sulfur isotopes in carbonate rocks record variations in seawater sulfate reservoirs associated with global oxygenation as well as depositional environments. During the Proterozoic, several evolutionary and biogeochemical events have occurred, with particular attention being paid to the rise in oxygen (GOE and NOE) in the Earth's atmosphere and oceans. These events appear to be closely linked to global sulfur geochemistry by both the production of weathered sulphates on land and the oxidation of sulfides and the evolution of disproportionated bacteria in the marine environment (Bottrell and Newton, 2006). Here, we report sulfur isotopic compositions of pyrite in metacarbonate rocks in order to estimate the depositional environment and gain insight into the effects of metamorphism. Metamorphic carbonate rocks containing pyrite from the Highland Complex in Sri Lanka was investigated in this study, which is believed to have been deposited in the Precambrian period. The δ³⁴S_V-CDT values of pyrite ranges from +6 to +26‰, and Δ³³S was slightly negative, most of which falls in the same quadrant in a Δ ³³S vs. δ ³⁴S diagram. This reflects a closed system sulfur isotope behavior caused by microbial sulfate reduction (MSR) and high sulfate demand, among other factors. In addition, while comparing the results of this study with un-metamorphic carbonate rocks in Proterozoic, the Δ³³S/δ³⁴S and Δ³³S/Δ³⁶S tendency of sulfur isotopes were similar. Therefore, it is considered that the change of sulfur isotopes due to metamorphism is negligible and the carbonate rocks act as perfect time capsule for preserving Precambrian ocean signatures. In addition, comparison with sulfur isotopic composition of pyrite in carbonate rocks of various ages indicate that the sulfur isotopic composition in the metacarbonate rocks in the Highland Complex were higher than in Archean, and similar to those in other Proterozoic eras. Therefore, we believe that the carbonate rocks provide key information in support of the hypothesized global ocean.

Reference
Bottrell, S. H., and Newton, R. J., 2006. Reconstructionofchangesin global sulfur cycling from marine sulfate isotopes. Earth science Reviews., 75, 59-83.