[S-MP27] Evolution of the early continents and the oceans understood by multi-geological proxies

[SMP27-P02] Sulfur and iron isotope study of metacarbonate rocks from the Highland Complex, Sri Lanka

*Madhusoodhan Satish-Kumar¹, Aoi Yoshioka¹, Gen Shimoda², Kosuke T. Goto², Sanjeewa P.K. Malaviarachchi³ (1.Department of Geology, Faculty of Science, Niigata University, Japan, 2.Geological Survey of Japan (AIST), Tsukuba, Japan, 3.University of Peradeniya, Sri Lanka)

Keywords:Sulfur isotopes, Iron isotopes, metacarbonate rocks, Gondwana

Sulfur isotope ratios in carbonate rocks record not only sedimentary environments, but also variations in seawater sulfate reservoirs. In addition, iron isotope ratios vary significantly with redox conditions, making them useful for understanding ocean redox conditions and iron biogeochemistry. Notable increase in oxygen in the Earth's atmosphere and oceans in the Proterozoic. These events appear to be closely linked to global sulfur and iron geochemistry, both by the formation of weathered sulfates on land and by sulfide oxidation and the evolution of disproportionate bacteria in the marine environment. We report the sulfur and iron isotopic compositions of pyrite in metamorphosed carbonate rocks, to infer the depositional environment from each source, and to evaluate the effects of metamorphism on these isotope systems. Samples considered here are metamorphosed carbonate rocks containing pyrite from the Highland Complex, Sri Lanka. The δ³⁴S_V-CDTvalues of pyrite range from +6 to +26‰ and δ³³S is slightly negative, most of which fall within the same quadrant in the Δ³³S vs δ³⁴S diagram. The δ⁵⁶Fe values of -0.28 to 1.05‰ have slight positive correlation with δ³⁴S. This may reflect a closed system sulfur supply due to microbial sulfate reduction (MSR) and high sulfate supply, as well as secondary iron supply from Fe(III). On the other hand, the presence of very thin pyrite lamella within a single carbonate mineral suggests that pyrite may have exsolved during retrograde metamorphism. The concentrations of sulfate and iron in the carbonate rocks are lower than in the unmetamorphosed ones. Therefore, metamorphism may have caused isotope equilibrium between carbonate and sulfide minerals. In addition, the presence of sulfide minerals such as galena and chalcopyrite suggests that the hydrothermal activity had no small effect on the sulfide minerals. Based on the above, we conclude that metamorphic carbonate rocks can provide important information to support the hypothesis of a global ocean.