TRIPLE OXYGEN AND HYDROGEN ISOTOPES IN SYN-GLACIAL HYDROTHERMALLY ALTERED ROCKS: COMPARISON BETWEEN MODERN ROCKS OF ICELAND AND SNOWBALL EARTH AGE ROCKS FROM THE BALTIC SHIELD

Hydrothermally altered rocks can be a powerful tool in studying environments of the deep past when other paleoclimate proxies are not available. In this study we use δ¹⁸O, Δ¹⁷O and δD of ancient hydrothermally altered rocks from 2.4-2.3 Ga rift zones of the Baltic Shield that are contemporaneous with the early Paleoproterozoic snowball Earth glaciations to infer stable isotopic composition of meteoric water and seawater at the time. The novel parameter Δ¹⁷O is used to reconstruct water-to-rock ratios and δ¹⁸O of meteoric water when original oxygen and hydrogen isotopic equilibria are lost. Using the early Paleoproterozoic subglacial hydrothermally altered rocks from the Belomorian Belt of the Baltic Shield we show that meteoric water had δ¹⁸O of about −35 ‰ VSMOW at low latitudes during the snowball Earth glaciations. The contemporaneous komatiitic basalts of the Vetreny Belt, Baltic Shield exhibit pervasive and well-preserved submarine alteration by seawater that had δ¹⁸O ≈ 0 ‰. To test the reliability of our findings, we compare the Paleoproterozoic rocks with modern hydrothermally altered rocks from continental and submarine hydrothermal systems using samples extracted from drill cores in Iceland (Krafla and Reykjanes) and modern seafloor (hole ODP 504B). We use isotopic equilibrium fractionation as well as Δ¹⁷O approach to compute isotopic composition of hydrothermal fluid and compare them to directly measured isotopic composition of the fluids. Samples from Geitafell eroded volcano which exhibits pervasive hydrothermal alteration with involvement of 5-6 Myr meteoric water were also analyzed for this study. Hydrothermally altered rocks from Iceland serve as a close analogue for the Paleoproterozoic rocks formed in rifting zones of the Baltic Shield that operated under glacial ice and seawater during snowball Earth glaciations. We search for snowball Earth age hydrothermally altered rocks elsewhere in the world. The early Paleoproterozoic Scourie dikes (δ¹⁸O as low as −2 ‰), Scotland are contemporaneous with the snowball Earth glaciations and their low-δ¹⁸O signature could be related to interaction between magma and glacial melt waters.