WHERE WATER MEETS MAGMA: A HIGH-TEMPERATURE RECORD OF SUBAERIALLY EXPOSED CONTINENTAL CRUST THROUGH TIME

Having an accurate archive of surface temperatures is essential for understanding the long-term evolution of Earth's systems. It is especially important to produce robust quantitative reconstructions of the surface temperatures for the first ½ of Earth’s history. The Archean Ocean temperatures remain poorly constrained and highly debated with estimates widely ranging from modern-day temperatures to as high as 100 °C. Meanwhile, sedimentary basins document intermittent glacial episodes between 2.9 and 2.7 Ga, and snowball Earth episodes between 2.4 and 2.2 Ga. Here I review the recent efforts to reconstruct the surface conditions of subaerially exposed blocks of continental crust using O-isotope geochemistry of magmatic-hydrothermal systems dating back to Neoarchean. In addition, I present an on-going effort to study modern-day hydrothermal exchange reactions in triple-O isotope space. The δ¹⁸O of subaerial precipitation is sensitive to the surface temperatures and paleogeography of the subaerially exposed crust. Due to the high-temperature reactive circulation of local ground waters, the meteoric water is recorded in refractory silicate minerals with δ¹⁸O values typically <0 ‰. Using magmatic-hydrothermal complexes from the Karelia and Kola cratons, my collaborators and I show that subaerially exposed continental crust was accompanied by an active hydrological cycle. The ¹⁸O-depleted meteoric waters are documented on these cratons between 2.7 and 2.3 Ga. Coupled with U-Pb zircon geochronology, we investigate in detail how the low-δ¹⁸O meteoric water is captured in magmas and host lithologies. The δ¹⁸O–Δ¹⁷O of altered host rocks and assimilated magmas unambiguously fingerprints the surface waters with δ¹⁸O = -18 ± 6‰ at 2674 Ma on Kola craton. Today such precipitation is found in the regions with temperatures reaching below 0 °C. Given the paleomagnetic data at the time, Kola craton was exposed at high latitudes between 60° and 90°, indicating that the climate was likely accompanied by temperatures below freezing. Our newly derived Neoarchean environmental proxy is in accord ca. 2.7 Ga glacial episodes, permitting the existence of cool climate at high latitudes. Further efforts should aim at reconciliating the evidences for apparent co-existence of hot Archean oceans and cool high-latitude climate.