Tracking Down the Paleoproterozoic Glaciations 2.5-2.2 Ga Using Coeval Subglacial Meteoric-Hydrothermal Systems and Estimates of δ18O Value in Seawater at ~2.45Ga
Friday, 19 December 2014
We continue investigation of the world’s lowest δ18O and δD (down to -27.3‰ and -235‰, respectively) rocks associated with high-Mg and high-Fe intrusions found within a 450-km-long zone of metamorphic rocks in the Belomorian Belt (Baltic shield, Russia). We suggest that extreme depletion occurred during premetamorphic high temperature interaction between shallow mafic intrusions and Paleoproterozoic glacial waters. Heat associated with intrusion of Paleoproterozoic mafic dikes allowed for circulation of glacial water in mafic dikes’ exterior causing isotopic exchange between ultra-low-δ18O water and host rocks. Based on correlation of Paleoproterozoic glaciogenic deposits scattered worldwide, it was inferred that that global glaciations occurred three times between 2.45 to 2.20 Ga, independently or systematically coincident with the rise of atmospheric oxygen. However, the exact ages of glaciations remain uncertain. Dating igneous cores within zircon grains from the intrusions that interacted with subglacial meteoric waters enables us to develop a new approach to determine the age of δ18O depletion and thus, glacial events. Recently published geochronology data for Fennoscandia indicates that there were at least three episodes of compositionally distinct mafic magmatism spanning between 2.5 and 2.2 Ga which may help us to precisely resolve the glaciation episodes. Initial efforts to date the Paleoproterozoic mafic intrusions in conjunction with newly published geochronology data suggest that the low-δ18O anomalies discovered at different localities were probably produced during different glacial episodes. We also present new data based on a study of the ~2.45 Ga Vetreny Belt rift, Karelia craton, where unmetamorphosed pillow basalts contain abundant genetically-related hydrothermal quartz-epidote veins (+4.7‰ and -0.44‰, for the two minerals respectively) that formed at temperature of ~350°C. Since hydrothermal minerals likely precipitated from seawater-based fluid, this direction will further provide us with insight into the isotopic composition of Paleoproterozoic seawater existed before the series of glaciations.