Paper No. 2
Presentation Time: 9:00 AM-6:30 PM
MICROANALYTICAL EVIDENCE FOR SLUSHBALL EARTH AT 2.4 GA IN SUBGLACIAL, HYDROTHERMALLY-ALTERED ROCKS AND COEVAL SUPERGENE MATERIALS
The rise of atmospheric oxygen in the Paleoproterozoic was accompanied by at least 3 global glaciations between 2.45 Ga, time of global superplume breakout event, and ~2.2-2.06 Ga time interval, marked by warm climate and Lomagundi δ13C excursion in seawater composition. Hard Snowball glaciation would fully restrict hydrologic cycle, whereas Slushball glaciaton would not (liquid ocean would permit accumulation of low-δ18O ice on the continents), although the latter is climatically unstable. We found evidence for accummulation of such ice at low latitudes in ultra-depleted δ18O values of hydrothermally-altered rocks at 10 localities, spanning 200 km across mid-grade metamorphic belt of Russian Karelia, Baltic Shield. The 2.9-2.55 Ga protolith of these rocks has undergone significant exchange with subglacial –27 to –35‰ meltwaters 2.4 Ga ago. As Karelia is considered to have been attached to the Superior craton and located near equator during the Paleoproterozoic, a global glaciation is required to explain these values. U-Pb zircon geochronology indicates that full-scale subglacial alteration happened in rift zones mainly during the first of these glaciations, in association with the 2.45 Ga superplume event. Isotopic countour maps for several of these localities indicate bulls-eye pattern of δ18O exchange. The most-studied locality has 6x1.5 km areal extent with 18O depletion below -10‰ and 200x100 m areal extent with δ18O values below -25‰, reaching the most negative δ18O value of -27.3‰. Coupled with δ18O depletion, δD values of amphiboles and micas are as low as –235‰. An extended range of δ18O values in studied rocks strictly adhere to mass-dependent fractionation of 17O vs 18O with the exponent of 0.5278, characteristic of equilibrium terrestrial fractionation rather than kinetic fractionation having lower exponent. Our search for unmetamorphosed 2.4-2.3 Ga sedimentary or volcanic rocks and their secondary (subglacial) alteration products is ongoing. We found that the bulk of 2.4 Ga shales and other materials are more 18O-depleted (down to +3 to +8‰) than younger and older rocks, but using bulk methods we have not yet found ultra-depleted values. The effort is underway to characterize Precambrian shales and authigenic rims of their detrital constituents for δ18O and δD values, and chemical composition.