Using Archean and Paleoproterozoic Shales and Tillites as a Window into Crustal Evolution and Surface Conditions

Abstract:

Precambrian shales and tillites have been insufficiently studied so far. We present oxygen and hydrogen isotope data for 103 bulk shale and tillites that were collected from drillholes on all continents from 3.2 to 1.4Ga. These samples have also been analyzed for total organic and inorganic carbon, total sulfur, δ¹³C_org values and by XRF for major and trace elements to calculate chemical index of alteration (CIA). Having uncompromised fresh samples from drillcores is a must for this kind of investigation. We have a particularly good coverage for the ca. 2.7-2.2 Ga time interval when Earth experienced 3-4 Snowball Earth glaciations associated with the rapid rise in atmospheric O₂ and fluctuations in CO₂, thus affecting weathering cycle and attainment of isotopic fractionation. All units have similar to Phanerozoic ranges in δ¹³C_org values (-23 to -33‰ PDB) and C_org content (0.1 to 10 wt. %). Compared to Phanerozoic shales, Precambrian shales have comparable ranges in δ¹⁸O values (+7 to +20‰), with slightly decreasing means with increasing age, and identical δ¹⁷O-δ¹⁸O slope (0.528). Shales in some drill holes display wide δ¹⁸O ranges over short stratigraphic intervals suggesting significant variability in the provenance. We however observe a significant, several permil downward shift and decrease in the range of δ¹⁸O values (7-9‰) in 2.2-2.5 Ga shales from several continents that are associated with the Paleoproterozoic glaciations. Scattered negative correlation of CIA with δ¹⁸O, for some of these shales broadly associated with the Paleoproterozoic glaciations suggest contact with glacial meltwater having ultra-low-δ¹⁸O values during deposition or diagenesis of these shales. The δD values of shales range from -50 to -75‰, and are comparable to Phanerozoic values, with the exception of the ~2.5-2.2 Ga shales that reach to -100‰. We also compare O isotope values of ultra-low-δ¹⁸O, +8 to -27‰ SMOW subglacial hydrothermal rocks recently discovered in Karelia (Russia), quartz amygdules in mafics and their relations to our global shale dataset. The overall conclusion is that despite first-order changes in areal mass, exposed surface conditions, pCO₂, pO₂ affecting chemical/physical weathering cycle, it was not dramatically different before and after the rise of atmospheric oxygen at ~2.3-2.4 Ga.