Insights into chemical weathering of the upper continental crust from the geochemistry of ancient glacial diamictites

摘要: Abstract Glacial diamictites, with ages ranging from ∼2900 to 0.01 Ma, record the changing composition of upper continental crust through time ( Gaschnig et al., 2014 ). Li concentrations and isotopic compositions, combined Pb chemical index alteration (CIA) values relative Sr are used here assess degree weathering recorded in these deposits origin this signature. The δ7Li most diamictites (ranging −3.9 +3.5) lower than those mantle-derived basalts (+3.7 ± 2, 2σ), low generally accompanied by high CIA Sr/Sr∗ (or depletion factor, Sr/Sr∗ = Sr/(Ce∗Nd)0.5), reflecting a signature that may have derived pre-depositional, syn-depositional, and/or post-depositional processes. Profiles three glacial relatively (a fresh road cut Neoproterozoic Nantuo Formation (CIA = 62–69), drill cores Paleoproterozoic Timeball Hill (CIA = 66–75) Duitschland Formations (CIA = 84–91)) do not show evidence significant weathering. High Th/U, loss uranium during oxidative weathering, is seen all Paleozoic few deposits. systematics suggest was largely inherited preexisting crust, although subset samples (the Konnarock, Dwyka, several samples) appears experienced U loss. Modern glaciomarine sediments little (CIA = 47, Sr/Sr∗ = 0.7, δ7Li = +1.8), consistent cold temperatures accompanying periods, suggesting limited syn-depositional has occurred. Thus, observed ancient be (UCC) over which glaciers traversed. strength signature, based on CIA, greatest Mesoarchean some weaker Phanerozoic diamictites. Combining data for Archean shales other types post-Paleoproterozoic sedimentary rocks (i.e., shales, mudstones, etc.), it less intense average, crust.