Paper No. 8-6
Presentation Time: 9:25 AM
LITHIUM AND OXYGEN ISOTOPIC COMPOSITIONS OF ANCIENT SHALES TRACE CHEMICAL WEATHERING ACROSS THE ARCHEAN-PROTEROZOIC BOUNDARY
To investigate the compositional evolution of continental crust and weathering conditions across the Archean-Proterozoic boundary, major and trace elements, Li and O isotopes were analyzed for thirty-four shales from the West Rand and Central Rand Groups of the Witwatersrand Supergroup and the Pretoria Group of the Transvaal Supergroup, Kaapvaal craton, South Africa, with depositional ages ranging from 2960 to 2250 Ma. The δ7Li values of the shales, ranging from -4.3 to +3.1, are lower than those of average MORB (+3.7±2, 2σ) and the low δ7Li values are generally accompanied by high CIA (Chemical Index of Alteration), δ18O and [Li] values, reflecting a weathering signature. A generally positive correlation between CIA and [Li] indicates uptake of lithium by clay minerals during chemical weathering. Lack of correlation between diagnostic trace element ratios (i.e., La/Yb, Th/La, Ni/Co and Th/Sc) and d7Li, d 18O, CIA values suggests that the mafic/felsic characteristics of the provenance has negligible influence on the isotopic compositions of the shales, though high Ni/Co shales (indicating mafic/ultramafic provenance) have uniformly high CIA. The δ7Li and [Li] averages (δ7Li = -0.9±3, 2σ; [Li] = 30 ± 22, 1σ) of the shales are lower than those of PAAS (post Archean Australian shales, δ7Li = -0.3 ± 2.6, 1σ; [Li] = 63 ± 25 ppmw, 1σ), and the CIA average of 82 for Archean shales is much higher than that of PAAS (CIA = 69), far above the range of fresh basalts (40-45) and the average upper continental crust (59-65). Combining shale data in this study with literature data for sedimentary rocks (e.g., shales, tillites, pelites), we observe that the Archean crust experienced more intense weathering than post-Archean crust (Li et al., 2016, GCA), but Archean shales have lower δ18O values (6.8 ± 1.4, 1σ) than those of post-Archean shales (7.5 ± 2.4, 1σ). As in previous works, we attribute the increasing trend of δ18O across the Archean – Proterozoic boundary to enhanced sedimentary recycling, possibly related to the wide-spread emergence of continents.