Weathering and alteration of volcanic ashes in various depositional settings during the Permian-Triassic transition in South China: Mineralogical, elemental and isotopic approaches

Abstract

To better understand the alteration of volcanic ash in different depositional environments, we measured the clay mineralogy, major and trace element geochemistry, and Sr and Nd isotopic composition of altered ashes in two Permian-Triassic boundary (PTB) successions in southern China. The Pengda and Xinmin sections, in Guizhou Province, represent different depositional settings, allowing us to investigate the role of the early burial environment on authigenic clay formation. The PTB ash beds in both sections consist predominantly of mixed-layer illite-smectite clays. Altered ashes in the relatively shallow Pengda section contain predominantly R2 and R3 I/S clays, with 70% to 85% illite layers, whereas their stratigraphic equivalents in the deeper Xinmin section contain only R3 I/S clays, which are of two types, with 88% and 99% illite layers, respectively. The two studied ash beds can be stratigraphically correlated between the Pengda and Xinmin sections, and show similar 143Nd/144Nd ratios (0.511991 for PD-1 and 0.511983 for XM-1; 0.512073 for PD-2 and 0.512074 for XM-2). REE distributions and immobile element cross plots are consistent in suggesting that the altered ashes may have originated via felsic volcanism. Altered ashes in the deep sea Xinmin section have notably high 87Sr/86Sr ratios (0.764209 and 0.795921) relative to those from equivalent horizons in shallower environments (0.728455 and 0.749953). This difference in 87Sr/86Sr ratios is likely attributable to different degrees of chemical weathering. Elemental ratios such as SiO2/Al2O3, K2O/Al2O3, Y/Sr and Zr/Sr all support more intensive chemical leaching in deep-water environments. The altered ashes at Xinmin contain substantially more K2O than those at Pengda, suggesting greater incorporation of K+ into clay minerals due to lower pH during early diagenesis. Clay mineral assemblages and stacking structures of I/S clays in the altered ashes are consistent with a general sequence of smectite illitization during diagenesis and burial metamorphism in both sections.