Facies dependence of the mineralogy and geochemistry of altered volcanic ash beds: An example from Permian-Triassic transition strata in southwestern China

Factors controlling variations in the clay mineralogy and geochemical composition of volcanic ash beds deposited in different types of depositional environments remain incompletely understood. The well-correlated, altered volcanic ash beds (tuffs or K-bentonites) of Permian-Triassic boundary (PTB) successions in South China provide an opportunity to better understand processes of alteration and diagenesis of volcanic materials in terrestrial, paralic, and marine sedimentary facies. The types of authigenic clay minerals produced vary among facies: mainly mixed-layer illite/smectite (I/S) clays in deep-marine, shallow-marine, and lacustrine facies, and kaolinite and mixed-layer I/S or kaolinite/smectite (K/S) clays in paludal and mixed marine-terrestrial facies. Furthermore, the stacking sequences of I/S clays in the ash beds are notably facies-dependent: mainly R3 with minor R1 in deep-marine facies, mainly R1 in shallow-marine facies, and mainly R3 in lacustrine and paludal facies. Ordering of I/S clays seems to have been controlled by porewater K+ availability, solution pH, and energy levels of the depositional environment. High-pH conditions favored the formation of feldspar and inhibited the illitization of smectite, and loss of K+ during early alteration of ash was related to environmental energy levels. However, the stacking sequences of I/S clays in terrestrial facies were dominantly controlled by burial conditions. The MgO and K2O contents of altered volcanic ash were related to sediment clay-mineral composition and thus also facies-dependent. Marine-facies ash beds contain relatively more MgO (i.e., MgO/K2O > 0.30; MgO/Al2O3 > 0.056) than terrestrial-facies ash beds (MgO/K2O < 0.30; MgO/Al2O3 < 0.056). MgO was generally retained during alteration of ash beds, and its concentration was largely a function of parent magma type and depositional and diagenetic conditions. K2O was almost completely leached out of volcanic glass, leading to formation of I/S clays by reaction with smectite.