Trace element compositions of iridium enriched illite-smectite assemblages from a K/Pg boundary section in the Anjar area of the Deccan volcanic province: palaeoenvironmental implications

Abstract

Clay minerals in the Cretaceous-Palaeogene (K/Pg) boundary section of the Anjar area comprise smectite, palygorskite, sepiolite, illite, and mixed layer illite – smectite. Elemental data plots show fractionation of heavy from light Rare Earth Elements (REE). The palygorskite-dominated upper succession shows high light REE concentrations compared to the illite-smectite-rich lower succession. REE patterns exhibit lower values in the lower and middle part of the succession, possibly related to leaching of the REE inter-element fractionation during weathering. Minor release of trace elements is coincident with an Ir-rich layer and anoxic conditions at the time of deposition. REE patterns in Ir-rich layers show a negative Eu anomaly. La/Yb ratios reflect heavy REE enrichment relative to light REE, and negative correlation with illite content in mixed layer illite-smectite. The increase in δCe is simultaneous with Ir spikes in the basal limestone, indicative of anoxic conditions. In the upper part of the succession, δCe shows more or less constant values, signifying an oxic environment of deposition. δCe and δEu are elevated in sepiolite-rich clays and negative in the palygorskite-dominated lower and upper successions respectively. The positive Ce anomalies and inter-element ratios of the illite-smectite reveal high Ir concentrations at three levels which have been attributed to abrupt changes in sedimentary redox conditions. It is inferred that the physico-chemical conditions during deposition of the Ir layers were different from those of other sedimentary units, as well as the Ir hosted fossiliferous siltstone.