Deccan volcanic contribution of Ir and other trace elements near the K/T boundary in India

Abstract

The concentrations of Ir and some other elements in the Takli intertrappean sediments, representing a 2-m-thick shallow continental deposit in the Deccan province around the K/T boundary (KTB) event, were measured to determine their volcanic contribution. Each unit of the intertrappeans, consisting of lava ash, clay and marl sandwiched between two basalt flows, was analysed. The concentrations of Fe and Co are found to be generally lower than in the adjacent basalts whereas Cr is similar but occasionally higher in the volcanic ash. Concentrations of eight rare-earth elements (REE), other than Ce, are also similar to their levels in basalts, except at a few horizons in the ash, where higher concentrations by a factor of 2-5 are found. Ir concentration is determined to be <27 pg g^-1 in flow basalts and ~50 pg g^-1 in intertrappeans. However, at two horizons in the ash, concentration about two times greater than the general level (50 pg g^-1) in the intertrappeans has been observed. The observed high concentration of REE, Cr and Ir in the intertrappeans could possibly arise as a result of fractionation processes occurring during or after the volcanic episodes if a uniform sedimentation rate is assumed. However, the highest Ir concentration observed in these intertrappeans (0.12 ppb) is lower by a factor of ~100 compared to concentrations in KTB clay in the marine section of Meghalaya (12.1 ppb), a few hundred kilometers away. The integrated amount of Ir in the intertrappeans also falls short by factor of ~3.5 compared to the Meghalaya section. This observation leads to the conclusion that the volcanic contribution is inadequate to explain the Ir inventory in the marine KTB layer.