ChemicalGeology, 103 (1993) 129-139 129 Elsevier Science Publishers B.V., Amsterdam [NA] Deccan volcanic contribution of Ir and other trace elements near the K/T boundary in India N. Bhandari, Mala Gupta and P.N. Shukla Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India (Received June 16, 1991; revised and accepted June 23, 1992) ABSTRACT Bhandari, N., Gupta, M. and Shukla, P.N., 1993. Deccan volcanic contribution of Ir and other trace elements near the K/T boundary in India. Chem. Geol., 103:129-139. The concentrations oflr 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- ~in flow basalts and ~ 50 pg g- ~in intertrappeans. However, at two horizons in the ash, concentration about two times greater than the general level (50 pg g ~ ) 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 inte- grated 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. I. Introduction The Cretaceous-Tertiary boundary (KTB) in most marine sections shows enhanced con- centrations of the siderophile elements Ir, Os, Au and Ni. Alvarez et al. (1980) proposed an asteroidal origin for these elements in the boundary. This hypothesis of collision of an asteroid with the Earth, ~ 65 Ma ago, is sup- ported by a number of other evidences, such as presence of shocked quartz (Bohor et al., 1984), sanidine spherules (Smit and Klaver, 1981 ), soot from forest fires (Wolbach et al., Correspondence to: N. Bhandari, Physical Research Lab- oratory, Navrangpura, Ahmedabad 380 009, India. 1985), but whether the severe extinction of land and marine species occurred due to such an event is still debated (Russell, 1979; Officer and Drake, 1985 ). An alternative scenario at- tributing the source of these elements to the mantle of the Earth, where they are highly abundant (Jagoutz et al., 1979), has been ad- vocated (McLean, 1985; Courtillot et al., 1986). These authors favour the intense vol- canism that formed the Deccan Plateau in cen- tral India at about the same time as the source of these elements. The Deccan basalt sequence overlies the thick and widespread Lameta beds which be- long to the Upper Cretaceous. At a few loca- tions in between the basalt flows some sedi- 0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.