Iridium anomaly associated with the Australasian tektite-producing impact: Masses of the impactor and of the Australasian tektites

Abstract

Recently published studies of Ocean Drilling Project (ODP) cores from near southeast Asia revealed microtektite contents much higher than those in previously studied cores, suggesting that Ir contents might be enhanced in the tektite-bearing horizons. We determined a positive Ir anomaly in ODP core 758B from the Ninetyeast Ridge, eastern Indian Ocean; the peak Ir concentration of 190 pg/ g was ~2× the continuum level. The net Ir fluence is 1.8 ± 0.5 ng cm −2 over the depth interval from 10.87 to 11.32 m; a small additional peak also associated with microtektites contributes another 0.5 ng Ir cm −2. Concentrations of Ir in core 769A show more scatter, but a small Ir enhancement is associated with the peak microtektite abundance; our best estimate of the poorly constrained fluence is 1.3 ± 0.7 ng cm −2. Data on deep-sea cores show that the microtektite fluence falls exponentially away from southeast Asia, the fluence dropping a factor of 2 in ~400 km. In southeast Asia the trend merges with a roughly estimated mass fluence of ~ 1.1 g cm −2 inferred from evidence of a melt sheet in northeast Thailand. Integration of the inferred distribution yields a total mass of Australasian tektites of 3.2 · 10 16 g, much higher than previous estimates. Assuming a similar fallout distribution for the impactor and a chondritic composition allows us to calculate its mass to be 1.5 · 10 15 g, about 3 orders of magnitude smaller than the minimum mass of the impactor responsible for the extinctions at the end of the Cretaceous.