Fresh pumice from the Central Indian Basin: A Krakatau 1883 signature

Abstract

The occurrence of pumice in the Central Indian Basin (CIB) has been a controversy. Some are of the opinion that these pumices are erupted from a volcano along the Ninetyeast ridge while others suggest the Indonesian are volcanism as their probable source. Recently, it is also suggested that the pumice are the products of in-situ silicic volcanism in the CIB. This study was initiated in order to resolve this controversy about the origin of the fresh pumice from the CIB. The fresh pumice pieces of different sizes and shapes were collected from the surficial sediments of the CIB from an average depth of about 5000 m (12 degrees S, 76 degrees E). Glass is the main phase comprising about 95% (volume) and resembles the matrix glass in Krakatau 1883 dacitic pumice which is also very poor (less than 5%) in the modal mineral composition. The mineral and glass analyses were carried out with the help of the electron microprobe. The orthopyroxenes, clinopyroxenes, plagioclases and spinels are the mineral phases present in these pumices. The pyroxenes have similar wo, en and fs components to those previously reported from the 1883 Krakatau eruption products. Geochemically, these fresh pumices have a high SiO sub(2) content (71%) and high total alkali content (8%) similar to 1883 eruption products of Kracatau. Thus based on the similarities in mineralogy and glass chemical composition it is inferred that these fresh pumices are the products of the fierce 1883 eruption of the Krakatau volcano in the Sunda strait of the Indonesian arc