FRESH PUMICE FROM THE CENTRAL INDIAN BASIN - A KRAKATAU 1883 SIGNATURE

The occurrence of pumice in the Central Indian Basin (GIB) has been a controversy. Some are of the opinion that these pumices are erupted fr om a volcano along the Ninetyeast ridge while others suggest the Indon esian are volcanism as their probable source. Recently, it is also sug gested that the pumice are the products of in-situ silicic volcanism i n the GIB. This study was initiated in order to resolve this controver sy about the origin of the fresh pumice from the GIB. The fresh pumice pieces of different sizes and shapes were collected from the surficia l sediments of the CIB from an average depth of about 5000 m (12 degre es S, 76 degrees E). Glass is the main phase comprising about 95% (vol ume) and resembles the matrix glass in Krakatau 1883 dacitic pumice wh ich is also very poor (<5%) in the modal mineral composition. The mine ral and glass analyses were carried out with the help of the electron microprobe. The orthopyroxenes, clinopyroxenes, plagioclases and spine ls are the mineral phases present in these pumices. The pyroxenes have similar wo, en and fs components to those previously reported from th e 1883 Krakatau eruption products. Geochemically, these fresh pumices have a high SiO2 content (71%) and high total alkali content (8%) simi lar to 1883 eruption products of Krakatau. Thus based on the similarit ies in mineralogy and glass chemical composition it is inferred that t hese fresh pumices are the products of the fierce 1883 eruption of the Krakatau volcano in the Sunda Strait of the Indonesian arc.