PRESENCE OF A SHALLOW MAGMA CHAMBER AT THE BARREN ISLAND VOLCANO, ANDAMAN SEA: EVIDENCE FROM GLASS SHARDS

Barren Island Volcano (BI), a Quaternary volcano in the Andaman Sea, is located on the Indonesian Volcanic Arc (IVA) that extends from the dormant volcanoes of Myanmar to the active volcanoes of Sumatra and Java in Indonesia. BI is located in a region where collision (without subduction) and subduction are concurrent due to oblique convergence of Indian and Burmese plates and it is the last of the active volcanoes of IVA, towards its northern end. There is an increase in the frequency of eruptions of BI during the last two decades since 1991, with eruptions in 1991, 1994-95, 2000(?) and 2005. It continues to be active since the great Sumatran earthquake (December 26, 2004), with continuous smoke emission and occasional pyroclastic and lava flows. BI volcanics have been studied by these authors since 2003, with two field visits in February 2003 and March 2008. Here we report the difference in chemical composition of the glass shards in the cinder and the lava flows of 1994-95 eruption. The composition of glass shards from cinder of 1994-95 eruption differs significantly in its major element chemistry than that of the lava flows of the same eruption. For both lava flows and glass shards, the variation of various oxides against silica show limited fractionation, but with a significant vertical shift, because of different levels of Al₂O₃, CaO, FeO and Na₂O. If we ignore this shift then lava flows and glass shards together would show a continuous fractionation tend, indicating a common source. However, the vertical shift indicates difference in the composition of the 'immediate' parental liquids, which gave rise to the lava flows and glass shards. This difference in the levels of Al₂O₃, CaO, FeO and Na₂O resulting in the vertical shift can be attributed to liquid differentiation due to density contrast, which could have caused the separation of ferro-magnesian rich and calc-alumina rich phases; whereby the later phase, being lighter, became part of the magmatic froth. This in turn implies the presence of a shallow magma chamber, where liquid differentiation took place prior to the eruption. During the eruption, calc-alumina rich froth likely gave rise to the glass shards of the reported composition.