Nature, source and composition of volcanic ash in sediments from a fracture zone trace of Rodriguez Triple Junction in the Central Indian Basin

Abstract

Volcanic glasses associated with pumice, micronodules and palagonite like lithic fragments were recovered from a volcanic terrain in a fracture zone defined as Rodriguez Triple Junction trace in the Central Indian Basin. Morphologically, the tephra consist predominantly of bubble wall shards with the co-occurrence of platy and blocky types. Rarity of vesicles in the shards suggests that they are formed in depths deeper than 4000 m, below the volatile fragmentation depth (VFD). Energy dispersive spectrometer (EDS) analyses have shown that the tephra have SiO 2 contents ranging from ∼74% to 77% suggesting that they are mainly rhyolitic and formed from melts of siliceous composition. The depositional age of the tephra layers determined by radiolarian assemblages and excess 230Th decay profile corresponds to an age older than 180 kyr. The tephra are rare in sediments younger than ∼239 kyr. The beginning of the volcanic eruption contributing this volcanic ash however, cannot be estimated. Occurrence of large sized tephra suggests proximal source, probably from a local volcanic source. Compositional and morphological contrasts, age constraints, occurrence of large sized tephra, and tectonic setting of the cored site/location suggest an intraplate volcanism, triggered by reactivation of tectonic activity in the fracture zone trace of the Indian Ocean Triple Junction during the Late Pleistocene.