SULFUR, FLUORINE AND CHLORINE DEGASSING OF PLIO-PLEISTOCENE VOLCANOES IN EAST AFRICA

Tephrostratigraphic research on the Turkana Basin of Northern Kenya has yielded more than two hundred forty silicic to intermediate, chemically and stratigraphically distinct tephra layers, spanning in age from 4 to 0.5 Ma, that had possible source volcanoes in the Ethiopian Highlands. These tephra layers have provided important links between widely separated fossil sites and contributed to our knowledge of the volcanic history of the region. In addition to occurrences over several million square kilometers on land, many of these tephra are also known from deep-sea cores in the Gulf of Aden, Indian Ocean and the Arabian Sea. The recovery of these silicic tephras from the over wide geographic area suggests that some of the eruptions were amongst the largest yet documented for the region. The oldest of these widespread tephra the Moiti Tuff, is estimated to have a minimum volume of 300 km². When they were active, these Plio-Pleistocene volcanoes must have added tremendous amount of tephra particles to the atmosphere loading it with sulfuric, hydrochloric and hydrofluoric acids. This study presents volatile analyses of these tephra layers. The degassing history and volatile budget of these volcanoes is determined by measurements of pre-eruption and residual contents of volatiles in the products of all phases of the eruption. Using the measured sulfur, fluorine and chlorine concentrations in glass shards, glass inclusions, phenocrysts and in glasses of several pumices, the amount of free volatiles under pre-eruption conditions of these Plio-Pleistocene volcanoes in East Africa is estimated and founded to be comparable to many well-characterized climatically significant volcanic events.