Evidence for an Active, Transcrustal Magma System in the Last 60 ka and Eruptive Degassing Budget (H2O, CO2, S, F, Cl, Br): The Case of Dominica

The Morne Trois Pitons-Micotrin volcanic complex on the island of Dominica (Lesser Antilles) emitted a series of plinian eruptions between 18 and 9 ka BP. We studied it to constrain magma storage conditions and volatile degassing balances, by comparison with the three previous ignimbrites (similar to 60-24 ka BP). Volatile concentrations in glass inclusions and mineral-melt thermobarometry indicate storage at <= 200 MPa (similar to 6-8 km) and 860-880 degrees C. The magmas feeding these plinian eruptions were stored at a shallower depth than those that older ignimbrites from the same volcanic complex and stored at similar to 16 km. Close magma composition and similar halogen ratios, however, suggest a common source for the magmas feeding both the plinian eruptions and the ignimbrites. The large eruptive fluxes of F, Cl, and Br to the atmosphere (up to 1.4-2.8 x 10(-1) Mt/km(3), 1.5-4.0 Mt/km(3), and 2-4 x 10(-2) Mt/km(3), respectively), estimated by the petrological method, support the potentially important role of volcanic halogens in modifying the chemistry of the atmosphere, though Cl is underestimated here because of buffering in a fluid phase. The behavior of S, potentially partitioned in the same fluid phase, prevents here the calculation of an eruptive outgassing budget.