Submarine magma degassing and explosive magmatism at Macdonald (Tamarii) seamount

Abstract

MACDONALD seamount is an active volcanic centre located at 29°98′ S, 140°25′ W in the south-central Pacific. Since its discovery¹ in 1968, a number of expeditions have surveyed and dredged rocks from the seamount. During a Scripps Institution of Oceanography expedition in October 1987, the first direct observation of volcanic activity at Macdonald was made when a small eruption occurred spewing forth scoriaceous basaltic rocks which briefly floated to the sea surface. The rocks were accompanied by huge bubbles of volcanic gas that created large sea surface slicks (H. Craig, personal communication). Similar activity at Macdonald was observed by a joint French-German team in January 1989 using the submersible Cyana². Here we look at the radio-geochemical features of the lone rock recovered from the October 1987 eruption. We present analyses of U-series nuclides from the first observed eruption of Macdonald (also known as Tamarii) seamount with interpretations focusing on the timescales of mag-matic events and the implications of uranium-series data for magma degassing. This highly gas-charged magma lost all of its volatile ²¹⁰Po upon eruption. Based on this, estimates of the fluxes of other volatile elements to sea water, which may contribute significantly to their oceanic budget, are calculated. High (²³⁰Th/²³²Th) and ⁸⁷Sr/⁸⁶Sr ratios, common signatures of altered crust, suggest that the explosive nature of the Macdonald volcan-ism results partly from the sealing of magma conduits by seawater circulation.