Pb-Ra disequilibria in young gas-laden magmas.

We present new U-Th-Ra-Pb and supporting data for young lavas from southwest Pacific island arcs, Eyjafjallajökull, Iceland, and Terceira, Azores. The arc lavas have significant U and Ra excesses, whereas those from the ocean islands have moderate Th and Ra excesses, reflecting mantle melting in the presence of a water-rich fluid in the former and mantle melting by decompression in the latter. Differentiation to erupted compositions in both settings appears to have taken no longer than a few millennia. Variations in the (Pb/Ra) values in all settings largely result from degassing processes rather than mineral-melt partitioning. Like most other ocean island basalts, the Terceira basalt has a Pb deficit, which we attribute to ~8.5 years of steady Rn loss to a CO-rich volatile phase while it traversed the crust. Lavas erupted from water-laden magma systems, including those investigated here, commonly have near equilibrium (Pb/Ra) values. Maintaining these equilibrium values requires minimal persistent loss or accumulation of Rn in a gas phase. We infer that degassing during decompression of water-saturated magmas either causes these magmas to crystallize and stall in reservoirs where they reside under conditions of near stasis, or to quickly rise towards the surface and erupt.