Pb-210-Ra-226 Disequilibria In Young Gas-Laden Magmas

We present new U-238-Th-230-Ra-226-Pb-210 and supporting data for young lavas from southwest Pacific island arcs, Eyjafjallajokull, Iceland, and Terceira, Azores. The arc lavas have significant U-238 and Ra-226 excesses, whereas those from the ocean islands have moderate Th-230 and Ra-226 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-210/Ra-226)(0) 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-210 deficit, which we attribute to -8.5 years of steady Rn-222 loss to a CO2-rich volatile phase while it traversed the crust. Lavas erupted from water-laden magma systems, including those investigated here, commonly have near equilibrium (Pb-210/Ra-226)(0) values. Maintaining these equilibrium values requires minimal persistent loss or accumulation of Rn-222 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.