210Pb-226Ra disequilibria in young gas-laden magmas

We present new 238 U- 230 Th- 226 Ra- 210 Pb and supporting data for young lavas from southwest Pacific island arcs, Eyjafjallajökull, Iceland, and Terceira, Azores. The arc lavas have significant 238 U and 226 Ra excesses, whereas those from the ocean islands have moderate 230 Th and 226 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 ( 210 Pb/ 226 Ra) 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 210 Pb deficit, which we attribute to ~8.5 years of steady 222 Rn loss to a CO 2 -rich volatile phase while it traversed the crust. Lavas erupted from water-laden magma systems, including those investigated here, commonly have near equilibrium ( 210 Pb/ 226 Ra) 0 values. Maintaining these equilibrium values requires minimal persistent loss or accumulation of 222 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.