210Pb-226Ra disequilibria in young gas-laden magmas.
We present new238U-230Th-226Ra-210Pb and supporting data for young lavas from southwest Pacific island arcs, Eyjafjallajökull, Iceland, and Terceira, Azores. The arc lavas have significant238U and226Ra excesses, whereas those from the ocean islands have moderate230Th and226Ra 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 (210Pb/226Ra)0values in all settings largely result from degassing processes rather than mineral-melt partitioning. Like most other ocean island basalts, the Terceira basalt has a210Pb deficit, which we attribute to ~8.5 years of steady222Rn 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 (210Pb/226Ra)0values. Maintaining these equilibrium values requires minimal persistent loss or accumulation of222Rn 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.
Department of Earth and Environmental Sciences, University of Iowa, Iowa City, Iowa, IA 52242, USA.
Mark
M
Reagan
Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia.
Simon
S
Turner
Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia.
Heather
H
Handley
Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia.
Michael
M
Turner
GeoZentrum Nordbayern, Friedrich-Alexander Universität Erlangen-Nürnberg, Schlossgarten 5, D-91054 Erlangen, Germany.
Christoph
C
Beier
Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia.|Trinity College Dublin, School of Natural Sciences, Department of Geology, Dublin 2, Ireland.
John
J
Caulfield
Department of Earth and Environmental Sciences, University of Iowa, Iowa City, Iowa, IA 52242, USA.
David
D
Peate