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

• Published in: Scientific Reports
• Main Authors: Reagan, Mark, Turner, Simon, Handley, Heather, Turner, Michael, Beier, Christoph, Caulfield, John, Peate, David
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Pacific; Eyjafjallajökull; Iceland; Ocean Island
• Online Access: https://researchers.mq.edu.au/en/publications/ffd9b3b1-82ad-4a71-a4ed-e7ca140905c3; https://doi.org/10.1038/srep45186; https://research-management.mq.edu.au/ws/files/54689131/Publisher_version.pdf; http://www.scopus.com/inward/record.url?scp=85016146381&partnerID=8YFLogxK; http://purl.org/au-research/grants/arc/DP0988658; http://purl.org/au-research/grants/arc/FT120100440

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.