October 30, 2012
Dr. T. Kawamoto
A new model for the magma generation in subduction zones is proposed by a research team of Drs. Tatsuhiko Kawamoto (Kyoto University), Masami Kanzaki (Okayama University), Kenji Mibe (University of Tokyo), Kyoko N. Matsukage (Ehime University) and Shigeaki Ono (JAMSTEC) and is published on the Proceedings of the National Academy of Sciences of U. S. A. Early Edition.
Summary
Subduction-zone magmatism is triggered by addition of H2O-rich slab-derived components: aqueous fluid (Figure A), hydrous partial melts (Figure B) or supercritical fluids (Figure C) from subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under high-pressure and high-temperature conditions beyond a critical end point.
Figure A、Figure B
Figure C
Drs. T. Kawamoto (Kyoto University), M. Kanzaki (Okayama University), K. Mibe (University of Tokyo), K. N. Matsukage (Ehime University) and S. Ono (JAMSTEC) propose a new hypothesis. They show the critical end points between aqueous fluids and sediment or high-Mg andesite (HMA) located at 80 and 90 km depth, respectively, using an in-situ observation technique at SPring-8, Japan. These depths are within mantle wedge underneath volcanic arcs, which are formed 90—200 km above subducting slabs (Figure C). The present data suggest that sedimental supercritical fluids are fed to the mantle wedge from subducting slab, and then they react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 90 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separation of slab-derived supercritical fluids into aqueous fluids and melts elucidates such double magmatism of mantle-derived andesites and basalts, and two slab-derived signatures observed in island arc basalts.
This study was supported by Japan Synchrotron Radiation Research Institute (JASRI), SPring-8 and by Kakenhi through the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science, and Technology.
Paper Information
[DOI] http://dx.doi.org/10.1073/pnas.1207687109
Kawamoto Tatsuhiko, Kanzaki Masami, Mibe Kenji, Matsukage Kyoko N., Ono Shigeaki. Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism. Proceedings of the National Academy of Sciences. October 29, 2012. doi: 10.1073/pnas.1207687109