Japan Geoscience Union Meeting 2016

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International Session (Oral)

Symbol S (Solid Earth Sciences) » S-GC Geochemistry

[S-GC16] Volatile Cycles in the Deep Earth - from Subduction Zones to the Mantle and Core

Wed. May 25, 2016 1:45 PM - 3:15 PM 304 (3F)

Convener:*Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Takeshi Hanyu(Japan Agency for Marine-Earth Science and Technology, Department of Solid Earth Geochemistry), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo), Colin Jackson(Geophysical Laboratory, Carnegie Institution of Washington), Chair:Jackson Colin(Carnegie Institution for Science Washington), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo)

2:30 PM - 2:45 PM

[SGC16-04] On the coexistence of high-Mg andesites and ultramafic volcanic rocks

★Invited papers

*Kenji Mibe1, Tatsuhiko Kawamoto2, Shigeaki Ono3 (1.Earthquake Research Institute, University of Tokyo, 2.Institute for Geothermal Sciences, Graduate School of Science, Kyoto University, 3.Japan Agency for Marine-Earth Science and Technology)

Keywords:high pressure and high temperature

In order to understand the origin of magmas in the hydrous upper mantle, we have been investigating the phase relation and compositions of phases in the system peridotite-H2O. Using X-ray radiography technique together with Kawai-type multi-anvil high-pressure apparatus (SPEED-1500, SPring-8, Japan), the second critical endpoint in the peridotite-H2O system was determined to be at around 3.8 GPa and 1000 ºC (corresponding to a depth of ~ 110 km) [1]. It is possible that two fluid phases (i.e., aqueous fluid and hydrous silicate melt) coexist up to 3.8 GPa. Above 3.8 GPa, however, hydrous silicate melt and aqueous fluid in upper mantle becomes indistinguishable from each other and the melting temperature of hydrous mantle peridotite can no longer be defined beyond this critical condition. Using the quenched recovered samples obtained by Mibe et al. [1], chemical compositions of aqueous fluid, silicate melt, and supercritical fluid magma in the vicinity of second critical endpoint were determined by the electron microprobe analyzer. In the run at 3.6 GPa, the composition of aqueous fluid was high-Mg andesitic, whereas the composition of hydrous silicate melt was komatiitic. Our experimental results indicate that high-Mg andesitic magma and komatiitic magma can be generated at the same time as the liquid-fluid immiscibility near the second critical endpoint in the peridotite-H2O system. Some natural examples on the coexistence of high-Mg andesites and ultramafic volcanic rocks from various tectonic settings and locations around the world will also be presented.
[1] Mibe, K., M. Kanzaki, T. Kawamoto, K. N. Matsukage, Y. Fei, and S. Ono (2007), Second critical endpoint in the peridotite-H2O system, J. Geophys. Res., 112, B03201, doi:10.1029/2005JB004125.