2007 Volume 41 Issue 2 Pages 63-75
An active convergent plate boundary comprises dynamic material circulation system where materials risen up from descending oceanic plate partly go back to the Earth's surface. An evaluation of the material flux in this system is important to assess future view of the planet Earth. In the mantle wedge, fluids circulate in various forms; i.e., aqueous fluids released from the descending oceanic lithosphere trigger partial melting of the mantle wedge, and the melt ascends through the mantle wedge leading arc volcanism. Investigations of mantle-derived xenoliths sampled from the active margin of a continent or an island arc are effective for identifying mantle processes, including partial melting and material circulation, in the mantle wedge. For the mantle xenoliths, however, various technical difficulties lie in the estimation of the depth where the xenoliths entrained by host magma. Since internal pressure (density) of CO2 inclusions in a mantle xenolith reflects both conditions of temperature and pressure where the xenolith existed, CO2 density in a fluid inclusion was estimated by Micro-Raman spectroscopy to calculate the depth (pressure) of origin applying to the equation of state of CO2 and the equilibration temperature. This newly developed method can serve as a new depth probe. Combination of the geobarometry and various micro-analyzing technique reveals the occurrence of subduction-related fluid in the mantle wedge with high spatial resolution. Four-dimensional (latitude, longitude, depth and geologic age) mapping of the fluid migration in the mantle wedge is theoretically possible to apply my developed techniques to mantle-derived xenolith, which remains the ancient record of the mantle.
