Japan Geoscience Union Meeting 2022

Presentation information

[E] Oral

S (Solid Earth Sciences ) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT18] GEOPHYSICAL PROPERTIES AND TRANSPORT PROCESSES IN THE DEEP CRUST AND MANTLE

Mon. May 23, 2022 9:00 AM - 10:30 AM 102 (International Conference Hall, Makuhari Messe)

convener:Bjorn Mysen(Geophysical Laboratory, Carnegie Inst. Washington), convener:Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Naoko Takahashi(Graduate School of Science, Tohoku University), convener:Saeko Kita(International Institute of Seismology and Earthquake Engineering, BRI), Chairperson:Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Naoki Takahashi(Natural History Museum and Institute, Chiba)


9:30 AM - 9:45 AM

[SIT18-03] Elasticity of Orthopyroxene at High Pressure and Temperature: Insights into the Metasomatism of mantle wedge by siliceous melts

*Xin Deng1, Jian Song1, Wangsheng Qian1, Zhongqing Wu1 (1.University of Science and Technology of China)


Keywords:elasticity, orthopyroxene, siliceous melts, subduction zone, low Vp/Vs ratio, first-principles calculations

Large amounts of silica held by slabs during subduction prefer to concentrate into the eclogite melts and further lead to the metasomatism of the overlying mantle wedge. However, the process of the metasomatism in mantle wedge by siliceous melts remains poorly constrained. Peridotite in the mantle wedge is sufficiently silica undersaturated. Therefore, SiO2-rich melts would react with olivine in the peridotite to form orthopyroxene (opx). The local enrichment of opx has also been proposed to account for the observed low VP/VS ratio in the mantle wedges of a few subduction zones, revealing the metasomatism by silica saturated melts there. But the anomalously low VP/VS regions in a few subduction zones are inconsistent with the widespread metasomatism of the mantle wedge. In this study, we determined the elastic properties of Fe-bearing opx (Mg0.875Fe0.125)SiO3 at high pressure and temperature. The VP/VS ratio of opx has a strong dependence on the pressure and temperature (P-T) conditions, and only exhibits low VP/VS at low pressure and high temperature. Although the enrichment of opx may be widespread, it can only produce a discernible low VP/VS ratio at subduction zones with proper thermal conditions. Together with the elasticities of other major minerals in the upper mantle, we investigated the effect of opx enrichment on the VP/VS ratio at mantle wedge conditions. The VP/VS ratio linearly decreases with the opx volume fraction (Ω), and the relationship can be described as VP/VS = 1.772 - 0.142Ω at 2 GPa and 1500 K, providing a simple way to access the enrichment degree of opx. Combined with seismic and geodynamic studies, we also constrained the mineral proportions of a mantle wedge in Alaska. The opx proportion maximal at the mantle wedge center suggests that the siliceous melts generated from the slab prefer to ascend and remain relatively isolated until they mix in the shallow hotter core of the mantle wedge.