Journal
GEOLOGY
Volume 39, Issue 4, Pages 411-414Publisher
GEOLOGICAL SOC AMER, INC
DOI: 10.1130/G31706.1
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Funding
- Ministry of Education, Culture, Sports, Science and Technology of Japan [21403010]
- Japanese Society for the Promotion of Science
- Grants-in-Aid for Scientific Research [21403010, 22109504, 23654185] Funding Source: KAKEN
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Magmatic processes during the earliest stage of subduction initiation are still not well understood. We examined peridotites recovered from an exhumed crust-mantle section exposed along the landward slopes of the northern Izu-Bonin Trench using the Japan Agency for Marine-Earth Science and Technology's remotely operated vehicle KAIKO7000II. Based on the Cr# [Cr/(Cr + Al) atomic ratio] of spinel, two distinctive groups, (1) high-Cr# (>0.8) dunite and (2) medium-Cr# (0.4-0.6) dunite, occur close to each other and are associated with refractory harzburgite. Two distinctive melts were in equilibrium with these dunites: a boninitic melt for the high-Cr# dunite and a mid-oceanic ridge basalt (MORB)-like melt for the medium-Cr# dunite. The TiO2 content of the latter melt is lower than typical MORB compositions. We suggest that the medium-Cr# dunite was a melt conduit for a basalt recently reported from the Mariana forearc that was erupted at the inception of subduction. The wide range of variation in the Cr#s of spinels in dunites from the Izu-Bonin-Mariana forearc probably reflects changing melt compositions from MORB-like melts to boninitic melts in the forearc setting due to an increase of slab-derived hydrous fluids and/or melts during subduction initiation.
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