H. Shukuno

Hot fingers in the mantle wedge : New insights into magma genesis in subduction zones

Abstract Quaternary volcanoes in the Northeast Japan arc can be grouped into 10 volcanic clusters striking transverse to the arc; these have an average width of 50 km, and are separated by parallel gaps 30–75 km wide. This clustering of volcanic centres, topographic profiles, low-velocity regions in the mantle wedge and local negative Bouguer gravity anomalies along the Japan Sea side of the volcanic arc are closely correlated. All these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined, 50 km wide fingers. Each of the 10 fingers recognised extends from deep mantle (>150 km) below the back-arc region towards the shallower mantle (∼50 km) beneath the volcanic front. Quaternary volcanoes are built immediately above the hot mantle fingers. The volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust, accompanied by Quaternary volcanic activity at the surface. Although volcanic activity is rare along the Japan Sea coast, tomographic results show that hot, low-velocity mantle fingers exist within the mantle wedge. The negative Bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers; these have not yet been erupted, but have accumulated at the Moho discontinuity.

Diversity of melt conduits in the Izu-Bonin-Mariana forearc mantle: Implications for the earliest stage of arc magmatism

ABSTRACT 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 Technology9s 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 TiO 2 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.