Effect of subduction components on production of basalts from Tateshina volcano, central Japan: geochemical calculation of dehydration of subducting oceanic crust and partial melting of overlying sediments, and subsequent fluid-mantle interaction

摘要

We report the major and trace element chemistry and Sr and Nd isotopic composition of basaltic rocks from the Quaternary Tateshina volcano, central Japan, and model the composition of these rocks in terms of the partial melting and melt-solid reactions controlled by the addition of slab-derived fluid and silicate melt to the original wedge mantle beneath the Tateshina area. Tateshina basaltic rocks have a SiO2 content ranging from 50.06 to 53.28 wt% and a MgO content ranging from 5 to 7 wt%, indicating that these rocks are less fractionated. These rocks have a narrow range of Sr and Nd isotopic compositions (~(87)Sr/~(86)Sr = 0.703619-0.703807, ~(143)Nd/~(144)Nd = 0.512893-0.512945). There is no correlation between Tateshina basaltic rocks and crustal materials in terms of ~(143)Nd/~(144)Nd-Srn/Nd, indicating that Tateshina basaltic rocks have no crustal contamination. The wedge mantle peridotite would have experienced accumulated subduction events, including the addition of subduction components and the extraction of basalt magmas during the Tertiary period. In addition, we have accounted for the mantle motion accompanying slab subduction within the wedge mantle. Thus, we assumed that the mantle wedge before the effects of Quaternary subduction that produced the volcanic rocks of Tateshina volcano could have blended with residual mantle after the extraction of Tertiary basalt magmas with depleted MORB-source mantle (DMM).rnThe model includes partial melting of the blended wedge mantle peridotite metasomatized by aqueous fluid and silicate melt derived from the dehydration of altered oceanic crust (AOC) and partial melting of oceanic sediments, respectively. The trace element and Sr isotope compositions of the basaltic rocks of Tateshina volcano can be reproduced by about 10% melting of the calculated wedge mantle peridotite modified by the addition of 0.8%-1.0% silicate melt from oceanic sediments and 0.2%-0.8% aqueous fluids from the AOC, respectively.