Takaaki Fukuoka

Boron contents of Japan Trench sediments and Iwate basaltic lavas, Northeast Japan arc: estimation of sediment-derived fluid contribution in mantle wedge

Mantle wedge below the Northeast Japan arc contains hydrous fluids that dehydrated from two parts of the descending slab: a sediment layer and an altered oceanic crust (AOC). In order to estimate the weight percent of a sediment-derived fluid in the mantle wedge, we determine boron and other element contents of Japan Trench sediments and Iwate basaltic lavas on the Northeast Japan volcanic front. Trace element contents of the trench sediments are used to estimate those of the sediment-derived fluid, and previously reported trace element contents of the AOC are used to estimate those of the AOC-derived fluid. Examination of mobile/immobile element ratios (B/Sm, Ba/Sm, K2O/Sm, B/Zr, Ba/Zr, K2O/Zr) of the two slab-derived fluids and Iwate basaltic lavas, indicates a mixing ratio between the AOC-derived fluid and the sediment-derived fluid is 90:10 in the mantle wedge beneath the Northeastern Japan arc. The mobile/immobile element ratios also suggest that the subducted sediment recycle to the mantle wedge beneath Northeast Japan as sediment-fluid rather than sediment-melt. Since the weight fraction of the total slab-derived fluid in the mantle wedge is estimated to be less than 1 wt%, an addition of a small amount (<0.1 wt%) of the sediment-derived fluid to the mantle wedge fulfills the boron and other trace element contents in the Iwate basaltic lavas.