12:00 〜 12:15
[SSS13-12] Distribution of slab-fluids around the edge of the Philippine Sea Plate in Central to Northeast Japan
キーワード：プレートエッジ、スラブ、フィリピン海プレート、中部日本
Marginal parts and edges of a plate and subducting slab may play important
roles in geodynanmics, because those are the places where the plate
interacts with other plates or with the mantle: Thermal, geochemical and
mechanical interactions are expected. The Philippine Sea slab (PHS slab)
that subducts beneath the Japan arcs has such an edge. To examine the
relationship between the arc magmatism and the slab edge in the transition
zone from Northeast Japan to Central Japan, we have investigated isotopic
systematics of the volcanic rocks in the area, including both the data from
literature and the new data for five isotopic ratios of Sr, Nd and Pb. The
new data include major element compositions of 22 samples from the back-arc
area where a few petrological data were available and five isotopic ratios
for the selected 6 samples from Pleistocene to early Quaternary epoch. As a
result, a detailed spatial variation of the isotopic ratios can be discussed
for Northeast to Central Japan. On the basis of the spatial variation of the
isotopic ratios and the estimated amount of slab-derived fluid, we found (1)
the amount of fluid derived from the two subducting slabs (i.e., the Pacific
slab and the Philippine Sea slab) decreases from a significantly high value
(~5 wt.% fluid added to the source mantle) to the north away from the
seismically determined edge of the PHS slab, (2) the proportion of the PHS
component in the total slab-derived fluid also decays northward, and (3) the
PHS component spreads to the north beyond the edge of PHS slab. These
observations strongly suggest that the aseismic Philippine Sea slab exists
beneath the southernmost Northeast Japan to deliver the PHS component to the
arc magmatism. In addition, the double subduction of the two slabs generate
enhanced suction force at the corner region near the edge of PHS slab, which
may account for the fluid focusing as described (1) above.
roles in geodynanmics, because those are the places where the plate
interacts with other plates or with the mantle: Thermal, geochemical and
mechanical interactions are expected. The Philippine Sea slab (PHS slab)
that subducts beneath the Japan arcs has such an edge. To examine the
relationship between the arc magmatism and the slab edge in the transition
zone from Northeast Japan to Central Japan, we have investigated isotopic
systematics of the volcanic rocks in the area, including both the data from
literature and the new data for five isotopic ratios of Sr, Nd and Pb. The
new data include major element compositions of 22 samples from the back-arc
area where a few petrological data were available and five isotopic ratios
for the selected 6 samples from Pleistocene to early Quaternary epoch. As a
result, a detailed spatial variation of the isotopic ratios can be discussed
for Northeast to Central Japan. On the basis of the spatial variation of the
isotopic ratios and the estimated amount of slab-derived fluid, we found (1)
the amount of fluid derived from the two subducting slabs (i.e., the Pacific
slab and the Philippine Sea slab) decreases from a significantly high value
(~5 wt.% fluid added to the source mantle) to the north away from the
seismically determined edge of the PHS slab, (2) the proportion of the PHS
component in the total slab-derived fluid also decays northward, and (3) the
PHS component spreads to the north beyond the edge of PHS slab. These
observations strongly suggest that the aseismic Philippine Sea slab exists
beneath the southernmost Northeast Japan to deliver the PHS component to the
arc magmatism. In addition, the double subduction of the two slabs generate
enhanced suction force at the corner region near the edge of PHS slab, which
may account for the fluid focusing as described (1) above.