HR: 0800h
AN: V41B-1440    [Abstracts]
TI: Spatial and temporal variations of Boron contents of basaltic rocks from northern Kyushu, SW Japan arc
AU: * Miyoshi, M
EM: miyoshim@es.sci.kumamoto-u.ac.jp
AF: Graduate school of Science and Technology, Kumamoto University, Kurokami 2-39-1, Kumamoto, Kumamoto, 860-8555 Japan
AU: Shimono, M
EM: sm826sm@yahoo.co.jp
AF: Department of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto, Kumamoto, 860-8555 Japan
AU: Fukuoka, T
EM: tfukuoka@ris.ac.jp
AF: Department of Environmental Systems, Rissho University, Magechi 1700, Kumagaya, 360-0194 Japan
AU: Sano, T
EM: sano@fuji-tokoha-u.ac.jp
AF: Collage of Environment and Disaster Research, Fuji Tokoha University, Ohbuchi 325, Fuji, 417-0801 Japan
AU: Hasenaka, T
EM: hasenaka@aster.sci.kumamoto-u.ac.jp
AF: Department of Science, Kumamoto University, Kurokami 2-39-1, Kumamoto, Kumamoto, 860-8555 Japan
AB: Boron (B) is a useful tracer to evaluate recycling of oceanic crust and sediment during subduction processes, because it is enriched in altered oceanic crust (AOC) and sea floor sediments. Evidence for recycling of sediments and /or AOC to the arc crust by magmatism is observed in higher B/Nb, B/Sm and B/Zr ratios of arc basaltic lavas compared with those of MORBs and OIBs. Therefore, boron is the best element to investigate the influence of subducting oceanic crust. We determined boron contents of basaltic rocks from Kyushu Island in order to investigate spatial and temporal variations of subduction component to the source materials in this part of SW Japan arc. Boron and other selected trace elements were determined by neutron induced prompt gamma-ray analysis (NPGA) at the thermal neutron beam guide of the JRR-3M reactor, Japan Atomic Energy Research Institute. Spatial change of boron content was investigated by comparing fore-arc basaltic rocks (from Aso, etc.) with back-arc basaltic rocks (from Kita-Matsuura and Iki Island, etc.). The former has higher B/Sm (1.2-4.8) and B/Nb (1.3-4.2) ratios than the latter (B/Sm, B/Nb <1). This probably reflects the irregular shape of subducting Philippine Sea Plate. In the northern and central Kyushu, the oceanic plate subduct beneath Aso, however it does not reach the area below back-arc volcanoes. Therefore, it is possible that the subduction metazomatism does not extend into the source materials of back-arc basaltic rocks. Temporal change of boron content was detected in Aso basaltic rocks. The pre-caldera basaltic products (2.2Ma) have lower B/Sm (1.2-1.4) and B/Nb (1.3-1.7) ratios than those of the caldera-forming (270-90ka) stage and the post-caldera (90ka-present) stage (B/Sm=2.9-4.8, B/Nb=2.2-4.2). B/Zr ratio of representative pre-caldera (0.06) and post-caldera basaltic rocks (0.12-0.17) during the slight change in Zr contents (90 to 130 ppm) would not be created by crystal fractionation or assimilation of basement granitic rocks (Zr=150ppm, B/Zr=0.1). These observations indicate that metazomatism of the source material beneath Aso was progressed by subduction of the Philippine Sea Plate from 2.2Ma to present. Basaltic rocks from Kyushu Island are characteristically low in subduction component in comparison with typical volcanic arc. They are low in B/Sm (<5) and B/K2O (<15) ratios, whereas those from NE Japan arc are high in B/Sm (3-12) and B/K2O (>30) ratios. The sharp contrast between them probably reflects the difference in the age of subducting plate. The Philippine Sea Plate (4-6Ma) is extremely young when compared with the Pacific Plate (90-118Ma) the underthrusts NE Japan arc. Young and hot Philippine Sea Plate probably contributes fewer amounts of AOC and/or sea floor sediment components than the old and cold Pacific Plate to the source mantle of basaltic magmas. Younger subducting slab perhaps carries less sediment than the older one. It is likely that hot slab releases hydrous fluid that contains boron and other subduction components before reaching the source area of arc magmas.
DE: 1025 Composition of the mantle
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005