伊豆島弧火山のSr,Nd同位体組成の特徴

―島弧マグマ形成に関する沈み込むスラブの役割―

抄録

Isotopic compositions of Sr and Nd were determined for the volcanic rocks from the Izu Islands Arc. The volcanic rocks from the volcanic front zone show variable Nd isotopes ranging from 0.513088 to 0.513125 of ¹⁴³Nd/¹⁴⁴Nd with an average ε_Nd of 9.0±0.3, and ⁸⁷Sr/⁸⁶Sr from 0.70318 to 0.70355 with an average ε_Sr of -16.1±0.7. The back arc side volcanics show slightly lower Nd and Sr isotopic ratios as ¹⁴³Nd/¹⁴⁴Nd from 0.512994 to 0.513066 with an average ε_Nd of 7.8±0.4 and ⁸⁷Sr/⁸⁶Sr form 0.70299 to 0.70325 with average ε_Sr of -19.9±0.4. Across arc isotopic variation of Sr and Nd from the volcanic front to the back-arc side is attributed to difference in the degree that the magmas of the subduction zones are generated by the addition of the fluid which is extracted from the hydrous minerals in the down-going slab. Magmas in the volcanic front are much more affected by the addition of the considerable amount of the fluid than the back-arc side magmas and altered Sr and Nd isotopic compositions from their original values of the mantle wedge. The role of the fluid from the down-going slab for magma generation is isotopically assessed in the intra-oceanic Izu arc, and the Nd and Sr isotopic compositions of the fluid extracted from the down-going slab is equivalent with those of the altered MORB.