This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro, and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge). Mantle source compositions beneath the Mariana Arc and the Mariana Trough indicate a mantle source that is depleted in high field strength elements relative to MORB (mid‐oceanic ridge basalt). Samples from the Mariana Arc, characterized by high ratios of Ba/Th, U/Th, ⁸⁴Kr/⁴He and ¹³²Xe/⁴He, are explained by addition of fluid from the subducted slab to the mantle wedge. Correlations of noble gas data, as well as large ion lithophile elements, indicate that heavy noble gases (Ar, Kr, and Xe) provide evidence for fluid fluxing into the mantle wedge. On the other hand, major elements and Sr, Nd, He, and Ne isotopic data of basalts from the Mariana Trough are geochemically indistinguishable from MORB. Correlations of ³He/⁴He and ⁴⁰Ar/³⁶Ar in the Mariana Trough samples are explained by mixing between MORB and atmosphere. One sample from the Central Graben indicates extreme enrichment in ²⁰Ne/²²Ne and ²¹Ne/²²Ne, suggesting incorporation of solar‐type Ne in the magma source. Excess ¹²⁹Xe is also observed in this sample suggesting primordial noble gases in the mantle source. The Mariana Trough basalts indicate that both fluid and sediment components contributed to the basalts, with slab‐derived fluids dominating beneath the Spreading Ridge, and that sediment melts, characterized by high La/Sm and relatively low U/Th and Zr/Nb, dominate in the source region of basalts from the Central Graben.

This study presents new major and trace element, mineral, and Sr, Nd, and noble gas isotope geochemical analyses of basalts, gabbro and clinopyroxenite from the Mariana Arc (Central Islands and Southern Seamount provinces) including the forearc, and the Mariana Trough (Central Graben and Spreading Ridge).

池田保夫・長尾敬介・石井輝秋・松本大介・Robert J. Stern・加々美寛雄・有馬　眞・Sherman H. Bloomer

本研究はマリアナ前弧を含むマリアナ弧及びマリアナトラフ(Central Grabenと Spreading Ridge)からの玄武岩,斑糲岩,単斜輝石岩の主成分・微量元素組成,鉱物組成,Sr,Nd及び希ガスの同位体組成を提示する.マリアナ弧とマリアナトラフ下のマントルソースはMORBのマントルソースに比べてHFS元素に乏しい.マリアナ弧の試料は,高Ba/Thと高U/Thで特徴づけられ,マントルウエッジに沈み込むスラブからの流体の付加で説明される.重希ガスの同位体組成(Ar, Kr, Xe)は,親石元素同様,マントルウエッジに流体が供給されていることを示す.一方,マリアナトラフからの試料の主成分元素組成及びSr,Nd,He,Neの同位体組成はMORBのものと区別できない.マリアナトラフの試料の³He/⁴He と ⁴⁰Ar/³⁶Arの相関はMORB組成と大気組成の混合で説明される.マリアナトラフのCentral Grabenからの一試料は²⁰Ne/²²Ne と ²¹Ne/²²Neに富み,マントルソースに太陽タイプネオンの存在が示唆される.さらに,過剰¹²⁹Xeも観察され,マントルソースに始原的希ガスの存在が示唆される.マリアナトラフの玄武岩は,Ba/Th‐La/SmとU/Th‐Zr/Nbでの相関から流体と堆積物両方の組成の寄与が示されるが,Spreading Ridgeからの玄武岩は,スラブからの流体組成の寄与が優勢で,Central Grabenからの玄武岩は堆積物溶融組成の寄与が優勢である.