16:30 〜 16:45
[SCG53-11] マリアナ前弧モホ遷移帯構成岩類の岩石学：マントル/ボニナイト質メルト相互作用
キーワード：ボニナイト、マリアナ前弧、モホ遷移帯、蛇紋岩泥火山
The Izu-Bonin-Mariana (IBM) arc is an immature oceanic island arc that has been formed since around 50 Ma, when the Pacific Plate started to subduct beneath the Philippine Sea Plate. Rock surveys of the inner slope of the Mariana Trench have revealed an igneous sequence, i.e., MORB-like basalt and overlying boninite, which were formed at the early stage of the IBM arc (Ishizuka et al., 2014). This finding suggests that the Mariana forearc region records the drastic change of magmatic regimes, although it is enigmatic how this change processed. Serpentinite mud volcanoes distributed in the Mariana forearc region, which are derived from the hydrated mantle wedge, bring deep-seated rocks of the Mariana forearc to the surface as clasts in the mud volcanoes. Investigation of mantle-derived serpentinite clasts in the mud volcanoes indicates that they are probably residues after the extraction of MORB or boninite melts (Parkinson and Pearce, 1998). However, the deep crustal processes beneath the Mariana forearc have been poorly understood due to the complete lack of rocks linking the forearc mantle and upper crust.
During the R/V Sonne (SO292/2) expedition in 2022, boninite, orthopyroxene-bearing plutons (including dunite, harzburgite, orthopyroxenite, norite, and gabbronorite), and residual harzburgite were dredged from Haga’I Tasi Seamount, which is one of the closest mud volcanoes to the Mariana Trench axis (approximately 30 km away). These orthopyroxene-bearing plutons are characterized by a crystallization sequence of olivine + Cr spinel, orthopyroxene, clinopyroxene, and plagioclase. The olivine Mg# and NiO content are consistent with those of mantle olivine. The Cr spinel Cr# (0.60-0.74) is similar to that found in low-Si boninite. Clinopyroxene (cpx) and orthopyroxene (opx) in the orthopyroxenite and norite exhibit a variety of textures and chemical compositions. Ca-rich opx irregularly surrounds Ca-poor opx. The former is compositionally similar to opx phenocryst in boninite, while the latter is comparable with opx in residual harzburgite. Cpx shows a constant Mg# and variable incompatible element contents. Interstitial and poikilitic cpx and cpx associated with Ca-poor opx can be in equilibrium with low-Si boninite-like melts. However, cpx associated with olivine can be in equilibrium with high-Si boninite-like melts. Geochemical modeling indicates that the cpx might have been formed through the interaction between residual harzburgite and low-Si boninitic melt. The lithological characteristics and formation processes of these orthopyroxene-bearing plutons suggest that they are derived from a rock suite of the Moho transition zone, as seen in ophiolites.
Mafic to intermediate plutons with boninitic compositions are collected from Deep Blue Seamount (Johnson et al., 2014), which is located at the same distance from the trench axis as Haga’I Tasi Seamount. Johnson et al. (2014) interpreted them as the Mariana forearc middle crust formed by boninitic magmatism. Boninitic sheeted dikes, overlain by pillow lavas, are drilled during IODP Expedition 352 (Reagan et al., 2015). These facts indicate that the ophiolitic sequence is present beneath the Mariana forearc and that boninitic magmatism occurred in a divergent environment. Ichiyama et al. (2021) proposed that deeper-seated rocks are brought from beneath the Mariana forearc according to the distance of serpentinite mud volcanoes from the trench axis, indicating that serpentinization beneath Haga’I Tasi Seamount reaches the forearc Moho.
During the R/V Sonne (SO292/2) expedition in 2022, boninite, orthopyroxene-bearing plutons (including dunite, harzburgite, orthopyroxenite, norite, and gabbronorite), and residual harzburgite were dredged from Haga’I Tasi Seamount, which is one of the closest mud volcanoes to the Mariana Trench axis (approximately 30 km away). These orthopyroxene-bearing plutons are characterized by a crystallization sequence of olivine + Cr spinel, orthopyroxene, clinopyroxene, and plagioclase. The olivine Mg# and NiO content are consistent with those of mantle olivine. The Cr spinel Cr# (0.60-0.74) is similar to that found in low-Si boninite. Clinopyroxene (cpx) and orthopyroxene (opx) in the orthopyroxenite and norite exhibit a variety of textures and chemical compositions. Ca-rich opx irregularly surrounds Ca-poor opx. The former is compositionally similar to opx phenocryst in boninite, while the latter is comparable with opx in residual harzburgite. Cpx shows a constant Mg# and variable incompatible element contents. Interstitial and poikilitic cpx and cpx associated with Ca-poor opx can be in equilibrium with low-Si boninite-like melts. However, cpx associated with olivine can be in equilibrium with high-Si boninite-like melts. Geochemical modeling indicates that the cpx might have been formed through the interaction between residual harzburgite and low-Si boninitic melt. The lithological characteristics and formation processes of these orthopyroxene-bearing plutons suggest that they are derived from a rock suite of the Moho transition zone, as seen in ophiolites.
Mafic to intermediate plutons with boninitic compositions are collected from Deep Blue Seamount (Johnson et al., 2014), which is located at the same distance from the trench axis as Haga’I Tasi Seamount. Johnson et al. (2014) interpreted them as the Mariana forearc middle crust formed by boninitic magmatism. Boninitic sheeted dikes, overlain by pillow lavas, are drilled during IODP Expedition 352 (Reagan et al., 2015). These facts indicate that the ophiolitic sequence is present beneath the Mariana forearc and that boninitic magmatism occurred in a divergent environment. Ichiyama et al. (2021) proposed that deeper-seated rocks are brought from beneath the Mariana forearc according to the distance of serpentinite mud volcanoes from the trench axis, indicating that serpentinization beneath Haga’I Tasi Seamount reaches the forearc Moho.