13:45 〜 15:15
[MIS13-P01] マリアナ前弧マントルの加水作用
★招待講演
キーワード：マリアナ前弧マントル、加水作用
Subduction zones are the most crucial regions for mass transfer from the Earth’s surface into the mantle. However, most mass transfer processes in subduction zones are poorly understood because they take place in the deep mantle beneath the Earth’s surface. In the Mariana forearc regions, mantle-derived serpentinized peridotites occur as clasts in serpentinite mud volcanoes and as basal rocks exposed in landward slopes of the trench. These forearc peridotites originated from the slab/mantle interface and provide excellent opportunities to understand mantle processes at the subduction initiation of the Mariana arc and the hydration and serpentinization processes of mantle peridotite by the percolating of slab-derived fluids.
Mafic and ultramafic clasts were recovered from three Mariana serpentinite mud volcanoes during IODP Expedition 366 (Fryer et al., 2018). Ichiyama et al. (2021) revealed that the mafic and ultramafic clasts change their petrological and mineralogical characteristics depending on the distances of the mud volcanoes from the trench. The antigorite- and amphibole-bearing ultramafic clasts and blueschist-facies mafic clasts occur at mud volcanoes farther from the trench. The trace-element patterns of amphibole in the ultramafic clasts show enrichment in Cs and Rb, implying the hydration by aqueous fluids released from the subducted slab. Ultramafic clasts were also collected at the mud volcano nearest to the trench during the R/V Sonne cruise (SO292/2; Menapace et al., 2022). The highly serpentinized ultramafic clasts do not include antigorite but lizardite/chrysotile as serpentine minerals. In addition to residual mantle harzburgite, poikilitic harzburgite associated with crustal rocks, such as orthopyroxenite, gabbro, diorite, and boninite, was also recovered at the mud volcano. This fact might imply that the serpentinization front is reaching the forearc crust/mantle boundary and that boninitic stratigraphy from the crust to the upper mantle is constructed below the Mariana forearc region.
Amphibole and antigorite are rare in ultramafic rocks exposed in the landward slopes of the Izu–Mariana trench. However, ultramafic rocks from the South Mariana trench are rich in amphibole and talc (Ohara & Ishii, 1998; Yanagida et al., 2007). Ca amphibole is classified as pargasite, edenite, magnesiohonblende, and tremolite. Tremolite is sometimes rimmed by cummingtonite, indicating retrogressive hydration at low pressures. The trace-element patterns of Ca amphibole exhibit light REE-enriched profiles without enrichment in Cs and Rb, unlike those of Ca amphibole in the serpentinite mud volcanoes. The seawater, instead of slab-derived fluids, was likely to be involved in the formation of amphibole in ultramafic rocks exposed in the South Mariana trench.
Mafic and ultramafic clasts were recovered from three Mariana serpentinite mud volcanoes during IODP Expedition 366 (Fryer et al., 2018). Ichiyama et al. (2021) revealed that the mafic and ultramafic clasts change their petrological and mineralogical characteristics depending on the distances of the mud volcanoes from the trench. The antigorite- and amphibole-bearing ultramafic clasts and blueschist-facies mafic clasts occur at mud volcanoes farther from the trench. The trace-element patterns of amphibole in the ultramafic clasts show enrichment in Cs and Rb, implying the hydration by aqueous fluids released from the subducted slab. Ultramafic clasts were also collected at the mud volcano nearest to the trench during the R/V Sonne cruise (SO292/2; Menapace et al., 2022). The highly serpentinized ultramafic clasts do not include antigorite but lizardite/chrysotile as serpentine minerals. In addition to residual mantle harzburgite, poikilitic harzburgite associated with crustal rocks, such as orthopyroxenite, gabbro, diorite, and boninite, was also recovered at the mud volcano. This fact might imply that the serpentinization front is reaching the forearc crust/mantle boundary and that boninitic stratigraphy from the crust to the upper mantle is constructed below the Mariana forearc region.
Amphibole and antigorite are rare in ultramafic rocks exposed in the landward slopes of the Izu–Mariana trench. However, ultramafic rocks from the South Mariana trench are rich in amphibole and talc (Ohara & Ishii, 1998; Yanagida et al., 2007). Ca amphibole is classified as pargasite, edenite, magnesiohonblende, and tremolite. Tremolite is sometimes rimmed by cummingtonite, indicating retrogressive hydration at low pressures. The trace-element patterns of Ca amphibole exhibit light REE-enriched profiles without enrichment in Cs and Rb, unlike those of Ca amphibole in the serpentinite mud volcanoes. The seawater, instead of slab-derived fluids, was likely to be involved in the formation of amphibole in ultramafic rocks exposed in the South Mariana trench.