Abstract
International Ocean Discovery Program Expeditions 352 and 351 drilled into the Western Pacific Izu‐Bonin forearc and rear arc. The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites and the rear arc consists of FAB‐like rocks. These rocks are pervaded by calcite veins. Blocky vein microtextures enclosing host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Significant diffusion‐fed and crystallization pressure‐driven antitaxial veining is restricted to the rear arc. The lack of faults and presence of an Eocene sedimentary cover in the rear arc facilitated antitaxial veining. Rare earth element and isotopic (δ18O, δ13C, 87Sr/86Sr, and Δ47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB‐hosted low‐T (<30 °C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35–33 and ~22 Ma. These intersections are interpreted as precipitation ages, which concur with Pacific slab rollback. Boninite‐hosted low‐T (<30 °C) vein calcites precipitated from seawater that was modified by fluid‐rock interactions. Mixing calculations yield a mixture of >95% seawater and <5% basaltic 87Sr/86Sr. In the rear arc, low‐T rock alteration lowered the circulating seawater in δ18O and 87Sr/86Sr. Thus, vein calcites precipitated from modified seawater with up to 20–30% basaltic 87Sr/86Sr at temperatures up to 74 ± 12 °C. These results show how the local geology and vein growth dynamics affect microtextures and geochemical compositions of vein precipitates.
Highlights
The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites and the rear arc consists of FAB‐like rocks
The knowledge of the stratigraphy, structure, and geochemical composition of the oceanic crust is based on ophiolites and drill cores from the International Ocean Discovery Program (IODP) and its predecessors
Drill cores that were recovered during the preceding IODP Expedition 351 (Figure 1) unveiled that the Izu‐Bonin rear arc crust is composed of FAB‐ like rocks but lacks boninites (Arculus, Ishizuka, Bogus, Gurnis, et al, 2015; Arculus, Ishizuka, Bogus, & the Expedition 351 Scientists, 2015; Hickey‐Vargas et al, 2018; Ishizuka et al, 2018)
Summary
The knowledge of the stratigraphy, structure, and geochemical composition of the oceanic crust is based on ophiolites and drill cores from the International Ocean Discovery Program (IODP) and its predecessors. Since ophiolites mostly experienced a complex history of uplift and emplacement on continental crust, their structures and geochemical compositions may have been overprinted and/or altered. With the aim of investigating the tectonomagmatic processes of subduction initiation, IODP Expedition 352 drilled into the Izu‐Bonin forearc crust in the Western Pacific (Figure 1) and successfully cored 1.22 km of volcanic rocks (Reagan et al, 2015). Drill cores that were recovered during the preceding IODP Expedition 351 (Figure 1) unveiled that the Izu‐Bonin rear arc crust is composed of FAB‐ like rocks but lacks boninites (Arculus, Ishizuka, Bogus, Gurnis, et al, 2015; Arculus, Ishizuka, Bogus, & the Expedition 351 Scientists, 2015; Hickey‐Vargas et al, 2018; Ishizuka et al, 2018)
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