Abstract
The Izu-Bonin-Mariana (IBM) island arc formed following initiation of subduction of the Pacific plate beneath the Philippine Sea plate at about 52 Ma. Site U1438 of IODP Expedition 351 was drilled to sample the oceanic basement on which the IBM arc was constructed, to better understand magmatism prior to and during the subduction initiation event. Site U1438 igneous basement Unit 1 (150 m) was drilled beneath 1460 m of primarily volcaniclastic sediments and sedimentary rock. Basement basalts are microcrystalline to fine-grained flows and form several distinct subunits (1a-1f), all relatively mafic (MgO = 6.5–13.8%; Mg# = 52–83), with Cr = 71–506 ppm and Ni = 62–342 ppm. All subunits are depleted in non-fluid mobile incompatible trace elements. Ratios such as Sm/Nd (0.35–0.44), Lu/Hf (0.19–0.37), and Zr/Nb (55–106) reach the highest values found in MORB, while La/Yb (0.31–0.92), La/Sm (0.43–0.91) and Nb/La (0.39–0.59) reach the lowest values. Abundances of fluid-mobile incompatible elements, K, Rb, Cs and U, vary with rock physical properties, indicating control by post-eruptive seawater alteration, but lowest abundances are typical of fresh, highly depleted MORBs. Mantle sources for the different subunits define a trend of progressive incompatible element depletion. Inferred pressures of magma segregation are 0.6–2.1 GPa with temperatures of 1280–1470 °C.New 40Ar/39Ar dates for Site U1438 basalts averaging 48.7 Ma (Ishizuka et al., 2018) are younger that the inferred age of IBM subduction initiation based on the oldest ages (52 Ma) of IBM forearc basalts (FAB) from the eastern margin of the Philippine Sea plate. FAB are hypothesized to be the first magma type erupted as the Pacific plate subsided, followed by boninites, and ultimately typical arc magmas over a period of about 10 Ma. Site U1438 basalts and IBM FABs are similar, but Site U1438 basalts have lower V contents, higher Ti/V and little geochemical evidence for involvement of slab-derived fluids. We hypothesize that the asthenospheric upwelling and extension expected during subduction initiation occurred over a broad expanse of the upper plate, even as hydrous fluids were introduced near the plate edge to produce FABs and boninites. Site U1438 basalts formed by decompression melting during the first 3 Ma of subduction initiation, and were stranded behind the early IBM arc as mantle conditions shifted to flux melting beneath a well-defined volcanic front.
Highlights
The process of subduction is an important influence on the evolution of the Earth’s surface and interior features, as well as present day human concerns such as geologic hazards and mineral resources (e.g., McGuire et al, 2017)
We considered three hypotheses for the origin of the variation in fluid mobile element enrichments: (1) it was a feature of primary magmas and reflected different amounts of slab-derived fluids as a trigger for melting for each subunit; (2) it was a feature of the individual magma batches forming the subunits, introduced by assimilation of hydrothermally altered oceanic crust in the magma feeder system; (3) it was produced by seawater-rock interaction with cooling basaltic flows, with rock physical properties and time controlling the extent of alteration
Basaltic basement drilled at Site U1438 in the Amami Sankaku basin formed concurrently with initiation of subduction along the Izu-Bonin-Mariana arc, and is not preexisting oceanic basement on which the IBM arc was constructed
Summary
The process of subduction is an important influence on the evolution of the Earth’s surface and interior features, as well as present day human concerns such as geologic hazards and mineral resources (e.g., McGuire et al, 2017). In 2014, the International Ocean Discovery Program (IODP) sponsored two expeditions, 351 and 352, with the specific goal of understanding the causes and effects of subduction initiation in the IzuBonin-Mariana (IBM) island arc system (Fig. 1, Arculus et al, 2015a, 2015b, 2015c; Pearce et al, 2015; Reagan et al, 2015, 2017). Prior research on this system concluded that subduction began at about 52 Ma, nearly simultaneously along its 1000 km length, and was marked by eruption of a specific magma type (Reagan et al, 2010; Ishizuka et al, 2011a). The age of the igneous seafloor of the Amami-Sankaku basin was unknown at the time of drilling, but was thought to be as old as Cretaceous based on ages of the adjacent Daito Ridges and Amami Plateau (Fig. 1) (Shiki et al, 1977, Hickey-Vargas, 2005; Ishizuka et al, 2011a)
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