Shervais, John and Reagan, Mark and Haugen, Emily and Almeev, Renat and Pearce, Julian and Prytulak, Julie and Ryan, Jeffrey and Whattam, Scott and Godard, Marguerite and Chapman, Timothy and Li, Hongyan and Kurz, Walter and Nelson, Wendy and Heaton, Daniel and Kirchenbaur, Maria and Shimizu, Kenji and Sakuyama, Tetsuya and Li, Yibing and Vetter, Scott (2019) 'Magmatic response to subduction initiation, Part I : Forearc basalts of the Izu-Bonin Arc from IODP Expedition 352.', Geochemistry, geophysics, geosystems., 20 (1). pp. 314-338.
The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.
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|Publisher Web site:||https://doi.org/10.1029/2018GC007731|
|Publisher statement:||© 2018. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.|
|Date accepted:||21 November 2018|
|Date deposited:||18 December 2018|
|Date of first online publication:||21 November 2018|
|Date first made open access:||No date available|
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