Magmatic Response to Subduction Initiation: Part 1. Fore-arc Basalts of the Izu-Bonin Arc From IODP Expedition 352.

John W Shervais,Wendy R Nelson,Emily Haugen,Julian A Pearce,Jeffrey G Ryan,Julie Prytulak,Daniel Heaton,Hongyan Li,Maria Kirchenbaur,Tetsuya Sakuyama,Timothy Chapman,Walter Kurz,Renat R Almeev,Mark Reagan,Kenji Shimizu,Yibing Li,Marguerite Godard,Scott K Vetter,Scott A Whattam

doi:10.1029/2018gc007731

Abstract

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.

Highlights

Subduction initiation is a singular and remarkable event in the life of a convergent margin and one that is necessary for plate tectonics to exist (e.g., Wakabayashi & Shervais, 2012)
We present here new data on the petrogenesis of fore-arc basalt (FAB) sampled by IODP (International Ocean Discovery Program) Expedition 352 (Reagan et al, 2015a, 2017)
Because Depleted FAB (D-FAB) and, to a lesser extent, some Primitive FAB (P-FAB) are more depleted in light rare earth elements (LREE) than mid-ocean ridge basalt (MORB), we explored the extraction of small melt fractions (1%–3%) in the garnet field prior to spinel field melting

Summary

Introduction

Subduction initiation is a singular and remarkable event in the life of a convergent margin and one that is necessary for plate tectonics to exist (e.g., Wakabayashi & Shervais, 2012). Initiating subduction in the absence of slab pull is challenging and requires special geodynamic circumstances, for example, nucleation on preexisting zones of weakness, gravitation failure of the slab, and far-field geodynamic forces (e.g., Crameri et al, 2012; Gurnis et al, 2004; Hall, 2002; Hall et al, 2003; Leng et al, 2012; Stern, 2004; Stern & Gerya, 2017). Stern and Bloomer (1992) proposed that subduction may initiate by gravitation failure spontaneously across fracture zones when one side is much older (denser) than the other, but modeling suggests that even in this situation, significant convergence is required before failure occurs (Gurnis et al, 2004; Hall et al, 2003; Leng et al, 2012; Schmeling et al, 2008). The IBM boninites are Eocene in age, older than other volcanic rocks of the IBM system (Ishizuka et al, 2006; Meijer et al, 1983)

Methods

Results

Discussion

Conclusion