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Magmatic Response to Subduction Initiation, Part II: Boninites and Related Rocks of the Izu-Bonin Arc From IOPD Expedition 352
Journal article   Open access

Magmatic Response to Subduction Initiation, Part II: Boninites and Related Rocks of the Izu-Bonin Arc From IOPD Expedition 352

John W Shervais, Mark K Reagan, Marguerite Godard, Julie Prytulak, Jeffrey G Ryan, Julian A Pearce, Renat R Almeev, Hongyan Li, Emily Haugen, Timothy Chapman, …
Geochemistry, geophysics, geosystems : G3, Vol.22(1), e2020GC009093
01/01/2021
DOI: 10.1029/2020GC009093
url
https://doi.org/10.1029/2020GC009093View
Published (Version of record) Open Access

Abstract

International Ocean Discovery Program Expedition 352 to the Izu-Bonin forearc cored over 800 m of basement comprising boninite and boninite-series lavas. This is the most extensive, well-constrained suite of boninite series lavas ever obtained from in situ oceanic crust. The boninites are characterized as high-silica boninite (HSB), low-silica boninite (LSB), or basaltic boninite based on their SiO2-MgO-TiO2 relations. The principal fractionation products of all three series are high-Mg andesites (HMA). Lavas recovered >250 meters below seafloor (mbsf) erupted at a forearc spreading axis and are dominated by LSB and HMA. Lavas recovered from <250 mbsf erupted off-axis and are dominated by HSB. The axial and off-axis lavas are characterized by distinct chemostratigraphic trends in their major, trace, and isotopic compositions. The off-axis lavas are chemically similar to boninite from the type locality at Chichijima, with concave-upward rare earth elements patterns. In contrast, the more abundant axial lavas have distinctly light rare earth element-depleted patterns and represent a new, previously unsampled precursor to the Chichijima-type boninite lavas. Petrogenetic modeling suggests that the axial lavas formed by fluxing of refractory mantle (likely the residue from forearc basalt extraction), with amphibolite-facies melt derived from subducting altered oceanic crust. The upper, off-axis lavas require an additional component of sediment-derived melt in addition. Both models are consistent with previously published isotopic data. Plain Language Summary Lavas erupted in the forearc of the Izu-Bonin-Mariana arc, and sampled by International Ocean Discovery Program Expedition 352, include unusual highly primitive compositions characterized by both high MgO and high SiO2 concentrations. These are termed "boninite," after their type locality in the nearby Bonin Islands. We divide these lavas into three suites distinguished by high-silica, low-silica, and very low (basaltic) silica contents. The basaltic boninite and low-silica boninite make up the deepest part of the volcanic section (>250 meters below sea floor[mbsf]) and are interpreted to have formed at an axial spreading center. Lavas less than 250 mbsf erupted later, in an off-axis setting, and consist of high-silica boninite. Both groups formed by melting of mantle depleted by a prior episode of melting, in response to the addition of water-rich melt derived from subducting oceanic crust. Formation of the off-axis high-silica boninite required the further input of melt derived from subducted sediment.
 Physical Sciences Geochemistry & Geophysics Science & Technology

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