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

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.

Magma Source Evolution Following Subduction Initiation: Evidence From the Element Concentrations, Stable Isotope Ratios, and Water Contents of Volcanic Glasses From the Bonin Forearc (IODP Expedition 352)

“…The resulting melt is rhyolitic, leaving a residue that has roughly equal proportions of hornblende and clinopyroxene, with lesser plagioclase and minor Fe‐Ti oxides (Figure 14; Table ). This residual assemblage is consistent with trace element compositions discussed above for Expedition 352 LSB (see also Shervais et al., 2020).…”

“…The implied temperatures in slab crust at the slab‐mantle interface are therefore high relative to those in normal subduction zones (see Maunder et al., 2020). The separate trace element (Shervais et al., 2020) and isotopic trends (H. Y. Li et al., 2019) for LSB and HSB illustrate that the compositions of slab melts involved in their genesis differed for these two magma suites. Isotopic fingerprinting provided evidence that the slab melts involved in LSB genesis were essentially restricted to subducted Pacific basaltic crust, whereas sediments were additionally involved in HSB genesis (H. Y. Li et al., 2019).…”

“…There is abundant textural evidence for magma mixing and mingling in boninite cores from Expedition 352 (M. K. Reagan et al., 2015), so these trends may be the result of mixing between simultaneously generated boninite and FAB. However, the trends toward FAB‐like compositions also may reflect variable source depletion (H. Y. Li et al., 2019) or FAB retention in pores or veins by the mantle sources for boninites (Shervais et al., 2020).…”

“…(2020) and boninite whole‐rock compositions from Shervais et al. (2020). Approximate fields for primitive melt compositions based on equilibrium with Fo 91–92 compositions are illustrated in solid (LSB) and dashed (HSB) outlines.…”

“…Correspondingly, whole rocks below 251 mbsf are LSB or LSB-related HMA and glasses have K/Ti < 1.8 ( Figure 7). Similarly, for Hole U1442A, whole rocks above 250 mbsf are of the HSB lineage and glasses have K/Ti > 1.8, whereas whole rocks below 250 mbsf have a LSB lineage (Shervais et al, 2020) and most have K/Ti < 1.8. However, there are exceptions for Hole U1442A.…”