Arc foundations: a lower crustal record of subduction initiation in the Izu-Bonin forearc

Our understanding of the processes at work in the crust-mantle transition during subduction initiation has suffered from a general paucity of in situ lower crustal samples. Two distinct compositional groups have been previously identified within 35 lower crustal samples (10 harzburgites, 12 dunites, 2 orthopyroxenites, 7 ol-gabbros, 1 gabbro, 3 wehrlites) collected from the inner trench slope of the Bonin Ridge (BR) [1]. Clinopyroxene (CPX) is absent from the BR harzburgites and dunites which are characterized by spinel Cr#s ranging from 64-74 and 57-93, respectively. This suggests that the BR peridotites underwent large degrees of partial melting beyond CPX exhaustion (Cr#>60) prior-to or during subduction initiation. Spinels in the dunites can be split into a moderate (Cr# 57-70, Mg# 22-56) and a high Cr# (Cr# 77-93, Mg# 9-66) group and, in some cases, exhibit moderate Cr# cores with high Cr# rims. In the orthopyroxenites, euhedral spinels range in Cr# and Mg# from 66-83 and 18-35 and orthopyroxenes have similar compositions to those in the harzburgites. We suggest that the high Cr# dunites, harzburgites, and orthopyroxenites are the result of melt-rock reaction and impregnation of a refractory peridotite with a boninitic liquid. The spinels in the wehrlites and gabbros range in Cr#, Mg# and TiO₂ from 44-63, 22-68, and 0.05-2.25, and exhibit a marked increase in TiO₂ and Cr# and decrease in Al₂O₃ and Mg# with increasing modal CPX. CPX is the dominant interstitial phase (i.e., >10%) in the wehrlites and gabbros and exhibits markedly low Ti and Zr and high Sc abundances in addition to relatively low Zr/Y and Ti/V ratios which are consistent with derivation from a FAB-related liquid [2]. Considering the timeline of melt evolution preserved in the volcanic record of the BR, we suggest that the wehrlites and gabbros are the result of melt-impregnation of a primary FAB magma into previously melt-depleted peridotites shortly after subduction initiation. We therefore suggest that the BR preserves a lower crustal record of the transition from MORB-like melts created by decompression melting at subduction initiation to arc-type flux melting and boninitic volcanism thereafter.

[1] Loocke et al., 2013, AGU Fall Meeting V31A-2668.

[2] Arculus et al., 2015, Nature Geoscience 8:728-733.