CONSTRAINTS ON MAGMA SERIES AND MELTING CONDITIONS FOR INTRUSIVE ROCKS FROM THE MARIANA FOREARC

Minerals from depleted peridotite, olivine gabbro, gabbro, and diabase samples collected during Dive 6K-1229 on the Izu-Bonin-Mariana (IBM) forearc were analyzed for major element compositions using an electron microprobe to constrain associated melt compositions and conditions of crystallization. These samples represent an ophiolitic sequence that was generated shortly after subduction initiation at the southern end of the Mariana forearc. Most of the samples have mineral compositions consistent with crystallization from forearc basalts (FAB). Olivine gabbros crystallized under nominally anhydrous conditions based on the Ca in olivine geohygrometer of Gavrilenko et al. (2016). Pyroxene based calculated temperatures for gabbro crystallization were in the range of 1060°C to 1212°C. Olivine gabbro temperatures were on the high end of this range. One analyzed gabbro sample had grains of orthopyroxene and higher-Mg clinopyroxenes suggesting a possible transition from FAB to boninite late in the genesis of the forearc crust. The depleted peridotite, a clinopyroxene harzburgite, displayed almost complete alteration of orthopyroxene with minor amounts of clinopyroxene in the interstices between the olivine and orthopyroxene grains. The high Mg numbers for minerals in this peridotite (olivine=0.92; cpx = 0.95) indicate it was the residue after high degrees of partial melting to generate FAB. Thus, this would represent an appropriate source for boninite. The results presented here characterize magma genesis during near trench spreading and the early stages of subduction where new, hot asthenosphere filled the void between the subducting plate and the overriding upper plate. Most magmas were generated in the infancy stage of subduction during stage 1 and perhaps stage 2 based on an idealized lifecycle of a suprasubduction zone (Shervais, 2001). Analyses of the samples show crystallization conditions for the plutonic rocks in this ophiolitic sequence similar to those for FAB lavas sampled by IODP Expedition 352 (Whattam et al., 2020), with potential late infiltration by boninite.