EVIDENCE FOR OPEN-SYSTEM PROCESSES DURING EARLY SUBDUCTION ZONE MAGMATISM

A subset of boninite samples recovered during IODP Expedition 352 exhibit highly variable mineral chemistries that require complex crystallization histories. Early-crystallized olivine show embayed and reacted margins, and some samples show zoning, with Fo89-91 cores and thin Fo78-84 rims. Cr-rich spinels are found as inclusions in both olivine and low-Ca pyroxene. The chemical composition of enclosed spinels are similar to Cr-spinels from podiform chromite deposits in ophiolites. Large, euhedral pyroxene crystals have low-Ca enstatite/clinoenstatite cores (En79±5 Fs12±1.5 Wo5±2) with augite rims (En42±9 Fs14±4 Wo36±4.5). Smaller pyroxene grains exhibit oscillatory zoning with cores and zones ranging from enstatite/clinoensatite to augite. Late crystallizing augite rims and groundmass crystals have highly variable Al₂O₃ contents (1.9-13.7 wt%.) and a Ca-Tschermak component (3-13 mol%), indicative of disequilibrium conditions. Crystal zoning appears to be the result of a multi-stage magma mixing processes which introduced additional Ca, Fe, Ti, Na, and Al into parental boninitic melts. Disequilibrium crystallization of pyroxene rims likely reflects rapid cooling during eruption. Shipboard observations document magma mingling textures in both boninite drill cores. These observations coupled with past petrologic studies, point to the presence of a dynamic magmatic system during the later stages of subduction initiation, in which multiple ephemeral magma bodies that may erupt simultaneously to form mingled lavas, or may intersect to form mixed magmas. These results argue against the presence of a homogenizing magma chamber found in fast-spreading ridges and earlier fore-arc basalt magmatism, and suggest a magmatic setting that may be more akin to a slow-spreading ridge. The changing characteristics of Izu-Bonin forearc magmatism over a short timescale (<1.5 Myrs) likely reflect the dynamic tectonic conditions encountered in response to subduction initiation where initial rapid seafloor spreading transitions into normal plate convergence.