Forearc basalt and boninite lineage: origin and evolution of magmas in the course of subduction initiation, Izu-Bonin Mariana arc

The IODP Expedition 352 conducted in 2014 along the Izu-Bonin-Mariana (IBM) arc is a unique opportunity to understand the geological processes during the initiation of subduction and the subsequent island arc evolution. During the expedition, both forearc basalts (FAB) and boninites have been collected and the transition from one to the other rock type is interpreted to result from a change of the mantle melting conditions from decompression-driven melting (FAB) to hydrous flux melting (boninite). This change of the melting regime during subduction initiation is also expected to be accompanied by changing redox conditions. The evolution of magma generation and magma differentiation conditions in the course of mantle reorganization during subduction initiation is the central issue of the current research project. A detailed petrological study of selected samples from 4 drilling cores will be used to clarify following fundamental questions: (1) what are the conditions of FAB and boninite primary magma generation, (2) what are the conditions of magma storage and differentiation at the transition between FAB and boninite volcanic activity (depth, fO2, regime of volatiles) and (3) how does the change of the mantle melting conditions (particularly fO2, volatiles and degree of melting) controls the budget and behavior of redox-sensitive siderophile (e.g. Au, Ag) and chalcophile (e.g. Cu, Zn) elements. These issues will be worked out using complementary analytical techniques, including microprobe (major element analyses of glasses and minerals), FTIR (volatile concentration in glasses) and Laser ablation techniques (trace element analyses in glasses), thermodynamic modeling of liquid lines of descent and high pressure experimental studies to clarify the formation and differentiation of Izu-Bonin-Mariana FAB and boninites. The results will also be of importance to establish the geochemical key characteristics expected in supra-subduction zone ophiolites, interpreted to be geodynamically related to a stage reflecting the initiation of subduction.