Subduction initiation and igneous petrogenesis: characterizing melt generation at a new convergent boundary through the geochemical analysis of volcanic glass

The impact of subduction initiation on regional to global tectonics and the compositions of major Earth reservoirs are topics of vigorous ongoing research. Here, pristine glasses extracted from ~51.9 Myr old basalts and younger boninites that erupted in the Izu-Bonin Mariana forearc immediately after subduction initiation were analyzed by microbeam techniques, with goals of characterizing the mantle sources and the conditions under which melting occurred to produce nascent arc crust. Forearc basalts (FAB) have relatively differentiated major element compositions. Thus, to determine melting conditions and source compositions, primitive melt compositions were restored through an inferred crystallization history based on melt liquidus associations. Subsequent modeling indicates that they were generated at high temperatures and low pressures relative to a mid ocean ridge basalt (MORB). Incompatible trace element compositions of FAB show that they are similar to MORB in that they were generated largely by decompression melting. Differences in several trace element ratios between MORB and FAB indicate that the mantle sources for FAB were unusually depleted. Differences between FAB sub-units indicate a range of petrogenetic histories. Upper FAB sub-units are weakly enriched in fluid-mobile elements which may indicate that fluids from the subducting Pacific plate contributed to melting. Boninites are separated into high and low silica types based on preexisting whole rock analyses. Glasses separated from these boninites are highly differentiated and thus classify as high-Mg andesites rather than boninites on MgO-SiO2-TiO2 diagrams. These glasses are also enriched in a suite of fluid mobile elements indicating that they are products of flux melting of the mantle involving fluids and melts from the subducting plate. Olivine calcium concentrations are consistent with hydrous parental boninite melts. Aluminum partitioning between olivine and hosted spinel inclusions constrains the temperatures of initial crystallization between 1170 and 1330 degrees Celsius. The change from decompression melting which generated forearc basalts to flux melting which generated high silica boninites illustrates an evolution of the subduction system over the course of the initiation process. Based on trace element ratio plots, mixing relationships between upper forearc basalts and highly enriched fluids probably released by the nascent subducting slab suggest that both decompression melting and fluid fluxing operated to produce low silica boninite during subduction initiation. This melt composition progressively becomes dominated by fluid flux melts with additional components derived from the slab to make high silica boninite. These late volcanic rocks record melting of a highly depleted mantle source. The fact that heavy rare earth element concentrations become increasingly depleted from FAB to low silica boninite to high silica boninite indicates that the mantle source changed in composition over time. The progressive decrease suggests that the initial mantle source for FAB remained the mantle source for the duration of subduction initiation related magmatism.