Trace element transport during dehydration processes in the subducted oceanic crust: 2. Origin of chemical and physical characteristics in arc magmatism

Abstract

The trace elements and Sr-Nd-Pb isotope compositions of arc magmas have been calculated from our estimates of: (1) the chemical composition of aqueous fluids emanating from the subducting basaltic oceanic crust, based on experimental data for element mobilization by aqueous fluids during the amphibolite/eclogite transition [1]; (2) the modal and chemical composition of the hydrous peridotite layer, which is formed by addition of the fluid derived from amphibolite dehydration to the base of the mantle wedge and the change in the hydrous mantle mode with increasing pressure and temperature; (3) the chemical composition of the fluid phase released by amphibole breakdown within the down-dragged hydrous peridotite layer, beneath the volcanic front; and (4) the mixing ratio of the this fluid with the pre-existing mantle wedge, as constrained by the H 2O content of arc magmas and appropriate degrees of partial melting. The resulting estimate for the chemical composition of primary arc basalt magma matches well with that of typical oceanic arc basalts. This supports the suggestion that aqueous fluids derived from the subducting crust play an essential role in generating arc magmatism and in producing their distinctive chemistry. Our calculations also indicate that more aqueous fluid enters the magma source region beneath the trench-side volcanic chain from amphibole breakdown than beneath sites of backarc-side volcanism due to phlogopite decomposition. This greater fluid flux beneath the volcanic front is consistent with the observed greater volume and/or number density of volcanoes at the volcanic front than behind the volcanic front — one of the most notable physical features of arc magmatism.