Tungsten isotopic fractionation at the Mariana arc and constraints on the redox conditions of subduction zone fluids

Abstract

Fluid release from subducting slabs during recycling of oceanic lithosphere plays an important role in concentrating select elements near the Earth’s surface. Yet the extent to which these fluids are oxidising or reducing (fO2), one of the key characteristics that controls element mobility, remains poorly understood. Additionally, it is unclear whether the fO2 signature of arc fluids is generated deep within the slab during subduction or if it reflects later, shallower subarc crustal processes. Previous experimental work has shown that the extent to which highly incompatible W partitions into aqueous fluids depends on the temperature and fO2 conditions of the fluids. In the subducting slab, W partitions into rutile with a different coordination number to that which it exhibits in fluids, a process which may generate stable W isotopic fractionation. Therefore, if the competition between partitioning into aqueous fluids versus rutile is controlled by the conditions within the subducting slab, the elemental and stable W isotopic systematics of arc lavas may provide a novel tool with which to investigate subduction zones. We present isotopic compositions of a well characterised suite of arc lavas from the central island province of the Mariana arc (W. Pacific), and rutile separates from exhumed fragments of subducted, depleted, mafic oceanic crust from the Raspas Complex, Ecuador. Our data show that lavas from Guguan that exhibit the strongest geochemical signature of subduction zone fluids (e.g., high Ba/Th, Mo/Ce, Pb/Ce), contain the highest abundances of W compared to similarly magmatically incompatible Th, in agreement with anticipated fluid mobile behaviour. These Guguan lavas have distinct, heavy stable W isotopic compositions (δ186/184W = +0.134 to +0.156 ‰) compared to MORB (δ186/184W = +0.078 to +0.099 ‰) and those from sediments-dominated islands like Uracas and Agrigan (δ186/184W = +0.080 to +0.111 ‰). The W isotopic compositions of sediments off-board the Mariana arc are more variable, with the volcaniclastic sediments having δ186/184W values closest to those of the sediment dominated arc lavas. We show that rutile from representative subducting mafic crust incorporates isotopically light W, consistent with a coordination change from tetrahedral in fluids and melts to octahedral in the structure of rutile. We suggest that isotopically heavy fluids, complementary to the residual rutile in the mafic crust, account for the high δ186/184W of the fluid-dominated Guguan lavas, a process previously invoked to control Mo systematics in the same samples. Combining our new elemental and isotopic W data with these existing Mo data, we model the influence of oxygen fugacity on fluid compositions in equilibrium with the mafic crust, and the fraction of such fluids required to be added to the mantle wedge to reproduce the Mo-W systematics of erupted arc lavas. Our models show that more fluid is required (F = 6 %) than can be generated internally in the mafic oceanic crust, and that this fluid must be oxidised, perhaps as high as FMQ +5. We suggest that these requirements document the interaction of subducted mafic crust with an external source of oxidising fluids sourced from dehydration of underlying serpentinites.