PREDICTIVE MODELING OF MAGNESIUM ISOTOPIC VARIATION IN ARC LAVAS WITH HYDRATION STATE OF SUB-ARC MANTLE SOURCES

Slab-derived fluids play a key role in arc magmatism by lowering the peridotite solidus and imparting a slab-like geochemical signature to arc lavas. High Mg isotopic values (δ²⁶Mg) in some arc lavas are suggested to have formed from sub-arc mantle sources that are hydrated by fluids released from high-δ²⁶Mg subducting slab (altered oceanic crust and abyssal peridotites) [1][2]. However, a key piece of missing evidence is direct measurements of peridotite samples from the sub-arc mantle. To provide a quantitative constraint on Mg isotope systematics in subduction zones, we build on published thermal structures and lava δ²⁶Mg data for three contrasting arcs (Kamchatka, Lesser Antilles, and Philippines), and explore the relationship between the extent of mantle hydration and δ²⁶Mg variation in the arc lava output. Toward this goal, we analyze a suite of fresh spinel harzburgite xenoliths from the Avacha volcano in southern Kamchatka arc, which is an end-member of sediment-barren sub-arc system. These samples have mantle-like δ²⁶Mg values with limited variation from -0.32 to -0.21‰. These values are similar to those reported for Kamchatka arc lavas [2], suggesting too little Mg in the reacting fluids to alter the δ²⁶Mg value of the mantle source. On the other hand, modeling of fluid-peridotite interactions at different fluid/rock ratios and with different initial fluid δ²⁶Mg values and Mg concentrations provides an estimate of the amount of fluids needed to match the high-δ²⁶Mg data observed for Antilles and Philippines. These geochemical constraints have, in turn, to be consistent with the thermal structures of the associated arcs, which predict possible dehydration reactions (altered crust versus mantle) occurring in the down-going slab at sub-arc depths, and is thus critical to the Mg mass balance for the fluid-peridotite interaction. Our findings so far suggest that low-degree fluid-peridotite interactions do not cause significant Mg isotopic variation in sub-arc mantle as represented by the Kamchatka harzburgites. Together with modeling results, this study provides insight into the feasibility of using Mg isotopes as a tracer for the extent of hydration in sub-arc mantle sources. [1] Teng et al., PNAS, 2016 [2] Li et al., NSR, 2017.