The Fluid Mobilities of K and Zr in Subduction Zones: Thermodynamic Constraints

Abstract

A subduction zone plays a critical role in forging continental crust via formation of arc magmas, which are characteristically enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs). This trace element pattern results from the different mobilities of LILEs and HFSEs during slab-to-wedge mass transfer, but the mechanisms of trace element transfer from subducting crusts are not fully understood. In this study, thermodynamic simulations are carried out to evaluate the mobilities of K and Zr, as representative cases of LILE and HFSE, respectively, in slab fluids. The fluids buffered by basaltic eclogite can dissolve > 0.1 molal of K at sub-arc depths (~3 to 5.5 GPa). However, only minor amounts of K can be liberated by direct devolatilization of altered oceanic basalt, because sub-arc dehydration mainly takes place at temperatures < 600 °C (talc-out), wherein the fluid solubility of K is very limited (<0.01 molal). Therefore, serpentinite-derived fluids are required to flush K from the eclogite. The solubility of K can be enhanced by the addition of NaCl to the fluid, because fluid Na+ can unlock phengite-bonded K via a complex ion exchange. Finally, it is further confirmed that Zr and other HFSEs are immobile in slab fluids.