The effect of fluorine and chlorine on trace element partitioning between apatite and sediment melt at subduction zone conditions

摘要

AbstractThe effect of F and Cl on trace element recycling during subduction-related sediment melting has been investigated by performing piston-cylinder experiments with a hydrous experimental pelite starting material (EPSM) with variable Cl (~0, 500, 1000, 2000, or 3000ppm) and F (~0, 700, or 1500ppm) concentrations, at 2.5GPa, 800°C. The variations of trace element concentrations in melt are systematically correlated with the variation of F (0.07–0.39wt%) and Cl (0.07–0.39wt%) contents. Trace elements Zn, V and Pb, and major elements Fe, Mg and Ca, show positive correlations with each other, and also with the Cl content in melt. The concentrations of light and medium rare earth elements (LMREE) increase with the Cl content in melt, whereas both F and Cl cause a decrease in the concentrations of high field strength elements (HFSE, such as Nb, Ta, Zr and Hf). Trace element (REE, Y, Sr, Th, U) concentrations in apatite are found to increase with the mole fraction of chlorapatite (ClAp). The preference for ClAp is stronger for cations with higher charge (e.g., Th4+, U4+3+) and larger ionic radii (e.g., LREE>HREE).Trace element partition coefficients between apatite and melt show up to 4 times variation between experiments, e.g.,DLaAp-melt=77–281;DSmAp-melt=176–519;DSrAp-melt=4–12 andD