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

The 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 3000 ppm) and F (~ 0, 700, or 1500 ppm) concentrations, at 2.5 GPa, 800 °C. The variations of trace element concentrations in melt are systematically correlated with the variation of F (0.07–0.39 wt%) and Cl (0.07–0.39 wt%) 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., Th^{4 +}, U^{4 +} > REE^{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., D_La ^Ap-melt = 77–281; D_Sm ^Ap-melt = 176–519; D_Sr ^Ap-melt = 4–12 and D_Th,U ^Ap-melt = 4–19. The REE partition coefficients between apatite and melt (D_REE ^Ap-melt) display a concave pattern with the peak at Sm/Nd and a negative Eu anomaly, and are significantly higher than previously reported values for partitioning experiments conducted at lower pressures and higher temperatures. The high values of D_LREE ^Ap-melt demonstrate the importance of apatite in terms of LREE partitioning during sediment melting, while allanite/monazite still dominates the partitioning of Th. In the absence of allanite/monazite, apatite-buffered melt is characterized by a significant enrichment of Th relative to La. Because of the contrasting behavior of LREE and HFSE in melt with the addition of Cl and F, the fractionation of these elements in slab-derived sediment melts will be enhanced by the presence of halogens.