Trace element partitioning between apatite and carbonatite melt at 800 °C and 200 MPa

Apatite is a pivotal mineral in carbonate rocks, because it can incorporate all of the major magmatic volatile species (H, P, F, S, Cl), as well as REE and HFSE into its structure. To quantify the effects of fractional crystallization of apatite crystals on the (H, P, F, S, Cl) volatiles and metal budget in the residue melt, the partition coefficients of a broad range of elements (F, Cl, S, REE, HFSE, Co, Ni, Sr, Mo, Ba, W, Pb, Th and U) between apatite and carbonatite melts were determined at 800 °C and 200 MPa using internally heated pressure vessels. The experimental results show that the partition coefficients of Sr, Y and REE (D) are in the range 2-10, and the partition coefficients of Sc, Mn, Fe, Co, Ni, Mo, W, U and HFSE are <<1. The effects of volatiles and oxygen fugacity on the partition coefficients are insignificant. This study defines the apatite-melt partition coefficients for the halogens: DF= 0.68-1.76; DCl=0.10-0.19; and DS= 0.016-0.05. The P2O5 solubility in carbonatite melt decreases from 6.8 to 1.4 wt% in ‘dry’ carbonate to volatile (H2O, F, Cl and S)-bearing carbonate. The results suggest that apatite is preferred to be saturated in hydrous carbonate, and thus cumulated apatite sequester more REE from the residue melts. Finally, we will show that the experimentally determined partition coefficients make apatite a potential indicator for the volatile and trace element abundances in carbonatite magmas.