The distribution of rare earth elements between monzogranitic melt and theaqueous volatile phase in experimental investigations at 800 degrees C and200 MPa

The partitioning of the rare earth elements between a peraluminous monzogra nitic melt and a chloride-bearing, sulfur- and carbon dioxide-free, aqueous volatile phase was examined experimentally as a function of chloride and m ajor element concentrations at 800 degreesC and 200 MPa. The light rare ear th elements (e.g. La, Ce) partition into the aqueous volatile phase to a gr eater extent than the heavy rare earth elements (e.g. Yb, Lu). Distribution of the rare earth elements and the major elements H, Na, K, Ca, and Al bet ween the melt phase (mp) and aqueous volatile phase tag) is a function of t he chlorine concentration in the system, and our data are consistent with t he rare earth and major elements occurring as chloride complexes in the aqu eous Volatile phase. Apparent equilibrium constants for experiments, at 800 degreesC and 200 MPa, K-REE1Na('aq/mp), expressed as the ratio as the rati o of the concentration of a given rare earth element in the aqueous volatil e phase to the concentration of the same element in the melt phase, divided by the cubed ratio of sodium in the aqueous volatile phase to the concentr ation of sodium in the melt phase, decrease systematically with increasing atomic number from K-La,Na('aq/mp) = 0.41(+/-0.03) to K-Lu,Na('aq/mp) = 0.1 1(+/-0.01), except for Eu. These experimentally derived apparent equilibriu m constants for the rare earth elements can be used in a numerical simulati on of magmatic volatile exsolution. The simulation gave results consistent with the elemental distribution in the potassic alteration zone of a deep p orphyry copper deposit, but higher concentrations of heavy rare earth eleme nts are released into the magmatic aqueous solution than are captured in th e secondary mineralization.