Synthetic Sr-Ca margarite, anorthite and slawsonite solid solutions and solid-fluid Sr-Ca fractionation

Calcium-strontium solid-solutions of margarite, anorthite, slawsonite, calcite and strontianite were synthesized from oxide-hydroxide-fluid mixtures by way of hydrothermal experiments at 400-500 degrees C and 390-500 MPa. The fractionation of Ca and Sr between the coexisting phases was investigated via electron microprobe and X-ray diffraction analyses of the solids and inductively coupled plasma-optical emission spectrometry of the fluids. A complete solid-solution series of margarite is indicated and the lattice parameters a, b, c, and V increase linearly with increasing Sr content. The lattice parameters of (Ca,Sr)-anorthite show a likewise linear increase with increasing Sr content. The formation of Sr-Ca slawsonite and Sr-Ca strontianite is restricted to high bulk Sr contents. The fractionation of Ca and Sr between minerals and fluid expressed as K (sub D(Sr-Ca)) (super mineral-fluid) increases in the following sequence: calcite-fluid<anorthite-fluid< or =margarite-fluid nearly equal 1< or =strontianite-fluid<slawsonite-fluid. At low X (sub Sr) (i.e., for common metamorphic bulk-rock compositions), K (sub D(Sr-Ca)) (super calcite-fluid) = 0.09, K (sub D(Sr-Ca)) (super anorthite-fluid) = 0.51, and K (sub D(Sr-Ca)) (super margarite-fluid) = 0.56; the Sr/Ca ratio of the fluid is therefore generally higher than that of the coexisting minerals. This suggests that during prograde metamorphism and probably continuous dehydration, the Sr/Ca ratio of the rock decreases continuously with time. But, owing to the roughly equal Sr-Ca mineral-fluid fractionation for margarite and anorthite, which was also determined for zoisite/clinozoisite and lawsonite, this decrease will result in only moderate changes of whole-rock and fluid Sr-composition as long as these phases are the main Sr-bearing minerals.