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

Authors: Liebscher, Axel; Thiele, Maik; Franz, Gerhard; Dörsam, Guido; Gottschalk, Matthias

Source: European Journal of Mineralogy, Volume 21, Number 2, March 2009 , pp. 275-292(18)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

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 °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^{mineral-fluid}_D(Sr-Ca) increases in the following sequence: calcite-fluid < anorthite-fluid ≤ margarite-fluid ≈ 1 ≤ strontianite-fluid < slawsonite-fluid. At low X_Sr (i.e., for common metamorphic bulk-rock compositions), K^{calcite-fluid}_D(Sr-Ca) = 0.09, K^{anorthite-fluid}_D(Sr-Ca) = 0.51, and K^{margarite-fluid}_D(Sr-Ca) = 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.

Keywords: MARGARITE; EXPERIMENTAL PETROLOGY; MINERAL-FLUID FRACTIONATION; XRD; STRONTIUM; GEOCHEMISTRY

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2009/0021-1917

Publication date: 2009-03-01

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