Ca-Sr fractionation between zoisite, lawsonite, and aqueous fluids: An experimental study at 2.0 and 4.0 GPa/400 to 800 C

Abstract

The Ca-Sr fractionation between zoisite and, respectively, lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients KD(Sr-Ca)A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/ aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl2 aqueous fluids are Kzo A1 -af(Sr-Ca)= 3.38 to 41.08 for the A1 site and Kzo A2 -af(Sr-Ca)= 0.45 to 6.51 for the A2 site. Assuming γCaaf = γSraf and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca2+(af) + SrA1(M2+)A2Al3[Si3O11(O/OH)] = Sr2+(af) + CaA1(M2+)A2Al3[Si3O11(O/OH)] and Ca2+(af) + (M2+)A1SrA2Al3[Si3O11(O/OH)] = Sr2+(af) + (M2+)A1CaA2Al3[Si3O11(O/OH)] yields Δμ0 = -29 kJ/mol and Wzo A1Sr-Ca = 5.5 kJ/mol for the A1 site and Δμ0 = -1.1 kJ/mol and Wza A2Sr-Ca = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P21/m form. Calculated lawsonite-aqueous fluidexchange coefficients indicate overall preference of Ca over Sr in the solid and are Klaw Cmc-afD(Sr-Ca) = 1.12 to 11.32 for orthorhombic and Klaw P21m-afD(Sr-Ca) = 1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca2+(af) + SrAl2Si2O7(OH)2·H2O = Sr2+(af) + CaAl2Si2O7(OH)2·H2O assuming γafCa = γafSr and a symmetric mixing model yields similar values of Δμ0 = -9 kJ/mol and Wlaw CmcmSr-Ca= 10 kJ/mol for orthorhombic and Δμ0 = -10 kJ/mol and Wlaw P21/mSr-Ca = 11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl2 aqueous fluids are Dzo-afSr = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and Dlaw Cmcm-af = 0.6 ± 0.2 for orthorhombic lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and lawsonite-bearing rocks. For low Sr-concentrations, xzoSr directly reflects xafSr and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.