• journal_title：American Mineralogist
• Contributor：Axel Liebscher ; Guido Dörsam ; Gerhard Franz ; Bernd Wunder ; Matthias Gottschalk
• Publisher：Mineralogical Society of America
• Date：2013-05-01
• Format：text/html
• Language：en
• Identifier：10.2138/am.2013.4279
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：98
• issue：5-6
• firstpage：955
• section：Articles

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 K_D(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 K_(Sr-Ca)^{zo A1-af} = 3.38 to 41.08 for the A1 site and K_(Sr-Ca)^{zo A2-af} = 0.45 to 6.51 for the A2 site. Assuming γ_Ca^af = γ_Sr^af and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca^2+(af) + Sr^A1(M²⁺)^A2Al₃[Si₃O₁₁(O/OH)] = Sr^2+(af) + Ca^A1(M²⁺)^A2Al₃[Si₃O₁₁(O/OH)] and Ca^2+(af) + (M²⁺)^A1Sr^A2Al₃[Si₃O₁₁(O/OH)] = Sr^2+(af) + (M²⁺)^A1Ca^A2Al₃[Si₃O₁₁(O/OH)] yields Δμ⁰ = −29 kJ/mol and W_Sr-Ca^{zo A1} = 5.5 kJ/mol for the A1 site and Δμ⁰ = −1.1 kJ/mol and W_Sr-Ca^{zo A2} = 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 fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K_D(Sr-Ca)^lawCmcm-af = 1.12 to 11.32 for orthorhombic and K_D(Sr-Ca)^lawP21m-af = 1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca^2+(af) + SrAl₂Si₂O₇(OH)₂·H₂O = Sr^2+(af) + CaAl₂Si₂O₇(OH)₂·H₂O assuming γ_Ca^af = γ_Sr^af and a symmetric mixing model yields similar values of Δμ⁰ = −9 kJ/mol and W_Sr-Ca^lawCmcm = 10 kJ/mol for orthorhombic and Δμ⁰ = −10 kJ/mol and W_Sr-Ca^lawP21/m = 11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl₂ aqueous fluids are D_Sr^zo-af = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D_Sr^lawCmcm-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, x_Sr^zo directly reflects x_Sr^af 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.