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

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)Cl (sub 2) aqueous fluid at 2.0 GPa/550, 600, and 700 degrees C and 4.0 GPa/800 degrees C for zoisite and 2.0 GPa/400 degrees C and 4.0 GPa/600 degrees 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 (sub D) (sub (Sr-Ca)) (super 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)Cl (sub 2) aqueous fluids are K (sub (Sr-Ca)) (super zo A1-af) = 3.38 to 41.08 for the A1 site and K (sub (Sr-Ca)) (super zo A2-af) = 0.45 to 6.51 for the A2 site. Assuming gamma (sub Ca) (super af) = gamma (sub Sr) (super af) and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca (super 2+(af)) + Sr (super A1) (M (super 2+) ) (super A2) Al (sub 3) [Si (sub 3) O (sub 11) (O/OH)]=Sr (super 2 + (af)) + Ca (super A1) (M (super 2+) ) (super A2) Al (sub 3) [Si (sub 3) O (sub 11) (O/OH)] and Ca (super 2+(af)) + (M (super 2+) ) (super A1) Sr (super A2) Al (sub 3) [Si (sub 3) O (sub 11) (O/OH)]=Sr (super 2+(af)) + (M (super 2+) ) (super A1) Ca (super A2) Al (sub 3) [Si (sub 3) O (sub 11) (O/OH)] yields Delta mu (super 0) =-29 kJ/mol and W (sub Sr-Ca) (super zo A1) =5.5 kJ/mol for the A1 site and Delta mu (super 0) =-1.1 kJ/mol and W (sub Sr-Ca) (super 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 P2 (sub 1) /m form. Calculated lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K (sub D) (sub (Sr-Ca)) (super law) (super Cmcm) (super -af) =1.12 to 11.32 for orthorhombic and K (sub D) (sub (Sr-Ca)) (super law) (super P) (super 21) (super m) (super -af) =1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca (super 2+(af)) + SrAl (sub 2) Si (sub 2) O (sub 7) (OH) (sub 2) .H (sub 2) O=Sr (super 2+(af)) + CaAl (sub 2) Si (sub 2) O (sub 7) (OH) (sub 2) .H (sub 2) O assuming gamma (sub Ca) (super af) =gamma (sub Sr) (super af) and a symmetric mixing model yields similar values of Delta mu (super 0) =-9 kJ/mol and W (sub Sr-Ca) (super law) (super Cmcm) =10 kJ/mol for orthorhombic and Delta mu (super 0) =-10 kJ/mol and W (sub Sr-Ca) (super law) (super P) (super 21) (super /m) =11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl (sub 2) aqueous fluids are D (sub Sr) (super zo-af) =2.8+ or -0.7 for zoisite at 2 GPa/600 degrees C and D (sub Sr) (super law) (super Cmcm) (super -af) =0.6+ or -0.2 for orthorhombic lawsonite at 4 GPa/600 degrees 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 (sub Sr) (super zo) directly reflects x (sub Sr) (super 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.