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Ca-Sr fractionation between zoisite, lawsonite, and aqueous fluids: An experimental study at 2.0 and 4.0 GPa/400 to 800 °C.
- Source :
- American Mineralogist; 2013, Vol. 98, p955-965, 11p
- Publication Year :
- 2013
-
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 K<subscript>D(Sr-Ca)</subscript><superscript>A1-A2</superscript> = 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<subscript>2</subscript> aqueous fluids are K<subscript>(Sr-Ca)</subscript><superscript>zo A1-af</superscript> = 3.38 to 41.08 for the A1 site and K<subscript>(Sr-Ca)</subscript><superscript>zo A1-af</superscript> = 0.45 to 6.51 for the A2 site. Assuming γ<subscript>Ca</subscript><superscript>af</superscript> = γ<subscript>Sr</subscript><superscript>af</superscript> and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca<superscript>2+(af)</superscript> + Sr<superscript>A1</superscript>(M<superscript>2+</superscript>)<superscript>A2</superscript> Al<subscript>3</subscript>[Si<subscript>3</subscript>O<subscript>11</subscript>(O/OH)] = Sr<superscript>2+(af)</superscript> + Ca<superscript>A1</superscript> (M<superscript>2+</superscript>)<superscript>A2</superscript>Al<subscript>3</subscript>[Si<subscript>3</subscript>O<subscript>11</subscript>(O/OH)] and Ca<superscript>2+(af)</superscript> + (M<superscript>2+</superscript>)<superscript>A1</superscript> Sr<superscript>A2</superscript> Al<subscript>3</subscript>[Si<subscript>3</subscript>O<subscript>11</subscript>(O/OH)] = Sr<superscript>2+(af)</superscript> + (M<superscript>2+</superscript>)<superscript>A1</superscript> Ca<superscript>A2</superscript> Al<subscript>3</subscript>[Si<subscript>3</subscript>O<subscript>11</subscript>(O/OH)] yields Δµ<superscript>0</superscript> = --29 kJ/mol and W<subscript>Sr-Ca</subscript><superscript>zo A1</superscript> = 5.5 kJ/mol for the A1 site and Δµ<superscript>0</superscript> = -1.1 kJ/mol and W<subscript>Sr-Ca</subscript><superscript>zo A2</superscript> = 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<subscript>1</subscript>/m form. Calculated lawsonite-aqueous fluid-exchange coefficients indicate overall preference of Ca over Sr in the solid and are K<subscript>D(Sr-Ca)</subscript><superscript>law Cm -cm-af</superscript> = 1.12 to 11.32 for orthorhombic and KD<subscript>D(Sr-Ca)</subscript><superscript>law Cm -cm-af</superscript> = 1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca<superscript>2+(af)</superscript> + SrAl<subscript>2</subscript>Si<subscript>2</subscript>O<subscript>7</subscript>(OH)<subscript>2</subscript>·H<subscript>2</subscript>O = Sr<superscript>2+(af)</superscript> + CaAl<subscript>2</subscript>Si<subscript>2</subscript>O<subscript>7</subscript>(OH)<subscript>2</subscript>·H<subscript>2</subscript>O assuming γ<subscript>Ca</subscript><superscript>af</superscript> = γ<subscript>Sr</subscript><superscript>af</superscript> and a symmetric mixing model yields similar values of Δµ<superscript>0</superscript> = --9 kJ/mol and W<subscript>Sr-Ca</subscript><superscript>law Cmcm-af</superscript> = 10 kJ/mol for orthorhombic and Δµ<superscript>0</superscript> = -10 kJ/mol and WW<subscript>Sr-Ca</subscript><superscript>law P21/m</superscript> = 11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl<subscript>2</subscript> aqueous fluids are D<subscript>Sr</subscript><superscript>zo-af</superscript> = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and D<subscript>Sr</subscript>l<superscript>aw Cmcm-af</superscript> = 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<subscript>Sr</subscript><superscript>zo</superscript> directly reflects x<subscript>Sr</subscript><superscript>af</superscript> 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. [ABSTRACT FROM AUTHOR]
- Subjects :
- AQUEOUS solutions
ZOISITE
LAWSONITE
CHEMICAL synthesis
THERMODYNAMICS research
Subjects
Details
- Language :
- English
- ISSN :
- 0003004X
- Volume :
- 98
- Database :
- Complementary Index
- Journal :
- American Mineralogist
- Publication Type :
- Academic Journal
- Accession number :
- 89132529
- Full Text :
- https://doi.org/10.2138/am.2013.4279