Multistage metasomatism during experimental reaction of axinite-(Fe) with F-, Cl-, OH-, Si-bearing fluids at 600 °C, 200 MPa and 800 °C, 500 MPa
Abstract Many studies have shown that geological processes, such as metamorphism and metasomatism, consist of multiple superimposed events developing over various timescales, with each stage overprinting the previous one and thus complicating reconstruction of the complete history. Here, we report the use of a natural calcium aluminum borosilicate mineral axinite-(Fe) [(Ca 2 Fe 2+ Al 2 BSi 4 O 15 (OH)], which occurs in skarn deposits, as a starting material in a series of experiments representing various metasomatic processes. The fluids used include NaF+H 2 O, NaCl+H 2 O, KCl+H 2 O, CaCl 2 +H...
Many studies have shown that geological processes, such as metamorphism and metasomatism, consist of multiple superimposed events developing over various timescales, with each stage overprinting the previous one and thus complicating reconstruction of the complete history. Here, we report the use of a natural calcium aluminum borosilicate mineral axinite-(Fe) [(Ca 2 Fe 2+ Al 2 BSi 4 O 15 (OH)], which occurs in skarn deposits, as a starting material in a series of experiments representing various metasomatic processes. The fluids used include NaF+H 2 O, NaCl+H 2 O, KCl+H 2 O, CaCl 2 +H 2 O, 2 M NaOH, 2 M KOH, Ca(OH) 2 +H 2 O, and Na 2 Si 2 O 5 +H 2 O, which are representative of common crustal fluids. The experiments were conducted at 600 °C; 200 MPa and at 800 °C; 500 MPa. The dominant experimental product was grandite garnet and, to a minor extent, wollastonite, titanite, and fluorite. Grandite garnet compositions vary significantly with respect to the fluid composition and the P-T conditions used, showing a wide range of grossular to andradite solid solutions. Under both P-T conditions, andradite-grossular zoning develops in experiments with metal-hydroxide–bearing fluids, whereas andradite-rich grandite garnet crystallizes in experiments utilizing Na 2 Si 2 O 5 +H 2 O. Despite varying physicochemical conditions, no differences are observed in the REE-chondrite normalized patterns from the product grandite garnet in each of the experiments. Wollastonite contains bustamite and ferrobustamite components whose composition is strongly controlled by the coexisting grandite garnet. These experiments provide important insights into understanding how first order skarn mineral assemblages may be altered during subsequent fluid-rock interaction under a variety of P-T conditions, highlighting how variations in the grandite garnet composition respond to variations in fluid composition.
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