Al-Fe3+ and F-OH substitutions in titanite and constraints on their P-T dependence

Abstract

Aluminous titanites occur in low-grade metasediments from the Salton Sea geothermal system, U.S.A., and in granitoids and associated fluorite-bearing skarns from southwest Japan. Average (Al+Fe 3+ ) content of the metamorphic titanites in the Salton Sea geothermal system decreases systematically with increasing temperature and depth from 0.21-0.37 per formula unit (pfu,) in the chlorite-calcite zone (T = 180-320°C), through 0,12-0.23 pk. in the biotite zone (320-340°C), to 0,12-0.13 pfu. in the clinopyroxene zone (340-360°C). Fluorine content varies from 0.0 to 0.20 pfu., and average X F [= F/(F+Cl+OH)] is less than 0.5 in all samples. Titanites in the granitoids are divided into magmatic titanites occurring as discrete grains in the matrix or as inclusions in plagioclase, and secondary titanites occurring as tiny granules in chlorite that alters biotite. The (Al+Fe 3+ ) content and F contents are, respectively, 0,18 ± 0.03 and 0.14 ± 0.04 pfu, for the magmatic titanites and 0,45 ± 0.05 and 0,35 ± 0.08 pfu, for the secondary titanites. The (Al+Fe 3+ ) content and F contents of titanites in fluorite-bearing skarns are 0.21 ± 0.05 and 0,16 ± 0.06 pfu., respectively. The titanites in the granitoids and skarns have high X F of 0.77 ± 0.17 and 0.75 ± 0.16, respectively. The low-grade metamorphic and secondary titanites have variable Al content and X F ; their compositions are controlled by both substitutions (Al, Fe 3+ )FTi −1 O −1 and (Al, Fe 3+ )(OH)Ti −1 O −1 . The Ca(Al, Fe 3+ )(OH)SiO 4 component of titanites was preferentially decomposed with increasing temperature and the (Al, Fe 3+ )FTi -1 O −1 substitution mainly controls Al content of titanite under high-temperature conditions. On the other hand, the Ca(Al, Fe 3+ )FSiO 4 component increases with increasing pressure at fixed temperature, and stabilizes titanite in high-temperature and high-pressure conditions such as eclogite-facies metamorphism