AL-FE3-OH SUBSTITUTIONS IN TITANITE AND CONSTRAINTS ON THEIR P-T DEPENDENCE( AND F)

Aluminous titanites occur in low-grade metasediments from the Salton S ea geothermal system, U.S.A., and in granitoids and associated fluorit e-bearing skarns from southwest Japan. Average (Al + Fe3+) content of the metamorphic titanites in the Salton Sea geothermal system decrease s systematically with increasing temperature and depth from 0.21-0.37 per formula unit (pfu.) in the chlorite-calcite zone (T = 180-320-degr ees-C), through 0.12-0.23 pfu. in the biotite zone (320-340-degrees-C) , to 0.12-0.13 pfu. in the clinopyroxene zone (340-360-degrees-C). Flu orine 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 granitoid s are divided into magmatic titanites occurring as discrete grains in the matrix or as inclusions in plagioclase, and secondary titanites oc curring 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 + Fe3+) content and F contents of titanites in fluorite-bearing skarns are 0.21 +/- 0.05 an d 0.16 +/- 0.06 pfu., respectively. The titanites in the granitoids an d skarns have high X(F) of 0.77 +/- 0.17 and 0.75 +/- 0.16, respective ly. The low-grade metamorphic and secondary titanites have variable Al content and X(F); their compositions are controlled by both substitut ions (Al, Fe3+)FTi-1O-1 and (Al, Fe3+)(OH)Ti-1O-1. The Ca(Al, Fe3+)(OH )SiO4 component of titanites was preferentially decomposed with increa sing temperature and the (Al, Fe3+)FTi-O1)-1 substitution mainly contr ols Al content of titanite under high-temperature conditions. On the o ther hand, the Ca(A], Fe3+)FSiO4 component increases with increasing p ressure at fixed temperature, and stabilizes titanite in high-temperat ure and high-pressure conditions such as eclogite-facies metamorphism.