Experimental study of titanite-fluorite equilibria in the A-type Mount Scott Granite: Implications for assessing F contents of felsic magma

Abstract

Titanite and fluorite stability in melt were experimentally evaluated at 850 °C, 200 MPa, f(O 2 ) ≈ NNO (nickel-nickel oxide oxygen buffer) as functions of total F and H 2 O content. Experiments employed the metaluminous Mount Scott Granite of the Wichita igneous province, Oklahoma. Over a large range of added H 2 O (∼1–7 wt%), melts containing 1 wt% F precipitated fluorite without titanite. In addition, at high F (≥ 1.2 wt%) plagioclase and hornblende reacted to form biotite. Thus, an increase in F during crystallization may explain the observed higher modal abundance of plagioclase and hornblende in titanite-dominant samples vs. higher modal biotite in fluorite-dominant samples within the Mount Scott Granite pluton. Coexistence of magmatic titanite and fluorite in the Mount Scott Granite pluton implies F m of ∼1 wt% at the point in its crystallization history where these minerals coprecipitated. We suggest that the presence of primary fluorite within high-temperature, shallowly emplaced, moderate f (O 2 ), subaluminous felsic rocks indicates high magmatic fluorine, whereas titanite without fluorite in such rocks indicates low initial fluorine.