ZONED TITANITE (SPHENE) IN THE MT. SCOTT GRANITE, SOUTHERN OKLAHOMA AULACOGEN, USA; IMPLICATIONS FOR PLUTON CRYSTALLIZATION AND COOLING

The Mount Scott Granite is the best-exposed unit within a series of A-type felsic rocks that dominate the near-surface expression of the early Cambrian Southern Oklahoma Aulacogen. Titanite grains within this porphyritic granite are typically clustered with mafic silicates, Fe-Ti oxides, and other accessory grains. The titanite crystals are 0.1- 0.3 mm long and appear to be partially resorbed. While most grains are chemically homogeneous, a few have sharp growth bands and patchy domains 10-25 microns in width that are elevated in F and Al (and correspondingly depleted in Ti); these domains are resolved in back-scattered electron, cathodoluminescence, and x-ray map imaging.

The lower-F, Al portions of zoned titanite are comparable to the more abundant homogenous grains with respect to major element composition. However, Zr is uniformly lower (100-700 ppm) in the zoned titanite grains (both in low- and high-F domains) compared to those in the homogenous titanite grains (1100-1400 ppm). Zr-in-titanite temperature calculations place the zoned titanite at 636 +8/-7 ºC, whereas homogenous titanite grains yield temperatures averaging 746 ± 6 ºC, assuming 0.2 GPa pressure (based on prior phenocryst geobarometry), silica saturation, and a TiO₂ activity of 0.51 (based on Fe-Ti oxides) during crystallization. Given the discrepancy in Zr, it is plausible that the zoned crystals are xenocrysts.

Recent work on diffusion reveals F to be one of the faster moving elements in titanite. Dodson modeling places the closure temperature for the smallest of these F-rich domains near 450 ºC. The preservation of the zones in these partially resorbed crystals suggests that titanite underwent chemical, but not thermal, breakdown. This dissolution has been previously attributed to fluorite and oxide crystallization at the expense of titanite due to increasing aF₂ during the late-stage crystallization of the magma. Furthermore the preserved domains indicate that the pluton was rapidly cooled, a finding consistent with field relationships that point to shallow final emplacement.