SOLUBILITY OF SPHENE IN SILICEOUS MELTS

The solubility of sphene (titanite; CaTiSiO₅) in SiO₂-rich melts was measured experimentally through growth experiments at 800-1000°C, 0.5 - 1.0 GPa, fO₂ = CCO, t = 72-168 h, and H₂O = 0 to 4 wt.%, and in dissolution experiments at 925-1300°C, 0.8 GPa, t = 18-118 h, and H₂O = 1-10 wt.% in a piston cylinder apparatus. We imaged run products using a Tescan Vega 3 LM SEM and analyzed run product glasses using an Oxford X-max EDS. For dissolution experiments we estimated saturation concentrations from measured diffusion profiles using the method of Harrison and Watson (1983, CMP v. 84 pp. 66-72). Run product glasses in growth experiments were homogeneous, and iron loss suppressed ilmenite crystallization. Sphene-saturated melts ranged from 62-78 wt.% SiO₂ and were metaluminous to moderately peraluminous, with aluminosity A/CNK from 0.38-1.22 (A/NK 1.09-1.57). Addition of Al₂O₃ to growth experiment starting materials suppressed sphene crystallization. Sphene solubility increases with increasing temperature, but is not affected by changes in pressure or bulk H₂O content. Multiple linear regression of glass composition data from 23 growth and dissolution experiments yielded the sphene solubility equation TiO₂ (wt.%) = 0.79*C - 7993/T(K) + 7.88, where C = (10* eCa)/(Al*Si), symbols are cation fractions, and excess calcium eCa = CaO - Al₂O₃ + Na₂O + K₂O with concentrations in moles (adj. r = 0.97). This equation should be useful for constraining the temperatures of melts, for determining whether sphene saturation in magmatic source regions is likely, and for determining when sphene can crystallize and begin to exert an influence on melt geochemistry.