T137 Work on the Zircon, Highlight the Apatite: Wielding the Power of Accessory Minerals and Honoring the Contributions of Bruce Watson, 2018 Roebling Medalist

Monday, 5 November 2018
1:30 PM - 5:30 PM
Room 240-241

SOLUBILITY OF SPHENE IN SILICEOUS MELTS

Paper number: T137-12
Program: Technical Sessions
Time: 4:10 PM – 4:25 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-320568

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