TITANOR: A TITANIUM-IN-POTASSIUM FELDSPAR THERMOMETER

The feldspars are appealing targets for geothermometry because of their prevalence in the Earth’s crust and the variety of temperature-dependent phenomena in which they participate. Many feldspars display zoning in CL, BSE, trace elements and sometimes visible light. Given the possibility that these zoning patterns might be attributed to temperature differences within a crystallizing system, a high spatial resolution thermometer is needed for the feldspar group. Titanium solubility in potassium feldspar was determined experimentally at 600°-1150°C and 5-15kb for the purpose of creating a Ti-in-kspar thermometer. Our experiments show that the concentration of titanium in potassium feldspar varies systematically with temperature and pressure when grown in equilibrium with rutile. Titanium concentrations range from tens to hundreds of ppm over the range of experimental P-T conditions. Natural potassium-rich feldspar crystals confirm this observation – all samples we have measured so far contain titanium at similar ppm levels. Titanium concentrations of this magnitude can be measured using EPMA, SIMS and LA-ICPMS. However, care must be taken using EPMA because of (1) the high order barium interference next to the titanium Kα peak and (2) the tendency for beam damage to occur in alkali feldspars, especially at beam currents greater than 50 nA. The partitioning of Ti between K-feldspar and quartz holds some promise as a thermobarometer that does not require estimates of TiO₂ activity in the system of interest.