Defect thermometry using rutile and feldspar

Abstract

Two potential trace-element/mineral pairs have been investigated for their potential use as thermobarometers. Al is a common trace/minor element found in rutile, typically on the order of 100’s of parts-per-million (ppm). Experimentally, when rutile is crystallized with Al2O3, the resulting rutile crystals contain 1000’s to 10,000’s of ppm Al, much higher than natural samples. Furthermore, oxygen activity plays a role in how much Al can be dissolved in rutile. To reconcile both of these differences, I have proposed a defect model of Al substitution where Al solubility is dependent on water activity and the amount of hydrous defects inside rutile. This model of Al dissolution in rutile has been used to address the low Al contents found in natural rutile crystals and has been used to create an Al-in-rutile thermobaromometer.

Within certain silicates, Ti can substitute for tetrahedral Si, a process that is highly temperature sensitive. I have explored the potential of using this substitution as the basis for a Ti-in-potassium feldspar (TitanOr) thermobarometer. Our experimental results show that Ti solubility in feldspar is highly temperature dependent and have calibrated its solubility over the range of 600-810°C and 0.5-1.0 GPa. Several rocks have been analyzed; the Ti contents from K-feldspar crystals of the Cathedral Peak Granodiorite from Yosemite National Park in California and the Fish Canyon Tuff of Colorado are consistent with models for their formation and match well with conditions obtained from using other thermobarometers. I also have applied TitanOr to the large K-feldspar megacrysts of the Ayer Crystalline Complex of Massachusetts.