T152 Creative Contributions in Volcanology and Petrology II: In Honor of Katharine V. Cashman, Recipient of the 2023 Distinguished Geological Career Award from the Mineralogy, Geochemistry, Petrology and Volcanology Division

Tuesday, 17 October 2023
8:00 AM - 12:00 PM
319

AN EXPLICIT SOLUTION FOR MAGMATIC TIO2 ACTIVITY FROM TANDEM ZIRCON AND TITANITE THERMOMETRY

Paper number: T152-13
Program: Technical Sessions
Time: 10:50 AM – 11:05 AM

Authors

Abstract

Thermobarometry is an important tool for tracing thermal histories in the geosciences. In granitoids and related rocks, two common thermometers are Ti-in-zircon1 and Zr-in-titanite2. One of the difficulties in implementing these and other 4+ cation thermometers (Ti-in-quartz and Zr-in-rutile1) is estimating various system variables, particularly the activities of TiO2 and SiO2 (aTiO2 and aSiO2). Constraints on the activity of TiO2 are critical to most thermometers, but it have been proposed to range from 0.2-13 using various methods. When rutile is present, the system can easily be assumed to have an aTiO2 of 1, yet rutile is commonly absent from calc-alkaline magmas, leading to a wide range of possible activities less than 14. Moreover, because the thermometers are functions of log(a), calculation with inaccurate activities results in large temperature disparities (50-100 °C)5. Here we present a new analytical framework that allows us to explicitly determine two of the more difficult parameters, temperature and aTiO2.

We analyzed zircons from five rock suites in the Tuolumne Intrusive Suite (TIS), Sierra Nevada, CA by laser ablation-split stream – inductively coupled plasma – mass spectrometry depth-profiling (LASS-DP). Each suite of zircon chemical profiles shows an inflection in the Yb/Dy ratio between 2-4 ppm Ti, which arises from the onset of titanite saturation. A zircon-derived titanite saturation allows for the use of Ti-in-zircon and Zr-in-titanite in tandem (using more reliable estimates on pressure and SiO2 activity), yielding a system of equations to explicitly solve for both temperature and TiO2 activity at the point of titanite saturation. For three of the TIS rocks, the solutions are 0.40-0.45 ± 0.05 aTiO2and 690-710 ± 20 . These values provide a robust point to extrapolate up- and down-temperature from the zircon-derived titanite saturation, and to evaluate other geochemical variables in relation to evolving magmatic temperature. This method can be applied to other rutile-absent, titanite-present systems to explicitly determine the activity of TiO2.

  1. Ferry & Watson, Contrib. Mineral. Petrol. (2007).
  2. Hayden et al., Contrib. Mineral. Petrol. (2008).
  3. Thomas & Watson, Reply to Contrib. Mineral. Petrol. (2012).
  4. Schiller & Finger, Contrib. Mineral. Petrol. (2019).
  5. Ackerson, & Mysen, Am. Mineral. (2020).

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-390241

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