Paper No. 152-11
Presentation Time: 10:50 AM
AN EXPLICIT SOLUTION FOR MAGMATIC TIO2 ACTIVITY FROM TANDEM ZIRCON AND TITANITE THERMOMETRY
SCHOONOVER, Erik1, ACKERSON, Michael2, GARBER, Joshua M.3, SMYE, Andrew J.1, KYLANDER-CLARK, Andrew R.C.4 and REIMINK, Jesse1, (1)Department of Geosciences, The Pennsylvania State University, Deike Building, University Park, PA 16802, (2)Dept. of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th St. and Constitution Ave. NW., Washington, DC 20560, (3)Department of Geosciences, Pennsylvania State University, University Park, PA 16802, (4)Department of Earth Science, University of California Santa Barbara, Santa Barbara, CA 93106
Thermobarometry is an important tool for tracing thermal histories in the geosciences. In granitoids and related rocks, two common thermometers are Ti-in-zircon
1 and Zr-in-titanite
2. One of the difficulties in implementing these and other 4+ cation thermometers (Ti-in-quartz and Zr-in-rutile
1) is estimating various system variables, particularly the activities of TiO
2 and SiO
2 (
aTiO
2 and
aSiO
2). Constraints on the activity of TiO
2 are critical to most thermometers, but it have been proposed to range from 0.2-1
3 using various methods. When rutile is present, the system can easily be assumed to have an
aTiO
2 of 1, yet rutile is commonly absent from calc-alkaline magmas, leading to a wide range of possible activities less than 1
4. 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
aTiO
2.
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.
- Ferry & Watson, Contrib. Mineral. Petrol. (2007).
- Hayden et al., Contrib. Mineral. Petrol. (2008).
- Thomas & Watson, Reply to Contrib. Mineral. Petrol. (2012).
- Schiller & Finger, Contrib. Mineral. Petrol. (2019).
- Ackerson, & Mysen, Am. Mineral. (2020).
![[Visit Client Website]](/img/gsa/banner.jpg)