Crystallization thermometers for zircon and rutile

• Published in: Contributions to mineralogy and petrology Vol. 151; no. 4; pp. 413 - 433
• Main Authors: Watson, E. B., Wark, D. A., Thomas, J. B.
• Format: Journal Article
• Language: English
• Published: Heidelberg Springer Nature B.V 01.04.2006
• Subjects: Crystallization; Crystals; Growth conditions; High pressure; Mineralogy; Petrology; Rocks; Thermometers; Titanium; Zirconium
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-006-0068-5

Zircon and rutile are common accessory minerals whose essential structural constituents, Zr, Ti, and Si can replace one another to a limited extent. Here we present the combined results of high pressure-temperature experiments and analyses of natural zircons and rutile crystals that reveal systematic changes with temperature in the uptake of Ti in zircon and Zr in rutile. Detailed calibrations of the temperature dependencies are presented as two geothermometers--Ti content of zircon and Zr content of rutile--that may find wide application in crustal petrology. Synthetic zircons were crystallized in the presence of rutile at 1-2 GPa and 1,025-1,450 degrees C from both silicate melts and hydrothermal solutions, and the resulting crystals were analyzed for Ti by electron microprobe (EMP). To augment and extend the experimental results, zircons hosted by five natural rocks of well-constrained but diverse origin (0.7-3 GPa; 580-1,070 degrees C) were analyzed for Ti, in most cases by ion microprobe (IMP). The combined experimental and natural results define a log-linear dependence of equilibrium Ti content (expressed in ppm by weight) upon reciprocal temperature: (formula omitted)In a strategy similar to that used for zircon, rutile crystals were grown in the presence of zircon and quartz (or hydrous silicic melt) at 1-1.4 GPa and 67-1,450 degrees C and analyzed for Zr by EMP. The experimental results were complemented by EMP analyses of rutile grains from six natural rocks of diverse origin spanning 0.35-3 GPa and 470-1,070 degrees C. The concentration of Zr (ppm by weight) in the synthetic and natural rutiles also varies in log-linear fashion with T -1: (formula omitted) The zircon and rutile calibrations are consistent with one another across both the synthetic and natural samples, and are relatively insensitive to changes in pressure, particularly in the case of Ti in zircon. Applied to natural zircons and rutiles of unknown provenance and/or growth conditions, the thermometers have the potential to return temperatures with an estimated uncertainty of +/- 10 degrees or better in the case of zircon and +/- 20 degrees or better in the case of rutile over most of the temperature range of interest (approximately 400-1,000 degrees C). Estimates of relative temperature or changes in temperature (e.g., from zoning profiles in a single mineral grain) made with these thermometers are subject to analytical uncertainty only, which can be better than +/- 5 degrees depending on Ti or Zr concentration (i.e., temperature), and also upon the analytical instrument (e.g., IMP or EMP) and operating conditions.[PUBLICATION ABSTRACT]