Ti-in-zircon thermometry: applications and limitations

The titanium concentrations of 484 zircons with U-Pb ages of *1 Ma to 4.4 Ga were measured by ion microprobe. Samples come from 45 different igneous rocks (365 zircons), as well as zircon megacrysts (84) from kimberlite, Early Archean detrital zircons (32), and zircon reference materials (3). Samples were chosen to represent a large range of igneous rock compositions. Most of the zircons contain less than 20 ppm Ti. Apparent temperatures for zircon crystallization were calculated using the Tiin-zircon thermometer (Watson et al. 2006, Contrib Mineral Petrol 151:413–433) without making corrections for reduced oxide activities (e.g., TiO2 or SiO2), or variable pressure. Average apparent Ti-in-zircon temperatures range from 500 to 850 C, and are lower than either zircon saturation temperatures (for granitic rocks) or predicted crystallization temperatures of evolved melts (*15% melt residue for mafic rocks). Temperatures average: 653 ± 124 C (2 standard deviations, 60 zircons) for felsic to intermediate igneous rocks, 758 ± 111 C (261 zircons) for mafic rocks, and 758 ± 98 C (84 zircons) for mantle megacrysts from kimberlite. Individually, the effects of reduced aTiO2 or aSiO2 , variable pressure, deviations from Henry’s Law, and subsolidus Ti exchange are insufficient to explain the seemingly low temperatures for zircon crystallization in igneous rocks. MELTs calculations show that mafic magmas can evolve to hydrous melts with significantly lower crystallization temperature for the last 10–15% melt residue than that of the main rock. While some magmatic zircons surely form in such late hydrous melts, low apparent temperatures are found in zircons that are included within phenocrysts or glass showing that those zircons are not from evolved residue melts. Intracrystalline variability in Ti concentration, in excess of analytical precision, is observed for nearly all zircons that were analyzed more than once. However, there is no systematic change in Ti content from core to rim, or correlation with zoning, age, U content, Th/U ratio, or concordance in U-Pb age. Thus, it is likely that other variables, in addition to temperature and aTiO2 , are important in controlling the Ti content of zircon. The Ti contents of igneous zircons from different rock types worldwide overlap significantly. However, on a more restricted regional scale, apparent Ti-in-zircon temperatures correlate with whole-rock SiO2 and HfO2 for plutonic rocks Communicated by T. L. Grove. Electronic supplementary material The online version of this article (doi:10.1007/s00410-008-0281-5) contains supplementary material, which is available to authorized users. B. Fu F. Z. Page J. Fournelle N. T. Kita J. W. Valley (&) Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA e-mail: [email protected] A. J. Cavosie Department of Geology, University of Puerto Rico, Mayagüez, PR 00681-9017, USA J. S. Lackey Geology Department, Pomona College, Claremont, CA 91711, USA S. A. Wilde Department of Applied Geology, Curtin University of Technology, Perth, WA 6845, Australia Present Address: B. Fu School of Earth Sciences, The University of Melbourne, Parkville, VIC 3010, Australia Present Address: F. Z. Page Geology Department, Oberlin College, Oberlin, OH 44074, USA 123 Contrib Mineral Petrol (2008) 156:197–215 DOI 10.1007/s00410-008-0281-5