page_title: Titanium in zircon geothermometry
section_title: Methods
hierarchical_section_title: Titanium in zircon geothermometry / Methods
context_page_description: Titanium in zircon geothermometry is a form of a geothermometry technique by which the crystallization temperature of a zircon crystal can be estimated by the amount of titanium atoms which can only be found in the crystal lattice. In zircon crystals, titanium is commonly incorporated, replacing similarly charged zirconium and silicon atoms. This process is relatively unaffected by pressure and highly temperature dependent, with the amount of titanium incorporated rising exponentially with temperature, making this an accurate geothermometry method. This measurement of titanium in zircons can be used to estimate the cooling temperatures of the crystal and infer conditions during which it crystallized. Compositional changes in the crystals growth rings can be used to estimate the thermodynamic history of the entire crystal. This method is useful as it can be combined with radiometric dating techniques that are commonly used with zircon crystals, to correlate quantitative temperature measurements with specific absolute ages. This technique can be used to estimate early Earth conditions, determine metamorphic facies, or to determine the source of detrital zircons, among other uses.
context_section_description: This section will review the process of measuring the titanium content of zircons, beginning with sample collection, mineral separation, mounting for microprobe analysis, and ending with the microquantitative element analysis. Once a rock has been collected, zircons are extracted using a series of techniques such as using a sieve, heavy liquid, shaking table, and magnetic separation to separate minerals based on differing densities and properties. Zircon crystals are then mounted to an epoxy or metal disc-shaped slide, where they can be shaved to about half thickness to reveal their internal structure. From here, they can be imaged using cathodoluminescence to make any zonations in the mineral visible. If zonation is apparent, multiple measurements of Ti abundance can be taken from the center to the rim to give the temperature evolution of the crystal.
  The final step involves measuring the abundance of titanium in a specific location on a zircon crystal with an ion microprobe. For this, the chemical compositions of the zircons is measure using secondary ion mass spectrometry. The sample is bombarded with a beam of primary ions, and the charge and mass of the ejected secondary ions are measured to determine the chemical composition at the point of contact. The quantitative value for titanium content is then compared to a known relationship of titanium incorporation and temperature to determine the crystallization temperature of that zone of the zircon. The titanium-to-temperature relationship was calculated using in situ radiometrically dated zircons with known melt temperatures from the surrounding rock. This Ti-in-zircon measurement can be done several times in zoned zircons, which may record the temperature evolution that resulted from many geologic events.
caption: Cartoon version of unzoned (left) and zoned (right) zircons. Red dots represent ion microprobe scan locations. White bar is about 50 μm.
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