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 (see zircon geochronology), 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.
Zircon
Zircon ((Zr1–y, REEy)(SiO4)1–x(OH)4x–y)) is an orthosilicate mineral that is commonly found as an accessory mineral throughout Earth's crust. Due to its crystal structure and geochemistry, zircon is a commonly analyzed mineral because of its utility for geologists as a geochronometer and geothermometer. Chemically, zircon is a particularly useful mineral because of its ability to incorporate many trace elements. Many of these elements can be used for radiometric dating to provide an age for the crystal. It is known to exchange uranium, thorium and rare earth elements (REE) such as yttrium, and lutetium. However, the chemical potential energies of these REE substitutions are not well understood, so they are not suitable for determining crystallization temperatures. Titanium is also incorporated into zircon, and its exchange rates has been studied in detail. Ti4+, a tetravalent ion, can replace Zr4+ or Si4+ in a temperature dependent mechanism. For zircons in the presence of TiO2, i.e. the mineral rutile, this substitution process is common and can be measured. Zircon is also useful because its incorporation of other elements like uranium, lutetium, samarium, and oxygen can be analyzed to provide further insight into the age and conditions the crystal grew under. Thermally, zircon is resistant to temperature changes and extremes. It is stable up to 1690 °C at ambient pressure and has a low thermal expansion rate. Zircon crystals are also some of the most incompressible silicate minerals. The high durability of zircons also allows them to crystallize around other silicate minerals, creating pockets, or inclusions, of surrounding melts that are indicative of magma at specific pressures and temperatures. This essentially forms a time-capsule giving a glimpse of past conditions in which the crystal formed. Zircons are known to be relatively retentive of their incorporated isotopes and thus very useful for microquantitative studies. Cations such as REE, U, Th, Hf, Pb, and Ti diffuse slowly out of zircons, and their measured quantities in the mineral are diagnostic of the melt conditions surrounding the crystal during growth. This slow rate of diffusion of many of the incorporated elements makes zircon crystals more likely to form compositional zoning, which may represent oscillatory zoning or sector zoning, as the melt composition or energy conditions change around the crystal over time. These zones show compositional differences between the core and rim of the crystal, providing observable evidence of changes in melt conditions. Slow diffusion rates also prevent contamination by leaking or loss of isotopes from the crystal, increasing the likelihood that chronologic and compositional measurements are accurate.
Methods
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 composition of the zircons is measured 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 titanium-in-zircon measurement can be done several times in zoned zircons, which may record the temperature evolution that resulted from many geologic events.
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