Titanium in zircon geothermometry
[3] Due to its crystal structure and geochemistry, zircon is a commonly analyzed mineral because of its utility for geologists as a geochronometer and geothermometer.
[1] Zircon is also useful because its incorporation of other elements like uranium, lutetium, samarium,[5] and oxygen[6] can be analyzed to provide further insight into the age and conditions the crystal grew under.
[3] 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.
Cations such as REE,[8] U, Th, Hf,[9] Pb,[10] and Ti[11] diffuse slowly out of zircons, and their measured quantities in the mineral are diagnostic of the melt conditions surrounding the crystal during growth.
[12] These zones show compositional differences between the core and rim of the crystal, providing observable evidence of changes in melt conditions.
[13] 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.
Zircon crystals are then mounted to an epoxy or metal disc-shaped slide,[14] where they can be shaved to about half thickness to reveal their internal structure.
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.
The titanium-to-temperature relationship was calculated using in situ radiometrically dated zircons with known melt temperatures from the surrounding rock.
This will provide previously unknown evidence for the conditions in early Earth and allow testing of ideas of how the planet evolved through the Hadean and Archean eons.
[1][15] Titanium-in-zircon geothermometry is considered to be a relatively reliable and accurate method of determining crystallization temperatures of zircons, with an error of only 10-16 degrees Celsius.
Recent studies have expressed concern over the gold coating on the surface of the ion microprobe mounts, which contains small amounts of titanium (~1 ppm) that could provide an error during measurement.
