Zircons: Age, Thermobarometry, and Source Inheritance

If the entirety of Earth history is considered, no other mineral yields more information about crust—from its primordial formation to present-day volcanic eruptions—than zircon (ZrSiO4). This information trove is borne out of a combination of key zircon properties, such as high physical hardness and a structure that can accommodate radioactive actinide isotopes like 235, 238U and 232Th that decay to different isotopes of Pb. Crystalline zircon has slow diffusion rates for U and Pb, which means that ages will not be perturbed by volume diffusion under most geologic conditions. The robust age data obtained from zircon anchors most of the geologic time scale in use today. For example, zircon geochronology obtained from a variety of techniques has been used to constrain the timing of igneous and metamorphic activity, to correlate biostratigraphic units and mass extinctions with absolute time though investigations of zircon-bearing volcanic ash beds, and to explore the assembly of supercontinents and the detrital record to almost 4.4Ga. Zircon also has two well-recognized crystallization thermometers, based on broadly different principles, which have found widespread use in the geological sciences. Chemical impurities (e.g., Li, Al, P, Ti, rare earth elements, Th, U), stable isotope ratios (e.g., Li-, O-, Si-, and Zr-isotopes), and mineral inclusions have constrained the source characteristics for in- and ex situ zircon. Evidence for assimilation of water-altered supracrustal material into silicate melts, melt evolution of plutons and volcanoes during fractional crystallization, the source of melt generation and crust-mantle interactions, and earliest evidence for biological activity on Earth have been explored though zircon chemistry and their primary inclusions.