Zircon age dating

Zircon is a common accessory to trace mineral constituent of most granite and felsic igneous rocks.Due to its hardness, durability and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands.Zircons contain trace amounts of uranium and thorium (from 10 ppm up to 1 wt%) and can be dated using several modern analytical techniques.

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Currently, zircons are typically dated by uranium-lead (U-Pb), fission-track, cathodoluminescence, and U Th/He techniques.

For instance, imaging the cathodoluminescence emission from fast electrons can be used as a prescreening tool for high-resolution secondary-ion-mass spectrometry (SIMS) to image the zonation pattern and identify regions of interest for isotope analysis.

This is done using an integrated cathodoluminescence and scanning electron microscope.

Zircons in sedimentary rock can identify the sediment source.

Zircon is rare within mafic rocks and very rare within ultramafic rocks aside from a group of ultrapotassic intrusive rocks such as kimberlites, carbonatites, and lamprophyre, where zircon can occasionally be found as a trace mineral owing to the unusual magma genesis of these rocks.

Zircon forms economic concentrations within heavy mineral sands ore deposits, within certain pegmatites, and within some rare alkaline volcanic rocks, for example the Toongi Trachyte, Dubbo, New South Wales Australia in association with the zirconium-hafnium minerals eudialyte and armstrongite.

The term U–Pb dating normally implies the coupled use of both decay schemes in the 'concordia diagram' (see below).

However, use of a single decay scheme (usually Pb) leads to the U–Pb isochron dating method, analogous to the rubidium–strontium dating method.

Where crystals such as zircon with uranium and thorium inclusions do not occur, a better, more inclusive, model of the data must be applied.

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