Zircon and monazite in granites

Grains of zircon from the Amitsoq Gneiss, one of the oldest rocks on Earth

Zircon (ZrSiO4) and monazite (CePO4) are common minor minerals in granites.  Although they are not abundant they are important because they are the host of many elements that are incompatible in the rock-forming minerals.  For example, the ability of zircon to incorporate U has made it the most important and reliable mineral for radiometric dating of the Earth's crust.  Zircon is also physically and chemically robust meaning that it is often preserved long after the rock in which it formed has eroded.  For example, the oldest known samples of the Earth are detrital zircons (with ages up to 4.4 Ga) in meta-sedimentary rocks of the Jack Hills in Western Australia.  We have recently shown that the trace element content of zircon is a fingerprint of the type of melt from which it crystallised and that the Jack Hills zircons formed from an "I-type" granite (Burnham and Berry, Nature Geoscience, 2017).  Granites are broadly classified as either I-type or S-type depending on whether their source was predominantly igneous or sedimentary.  This classification was based on the rocks of the Lachlan Fold Belt, in which Canberra is situated, and was largely developed at ANU (see the memorial bench to Bruce Chappell and Alan White between Jaeger 8 and the Old Hospital Building).  Zircon and monazite both crystallise early in granites and are hosts for the rare earth elements (REE), however, the REE signature in zircon will depend on whether monazite or zircon crystallise first.  This is related to the solubility of these minerals in a granitic melt, which will vary with granite type and temperature.

In this project the solubility of zircon and monazite in I- and S-type granites will be determined experimentally.  Synthetic granite compositions containing excess zircon and monazite will be prepared and then equilibrated as a function of pressure and temperature.  The products will be a mixture of zircon, monazite, and quenched melt (glass).  These three phases will be analysed to determine their trace element contents, and hence the Zr and Ce contents at saturation.  Partition coefficients of other REE may also be determined.  The project will provide training in the methods of experimental petrology, including the operation of piston-cylinder devices.  The student would also receive training in sample preparation and polishing, scanning electron microscopy, electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry, and Raman spectroscopy.

For more information about this potential research topic or activity, or to discuss any related research area, please contact the supervisor.

Andrew Berry and Antony Burnham