Thermobarometry of chromite

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Chrome spinel (chromite) is an important mineral in peridotites, and may be stable up to over 7 GPa (> 200 km depth) in the mantle. It occurs commonly as an inclusion in diamonds (indeed, at the Murowa mine in Zimbabwe it is the commonest inclusion type) and thus may provide valuable insights into their formation. It is also the primary indicator mineral for locating kimberlites in the Australian outback, where heavy weathering destroys other conventional minerals such as garnet. However, a poor understanding of chromite trace element systematics has meant it is often been neglected as a source of information.

 

This project will use high pressure experimental techniques to look at two aspects of spinel mineral chemistry.

 

1) The exchange of Zn between olivine and spinel has been proposed as a thermometer. The only existing calibration is empirical, i.e. it has been determined from xenoliths whose P,T conditions were estimated using other geothermometer techniques. For example, the calibration of Ryan et al. (1996) used temperatures calculated using Ni in garnet, but the calibration of this thermometer is itself disputed (Canil, 1999).

 

2) A second significant trace element in spinel is Si. Mg-chromite inclusions from diamond have variable SiO2 concentrations, from 0 – 2 % but averaging 0.16 ± 0.12 (1σ). The substitution occurring is likely to be 2Al ↔ MgSi (i.e. solid solution towards Mg2SiO4). Mg2SiO4 in the spinel structure is better known as the ultra-high pressure mineral ringwoodite, which is stable in the mantle between 500 and 660 km depth. Therefore, there is likely to be a pressure dependence of this reaction but this has not been explored experimentally before.

 

The aim of the project will be to determine the partitioning of Zn and Si between olivine and spinel at a range of pressures and temperatures. The student carrying out this project will receive training in high pressure experimentation, and in a wide range of analytical techniques including scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and laser ablation mass spectrometry (LA-ICP-MS). The results will be applicable to mineral inclusions from diamonds and chromite-bearing xenoliths.

 

References:

Canil, D. (1999). The Ni-in-garnet geothermometer: calibration at natural abundances. Contributions to Mineralogy and Petrology 136, 240-246.

Ryan et al. (1996). Garnet geotherms: pressure-temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. Journal of Geophysical Research 101, 5611-5625.

For more information about this potential research topic or activity, or to discuss any related research area, please contact the supervisors (Antony Burnham and Greg Yaxley).

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