Chromite as a petrogenetic indicator in ultramafic rocks

S. Yao, W.L. Griffin and S.Y. O'Reilly, GEMOC, Macquarie

Chromite [(Mg,Fe)(Al,Cr,Fe)2O4] is a common accessory mineral in ultramafic volcanic rocks. It has long been used as an important indicator mineral by diamond exploration companies because it is very resistant to weathering and remains in the regolith after other indicators such as garnets are destroyed. However, only some of the rocks that contain chromites (e.g. kimberlites and lamproites) are potential diamond sources. Other ultramafic rocks such as komatiites also contain abundant chromite, but they are definitely diamond free. Many attempts have been made to recognise the chromites from potentially diamondiferous source rocks by using their major element chemistry, but ambiguity remains. The high resolution and low detect limits of Laser-Ablation Microprobe (LAM) analyses enable us to measure minor and trace elements on a very small spot of chromites. This technique broadens our knowledge about chemistry of chromites.

Kimberlites, lamproites and some ultramafic lamprophyres are directly mantle-derived rocks in which xenocrystic and early-crystallisation chromites are equilibrated with mantle olivine. The chemistry of chromites in these rocks reveals the nature of, and processes in, the mantle. All chromites from these mantle-derived rocks show a good trend of Co with Zn. We define this trend as a "Mantle Array", which represents chromites equilibrated with mantle olivine. Zn contents are temperature-dependent (Griffin et al., 1994; Ryan et al., 1996), so Co shows a good correlation with TZn (_C) derived from Zn composition, therefore suggesting the Mantle Array is controlled by temperature and Co content may prove to be a new thermometer for chromites. Ni contents are roughly negatively correlated with Zn, implying Ni contents are also controlled by temperature. Mn contents in Mantle Array chromites are relatively low and constant (1500 ppm to 3000 ppm) and not correlated with temperature. All the behaviour of these elements in Mantle Array chromites is attributed to the partitioning of these elements between chromite and mantle olivine, which serves as a reservoir of these elements. Thus the Mantle Array chromites can be defined on 4 axes: Zn, Co, Ni and Mn.

Chromites from komatiite, which is an ultramafic volcanic rock erupted predominantly in Archaean, do not follow the Mantle Array, but plot on 2 sides of this Array in the plot of Co against Zn, implying that chromites in komatiite are not equilibrated with mantle olivine. Komatiite chromites also have much higher Mn and lower Ni than most of the Mantle Array chromites.

Discrimination of chromites between Mantle Array rocks and komatiite is important because Mantle Array rocks are potentially diamondiferous sources but komatiite is not.

Other trace elements also appear to be useful for discriminating chromites from different rock types. For example, chromites from kimberlites and lamproites contain much higher Nb, Ta, Zr and Sc than those from lamprophyres and komatiites. Further statistical analysis can help in setting up a discrimination tree to recognise chromites from different sources (kimberlites, lamproites, lamprophyres and komatiites).

References

Griffin, W.L., Ryan, C.G., Gurney, J.J., Sobolev, N.V. and Win, T.T. (1994), Chromite macrocrysts in kimberlites and lamproites: Geochemistry and origin, in H.O.A. Meyer and O.H. Leonardos (Eds) Kimberlites, Related Rocks and Mantle Xenoliths. CPRM Special Publication 1/A, pp.366-377, Companhia de Pesquisa de Recursos Minerais, Brasil.

Ryan, C.G., Griffin, W.L. and Pearson, N.J., (1996), Garnet geotherms: Pressure-temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks, Journal of Geophysical Research, Vol.101, No.B3, pp.5611-5625.

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