GEMOC ARC National Key Centre

TRACE ELEMENTS IN CHROMITES FROM KIMBERLITES AND RELATED ROCKS: RELATION TO TEMPERATURE AND MANTLE COMPOSITION

Shixin Yao¹, William L. Griffin^1,2 and Suzanne Y. O'Reilly¹

1 GEMOC Macquarie,
2 CSIRO Exploration and Mining

About 1500 chromites from 47 kimberlite, lamproite and lamprophyre localities world-wide have been analysed to determine their major and trace element contents by using electron microprobe and laser ablation microprobe ICP-MS, respectively, in the GEMOC National Key Centre at Macquarie University. Chromites analysed are heavy mineral concentrates embedded in epoxy resin and polished and their sizes are typically 0.5-1 mm in diameter, with a few larger than 1 mm. Chemical compositions reported here represent analyses of the cores of grains.  Chromites from xenoliths have been analysed in-situ using thick polished sections.

There are some good inter-element correlations observed in chromites from kimberlites and related rocks. All chromites show a positive correlation between Co and Zn and a negative correlation between Co and Ni. A negative correlation also exists between Zn and Ni.

Some analyses of Cr-spinel (low Cr) in xenoliths from Nushan alkaline basalt, southeastern China and chromites in xenoliths from Wesselton kimberlite, South Africa show the same correlations, within single suites. This implies that most chromite macrocrysts in kimberlites and related rocks are xenocrysts derived by disaggregation of mantle peridotites and captured by magmas ascending through the mantle sections. Those scattered off the main trends are considered to be magmatic or modified (metasomatised or metamorphosed) chromites. Chromites along the trends are defined as a "Mantle Array",  representing chromites equilibrated with mantle olivine.

Zn contents are temperature-dependent (Griffin et al., 1994; Ryan et al., 1996), and Co shows a good correlation with TZn (°C) derived from Zn composition, therefore suggesting the locus of Mantle Array is controlled by temperature. The overall negative correlation of Ni with Zn indicates that Ni contents in chromites are also controlled by temperature. The behaviour of Mantle Array chromites is attributed to the partitioning of these elements between chromite and mantle olivine, which serves as a reservoir of these elements. Generally, the Mantle Array chromites can be defined on 3 axes: Zn, Co and Ni.

In order to test whether these Mantle Array chromites record differences in  the chemical composition of the lithosphere related to age or tectonic position, we have classified the localities according to the age of the last tectonothermal modification of the crust through which these kimberlites and related rocks penetrated, using a version of the scheme proposed by Janse (1984).  "Archon" represents a craton stabilised in Archean time with latest crustal modification >2.5 Ga. "Protons" are cratons with latest crustal modification before 1000 Ma. "Tectons" are basically younger tectonic units, mainly Phanerozoic in age.

The trace elements of mantle-array chromites show differences among different tectonic environments. Chromites from Archons contain significantly higher V than those from Protons and Tectons, and chromites from Tectons have relatively higher Ga than those from Protons although they have similar V contents. Ga is positively correlated with Al#, and the higher Al and Ga in Tecton chromites reflects the enrichment of Phanerozoic mantle in Al  relative to Proterozoic and Archean mantles. The higher V in Archon chromites correlates positively with Cr#, which is an indicator of the degree of depletion of the mantle.  Chromites from Archons tend to have high Nb and Nb/Zr, while those from Protons show a much greater spread in Zr contents and Nb/Zr.  Chromites from Tecton environments, and all chromites from lamprophyres, tend to have low Nb and Zr.  These differences may be related to time-integrated differences in metasomatic style, related to tectonic setting; chromites from xenoliths in the Wesselton kimberlite (S. Africa) have high Nb contents which can be related to observed phlogopite-related metasomatism in the mantle.

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.

Janse, A.J.A., 1994.  Is Clifford's Rule still valid? Affirmative  examples from around the world.  In H.O.A.
Meyer and O. Leonardos  (eds) Diamonds: characterization, genesis and exploration, CPRM Spec. Publ. 1A/93, Dept. Nacional da Prod. Mineral., Brazilia,  215-235.

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.