Pearson, N.J.1, Griffin, W.L.1,2, Doyle, B.J.3, O'Reilly, S.Y.1, van Achterbergh, E.1 and Kivi, K.4

1. GEMOC National Key Centre, School of Earth Sciences, Macquarie University, NSW 2109, Australia

2. CSIRO Exploration and Mining, PO Box 126, North Ryde, NSW 2113, Australia

3. Kennecott Canada Exploration Inc., 200 Granville St, Vancouver, BC V6C 1S4, Canada

4. Kennecott Canada Exploration Inc., 1300 Walsh St, Thunder Bay, Ontario P7E 4X4, Canada

The composition, structure and thermal state of the lithosphere beneath the Lac de Gras area in the Slave Craton have been determined from a suite of mantle-derived xenoliths. The xenoliths in this study come from kimberlite pipes DO-18, DO-27 and A154 and have been recovered after the crushing stage during the processing of the kimberlite. They are generally small fragments 1-2 cm3 in volume that have survived the jaw crusher, whereas the larger xenoliths (up to 20 cm in diameter) seen in drill core are extremely friable and have disaggregated. This sampling bias limits the the conclusions that can be made on the proportions of rock types, while the small size makes it difficult to obtain robust modal or bulk chemical information.

Several lithological groups have been recognised in the xenolith sample population.

Lherzolites (ol+opx+cpx+grt±crt): Grt lherzolites show a broad spectrum of microstructures (granoblastic, porphyroclastic, mylonitic) and range in grain size (<1mm up to >1cm). Fo contents in olivine range from an average of 91.5 in sheared lherzolites to 92.8 in fine-grained, cpx-poor samples. The overall range in garnet composition is XMg 80.1 to 85.1; CaO wt % 4.5-7.9; Cr2O3 wt % 2.4-12.2 (Fig. 1). Zoning in a number of samples is characterised by increasing CaO, Cr2O3 and decreasing XMg from core to rim. Other samples show the reverse of this trend, although in both cases, the zoning is parallel to lherzolite trend in a plot of CaO vs Cr2O3. Cpx (Cr-diopside) modal abundance is low <5% and in several samples with extremely low abundances, cpx occurs only in intimate association with Cr-spinel.

Fig.2 P-T plots for Lac de Gras xenoliths for selected geo-thermometer and geobarometer combinations.

Acknowledgment: We thank the staff of Diavik Diamond Mines Inc. and Aber Resources Ltd. for their efforts in supplying some of the xenolith material used in this work, and Joe Boyd for access to unpublished data.

References

Boyd, F.R., 1987. in Mantle Xenoliths, P.H. Nixon (editor), 403-412.

Boyd, F.R. and Canil, D., 1997. Goldschmidt Conf Abstracts, 34-35.

Boyd, F.R., Pokhilenko, N. P., Pearson, D.G., Mertzman, S.A., Sobolev, N.V. and Finger, L.W. 1997. Contrib. Mineral. Petrol 128, 228-246

Griffin, W.L., Doyle, B.J., Ryan, C.J., Pearson, N.J., O'Reilly, S.Y., Davies, R., Kivi, K. and van Achterbergh, E., 1998. Jour. Petrol., subm.

Nixon, P.H., 1987 in Mantle Xenoliths, P.H. Nixon (editor), 215-239.

Pearson, N.J., O'Reilly, S.Y., and Griffin, W.L., 1991. Eur. J. Mineral., 3, 293-322.

Pearson, N.J., O'Reilly, S.Y., and Griffin, W.L., 1995. Lithos, 36, 257-287.

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