GEMOC ARC National Key Centre

M.P. Grégoire¹, B.N. Moine^1,2, S. Y. O'Reilly¹, J.-Y. Cottin² and A. Giret²

1. GEMOC Macquarie
2. Department of Geology-UMR 6524, University J. Monnet, 23 rue P. Michelon, 42023 St-Etienne, France

The Kerguelen archipelago was first originated in the South East Indian Ridge. This early geodynamic setting would have resembled that of Iceland. Then plate separation apparently moved the archipelago off the ridge. For the last 26 Ma, the Kerguelen islands have occupied a within-plate setting similar to that of Hawaii. The islands have been built up by successive magmatic eruptions showing either tholeiitic-transitional or either alkaline affinities and related to the Kerguelen mantle plume.

Mantle peridotite xenoliths are abundant in the most alkaline lavas from the Kerguelen islands. They are mainly clinopyroxene (cpx)-spinel-bearing harzburgites and spinel- and cpx -bearing dunites with minor cpx -poor lherzolites.

The spinel harzburgites can be subdivided into protogranular Cr-diopside-bearing harzburgites and poikilitic magnesian-augite-bearing harzburgites. The magnesian-augite harzburgites are higher in Cr2O3, Na2O, Al2O3 and TiO2 and incompatible trace elements than the Cr-diopside. Both types of harzburgites and the cpx-poor lherzolites bear the imprint of two main mantle processes: (i) a high-degree (15-25 %) partial melting event inferred to be related to stabilization of the oceanic lithosphere in the South East Indian Ridge, (ii) reaction of the resultant harzburgitic residues with highly alkaline magmas (alkaline lamprophyres) related to the activity of the Kerguelen mantle plume (Grégoire et al., 1997 and 1999).

Dunites always contain clinopyroxene, spinel and phlogopite with rare amphibole. They are interpreted as reaction products of original harzburgites with tholeiitic-transitional and alkaline melts (Grégoire et al., 1997). They have been latter percolated by highly alkaline (alkaline lamprophyres) or CO2-rich silicate (carbonatitic) melts (Grégoire et al., 1999).

This study provides a new consistent dataset of measured two-mineral partition coefficients and calculated mineral/melt partition coefficients for a significant number of trace elements. We show that trace elements can be accounted for in the constituent minerals for most of the samples. Clinopyroxene is the major host for many trace elements; spinel is important for V and Ti; orthopyroxene for Ti, Zr, HREE, Y, Sc and V; and olivine hosts significant Sc.

Grégoire M., Lorand J.P., Cottin J.Y., Giret A., Mattielli N., Weis D. 1997. Eur. J. Mineral. 9, 1085-1100.
Grégoire M., Moine B.N., Cottin J.Y., Giret A 1999 submitted to J. Petrol.