GEMOC ARC National Key CentreAnomalous oceanic crust beneath Kerguelen process from petrological and petrophysical evidence: result of a Plume-Ridge interaction
J.Y. Cottin1, M. Grégoire2, B.N. Moine3 , G. Michon1, S. Sheppard4, S.Y. O’Reilly5, A. Giret1
1. Département de Géologie-Pétrologie-Géochimie, Université Jean Monnet, St Etienne, France
2. Department of Geological Sciences, University of Cape Town, South Africa
3. Laboratoire de Géochimie des Systèmes Volcaniques, Université P.&M. Curie, Paris, France
4. Département des Sciences de la Terre, ENS-Lyon, France
5. GEMOC Macquarie
Deep-seated meta-igneous xenoliths sampled by alkali basalts from the Kerguelen Islands reveal that basaltic magmas have intruded the upper mantle beneath the Kerguelen Islands. These record volcanic activity associated with their mid-ocean ridge (SEIR) location and subsequent translation away from the ridge to an intraplate setting over the Kerguelen Plume. The meta-igneous xenoliths sample two distinctive geochemical episodes distinguished by major, trace element and isotopic compositions: one is tholeiitic transitional and one is alkali basaltic. Calculated D-H2O values of fluids in equilibrium with amphibole and mica occurring in some meta-igneous rocks have a bimodal distribution indicating large heterogeneities which could be related to the magmatic history of the Kerguelen oceanic crust and upper mantle. Ultrasonic measurements of compressional wave speed Vp have been carried out, and densities measured, for representative samples of the two types of meta-igneous xenoliths. A harzburgite that represents the Kerguelen upper mantle wall-rock has been incorporated. Vp and density have also been calculated using modal proportions of minerals in xenoliths and appropriate elastic properties of end-members for the constituent minerals. Pyroxene granulites have Vp within the range 7.37—7.52 km/s; calculated velocities for the garnet granulites and pyroxenites range from 7.69 to 7.99 km/s, whereas measured and calculated Vp for a mantle harzburgite are 8.45 and 8.29 km/s, respectively. The seismic structure of the Kerguelen Islands can be explained by (i) a mixture of underplated pyroxene granulites and ultramafic rocks responsible for the low velocity transitional zone below the oceanic layer 3, (ii) varying proportions of granulites and pyroxenites within the upper mantle producing lateral heterogeneities, and (iii) intercalation of granulites and pyroxenites throughout the entire upper mantle column, along with high temperatures, accounting for the relatively low mantle velocities detected seismically (7.70 — 7.95 km/s).