The lithospheric mantle beneath the Kerguelen Islands (Indian Ocean): petrological and petrophysical characteristics of mantle mafic rock types and correlation with seismic profiles

Deep-seated meta-igneous xenoliths brought to the surface by alkali basalti c magmas from the Kerguelen Islands reveal that basaltic magmas have intrud ed the upper mantle throughout their geological evolution. These xenoliths record volcanic activity associated with their early South East Indian Ridg e location and subsequent translation to an intraplate setting over the Ker guelen Plume. The meta-igneous xenoliths sample two distinctive geochemical episodes: one is tholeiitic transitional and one is alkali basaltic. Geoth ermobarometry calculations provide a spatial context for the rock type sequ ence sampled and for interpreting petrophysical data. The garnet granulites equilibrated over a pressure range of 1.15 to 1.35 GPa and the garnet pyro xenite at 1.8 GPa. Ultrasonic measurements of compressional wave speed V-P have been carried out at pressures up to I GPa, and densities measured for representative samples of meta-igneous xenoliths and for a harzburgite that represents the peridotitic mantle. V-P and density have also been calculat ed using modal proportions of minerals and appropriate elastic properties f or the constituent minerals. Calculated and measured V-P agree well for roc k types with microstructures not complicated by kelyphitic breakdown of gar net and/or pervasive grain-boundary cracking. Pyroxene granulites have meas ured and calculated V-P within the range 7.37-7.52 km/s; calculated velocit ies for the garnet granulites and pyroxenites range from 7.69 to 7.99 km/s. whereas measured and calculated V-P for a mantle harzburgite Lire 8.45 and 8.29 km/s respectively. The Seismic Structure observed beneath the Kerguel en Islands can be explained by (1) a mixture or underplated pyroxene granul ites and ultramafic rocks responsible for the 2-3 km low velocity transitio nal zone below the oceanic layer 3, (2) varying proportions of granulites a nd pyroxenites in different regions within the upper mantle producing the l ateral heterogeneities, and (3) intercalation of the granulites and pyroxen ites throughout the entire upper Mantle Column, along with elevated tempera tures, accounting for the relatively low mantle velocities (7.70-7.95 km/s) .