Oceanic plateaus form by mantle processes distinct from those forming oceanic crust at divergent plate boundaries. Eleven drillsites into igneous basement of Kerguelen Plateau and Broken Ridge, including seven from the recent Ocean Drilling Program Leg 183 (1998^99) and four from Legs 119 and 120 (1987^88), show that the dominant rocks are basalts with geochemical characteristics distinct from those of mid-ocean ridge basalts. Moreover, the physical characteristics of the lava flows and the presence of wood fragments, charcoal, pollen, spores and seeds in the shallow water sediments overlying the igneous basement show that the growth rate of the plateau was sufficient to form subaerial landmasses. Most of the southern Kerguelen Plateau formed at V110 Ma, but the uppermost submarine lavas in the northern Kerguelen Plateau erupted during Cenozoic time. These results are consistent with derivation of the plateau by partial melting of the Kerguelen plume. Leg 183 provided two new major observations about the final growth stages of the Kerguelen Plateau. 1: At several locations, volcanism ended with explosive eruptions of volatilerich, felsic magmas; although the total volume of felsic volcanic rocks is poorly constrained, the explosive nature of the eruptions may have resulted in globally significant effects on climate and atmospheric chemistry during the late-stage, subaerial growth of the Kerguelen Plateau. 2: At one drillsite, clasts of garnet^biotite gneiss, a continental rock, occur in a fluvial conglomerate intercalated within basaltic flows. Previously, geochemical and geophysical evidence has been used to infer continental lithospheric components within this large igneous province. A continental geochemical signature in an oceanic setting may represent deeply recycled crust incorporated into the Kerguelen plume or continental fragments dispersed during initial formation of the Indian Ocean during breakup of Gondwana. The clasts of garnet^biotite gneiss are the first unequivocal evidence of continental crust in this oceanic plateau. We propose that during initial breakup between India and Antarctica, the spreading center jumped northwards transferring slivers of the continental Indian plate to oceanic portions of the Antarctic plate. ß

Mantle plumes are expected to be affected by large‐scale flow in the Earth's mantle related to plate motions, subducted slabs, and possibly large‐scale upwellings. Motion of plume conduits will depend on both large‐scale flow and buoyant rising speed of the conduit through the mantle. Here we present a model of depth‐dependent plume conduit temperature, viscosity, radius, and buoyant rising speed and use it to compute plume and hot spot motion. Results support a temperature anomaly of about 500 K at the plume base. In this case, sublithospheric temperature anomaly is about 150 K for the Iceland plume; transition zone anomaly is between 150 and 200 K for Iceland, about 250 K for Samoa, and about 300 K for Hawaii. Thermal plume radii are about 100 km in the upper mantle, increasing to about 200 km in the lower mantle. Beneath the lithosphere, viscosity in the vicinity of plumes is substantially reduced compared to the underlying mantle, and fast‐moving plates deflect plumes by 200 km or less, corresponding to a plume conduit buoyant rise time of about 3 Myr between 400 and 100 km depth, where most of the shear flow due to plate motions occurs. During 100 Myr, plume conduits rise buoyantly from about 1500–2000 km depth. In many cases, computed hot spot motion agrees with previous computations: south‐southeastward motion of the Hawaiian hot spot during the past 100 Myr, Louisville hot spot motion in a similar direction but at slower speed during the past 50 Myr, and westward motion of the Easter hot spot relative to Hawaii and Louisville. However, the previously computed substantial southward motion of the Kerguelen hot spot is not confirmed. If plume sources in the lowermost mantle are assumed to move with large‐scale flow, they are predicted to be displaced toward the two large‐scale upwellings beneath Africa and the Pacific relative to hot spot surface locations. For fixed sources, predicted tilts of the lower part of conduits tend to be opposite to that.

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The Kerguelen Plateau, a Large Igneous Province in the southern Indian Ocean, was formed as a product of the Kerguelen hotspot in several eruptive phases during the last 120 Myr. We obtained new paleolatitudes for the central and northern Kerguelen Plateau from paleomagnetic investigations on basalts, which were drilled during ODP Leg 183 to the Kerguelen Plateau^Broken Ridge. The paleolatitudes coincide with paleolatitudes from previous investigations at the Kerguelen Plateau and Ninetyeast Ridge (the track of the Kerguelen hotspot) and indicate a difference between paleolatitudes and present position at 49 ‡S of the Kerguelen hotspot. We show that true polar wander, the global motion between the mantle and the rotation axis, cannot explain this difference in latitudes. We present numerical model results of plume conduit motion in a large-scale mantle flow and the resulting surface hotspot motion. A large number of models all predict southward motion between 3 ‡ and 10 ‡ for the Kerguelen hotspot during the last 100 Myr, which is consistent with our paleomagnetic results. ß

[1] The natural remanent magnetization (NRM) of the Martian meteorite ALH84001 is predominantly carried by fine magnetite, which is found in association with carbonate. The magnetite is in epitaxial and topotactic relation with the carbonate and formed from the carbonate in the major impact event at 4.0 Ga. The NRM will therefore record this field. The local preferential crystallographic and shape alignment of the magnetite defines local easy directions of magnetization may account for the observed inhomogeneity of the NRM on a microscopic scale. Normalizing the intensity of the NRM by the saturation isothermal remanence (IRMs) then gives an estimate for the 4.0 Ga Martian field one order smaller than the present geomagnetic field. Such a field is unlikely to be strong enough to generate the high-intensity Martian magnetic anomalies. ALH 84001 in its pristine state as an orthopyroxenite is not a plausible source rock for the Martian anomalies because its magnetite was not formed until the 4.0 Ga event.