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Abstract: Kevin Burke's original and thought-provoking contributions have been published steadily for the past 60 years, and more than a decade ago he set out to resolve how plate tectonics [...]

Author(s): Clinton P. Conrad (corresponding author) [1]; Bernhard Steinberger [2, 3]; Trond H. Torsvik [3, 4, 5] Viscous convection within the mantle is linked to tectonic plate motions [1, 2, [...]

Diamond distribution Diamond formation occurs in high-pressure conditions more than 150 kilometres deep in the Earth's mantle. The diamonds make it to the surface in vertical pipe-like structures made up of volcanic rocks called kimberlites. Several thousand such kimberlite pipes have been mapped so far, but research has focused on very old cratons, the areas of oldest continental crust, as this is where most economically viable diamonds are found. Trond Torsvik and colleagues use a plate-tectonic reconstruction for the past 540 million years to locate the positions of these cratons relative to the deep mantle at times when kimberlites were erupted. The kimberlites are shown to have been associated with the edges of large-scale heterogeneities in the deepest mantle, which the authors infer were zones at the core-mantle boundary where magma upwelling generated the mantle plumes that led to the formation of the kimberlites. These plumes may have controlled the distribution of almost all kimberlites that have erupted in the past 540 million years. Diamonds are formed under high pressure more than 150 kilometres deep in the Earth's mantle, and are brought to the surface mainly by volcanic rocks called kimberlites. Here, plate reconstructions and tomographic images have been used to show that the edges of the largest heterogeneities in the deepest mantle, stable for at least 200 million years and possibly for 540 million years, seem to have controlled the eruption of most Phanerozoic kimberlites. This has implications for future exploration for kimberlites. Diamonds are formed under high pressure more than 150 kilometres deep in the Earth's mantle and are brought to the surface mainly by volcanic rocks called kimberlites. Several thousand kimberlites have been mapped on various scales.sup.1,2,3,4, but it is the distribution of kimberlites in the very old cratons (stable areas of the continental lithosphere that are more than 2.5 billion years old and 300 kilometres thick or more.sup.5) that have generated the most interest, because kimberlites from those areas are the major carriers of economically viable diamond resources. Kimberlites, which are themselves derived from depths of more than 150 kilometres, provide invaluable information on the composition of the deep subcontinental mantle lithosphere, and on melting and metasomatic processes at or near the interface with the underlying flowing mantle. Here we use plate reconstructions and tomographic images to show that the edges of the largest heterogeneities in the deepest mantle, stable for at least 200 million years and possibly for 540 million years, seem to have controlled the eruption of most Phanerozoic kimberlites. We infer that future exploration for kimberlites and their included diamonds should therefore be concentrated in continents with old cratons that once overlay these plume-generation zones at the core-mantle boundary., Author(s): Trond H. Torsvik [sup.1] [sup.2] [sup.3] , Kevin Burke [sup.3] [sup.4] , Bernhard Steinberger [sup.1] [sup.2] [sup.5] , Susan J. Webb [sup.3] , Lewis D. Ashwal [sup.3] Author Affiliations: [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.12.025 Byline: Bernhard Steinberger (a)(b)(c), Stephanie C. Werner (c), Trond H. Torsvik (a)(c)(d) Keywords: Earth; Venus, Interior; Mars, Interior; Volcanism Abstract: The relation between gravity anomalies, topography and volcanism can yield important insights about the internal dynamics of planets. From the power spectra of gravity and topography on Earth, Venus and Mars we infer that gravity anomalies have likely predominantly sources below the lithosphere up to about spherical harmonic degree l=30 for Earth, 40 for Venus and 5 for Mars. To interpret the low-degree part of the gravity spectrum in terms of possible sublithospheric density anomalies we derive radial mantle viscosity profiles consistent with mineral physics. For these viscosity profiles we then compute gravity and topography kernels, which indicate how much gravity anomaly and how much topography is caused by a density anomaly at a given depth. With these kernels, we firstly compute an expected gravity-topography ratio. Good agreement with the observed ratio indicates that for Venus, in contrast to Earth and Mars, long-wavelength topography is largely dynamically supported from the sublithospheric mantle. Secondly, we combine an empirical power spectrum of density anomalies inferred from seismic tomography in Earth's mantle with gravity kernels to model the gravity power spectrum. We find a good match between modeled and observed gravity power spectrum for all three planets, except for 2a[c]1/2la[c]1/24 on Venus. Density anomalies in the Venusian mantle for these low degrees thus appear to be very small. We combine gravity kernels and the gravity field to derive radially averaged density anomaly models for the Martian and Venusian mantles. Gravity kernels for l[greater than or equal to]5 are very small on Venus below [approximately equal to]800km depth. Thus our inferences on Venusian mantle density are basically restricted to the upper 800km. On Mars, gravity anomalies for 2a[c]1/2la[c]1/25 may originate from density anomalies anywhere within its mantle. For Mars as for Earth, inferred density anomalies are dominated by l=2 structure, but we cannot infer whether there are features in the lowermost mantle of Mars that correspond to Earth's Large Low Shear Velocity Provinces (LLSVPs). We find that volcanism on Mars tends to occur primarily in regions above inferred low mantle density, but our model cannot distinguish whether or not there is a Martian analog for the finding that Earth's Large Igneous Provinces mainly originate above the margins of LLSVPs. Author Affiliation: (a) Center for Geodynamics, Geological Survey of Norway, NO-7491 Trondheim, Norway (b) Helmholtz Centre Potsdam, German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany (c) Physics of Geological Processes, Univ. of Oslo, NO-0316 Oslo, Norway (d) School of Geosciences, Univ. of the Witwatersrand, Wits 2050, South Africa Article History: Received 5 February 2009; Revised 16 October 2009; Accepted 22 December 2009

The motion of continents relative to the Earth's spin axis may be due either to rotation of the entire Earth relative to its spin axis--true polar wander (1,2)--or to the [...]

We use plate reconstructions to show that parts of the Hawaiian hotspot track of ca. 80-90 Ma age could be preserved in the Bering Sea. Based on these reconstructions, the Hawaiian hotspot was beneath the Izanagi plate before ca. 83 Ma. Around that time, the part of the plate carrying the hotspot track was transferred to the Kula plate. After 75-80 Ma the Hawaiian hotspot underlay the Pacific plate. Circa 40-55 Ma, subduction initiated in the Aleutian Trench. Part of the Kula plate was attached to the North American plate and is preserved as the oceanic part of the Bering Sea. We show that for a number of different plate reconstructions and a variety of assumptions covering hotspot motion, part of the hotspot track should be preserved in the Bering Sea. The predicted age of the track depends on the age of Aleutian subduction initiation. We speculate that Bowers and Shirshov Ridges were formed by paleo-Hawaiian hotspot magmatism. Keywords: Hawaii, hotspots, plate motion, Kula plate, Bering Sea, Bowers Ridge.