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Mookherjee M.,Cornell University | Bezacier L.,CNRS Nantes Laboratory of Planetology and Geodynamics
Physics of the Earth and Planetary Interiors | Year: 2012

We investigated the structure, equation of state, and elasticity of glaucophane [Na 2Mg 3Al 2Si 8O 22(OH) 2], up to 9GPa, which encompasses its experimentally observed stability field. We find that the pressure-volume results for glaucophane are well represented by a third order Birch-Murnaghan formulation, with K 0=81GPa, K0'=4.5 and V 0=899.4å 3. The full elastic constant tensor reveals significantly larger stiffness along the (100) plane. The [100] direction is the relatively softer. This could be rationalized in terms of the stacking of the stiffer tetrahedral units along [010] and [001] directions within the crystal structure. Glaucophane is a dominant mineral constituent of blueschist facies rock, and has significantly lower velocities compared to garnet bearing eclogites. In addition, glaucophane is anisotropic and could account for the observed low velocity layer in the subducting slabs at depth range within the thermodynamic stability of glaucophane. At high-pressures, beyond stability of glaucophane, hydrous phase such as lawsonite could account for the observed low velocity layers in certain subduction zones. © 2012 Elsevier B.V..

Mege D.,CNRS Nantes Laboratory of Planetology and Geodynamics | Mege D.,Polish Academy of Sciences | Bourgeois O.,CNRS Nantes Laboratory of Planetology and Geodynamics
Earth and Planetary Science Letters | Year: 2011

Martian global climate models that account for evidence of past glaciations reported in tropical to mid-latitude regions of Mars predict accumulation of water ice in Valles Marineris during past periods of high obliquity. Observational evidence for such glaciations is given here. Topographic basement ridges of tectonic origin are common in Valles Marineris, and display sackung features, an assemblage of tectonic patterns that are diagnostic of deep-seated gravitational slope deformation. This deformation is most easily explained by paraglacial ridge failure subsequent to ridge wall debuttressing and decohesion following the retreat of glaciers. This interpretation is supported by extensive bibliographic analysis of sackung triggers on Earth, by morphological evidence of subglacial erosion of the lower parts of Valles Marineris wallslopes, of periglacial erosion of their upper parts and by the presence of various types of glacial landforms on the floors of Valles Marineris troughs. The age of these equatorial glaciations is found to be older than 1.4. Gy and younger than 3.5. Gy. © 2011 Elsevier B.V.

Finlay C.C.,ETH Zurich | Amit H.,CNRS Nantes Laboratory of Planetology and Geodynamics
Geophysical Journal International | Year: 2011

We present a method to estimate the typical magnitude of flow close to Earth's core surface based on observational knowledge of the geomagnetic main field (MF) and its secular variation (SV), together with prior information concerning field-flow alignment gleaned from numerical dynamo models. An expression linking the core surface flow magnitude to spherical harmonic spectra of the MF and SV is derived from the magnetic induction equation. This involves the angle γ between the flow and the horizontal gradient of the radial field. We study γ in a suite of numerical dynamo models and discuss the physical mechanisms that control it. Horizontal flow is observed to approximately follow contours of the radial field close to high-latitude flux bundles, while more efficient induction occurs at lower latitudes where predominantly zonal flows are often perpendicular to contours of the radial field. We show that the amount of field-flow alignment depends primarily on a magnetic modified Rayleigh number Raη=αg0ΔTD/ηΩ, which measures the vigour of convective driving relative to the strength of magnetic dissipation. Synthetic tests of the flow magnitude estimation scheme are encouraging, with results differing from true values by less than 8 per cent. Application to a high-quality geomagnetic field model based on satellite observations (the xCHAOS model in epoch 2004.0) leads to a flow magnitude estimate of 11-14 kmyr-1, in accordance with previous estimates. When applied to the historical geomagnetic field model gufm1 for the interval 1840.0-1990.0, the method predicts temporal variations in flow magnitude similar to those found in earlier studies. The calculations rely primarily on knowledge of the MF and SV spectra; by extrapolating these beyond observed scales the influence of small scales on flow magnitude estimates is assessed. Exploring three possible spectral extrapolations we find that the magnitude of the core surface flow, including small scales, is likely less than 50 kmyr-1. © 2011 The Authors. Geophysical Journal International © 2011 RAS.

Konig S.,University of Bonn | Lorand J.-P.,CNRS Nantes Laboratory of Planetology and Geodynamics | Luguet A.,University of Bonn | Graham Pearson D.,University of Alberta
Earth and Planetary Science Letters | Year: 2014

The chalcophile and highly siderophile elements Se and Te, like the other Highly Siderophile Elements (HSE) in the terrestrial mantle, may constitute powerful key tracers for meteoritic materials that hit the Earth in its latest accretionary stages ("Late Veneer"). Here the Se and Te systematics of mantle-derived peridotites (orogenic peridotites, ophiolites, cratonic peridotite xenoliths) are assessed. Combined with published in-situ analyses of HSE host minerals, whole-rock data are modelled with respect to current petrogenetic models that affect mantle composition, for example partial melting and magmatic refertilisation. We demonstrate that the near-chondritic Se/Te signature (SeN/TeN ≈ 9 ± 4; N = CI-chondrite normalised) of "fertile" ophiolitic and orogenic lherzolites cannot be a primitive signature of the Earth's mantle. This signature can however be explained by simple refertilisation models. The HSE-Se-Te budget of these fertile rocks can be modelled by mixing various proportions of a residual assemblage of Fe-Ni monosulphide solid solutions (Mss) and/or refractory platinum group minerals (PGMs - Ru-Os-Ir sulphides + Pt-Ir-Os alloys) with a metasomatic assemblage comprising low-temperature Pt-Pd-Te phases and Cu-Ni-rich sulphides. On the other hand, the reported Se and Te ratios in fertile peridotites are not consistent with melt depletion alone. Additions of late-stage metasomatic S-Se-Te-HSE-rich phases render Primitive Upper Mantle (PUM) estimates for Se and Te highly debatable, especially without appropriate consideration of refertilisation and metasomatism. Our results indicate that there is currently no firm evidence for chondritic S-Se-Te signatures in the Primitive Upper Mantle. This conclusion challenges the simplistic perception that near-chondritic Se/Te ratios may readily trace the Late Veneer composition. © 2013 Elsevier B.V.

Husson L.,CNRS Geosciences Laboratory of Rennes | Husson L.,CNRS Nantes Laboratory of Planetology and Geodynamics
Physics of the Earth and Planetary Interiors | Year: 2012

Trench motion and upper plate deformation ultimately respond to mantle flow. Herein I build upon the mantle flow model results of Conrad and Behn (2010) and compute the drag forces underneath all plates, and show that they control the dynamics of plates and plate boundaries. The small misfit angle between between the traction azimuths of mantle traction and absolute plate motion corroborates the idea that convective mantle drag is a prominent driver of plate tectonics. Less intuitive is the fact that the interplay between the drag forces from the upper and lower plates, that amounts to -5 to 8.5×1012Nm-1 (per unit trench length), dictates both trench migration rates and upper plate deformation. At odds with the classic view that assigns the prime role to the idiosyncrasies of subduction zones (slab age, interplate friction, water content etc), I find that the intrinsic properties of subduction zones in fact only modulate this behavior. More specifically, the mean value of the integrated trenchward mantle drag force from the lower and upper plates (from -2 to 6.5×1012Nm-1) controls upper plate deformation. Conversely, it is the difference between the lower and upper plates mantle drag forces (from -3 to 10×1012Nm-1) that controls trench migration rates. In addition, I find that a minimum trenchward force of ∼2.5×1012Nm-1 must be supplied by mantle drag before trenches can actually advance, and before upper plates undergo compression. This force results from the default tendency of slabs to rollback when solely excited by their own buoyancy, and is thus the effective tensional force that slab pull exerts on the plate interface. © 2012 Elsevier B.V.

Lorand J.-P.,CNRS Nantes Laboratory of Planetology and Geodynamics | Luguet A.,University of Bonn | Alard O.,Montpellier University
Lithos | Year: 2013

The platinum-group element (PGE) systematics of continental mantle peridotites show large variability, reflecting petrogenetic processing of the upper mantle during partial melting and melt/fluid percolation inside the lithosphere. By removing Pd-Cu-Ni rich sulfides, partial melting events that have stabilized the sub-continental mantle lithosphere fractionated PPGEs (Palladium-group PGE; Pt, Pd) relative to IPGEs (Iridium-group PGE; Os, Ir, Ru, Rh). Residual base-metal sulfides (BMS) survive as enclosed IPGE-enriched Monosulfide Solid Solutions (Mss), which otherwise decompose into Ru-Os-Ir-rich refractory platinum-group minerals (PGMs) once the partial melts become S-undersaturated. The small-scale heterogeneous distribution of these microphases may cause extreme nugget effects, as seen in the huge variations in absolute PGE concentrations documented in cratonic peridotites. Magmas fluxing through the lithospheric mantle may change the initial PGE budgets inherited from the melting events, resulting in the great diversity of PGE systematics seen in peridotites from the sub-continental lithosphere. For instance, melt-rock reactions at increasing melt/rock ratios operate as open-system melting processes removing residual BMS/PGMs. Highly percolated peridotites are characterized by extreme PGE depletion, coupled with PGE patterns and Os-isotope compositions that gradually evolve toward that of the percolating melt. Reactions at decreasing melt-rock ratios (usually referred to as «mantle metasomatism») precipitate PPGE-enriched BMS that yield suprachondritic Pd/Ir and occasionally affect Pt/Ir and Rh/Ir ratios as well. Moreover, volatile-rich, small volume melts fractionate Os relative to Ir and S relative to Se, thereby producing rocks with supra-chondritic Os/Ir and S/Se coupled with supra-chondritic Pd/Ir and Pt/Ir. Major magmatic inputs at the lithosphere-asthenosphere boundary may rejuvenate the PGE systematics of the depleted mantle. Integrated studies of «refertilized» peridotites with worldwide provenance provide evidence for mixing between old PGM-rich harzburgitic protoliths and newly-precipitated BMS. Long-lived PGMs carry the Os-isotope compositions of ancient melt-depletion events into seemingly undepleted fertile lherzolites. Another diagnostic feature of major refertilization processes is the increasing modal abundance of Pt-Pd-Te-Bi or Pt-As-S microphases. Due to regional-scale refertilization processes, sizeable (>. 100. km) domains of the upper lithospheric mantle are now significantly enriched in Pd, Au, Cu, Se, and other incompatible chalcophile elements that are of considerable importance in PGE-ore forming events. © 2012 Elsevier B.V.

Amit H.,CNRS Nantes Laboratory of Planetology and Geodynamics | Pais M.A.,University of Coimbra
Geophysical Journal International | Year: 2013

Core flows inverted from time-dependent geomagnetic field models image the geodynamo at the top of its generation region, the Earth's outer core. Physical assumptions incorporated in these inversions affect the resulting flows. Based on rapid rotation dominance, two assumptions similar in form yet different in essence have been proposed: tangential geostrophy (TG, LeMouël 1984) and columnar flow (CF, Amit&Olson 2004).We recall that CF is theoretically consistent with the quasi-geostrophy (QG) theory for an incompressible fluid with spherical solid boundaries whereas TG is not. As such, we highlight the importance of applying the CF assumptionwhen inverting geomagnetic data for interior core (columnar) flows that can be used in kinematic dynamo and thermal convection models in the Boussinesq approximation. Next we evaluate the non-uniqueness associated with CF flows. The areas of ambiguous patches at the core surface where invisible TG or CF flows reside are roughly comparable. The spatial distribution of ambiguous patches for both TG and CF is quite asymmetric about the equator, so assuming equatorial symmetry is expected to reduce the non-uniqueness significantly. In fact, for assumed equatorial symmetry, the only possible non-unique flows will be those along hypothetical -contours in the opposite hemispheres that their equatorial plane projections are parallel. TG flows exhibit a strong Atlantic/Pacific hemispheric dichotomy and a well-defined eccentric gyre whereas in CF flows the dichotomy between these two hemispheres is weaker and the gyre is less clear suggesting that the eccentric gyre might not conserve mass. Both TG and CF upwelling/downwelling patterns are strongly localized in the equatorial region. In addition, in both cases upwelling/downwelling is correlated with equatorward/poleward flow respectively, as expected for QG convection. CF upwelling is more intense than TG upwelling but the magnitude ratio is smaller than the factor 2 distinguishing the analytical expressions of the two assumptions. This smaller magnitude ratio is due to the fact that presently observed geomagnetic secular variation features are mostly explained by magnetic field advection by toroidal core flow in the frozen-flux approximation. Robust upwelling features below India/Indonesia may be viewed as geomagnetic evidence for whole core convection. © The Authors 2013. Published by Oxford University Press on behalf of The Royal Astronomical Society.

Capdeville Y.,CNRS Nantes Laboratory of Planetology and Geodynamics | Marigo J.-J.,Ecole Polytechnique - Palaiseau
Geophysical Journal International | Year: 2013

We propose a two scale asymptotic method to compute the effective effect of a free surface topography varying much faster than the minimum wavelength for 2-D P-SV elastic wave propagation. The topography variation is assumed to be non-periodic but with a deterministic description and, in this paper, the elastic body below the topography is assumed to be homogeneous. Two asymptotic expansions are used, one in the boundary layer close to the free surface and one in the volume. Both expansions are matched appropriately up to the order 1 to provide an effective topography and an effective boundary condition. We show that the effective topography is not the averaged topography but it is a smooth free surface lying below the fast variations of the real topography. Moreover, the free boundary condition has to be modified to take account of the inertial effects of the fast variations of the topography above the effective topography. In other words, the wave is not propagating in the fast topography but below it and is slowed down by the weight of the fast topography. We present an iterative scheme allowing to find this effective topography for a given minimum wavelength.We do not attempt any mathematical proof of the proposed scheme, nevertheless, numerical tests show good results. © The Authors 2012.

Olson P.,Johns Hopkins University | Amit H.,CNRS Nantes Laboratory of Planetology and Geodynamics
Physics of the Earth and Planetary Interiors | Year: 2014

Scaling relationships are derived for the frequency of magnetic polarity reversals in numerical dynamos powered by thermochemical convection. We show that the average number of reversals per unit of time scales with the local Rossby number Roℓ of the convection. With uniform core-mantle boundary (CMB) heat flux, polarity reversals are absent below a critical value Roℓcrit≃0.05, beyond which reversal frequency increases approximately linearly with Roℓ. The relative standard deviation of the dipole intensity fluctuations increases with reversal frequency and Roℓ. With heterogeneous CMB heat flux that models the large-scale seismic heterogeneity in Earth's lower mantle, reversal frequency also exhibits linear dependence on Roℓ, and increases approximately as the square root of the amplitude of the CMB heterogeneity. Applied to the history of the geodynamo, these results imply lower CMB heat flux with Roℓ≤Roℓcrit during magnetic superchrons and higher, more heterogeneous CMB heat flux with Roℓ>Roℓcrit when geomagnetic reversals were frequent. They also suggest that polarity reversals may have been commonplace in the early history of other terrestrial planets. We find that zonal heterogeneity in CMB heat flux produces special effects. Close to Roℓcrit enhanced equatorial cooling at the CMB increases reversal frequency by concentrating magnetic flux at low latitudes, whereas far beyond Roℓcrit enhanced polar cooling at the CMB increases reversal frequency by amplifying outer core convection. © 2014 Elsevier B.V.

Amit H.,CNRS Nantes Laboratory of Planetology and Geodynamics
Physics of the Earth and Planetary Interiors | Year: 2014

It has been argued based on recent seismic and mineral physics studies that the top of Earth's liquid outer core is stably stratified. Here I analyze persistent geomagnetic secular variation features on the core-mantle boundary to examine whether a kinematic signature of core fluid upwelling/downwelling can be detected. I focus on regions of intense high-latitude geomagnetic flux patches that may be maintained by fluid downwelling. In order to identify persistent patterns, the radial field and its secular variation are stacked in the flux patch moving reference frame. These stacked images are compared with forward solutions to the radial induction equation based on idealized field-flow models. Clear advective secular variation signature below North America indicates that these intense flux patches may exhibit significant mobility. Stretching signature in the form of persistent positive secular variation correlated with the intense flux patch below the Southern Indian Ocean may be considered as regional scale geomagnetic evidence for whole core convection, although pure toroidal flow cannot be outruled. © 2014 Elsevier B.V.

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