METSTOR: A GIS to look for potential CO2 storage zones in France

AbstractThe METSTOR project offers a methodology to look for potentially interesting CO2 storage areas in France at the initial stage, before the “site selection” step. Our tool, embodied in a Geographic Information System, is based on an interactive map of CO2 storage capacities. Other relevant information layers are included. The geographic layers are complemented with a series of online technical notices. It seems to be the first open online GIS that offers policy makers, businesses and the public at large an integrated access to that necessary information. Our prototype, limited mainly to the Paris Basin, is released online at www.metstor.fr

Structure and density of Fe-C liquid alloys under high pressure

International audienceThe density and structure of liquid Fe-C alloys have been measured up to 58 GPa and 3,200 K by in situ X-ray diffraction using a Paris-Edinburgh press and laser-heated diamond anvil cell. Study of the pressure evolution of the local structure inferred by X-ray diffraction measurements is important to understand the compression mechanism of the liquid. Obtained data show that the degree of compression is greater for the first coordination sphere than the second and third coordination spheres. The extrapolation of the measured density suggests that carbon cannot be the only light element alloyed to iron in the Earth's core, as 8-16 at % C (1.8-3.7 wt % C) would be necessary to explain the density deficit of the outer core relative to pure Fe. This concentration is too high to account for outer core velocity. The presence of other light elements (e.g., O, Si, S, and H) is thus required

Seismic evidence for large-scale compositional heterogeneity of oceanic core complexes

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q08002, doi:10.1029/2008GC002009.Long-lived detachment faults at mid-ocean ridges exhume deep-seated rocks to form oceanic core complexes (OCCs). Using large-offset (6 km) multichannel seismic data, we have derived two-dimensional seismic tomography models for three of the best developed OCCs on the Mid-Atlantic Ridge. Our results show that large lateral variations in P wave velocity occur within the upper ~0.5–1.7 km of the lithosphere. We observe good correlations between velocity structure and lithology as documented by in situ geological samples and seafloor morphology, and we use these correlations to show that gabbros are heterogeneously distributed as large (tens to >100 km2) bodies within serpentinized peridotites. Neither the gabbros nor the serpentinites show any systematic distribution with respect to along-isochron position within the enclosing spreading segment, indicating that melt extraction from the mantle is not necessarily focused at segment centers, as has been commonly inferred. In the spreading direction, gabbros are consistently present toward the terminations of the detachment faults. This suggests enhanced magmatism during the late stage of OCC formation due either to natural variability in the magmatic cycle or to decompression melting during footwall exhumation. Heat introduced into the rift valley by flow and crystallization of this melt could weaken the axial lithosphere and result in formation of new faults, and it therefore may explain eventual abandonment of detachments that form OCCs. Detailed seismic studies of the kind described here, when constrained by seafloor morphology and geological samples, can distinguish between major lithological units such as volcanics, gabbros, and serpentinized peridotites at lateral scales of a few kilometers. Thus such studies have tremendous potential to elucidate the internal structure of the shallow lithosphere and to help us understand the tectonic and magmatic processes by which they were emplaced.This research was supported by grants from the U.S. NSF-IODP Program

Response of a multi-domain continental margin to compression: study from seismic reflection-refraction and numerical modelling in the Tagus Abyssal Plain

The effects of the Miocene through Present compression in the Tagus Abyssal Plain are mapped using the most up to date available to scientific community multi-channel seismic reflection and refraction data. Correlation of the rift basin fault pattern with the deep crustal structure is presented along seismic line IAM-5. Four structural domains were recognized. In the oceanic realm mild deformation concentrates in Domain I adjacent to the Tore-Madeira Rise. Domain 2 is characterized by the absence of shortening structures, except near the ocean-continent transition (OCT), implying that Miocene deformation did not propagate into the Abyssal Plain, In Domain 3 we distinguish three sub-domains: Sub-domain 3A which coincides with the OCT, Sub-domain 3B which is a highly deformed adjacent continental segment, and Sub-domain 3C. The Miocene tectonic inversion is mainly accommodated in Domain 3 by oceanwards directed thrusting at the ocean-continent transition and continentwards on the continental slope. Domain 4 corresponds to the non-rifted continental margin where only minor extensional and shortening deformation structures are observed. Finite element numerical models address the response of the various domains to the Miocene compression, emphasizing the long-wavelength differential vertical movements and the role of possible rheologic contrasts. The concentration of the Miocene deformation in the transitional zone (TC), which is the addition of Sub-domain 3A and part of 3B, is a result of two main factors: (1) focusing of compression in an already stressed region due to plate curvature and sediment loading; and (2) theological weakening. We estimate that the frictional strength in the TC is reduced in 30% relative to the surrounding regions. A model of compressive deformation propagation by means of horizontal impingement of the middle continental crust rift wedge and horizontal shearing on serpentinized mantle in the oceanic realm is presented. This model is consistent with both the geological interpretation of seismic data and the results of numerical modelling. (C) 2008 Elsevier B.V. All rights reserved.Instituto Nacional de Engenharia, Tecnologia e Inovacao(INETI); Landmark Graphics Corporation; Landmark University Grant Program; LATTEX/IDL [ISLF-5-32]; FEDERinfo:eu-repo/semantics/publishedVersio

High-pressure behaviour of serpentine minerals: a Raman spectroscopic study

International audienc

Melting of Peridotite to 140 Gigapascals

International audienceInterrogating physical processes that occur within the lowermost mantle is a key to understanding Earth's evolution and present-day inner composition. Among such processes, partial melting has been proposed to explain mantle regions with ultralow seismic velocities near the core-mantle boundary, but experimental validation at the appropriate temperature and pressure regimes remains challenging. Using laser-heated diamond anvil cells, we constructed the solidus curve of a natural fertile peridotite between 36 and 140 gigapascals. Melting at core-mantle boundary pressures occurs at 4180 T 150 kelvin, which is a value that matches estimated mantle geotherms. Molten regions may therefore exist at the base of the present-day mantle. Melting phase relations and element partitioning data also show that these liquids could host many incompatible elements at the base of the mantle

An alternative model for the formation of hydrous Mg/Ni layer silicates ("deweylite"/"garnierite") in faulted peridotites of New Caledonia : II. Petrography and chemistry of white and green clay infillings

This study, which complements a first mineralogical work, presents detailed petrographic and chemical data on the sequences of clay infillings commonly found in serpentine veins of reactivated faults from the New Caledonian peridotite formation. Chemical trends and transfers established from the outer serpentine fringe to the inner clay infilling, as well as from the white (deweylite) to the greenish (garnierite) parts of the veins, enable us to decipher the processes and conditions involved in the redistribution of Mg and Ni along reactivated faults. As commonly reported from studies of peridotite formations worldwide, two main chemical trends are distinguished. In New Caledonia, these trends belong to distinct periods of tectonic activities associated with the dislocation and early alteration of the ophiolite nappe. They result from two kinds of Ni-ore-forming processes and reveal a significant decrease in the mobility of Ni over time. The first process relates the step-by-step alteration of serpentine species into talclike (TL) minerals to the sequential leaching of Mg and Fe, together with their local replacement by Ni in octahedral sites of the newly formed TL minerals. The TL minerals are thus considered as the main Ni-bearing phases (pimelite) of the ore. The large-scale redistribution of Mg and Ni during a first period of tectonic activity and alteration leads to the differentiation of white (Ni-free) and olive-green (Ni-rich) clay infillings along the reactivated faults. The second process belongs to a second period of tectonic activity and alteration where most of the serpentine species have been converted into TL minerals without major Ni enrichment (formation of a second sequence of milky white and turquoise clay infillings). Redistribution of Mg and Ni occurs over shorter distances from the transition of Mg-rich to Ni-rich clay infillings (or breccias). It results from oscillatory phenomena and self-organised precipitation processes leading to cyclic and inverse Mg and Ni variations in alternating bands of contrasted optical anisotropy. The early redistribution of Ni and the significant decrease in the mobility of this element from the first to the second period of tectonic activity provide better constraints on an alternative model for the genesis of the Ni-silicate ore

Modern and past volcanic degassing of iodine

International audienceWe have monitored iodine degassing from a melt to a water vapor during decompression (i.e. magma ascent). Experimentshave been performed by combining diamond anvil cells experiments with synchrotron X-rays fluorescence analysis. Partitioncoefficients DIfluid/melt measured for a pressure and temperature range of 0.1–1.8 GPa and 500–900 C, range from 41 to 1.92,values for room conditions DIfluid/glass (quenched samples) are equal to or higher than 350. We show that iodine degassing withwater is earlier and much more efficient than for lighter halogen elements, Cl and Br. Iodine is totally degassed from the silicatemelt at room conditions. By applying these results to modern volcanology, we calculate an annual iodine flux for subductionrelated volcanism of 0.16–2.4 kt yr1. We suggest that the natural iodine degassing may be underestimated, havingpossible consequences on the Earth’s ozone destruction cycle. By applying this results to the Early Earth, we propose a processthat may explain the contrasted signature of I, Br and Cl, strongly depleted in the bulk silicate Earth, the most depletedbeing iodine, whereas fluorine is almost enriched. The Earth may have lost heavy halogen elements during an early waterdegassing process from the magma ocean

New host for carbon in the deep Earth

The global geochemical carbon cycle involves exchanges between the Earth’s interior and the surface. Carbon is recycled into the mantle via subduction mainly as carbonates and is released to the atmosphere via volcanism mostly as CO2. The stability of carbonates versus decarbonation and melting is therefore of great interest for understanding the global carbon cycle. For all these reasons, the thermodynamic properties and phase diagrams of these minerals are needed up to core mantle boundary conditions. However, the nature of C-bearing minerals at these conditions remains unclear. Here we show the existence of a new Mg-Fe carbon-bearing compound at depths greater than 1,800 km. Its structure, based on three-membered rings of corner-sharing (CO4)4- tetrahedra, is in close agreement with predictions by first principles quantum calculations [Oganov AR, et al. (2008) Novel high-pressure structures of MgCO3, CaCO3 and CO2 and their role in Earth’s lower mantle. Earth Planet Sci Lett 273:38–47]. This high-pressure polymorph of carbonates concentrates a large amount of Fe(III) as a result of intracrystalline reaction between Fe(II) and (CO3)2- groups schematically written as 4FeO + CO2 → 2Fe2O3 + C. This results in an assemblage of the new high-pressure phase, magnetite and nanodiamonds