Inventory of the Economic Zones of the French Territories in the Pacific The ZoNéCo and ZEPOLYF Programmes

Until recent years, no systematic campaign of charting or evaluation of potential had been organized in the immense maritime economic zones of the French territories in the Pacific. The ZoNéCo Programme, for the economic zone of New Caledonia, and the ZEPOLYF Programme, for that of French Polynesia, have instituted a systematic process of drawing up an inventory or assessment of these regions, the method and the first results of which are described here

The Monviso ophiolitic massif (Western Alps), a section through a serpentinite subduction channel.

The exhumation of subducted lithosphere requires a mechanically weak zone at the interface between the subduction plane and the rigid overlying mantle peridotites with a viscosity greater than 10 20 Pa.s. At shallow depths (< 40-50 km) blueschists are exhumed in accretionary wedge along the interface between the subducting plate and the overriding plate (Platt, 1993). At greater depth, serpentinites plays the role of mechanically weak layer in cool continental subduction and act as the lubricant and produce a return flow for the exhumation of eclogitic rocks. The close association of serpentinites and eclogites in the Monviso massif (Western Alps) allow to discuss the concept of subduction serpentinite channel. We propose that the Monviso ophiolitic massif corresponds to a section of a 50 km long serpentinite channel in where eclogitic blocks were exhumed between 60 and 45 Ma and ended whEuropean continental margin was involved in the southeast dipping subduction zone

Tectonique intraocéanique décrochante à l'ouest des îles Fidji (bassin nord-fidjien) : campagne SEAPSO III du N.O. Jean-Charcot

Un levé bathymétrique au moyen du sondeur multifaisceaux (Seabeam) et un levé de sismique réflexion continue du secteur oriental du Bassin Nord-Fidjien ont été réalisés lors du leg III de la campagne SEAPSO. Une exploitation préliminaire des données montre que ce secteur, précédemment interprété comme axe d'accrétion, est en réalité une zone de déformation intraocéanique décrochante. (Résumé d'auteur

Micro-scale investigation of carbonation process in partially serpentinized peridotites

The carbonation of ultramafic rocks is, theoretically, the most efficient reaction to trap CO2 irreversibly in the form of solid carbonates, as predicted by equilibrium thermodynamic calculations. However, the success of industrial or natural carbonation in large ultramafic aquifers or oceanic ultramafic exposures does not only rely on the thermodynamic conditions of chemical reactions, but also on their feedback effects on the reactive surface area and on the local porosity and permeability. In addition, side processes like serpentinization, redox reactions, abiotic catalytic effects, and biological activity, can be expected in such complex natural system. Their occurrence and implications on carbon speciation and carbon transfers during hydrothermal alteration of oceanic peridotites have not been explored yet and requires detailed study of natural and/or experimental carbonation zones. We have combined petrographic and electron microscopy with SIMS, Raman and FTIR microspectroscopy on partially serpentinized peridotites drilled during the IODP leg 304 (30 N, MAR) in order to characterize the mechanisms of peridotite carbonation at the fluid-mineral interface and identify the associated speciation of carbon (inorganic and organic carbon occurrences). We present first results on zones located close to talc-tremolite sheared veins in holes 1309B and D. Associations of carbonates, porous phyllosilicates and oxides are observed in close vicinity of relict olivines that underwent a previous stage of serpentinization. The olivine-carbonate interface is nanoporous which facilitates mass transfer between fluid and mineral. The phyllosilicate identified as saponite results from the metasomatic replacement, during the carbonation stage, of previously formed serpentine. These observations do not favour reaction-induced cracking but rather a transfer-controlled process in an open system. Among the submicrometric dark clusters widely-distributed in saponite and in serpentine, vibrational microspectroscopy reveals the presence of various types of organic compounds that tend to be located close to micrometric sulphides grains. Those results underline the microscale variability of carbon speciation within hydrothermally altered peridotites. The association of reduced carbon phases with the carbonation texture suggests that CO2 conversion may not be limited to solid carbonate formation in natural systems and that biological activity and/or abiotic CO2 reduction, possibly catalyzed by Ni-rich sulphides, can occur contemporaneously. This complex association of reactions has to be unravelled further to determine the respective contribution of abiotic versus biological processes and integrate them in carbon transfers modelling through the oceanic lithosphere

Experimental investigation of the stability of Fe-rich carbonates in the lower mantle

International audienceThe fate of carbonates in the Earth's mantle plays a key role in the geodynamical carbon cycle. Although iron is a major component of the Earth's lower mantle, the stability of Fe-bearing carbonates has rarely been studied. Here we present experimental results on the stability of Fe-rich carbonates at pressures ranging from 40 to 105 GPa and temperatures of 1450-3600 K, corresponding to depths within the Earth's lower mantle of about 1000-2400 km. Samples of iron oxides and iron-magnesium oxides were loaded into CO2 gas and laser heated in a diamond-anvil cell. The nature of crystalline run products was determined in situ by X-ray diffraction, and the recovered samples were studied by analytical transmission electron microscopy and scanning transmission X-ray microscopy. We show that Fe-(II) is systematically involved in redox reactions with CO2 yielding to Fe-(III)-bearing phases and diamonds. We also report a new Fe-(III)-bearing high-pressure phase resulting from the transformation of FeCO3 at pressures exceeding 40 GPa. The presence of both diamonds and an oxidized C-bearing phase suggests that oxidized and reduced forms of carbon might coexist in the deep mantle. Finally, the observed reactions potentially provide a new mechanism for diamond formation at great depth

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

Plutonic foundation of a slow-spreading ridge segment : oceanic core complex at Kane Megamullion, 23°30′N, 45°20′W

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): Q05014, doi:10.1029/2007GC001645.We mapped the Kane megamullion, an oceanic core complex on the west flank of the Mid-Atlantic Ridge exposing the plutonic foundation of a ∼50 km long, second-order ridge segment. The complex was exhumed by long-lived slip on a normal-sense detachment fault at the base of the rift valley wall from ∼3.3 to 2.1 Ma (Williams, 2007). Mantle peridotites, gabbros, and diabase dikes are exposed in the detachment footwall and in outward facing high-angle normal fault scarps and slide-scar headwalls that cut through the detachment. These rocks directly constrain crustal architecture and the pattern of melt flow from the mantle to and within the lower crust. In addition, the volcanic carapace that originally overlay the complex is preserved intact on the conjugate African plate, so the complete internal and external architecture of the paleoridge segment can be studied. Seafloor spreading during formation of the core complex was highly asymmetric, and crustal accretion occurred largely in the footwall of the detachment fault exposing the core complex. Because additions to the footwall, both magmatic and amagmatic, are nonconservative, oceanic detachment faults are plutonic growth faults. A local volcano and fissure eruptions partially cover the northwestern quarter of the complex. This volcanism is associated with outward facing normal faults and possible, intersecting transform-parallel faults that formed during exhumation of the megamullion, suggesting the volcanics erupted off-axis. We find a zone of late-stage vertical melt transport through the mantle to the crust in the southern part of the segment marked by a ∼10 km wide zone of dunites that likely fed a large gabbro and troctolite intrusion intercalated with dikes. This zone correlates with the midpoint of a lineated axial volcanic high of the same age on the conjugate African plate. In the central region of the segment, however, primitive gabbro is rare, massive depleted peridotite tectonites abundant, and dunites nearly absent, which indicate that little melt crossed the crust-mantle boundary there. Greenschist facies diabase and pillow basalt hanging wall debris are scattered over the detachment surface. The diabase indicates lateral melt transport in dikes that fed the volcanic carapace away from the magmatic centers. At the northern edge of the complex (southern wall of the Kane transform) is a second magmatic center marked by olivine gabbro and minor troctolite intruded into mantle peridotite tectonite. This center varied substantially in size with time, consistent with waxing and waning volcanism near the transform as is also inferred from volcanic abyssal-hill relief on the conjugate African plate. Our results indicate that melt flow from the mantle focuses to local magmatic centers and creates plutonic complexes within the ridge segment whose position varies in space and time rather than fixed at a single central point. Distal to and between these complexes there may not be continuous gabbroic crust, but only a thin carapace of pillow lavas overlying dike complexes laterally fed from the magmatic centers. This is consistent with plate-driven flow that engenders local, stochastically distributed transient instabilities at depth in the partially molten mantle that fed the magmatic centers. Fixed boundaries, such as large-offset fracture zones, or relatively short segment lengths, however, may help to focus episodes of repeated melt extraction in the same location. While no previous model for ocean crust is like that inferred here, our observations do not invalidate them but rather extend the known diversity of ridge architecture.NSF Grants OCE-0118445, OCE-0624408 and OCE-0621660 supported this research. B. Tucholke was also supported by the Henry Bryant Bigelow Chair in Oceanography at Woods Hole Oceanographic Institution

Fe–FeO and Fe–Fe₃C melting relations at Earth's core–mantle boundary conditions: Implications for a volatile-rich or oxygen-rich core

International audienceEutectic melting temperatures in the Fe–FeO and Fe–Fe3C systems have been determined up to 150 GPa. Melting criteria include observation of a diffuse scattering signal by in situ X-Ray diffraction, and textural characterisation of recovered samples. In addition, compositions of eutectic liquids have been established by combining in situ Rietveld analyses with ex situ chemical analyses. Gathering these new results together with previous reports on Fe–S and Fe–Si systems allow us to discuss the specific effect of each light element (Si, S, O, C) on the melting properties of the outer core. Crystallization temperatures of Si-rich core compositional models are too high to be compatible with the absence of extensive mantle melting at the core–mantle boundary (CMB) and significant amounts of volatile elements such as S and/or C (>5 at%, corresponding to >2 wt%), or a large amount of O (>15 at% corresponding to ∼5 wt%) are required to reduce the crystallisation temperature of the core material below that of a peridotitic lower mantle

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