Éditeur(s) : HAL CCSD
Résumé : International audience
Tsavorite is exclusively hosted in the Neoproterozoic Metamorphic Mozambique Belt (NMMB). The gemstone mines, widespread between Kalalani (Tanzania) and Mgama Ridge (Kenya), define a continuous corridor over a hundred kilometers in length. The tsavorite is hosted by a metasedimentary sequence defined as the Kurase tsavorite-bearing metasediments (Kurase-TB metasediments) that also hosts rubies. These metasediments underwent amphibolite-facies metamorphism and are surrounded by granulitic gneisses that are also of sedimentary origin (the Kurase high-temperature gneisses). All these rocks lie below the Kasigau Group, a unit dominated by granulite-facies metamagmatic rocks.To constrain the timing of events that led to this peculiar occurrence of tsavorite, we have performed geochronological analyses of thin sections and of separated grains of zircon, monazite, and rutile using LA-ICP-MS and ID-TIMS, as well as 40Ar/39Ar of muscovite and phlogopite from various lithologies. The results show that the different terranes were metamorphosed synchronously between 620–580 Ma but under different P-T strain conditions. The Kurase-HT gneisses and the rocks from the Kasigau Group are highly strained and underwent granulite-facies metamorphism with abundant partial melting and emplacement of felsic melts between 620 and 600 Ma. Textural observations also underlined a late regional water flux controlling the occurrence of V-free muscovite and monazite mineralizations at 585 Ma. The latter event can be related to the activity of the Galana shear zone, in the east. The Kurase-TB metasediments escaped strain and partial melting. They record amphibolite-facies conditions with static heating, since initial sedimentary structures were locally preserved. The age of the tsavorite mineralization was inferred at 600 Ma from metamorphic zircon rims and monazite from the closest host-rocks, sampled in the mines. Hence, tsavorite crystallization occurred statically at the end of the metamorphic event, probably when the temperature and the amount of volatiles were at maximum levels.Conversely, the ruby formed by local metasomatism of felsic dikes and isolated ultramafic bodies. The rubies are older and zircons and monazites from a ruby-bearing felsic dike (plumasite) were dated at 615 Ma. Finally, data from rutile and micas indicate a global cooling below 430 °C of the whole region between 510 and 500 Ma.
The Canadian Mineralogist

hal-01616037
https://hal.archives-ouvertes.fr/hal-01616037
https://hal.archives-ouvertes.fr/hal-01616037/document
https://hal.archives-ouvertes.fr/hal-01616037/file/JEM.pdf
DOI : 10.3749/canmin.1700019

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The complex electrical impedance of a garnet-biotite-sillimanite residual enclave in the Neogene dacite of El Hoyazo (SE Spain) has been determined up to 978 °C at 200-300 MPa. This well studied material represents a direct sampling of the Alborán Domain thinned lower crust undergoing partial melting. The paragenesis is garnet + biotite + sillimanite + plagioclase + graphite ± cordierite coexisting with widespread (~ 10 wt.%) rhyolitic melt occurring as inclusions and interstitial glass and developed during regional anatexis at 850 ± 50 °C and 500-700 MPa. The samples were used in cyclic measurements consisting in heating-cooling ramps at progressively higher maximum temperature to observe the effect of reactions on conductivity. In the first cycle up to 850 °C at 2-3 kbar, re-melting of the interstitial glass is achieved with no additional reactions and logarithmic conductivity (S/m) is up to − 1.5. At T > 950 °C new melt with orthopyroxene + ilmenite is produced from biotite partial breakdown and the logarithmic conductivity (S/m) is up to − 0.7. Application of mixing models as Hashin-Shtrikman bounds or Archie's Law, shows that once interconnected, melt controls the electrical conductivity of the enclave. The electrical conductivity of the rock can be simulated with the electrical conductivity data obtained on the starting Matrix glass of the enclave and with the new melt. Our experimental results obtained on a graphitic metasedimentary material evidence show that graphite does not contribute to the conductivity enhancement and that the electrical anomaly observed below the Betics can be explained as effect of partial melting of a residual crust. Comparison with previous works on mafic granulites shows that similar conductivity values can be achieved both by melting of a residual felsic crust or by melting of mafic materials and suggests that pelitic rocks can be more common at deep levels than expected.
ISSN: 0040-1951

hal-00823063
https://hal.archives-ouvertes.fr/hal-00823063
DOI : 10.1016/j.tecto.2012.11.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Sedimentary rocks and detrital minerals sample large areas of the continental crust, and they are increasingly seen as a reliable archive for its global evolution. This study presents two approaches to model the growth of the continental crust through the sedimentary archive. The first builds on the variations in U-Pb, Hf and O isotopes in global databases of detrital zircons. We show that uncertainty in the Hf isotope composition of the mantle reservoir from which new crust separated, in the 176Lu/177Hf ratio of that new crust, and in the contribution in the databases of zircons that experienced ancient Pb loss(es), adds some uncertainty to the individual Hf model ages, but not to the overall shape of the calculated continental growth curves. The second approach is based on the variation of Nd isotopes in 645 worldwide fine-grained continental sedimentary rocks with different deposition ages, which requires a correction of the bias induced by preferential erosion of younger rocks through an erosion parameter referred to as K. This dimensionless parameter relates the proportions of younger to older source rocks in the sediment, to the proportions of younger to older source rocks present in the crust from which the sediment was derived. We suggest that a Hadean/Archaean value of K = 1 (i.e., no preferential erosion), and that post-Archaean values of K = 4–6, may be reasonable for the global Earth system. Models built on the detrital zircon and the fine-grained sediment records independently suggest that at least 65% of the present volume of continental crust was established by 3 Ga. The continental crust has been generated continuously, but with a marked decrease in the growth rate at ~ 3 Ga. The period from > 4 Ga to ~ 3 Ga is characterised by relatively high net rates of continental growth (2.9–3.4 km3 yr− 1 on average), which are similar to the rates at which new crust is generated (and destroyed) at the present time. Net growth rates are much lower since 3 Ga (0.6–0.9 km3 yr− 1 on average), which can be attributed to higher rates of destruction of continental crust. The change in slope in the continental growth curve at ~ 3 Ga is taken to indicate a global change in the way bulk crust was generated and preserved, and this change has been linked to the onset of subduction-driven plate tectonics. At least 100% of the present volume of the continental crust has been destroyed and recycled back into the mantle since ~ 3 Ga, and this time marks a transition in the average composition of new continental crust. Continental crust generated before 3 Ga was on average mafic, dense, relatively thin (< 20 km) and therefore different from the calc-alkaline andesitic crust that dominates the continental record today. Continental crust that formed after 3 Ga gradually became more intermediate in composition, buoyant and thicker. The increase in crustal thickness is accompanied by increasing rates of crustal reworking and increasing input of sediment to the ocean. These changes may have been accommodated by a change in lithospheric strength at around 3 Ga, as it became strong enough to support high-relief crust. This time period therefore indicates when significant volumes of continental crust started to become emergent and were available for erosion and weathering, thus impacting on the composition of the atmosphere and the oceans.
ISSN: 0037-0738

hal-01622696
https://hal.archives-ouvertes.fr/hal-01622696
DOI : 10.1016/j.sedgeo.2017.06.001

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We compile field data collected along the eastern part of the North Pyrenean Zone (NPZ) to point to a tectonic evolution under peculiar thermal conditions applying to the basin sediments in relation with the opening of the Cretaceous Pyrenean rift. Based on this compilation, we show that when thinning of the continental crust increased , isotherms moved closer to the surface with the result that the brittle-ductile transition propagated upward and reached sediments deposited at the early stage of the basin opening. During the continental breakup, the pre-rift Mesozoic cover was efficiently decoupled from the Paleozoic basement along the Triassic evaporite level and underwent drastic ductile thinning and boudinage. We suggest that the upper Albian and upper Creta-ceous flysches acted as a blanket allowing temperature increase in the mobile pre-rift cover. Finally, we show that continuous spreading of the basin floor triggered the exhumation of the metamorphic, ductily sheared pre-rift cover, thus contributing to the progressive thinning of the sedimentary pile. In a second step, we investigate the detailed geological records of such a hot regime evolution along a reference-section of the eastern NPZ. We propose a balanced restoration from the Mouthoumet basement massif (north) to the Boucheville Albian basin (south). This section shows a north to south increase in the HT Pyrenean imprint from almost no metamorphic recrystallization to more than 600 °C in the pre-and syn-rift sediments. From this reconstruction, we propose a scenario of tectonic thinning involving the exhumation of the pre-rift cover by the activation of various detachment surfaces at different levels in the sedimentary pile. In a third step, examination of the architecture of current distal passive margin domains provides confident comparison between the Pyrenean case and modern analogs. Finally, we propose a general evolutionary model for the pre-rift sequence of the Northeastern Pyrenean rifted margin.
ISSN: 0040-1951

Droits : http://creativecommons.org/licenses/by-nc-nd/
insu-01355131
https://hal-insu.archives-ouvertes.fr/insu-01355131
https://hal-insu.archives-ouvertes.fr/insu-01355131/document
https://hal-insu.archives-ouvertes.fr/insu-01355131/file/TECTO10807-Incorporated%20changes-rev2.pdf
DOI : 10.1016/j.tecto.2016.07.022

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Gulf of Cadiz spans the plate boundary between Africa and Eurasia west of the Betic–Rif mountain belt. A narrow east dipping subduction zone descends beneath the Gulf of Cadiz and the straits of Gibraltar. The deep crustal structure of the Gulf and the adjacent SW Iberian and Moroccan margins is constrained by numerous multi-channel seismic reflection and wide-angle seismic surveys. A compilation of these existing studies is presented in the form of depth to basement, sediment thickness, depth to Moho and crustal thickness maps. These structural maps image an E–W trending trough, with thin (< 10 km) crust beneath the Gulf of Cadiz. This trough is filled by an eastward thickening wedge of sediments, reaching a thickness of 10–15 km in the eastern Gulf. These sediments are tectonically deformed, primarily along a series of westward-vergent thrust faults and represent a 200–250 km wide accretionary wedge. The northern and especially the southern limits of the accretionary wedge are marked by sharp morphological lineaments showing evidence of recent deformation. These tectonic limits are situated in an internal position with respect to the Miocene deformation front (external Betic and Rif allocthons), which has been abandoned. At the western boundary of the accretionary wedge, near the adjacent Seine and Horseshoe abyssal plains, an E–W trending basement high (Coral Patch Ridge) can be seen indenting the deformation front in an asymmetric manner. Analog modeling is performed using granular materials accreted against a semicircular backstop (representing the basement of the Rif and Betic mountain belts). The modeling initially produces a symmetric, arcuate accretionary wedge. The ensuing collision of an oblique rigid indenter retards accretion on one side, resulting in an embayment and a locally steeper deformation front. The deformation pattern observed in morphology and high-resolution seismic profiles suggests the accretionary wedge and underlying subduction system is still active. The implications of active subduction for the source region of the 1755 Lisbon earthquake and the regional seismic hazard assessment are discussed.
ISSN: 0040-1951

hal-00424922
http://hal.univ-brest.fr/hal-00424922
http://hal.univ-brest.fr/hal-00424922/document
http://hal.univ-brest.fr/hal-00424922/file/Gutscher_09Tphys_rev.pdf
DOI : 10.1016/j.tecto.2008.11.031

Éditeur(s) : HAL CCSD Elsevier Masson
Résumé : International audience
The Chaînons Béarnais ranges (North-Pyrenean Zone, west-central Pyrenees) display a fold-and-thrust structure involving the Mesozoic sedimentary cover, decoupled from its substratum at the Keuper evaporites level and associated with a few peridotite bodies and scarce Palaeozoic basement lenses. In the western part of the Chaînons Béarnais, the newly described recumbent fold of the Saraillé massif comprises a peridotite body and several lenses of Palaeozoic basement wrapped by the Triassic to Aptian sedimentary cover. This structure represents a remnant of the distal portion of the Pyrenean paleo-rifted margin where mantle rocks have been exhumed during Albian–Cenomanian times. In this paper, we present the first detailed mapping and microstructural analysis of the Saraillé massif, providing new geological basis for reconstructing the evolution of this part of the paleo-margin. Our mapping (i) shows that the pre-rift Mesozoic cover forms a recumbent fold cored by mantle and crustal rocks and (ii) confirms that the prerift cover was detached from its bedrock along a layer of Triassic evaporites and slid onto the exhumed mantle rocks. Sliding of the prerift cover was associated with extreme crustal thinning and mantle exhumation along a major detachment fault, together with intense metasomatism affecting both the continental basement and the sedimentary cover. We show for the first time (1) that the Mesozoic pre-rift sediments experienced syn-metamorphic ductile thinning during mantle exhumation, and (2) that during its extreme attenuation, the continental basement was reduced to tectonic lenses some ten meters thick by ductile shearing.
ISSN: 1631-0713

insu-01265777
https://hal-insu.archives-ouvertes.fr/insu-01265777
https://hal-insu.archives-ouvertes.fr/insu-01265777/document
https://hal-insu.archives-ouvertes.fr/insu-01265777/file/corre-CRGeosc-2016.pdf
DOI : 10.1016/j.crte.2015.11.007

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Orogenic lherzolites exhibit a specific (although reproducible) platinum-group element signature characterized by slight positive deviations of Pd/Ir, Rh/Ir and Ru/Ir ratios from the canonical chondritic model. Such a signature was alternatively considered to be a compositional feature of the primitive upper mantle or resulting from sulfide melt addition by refertilisation reactions in the continental lithosphere. To shed additional light on this conundrum, the distribution of PGE in FON B 93, an unserpentinized, fertile (3.24% Al2O3) orogenic lherzolite (French Pyrenees) used as in-house standard by our group, has been studied to different scales, from the bulk rock to trace minerals. In addition to new ICPMS analyses after separation into Nis and Te coprecipitation, its whole-rock PGE concentrations were redetermined by three different laboratories using very precise ID-ICP-MS methods after digestion of the sample in Carius tube at T=250 degrees C and 320 degrees C or in high-pressure asher (HPA-S). The four methods produce reproducible Ru, Rh and Pd contents (7.1 +/- 0.18 ppb; 1.43 +/- 0.05 ppb; 7.1 +/- 0.30 ppb) whereas the non-ID NiS-fire assay method underestimates the Os and Ir concentrations by c.a. 10 and 15% compared to ID-ICP-MS analyses (4.40 +/- 0.07 and 4.00 +/- 0.17 ppb, respectively). Platinum was the most difficult to analyse. If performed on powder aliquots smaller than 3 g, the ID-ICP-MS analyses generate strong nugget effects while the non-ID NiS-fire assay method yields statistically lower (but highly reproducible) Pt concentrations (6.92 +/- 0.26 ppb). These features reflect the strong partitioning of Pt into trace phases that SEM and laser ablation-ICP-MS analyses on thin sections identify as both high-temperature Pt-Ir-Os alloys and Pt-(Pd) tellurides of likely subsolidus origin; ten to fifteen grains ranging in maximum dimension from a few micrometres to a few hundreds of nanometers, were identified by standard polished thin section. LA-ICP-MS data on base metal sulfides (25 grains analysed), coupled with the whole-rock S concentration (277 +/- 10 ppm) and modal composition of the BMS (90% pentlandite+accessory pyrite and secondary pyrrhotite + 10% chalcopyrite) allowed the contribution of the BMS phase to the PGE budget of FON B 93 to be estimated. Except Pt that exhibits a 95% deficit in the BMS phase, the PGE concentrations measured by ID-ICP-MS can be balanced by BMS while Al-spinel is a negligible contributor, accounting for less than 0.5% of the Ru budget. The occurrence of Pt in trace phases may bias the whole-rock Pt concentrations because 1) mechanical collection of Pt-rich trace phases remains problematic with the Nis button, and 2) Pt-Ir-Os alloys may be prone to digestion problems in conventional Carius tube procedures. Since they could be stable at mantle depth, such refractory alloys that contain Os and Ir may also have enhanced the heavy PGEs/light PGEs fractionation. Our observations likely pertain to orogenic lherzolites as a whole because BMS assemblages in these mantle rocks record evolution at low sulfur fugacity, which prevents Pt-Ir-Os alloy from entering the Mss in the mantle; moreover, at subsolidus temperature, pentlandite, the major BMS, cannot accommodate 0 valence state Pt.
ISSN: 0009-2541

hal-00411486
https://hal.archives-ouvertes.fr/hal-00411486
DOI : 10.1016/j.chemgeo.2007.06.030

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Detailed structural and petrological mapping in the Beni Bousera peridotite (Rif Belt, northern Morocco) shows that this orogenic peridotite massif is composed of four tectono-metamorphic domains with consistent kinematics, marked by a pervasive, shallowly dipping foliation with a NW-SE stretching lineation that progressively rotates towards a NNE-SSW orientation in the lowermost part of the massif. From top to bottom, these domains are garnet-spinel mylonites, Ariègite subfacies fine-grained porphyroclastic spinel peridotites, Ariègite-Seiland subfacies porphyroclastic, and Seiland subfacies coarse-porphyroclastic to coarse-granular spinel peridotites. Microstructures and crystal preferred orientations point to deformation dominantly by dislocation creep in all domains, but the continuous increase in average olivine grain size indicates decreasing plastic work rates from top to bottom. This evolution in deformation conditions is consistent with the change in synkinematic pressure and temperature conditions, from 900°C at 2*0 GPa in garnet-spinel mylonites to 1150°C at 1*8 GPa in the Seiland domain. A pervasive diffuse dunitic-websteritic layering subparallel to the foliation suggests deformation in the presence of melt in the Seiland domain. Gravitational instabilities owing to local melt accumulation may account for <200 m wide areas exhibiting a vertical lineation in this domain. To account for the consistent kinematics and the tectono-metamorphic evolution, which implies a temperature gradient of c. 125°C km−1 preserved across the Beni Bousera massif, we propose that the entire massif records the functioning of a low-angle shear zone, a few kilometres wide, which accommodated exhumation of the base of the lithosphere from ∼90 to ∼60 km depth. Partial melting in the Seiland domain may be explained by fast decompression of the footwall, without the need for exotic heat sources. Moreover, if the present-day orientation of the shear zone is similar to that when it was active in the mantle, the stretching lineations at high angle to the metamorphic gradient imply that shearing parallel to the trend of the belt accompanied thinning; that is, a transtensional deformation of the margin.
ISSN: 0022-3530

hal-00950056
https://hal.archives-ouvertes.fr/hal-00950056
DOI : 10.1093/petrology/egt067

Éditeur(s) : HAL CCSD Royal Society of Chemistry
Résumé : International audience
A versatile sol-gel route has been developed for the preparation of hierarchical porous silica membrane material with highly dispersed platinum nanoparticles (Pt-NPs). The outstanding feature of the "one pot" synthesis developed in this work lies in the successful preparation of a stable complex suspension made of silica sol, multi-scale porogens and Pt nanoparticles. The multi-scale porogens were based on non-ionic triblock copolymers and aqueous latex suspension to create mesopores and macropores, respectively, with finely tuned pore size and organization. For further functionalization of the membranes, Pt nanoparticles of about 4 nm in diameter were prepared by irradiating with microwaves (MW) the Pt precursors added in the starting suspension. This original path is suggested to enable a pre-organization of Pt nanoparticles in the ordered mesoporous structure. The suspensions were then deposited as thin films on either dense or macroporous supports, and further thermally treated at 450°C to remove the porogen units. Hierarchical porous layers composed of micropores (<2 nm), ordered mesopores (∼4 nm) and macropores (∼70 nm) with or without Pt were prepared and characterized. The resulting membranes are foreseen to exhibit great potential as multifunctional membranes for gas separation coupled with catalytic reaction. © The Royal Society of Chemistry 2008.
ISSN: 0959-9428

hal-01680586
https://hal.univ-antilles.fr/hal-01680586
DOI : 10.1039/b807029f

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
This study describes the structure of the digestive gland of Strombus gigas in individuals from Guadeloupe and discusses the function of its cell types and their relationship with intracellular Apicomplexa-like parasites. Three cellular types were found in the epithelium of the blind-ending tubules of the digestive gland according to histological and transmission electron microscopy (TEM) observations; these were: digestive cells, pyramidal crypt cells and vacuolated cells. Columnar digestive cells were characterized by large Alcian blue-positive granules, which have not been previously described in digestive cells of other caenogastropods. Such granules contain large quantities of proteoglycans that are exported to the stomach through the physiological destruction of the digestive cells, which undergo a holocrine secretion. Their cytoplasm appears vacuolar due to lipid extraction by solvents used for tissue preparation. Vacuolated cells also appear to be lipid-storage cells. Small triangular-shaped crypt cells, on the other hand, appear to be metabolically active as suggested by a strong positive in situ hybridization of eukaryotic ribosomes, which was confirmed by their large content of ribosomes and rough endoplasmic reticulum compared to the other cell types. These observations suggest that crypt cells may be immature cells that are involved in the replacement of eliminated digestive cells. However, their spherocrystal inclusions indicate that they may be excretory cells or calcium cells. Large brown inclusions were frequently observed in vacuolated cells; these were identified as parasitic protozoans and were present in the digestive gland of all sampled specimens. These protozoans have previously been described from a queen conch population in the San Andres Archipelago (Colombia). Several life cycle stages of the parasite were identified by scanning electron microscopy and TEM; trophozoites were characterized by their conoid-like structure, sporocysts by their thick walls, and gamonts by their thin walls. These observations suggest that this parasite completes its entire life cycle within the same host and type of tissue. Although previous investigations place this parasite within the Apicomplexa group, further investigations are necessary in order to confirm the identification of the parasite.
ISSN: 0260-1230

hal-00755287
https://hal.univ-antilles.fr/hal-00755287
DOI : 10.1093/mollus/eyn041

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Tension wood of Laetia procera (Poepp.) Eichl. (Flacourtiaceae), a neo-tropical forest species, shows a peculiar secondary wall structure, with an alternance of thick and thin layers, while opposite wood of this species has a typical secondary wall structure (S1 + S2 + S3). Samples for the study of microstructural properties were collected upon the estimation of growth stresses in the living tree, in order to analyze the correlation of the former with the latter. Investigation using optical microscopy, scanning electron microscopy and UV microspectrophotometry allowed the description of the anatomy, ultra-structure and chemistry of this peculiar polylaminate secondary wall. In the thick layers, cellulose microfibril angle is very low (i.e., microfibril orientation is close to fibre axis) and cellulose microfibrils are well organized and parallel to each other. In the thin layers, microfibrils (only observable in the inner layer) are less organized and are oriented with a large angle relative to the axis of the cell. Thick layers are lightly lignified although thin layers show a higher content of lignin, close to that of opposite wood secondary wall. The more the wood was under tensile stress, the less the secondary wall was lignified, and lower the syringyl on guaiacyl lignin units' ratio was. The innermost layer of the secondary wall looks like a typical S3 layer with large microfibril angle and lignin occurrence. The interest of this kind of structure for the understanding of stress generation is discussed.
ISSN: 0931-1890

hal-00194923
https://hal.archives-ouvertes.fr/hal-00194923
https://hal.archives-ouvertes.fr/hal-00194923/document
https://hal.archives-ouvertes.fr/hal-00194923/file/Trees_RuelleYoshidaClairThibaut_submitted2006.pdf
DOI : 10.1007/s00468-007-0128-0

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : A differential effective medium (DEM) model is used to predict elastic properties for a set of porous and anisotropic aggregates, comprised of mixtures of calcite and muscovite. The DEM takes into consideration an anisotropic background medium with triclinic or higher symmetry, in which inclusions of idealized ellipsoidal shape are added incrementally. In general, the calculated elastic properties of a solid that contains inclusions representing "dry" pores/cracks are strongly dependent on the orientation and aspect ratio of the inclusions. Aspect ratios of inclusions in the synthetic aggregates, which consist of air-filled pores, are estimated from anisotropy of magnetic susceptibility (AMS) of samples whose pore space has been impregnated with a colloidal ferrofluid. The AMS derived pore shape geometry is used as an input value for inclusions in the DEM. Modeling results are compared with laboratory determined elastic properties, measured with ultrasonic waves. Calculated shear wave velocities agree in general well with laboratory measured S wave velocities, whereas calculated P wave velocities are typically 0.5-1.1 km/s higher than measured values. Differences between calculated and measured P wave velocities are attributed mainly to incomplete and biased ferrofluid saturation of pores. Spherical pores are preferably filled during imbibition, in comparison to thin cracks, which leads to overprediction of the calculated P wave velocities. The amount of ferrofluid that fills the pore space is dependent on the ratio of calcite to muscovite and the load used for compaction during sample synthesis. The permeability decreases with increasing muscovite content and increasing compaction load. Incomplete saturation of samples with high-muscovite content is confirmed by X-ray microtomography density contrast imaging of dry and ferrofluid saturated specimens.
ISSN: 0148-0227

hal-00617717
https://hal.archives-ouvertes.fr/hal-00617717
DOI : 10.1029/2010JB007750

Éditeur(s) : HAL CCSD Institute of Electrical and Electronics Engineers (IEEE) Institute of Electrical and Electronics Engineers
Résumé : International audience
Conventional quantitative electron probe microanalysis of actinides requires the use of reference standard samples. However, for such elements, standards are generally not available. To overcome this difficulty, standardless methods of analysis are used, in which the x-ray intensity emitted by the standard is calculated. To be reliable, such calculations require accurate knowledge of physical data such as the x-ray production cross section. However, experimental data of this quantity are not always available for actinide elements. In the present work, experimental L and M x-ray production cross sections were measured for elements uranium and lead. Measurements were performed with two electron microprobes using wavelength-dispersive spectrometers using thin self-supporting targets. Experimental results are compared with calculated cross sections obtained from different analytical formulae, and, whenever possible, with experimental data obtained from the literature.
ISSN: 0018-9499

hal-01122820
https://hal.archives-ouvertes.fr/hal-01122820
DOI : 10.1109/TNS.2014.2321016

Éditeur(s) : American Geophysical Union
Résumé : [1] During the Zoneco 11 marine geophysical survey (September 2004), two deep reflection seismic profiles recorded by ocean bottom seismometers were acquired in the offshore domain west of New Caledonia. The northern profile crosses the New Caledonia Basin, the Fairway Ridge, the Fairway Basin, and the Lord Howe Rise. The southern profile crosses the Norfolk Rise south of New Caledonia, the New Caledonia Basin, the Fairway Ridge and Basin, and ends at the foot of Lord Howe Rise. On the northern profile the Lord Howe Rise has a crustal thickness of 23 km and exhibits seismic velocities and velocity gradients characteristic of continental crust. The crust thins to 12-15 km in the neighboring Fairway Basin, which is interpreted to be of thinned continental origin based on the seismic velocities. The crustal thickness of the Fairway Rise is 22 km, and it is also interpreted to be of continental origin. The New Caledonian Basin is underlain by crust of 10 km thickness, which shows unusally high velocities (between 7.0 and 7.4) uncharacteristic for either thinned continental or oceanic crust. On the southern profile the Norfolk Rise is also found to be of continental nature. Here, the New Caledonia Basin shows velocities, crustal thickness, and basement roughness characteristic of typical oceanic crust. The crust in the Fairway Basin shows higher velocities than on the northern profile, which could be caused by volcanic intrusions into the crust during extension. A deep reflector in the upper mantle was imaged underneath the New Caledonian Basin on the northern profile.
Journal of Geophysical Research - Solid earth (0148-0227) (American Geophysical Union), 2007-11 , Vol. 112 , N. B11102 , P. NIL_71-NIL_88

Droits : 2007 American Geophysical Union
http://archimer.ifremer.fr/doc/2007/publication-3543.pdf
DOI:10.1029/2007JB005093
http://archimer.ifremer.fr/doc/00000/3543/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
At the North-Ecuadorian convergent margin (1°S–1.5°N), the subduction of the rough Nazca oceanic plate leads to tectonic erosion of the upper plate and complex seismogenic behavior of the megathrust. We used three selected pre-stack depth migrated, multi-channel seismic reflection lines collected during the SISTEUR cruise to investigate the margin structure and decipher the impact of the subducted Atacames seamounts on tectonic erosion, interseismic coupling, and seismogenesis in the region of the 1942 Mw7.8 earthquake.This dataset highlights a subducted ∼30×40 km∼30×40 km, double-peak seamount that belongs to the Atacames seamount chain and that is associated with a deep morphologic re-entrant containing mass transport deposits.The seamount subduction uplifted the margin basement by ∼1.6 km and pervasively broke the margin by deep and intense reverse faulting ahead of the seamount, a process that is likely to weaken considerably the margin. In the seamount wake, the basement reverse fault system rotated counter-clockwise. This faulted basement is overlain with slope sediment sliding along listric normal faults that sole out onto the BSR. This superposition of deep tectonic contraction within the basement and shallow gravitational extension deformation within the sediment highlights the key role of gas hydrate on outer slope erosion.In addition to long-term regional basal erosion, the margin basement has thinned locally by an extra 0.8–1 km in response to the subduction of the Atacames seamount chain and hydrofracturing by overpressured fluids at the margin toe. This pervasively and deeply fractured margin segment is associated with a seismically quiet and GPS-modeled low interseismic coupling corridor that terminates downdip near the 1942 epicenter and locked zone. We suggest that the deeply buried double-peak Atacames seamount triggered the 1942 earthquake ahead of its leading flank. This result supports previous studies proposing that subducted seamounts provide unfavorable conditions for locking the updip segment of the plate boundary limiting the updip extent of seismogenic zones, but may favor large subduction earthquakes at greater depths.
ISSN: 0012-821X

hal-01256399
https://hal.archives-ouvertes.fr/hal-01256399
DOI : 10.1016/j.epsl.2015.10.043

Éditeur(s) : Amer Geophysical Union
Résumé : The origin of the Algerian margin remains one of the key questions still discussed in the Western Mediterranean sea, due to the imprecise nature and kinematics of the associated basin during the Neogene. For the first time, the deep structure of the Maghrebian margin was explored during the SPIRAL seismic survey. In this work, we present a N-S transect off Tipaza (west of Algiers), a place where the margin broadens due to a topographic high (Khayr-al-Din Bank). New deep penetration seismic profiles allow us to image the sedimentary sequence in the Algerian basin and the crustal structure at the continent-ocean boundary. Modeling of the wide-angle data shows thinning of the basement, from more than 15km in the continental upper margin to only 5–6km of oceanic-type basement in the Algerian basin, and reveals a very narrow or absent transitional zone. Analysis of the deep structure of the margin indicates features inherited from its complex evolution: (1) an oceanic-type crust in the deep basin, (2) similarities with margins formed in a transform-type setting, (3) a progressive deepening of the whole sedimentary cover, and the thickening of the Plio-Quaternary sediments at the margin foot, coeval with (4) a downward flexure of the basement in the basin. These features argue for a multiphased evolution of the margin, including (1) an early stage of rifting and/or spreading, (2) a late transcurrent episode related to the westward migration of the Alboran domain, and (3) a diffuse Plio-Quaternary compressional reactivation of the margin.
Journal Of Geophysical Research-solid Earth (0148-0027) (Amer Geophysical Union), 2013-08 , Vol. 118 , N. 8 , P. 3899-3916

Droits : 2013. American Geophysical Union. All Rights Reserved.
http://archimer.ifremer.fr/doc/00152/26333/24412.pdf
DOI:10.1002/jgrb.50318
http://archimer.ifremer.fr/doc/00152/26333/

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Numerical models show that small-scale convection (SSC) occurring atop a mantle plume is a plausible mechanism to rejuvenate the lithosphere. The triggering of SSC depends on the density contrast and on the rheology of the unstable layer underlying the stagnant upper part of the thermal boundary layer (TBL). Partial melting may change both properties. We analyze, using 2-D numerical simulations, how partial melting influences the dynamics of time-dependent SSC instabilities and the resulting thermo-mechanical rejuvenation of an oceanic plate moving atop of a plume. Our simulations show a complex behavior, with acceleration, no change, or delay of the SSC onset, due to competing effects of the latent heat of partial melting, which cools the plume material, and of the buoyancy increase associated with both melt retention and depletion of residue following melt extraction. The melt-induced viscosity reduction is too localized to affect significantly SSC dynamics. Faster SSC triggering is promoted for low melting degrees (low plume temperature anomalies, thick lithosphere, or fast moving plates), which limit both the temperature reduction due to latent heat of melting and the accumulation of depleted buoyant residue in the upper part of the unstable layer. In contrast, high partial melting degrees lead to a strong temperate decrease due to latent heat of melting and development of a thick depleted layer within the sublithospheric convecting layer, which delay the development of gravitational instabilities. Despite differences in SSC dynamics, the thinning of the lithosphere is not significantly enhanced relatively to simulations that neglect partial melting.
ISSN: 1525-2027

hal-01277175
https://hal.archives-ouvertes.fr/hal-01277175
DOI : 10.1002/2015GC005967

Éditeur(s) : HAL CCSD Royal Society of Chemistry
Résumé : International audience
Abstract View references (60)Here we show the long term performance at high temperatures of a multi-tube module containing 8 membranes in 4 parallel lines with a total of 545 cm 2 area. The membranes were prepared via thin film dip coating of cobalt oxide silica (CoO xSi) sol-gel on tubular alumina supports. A preliminary study found that the sol-gel containing 20 mol% cobalt oxide formed the best microporous structure with the highest surface area and pore volume. All resulting membranes delivered permeances of ∼1 × 10 -7 mol m -2 s -1 Pa -1 at 500 °C, indicating a high repeatability for the membrane fabrication process. The permselectivities of helium (He) and hydrogen (H 2) over carbon dioxide (CO 2) and nitrogen (N 2) increased from 10-20 at 100 °C to values close to 1000 at 500 °C. Additionally, the apparent energies of activation (E act) for the smaller kinetic diameter gases He and H 2 at 12.2 and 19.5 kJ mol -1 were high and contrary to the negative values for larger gases N 2 and CO 2 at -1.8 and -7.4 kJ mol -1. These remarkable results were attributed to the molecular sieving mechanism of the microporous silica which was enhanced by the embedding of cobalt oxide into the matrix, delivering structural control with an average pore size of 3 Å. The E act for H 2 permeance was higher than that of He, indicating that the cobalt oxide played an important role in H 2 transport. Two membrane lines performed exceptionally well for binary gas mixture processing with H 2 purity reaching values close to 100% in the permeate stream for argon (Ar) concentrations of up to 80% in the retentate stream. A major finding here is that the binary gas selectivity was independent of temperature, contrary to the permselectivity observed for single gas permeance. Further, the H 2 flow rate was greatly affected by the concentration of Ar in the mixture, while the temperature dependency played only a marginal role. In particular, competitive adsorption in the percolative pathways containing pore constrictions or bottlenecks of the anisotropic CoO xSi matrix allowed Ar to impede H 2 diffusion. Finally, the CoO xSi membranes proved thermally stable and robust for 2000 h of testing for various thermal cycles up to 500 °C. © The Royal Society of Chemistry 2012.
Energy & Environmental Science

hal-01683199
https://hal.univ-antilles.fr/hal-01683199
DOI : 10.1039/c2ee03247c

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We report structural evidence of ductile strain localization in mantle pyroxenite from the spinel to plagioclase websterite transition in the Ronda Peridotite (southern Spain). Mapping shows that, in this domain, small-scale shear zones occurring at the base of the lithospheric section are systematically located within thin pyroxenite layers, suggesting that the pyroxenite was locally weaker than the host peridotite. Strain localization is associated with a sudden decrease of grain size and increasing volume fractions of plagioclase and amphibole as a result of a spinel to plagioclase phase transformation reaction during decompression. This reaction also fostered hydrogen extraction ('dehydroxylation') from clinopyroxene producing effective fluid saturation that catalyzed the synkinematic net-transfer reaction. This reaction produced fine-grained olivine and plagioclase, allowing the onset of grain-size sensitive creep and further strain localization in these pyroxenite bands. The strain localization in the pyroxenites is thus explained by their more fertile composition, which allowed earlier onset of the phase transition reactions. Geothermobarometry undertaken on compositionally zoned constituent minerals suggests that this positive feedback between reactions and deformation is associated with cooling from at least 1000°C to 700°C and decompression from 1*0 to 0*5 GPa.
ISSN: 0022-3530

hal-00941293
https://hal.archives-ouvertes.fr/hal-00941293
DOI : 10.1093/petrology/egt039

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10 kbar and 780–820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550 °C and 1–3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension.
ISSN: 0040-1951

insu-01682810
https://hal-insu.archives-ouvertes.fr/insu-01682810
DOI : 10.1016/j.tecto.2017.11.028