Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg+Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr+Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites,as their formation requires hot, hydrous, shallow mantle (> 1250 degrees C at < 30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880-1030 degrees C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions. (c) 2008 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00412594
https://hal.archives-ouvertes.fr/hal-00412594
DOI : 10.1016/j.epsl.2008.06.014

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study presents microstructural evidence for deformation-controlled melt organization and for changes in olivine deformation associated with the presence of melt in an 80 m vertical section of the crust-mantle transition zone in the Oman ophiolite. This zone represents an 'end member' case for analyzing feedbacks between deformation and melt distribution in the upper mantle, since it experienced strong shear strains in presence of large melt fractions. It is characterized by a subhorizontal compositional layering at the mm to meter scale, from weakly impregnated dunites to olivine-rich gabbros, which parallels a pervasive foliation containing a strong stretching lineation. The parallelism between the compositional layering and the foliation, the diffuse limits of the layers, the alignment of elongated plagioclase-rich aggregates devoid of internal deformation structures with the elongation of olivine crystals in the dunitic layers, and the sharp compositional changes across some, but not all layer limits suggest deformation plays an essential role on the development of the layering. The variation on a mm-scale of the olivine crystal preferred orientation (CPO) symmetry as a function of the modal content: from axial-[100] symmetry in layers with <70% modal olivine to axial-[010] in more gabbroic levels (<40% olivine), which is repeated over the entire section, implies deformation in presence of variable melt fractions. Axial-[100] olivine CPO in olivine-rich layers is consistent with deformation by dislocation creep under high temperature, low pressure, dry conditions. Axial-[010] olivine CPO patterns imply additional sliding along preferentially wetted (010) grain boundaries, increase in the activity of [001] glide, or transpression localized in the melt-rich layers. Since the change in CPO symmetry is not accompanied by dispersion, instantaneous melt fractions must have remained <30-40% in all layers. The continuous variation in olivine CPO symmetry with decreasing olivine content implies therefore that the former depends on the cumulated strain in presence of melt rather than on the instantaneous melt fraction.
ISSN: 0012-821X

hal-00795508
https://hal.archives-ouvertes.fr/hal-00795508
DOI : 10.1016/j.epsl.2012.10.003

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
We report new structural, microstructural, petrological, and major- and trace-element data on ultramafic rocks from the Kondyor zoned ultramafic complex in Far-East Russia. The ultramafic rocks are subdivided into three subconcentric lithologies, from core to rim: (1) a metasomatic domain where generally phlogopite-rich dykes pervasively intrude dunite; (2) a main dunite core; (3) a pyroxenite rim. The ultramafic rocks have nearly vertical contacts with the surrounding Archaean basement (gneisses, quartzites and marbles) and hornfelsed Riphean sediments. The hornfelsed sediments show a relatively steep ( 60), outward dipping layering, which rapidly flattens to horizontal away from the inner contact. Although the Riphean sediments define a dome-like structure, the inward, shallow dipping foliation of the dunites indicates a synformal structure. Detailed petro-structural investigations indicate that the Kondyor dunites were deformed by solid-state flow under asthenospheric mantle conditions. The outward textural change from coarse- to fine-grained equigranular dunite and the outward-increasing abundance of subgrains and recrystallized olivine grains suggest dynamic recrystallization while fluid circulation was channelized within the core metasomatic zone, with a decreasing melt fraction from core to rim, and also suggest that solid-state deformation induced grain-size reduction towards the cooling border of the Kondyor massif. Based on their geochemistry, the dunites are interpreted as mantle rocks strongly affected by reaction with melts similar to the JurassicCretaceous Aldan Shield lamproites. Rim pyroxenites were formed by a melt-consuming peritectic reaction, implying the existence of at least a small, conductive thermal gradient around the dunite body while the latter was still at near-solidus temperature conditions. This suggests that the zoned structure of Kondyor was initiated at mantle depths, most probably within the subcontinental lithosphere. Upon cooling, the lamproitic melts were progressively focused in the central part of the massif and drained into vein conduits where they reacted with the wall-rock dunite. Two-dimensional numerical modelling based on finite-differences with a marker-in-cell technique incorporates temperature-dependent rheologies for both molten and non-molten host rocks. The modelling consolidates the structural, petrological and geochemical interpretations, which show that the dunites represent the synformal, flat-lying apex of an asthenospheric mantle diapir, triggered by fluid pressure channelized in the core, which nearly reached the Earths surface. We conclude that translithospheric mantle diapirism is an important mode of mass transfer in the Earth.
ISSN: 0022-3530

hal-00424508
https://hal.archives-ouvertes.fr/hal-00424508
DOI : 10.1093/petrology/egn083

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Peridotite xenoliths brought up to the surface by the volcanism of the Kerguelen Islands represent a mantle that has been affected by a high degree of partial melting followed by intense melt percolation above the Kerguelen plume. These xenoliths are therefore particularly suitable to investigate effects of melt-rock interaction on crystallographic fabrics (lattice-preferred orientation (LPO)) of peridotite minerals and on the LPO-induced seismic properties of peridotites above a mantle plume. We have studied a suite of 16 ultramafic samples representative of different degrees of partial melting and magma-rock interaction among which the protogranular harzburgites are the least metasomatised xenoliths and dunites are the ultimate stage of metasomatism. Olivine LPO is characterized by high concentration of [010] axes perpendicular to the foliation and [100] axes close to the lineation or distributed in the foliation plane in harzburgites, whereas the high concentration of [100] axes is parallel to the lineation and [010] axes is perpendicular to the assumed foliation in dunites. Olivine LPO in harzburgites is interpreted as being due to a deformation regime in axial compression or transpression. The fabric strength of olivine decreases progressively from protogranular to poikilitic harzburgites and finally to dunites, for which it remains nevertheless significant (J index 3.8). Seismic properties calculated from LPO of minerals indicate that metasomatism at higher melt/rock ratio lowers the P wave velocities. The most significant difference between harzburgites and dunites corresponds to the distribution of S wave anisotropy. Harzburgites display the maximum of anisotropy within the foliation plane and the minimum of anisotropy perpendicular to the foliation plane, whereas the lowest anisotropy is parallel to the lineation for dunites. These modifications of seismic properties as a result of metasomatic processes may induce seismic heterogeneities in the mantle above the Kerguelen plume. In addition, assuming a lithospheric mantle primarily harzburgitic and structured with a horizontal foliation, the seismic properties calculated for the Kerguelen xenoliths reconcile the rather high anisotropy evidenced by the horizontally propagating surface waves with the apparent isotropy revealed by the absence of splitting of vertically propagating teleseismic SKS waves recorded by the GEOSCOPE Kerguelen station.
ISSN: 1525-2027

hal-00366222
https://hal.archives-ouvertes.fr/hal-00366222
https://hal.archives-ouvertes.fr/hal-00366222/document
https://hal.archives-ouvertes.fr/hal-00366222/file/bascou_Kerguelen_2007GC001879_hal.pdf
DOI : 10.1029/2007GC001879

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Massif du Sud is a large ophiolitic complex that crops out in the southern region of New Caledonia (SW Pacific). It is dominated by harzburgite tectonite that locally shows a transitional gradation to massive dunite up section. Clinopyroxene, orthopyroxene and plagioclase progressively appear in dunite up to the transition to layered wehrlite and orthopyroxene-gabbro. The dunite-wehrlite and wehrlite-gabbro contacts are parallel and the latter defines the paleo-Moho.;Highly depleted modal, mineral and bulk rock compositions indicate that harzburgites are residues after high degrees (20-30%) of partial melting mainly in the spinet-stability field. Their relative enrichment in HFSE, LREE and MREE is due to re-equilibration of melting residues with percolating melts. Dunite formed in the Moho transition zone by reaction between residual mantle harzburgite and olivine-saturated melts that led to pyroxene dissolution and olivine precipitation. Rare clinopyroxene and plagioclase crystallized in interstitial melt pores of dunite from primitive, low-TiO2, ultra-depleted liquids with a geochemical signature transitional between those of island arc tholeiites and boninites.;Ascending batches of relatively high-SiO2, ultra-depleted melts migrated through the Moho transition zone and generated wehrlite by olivine dissolution and crystallization of clinopyroxene, orthopyroxene and plagioclase in variable amounts. These liquids were more evolved and were produced by higher degrees of melting or from a more depleted source compared with melts that locally crystallized clinopyroxene in dunite. Ultra-depleted magmas, non-cogenetic with those that formed the Moho transition zone, ascended to the lower crust and generated gabbroic cumulates with subduction-related affinity. Thus, the ultramafic and mafic rocks in the Moho transition zone and lower crust of the Massif du Sud ophiolite are not products of fractional crystallization from a single magma-type but are the result of migration and accumulation of different melts in a multi-stage evolution. The record of high partial melting in the mantle section, and migration and accumulation of ultra-depleted subduction-related melts in the Moho transition zone and lower crust support that the Massif du Sud ophiolite is a portion of forearc lithosphere generated in an extensional regime during the early phases of the subduction zone evolution. Our results show the existence of different types of ultra-depleted melt compositions arriving at the Moho transition zone and lower crust of an infant intraoceanic paleo-arc. Ultra-depleted melts may thus be a significant component of the melt budget generated in oceanic spreading forearcs prior to aggregation and mixing of a large range of melt compositions in the crust.
ISSN: 0009-2541

hal-00429333
https://hal.archives-ouvertes.fr/hal-00429333
DOI : 10.1016/j.chemgeo.2009.06.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Sapat peridotite in Kohistan (NW Pakistan) is a 12x1 km sized body of meta-harzburgites, dunites, pyroxenites and meta-gabbros in the hanging wall of the Indus Suture Zone. Field relations, micro-textures, bulk and mineral chemistry document melt-rock reactions producing dunites at the expense of meta-harzburgites, and a consecutive olivine-clinopyroxenite formation from the same melt type. The association of mesa-gabbros, dunites, and pyroxenites is interpreted as a crust-mantle transition zone produced by the percolation of highly depleted primitive arc melt. The meta-harzburgites represent large portions of refractoty mantle that have suffered melt extraction and metasomatism prior to their interaction with the dunite-forming primitive arc melt and the establishment of the crust-mantle transition zone. Uncommon vein assemblages in the dunites that include Cr-clinochlore, calcite and olivine document a late percolation of slab-related, volatile-rich fluids. Taking into account the highly refractory composition of the Sapat mantle, its structural position, the evidence for interaction with highly depleted arc melts and the sub-solidus interaction with volatile-rich arc fluids, the Sapat ultramafic rocks are interpreted as being derived from the frontal Kohistan arc, representing a crust-mantle transition formed by arc magmatism.
ISSN: 0024-4937

hal-00424515
https://hal.archives-ouvertes.fr/hal-00424515
DOI : 10.1016/j.lithos.2008.07.004

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Two types of olivine occur in kimberlites from Greenland, Canada and southern Africa. The first, xenocrystic olivine, displays several different forms. Most distinctive are 'nodules', a term we use to describe the large (1-5 mm), rounded, single crystals or polycrystalline aggregates that are a common constituent of many kimberlites. Olivine compositions are uniform within single nodules but vary widely from nodule to nodule, from Fo81 to 93. Within many nodules, sub- to euhedral asymmetric tablets have grown within larger anhedral olivine grains. Dislocation structures, particularly in the anhedral grains, demonstrate that this olivine was deformed before being incorporated into the kimberlite magma. Olivine grains in the kimberlite matrix between the nodules have morphologies similar to those of the tablets, suggesting that most matrix olivine grains are parts of disaggregated nodules. We propose that a sub- to euhedral form is not sufficient to identify phenocrysts in kimberlites and provide some criteria, based on morphology, internal deformation and composition, that distinguish phenocrysts from xenocrysts. The second type of olivine is restricted to rims surrounding xenocrystic grains. Only this olivine crystallized from the kimberlite magma. Major and trace element data for the rim olivine are used to calculate the composition of the parental kimberlite liquid, which is found to contain between about 20 and 30% MgO. The bulk compositions of many kimberlites contain higher MgO contents as a result of the presence of xenocrystic olivine. The monomineralic, dunitic, character of the nodules, and the wide range from Fo-rich to Fo-poor olivine compositions, provide major constraints on the origin of the nodules. Dunite is a relatively rare rock in the mantle and where present its olivine is persistently Fo-rich. The dunitic source of the nodules in kimberlites lacked minerals such as pyroxene and an aluminous phase, which make up about half of most mantle-derived rocks. It appears that these minerals were removed from the mantle peridotite that was to become the source of the nodules, and the Fo content of the retained olivine was modified during interaction with CO2-rich fluids whose arrival at the base of the lithosphere immediately preceded the passage of the kimberlite magmas. Fragments of the resultant dunite were entrained into the kimberlite, where they were retained both as intact nodules and as disaggregated grains in the matrix.
ISSN: 0022-3530

insu-00543321
https://hal-insu.archives-ouvertes.fr/insu-00543321
BIBCODE : 10.1093/petrology/egp080
DOI : 10.1093/petrology/egp080

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Most kimberlites contain abundant dunitic nodules. These are centimetre-sized, rounded and multi-grained assemblages of xenocrystic olivine with a wide range of compositions (Fo83 to Fo94). The absence of orthopyroxene and other mantle minerals and the range of olivine compositions have been attributed to reaction between mantle peridotite and (proto)kimberlitic fluid or melt, but the timing of the reaction is a subject of debate. In a kimberlite from the Kangamiut region of Greenland, nodule cores are surrounded by fine-grained outer margins with near-constant Fo contents (∼Fo88) but highly variable minor element contents (e.g. 500–2500 ppm Ni). These margins crystallized from the kimberlite melt and we show that their compositions can be explained by crystallization of olivine alone, if a high partition coefficient for Ni between melt and olivine (DNi > 20) is assumed. Orthopyroxene assimilation is not required, removing the constraint that its dissolution occurred during ascent of the kimberlite magma. Within some nodules, in addition to the usual core-to-margin gradients, we observe asymmetric compositional changes (variable Fo but near-constant minor element contents) across mobile grain boundaries. These changes document fluid percolation at the grain scale that occurred during dynamic recrystallization in the deforming lithospheric mantle. We note that chemical gradients associated with mobile grain boundaries are observed in olivines that cover the entire compositional range of the nodules, and propose that fluid-assisted dynamic recrystallization took place in dunite that was already compositionally heterogeneous. Reaction between peridotite and protokimberlitic melt or fluid and dissolution of orthopyroxene thus occurred within the lithospheric mantle, immediately (a few days) prior to the ascent of the kimberlite melt and the entrainment of the dunite nodules. We propose that the grain boundary zones probably mimic, at a fine scale, the fluid–peridotite interaction that caused, at a larger scale, orthopyroxene dissolution and formation of compositionally diverse olivine in kimberlites.
ISSN: 0022-3530

hal-01277963
https://hal.archives-ouvertes.fr/hal-01277963
DOI : 10.1093/petrology/egv054

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Jacupiranga Complex is one of several Meso-Cenozoic alkaline intrusive complexes along the margins of the intracratonic Paraná Basin in southern Brazil. The complex encompasses a wide range of rock-types, including dunites, wehrlites, clinopyroxenites, melteigites-ijolites, feldspar-bearing rocks (diorites, syenites, and monzonites), lamprophyres and apatite-rich carbonatites. While carbonatites have been extensively investigated over the last decades, little attention has been paid to the silicate rocks. This study presents new geochonological and geochemical data on the Jacupiranga Complex, with particular emphasis on the silicate lithotypes. 40Ar/39Ar ages for different lithotypes range from 133.7 ± 0.5 Ma to 131.4 ± 0.5 Ma, while monzonite zircon analyzed by SHRIMP yields a U-Pb concordia age of 134.9 ± 1.3 Ma. These ages indicate a narrow time frame for the Jacupiranga Complex emplacement, contemporaneous with the Paraná Magmatic Province. Most of the Jacupiranga rocks are SiO2-undersaturated, except for a quartz-normative monzonite. Based on geochemical compositions, the Jacupiranga silicate lithotypes may be separated into two magma-evolution trends: (1) a strongly silica-undersaturated series, comprising part of the clinopyroxenites and the ijolitic rocks, probably related to nephelinite melts and (2) a mildly silica-undersaturated series, related to basanite parental magmas and comprising the feldspar-bearing rocks, phonolites, lamprophyres, and part of the clinopyroxenites. Dunites and wehrlites are characterized by olivine compositionally restricted to the Fo83-84 interval and concentrations of CaO (0.13–0.54 wt%) and NiO (0.19–0.33 wt%) consistent with derivation by fractional crystallization, although it is not clear whether these rocks belong to the nephelinite or basanite series. Lamprophyre dikes within the complex are considered as good representatives of the basanite parental magma. Compositions of calculated melts in equilibrium with diopside cores from clinopyroxenites are quite similar to those of the lamprophyres, suggesting that at least a part of the clinopyroxenites is related to the basanite series. Some feldspar-bearing rocks (i.e. meladiorite and monzonite) show petrographic features and geochemical and isotope compositions indicative of crustal assimilation, although this may be relegated to a local process. Relatively high CaO/Al2O3 and La/Zr and low Ti/Eu ratios from the lamprophyres and calculated melts in equilibrium with cumulus clinopyroxene point to a lithospheric mantle metasomatized by CO2-rich fluids, suggesting vein-plus-wall-rock melting mechanisms. The chemical differences among those liquids are thought to reflect both variable contributions of melting resulting from veins and variable clinopyroxene/garnet proportions of the source.
ISSN: 0895-9811

hal-01583547
https://hal.archives-ouvertes.fr/hal-01583547
DOI : 10.1016/j.jsames.2017.05.009

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : If the net flux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafic keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo–island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafic crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display flat or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafic crust that have higher REE concentrations and are derived from light rare earth element (LREE)–enriched melts similar to island arc basalt. This observation, along with the lower 208Pb/204Pb and 206Pb/204Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafic rocks derive from a common melt via crystal fractionation. In the 208Pb/204Pb versus 206Pb/204Pb diagram, ultramafic rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafic rocks. These data are consistent with a scenario whereby the Jijal ultramafic section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafic cumulates complementary to the evolved mafic plutonic rocks implies either (1) that a substantial volume of such ultramafic cumulates was delaminated or torn out by subcrustal mantle flow from the base of the arc crust in extraordinarily short time scales (0.10–0.35 cm/yr), or (2) that the net flux to the Kohistan arc crust was more evolved than primitive arc basalt.
ISSN: 0091-7613

hal-00406688
https://hal.archives-ouvertes.fr/hal-00406688
DOI : 10.1130/G23675A.1

Éditeur(s) : HAL CCSD
Résumé : In order to minimize CO2 atmospheric concentration, a solution consists in sequestrating CO2 in geological reservoirs. To estimate long term risks, it is necessary to quantify the couplings between reaction processes as well as structural and hydrodynamical modifications. We realised two experimental benches enabling injecting CO2-enriched-brine in conditions corresponding to in situ storage (T < 200 oC and P < 200 bar) and developed
an experimental protocol using X-Ray microtomography and fluid and rock analyses in order to measure the variations of physical and chemical parameters.
The study of carbonated reservoirs near the injection well, allows quantifying different k-phi relationships depending on the dissolution processes and triggered by the local fluid chemical composition and initials conditions. Away from the injection well, we observe carbonate precipitation decreasing the permeability. The study of fracturated caprock samples shows that alternative percolation of CO2-enriched-brine and CO2 gas increases the fracture permeability.
The study of silicated rocks indicates carbonate precipitation in zeolite sandstone and sintered dunite grains. Nevertheless, in zeolite sandstone we also observe the precipitation of clay particles located in the uid pathways which decrease strongly the permeability.
Afin de minimiser la concentration en CO2 dans l'atmosphère une solution consiste à le séquestrer dans les réservoirs géologiques. Pour évaluer les risques à long terme, il est nécessaire de quantifier les couplages entre les processus réactionnels et les modifications structurales et hydrodynamiques. Dans cette optique, nous avons construit deux dispositifs expérimentaux permettant d'injecter des saumures chargées en CO2 dans les conditions de stockage (T < 200 °C et P < 200 bar) et mis en oeuvre un protocole expérimental utilisant la microtomographie RX et l'analyse des roches et des fuides pour quantifier les variations des paramètres physiques et chimiques. Dans le cas des réservoirs carbonatés, on observe, près du puits d'injection, une forte variabilité des relations k-phi en fonction du régime de dissolution contrôlé par la chimie locale du fluide, ainsi que par les conditions initiales. A plus grande distance, on observe des processus de précipitation diminuant fortement la perméabilité. L'étude des roches de couverture fracturées (argilite) a montré qu'une percolation alternée d'une saumure chargée en CO2 et de CO2 gaz augmentait la perméabilité de fracture. Dans le cas des roches silicatées, la précipitation de carbonates est mise en évidence aussi bien dans les grès à zéolites que dans les frittés de dunite de San Carlos. Cependant, dans les grès à zéolites, la précipitation d'une phase argileuse est observée dans les chemins d'écoulement et engendre une forte diminution de la perméabilité.
https://tel.archives-ouvertes.fr/tel-00416408

tel-00416408
https://tel.archives-ouvertes.fr/tel-00416408
https://tel.archives-ouvertes.fr/tel-00416408/document
https://tel.archives-ouvertes.fr/tel-00416408/file/These_Luquot.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We performed torsional deformation experiments on pre-hydrated fine-grained olivine aggregates using an innovative experimental assembly to investigate water weakening in mantle rocks at high shear strains. San Carlos olivine powder was cold-pressed and then hot-pressed under hydrous conditions, producing aggregates with average grain sizes of 7 or 15 μm. Deformation experiments were performed in a high-resolution gas-medium apparatus equipped with a torsional actuator, under a confining pressure of 300 MPa, a temperature of 1200 °C, and constant shear strain rates ranging from 8 × 10−5 to 1.4 × 10−4 s−1. Maximum shear stresses range from 150 to 195 MPa. These values are 30% lower relative to those determined in previous torsion experiments on dry, fined-grained dunites under similar conditions. Textures and microstructures of the starting and deformed specimens were characterized by scanning and transmission electron microscopy. All deformed aggregates exhibit a shape-preferred orientation marking a foliation and lineation, as well as a reduction in mean grain size from 15 μm down to 3-4 μm due to dynamic recrystallization. Olivine crystallographic fabrics developed rapidly (γ < 0.1), but their strength, characterized by the J-index, is low compared to naturally deformed peridotites or to polycrystalline olivine deformed at similar finite shear strains under dry conditions. The crystallographic fabrics are consistent with deformation by a dislocation accommodated creep mechanism with activation of multiple {0 k l}[1 0 0] systems, among which the (0 1 0)[1 0 0] slip system is dominant, and minor participation of the (0 1 0)[0 0 1] slip system. Transmission electron microscopy confirmed the occurrence of dislocations with [1 0 0] and [0 0 1] Burgers vectors in most grains. Analysis of unpolarized infrared spectra indicates that hydrogen concentration in the olivine lattice is below the saturation level of 18 ppm wt H2O, which is similar to those typically observed in spinel-bearing peridotite xenoliths, and also provide evidence for water-rich inter-granular material trapped in pores and grain boundaries. Seismic properties computed from the CPO observations correspond to those most commonly observed in naturally deformed mantle peridotites with fast P-wave propagation and S-wave polarization subparallel to the shear direction. These torsion experiments on fine-grained olivine polycrystals under hydrous conditions indicate that water weakening under lithospheric conditions is linked to various defects with hydrogen in the olivine structure, as well as with water-derived species in grain boundaries or pores.
ISSN: 0031-9201

hal-00745170
https://hal.archives-ouvertes.fr/hal-00745170
DOI : 10.1016/j.pepi.2012.05.001

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
La Cabaña peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabaña peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3 kbar and 380°–420 °C. Within La Cabaña peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe2+-rich chromite with a variation trend of major elements from chromite to Fe2+-rich chromite similar to those observed in other locations, i.e., an increase in Fe2O3 and FeO, a decrease in Al2O3 and MgO. Cr2O3 content increases from chromite to Fe2+-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4 wt.%, with the exception of MnO (<0.9 wt.%), which shows a correspondence with increasing degree of alteration. Cr# (Cr/Cr + Al) versus Mg# (Mg/Mg + Fe2+) and Fe3+/Fe3++Fe2+ versus Mg# plots are used to illustrate the Cr-spinel alteration process. Overall, the Cr-spinels from Lavanderos (chromitite pods and disseminated grains) exhibit Cr# values ranging from 0.6 to 1.0, Mg# (Mg/Mg + Fe2+) below 0.5, and (Fe3+/Fe3++Fe2+) <0.4. Cr-spinels from chromitites in Centinela Bajo have Cr# and Mg# values that range from 0.65 to 1.0, and 0.7-0.3, respectively, and (Fe3+/Fe3++Fe2+) < 0.4. Accessory Cr-spinels from Centinela Bajo have Cr# and Mg# values that range from 0.55 to 1.0, and 0.6- < 0.1, respectively. The (Fe3+/Fe3++Fe2+) ratio is less than 0.4 in chromite cores and Fe2+-rich chromite, and >0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe2+-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe2+-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe3+ is incorporated in chromite/Fe2+-rich chromite shifting its composition to an Fe3+-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe2+-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabaña reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe2+-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.
ISSN: 0930-0708

hal-01172266
https://hal.archives-ouvertes.fr/hal-01172266
DOI : 10.1007/s00710-014-0335-5

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The ultramafic rocks of the Collo region in northeastern Algeria crop out as “stratified” masses that cut across older metamorphic formations of the Petite Kabylie basement. Based on petrological compositions and mineralogical observations, these rocks are mainly peridotites and serpentinites. The peridotites are identified as lherzolites, but dunites may occur rarely. The lherzolites are composed of olivine, orthopyroxene, clinopyroxene and chromian spinel. Their chemical composition shows high MgO (34.4–37.5 wt%), Cr (0.14–0.27 wt%), Ni (0.14–0.26 wt%) and Co (34–133 ppm) contents and low CaO and Al2O3 concentrations (0.02–2.2 wt% and 0.5 to 2.8 wt%, respectively). The chromite, which represents approximately 1–3% of the rock, is ubiquitous and shows two different generations: primary millimetric euhedral crystals and secondary fine xenomorphic grains and interstitial aggregates. The primary chromites are alumino-ferro-magnesian crystals that show high Al2O3 (25.77%–27.36%) and MgO (10.70%–13.36%). Cr# (100 × Cr/(Al + Cr)) ranges from 45 to 48, and Mg# (100 × Mg/(Mg + Fe2+)) from 49 to 59. The secondary interstitial grains are iron-rich chromites. They show low Al2O3 (4.67%–9.54%) and MgO (4.60%–4.65%). Cr# is relatively high (77–88), whereas Mg# shows relatively low values, ranging from 22 to 25. Primary chromite and whole-rock chemistry show that the Collo ultramafic rocks belong to Alpine-type peridotites that were emplaced within an orogenic setting. The oxygen isotopic composition of both peridotites and chromites is consistent with their derivation from a mantle source (δ18O ranges from +3.0 to +5.9‰). Low δ18O values (<+4.4‰) are recorded in serpentinites and are attributed to the effect of serpentinization processes through high-temperature metasomatic fluids. Magnesite-bearing serpentinites show the lowest δ18O values. These are interpreted as the result of surface water input.
ISSN: 0899-5362

hal-01468003
https://hal.archives-ouvertes.fr/hal-01468003
DOI : 10.1016/j.jafrearsci.2016.10.012

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report gabbroic dikes in the plagioclase tectonite domains of the Ojén and Ronda massifs (Betic Cordillera, southern Spain), which record crystallization at low-pressure syn-, or slightly postkinematic to the late ductile history of the Betic Peridotite in the westernmost Mediterranean. We present mineral major and trace element compositional data of discordant gabbroic dikes in the Ojén massif and gabbroic patches in the Ronda massif, complemented by the whole rock and electron backscattered diffraction (EBSD) data of the Ojén occurrence. In the Ojén massif, gabbro occurs as 1–3 centimeter wide discordant dikes that crosscut the plagioclase tectonite foliation at high angle. These dikes are composed of cm-scale igneous plagioclase and clinopyroxene crystals that show shape preferred orientations subparallel to the lineation of the host peridotite and oblique to the trend of the dike. Intrusion of Ojén gabbro dikes is coherent with the stress field that formed the high temperature, ductile plagioclase tectonite foliation and then attests for a mantle igneous event prior to the intracrustal emplacement of the massif. In the Ronda massif, gabbroic rocks crystallized in subcentimeter wide anastomozing veins, or as interstitial patches in the host dunite. They are mostly composed of plagioclase and clinopyroxene. Plagioclase composition is bytownitic in the Ojén, and andesinic in the Ronda massif. Clinopyroxene in both places shows identical, light Rare-Earth Element (LREE) depleted trace element patterns. The calculated trace element composition of melts in exchange equilibrium with the studied igneous clinopyroxenes reflects LREE-enriched character coupled with negative Eu anomaly, and indicates that gabbro-forming melts in Ronda and Ojén share a common melt source with an island arc tholeiitic affinity. Geothermobarometric data and liquidus mineralogy indicate that gabbro crystallization occurred at shallow depths (0.2–0.5 GPa) in a 7–16 km thick lithospheric section. These data suggest that gabbro-forming melts in the Betic Peridotite record a mantle igneous event at very shallow depths and provide evidence for the hyperextension of the continental lithosphere compatible with extreme backarc basin extension induced by the slab rollback of the Cenozoic subduction system in the westernmost Mediterranean.
ISSN: 0040-1951

hal-01172246
https://hal.archives-ouvertes.fr/hal-01172246
DOI : 10.1016/j.tecto.2015.03.011

Éditeur(s) : HAL CCSD Elsevier
Résumé : Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation.Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith (‘hybrid mantle wedge’) composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid compositions extracted from these hybrid sources are higher in normative quartz and hypersthene (i.e., they have a more silica-saturated character) in comparison with basalts derived from prior melt-depleted asthenospheric mantle beneath ridges. These primary arc melts range from silica-rich picrite to boninite and high-Mg basaltic andesite along a residual spinel harzburgite cotectic. Silica enrichment in the mantle sources of arc-related, subalkaline picrite-boninite-andesite suites coupled with the amount of water and depth of melting, are important for the formation of medium-Fe (‘calc-alkaline’) andesite-dacite-rhyolite suites, key lithologies forming the continental crust.
ISSN: 0016-7037

hal-01553520
https://hal.archives-ouvertes.fr/hal-01553520
DOI : 10.1016/j.gca.2016.09.030

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 Oxford University Press (OUP)
Résumé : The Oman ophiolite consists of several massifs cropping out along a 500 km long band trending NW-SE along the coast of Oman; it is one of the best exposed sections of oceanic crust and mantle in the world. There is a gradient in igneous processes and composition in the ophiolite, with the northern massifs recording a polygenetic igneous history involving an increasingly important subduction component, whereas the southern massifs were formed primarily via a mid-ocean ridge basalt (MORB)-like, single-stage process at a submarine spreading ridge. In this study we use geochemical data from Wadi Tayin, which is one of the southern massifs of the Oman ophiolite, to constrain the composition and genesis of oceanic crust and upper mantle. The Wadi Tayin harzburgites are residues of partial melting that are as depleted as the most depleted mid-ocean ridge peridotites. They have low middle- to heavy rare earth element ratios, most probably reflecting melting close to and beyond the exhaustion of clinopyroxene. Like many abyssal peridotites, the Wadi Tayin samples show enrichment in highly incompatible elements, giving rise to U-shaped MORB-normalized trace element patterns. We favor the idea that this is caused by enrichment of highly incompatible elements along grain boundaries in peridotite, which may be the result of near-equilibrium partitioning between grain boundaries and grain interiors, rather than disequilibrium processes. Equilibrium partitioning between crystals and grain boundaries during melting and melt extraction is also our preferred explanation for ubiquitous high Pb contents relative to Ce and La in our samples as well as in most abyssal peridotites. The Wadi Tayin samples record substantial variability in terms of osmium isotopic composition and platinum group element (PGE) concentrations. The more radiogenic nature of the dunites and impregnated peridotites compared with the residual harzburgites may be due to relatively high Os-187/Os-188 in melt transported through the dunites. The presence of residual peridotites with whole-rock osmium isotopic compositions less radiogenic than MORB may be explained by melting of a veined (upwelling) mantle forming mixed melts in which much of the Os derives from veins with high Re/Os in a matrix of previously depleted peridotite. An important result of this study is that the shallowest samples are refertilized (i.e. anomalously enriched in incompatible elements), and record relatively high metamorphic closure temperatures. These observations suggest that migrating melt underwent crystallization during rapid cooling in the uppermost mantle during and immediately after ridge magmatism. A variety of geothermometers all yield the result that the stratigraphically highest harzburgites equilibrated at higher temperature than the deeper ones. The systematic decrease in closure temperature with increasing depth below the Moho transition zone probably reflects systematic variation in cooling rate as a function of depth in the mantle section. We hypothesize that the refertilization and higher closure temperatures recorded by the uppermost mantle samples are linked. More rapid cooling led to higher closure temperatures, and to partial crystallization of migrating melts in the shallowest part of the mantle section, yielding slightly elevated abundances of elements such as Ca and Na, and incompatible trace elements.
ISSN: 0022-3530

hal-00456056
https://hal.archives-ouvertes.fr/hal-00456056
DOI : 10.1093/petrology/egp077

Éditeur(s) : HAL CCSD American Chemical Society
Résumé : International audience
Carbonation of ultramafic rocks in geological reservoirs is, in theory, the most efficient way to trap CO2 irreversibly; however, possible feedback effects between carbonation reactions and changes in the reservoir permeability must be considered to realistically assess the efficiency and sustainability of this process. We investigated changes in the hydrodynamic properties of sintered dunite samples by means of percolation experiments, under conditions analogous to that of in situ carbonation. Our results show that carbonation efficiency is controlled by the local renewal of the reactants and the heterogeneity of the pore structure. Preferential flow zones are characterized by the formation of magnetite and of a silica-rich layer at the olivine surfaces, which eventually inhibits olivine dissolution. Conversely, sustainable olivine dissolution together with coprecipitation of magnesite, siderite, and minor Mg-TOT-phyllosilicates, occur in reduced-flow zones. Thus carbonate precipitation only decreases porosity in zones where diffusion-controlled transport is dominant. Consequently, while high flow rates will decrease the carbonation efficiency of the reservoir and low flow rates may reduce the permeability irreversibly close to the injection point, moderate injection rates will ensure a partial carbonation of the rock and maintain the reservoir permeability.
ISSN: 0013-936X

hal-00424540
https://hal.archives-ouvertes.fr/hal-00424540
DOI : 10.1021/es8018429

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The 1415 m deep IODP Hole U1309D (Mid-Atlantic Ridge, 30 degrees N) is the second deepest hole drilled into slow spreading oceanic lithosphere. The recovered section comprises essentially gabbroic rocks, with a large range of compositions. The most primitive end-members of the gabbroic sequence, herein referred to as olivine-rich troctolites (ol > similar to 70%), have textures and geochemical compositions intermediate between that of mantle peridotites and primitive cumulates, indicative of melt impregnation processes. We carried out a detailed microstructural study to further characterize the petrogenetic processes leading to their formation, as well as discuss their mode of emplacement and relationship with neighboring mantle lithosphere. In olivine-rich troctolites, olivines range from coarse-grained subhedral crystals, commonly containing well-developed subgrains, to medium-grained rounded crystals with fewer or no substructures. They are embedded in large, undeformed pyroxene and plagioclase poikiloblasts. Olivine substructures reveal dislocation creep that is consistent with activation of the main high-temperature slip systems, dominantly (010)[100]. Olivine crystallographic preferred orientation is very weak but generally shows a relatively stronger, uncommon [001] concentration. These unusual olivine fabrics are interpreted as resulting from melt impregnation of a previously deformed olivine matrix: the solid olivine framework is disrupted by olivine corrosion along grain and subgrain boundaries, and the high-temperature plastic fabric is modified in a liquid-dominated regime. Based on mineral composition and fabrics and in comparison with what is observed in impregnated mantle rocks elsewhere, we posit that olivine represents relicts of mantle peridotites disaggregated by large melt influx, although the mantle origin of olivine is not unequivocally demonstrated yet. Whatever the initial lithology, impregnation by large volumes of melt has strongly modified the original composition and microstructure. If the mantle origin hypothesis is correct, the original olivine fabric could have been efficiently weakened by dunitization prior to disruption of the olivine framework by melt impregnation. Incorporation, at the base of the lithosphere, of small slivers of impregnated dunite into gabbroic sections, trapped between successive igneous units, may be a common mechanism of lower crustal accretion at slow spreading ridges. Extensive melt-rock interaction processes are expected to contribute significantly to the final chemical composition of erupted lavas.
ISSN: 1525-2027

hal-00496391
https://hal.archives-ouvertes.fr/hal-00496391
DOI : 10.1029/2009GC002995