Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : Mid-ocean ridges magmatism is, by and large, considered to be mostly dry. Nevertheless, numerous works in the last decade have shown that a hydrous component is likely to be involved in ocean ridges magmas genesis and/or evolution. The petrology and geochemistry of peculiar coarse grained gabbros sampled in the upper part of the gabbroic sequence from the northern Oman ophiolite (Wadi Rajmi) provide information on the origin and fate of hydrous melts in fast-spreading oceanic settings. Uncommon crystallization sequences for oceanic settings (clinopyroxene crystallizing before plagioclase), extreme mineral compositions (plagioclase An% up to 99, and clinopyroxene Mg # up to 96), and the presence of magmatic amphibole, imply the presence of a high water activity during crystallization. Various petrological and geochemical constraints point to hydration, resulting from the recycling of hydrothermal fluids. This recycling event may have occurred at the top of the axial magma chamber where assimilation of anatectic hydrous melts is recurrent along mid-ocean ridges or close to segments ends where fresh magma intrudes previously hydrothermally altered crust. In ophiolitic settings, hydration and remelting of hydrothermally altered rocks producing hydrous melts may also occur during the obduction process. Although dry magmatism dominates oceanic magmatism, the dynamic behavior of fast-spreading ocean ridge magma chambers has the potential to produce the observed hydrous melts (either in ophiolites or at spreading centers), which are thus part of the general mid-ocean ridges lineage.
ISSN: 1525-2027

hal-00939400
https://hal.archives-ouvertes.fr/hal-00939400
DOI : 10.1002/ggge.20137

Éditeur(s) : HAL CCSD IODP
Résumé : The Superfast Spreading Crust campaign, echoing long-standing ocean lithosphere community endeavors, was designed to help us understand the formation, architecture, and evolution of ocean crust formed at fast spreading rates. Integrated Ocean Drilling Program (IODP) Expedition 335, "Superfast Spreading Rate Crust 4" (13 April-3 June 2011), was the fourth scientific drilling cruise of the Superfast Spreading Crust campaign to Ocean Drilling Program (ODP) Hole 1256D. The expedition aimed to deepen this basement reference site several hundred meters into the gabbroic rocks of intact lower oceanic crust to address the following fundamental scientific questions: Does the lower crust form by subsidence of a crystal mush from a high-level magma chamber (gabbro glacier), by intrusion of sills throughout the lower crust, or by some other mechanism? How does melt percolate through the lower crust, and what are the reactions and chemical evolution of magmas during migration? Is the plutonic crust cooled by conduction or hydrothermal circulation? What are the role and extent of deeply penetrating seawater-derived hydrothermal fluids in cooling the lower crust and the chemical exchanges between the ocean crust and the oceans? What are the relationships among the geological, geochemical, and geophysical structure of the crust and, in particular, the nature of the seismic Layer 2-3 transition? What is the magnetic contribution of the lower crust to marine magnetic anomalies?
https://hal.archives-ouvertes.fr/hal-00688990

hal-00688990
https://hal.archives-ouvertes.fr/hal-00688990
DOI : 10.2204/iodp.pr.335.2011

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Search String Advanced > Saved Searches >ARTICLE TOOLS Get PDF (3653K) Save to My Profile E-mail Link to this Article Export Citation for this Article Get Citation Alerts Request PermissionsMore Sharing ServicesShare|Share on citeulikeShare on facebookShare on deliciousShare on www.mendeley.comShare on twitter Abstract Article References Cited ByView Full Article (HTML) Enhanced Article (HTML) Get PDF (3653K)AbstractFor the first time, the crystallized remnant of an oceanic ridge magma chamber is documented in the Oman ophiolite. It exists in the centre of a 40 km long monoclinal ridge (Jebel Dihm, Wadi Tayin massif), exposing a full crustal section perpendicular to the spreading direction. New detailed mapping supported by U-Pb zircon geochronology suggests that the active, fast-spreading ridge that died just prior to detachment of the ophiolite is preserved and largely intact. Our observations provide insights into the crystallizing mush zone of a magma chamber, before it crosses the external walls and solidifies as deformed gabbros. Our data provide new constraints on the shape and internal dynamics of a magma chamber, including gabbro subsidence from the floor of a perched melt lens and the limited contribution of sills to crustal accretion. By locating precisely the palaeo-ridge axis, prior full spreading rate estimates can be increased to ~140 km Ma−1.
ISSN: 0954-4879

hal-01121586
https://hal.archives-ouvertes.fr/hal-01121586
DOI : 10.1111/ter.12130

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Oman ophiolite includes both a fossil fast spreading axis, defined by five mantle diapirs, and an off-axis mantle diapir emplaced 30 km from the axis, providing a natural laboratory for the study of off-axis magmatic processes. We compare field and petrological observations coupled with geochemical and isotopic analyses of samples from the off-axis diapir with those of the nearest on-axis diapir, with a particular focus on the Moho Transition Zone (MTZ). Both diapirs are defined by the presence of steeply plunging lineations, but in the on-axis case, these lineations rotate gradually into parallelism with the horizontal magmatic lineations of the overlying crust, while in the off-axis case, a shear zone separates the steeply plunging lineations from the horizontal lineations of the surrounding mantle. In the on-axis diapir, the MTZ is 50 to 500 m thick and composed of dunite with layered gabbro lenses whereas in the off-axis diapir, the MTZ is thicker and composed of dunite with massive (∼20% of MTZ) clinopyroxenite lenses and a notable absence of plagioclase. Moreover, the off-axis diapir is associated with amphibole-bearing intrusions, consisting of Mg-rich gabbroic sills in the mantle peripheral to the diapir, and microgabbroic lenses of broadly basaltic composition in the overlying crust. The ε Nd values of the pyroxenites in the MTZ of the off-axis diapir fully overlap with those of the intrusions in the surrounding mantle and crust, suggesting that they are genetically related. Calculated rare earth element (REE) abundances of liquids in equilibrium with clinopyroxene imply that the magmas that traversed the MTZ of the off-axis diapir were more depleted in highly incompatible elements than their counterparts in the MTZ of the on-axis diapir. On the other hand, Nd isotopic compositions of the off-axis samples (εNd=6.2–7.9εNd=6.2–7.9 in 18 of 19 samples) indicate derivation of their parental magmas from a less depleted source than that which produced the magma associated with the on-axis gabbro (εNd=7.8–9.2εNd=7.8–9.2, 10 analyses).To explain these observations, we suggest that the earliest magmas in the uprising off-axis diapir formed from the partial melting of pyroxenite veins with less radiogenic Nd isotopic compositions than those of the ambient peridotite. As the diapir traversed the cool, hydrated lithosphere these early melts interacted with depleted harzburgites, lowering the incompatible element contents of the melt products while having little effect on their Nd isotopic compositions. The great abundance of clinopyroxene in the off-axis MTZ might be explained by the high pyroxene component in the original melt but perhaps also by the presence of water in the lithosphere, which would favor the crystallization of clinopyroxene while inhibiting that of plagioclase. The intrusions in the overlying crust could represent, to first order, the secondary melts produced by the melt–harzburgite reaction, while the sills in the surrounding mantle may be cumulates from such secondary melts.These results shed light on processes occurring during interaction between rising off-axis material and depleted, hydrated lithospheric mantle. Furthermore, if our interpretation is correct, the low εNd values of the off-axis samples contribute to the growing body of evidence for the presence of pyroxenite veins in the MORB mantle source.
ISSN: 0012-821X

hal-01306575
https://hal.archives-ouvertes.fr/hal-01306575
DOI : 10.1016/j.epsl.2015.12.005

Éditeur(s) : HAL CCSD
Résumé : National audience
New experimental data on electrical and anelastic properties of oceanic rocks at high pressures and temperatures are compared with the field data of seismic and electrical anomalies observed at oceanic spreading centers. The laboratory results of electrical conductivity and internal friction on gabbro and peridotites permit to estimate a degree of partial melting and maximum temperature in axial magma chambers (12-15% of melting at 1155-1165°C) and at the upper boundary of a source zone at the depth of 40-90 km (less than 5% of melting in the absence of pyroxene crystals and up to 11-14% in the pyroxene reach rocks at 1360-1370°C) at mid-oceanic ridges.
ISSN: 0934-6554

hal-01322920
http://hal.univ-reunion.fr/hal-01322920

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Geochronological studies of samples from continuous crustal sections of fossil intra-oceanic arcs are paramount for determining the precise timing of major processes that took place during the building of an oceanic island-arc crust. In this study, laser ablation ICP-MS U-Pb zircon (and conventional Rb-Sr) analyses pinpoint the timing of major events responsible for crustal growth of the Kohistan paleo-island arc (Northern Pakistan). Inheritance in magmatic rocks is evidence for recycling of intra-arc material and indicates that the Kohistan arc may have formed as early as 135 +/- 4 Ma. One older inherited grain indicates that the arc developed on a young oceanic lithosphere whose remnants include a c. 175 Ma old component. An interesting consequence is that intra-oceanic arc magmas can yield inherited zircons, which can be much older than the arc system itself without requiring recycling back into the mantle by subduction processes. In the case of the Kohistan arc, ante-arc Neotethys oceanic lithosphere was tapped by arc magmas during their way upward into the arc crust. The oldest magmatic ages measured in this study fall in the range 101-102 Ma (100.9 +/- 0.6 and 102.1 +/- 0.4 Ma). This period corresponds to arc build-up and thickening of the arc crust. Leucogranitic melts dated at 89.9 +/- 0.4 Ma and 90.9 +/- 1.0 Ma are considered as produced by dehydration/melting reaction accompanying granulitisation of the thickened lower arc crust. A maximum age for this event is 97.7 +/- 0.7 Ma, the age of emplacement of a garnet meta-tonalite affected by granulite facies metamorphism. Magmatic activity was still ongoing during and after the granulitisation as testified by emplacement of a diorite and a gabbro dated at 89.1 +/- 0.6 and 88.2 +/- 0.9 Ma respectively. The upper part of the metaplutonic sequence contains diorite samples dated at 84.6 +/- 0.5 Ma and 84.3 +/- 0.5 Ma and as young as 81.1 +/- 0.7 Ma. During this period, and based on the present day outcropping sequences, the mean crustal growth rate varied from 32 to 65 km(3)/km/Ma (volume per unit width along the strike of the arc) which is comparable to the range for present day arcs of the Western Pacific region. The correspondence of crustal growth rates observed for present-day island arcs and the good preservation of the crustal section in the Kohistan arc, make this area an exceptional natural laboratory to study arc related processes and to check models of continental crust formation by arc accretion.
ISSN: 0012-821X

hal-00644555
https://hal.archives-ouvertes.fr/hal-00644555
DOI : 10.1016/j.epsl.2011.07.016

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Highly siderophile element concentrations (HSE: Re and platinum-group elements (PGE)) are presented for gabbros, gabbroic eclogites and basaltic eclogites from the high-pressure Zermatt-Saas ophiolite terrain, Switzerland. Rhenium and PGE (Os, Ir, Ru, Rh, Pt, Pd) abundances in gabbro- and eclogite-hosted sulphides, and Re-Os isotopes and elemental concentrations in silicate phases are also reported. This work, therefore, provides whole rock and mineral-scale insights into the PGE budget of gabbroic oceanic crust and the effects of subduction metamorphism on gabbroic and basaltic crust. Chondrite-normalised PGE patterns for the gabbros are similar to published mid-ocean ridge basalts (MORB), but show less inter-element fractionation. Mean Pt and Pd contents of 360 and 530 pg/g, respectively, are broadly comparable to MORB, but gabbros have somewhat higher abundances of Os, Ir and Ru (mean: 64, 57 and 108 pg/g). Transformation to eclogite has not significantly changed the concentrations of the PGE, except Pd which is severely depleted in gabbroic eclogites relative to gabbros (similar to 75% loss). In contrast, basaltic eclogites display significant depletion of Pt (>= 60%), Pd (>85%) and Re (50-60%) compared with published MORB, while Os, Ir and Ru abundances are broadly comparable. Thus, these data suggest that only Pt, Pd and Re, and not Os, Ir and Ru, may be significantly fluxed into the mantle wedge from mafic oceanic crust. Re-Os model ages for gabbroic and gabbroic eclogite minerals are close to age estimates for igneous crystallisation and high-pressure metamorphism, respectively, hence the HSE budgets can be related to both igneous and metamorphic behaviour. The gabbroic budget of Os, Ir, Ru and Pd (but not Pt) is dominated by sulphide, which typically hosts >90% of the Os, whereas silicates account for most of the Re (with up to 75% in plagioclase alone). Sulphides in gabbroic eclogites tend to host a smaller proportion of the total Os (10-90%) while silicates are important hosts, probably reflecting Os inheritance from precursor phases. Garnet contains very high Re concentrations and may account for >50% of Re in some samples. The depletion of Pd in gabbroic eclogites appears linked, at least in part, to the loss of Ni-rich sulphide. Both basaltic and gabbroic oceanic crust have elevated Pt/Os ratios, but Pt/Re ratios are not sufficiently high to generate the coupled Os-186-Os-187 enrichments observed in some mantle melts, even without Pt loss from basaltic crust. However, the apparent mobility of Pt and Re in slab fluids provides an alternative mechanism for the generation of Pt- and Re-rich mantle material, recently proposed as a potential source of Os-187-Os-186 enrichment.
ISSN: 0016-7037

hal-00420072
https://hal.archives-ouvertes.fr/hal-00420072
DOI : 10.1016/j.gca.2008.11.036

Éditeur(s) : HAL CCSD
Résumé : Ce travail est composé de trois chapitres, qui correspondent respectivement à trois publications : -le premier présente le cadre géologique et structural du massif de Najac (Bodinier et Burg,1981) et discute ses relations avec l 'ensemble des séries cristallophyliennes du Rouergue occidental; -le deuxième concerne l'étude géochimique de ce massif, menée afin de définir son affinité géochimique et d'élucider les relations pétrogénétiques entre les différents termes qui le constituent (Bodinier,1981); -enfin,le troisième chapitre concerne l'étude strictement géochimique du massif de Chamrousse , avec des objectifs identiques à ceux envisagés pour le massif de Najac .Pour une meilleure compréhension du problè, les données géologiques mises aimablement à notre disposition par F. Carme y sont présentées succintement (Bodinier et al.,1981) . Le but de cette étude, les résultats acquis et les implications majeures sont résumés dans les premières pages.
pas de résumé
https://tel.archives-ouvertes.fr/tel-00803436

tel-00803436
https://tel.archives-ouvertes.fr/tel-00803436
https://tel.archives-ouvertes.fr/tel-00803436/document
https://tel.archives-ouvertes.fr/tel-00803436/file/These-Bodinier-1981.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
At oceanic spreading centers, interactions between magma and hydrothermal convecting systems trigger major physical, thermal, and chemical exchanges. The two-pyroxene hornfels recovered from the base of the sheeted dike sequence at Integrated Ocean Drilling Program (IODP) Site 1256 (equatorial Eastern Pacific) are interpreted as a conducting boundary layer between the underlying axial melt lens and the hydrothermally cooled sheeted dikes. They are cut by numerous small, felsic veins, which were recently interpreted as a product of hydrous partial melting of sheeted dikes. Here, we present trace element compositions of products (melts and residues) of hydrous partial melting experiments using basalts and hornfels from IODP Site 1256 as starting material. The experimental products generated between 910 °C and 970 °C match the natural lithologies from Site 1256 in terms of major and trace element compositions. The compositions of the anatectic melts correspond to the compositions of the felsic veins, while the residual minerals match the compositions of the two-pyroxene hornfels, evidencing that hydrous partial melting is an important magmatic process in the gabbro/dike transition of fast-spreading mid-oceanic ridges. Our results complement previous experimental studies on anatectic processes occurring at the roof of the magma chambers from fast-spreading mid-ocean ridges. Moreover, calculations of mixing and assimilation fractional crystallization using the experimental partial melts as contaminant/assimilant showed that anatectic melts can only be a minor contributor to the contamination process.
ISSN: 0024-4937

hal-01419949
https://hal.archives-ouvertes.fr/hal-01419949
DOI : 10.1016/j.lithos.2016.05.001