Éditeur(s) : HAL CCSD Integrated Ocean Drilling Program
Résumé : In this report we present the results of laboratory measurements carried out to explore the electrical properties of gabbroic and troctolitic samples from Integrated Ocean Drilling Program (IODP) Hole U1309D in the Atlantis Massif, an oceanic core complex located at 30°N on the Mid-Atlantic Ridge. Oceanic core complexes are presumably localized in portions of heterogeneous oceanic crust that are relatively rich in igneous rocks and are the locus of significant hydrothermal activity. Electrical properties of igneous rocks are used to discuss the rocks' porosity structure as a function of mineralogy, alteration processes, and deformation. Electrical properties of the gabbroic suites sampled during IODP Expedition 304/305 can contribute to the understanding of these processes in the Atlantis Massif. We present the full suite of electrical data together with density and porosity measurements on the same suite of samples.
ISSN: 1930-1014

hal-00464179
https://hal.archives-ouvertes.fr/hal-00464179
DOI : 10.2204/iodp.proc.304305.204.2009

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (∼0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt–rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equilibration with residual harzburgites. Electronic supplementary material Supplementary material is available in the online version of this article at ;; and is accessible for authorized users
ISSN: 0010-7999

hal-00404655
https://hal.archives-ouvertes.fr/hal-00404655
DOI : 10.1007/s00410-006-0148-6

Éditeur(s) : HAL CCSD
Résumé : IODP Hole U1309D (IODP Exp. 304-305, 30°N) and ODP Site 1275 (ODP Lag 209, 15°45?N) sampled two oceanic core complexes, at the Mid-Atlantic Ridge. The recovered rocks are mostly gabbroic with some very primitive and olivine rich (Ol>70%). In Hole U1309D, trace element compositions of clinopyroxene and plagioclase poikiloblasts from olivine-rich rocks ndicate that they crystallized from the same melt in all lithologies. Olivine trace element compositions are in disequilibrium with the two other minerals. Olivine crystallographic preferred orientations are weak, with a relatively strong uncommon [001] concentration but consistent with deformation by dislocation creep with activation of the high temperature (010) [100] slip system, commonly described in asthenospheric mantle. The joint study of geochemical processes and microstructures in these rocks suggest a complex crystallization history in an open system with percolation of large volume of MORBtype melt and interaction with the depleted shallow mantle. Olivine-rich rocks are interpreted as the ultimate residue of these melt-mantle reaction processes. At Site 1275, the formation of the more evolved rocks of the gabbroic series is not related to the impregnation event creating olivine-rich rocks.
These rocks represent late magmatic injections, which are completely crystallized at depth as intrusivegabbroic bodies. The results presented in this thesis are consistent with the formation of oceanic core complexes associated with relatively strong magmatic activity, and with the crystallization of most of melt in the lithosphere without basaltic counterpart erupted on the seafloor.
Le puits IODP U1309D (Exp. IODP 304-305, 30°N) et le Site ODP 1275 (Leg ODP 209, 15°45'N) ont permis d'échantillonner deux core complexes océaniques de la dorsale Médio-Atlantique. Les roches récupérées sont principalement gabbroïques dont certaines très primitives et riches en olivine (ol >70%). Dans le puits U1309D, les compositions en éléments en trace des poeciloblastes de clinopyroxène et de plagioclase des roches riches en olivine indiquent qu'ils précipitent depuis le même magma dans toutes les lithologies. La composition en éléments en trace des olivines est en déséquilibre avec ces deux minéraux. Les fabriques cristallographiques de l'olivine sont faibles avec une concentration sur [001] inhabituelle, néanmoins compatibles avec une déformation plastique de haute température, avec l'activation du système de glissement (010) [100] communément décrit dans le manteau asthénosphérique. L'étude conjointe des caractéristiques géochimiques et microstructurales de ces roches met en lumière une histoire complexe de cristallisation dans un système ouvert où de larges volumes de magma de type MORB ont percolé et interagi avec le manteau appauvri superficiel. Ces roches riches en olivine représenteraient le résidu ultime de ces réactions liquide-manteau. Au site 1275, la formation des roches les plus évoluées de la série n'apparaît pas liée à l'événement d'imprégnation formant les roches riches en olivine. Ces roches correspondent à des injections tardives de magma qui ont entièrement cristallisé en profondeur sous forme de plutons intrusifs. Les résultats présentés dans ce mémoire sont compatibles avec une formation des core complexes océaniques associée à une activité magmatique relativement importante, et à une cristallisation complète de tout ou partie de ces magmas dans la lithosphère sans contre-partie volcanique en surface.
https://tel.archives-ouvertes.fr/tel-00354807

tel-00354807
https://tel.archives-ouvertes.fr/tel-00354807
https://tel.archives-ouvertes.fr/tel-00354807/document
https://tel.archives-ouvertes.fr/tel-00354807/file/these_MD.pdf

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Expeditions 304 and 305 of the Integrated Ocean Drilling Program cored and logged a 1.4 km section of the domal core of Atlantis Massif. Postdrilling research results summarized here constrain the structure and lithology of the Central Dome of this oceanic core complex. The dominantly gabbroic sequence recovered contrasts with predrilling predictions; application of the ground truth in subsequent geophysical processing has produced self-consistent models for the Central Dome. The presence of many thin interfingered petrologic units indicates that the intrusions forming the domal core were emplaced over a minimum of 100-220 kyr, and not as a single magma pulse. Isotopic and mineralogical alteration is intense in the upper 100 m but decreases in intensity with depth. Below 800 m, alteration is restricted to narrow zones surrounding faults, veins, igneous contacts, and to an interval of locally intense serpentinization in olivine-rich troctolite. Hydration of the lithosphere occurred over the complete range of temperature conditions from granulite to zeolite facies, but was predominantly in the amphibolite and greenschist range. Deformation of the sequence was remarkably localized, despite paleomagnetic indications that the dome has undergone at least 45 degrees rotation, presumably during unroofing via detachment faulting. Both the deformation pattern and the lithology contrast with what is known from seafloor studies on the adjacent Southern Ridge of the massif. There, the detachment capping the domal core deformed a 100 m thick zone and serpentinized peridotite comprises similar to 70% of recovered samples. We develop a working model of the evolution of Atlantis Massif over the past 2 Myr, outlining several stages that could explain the observed similarities and differences between the Central Dome and the Southern Ridge.
ISSN: 0148-0227

hal-00617528
https://hal.archives-ouvertes.fr/hal-00617528
https://hal.archives-ouvertes.fr/hal-00617528/document
https://hal.archives-ouvertes.fr/hal-00617528/file/2010JB007931.pdf
DOI : 10.1029/2010JB007931

Éditeur(s) : HAL CCSD Integrated Ocean Drilling Program
Résumé : This contribution is a compilation of electron microprobe analyses of primary minerals in mafic and ultramafic oceanic crustal rocks from Atlantis Massif. The samples were recovered during Integrated Ocean Drilling Program Expedition 304/305, which was designed to investigate the formation and evolution of oceanic core complexes. The analyses were performed at five different institutions, and while common standard reference materials were not utilized at all institutions, internal standards and routine operational protocols were undertaken to assure analysis quality. More than 5000 analyses of plagioclase, pyroxene, olivine, and spinel are compiled here as a collaborative effort to provide mineral analyses to the expedition shipboard science party and other interested researchers.
ISSN: 1930-1014

hal-00464225
https://hal.archives-ouvertes.fr/hal-00464225
DOI : 10.2204/iodp.proc.304305.202.2009

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
IODP Hole U1309D (Atlantis Massif, Mid-Atlantic Ridge 30 degrees N) is the second deepest hole drilled into slow spread gabbroic lithosphere. It comprises 5.4% of olivine-rich troctolites (->70% olivine), possibly the most primitive gabbroic rocks ever drilled at mid-ocean ridges. We present the result of an in situ trace element study carried out on a series of olivine-rich troctolites, and neighbouring troctolites and gabbros, from olivine-rich intervals in Hole U1309D. Olivine-rich troctolites display poilkilitic textures; coarse-grained subhedral to medium-grained rounded olivine crystals are included into large undeformed clinopyroxene and plagioclase poikiloblasts. In contrast, gabbros and troctolites have irregularly seriate textures, with highly variable grain sizes, and locally poikilitic clinopyroxene oikocrysts in troctolites. Clinopyroxene is high Mg# augite (Mg# 87 in olivine-rich troctolites to 82 in gabbros), and plagioclase has anorthite contents ranging from 77 in olivine-rich troctolites to 68 in gabbros. Olivine has high forsterite contents (82-88 in olivine-rich troctolites, to 78-83 in gabbros) and is in Mg-Fe equilibrium with clinopyroxene. Clinopyroxene cores and plagioclase are depleted in trace elements (e.g., Yb-cpx similar to 5-11 x Chondrite), they are in equilibrium with the same MORB-type melt in all studied rock-types. These compositions are not consistent with the progressively more trace element enriched (evolved) compositions expected from olivine rich primitive products to gabbros in a MORB cumulate sequence. They indicate that clinopyroxene and plagioclase crystallized concurrently, after melts having the same trace element composition, consistent with crystallization in an open system with a buffered magma composition. The slight trace element enrichments and lower Cr contents observed in clinopyroxene rims and interstitial grains results from crystallization of late-stage differentiated melts, probably indicating the closure of the magmatic system. In contrast to clinopyroxene and plagioclase, olivine is not in equilibrium with MORB, but with a highly fractionated depleted melt, similar to that in equilibrium with refractory oceanic peridotites, thus possibly indicating a mantle origin. In addition, textural relationships suggest that olivine was in part assimilated by the basaltic melts after which clinopyroxene and plagioclase crystallized (impregnation). These observations suggest a complex crystallization history in an open system involving impregnation by MORB-type melt(s) of an olivine-rich rock or mush. The documented magmatic processes suggest that olivine-rich troctolites were formed in a zone with large magmatic transfer and accumulation, similar to the mantle-crust transition zone documented in ophiolites and at fast spreading ridges.
ISSN: 0009-2541

hal-00420916
https://hal.archives-ouvertes.fr/hal-00420916
DOI : 10.1016/j.chemgeo.2009.02.013

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

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The present study is based on a set of lavas and crosscutting dikes collected by dives along detailed vertical transects on the northern flank of the western part of the Blanco Transform Fault, Northeast Pacific. The studied area consists of a small basin, the Western Blanco Depression (WBD), extending from the southern end of the Juan de Fuca ridge to a pseudofault trace 60 km eastward. The Northern Scarp of the WBD comprises a volcanic unit overlying a sheeted-dike complex. Major and trace element data, coupled with Sr–Nd isotope ratios, reveal a two-component mantle source, composed by an isotopically depleted matrix variably veined by more enriched material. One chemical group (NS2), indistinguishable from the other Northern Scarp samples on the basis of trace element data, has an unusually depleted isotopic composition typical of a nearly pure mantle end-member. Some cogenetic samples of the Northern Scarp have been used to constrain the differentiation modalities. Anorthite and MgO content profiles in plagioclase xenocrysts and phenocrysts reveal (i) the existence of H2O-bearing evolved melts in the mushy zones and (ii) the occurrence of mixing process between these melts and anhydrous mafic liquids. The hydration is supported by other petrographic features such as high magmatic fO2 values, calculated from Fe–Ti oxide pairs, and the presence of pyroxene inclusions in plagioclase phenocrysts. Mixing, consistent with the existence of Ni-rich ferrobasalts, is interpreted to be the consequence of the reservoir refilling by mafic liquids (Mg# = 70). These petrological and geochemical evidences are combined with the evolution of Mg# with depth to suggest a periodic open-system magma chamber evolution beneath the southern end of the Juan de Fuca ridge.
ISSN: 0024-4937

insu-00213426
https://hal-insu.archives-ouvertes.fr/insu-00213426
DOI : 10.1016/j.lithos.2007.06.009