Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
In the Oman ophiolite crustal section, a contact zone between the gabbro unit and the volcanics and diabase sheeted dikes, called the root zone of the sheeted dike complex, has been recently mapped at a fine scale in a selected area. The Oman ophiolite is derived from a fast spreading ridge which had a melt lens located between the main gabbro unit and the root zone of the sheeted dike complex. With a few exceptions accounted for, this horizon has a fairly constant thickness, similar to 100 m, and a crude internal pseudo-stratigraphy. At the base of the root zone are isotropic ophitic gabbros interpreted as a thermal boundary layer. This layer is transitional between the magmatic system of the melt lens, convecting at 1200 degrees C, and a high-temperature (< 1100 degrees C) hydrothermal system, convecting within the root zone. Above this level, the isotropic gabbros have been, locally, largely molten due to an influx of seawater, at similar to 1100 degrees C, thus generating varitextured ophitic and pegmatitic gabbros. These latter gabbros constitute the upper part of the root zone and are associated with trondjhemitic intrusions as screens in the lower sheeted dikes. Diorites and trondjhemites were also generated by hydrous melting, at temperatures below 1000 degrees C. The whole root zone is a domain of very sharp average thermal gradient (similar to 7 degrees C/m). At the top of the root zone, a new thermal boundary layer, with diabase dikes hydrated in amphibolite facies conditions, separates the preceding high- temperature convective system from the well-known greenschist facies (< 450 degrees C) hydrothermal system operating throughout the sheeted dike complex, up to the seafloor. The isotropic gabbros near the base of the root zone are intruded by protodikes with distinctive microgranular margins and an ophitic center. Protodike swarms are exceptional because, intruding a medium at similar to 1100 degrees C, they are largely destroyed by dike-in-dike intrusions and by hydrous melting. However, they demonstrate that this zone was generated by melt conduits issued from the underlying melt lens. Each dike of the sheeted dike complex is thus fed by one protodike. As this zone has been recently drilled by IODP in the eastern Pacific Ocean, a brief comparison is proposed.
ISSN: 1525-2027

hal-00411331
https://hal.archives-ouvertes.fr/hal-00411331
DOI : 10.1029/2007GC001918

Éditeur(s) : HAL CCSD
Résumé : A section of ancient fast-spreading ocean crust was sampled at Wadi Gideah, located in the Wadi Tayin Massif of the Oman ophiolite. This sample suite was used for generating a coherent data set combining various petro-geochemical and structural investigations, with the aim of building a reference section for advancing our understanding of crustal accretion processes at fast-spreading mid-ocean ridges. Additionally, this study focuses on one outcrop that represents part of the fossil axial melt lens (AML).Major and trace element analyses reveal a trend of chemical evolution up section in the lower crust, with a marked difference between the layered gabbros and the foliated gabbros above. Petrological modeling shows that the chemical evolution up section can be produced by hydrous fractionation crystallization, with melt H2O contents of ~0.8 wt% and 0.8 to 1.2 wt% for the lower and upper crust, respectively. Together with the observed very steep bulk Zr/Hf vs. Zr gradient, high F and Cl content of magmatic amphibole, general Nb-Ta depletion of melts compared with NMORB, and high Sr87/Sr86 ratio compared with modern fast-spread crust, this suggests that Wadi Gideah layered gabbros were accreted by crystallization of ascending melts in sills, in a context of subduction initiation. The distinct changes in chemical trends and average grain size at the layered to foliated gabbro transition are tentatively explained by enhanced hydrothermal cooling deeper than the AML. The gabbro/dike transition displays complex structural and lithological relationships that, together with fractional crystallization modeling, point to episodic vertical movements of the AML. Plagioclase crystallographic preferred orientations (CPO) measured over the whole gabbro section are always consistent with magmatic flow. The CPO strength generally increases downward, with more scattering in the layered gabbro section. From the top to the bottom of the layered gabbros, CPO become progressively more prolate, possibly reflecting increasing shear induced by active mantle flow underneath.Wadi Gideah lower crust recorded a formation history consistent with a hybrid accretion model, combing several processes such as downward magmatic flow and/or upward melt migration in the upper foliated gabbros, sill intrusions in the layered gabbros, and deep hydrothermal circulation. The latter was presumably focused in channels, preserved today as several, up to 100 m wide zones of extensively altered former layered gabbro, cross-cutting the magmatic layering. These metagabbros display significantly higher Sr87/Sr86 ratio, late stage magmatic phases, and evidence for high temperature partial melting.
Une coupe d'ancienne croute océanique rapide a été échantillonnée dans le Wadi Gideah, situé dans le Massif de Wadi Tayin de l'Ophiolite d'Oman. Cette série d'échantillons a permis de générer un jeu de données cohérent, combinant des études pétro-géochimiques et structurales, dans le but de construire une coupe de référence pour mieux contraindre notre compréhension des processus d'accrétion crustale aux dorsales océaniques rapides. Ce travail intègre aussi l'étude détaillée d'un affleurement représentant en partie la lentille magmatique axiale (LMA) fossile.Les analyses d'éléments majeurs et traces révèlent une évolution chimique dans la croûte inférieure, avec une nette différence entre les gabbros lités et les gabbros foliés sus-jacents. La modélisation pétrologique montre que l'évolution chimique vers le haut peut être produite par cristallisation fractionnée hydratée, avec des teneurs en H2O des magmas de ~0.8 wt% dans la croute inférieure et 0.8 to 1.2 wt% dans la croute supérieure. Combiné aux forts gradients de Zr/Hf vs. Zr, aux teneurs élevées de F et Cl dans les amphiboles magmatiques, à l'appauvrissement général en Nb-Ta des magmas comparé aux NMORB, et aux rapports Sr87/Sr86 élevés comparés à la croûte rapide moderne, ceci suggère que les gabbros lités du Wadi Gideah ont été formés par la cristallisation des magmas ascendants dans des sills, dans un contexte d'initiation de subduction. Les changements de signature géochimique et de taille de grain à la transition entre gabbros lités et gabbros foliés témoignent possiblement d'un fort refroidissement hydrothermal plus profond que la LMA. La transition complexe filonien/gabbro montre des relations structurale et lithologiques complexes, qui, combinées o la modélisation de la cristallisation fractionnée, traduisent les mouvements verticaux épisodiques de la LMA. Les orientations préférentielles cristallographiques (OPC) des plagioclases mesurées sur l'ensemble de la coupe dans les gabbros témoignent toujours d'une déformation magmatique. L'intensité des OPC a tendance à augmenter vers le bas, avec une plus forte variabilité dans les gabbros lités. Dans les gabbros lités, les OPC sont de plus en plus linéaires vers le bas, résultant possiblement d'un cisaillement plus fort induit par l'écoulement actif du manteau sous-jacent.La croûte inférieure dans le Wadi Gideah a enregistré une histoire compatible avec un modèle d'accrétion hydride, combinant plusieurs processus tels que l'écoulement magmatique vers le bas et/ou la migration de liquides magmatiques vers le haut dans les gabbros foliés supérieurs, l'intrusion de sills dans les gabbros lités, et une circulation hydrothermale profonde. Cette dernière était probablement concentrée dans des chenaux, préservés aujourd'hui sous la forme de plusieurs zones, dont l'épaisseur atteint 100 m, de gabbros lités fortement altérés, recoupant le litage magmatique. Ces métagabbros ont des rapports Sr87/Sr86 élevés, comportent des phases magmatiques tardives et montrent des évidences de fusion partielle.
https://hal.archives-ouvertes.fr/tel-01302675

tel-01302675
https://hal.archives-ouvertes.fr/tel-01302675
https://hal.archives-ouvertes.fr/tel-01302675/document
https://hal.archives-ouvertes.fr/tel-01302675/file/Th%C3%A8se_Tim-Mueller_2015.pdf

Éditeur(s) : HAL CCSD American Geophysical Union - AGU
Résumé : IODP Expedition 335 "Superfast Spreading Rate Crust 4" returned to ODP Hole 1256D with the intent of deepening this reference penetration of intact ocean crust several hundred meters into cumulate gabbros. This was the fourth cruise of the superfast campaign to understand the formation of oceanic crust accreted at fast spreading ridges, by exploiting the inverse relationship between spreading rate and the depth to low velocity zones seismically imaged at active mid-ocean zones, thought to be magma chambers. Site 1256 is located on 15-million-year-old crust formed at the East Pacific Rise during an episode of superfast ocean spreading (>200 mm/yr full rate). Three earlier cruises to Hole 1256D have drilled through the sediments, lavas and dikes and 100 m into a complex dike-gabbro transition zone. The specific objectives of IODP Expedition 335 were to: (1) test models of magmatic accretion at fast spreading ocean ridges; (2) quantify the vigor of hydrothermal cooling of the lower crust; (3) establish the geological meaning of the seismic Layer 2-3 boundary at Site 1256; and (4) estimate the contribution of lower crustal gabbros to marine magnetic anomalies. It was anticipated that even a shortened IODP Expedition could deepen Hole 1256D a significant distance (300 m) into cumulate gabbros. Operations on IODP Expedition 335 proved challenging from the outset with almost three weeks spent re-opening and securing unstable sections of the Hole. When coring commenced, the destruction of a hard-formation C9 rotary coring bit at the bottom of the hole required further remedial operations to remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets returned large samples of a contact-metamorphic aureole between the sheeted dikes and a major heat source below. These large (up to 3.5 kg) irregular samples preserve magmatic, hydrothermal and structural relationships hitherto unseen because of the narrow diameter of drill core and previous poor core recovery. Including the ~60 m-thick zone of granoblastic dikes overlying the uppermost gabbro, the dike-gabbro transition zone at Site 1256 is over 170 m thick, of which more than 100 m are recrystallized granoblastic basalts. This zone records a dynamically evolving thermal boundary layer between the principally hydrothermal domain of the upper crust and a deeper zone of intrusive magmatism. The recovered samples document a sequence of evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal, contact-metamorphic, partial melting and retrogressive processes. Despite the operational challenges, we achieved a minor depth advance to 1522 m, but this was insufficient penetration to complete any of the primary objectives. However, Hole 1256D has been thoroughly cleared of junk and drill cuttings that have hampered operations during this and previous Expeditions. At the end of Expedition 335, we briefly resumed coring and stabilized problematic intervals with cement. Hole 1256D is open to its full depth and ready for further deepening in the near future.
InterRidge News

hal-00688821
https://hal.archives-ouvertes.fr/hal-00688821

Éditeur(s) : HAL CCSD IODP
Résumé : Integrated Ocean Drilling Program (IODP) Expedition 335 (14 April–4 June 2011) will be the fourth ocean cruise of the "Superfast" campaign to drill a deep hole into intact oceanic basement and will return to Ocean Drilling Program (ODP) Hole 1256D to deepen this scientific reference penetration a significant distance into cumulate gabbros. Cores and data recovered during the Superfast 4 expedition will provide hitherto unavailable observations that will test models of the accretion and evolution of the oceanic crust. Site 1256 was specifically located on oceanic crust that formed at a superfast spreading rate (>200 mm/y) to exploit the observed relationship between spreading rate and depth to axial low velocity zones, thought to be magma chambers, seismically imaged at active mid-ocean ridges. This was a deliberate strategy to reduce the drilling distance to gabbroic rocks because thick sequences of lavas and dikes have proved difficult to penetrate in past. ODP Leg 206 (2002) initiated operations at Site 1256, including the installation in Hole 1256D of a reentry cone with 16 inch casing inserted through the 250 m thick sedimentary cover and cemented into basement to facilitate deep drilling. The hole was then cored ~500 m into basement. IODP Expeditions 309 and 312 (2005) successfully completed the first sampling of an intact section of upper oceanic crust from lavas, through the sheeted dikes, and into the upper gabbros. Hole 1256D now penetrates >1500 meters below seafloor (mbsf) and >1250 m subbasement and currently resides in the dike–gabbro transition zone. The first gabbroic rocks were encountered at 1407 mbsf. Below this lies a ~100 m complex zone of fractionated gabbros intruded into contact metamorphosed dikes. Although previous cruises achieved the benchmark objective of reaching gabbro in intact ocean crust, critical scientific questions remain. These include the following: 1. Does the lower crust form by the recrystallization and subsidence of a high-level magma chamber (gabbro glacier), crustal accretion by intrusion of sills throughout the lower crust, or some other mechanism? 2. Is the plutonic crust cooled by conduction or hydrothermal circulation? 3. What is the geological nature of Layer 3 and the Layer 2/3 boundary at Site 1256? 4. What is the magnetic contribution of the lower crust to marine magnetic anomalies? Hole 1256D is poised at a depth where samples that should conclusively address these questions can be obtained, possibly with only a few hundred meters of drilling. Importantly, as of the end of Expedition 312, the hole was clear of debris and open to its full depth. Increased rates of penetration (1.2 m/h) and enhanced core recovery (>35%) in the gabbros indicate that this return to Hole 1256D could deepen the hole >300 m into plutonic rocks, past the transition from dikes to gabbro, and into a region of solely cumulate gabbroic rocks.
https://hal.archives-ouvertes.fr/hal-00564953

hal-00564953
https://hal.archives-ouvertes.fr/hal-00564953
DOI : 10.2204/iodp.sp.335.2010

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Oman ophiolite, a fossil fast-spreading oceanic ridge had a melt lens perched on top of the magma chamber wherein the gabbro unit was crystallizing. In ophiolites, this melt lens is reduced to a horizon that has been identified in the field. The gabbros that subsided from the lens horizon and drifted out of the magma chamber, while developing a steep magmatic foliation, provide insights into the active melt lens processes. A gabbro, sampled right below the horizon where the lens closed, has virtually not subsided and it offers a direct glance at the floor of the lens. Below, the deeper the gabbros are sampled, the more they have subsided before drifting out of the magma chamber, and the closer they derive from the center of the lens floor. Cross sections in these gabbros record successive events occurring within the lens. In particular, anorthosite layers and lenses that are interlayered within the foliated gabbros record the intrusion of a new melt where plagioclase is the first mineral to crystallize on liquidus. Detailed mapping of anorthosite swarms suggest that melt intrusions are massive, lasting less than 4 yr and that their frequency is similar to 100 yr. Their large volume controls the stability of melt lenses. A new melt surge induces rapid and large inflation of the lens. Melt lens deflation follows, that is due to the ongoing process of crustal accretion both in the gabbro unit by continuous subsidence of a thick gabbroic mush from the lens floor, and in the lid above the lens roof by sheeted dike intrusions feeding basalt extrusions at the higher frequency of similar to 10 yr.
ISSN: 0012-821X

hal-00617444
https://hal.archives-ouvertes.fr/hal-00617444
DOI : 10.1016/j.epsl.2011.01.029

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 European Geosciences Union
Résumé : We designed a thermo-mechanical numerical model for fast-spreading mid-ocean ridge with variable viscosity, hydrothermal cooling, latent heat release, sheeted dyke layer, and variable melt intrusion possibilities. The model allows for modulating several accretion possibilities such as the "gabbro glacier" (G), the "sheeted sills" (S) or the "mixed shallow and MTZ lenses" (M). These three crustal accretion modes have been explored assuming viscosity contrasts of 2 to 3 orders of magnitude between strong and weak phases and various hydrothermal cooling conditions depending on the cracking temperatures value. Mass conservation (stream-function), momentum (vorticity) and temperature equations are solved in 2-D cartesian geometry using 2-D, alternate direction, implicit and semi-implicit finite-difference scheme. In a first step, an Eulerian approach is used solving iteratively the motion and temperature equations until reaching steady states. With this procedure, the temperature patterns and motions that are obtained for the various crustal intrusion modes and hydrothermal cooling hypotheses display significant differences near the mid-ocean ridge axis. In a second step, a Lagrangian approach is used, recording the thermal histories and cooling rates of tracers travelling from the ridge axis to their final emplacements in the crust far from the mid-ocean ridge axis. The results show that the tracer's thermal histories are depending on the temperature patterns and the crustal accretion modes near the mid-ocean ridge axis. The instantaneous cooling rates obtained from these thermal histories betray these discrepancies and might therefore be used to characterize the crustal accretion mode at the ridge axis. These deciphering effects are even more pronounced if we consider the average cooling rates occurring over a prescribed temperature range. Two situations were tested at 1275-1125 °C and 1050-850 °C. The first temperature range covers mainly the crystallization range that is characteristic of the high temperature areas in the model (i.e. the near-mid-oceanic-ridge axis). The second temperature range corresponds to areas in the model where the motion is mainly laminar and the vertical temperature profiles are closer to conductive. Thus, this situation results in less discriminating efficiency among the crustal accretion modes since the thermal and dynamic properties that are described are common to all the crustal accretion modes far from the ridge axis. The results show that numerical modeling of thermo-mechanical properties of the lower crusts may bring useful information to characterize the ridge accretion structure, hydrothermal cooling and thermal state at the fast-spreading ridges and may open discussions with petrological cooling rate results.
ISSN: 1991-959X

hal-00907012
https://hal.archives-ouvertes.fr/hal-00907012
DOI : 10.5194/gmd-6-1659-2013

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 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 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