Benoit Ildefonse

A long in situ section of the lower ocean crust: results of ODP Leg 176 drilling at the Southwest Indian Ridge

Ocean Drilling Program Leg 176 deepened Hole 735B in gabbroic lower ocean crust by 1 km to 1.5 km. The section has the physical properties of seismic layer 3, and a total magnetization sufficient by itself to account for the overlying lineated sea-surface magnetic anomaly. The rocks from Hole 735B are principally olivine gabbro, with evidence for two principal and many secondary intrusive events. There are innumerable late small ferrogabbro intrusions, often associated with shear zones that cross-cut the olivine gabbros. The ferrogabbros dramatically increase upward in the section. Whereas there are many small patches of ferrogabbro representing late iron- and titanium-rich melt trapped intragranularly in olivine gabbro, most late melt was redistributed prior to complete solidification by compaction and deformation. This, rather than in situ upward differentiation of a large magma body, produced the principal igneous stratigraphy. The computed bulk composition of the hole is too evolved to mass balance mid-ocean ridge basalt back to a primary magma, and there must be a significant mass of missing primitive cumulates. These could lie either below the hole or out of the section. Possibly the gabbros were emplaced by along-axis intrusion of moderately differentiated melts into the near-transform environment. Alteration occurred in three stages. High-temperature granulite- to amphibolite-facies alteration is most important, coinciding with brittle^ductile deformation beneath the ridge. Minor greenschist-facies alteration occurred under largely static conditions, likely during block uplift at the ridge transform intersection. Late post-uplift low-temperature alteration produced locally abundant smectite, often in previously unaltered areas. The most important features of the high- and low-temperature alteration are their respective

Oceanic core complexes and crustal accretion at slow-spreading ridges

Oceanic core complexes expose intrusive crustal rocks on the seafloor via detachment faulting and are often associated with significant extents of serpentinized mantle peridotite at the seafloor. These serpentinite units have unknown thickness in most cases. Assuming that steep slopes surrounding the domal core provide a cross section, one would infer that they comprise much of the section for depths of at least several hundreds of meters. IODP expeditions 304-305 results at the Mid-Atlantic Ridge 30 N (Atlantis Massif), taken together with previous ODP drilling results from the Atlantic and Indian Oceans, suggest that a revised model of oceanic core complex (OCC) development should be considered. All of the ODP/IODP drilling at 4 different core complexes and/or inside corner highs so far have recovered only gabbroic sections, with almost no serpentinized peridotite.

Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15°N (ODP Leg 209, Site 1274)

[1] The results of detailed textural, mineral chemical, and petrophysical studies shed new light on the poorly constrained fluid-rock reaction pathways during retrograde serpentinization at mid-ocean ridges. Uniformly depleted harzburgites and dunites from the Mid-Atlantic Ridge at 15� N show variable extents of static serpentinization. They reveal a simple sequence of reactions: serpentinization of olivine and development of a typical mesh texture with serpentine-brucite mesh rims, followed by replacement of olivine mesh centers by serpentine and brucite. The serpentine mesh rims on relic olivine are devoid of magnetite. Conversely, domains in the rock that are completely serpentinized show abundant magnetite. We propose that low-fluid-flux serpentinization of olivine to serpentine and ferroan brucite is followed by later stages of serpentinization under more open-system conditions and formation of magnetite by the breakdown of ferroan brucite. Modeling of this sequence of reactions can account for covariations in magnetic susceptibility and grain density of the rocks. Citation: Bach, W., H. Paulick, C. J. Garrido, B. Ildefonse, W. P. Meurer, and S. E. Humphris (2006), Unraveling the sequence of serpentinization reactions: petrography, mineral chemistry, and petrophysics of serpentinites from MAR 15� N (ODP Leg 209, Site 1274), Geophys. Res. Lett., 33, L13306,

Effect of mechanical interactions on the development of shape preferred orientations: a two-dimensional experimental approach

Abstract Modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions. Experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method. For increased concentration of rigid particles, the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours. In coaxial flow, the fabric intensity is consequently reduced. In simple shear flow, the fabric evolution is no longer cyclic; the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane. In the case of preferred orientations of particles having a low aspect ratio (2.5), the fabric rotates through the shear plane, then undergoes a reverse rotation and rotates toward the shear plane again. In the light of these experimental results, we emphasize that: • -quantification of finite strain based on such fabrics may lead to significant underestimates, and calculated values should be taken as minimum estimates; • -the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles. Techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows. The concentrated population fabrics show an angular behaviour close to that of passive markers: they tend to align parallel to the shear plane in simple shear flow.

Deformation around rigid particles: The influence of slip at the particle/matrix interface

Abstract Theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent. In natural rocks, the boundary behaviour may cover the whole range from free-slip to complete coherence. Examples where slip may be promoted include crack-seal deformation during the development of pressure shadows, core-mantle structures, where the mantle is composed of much weaker, fine recrystallized material, and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase. Analogue scale-model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non-slipping particle/matrix interfaces. When slip does occur, particles rotate faster in pure shear and slower in simple shear than for non-slip, leading to the more rapid development of a shape preferred orientation. The preferred orientation tends to be bimodal in simple shear flow. The flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses, dependent on the original particle orientation. In multi-particle experiments, slip on the interfaces enhances strong strain localization in non-linear viscous material, to produce patterns very similar to natural S-C fabrics.

Mechanical interactions between rigid particles in a deforming ductile matrix. Analogue experiments in simple shear flow

Abstract The mechanical interaction between two or more particles is investigated in Newtonian simple shear flow. The experimental models allow observation of both the particle rotation and the deformation pattern around the particles. The finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric, with high finite strain zones aligned to the direction of maximum finite stretch. These correspond to local flow with low vorticity number. Heterogeneous strain patterns around rigid particles spread over a distance of 1–2 times the particle length and this distance increases with increasing bulk strain. Where rigid particles are more concentrated, these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions. In rocks, the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion, which becomes unreliable with high concentrations of rigid particles. Particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length, that is only in very concentrated suspensions of rigid particles. In a composite shape fabric, the development of the sub-fabric corresponding to the smaller minerals will be more disturbed.

Root zone of the sheeted dike complex in the Oman ophiolite

The root zone of the sheeted dike complex representing a thin zone (hundred meters thick) of extreme thermal gradient (∼5°C/m) is regarded as a thermal boundary between the convective magma chamber system below, and the main convective hydrothermal circuit which closes above, at the base of this root zone. The root zone of the sheeted dike complex is located at the top of the high level foliated gabbro unit, where the foliation steepens, and where the first diabase dikes appears. It is a complex zone characterized by mutual intrusions of microgabbros dikes (that we call protodikes) with brownish microgranular contacts against the gabbro matrix. Upward, viscous flow in the protodikes and in the reheated enclosing gabbros generate a diffuse transition to the sheeted complex. Protodike margins stretched in the enclosing flowing doleritic gabbros form a complicated network which can be depicted thanks to microstructural analysis. Later diabase dikes cross-cut the section. These relationships are obscured by the hydrothermal circulation which has generated, in particular, isotropic amphibole gabbro veins. These veins tend to propagate horizontally; they may be interpreted as the downward closure of the main hydrothermal convective circuit.

Variable crustal thickness in the Oman ophiolite: Implication for oceanic crust

From 32 detailed cross sections through the gabbro unit of the Oman ophiolite, it is concluded that the thickness of this unit is on average 3.6 km. The lower layered gabbros represent two thirds, and the upper homogeneous foliated gabbros represent one third the gabbro unit. Assuming that the overlying basaltic lid (sheeted dikes and extrusives) is 1.5–2 km thick, the average crustal section in the Oman ophiolite is 0.5–1 km thinner than the standard 6-km-thick oceanic crust usually considered to be produced at fast spreading ridges, a point which is discussed. Variations in gabbro thickness between 5.4 km and 1.5 km are recorded. There is a general correlation throughout the ophiolite belt, particularly in the southeastern massifs, such that the thinnest gabbro units (2.2–2.5 km thick) overlay the thickest (300–700 m) transition zones which separate them from the mantle harzburgites and the thickest gabbro units (3.6–3.9 km thick) overlay the thinnest (5–100 m) transition zones. The combination of thinner gabbro units and thicker transition zones is observed above mantle diapirs or in domains which, following our structural models, were accreted above diapirs and have drifted in the spreading direction. If it is assumed that the extrusive basalt and the sheeted dike complex units have a constant thickness, such large variations indicate similar variations in the Moho level below the ridge of origin; in particular, the Moho above mantle diapirs should be some 1–1.5 km shallower than away from diapirs. As the Oman ophiolite is considered to derive from a fast spreading paleoridge, this doming should be detected in actual fast spreading ridges, as suggested by Barth and Mutter [this issue] and Wang et al. [this issue].

Flow mechanism and viscosity in basaltic magma chambers

Magmatic flow in the dense suspension of crystallizing gabbros below the free surface of basaltic magma chambers is considered from the point of view of flow mechanisms and rheology. Hyperdense suspensions (∼20% melt fraction) may arise if flat plagioclase crystals develop a strong preferred orientation induced by magmatic flow. With the help of Nomarski differential interference contrast and back scattered electron figures, we show that suspension flow is possible even for smaller melt fractions if impingements between moving crystals are reduced by chemical dissolution at their contact points. This dissolution process is rate controlling. With strain rates near 10−9 s−1 and viscosities near 1014–16 Pa.s, such crystalline mushes should be closer to plastically deforming solids than to the overlying basaltic suspension. If we characterize magma chambers by suspension flow, no matter how small the melt fraction, magma chambers below oceanic fast spreading centers should not be restricted to a perched melt lens, but should extend to the Moho and comprise the entire volume of observed strong seismic attenuation.

Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D: Fossilization of a dynamic melt lens at fast spreading ridges

The transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system. It is therefore critical to our understanding of fast spreading ridge accretion processes. We present maps of two areas of the Oman ophiolite where this transition zone is observed as continuous outcrops. Our observations, which include the base of the sheeted dike being crosscut by gabbros, are consistent with episodic dike injections in a steady state model but also suggest that the root of these dikes is commonly erased by vertical movements of the top of the melt lens. Dike assimilation is a possible mechanism for incorporating hydrated phases, which result from hydrothermal alteration, to the melt lens during upward migrations of its upper boundary. Upward migrations are also responsible for a granoblastic overprint of the root of the dikes that is also observed in the stoped diabase xenoliths. This granoblastic overprint attests to reheating of previously hydrothermally altered lithologies which can even trigger hydrous partial melting due to the lowering of the solidus of mafic lithologies by the presence of a water activity. Clinopyroxenes present in these granoblastic lithologies are typically low in Ti and Al content, thus strongly contrasting with corresponding magmatic clinopyroxene. This may attest to the recrystallization of clinopyroxenes after amphiboles under the peculiar conditions present at the root zone of the sheeted dike complex. Downward migrations of the top of the melt lens result in the crystallization of the isotropic gabbros at its roof, which represent the partly fossilized melt lens. Melt lens fossilization eventually occurs when magma supply is stopped or at the melt lens margins where the thermal conditions become cooler. Melt lens migration, recrystallization of hydrothermally altered sheeted dikes during reheating stages, and assimilation processes observed in the Oman ophiolite are consistent with the observations made in IODP Hole 1256D. We propose a general dynamic model in which the melt lens at fast spreading ridges undergoes upward and downward movements as a result of either eruption/replenishment stages or variations in the hydrothermal/magmatic fluxes.