Abyssal peridotite from the 15°20'N area of the Mid-Atlantic Ridge show complex geochemical variations among the different sites drilled during ODP Leg 209. Major element compositions indicate variable degrees of melt depletion and refertilization as well as local hydrothermal metasomatism. Strongest evidence for melt-rock interactions are correlated Light Rare Earth Element (LREE) and High Field Strength Element (HFSE) additions at sites 1270 and 1271. In contrast, hydrothermal alteration at Sites 1274, 1272, and 1268 causes LREE mobility associated with minor HFSE variability, reflecting the low solubility of HFSE in aqueous solutions. Site 1274 contains the least-altered, highly refractory, peridotite with strong depletion in LREE and shows a gradual increase in the intensity of isochemical serpentinization; except for the addition of H 2 O which causes a mass gain of up to 20 g/100 g. The formation of magnetite is reflected in decreasing Fe 2+ /Fe 3+ ratios. This style of alteration is referred to as rock-dominated serpentinization. In contrast, fluid-dominated serpentinization at Site 1268 is characterized by gains in sulfur and development of U-shaped REE pattern with strong positive Eu anomalies which are also characteristic for hot (350 to 400 °C) vent-type fluids discharging from black smoker fields. Serpentinites at Site 1268 were overprinted by talc alteration under static conditions due to interaction with high a SiO 2 fluids causing the development of smooth, LREE-enriched patterns with pronounced negative Eu anomalies. These results show that hydrothermal fluid-peridotite and fluid-serpentinite interaction processes are an important factor regarding the budget of exchange processes between the lithosphere and the hydrosphere in slow spreading environments.

[1] 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 B071031 of 25 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°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 ∼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.

[1] Hole U1309D (Integrated Ocean Drilling Program (IODP) Legs 304/305) penetrated 1415 m into the seafloor of the Atlantis Massif, an oceanic core complex at 30°N, Mid-Atlantic Ridge. More than 96% of all recovered rocks are gabbroic. On the basis of a mineral chemical overview, we suggest that between 800 and 1100 m below sea floor (mbsf), a magmatic unit occurs, ranging from olivine gabbro and troctolite in the lower part to gabbronorite and oxide gabbro in the upper part. Below 1235 mbsf, massive gabbronorites/oxide gabbros were drilled and they may represent the roof of an underlying magmatic unit. The focus here is on the zone where both units interact and screens, totaling 50 m, of a microstructurally distinct, olivine-rich troctolite occur. We argue that the olivine-rich troctolite is a former mantle rock which was converted to a crust-mantle transition zone dunite at the base of the upper magmatic unit. Later, as melts derived from the lower magmatic unit percolated through it, it was equilibrated to a more evolved chemistry and transformed to a fine-grained, olivine-rich troctolite. Our main arguments against a possible cumulate nature of the olivine-rich troctolite are the lack of a systematic downhole trend in compatible elements within the olivine-rich troctolite, its distinctly fine-grained microstructure, the high Cr content of cpx, and its Ni-rich olivine composition. The high NiO for a given Mg/(Mg + Fe) in the olivine-rich troctolite can be modeled by simple equilibration of relict mantle olivine with a mildly evolved melt. Evidence for the percolation of evolved melts through the olivine-rich troctolites are Ti-rich, interstitial pyroxenes and, as inclusions in Cr-spinel, highly evolved amphiboles and orthopyroxenes plus the occurrence of millimeter-scale noritic veins. The percolation by evolved melts would also be the major difference to otherwise conceptually similar rocks from the ophiolitic crust-mantle transition zone.

Constraints provided by the Bay of Islands ophiolite complex (BOIC), western Newfoundland, suggest that its generation, as well as its obduction, is related to collision of the Dunnage Zone island arc system with the irregular continental margin of eastern North America. The constraints include the supra‐subduction zone setting of the BOIC; its extensional environment of generation; the synchronous occurrence of ophiolite generation with initiation of drowning and deformation of the North American continental margin; and the restriction of the BOIC, as well as other Appalachian ophiolites and associated Taconian allochthons, to reentrants in the North American margin. During the early phases of collision of the Dunnage arc with the North American margin, subduction of old oceanic lithosphere trapped in reentrants will continue and allow the development of two potential sites of ophiolite generation. The first develops through trench rollback, leading to ophiolite generation in the overriding arc plate outboard of the reentrant, whereas the second develops in a transtensional environment with spreading centers offset by strike‐slip faults which propagate through the overriding plate away from the indenter promontory. The configuration of the BOIC spreading ridge normal to the continental margin, and the direction of initial obduction parallel to the margin, favor generation of the ophiolite in an overall strike‐slip regime. With complete subduction of the oceanic remnant, extension in the overriding plate ceases, and the buoyant newly formed oceanic lithosphere is obducted onto the continental margin. Diachronous collision of the arc with the irregular continental margin accounts for the range of ages for ophiolite generation and obduction observed within the Dunnage Zone, and for termination of arc volcanism along the North American margin.

[1] The origin of large mantle dunites is considered critical for models of melt migration in the mantle. Their presence is not compatible with formation synchronous to a fracture-related melt transport event. In models of porous channel systems for melt transport, they represent a strongly coalesced, high-flux conduit. Dunites from the lower parts of the mantle sections in the Bay of Islands Ophiolite are investigated by detailed geochemical traverses and with single samples. Dunites tend to cluster in the sense that several smaller dunites are associated with larger dunites or several dunites occur together. The chemistry of the large bodies is very depleted (Mg# in olivine 92-94, CaO in olivine 0.05-0.08%, Cr# [100 Cr/(Cr + Al)] in spinel 65-85, TiO 2 in clinopyroxene 0.01-0.04%, Sm/Yb 0.2 to 0.7 relative to N-MORB). Detailed traverses across the dunites commonly show a decrease of NiO in olivine associated with an increase in the Mg# along the harzburgite-dunite boundary. Internally, dunite bodies are nearly homogeneous. Thickness of dunite bodies correlates with chemistry, in particular Mg# in olivine and probably Cr# and ferric iron in spinel, but not NiO in olivine. Incompatible element data for the largest dunites argue for their formation by an extremely depleted, high Mg# (boninitic?) melt. We suggest that integrated refractory melt: rock ratios in the largest dunites (up to 40 m) were below 8, because of a low abundance of refractory melts in the crust, and a lack of a systematic change of NiO in olivine with dunite width or across single dunites in detailed chemical traverses. Tectonically, the formation of depleted melts in a late stage of the spreading center is indicated. Their melt feeders failed when approaching the base of the mantle lithosphere and generated large dunites as replacive bodies. The latest expression of this magmatism are orthopyroxenite dykes, in part draining the large dunites. Since the large majority of all deeper mantle dunites are of refractory chemical nature and not akin to MORB, we caution as universally taking large dunite bodies to represent deep-reaching channels with high melt flux and to take the abundance and size distribution of all dunites in an ophiolitic mantle section to infer melt migration mechanisms. In the Bay of Islands Ophiolite, the largest dunites in the mantle section appear to have little to do with the main constructional stage of the spreading center.