[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.

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Zircon OD-3 from the Paleogene Kawamoto Granodiorite (Mihara body) in Japan has been identified as a potential multi-grain secondary standard for U-Pb dating. We have carried out an inter-laboratory evaluation in order to evaluate possible heterogeneity amongst the OD-3 zircon grains. U-Pb ages were obtained using two analytical techniques (a sensitive high-resolution ion microprobe and a laser ablation-inductively coupled plasma-mass spectrometry) in eight laboratories. All the 238 U-206 Pb ages show good agreement, with an overall weighted average 238 U-206 Pb age of 33.0 Ϯ 0.1 Ma (2s). The U-Pb age results revealed no significant variation or heterogeneity in the U-Pb ages of the OD-3 grains. Twelve fission-track (FT) ages from three laboratories are also reported, and have a weighted average of 32.6 Ϯ 0.6 Ma (2s). Despite the different closure and annealing temperatures of the U-Pb and FT chronometers, respectively, the FT age is in good agreement with the U-Pb age. This suggests that the OD-3 zircon had a relatively fast cooling history and has not experienced later thermal annealing. The chronological dataset reported here clearly demonstrates that the OD-3 zircon could be a useful and reliable secondary standard for use during U-Pb dating studies of Cenozoic zircons.

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10Primitive melt inclusions in chrome spinel from the Ogasawara Archipelago comprise 11 two discrete groups of high-SiO2, MgO (high-Si) and low-SiO2, MgO (low-Si) boninitic suites 12 with ultra-depleted dish-and V-shaped, and less depleted flat rare earth element (REE) patterns. 13The most magnesian melt inclusions of each geochemical type were used to estimate the T-P 14 conditions for primary boninites, that range from 1345°C-0.56 GPa to 1421°C-0.85 GPa for the 15 48-46 Ma high-Si and low-Si boninites, and 1381°C-0.85 GPa for the 45 Ma low-Si boninite. 16