Leg 147 of the Ocean Drilling Program recovered sections of the East Pacific Rise lower crust and shallow mantle ( l Ma), tectonically exposed at the western end of the Cocos-Nazca propagator of the Hess Deep Rift Valley. These rocks record a polyphase history of hydrothermal alteration and provide new constraints on the depth and mechanisms of hydrothermal circulation at fast-spreading ridges. A complex sequence of harzburgite-dunite-troctolite-gabbro recovered at Site 895 is considered to be the result of processes of melt migration and wall-rock reaction close to the mantle/crust boundary. The peridotites are extensively serpentinized (50%-100%) and are cut by multiple generations of fracture-filling veins. In the gabbros, progressive alteration under greenschist to zeolite facies conditions is characterized by tremolite + chlorite + diopside + anorthjte ± prehnite assemblages in the least altered samples, and incipient rodingitization to prehnite + hydrogrossular + zeolite + clays as Cataclastic deformation and veining increases.Oxygen isotope ratios of mineral separates from the gabbros and peridotites from Site 895 show a depletion in I8O relative to mantle values and are consistent with high-temperature exchange with aqueous fluids. Dunite/harzburgite ratios of chlorite, serpentine, and tremolite, together with δ' 3 C values of CO 2 extracted from completely serpentinized dunites, suggest at least two, but possibly three, components of the hydrothermal fluids: hydrothermally altered seawater; magmatic volatiles; and H 2 released during serpentinization. These data combined with structural data imply that penetration of seawater and high-temperature hydrothermal alteration produced a low 18 O shallow mantle sequence at some distance off-axis of the East Pacific Rise, but at an early stage in the propagation of the Cocos-Nazca rift and formation of the Hess Deep Rift Valley. Mineral assemblages in the gabbroic rocks and the presence of antigorite at Hess Deep, combined with oxygen isotope ratios, suggest that faulting associated with the Cocos-Nazca propagator enhanced seawater penetration and hydrothermal alteration at temperatures above 350°C in this segment of the East Pacific Rise oceanic lithosphere. The results of this study suggest that seawaterperidotite interactions and high-temperature serpentinization processes may be an important contribution to the overall 18 Obudget in the oceanic lithosphere nd may represent a significant sink for mantle CO 2 and source of H 2 .

Contents AbstractInternational Ocean Discovery Program (IODP) Expedition 357 successfully cored an east-west transect across the southern wall of Atlantis Massif on the western flank of the Mid-Atlantic Ridge (MAR) to study the links between serpentinization processes and microbial activity in the shallow subsurface of highly altered ultramafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. The primary goals of this expedition were to (1) examine the role of serpentinization in driving hydrothermal systems, sustaining microbial communities, and sequestering carbon; (2) characterize the tectonomagmatic processes that lead to lithospheric heterogeneities and detachment faulting; and (3) assess how abiotic and biotic processes change with variations in rock type and progressive exposure on the seafloor. To accomplish these objectives, we developed a coring and sampling strategy centered on the use of seabed drills-the first time that such systems have been used in the scientific ocean drilling programs. This technology was chosen in the hope of achieving high recovery of the carbonate cap sequences and intact contact and deformation relationships. The expedition plans also included several engineering developments to assess geochemical parameters during drilling; sample bottom water before, during, and after drilling; supply synthetic tracers during drilling for contamination assessment; acquire in situ electrical resistivity and magnetic susceptibility measurements for assessing fractures, fluid flow, and extent of serpentinization; and seal boreholes to provide opportunities for future experiments.Seventeen holes were drilled at nine sites across Atlantis Massif, with two sites on the eastern end of the southern wall (Sites M0068 and M0075), three sites in the central section of the southern wall north of the Lost City hydrothermal field (Sites M0069, M0072, and M0076), two sites on the western end (Sites M0071 and M0073), and two sites north of the southern wall in the direction of the central dome of the massif and Integrated Ocean Drilling Program Site U1309 (Sites M0070 and M0074). Use of seabed drills enabled collection of more than 57 m of core, with borehole penetration ranging from 1.30 to 16.44 meters below seafloor and core recoveries as high as 74.76% of total penetration. This high level of recovery of shallow mantle sequences is unprecedented in the history of ocean drilling. The cores recovered along the southern wall of Atlantis Massif have highly heterogeneous lithologies, types of alteration, and degrees of deformation. The ultramafic rocks are dominated by harzburgites with intervals of dunite and minor pyroxenite veins, as well as gabbroic rocks occurring as melt impregnations and veins, all of which provide information about early magmatic processes and the magmatic evolution in the southernmost portion of Atlantis Massif. Dolerite dikes and basaltic rocks represent the latest stage of magmatic activity. Overall, the ultramafic rocks recovered ...

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Leg 147 of the Ocean Drilling Program recovered sections of the East Pacific Rise lower crust and shallow mantle ( l Ma), tectonically exposed at the western end of the Cocos-Nazca propagator at the Hess Deep Rift Valley. At Site 894, variably metamorphosed, isotropic gabbros and gabbronorites from the upper part of the plutonic section were recovered. In this study, we present petrologic and stable isotope data that document a complex polyphase history of fluid infiltration, metamorphism, and deformation from late magmatic activity through upper amphibolite facies to zeolite facies conditions. Alteration occurred through several stages of fracturing and fluid infiltration during progressive transport of the oceanic crust away from the axis of the East Pacific Rise and ultimate intersection with the Cocos-Nazca propagator. Alteration was controlled by fracture permeabilities, grain geometries, and the chemical composition of progressive pulses of hydrothermal fluids. Early, near-axis high temperature (>500°C) fluid infiltration in microveins and along grain boundaries produced amphibolite-facies mineral assemblages. Subsequent off-axis cooling and Cocos-Nazca-related uplift and faulting enhanced fluid penetration, resulting in variable overprinting under greenschist to zeolite facies conditions in association with discrete veins and local Cataclastic shear zones.Oxygen isotope ratios of amphibole separates are depleted in 18O relative to unaltered gabbro compositions and are consistent with high temperature exchange with aqueous fluids. Plagioclase compositions form two groups: a high temperature altered group (δ l8 θ between 3.0%o and 6.3%o); and a group with δ 18 θ between 8.3%o and 10.2‰, indicative of local lower temperature (150°-250°C) overprinting associated with discrete veining. D/H ratios of amphibole suggest two components in the early hydrothermal fluids: an altered seawater component (δD O‰) and a mixed magmatic-derived/altered seawater component (δD -30%c to -20‰). These data, combined with microstructural data, suggest that penetration of seawater at high temperatures (>500°C), possibly mixing with CO 2 -rich magmatic volatiles, resulted in a low 18 O sequence of upper-amphibolite facies oceanic lithosphere at an early stage in the spreading history of the EPR. Fluid mixing at magma chamber/oceanic crust boundaries and fracture-controlled, high-temperature metamorphism may be characteristic of alteration processes at this fast-spreading ridge environment.

Operations 3 Lithology, alteration, and structure 15 Bulk rock geochemistry 20 Fluid chemistry 23 Microbiology 24 Sensor package data 29 Physical properties 28 Borehole plugs 32 Downhole logging 34 Paleomagnetism 35 References Operations During Expedition 357, cores were recovered from two sites in the eastern area of Atlantis Massif: Sites M0068 and M0075 (Figure F1; Table T1). Newly acquired multibeam data, combined with preexisting data sets, were evaluated prior to each site to guide the drill teams with regard to anticipated seabed conditions and slope. Site M0068 Cores were recovered from two holes at Site M0068 (proposed Site AM-02A), with an average site recovery of 58.88%. The mean water depth for the two holes was 1102.35 m with no tidal range. The total time spent on station was 2.36 days. Hole M0068A The RSS James Cook moved onto Site M0068 at 2208 h on 5 November 2015, with the site position centered over the British Geological Survey RockDrill2 (RD2) port-side deployment position. The vessel settled into dynamic positioning (DP) mode. Prior to coring at each location, a conductivity, temperature, and depth (CTD) cast was conducted to quantify the background geochemistry and microbiology within the water column. The CTD was deployed at 2248 h and held at 10 m water depth for a full systems check. At 2332 h, the CTD was 1.6 m off the bottom, and the first three Niskin bottles were fired. Two more Niskin bottles were fired on the way up, with a final one fired near the surface (see Table T5 in the Expedition 357 summary chapter [Früh-Green et al., 2017c]). The CTD was recovered to deck at 0005 h on 6 November. During the ship CTD cast, the RD2 was made ready for deployment, but a communications problem with the CTD sensor mounted on the drill delayed launch. The RD2 was deployed at 0124 h; however, problems with the winch spooling mechanism and the methane sensor on the water sensor package meant the drill was recovered at 0146 h before landing on the seafloor. At 0234 h, the RD2 was launched again, landing on the seafloor at 0356 h after a full mid-water column systems check. Coring operations commenced at 0418 h. The first core run was completed at 0602 h. However, a problem with the arm shift meant the RD2 had to be recovered after a total penetration of 1.97 m. Prior to recovery, the Niskin bottles on the water sampling package were fired. The RD2 was on deck and secure by 0835 h. The vessel began to transit to Site M0069 at 0840 h on 6 November. Preparations began for deploying the Meeresboden-Bohrgerät 70 (MeBo) drill. One coring attempt was made in Hole M0068A to a maximum depth of 1.97 m with 23.92% recovery. Hole M0068B The vessel arrived on station at 1545 h on 23 November 2015. RD2 preparations continued until 1757 h, when the drill was deployed. At 1836 h, the RD2 was brought back on deck after a midwater column systems check revealed two sensor inputs were G.L. Früh-Green et al. Eastern sites IODP Proceedings 2 V o l u m e 3 5 7

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Operations 3 Lithology, alteration, and structure 20 Bulk rock geochemistry 21 Fluid chemistry 21 Microbiology 23 Sensor package data 29 Borehole plugs 31 Physical properties 35 Downhole logging 36 Paleomagnetism 38

Portions of this work may have been published in whole or in part in other International Ocean Discovery Program documents or publications. This IODP Scientific Prospectus is based on precruise JOIDES Resolution Facility advisory panel discussions and scientific input from the designated Co-Chief Scientists on behalf of the drilling proponents. During the course of the cruise, actual site operations may indicate to the Co-Chief Scientists, the Staff Scientist/Expedition Project Manager, and the Operations Superintendent that it would be scientifically or operationally advantageous to amend the plan detailed in this prospectus. It should be understood that any proposed changes to the science deliverables outlined in the plan presented here are contingent upon the approval of the IODP JRSO Director.