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SummaryThis workshop report describes plans for scientific drilling in the Samail ophiolite in Oman in the context of past, current, and future research. Long-standing plans to study formation and evolution of the Samail crust and upper mantle, involving igneous and metamorphic processes at an oceanic spreading center, have been augmented by recent interest in ongoing, low temperature processes. These include alteration and weathering, and the associated sub-surface biosphere supported by chemical potential energy due to disequilibrium between mantle peridotite and water near the surface. This interest is motivated in part by the possibility of geological carbon capture and storage via engineered, accelerated mineral carbonation in Oman. Our International Continental Drilling Program (ICDP)proposal led to the Workshop on Scientific Drilling in the Samail Ophiolite in Palisades, New York, on 13-17 September 2012. There were seventy-seven attendees from eleven countries, including twenty early career scientists.After keynote presentations on overarching science themes, participants in working groups and plenary sessions outlined a ~U.S.$2 million drilling plan that practically addresses testable hypotheses and areas of frontier discovery in the following areas.• understanding the subsurface biosphere • characterizing the rates and mechanisms of ongoing mineral hydration and carbonation • characterizing chemical and physical processes of mass transfer across a subduction zone • evaluating well-posed hypotheses on hydrothermal circulation, cooling, and emplacement mechanisms of igneous rocks in the lower crust • investigating key problems in the dynamics of mantle flow and melt transport beneath oceanic spreading ridgesThis report places these goals in the context of complementary research via ocean drilling and ongoing studies of active processes at oceanic spreading centers, subduction zones, and peridotite-hosted hydrothermal systems. We end with an outline of the synergy between Oman drilling and the specific drilling proposed in the Integrated Ocean Drilling Program (IODP) proposal "Mohole to Mantle Project (M2M)", IODP Proposal 805-MDP. Workshop Proceedings and ResultsKeynote speakers outlined hypotheses and areas of frontier scientific exploration to be addressed via drilling, including• the nature of mantle upwelling, • the chemical and physical mechanisms of mantle melt transport, • the processes of lower crustal accretion and cooling, • the frequency and magnitude of microseismicity during weathering, • the rate and location of ongoing alteration, and • the composition, density, and spatial distribution of subsurface microbial communities.Additional keynote talks covered state-of-the-art geological logging of drillcore, geophysical logging in boreholes, and data management.Breakout groups considered overarching science themes, then designed idealized projects to address these themes, and finally considered practical constraints. We agreed to focus on studies relevant to global processes. There was a consen...

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In the absence of a complete profile through fast‐spreading modern oceanic crust, we established a reference profile through the whole paleo crust of the Samail ophiolite (Sultanate of Oman), which is regarded as the best analogue for fast‐spreading oceanic crust on land. To establish a coherent data set, we sampled the Wadi Gideah in the Wadi‐Tayin massif from the mantle section up to the sheeted dikes and performed different analytical and structural investigations on the same suite of samples. This paper reports our studies of the lower crust, a 5 km thick pile of gabbros, focusing on petrographic features and on the results of mineral analyses. Depth profiles of mineral compositions combined with petrological modeling reveal insights into the mode of magmatic formation of fast‐spreading lower oceanic crust, implying a hybrid accretion mechanism. The lower two thirds of the crust, mainly consisting of layered gabbros, formed via the injection of melt sills and in situ crystallization. Here, upward moving fractionated melts mixed with more primitive melts through melt replenishments, resulting in a slight but distinct upward differentiation trend. The upper third of the gabbroic crust is significantly more differentiated, in accord with a model of downward differentiation of a primitive parental melt originated from the axial melt lens located at the top of the gabbroic crust. Our hybrid model for crustal accretion requires a system to cool the deep crust, which was established by hydrothermal fault zones, initially formed on‐axis at very high temperatures.

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