Serpentinization at mid-ocean ridges occurs in slow spreading rate contexts and affects mantle-derived peridotites next to large-offset axial normal faults (detachment faults). Microstructural, mineralogical and oxygene isotope studies of serpentinized samples from the footwall of these detachments indicate that their serpentinization involved an initial stage of formation of the serpentine mesh texture, followed by several stages of veining and recrystallization of the initial serpentine, and occurred at relatively high temperatures (200-350°C), within the range that is most kinetically favorable for olivine serpentinization. Microcracks that are used as pathways for hydrothermal fluids at the mesh texture formation stage are tightly-spaced (< a few hundred microns) and kinetic constrains indicate that this stage probably occurs at low fluid-rock ratio within a few years of hydrothermal fluids gaining access to the peridotite. Mesh texture serpentinization at mid-ocean ridges is therefore expected to be fast relative to geological displacement rates. This leads to a conceptual model of mesh texture formation in the axial lithosphere at depths corresponding to the 200-350°C range of temperature, in and next to axial detachment fault zones and in association with deep and weak hydrothermal convection. Seismic constraints suggest that this may occur at depths down to 15 km below seafloor at very slow spreading rates, and up to 4-5 km into the detachment footwall. Episodes of extensive serpentinization involving black-smoker type fluids also occur locally in more superficial tectonically damaged domains that channel the deep parts and the upwelling limbs of vigourous hydrothermal cells cooling magmatic intrusions. These two modes of serpentinization are expected to combine in detachment-dominated ridge regions to create an upper lithosphere layer made of partially serpentinized peridotites and isolated gabbroic bodies, with crustal-type seismic velocities and densities.

Dr Mathilde Cannat, Institut de Physique du Globe de Paris, France

Dr Mathilde Cannat, Institut de Physique du Globe de Paris, France

After a PhD at the University of Nantes and a post doc at the University of Durham, Dr Cannat joined CNRS as a junior researcher at the university of Brest. There, she has participated on her first research cruise, ODP leg 118, then to several geological exploration cruises, using submersibles, in the Atlantic and Pacific oceans, focusing her research on the composition and structure of slow spread oceanic crust, and documenting the process of tectonic exhumation of serpentinized mantle-derived peridotites. In the early 2000, as a senior CNRS researcher at the University Pierre et Marie Curie in Paris, then at Institut de Physique du Globe, also in Paris, she addressed two complementary research questions: the role of magma in mantle exhumation processes, both at mid ocean ridges and at distal divergent continental margins, and the role of hydrothermal circulation and cooling in these same contexts. To tackle these questions, Dr Cannat joined with colleagues to develop two long-term field projects. The first is targeted at an anomalously low melt supply region of the global mid-ocean ridge system, in the eastern part of the Southwest Indian Ridge, and the second is the Lucky Strike observatory for the monitoring of hydrothermal circulations and associated ecosystems, at the Mid Atlantic ridge. In addition to her direct involvement as a researcher, she has developed this last project at the broader, community scale, that eventually led to this observatory becoming part of the EMSO (European Multidisciplinary Seafloor and water column Observatory) research infrastructure.

Serpentinization of forearc mantle in nonaccretionary convergent plate margins provides a mechanism for cycling of oceanic plate constituents. Fluid seeps and serpentinite mud volcanism in such environments opens a window into subduction channel processes at depths far too deep to be accessed by any known technology. Samples from the subducted oceanic plate, from guyots subsiding with it, and from forearc lithosphere have all been recovered in cores, dredges, and submersible/ROV dives on Mariana forearc serpentinite mud volcanoes and fluid seeps. Recently IODP drilling on Expedition 366 recovered cores from the summits and flanks of three mud volcanoes that extended sampling of subduction channel materials from ~13 to 19 km depth to slab, from ~55 to 86 km west of the Mariana Trench, from ~80°C to 350°C, and from an along-strike distance of over 650 km. These samples show a pattern of compositional variation consistent with models of subducting slab dehydration. Extensional tectonic deformation in the Mariana forearc, caused by rollback of the Pacific plate, creates fault-controlled conduits for escape of slab-derived fluids. The seeps offer a foothold for microbial and macrofauna communities and the mudflows record a history of eruption as old as 50 Ma. Similar serpentinite deposits worldwide, as old as 3.5 Ga, suggest an even greater history of convergent margin cycling. The details of dynamics of such processes are still poorly known. IODP Expedition 366 emplaced cased boreholes at the active summits of three serpentinite mud volcanoes to be instrumented and thus provide future long-term monitoring.

Professor Patricia Fryer, University of Hawai`i at Mānoa, USA

Professor Patricia Fryer, University of Hawai`i at Mānoa, USA

Dr Patricia Fryer is a Research Professor in marine geology at the University of Hawai`i at Mānoa. She is interested in all aspects of subduction. Her research focus in the Mariana convergent plate margin has included dives in Alvin and Shinkai 6500 submersibles, expeditions with remotely operated vehicles, and three deep ocean drilling expeditions in the Mariana forearc region. She is currently working with core samples from this region to understand not only the physical and chemical processes of their origins, but also to collaborate with fluid geochemists and microbiologists on the implications of microbial communities thriving on the serpentinite mud volcanoes of the Mariana forearc and exposures in the inner trench slope.

Approximately 15,000 km2 of partially serpentinised peridotite of the Samail ophiolite is actively undergoing serpentinization at temperatures <60°C. This is evidenced by the presence of highly reducing, pH>11, H2, CH4 and Ca-OH-bearing fluids in springs and in water wells, and calcite travertine terraces on the surface. The progress of serpentinization depends on the ingress capability of water into the rock formation, which is a function of permeability, porosity, and connectivity of the rocks as well as the fluid pathways and residence time of fluids in the rocks. There is little information about these parameters in the mantle peridotite aquifers of the Samail ophiolite. Previous studies developed a conceptual hydrogeological and reaction-path model of the peridotite [1,2], describing an intensely fissured, permeable zone of about 50 m below surface and a deeper, less permeable zone, dominated by rare discrete fractures. The upper zone is characterized by a Mg-HCO3-rich, pH 8 to 9 groundwater and the deeper zone by a Ca-OH-rich, pH 11-12 fluids. Although this general hydrogeological model is widely accepted, it is not quantitative and has not been tested by direct observations. We are investigating the different hydrogeological regimes and the coupling between these regimes and water-rock reactions in order to quantify the permeability, porosity and connectivity of fluid pathways as well as the residence times of fluids in the peridotite aquifers. Borehole pumping and packer testing, combined with wireline logging and detailed geochemical analysis of fluids and dissolved gases reveal a general stratification of the aquifers with regard to permeability, fluid types and fluid residence times as a function of depth. Well-defined transitions zones, with steep geochemical gradients were detected that coincide with changes in permeability. More important, these transition zones may be key locations for thriving subsurface microbial communities, deriving energy from ongoing water-rock reactions.

Professor Juerg M Matter, University of Southampton, UK

Professor Juerg M Matter, University of Southampton, UK

Juerg M Matter is Associate Professor in Geoengineering at the University of Southampton, UK since spring 2013. Matter was previously an Associate Research Professor at Lamont-Doherty Earth Observatory, and an Adjunct Assistant Professor at the School of International and Public Affairs at Columbia University, New York. Matter studies the physical and geochemical processes of gas-water-rock reactions. He has worked on the geochemical and isotopic characterization of groundwater systems applying field techniques and numerical simulations. His primary focus now is on geologic carbon capture and storage. Matter has made fundamental advances in understanding permanent carbon storage in unconventional geologic reservoirs, such as basaltic rocks and mantle peridotites. He also develops new tracer techniques for monitoring and accounting of CO₂ transport, reactivity and storage in geologic systems. He received his MSci and PhD from ETH-Zurich in Switzerland.

Fischer-Tropsch type (FTT) processes are commonly invoked to explain abiotic CH₄ formation in serpentinization systems; however, established models fail to explain the occurrence of radiocarbon-free CH₄ in seafloor hydrothermal systems and ophiolitic gas seeps, which suggests that CH₄ formation is decoupled from dissolved inorganic carbon in circulating fluids^1-3. This study examines serpentinization processes and the formation of CH₄ in olivine-hosted secondary fluid inclusions from different geologic settings. Raman spectroscopy of inclusion contents reveals chrysotile, brucite, and magnetite as the dominant alteration mineral assemblage and CH₄ or H₂ (or both) as the dominant volatiles in most samples. The formation of CH₄ is envisioned as follows: 1) Cooling of igneous rocks to below the ductile-brittle transition temperature allows fracturing and creates pathways for hydrothermal and magmatic fluids. 2) At temperatures > ~ 400°C olivine is stable in the presence of H₂O, which can cause healing of fractures and entrapment of fluids. 3) Cooling to lower greenschist facies conditions destabilizes olivine, which reacts with trapped H₂O to form daughter minerals and H₂ ⁴. 4) During this process, aH₂O and f_O2 decrease within the inclusion, creating conditions conducive to CH₄ formation. The olivine host appears to be sufficiently impermeable with respect to CH₄ and H₂ at low temperatures to store these volatiles over geologic timescales until they are released by fracturing or dissolution of their host. The range of δ¹³C of trapped CH₄ overlaps with the measured range of δ¹³C_CH4 in submarine vent fluids and continental gas seeps in both mafic and ultramafic substrates5. The concept of CH₄ synthesis in fluid inclusions, storage  over geological timescales, and subsequent mining of trapped CH₄ provides a plausible mechanism for the occurrence of radiocarbon-dead abiotic CH₄ in a wide range of geological settings, including mafic and ultramafic seafloor hydrothermal systems and ophiolitic gas seeps.

Professor Frieder Klein, Woods Hole Oceanographic Institution, USA

Professor Frieder Klein, Woods Hole Oceanographic Institution, USA

Frieder Klein is a scientist in the Marine Chemistry and Geochemistry Department at the Woods Hole Oceanographic Institution where he works on fluid-rock interactions in hydrothermal systems. He attended the Philipps University, Marburg and received a PhD in Earth Sciences from the University of Bremen, Germany. He also worked at the Instituto Andaluz de Ciencias de la Tierra (Granada, Spain) and the Laboratory for Atmospheric & Space Physics, University of Colorado (Boulder, CO). Using field observations, laboratory experiments, and theoretical models his current research centers on reaction pathways during serpentinization and related processes including abiotic organic synthesis and carbonate formation, habitability of subseafloor environments, aseismic creep in tectonic fault zones, and subduction of slow-spreading lithosphere.

Molecular hydrogen (H2) and magnetite are two of the most significant products of serpentinisation, both of which result from oxidation of iron as the reaction proceeds. Natural serpentinites exhibit substantial variability in the extent of iron oxidation and magnetite production, implying considerable variation in hydrogen production as well. However, the underlying reason for this variability remains highly uncertain. I will present results of a series of laboratory experiments designed to investigate how different environmental parameters impact the extent of iron oxidation during serpentinisation and the reaction pathways involved. Rates of hydrogen generation drop off significantly with decreasing temperature, both because the overall reaction slows and because iron increasingly partitions into brucite rather than magnetite, which does not involve oxidation. Conversely, higher pH increases rates of reaction and hydrogen production in most cases. Conditions that result in a greater contribution from orthopyroxene relative to olivine can limit magnetite production as iron gets preferentially partitioned into serpentine minerals, but this does not necessarily decrease hydrogen generation since ferric iron can be incorporated into serpentines. While chemical thermodynamics appear to be a major contributor to controlling the fate of iron during serpentinisation, many reactions in experimental systems remain out of thermodynamic equilibrium indicating kinetic factors must also contribute.

Dr Tom McCollom, University of Collorado, USA

Dr Tom McCollom, University of Collorado, USA

Dr McCollom is an aqueous geochemist whose primary research interests include sources of geochemical energy that support chemosynthetic  microbial growth and on abiotic synthesis of organic compounds.   Current research is focused on the chemical pathways and isotopic signatures of compounds formed in hydrothermal environments, particularly those formed during serpentinization of ultramafic rocks.  The ultimate goals of this research are to understand how life originated on Earth and how it persists today in subsurface environments that are devoid of sunlight, which will impact the search for places where life might exist elsewhere in our solar system.   Ongoing subjects of study include: formation of hydrogen and hydrocarbons in serpentinizing environments, abiotic synthesis and stability of amino acids in hydrothermal systems, and the role of organosulfur compounds in prebiotic organic synthesis pathways.   Dr McCollom’s research primarily employs a combination of laboratory experimental simulation of hydrothermal environments and development of numerical models, but has also included field work at several continental and deep-sea hydrothermal systems.  For the last ten years, Dr McCollom has been a research scientist in the Laboratory for Atmospheric and Space Physics at the University of Colorado.

Iron is a key component of the dynamic chemistry of serpentinizing systems. Oxidation of Fe(II) in primary minerals and accommodation of Fe(III) in secondary phases drives production of energy-rich H2 gas and the creation of reducing conditions. The effect of reaction conditions (e.g. protolith composition, temperature, fluid composition/flux) on the transformation and accommodation o Fe in the alteration mineral assemblage is not yet clearly understood. This is despite the influence that Fe chemistry has on many geological and (astro)biological processes including: the potential for and activity of microbial life in the subsurface ocean crust and hydrothermal vents, the potential for sequestration of carbon in ultramafic rocks, the origin of life, and the interpretation of the significance of mineral signatures detected on other planets. Integrated Raman hyperspectral, quantitative elemental, and x-ray absorption (sensitive to Fe oxidation state) spectral images and point analyses illustrate that serpentinized rocks obtained from IODP Expedition 357 to the Atlantis Massif have undergone multiple episodes of water/rock interaction; producing a variety of alteration textures and phases including Fe-rich and Fe-poor varieties of serpentine that have distinct distributions. The difference in Fe content may be indicative of different reaction conditions associated with each episode of water/rock interaction. The ferric component of all serpentines ranges from Fe3+/FeTotal ~40-80%, suggesting H2 could have been produced during serpentine formation. However, the difference in reaction conditions could affect the potential development of a subsurface, in-situ microbial biosphere supported by the serpentinization process.

Dr Lisa Mayhew, University of Colorado Boulder, USA

Dr Lisa Mayhew, University of Colorado Boulder, USA

Dr Mayhew is a geochemist and geomicrobiologist in the Templeton Laboratory at the University of Colorado Boulder. Dr Mayhew’s expertise is in the development and application of microscale, geochemical analytical tools to investigate mineral assemblages and mineral chemistry of igneous and metamorphic rocks, particularly peridotites and serpentinites. Currently, Dr Mayhew is investigating the iron mineralogy and speciation of subsurface serpentinites from the Atlantis Massif (IODP Expedition 357) and the Samail Ophiolite, Sultanate of Oman (Oman Drilling Project). Use of techniques such as synchrotron radiation spectroscopy, Raman spectroscopy, and electron microscopy to understand the chemistry of these rocks can help to unravel the alteration processes these rocks have undergone and perhaps aid in identifying the influence of biotic processes on this alteration.

Mantle peridotite is far from equilibrium in near surface conditions leading to rapid, extensive serpentinization, carbonation and oxidation, as well as other geochemical changes. In the Samail ophiolite in the Sultanate of Oman this geochemical changes have likely occurred through the entire history of the ophiolite, from formation near the axis of a spreading ridge to present day low temperature serpentinization/carbonation. Alteration products occur across the entire mantle section of the ophiolite, primarily as serpentine minerals, brucite and carbonates varying in composition from magnesium rich to calcium rich as water flows through the peridotites. The nature of these alteration products suggests that alteration is not entirely isochemical and provides evidence of magnesium mobility during carbonation and serpentinization. Each of the alteration products exhibits different fluid-mineral Mg isotopic fractionation deviating significantly from an initially homogenous mantle Mg isotopic composition (26Mg = −0.25‰ DSM3). Partially serpentinized harzburgites and dunites have mantle-like 26Mg (-0.23‰). Weathered serpentinized peridotites with magnesium depletion have heavy 26Mg (0.94‰), among the heaviest 26Mg values that have been reported in altered peridotite. Massive magnesite veins with very light compositions (-3.3‰) distributed within peridotite massifs are potential sinks for light magnesium isotopes removed from altered peridotites. The fractionation of Mg isotopes observed in the mantle section of the ophiolite spans more than 50% of the known terrestrial fractionation.

Mr Juan Carlos de Obeso, Columbia University, USA

Mr Juan Carlos de Obeso, Columbia University, USA

Juan Carlos de Obeso is a PhD candidate in the Department of Earth and Environmental Sciences of Columbia University. He grew up in Guadalajara, Mexico and obtained his undergraduate degree in Chemical Engineering in ITESO. He has a master in Climate and Society from Columbia University. Since 2013 he has been working in serpentinization and carbonation of peridotites in Prof. Peter Kelemen lab at Lamont-Doherty Earth Observatory. His research uses a variety of analytical techniques along with thermodynamic modeling to understand conditions of low temperature alteration of mantle rocks mainly from samples of the Samail ophiolite in the Sultanate of Oman. He has taught introductory geology classes in Columbia University Science Honor Program. He has served as tutor in Clubes de Ciencia Mexico as well as technical advisor for their online program. He enjoys running and rock climbing.