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Serpentinite in the Earth System

Discussion meeting

Location

The Royal Society, London, 6-9 Carlton House Terrace, London, SW1Y 5AG

Overview

Scientific Discussion meeting organised by Dr Andrew McCaig, Professor Peter Kelemen, Professor Gretchen Früh-Green and Professor Damon Teagle

A serpentinite hosted hydrothermal chimney, image credit: Bénédicte Ménez and the Geomicrobiology group, IPGP

This meeting will bring together international scientists working on all aspects of serpentinisation, a process that may have been important for the origin of life on Earth and perhaps other planets. Serpentine is also a key carrier of water to depth in subduction zones, leading to intermediate depth earthquakes and the formation of island arc volcanoes.

The schedule of talks and speaker biographies are available below. Recorded audio of the presentations will be available on this page after the meeting has taken place.

Poster session

Deadline extension: 22 October 2018

There will be a poster session at 17:00 on Monday 19 November. If you would like to apply to present a poster please submit your proposed title, abstract (not more than 200 words and in third person), author list, name of the proposed presenter and institution to the Scientific Programmes team no later than Monday 22 October 2018. Please note that places are limited and posters are selected at the scientific organiser's discretion. Poster abstracts will only be considered if the presenter is registered to attend the meeting.

Attending the event

This meeting is intended for researchers in relevant fields.

  • Free to attend
  • Limited places, advanced registration is essential
  • An optional lunch can be purchased during registration

An evening poster session and drinks reception will be held following the close of the meeting on Monday 19 November 2018. Whilst the posters are free to view for all registered participants, the corresponding optional drinks reception is ticketed. Drinks reception tickets can be purchased in advance during registration.

Enquiries: Contact the Scientific Programmes team.

Event organisers

Select an organiser for more information

Schedule of talks

19 November

Session 1 09:00-12:30

Settings of serpentinisation of Earth

6 talks Show detail Hide detail

Chairs

Professor Christopher MacLeod, Cardiff University, UK

09:00-09:05 Welcome by the Royal Society & Andrew McCaig

09:05-09:30 Tectonic and magmatic controls on serpentinization at mid-ocean ridges

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

Abstract

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.

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09:30-09:45 Discussion

09:45-10:15 Serpentinisation in the nonaccretionary Mariana convergent plate margin: Lessons from deep ocean drilling and seafloor observations

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

Abstract

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.

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10:15-10:30 Discussion

10:30-11:00 Coffee break

11:00-11:30 Background serpentinisation versus focused hydrothermalism in oceanic core complexes

Professor Muriel Andreani, Université de Lyon 1, France

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11:30-11:45 Discussion

11:45-12:15 The hydrogeology of the active serpentinisation system in the Samail ophiolite, Oman

Professor Juerg M Matter, University of Southampton, UK

Abstract

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.

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12:15-12:30 Discussion

12:30-13:30

Lunch

Session 2 13:30-18:15

Mineralogy and geochemistry of serpentinisation

6 talks Show detail Hide detail

Chairs

Professor Gretchen Früh-Green, ETH Zurich, ETH Zurich, Switzerland

13:30-14:00 Fischer-Tropsch-Type Synthesis in Olivine-Hosted Fluid Inclusions: The Origin of Methane in Serpentinisation Systems?

Professor Frieder Klein, Woods Hole Oceanographic Institution, USA

Abstract

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 fluids1-3. 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 fO2 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 δ13CCH4 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.

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14:00-14:15 Discussion

14:15-14:45 Experimental perspectives on hydrogen generation, Fe partitioning, and magnetite production during serpentinisation of ocean crust

Dr Tom McCollom, University of Collorado, USA

Abstract

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.

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14:45-15:00 Discussion

15:00-15:30 Tea break

15:30-16:00 Iron speciation in subsurface serpentinites from the Atlantis Massif (IODP Exp.357)

Dr Lisa Mayhew, University of Colorado Boulder, USA

Abstract

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.

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16:00-16:15 Discussion

16:15-16:45 Carbon mineralisation accompanying serpentinisation in the Oman ophiolite

Mr Juan Carlos de Obeso, Columbia University, USA

Abstract

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.

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16:45-17:00 Discussion

17:00-18:15 Poster session

20 November

Session 3 09:00-12:30

Serpentinisation and life

5 talks Show detail Hide detail

Chairs

Professor Damon A H Teagle, University of Southampton, UK

09:00-09:30 Microbial methanogenesis within hydrating peridotite

Professor Alexis Templeton, University of Colorado at Boulder, USA

Abstract

The peridotite aquifers in Oman are actively undergoing low-temperature hydration and carbonation at near-surface temperatures. Several distinct fluid regimes exist that harbor distinct microbial communities and functional capabilities. Organisms such as Methanobacterium sp., methanogens who possess the genetic capability for the reduction of CO2 using molecular hydrogen, are notably widespread. However, the thermodynamic feasibility of CO2 reduction to methane is spatially and temporally variable, depending upon the extent of water rock reaction, fluid mixing, fluid pH, carbon availability and flux of dissolved hydrogen. Thus, it is challenging to determine the conditions under which rock-hosted methanogens convert dissolved inorganic carbon into methane, and to predict the scale of carbon fluxes controlled by in-situ methanogenesis. We are investigating the in situ coupling between water/rock reaction and methanogenic activity, in order to interpret the isotopic, lipid and mineralogical biosignatures of the peridotite-hosted subsurface biosphere. Detailed geochemical analysis of fluids and dissolved gases derived from subsurface sampling, coupled with methane production assays, genetic analysis of the methanogens, and laboratory experiments, together show that biological methane production can occur across an enormous range of conditions within the peridotite-water-CO2 system. Distinct isotopic and isotopologue compositions of the methane are produced that can be used to infer the carbon availability and energy state of the microbial communities. In particular, we can demonstrate that carbonate minerals might be critically important for sustaining slow biological methane production in hyperalkaline systems. Such findings are important in focusing our search for rock-hosted life on Earth and other rocky bodies in our solar system. Continuous biological methane production under hyperalkaline conditions should also be a quantitatively significant component of the carbon flux within serpentinites, shifting the balance from mineral carbonation to methanation.

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09:30-09:45 Discussion

09:45-10:15 Carbon sources shaping deep ecosystems in oceanic serpentinites

Professor Bénédicte Ménez, Institut de Physique du Globe de Paris/Université Paris Diderot, France

Abstract

In serpentinizing alkaline environments, the lack of inorganic carbon sources and the reduced conditions are challenging for the development of deep ecosystems. Alternatively, abiotic organic compounds deriving from the serpentinization process could serve as carbon or energy sources, making organoheterotrophy a valuable strategy for microbes in serpentinization-related environments (1-3). Up to now, effective abiotic synthesis was only demonstrated for a restricted number of low-molecular-weight organic compounds in hydrothermal systems associated with serpentinization on Earth. These compounds include methane, short-chain alkanes and formate. In this talk, recent evidences for the presence of a larger diversity of abiotic organic compounds will be discussed. Special emphasis will be put on condensed carbonaceous matter predicted to be thermodynamically favored at low temperature where the formation of methane is kinetically inhibited (4). Condensed carbonaceous matter was recently shown to form simultaneously to the hydrothermal alteration of the oceanic crust and the growth of the host mineralogical assemblage. The local production of H2 associated with mineral formation and the presence of various catalysts appear to represent key parameters controlling the production rate and the chemistry of the associated carbonaceous matter. New pathways and their relevancy at the crust scale will be addressed during the talk. Consequences for ancestral metabolisms and microbial life strategies in the present-day deep biosphere will be discussed in light of the metabolic capabilities of microbes inhabiting such environments.

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10:15-10:30 Discussion

10:30-11:00 Coffee break

11:00-11:30 Identifying microbial activity in serpentinization systems using organics and isotopes

Assistant Professor Susan Q Lang, University of South Carolina, USA

Abstract

The hydrogen produced during water-rock serpentinization reactions can fuel life in the rocky oceanic subsurface, and promote the abiotic synthesis of organic molecules. Microbial activity may be limited, however, by a combination of elevated temperatures, alkaline pHs, and limited bioavailable inorganic carbon. Our goal is to develop a mechanistic understanding of the relationship between microbial activity and the physical and geochemical characteristics of the rocky subsurface within serpentinizing environments. These associations can provide insights into where life may have developed on Earth or exist on other planetary bodies. Recent expeditions to the Atlantis Massif and to the Lost City Hydrothermal field (30°N, Mid-Atlantic Ridge) provide the opportunity to investigate carbon cycling and the presence of life in a zone of active serpentinization. The abundances, distributions, and isotopic compositions of carbon compounds are highly heterogeneous, and reflect varying degrees of subsurface water-rock reactions and microbial activity. In the subseafloor rocks, high concentrations of small organic acids are associated with serpentinized harzburgite and metadolerite lithologies in particular. Indications of biological activity, such as biologically derived amino acids and cellular abundances, have distribution patterns distinct from those of organic acids. Active metabolic pathways can be identified through a combination of incubation experiments and by comparing the radiocarbon content of lipid biomarkers to that of potential carbon sources. These findings may shed light on the feasibility of current theories that propose that the earliest metabolisms, and the Acetyl-CoA pathway in particular, may have developed by a transition from geoenergetics to bioenergetics within alkaline hydrothermal systems.

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11:30-11:45 Discussion

11:45-12:15 Serpentine and the search for life beyond Earth

Dr Steve Vance, California Institute of Technology, USA

Abstract

Hydrogen from serpentinization is a potential source of chemical energy for life in the subsurface of Mars, and in the icy ocean worlds in the outer solar system. Much of the context of planetary serpentinization relies on inference from modeling and studies on Earth. While there is good evidence that serpentinization has occurred on Mars, the extent and duration of that activity has not been constrained. Similarly, current serpentinization might help to explain the abundant hydrogen in the ocean of the tiny saturnian moon Enceladus, but this raises questions of how long such activity has persisted. Titan’s hydrocarbon rich atmosphere may have derived from ancient or even present-day serpentinization at the bottom of its ocean, but this is difficult to confirm, and difficult to conceive under the high pressures in Titan’s interior. In Europa, volcanic activity and serpentinization may provide complementary sources of hydrogen as a redox couple to oxygen generated at the moon’s surface. Planned robotic exploration missions to these places can aid in the quest to understand the planetary context of serpentinization

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12:15-12:30 Discussion

12:30-13:30

Lunch

Session 4 13:30-17:00

Serpentinite in subduction zones and deformation

5 talks Show detail Hide detail

Chairs

Professor Peter Kelemen, Columbia University, USA

13:30-14:00 On the pH of serpentinizing fluids in the crust and mantle

Professor Craig Manning, University of California, USA

Abstract

Fluids issuing from serpentinite bodies may exhibit alkaline to extremely alkaline pH, both on land and in submarine settings (e.g., Barnes and O’Neil 1969; Barnes et al. 1972; Kelley et al. 2001, 2005). Production of highly alkaline fluids is associated transformation of anhydrous olivine and orthopyroxene to hydrous serpentine and brucite, and reactions describing this process are typically written such that H+ is consumed as a reactant. However, several considerations suggest that the extremes in pH may be limited to shallow, near-surface environments: (1) hydrous minerals become unstable at high T, (2) self-dissociation of H2O is more extensive at high P and T, and (3) the relative stabilities of aqueous solutes change with P and T. To address this possibility, we modeled equilibrium compositions and pH of fluids associated with model and natural ultramafic bulk compositions over a range of crustal and mantle conditions using high-P thermodynamic data for minerals and aqueous species (Holland and Powell 1998; Sverjensky et al. 2014). Results show that, for the MgO-SiO2-H2O (MSH) system, H2O equilibrated with a model mantle rock (90% forsterite 10% enstatite) possesses alkaline pH at all P and T in the range 0.05 to 3 GPa and 100 to 800°C. However, the difference between calculated and neutral pH (pH) is greater than 2 only below 150°C. Along isobars, pH decreases with rising T; pH < 1 is typical at >450°C at all P. This is due neither to the declining stability of hydrous minerals, nor to changes in KH2O; rather, it appears to be a consequence of the increasing stability of anionic aqueous species relative to cations, which requires increasing H+ concentration (activity) to balance charge. Addition of other sources of alkalinity such as carbonate or sulfate yields the same behavior. Similar patterns are seen in more complex, natural bulk compositions (i.e., with Fe, Al, Ca). Support for the hypothesis can be seen in other model systems (e.g., CaO-SiO2-H2O). Suppressed excursions from neutral pH to be a general feature of metamorphic fluids of the crust and mantle, and the familiar high alkalinity of serpentinite fluids is likely unique to near-surface environments.

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14:00-14:15 Discussion

14:15-14:45 Antigorite deformation, dehydration and embrittlement

Dr Nicolas Brantut, University College London, UK

Abstract

Antigorite is the stable form of serpentine at elevated pressure, and is thought to be a key mineral controlling the strength and the source of intermediate depth earthquakes in the subducted oceanic lithosphere. Laboratory deformation experiments consistently show that antigorite-rich rocks are strong and brittle, including at elevated pressure and temperature. In the low pressure, low temperature regime (<0.2 GPa, room temperature), brittle deformation of antigorite is not accompanied with dilatancy or stress-induced anisotropy, unlike most other silicate rocks. Failure produces complete dynamic strength drops, and melting occurs at asperities on the fault plane. At elevated pressure and temperature (~1 GPa, 500C), antigorite deformation is localised in thin shear bands with intense grain size reduction, and intracrystalline plasticity is limited. The peculiar behaviour of antigorite can be attributed to (1) its complex mineral structure, which has a very large unit cell, is highly anisotropic, and exhibits a corrugation along its cleavage plane, and (2) to the small grain size and prolate grain shapes typically found in antigorite-rich rocks. Antigorite weakens dramatically upon dehydration at elevated temperature, and forms a (transiently) porous aggregate. The coupling between dehydration and compaction can produce deformation instabilities which could contribute to the weakening of the surrounding rocks and the potential generation of earthquake instabilities.

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14:45-15:00 Discussion

15:00-15:30 Tea break

15:30-16:00 Fluid escape from dehydrating serpentinites in subduction zones

Professo Oliver Plümper, Utrecht University, Netherlands

Abstract

At subduction zones seawater-altered oceanic lithosphere is returned to the Earth’s mantle, where increasing temperatures and pressures result in the progressive destabilization of hydrous minerals and thereby the release of aqueous fluids. This cycling of volatiles is one of the most distinctive features of subduction zones and has fundamental consequences for Earth’s geodynamics and geochemical cycles. Fluids released from the subducting slab trigger sub-arc mantle melting and induce petrophysical changes that are a potential source of intermediate-depth seismicity. However, the mechanisms of fluid escape from static and deforming dehydrating serpentinites are currently under debate and the impact of dehydration and fluid escape on rheological properties remains poorly constrained. Using a combined approach of field observations, microstructural analysis down to the nanoscale, and numerical modelling the authors investigated meta-serpentinites from the Erro-Tobbio unit, Ligurian Alps (Italy). This unit was subducted to peak metamorphic conditions of 2.0–2.5 GPa and 550–600 ºC, resulting in the breakdown of antigorite+brucite to form olivine+H2O. The multi-scale analysis shows that initiation of fluid flow is controlled by intrinsic chemical heterogeneities, localizing dehydration at specific microsites. Porosity generation is directly linked to the dehydration reactions and resultant fluid pressure variations force the reactive fluid release to organize into vein networks across a wide range of spatial scales. This channelization results in large-scale fluid escape with sufficient fluxes to drain subducting plates. Microstructural observations of meta-serpentinites that have undergone dehydration during deformation display evidence for complex relationships between crystal growth, grain-boundary sliding, and compaction. None of the investigated meta-serpentinites preserve evidence of dehydration embrittlement or shear instability indicative of intermediate-depth seismicity.

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16:00-16:15 Discussion

16:15-17:00 Panel discussion

Serpentinite in the Earth System

19 - 20 November 2018

The Royal Society, London 6-9 Carlton House Terrace London SW1Y 5AG UK
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