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Javier Escartin - One of the best experts on this subject based on the ideXlab platform.
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investigating fine scale permeability structure and its control on hydrothermal activity along a fast spreading ridge the east pacific rise 9 43 53 n using seismic velocity poroelastic response and numerical modeling
Geophysical Research Letters, 2019Co-Authors: M Marjanovic, Fabrice J Fontaine, Thibaut Barreyre, Javier EscartinAbstract:Along with the intracrustal heat source, crustal permeability is considered as the controlling factor for hydrothermal circulation within zero-age oceanic crust. To obtain fine-scale, 2-D models of upper crustal permeability along the East Pacific Rise 9°50′N, known for prolific hydrothermal activity, we use recently derived high-resolution seismic velocity and examine a number of the existing velocity-permeability relationships. To constrain our preferred permeability model, we compare thus derived permeability models with collocated permeability estimates from poroelastic response to tidal loading at L-vent. Furthermore, using the preferred permeability result, we model hydrothermal convection in 2-D and find that the distributions of recharge and discharge zones are in good agreement with seafloor observations, including locations of the vent fields. Our results suggest that seismic velocities can be used as a tool for deriving spatial variation of permeability, which must be considered in modeling of hydrothermal flow. Plain Language Summary Crustal permeability represents one of the main controlling factors for development and persistence of hydrothermal circulation at Mid-Ocean Ridges. However, this parameter remains poorly constrained. Using recently obtained seismic velocity model and available velocity-permeability relationships, we calculate a number of permeability models for East Pacific Rise 9°50′ N region, known for vigorous venting sites, including L-vent. To narrow down a wide range of upper crustal permeabilities and constrain our preferred permeability model, we use measurements obtained from poroelastic response of crustal lithologies to tides at L-vent. Our results suggest that average permeability for the first~100 m of the upper oceanic crust is 10 −11.2 m 2 , whereas 10 −14 m 2 characterizes the remaining part. We further evaluate our results with numerical models of hydrothermal circulation. The model that uses our preferred permeability field predicts locations of hydrothermal fluid recharge and discharge zones that are consistent with seafloor observations. Our study suggests that a growing number of high-resolution seismic velocity models can be further used to provide first-order estimates of permeability that will help us to advance our understanding behind hydrothermal processes, fluid circulation, and associated exchanges along Mid-Ocean Ridges.
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sensitivity of seafloor bathymetry to climate driven fluctuations in mid ocean ridge magma supply
Science, 2015Co-Authors: J A Olive, Javier Escartin, Mark D Behn, Garrett Ito, W R Buck, S M HowellAbstract:Recent studies have proposed that the bathymetric fabric of the seafloor formed at Mid-Ocean Ridges records rapid (23,000 to 100,000 years) fluctuations in ridge magma supply caused by sealevel changes that modulate melt production in the underlying mantle. Using quantitative models of faulting and magma emplacement, we demonstrate that, in fact, seafloor-shaping processes act as a low-pass filter on variations in magma supply, strongly damping fluctuations shorter than about 100,000 years. We show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input. This provides a robust framework for deciphering the footprint of mantle melting in the fabric of abyssal hills, the most common topographic feature on Earth.
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Along-axis hydrothermal flow at the axis of slow spreading Mid-Ocean Ridges: Insights from numerical models of the Lucky Strike vent field (MAR)
Geochemistry Geophysics Geosystems, 2014Co-Authors: Fabrice Fontaine, Javier Escartin, Mathilde Cannat, Wayne CrawfordAbstract:The processes and efficiency of hydrothermal heat extraction along the axis of Mid-Ocean Ridges are controlled by lithospheric thermal and permeability structures. Hydrothermal circulation models based on the structure of fast and intermediate spreading Ridges predict that hydrothermal cell organization and vent site distribution are primarily controlled by the thermodynamics of high-temperature Mid-Ocean ridge hydrothermal fluids. Using recent constraints on shallow structure at the slow spreading Lucky Strike segment along the Mid-Atlantic Ridge, we present a physical model of hydrothermal cooling that incorporates the specificities of a magma-rich slow spreading environment. Using three-dimensional numerical models, we show that, in contrast to the aforementioned models, the subsurface flow at Lucky Strike is primarily controlled by across-axis permeability variations. Models with across-axis permeability gradients produce along-axis oriented hydrothermal cells and an alternating pattern of heat extraction highs and lows that match the distribution of microseismic clusters recorded at the Lucky Strike axial volcano. The flow is also influenced by temperature gradients at the base of the permeable hydrothermal domain. Although our models are based on the structure and seismicity of the Lucky Strike segment, across-axis permeability gradients are also likely to occur at faster spreading Ridges and these results may also have important implications for the cooling of young crust at fast and intermediate spreading centers.
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the rheology of the lower oceanic crust implications for lithospheric deformation at mid ocean Ridges
Geophysical monograph, 2013Co-Authors: Greg Hirth, Javier EscartinAbstract:An analysis of experimentally determined flow laws for gabbroic rocks indicates that the lower oceanic crust is considerably stronger than commonly assumed in many modeling studies. A combination of experimental, geochemical and petrological constraints suggest that the majority of the lower oceanic crust is almost dry prior to hydrothermal alteration. Several geological and geophysical observations also indicate that the strength of the lower ocean crust approaches that of dry gabbroic rock. These observations are inconsistent with commonly used rheological models for the oceanic lithosphere that incorporate a weak lower crust that decouples a strong upper mantle from a brittle upper crust.
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detachments in oceanic lithosphere deformation magmatism fluid flow and ecosystems
Eos Transactions American Geophysical Union, 2011Co-Authors: Javier Escartin, J. Pablo CanalesAbstract:[1] AGU Chapman Conference on Oceanic Detachments; Agros, Cyprus, 8–15 May 2010; Oceanic detachments are large-offset normal faults along the flanks of Mid-Ocean Ridges. They represent a mode of accretion of the oceanic lithosphere that is fundamentally different from classical “magmatic” models, resulting in lithospheric composition and structure that are strikingly different from the Penrose model established 40 years ago of a layered magmatic crust. Oceanic detachments, which exhume deep lithosphere, forming oceanic core complexes (OCCs), are scientifically interesting because they represent tectonic windows to deep-seated rocks and processes (mantle flow, melt generation and migration, strain localization, and crustal accretion) at Mid-Ocean Ridges; a fundamental process in the generation of oceanic lithosphere along sizeable sections of slow and ultraslow spreading centers; a system sustaining both long-lived, high-temperature hydrothermal circulation and low-temperature, hydrogen-rich, serpentinite-related systems with their associated mineral deposits and ecosystems; a fault zone, containing weak hydrous alteration phases, that efficiently localizes strain, and that has associated footwall flexure and rotation; and a key to understanding continental core complexes as well as detachments at extensional, magma-poor continental margins.
Mark D Behn - One of the best experts on this subject based on the ideXlab platform.
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sensitivity of seafloor bathymetry to climate driven fluctuations in mid ocean ridge magma supply
Science, 2015Co-Authors: J A Olive, Javier Escartin, Mark D Behn, Garrett Ito, W R Buck, S M HowellAbstract:Recent studies have proposed that the bathymetric fabric of the seafloor formed at Mid-Ocean Ridges records rapid (23,000 to 100,000 years) fluctuations in ridge magma supply caused by sealevel changes that modulate melt production in the underlying mantle. Using quantitative models of faulting and magma emplacement, we demonstrate that, in fact, seafloor-shaping processes act as a low-pass filter on variations in magma supply, strongly damping fluctuations shorter than about 100,000 years. We show that the systematic decrease in dominant seafloor wavelengths with increasing spreading rate is best explained by a model of fault growth and abandonment under a steady magma input. This provides a robust framework for deciphering the footprint of mantle melting in the fabric of abyssal hills, the most common topographic feature on Earth.
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grain size dynamics beneath mid ocean Ridges implications for permeability and melt extraction
Geochemistry Geophysics Geosystems, 2015Co-Authors: Andrew Turner, Richard F Katz, Mark D BehnAbstract:Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two-dimensional, single phase model for the steady state mean grain size beneath a Mid-Ocean ridge. The mantle rheology is modeled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the paleowattmeter relationship of Austin and Evans (2007). We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading-rate, and mantle hydration. We interpret the mean grain-size field in terms of its permeability to melt transport. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions occurs across a boundary that is steeply inclined toward the ridge axis. We hypothesize that such a permeability structure generated from the variability of the mean grain size may focus melt toward the ridge axis, analogous to Sparks and Parmentier (1991)-type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain-size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise.
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grain size dynamics beneath mid ocean Ridges implications for permeability and melt extraction
arXiv: Geophysics, 2014Co-Authors: Andrew Turner, Richard F Katz, Mark D BehnAbstract:Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two-dimensional, single phase model for the steady-state mean grain size beneath a Mid-Ocean ridge. The mantle rheology is modelled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the piezometric relationship of Austin and Evans [2007]. We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading-rate, and mantle hydration. We interpret the mean mean grain-size field in the context of permeability. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions forms a boundary that is steeply sloped toward the ridge axis. We hypothesise that such a permeability structure generated from the variability of the mean grain size may be able to focus melt towards the ridge axis, analogous to a Sparks and Parmentier [1991]-type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain-size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise [Baba et al., 2006, Key et al., 2013].
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the structure of oceanic core complexes controlled by the depth distribution of magma emplacement
Nature Geoscience, 2010Co-Authors: Jeanarthur Olive, Mark D Behn, Brian E TucholkeAbstract:Extension at Mid-Ocean Ridges can be accommodated by detachment faults, forming oceanic core complexes that develop under low rates of magma intrusion. Modelling reveals that oceanic core complexes can also form under high rates of magma intrusion, if the magma is injected into the lower ductile layer of the crust.
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magmatic and tectonic extension at mid ocean Ridges 2 origin of axial morphology
Geochemistry Geophysics Geosystems, 2008Co-Authors: Garrett Ito, Mark D BehnAbstract:[1] We investigate the origin of Mid-Ocean ridge morphology with numerical models that successfully predict axial topographic highs, axial valleys, and the transition between the two. The models are time-dependent, simulating alternating tectonic and magmatic periods where far-field extension is accommodated by faulting and by magmatism, respectively. During tectonic phases, models predict faults to grow on either side of the ridge axis and axial height to decrease. During magmatic phases, models simulate magmatic extension by allowing the axial lithosphere to open freely in response to extension. Results show that fault size and spacing decreases with increasing time fraction spent in the magmatic phase FM. Magmatic phases also simulate the growth of topography in response to local buoyancy forces. The fundamental variable that controls the transition between axial highs and valleys is the “rise-sink ratio,” (FM/FT)(τT/τM), where FM/FT is the ratio of the time spent in the magmatic and tectonic periods and τT/τM is the ratio of the characteristic rates for growing topography during magmatic phases (1/τM) and for reducing topography during tectonic phases (1/τT). Models predict the tallest axial highs when (FM/FT)(τT/τM) ≫ 1, faulted topography without a high or valley when (FM/FT)(τT/τM) ∼ 1, and the deepest median valleys when (FM/FT)(τM/τT) < 1. New scaling laws explain a global negative correlation between axial topography and lithosphere thickness as measured by the depths of axial magma lenses and microearthquakes. Exceptions to this trend reveal the importance of other behaviors such as a predicted inverse relation between axial topography and spreading rate as evident along the Lau Spreading Center. Still other factors related to the frequency and spatial pervasiveness of magmatic intrusions and eruptions, as evident at the Mid-Atlantic and Juan de Fuca Ridges, influence the rise-sink-ratio (FM/FT)(τT/τM) and thus axial morphology.
Fabrice J Fontaine - One of the best experts on this subject based on the ideXlab platform.
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investigating fine scale permeability structure and its control on hydrothermal activity along a fast spreading ridge the east pacific rise 9 43 53 n using seismic velocity poroelastic response and numerical modeling
Geophysical Research Letters, 2019Co-Authors: M Marjanovic, Fabrice J Fontaine, Thibaut Barreyre, Javier EscartinAbstract:Along with the intracrustal heat source, crustal permeability is considered as the controlling factor for hydrothermal circulation within zero-age oceanic crust. To obtain fine-scale, 2-D models of upper crustal permeability along the East Pacific Rise 9°50′N, known for prolific hydrothermal activity, we use recently derived high-resolution seismic velocity and examine a number of the existing velocity-permeability relationships. To constrain our preferred permeability model, we compare thus derived permeability models with collocated permeability estimates from poroelastic response to tidal loading at L-vent. Furthermore, using the preferred permeability result, we model hydrothermal convection in 2-D and find that the distributions of recharge and discharge zones are in good agreement with seafloor observations, including locations of the vent fields. Our results suggest that seismic velocities can be used as a tool for deriving spatial variation of permeability, which must be considered in modeling of hydrothermal flow. Plain Language Summary Crustal permeability represents one of the main controlling factors for development and persistence of hydrothermal circulation at Mid-Ocean Ridges. However, this parameter remains poorly constrained. Using recently obtained seismic velocity model and available velocity-permeability relationships, we calculate a number of permeability models for East Pacific Rise 9°50′ N region, known for vigorous venting sites, including L-vent. To narrow down a wide range of upper crustal permeabilities and constrain our preferred permeability model, we use measurements obtained from poroelastic response of crustal lithologies to tides at L-vent. Our results suggest that average permeability for the first~100 m of the upper oceanic crust is 10 −11.2 m 2 , whereas 10 −14 m 2 characterizes the remaining part. We further evaluate our results with numerical models of hydrothermal circulation. The model that uses our preferred permeability field predicts locations of hydrothermal fluid recharge and discharge zones that are consistent with seafloor observations. Our study suggests that a growing number of high-resolution seismic velocity models can be further used to provide first-order estimates of permeability that will help us to advance our understanding behind hydrothermal processes, fluid circulation, and associated exchanges along Mid-Ocean Ridges.
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Hydrothermal circulation at slow-spreading Mid-Ocean Ridges: The role of along-axis variations in axial lithospheric thickness
Geology, 2008Co-Authors: Fabrice J Fontaine, Mathilde Cannat, Javier EscartinAbstract:At several ridge segments along the slow-spreading Mid-Atlantic Ridge, the lithosphere appears to be cooled by centrally located, isolated hydrothermal fields, hundreds of meters wide, extracting as much as 1000 MW from the lithosphere and hosting very large (>106 m3) sulfide edifices. These fields are possibly fueled by subseafloor hydrothermal cells cooling and leaching the lithosphere up to a few tens of kilometers along axis. However, the detailed mechanisms by which such hydrothermal heat extraction takes place are not well constrained. It is postulated that melt focusing and preferred cooling near transforms result in a thinner lithosphere at the center of slow-spreading ridge segments. In this configuration, and with a depth of penetration controlled by brittle lithospheric thickness, the base of the hydrothermal system is not at constant depth. Here we present models of along-axis hydrothermal circulation showing that pressure gradients generated along this basal slope influence flow dynamics. We show that the size of hydrothermal cells increases with the basal slope α. For α 15°–20°, the circulation reaches steady state and is composed of a single cell with a broad recharge and a focused discharge. Although our models make several simplifying assumptions, we propose that along-axis variations in lithosphere thickness associated with the magmatic and tectonic segmentation of slow-spreading Ridges should favor the formation of large and centrally located vent fields, mining heat on several kilometers along axis. We also predict that more short-lived and weaker vent fields may develop away from the segment center.
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permeability changes due to mineral diagenesis in fractured crust implications for hydrothermal circulation at mid ocean Ridges
Earth and Planetary Science Letters, 2001Co-Authors: Fabrice J Fontaine, Michel Rabinowicz, Jacques BoulegueAbstract:Abstract The hydrothermal processes at ridge crests have been extensively studied during the last two decades. Nevertheless, the reasons why hydrothermal fields are only occasionally found along some ridge segments remain a matter of debate. In the present study we relate this observation to the mineral precipitation induced by hydrothermal circulation. Our study is based on numerical models of convection inside a porous slot 1.5 km high, 2.25 km long and 120 m wide, where seawater is free to enter and exit at its top while the bottom is held at a constant temperature of 420°C. Since the fluid circulation is slow and the fissures in which seawater circulates are narrow, the reactions between seawater and the crust achieve local equilibrium. The rate of mineral precipitation or dissolution is proportional to the total derivative of the temperature with respect to time. Precipitation of minerals reduces the width of the fissures and thus percolation. Using conventional permeability versus porosity laws, we evaluate the evolution of the permeability field during the hydrothermal circulation. Our computations begin with a uniform permeability and a conductive thermal profile. After imposing a small random perturbation on the initial thermal field, the circulation adopts a finger-like structure, typical of convection in vertical porous slots thermally influenced by surrounding walls. Due to the strong temperature dependence of the fluid viscosity and thermal expansion, the hot rising fingers are strongly buoyant and collide with the top cold stagnant water layer. At the interface of the cold and hot layers, a horizontal boundary layer develops causing massive precipitation. This precipitation front produces a barrier to the hydrothermal flow. Consequently, the flow becomes layered on both sides of the front. The fluid temperature at the top of the layer remains quite low: it never exceeds a temperature of 80°C, well below the exit temperature of hot vent sites observed at black or white ‘smokers’. We show that the development of this front is independent of the Rayleigh number of the hydrothermal flow, indicating that the mineral precipitation causes cold, diffusive vents. Finally, we present a model suggesting that the development of smokers is possible when successive tectonic/volcanic events produce a network of new permeable fissures that can overcome the permeability decrease caused by mineral precipitation. Such a model is consistent with recent seismic data showing hydrothermal vents located at seismologically active ridge segments.
Mathilde Cannat - One of the best experts on this subject based on the ideXlab platform.
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serpentinization of mantle derived peridotites at mid ocean Ridges mesh texture development in the context of tectonic exhumation
Geochemistry Geophysics Geosystems, 2014Co-Authors: Stephane Roumejon, Mathilde CannatAbstract:At slow spreading Ridges, axial detachment faults exhume mantle-derived peridotites and hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault. At the microscopic scale, serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture. We present a petrographic study (SEM, EBSD, and Raman) of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the Mid-Atlantic and Southwest Indian Ridges. We show that serpentinization initiated along two intersecting sets of microfractures that have consistent orientations at the sample scale, and in at least one studied location, at the 100 m scale. We propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic stresses and subsequently served as channels for serpentinizing fluids. Additional reaction-induced cracks developed for serpentinization extents <20%. The resulting microfracture network has a typical spacing of ∼60 µm but most serpentinization occurs next to a subset of these microfractures that define mesh cells 100–400 µm in size. Apparent mesh rim thickness is on average 33 ± 19 µm corresponding to serpentinization extents of 70–80%. Published laboratory experiments suggest that mesh rims formation could be completed in a few years (i.e., quasi instantaneous at the plate tectonic timescale). The depth and extent of the serpentinization domain in the detachment fault's footwall are probably variable in time and space and as a result we expect that the serpentine mesh texture at slow spreading Ridges forms at variable rates with a spatially heterogeneous distribution.
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Along-axis hydrothermal flow at the axis of slow spreading Mid-Ocean Ridges: Insights from numerical models of the Lucky Strike vent field (MAR)
Geochemistry Geophysics Geosystems, 2014Co-Authors: Fabrice Fontaine, Javier Escartin, Mathilde Cannat, Wayne CrawfordAbstract:The processes and efficiency of hydrothermal heat extraction along the axis of Mid-Ocean Ridges are controlled by lithospheric thermal and permeability structures. Hydrothermal circulation models based on the structure of fast and intermediate spreading Ridges predict that hydrothermal cell organization and vent site distribution are primarily controlled by the thermodynamics of high-temperature Mid-Ocean ridge hydrothermal fluids. Using recent constraints on shallow structure at the slow spreading Lucky Strike segment along the Mid-Atlantic Ridge, we present a physical model of hydrothermal cooling that incorporates the specificities of a magma-rich slow spreading environment. Using three-dimensional numerical models, we show that, in contrast to the aforementioned models, the subsurface flow at Lucky Strike is primarily controlled by across-axis permeability variations. Models with across-axis permeability gradients produce along-axis oriented hydrothermal cells and an alternating pattern of heat extraction highs and lows that match the distribution of microseismic clusters recorded at the Lucky Strike axial volcano. The flow is also influenced by temperature gradients at the base of the permeable hydrothermal domain. Although our models are based on the structure and seismicity of the Lucky Strike segment, across-axis permeability gradients are also likely to occur at faster spreading Ridges and these results may also have important implications for the cooling of young crust at fast and intermediate spreading centers.
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Hydrothermal circulation at slow-spreading Mid-Ocean Ridges: The role of along-axis variations in axial lithospheric thickness
Geology, 2008Co-Authors: Fabrice J Fontaine, Mathilde Cannat, Javier EscartinAbstract:At several ridge segments along the slow-spreading Mid-Atlantic Ridge, the lithosphere appears to be cooled by centrally located, isolated hydrothermal fields, hundreds of meters wide, extracting as much as 1000 MW from the lithosphere and hosting very large (>106 m3) sulfide edifices. These fields are possibly fueled by subseafloor hydrothermal cells cooling and leaching the lithosphere up to a few tens of kilometers along axis. However, the detailed mechanisms by which such hydrothermal heat extraction takes place are not well constrained. It is postulated that melt focusing and preferred cooling near transforms result in a thinner lithosphere at the center of slow-spreading ridge segments. In this configuration, and with a depth of penetration controlled by brittle lithospheric thickness, the base of the hydrothermal system is not at constant depth. Here we present models of along-axis hydrothermal circulation showing that pressure gradients generated along this basal slope influence flow dynamics. We show that the size of hydrothermal cells increases with the basal slope α. For α 15°–20°, the circulation reaches steady state and is composed of a single cell with a broad recharge and a focused discharge. Although our models make several simplifying assumptions, we propose that along-axis variations in lithosphere thickness associated with the magmatic and tectonic segmentation of slow-spreading Ridges should favor the formation of large and centrally located vent fields, mining heat on several kilometers along axis. We also predict that more short-lived and weaker vent fields may develop away from the segment center.
Richard F Katz - One of the best experts on this subject based on the ideXlab platform.
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volatiles beneath mid ocean Ridges deep melting channelised transport focusing and metasomatism
Earth and Planetary Science Letters, 2017Co-Authors: Tobias Keller, Richard F Katz, Marc M HirschmannAbstract:Deep-Earth volatile cycles couple the mantle with near-surface reservoirs. Volatiles are emitted by volcanism and, in particular, from Mid-Ocean Ridges, which are the most prolific source of basaltic volcanism. Estimates of volatile extraction from the asthenosphere beneath Ridges typically rely on measurements of undegassed lavas combined with simple petrogenetic models of the mean degree of melting. Estimated volatile fluxes have large uncertainties; this is partly due to a poor understanding of how volatiles are transported by magma in the asthenosphere. Here, we assess the fate of mantle volatiles through numerical simulations of melting and melt transport at Mid-Ocean Ridges. Our simulations are based on two-phase, magma/mantle dynamics theory coupled to an idealised thermodynamic model of mantle melting in the presence of water and carbon dioxide. We combine simulation results with catalogued observations of all ridge segments to estimate a range of likely volatile output from the global Mid-Ocean ridge system. We thus predict global MOR crust production of 66–73 Gt/yr (22–24 km3/yr) and global volatile output of 52–110 Mt/yr, corresponding to mantle volatile contents of 100–200 ppm. We find that volatile extraction is limited: up to half of deep, volatile-rich melt is not focused to the axis but is rather deposited along the LAB. As these distal melts crystallise and fractionate, they metasomatise the base of the lithosphere, creating rheological heterogeneity that could contribute to the seismic signature of the LAB.
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grain size dynamics beneath mid ocean Ridges implications for permeability and melt extraction
Geochemistry Geophysics Geosystems, 2015Co-Authors: Andrew Turner, Richard F Katz, Mark D BehnAbstract:Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two-dimensional, single phase model for the steady state mean grain size beneath a Mid-Ocean ridge. The mantle rheology is modeled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the paleowattmeter relationship of Austin and Evans (2007). We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading-rate, and mantle hydration. We interpret the mean grain-size field in terms of its permeability to melt transport. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions occurs across a boundary that is steeply inclined toward the ridge axis. We hypothesize that such a permeability structure generated from the variability of the mean grain size may focus melt toward the ridge axis, analogous to Sparks and Parmentier (1991)-type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain-size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise.
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grain size dynamics beneath mid ocean Ridges implications for permeability and melt extraction
arXiv: Geophysics, 2014Co-Authors: Andrew Turner, Richard F Katz, Mark D BehnAbstract:Grain size is an important control on mantle viscosity and permeability, but is difficult or impossible to measure in situ. We construct a two-dimensional, single phase model for the steady-state mean grain size beneath a Mid-Ocean ridge. The mantle rheology is modelled as a composite of diffusion creep, dislocation creep, dislocation accommodated grain boundary sliding, and a plastic stress limiter. The mean grain size is calculated by the piezometric relationship of Austin and Evans [2007]. We investigate the sensitivity of our model to global variations in grain growth exponent, potential temperature, spreading-rate, and mantle hydration. We interpret the mean mean grain-size field in the context of permeability. The permeability structure due to mean grain size may be approximated as a high permeability region beneath a low permeability region. The transition between high and low permeability regions forms a boundary that is steeply sloped toward the ridge axis. We hypothesise that such a permeability structure generated from the variability of the mean grain size may be able to focus melt towards the ridge axis, analogous to a Sparks and Parmentier [1991]-type focusing. This focusing may, in turn, constrain the region where significant melt fractions are observed by seismic or magnetotelluric surveys. This interpretation of melt focusing via the grain-size permeability structure is consistent with MT observation of the asthenosphere beneath the East Pacific Rise [Baba et al., 2006, Key et al., 2013].
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magma dynamics with the enthalpy method benchmark solutions and magmatic focusing at mid ocean Ridges
Journal of Petrology, 2008Co-Authors: Richard F KatzAbstract:Magma genesis and transport link mantle convection with surface volcanism and hence with the long-term chemical and morphological evolution of the Earth's; crust. Modeling the dynamics of magma-mantle interaction in tectonic settings remains a challenge, however, because of the complexity of multi-component thermodynamics and melt segregation in a permeable, compactible, and actively deforming mantle matrix. Here I describe a flexible approach to formulating the thermochemistry of such models based on the Enthalpy Method, a technique commonly used in simulations of alloy solidification. This approach allows for melting and freezing based on a familiar binary phase diagram, consistent with conservation of energy and two-phase compaction and flow. I present an extension of the Enthalpy Method to more than two thermodynamic components. Simulation of a one-dimensional upwelling and melting column provides a benchmark for the method. Two-dimensional simulations of the melting region that feeds magma to a rapidly spreading Mid-Ocean ridge demonstrate the utility of the Enthalpy Method. These calculations provide a new estimate of the efficiency of magmatic focusing along the base of the oceanic lithosphere. Modeled focusing efficiency varies with mantle permeability and resistance to compaction. To yield 5-7 km of oceanic crust with ∼20% melting of a homogeneous, sub-ridge mantle, a focusing efficiency of greater than 70% is required. This, in turn, suggests that matrix permeability and bulk viscosity are at the high end of previously estimated values.