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  • 1
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    Faulting and off-axis submarine massive sulfide accumulation at slow spreading mid-ocean ridges: A numerical modeling perspective (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 18 (6). 2305-2320 .
    Details
    Publication Date: 2020-02-06
    Description: The potential of mining seafloor massive sulfide deposits for metals such as Cu, Zn, and Au is currently debated. One key challenge is to predict where the largest deposits worth mining might form, which in turn requires understanding the pattern of subseafloor hydrothermal mass and energy transport. Numerical models of heat and fluid flow are applied to illustrate the important role of fault zone properties (permeability and width) in controlling mass accumulation at hydrothermal vents at slow spreading ridges. We combine modeled mass-flow rates, vent temperatures, and vent field dimensions with the known fluid chemistry at the fault-controlled Logatchev 1 hydrothermal field of the Mid-Atlantic Ridge. We predict that the 135 kilotons of SMS at this site (estimated by other studies) can have accumulated with a minimum depositional efficiency of 5% in the known duration of hydrothermal venting (58,200 year age of the deposit). In general, the most productive faults must provide an efficient fluid pathway while at the same time limit cooling due to mixing with entrained cold seawater. This balance is best met by faults that are just wide and permeable enough to control a hydrothermal plume rising through the oceanic crust. Model runs with increased basal heat input, mimicking a heat flow contribution from along-axis, lead to higher mass fluxes and vent temperatures, capable of significantly higher SMS accumulation rates. Nonsteady state conditions, such as the influence of a cooling magmatic intrusion beneath the fault zone, also can temporarily increase the mass flux while sustaining high vent temperatures.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2017GC006880
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Global rates of mantle serpentinization and H2 production at oceanic transform faults in 3-D geodynamic models (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 44 (13). 6726-6734 .
    Details
    Publication Date: 2020-02-06
    Description: Previous studies have estimated that mantle serpentinization reactions generate H2 at a rate of 1010–1012 mol/yr along the global mid-ocean ridge (MOR) system. Here we present results of 3-D geodynamic simulations that predict rates of additional mantle serpentinization and H2 production at oceanic transform faults (OTF). We find that the extent and rate of mantle serpentinization increases with OTF length and is maximum at intermediate slip rates of 5 to 10 cm/yr. The additional global OTF-related production of H2 is found to be between 6.1 and 10.7 × 1011 mol/yr, which is comparable to the predicted background MOR rate of 4.1–15.0 × 1011 mol H2/yr. This points to oceanic transform faults as potential sites of intense fluid-rock interaction, where chemosynthetic life could be sustained by serpentinization reactions.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2017GL072893
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    The effect of obliquity-driven changes on paleoclimate sensitivity during the late Pleistocene (2018)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 45 (13). pp. 6661-6671.
    Details
    Publication Date: 2021-02-08
    Description: We reanalyze existing paleodata of global mean surface temperature ΔTg and radiative forcing ΔR of CO2 and land ice albedo for the last 800,000 years to show that a state‐dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔTg is based on reconstructions, and not when ΔTg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land‐ice sheet growth and sea level fall) the multi‐millennial component of reconstructed ΔTg diverges from CO2, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction‐based extrapolation of S for the future we exclude intervals with strong ΔTg‐CO2 divergence and find that S is less state‐dependent, or even constant (state‐independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO2.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2018GL077717
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    Modeling fluid flow in sedimentary basins with sill intrusions: Implications for hydrothermal venting and climate change (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 14 (12). pp. 5244-5262.
    Details
    Publication Date: 2018-02-28
    Description: Large volumes of magma emplaced within sedimentary basins have been linked to multiple climate change events due to release of greenhouse gases such as CH4. Basin-scale estimates of thermogenic methane generation show that this process alone could generate enough greenhouse gases to trigger global incidents. However, the rates at which these gases are transported and released into the atmosphere are quantitatively unknown. We use a 2D, hybrid FEM/FVM model that solves for fully compressible fluid flow to quantify the thermogenic release and transport of methane and to evaluate flow patterns within these systems. Our results show that the methane generation potential in systems with fluid flow does not significantly differ from that estimated in diffusive systems. The values diverge when vigorous convection occurs with a maximum variation of about 50%. The fluid migration pattern around a cooling, impermeable sill alone generates hydrothermal plumes without the need for other processes such as boiling and/or explosive degassing. These fluid pathways are rooted at the edges of the outer sills consistent with seismic imaging. Methane venting at the surface occurs in three distinct stages and can last for hundreds of thousands of years. Our simulations suggest that although the quantity of methane potentially generated within the contact aureole can cause catastrophic climate change, the rate at which this methane is released into the atmosphere is too slow to trigger, by itself, some of the negative δ13C excursions observed in the fossil record over short time scales (〈 10,000 years).
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2013GC005012
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 5
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    Interrelation between rifting, faulting, sedimentation, and mantle serpentinization during continental margin formation-including examples from the Norwegian Sea (2013)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 14 (10). pp. 4351-4369.
    Details
    Publication Date: 2018-02-28
    Description: The conditions permitting mantle serpentinization during continental rifting are explored within 2-D thermotectonostratigraphic basin models, which track the rheological evolution of the continental crust, account for sediment blanketing effects, and allow for kinetically controlled mantle serpentinization processes. The basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur. The isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions. A systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs. Increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures. Sediment supply has therefore, through the temperature-dependence of the viscous flow laws, strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high. In a case study for the Norwegian margin, we test whether the inner lower crustal bodies (LCB) imaged beneath the Møre and Vøring margin could be serpentinized mantle. Multiple 2-D transects have been reconstructed through the 3-D data set by Scheck-Wenderoth and Maystrenko (2011). We find that serpentinization reactions are possible and likely during the Jurassic rift phase. Predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on LCBs from seismic and gravity data. We conclude that some of the inner LCBs beneath the Norwegian margin may be partially serpentinized mantle.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/ggge.20268
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 6
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    Controls of bathymetric relief on hydrothermal fluid flow at mid-ocean ridges (2012)
    AGU (American Geophysical Union)
    In:  Geochemistry, Geophysics, Geosystems, 13 . Q05002.
    Details
    Publication Date: 2019-09-23
    Description: We present quantitative modeling results for the effects of surface relief on hydrothermal convection at ocean-spreading centers investigating how vent site locations and subsurface flow patterns are affected by bathymetry induced sub-seafloor pressure variations. The model is based on a 2-D FEM solver for fluid flow in porous media and is used to simulate hydrothermal convection systematically in 375 synthetic studies. The results of these studies show that bathymetric relief has a profound effect on hydrothermal flow: bathymetric highs induce subsurface pressure variations that can deviate upwelling zones and favor venting at structural highs. The deviation angle from vertical upwelling can be expressed by a single linear dependence relating deviation angle to bathymetric slope and depth of the heat source. These findings are confirmed in two case studies for the East Pacific Rise at 9°30′N and Lucky Strike hydrothermal fields. In both cases, it is possible to predict the observed vent field locations only if bathymetry is taken into account. Our results thereby show that bathymetric relief should be considered in simulations of submarine hydrothermal systems and plays a key role especially in focusing venting of across axis hydrothermal flow onto the ridge axis of fast spreading ridges.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2012GC004041
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 7
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    Modes of crustal accretion and their implications for hydrothermal circulation (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Geophysical Research Letters, 43 (3). pp. 1124-1131.
    Details
    Publication Date: 2019-02-01
    Description: Hydrothermal convection at mid-ocean ridges links the ocean's long-term chemical evolution to solid earth processes, forms hydrothermal ore deposits, and sustains the unique chemosynthetic vent fauna. Yet the depth extent of hydrothermal cooling and the inseparably connected question of how the lower crust accretes remain poorly constrained. Here based on coupled models of crustal accretion and hydrothermal circulation, we provide new insights into which modes of lower crust formation and hydrothermal cooling are thermally viable and most consistent with observations at fast-spreading ridges. We integrate numerical models with observations of melt lens depth, thermal structure, and melt fraction. Models matching all these observations always require a deep crustal-scale hydrothermal flow component and less than 50% of the lower crust crystallizing in situ.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2015GL067335
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 8
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    Modeling of craton stability using a viscoelastic rheology (2010)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Solid Earth, 115 (B11). B11413.
    Details
    Publication Date: 2019-09-23
    Description: Archean cratons belong to the most remarkable features of our planet since they represent continental crust that has avoided reworking for several billions of years. Even more, it has become evident from both geophysical and petrological studies that cratons exhibit deep lithospheric keels which equally remained stable ever since the formation of the cratons in the Archean. Dating of inclusions in diamonds from kimberlite pipes gives Archean ages, suggesting that the Archean lithosphere must have been cold soon after its formation in the Archean (in order to allow for the existence of diamonds) and must have stayed in that state ever since. Yet, although strong evidence for the thermal stability of Archean cratonic lithosphere for billions of years is provided by diamond dating, the long-term thermal stability of cratonic keels was questioned on the basis of numerical modeling results. We devised a viscoelastic mantle convection model for exploring cratonic stability in the stagnant lid regime. Our modeling results indicate that within the limitations of the stagnant lid approach, the application of a sufficiently high temperature-dependent viscosity ratio can provide for thermal craton stability for billions of years. The comparison between simulations with viscous and viscoelastic rheology indicates no significant influence of elasticity on craton stability. Yet, a viscoelastic rheology provides a physical transition from viscously to elastically dominated regimes within the keel, thus rendering introduction of arbitrary viscosity cutoffs, as employed in viscous models, unnecessary.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2009JB006482
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 9
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    Implications of subduction rehydration for earth's deep water cycle (2006)
    AGU (American Geophysical Union)
    In:  In: Earth's Deep Water Cycle. , ed. by Jacobsen, S. D. and Lee, S. F. M. v. d. Geophysical Monograph Series, 168 . AGU (American Geophysical Union), Washington, DC, pp. 163-276. ISBN 978-0-87590-433-7
    Details
    Publication Date: 2017-05-16
    Description: The "standard model" for the genesis of the oceans is that they are exhalations from Earth's deep interior continually rinsed through surface rocks by the global hydrologic cycle. No general consensus exists, however, on the water distribution within the deeper mantle of the Earth. Recently Dixon et a/. [2002] estimated water concentrations for some of the major mantle components and concluded that the most primitive (FOZO) are significantly wetter than the recycling associated EM or HIMU mantle components and the even drier depleted mantle source that melts to form MORB. These findings are in striking agreement with the results of numerical modeling of the global water cycle that are presented here. We find that the Dixon et a/. [2002] results are consistent with a global water cycle model in which the oceans have formed by efficient outgassing of the mantle. Present-day depleted mantle will contain a small volume fraction of more primitive wet mantle in addition to drier recycling related enriched components. This scenario is consistent with the observation that hotspots with a FOZO-component in their source will make wetter basalts than hotspots whose mantle sources contain a larger fraction of EM and HIMU components.
    Type: Book chapter , PeerReviewed
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    DOI: 10.1029/168GM20
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    OceanRep: Book chapter
  • 10
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    Fluid release from the subducted Cocos plate and partial melting of the crust deduced from magnetotelluric studies in southern Mexico: implications for the generation of volcanism and subduction dynamics (2006)
    AGU (American Geophysical Union)
    In:  Journal of Geophysical Research: Solid Earth, 111 . B08102.
    Details
    Publication Date: 2018-04-25
    Description: In order to study electrical conductivity phenomena that are associated with subduction related fluid release and melt production, magnetotelluric (MT) measurements were carried out in southern Mexico along two coast to coast profiles. The conductivity-depth distribution was obtained by simultaneous two-dimensional inversion of the transverse magnetic and transverse electric modes of the magnetotelluric transfer functions. The MT models demonstrate that the plate southern profile shows enhanced conductivity in the deep crust. The northern profile is dominated by an elongated conductive zone extending 〉250 km below the Trans-Mexican Volcanic Belt (TMVB). The isolated conductivity anomalies in the southern profile are interpreted as slab fluids stored in the overlying deep continental crust. These fluids were released by progressive metamorphic dehydration of the basaltic oceanic crust. The conductivity anomalies may be related to the main dehydration reactions at the zeolite → blueschist → eclogite facies transitions and the breakdown of chlorite. This relation allows the estimation of a geothermal gradient of ∼8.5°C/km for the top of the subducting plate. The same dehydration reactions may be recognized along the northern profile at the same position relative to the depth of the plate, but more inland due to a shallower dip, and merge near the volcanic front due to steep downbending of the plate. When the oceanic crust reaches a depth of 80–90 km, ascending fluids produce basaltic melts in the intervening hot subcontinental mantle wedge that give rise to the volcanic belt. Water-rich basalts may intrude into the lower continental crust leading to partial melting. The elongated highly conductive zone below the TMVB may therefore be caused by partial melts and fluids of various origins, ongoing migmatization, ascending basaltic and granitic melts, growing plutons as well as residual metamorphic fluids. Zones of extremely high conductance (〉8000 S) in the continental crust on either MT profile might indicate extinct magmatism.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1029/2005JB003739
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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