Björn Holger Heyn

• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2020). Core-mantle boundary topography and its relation to the viscosity structure of the lowermost mantle. Earth and Planetary Science Letters.  ISSN 0012-821X.  543 . doi: 10.1016/j.epsl.2020.116358 Full text in Research Archive. Show summary

  Two large areas of anomalously low seismic velocities are visible in all tomographic models of the lowermost mantle. Depending on the density structure of these Large Low Shear Velocity Provinces (LLSVPs), the core-mantle boundary (CMB) will deform upwards or downwards due to isostatic and dynamic topography, the latter being sensitive to the viscosity structure of the lowermost mantle. Heterogeneities in the viscosity structure, although difficult to constrain, might be especially important if the LLSVPs are thermochemical piles with elevated intrinsic viscosity as suggested by mineral physics. Based on numerical models, we identify a short-wavelength (about 80-120 km wide, up to a few km deep) topographic depression that forms around the pile edges if the pile is more viscous than the surrounding mantle. The depression forms when a wedge of thermal boundary layer material becomes compressed against the viscous pile, and is enhanced by relative uplift of the CMB beneath the pile by plumes rising above it. The depth and asymmetry of the depression constrain the magnitude of the viscosity contrast between pile and the surrounding mantle. Furthermore, (periodic) plume initiation and pile collapse at the pile margin systematically modify the characteristic depression, with a maximum in asymmetry and depth at the time of plume initiation. Core-reflected waves or scattered energy may be used to detect this topographic signature of stiff thermochemical piles at the base of the mantle.

• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2020). How Thermochemical Piles Can (Periodically) Generate Plumes at Their Edges. Journal of Geophysical Research (JGR): Solid Earth.  ISSN 2169-9313.  125(6) . doi: 10.1029/2019JB018726 Full text in Research Archive. Show summary

  Deep-rooted mantle plumes are thought to originate from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle. Visible in seismic tomography, the LLSVPs are usually interpreted to be intrinsically dense thermochemical piles in numerical models. Although piles deflect lateral mantle flow upward at their edges, the mechanism for localized plume formation is still not well understood. In this study, we develop numerical models that show plumes rising from the margin of a dense thermochemical pile, temporarily increasing its local thickness until material at the pile top cools and the pile starts to collapse back toward the core-mantle boundary (CMB). This causes dense pile material to spread laterally along the CMB, locally thickening the lower thermal boundary layer on the CMB next to the pile, and initiating a new plume. The resulting plume cycle is reflected in both the thickness and lateral motion of the local pile margin within a few hundred km of the pile edge, while the overall thickness of the pile is not affected. The period of plume generation is mainly controlled by the rate at which slab material is transported to the CMB, and thus depends on the plate velocity and the sinking rate of slabs in the lower mantle. A pile collapse, with plumes forming along the edges of the pile's radially extending corner, may, for example, explain the observed clustering of Large Igneous Provinces (LIPs) in the southeastern corner of the African LLSVP around 95–155 Ma.

• Trønnes, Reidar G; Baron, Marzena Anna; Eigenmann, Katarina R.; Guren, Marthe Grønlie; Heyn, Björn Holger; Løken, Andreas & Mohn, Chris Erik (2019). Core formation, mantle differentiation and core-mantle interaction within Earth and the terrestrial planets.. Tectonophysics.  ISSN 0040-1951. . doi: 10.1016/j.tecto.2018.10.021 Full text in Research Archive.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2018). Stabilizing effect of compositional viscosity contrasts on thermochemical piles. Geophysical Research Letters.  ISSN 0094-8276.  45(15), s 7523- 7532 . doi: 10.1029/2018GL078799 Full text in Research Archive. Show summary

  The large low shear velocity provinces (LLSVPs) observed in the lowermost mantle are widely accepted as chemically distinct thermochemical 'piles', but their origin and long-term evolution remain poorly understood. The survival time and shape of the LLSVPs are thought to be mainly controlled by their compositional density, while their viscosity has beeen considered less important. Based on recent constraints on chemical reactions between mantle and core, a more complex viscosity structure of the lowermost mantle, possibly including high viscosity thermochemical pile material, seems reasonable. In this study, we use numerical models to identify a trade-off between compositional viscosity and density contrasts required for long-term stability of thermochemical piles, which permits lower-density and higher-viscosity piles. Moreover, our results indicate more restrictive stability conditions during periods of strong deformation-induced entrainment, e.g. during initial pile formation, which suggests long-term pile survival.

• Reiss, Anne-Sophie; Thomas, Christine; van Driel, Jac & Heyn, Björn Holger (2017). A hot midmantle anomaly in the area of the Indian Ocean Geoid Low. Geophysical Research Letters.  ISSN 0094-8276.  44(13), s 6702- 6711 . doi: 10.1002/2017GL073440

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• Conrad, Clinton Phillips; Domeier, Mathew; Selway, Kate & Heyn, Björn Holger (2020). A link between seamount volcanism and thermochemical piles in the deepest mantle.
• Heyn, Björn Holger (2020). Geodynamics of Earth's Large Low Shear Velocity Provinces Interaction with mantle flow, plume formation, and core-mantle boundary deformation. Series of dissertations submitted to the Faculty of Mathematics and Natural Sciences, University of Oslo.. 2221. Full text in Research Archive. Show summary

  Based on evidence from the geochemistry of ocean island volcanism, we know that the Earth’s mantle must be heterogeneous on various scales, despite active convection for billions of years. Seismic tomography has shown that there are two continent-sized regions of anomalously low seismic velocities at the base of the mantle, the so-called Large Low Shear Velocity Provinces (LLSVPs), which likely represent piles of chemical heterogeneities resulting from magma ocean crystallisation at the beginning of Earth’s history. Large-scale upwellings, so-called mantle plumes, and the associated hotspot volcanism seem to cluster around these piles, which are presumably dense and potentially viscous due to their composition. This thesis explores the importance of viscosity contrasts on the interaction between mantle flow, mantle plumes and dense thermochemical piles. It demonstrates the importance of viscosity for entrainment of pile material by plumes, and shows how piles interact with mantle flow to periodically generate plumes around their margins. This interaction produces short-scale core-mantle boundary topography, which, if detected, should constrain the nature and properties of LLSVPs.

• Heyn, Björn Holger; Conrad, Clinton Phillips & Selway, Kate (2020). Numerical constraints on heat flux variations and lithospheric thinning associated with passage of the Iceland plume beneath Greenland.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2020). How thermochemical piles initiate plumes at their edges. European Geosci. Union, Gen. Assembly, Geophys. Res. Abstr..
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2020). How thermochemical piles initiate plumes at their edges.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2020). How thermochemical piles initiate plumes at their edges.
• Selway, Kate; Conrad, Clinton Phillips; Ramirez, Florence; Karlsson, Nanna B; Weerdesteijn, Maaike Francine Maria & Heyn, Björn Holger (2020). How magnetotellurics can aid cryosphere studies: mantle rheology, GIA, surface heat flow, and basal melting.
• Conrad, Clinton Phillips; Domeier, Mathew; Selway, Kate & Heyn, Björn Holger (2019). A link between seamount volcanism and thermochemical piles in the deepest mantle.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2019). Core-mantle boundary topography and its relation to lowermost mantle viscosity structure. Ada Lovelace Workshop on Modelling Mantle and Lithosphere Dynamics. Abstr..
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2019). Core-mantle boundary topography and its relation to lowermost mantle viscosity structure.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2019). Linking surface volcanism and deep Earth: Piles, plumes, and dynamic topography.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2019). Periodic plume generation at the edges of thermochemical piles. European Geosci. Union, Gen. Assembly, Geophys. Res. Abstr..
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2019). Periodic plume generation at the edges of thermochemical piles.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2019). What core-mantle boundary topography can tell us about plume locations and the viscosity and density structure of thermochemical piles. European Geosci. Union, Gen. Assembly, Geophys. Res. Abstr..
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2019). What core-mantle boundary topography can tell us about plume locations and the viscosity and density structure of thermochemical piles.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2018). Stabilization of thermochemical piles by compositional viscosity contrasts.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2018). Stabilization of thermochemical piles by compositional viscosity contrasts. EGU Gen. Assembly, EGU2018-12950.
• Heyn, Björn Holger; Conrad, Clinton Phillips & Trønnes, Reidar G (2017). Stabilizing effect of a chemical viscosity contrast on LLSVP structures Abstr., Gordon Res. Conf. on Chemical and dynamical evolution of Earth's deep interior, from formation to today. Mount Holyoke College, South Hadley, MA.

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