The Central-East Atlantic Anomaly: its role in the genesis of the Canary and Madeira volcanic provinces

The Canary and Madeira provinces, located in the central-east Atlantic Ocean, are characterized by irregularly distributed hotspot tracks displaying large age differences and variable distances between volcanoes. For this reason, the geodynamic mechanism(s) that control the spatio-temporal patterns of volcanism are still unclear. Here, we use results from seismic tomography, shear-wave splitting, and gravity to show that the Central-East Atlantic Anomaly (CEAA), rising from the African large low-shear-velocity province and stalled in the topmost lower mantle, is the source of distinct upper-mantle diapirs feeding those provinces. The diapirs detach intermittently from the CEAA and seem to be at different evolutionary stages. Geochemistry data confirm the lower-mantle origin of the diapirs, and plate reconstructions constrain their temporal evolution. Our observations suggest that the accumulation of deep plume material in the topmost lower mantle can play a significant role in governing the spatio-temporal distribution of hotspot volcanism.This is a contribution to project SIGHT (Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 &#8211; IDL.

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Estimates of true Polar wander since 300Ma

10.5194/egusphere-egu2020-9789 ◽

2020 ◽

Author(s):

Jean Besse ◽

Marianne Greff ◽

Sophie Vicente de Gouveia

Keyword(s):

Maximum Degree ◽

Temporal Evolution ◽

Shear Velocity ◽

Present Value ◽

Polar Wander ◽

True Polar Wander ◽

Plate Reconstructions ◽

Volcanic Data ◽

Low Shear ◽

Axis Passing

We investigate true polar wander (TPW) since 300Ma. We construct a hotspot reference frame using an updated list of active hotspots with improved criteria aimed at detecting their depth origin, a compilation of terrestrial volcanic data suspected to reveal hotspot activity, and a set of plate reconstructions, based initially on paleomagnetism corrected with respect to hotspots under the assumption of hotspot fixity. The polar motion curves (representing the motion of the mantle taken as a whole) during the periods t=[0 and 150-170] and [150-170 to 280Ma] roughly aligns along two great circles which poles&#160; are both located close to the equator, with a &#160;longitude differing&#160; by some 50&#176;, and positioned close to an axis passing through the Large Low Shear Velocity Provinces (LLSVPs), and close to the maximum degree 2 geoid high under Africa. The TPW rate is slowly decreasing with respect to time but remains close or below the observed 10cm/yr present value.&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; We compare our TPW data with those obtained from a mantle density heterogeneities model which computes the temporal evolution of the Principal Inertia Axis (PIA).&#160; The minimum PIA is shown to be in agreement with the two poles previously determined, while the maximum PIA&#160; path (which represents the evolution of the geographic pole) displays strong similarities with the observed TPW (directions, cusps). The sudden changes of TPW direction (i.e., cusps) can be explained&#160; by mass reorganizations within the mantle principally linked to changes in subductions, while the domes greatly stabilize the system.&#160;

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Competing Deformation Mechanisms in Periclase: Implications for Lower Mantle Anisotropy

Minerals ◽

10.3390/min9110650 ◽

2019 ◽

Vol 9 (11) ◽

pp. 650 ◽

Cited By ~ 2

Author(s):

Feng Lin ◽

Samantha Couper ◽

Mike Jugle ◽

Lowell Miyagi

Keyword(s):

Lower Mantle ◽

Seismic Anisotropy ◽

Shear Velocity ◽

Deformation Mechanisms ◽

Effect Of Temperature ◽

Lattice Strains ◽

Slip Activity ◽

Consistent Method ◽

Mantle Anisotropy ◽

Low Shear

Seismic anisotropy is observed above the core-mantle boundary in regions of slab subduction and near the margins of Large Low Shear Velocity Provinces (LLSVPs). Ferropericlase is believed to be the second most abundant phase in the lower mantle. As it is rheologically weak, it may be a dominant source for anisotropy in the lowermost mantle. Understanding deformation mechanisms in ferropericlase over a range of pressure and temperature conditions is crucial to interpret seismic anisotropy. The effect of temperature on deformation mechanisms of ferropericlase has been established, but the effects of pressure are still controversial. With the aim to clarify and quantify the effect of pressure on deformation mechanisms, we perform room temperature compression experiments on polycrystalline periclase to 50 GPa. Lattice strains and texture development are modeled using the Elasto-ViscoPlastic Self Consistent method (EVPSC). Based on modeling results, we find that { 110 } ⟨ 1 1 ¯ 0 ⟩ slip is increasingly activated with higher pressure and is fully activated at ~50 GPa. Pressure and temperature have a competing effect on activities of dominant slip systems. An increasing { 100 } ⟨ 011 ⟩ : { 110 } ⟨ 1 1 ¯ 0 ⟩ ratio of slip activity is expected as material moves from cold subduction regions towards hot upwelling region adjacent to LLSVPs. This could explain observed seismic anisotropy in the circum-Pacific region that appears to weaken near margins of LLVSPs.

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Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities

Nature Communications ◽

10.1038/s41467-019-13720-2 ◽

2020 ◽

Vol 11 (1) ◽

Cited By ~ 2

Author(s):

Wenzhong Wang ◽

Yinhan Xu ◽

Daoyuan Sun ◽

Sidao Ni ◽

Renata Wentzcovitch ◽

...

Keyword(s):

Phase Transition ◽

Oceanic Crust ◽

Seismic Tomography ◽

Lower Mantle ◽

Shear Velocity ◽

Calcium Ferrite ◽

Velocity Anomaly ◽

Mantle Heterogeneities ◽

Subducted Oceanic Crust ◽

Low Shear

AbstractSeismic heterogeneities detected in the lower mantle were proposed to be related to subducted oceanic crust. However, the velocity and density of subducted oceanic crust at lower-mantle conditions remain unknown. Here, we report ab initio results for the elastic properties of calcium ferrite‐type phases and determine the velocities and density of oceanic crust along different mantle geotherms. We find that the subducted oceanic crust shows a large negative shear velocity anomaly at the phase boundary between stishovite and CaCl2-type silica, which is highly consistent with the feature of mid-mantle scatterers. After this phase transition in silica, subducted oceanic crust will be visible as high-velocity heterogeneities as imaged by seismic tomography. This study suggests that the presence of subducted oceanic crust could provide good explanations for some lower-mantle seismic heterogeneities with different length scales except large low shear velocity provinces (LLSVPs).

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Lateral variations of electrical conductivity in the lower mantle constrained by Swarm and CryoSat-2 missions

Earth Planets and Space ◽

10.1186/s40623-020-01334-8 ◽

2021 ◽

Vol 73 (1) ◽

Author(s):

Jakub Velímský ◽

Ondřej Knopp

Keyword(s):

Electrical Conductivity ◽

Lower Mantle ◽

Shear Velocity ◽

Earth’S Mantle ◽

Geophysical Parameter ◽

Em Induction ◽

Mantle Conductivity ◽

Lateral Variations ◽

Low Shear ◽

Conductivity Models

AbstractThe electrical conductivity is an important geophysical parameter connected to the thermal, chemical, and mineralogical state of the Earth’s mantle. In this paper, we apply the previously developed methodology of forward and inverse EM induction modeling to the latest version of satellite-derived spherical harmonic coefficients of external and internal magnetic field, and obtain the first 3-D mantle conductivity models with contributions from Swarm and CryoSat-2 satellite data. We recover degree 3 conductivity structures which partially overlap with the shape of the large low-shear velocity provinces in the lower mantle.

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Formation of large low shear velocity provinces through the decomposition of oxidized mantle

Nature Communications ◽

10.1038/s41467-021-22185-1 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

Wenzhong Wang ◽

Jiachao Liu ◽

Feng Zhu ◽

Mingming Li ◽

Susannah M. Dorfman ◽

...

Keyword(s):

High Pressure ◽

Lower Mantle ◽

Single Phase ◽

Ferric Iron ◽

Thermal Structure ◽

Shear Velocity ◽

Mantle Mineral ◽

Deep Earth ◽

Low Shear ◽

Lowermost Mantle

AbstractLarge Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle are key to understanding the chemical composition and thermal structure of the deep Earth, but their origins have long been debated. Bridgmanite, the most abundant lower-mantle mineral, can incorporate extensive amounts of iron (Fe) with effects on various geophysical properties. Here our high-pressure experiments and ab initio calculations reveal that a ferric-iron-rich bridgmanite coexists with an Fe-poor bridgmanite in the 90 mol% MgSiO3–10 mol% Fe2O3 system, rather than forming a homogeneous single phase. The Fe3+-rich bridgmanite has substantially lower velocities and a higher VP/VS ratio than MgSiO3 bridgmanite under lowermost-mantle conditions. Our modeling shows that the enrichment of Fe3+-rich bridgmanite in a pyrolitic composition can explain the observed features of the LLSVPs. The presence of Fe3+-rich materials within LLSVPs may have profound effects on the deep reservoirs of redox-sensitive elements and their isotopes.

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Lateral Variations of Electrical Conductivity in the Lower Mantle Constrained by Swarm and CryoSat-2 Missions

10.21203/rs.3.rs-96902/v1 ◽

2020 ◽

Author(s):

Jakub Velímský ◽

Ondřej Knopp

Keyword(s):

Electrical Conductivity ◽

Lower Mantle ◽

Shear Velocity ◽

Earth’S Mantle ◽

Geophysical Parameter ◽

Em Induction ◽

Mantle Conductivity ◽

Lateral Variations ◽

Low Shear ◽

Conductivity Models

Abstract The electrical conductivity is an important geophysical parameter connected to the thermal, chemical, and mineralogical state of the Earth's mantle. In this paper we apply the previously developed methodology of forward and inverse EM induction modelling to the latest version of satellite-derived spherical harmonic coefficients of external and internal magnetic field, and obtain the first 3-D mantle conductivity models with contributions from Swarm and CryoSat-2 satellite data. We recover degree 3 conductivity structures which partially overlap with the shape of the large low-shear velocity provinces in the lower mantle.

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Geochemical constraints on the structure of the Earth’s deep mantle and the origin of the LLSVPs

10.31223/x5js7z ◽

2021 ◽

Author(s):

Matthew Gleeson ◽

Caroline Soderman ◽

Simon Matthews ◽

Sanne Cottaar ◽

Sally Gibson

Keyword(s):

Mantle Source ◽

Lower Mantle ◽

Source Region ◽

Shear Velocity ◽

Mantle Plumes ◽

The North ◽

Deep Mantle ◽

The Pacific ◽

Heterogeneous Mantle ◽

Low Shear

Geophysical analysis of the Earth’s lower mantle has revealed the presence of two superstructures characterized by low shear wave velocities on the core-mantle boundary. These Large Low Shear Velocity Provinces (LLSVPs) play a crucial role in the dynamics of the lower mantle and act as the source region for deep-seated mantle plumes. However, their origin, and the characteristics of the surrounding deep mantle, remain enigmatic. Mantle plumes located above the margins of the LLSVPs display evidence for the presence of this deep-seated, thermally and/or chemically heterogeneous mantle material ascending into the melting region. As a result, analysis of the spatial geochemical heterogeneity in OIBs provides constraints on the structure of the Earth’s lower mantle and the origin of the LLSVPs. In this study, we focus on the Galápagos Archipelago in the eastern Pacific, where bilateral asymmetry in the radiogenic isotopic composition of erupted basalts has been linked to the presence of LLSVP material in the underlying plume. We show, using spatial variations in the major element contents of high-MgO basalts, that the isotopically enriched south-western region of the Galápagos mantle – assigned to melting of LLSVP material – displays no evidence for lithological heterogeneity in the mantle source. As such, it is unlikely that the Pacific LLSVP represents a pile of subducted oceanic crust. Clear evidence for a lithologically heterogeneous mantle source is, however, found in the north-central Galápagos, indicating that a recycled crustal component is present near the eastern margin of the Pacific LLSVP, consistent with seismic observations.

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