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

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.

scholarly journals IODP Expedition 330: Drilling the Louisville Seamount Trail in the SW Pacific

2013 ◽  
Vol 15 ◽  
pp. 11-22 ◽  
Author(s):  
A. A. P. Koppers ◽  
T. Yamazaki ◽  
J. Geldmacher ◽  
Keyword(s):  
Mantle Plume ◽  
Mantle Source ◽  
Isotope Geochemistry ◽  
Oceanic Lithosphere ◽  
Similar Amount ◽  
Mantle Plumes ◽  
Large Variability ◽  
Sw Pacific ◽  
Integrated Ocean Drilling Program ◽  
The Pacific

Deep-Earth convection can be understood by studying hotspot volcanoes that form where mantle plumes rise up and intersect the lithosphere, the Earth's rigid outer layer. Hotspots characteristically leave age-progressive trails of volcanoes and seamounts on top of oceanic lithosphere, which in turn allow us to decipher the motion of these plates relative to "fixed" deep-mantle plumes, and their (isotope) geochemistry provides insights into the long-term evolution of mantle source regions. However, it is strongly suggested that the Hawaiian mantle plume moved ~15° south between 80 and 50 million years ago. This raises a fundamental question about other hotspot systems in the Pacific, whether or not their mantle plumes experienced a similar amount and direction of motion. Integrated Ocean Drilling Program (IODP) Expedition 330 to the Louisville Seamounts showed that the Louisville hotspot in the South Pacific behaved in a different manner, as its mantle plume remained more or less fixed around 48°S latitude during that same time period. Our findings demonstrate that the Pacific hotspots move independently and that their trajectories may be controlled by differences in subduction zone geometry. Additionally, shipboard geochemistry data shows that, in contrast to Hawaiian volcanoes, the construction of the Louisville Seamounts doesn’t involve a shield-building phase dominated by tholeiitic lavas, and trace elements confirm the rather homogenous nature of the Louisville mantle source. Both observations set Louisville apart from the Hawaiian-Emperor seamount trail, whereby the latter has been erupting abundant tholeiites (characteristically up to 95% in volume) and which exhibit a large variability in (isotope) geochemistry and their mantle source components. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.15.02.2013" target="_blank">10.2204/iodp.sd.15.02.2013</a>

scholarly journals Competing Deformation Mechanisms in Periclase: Implications for Lower Mantle Anisotropy

Minerals ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 650 ◽  
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.

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

2020 ◽  
Vol 11 (1) ◽  
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).

The North America mid-Cretaceous kimberlite corridor: Wet, edge-driven decompression melting of an OIB-type deep mantle source

2017 ◽  
Vol 18 (7) ◽  
pp. 2727-2747 ◽  
Author(s):  
B. A. Kjarsgaard ◽  
L. M. Heaman ◽  
C. Sarkar ◽  
D. G. Pearson
Keyword(s):  
North America ◽  
Mantle Source ◽  
The North ◽  
Deep Mantle

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

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.

Sm-Nd and Lu-Hf isotope and trace-element systematics of Mesoarchaean amphibolites, inner Ameralik fjord, southern West Greenland

2015 ◽  
Vol 79 (4) ◽  
pp. 857-876 ◽  
Author(s):  
Kristoffer Szilas ◽  
J. Elis Hoffmann ◽  
Christina Hansmeier ◽  
Julie A. Hollis ◽  
Carsten Münker ◽  
...  
Keyword(s):  
Trace Element ◽  
Mantle Source ◽  
Source Region ◽  
Hf Isotope ◽  
Apparent Paradox ◽  
West Greenland ◽  
The North ◽  
Mantle Sources ◽  
Sample Set ◽  
North Atlantic Craton

AbstractFragmented supracrustal rocks are typical components of Archaean high-grade gneiss terranes, such as those in the North Atlantic Craton. Here we present the first major, trace element and Nd-Hf isotope data for amphibolites collected in the yet poorly studied southern inner Ameralik fjord region of southern West Greenland. In addition, new U-Pb zircon ages were obtained from the surrounding TTG gneisses.Based on their trace-element patterns, two different groups of amphibolites can be distinguished. Following screening for post-magmatic alteration and outlying ε values, a reduced sample set defines a147Sm/143Nd regression age of 3038 Ma ±310 Ma (MSWD = 9.2) and a176Lu/176Hf regression age of 2867 ±160 Ma (MSWD = 5.5). Initial εNd2970Mavalues of the least-altered amphibolites range from 0.0 to +5.7 and initial εHf2970Ma range from +0.7 to +10.4, indicating significant isotopic heterogeneity of their mantle sources with involvement of depleted domains as well as crustal sources.Surprisingly, the amphibolites which are apparently most evolved and incompatible element-rich have the most depleted Hf-isotope compositions. This apparent paradox may be explained by the sampling of a local mantle source region with ancient previous melt depletion, which was re-enriched by a fluid component during subduction zone volcanism or alternatively by preferential melting of an ancient pyroxenite component in the mantle source of the enriched rocks.

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

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.

scholarly journals Do mantle plumes preserve the heterogeneous structure of their deep-mantle source?

2016 ◽  
Vol 434 ◽  
pp. 10-17 ◽  
Author(s):  
T.D. Jones ◽  
D.R. Davies ◽  
I.H. Campbell ◽  
C.R. Wilson ◽  
S.C. Kramer
Keyword(s):  
Mantle Source ◽  
Heterogeneous Structure ◽  
Mantle Plumes ◽  
Deep Mantle

scholarly journals Origin of the vast height difference between the African and Pacific LLSVPs

2020 ◽  
Author(s):  
Qian Yuan ◽  
Mingming Li
Keyword(s):  
Shear Velocity ◽  
Maximum Height ◽  
Height Difference ◽  
Core Mantle Boundary ◽  
The Core ◽  
Mantle Viscosity ◽  
The Pacific ◽  
Wide Range ◽  
The Pacific Ocean ◽  
Low Shear

Abstract Seismic observations have revealed two Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific Ocean in the lowermost mantle1–4. However, the detailed morphology and stability of the LLSVPs and their interaction with the surrounding mantle remain unclear. Here we show that, whereas the maximum height of the Pacific LLSVP is ~500 – 700 km, the African LLSVP reaches a height of ~1,500 – 1,800 km from the core-mantle boundary. With numerical modeling experiments, we find that the height of the Pacific LLSVP can be reconciled by a wide range of the LLSVP density and surrounding mantle viscosity. However, the large height of the African LLSVP indicates that it may be less dense and thus less stable than the Pacific LLSVP.

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