Large low-shear-velocity provinces

Large low-shear-velocity provinces, LLSVPs, also called LLVPs or superplumes, are characteristic structures of parts of the lowermost mantle (the region surrounding the outer core) of Earth. These provinces are characterized by slow shear wave velocities and were discovered by seismic tomography of deep Earth. There are two main provinces: the African LLSVP and the Pacific LLSVP. Both extend laterally for thousands of kilometers and possibly up to 1,000 kilometres vertically from the core–mantle boundary. The Pacific LLSVP is across, and underlies four hotspots that suggest multiple mantle plumes underneath. These zones represent around 8% of the volume of the mantle (6% of Earth). Other names for LLSVPs include "superswells", "thermo-chemical piles", or "hidden reservoirs". Most of these names, however, are more interpretive of their proposed geodynamical or geochemical effects. For example, the name "thermo-chemical pile" interprets LLSVPs as lower-mantle piles of

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Effect of Fe-enrichment on seismic properties of perovskite and post-perovskite in the deep lower mantle

Susannah McGregor Dorfman

Recent experimental measurements of the equation of state of perovskites and post-perovskites in the (Mg,Fe)SiO3 and (Mg,Fe,Al)(Fe,Al,Si)O-3 systems over a wide range of iron contents are used to constrain the effects of Fe and Al on density and bulk modulus of these phases at deep mantle pressures. The density of Fe-bearing perovskite follows a linear relationship with Fe-content at a representative mid-mantle depth of 1850 km (80 GPa): (80) (g cm(- 3)) = 5.054(1) + 1.270(3)X-Fe. The bulk modulus of silicate perovskite is not sensitive to Fe-content and follows the relationship, K-80 (GPa) = 546(2) + 12(25)X-Fe. The velocity heterogeneity parameter, partial derivative ln V-B/partial derivative X-Fe, determined by experimental values for the bulk sound speed is 0.10(1), in agreement with theory and the behaviour of other Fe-bearing silicates. Near the core-mantle boundary, Fe-rich post-perovskite is observed to be more compressible than the Mg-end-member, in contrast to theoretical predictions. From experimental data, the densities of perovskite and post-perovskite at 125 GPa (2700 km depth) are (125,Pv) (g cm(-3)) = 5.426(11) + 1.38(4)X-Fe and (125,pPv) (g cm(-3)) = 5.548(1) + 1.41(3)X-Fe. The density contrast across the post-perovskite transition is similar to 2 per cent, irrespective of Fe-content, but the contrast in bulk sound speed increases with Fe-content. Al-rich silicates exhibit no significant differences in density or compressibility relative to Al-free silicates, but may be responsible for seismic heterogeneities due to differences in the depth and width of the post-perovskite transition. Observations of increased densities in large low shear velocity provinces and ultra-low-velocity zones may be consistent with local iron enrichment from Mg#90 to Mg# 78-88 and Mg# < 50, respectively.

Oxford University Press2014

Seismic tomography models reveal two large low shear velocity provinces (LLSVPs) that identify large-scale variations in temperature and composition in the deep mantle. Other characteristics include elevated density, elevated bulk sound speed, and sharp boundaries. We show that properties of LLSVPs can be explained by the presence of small quantities (0.3-3%) of suspended, dense Fe-Ni-S liquid. Trapping of metallic liquid is demonstrated to be likely during the crystallization of a dense basal magma ocean, and retention of such melts is consistent with currently available experimental constraints. Calculated seismic velocities and densities of lower mantle material containing low-abundance metallic liquids match the observed LLSVP properties. Small quantities of metallic liquids trapped at depth provide a natural explanation for primitive noble gas signatures in plume-related magmas. Our model hence provides a mechanism for generating large-scale chemical heterogeneities in Earth's early history and makes clear predictions for future tests of our hypothesis.

Amer Geophysical Union2016

Chemical Characterisation of Deep Earth Reservoirs

Hélène Marie Piet

Seismic imaging of the internal Earth has emphasized the inhomogeneity of its mantle. This interface layer of nearly 3000 km surrounding the central metallic core conducts heat released from the latter, which played a significant role in shaping the actual mantle as it currently is. The present state of the deep Earth (mainly the lower mantle and the core) is also a consequence of the physical properties of the material composing them. Large-low shear velocity provinces atop the core-mantle boundary, for example, are composed of a denser material than their surrounding environment but their origin, formation and survival over billions of years to mantle convection, is still largely debated. Here we use chemical partitioning to characterise the current composition of those deep Earth reservoirs and better understand the processes leading to their formation. In the first part, experiments were conducted in the laser-heated diamond anvil cell and the latest advances in microanalysis were used to refine iron partitioning between minerals of the lower mantle. We reconcile long-standing discrepancies in previous partitioning data and show that significant variations of silicate iron content are associated to changing ferric iron incorporation. We also note that such chemical variations might significantly affect the physical properties of the silicate, which might in turn provoke mantle rheology variations. Our results and observations thus provide a mineral physics based argument for the stabilization of deep Earth reservoirs and especially large structures like large low-shear-velocity provinces (LLSVPs). In the second and numerical part of the thesis, previous results on siderophile (iron-loving) element partitioning between metal and silicate were used to constrain some aspects of core formation. We constrain the chemical and physical state of the magma ocean during core formation explaining both the siderophile composition of the mantle and the light element composition of the core. We simulate the Moon-Forming Giant Impact in the last steps of Earth accretion, where we constrain the size of the impactor to be smaller than 15% of Earth's total mass, a roughly Mars-sized object, and further show that melting of the large fractions of the mantle upon impact by the giant impactor is consistent with siderophile composition of the bulk silicate Earth, which can have great implications for the geodynamical evolution of the mantle. In conclusions, given the fact that the mantle may have been almost fully molten, further crystallization may have isolated a layer of liquid primitive material at the base of the mantle possibly explaining the seismic signature of the LLSVPs associated with a strong thermochemical contrast with the surrounding mantle. The layered composition in iron of the actual mantle also provides a chemical argument for a viscously layered mantle, which could have helped sustain LLSVPs for billions of years from efficient convection.

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