Compositional heterogeneity near the base of the mantle transition zone beneath Hawaii
Chunquan YuElizabeth DayMaarten V. de HoopMichel CampilloSaskia GoesRachel A. BlytheRobert D. van der Hilst
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Abstract Global seismic discontinuities near 410 and 660 km depth in Earth’s mantle are expressions of solid-state phase transitions. These transitions modulate thermal and material fluxes across the mantle and variations in their depth are often attributed to temperature anomalies. Here we use novel seismic array analysis of SS waves reflecting off the 410 and 660 below the Hawaiian hotspot. We find amplitude–distance trends in reflectivity that imply lateral variations in wavespeed and density contrasts across 660 for which thermodynamic modeling precludes a thermal origin. No such variations are found along the 410 . The inferred 660 contrasts can be explained by mantle composition varying from average (pyrolitic) mantle beneath Hawaii to a mixture with more melt-depleted harzburgite southeast of the hotspot. Such compositional segregation was predicted, from petrological and numerical convection studies, to occur near hot deep mantle upwellings like the one often invoked to cause volcanic activity on Hawaii.Keywords:
Hotspot (geology)
Classification of discontinuities
Core–mantle boundary
The seismic structure of the transition-zone discontinuities was studied beneath the forty-nine hotspot locations of the catalog of Courtillot et al. (2003), using a global data set of SS precursors. Some of these hotspots are proposed to originate from plumes rising in the upper mantle or from the core-mantle boundary region. I found thin transition zones in approximately two-thirds of the twenty-six hotspot locations for which precursor observations could be made. This observation agrees with the expectation for the olivine phase transition of a systematically thin transition zone in high-temperature regions. Other hotspot locations showed a clear deepening of both the 410- and 660-km discontinuities, which is consistent with a phase transition from majorite garnet to perovskite at a depth of 660 km. Predictions from mineral physics suggest that this transition is more important than the olivine phase transition in regions with high mantle temperatures. So, a hotspot location with a deep 410-km discontinuity in combination with either a shallow or deep 660-km discontinuity might be consistent with hot upwellings rising from the lower into the upper mantle. Hotspot locations with a shallow 410-km discontinuity are not in agreement with a positive thermal anomaly from the surface down to the mantle transition zone. This new interpretation of seismic discontinuities in the transition zone has important implications for our understanding of geodynamics in potential mantle plume locations.
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Classification of discontinuities
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The core‐mantle boundary is a fundamental compositional discontinuity in the Earth, where molten iron alloy from the core meets solid silicate minerals from the mantle. Heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle. At the same time, chemical reactions may create a layer of different composition and density than the overlying mantle. Using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell, we investigate how a layer forms at the base of the mantle. Development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core‐mantle boundary and on the rate at which the material diffuses into the mantle. If the material is less than 3–6% denser than the overlying mantle, assuming reasonable choices of lower mantle thermodynamic parameters, the reactant material will be swept away by upwelling plumes. More dense material, on the other hand, forms an internally convecting layer that entrains material from above. The varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle.
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Abstract Seismic observations suggest (1) significant accumulation of subducted slabs above the 670‐km discontinuity in many subduction zones, (2) possible structure change at ~1,000‐km depth, and (3) the large low shear wave velocity provinces above the core‐mantle boundary in the African and Pacific lower mantle be associated with chemical heterogeneity. Global mantle convection models with realistic plate motion history reproduce most of these structures. However, it remains unclear how the convection models compare with seismic models at different spatial wavelengths and depths. By conducting quantitative analysis between mantle convection and seismic models, we found that mantle convective structures show significant correlations with seismic structures in the upper mantle and mantle transition zone for wavelengths up to spherical harmonic degree 20. However, the global correlation is weak at intermediate to short wavelengths (for degrees 4 and higher) in the lower mantle below ~1,000‐km depth. A weak layer beneath the spinel‐to‐postspinel phase change help consistently reproduce stagnant slabs in the western Pacific, while having insignificant effects elsewhere, that is, the large low shear wave velocity province structures. The cold slab structures and their correlations with the seismically fast anomalies are nearly identical for our convection models with and without the plumes, indicating that seismically fast anomalies in the mantle mainly result from the subducted slabs. Models with viscosity increase at 1,000‐km depth and the 670‐km depth phase change may reproduce seismic slab structures including the stagnant slabs in the mantle transition zone equally well as models with a thin weak layer below the 670‐km phase boundary.
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We discuss the space relationship between upper mantle plumes revealed earlier from analysis of long-wavelength isostatic gravity anomalies and the subducting Pacific slab. According to global seismic tomography, the oceanic slab in its segments corresponding to the Japan and Izu–Bonin island arcs flattens out at the bottom of the mantle transition zone, extends horizontally far beneath Eurasia, and then resumes sinking into the lower mantle. The upper mantle plumes are located beyond the western endpoint of the slab sector that advances the farthest beneath the continent. A considerable part in the plume material may belong to fertilized (enriched with incompatible elements) peridotite. A layer of fertilized peridotite forms at depths between 200 and 600 km under the effect the melts produced by partial melting of the slab oceanic crust cause on the overlying depleted mantle. The peridotite layer integrates into the slab and heats up by friction along the slab top during the horizontal motion of the latter in the transition zone where the mantle material is of relatively high strength. Portions of hot fertilized peridotite detach from the slab as it sinks into the lower mantle, rise by buoyancy through the upper part of the transition zone, and become entrained into an elongate asthenospheric convection cell which arises beneath the continent behind the subduction zone. The ascending convection flow splits into separate streams which are the upper mantle plumes. Upper mantle plumes, subducting slab, mantle transition zone, fertilized peridotite, asthenospheric convection
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The D″ region of the mantle has been interpreted as a variable‐thickness layer of hot, chemically heterogeneous material at the base of the mantle. This paper reports the results of numerical simulations testing whether growth of a chemically distinct layer at the base of the mantle is possible by influx of molten iron alloy from the core to the mantle. A finite‐element model of thermo‐chemical mantle convection was used to simulate exchange of material between the mantle and core. Growth of a significant layer at the base of the mantle depends on several factors; we focus on the effect of density of the material in the chemical boundary layer. Relatively low density material was swept away from the core‐mantle boundary by mantle upwellings. Very high density material, in contrast, was not swept away, but flowed into the mantle too slowly to form a layer as thick as D″. For moderate density contrasts between material in a chemical boundary layer and the overlying mantle, a laterally heterogeneous layer formed. The stable layer results in an elevated temperature contrast between the core and the mantle. High temperatures in this layer are associated with broad regions of elevated temperature in the overlying mantle.
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