The role of water in Earth's mantle
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Geophysical observations suggest that the transition zone is wet locally. Continental and oceanic sediment components together with the basaltic and peridotitic components might be transported and accumulated in the transition zone. Low-velocity anomalies at the upper mantle-transition zone boundary might be caused by the existence of dense hydrous magmas. Water can be carried farther into the lower mantle by the slabs. The anomalous Q and shear wave regions locating at the uppermost part of the lower mantle could be caused by the existence of fluid or wet magmas in this region because of the water-solubility contrast between the minerals in the transition zone and those in the lower mantle. δ-H solid solution AlO2H-MgSiO4H2 carries water into the lower mantle. Hydrogen-bond symmetrization exists in high-pressure hydrous phases and thus they are stable at the high pressures of the lower mantle. Thus, the δ-H solid solution in subducting slabs carries water farther into the bottom of the lower mantle. Pyrite FeO2H x is formed due to a reaction between the core and hydrated slabs. This phase could be a candidate for the anomalous regions at the core-mantle boundary.Keywords:
Core–mantle boundary
Abstract Low‐δ 26 Mg basalts are commonly interpreted to represent melts derived from carbonated mantle sources. The mantle domain feeding low‐δ 26 Mg Cenozoic basalts in eastern China overlaps the so‐called Big Mantle Wedge (BMW) above the stagnant Pacific slab in the mantle transition zone, which indicates that the BMW is an important carbon reservoir generated by the slab. However, Mg isotopic composition in the nearby mantle beyond the BMW and, thus, the spatial extent of carbonated components in the mantle beneath eastern Asia have not yet been extensively characterized. Therefore, it remains largely unconstrained if additional or alternative carbon reservoirs exist. Here we carried out a geochemical study on Cenozoic Huihe nephelinites, which crop out ~500 km west of the present‐day BMW. These rocks are characterized by negative K, Zr, Hf, and Ti anomalies, high Zr/Hf, Ca/Al ratios, and low δ 26 Mg values, which suggest that they are derived from a carbonated mantle source. The composition of the nephelinites demonstrates that low δ 26 Mg mantle components exist at significant distances from the present‐day BMW, which highlights that in addition to the stagnant Pacific slab, other oceanic slab(s) also contribute(s) carbonate‐bearing crustal materials to the mantle sources of Cenozoic volcanism in eastern Asia.
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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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A three-dimensional P-velocity model of the European mantle built on the Taylor approximation made it possible to analyze the velocity structure of the mantle under the Mediterranean and, in particular, under Central Italy, to a depth of 2500 km.It is shown that the crust earthquakes in Central Italy, characterized by a magnitude up to 7,0, are associated with super-deep fluid processes of the mantle. A possible seismic channel was found, linking the propagation of the fluid process from the lower mantle to the crust inclusive. The manifestations of the super-deep fluid process are isolated at the depths of the lower and middle mantle. In the upper mantle and transition zone of the upper mantle, the channel is determined by the distinguished seismic boundaries of the 2th-generation, which are determined by the transition from the increase of gradients of velocity from depth to descent or vice versa. These seismic boundaries correspond to phase transitions.Consideration of the deep structure of the mantle under Central Italy has shown the presence of low velocities in the area under consideration from the lower mantle to the zone of division-2. The analysis of the structure of tops of the upper mantle showed the presence of the mantle section in area of 13°±0,5 lon.Ч43°±0,5 lat., where the earthquakes with a magnitude up to 7,0 stand out in the crust. A section is timed to the area of thrust Moho boundary of and correlated with its crossing of Ankona-Ancio fault, dissociating the Central Apennines from the North. This region corresponds to a triple intersection of faults and an increased heat flux, and there is also an increased fission of the upper mantle (7 seismic boundaries of the 2th-generation). Depth of occurrence of the main geodynamic boundary is less than 670 km.
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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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Seismic discontinuities in the mantle are indicators of its thermo-chemical state and offer clues to its dynamics. Ray-based seismic methods, though limited by the approximations made, have mapped mantle transition zone discontinuities in detail, but have yet to offer definitive conclusions on the presence and nature of mid-mantle discontinuities. Here, we show how to use a wave-equation-based imaging method, reverse-time migration of precursors to surface-reflected seismic body waves, to uncover both mantle transition zone and mid-mantle discontinuities, and interpret their physical nature. We observe a thinned mantle transition zone southeast of Hawaii, and a reduction in impedance contrast around 410 km depth in the same area, suggesting a hotter-than-average mantle in the region. Here, we furthermore reveal a 4000-5000 km-wide reflector in new images of the mid mantle below the central Pacific, at 950-1050 km depth. This deep discontinuity exhibits strong topography and generates reflections with polarity opposite to those originating at the 660 km discontinuity, implying an impedance reversal near 1000 km. We link this mid-mantle discontinuity to the upper reaches of deflected mantle plumes upwelling in the region. Reverse-time migration full-waveform imaging is a powerful approach to imaging Earth's interior, capable of broadening our understanding of its structure and dynamics and shrinking modeling uncertainties.
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