Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle
361
Citation
105
Reference
10
Related Paper
Citation Trend
Keywords:
Hotspot (geology)
Core–mantle boundary
Crustal recycling
Cite
Crustal recycling
Planetary differentiation
Hotspot (geology)
Underplating
Cite
Citations (5)
The structure of mantle and mode of convection is still debatable in the scientific community. Geochemical approaches strongly suggest layered mantle structure comprising of depleted upper mantle (DM, source of mid-ocean ridge basalts) and the lower mantle reservoir (LM, source of ocean island basalts); the depth of compositional barrier is at~ 1700 km (Kellogg et al., 1999). However, global seismic tomographic images showing slabs penetrating the 660-km seismic discontinuity up to the core-mantle boundary (D” layer) is argued in favour of whole mantle convection. We use present-day observed global surface heat flow (46±3 TW) and concentration of heat producing elements U, Th, and K in silicate Earth reservoirs and perform mass-heat energy balance estimates to constrain compositional layering and style of convection in the mantle. Our preferred model gives convective Urey ratio (Ur, fraction of the mantle heat loss attributed to the radiogenic heat in the mantle) Ur ~ 0.7, close to that preferred by parameterized models of mantle convection and cooling history. This agrees with numerical estimates of mantle cooling rate (100 K.Gyr−1 for the present-day) that attribute ∼ 70% of the present-day surface heat flow to the radiogenic heat. Considering whole mantle to be DM composition given by Salters and Stracke (2004), Ur becomes 0.12±0.04, suggesting ~ 90% of heat from secular cooling only and a present-day mantle cooling rate of ∼ 280 K.Gyr−1. This indicates a catastrophic thermal history of the earth and predicts an incredibly hot Earth during the early history. The implication is that whole mantle convection is only possible if HPE concentrations are substantially underestimated for the MORB source, which seems unlikely. Our results suggest significant compositional stratification in the mantle similar to that proposed by Kellogg et al. (1999).
Radiogenic nuclide
Core–mantle boundary
Planetary differentiation
Hotspot (geology)
Cite
Citations (0)
Seismic and geochemical observations indicate a compositionally heterogeneous mantle in the lower mantle, suggesting a layered mantle. The volume and composition of each layer, however, remain poorly constrained. This study seeks to constrain the layered mantle model from observed plume excess temperature, plume heat flux, and upper mantle temperature. Three‐dimensional spherical models of whole mantle and layered mantle convection are computed for different Rayleigh number, internal heat generation, buoyancy number, and bottom layer thickness for layered mantle models. The model results show that these observations are controlled by internal heating rate in the layer overlying the thermal boundary layer from which mantle plumes are originated. To reproduce the observations, internal heating rate needs ∼65% for whole mantle convection, but for layered mantle models, the internal heating rate for the top layer is ∼60–65% for averaged bottom layer thicknesses <∼1100 km. The heat flux at the core‐mantle boundary (CMB) is constrained to be ∼12.6 TW for whole mantle convection. For layered mantle, an upper bound on the CMB heat flux is ∼14.4 TW. For mantle secular cooling rate of ∼80 K/Ga, the current study suggests that the top layer of a layered mantle is relatively thick (>2520 km) and has radiogenic heat generation rate >2.82 × 10 −12 W/kg that is >3 times of that for the depleted mantle source for mid‐ocean ridge basalts (DMM). For the top layer to have the radiogenic heat generation of the DMM, mantle secular cooling rate needs to exceed 145 K/Ga. The current study also shows that plume temperature in the upper mantle is about half of the CMB temperature for whole mantle convection or ∼0.6 of temperature at compositional boundary for a layered mantle, independent of internal heating rate and Rayleigh number. Finally, the model calculations confirm that mantle plumes accounts for the majority (∼80%) of CMB heat flux in whole mantle convection models. However, plume heat flux decreases significantly by as much as a factor of 3, as plumes ascend through the mantle to the upper mantle, owing to the adiabatic and possibly diffusive cooling of the plumes and owing to slight (∼180 K) subadiabaticity in mantle geotherm.
Hotspot (geology)
Core–mantle boundary
Planetary differentiation
Mantle plume
Internal heating
Radiogenic nuclide
Cite
Citations (219)
Abstract Self‐consistent numerical models are developed for a coupled magmatism‐mantle convection system with tectonic plates in a two‐dimensional rectangular box to understand the Earth's mantle evolution. The mantle evolves in two stages owing to decaying internal and basal heating, provided that the lithosphere is mechanically strong enough to inhibit spontaneous formation of new subduction zones by ridge push force. On the earlier stage that continues for the first 1–2 Gyr, the deep mantle is strongly heated, and hot materials there frequently ascend to the surface as bursts. The mantle bursts cause vigorous magmatism and make the lithosphere move chaotically. The thermostat effect of the vigorous magmatism keeps the average temperature in the upper mantle below about 1800 K no matter how strongly the mantle is heated. As the heating rate of the mantle declines, however, the mantle evolves into the later stage where mantle bursts subside, rigid tectonic plates emerge to move rather steadily, and subducted basaltic crusts accumulate on the core‐mantle boundary to form compositionally dense piles. Hot plumes occasionally ascend from the basaltic piles to cause magmatism. It takes time on the order of one billion years for the slabs that sink into the lower mantle to return back to the upper mantle, and the long overturn time makes the thermal history of the upper mantle, which has been petrologically constrained for the Earth, distinct from that of the whole mantle. The long overturn time also makes water injected into the mantle by slabs distribute heterogeneously.
Planetary differentiation
Hotspot (geology)
Crustal recycling
Cite
Citations (15)
Hotspot (geology)
Core–mantle boundary
Crustal recycling
Planetary differentiation
Mantle plume
Cite
Citations (38)
Data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow. Such an approach is essential in mantle circulation models, where we seek to constrain an unknown initial condition some time in the past, and thus cannot hope to use first-principles convection calculations to infer the flow history of the mantle. One of the most important observables for mantle-flow history comes from models of Mesozoic and Cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle-buoyancy forces due to subducted oceanic slabs. Here we present five mantle circulation models with an assimilated plate-motion history spanning the past 120 Myr, a time period for which reliable plate-motion reconstructions are available. All models agree well with upper- and mid-mantle heterogeneity imaged by seismic tomography. A simple standard model of whole-mantle convection, including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation, reveals downwellings related to Farallon and Tethys subduction. Adding 35% bottom heating from the core has the predictable effect of producing prominent high-temperature anomalies and a strong thermal boundary layer at the base of the mantle. Significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep, factor 100, viscosity rise from the upper to the lower mantle results in substantial transition-zone heterogeneity, enhanced by the effects of trench migration implicit in the assimilated plate-motion history. An expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km, which is influenced by Jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the Cretaceous. This result implies that sequential assimilation of past plate-motion models is ineffective in studying the temporal evolution of core-mantle-boundary heterogeneity, and that a method for extrapolating present-day information backwards in time is required. For short time periods (of the order of perhaps a few tens of Myr) such a method exists in the form of crude 'backward' convection calculations. For longer time periods (of the order of a mantle overturn), a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model.
Hotspot (geology)
Core–mantle boundary
Seismic Tomography
Crustal recycling
Internal heating
Cite
Citations (107)
Core–mantle boundary
Crustal recycling
Hotspot (geology)
Planetary differentiation
Post-perovskite
Cite
Citations (375)
Radiogenic nuclide
Crustal recycling
Cite
Citations (2)