Stirring in three‐dimensional mantle convection models and implications for geochemistry: Passive tracers

Huang, Jia; Davies, Geoffrey F.

doi:10.1029/2006gc001312

Cited by 27 publications

(74 citation statements)

References 61 publications

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“…Also the model vigour is constant, while on Earth it is expected to be more vigorous in the hotter past. The combined effect is that the number of overturns in the simulated case will be many times less than for Earth (Huang and Davies, 2007a). Therefore the number of passages through melting zones will also be much lower in this simulation than Earth.…”

Section: Discussionmentioning

confidence: 92%

Global-scale modelling of melting and isotopic evolution of Earth's mantle: melting modules for TERRA

et al. 2016

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Abstract. Many outstanding problems in solid-Earth science relate to the geodynamical explanation of geochemical observations. Currently, extensive geochemical databases of surface observations exist, but satisfying explanations of underlying mantle processes are lacking. One way to address these problems is through numerical modelling of mantle convection while tracking chemical information throughout the convective mantle.We have implemented a new way to track both bulk compositions and concentrations of trace elements in a finiteelement mantle convection code. Our approach is to track bulk compositions and trace element abundances via particles. One value on each particle represents bulk composition and can be interpreted as the basalt component. In our model, chemical fractionation of bulk composition and trace elements happens at self-consistent, evolving melting zones. Melting is defined via a composition-dependent solidus, such that the amount of melt generated depends on pressure, temperature and bulk composition of each particle. A novel aspect is that we do not move particles that undergo melting; instead we transfer the chemical information carried by the particle to other particles. Molten material is instantaneously transported to the surface layer, thereby increasing the basalt component carried by the particles close to the surface and decreasing the basalt component in the residue.The model is set to explore a number of radiogenic isotopic systems, but as an example here the trace elements we choose to follow are the Pb isotopes and their radioactive parents. For these calculations we will show (1) the evolution of the distribution of bulk compositions over time, showing the buildup of oceanic crust (via melting-induced chemical separation in bulk composition), i.e. a basalt-rich layer at the surface, and the transportation of these chemical heterogeneities through the deep mantle; (2) the amount of melt generated over time; (3) the evolution of the concentrations and abundances of different isotopes of the trace elements (U, Th, K and Pb) throughout the mantle; and (4) a comparison to a semi-analytical theory relating observed arrays of correlated Pb isotope compositions to melting age distributions (Rudge, 2006).

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Section: Discussionmentioning

confidence: 92%

Global-scale modelling of melting and isotopic evolution of Earth's mantle: melting modules for TERRA

et al. 2016

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“…This can be taken into account in the sampling theory by excluding the younger part of the distribution and calculating proportions of the rest in each age range. The result (Huang and Davies, 2007a) is shown in Fig. 10 as the dashed grey line, with the sampling delay adjusted to match the numerical gap.…”

Section: Sampling Theorymentioning

confidence: 99%

“…The conversion to Earth time must be done for each age range before a mean is calculated. The expression with sampling delay is more complicated (Huang and Davies, 2007a). The variation of this mean age with Earth time is included in Fig.…”

Section: Sampling Theorymentioning

confidence: 99%

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Dynamical geochemistry of the mantle

Davies

2011

Solid Earth

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Abstract. The reconciliation of mantle chemistry with the structure of the mantle inferred from geophysics and dynamical modelling has been a long-standing problem. This paper reviews three main aspects. First, extensions and refinements of dynamical modelling and theory of mantle processing over the past decade. Second, a recent reconsideration of the implications of mantle heterogeneity for melting, melt migration, mantle differentiation and mantle segregation. Third, a recent proposed shift in the primitive chemical baseline of the mantle inferred from observations of non-chondritic 142 Nd in the Earth. It seems most issues can now be resolved, except the level of heating required to maintain the mantle's thermal evolution.A reconciliation of refractory trace elements and their isotopes with the dynamical mantle, proposed and given preliminary quantification by Hofmann, White and Christensen, has been strengthened by work over the past decade. The apparent age of lead isotopes and the broad refractory-element differences among and between ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) can now be quantitatively accounted for with some assurance.The association of the least radiogenic helium with relatively depleted sources and their location in the mantle have been enigmatic. The least radiogenic helium samples have recently been recognised as matching the proposed nonchondritic primitive mantle. It has also been proposed recently that noble gases reside in a so-called hybrid pyroxenite assemblage that is the result of melt from fusible pods reacting with surrounding refractory peridotite and refreezing. Hybrid pyroxenite that is off-axis may not remelt and erupt at MORs, so its volatile constituents would recirculate within the mantle. Hybrid pyroxenite is likely to be denser than average mantle, and thus some would tend to settle in the D Correspondence to: G. F. Davies (geoff.davies@anu.edu.au) zone at the base of the mantle, along with some old subducted oceanic crust. Residence times in D are longer, so the hybrid pyroxenite there would be less degassed. Plumes would sample both the degassed, enriched old oceanic crust and the gassy, less enriched hybrid pyroxenite and deliver them to OIBs. These findings can account quantitatively for the main He, Ne and Ar isotopic observations. It has been commonly inferred that the MORB source is strongly depleted of incompatible elements. However it has recently been argued that conventional estimates of the MORB source composition fail to take full account of mantle heterogeneity, and in particular focus on an ill-defined "depleted" mantle component while neglecting less common enriched components. Previous estimates have also been tied to the composition of peridotites, but these probably do not reflect the full complement of incompatible elements in the heterogeneous mantle. New estimates that account for enriched mantle components suggest the MORB source complement of incompatibles could be as much as 50-100 % larger than previous estimates.A maj...

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“…The thermochemical characteristics of the LLSVPs may also reconcile geochemical inferences of chemically distinct “reservoirs” that explain the difference between the oceanic island basalt and the mid‐ocean ridge basalt [e.g., Hofmann , ; Kellogg et al ., ; Kunz et al ., ]. However, it should be pointed out that a number of recent studies have suggested that these seismic and geochemical observations could be explained as purely thermal and postperovskite phase change effects [ Davies et al ., ; Schuberth et al ., , ; Huang and Davies , ], thus questioning the necessity of interpreting the LLSVPs as chemical piles.…”

Section: Introductionmentioning

confidence: 99%

The long‐wavelength geoid from three‐dimensional spherical models of thermal and thermochemical mantle convection

Liu

Zhong

2015

JGR Solid Earth

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The Earth's long‐wavelength geoid anomalies have long been used to constrain the dynamics and viscosity structure of the mantle in an isochemical, whole mantle convection model. However, there is strong evidence that the seismically observed large low shear velocity provinces (LLSVPs) in the lower mantle underneath the Pacific and Africa are chemically distinct and likely denser than the ambient mantle. In this study, we have formulated dynamically self‐consistent 3‐D spherical mantle convection models to investigate how chemically distinct and dense piles above the core‐mantle boundary may influence the geoid. Our dynamic models with realistic mantle viscosity structure produce dominantly spherical harmonic degree‐2 convection, similar to that of the present‐day Earth. The models produce two broad geoid and topography highs over two major thermochemical piles in the lower mantle, consistent with the positive geoid anomalies over the Pacific and African LLSVPs. Our geoid analysis showed that the bottom layer with dense chemical piles contributes negatively to the total geoid, while the layer immediately above the chemical piles contributes positively to the geoid, canceling out the effect of the piles. Thus, the bottom part of the mantle, as a compensation layer, has zero net contribution to the total geoid, and the thickness of the compensation layer is ~1000 km or 2 to 3 times as thick as the chemical piles. Our results help constrain and interpret the large‐scale thermochemical structure of the mantle using surface observations of the geoid and topography, as well as seismic models of the mantle.

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Stirring in three‐dimensional mantle convection models and implications for geochemistry: Passive tracers

Cited by 27 publications

References 61 publications

Global-scale modelling of melting and isotopic evolution of Earth's mantle: melting modules for TERRA

Global-scale modelling of melting and isotopic evolution of Earth's mantle: melting modules for TERRA

Dynamical geochemistry of the mantle

The long‐wavelength geoid from three‐dimensional spherical models of thermal and thermochemical mantle convection

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