A chemical Earth model with whole mantle convection: The importance of a core–mantle boundary layer (D″) and its early formation

Tolstikhin, Igor; Kramers, Jan D.; Hofmann, Albrecht W.

doi:10.1016/j.chemgeo.2005.09.015

Cited by 81 publications

(64 citation statements)

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“…Faster degassing has previously been inferred from argon and xenon observations (Allegre et al, 1987;Turner, 1989;Tolstikhin et al, 2006). The evolution of 4 He is then calculated from the abundances of U and Th through time and the degassing inferred from 3 He.…”

Section: Noble Gases In the Heterogeneous Mantlementioning

confidence: 99%

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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Section: Noble Gases In the Heterogeneous Mantlementioning

confidence: 99%

Dynamical geochemistry of the mantle

Davies

2011

Solid Earth

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“…The detailed nature of chemical heterogeneities is still under debate, the two main hypothesis being that they result either from the evolution of primordial reservoirs of dense materials formed by differentiation early in the Earth's early history (e.g. Solomatov & Stevenson 1993;Labrosse et al 2007;Lee et al 2010;Tolstikhin et al 2006), or from accumulation of subducted oceanic crust above the core-mantle boundary (CMB, e.g. Gurnis & Davies 1986;Christensen 1989;Christensen & Hofmann 1994).…”

Section: Introductionmentioning

confidence: 99%

The stability and structure of primordial reservoirs in the lower mantle: insights from models of thermochemical convection in three-dimensional spherical geometry

Deschamps

Tackley

2014

Geophysical Journal International

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S U M M A R YLarge-scale chemical lateral heterogeneities are inferred in the Earth's lowermost mantle by seismological studies. We explore the model space of thermochemical convection that can maintain reservoirs of dense material for a long period of time, by using similar analysis in 3-D spherical geometry. In this study, we focus on the parameters thought to be important in controlling the stability and structure of primordial dense reservoirs in the lower mantle, including the chemical density contrast between the primordial dense material and the regular mantle material (buoyancy ratio), thermal and chemical viscosity contrasts, volume fraction of primordial dense material and the Clapeyron slope of the phase transition at 660 km depth. We find that most of the findings from the 3-D Cartesian study still apply to 3-D spherical cases after slight modifications. Varying buoyancy ratio leads to different flow patterns, from rapid upwelling to stable layering; and large thermal viscosity contrasts are required to generate long wavelength chemical structures in the lower mantle. Chemical viscosity contrasts in a reasonable range have a second-order role in modifying the stability of the dense anomalies. The volume fraction of the initial primordial dense material does not effect the results with large thermal viscosity contrasts, but has significant effects on calculations with intermediate and small thermal viscosity contrasts. The volume fraction of dense material at which the flow pattern changes from unstable to stable depends on buoyancy ratio and thermal viscosity contrast. An endothermic phase transition at 660 km depth acts as a 'filter' allowing cold slabs to penetrate while blocking most of the dense material from penetrating to the upper mantle.

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“…While originally the plume source was assumed to be the whole lower mantle, the basic 419 framework could still be viable if instead of the whole lower mantle, the plume source was much 420 smaller, such as D" (e.g., Tolstikhin et al, 2006).…”

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confidence: 99%