Formation of Thermochemical Heterogeneities by Core‐Mantle Interaction. Issue 2 (17th February 2023)

• Record Type: Journal Article
• Authors: Stein, Claudia; Hansen, Ulrich
• Is Part Of: Journal of geophysical research. Volume 128:Issue 2(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-02-17
• Subjects: core‐mantle interaction -- mantle convection -- LLSVPs -- ULVZs; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2022JB025689
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Earth's core‐mantle boundary (CMB) shows a complex structure with various seismic anomalies such as the large low shear‐wave velocity provinces (LLSVPs) and ultra‐low velocity zones (ULVZs). As these structures are possibly induced by chemically distinct material forming a layer above the CMB, models of mantle convection made ad hoc assumptions to simulate the dynamics of this layer. In particular, density and mass were prescribed. Both conditions are critical for the dynamics but hardly constrained. Core‐mantle interaction is considered as one possible origin for this dense layer. For example, diffusion‐controlled enrichment of iron has been proposed. We here apply a chemical gradient between the mantle and the denser core to analyze the penetration of dense material into the mantle. As such, we employ 2D Cartesian models where a thermochemical layer at the base of the mantle develops self‐consistently by a diffusive chemical influx. Our simulations indicate that chemical diffusion is strongly affected by the convective mantle flow. This convection‐assisted diffusion yields a compositional influx mainly in the areas where slabs spread over the bottom boundary and sweep dense material aside to form accumulations with rising plumes atop. Like for a prescribed dense layer this process leads to chemically distinct piles, which are typically smaller (therefore more suited to explain ULVZs) but more persistent due to the constant chemical influx. Combining the influx scenario with the primordial layer can possibly explain the simultaneous existence of LLSVPs and ULVZs along with the observation of a core‐like isotopic composition in the mantle. Plain Language Summary: The core‐mantle boundary (CMB) shows a complex structure. Seismologists have observed features that are possibly denser than their surroundings. These structures form from a dense layer above the CMB. Therefore typical mantle convection models have assumed an initial dense basal layer. The thickness and density of this prescribed layer are crucial but hardly constrained. Here we investigate core‐mantle interaction as one possible origin for this layer and employ 2D Cartesian models of mantle convection that consider a diffusive chemical gradient between the iron‐rich core and the silicate mantle. Our simulations show that the diffusive influx is coupled to the convective mantle flow. Convection‐assisted diffusion gives a larger influx beneath slabs spreading over the CMB. Additionally, as in the models with a prescribed layer, the rising plumes pull dense material up and form piles. In this study, however, the constant chemical influx leads to piles existing for longer times. The piles are typically smaller but can maybe in combination with a primordial layer explain different seismologically observed structures and the presence of core material in the mantle. Key Points: We analyze convection‐assisted core‐mantle interaction in thermochemical mantle convection models Dense material penetrates into the mantle as a result of a basal diffusive chemical influx, where penetration is promoted by convection Small piles form with some of the dense core material being entrained by plumes