Numerical study of the origin and stability of chemically distinct reservoirs deep in Earth's mantle

Abstract

Seismic tomography is providing mounting evidence for large scale compositional heterogeneity deep in Earth's mantle, and also the diverse geochemical and isotopic signatures observed in oceanic basalts suggest that the mantle is not chemically homogeneous. Isotopic studies on Archean rocks indicate that mantle inhomogeneity may have existed for most of the Earth's history. One important component may be recycled oceanic crust, residing at the base of the mantle. We investigate, by numerical modeling, if such reservoirs may have been formed in the early Earth, before plate tectonics (and subduction) were possible, and how they have survived -and evolved -since then. During Earth's early evolution, thick basaltic crust may have sunk episodically into the mantle in short but vigorous diapiric resurfacing events. These sections of crust may have resided at the base of the mantle for very long times. Entrainment of material from the enriched reservoirs thus produced may account for EM and HIMU signatures in oceanic basalts, whereas deep subduction events may have shaped and replenished deep mantle reservoirs. Our modeling shows that (1) convective instabilities and resurfacing may have produced deep enriched mantle reservoirs prior to the era of plate tectonics, that (2) such formation is qualitatively consistent with the geochemical record showing multiple distinct OIB sources, and that (3) reservoirs thus produced may be stable for billions of years.

The importance of slab pull and a global asthenosphere to plate motions

“…At depth, upper mantle slabs can be dynamically supported by viscous stresses, which may explain a transition of earthquake focal mechanisms from down‐dip extension to compression in the mid‐upper mantle [ Isacks and Molnar , 1971; Alpert et al , 2010]. Locally, magnification of pull forces is possible due to compositional effects, for example, in regions where density increases due to extensive eclogite formation [ Cloos , 1993; van Thienen et al , 2005]. Because of the uncertain significance of the above mechanisms, we treat the slab pull fraction ( f sp ) as a free parameter in our models.…”

Mantle Geochemical Geodynamics

“…An important point shown by van Thienen et al, (2005) is that any layering around 660 km induced by the post-spinel phase transition makes the layer more stable over long time periods, presumably because it reduces the temperature drop across the layer and the convective velocities.…”

Large-Scale Thermo-chemical Structure of the Deep Mantle: Observations and Models

“…Several mechanisms have been proposed for the early partial differentiation of the Earth's mantle, including fractional crystallization of a regular magma ocean (Solomatov and Stevenson 1993;Solomatov 2007), crystallization of a basal magma ocean (Labrosse et al 2007), which also explains the ultra-low seismic velocity zones (ULVZ) observed locally at the bottom of the mantle (Garnero and Helmberger 1995;Williams and Garnero 1996), 'upside-down' differentiation (Lee et al 2010), which consists of the formation, crystallization, and sinking of dense liquids in the upper mantle during the Earth's first billion year, and the recycling of a primitive crust (Tolstikhin et al 2006), which is supported by geochemical data (Boyet and Carlson 2006;Caro et al 2003Caro et al , 2004 and numerical models of convection (van Thienen et al 2004(van Thienen et al , 2005. Alternatively, models of mantle composition based on E-chondrite predict that the composition in main oxides of the lower mantle is different from that of the upper mantle (Javoy et al 2010).…”