Mantle Convection and Surface Expressions. Группа авторов

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СКАЧАТЬ substantially alter the computed velocity profiles.

      Despite these uncertainties, some general and potentially more robust features can be observed and related to concepts of material transport in Earth’s mantle. P‐ and S‐wave velocities of metabasalt differ substantially from those of PREM throughout the lower mantle and for all compositional scenarios considered here. Deep recycling of oceanic lithosphere can therefore be expected to create strong contrasts in elastic properties between former basaltic crust and surrounding mantle rocks and to affect seismic wave propagation in the lower mantle. Since fragments of subducted oceanic crust can survive several hundred million years of convective stirring while retaining dimensions of several kilometers (Stixrude & Lithgow‐Bertelloni, 2012), they would create perturbations in elastic properties of sufficient magnitude and suitable size to efficiently scatter seismic energy. The interpretation of seismic scatterers in Earth’s lower mantle in terms of deep recycling of oceanic crust (Frost et al., 2017; Kaneshima and Helffrich, 2009; Rost et al., 2008; Waszek et al., 2018) is therefore compatible with mineral‐physical models and further supported by geodynamical simulations (Ballmer et al., 2015; Brandenburg & van Keken, 2007) and geochemical observations (Hofmann, 1997; Stracke, 2012). Modeled P‐ and S‐wave velocities of metabasalt are lower than those of PREM and, with few exceptions, lower than those of most scenarios of pyrolite and harzburgite (Figure 3.9). The low P‐ and S‐wave velocities of metabasalt reflect the low sound wave velocities of calcium silicate perovskite found in recent experimental studies (Gréaux et al., 2019; Thomson et al., 2019) and challenge earlier modeling results that predicted a fast seismic signature for recycled basaltic crust (Davies et al., 2012; Deschamps et al., 2012; Stixrude & Lithgow‐Bertelloni, 2012). As a consequence, the hypothesis that recycled oceanic crust might contribute to the velocity reductions of large low‐velocity provinces (LLVP) received new attention (Garnero et al., 2016; McNamara, 2019; Thomson et al., 2019). The example of calcium silicate perovskite demonstrates, however, how current mineral‐physical models for the lower mantle hinge on the elastic properties of individual mineral phases that, when revised, may exert a strong leverage on computed wave velocities.

      For most of the compositional scenarios explored for pyrolite and harzburgite shown in Figure 3.9, S‐wave velocities show steeper gradients with depth than PREM while P‐wave velocity gradients are either similar to those of PREM or only slightly steeper than PREM. P‐ and S‐wave velocities are lower than those of PREM at 800 km depth for all scenarios, and only S‐wave velocities eventually rise above those of PREM for the majority of scenarios and at depths that depend on the combination of compositional parameters. P‐wave velocities remain below those of PREM for all pyrolite and harzburgite scenarios and at all depths considered here. Note that steep S‐wave velocity gradients and low P‐wave velocities remain characteristic signatures of pyrolite and harzburgite along hotter or colder adiabatic compression paths (Figure 3.8). P‐ and S‐wave velocity gradients that exceed those of seismic reference models have also been found by earlier mineral‐physical models for adiabatic compression paths of pyrolite (Cammarano et al., 2005b; Cobden et al., 2009; Matas et al., 2007) with the spin transition of ferropericlase bringing computed P‐wave velocity gradients closer to those of seismic reference models (Cammarano et al., 2010). Regions of the lower mantle that exhibit higher‐than‐average S‐wave velocity gradients have been found to coincide with the distribution of geoid lows and locations of former subduction (Richards & Engebretson, 1992; Spasojevic et al., 2010; Steinberger, 2000). In view of the mineral‐physical models shown in Figure 3.9, these observations are consistent with the storage of former slabs in the lower mantle that are dominated by peridotitic rocks from the upper mantle.

If we accept steeper‐than‐PREM S‐wave velocity gradients and lower‐than‐PREM P‐wave velocities as signatures of peridotitic rocks in the lower mantle, we find that a complementary lithology with higher‐than‐PREM P‐wave velocities and lower‐than‐PREM S‐wave velocity gradients would be required to match PREM as an average between these two lithologies. Based on the here‐used mineral‐physical data set (Table 3.1 and Figure 3.7), iron‐free bridgmanite compositions, i.e., MgSiO₃ and (Mg_0.96Al_0.04)(Si_0.96Al_0.04)O₃, would satisfy these criteria. Rocks that are dominated by iron-poor bridgmanite could therefore compensate for the steep S‐wave velocity gradients and low P‐wave velocities of former upper‐mantle rocks, such as pyrolite and harzburgite, that sink into the lower mantle as cold slabs imaged by seismic tomography (Fukao & Obayashi, 2013; Sigloch et al., 2008; Simmons et al., 2015; van der Hilst et al., 1997). The results of recent geodynamic simulations suggest that domains of bridgmanite‐rich rocks may remain isolated by channeling mantle flow around themselves when viscosity contrasts between such highly rigid domains and weaker pyrolitic rocks are high enough (Ballmer et al., 2017a). Mantle domains rich in bridgmanite but at the same time depleted in iron could have formed by fractional crystallization of a magma ocean, from which iron-poor bridgmanite would crystallize first as the liquidus phase (Andrault et al., 2012; Boukaré et al., 2015; Fiquet et al., 2010). Fe‐enriched fractionation products could have been separated from bridgmanite‐rich domains by accumulating at the base of the mantle (Ballmer et al., 2017b; Labrosse et al., 2015) and evolving to iron-rich mineral assemblages that have been invoked to explain geodynamic and seismic properties of LLVPs (Deschamps et al., 2012; Garnero et al., 2016) and ultra‐low velocity zones (ULVZ) (Bower et al., 2011; Muir and Brodholt, 2015b, 2015a; Wicks et al., 2017, 2010; Yu & Garnero, 2018).

      While the idea of a lower mantle composed of complementary peridotitic and bridgmanite‐dominated rocks may seem consistent with a number of geophysical observations as discussed above and would also bring the Mg/Si ratio of the mantle as a whole closer to chondritic values (McDonough & Sun, 1995), this interpretation of the modeled P‐ and S‐wave velocity profiles is by no means unique. As mentioned above, S‐wave velocities for a pyrolitic bulk composition can in principle be reconciled with those of PREM when accepting deviations from an adiabatic temperature profile (Figure 3.8). The here‐modeled S‐wave velocity gradients for pyrolite and harzburgite would match PREM along superadiabatic temperature profiles. Superadiabatic thermal gradients have also been inferred from earlier comparisons of mineral‐physical models with seismic velocity profiles for the lower mantle and for a range of different compositions (Cammarano et al., 2010, 2005b; Cobden et al., 2009; Deschamps and Trampert, 2004; Khan et al., 2008; Matas et al., 2007). In contrast, an adiabatic pyrolitic lower mantle has been found to be consistent with PREM based on a data set for mineral elasticity derived exclusively from DFT computations (Wang et al., 2015). A recent attempt to reconcile existing data of Fe‐Mg exchange between bridgmanite and ferropericlase also found agreement between adiabatic compression of pyrolite and PREM based on S‐wave velocities (Hyung et al., 2016).

      It is interesting to note that a significant reduction of the Fe‐Mg exchange coefficient between bridgmanite and ferropericlase with increasing pressure as indicated by recent thermodynamic models (Nakajima et al., 2012; Xu et al., 2017) could entail substantial reductions of P‐ and S‐wave velocities for peridotitic bulk rock compositions. The resulting low P‐ and S‐wave velocities could help to explain the uppermost protrusions of LLVPs that locally extend more than 1000 km upwards above the core–mantle boundary (Durand et al., 2017; Hosseini et al., 2020; Koelemeijer et al., 2016). Because P‐wave velocities of both bridgmanite and ferropericlase seem to be less sensitive to changes in the iron content than S‐wave velocities (Figure 3.7), СКАЧАТЬ