The fate of subducted oceanic crust: a mineral segregation model

Abstract

Plate subduction and mantle plumes are two of the most important material transport processes of the silicate Earth. Currently, a debate exists over whether the subducted oceanic crust is recycled back to the Earth's surface through mantle plumes, and can explain their derivation and major characteristics. It is also puzzling as to why plume heads have huge melting capacities and differ dramatically from plume tails both in size and chemical composition. We present data showing that both ocean island basalt and mid-ocean ridge basalt have identical supra-primitive mantle mean Nb/U values of ∼46.7, significantly larger than that of the primitive mantle value. From a mass balance calculation based on Nb/Uwe have determined that nearly the whole mantle has evolved by plate subduction-induced crustal recycling during formation of the continental crust. This mixing back of subducted oceanic crust, however, is not straightforward, because it generally would be denser than the surrounding mantle, both in solid and liquid states. A mineral segregation model is proposed here to reconcile different lines of observation. First of all, subducted oceanic crustal sections are denser than the surrounding mantle, such that they can stay in the lower mantle, for billions of years as implied by isotopic data. Parts of subducted oceanic crust may eventually lose a large proportion of their heavy minerals, magnesian-silicate-perovskite and calcium-silicateperovskite, through density segregation in ultra-low-velocity zones as well as in verylow-velocity provinces at the core-mantle boundary due to low viscosity. The remaining minerals would thus become lighter than the surrounding mantle, and could rise, trapping mantle materials, and forming mantle plumes. Mineral segregation progressively increases the SiO 2 content of the ascending oceanic crust, which enhances flux melting, and results in giant Si-enriched plume heads followed by dramatically abridged plume tails. Therefore, ancient mineral-segregated subducted oceanic crust is likely to be a major trigger and driving force for the formation of mantle plumes.

Carbonated mantle domains at the base of the Earth's transition zone

“…Mineral segregation in these low velocity regions is likely to occur when there are relatively high degrees of partial melting (Sun et al 2011). Diamond is much less dense than the surrounding mantle and silicate melts at depths below the 660-km discontinuity, such that diamond in subducted oceanic slabs may be released and carried upwards by mantle plumes.…”

Geochronology, Petrology, and Genesis of Two Granitic Plutons of the Xianghualing Ore Field in South Hunan Province: Constraints from Zircon U–Pb Dating, Geochemistry, and Lu–Hf Isotopic Compositions

“…As a part of Jurassic tectonic-magmatic activity in South China, the Laiziling and Jianfengling plutons might be formed in the same tectonic setting with other coeval granitic plutons, such as Qitianling, Guposhan, and Jiuyishan plutons [62,93,94]. However, the geodynamic mechanism triggering the extensional setting and magma activity in South China has long been in debate for decades [59,[97][98][99][100][101][102][103][104][105][106][107]. Several models have been proposed to illustrate the geodynamic mechanism, for example, westward subduction of the paleo-pacific plate, mantle plume, post-collision, and so on [97][98][99][100][101]104,105,107].…”

Strong seismic scatterers near the core–mantle boundary north of the Pacific Anomaly

“…Moreover, the rheological differences between the recycled crust and surrounding mantle may defer the homogenization and lead to incomplete mixing (Metcalfe et al, 1995;Helffrich and Wood, 2001;Van Keken et al, 2002), making it possible for the subducted slab to remain in the CMB region for a long time. Furthermore, during the homogenization, the segregation of different minerals within the crust could also result in compositional heterogeneities (Sun et al, 2011) because of low viscosity effects (here the Pacific Anomaly may serve this purpose). Thus, different compositions from a segregation process can explain our observed lateral variations within the fast anomalies.…”