Late Triassic high Mg diorites of the Wulong pluton in the South Qinling Belt, China: Petrogenesis and implications for crust-mantle interaction
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Abstract Subducted oceanic crust is enriched in free silica. Although being one of the silica polymorphs at lower‐mantle pressures, niccolite‐type phase (Nt‐phase) has not been documented in multicomponent metabasaltic or metasediment compositions relevant to subducting oceanic crust. Here, we report the formation of an Al‐rich Nt‐phase (∼24.4 to 32.4 wt% Al 2 O 3 ), coexisting with Al‐depleted bridgmanite (∼6.4 to 7.6 wt% Al 2 O 3 ), δ‐phase, and iron‐rich phase in model hydrated basalts over the pressure‐temperature range of 84–113 GPa and 1,800–2,200 K. Infrared spectroscopy of a pure synthetic Al‐rich Nt‐phase shows OH bending and stretching vibrations at high pressures, indicative of its hydrous nature. This study suggests that Al‐rich Nt‐phase can serve as a potential water carrier in subducted oceanic crust to the deep lower mantle.
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Some of the fundamental features of plate tectonics are interpreted in connection with the behavior of oceanic crust. It is shown to be likely that the oceanic crust which is produced at the mid-ocean ridge by chemical differentiation may be removed from the downgoing slab by melting at the depth of asthenosphere behind the deep-sea trench. The melting of crustal material after the subduction is made possible by an efficient supply of heat through the well-developed asthenosphere with a low-velocity and high-attenuation of seismic waves. The removal of subducted oceanic crust from the slab is consistent with the positive gravity anomaly behind trenches and the double Benioff zone recently discovered. We propose new type of driving forces of plate motion, which arises from the density contrast between the crust and mantle when the oceanic crust is either created or destructed. The proposed driving mechanism is consistent with the non-uniform size and shape of individual plates, the migration of mid-ocean ridges and compressional intraplate stress, while these facts are difficult to understand in the framework of conventional models. A continuous accumulation of basaltic magma beneath the trench-arc system results in a catastrophic overflow of material, which corresponds to back-arc spreading. The picture presented in this paper explains the evolution of marginal basins that is characterized by the presence of remnant arcs, the changes in stress field and the dip angle of the slab, and the anomalous depth-age relationships.
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Abstract The composition of Earth's mantle, continental crust, and oceanic crust continuously evolve in response to the dynamic forces of plate tectonics and mantle convection. The classical view of terrestrial geochemistry, where mid‐ocean ridges sample mantle previously depleted by continental crust extraction, broadly explains the composition of the oceanic and continental crust but is potentially inconsistent with observed slab subduction to the lower mantle and oceanic crust accumulation in the deep mantle. We develop a box model to explore the key processes controlling crust‐mantle chemical evolution. The model mimics behaviors observed in thermochemical convection simulations including subducted oceanic crust separating and accumulating in the deep mantle. We demonstrate that oceanic crust accumulation strongly depletes the mantle independently of continental crust extraction. Slab stalling depths and continental crust recycling rates also affect the extent and location of mantle depletion. We constrain model regimes that reproduce oceanic and continental crust compositions using Markov chain Monte Carlo sampling. Some regimes deplete the lower mantle more than the upper mantle, contradicting the assumption of a more enriched lower mantle. All regimes require oceanic crust accumulation in the mantle. Though a small fraction of the mantle mass, accumulated oceanic crust can sequester trace element budgets exceeding the continental crust, depleting the mantle more than continental crust extraction alone. Oceanic crust accumulation may therefore be as important as continental crust extraction in depleting the mantle, contradicting the paradigmatic complementarity of depleted mantle and continental crust. Instead, depleted mantle is complementary to continental crust plus sequestered oceanic crust.
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Recycled ancient oceanic crust with variable amounts of aging, or inclusion of sediments of differing types and origins has often been invoked as a source for present‐day ocean island basalts (OIB), but the current evidence remains largely qualitative. Previous quantitative modeling has shown that much has to be learned in order to better understand the implications of crustal recycling on mantle heterogeneity. Here, we present new model calculations incorporating recent constraints on subduction‐zone processes and the composition of subducted sediments. Modeled compositions of the recycled oceanic crust vary widely as a function of the recycling age and composition of the oceanic crust. HIMU‐type sources can only be created by recycling igneous oceanic crust if it has undergone substantial modification during subduction. Although the required modifications are qualitatively consistent with dehydration processes in subduction zones, the many uncertainties prevent a precise estimate of the isotopic composition of ancient recycled igneous crust. Inclusion of sediments increases the isotopic variability and although the resulting Sr and Nd isotopic signatures can be similar to enriched mantle (EM) signatures, the Pb isotopic composition of EM‐type OIB is difficult to reconcile with the presence of sediment in their sources. The large variability of modeled compositions of the subducted crust suggests that if mantle heterogeneity is largely formed by crustal recycling, each OIB is likely to have a unique isotopic composition resulting from specific combinations of composition, age and subduction modification of the subducted crust. Given the variability of the recycled components, a small number of relatively well‐defined enriched compositions can only be explained if either the subduction processing of oceanic crust is a far better defined process than observation would seem to indicate, or, the intramantle disaggregation and mixing of compositionally diverse recycled materials is surprisingly efficient.
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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/U we 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-silicate-perovskite, through density segregation in ultra-low-velocity zones as well as in very-low-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 SiO2 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.
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