The Influence of Ridge Subduction on the Geochemistry of Vanuatu Arc Magmas
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Abstract The effects of buoyant ridge subduction have been researched for decades. However, it remains unclear how this process influences magma chemistry. Here we use a compilation of geochemical data, well‐established geochemical proxies (i.e., Ba/Nb, Th/Nb) and mantle redox modeling (i.e., V/Sc, Cu) to propose that the subduction of a ridge underneath the central portion of the Vanuatu arc causes shallow‐angle subduction and the development of a slab tear. We suggest that the shallow slab pinches out the asthenospheric mantle and bulldozes ancient metasomatized lithospheric mantle from the forearc toward the main‐arc. Slab‐fluxed melting of this bulldozed material could account for the along‐arc 87 Sr/ 86 Sr‐ 143 Nd/ 144 Nd variations of the Vanuatu magmas. The influx of hot sub‐slab material into the Vanuatu arc mantle wedge through a slab tear produces magmatism within the forearc. Modeling of V/Sc and Cu systematics suggest that the mantle source of the forearc magmas has a higher f O 2 and Cu content than the source of the main‐arc and rear‐arc samples. The main‐arc and rear‐arc mantles were metasomatized by both slab‐derived fluids and melts. Whereas release of high Cu‐SO 2 slab‐derived fluids caused oxidation and Cu enrichment of the forearc mantle. These systematics indicate a decrease in the f O 2 of slab fluxes with increasing depth‐to‐slab and distance‐from‐trench. Our findings highlight the role of ridge subduction in controlling the along‐arc and across‐arc variations in the chemistry of Vanuatu arc magmas. This updated geodynamic model, based on geochemistry, is consistent with recent geophysical constraints and 3D numerical modeling.Keywords:
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SUMMARY Lithospheric plates on the Earth's surface interact with each other, producing distinctive structures comprising two descending slabs. Double-slab subduction with inward-dipping directions represents an important multiplate system that is not yet well understood. This paper presents 2-D numerical models that investigate the dynamic process of double-slab subduction with inward dipping, focussing on slab geometry and mantle transition zone upwelling flow. This unique double-slab configuration limits trench motion and causes steep downward slab movement, thus forming fold piles in the lower mantle and driving upward mantle flow between the slabs. The model results show the effects of lithospheric plate properties and lower-mantle viscosity on subducting plate kinematics, overriding plate stress and upward mantle flow beneath the overriding plate. Appropriate lower-mantle strength (such as an upper–lower mantle viscosity increase with a factor of 200) allows slabs to penetrate into the lower mantle with periodical buckling. While varying the length and thickness of a long overriding plate (≥2500) does not have a substantial effect on slab geometry, its viscosity has a marked impact on slab evolution and mantle flow pattern. When the overriding plate is strong, slabs exhibit an overturned geometry and hesitate to fold. Mantle transition zone upwelling velocity depends on the speed of descending slabs. The downward velocity of slabs with a large negative buoyancy (caused by thickness or density) is very fast, inducing a significant transition zone upwelling flow. A stiff slab slowly descends into the deep mantle, causing a small upward flow in the transition zone. In addition, the temporal variation of mantle upwelling velocity shows strong correlation with the evolution of slab folding geometry. In the double subduction system with inward-dipping directions, the mantle transition zone upwelling exhibits oscillatory rise with time. During the backward-folding stage, upwelling velocity reaches its local maximum. Our results provide new insights into the deep mantle source of intraplate volcanism in a three-plate interaction system such as the Southeast Asia region.
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Although slab-derived fluid significantly affects melt generation and dynamics within subduction zones, its amount and distribution are not sufficiently constrained at present. Herein, we use isotopic systematics of arc volcanic rocks, subducting materials, and intrinsic mantle components prior to metasomatism, to quantify the contribution of the slab-derived fluid that metasomatizes the overlying mantle wedge beneath the entire area of Japan arcs. Simultaneous application of several multivariate statistical analyses (clustering analysis and principal/independent component analyses) to the isotopic data set allows Japan arcs to be broadly divided into eastern and western parts at the first order. Moreover, a clear higher-order inter-arc segmentation is observed, together with some intra-arc variations that possibly correspond to the heterogeneity of incoming plates. Inter-arc segmentation is shown to be primarily controlled by the geometrical parameters of the slab and the arc (e.g., subduction of a single plate or double plates beneath either oceanic or continental crust), which results in differences between mantle wedge and slab thermal conditions. Accordingly, the Kuril and Izu arcs, which have thin arc crusts (~20 km), exhibit the lowest extent of slab-derived fluid addition (0.1 wt%) to the mantle wedge, while the NE Japan arc, with a thicker arc crust (up to 36 km), features a higher value of 0.2 wt%, although the slab thermal parameters for these three arcs are essentially the same. The Central Japan arc shows the highest extent of slab-derived fluid addition (>1.0 wt%) because of the overlapping subduction of Pacific and Philippine Sea slabs, while the SW Japan and Ryukyu arcs feature moderate values of ~0.5 wt%. Moreover, a clear exotic plume zone and spots are observed in SW Japan and the Japan Sea. In addition to the variability of slab-derived fluid composition, the intrinsic mantle composition (before slab-derived fluid–induced metasomatism) shows a clear along-arc variation that is possibly caused by a large-scale mantle flow from the continental side. Thus, slab-derived fluid addition and mantle composition variability equally contribute to inter-arc segmentation, which highlights the importance of both local and regional thermal flow structures of slab-mantle systems.
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Plate tectonic history, geological, geochemical (element and isotope ratios), and seismological (P-wave tomography and SKS splitting) data are combined with laboratory modeling to present a three-dimensional reconstruction of the subduction history of the central Mediterranean subduction. We find that the dynamic evolution of the Calabrian slab is characterized by a strong episodicity revealed also by the discrete opening of the Tyrrhenian Sea. The Calabrian slab has been progressively disrupted by means of mechanical and thermal erosion leading to the formation of large windows, both in the southern Tyrrhenian Sea and in the southern Apennines. Windows at lateral slab edges have caused a dramatic reorganization of mantle convection, permitting inflow of subslab mantle material and causing a complicated pattern of magmatism in the Tyrrhenian region, with coexisting K- and Na-alkaline igneous rocks. Rapid, intermittent avalanches of large amounts of lithospheric material at slab edges progressively reduced the lateral length of the Calabrian slab to a narrow (200 km) slab plunging down into the mantle and enhancing the end of the subduction process.
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Advances in seismic tomography have revealed that the Earth’s mantle is more complex than previously thought and hosts a plethora of slab morphologies. These morphologies can be broadly grouped into three main types; penetrating slabs, deflected slabs and broken, or orphan, slabs. Slab orphaning is a newly discovered phenomenon whereby slabs break directly at mid-mantle depths. This produces a flattened parent slab above 660 km and an orphan slab below it. As the orphan slab slowly sinks towards the core-mantle boundary, subduction continues through the lateral motion of the parent above 660 km. In nature, the Tonga, Arabian, Japan and Central American slabs are possible candidates for slab orphaning. Orphaning has significant implications for the interpretation of slab remnants and their inferred ages, with consequences for tectonic reconstructions. The subduction of slabs, however, does not take place in isolation and slab dynamics must be influenced to some degree by the overriding plate. The nature of the overriding plate plays a major role in the evolution of deep slab morphologies at mid-mantle depths. 2-D numerical simulations of subduction indicate that the presence of continental lithosphere at subduction zones produces markedly different slab behaviour at depth. In particular, a continental overriding plate results in bigger orphans and encourages the mid-mantle penetration of slabs that are otherwise inclined to flatten. It is therefore clear that, in contrast to the slabs of the upper mantle, the deeper slab morphologies are the result of a complex interaction between the overriding plate forcing and the changes in the relative strength ratios of the slab and mantle.
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Seismic tomography shows that slow velocity anomalies exist in the mantle wedge and extend to the forearc region down to the subducting Philippine Sea slab under northern Kyushu. We also conducted numerical simulations with petrologic data to estimate the fluid distribution in the mantle wedge. The seismic and simulation results indicate that regimes of melting and magmatism in a subduction zone with a young slab are different from those with old slabs. Dehydration and melting occur beneath the arc and forearc above a young (and warm) slab such as in northern Kyushu (≤ 26 Ma), while fluids (aqueous solution and melt) occur mainly beneath the back arc above old slabs.
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