In the s it was thought that all subduction-related water is released beneath volcanic arcs and none passes into the deeper mantle. It is known today that such a view is incorrect. Experimental discoveries of hydrous phases and evidence for high water content in nominally anhydrous minerals, together with numerical modeling, show that at high rates of subduction significant amounts of water can subduct as deep as the transition zone and probably deeper e.
Modeling Bina et al. Thus, there is a physical and natural explanation for volcanism in the inner parts of continents, in particular in back-arc tectonic settings in relation to active at the time of volcanism subduction. The paleotectonic setting of Karoo. Consider the paleotectonic setting of the Karoo flood basalt province Figure 1. The diameter of the province is about km.
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Its southern in present day coordinates end was about km from an active subduction trench in the Jurassic. Considering prolonged Pantalassa paleoPacific subduction beneath this part of Gondwana we can assume that the mantle transition zone was full of slabs and therefore highly hydrated see paleotectonic reconstructions: Early Carboniferous — Late Carboniferous — Late Permian — Early Triassic — Jurassic.
Figure 1. Paleotectonic reconstruction of Gondwana in Early Jurassic time from www.
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The location of the Karoo flood basalt province is added. It is located in area beneath which prolonged subduction occurred starting in the Carboniferous. Subducting slabs probably stagnated and were trapped in the mantle transition zone. Such slabs brought significant amounts of water and dehydrated with time. Dehydration led to volcanism similar in a sense to arc volcanism but induced by much deeper dehydration. Figure 2 shows experimental data on water stability fields of mineral phases within mantle peridotite and numerical calculations of slab buoyancy from Ivanov et al.
This figure shows that fast cold subducting slabs can bring significant amounts of water into the deep mantle and that such slabs tend to stagnate in the transition zone.
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With time such slabs heat up to ambient mantle temperature and dehydrate. Released water induces melting. If the released water is not enough to induce melting because it is dissolved in olivine up to 0.
Figure 2. Saturation of the transition zone with water via subduction and possible regions of melting of water-saturated, advected mantle peridotite after Ivanov et al. Left: a fast subducting slab attains positive buoyancy while crossing km discontinuity. Both fast- and slow-subducting slabs attain positive buoyancy while crossing km discontinuity after Bina et al. Straight lines show boundaries between different mineral assemblages not included for the sake of simplicity. Curved dotted line shows limits of clinopyroxene stability field cpx in and out. H 2 O solubility of olivine decreases with increasing temperature Smyth et al.
White ellipses mark possible depths of melting of water-saturated mantle peridotite advected from the transition zone.
The depths of the regions of melting correspond well to those suggested by Hirschman et al. At intermediate depth between the two regions of melting in the upper mantle is characterized by the highest water capacity that prevents melting Hirschman et al. Now consider why the subduction environment is characterized by volcanic rocks with HFSE-negative anomaly. It is almost universally agreed that the source of melting beneath arcs above subducting slabs is hydrated MORB mantle referred to as mantle wedge e.
Previously dry MORB-type mantle is hydrated by water fluids derived from a subducting slab.
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