Australia: The Land Where Time Began

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Slab Melting Beneath the Cascade Arc Driven by Dehydration of Altered Oceanic Peridotite

At subduction zones water is returned to the interior of the Earth. The processes and pathways by which water leaves the plates that are being subducting and causes melting beneath volcanic arcs are complex; the subducting sediment, altered oceanic crust, or hydrated mantle in the subducting plate, that are the source of this water, is debated; and the role played by the temperature of the slab is not clear. In this paper Walowski et al.¹ analyse the hydrogen-isotope and trace-element signature of melt inclusions in samples of ash from the Cascade Arc, where lithosphere that is young and hot is being subducted. When they compared these data with analyses that have been published they found that the fluids contained in the Cascade magmas are sourced from deeper parts of the subducting slab – hydrated mantle peridotite in the interior of the slab – compared with fluids in magmas from the Marianas Arc, where lithosphere that is older and colder is being subducted. They used geodynamic modelling to demonstrate that the upper crust of the subducting slab in the hotter subduction zone dehydrates rapidly at shallow depths. As subduction continues, fluids that have been released from the interior of the deeper plate migrate into the dehydrated parts where they cause melting in these parts that had previously been dehydrated. Further melting is triggered when these melts migrate into the overlying mantle wedge. According to Walowski et al.¹ their results provide a physical model to explain the melting in the plate that has been subducted and mass transfer from the slab to the mantle beneath arcs where oceanic lithosphere that is relatively young is subducted.

Sources & Further reading

1. Walowski, K. J., P. J. Wallace, E. H. Hauri, I. Wada and M. A. Clynne (2015). "Slab melting beneath the Cascade Arc driven by dehydration of altered oceanic peridotite." Nature Geosci 8(5): 404-408.