The Role of the Dynamics of the Subducting Plate in Generating Arc Magmatism

Abstract

The thermal state of subducting plates is thought to be of great importance in the generation of the arc magmatism above them. The temperature of slabs affects dehydration, the primary driver of arc magmatism, but may also determine whether the slab itself undergoes partial melting. The focus of this study has therefore been slab temperature: how sensitive it is to the dynamic properties of the slab, and exactly what effect it has on arc magmatism today and back in Earth’s early history.
Through the use of numerical models, we improve on existing proxies for slab temperature. Our models demonstrate that the temperature of different parts of the slab depend variably on slab age, trench normal convergence rate, slab dip and the decoupling depth of the subducting and over-riding plates. In addition to forward modelling, we worked backwards from real-world data. To this end we developed a tool to search for statistically significant correlations between the same slab parameters, as well as over-riding crustal thickness, and the trace element characteristics of arc lavas in global databases.
We also investigate a recently proposed dynamic process which may have an effect on arc processes: the formation of plumes of slab derived material (relamination). We find that the relamination of mafic oceanic crust is more likely under Archean conditions, potentially explaining systematic differences between the chemistry of Archean rocks and more modern continental crust and arc rock. We also undertook a preliminary investigation on the feasibility and effect that episodically driven subduction could have on the thermal profile of subducting slabs.
We demonstrate here that through forward numerical modelling and analysis of the chemistry of arc lavas, we can gain valuable insight into the thermal state and complex dynamics of the slab in the critical sub-arc region.