The Subducting Slab as a Chromatographic Column: Regimes of Sub-Solidus Mass Transport as a Function of Lithospheric Hydration State, With Special Reference to the Fate of Carbonate

Abstract

We investigate the chemical budget of subduction zones at sub-solidus conditions using a thermodynamic-numerical simulation in which all major rock components are treated as soluble and potentially mobile in aqueous fluids. This new strategy significantly improves the accuracy of predicted fluid-rock equilibrium compositions in open petrological systems. We show that all slabs release volatiles and nonvolatiles to the mantle wedge, contributing to its refertilization. But some mobile constituents, such as alkali and alumina, may be trapped along layer boundaries or traverse without interaction depending on chemical contrasts between adjacent lithologies. The accumulation of igneous alumina and silica in the limestones of the central-eastern Pacific slabs drives their decarbonation and is marked by metasomatic garnet growth. Those slabs are also predicted to lose much of their alkalis before sub-arc depth. Even when they are produced in the altered mafic and ultramafic layers, fluids reach the slab/mantle wedge interface with distinct compositional signatures that are typical of the sedimentary cover. We distinguished supply and transport limited regimes of element subduction by testing the sensitivity of our mass balance to changes in slab hydration state (HS). Transport limited slabs sensitive to HS include notably a hotspot of carbon release to the mantle wedge (e.g., Costa Rica). Finally, we show that the quantitative budgets do depend on the geometry of fluid flows, and on assuming that slabs are mechanically continuous structures, which is questionable. Taken together, these insights will help better constrain the long-term chemical evolution of the shallow planetary interior, and the thermomechanical behavior of the subduction interface.