A REVISED MODEL FOR THE SUBDUCTION OF OCEANIC CRUST: A COMBINED MONTE CARLO ANALYSIS AND PHASE EQUILIBRIA MODELLING APPROACH

Subduction of oceanic lithosphere at convergent margins drives plate tectonic motion on Earth, and leads to deformation and high-grade metamorphism in both the descending slab and over-riding arc. Determining the petrophysical changes that occur to subducted crust along the downgoing slab–asthenospheric mantle interface is critical to defining mass and energy transport between the hydrosphere and deep interior, and for constraining long-term geochemical cycles. Numerous studies over the past decade have relied upon thermo-mechanical models describing how slab-top P–T conditions change over time, although recent evaluation of such datasets have shown that geodynamic predictions are often 100–500 °C colder than P–T conditions recorded by exhumed metamorphic rocks. As such, geochemical, petrological, and geophysical interpretations formulated using such “cold” assumptions may be subject to error.

Here, we combined a Monte Carlo approach with thermodynamic phase equilibrium modelling to constrain how predicted phase assemblages, the P–T conditions of key devolatilization reactions, and the effect of densification with depth vary for typical MORB along these newly defined “hotter” subduction zone geotherms. Variation in bulk-rock compositions within the range of uncertainty provided for typical MORB predict that phase proportions along any given P–T path may vary by up to 20 vol.%, including key indicators of subduction, such as omphacite, actinolite, and glaucophane. The proportion of amphibole –and to a lesser extent clinopyroxene– is strongly sensitive to both changes in the bulk-rock composition and variation in slab-top surface P–T conditions. The abundance of mica also shows notable variation, having implications for volatile transport to sub-arc depths. In particular, we show that lawsonite does not stabilize in typical subduction zones, which provides a simple but important solution to the mismatch between its predicted abundance in experiments and its rarity in nature – often termed the lawsonite paradox.