Reconciling mantle wedge thermal structure with arc lava thermobarometric determinations in oceanic subduction zones

Alexander Perrin,Julie Prytulak,Stephan Kramer,Saskia Goes,D Rhodri Davies,Cian Wilson

doi:10.1002/2016gc006527

Abstract

AbstractSubduction zone mantle wedge temperatures impact plate interaction, melt generation, and chemical recycling. However, it has been challenging to reconcile geophysical and geochemical constraints on wedge thermal structure. Here we chemically determine the equilibration pressures and temperatures of primitive arc lavas from worldwide intraoceanic subduction zones and compare them to kinematically driven thermal wedge models. We find that equilibration pressures are typically located in the lithosphere, starting just below the Moho, and spanning a wide depth range of ∼25 km. Equilibration temperatures are high for these depths, averaging ∼1300°C. We test for correlations with subduction parameters and find that equilibration pressures correlate with upper plate age, indicating overriding lithosphere thickness plays a role in magma equilibration. We suggest that most, if not all, thermobarometric pressure and temperature conditions reflect magmatic reequilibration at a mechanical boundary, rather than reflecting the conditions of major melt generation. The magma reequilibration conditions are difficult to reconcile, to a first order, with any of the conditions predicted by our dynamic models, with the exception of subduction zones with very young, thin upper plates. For most zones, a mechanism for substantially thinning the overriding plate is required. Most likely thinning is localized below the arc, as kinematic thinning above the wedge corner would lead to a hot fore arc, incompatible with fore‐arc surface heat flow and seismic properties. Localized subarc thermal erosion is consistent with seismic imaging and exhumed arc structures. Furthermore, such thermal erosion can serve as a weakness zone and affect subsequent plate evolution.

Highlights

Subduction zones are the location of the greatest mass and heat fluxes at Earth’s surface and, are critical to understanding a wide array of Earth processes
We explore whether: (a) primitive melt P, T conditions agree with wedge melting conditions in a global sense and (b) if P, T conditions correlate with any physical subduction parameters that may provide insight to whether the subducting slab, the upper plate or mantle wedge’s thermal structure is the dominant control on magma generation and transport
A plot of equilibration pressure versus MgO for all MgO contents yields some sense of lower MgO lavas tending to have lower calculated pressures in some subduction zones (Figure S1)

Summary

Introduction

Subduction zones are the location of the greatest mass and heat fluxes at Earth’s surface and, are critical to understanding a wide array of Earth processes. The mantle wedge is a key part of the subduction system, where magmas are generated and transferred to the arc, likely instigated by volatile release from the downgoing plate [Tatsumi, 1986; Schmidt and Poli, 1998; Grove et al, 2012; Gill, 2012]. Transport and the position of the arc are affected by (Figure 1): (a) subducting plate conditions, (b) mantle wedge conditions, and (c) the upper plate. The relative importance of each of these factors remains debated, with geochemical and geophysical studies providing independent constraints that support various scenarios.

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Discussion

Conclusion