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

Abstract

Subduction 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 13008C. 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.

Magmatic Response to Subduction Initiation: Part 1. Fore‐arc Basalts of the Izu‐Bonin Arc From IODP Expedition 352

“…The high melt fractions derived from shallow depths in previously depleted mantle suggest an abnormally hot mantle source region beneath the spreading axis. This is supported by thermobarometry calculations on the least evolved FAB samples, which document relatively high temperatures (~1,400 °C) at low pressures (1.4–1.6 GPa or 46–53‐km depth) relative to most MORB and to primitive arc lavas (e.g., Perrin et al, ).…”

“…We explored Fe 3+ /Fe‐total ratios of 0.10 and 0.15 and varied the Mg#s of the assumed residual mantle from 90 to 91. Regardless of chosen conditions, equilibration temperatures are slightly high (~1400 °C) and pressures low (1.4–1.6 GPa or 46–53 km depth) relative to most MORB and to primitive arc lavas (e.g., Perrin et al, ). There is negligible difference in results between the anhydrous and 0.5 wt% H 2 O cases.…”

“…The high melt fractions derived from shallow depths in previously depleted mantle suggest an abnormally hot mantle source region beneath the spreading axis. This is supported by thermobarometry calculations on the least evolved FAB samples, which document relatively high temperatures (~1,400°C) at low pressures (1.4-1.6 GPa or 46-53-km depth) relative to most MORB and to primitive arc lavas (e.g.,Perrin et al, 2016).The Manus plume may have played a role in this process by inducing the upflow of deeper, hotter mantle along the West Philippine Ridge, driving high degrees of partial melting of depleted Indian lithosphere or plume-residue mantle during the early stages of subduction(Ishizuka et al, 2013;Macpherson & Hall, 10.1029/2018GC007731…”

The global systematics of primitive arc melts

“…Perrin et al . [], based on the Lee et al . [] geobarometer, underlined that many of the last equilibration conditions of primitive arc melts lie within the subarc lithosphere.…”

“…In general, the andesites yield unrealistically low pressures, half of their calculated last mantle equilibration points would lie in the overriding crust. Perrin et al [2016], based on the Lee et al [2009] geobarometer, underlined that many of the last equilibration conditions of primitive arc melts lie within the subarc lithosphere. If the last equilibration pressures of primitive arc basalts represent the hottest nose in the subarc mantle wedge, this would require significant subarc lithosphere erosion as already predicted by Toks€ oz and Hsui [1978].…”