Spatial variation of subduction zone fluids during progressive subduction: Insights from Serpentinite Mud Volcanoes

Catriona D Menzies,Roy E Price,Jeffrey Ryan,Olivier Sissmann,Ken Takai,C Geoffrey Wheat

doi:10.1016/j.gca.2021.10.030

Abstract

Geological processes at subduction zones control seismicity, plutonism and volcanism, and geochemical cycling between the oceans, crust, and mantle. The down-going plate experiences metamorphism, and the associated dehydration and fluid flow alters the physical properties of the plate interface and mantle wedge, as well as controlling the composition of material descending into the mantle. Any direct study of slab evolution during subduction is inhibited by the prohibitive depths at which these processes occur. To examine these processes we use serpentinite mud volcanoes in the Mariana forearc, that permit sampling of serpentinite materials and their pore waters that ascend from the subduction channel. We present new pore water chemical data from the summit and flanks of three serpentinite mud volcanoes that were drilled during International Ocean Discovery Program Expedition 366 which are reflective of reactions within the crust and mantle during the early, shallow (<20 km) stages of subduction. We show, via thermodynamic modelling, that our new data on the evolution of pore water chemical compositions reflect mineralogical characteristics of a predominately basaltic source from the downgoing Pacific Plate. However, a component from sedimentary sources is likely, especially for those mud volcanoes near the trench. Other potential slab-derived constituents, such as lithospheric serpentinite, carbonate-rich sediments, or seamount basalts with an intraplate geochemical character, are not required to explain our results. Our results indicate that with progressive subduction the lawsonite-epidote mineral transformation boundary at ∼250 °C may help drive slab carbonate destabilisation, despite its apparent thermodynamic stability at such temperatures and projected pressures (∼300 °C and ∼0.6 GPa). New dissolved gas data also point to primary thermodynamic controls over methane/ethane production within the subduction channel as depths-to-slab increase. Our findings provide direct evidence for the progressive mineralogical and chemical evolution of a subducting oceanic plate, which liberates a progressively evolving fluid phase into the subduction channel.

Highlights

Subduction of oceanic crust is one of the primary processes that shape the seafloor, return elements to the mantle (Bebout, 2007), and trigger some of the largest earthquakes on Earth (Moore and Saffer, 2001; Henstock et al, 2006)
Across- and along-strike extension and vertical tectonic deformation produce faults, some of which penetrate the overriding Philippine Plate into the subduction channel below (Fryer and Salisbury, 2006; Fryer et al, 2020). Conduits form where such faults intersect, forming a pathway for fluids, clasts of rock from the subduction channel and mantle wedge, and a serpentinite matrix to be transported from the subduction channel to the seafloor
Data from Asut Tesoru provide a closer match to data from previous drilling expeditions on South Chamorro and Conical Seamounts (Mottl, 1992; Mottl et al, 2003; Wheat et al, 2020), both to the west of the transect of seamounts drilled during International Ocean Discovery Program (IODP) Exp. 366 and further from the trench

Summary

Introduction

Subduction of oceanic crust is one of the primary processes that shape the seafloor, return elements to the mantle (Bebout, 2007), and trigger some of the largest earthquakes on Earth (Moore and Saffer, 2001; Henstock et al, 2006). Between the trench and arc there is generally a void of physical samples to assess the state of water-rocksediment reactions, thermal and pressure conditions, and physical properties of materials within the subduction channel Our understanding of this transition zone at a nonaccretionary convergent margin is possible through studies of the Mariana forearc (Fryer, 1996). Across- and along-strike extension and vertical tectonic deformation produce faults, some of which penetrate the overriding Philippine Plate into the subduction channel below (Fryer and Salisbury, 2006; Fryer et al, 2020) Conduits form where such faults intersect, forming a pathway for fluids, clasts of rock from the subduction channel and mantle wedge, and a serpentinite matrix to be transported from the subduction channel to the seafloor. These serpentinite mud volcanoes provide a window into the chemical, thermal and physical conditions within the subduction channel during the initial to intermediate stages (10–20 km depth) of subduction (Fryer et al, 1999)

Results

Discussion

Conclusion