Unveiling The Mechanisms Of Mass Transfer In Modern Subduction Zones

Abstract

Subduction channels are crucial pathways for recycling slab mass into the deep mantle. A portion of this material is returned to the mantle wedge, contributing to the remarkable diversity of arc lavas from mafic to rhyolitic forms, unlike in other geological settings. Despite this variation, the lavas found in both continental and oceanic arcs generally display consistent trace element patterns. This uniformity highlights a dominant mass transfer mechanism that defines the signature of global arc volcanism. Aqueous fluids and partial melts from slab sources are key mechanisms for this consistency, but they must avoid mantle interaction to preserve their signature. Buoyant mélange diapirs originating from the slab can prevent this interaction, although their prevalence and formation conditions remain largely unknown. This thesis integrates high-pressure, high-temperature experiments combining thermodynamic, geochemical, and geodynamic modeling. It aims to uncover (a) the mechanisms of mass transfer agents in modern subduction zones, (b) their role in elemental cycling, and (c) their contributions to the diversity and distinct signatures of volcanic arcs.Reactions in the mantle, driven by subduction slab partial melting, have been investigated under subarc depth conditions, revealing a key mechanism for preserving the geochemical signatures of slabs (Chapter 1). The findings reveal that mica-rich, olivine-free pyroxenites form due to the high silica content in the slab's partial melt, reaching a “melt-buffer” state. This allows subsequent slab melts to flow freely through these pyroxenites, preserving their element compositions while minimizing interactions with the surrounding mantle rock. Additionally, being less dense than the mantle, these pyroxenites can create instabilities in the mantle. Previous research only focused on chlorite-rich mélanges and pure sediments to showcase a diverse range of subducted lithologies. This study provides phase equilibria of unexplored serpentinite-rich mélanges (Chapter 2) and shaly-rich mélanges (Chapter 3) under conditions of deep forearc to subarc depths. Covering the full spectrum of ultramafic and sedimentary-rich mélanges. Serpentine-rich mélanges transform into peridotite-like rocks with minor hydrous minerals and coexist with aqueous fluids and basaltic melts, while shaly-rich mélanges transform to olivine-free pyroxenite with abundant hydrous minerals and coexist with dacitic to rhyolitic melts. Key findings reveal that mantle viscosity, slab geotherm, and subduction rates significantly influence diapir growth, regardless of mélange characteristics. Fast, cold subduction limits diapirism and leads to effective volatile sequestration in hydrous minerals, facilitating their transfer into the mantle and progressively releasing aqueous fluids that carry trace element signatures into arc magma sources. Conversely, warm, slowly subducting slabs can promote diapirism in thinner ultramafic or sediment-rich channels. Aqueous fluids dominate in ultramafic channels, while low-degree partial melts prevail in sediment-rich channels. Both agents help transfer distinct trace signatures to magma sources. Diapirism can occur in tectonic slabs with heat sources, such as nearby slab tears or plumes. However, those mélanges lose buoyancy upon reaching thermal equilibrium at temperatures above 850 °C. Smaller diapirs may stagnate near the slab-mantle interface, while larger ones can retain buoyancy and remelt in hotter mantle regions. High degree melting of diapirs explains some arc lava diversity, but it does not account for the consistent arc trace element patterns. Overall, diapirism is contingent on hot slabs, while aqueous fluids and partial melts remain as the dominant agents of mass transfer from the slab to arc magma sources.