25 - Subduction and island-arc magmatism

Summary

Introduction: subduction, associated processes and the crucial role of water

The subduction factory reworks a huge amount of material, similar to that involved in ridge magmatism, and it redistributes trace elements in uniquely fractionated relative abundances between three principal terrestrial reservoirs, the mantle, the continental crust and the oceans.

The bulk present-day mass flux of subducted material is impressive, ∼10¹⁸ g yr⁻¹, of which sedimentary (mainly terrigenous) and basaltic subfluxes contribute 1.3 × 10¹⁵ g yr⁻¹ and 6.7 × 10¹⁶ g yr⁻¹ respectively and the remainder is material of the subcrustal lithospheric mantle (Crisp, 1984; Veizer and Jansen, 1985; Rea and Ruff, 1996; Plank and Langmuir, 1998).

On its way to a convergent boundary, the oceanic plate is cooled and becomes thicker as the lithosphere freezes onto its base. The pre-subducted oceanic plate thus consists of several rock types: sediments highly enriched in certain trace elements, basaltic crust, gabbros and peridotites. The intense water flux through the ocean floor (∼ 10¹⁷ g yr⁻¹) alters all these rocks (particularly the basalts), which then carry specific ocean-related chemical and isotopic signatures, and it also changes the sea-water chemistry via the delivery of mantle-derived elements into the ocean (Fig. 27.20; Staudigel, 2003).

Subduction occurs at the convergent boundaries where plates collide (Fig. 23.2). When approaching the collision zone, the stresses applied to the cold rigid plate cause its faulting and bending and its movement downward, generating a deep trench between the subducting and overriding plates.