Chapter 21 - Roles of Serpentinization in Plate Tectonics and the Evolution of Earth's Mantle

Abstract

Here we review the roles of serpentinization in Plate Tectonics and the evolution of Earth's mantle, water, carbon, and life. We see that Harry Hess was likely right in his vision that chemical reactions between Earth's mantle and oceans play a large role in Earth evolution; the serpentinization of oceanic lithospheric mantle has had a major influence on Earth's chemical, mechanical, and biological evolution, including its deep water and carbon cycles. It now appears that serpentinization occurs less during plate extension at mid-ocean ridges and deformation along transform faults and their fracture zone continuations than it does during the plate's bending just prior to subduction at a trench. Bend-faulting during plate subduction allows seawater to penetrate tens of km into cool lithospheric mantle. It partially serpentinizes the mantle along these faults as the plate bends. Subsequent deserpentinization and eclogitization of the overlying subducted crust causes the plate to unbend, with the fluid release from these dehydration reactions driving magmatism in the overlying volcanic arc. These volume changes during serpentinization and deserpentinization are what allow the slab to maintain a strong stress-guide as it bends, then unbends to sink into the deep mantle at a subduction zone. Although we remain confident that this basic scenario will be proven correct, we are surprised at how difficult it has been to better constrain the magnitude of this critical Earth process. Improved observations could greatly improve our level of understanding, for example, conductivity measurements before and after a large bend-fault earthquake may be able to determine how long and how deep a bend-fault stays open after the initial ∼1% fault zone porosity increase associated with typical earthquake slip. Serpentine's roles in aiding plate bending and unbending during subduction seem particularly ripe for exploration during the next decade.