Shallow vs. Deep subduction in Earth history: Contrasting regimes of water recycling into the mantle

Abstract

Water storage capacity of deep Earth mantle minerals allows storing several ocean masses. However, the amount of water delivered to the mantle through geological history remains uncertain because this depends on tectonic style. It also remains unclear how the water storage capacity of oceanic plates and the mode of delivery changed through time and how these variations controlled mantle water content and distribution. Here, we use 2D numerical modeling of subduction styles at different mantle potential temperatures (T_p) corresponding to different stages of the Earth’s evolution to show two principal regimes of subduction – shallow flat subduction in the early Earth vs. deep and steep subduction in the modern Earth − that provide contrasting regimes of water recycling into the mantle. Deep and steep subduction occurs at lower mantle potential temperatures (ΔT=0-∼100 °C, T_p compared to today). During the modern regime, abundant water is transported to the mantle transition zone, where some is released from subducted slabs and retained in nominally anhydrous minerals (NAMs, wadsleyite and ringwoodite). Shallow, low angle subduction is obtained in the experiments corresponding to the hotter Precambrian mantle (ΔT=∼150–275 °C, ∼1.5–3.0 Ga), which recycled most water into the shallow (<120 km) mantle. This regime provided cool conditions along the flattened subduction interface with limited slab melting of the mantle wedge. Shallow, flat subduction caused strong hydration of the overriding lithosphere, with water predominantly stored in hydrous minerals (e.g. serpentine, chlorite). Due to the breakdown of hydrous minerals, most of this water was easily returned to the surface and/or was stored in the continental crust. We conclude that if the secular cooling of the Earth’s mantle was accompanied by changes in the subduction style from shallow to deep, then it decreased the total volume of oceanic water recycled by subduction but increased the depth of recycling and the average water content in the Mantle Transition Zone. These conclusions pertain to episodes of plate tectonics in Earth history; other modes of water delivery must be considered for single lid tectonic episodes.