Deep water recycling through time

Abstract:

The water cycle through degassing and deep recycling via subduction zones is of primary importance for our understanding of the long-term exchange of water between the earth’s interior and the exosphere. Moreover, the transport of water bound in hydrous minerals at great depths have a major impact on the Earth’s volatile budget, on the chemical evolution of the Earth, and on the deep mantle composition and rheology.

Here, we use a numerical tool that combines thermo-mechanical models with a thermodynamic database to examine slab dehydration for present-day and early Earth settings and its consequences for the deep water recycling. We parameterize the amount of water that can be carried deep into the mantle, W (x10⁵ kg/m²), as a function of subduction velocity v_s (cm/yr), slab age a (Myrs), and mantle potential temperature T_m (°C):

W = 1.06v_s + 0.14a - 0.023 T_m + 17

We generally observe that a 1) 100°C increase in the mantle temperature, or 2) ~15 Myr decrease of plate age, or 3) decrease in subduction velocity of ~2 cm/yr all have the same effect on the amount of water retained in the slab at depth, corresponding to a decrease of ~2.2x10⁵ kg/m² of H₂O. We estimate that for present-day conditions ~26% of the global influx water, or 7x10⁸ Tg/Myr of H₂O, is recycled into the mantle. Using a realistic distribution of subduction parameters, we illustrate that deep water recycling might still be possible in early Earth conditions, although its efficiency would generally decrease. Indeed, 0.5-3.7x10⁸ Tg/Myr of H₂O could still be recycled in the mantle at 2.8 Ga.