Deep water recycling through time

Tuesday, 16 December 2014
Valentina Magni, University of Durham, Durham, DH1, United Kingdom, Pierre Bouilhol, Durham University, Durham, United Kingdom and Jeroen Van Hunen, University of Durham, Durham, United Kingdom
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 vs (cm/yr), slab age a (Myrs), and mantle potential temperature Tm (°C):

W = 1.06vs + 0.14a - 0.023 Tm + 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.