Slab dehydration and deep water recycling through time


Valentina Magni

Centre for Earth Evolution and Dynamics

The transport of water to large depths via subduction zones has a major impact on the Earth’s volatile budget, on the chemical evolution of the Earth, and on the deep mantle composition and rheology. The amount of water that is retained in slabs and carried deep into the mantle is difficult to constrain for the present-day, and, arguably, even more so for early Earth conditions. 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 recyclcing throughout time. Our results show that faster slabs dehydrate over a wide area: they start dehydrating shallower and they carry water deeper into the mantle. Morover, we parameterize the amount of water that can be carried deep into the mantle as a function of subduction velocity, slab age, and mantle temperature. 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, 0.5–3.7 3 108 Tg/Myr of H2O could still be recycled in the mantle at 2.8 Ga.

Published Aug. 3, 2016 4:34 PM - Last modified Aug. 9, 2016 1:30 PM