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Noble gases recycled into the mantle through cold subduction zones

Author:
Andrew J. Smye  Colin R.M. Jackson  Matthias Konrad-Schmolke  Marc A. Hesse  Steve W. Parman  David L. Shuster  Chris J. Ballentine  


Journal:
Earth and Planetary Science Letters


Issue Date:
2017


Abstract(summary):

Abstract Subduction of hydrous and carbonated oceanic lithosphere replenishes the mantle volatile inventory. Substantial uncertainties exist on the magnitudes of the recycled volatile fluxes and it is unclear whether Earth surface reservoirs are undergoing net-loss or net-gain of H 2 O and CO 2 . Here, we use noble gases as tracers for deep volatile cycling. Specifically, we construct and apply a kinetic model to estimate the effect of subduction zone metamorphism on the elemental composition of noble gases in amphibole – a common constituent of altered oceanic crust. We show that progressive dehydration of the slab leads to the extraction of noble gases, linking noble gas recycling to H 2 O. Noble gases are strongly fractionated within hot subduction zones, whereas minimal fractionation occurs along colder subduction geotherms. In the context of our modelling, this implies that the mantle heavy noble gas inventory is dominated by the injection of noble gases through cold subduction zones. For cold subduction zones, we estimate a present-day bulk recycling efficiency, past the depth of amphibole breakdown, of 5–35% and 60–80% for 36 Ar and H 2 O bound within oceanic crust, respectively. Given that hotter subduction dominates over geologic history, this result highlights the importance of cooler subduction zones in regassing the mantle and in affecting the modern volatile budget of Earth's interior. Highlights • We model the transport of noble gases during subduction of oceanic crust. • Fractionation of noble gases is controlled by the availability of free fluid. • Hot subduction zones fractionate slab-bound noble gases. • Mantle heavy noble gas signature is consistent with recent increase in recycling efficiency of noble gases and water.


Page:
65-65


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