April 10th - Nils B. Gies (Postdoctoral Researcher, Institute of Geological Sciences, University of Bern).
Title: Investigating H2O Recycling in Subduction Zones: Combining Observations in Natural Samples with Numerical Modelling
Time: 12:30
Place: Askja, 3rd floor meeting room (room 367) and streamed virtually via Zoom:
List of Friday seminars of Institute of Earth Sciences and Nordic Volcanological Center
Abstract:
How the balance between subduction input and the output of H2O in arcs, oceanic island basalts (OIB) and mid-ocean ridge basalts (MORB) has been maintained over 4 billion years is essential to understand the habitability of Earth.
Constraining the global budget of H2O transported deep into the mantle by subduction zones, we modelled global H2O fluxes in hydrous phases in a gridded, two-dimensional, thermodynamic model that includes intra-slab fluid migration and reaction. These models show that the amount of H2O stored in hydrous phases in subduction zone mantle, as well as the different dehydration patterns, depend strongly on the thermal structure of the subduction zone, and on the intensity and depth of initial slab mantle hydration. During hotter subduction in Precambrian times, H2O transport to the deep mantle through hydrous phases is less likely. Further, we evaluated the role of nominally anhydrous minerals, such as olivine and clinopyroxene, which can incorporate H as a trace element under subduction zone conditions. Clinopyroxenes are of particular interest due to their ability to incorporate several hundred μg/g of H2O, and their stability over a wide pressure and temperature range in metasedimentary, mafic, and ultramafic rocks. Based on a combination of FTIR spectroscopic data on eclogites and literature values, the global subduction transport capacity of NAMs was estimated and H2O fluxes based on NAMs modelled. In ultramafic rocks, olivine dominates the H2O budget, whereas in mafic rocks, clinopyroxene is the most important carrier of H2O. For intermediate and cold subduction zones, the H2O content in NAMs is insignificant. Conversely, in hot subduction zones, the H2O transport capacity of NAMs exceeds that of hydrous phases. Consequently, in the past, when the Earth was hotter and subduction was faster, NAMs dominated the deep water cycle. The modern global H2O ingassing via NAMs is in the range of the global H2O MORB outgassing and might have been the main mechanism of supplying H2O to the deep mantle in Precambrian times.
All are welcome.