The fast and the faint: Seismic signatures of water and silica in Earth’s mantle from mineral elasticity measurements - Johannes Buchen

Johannes Buchen - Bayerisches Geoinstitut, Bayreuth University

Jeudi 22 février 2018 à 10 h 30

IMPMC - Sorbonne Université - 4 place Jussieu, 75005 Paris, tour 23, 4e étage, couloir 22-23, salle 401

Abstract

Deep cycling of volatile elements and crustal material in Earth’s mantle may affect global geodynamics by altering physical properties of mantle rocks or by inducing melting. Large amounts of hydrogen, for example, can be stored in nominally anhydrous minerals of the transition zone such as wadsleyite. Basaltic oceanic crust is forced into the upper mantle at subduction zones and may continue sinking to greater depths. At conditions of the lower mantle, free silica phases are predicted to contribute with more than 10 vol-% to basaltic lithologies. Geophysical detection of hydrous regions within the transition zone as well as tracing of subducted crust in the lower mantle remain key tasks to understand deep geochemical cycles. Here I present the results of high-pressure experiments on the elastic properties of wadsleyite single crystals and stishovite polycrystals. We used a combination of X-ray diffraction and Brillouin spectroscopy to derive internally consistent descriptions for the high-pressure elasticity of these minerals. In contrast to previous inferences, we found a low sensitivity of sound wave velocities to hydration for iron-bearing wadsleyite and a smooth increase of shear wave velocities across the tetragonal-to-orthorhombic phase transition in polycrystalline stishovite. By complementing our results with literature data, we modeled the elastic properties of mantle rocks and evaluated the seismic signatures of hydration in the shallow transition zone as well as of silica-rich material in the lower mantle. When linked to seismic observations, our findings may contribute to detect reservoirs and to trace pathways of deep geochemical cycles.