Constraining the kinetics of mantle mineralogical phase transformation using the Earth's response to surface mass redistribution

Abstract:

Over the past decades, geodetic observations have provided crucial constraints on the Earth's rheology on time scales of a decade or less. We focus on determining mantle rheological properties by investigating the Earth's response to surface mass redistribution over seasonal to millennial time scales, using new geodetic observables. In particular, we investigate variations of mechanical properties within the mantle transition zone caused by potential mineralogical phase transformations induced by surface loading at various frequencies.

The response of mineralogical transformations involving the olivine or pyroxenes and their polymorphs to pressure variations induced by surface loading is unknown. However, in regions with a two-phase equilibrium, where high and low-pressure phases coexist, the induced pressure variations at depth may disrupt the equilibrium, inducing re-equilibration processes which are expected to modify the phase and amplitude of surface deformation induced by surface loading.

We set a theoretical frame to model phase transformations linked to grain-scale processes within the mantle transition zone undergoing sinusoidal pressure variations induced by surface loading. We describe the bulk and shear moduli as frequency dependent complex numbers, varying between a seismic value at high frequencies and a relaxed one at low frequencies. We predict surface deformation induced by surface mass redistribution at various frequencies for models including mineralogical phase transformations and compare results to purely (visco-)elastic Earth models.

We discuss constraints on the kinetics of mantle phase transformations using space geodetic measurements. We compute geodetic strain induced by seasonal hydrological, oceanic and atmospheric surface loading, derived from the Gravity and Recovery Climate experiment (GRACE) for various rheologies and compare models results to seasonal deformation recorded by a set of 700 globally distributed GNSS sites. We show that the observed displacements at the Earth's surface are close to the ones predicted for an elastic Earth, suggesting that mantle phase transformation kinetics are longer than a year on global average. Finally, we discuss the potential importance of mantle phase transformations in Glacial Isostatic Adjustment studies.