Seismic signature of water in the mantle transition zone

Abstract

Though water has a major influence on tectonic and other geodynamic processes, little is known about its quantity and distribution within the deep Earth. In the last few decades, laboratory experiments on nominally anhydrous minerals (NAMs) of the transition zone have shown that these minerals can contain significant amounts of water, up to 3.3 wt%. In this study, we investigate if it is possible to use seismic observations to distinguish between a hydrous and anhydrous transition zone, by studying the relationship between mineral experimental data and seismic structures of the transition zone. We perform an extensive literature search, to generate a compilation of the water storage capacities, elastic parameters and phase boundary data for potentially hydrous minerals in the transition zone, and use thermodynamic modelling to compute synthetic seismic profiles of density, V P and V S at transition zone temperatures and pressures. We find that large uncertainties on the mineral phase equilibria (ca. 2 GPa) and elastic properties produce a wide range of seismic profiles. In particular, there is a lack of data at temperatures expected at transition zone pressures. Comparing our hydrous transition zone models with equivalent profiles at anhydrous conditions, we see that the depths of the 410 and 660 discontinuities cannot at present be used to map the water content of the transition zone due to these uncertainties. A better constraint may be given by the average velocities and in particular the average velocity gradients of the mantle transition zone, which decrease and increase, respectively, with increasing water content. We also find that, given our inability to constrain the depths of the 410 and 660 discontinuities at high temperatures, it is not possible to distinguish thermal from water effects, because in general their other seismic properties trade off with each other perfectly. This implies that the conventional view of a slow and thick transition zone for water and slow and thin transition zone for high temperature may not be correct. We suggest that further experiments in mineral physics should primarily focus on better constraining the phase transitions between hydrous olivine and its high-pressure polymorphs. Additionally, the uncertainties on the elastic mineral properties could be reduced significantly if data on the correlations between bulk and shear moduli and their corresponding pressure derivatives would be published.