Seismic Signature of a Hydrous Mantle Transition Zone

Abstract:

The quantity and distribution of water inside the deep Earth has major consequences for tectonic and geodynamic processes, yet remains essentially unconstrained. Laboratory experiments on nominally anhydrous minerals (NAMs) of the mantle transition zone have indicated that these minerals in particular are capable of storing significant amounts of water. We pose the question of whether seismology can be used to distinguish a hydrous from an anhydrous transition zone. We perform an extensive literature search of the experimentally-determined elastic properties and phase equilibria of hydrous NAMs, and use thermodynamic modelling to predict the seismic properties (Vp, Vs and density) of these minerals at transition zone pressures and temperatures. We compare these models with their anhydrous counterparts, taking into account the (often large) uncertainties in the mineralogical data. We find that much experimental work remains to be done for completely defining the elastic parameters and phase equilibria of the hydrous minerals, and large uncertainties produce a wide range of possible seismic structures. This uncertainty makes it difficult to use metrics such as the depths of the 410 and 660 discontinuities for mapping transition zone water content. At the same time, average P and S wave velocities inside the transition zone are not helpful, since there is a near-perfect trade-off between changes in water content, changes in temperature and changes in iron content. Average velocity gradients, density and density gradients appear to be both sensitive to, and diagnostic of, the presence of water. However these parameters are difficult to resolve seismically. Potentially the impedance contrast at 410 may also be useful, depending on the partitioning of water between olivine and wadsleyite. From a seismological perspective, it would be most helpful for future experimental studies to focus on better constraining the phase boundaries in hydrous (Mg,Fe)2SiO4, since these control the depths of the 410 and 660 discontinuities, which themselves are readily imaged by a number of seismic probes. Phase equilibria at the high temperatures appropriate to mantle transition zone conditions (c. 1700-2000 K) would be especially useful.