Abstract

The rate and style of chemical interchange between the Earth’s upper and lower mantle is still poorly understood, yet knowing this is essential to understanding how the planet’s composition has evolved since its formation. One fundamental question concerns whether tectonic plates can transport significant volumes of volatile compounds, water in particular, to great depths. If so, this provides a way to recycle this material back into the Earth.

To address this question, I examine the seismic signals from earthquakes at depths of over 400 km, whose origin is itself highly uncertain. Using the the method of ‘source-side shear wave splitting’, I examine the anisotropy in the region of these events, looking at subduction zones around the Pacific. I find that in these regions at least, the transition zone (410 to 660 km depth) shows seismic anisotropy that is as strong as the upper or lowermost mantle. By testing a range of possible causative mechanisms, I show that the most likely is the alignment of dense hydrous magnesium silicate crystals, known sometimes as the ‘alphabet phases’ because they are called ‘A’, ‘B’, ‘D’, and so on. This implies water is carried at least to 660 km depth by subducting slabs. My results also suggest that the earthquakes may not be caused by the transition of metastable olivine in the slab interior.