Backhouse, Nicholas; (2025) The seismic velocity of hydrous phases in subduction zones: a new lab-based approach in a Paris-Edinburgh press. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Subduction zones are a vital step in Earth’s water cycle, as they represent the only method by which water is re-introduced into the mantle. Mafic minerals in the oceanic lithosphere become serpentinised; then, upon subduction, the serpentine dehydrates and releases water into the overlying mantle wedge, leading to the formation of further hydrous phases in the wedge such as chlorite. This process is fundamental to arc volcanism and seismicity, and thus has been investigated with seismic tomography. However, interpretation of these models requires measurements of the seismic velocity of these hydrous phases at subduction zone conditions, and currently there are very few such measurements of antigorite or chlorite. Currently high pressure and temperature seismic velocity measurements of minerals require a synchrotron X-ray source. Such sources are difficult to access, with awarded beamtime usually accounting for a few days per year, and thus a method of carrying out such experiments without a synchrotron source was developed. This method uses a lab-based X-ray source and camera-scintillator system to image the sample in-situ. The seismic velocity of the alumina buffer rod was used as an in-situ pressure standard. This method can now consistently carry out seismic velocity measurements of polycrystalline materials up to >5 GPa with an uncertainty <1 GPa. The high pressure and temperature seismic velocity of antigorite and chlorite were measured up to ∼5 GPa and 773 K. These measurements were then fit to high-pressure and temperature equations of state, and the resulting models were compared to seismic tomography models of subduction zones. In general, the measurements of antigorite suggest that serpentinisation alone cannot explain elevated Vp /Vs ratios in subduction zones, but the dehydration of the subducting slab is expected to be associated with a large increase in both Vp and Vs. Measurements of chlorite suggest that Vs anisotropy is larger than Vp, and that preferential alignment of chlorite could be a contributing factor to mantle wedge anisotropy. Most strikingly, Vp /Vs ratio of chlorite is extremely high, and therefore a small proportion of chlorite in mantle peridotite can explain elevated Vp /Vs in subduction zones.

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