X Marks the Spot: Seismic Signals of Silica and Hidden Hawaiian Heterogeneities

Abstract:

The Hawaiian Island chain in the middle of the Pacific Ocean is a well-studied example of hotspot volcanism caused by an underlying upwelling mantle plume. However, the thermal and compositional nature of the plume is still uncertain. The depth and amplitude of seismic discontinuities can show how the plume effects phase transitions in mantle minerals, providing insights into the plume's thermo-chemical properties. This study utilises >5000 high quality receiver functions from Hawaiian island stations to detect P-to-s converted phases. These receiver functions are stacked in a variety of ways in order to image seismic discontinuities between 200 to 800 km depth.

In the mantle transition zone, we find that to the southwest of the Big Island the 660 discontinuity is split. This is inferred to represent the position of the hot plume at depth, with the upper discontinuity caused by an olivine phase transition and the lower by a garnet phase transition.

In the upper mantle, the so-called X-discontinuity, which has an enigmatic origin, is found across the region at depths varying between 290 to 350 km. To the east of the Big Island the X-discontinuity lies around 336 km and is particularly strong in amplitude, to such an extent that the discontinuity around 410 km disappears. Synthetic modelling reveals that such observations can be explained by a silica phase transition from coesite to stishovite. This suggests there is widespread ponding of silica-saturated material (such as eclogite, which is silica-rich relative to pyrolite) spreading out from the plume to the east, a hypothesis which is consistent with dynamical models. We suggest that this seemingly thermochemical plume could be sampling recycled basalt, now in the form of eclogite, from lower in the mantle.

Therefore these results support the presence of a significant garnet and eclogite component within the Hawaiian mantle plume. We will briefly highlight further work comparing Hawaii with other hotspot locations around the world to consider whether this is also occurring in other plumes and what heterogeneous plumes may imply about the recycling of material in the mantle.