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.