Ultra-low velocity zone beneath the Atlantic near St. Helena
Abstract
There are various hotspots in the Atlantic ocean, which are underlain
by mantle plumes that likely cross the mantle and originate at the
core-mantle boundary. We use teleseismic core-diffracted shear waves
to look for an Ultra-Low Velocity Zone (ULVZ) at the potential base of
central Atlantic mantle plumes. Our data set shows delayed postcursory
phases after the core-diffracted shear waves. The observed patterns are
consistent in frequency dependence, delay time, and scatter pattern
with those caused by mega-ULVZs previously modelled elsewhere.
Synthetic modelling of a cylindrical structure on the core-mantle
boundary below St. Helena provides a good fit to the data. The
preferred model is 600 km across and 20 km high, centred at
approximately 15° South, 15° West, and with a 30% S-wave
velocity reduction. Significant uncertainties and trade-offs do remain
to these parameters, but a large ULVZ is needed to explain the data. The
location is west of St. Helena and south of Ascension. Helium and neon
isotopic systematics observed in samples from this region could point
to a less-outgassed mantle component mixed in with the dominant
signature of recycled material. These observations could be explained
by a contribution from the Large Low Shear Velocity Province (LLSVP).
Tungsten isotopic measurements would be needed to understand whether a
contribution from the mega-ULVZ is also required at St. Helena or
Ascension.