Ultra‐Low Velocity Zone Beneath the Atlantic Near St. Helena

Abstract

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 modeled elsewhere. Synthetic modeling 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, centered 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. Plain Language Summary Nearly 3,000 km beneath the Atlantic to the West of the island of St. Helena, on the boundary between Earth's metal core and rocky mantle, we have discovered a new area where seismic waves diffracting along that boundary travel significantly slower than expected. This area is called an ultra‐low velocity zone. In this study, we use seismic waves which propagate along the core‐mantle boundary. The waves that interact with the ultra‐low velocity zone are scattered and become severely delayed. Using the observations, we have constrained the ultra‐low velocity zone to a broadly cylindrical structure, 600 km across, 20 km high and centered at 15° South, 15° West. The material inside is reduced by 30% in seismic shear wave velocity compared to outside. We confirmed this model by computing and comparing synthetic waveforms for a range of different ultra‐low velocity zone models. This ULVZ location is right beside or just inside a much larger region of low velocity, dubbed the African Large Low Shear‐Velocity Province (LLSVP). The observed ultra‐low velocity zone could be the base of the upwelling or mantle plume rising through the mantle and causing the hotspots of St. Helena and/or Ascension at the surface. Key Points Observation of significant S diff postcursors sampling the CMB beneath the Atlantic Modeling of postcursors reveals a previously unknown mega‐ULVZ situated on the CMB to the West of St. Helena Further measurements of St. Helena and Ascension samples are needed to identify a potential ULVZ‐associated geochemical signature