Abstract
Age-progressive volcanism is generally accepted as the surface expression of deep-rooted mantle plumes, which are enigmatically linked with the African and Pacific large low-shear velocity provinces (LLSVPs). We present geochemical and geochronological data collected from the oldest portions of the age-progressive enriched mantle one (EMI)-type Tristan-Gough track. They are part of a 30- to 40-million year younger age-progressive hotspot track with St. Helena HIMU (high time-integrated 238U/204Pb) composition, which is also observed at the EMI-type Shona hotspot track in the southernmost Atlantic. Whereas the primary EMI-type hotspots overlie the margin of the African LLSVP, the HIMU-type hotspots are located above a central portion of the African LLSVP, reflecting a large-scale geochemical zonation. We propose that extraction of large volumes of EMI-type mantle from the margin of the LLSVP by primary plume heads triggered upwelling of HIMU material from a more internal domain of the LLSVP, forming secondary plumes.
Highlights
The geochemical variability of oceanic lavas primarily reflects the heterogeneity of the Earth’s mantle, which requires at least four end members to explain the observed compositional range in Sr-Nd-PbHf isotope ratios, known as depleted mantle (DM), enriched mantle one and two (EMI and EMII), and high = 238U/204Pb [HIMU; [1]]
The groundmass 40Ar/39Ar weighted mean ages (WMAs) of the four new Walvis Ridge seamount samples range from 65.50 ± 0.77 million year (Ma) to 73.4 ± 1.2 Ma and lie within the age range of previously published Walvis HIMU volcanism
Our geochemical and geochronological data from the Agate Mountain complex are consistent with it forming a continental extension of the submarine HIMU volcanism of the Walvis Ridge onto the Namibian coast
Summary
Whereas other lower mantle low-velocity anomalies are dome-like and appear to ascend only to depths of ~1000 km, such as beneath the Tristan-Gough hotspot [4] These dome-like lower-mantle structures, smaller conduit-like structures can be distinguished at shallower depths, such as beneath Tristan da Cunha to depths of ~500 km [5]. These low-velocity seismic anomalies are believed to reflect active upwellings derived from the base of the lower mantle.
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