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Plume source regions in the South Atlantic – spatial and temporal variability and implications for the LLSVP source region

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Class, C. , Le Roex, A. , O'Connor, J. and Jokat, W. (2012): Plume source regions in the South Atlantic – spatial and temporal variability and implications for the LLSVP source region , AGU Fall Meeting 2012, San Francisco, CA, USA., 3 December 2012 - 7 December 2012 .
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Abstract:

To the extent that a lower mantle origin is accepted for individual mantle plumes, they are our only means of investigating the chemical variability of lower mantle regions in space and time. Ultimately such mapping of the lower mantle should provide important constraints on the geological processes that led to the formation of these plume source regions. It is generally accepted that mantle plume sources contain differentiated recycled material from the surface of the Earth, but uncertainties remain as to the nature, composition and age of these recycled components. In addition, in the southern hemisphere plumes preferentially rise from the edges of ‘large low shear velocity provinces’ (LLSVP) (Thorne et al., 2004). It remains to be shown whether LLSVPs contribute material to rising mantle plumes and what their geochemical composition might be. The South Atlantic with four closely spaced mantle plumes in the vicinity to the African LLSVP could provide insights into these questions. Criteria in support of a lower mantle origin of these plumes are (1) presence of a flood basalt province (Tristan-Gough), (2) longevity of age-progressive volcanism (Tristan-Gough 130Ma, Shona 80 Ma, Discovery 40 Ma), as well as (3) enrichment of primordial 3He relative to MORB mantle (Discovery, Shona, Bouvet). The South Atlantic plumes are aligned and produce volcanism synchronously, consistent with their origin at the western edge of the African LLSVP. Geochemically the South Atlantic plumes are heterogeneous, spanning compositions in isotope space from EMI to Stracke’s FOZO (Stracke et al., 2005) and the extreme DUPAL signature with high delta 74 and delta 84 as represented by Gough Island. The extreme DUPAL is found in 3 of the 4 plume systems, indicating a common mantle source. The extreme DUPAL contributed to the Tristan-Gough plume system since 70 Ma and represents the southern component of the laterally zoned plume conduit (Rhode, personal comm. 2012). The Discovery plume is laterally zoned since 40 Ma and the extreme DUPAL signature represents the northern component. In the Shona plume system, the extreme DUPAL component is found in the seamounts east of the Meteor Rise (31-43Ma) as well as the Agulhas Ridge (70-80 Ma). The EMI plume component is found on the Walvis Ridge (80Ma) and the Discovery seamounts (40Ma) and spreading ridge anomaly (0Ma) and shows a high variability in 207Pb/204Pb indicating an old age for this component. Plume dynamics suggest far-reaching sampling of lower mantle sources by plumes (Farnetani et al., 2012), however, the dynamics of the closely space South Atlantic plumes rising from a dense LLSVP remains to be investigated. The distribution of geochemical components along the South Atlantic plume tracks suggests that the DUPAL and EMI signatures are restricted to the African LLSVP and, moreover, a direct contribution of Pb from the LLSVP is indicated by Pb components formed during early Earth.

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