Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1

Details Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1 Melt-generation processes associated with the Tristan mantle plume: Constraints on the origin of EM-1

Sep 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005e%26psl.237..744g&link_type=abstract

Earth and Planetary Science Letters, Volume 237, Issue 3-4, p. 744-767.

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Scientific paper

The enriched mantle 1 (EM-1) component in ocean island basalts (OIB, e.g., Kerguelen, Pitcairn and Walvis Ridge) has been attributed to melting in upwelling mantle plumes of either: (i) shallow-recycled delaminated subcontinental-lithospheric-mantle or crust; or (ii) deep-recycled metasomatised lithosphere, oceanic plateau or oceanic crust plus a few percent of pelagic sediment. We present new geochemical data for OIB samples from the central South Atlantic; these include 100 to 30 Ma alkali and tholeiitic basalts from the Walvis Ridge and Rio Grande Rise and < 3 Ma basanites and basalts from Tristan da Cunha, Inaccessible and Gough. Additionally, we have analysed Cretaceous mafic potassic magmas from south-west Africa and eastern South America in order to establish the compositional variation of metasomatised lithospheric mantle that may have been delaminated during Gondwana break-up. The results of our rare-earth-element inversion and Sr-, Nd- and Pb-isotopic mixing models suggest that the ‘depleted’ mantle plume component resembles FOZO and that the composition of the ‘enriched’ mantle component in central South Atlantic OIB has varied both spatially and temporally. At least three different enriched mantle end-members are required to explain the compositional range of 100 to 30 Ma OIB magmas. These resemble the source regions of mafic potassic magmas from: (i) the Congo craton and Damara belt of south-west Africa; (ii) the São Francisco craton and Brasilia belt of south-east Brazil; and (iii) the Rio Apa Luis Alves craton of southern Brazil and Paraguay. The most isotopically enriched EM-1 basalts (ɛNd = - 0.8 to - 4.5), generated on the Walvis Ridge and Rio Grande Rise between 89 and 78 Ma, appear to contain a 10% to 15% contribution from a melt source region with low ɛNd, 206Pb / 204Pb and high [La / Nb]n, similar in composition to metasomatised subcratonic lithospheric mantle beneath southern Brazil and Paraguay. Reconstructions of plate motions indicate that, at the time of continental break-up, this would have been located above the sites of subsequent EM-1 melt generation. Shallow-mantle recycling of metasomatised lithosphere delaminated from south-west Africa and south-east Brazil may explain the further variations in trace-element and isotopic ratios of 100 and 80 to 30 Ma basalts from the Walvis Ridge, respectively. Recent magmas from Tristan, Inaccessible and Gough contain a melt contribution from an enriched mantle source with high 206Pb / 204Pb, 208Pb / 204Pb and 87Sr / 86Sr ratios and low ɛNd, relative to bulk-Earth. This does not resemble the isotopic composition of mafic potassic magmas from continents adjacent to the South Atlantic and we therefore propose that it is a deep-recycled mantle component. The high Pb-isotopic ratios of recent Tristan plume magmas are inconsistent with a melt contribution from recycled oceanic crust plus a few percent pelagic sediments. We suggest that the deep mantle component is recycled metasomatised lithospheric mantle. Recycled metasomatised lithospheric mantle, of variable composition, only appears to have been an intrinsic part of the sub-oceanic Tristan mantle plume and the bulk-rock composition of present-day Tristan plume related melts are therefore inappropriate for modelling the contribution of plume-derived melts to the Paraná Etendeka continental flood-basalts. This relationship may also apply to other large igneous provinces and ocean islands along their associated hot spot tracks.

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