South Atlantic DUPAL anomaly - Dynamic and compositional evidence against a recent shallow origin

摘要

The DUPAL anomaly in the South Atlantic is present in intra-plate volcanism associated with the Tristan-Gough, Discovery and Shona plumes, and along adjacent sections of the southern Mid-Atlantic Ridge, and has been variably attributed to either shallow continental or deep recycled sources. Based on geodynamic considerations, we identify the Namaqua-Natal Belt of southern Africa as the most likely origin of any subcontinental lithospheric mantle (SCLM) or lower continental crust (LCC) recently introduced into the source of DUPAL oceanic basalts by processes of plume erosion of the SCLM, general erosion of the SCLM by the mantle flow field, or tectonic detachment of SCLM and LCC related to continental breakup. Sr and Pb isotopic compositions of clinopyroxenes of SCLM xenoliths from the Proterozoic Namaqua-Natal Belt (Uintjiesberg, Markt, Hebron) are measured by laser MC-ICPMS. The inferred isotopic composition of the SCLM spans a large range with ~(206)Pb/~(204)Pb ~18.2 to 19.4, ~(208)Pb/~(204)Pb ~37.5 to 39.2 and ~(87)Sr/~(86)Sr ~0.704 to 0.716. In contrast, lower crustal samples from the Proterozoic Namaqua-Natal Belt (Uintjiesberg, Markt, Hebron, Cordaatskuil, and Lovedale) are unradiogenic in Pb isotopic composition (whole rock data) with ~(206)Pb/~(204)Pb ~14.2 to 18.5, ~(208)Pb/~(204)Pb ~34.5 to 37.8 and ~(87)Sr/~(86)Sr ~0.703 to 0.715. Sr-Nd-Pb isotopic compositions of samples from Vema and the 7°E seamounts constrain the upper mantle composition away from lower mantle plumes. None of the measured SCLM, LCC and seamount samples show DUPAL signatures.The new results, in combination with literature data, provide no evidence for an origin of the DUPAL signature from recent introduction of continental material as a shallow component in the source of DUPAL oceanic basalts. Rather, the distribution of a more diluted DUPAL signature in plume-influenced MORB along sections of the southern Mid-Atlantic Ridge spatially associated with intra-plate volcanism carrying the DUPAL signature supports an origin related to the activity of the Tristan-Gough, Discovery and Shona mantle plumes. The common geochemical signatures of the closely spaced Tristan-Gough, Discovery and, to a lesser extent, Shona plumes support a common plume source formation history.