Shallow origin for South Atlantic Dupal Anomaly from lower continental crust: Geochemical evidence from the Mid-Atlantic Ridge at 26°S

摘要

We measured trace element concentrations and Pb isotope compositions of fresh volcanic glass samples from the Mid-Atlantic Ridge at 26°S, and from nearby off-axis seamounts. The samples have previously been studied for major elements and Sr-Nd-He isotopes. All samples are depleted MORB, and include some of the most incompatible trace element depleted lavas yet reported from the Atlantic. The seamount lavas are more depleted in highly incompatible elements than the axial lavas, but have high Sr, Pb and Eu concentrations, relative to REE of similar incompatibility. The lavas with the highest Sr/Nd, Pb/Ce and Eu/Eu* have the highest ~3He/~4He (up to 11.0 R/RA) ratios and the lowest incompatible trace element concentrations. They also have the highest ~(87)Sr/~(86)Sr (up to 0.7036) and ~(208)Pb/~(204)Pb for a given ~(206)Pb/~(204)Pb ratio, which are characteristics of lavas from the Dupal Anomaly in the South Atlantic, and of many EM-1 type intraplate lavas generally.Our data place constraints on the origin of the Dupal Anomaly. The enrichments in Sr, Pb and Eu, together with the low Ca/Al ratios of the seamount lavas indicate that their mantle source consists of material that at one time contained plagioclase, and must therefore have resided at crustal pressures. We argue that the trace element and isotopic compositions of the seamount lavas are best explained by derivation from a mantle source contaminated with lower continental crust, which was introduced into the upper mantle during continental rifting and breakup in the South Atlantic Our results support previous suggestions that the Dupal Anomaly in the South Atlantic has a relatively recent, shallow origin in lower continental crust and continental lithospheric mantle, rather than in recycled material supplied from the deeper mantle by plumes. Plate reconstructions place the Parana-Etendeka flood basalt province over the central part of the Dupal Anomaly at the time of rifting of South America and Africa at 134 Ma. The flood basalts which have undergone the least crustal-level contamination also have extreme Dupal compositions. We speculate that delamination of dense lower continental crust during continental rifting causes flood basalt magmatism, whilst variably polluting the upper oceanic mantle with continental material.