Mantle plume heterogeneity versus shallow-level interactions: A case study, the Sao Nicolau Island, Cape Verde archipelago

摘要

We present new Sr-Nd-Pb isotopic data, as well as major- and trace-element concentrations, for 19 basaltic samples from Sao Nicolau Island, Cape Verde archipelago. Fine-scale study of the island argues in favor of mixing between four endmembers to explain isotopic variations of collected samples: 1) a radiogenic endmember (~(87)Sr/~(86)Sr-0.7034; ~(143)Nd/~(144)Nd-0.51285; ~(206)Pb/~(204)Pb-20.0; ~(207)Pb/~(204)Pb~ 15.65; ~(208)Pb/ ~(204)Pb~39.8) representative of the Cape Verde plume deep source; 2) an unradiogenic endmember having isotopic compositions resembling those of Atlantic MORB dredged at the same latitude; 3) a low Sr-high Nd and Pb endmember identified as the Jurassic MORB basement of the archipelago; and 4) Sao Vicente-like Cape Verde carbonatites. Compositional and isotopic results show that most of the measured variations can be related to mixing of plume-derived melts with shallow-level reservoirs. Therefore the source heterogeneity of the Cape Verde plume is much smaller than the one sampled in basaltic samples. This observation illustrates how caution is required when interpreting global OIB data in terms of mantle topology without filtering from the contribution of shallow-level reservoirs.