Primary and secondary processes constraining the noble gas isotopic signatures of carbonatites and silicate rocks from Brava Island: evidence for a lower mantle origin of the Cape Verde plume

We present and discuss noble gas compositions of minerals from silicate rocks (olivines) and carbonatites (apatites and calcites) from Brava Island. The presence of an almost ubiquitous atmosphere-derived fingerprint is explained as reflecting contamination by seawater. Because of the high U and Th content in apatites, which are responsible for ⁴He production by α-decay, the high measured ⁴He/³He ratios do not represent magmatic signatures. In contrast, low values of ⁴He/³He in calcites (≥61,223; R/R _a ≤ 11.80) and olivines (≥56,240; R/R _a ≤ 12.85) are considered to be representative of signatures trapped at the time of crystallization, given that there are no evidences for significant cosmogenic additions. These relatively low ⁴He/³He ratios depicted by silicate and carbonatite rocks imply the contribution of a reservoir that evolved under low (U + Th)/³He; this is considered a strong evidence for the genesis of Brava by a mantle plume deeply anchored in the lower mantle. The inferred low ⁴He/⁴⁰Ar* ratio (≈0.3), before degassing, is thought to reflect the contribution to the carbonatites source of a mantle domain evolving under high K/U, which cannot be explained by recycling of crustal components. The possible link between the low ⁴He/⁴⁰Ar* source and the “missing Ar reservoir” is discussed. The usually referred geochemical dichotomy between Northern and Southern Cape Verde islands, which is markedly evident from Sr, Nd, and Pb isotope signatures, is not apparent from Brava Island (Southern Cape Verde), where some samples present relatively unradiogenic ⁴He/³He signatures, similar to those reported for the Northern islands of the archipelago.