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

摘要

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 á-decay, the high measured ~4He/~3He ratios do not represent magmatic signatures. In contrast, low values of ~4He/~3He in calcites (>61,223; < 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.