Noble gas and carbon isotopic signatures of Cape Verde oceanic carbonatites: Implications for carbon provenance

Abstract

We discuss noble gas (He, Ne, Ar, Kr, and Xe) and C isotope signatures of carbonatites from the Cape Verde Archipelago. These are the first noble gas compositions ever reported for oceanic carbonatites. The noble gas analyses were performed by crushing the calcite and apatite separates. Some of the analyzed calcites present low 4He/ 3He ratios (down to 46,700; R/Ra up to 15.5) that cannot be explained by the addition of cosmogenic 3He, demonstrating that carbonatite magmas came from a reservoir characterised by low time-integrated (U + Th)/ 3He. Such a reservoir is thought to be localised in the deep lower mantle, constraining the depth of origin of the Cape Verde plume. In contrast, apatite samples return highly radiogenic 4He/ 3He signatures due to their high Th and U contents. An in situ source for 4He in these apatites is further supported by air-like or lower 20Ne/ 22Ne ratios and relatively high 21Ne/ 22Ne ratios (up to 0.0485), which result from 21Ne (and 22Ne) formation by nucleogenic reactions. Some apatites plot to the left of the MORB line in the neon three-isotope diagram. This is explained by mass-fractionation processes since these apatites are also characterised by 38Ar/ 36Ar ratios lower than the air value or even than the range of values usually used to characterise the MORB-OIB field. Considering that carbonate recycling, which could be characterised by high levels of Te and Ba, would simultaneously increase with time the 129Xe and 130Xe, the observed 129Xe anomalies ( 129Xe/ 130Xe up to 6.84) cannot be explained by models calling upon crustal carbonate recycling. We attribute these anomalies to an ancient mantle origin by decay of the now extinct 129I. Moreover, experimental work has suggested that crustal carbonates are unlikely to be transported to deep lower mantle depths as a consequence of their removal by melting reactions at subduction. Thus, our noble gas data are indicative of a non-recycled origin for carbon, endorsing the role of primordial carbon in the genesis of Cape Verde carbonatites. This conclusion is supported by typical mantle δ 13 C values (− 8.0 to − 4.2‰) that are lighter than those characterising crustal inorganic carbonates. The fact that He is the noble gas element characterised by the highest solubility in magmas, the faster diffusion and the lower mineral/melt partition coefficients is taken as an explanation for its decoupling from the other noble gases, with He being the one better retaining a deep lower mantle signal.