We discuss noble gas (He, Ne, At, Kr, and Xe) and C isotope signatures of carbonatites from the Cape verae 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 He-4/He-3 ratios (down to 46,700; R/Ra up to 15.5) that cannot be explained by the addition of cosmogenic He-3, demonstrating that carbonatite magmas came from a reservoir characterised by low time-integrated (U + Th)/He-3. 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 He-4/He-3 signatures due to their high Th and U contents. An in situ source for 4 He in these apatites is further supported by air-like or lower Ne-20/Ne-22 ratios and relatively high Ne-21/Ne-22 ratios (up to 0.0485), which result from Ne-21 (and Ne-22) 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 Ar-38/Ar-36 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 Xe-129 and Xe-130, the observed 129Xe anomalies (Xe-129/Xe-130 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 I-129. 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 delta C-13 values (-8.0 to -4.2 parts per thousand) 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. (c) 2010 Elsevier B.V. All rights reserved.
Mata, Joao Moreira, Manuel Doucelance, Regis Ader, Magali Silva, Luis C.