Refining the noble gas record of the Réunion mantle plume source: Implications on mantle geochemistry

Abstract

Abstract We report isotope analyses of helium, neon, argon, and xenon using different extraction techniques such as stepwise dynamic and static crushing, and high-resolution stepwise heating of three mantle xenoliths from Reunion Island. He and Ne isotopic compositions were similar to previously reported Reunion data, yielding a more radiogenic composition when compared to the Hawaiian or Icelandic mantle plume sources. We furthermore observed correlated 129 Xe/ 130 Xe and 136 Xe/ 130 Xe ratios following the mantle trend with maximum values of 6.93 ± 0.14 and 2.36 ± 0.06, respectively. High-resolution argon analyses resulted in maximum 40 Ar/ 36 Ar ratios of 9000–11,000, in agreement with maximum values obtained in previous studies. We observed a well-defined hyperbolic mixing curve between an atmospheric and a mantle component in a diagram of 40 Ar/ 36 Ar vs. 20 Ne/ 22 Ne. Using a mantle 20 Ne/ 22 Ne of 12.5 (Ne–B) a consistent 40 Ar/ 36 Ar value of 11,053 ± 220 in sample ILR 84-4 was obtained, whereas extrapolations to a higher mantle 20 Ne/ 22 Ne ratio of 13.8 (solar wind) would lead to a much higher 40 Ar/ 36 Ar ratio of 75,000, far above observed maximum values. This favours a mantle 20 Ne/ 22 Ne of about 12.5 considered to be equivalent to Ne–B. Extrapolated and estimated 40 Ar/ 36 Ar ratios of the Reunion, Iceland, Loihi, and MORB mantle sources, respectively, tend to be linearly correlated with air corrected 21 Ne/ 22 Ne and show the same systematic sequence of increasing relative contributions in radiogenic isotopes (Iceland–Loihi–Reunion–MORB) as observed for 4 He/ 3 He. In general, He–Ne–Ar isotope systematics of the oceanic mantle can be explained by following processes: (i) different degree of mixing between pure radiogenic and pure primordial isotopes generating the MORB and primitive plume (Loihi-type) endmembers; (ii) relatively recent fractionation of He relative to Ne and Ar, in one or both endmembers; (iii) after the primary fractionation event, different degrees of mixing between melts or fluids of MORB and primitive plume affinity generate the variety of observed OIB data, also on a local scale; (iv) very late-stage secondary fractionation during magma ascent and magma degassing leads to further strong variation in He/Ne and He/Ar ratios.