Magma residence time beneath the Piton de la Fournaise Volcano, Reunion Island, from U-series disequilibria

The U, Th and Ba concentrations, ²³⁸U-²³⁰Th- ²²⁶Ra-²¹⁰Pb and ²²⁸Ra -²³²Th disequilibria have been measured in a suite of basalts and oceanites erupted during the last two millennia at Piton de la Fournaise (Réunion Island, Indian Ocean). Most of the variation in the concentration of all three incompatible elements is due to crystal fractionation or accumulation (and incorporation of olivine xenocrysts in the oceanites), but contamination by hydrothermally altered rocks or secondary minerals might explain significant variations in Ba/Th. Basalts with Th contents lower than 2.25 ppm have only been produced in eruptions outside the Enclos Fouqué caldera, whereas more evolved basalts are predominantly observed close to the summit craters, inside the Enclos caldera and the rift-zones. The samples have almost uniform Th/U of 3.95, identical Th isotope ratios ((²³⁰Th/ ²³²Th) = 0.934) and, therefore, approximately 20% excess of ²³⁰Th over ²³⁸U. All analysed samples are in ²²⁸Ra-²³²Th radioactive equilibrium, showing the absence of recent (< 30 years) Ra-Th fractionation. Lava flows erupted inside the Enclos caldera and the rift-zones have (²²⁶Ra/²³⁰Th)₀ varying from 1.18 to 1.34, whereas the basalts from craters outside the Enclos caldera and the rift-zones, in addition to the oceanites and the 1945 basalt, have higher (²²⁶Ra/²³⁰Th)₀ around 1.40. This value is assumed to represent that of the deep magma entering the central plumbing system, beneath the Enclos caldera. The evolution through time of (²²⁶Ra/²³⁰Th)₀ is attributed to alternating episodes of near closed-system evolution in one or several, poorly replenished, deep reservoirs, on timescales of the order of 1000 yr, and episodes of increased re-injection and mixing of the deep magma into older magmas of the previous period. Modelling of such a recent re-injection episode, from 1960 to 1998, suggests a short residence time of about 25 yr and a volume of 0.35 km³ for the shallow reservoir. At the time of eruption, (²¹⁰Pb/²²⁶Ra)₀ in both oceanites and basalts from the Enclos and rift-zones spans a narrow range with a mean value of 0.68, with the exception of the 1945 basalt which has an unusually low ratio of 0.20. The value of 0.68 can be explained by Rn degassing from the shallow magma chamber on a timescale comparable to that deduced from the evolution of (²²⁶Ra/²³⁰Th)₀ in the 1960-1998 period.