Pb isotope geochemistry of Piton de la Fournaise historical lavas
Résumé
Variations of Pb isotopes in historical lavas (1927–2007) from Piton de la Fournaise are investigated based on new (116 samples) and published (127 samples) data. Lead isotopic signal exhibits smooth fluctuations (18.87 < 206Pb/204Pb < 18.94) on which superimpose unradiogenic spikes (206Pb/204Pb down to 18.70). Lead isotopes are decoupled from 87Sr/86Sr and 143Nd/144Nd, which display small and barely significant variations, respectively.
No significant change of Pb isotope composition occurred during the longest (> 3 years) periods of inactivity of the volcano (1939–1942, 1966–1972, 1992–1998), supporting previous inferences that Pb isotopic variations occur mostly during and not between eruptions. Intermediate compositions (18.904 < 206Pb/204Pb < 18.917) bracket the longest periods of quiescence. In this respect, the highly frequent occurrence of an intermediate composition (18.90 < 206Pb/204Pb < 18.91), which clearly defines an isotopic baseline during the most recent densely sampled period (1975–2007), either suggests direct sampling of plume melts or sampling of a voluminous magma reservoir that buffers Pb isotopic composition. Deviations from this prevalent composition occurred during well-defined time periods, namely 1977–1986 (radiogenic signature), 1986–1990 and 1998–2005 (unradiogenic signatures). The three periods display a progressive isotopic drift ending by a rapid return (mostly during a single eruption) to the isotopic baseline. The isotopic gradients could reflect progressive emptying of small magma reservoirs or magma conduits, which are expected to be more sensitive to wall-rock interactions than the main magma chamber. These gradients provide a lower bound ranging from 0.1 to 0.17 km3 for the size of the shallow magma storage system. The isotopic shifts (March 1986, January 1990 and February 2005) are interpreted as refilling the plumbing system with deep melts that have not interacted with crustal components. The volume of magma erupted between the two major refilling events of March 1986 and February 2005 (0.28 km3) could provide a realistic estimate of the magma reservoir size. Unradiogenic anomalies appear to be linked, more or less directly, to the eruption of olivine-rich lavas. The related samples have low 206Pb/204Pb and 208Pb/204Pb but normal 207Pb/204Pb, suggesting a recent decrease of U/Pb and Th/Pb, for instance through sequestration of Pb into sulfides. Olivine and sulfides, which are both denser than silicate melts, could be entrained with magma pulses, which give rise to high-flux oceanite eruptions.
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