210Pb-226Ra radioactive disequilibria in recent lavas and radon degassing:: inferences on the magma chamber dynamics at Stromboli and Merapi volcanoes

U-series and especially Pb-210-Ra-226 disequilibria have been measured in recently erupted lavas from the Stromboli (Aeolian Islands, Italy) and Merapi (Java, Indonesia) volcanoes. Both volcanoes display Th-230-U-238 disequilibria (with a Th enrichment for Stromboli and a U enrichment for Merapi, respectively), strong Ra-226 enrichments over Th-230 (whereas Ra-228-Th-232 equilibrium is ubiquitous), and a contrasted evolution of their (Pb-210/Ra-226) activity ratios calculated at the time of eruption. While these ratios are always close to the equilibrium value of 1 at Stromboli, they show significant variations at Merapi (from 0.75 to 1). It is suggested that the low (Pb-210/Ra-226) activity ratios result from continuous degassing of Rn-222, the precursor of Pb-210 in the decay series, from a shallow magma chamber. A model assuming a physical steady-state of the shallow magma reservoir is proposed, linking the (Pb-210/Ra-226) ratio of the magma to the fraction of Rn-222 lost and to the renewal rate (or residence time) of the magma in the shallow reservoir. The near equilibrium (Pb-210/Ra-226) ratios at Stromboli imply a high renewal rate of the shallow reservoir (in the range 0.7-3 yr(-1)) in agreement with its persistent explosive activity. The volume of the reservoir is estimated at 1.3 +/- 1.2 x 10(7) m(3). In contrast, the variability of (Pb-210/Ra-226) ratios at Merapi is interpreted by successive phases of nearly closed-system evolution of the shallow magma chamber (with continuous Rn degassing) and episodes of reinjections and mixing of a deep undegassed magma in this reservoir. Its volume is estimated at 1.6 x 10(7) m(3). (C) 1999 Elsevier Science B.V. All rights reserved.