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

Abstract

U-series and especially 210Pb- 226Ra disequilibria have been measured in recently erupted lavas from the Stromboli (Aeolian Islands, Italy) and Merapi (Java, Indonesia) volcanoes. Both volcanoes display 230Th- 238U disequilibria (with a Th enrichment for Stromboli and a U enrichment for Merapi, respectively), strong 226Ra enrichments over 230Th (whereas 228Ra- 232Th equilibrium is ubiquitous), and a contrasted evolution of their ( 210Pb/ 226Ra) 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 ( 210Pb/ 226Ra) activity ratios result from continuous degassing of 222Rn, the precursor of 210Pb 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 ( 210Pb/ 226Ra) ratio of the magma to the fraction of 222Rn lost and to the renewal rate (or residence time) of the magma in the shallow reservoir. The near equilibrium ( 210Pb/ 226Ra) 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 × 10 7 m 3. In contrast, the variability of ( 210Pb/ 226Ra) 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 × 10 7 m 3.