Physics
Scientific paper
Oct 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999e%26psl.172..111g&link_type=abstract
Earth and Planetary Science Letters, Volume 172, Issue 1-2, p. 111-126.
Physics
19
Scientific paper
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+/-1.2×107 m3. 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 1.6×107 m3.
Condomines Michel
Gauthier Pierre-J.
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