238U–230Th–226Ra disequilibria in andesitic lavas of the last magmatic eruption of Guadeloupe Soufriere, french Antilles: Processes and timescales of magma differentiation
Résumé
Andesitic products of the last magmatic eruption at La Soufrière in Guadeloupe (1530 AD) were analyzed for 238U–230Th–226Ra in order to constrain the timescales of magmatic processes preceding the eruption. Magma composition evolved during eruption from acid andesitic scoria falls and flows to andesitic lavas during the final phase of dome extrusion, with some evidence of mixing in banded pumices at the base of the first eruptive unit. Most of the least differentiated products show 238U and 226Ra excesses relative to 230Th, like many other arc lavas. The (226Ra/230Th) activity ratios of whole rocks then decrease with the degree of differentiation from 1.507 to 0.983, whereas the (230Th/238U) ratios increase from 0.942 to 1.003. The groundmasses of the least differentiated lava show a (226Ra/230Th) ratio higher than 1, but all the other samples are in secular equilibrium. These variations can be modeled by time-dependent differentiation from a common parental melt. Assuming a closed-or open-magma chamber behaviour (closed or open system), the duration of magma differentiation ranges from 30 to 70 kyr with crystallization rates ranging from 0.012 to 0.019 kyr− 1. 238U–230Th disequilibria have also been measured in minerals from one of the least differentiated products and provide crucial information on their age of crystallization, their origin and the crystal dynamics in the magma chamber. A crystallization age of 34.9 ± 8.3 kyr (2σ) is obtained using U–Th internal isochrons on pyroxenes, which is older than the differentiation time of their host lava given by time-dependant differentiation models, but younger than the differentiation time of the most evolved magma compositions. Older cumulate crystals were thus recycled in the erupted magmas, consistent with petrologic evidence for crystal-melt disequilibria. The calculated (226Ra/230Th) ratios of mineral assemblages (> 1 for most samples) suggest the presence of plagioclases younger than 8000 yr, in agreement with the fact that plagioclases do not plot on the U–Th isochron defined by pyroxenes. This study thus exemplifies the hybrid nature of arc andesitic magmas such as the 1530 AD Soufriere magma, which is generated by mixing of crystals and melts produced by fractional crystallization of a common parental magma. Our results also confirm that the magma chamber below large volcanoes such as La Soufrière of Guadeloupe is long-lived (up to tens of thousands of years). This timescale is much longer than the cooling times predicted by a closed-system thermal model, suggesting that the magma chamber was sustained by inputs of fresh and hot magma and behaves as an open system.