HR: 10:50h
AN: V12A-03    [Abstracts]
TI: Phenocryst Dynamics in the Magma Chamber of the Guadeloupe Soufriere From U-Th-Ra Disequilibria in 1440A.D. Lavas
AU: * Touboul, M
EM: touboul@ipgp.jussieu.fr
AF: IPGP-Universite Paris 7-Laboratoire de Geochimie-Cosmochimie, 4, place Jussieu, Paris cedex 05, 75252 France
AU: Bourdon, B
EM: bourdon@ipgp.jussieu.fr
AF: IPGP-Universite Paris 7-Laboratoire de Geochimie-Cosmochimie, 4, place Jussieu, Paris cedex 05, 75252 France
AB: U-Th and Ra analyses of andesitic to dacitic products of the Soufriere last eruption (1440+/-50 A.D.) were performed in order to constraint the time scales and the nature of magmatic processes preceeding eruption. The stratigraphic sequence indicates that, dacitic lavas were first erupted followed by more andesitic lavas, similar to that of the actual dome. Most whole rocks show ²³⁸U and ²²⁶Ra excess relative to ²³⁰Th, like many others arc lavas. Their (²²⁶Ra/²³⁰Th) activity ratios decrease with differentiation degree 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 others are at secular equilibrium. Time dependent fractional crystallization models assuming a common andesitic parental liquid in a vertically zoned magma chamber can explain the compositional variations and U-Th-Ra disequilibria of 1440 Soufriere lavas and groundmasses. The timescales of magma differentiation inferred by fractional crystallization models are of a few ten thousands years for a crystallization rate ranging from 1 to 2.10^-5 y^-1.
The 1440 Soufriere lavas show phenocrysts out of equilibrium with their host melt (complex zoning and overgrowth rims). U-Th disequilibria have been measured in one of the least differentiated lava and provide precious information on their age of crystallization, their origin and the solids dynamics in magma chamber. The crystallization age of 35.0ñ5.9 ky obtained for pyroxenes using U-Th internal isochron is older than the differentiation time of their host lava given by crystallization models but younger than the differentiation time required to get the most evolved composition. That suggests old cumulates remobilization (recycling), whereas the liquids must have evolved separately.
Moreover, the magnetite and plagioclase separates attest of a U-Th disturbance. In fact, they do not plot on the isochron defined by the whole rock, the groundmass, the clinopyroxenes and the orthopyroxenes. The magnetite phenocrysts show a very particular U-Th signature. Taken published partitioning coefficient data, they have too high U/Th ratio to have crystallized from any magma of the Soufriere or of Lesser Antilles arc. It thus requires an episode of U-enrichment after crystallization, sufficiently old to allow a strong increase of (²³⁰Th/²³²Th) ratio. Probably dragged from an altered wall-rock, the magnetites suggest an assimilation process. By comparing the (²²⁶Ra/²³⁰Th) ratios of whole rocks and groundmasses, (²²⁶Ra/²³⁰Th) ratios of mineral assemblages can be inferred. Greater than 1 for most lavas, they show that some plagioclases have crystallization ages younger than 8000 years which is shorter than the differentiation timescale of their host lavas. It must result of a late crystallisation in their host lava or of a crystallisation from a magma batch injected later than their host lava and followed by their ascent through chamber by flotation. The U-Th-Ra disequilibria of minerals also show evidence for minerals exchanges between the different magma batches by sinking or flotation and of surrounding assimilation.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1036 Magma chamber processes (3618)
DE: 1120 Isotopic disequilibrium dating
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005