We present new electron microprobe and Sr-isotope analytical results from stirring/mixing experiments using natural volcanic samples. In order to constrain the dynamics of such mingling / mixing events, we applied a Taylor–Couette flow, simulating forced convection under very low Reynolds numbers, in a time series ranging from 1 hour up to 1 week. The end-member melts derive from samples of the Campanian Ignimbrite (CI), in Italy. The CI is thought to represent a layered reservoir formed in 3 stages: 1) a resident phono-trachytic magma reservoir (end-member A); replenished by 2) a less evolved trachybasaltic-trachytic magma (end-member B of trachytic composition); 3) short-term pre-eruptive mixing in the shallow chamber between a new trachytic and the phono-trachytic resident magmas. Our experiments are motivated by this hypothesis. In our experiments the two end-members are stirred together. Stirring under constant low flow velocity (0.5 rotations per minute) initially generates single convection cells, which causes progressive homogenization of some major components. This is the case after 1, 4 and 9 hours. After 16 hours the 87Sr/86Sr-isotopic system is homogenized and the starting compositions are fully mixed. Then separate convection cells and compositional layering for major and minor elements emerged. Based on microprobe measurements of quenched melts (glass) from the 16-hour, 25-hour and 1-week long experiments, we confirm the separation of layers having different densities. This phenomenon is locally complicated by the production of micro-volumes of unmixed melts. Our results support the effectiveness of the interplay between convection and diffusion, enhanced by a double-diffusive-convection-driven differentiation for moderately high-silica magmas under high (near-liquidus) temperatures, attesting that differentiation initiates in the liquidus before the onset of fractional crystallization.
Heterogeneities in Magma Chambers: insights from the behavior of major and minor elements during mixing experiments with natural alkaline melts
PERUGINI, Diego;
2008
Abstract
We present new electron microprobe and Sr-isotope analytical results from stirring/mixing experiments using natural volcanic samples. In order to constrain the dynamics of such mingling / mixing events, we applied a Taylor–Couette flow, simulating forced convection under very low Reynolds numbers, in a time series ranging from 1 hour up to 1 week. The end-member melts derive from samples of the Campanian Ignimbrite (CI), in Italy. The CI is thought to represent a layered reservoir formed in 3 stages: 1) a resident phono-trachytic magma reservoir (end-member A); replenished by 2) a less evolved trachybasaltic-trachytic magma (end-member B of trachytic composition); 3) short-term pre-eruptive mixing in the shallow chamber between a new trachytic and the phono-trachytic resident magmas. Our experiments are motivated by this hypothesis. In our experiments the two end-members are stirred together. Stirring under constant low flow velocity (0.5 rotations per minute) initially generates single convection cells, which causes progressive homogenization of some major components. This is the case after 1, 4 and 9 hours. After 16 hours the 87Sr/86Sr-isotopic system is homogenized and the starting compositions are fully mixed. Then separate convection cells and compositional layering for major and minor elements emerged. Based on microprobe measurements of quenched melts (glass) from the 16-hour, 25-hour and 1-week long experiments, we confirm the separation of layers having different densities. This phenomenon is locally complicated by the production of micro-volumes of unmixed melts. Our results support the effectiveness of the interplay between convection and diffusion, enhanced by a double-diffusive-convection-driven differentiation for moderately high-silica magmas under high (near-liquidus) temperatures, attesting that differentiation initiates in the liquidus before the onset of fractional crystallization.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
