Experimental cannibalization of plagioclase by alkaline basalt magmas

Time-series crystallization/dissolution experiments were conducted on a natural potassicbasalt seeded with bytownitic plagioclases (Plg) at atmospheric pressure, in air, at 1180-1240 °C and isothermal dwell time up to 20 hours. Plg-seed presence promotes the earlyformation of new-Plg, dampening the clinopyroxene (Cpx) crystallization. New-Plgsgrow at a rate from 10-6 up to 10-8 cm·s-1 as the dwell time increases. Seeds overgrowat similar rate. Cpx crystallizes with a delay of at least 3 hours; this has a significantimpact on the composition of both residual melt and new-Plgs. For undercooling>35 °C the Cpx delay causes a strong supersaturation of this phase in the melt resulting ina decrease in the new-Plg nucleation rate by 2 orders of magnitude in the 3 h-experiment.In the 15h-run, Cpx coarsening and the decrease of crystallinity suggest the achievementof a near-equilibrium conditions. Cpx growth rate is in the order of 10-7 cm·s-1 showingvery limited variation. Finally, for the investigated superheating (5-15 °C) only the longlasting experiments allows an estimation of Plg dissolution rate (10-9 cm·s-1) althoughchanges in the melt composition are already detectable in the 3h-runs.As a whole our results suggest that in natural systems, the takeover of antecrysts/xenocrysts by a magma can induce on a short time scales, changes in its initial nucleationbehavior with remarkable petrological implications for the solidification paths anderuptive dynamics of potassic magmatic systems.