A set up of an atmosphere-controlled furnace with stirring device, and effect of stirring on the crystallization behavior of basaltic melts.

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An atmosphere-controlled furnace with stirring device was constructed. The heater is a Siliconit DSP-26 (spiral SiC heater) with radius of 56mm and length of 300mm. The furnace is characterized by small temperature gradient within the sample (30ml platinum crusible, with upper diameter of 36mm). The furnace atmosphere is controlled by mixed gas of H2 and CO2, and the temperature is controlled by an automatic thyristor furnace controller using a Pt-PtRh thermocouple. The furnace temperature is controlled within 2ﾟC and oxygen fugacity is accurate within 0.5 log unit of bars at the sample site. The stirring Pt pipe is 11mm in diameter and is attached to ceramic rod, which is rotated at a rate of 5-57 rpm. The melt in the crusible is observed and sampled from the top of the furnace. The sampling was made manually by a ceramic rod, and quenched in air.We used a 1778A. D. lava of Izu-Oshima, Japan, as a starting material. The crystallization behavior and element partition relation was investigated by static wire-loop method previously. The sample is first melted at 1250ﾟC (about 50ﾟC above the liquidus temperature) for 10 hours without stirring, and then cooled at constant rate with stirring. Delay of plagioclase nucleation from overheated basaitic melts has been well documented (e. g. Gibb, 1974 Min. Mag.). In the present experiment, ca. 35ﾟC undercooling was requirred for plagioclase nucleation for rotation rate of 57 rpm at a cooling rate of 10ﾟC/hr. Plagioclase population density at 1150ﾟC sample is 4*10^4cm^<-3> for a run without stirring, and 7*10^5cm^<-3> for a run with rotation rate of 57rpm, indicating an effect of stirring on the nucleation of plagioclase in a basaitic melt.