Supercooling and the crystallization of plagioclase from a basaltic magma

Abstract

SUMMARY. The liquidus temperature 0198 ~ and equilibrium phase relations of a sample of Columbia River basalt from the Picture Gorge section have been determined at I atmosphere by heating in a controlled atmosphere. When this basalt is cooled from above its liquidus temperature the liquidus phase (plagioclase) may fail to crystallize depending on the degree of undercooling and the duration of the experiment. A field in temperature-time space in which plagioclase fails to crystallize on cooling is separated from another in which plagioclase always crystallizes by a third in which the nucleation of plagioclase is unpredictable in terms of temperature and time. The extent to which this basaltic liquid can be supercooled without the crystallization of plagioclase is independent of the time it is held above the liquidus or the temperature in excess of the liquidus to which it is heated.The exceptionally long times required to ensure the nucleation of plagioclase at or near the liquidus temperature suggest that many so-called 'equilibrium' phase relations determined from experiments of a few hours' duration could be in serious error if the 'equilibration' involves a nucleation process.It is demonstrated that, over a range of cooling rates, the temperature at which plagioclase begins to crystallize on cooling varies markedly and the temperature and times required for both possible and certain nucleation of plagioclase are calculated for a range of constant cooling rates. The range of cooling rates over which the nucleation temperature of plagioclase varies is likely to occur in nature only in certain lava flows and small minor intrusions. In such cases this could lead to changes in the order in which the minerals appear on cooling and other petrologically significant effects.MANY silicate liquids can be cooled below their equilibrium liquidus temperature without crystallization of the liquidus phase. This tendency to supercool is generally recognized (Schairer, ~959) and necessitates some caution in interpreting crystalliquid relations in experimental charges that have been above the intended equilibration temperature at any time before or during the experiments.Whether or not a silicate liquid can be supercooled below its liquidus temperature seems to depend largely on the composition of the liquid and the nature of the liquidus phase. For example, in systems where forsteritic olivine is the liquidus phase there are no recorded instances of supercooling: on the contrary, olivine appears to crystallize very readily as soon as its saturation level is reached, as evidenced by the difficulties encountered in quenching Mg-rich liquids (Yoder, I952; Kushiro and Yoder, ~969; Green, I973). On the other hand, there are several examples of silicate melts failing to crystallize pyroxene or feldspar below the temperature at which the solid phase is in equilibrium with the liquid. In studying the melting relations of some calc-alkaline volcanic rocks Brown and Schairer (1968) completely melted their samples at I3oo ~ in order to ...

Viscosity measurement of subliquidus magmas: 1707 basalt of Fuji volcano

“…The basaltic sample is composed of scoria fragments with phenocrysts of olivine (0.9 vol%) and plagioclase (0.4%) in the matrix of fine microlites of plagioclase, pyroxene, titanomagnetite and tachylyte glass. Without the seeding of nuclei to the melt, delays in nucleation may occur for several tens of centi degrees below the liquidus temperature, as experimentally demonstrated by Gibb (1974).…”

The Origin and Significance of Reverse Zoning in Melilite from Allende Type B Inclusions

“…Second, experimental run products containing both pyroxene and anorthite sometimes have reverselyzoned melilite and sometimes do not, even though the experiments were run under iden-· tical conditions. As discussed below, these differences may be related to the random nature of anorthite nucleation (e.g., Gibb 1974) so that, in some experiments, anorthite begins crystallizing before pyroxene at slow cooling rates, producing melilites with only normal zoning, whereas, in other experiments run at the same cooling rates, pyroxene begins crystallizing first and reverselyzoned melilites result.…”

Mg-Fe Partitioning between Plagioclase and Liquid in Basalts of Hole 504B, ODP Leg 111: A Study of Melting at 1 ATM

“…2). The equilibrium liquidus temperature of Plagioclase was determined with the fusion (or heating) experiment (Gibb, 1974), in which the temperature is raised to a fixed value without overshooting and then maintained for a certain period before quenching the charges in distilled water. Because of the lack of fresh olivine crystals in the starting materials, the temperature of olivine appearance could not be determined.…”

“…Because supercooling effects are inevitable in the growth of crystals in basaltic melts (Gibb, 1974;Donaldson, 1979;Baker and Grove, 1985), seeded runs were made with fusion time schedules to grow Plagioclase at a small degree of undercooling (AT = 15°-25°C) for 43-46 hr. The initial pellets consisted of fused glass powder and Plagioclase seed crystals; however, the Plagioclase growth rate was too low to produce overgrowth on the seeded Plagioclase for electronprobe microanalyses.…”