CRYSTALLIZATION PROCESSES IN AN ARTIFICIAL MAGMA - VARIATIONS IN CRYSTAL SHAPE, GROWTH-RATE AND COMPOSITION WITH MELT COOLING HISTORY

A large (4.8 m(3), 1.3 x 10(7) g) artificial mafic melt with a bulk co mposition similar to that of a basalt (but with a high CaO content of 17 wt %) was generated during a demonstration of in situ vitrification and was allowed to cool naturally. During the melting process, convec tion was vigorous, resulting in a chemically and thermally homogeneous melt body. Once heating was complete, the cooling rate was rapid with the temperature dropping from 1500 degrees C to 500 degrees C in simi lar to 6 days within the interior of the 3 m diameter, 1.5 m thick bod y. A similar to 20 h period of constant temperature (1140 degrees C) o bserved during cooling was the result of latent heat released by wides pread crystallization. The final crystalline assemblage consists of di opsidic to hedenbergitic pyroxene and anorthitic feldspar, with a subo rdinate amount of potassic feldspar, plus a small amount of evolved gl ass. The compositions and proportions of phases agree well with those predicted by the MELTS thermodynamic model. Thermal and textural evide nce suggest that convection within the melt ceased coincident with for mation of the first crystals. Textural investigation of core samples r eveals large (up to 1 cm in length) acicular diopsidic pyroxenes in a matrix of smaller feldspar and zoned pyroxene crystals (similar to 500 mu m in length). Crystal shape and pyroxene composition vary as a fun ction of position within the solidified body, as a function of cooling rate. Both crystal size and degree of crystallinity are highest in th e central, most slowly-cooled parts of the rock. Crystal shape is char acterized by tabular, equilibrium-growth forms in the slowly-cooled ar eas, grading to highly skeletal, dendritic forms at the rapidly-cooled edges of the body. The pyroxene crystals are dominantly homogeneous d iopside, but crystals are characterized by thin Fe-rich hedenbergitic rims. These rims were deposited when Mg in the melt was depleted by di opside growth, and melt temperature had cooled sufficiently to allow F e-rich pyroxene growth. Crystal growth rates can be calculated based o n thermal behavior of the melt, reinforced by thermodynamic modelling, and are determined to be between 10(-7) and 10(-8) cm/s in the centra l part of the melt. These estimates agree well with growth rates in na tural systems with similar cooling rates. Pyroxene crystals that forme d at a higher cooling rates are characterized by higher Al and lower M g contents relative to tabular equilibrium crystalline forms, presumab ly as a result of disequilibrium melt compositions at the crystal-melt interface.