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

DUNBAR NW, JACOBS GK, NANEY MT

CONTRIBUTIONS TO MINERALOGY AND PETROLOGY
v. 120(#3-4) pp. 412-425 1995
(Title at LANL MAIN.)

Institutions:

Abstract:

A large (4.8 m(3), 1.3 x 10(7) g) artificial mafic melt with a bulk composition 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, convection 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 similar to 6 days within the interior of the 3 m diameter, 1.5 m thick body. A similar to 20 h period of constant temperature (1140 degrees C) observed during cooling was the result of latent heat released by widespread crystallization. The final crystalline assemblage consists of diopsidic to hedenbergitic pyroxene and anorthitic feldspar, with a subordinate amount of potassic feldspar, plus a small amount of evolved glass. The compositions and proportions of phases agree well with those predicted by the MELTS thermodynamic model. Thermal and textural evidence suggest that convection within the melt ceased coincident with formation of the first crystals. Textural investigation of core samples reveals 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 function of position within the solidified body, as a function of cooling rate. Both crystal size and degree of crystallinity are highest in the central, most slowly-cooled parts of the rock. Crystal shape is characterized by tabular, equilibrium-growth forms in the slowly-cooled areas, grading to highly skeletal, dendritic forms at the rapidly-cooled edges of the body. The pyroxene crystals are dominantly homogeneous diopside, but crystals are characterized by thin Fe-rich hedenbergitic rims. These rims were deposited when Mg in the melt was depleted by diopside growth, and melt temperature had cooled sufficiently to allow Fe-rich pyroxene growth. Crystal growth rates can be calculated based on thermal behavior of the melt, reinforced by thermodynamic modelling, and are determined to be between 10(-7) and 10(-8) cm/s in the central part of the melt. These estimates agree well with growth rates in natural systems with similar cooling rates. Pyroxene crystals that formed at a higher cooling rates are characterized by higher Al and lower Mg contents relative to tabular equilibrium crystalline forms, presumably as a result of disequilibrium melt compositions at the crystal-melt interface.

Keywords:

ROCKS; NUCLEATION; SYSTEMS; LAVA

Categories:

GEOLOGY; MINERALOGY

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