Abstract
Crystallization experiments using a synthetic igneous analog have
shown that grain boundary migration can take place in melt-present
environments (Means and Park, 1994). In order to evaluate the
importance of metamorphic processes during the textural evolution
of a crystal mush, microstructural evidence is needed to link them
with final igneous textures. In natural magmatic systems,
plagioclase may provide a microstructural link; plagioclase solid
solution enables the process of chemical zoning and preserves a
record of crystal morphology (and chemistry) during growth. If
metamorphic processes are common, they should affect the zoning
patterns of plagioclase crystals.
A petrographic stage heater apparatus was designed and built for
the purpose of observing crystallization processes with a new
plagioclase analog. The analog utilized synthetic compounds within
the (K, NH4)SCN
system and was crystallized at low temperatures (< 172°C) for
observation in situ with an optical microscope. The melting points
of compounds containing XK = (0, 0.25, 0.50, 0.75, 1)
were measured in both thin section and in sealed capillary tubes
in order to construct a phase diagram. Compounds of intermediate
composition XK = (0.25, 0.50, 0.75) created a
continuous solid solution with initial and final melting points
between 82°C and 171-175°C in thin section, and between 108°C and
144°C (±1) in sealed capillary tubes. The differences between the
two data sets are probably due to the absorption of atmospheric
water and differing abilities of the two systems to contain the
water.
Textural studies of quenched compounds with composition XK
= 0.8 and XK = 0.75 resulted in the formation of
crystals with concentric extinction patterns (under cross
polarized light) that resemble patterns produced by chemical
zoning in plagioclase. Grain boundary migration was also observed
between chemically homogenous grains within the solid solution.
However, relatively slow rates of cooling were required to produce
grain boundary migration. While grain boundary migration was only
observed in unzoned solid solution crystals, further
crystallization experiments utilizing faster rates of cooling may
produce grain boundary migration in chemically zoned crystals.
Scott, E., 2002. The use of a synthetic binary solid solution to
model igneous textural evolution.
Unpublished MSc. thesis, State University of New York at Albany.
83 pp., +x
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 40 Z899 2002 S26
MS thesis digital
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