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Joint project with Durmus Boztug, Sivas University.
Some results were presented at the Hutton conference in Japan
September 2003
SOLIDIFICATION OF FELSIC MAGMAS: EVIDENCE FROM CRYSTAL SIZE
DISTRIBUTIONS (CSD) OF FELDSPARS.
Michael D HIGGINS1, Gabriel Machado1, 2, Dominique Meilleur1,
Réjean
Godin1, Durmuş Boztuğ3, Nazmi Otlu3, Sibel Tatar3 and Sabah Y Şahin4.
1. Sciences de la Terre, Université du Québec à
Chicoutimi,
Chicoutimi, G7H 2B1, Canada. 2. Dept. of Earth Sciences, University of
Ottawa,
Ottawa, Canada. 3. Dept. of Geological Engineering, Cumhuriyet
University,
58140 Sivas, Turkey. 4. Dept. Geophysical Engineering, Istanbul
University,
34850 Istanbul, Turkey.
The solidification of magma is a complex process involving nucleation,
growth
and solution of crystals. Most rocks only preserve the final stage of
this
process, but aspects of intermediate steps can be acquired from studies
of
sample ensembles, volcanic rocks or textures in oikocrysts.
Solidification
is the development of texture; hence by quantitative analysis of
crystal
sizes we can hope to gain insight into the solidification process.
Here,
we will discuss the solidification of felsic magmas, using CSD analysis
of
plutonic and volcanic rocks from California, Greece, Turkey, Finland
and
Nova Scotia. CSD data were acquired by direct measurement of megacrysts
on
outcrops, from slabs and thin sections by staining and automatic image
analysis.
The data were combined and transformed from intersection data to CSDs
using
the program CSDCorrections 1.3. CSDs define four different situations.
1) Many granitoids and rhyolites have a continuous distribution of
crystals
down to nuclei. That is, the lower size limit of the CSD is only
limited
by the measurement technique. These rocks commonly have a straight CSD
for
small sizes, which is consistent with nucleation and growth in an
environment
of exponentially increasing undercooling. This could be produced by
eruption,
uplift or rapid cooling caused by hydrothermal circulation.
2) Some porphyritic rocks completely lack small crystals and have a
hump-shaped
CSD (Keban, Turkey). This can be produced from a straight CSD by
extensive
textural coarsening (also known as Ostwald ripening, annealing or
textural
maturation). This process involves the solution of small crystals and
the
simultaneous growth of large crystals. It is driven by minimization of
surface
energy and occurs when the magma is maintained close to the liquidus
for
a long time. This situation can occur when the magma chamber is kept
warm
by injection of mafic magma, or when the felsic magma is emplaced into
very
warm rocks. The same pattern is also seen is megacrysts from some
granitoids
(Tuolumne suite, California; Eurajoki Stock, Finland).
3) Many felsic volcanic and plutonic rocks have a continuous concave up
CSD.
This can be generated by physical mixing of two magmas with straight
CSDs
like those produced by process 1 (dacites, Thera, Greece). Or it can
also
be produced by solidification under two different cooling regimes (for
instance
processes 1 and 2; Tuolumne, California). Finally it may be produced by
alternate
cycles of nucleation and growth (process 1) followed by textural
coarsening
(process 2; andesites, Soufriere Hills, Montserrat; Eurajoki, Finland).
4) The products of processes 1 and 3 can be further modified by
textural
coarsening. This will rotate the CSDs counter-clockwise as
small
crystals
are dissolved and large crystals grow (granitoids, Yozgat, Turkey).
Hence,
CSD analysis of rhyolites and granitoids can give some ideas on the
thermal
and physical history of a magmatic unit during solidification.
.