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Experimental study of zircon coarsening in quartzite + or -H (sub 2) O at 1.0 GPa and 1000 degrees C, with implications for geochronological studies of high-grade metamorphism

John C. Ayers, Katherine de la Cruz, Calvin Miller and Oran Switzer
Experimental study of zircon coarsening in quartzite + or -H (sub 2) O at 1.0 GPa and 1000 degrees C, with implications for geochronological studies of high-grade metamorphism
American Mineralogist (March 2003) 88 (2-3): 365-376

Abstract

The rate and mechanism of zircon coarsening in quartzite + or -H (sub 2) O at 1.0 GPa and 1000 degrees C were characterized by performing time-series experiments in a piston-cylinder apparatus. Anhydrous experiments show no significant growth of zircon or textural modifications after 24h. In the presence of 1-2 wt% H (sub 2) O, zircon (2 wt%) in coarsely powdered quartz recrystallized in the first 8 hours so that mean crystal size decreased relative to the starting material. After 8 h zircon grew by Ostwald ripening and by coalescence of crystals, and maintained its position on quartz grain boundaries, even while quartz crystals coarsened, by recrystallizing in the fluid. Fitting the experimental data produced the growth-rate equation log (<D> (super n) -<D (sub 0) > (super n) ) = log (<D> (super 2) -<D (sub 0) > (super 2) ) = log (t-t (sub 0) ) + log K (sub n) = log (t-t (sub 0) )-5.9, where <D> is mean diameter in micrometers, the subscript 0 denotes initial value at time zero, t is experiment duration in seconds, and K (sub n) is the growth-rate constant with units of mu m (super 2) /s (five experiments, r (super 2) = 0.988). A value of n = 2 suggests that growth is interface controlled, with the rate limited by dissolution and precipitation on the crystal surface. An equivalent growth-rate equation is <D>-<D (sub 0) > = K (sub 1/n) t (super 1/n) = K (sub n) (super 1/n) t (super 1/n) = (1.12X10 (super -3) mu m/s (super 1/2) )t (super 1/2) , a parabolic rate law equation that can be used to demonstrate that zircon in the presence of H (sub 2) O at 1000 degrees C grows fast enough by Ostwald ripening to produce micrometer-thick overgrowths in <10 y. Rapid coarsening of zircon by Ostwald ripening will occur during thermal metamorphism when fluid is present. Addition of fluid (and possibly melt) greatly increases the rate of coarsening, suggesting that most metamorphic rims form nearly instantaneously during fluid influx (or anatexis), and that U-Pb rim ages date such events.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 88
Serial Issue: 2-3
Title: Experimental study of zircon coarsening in quartzite + or -H (sub 2) O at 1.0 GPa and 1000 degrees C, with implications for geochronological studies of high-grade metamorphism
Affiliation: Vanderbilt University, Department of Geology, Nashville, TN, United States
Pages: 365-376
Published: 200303
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 33
Accession Number: 2003-027614
Categories: General geophysicsGeochronology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200309
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