Crystal size distributions in Metamorphic rocks

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6651500

Cashman, K V; Ferry, J M

Crystal size distributions (CSDs) for metamorphic rocks yield quantitative information about crystal nucleation and growth rates, growth times, and the degree of overstepping (..delta..T) of reactions during metamorphism. CSDs were interpreted with an equation that relates a population density function (n) to crystal size (L), growth rate (G), and growth time (tau): n = n/sup 0/exp(-L/G tau). Plots of ln(n) vs. L for olivine + pyroxene and magnetite in high-temperature (1000/sup 0/C) basalt hornfelses from the Isle of Skye define linear arrays with slopes -1/(G tau) and intercepts n/sup 0/ = (J/G). Results are consistent with continuous nucleation and growth of olivine, pyroxene, and magnetite at the same rates of nucleation and growth. The CSDs are not consistent with near-instantaneous nucleation of minerals followed by growth at a constant rate. Nucleation rates were 1 crystal/second-6 hours per cm/sup 3/. CSDs of large magnetites and garnets in regionally metamorphosed pelitic schists from south-central Maine also define linear arrays on ln(n) vs. L plots consistent with continuous nucleation and growth during metamorphism. The degree of ripening, quantitatively measured by the largest crystal size that exhibits anomalous depletion, increases with increasing grade between the chlorite and sillimanite zones. CSDs for the hornfelses exhibit no evidence for ripening. Using the W/sup 2/ growth rate law, we infer that garnets grew in 3-13,000 years at ..delta..T = 0.01/sup 0/-10/sup 0/C and that nucleation rates were 1 crystal/25 minutes-50 years per cm/sup 3/. CSDs show that minerals in both regional and contact metamorphosed rocks are far from textural equilibrium.

• Research Organization: Johns Hopkins Univ., Baltimore, MD (USA)
• OSTI ID: 6651500
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580300* -- Mineralogy
Petrology
& Rock Mechanics-- (-1989)
ALKALINE EARTH METAL COMPOUNDS
BASALT
CHALCOGENIDES
CRYSTAL GROWTH
CRYSTALLIZATION
CRYSTALLOGRAPHY
IGNEOUS ROCKS
IRON COMPOUNDS
IRON ORES
IRON OXIDES
IRON SILICATES
MAGNESIUM COMPOUNDS
MAGNESIUM SILICATES
MAGNETITE
METAMORPHIC ROCKS
METAMORPHISM
MINERALOGY
MINERALS
NUCLEATION
OLIVINE
ORES
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
PHASE TRANSFORMATIONS
PYROXENES
ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
TRANSITION ELEMENT COMPOUNDS
VOLCANIC ROCKS