Abstract
Domains of local reaction and their influence on nucleation in metamorphic rocks
Abstracts with programs - Geological Society of America, Vol.30(7), p.280
Geological Society of America, 1998 annual meeting
1998
Abstract
Variation in the size of local reaction domains in metamorphic rocks has a profound influence on the reaction overstepping that controls the nucleation of a new phase. Rocks with compositional heterogeneities such as porphyroblasts can develop a variety of local overstepping rates, depending upon the scale of effective transport. For example, consider a metapelite with staurolite and garnet porphyroblasts set in a matrix of biotite, chlorite, muscovite, plagioclase and quartz that is heated so that it oversteps a sillimanite-forming reaction. If the scale of material transport in the rock is large enough to allow domains with different porphyroblasts to equilibrate with each other, the chemical potential of Al (sub 2) SiO (sub 5) will be uniform through out the rock and depend on metastable equilibria among all the minerals in the rock. However, if transport is more limited, so that domains of equilibration are smaller than the distance between porphyroblasts, the chemical potential of Al (sub 2) SiO (sub 5) will have different values that depend upon local metastable mineral assemblages. In the example considered here, domains with staurolite have the lowest amount of overstepping of the sillimanite-forming reaction, those with garnets are intermediate, and domains without porphyroblasts have the highest amount of overstepping. The reason for this behavior is that domains with porphyroblasts develop reactions among the local metastable assemblages which lower the rate of increase in the chemical potential of Al (sub 2) SiO (sub 5) as the sillimanite-forming reaction is overstepped. Local reactions producing metastable staurolite are more effective in reducing the overstepping rate than those producing garnet. This means that matrix domains away from porphyroblasts are the most favorable areas for sillimanite nuclei to form, domains near garnet are less favored, and domains near staurolite are the least favored sites for sillimanite nucleation. This may explain why sillimanite segregations commonly appear to nucleate and grow in the matrix of metapelites, rather than by direct replacement of pre-existing porphyroblasts.
Details
- Title: Subtitle
- Domains of local reaction and their influence on nucleation in metamorphic rocks
- Creators
- C. T Foster - University of Iowa
- Resource Type
- Abstract
- Publication Details
- Abstracts with programs - Geological Society of America, Vol.30(7), p.280
- Conference
- Geological Society of America, 1998 annual meeting
- Publisher
- Geological Society of America (GSA)
- ISSN
- 0016-7592
- Alternative title
- Geological Society of America, 1998 annual meeting
- Language
- English
- Date published
- 1998
- Academic Unit
- Earth and Environmental Sciences
- Record Identifier
- 9984240800102771
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