Mathematics – Logic
Scientific paper
Mar 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983gecoa..47..597g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 47, Issue 3, pp.597-616
Mathematics
Logic
9
Scientific paper
Compositional differences between granulite facies rocks and equivalent amphibolite facies rocks and the observation of CO 2 -rich fluid inclusions in granulites, have led to the suggestion that CO 2 must play a role in modifying the composition of deep continental crust. How CO 2 effects this change has remained unclear. Using the thermodynamic properties of aqueous ions in a fluid of evolving CO 2 / H 2 O ratio, it is possible to model the incongruent dissolution of feldspars under conditions appropriate for granulite facies metamorphism. The results demonstrate that dissolution will be strongly enhanced at high CO 2 / H 2 O ratios, with ion solubilities being Na + > K + >> Ca ++ . This enhancement is compatible with the reported compositional contrasts between granulite and amphibolite facies rock, but requires large fluid volumes. To test the dissolution model, a detailed field and petrologic study was conducted in a well exposed granulite facies terrane in West Greenland. Strong correlation between fluid composition and bulk rock chemistry can be documented; CO 2 -rich regions contain rocks which consistently have low a Na 2 O / a CaO ratios, while H 2 O-rich regions consistently have high a Na 2 O / a CaO ratios. Magnetite rims on sulfide grains are ubiquitous in high f Co 2 regions and are absent in high f H 2 O regions, and they provide evidence that CO 2 was introduced into the region. These correlations and observations are predictable from the properties of the dissolution process. These considerations, along with observations regarding graphite petrogenesis, provide strong arguments that the total fluid volume interacting with the rock during metamorphism was very large, in some cases equaling or exceeding total rock volume. Such large fluid volumes can lead to significant compositional modification of the crust, and will mask the original protolith chemistry. Such processes should lead to Ca- and Al-enriched, Na-, K-, S- and Si-depleted residues in the deep crust.
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