Title: Systematics and problems in isotope work on eclogites
Abstract: The acquisition and use of isotopic results from plutonic and volcanic rocks are relatively unambiguous. However, in the more complex field of metamorphic petrology, the interpretation of isotopic data may become more difficult. As exchange distances are extremely short, equilibration depends upon the inter-relationship of a number of different parameters, including temperature, fluid transport and tectonic deformation. Since under normal metamorphic conditions increasing temperature and fluid pervasion facilitate exchange reactions, answers are rather more conclusive in the amphibolite facies than for the greenschist facies. In granulite-facies terrains, fluids being very scarce, equilibration is rendered somewhat more difficult. Under eclogite-facies conditions, disequilibrium amongst the major and trace elements is common and therefore isotopic systems very often do not reach equilibrium. In addition, many high-pressure terrains outcrop discontinuously and geological control becomes difficult. In high-pressure rocks, KAr and 40Ar39Ar measurements may be handicapped by the presence of excess Ar, although not always convincingly revealed. Low Rb/Sr ratios in eclogites are unfavourable for whole-rock isotopic analyses. In addition, since the Rb content of the protolith is generally very low, any small gain or loss of Rb during the metamorphism significantly affects the Rb&z.sbnd;Sr system. Minerals rarely reach equilibrium in low-T eclogites in contrast to high-T eclogites. SmNd systematics on minerals from high-T eclogites are therefore a very good tool for determining the age of the eclogite paragenesis. UPb systematics on zircons from eclogites is also very promising, since, despite the difficult analytical task, the results are easier to interpret than in acidic rocks and can accurately provide both the age of the protolith and the age of high-P metamorphism. H and O isotopes represent a powerful tool to reconstruct crystallization conditions whilst C isotopes help in the interpretation of carbonate eclogites.
Publication Year: 1985
Publication Date: 1985-07-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 12
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