The evolution of illite to muscovite: An example of the behaviour of isotopes in low-grade metamorphic terrains
1986; Elsevier BV; Volume: 57; Issue: 1-2 Linguagem: Inglês
10.1016/0009-2541(86)90092-6
ISSN1872-6836
Autores Tópico(s)Clay minerals and soil interactions
ResumoA review of the possibilities and limitations of the use of radiogenic isotopes in low-grade terrains, i.e. in the temperature range below the blocking temperature of potassic white mica, is discussed using a background of a data base of over 500 isotope analyses on more than 250 illitic-muscovitic samples published in various papers. Special stress is laid on the main problems: (1) distinction between detrital and neoformed minerals; (2) the slow evolution of minerals without pronounced breaks; and (3) retention problems of radiogenic daughter products. A brief description of low-grade metamorphism is given, and criteria enabling a subdivision into low- to very low-grade metamorphism are discussed, comprising illite crystallinity, mineralogy, fluid inclusions and coal rank measurements. The useful type of lithologies for this kind of work, ranges from shales-slates through volcaniclastic tuffs, with high primary glass contents and limestone/dolomites-marbles to mylonites. It can be concluded, that carefully selected illitic muscovites yield geologically meaningful ages, provided that more than one isotopic method is applied.
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