Argon composition of metamorphic fluids: implications for 40 Ar/ 39 Ar geochronology

R. J. Cumbest, E. L. Johnson, Tullis C. Onstott

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The concentration and isotopic composition of argon trapped in fluid inclusions within a mylonitic amphibolite were determined. Such information provides constraints on the origin of metamorphic fluids. In this study, fluid in inclusions formed during ductile deformation was probably derived locally from Paleozoic, Caledonian sequences, whereas brines, emplaced in cross-cutting fractures, were derived from Proterozoic footwall rocks several kilometers from the mylonite, By utilizing 38 Ar Cl to correct for "excess 40 Ar,' we estimate a 370 Ma closure age for the amphibole, which is distinctly younger than the 440-480 Ma plateau ages indicated by step-heating spectra. -Authors

Original languageEnglish (US)
Pages (from-to)942-951
Number of pages10
JournalGeological Society of America Bulletin
Issue number7
StatePublished - Jan 1 1994

All Science Journal Classification (ASJC) codes

  • Geology


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