Argon composition of metamorphic fluids. Implications for [sup 40]Ar/[sup 39] geochronology

Journal Article · Fri Jul 01 04:00:00 EDT 1994 · Geological Society of America, Bulletin; (United States)

DOI:https://doi.org/10.1130/0016-7606(1994)106<0942:ACOMFI>2.3.CO;2· OSTI ID:7198613

Cumbest, R J; Johnson, E L; Onstott, T C ^[1]

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1. Princeton Univ., NJ (United States)

The concentration and isotopic composition of argon trapped in fluid inclusions within a mylonitic amphibolite were determined by combining electron microprobe, [sup 40]Ar/[sup 39] Ar laser microprobe, and fluid inclusion analyses on the same samples. In our study, fluid in inclusions formed during ductile deformation was probably derived locally from the Paleozoic, Caledonian sequences, whereas brines, emplaced in cross-cutting fractures, were derived from Proterozoic footwall rocks several kilometers from the mylonite. By utilizing [sup 38]Ar[sub Cl] to correct for [open quotes]excess [sup 40]Ar,[close quotes] 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. The [open quotes]excess [sup 40]Ar[close quotes] responsible for the anomalous plateaus probably resulted from diffusive re-equilibration of the amphibole with the fluids present during ductile deformation of the mylonite. The significantly younger cooling age determined by our method is concordant with ages determined from more recent geochronological studies in the region. These ages indicate that the western gneiss terrain, in our study area, records deformation and cooling associated with Siluro-Devonian tectonothermal metamorphism rather than an earlier Caledonian tectonism as proposed in previous [sup 40]Ar/[sup 39]Ar investigators. 46 refs., 8 figs.

• DOE Contract Number: FG02-91ER14197
• OSTI ID: 7198613
• Journal Information: Geological Society of America, Bulletin; (United States), Journal Name: Geological Society of America, Bulletin; (United States) Vol. 106:7; ISSN 0016-7606; ISSN BUGMAF
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400102 -- Chemical &amp; Spectral Procedures
58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
AMPHIBOLE
AMPHIBOLITES
ARGON 36
ARGON 37
ARGON 38
ARGON 39
ARGON 40
ARGON ISOTOPES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
BRINES
DAYS LIVING RADIOISOTOPES
DEVELOPED COUNTRIES
ELECTRON CAPTURE RADIOISOTOPES
ELECTRON MICROSCOPY
EUROPE
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC AGES
INCLUSIONS
ISOTOPES
LASERS
LIGHT NUCLEI
MAGNETIC PROPERTIES
MAGNETIC SUSCEPTIBILITY
METAMORPHIC ROCKS
METAMORPHISM
MICROSCOPY
MINERALS
NORWAY
NUCLEI
PALEOZOIC ERA
PHYSICAL PROPERTIES
PROBES
RADIOISOTOPES
ROCKS
SCANDINAVIA
SILICATE MINERALS
STABLE ISOTOPES
TECTONICS
TRANSMISSION ELECTRON MICROSCOPY
WESTERN EUROPE
YEARS LIVING RADIOISOTOPES