Laser [sup 40]Ar/[sup 39]Ar dating of polymetamorphic 2M[sub 1] muscovite, an example from northern Vermont

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5690663

Hames, W E ^[1]

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1. Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Earth, Atmospheric, and Planetary Sciences

Rocks which are subject to superimposed metamorphic events commonly contain texturally and chemically distinct generations of white micas. Laird and Albee (1974) made a pioneering Ar-40/Ar-39 study of the age relationships among polymetamorphic micas from northern Vermont; they found that coarse muscovite associated with a chloritoid-kyanite assemblage gave incremental-heating plateau and isochron ages of about 440 Ma, whereas finely-grained muscovite (as pseudomorphs after kyanite) yielded plateau and isochron ages of about 365 Ma, respectively. Coarse muscovite from the same sample studied by Laird and Albee (1974) has been reexamined using Ar-40/Ar-39 laser probe techniques. Laser-spot ages of the core of the muscovite range from 451 to 457 Ma; laser spot ages along the rims of the muscovite range from about 400 to 430 Ma. (Individual laser spot ages have a 2 sigma error of about 1%). The distribution of radiogenic Ar-40 in the muscovite suggests the following: the effective grain size for argon diffusion in this sample is substantially greater than the laser melt-pit size (about 80[mu]m in diameter), and appears to be on the scale of the actual grain size (about 2 mm diameter); the incremental heating procedure used by Laird and Albee (1974) homogenized the argon isotopic gradients in the sample, and yielded an age intermediate to the muscovite core and rim closure ages; the initial argon isotopic closure of the muscovite occurred at about 455 Ma following the metamorphism which produced the chloritoid-kyanite assemblage; and, near-edge portions of the muscovite lost at least 50% of their constituent radiogenic argon during Devonian metamorphism.

• OSTI ID: 5690663
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ARGON 39
ARGON 40
ARGON ISOTOPES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
DEVELOPED COUNTRIES
ELECTROMAGNETIC RADIATION
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
GEOLOGIC FORMATIONS
ISOTOPE DATING
ISOTOPES
LASER RADIATION
LIGHT NUCLEI
METAMORPHIC ROCKS
METAMORPHISM
MICA
MINERALS
MUSCOVITE
NORTH AMERICA
NUCLEI
RADIATIONS
RADIOISOTOPES
ROCKS
SILICATE MINERALS
STABLE ISOTOPES
USA
VERMONT
YEARS LIVING RADIOISOTOPES