Argon retentivity of hornblendes: A field experiment in a slowly cooled metamorphic terrane

Journal Article · Sun Nov 01 04:00:00 EST 1987 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(87)90365-6· OSTI ID:5152456

Onstott, T C; Peacock, M W ^[1]

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1. Princeton Univ., NJ (USA)

Hornblende from samples of amphibolite and granitic gneiss, collected within a single outcrop in the central Adirondacks, yield significantly different {sup 40}Ar*/{sup 39}Ark{sub K} dates of 948 {plus minus} 5 and 907 {plus minus} 5 Ma. Assuming that this terrane cooled slowly following high-grade metamorphism and that the samples have experienced the same thermal history, the difference in dates apparently reflects a corresponding difference in blocking temperature for diffusion of radiogenic argon in these hornblende samples. The Fe/(Fe + Mg + Mn) of the hornblende samples are 0.8 and 0.6, the higher ratio corresponding to the younger {sup 40}Ar*/{sup 39}Ar{sub k} date. Transmission electron microscopy observations indicate that both hornblende samples are homogeneous and devoid of any exsolution, but contain zones of fibrous phyllosilicates {approximately} 0.1 to 2 {mu}m wide parallelling (100) and (110). These alteration zones probably formed during post-metamorphic cooling as a result of the migration of fluids through the hornblendes, and are obvious pathways for argon escape from hornblende. As these features are more abundant in the hornblende sample with the younger {sup 40}Ar/{sup 39}Ar date and higher Fe/(Fe + Mg + Mn), they may influence the argon blocking temperature by effectively partitioning the hornblende grains into diffusion domains of varying size. Biotite from the granitic gneiss yields an {sup 40}Ar*/{sup 39}Ar{sub k} date of 853 {plus minus} 2 Ma, with a mildly discordant step-heating spectrum that in part reflects the degassing of submicroscopic inclusions precipitated during alteration of the host biotite. Plagioclase from the amphibolite yields a {sup 40}Ar/{sup 39}Ar{sub k} integrated date of 734 {plus minus} 3 Ma.

• OSTI ID: 5152456
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Journal Name: Geochimica et Cosmochimica Acta; (USA) Vol. 51:11; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ADIRONDACK MOUNTAINS
AGE ESTIMATION
AMPHIBOLE
AMPHIBOLITES
APPALACHIAN MOUNTAINS
ARGON
ARGON ISOTOPES
CHEMICAL COMPOSITION
CHEMISTRY
DISSOLUTION
ELEMENTS
FEDERAL REGION II
FLUIDS
GASES
GEOCHEMISTRY
GEOLOGIC HISTORY
GNEISSES
HORNBLENDE
HYDROTHERMAL ALTERATION
ISOTOPE DATING
ISOTOPES
METAMORPHIC ROCKS
METAMORPHISM
MINERALOGY
MINERALS
MOUNTAINS
NEW YORK
NONMETALS
NORTH AMERICA
RARE GASES
ROCKS
SILICATE MINERALS
USA