sup 40 Ar/ sup 39 Ar results for alkali feldspars containing diffusion domains with differing activation energy

Journal Article · Wed May 01 04:00:00 EDT 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90319-Z· OSTI ID:6140253

Harrison, T M; Heizler, M T ^[1]; Lovera, O M ^[2]

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1. Univ. of California, Los Angeles (United States)
2. California Inst. of Tech., Pasadena (United States)

Extension of the single diffusion domain/activation energy closure model of Dodson to apply to minerals with a discrete distribution of domain sizes appears to reconcile {sup 40}Ar/{sup 39}Ar age spectra of alkali feldspars with their associated Arrhenius plots. However, small remaining discrepancies, particularly apparent in log (r/r{sub o}) plots, suggest that some, and perhaps most, alkali feldspars contain diffusion domains with activation energies that may vary by as much as 8 kcal/mol. An important consequence of even relatively small variations in activation energy between domains is that the shape of an age spectrum can change dramatically by varying the laboratory heating schedule. We find that Arrhenius and log (r/r{sub o}) plots have the potential to reveal even small differences in activation energy ({approximately}2 kcal/mol) between domains, at least in cases where the domains are well separated in size. Variations in activation energy of {approximately}5 kcal/mol can result in differences in calculated closure temperature of up to 30C from that obtained assuming equal activation energies for all domains. Overestimates of apparent activation energy and other inconsistencies resulting from reversed heating experiments may reflect annealing of subgrain features which define the smallest diffusion domain size. Use of the diffusion compensation relationship may provide a way to assess the relative distribution of diffusion domain sizes in a sample containing domains with multiple activation energies.

• OSTI ID: 6140253
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 55:5; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ACTIVATION ENERGY
AGE ESTIMATION
ARGON 39
ARGON 40
ARGON ISOTOPES
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
ENERGY
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FELDSPARS
GEOLOGIC HISTORY
ISOTOPE DATING
ISOTOPE RATIO
ISOTOPES
KINETICS
LIGHT NUCLEI
MINERALS
NUCLEI
RADIOISOTOPES
REACTION KINETICS
SILICATE MINERALS
STABLE ISOTOPES
VARIATIONS
YEARS LIVING RADIOISOTOPES