Systematic analysis of K-feldspar {sup 40}Ar/{sup 39}Ar step heating results: I. Significance of activation energy determinations

Journal Article · Fri Aug 01 04:00:00 EDT 1997 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/S0016-7037(97)00147-6· OSTI ID:585148

Lovera, O M; Grove, M; Harrison, T M; Mahon, K I ^[1]

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1. Univ. of California, Los Angeles, CA (United States)

To better understand the argon retention properties of basement K-feldspars, {sup 40}Ar/{sup 39}Ar step-heating results for 115 specimens representing a wide range of temperature-time evolution from a diverse array of geologic environments have been systematically evaluated. In carrying out the measurements, we instituted nonconventional analysis routines including: (1) duplicate isothermal steps; (2) multiple isothermal steps at 1100{degrees}C to extract as much gas as possible prior to melting; and (3) temperature cycling. To maintain a self-consistent approach, we systematically applied a weighted least square regression to determine the activation energies (E) and log(D{sub 0}/r{sub 0}{sup 2}) values. Activation energies were found to define a normal distribution (46 {+-} 6 kcal/mol) spanning over 30 kcal/mol. Corresponding log(D{sub 0}/r{sub 0}{sup 2}) values (5 {+-} 3 s{sup -1}) were highly correlated with E, yielding a slope that reproduces the previously documented feldspar compensation relationship. Numerical analysis of this correlation permits us to rule out large systematic laboratory errors in our data. In applying the multiple diffusion domain (MDD) model, we observed general tendencies in the domain distribution of basement K-feldspars. For the majority of samples, most {sup 39}Ar resides in the larger domains. The smallest domains generally constitute <5% volume fraction of the sample and tend to plot {approximately}2 orders of magnitude above log (D{sub 0}/r{sub 0}{sup 2}). Alternatively, the largest domains constrained by {sup 39}Ar released below melting have log (D/r{sup 2}) values that are typically {approximately}3 orders of magnitude smaller than log(D{sub 0}/r{sub 0}{sup 2}). Systematic analysis of the database demonstrates that the diffusion behavior predicted by the MDD model prevails for virtually all samples. 44 refs., 14 figs., 2 tabs.

• OSTI ID: 585148
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 15 Vol. 61; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
58 GEOSCIENCES
ACTIVATION ENERGY
ARGON 40
FELDSPARS
GEOTHERMOMETRY
TECTONICS