SYSTEMATIC ANALYSIS OF K-FELDSPAR AR-40 AR-39 STEP HEATING RESULTS .1. SIGNIFICANCE OF ACTIVATION-ENERGY DETERMINATIONS/

To better understand the argon retention properties of basement K-feld spars, Ar-40/Ar-39 step-heating results for 115 specimens representing a wide range of temperature-time evolution from a diverse array of ge ologic environments have been systematically evaluated. In carrying ou t the measurements, we instituted nonconventional analysis routines in cluding: (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 determ ine the activation energies (E) and log(D-0/r(0)(2)) values. Activatio n energies were found to define a normal distribution (46 +/- 6 kcal/m ol) spanning over 30 kcal/mol. Corresponding log(D-0/r(0)(2)) values ( 5 +/- 3 s(-1)) were highly correlated with E, yielding a slope that re produces the previously documented feldspar compensation relationship. Numerical analysis of this correlation permits us to rule out large s ystematic laboratory errors in our data. In applying the multiple diff usion domain (MDD) model, we observed general tendencies in the domain distribution of basement K-feldspars. For the majority of samples, mo st Ar-39 resides in the larger domains. The smallest domains generally constitute <5% volume fraction of the sample and tend to plot similar to 2 orders of magnitude above log (D-0/r(0)(2)). Alternatively, the largest domains constrained by Ar-39 released below melting have log(D /r(2)) values that are typically similar to 3 orders of magnitude smal ler than log(D-0/r(0)(2)). Systematic analysis of the database demonst rates that the diffusion behavior predicted by the MDD model prevails for virtually all samples. However, two kinds of anomalous degassing b ehavior were observed. The first appears to be due to our inability to isolate effects resulting from distributions characterized by very sm all domains while the second may result from inhomogeneous K-distribut ions. Although these phenomena are capable of producing a wide range o f calculated diffusion parameters, close inspection reveals that impor tant systematics of the K-feldspar results cannot be accounted for the se factors and remain best explained by real intrasample differences i n diffusion properties. Finally, while uncertainties in E determined f rom Ar-40/Ar-39 step-heating experiments can produce significant dispe rsion in calculated temperatures (similar to 10 degrees C/kcal/mol), t he overall form of the cooling history, usually the most important res ult for tectonic applications, is preserved. Copyright (C) 1997 Elsevi er Science Ltd.