The thermal significance of potassium feldspar K-Ar ages inferred from 40Ar 39Ar age spectrum results

Abstract

40Ar 39Ar age spectrum analyses of three microcline separates from the Separation Point Batholith, northwest Nelson, New Zealand, which cooled slowly (~5°C-Ma-1) through the temperature zone of partial radiogenic 40Ar accumulation are characterized by a linear age increase over the first 65 percent of gas release with the lowest ages (~80 Ma) corresponding to the time that the samples cooled below about 100°C. The last 35 percent of 39Ar released from the microclines yields plateau ages (103,99 and 93 Ma) which reflect the different bulk mineral ages, and correspond to cooling temperatures between about 130 to 160°C. Theoretical calculations confirm the likelihood of diffusion gradients in feldspars cooling at rates ≤5°C-Ma-1. Diffusion parameters calculated from the 39Ar release yield an activation energy, E = 28.8 ± 1.9 kcal-mol-1, and a frequency factor/grain size parameter, D0 l2 = 5.6-3.9+14 sec-1. This Arrhenius relationship corresponds to a closure temperature of 132 ± 13°C which is very similar to the independently estimated temperature. From the observed diffusion compensation correlation, this D0 l2 implies an average diffusion half-width of about 3 μm, similar to the half-width of the perthite lamellae in the feldspars. The range in microcline K-Ar ages from the Separation Point Batholith is the result of relatively small temperature differences within the pluton during cooling. Comparison of the diffusion laws determined for microcline with those for anorthoclases and other homogeneous K-feldspars (E = 40 to 52 kcal-mol-1) reveals that Ar diffusion is more highly temperature dependent in the disordered structural state than in the ordered structural state. Previously published U-shaped age spectra are probably the result of the superimposition of excess 40Ar upon diffusion profiles of the kind described here. Show more