Investigations of an intrusive contact, northwest Nelson, New Zealand-II. Diffusion of radiogenic and excess 40Ar in hornblende revealed by 40Ar39Ar age spectrum analysis

Abstract

The Rameka Gabbro, emplaced 367 Ma ago, experienced a well documented reheating on intrusion of the Separation Point Batholith 114 Ma ago. 40Ar39Ar age spectrum analyses of hornblende from the Rameka Gabbro show diffusion gradients which provide information on the 40Ar boundary concentration during reheating. Three samples of hornblende exhibit age spectra that conform to a model of 40Ar loss by diffusion, implying a zero 40Ar boundary concentration during heating. The calculated 40Ar loss from these samples, together with a model of heat flow in the aureole, provide estimates of diffusion coefficients of 40Ar in Mg-rich hornblende which correspond to an activation energy, E, of ~60 kcal-mol-1 and a frequency factor. D0, of ~ 10-3 cm2-sec-1. When combined with laboratory diffusion results, these data yield a well defined diffusion law (E = 63.3 ± 1.7 kcal-mol-1, D0 = 0.022 +0.048-0.010 cm2-sec-1). The age spectra of the eight other samples record steep gradients of excess 40Ar over the first few percent of gas release. Although this effect causes high apparent conventional K-Ar ages, the plateau segments of many sampes still record the crystallization age of 367 ± 5 Ma. These measurements show that the excess 40Ar phase developed locally in the intergranular regions of the gabbro, following intrusion of the batholith. on time scales that varied from 104 to 106years. The minimum average 40Ar36Ar ratio of this component was found to be 1300 ± 400. The partial pressure of Ar was at least 10-2 bars in some places. A single 40Ar39Ar age spectrum analysis of plagioclase reveals a 'saddle-shaped" release pattern with a minimum at 140 Ma. In conjunction with theoretical diffusion models and a diffusion law, 40Ar39Ar age spectrum analysis of hornblende that has experienced a post-crystallization heating can provide close estimates of the maximum temperature of the thermal event as well as both age of crystallization and reheating. © 1980.