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

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