Argon behaviour in gem-quality orthoclase from Madagascar: Experiments and some consequences for [formula omitted] geochronology

Abstract

In order to investigate argon diffusion in as simple a K-feldspar structure as possible, a single crystal of the gem-quality Itrongay K-feldspar from Madagascar has been studied using cycled step heating, ultra-violet (UV) laser depth profiling, in vacuo crushing, Electron Microprobe Analysis (EMPA) and X-ray diffraction (XRD) techniques. The results have been modelled using both the multidomain model and a multipath model invoking pipe or short circuit (SC) diffusion. Cycle heating (both forward and reversed) indicates a very retentive K-feldspar, in which the great majority of argon release conforms to a simple model of volume diffusion with a single domain size, a single activation energy, and is associated with a plateau age of 435 ± 8 Ma. However, the first 1% of argon release exhibits younger ages and the same type of complex behaviour seen in other K-feldspars. Modelling the argon release in terms of multidomains yields good fits to data with four domains of varying activation energies. An alternative model for the argon release, involving not only volume diffusion through the lattice but also SC or pipe diffusion and mass transfer between lattice and rapid diffusion paths, provides a good though less sophisticated model to explain the argon release. Argon concentration/depth profiles of a previously outgassed sample, measured using the UV laser ablation technique, exhibited argon loss in only the upper 5–10 μm. Argon loss calculated from the measured profiles suggests that the low temperature domains outgassed during step heating were within 5–10 μm of the grain surfaces and thus may be artefacts of sample preparation and surface texture effects. In vacuo crushing of an untreated sample released argon with an older age than the plateau value, which was not detected by any of the other analytical techniques. However, previously cycled step heated samples contained very little excess argon, suggesting the existence of traps within the feldspar structure into which 40Ar diffused in nature to yield apparent excess argon, but which were then filled with corresponding 39Ar during cycle heating. The K-feldspar structure illuminated by these studies is one in which argon diffuses through the lattice over distances of at least 100 pm probably via a volume diffusion mechanism to the grain boundaries. However, close to the surface argon may also diffuse along fast diffusion paths resulting from natural traps opened during sample preparation. Though restricted to very small volumes of gas in the case of the Itrongay orthoclase gem, the existence of the traps has potentially important consequences for 40 Ar 39 Ar geochronology applied to K-feldspars.