HIGH SPATIAL RESOLUTION 40AR/39AR INVESTIGATIONS USING AN ULTRA-VIOLET LASER PROBE EXTRACTION TECHNIQUE

Ultra-violet laser extraction offers a significant new approach to 40Ar39Ar dating. The technique surmounts two important drawbacks to the existing visible and near IR laser probe 40Ar-39Ar techniques; the difficulty in analysing minerals such as quartz, plagioclase, and K-feldspar which are transparent at these wavelengths, and the spatial resolution (~50 μm) limited by argon loss due to beam reflection/ refraction into the surrounding minerals. Three experiments demonstrate the advantages of UV laser ablation using a quadrupled Nd-YAG laser (λ = 266 nm) in applications to geological problems. 1. (1) A traverse across a biotite/quartz boundary demonstrates that UV laser ablation extracts negligible amounts of argon from beyond the visible laser pit, even in quartz. 2. (2) Analyses of a large plagioclase phenocryst from the Parana volcanic field, Brazil, differentiated pristine and altered areas. Analysis of the feldspar grain margin caused no detectable argon loss from groundmass only 20 μm distant. 3. (3) A depth profile in K-feldspar produced by repeatedly rastering the UV laser beam across an area achieves yet higher spatial resolution. Argon diffusion profiles around 20 μm deep, were measured with a spatial resolution of 2–3 μm in a sample of gem quality K-feldspar, thermally treated at 700°C and 2 kbar argon pressure.