High spatial resolution 40 Ar / 39 Ar investigations using an ultra-violet laser probe extraction technique

Details High spatial resolution 40 Ar / 39 Ar investigations using an ultra-violet laser probe extraction technique High spatial resolution 40 Ar / 39 Ar investigations using an ultra-violet laser probe extraction technique

Aug 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.3519k&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 16, pp.3519-3525

Mathematics

Logic

13

Scientific paper

Ultra-violet laser extraction offers a significant new approach to 40 Ar / 39 Ar dating. The technique surmounts two important drawbacks to the existing visible and near IR laser probe 40 Ar- 39 Ar techniques; the difficulty in analysing minerals such as quartz, plagioclase, and K-feldspar which are transparent at these wavelengths, and the spatial resolution (~50 m) 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 ( = 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 Paraná volcanic field, Brazil, differentiated pristine and altered areas. Analysis of the feldspar grain margin caused no detectable argon loss from groundmass only 20 m 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 m deep, were measured with a spatial resolution of 2-3 m in a sample of gem quality K-feldspar, thermally treated at 700°C and 2 kbar argon pressure.

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