The rapid heating, defocused beam technique: A CO₂-laser-based method for highly precise and accurate determination of δ¹⁸O values of quartz

We describe a laser fluorination technique specifically for quartz that allows both high-accuracy and high-precision determination of oxygen isotope ratios, regardless of grain size. The rapid heating, defocused beam technique (RHD) utilizes a defocused 32 W CO₂ laser at full power which delivers 17 W (∼ 20 W/mm²) to the sample to quickly react quartz. In contrast, other recent laser fluorination studies use a tightly focused laser beam and slow heating (i.e. Sharp and Kirschner, 1995; Fouillac and Girard, 1996; Kirschner and Sharp, 1997), and find laser analysis of fine-grained quartz to be inaccurate. Three quartz standards, NBS-28, QZ-BRA, and QZ-CWRU were analyzed using the RHD technique at the University of Wisconsin to test for grain size effects. RHD analyses show no correlation between grain size and δ¹⁸O values and are in excellent agreement with δ¹⁸O values obtained using conventional fluorination techniques. Additional analyses of the same quartz standards performed at University of Wisconsin using slow heating and a focused beam yield δ¹⁸O values that are both less precise and significantly (up to 0.8‰) lower than the accepted values. Attempts to use the RHD technique with a 20 W CO₂ laser which delivers 8 W (∼ 10 W/mm²) to the sample at BRGM were unsuccessful, probably due to insufficient power density of the defocused beam. We conclude that the RHD technique yields excellent accuracy and precision, but that power densities of > 15-20 W/mm² may be necessary across a large part of the sample surface. We recommend the use of lasers with at least 30 W of power.