Two diffusion pathways in quartz: A combined UV-laser and RBS study

The diffusive behavior of argon in quartz was investigated with three analytical depth profiling methods: Rutherford Backscattering Spectroscopy (RBS), 213 nm laser ablation, and 193 nm (Excimer) laser ablation on the same set of experimental samples. The integration of multiple depth profiling methods, each with different spatial resolution and sensitivity, allows for the cross-checking of methods where data ranges coincide. The use of multiple methods also allows for exploration of diffusive phenomena over multiple length-scales. Samples included both natural clear rock crystal quartz and synthetic citrine quartz. Laser analysis of clear quartz was compromised by poor coupling with the laser, whereas the citrine quartz was more easily analyzed (particularly with 193 nm laser). Diffusivity measured by both RBS and 193 nm laser ablation in the outermost 0.3 μm region of citrine quartz are self-consistent and in agreement with previously published RBS data on other quartz samples (including the clear quartz measured by RBS in this study). Apparent solubilities (extrapolated surface concentrations) for citrine quartz are in good agreement between RBS, 213 nm, and 193 nm laser analyses. Deeper penetration of argon measured up to 100 μm depth with the 213 nm laser reveal contributions of a second, faster diffusive pathway, effective in transporting much lower concentrations of argon into the crystal interiors of both clear and citrine quartz. By assuming such deep diffusion is dominated by fast pathways and approximating them as a network of planar features, the net diffusive uptake can be modeled and quantified with the Whipple-LeClaire equation, yielding δD_b values of 1.32 × 10⁻¹⁴ to 9.1 × 10⁻¹⁷ cm³/s. While solubility values from the measured profiles confirm suggestions that quartz has a large capacity for argon uptake (making it a potentially important sink for argon in the crust), the slow rate of lattice diffusion may limit its capability to take up argon in shorter lived geologic environments and in experiments. In such shorter-lived systems, bulk argon diffusive uptake will be dominated by the fast pathway and the quartz lattice (including natural isolated defects that may also be storing argon) may never reach its equilibrium capacity.