Lattice diffusion of Ar in quartz, with constraints on Ar solubility and evidence of nanopores

Abstract

Diffusion and solubility of Ar in optically clear natural and synthetic quartz crystals were examined at ∼500 to 1200 °C by treating polished specimens in pressurized Ar (1–185 MPa) and characterizing the resulting diffusive-uptake (or subsequent diffusive-loss) profiles using Rutherford backscattering spectroscopy (RBS). Analytical uncertainty leads to significant scatter in the data, but the Ar diffusivity, D, is reasonably well constrained by the following Arrhenius relationship:D = 8.2−4.2+8.8 × 10 −19 exp[(−6150 ± 750)/T(K)] m 2/sNo effects of crystallographic orientation or quartz structural form (α or β) are discernible. Apparent solubilities typically fall between 1000 and 3000 ppm (by mass), with large uncertainties (±50–60% 2σ), but some lower values (∼700 ppm) are observed near the low end of the Ar pressure range investigated. Occasional high-concentration outlier values fall between 5000 ppm and 3.8 wt.% Ar. These do not correlate with Ar pressure, suggesting extrinsic (non-lattice) siting of Ar in some cases. Field-emission SEM images and numerical simulations of the diffusion process document isolated nanopores as the hosts for the occasional very high concentrations of Ar (observable pores range down to ∼10 nm in diameter; indirect evidence points to smaller ones as the more common sinks for Ar). The systematics of the data suggest an actual (lattice) solubility of ∼2000 ppm at 100- to 200-MPa Ar pressure, which is equivalent to a partition coefficient of ∼0.001cm 3STP/g · atm. Using either organic clathrate or fullerene as the Ar sources, 1-GPa experiments in a piston-cylinder apparatus result in similar uptake of Ar into quartz, in this case through partitioning equilibrium with C-O-H fluid (clathrate source) or amorphous carbon (fullerene source). The ability of quartz, relative to other minerals, to incorporate significant amounts of Ar may allow this ubiquitous and abundant mineral to serve as a local sink for Ar in crustal rocks lacking a free fluid phase. The diffusion data permit open-system behavior of Ar in quartz below the closure temperature of biotite and other 40Ar source minerals.