Argon retention properties of silicate glasses and implications for 40Ar/39Ar age and noble gas diffusion studies

Sized aggregates of glasses (47-84 wt% SiO2) were fused from igneous-derived cohesive fault rock and igneous rock, and step-heated from similar to400 to >1,200 degreesC to obtain their 39Ar diffusion properties (average E = 3 3,400 cal mol(-1); D-o=4.63x10(-3) cm(2) s(-1)). At T < similar to1,000 degreesC, glasses containing < similar to69 wt% SiO2 and abundant network-forming cations (Ca, Fe, Mg) reveal moderate to strong non-linear increases in D and E, reflecting structural modifications as the solid transitions to melt. Extrapolation of these Arrhenius properties down to typical geologic T-t conditions could result in a 1.5 log(10) unit underestimation in the diffusion rate of Ar in similar materials. Numerical simulations based upon the diffusion results caution that some common geologic glasses will likely yield 40Ar/39Ar cooling ages rather than formation ages. However, if cooling rates are sufficiently high, ambient temperatures are sufficiently low (e.g., < 65-175 degreesC), and coarse particles (e.g., radius (r) > similar to1 mm) are analyzed, glasses with compositions similar to ours may preserve their formation ages.