Ar diffusion and solubility measurements in plagioclases using the ultra-violet laser depth-profiling technique

We describe the first direct measurements of Ar diffusion and solubility in plagioclases using ultra-violet (UV) laser ablation depth-profiling and noble gas mass spectrometer analyses of experimentally treated (599–1000 °C, 50–200 MPa of Ar) crystal fragments of labradorite and oligoclase. Labradorite ⁴⁰Ar gain diffusion profiles were measured, yielding an activation energy of 26.72±4.58 kcal mol⁻¹ (118.0±19.16 kJ mol⁻¹) and a frequency factor of 9.77×10⁻⁹ (+8.79×10⁻⁸, −8.79×10⁻⁹) cm² s⁻¹ (95% confidence). The Ar solubility in labradorite was measured yielding a value of <0.2 ppb bar⁻¹, which is similar to or lower than many rock forming minerals.

The labradorite diffusion parameters indicate Ar closure temperatures of 211 °C for a spherical diffusion geometry, and 243 °C for a planar diffusion geometry (for 100 µm-diameter grains, with cooling rates of 10 °C Ma⁻¹). The data indicate that labradorite is less Ar retentive than K-feldspar at low temperatures, but more Ar retentive than K-feldspar at high temperatures, corroborating previous work on plagioclase. The relatively slow Ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems.