Argon diffusion in pyroxenes: Implications for thermochronometry and mantle degassing

Abstract

The rate at which argon diffuses in pyroxenes is relevant to several problems in the Earth and planetary sciences. Pyroxenes are among the most abundant minerals in the Earth's upper mantle and are important hosts for potassium and radiogenic 40Ar ( 40Ar*). Additionally, pyroxenes commonly contain > 25% of the total 40Ar* within meteorites, and therefore hold great potential for quantifying the timing, duration, and thermal conditions of events experienced by meteorites and their parent bodies. We conducted detailed, feedback-controlled laser heating diffusion experiments using synthetically produced 37Ar in gem-quality clinopyroxene (CPX) and orthopyroxene (OPX) crystals. Stepwise heating between 850 and 1350 °C yielded linear Arrhenius arrays from which activation energies ( E a ) of 379.2 ± 4.1 and 371.0 ± 6.0 kJ/mol and pre-exponential factors ( D o ) of (1.36 × 10 − 4 ) − 6.15 × 10 − 5 + 1.12 × 10 − 4 and (5.73 × 10 − 2 ) − 2.25 × 10 − 2 + 3.72 × 10 − 2 were determined for CPX and OPX, respectively; these E a 's are an order of magnitude higher than previously reported values obtained with a different experimental approach ( Watson et al., 2007) and indicate that Ar diffusion within pyroxenes is strongly temperature dependent. At high temperatures Ar diffusion in pyroxenes is rapid, which indicates that diffusive 40Ar distributions observed within meteoritic pyroxenes may provide information about brief, high-temperature shock heating events. In contrast, subsequent low-T conditions should minimally influence these concentration gradients. The experimental Ar diffusion kinetics in OPX and CPX correspond to closure temperatures of 600–700 and 700–800 °C, respectively (for 10 °C/Ma cooling). These results suggest that diffusive equilibration through 0.1–0.5 mm pyroxene grains occurs in minutes to hours at basaltic melt generation temperatures (~ 1300 °C), and is therefore unlikely to inhibit mantle degassing.