A viable transport mechanism to degas the continental Crust

Diffusion A viable transport mechanism to degas the continental Crust  

Estimating the diffusivity of noble gases through crustal rocks—as opposed to 

The pattern of excess °Ar trapped in minerals can be used to estimate the bulk diffusivity through the rock. In theory, the concentration of °Ar in a mineral above its closure temperature should be close to zero. However, if the Ar is produced in the rock faster than it can diffuse out, then the mineral is likely to retain some excess °Ar. The build-up of °Ar in a mineral above its closure temperature is a function of the relative rates of Ar production and diffusion out of the rock, not the mineral. Utilizing outcrop-scale gradients in °Ar concentrations in an amphibolite from the Simplon pass Switzerland, Baxter et al. (2001) calculate a bulk Ar diffusivity of the order 10 cm s at a temperature of 500°C (the conditions of their study). These results probably define the upper limit on the bulk Ar diffusivity in the upper crust (at least, for the immediate lithology of amphibolite) as diffusion rates are likely to decrease at lower temperatures. Studies by Poland (1979) and Scailliet (1996) also correlate excess °Ar buildup with limited bulk diffusivity. 

Torgersen (1989) formulates the steady-state He flux (F) as a function of the production rate (G) and diffusion geometry of the system. In an homogeneous slab 

In crystalline rocks, this transition may occur over a broad temperature range, but typically occurs between 300-450°C (Manning and Ingebritsen 1999 Simpson 1999). For typical crustal gradients of 30°C/km, transition from the brittle to the ductile region occurs between 10- to 15-km depth. 

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