Hot fluids or rock in eclogite metamorphism? (Reply): Geophysics

Bjørnerud and Austrheim 1 interpret the geological evidence in the rocks of Holsnøy at Lindås nappe, Norway, to be inconsistent with our cold-crust model 2 , but do not question our new argon isotopic data, on which we base the thermal history of the terrain. A critical flaw underlying their arguments 1 , 3 is the implicit assumption that element diffusion does not occur in dry environments, although there is clear evidence to the contrary 2 , 4 , 5 . Counter to earlier claims 3 , 6 of element and isotope immobility in dry rocks, we have demonstrated the existence of diffusion profiles in phlogopite associated with the uptake of argon during the Caledonian in ‘unreacted’ protolith of the Lindås nappe. Diffusion has taken place in these dry rocks and cannot be ignored.