Chemical aspects of viscous flow in planetary mantles

抄録

A critique is presented of the various mathematical rheological models, and their applications to the mantles of the Earth and the terrestrial planets is discussed. One of the models considered is derived from absolute reaction rate theory of chemical kinetics and allows for automatic transition between Newtonian (strain rate proportional to stress) and non-Newtonian viscous flow depending on the differential stress and the strain rate. A good fit to data is obtained when this model is applied to the post glacial Fennoscandian uplift. The deduced strain rate, as obtained from the assumption of proportionality between strain rate and rate of uplift, is in the range of 10^-16 to 10^-15sec^-1, while the deduced viscosities fall in the range of 10²² and 10²³ poises with uncertainties of an order of magnitude. Flow is found to be non-Newtonian but is becoming increasingly Newtonian with time. The various thermodynamic parameters of viscous flow, including pressure and temperature sensitivity, are also discussed and the viscous relaxation times of the different terrestrial planets are compared in terms of their temperatures. From this analysis it is concluded that the relatively low relief and the recently discovered circular depressions on Venus are probably a result of high lithospheric temperatures on this planet. The depressions are probably due to subsidence around basic volcanic centers and correspond to shield volcanos on Earth and Mars. Application of the same theory to terrestrial global plate tectonics leads to the conclusion that there should be viscous accretion of mantle material on the postulated cool descending slab of oceanic crust and that the viscosity of planetary mantles should be increasing with time.