Predicting Weathering Rind Thicknesses and Weathering Rates

Abstract:

Weathering rind thicknesses have long been used in geochronology, even though the rate at which they develop has never been established. Experimental studies show that the thickness of the weathered layer increases as an approximate power of the time, with an exponent between 0.5 (commonly considered diffusion controlled) and 1.0. We use percolation theory to show that the observed time dependences result from advective transport limitations. Calculations for non-reactive transport suffice, but Gaussian transport theory must be abandoned. Two-dimensional (2D) flow connectivity under unsaturated conditions generates an exponent of 0.82, 3D unsaturated flow the exponent 0.69, 2D saturated flow, 0.625, and 3D saturated conditions 0.53. As it turns out, these are the values observed, under conditions that match expectations, and with R² values often near 1 (0.999). The time derivative of the weathering rind thickness gives the weathering rate, and the corresponding powers in this case are – 0.18, -0.31, - 0.375, and – 0.47. At this time we can report studies with the first and the last values. The same scaling of length and time is observed in non-Gaussian transport in semiconductors and polymers. Of the 25 data sets that we have so far investigated, every one fits securely into one of the above classifications. Thus, we conclude that our current framework describes the known data on silicate weathering rates.