Weathering process of Nabari gabbroic body 2

Abstract

Weathering process of plagioclase in the middle depth of weathering crust which develops at the top of Nabari gabbroic body, Southwest Japan was examined mineralogically and geochemically. Unweathered core stones are coated, first by shell and then by saprolite in the upper depth (Zone A) and in the middle depth (Zone B), whereas they are directly coated by saprolite in the lower depth (Zone C). In the Zone B, the shell shows a sequence of different types of weathering products of plagioclase, which occur in layering from outer part to inner fresh part within the shell as follows:-compact kaolinite (Type 1); -sparse halloysite (Type 2); -void space with minor amount of gel (Type 3); -unweathered plagioclase. As for the formation of void space (Type 3), it is inferred that plagioclase dissolves congruently by chemical weathering at the middle depth of the weathering crust. As plagioclase dissolves, released constituent cations, including aluminum, are almost removed out from the shell, and no secondary minerals neoform in the immediate vicinity of dissolving plagioclase. The congruent dissolution indicates that the dissolution process is controlled kinetically by mineral-water interfacial reaction in non-equilibirium and that the released constituents are all removed out by transport of pore solution and/or by ion diffusion in aqueous media in open system. The lack of secondary minerals shows the need to introduce kinetics of non-equilibrium process also in precipitation.