Micromorphological Characterization and Microchemical Quantification of Weathering in an Alkali Basalt Pebble

Abstract

Optical and submicroscopical techniques enable in situ chemical and mineralogical quantification of weathering and neoformation of minerals. A mass balance can be set up if the neoformed minerals can be related to their source. In thin section, a partially weathered basalt pebble enclosed by a weakly anisotropic, partially isotropic nonlaminated clay coating was examined. Such coatings were absent around any other adjacent grain surface and voids. Both observations indicated that the clay coating formed from precipitation of constituents weathered from the pebble. Weathering was considered isovolumetric because original shapes and volumes of the dissolved phenocrysts and groundmass of the pebble are maintained. The weathering rind closest to the unaltered core was characterized by weathering of glass. The strongly altered outer weathering rind was characterized by intensive pyroxene alteration. Chemical trends of various zones were in good agreement with micromorphological observations and mineralogical calculations. Mass balance calculations indicated that all elements except Fe were leached from the entire weathering rind. Essentially all Na and K, a considerable part of the Mg (44%), Ca (58%), and Si (29%), and a small amount of the Ti (15%) were leached from the rind. Only Si, Al, and a small amount of Ca were found in the neoformed coating. More Al was found in the coating than may have been leached from the rind, suggesting an external Al source when isovolumetric weathering is assumed. Sensitivity analysis of the calculation method concerning the consequences of changes in bulk density of core, rind, and coating showed similar trends in the elemental chemical amounts, except for Al. The isovolumetric method was successfully applied in this microscale weathering study.