In situ monitoring of elemental losses and gains during weathering using the spatial element patterns obtained by portable XRF

Abstract

Weathering would produce damage to objects. This paper proposed a method to monitor geochemical variations on outcrop surfaces, through in situ measurement using portable X-ray fluorescence (pXRF) analyser at high spatial resolution, and qualitative and quantitative determination of elemental gains and losses based on spatial patterns of elemental concentrations and calculated enrichment factors. Taking into account the representativeness of sample matrices, a weathering diabase profile consisting of saprolite and the corresponding rock was selected. Comparisons were made between the laboratory results and the pXRF results, besides, the difference between the two test modes based on different calibration principles (mining mode: Fundamental Parameters calibration; soil mode: Compton normalization) in measuring multi-matrices was evaluated. The pXRF results were comparable to the laboratory results for most elements (excellent correlation: Ca, K, Mn, Mo, Zn, and V; good correlation: Ti, Rb, and Sr; moderate correlation: Fe, Si, and Zr; bad correlation: Ba and Al). Much lower readings for light elements would be obtained from the mining mode when loose samples were analysed, and the soil mode was more suitable for measuring a weathering profile, and if light elements such as Mg, Al, and Si are involved, the mining mode can be used, and the reading ranges of the light elements will require verification. Elemental enrichment and depletion were discussed based the spatial pattern of elemental concentrations, and four groups of elements of continuous emigration, slight emigration, local immigration caused by later veins, and continuous immigration with weathering were effectively discriminated based on the spatial pattern of elemental enrichment factors in the profile. Using this method we could rapidly ascertain the migration characteristics of multielements and locate the spatial positions of intense weathering, providing a new insight into the measuring and monitoring of chemical changes with weathering in rock outcrop, building stone, and so on.