Evaluating effects of chemical weathering and surface contamination on the in situ provenance analysis of building stones in the Cuzco region of Peru with portable XRF

Abstract

As the only type of instrument capable of determining artifact geochemistry in a wide variety of settings, portable X-Ray Fluorescence (PXRF) may be frequently utilized as probably the best option for collecting data when removal of objects in part or in whole is unfeasible. However, using PXRF in circumstances where sample selection and preparation diverge from standards developed for lab-based instrumentation requires assessing the impact of new and potentially limiting factors to establish the validity of the method in these new contexts. Here we examine the effects of surface contamination and chemical weathering on the ability to assess provenance of igneous building stones used in the Cuzco region of Peru. Surface contamination was assessed through comparing low-impact cleaning methods on diorite and andesite blocks, and weathering was examined by comparing weathered vs. fresh surfaces of samples from two andesite quarries. In general, the lightest elements were most impacted by contamination and weathering while measurements of many of the heavier elements commonly used in provenance studies were relatively unaffected. Concentrations of Pb and Zn were problematic in the urban context of Cuzco because of pollution. Ability to determine provenance of building stones within those limitations (i.e., the validity of this application of PXRF) was demonstrated in a preliminary study by comparing measurements from uncleaned, exposed building stones in the region to fresh quarry samples.

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