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Silcrete Geological Source Discrimination with LA-ICP-MS
Silcrete Geological Source Discrimination with LA-ICP-MS for Minimally Destructive Archaeological Stone Tool Provenience Studies
Archaeological provenience studies are constrained by both the ethical obligation to use the least destructive techniques possible to preserve specimens for future analysis and by regulatory regimes that restrict how material culture objects may be studied.
Silcrete is a soil duricrust that has been used as a toolstone since at least the Middle Stone Age and is one of the earliest mineral resources to have been heat treated to improve material properties. In the last two decades, there have been substantial advances in the detection of heat-treated silcrete artifacts, as well as promising developments in geochemical provenience research. Provenience,
Figure 1: Three South African Silcrete Sources in this Study
or sourcing, studies of stone artifacts typically rely on measuring concentrations of chemical elements in geological source samples from known locations and archaeological artifacts of unknown origin. Elemental concentrations are a form of “closed” compositional data in which an increase in one element requires an equivalent decrease in the concentrations of the other elements present. This can yield unique geochemical “fingerprints” that permit multivariate statistical discrimination among sources and predictive assignment of artifacts to a source.
Figure 2: 5 ablation scan lines
Here, we present pilot research demonstrating the feasibility of using a powerful and minimally invasive technique, Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS), to measure elemental concentrations in 30 silcrete samples from three geological sources in coastal South Africa and discriminate among those sources. Solution ICP-MS and ICP-Optical Emission Spectroscopy have been used successfully for silcrete provenience studies but are destructive techniques that require laborious mechanical and chemical preparation of the digestion-resistant silcrete. For this pilot study, we sub-sampled each piece of silcrete for solution ICP-MS. Concentration data for a sub-set of the measured elements were used to discriminate among the three sources. The reserved undigested portion of each sample was
embedded in epoxy, polished, and ablated with five replicate scan lines. Rather than measuring fully quantitative concentrations by LA-ICP-MS, which requires knowing the concentration of a major element for use as an internal standard, we acquired semi-quantitative data externally calibrated to silicate glass reference materials. Concentrations for trace elements were then ratioed to the measured silicon concentration in each sample, effectively normalizing the data without a true internal standard. Statistical analysis of the ratio data was able to recapitulate the source discrimination achieved with the solution ICP-MS data set.
Authors:
- Andrew M. Zipkin, PhD, RPA, Associate Scientist, ICP Services at Eurofins EAG Laboratories
- Jayde N. Hirniak, School of Human Evolution and Social Change, Arizona State University, Tempe, AZ, USA
- John K. Murray, School of Human Evolution and Social Change, Arizona State University, Tempe, AZ, USA
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