PORTABLE HANDHELD X-RAY FLUORESCENCE (XRF) IN PROVENANCE STUDIES OF YAPESE STONE MONEY

The stone money of Yap (Federated States of Micronesia) are large disks regarded by Yapese islanders as culturally and economically valuable objects. Most stone money was carved out of flowstone precipitates from caves on an uplifted limestone terrain that forms the Rock Islands of the Republic of Palau and was then transported across land and sea >400 km to Yap. Some Yapese stone money is purported to have come from even more distant western Pacific islands: Guam, the Philippines, and Taiwan. Sampling of these objects for petrographic or geochemical analyses is not an option, so this research evaluated use of a non-destructive portable handheld XRF technique in provenance studies of stone money.

Flowstone samples from four locations on Palau were examined and compared to a sample of broken stone money acquired from Yap Historical Preservation Office staff. Fresh interior and weathered exterior surfaces of all samples were analyzed in the laboratory using a Bruker Tracer III-V XRF elemental analyzer in the “lab rat” mode, which involved screening for all elements without filters at 40 kV and 1.3 µA.

XRF graphs of all analyzed fresh sample interiors are very similar with a prominent Ca peak of calcite and a small Cl peak in one of the Palau samples that was collected in seawater. Elevated Fe, Sr, Cu and Zn concentrations made the exterior of the Yapese stone money sample significantly different from the Palau flowstone sample exteriors. On the other hand, exteriors of all Palau samples were very similar to each other with large Ca and small Fe peaks, while the sample collected in water also had a small Cl peak.

This study concluded that samples from Palau are compositionally identical to the analyzed stone money piece from Yap and that due to surficial contamination by soil and organic material there is more compositional variation between the interior and exterior of individual samples than between samples from different localities. This analysis could not discriminate between the four source rock sites on Palau and thus did not prove very useful. Given the lack of other non-destructive analytical techniques, to further test the usefulness of portable handheld XRF in discerning the provenance of Yapese stone money future studies would need to include cave flowstones from other Pacific islands and in situ analysis of more stone money.