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DETAILED GEOCHEMICAL EVALUATION OF THE HETEROGENEITY OF A SINGLE PETRIFIED WOOD LOG – IMPLICATIONS FOR USE AS A PROVENIENCE TOOL IN ARCHAEOLOGY
The log was sawed into 18, 1/2 cm thick disk-shaped sections, which were further subdivided into 4 quadrant slabs. Semi-quantitative elemental abundances for these slabs were obtained by ED-XRF and LA-ICP-MS directly on the slabs themselves. In addition, 3 out of 4 quadrants for each disk were pulverized, and quantitative data for more than 30 elements from these 54 powders were obtained by ICP-MS.
Virtually all of the elements exhibited a range of abundances for this single log of at least a factor of 4, and in many cases more than an order of magnitude. The semi-quantitative LA-ICP-MS results demonstrate that this heterogeneity occurs on a millimeter scale. Standard deviations for most elements were greater than 20% relative, and many were more than 40% relative. The lone exception was U [13 to 25 ppm, standard deviation 2.6 ppm (15% relative)].
To evaluate the efficacy of applying 87Sr/86Sr ratios, 87Rb/86Sr ratios were calculated from the ICP-MS data (0.43 to 2.9, average 1.5, standard deviation 0.46). These results indicate that using measured 87Sr/86Sr, without correction for in-situ decay, would also yield highly variable results.
These results compare unfavorably with those typically obtained for obsidian (standard deviations for samples from a single locality are generally <10% relative), and suggest that, with the possible exception of U, elemental abundances may not be an efficacious tool for sourcing petrified wood artifacts from archaeological sites.
The financial support of the Colorado State Historical Fund, Project #2002-02-039, is gratefully acknowledged.