Laboratory Equipment and Methods
Non-destructive quantitative
analyses of obsidian are performed at Geochemical Research Laboratory (GRL)
on a QuanX EC (Thermo Electron Scientific Instruments Corporation) energy
dispersive x-ray fluorescence (edxrf) spectrometer. X-ray spectra are acquired
and elemental intensities extracted for each peak region of interest, after
which matrix correction algorithms are applied to specific regions of the
x-ray energy spectrum to compensate for inter-element absorption and enhancement
effects. Following these corrections, intensities are converted to concentration
estimates by employing least-squares calibration lines established for each
element from analysis of up to 30 international rock standards certified by
the U.S. Geological Survey, the U.S. National Institute of Standards and Technology,
the Geological Survey of Japan, the Centre de Recherches Petrographiques et
Geochimiques (France), and the South African Bureau of Standards.
Trace element measurements are expressed in quantitative units (i.e. parts per million [ppm]
and/or weight percent composition [%]), and matches between unknowns (i.e.
archaeological artifacts) and known geologic obsidian chemical groups are
made on the basis of correspondences (at the 2-sigma level) in diagnostic
trace element concentration values (typically ppm values for Rb, Sr, Y, Zr,
Nb and, when necessary, K, Ca, Ba, Ti, Mn and Fe2O3T) or Fe/Mn ratios. Artifact-to-obsidian
source (geochemical type) correspondences are considered reliable if diagnostic
mean measurements for artifacts fall within 2 standard deviations of mean
values for geologic source standards. Composition measurements are reported
to the nearest ppm (or, for Fe2O3T, to nearest hundredth wt. %) to reflect
calibration-imposed resolution capabilities of non-destructive edxrf spectrometry.
Current resolution limits at GRL for the determination of Rb are about 4 ppm;
Sr about 3 ppm; Y about 3 ppm; Zr about 4 ppm; and Nb about 3 ppm; Ba about
13 ppm; Ti about 15 ppm; Mn about 10 ppm; and Fe2O3T about .02%. When counting
and fitting error uncertainty estimates (± values) for an obsidian
sample exceed the element-specific resolution limits given above, the larger
number incorporates both composition variation and measurement error arising
from differences in sample size, surface and x-ray reflection geometry.