M.A. Theses: Alison Barbara Cormie, 1981

Chemical Correlation of Volcanic Ashes For Use As Stratigraphic Markers in Archaeology

Volcanic ashes from three Holocene eruptions in the Pacific Northwest (Mazama, 6,600 y.b.p., Bridge River 2,440 y.b.p. and Mt. St. Helens Yn 3,400 y.b.p.) are distributed throughout south central British Columbia. These tephras are often found in local (B.C.) archaeological deposits and once identified could provide archaeologists working in this region with excellent time-stratigraphic markers. In this work, we investigated the use of energy dispersive X-ray fluorescence (XES) analysis and alpha counting analysis of glass concentrates for identifying tephras on the basis of their major and trace element chemistry. Neutron activation analysis (NAA) was also used in order to verify results obtained by the other techniques.

As sample purification proves to be the greatest barrier to the rapid use of these instrumental techniques for routine analysis, a great deal of effort was directed towards simplifying sample preparation procedures. By extensively studying the effects of sample preparation on composition, we discovered that the <62μm size fraction of sieved ashes is mostly glass and can be used to identify the B.C. tephras with only simple pre-treatments. With such pre-treatments both XES and alpha-counting proved to be simple instrumental techniques that allowed the identification of the B.C. tephras with high degrees of reliability and a minimum of effort. With XES, Zr and K were useful elements for distinguishing among the three B.C. tephras; alpha counting permitted the ready identification of Mt. St. Helens Yn on the basis of U and Th concentrations. XES offered the advantage of rapid laboratory analysis while alpha-counting offered simple analysis with the potential of identifying tephras in the field.

In addition to developing these routine methods for identifying the B.C. tephras we used the three instrumental techniques to analyze glass separates from the B.C. tephras plus three additional sources. The results showed that by using the three instrumental techniques either singly or in combination one can readily distinguish most or all of these tephras. These methods could therefore be useful for characterizing tephras in an unknown region.