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Trace element chemistry of individual apatite phenocrysts as a tool for fingerprinting altered volcanic ash beds: Assessing interbed and intrabed variation at local and regional scales

George H. Shaw^,1

¹ Geology Department, Union College, Schenectady, New York 12308, USA

Trace element analyses of groups of single apatite microphenocrysts separated from K-bentonites define the intrabed homogeneity and interbed variability of apatites in a set of K-bentonites from the Gaspé Peninsula and the regional-scale homogeneity of the Tioga B K-bentonite. By analyzing many single grains from each bed it is possible to quantify the homogeneity of beds for statistical comparisons of similarity and differences. This approach leads directly to tests that can be used to chemically correlate (or distinguish) K-bentonite beds. The results from multiple K-bentonite beds from Gaspé demonstrate that many of them may be distinguished from one another. The results from the Tioga B K-bentonite demonstrate that there is little variation in apatite chemistry over distances of several hundred kilometers. This technique is advantageous for use in K-bentonite correlation because it provides a direct test of sample contamination as well as the necessary data on variance of each sample, allowing for robust statistical analysis.

Key Words: K-bentonite • Gaspé • Tioga • correlation • apatite • rare earth elements