THE UNIQUE LAHARS FROM THE SAME TECTONIC SOURCE: GEOCHEMICAL AND PETROLOGICAL DISCRIMINATORS

This study addresses whether volcanoes located the same tectonic setting produce chemically and petrologically unique lahars and which specific elements contribute to the differentiation of these lahars. This research includes analysis of geochemical signatures and a petrological investigation of the lahars as well as a complimentary analysis of linked glacial deposits. During field work samples of lahars were collected from Mt. Hood, Mt. St. Helens, Mt. Rainier, and Mt. Adams all located in the Cascade Range of Oregon. Glacial samples were collected in the Hood River, Toutle River, and the drainage of Mt. Rainier. Preliminary research used X-ray diffraction (XRD) to identify a predominantly clay rich and potassium rich feldspathic mineralogy across all lahars. Initial petrological analysis of select samples show Mt. Hood and Mt. St. Helens possessing similar petrologies with fine grained matrices supporting large angular grains of both detrital minerals (Kspar, Qtz, Cpx, Opx, and rare Bt) and volcanic lithics. In these samples Cpx occurs significantly more often than Opx. The lithics present are of two types either microlitic and pilotaxitic or vitreous. Analysis of the other mountains included in the study is ongoing. Information on the chemistry of the lahars and glacial samples was obtained through X-ray fluorescence (XRF) and Scanning Electron Microscopy (SEM-EDAX). Statistical analysis of the data included a Bray-Curtis Matrix, the application of discrimination plots to data from EDAX and XRF analyses, ANOVA tests were run on all the data, as was a SIMPER analysis. Initial analyses of these data suggest that while these lahars may appear similar there are strong differences in their trace element chemistry. Here it is seen that strontium and barium can be used to differentiate the lahars despite their shared tectonic source. Through the initial SIMPER analysis it is shown that within each sample set consisting of lahars from a single volcano Sr and Ba contribute to average similarities above 90% when comparing the lahars with ones from other volcanoes the abundances of Sr and Ba contributed to strong (greater than 70%) dissimilarity. Other elements serving as possible discriminators include V, Zn, Cr, and Ni.