A detailed Pb isotopic study of crustal contamination/ assimilation: the Edgecumbe volcanic field, SE Alaska

To better understand the process of crustal contamination/ assimilation, 23 Pb isotopic compositions and 12 concentrations have been measured on lavas and basement rocks from the Edgecumbe volcanic field, SE Alaska. Measured isotopic ratios have the following ranges: 206Pb/204Pb = 18.477 to 19.161; 207Pb/204Pb = 15.562 to 15.679; 208Pb/204Pb = 38.17 to 38.85. While the data form well-constrained linear arrays on Pb-Pb diagrams, no simple correlation exists with major element composition. Basaltic lavas (<= 51 wt% SiO2) are characterized by two isotopic groups. The olivine basalt (<= 48% SiO2) is more radiogenic than the plagioclase basalt (48 to 51%) which also shows more heterogeneity. In the silica range 52 to 55%, Pb isotopic ratios increase significantly but remain fairly constant in the range 55 to 70% SiO2. Lead concentrations vary from 1 ppm in the basalts to 7 ppm in the rhyodacites. Analyzed basement rocks are more radiogenic than any of the lavas (206Pb/ 204Pb = 19.20; 207Pb/204Pb = 15.65; 208Pb/204Pb = 38.86). The results are discussed. (author)