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

Abstract

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 = 18.477–19.161 ; 207Pb 204Pb = 15.562–15.679 ; 208Pb 204Pb = 38.17–38.85 . While the data form well-constrained linear arrays on Pb-Pb diagrams, no simple correlation exists with major element composition. Basaltic lavas (≤ 51 wt% SiO 2) are characterized by two isotopic groups. The olivine basalt (≤ 48% SiO 2) is more radiogenic than the plagioclase basalt (48–51%) which also shows more heterogeneity. In the silica range 52–55%, Pb isotopic ratios increase significantly but remain fairly constant in the range 55–70% SiO 2. 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 Pb isotopic data are qualitatively consistent with the contamination process described by Myers et al. (1984). However, because of fundamental differences in the mixing relations between the Sr system studied earlier and the Pb system, the new Pb data have revealed details of the process not apparent from the Sr data alone. In particular, it has been shown that the parent magma was more primitive than originally assumed, and that two contamination events are recorded in the lavas. The first event, involving a mafic parent and different crustal contaminants, produced the intermediate and siliceous hybrids in cupolas located above the main basaltic chamber. The types of country rock intruded as well as the degree of partial fusion achieved in individual cupolas controlled the range of hybrid compositions produced while the eruption sequence was determined by the order in which the cupolas were tapped. The second contamination event produced the plagioclase basalt, the most voluminous basaltic unit, by mixing the mafic parent with the olivine basalt, an independent, primary magma. Our results suggest crustal contamination models that assume bulk assimilation of crustal end members may be too simplistic.