PB AND HF ISOTOPE DATA FOR MAFIC LAVAS FROM THE THREE SISTERS AND NEWBERRY VOLCANO REGIONS OF THE CENTRAL OREGON CASCADES
Pb and Hf isotope data were collected for the same samples previously analyzed for major and trace elements, U-series (U-Th-Ra-Pa), and Sr-Nd isotopes, using a MC-ICP-MS at the University of New Mexico. Ranges in Pb and Hf ratios, the first Hf data from the Oregon Cascades, are consistent with existing Cascade arc data: 206Pb/204Pb = 18.821–19.022, 207Pb/204Pb = 15.576–15.617, 208Pb/204Pb = 38.391–38.424, and 176Hf/177Hf = 0.283032–0.283094 (εHf = 9.19–11.38). The most primitive lavas define a narrower range of 176Hf/177Hf values, with εHf > 10.28. The most primitive lavas also define narrower ranges in Pb isotope ratios, intermediate between the Three Sisters and Newberry basaltic andesites. Pb and Hf isotope ratios require a small contribution from subducting sediments, consistent with the Sr-Nd data.
The difference in Pb isotope ratios between basaltic andesites from the Three Sisters and Newberry confirms the interpretation that differentiation of primitive magmas occurs as a result of mixing/assimilation at distinct levels within the crust in different parts of the arc. The similarity in Pb isotope ratios between primitive and more evolved lavas in the Three Sisters region is consistent with interaction between primitive melts and mafic lower crust having similar, but slightly less radiogenic, Pb values. In contrast, basaltic andesites at Newberry have Pb isotope ratios higher than the most primitive lavas, consistent with assimilation of felsic upper crust.