PB AND HF ISOTOPE DATA FOR MAFIC LAVAS FROM THE THREE SISTERS AND NEWBERRY VOLCANO REGIONS OF THE CENTRAL OREGON CASCADES

The Cascade arc is an end member convergent margin in which the young and slowly subducting slab is hotter than in most subduction zones. This should affect the timing and location of slab dehydration, with implications for the processes of magma generation in the overlying mantle wedge. Radiogenic isotope ratios have the potential to constrain the extent of involvement of slab-derived components (e.g. crustal and sediment fluids, melts, etc.). Prior work on a suite of basaltic and basaltic andesite lavas from the central Oregon Cascades suggested limited involvement of subducted sediments during magma generation. Additionally, interaction with different crustal components has been invoked to explain the generation of the more evolved basaltic andesite lavas beneath the main arc (Three Sisters region) and back-arc (Newberry region).

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 (ε_Hf = 9.19–11.38). The most primitive lavas define a narrower range of ¹⁷⁶Hf/¹⁷⁷Hf values, with ε_Hf > 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.