MAGNETITE (U-TH)/HE GEOCHRONOLOGY CALIBRATION AND INVESTIGATION OF THE COLUMBIA RIVER BASALT GROUP

The development of the magnetite (U-Th)/He dating technique offers an alternative method to traditional geochronometric tools for high-precision age constraints of extrusive, mafic volcanics. Difficulty in determining absolute eruption ages of basaltic, volcanic rocks by groundmass and plagioclase K-Ar and ⁴⁰Ar/³⁹Ar geochronology is rooted in their excess ⁴⁰Ar and/or recoil, lack of suitable mineral phases such as sanidine and zircon, and susceptibility to alteration. Because of these geological and analytical complications, established geochronometers may not provide accurate and precise ages; recent studies have shown, that magnetite (U-Th)/He dating presents great promise as an alternative volcanic geochronometer. Magnetite is a pervasive mineral found within such rocks, therefore it is possible to measure the accumulation of radiogenic ⁴He from U, Th, Sm decay to determine an eruption age for aphanitic and/or altered basalts. In an effort to refine and test this method, the Columbia River Basalt Group (CRBG) in the NW U.S.A provides an ideal area to systematically study this geochronometer given the vast amount of existing petrologic and geologic data.

With this study, we expect to establish a detailed protocol for obtaining high-resolution magnetite (U-Th)/He and to provide a rigorous evaluation of the reliability and reproducibility of age data in comparison with existing chrono-, chemo-, and magnetostratigraphic framework of the CRBG including the Steens Basalt of southeastern Oregon. This will help establish guidelines for data quality control of magnetite He dating and for inter-comparison with traditionally used dating methods such as ⁴⁰Ar/³⁹Ar. Besides further establishing magnetite (U-Th)/He dating as a reliable basalt geochronometer, the ultimate objective of the proposed study is to provide reliable new age constraints for the CRBG members, with special emphasis on the basal portion of the CRBG (Imnaha) and its correlation with the Upper Steens Basalt. Doing so will both improve the regional correlation and help elucidate the nature, spatial distribution, and geodynamic significance of the massive Cenozoic flood basalt outpouring in the Pacific Northwest.