New approaches to quaternary geochronology: U-Th/He dating of basalt and stable isotope mapping of polar glaciers

Constraining absolute timescales of Quaternary events, such as volcanic eruptions and climatic changes, remains a difficult problem. This work demonstrates two dating techniques that can be applied to the Quaternary period: the U-Th/He radiometric method and stable isotope mapping of glacial ice.; The investigation of the U-Th/He method followed a progression from initial testing on a sample of known age to application of the technique to basalt samples of unknown age in Idaho and Hawaii. Chapter 2 describes an initial test of the technique on garnet phenocrysts from the 79 AD eruption of Mt. Vesuvius. The determined age, corrected for diffusive loss of He, alpha ejection, and initial U series disequilibrium, is 1885 +/- 188 yr which compares well with the known age of 1923 years. The Vesuvius garnets are unusual in that they have high U and Th concentrations (∼20ppm). The second experiment involved measurements of minerals with low U and Th concentrations and included measurement of U, Th, and He concentrations on the same sample aliquot (Chapters 3, 4, and 5), comparison of the U-Th/He and U-Th disequilibrium techniques, and the concurrent determination of the U-Th/He age and He-3 cosmogenic exposure ages on the same samples. Olivine phenocrysts from basalt lavas of the Snake River Plain, Idaho, and Mauna Kea volcano, Hawaii were used for this study. Combined U-Th/He crystallization age and 3He exposure age data on lava outcrops and boulders from Box Canyon, near Hagerman, Idaho, indicate that the basalts are 81 +/- 9 ka to 107 +/- 10 ka old, and suggest two periods of eruption, both clearly distinguishable from nearby 395 ka lavas previously dated using Ar-Ar. Exposure ages of boulders and scoured bedrock are 20.7 +/- 0.9 to 49.5 +/- 3 ka, indicating that Box Canyon formed prior to the Bonneville Flood. Post-shield lavas from Mauna Kea were analyzed to probe the limits of the U-Th/He technique on samples with low U and Th concentrations. The U-Th/He ages conform to the stratigraphy and previously published C-14 and K-Ar ages. Improvements in sample preparation and He extraction methods required for the U-Th-He-He technique are outlined in Chapter 5.; Timescales of glacial ice motion are of considerable interest because of the relationship between ice dynamics and climate. Comparison of the stable isotopic composition of ice exposed at the margins of ice sheets and the surfaces of glaciers, to ice core records taken from their accumulation areas, has the potential for constraining the geometry and timescales of ice flow. Chapter 6 presents oxygen and hydrogen isotopic data from a longitudinal traverse of Taylor Glacier, Antarctica. The data indicate at least 65 kyr of age difference over 30 km of glacier length, and provide new constraints on the internal structure of the glacier.