HR: 13:40h
AN: U23B-01    [Abstracts]
TI: Using Lava Flows to Constrain Cosmogenic Nuclide Production Rates: Lessons from Hawaii
AU: * Desilets, D
EM: ddesilet@hwr.arizona.edu
AF: Department of Hydrology and Water Resources, University of Arizona, 1133 East North Campus Drive , Tucson, AZ 85721 United States
AU: Zreda, M
EM: marek@hwr.arizona.edu
AF: Department of Hydrology and Water Resources, University of Arizona, 1133 East North Campus Drive , Tucson, AZ 85721 United States
AB: A potentially major systematic uncertainty in cosmogenic dating is in scaling a calibrated production rate to the altitude of another field site. Despite much recent work aimed at improving scaling models, the scaling remains uncertain because measurements of cosmic-ray intensity, which are used as a proxy to describe the elevation dependence of cosmogenic production rates, have not been checked against measurements of cosmogenic nuclides in geologic samples. Well-preserved, subaerially exposed, Quaternary lava flows on the island of Hawaii have several characteristics that make them ideal for such a comparison: (1) flows cover a large range of elevations, with little change in latitude; (2) many flows are old enough for sufficient cosmogenic nuclide accumulation, yet young enough that significant erosion has not occurred; (3) primary flow surfaces are well-preserved, even in flows that are >60 ka, owing to the semi-arid to arid rain shadows of the volcanoes; (4) extensive measurements of cosmic-ray fluxes have been conducted on Mauna Kea and on the nearby island of Maui. We measured ³⁶Cl in 40 hawaiite samples collected from an altitude transect on the dry, southwest flank of Mauna Kea, Hawaii (19.8° N, 155.5° W). The elevation dependence of ³⁶Cl production is best described by an atmospheric attenuation length of 142±5 g cm^-2, which is close to the value of 140 g cm^-2 predicted from cosmic-ray measurements. The good agreement between the ³⁶Cl elevation profile and the cosmic-ray surveys suggests that cosmic-ray measurements can be used to scale production rates. But difficulties in delineating flow boundaries, together with potential systematic errors due to geological complications, such as erosion or burial, mean that interpreting cosmogenic nuclide data from lava flows can be problematic. Our experience points to the importance of accurate stratigraphic studies and mapping if lava flows are to be used as calibration sites. This reasearch was supported by the National Science Foundation under grants EAR-0001191, EAR-0126209 and ATM-0081403 and by Packard Fellowship in Science and Engineering 95-1832.
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
SC: Union [U]
MN: Fall Meeting 2005