9526126 Zreda Sequences of mafic lavas contain records of the cosmic ray intensity on earth. Such lavas, however, are difficult to accurately date, especially beyond the 14C range, because of their fine-crystalline texture an low potassium content. The in situ accumulation of cosmogenic nuclides can be potentially used to approach this problem. I propose to test a new cosmogenic approach to determine the age of now-buried mafic lava flows and/or the cosmic-ray intensity in the past. It uses two cosmogenic nuclides with different half-lives. The accumulation up of a longer-lived or stable nuclide is used to determine how long the lava had been exposed at the surface. the decay of a shorter-lived nuclide gives the burial duration. The feasibility of this novel approach will be tested using selected samples from the 1056-m core at Hilo, Hawaii. This preliminary study has three objectives: (1) to evaluate the retention of cosmogenic noble gases; (2) to evaluate the utility of long-lived cosmogenic radionuclides as a measure of the exposure duration; and (3) to assess the utility of shorter-lived cosmogenic nuclides as a measure of the burial duration. If the approach is successful, it will enable us to simultaneously determine cosmogenic ages of lava flows and the cosmic ray intensity during their exposure to the atmosphere. Preliminary results in this project will be used in future proposals to perform a comprehensive cosmogenic study on this and other volcanic sequences.
