Cosmogenic Isotopes Produced In Situ in Terrestrial Rocks: Quantifying the Effect of Elevation on the Production Rates

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EAR-0001191ZredaThe goal of this proposal is to determine how production rates of in-situ cosmogenic isotopes change with elevation. This goal will be achieved by measuring cosmogenic 36Cl and/or 3He in samples from lava flows that extend over long distances in the vertical. Hawaii is the ideal location for this project because: (1) many very long lava flows have been mapped (some extend over 2 km in the vertical); (2) the stratigraphy of lava flows is well established; (3) individual flows can be traced in the field, thus making it possible to ensure that samples are collected from the same lava flow; and (4) many flows are well dated, thus making it possible to combine attenuation lengths from two or more lava flows into a composite record. At least ten samples from different elevations will be measured in each lava flow, and the concentration of accumulated cosmogenic isotopes will be expressed as a func-tion of elevation. Data from individual lava flows will then be combined and a composite function describing the dependence of production rates on elevation will be developed. This function will be compared with the measured neutron intensity, and the applicability of the measured neutron atten-uation length over geological time scales will be assessed. Finally, improved altitudinal scaling for-mulation for cosmogenic production rates will be constructed, which should be an important contribution to the improvement of all cosmogenic dating methods.