Cosmogenic chlorine-36 production in K-feldspar

The high production rate of in-situ cosmogenic ³⁶Cl from potassium allows K-rich minerals to be dated with high sensitivity and precision. K-rich minerals, such as K-feldspar and biotite are commonly associated with quartz in which ²⁶Al and ¹⁰Be are produced, allowing three cosmogenic nuclides, with half-lives ranging from 0.3 Ma to 1.5 Ma, to be measured in a single sample. To calibrate the production rate of ³⁶Cl from potassium we have measured K-feldspar samples from ice-scoured bedrock from three separate sites, covering a range of altitudes, latitudes and exposure ages. The results from each site are internally consistent, with eight glacial pavements from the Sierra Nevada giving a production rate of 1290 ± 220 atom/(gK)/yr at 3000 m and 38°N, three Scottish glacial pavement samples giving 301 ± 27 atom/(gK)/yr at 520 m and 58°N, and two Antarctic bedrock samples giving 1310 ± 50 atom/(gK)/yr at 2000 m and 70°S. Normalising these production rates to sea level and high latitude, results from the Sierra Nevada and Scotland are found to be in good agreement with each other, but the production rate derived from the Antarctic samples is found to be approximately 35% higher. Repeat measurements and stepwise dissolution indicate that the high ³⁶Cl concentration of the Antarctic samples is real and cannot be attributed to meteoric ³⁶Cl. We must therefore conclude that either the ³⁶Cl production rate differ on the 10⁴ and 10⁶ yr time scales, or that the current altitude scaling factor applied to the Antarctic results underestimates the true scaling factor for the Antarctic atmosphere. The production rates derived above are based principally on K-feldspars with low chloride contents. Samples with high chloride contents required corrections for ³⁶Cl produced by neutron capture on ³⁵Cl. The {³⁶Cl}/{Cl} ratio of this component was measured directly in four samples from the Sierra Nevada after crushing the K-feldspar to release Cl-rich fluid from inclusions. The {³⁶Cl}/{Cl} ratios measured appear to be in reasonable agreement with values predicted by recent calibrations of the production by neutron capture on ³⁵Cl, but the final comparison awaits measurements of uranium, thorium and neutron absorber (B, Gd, Sm) concentrations in our samples.