Cosmogenic chlorine-36 production rates in terrestrial rocks

Chlorine-36 is produced in rocks exposed to cosmic rays at the earth surface through thermal neutron activation of ³⁵Cl, spallation of ³⁹K and ⁴⁰Ca, and slow negative moun capture by ⁴⁰Ca. We have measured the ³⁶Cl content of ¹⁴C-dated glacial boulders from the White Mountains in eastern California and in a ¹⁴C-dated basalt flow from Utah. Effective, time-intergrated production parameters were calculated by simultaneous solution of the ³⁶Cl production equations. The production rates due to spallation are 4160 ± 310 and 3050 ± 210 atoms ³⁶Cl yr^-1 mol^{-1 39}K and ⁴⁰Ca, respectively. The thermal neutron capture rate was calculated to be (3.07 ± 0.24) × 10⁵ neutrons (kg of rock)^-1 yr^-1. The reported values are normalized to sea level and high geomagnetic latitudes. Production of ³⁶Cl at different altitudes and latitudes can be estimated by appropriate scaling of the sea level rates. Chlorine-36 dating was performed on carbonate ejecta from Meteor Crater, Arizona, and late Pleistocene morainal boulders from the Sierra Nevada, California. Calculated ³⁶Cl ages are in good agreement with previously reported ages obtained using independent methods.