COSMOGENIC CHLORINE-36 PRODUCTION IN CALCITE BY MUONS - EXPOSURE AND EROSION HISTORY

Abstract

At depths below a few metres, 36Cl production in calcite is initiated almost entirely by cosmic ray muons. The principal reactions are (1) direct negative muon capture by Ca; 40Ca(μ-,α)36Cl, and (2) capture by 35Cl of secondary neutrons produced in muon capture and muon-induced photodisintegration reactions. We have determined rates for 36Cl and neutron production due to muon capture in calcite from a 20 m (5360 g cm-2) depth profile in limestone. The 36Cl yield from muon capture by Ca in pure calcite is 0.012 +/- 0.002 atom per stopped negative muon. The surface production rate of 36Cl by muon capture on Ca in calcite is, therefore, 2.1 +/- 0.4 atom g-1a-1 at sea level and high latitude, approximately 11% of the production rate by Ca spallation. If it is assumed that 34% of the negative muons are captured by the Ca atom in calcite, the α-yield from 40Ca following muon capture is 0.043 +/- 0.008, somewhat lower than the result of a recent muon irradiation experiment (0.062 +/- 0.020), but well within the extremes of existing theoretical predictions (0.0033-0.15). The average neutron yield following muon capture in pure calcite is 0.44 +/- 0.15 secondary neutrons per stopped negative muon, in good agreement with existing theoretical predictions.Cosmogenic isotope production by muons must be taken into account when dating young geomorphic surfaces, especially those created by excavation of only a few metres of overlying rock. Attention to isotope production by muons is also crucial to determining surface erosion rates accurately. Due to the deep penetration of muons compared to cosmic ray hadrons, the accumulation of muon-produced 36Cl is less sensitive to erosion than that of spallogenic 36Cl. Although production by muons at the surface is only a small fraction of production by spallation, the fraction of muon-produced 36Cl in rapidly eroding limestone surfaces can approach 50%. In such cases, erosion rates estimated using conventional models which attribute production solely to spallation will be in error by up to 40%. The difference in sensitivity to erosion of spallogenic and muon-produced 36Cl suggests methods for dating deeply eroded surfaces, checking the assumption of steady-state when calculating erosion rates, and unravelling multi-stage exposure and erosion histories.