Sources of in-situ 36Cl in basaltic rocks. Implications for calibration of production rates

In-situ cosmogenic Cl-36 production rates from spallation of Ca and K determined in several previously published calibration studies differ by up to 50%. In this study we compare whole rock Cl-36 exposure ages with Cl-36 exposure ages evaluated in Ca-rich plagioclase in the same 10 +/- 3 ka lava sample taken from Mt Etna (Sicily, 38 degrees N). The exposure age of the sample was determined by K-Ar and corroborated by He-3 measurements on cogenetic pyroxene phenocrysts. Sequential dissolution experiments cosmogenic showed that high Cl concentrations in plagioclase grains could be reduced from 450 ppm to less than 3 ppm after 16% dissolution. Cl-36 exposure ages calculated from the successive dissolution steps of this leached plagioclase sample are in good agreement with K-Ar and He age. Stepwise dissolution of whole rock grains, on the other hand, is not as effective in reducing high Cl concentrations as it is for the plagioclase. 330 ppm Cl still remains after 85% dissolution. The Cl-36 exposure ages derived are systematically about 30% higher than the ages calculated from the plagioclase. We could exclude contamination by atmospheric Cl-36 as an explanation for this overestimate. Magmatic Cl-36 was estimated by measuring a totally shielded sample, but was found to account for only an insignificant amount of Cl-36 in the case of the 10 ka whole rock sample. We suspect that the overestimate of the whole rock exposure age is due to the difficulty in accurately assessing all the factors which control production of Cl-36 by low-energy neutron capture on Cl-35, particularly variable water content and variable snow cover. We conclude that some of the published Cl-36 spallation production rates might be overestimated due to high Cl concentrations in the calibration samples. The use of rigorously pretreated mineral separates reduces Cl concentrations, allowing better estimates of the spallation production rates. In the Appendix of this paper we document in detail the equations used. These equations are also incorporated into a Cl-36 calculation spreadsheet made available in the supplementary data. (C) 2009 Elsevier B.V. AM rights reserved.