Production rates of 36Cl in basalts from the calibration site of Fuerteventura, Canary Islands

Abstract

Age determination based on cosmogenic nuclides is an important tool to investigate landscape development and age relations of geologically very young materials. The aim of this study is to contribute data to establish age determination of the basis of cosmogenic 36Cl production as a generally reliable method. 36Cl is a radionuclide that is in situ produced by cosmic radiation at the earth surface. It is formed by spallation from Ca, K, Ti, and Fe, by thermal neutron capture in 35Cl, and by muogenic production from Ca and K. The concentration of the cosmogenic nuclide provides a measure of the exposure age of the surface, but also of the exposure history which may include periods of burial or erosion. Several factors such as the geographic position of the site, the topographic shielding of the surrounding hillside and the sample thickness or sampling depth influence the effective amount of radiation hitting the surface and are taken into account by applying appropriat e scaling factors. Basalt samples from the mid-latitude, low altitude calibration site of Fuerteventura, Canary Islands were collected and the production rates of 36Cl were determined. Geologically young samples covering an age range of approximately 50 to 400 ka could be collected from a number of flows, which suit the time span that can be covered with 36Cl age determination. The age was independently determined with the 40Ar/39Ar method. From nine lava flows 7 or 8 samples were collected whose surface structures indicated as little erosion as possible. ICP and XRF measurements proved that the basalts were very similar in chemical composition. The preparation of the AMS samples followed generally the procedure established by Stone et al. (1996b). The measurements were performed at the AMS facility at the University of Utrecht. From the results of the measurements total chlorine concentrations the amount of 36Cl, and the production rates were deduced. The high variability of the produ ction rates for the individual production mechanisms given in the literature make the estimation of expected total production rates rather difficult. Production rates for individual production mechanisms of 36Cl could not be obtained due to the lack of suitable phenocrysts. The production rates for the total rock are somewhat higher, except for erosion affected flows, but still compare fairly well with data presented elsewhere in the literature. For the samples of 36Cl concentrations higher than expected a number of possible reasons is evaluated which in combination may cause increased 36Cl build-up, such as thin water layers trapped in the uppermost vesicles in the rock, or a weaker earth magnetic field or problems with the Ar ages. The calculation of the production rate of 36Cl from Ca and of the thermal neutron flux had to be based on literature data for the other production paths involved in the total production. These data indicate loss of 36Cl in some of the samples while other s amples even show increased 36Cl contents compared to the expected amounts.