Search

There is growing interest in geochronological applications of terrestrial in situ-produced cosmogenic nuclides, with the most commonly measured being ¹⁰Be and ²⁶Al in quartz. To extract and then separate these radionuclides from quartz and prepare them in the oxide form suitable for accelerator mass spectrometry (AMS) requires extensive and careful laboratory processing. Here we discuss the suitability of a crushed, sieved and etched, sub-aerially exposed vein quartz specimen (CoQtz-N) to act as a reference material for chemical laboratory preparation and AMS measurements. Splits of CoQtz-N were distributed to eleven target preparation laboratories. The CoQtz-N ¹⁰Be targets were then measured at seven different AMS facilities and five of the preparation laboratories had their ²⁶Al targets measured at four different AMS facilities. We show that CoQtz-N splits are sufficiently homogeneous with regard to nuclide concentrations, that it has been cleaned of any atmospheric derived (i.e. meteoric) ¹⁰Be and that it has low concentrations of the major elements that can interfere with Be and Al extraction chemistry and AMS measurements. We derive preliminary concentrations for ¹⁰Be and ²⁶Al in CoQtz-N as 2.53 ± 0.09 × 106 at/g and 15.6 ± 1.6 × 106 at/g, respectively, at the 95% confidence limit.

Cosmic ray exposure (CRE) dating was performed on the caldera cliffs of Santorini with the aim of detecting cliff segments predating the Minoan eruption (17th century BCE). The methodology involved the determination of in situ-produced cosmogenic 36Cl concentration in basaltic-to-rhyodacitic whole rocks cropping out in the cliffs. After the samples were processed following the chemical protocol of 36Cl preparation for silicate rocks, 36Cl concentrations were measured by accelerator mass spectrometry (AMS). Important challenges during the implementation procedure were related to large amounts of radiogenic 36Cl, complex modeling of inherited 36Cl, and dominance of the thermal and epithermal (low-energy) neutron capture production pathway. Nevertheless, quantitative assessments on the basis of the contribution of the low-energy neutron capture pathway percent to the total production rate validated the calculated CRE dates. Current CRE ages demonstrate that an ancient caldera existed on pre-Minoan Santorini, occupying at least the northern half of the modern-day caldera. © 2016, Springer-Verlag Berlin Heidelberg.

Cosmonuclide concentrations in surface samples are function of both duration of exposure to cosmic radiation and surface stability. From the measurement of a single cosmonuclide concentration in a single surface sample, it is thus impossible to quantify simultaneously the exposure duration and the denudation rate affecting the studied object. In the case of a simple exposure history, measurements of two cosmonuclide concentrations within the same surface sample may however theoretically be used to estimate both unknowns. A more constraining approach to accurately quantify both exposure time and denudation rate is to take advantage of the fact that the effective production attenuation length of neutrons is significantly shorter than that of muons. The neutron-induced cosmonuclide concentrations thus reach steady-state with respect to denudational loss much more rapidly than the muon induced ones. Consequently, the near-surface cosmonuclide concentrations mainly resulting from interactions with neutrons might be used to estimate the denudation rates while the several meters depth concentrations mainly resulting from interactions with muons might be used to estimate the exposure duration. A unique well constrained depth profile thus permits determination of both the exposure time and the denudation rate. In the case of abandoned material, inheritance due to previous exposition to cosmic rays can be revealed. Multi-nuclide depth profiles are also excellent tools for better deducing the physical parameters of the particles involved in the production of these cosmogenic radionuclides. This approach has been applied twice: 1.) 10Be and 26Al along a 25 meters pure quartz core from Galicia, Spain, and 2.) 10Be, 26Al and 36Cl along a ~11 meters carbonate and quartzose conglomerates core from La Ciotat, S.E. France. Both projects confirm that the density of the material is one of the most sensitive parameters. The latter study also allows to precise the still debatable spallation production rate of 36Cl from Ca.