Linus Ros

Hydrogen concentration analysis in clinopyroxene using proton–proton scattering analysis

Traditional methods to measure water in nominally anhydrous minerals (NAMs) are, for example, Fourier transformed infrared (FTIR) spectroscopy or secondary ion mass spectrometry (SIMS). Both well-established methods provide a low detection limit as well as high spatial resolution yet may require elaborate sample orientation or destructive sample preparation. Here we analyze the water content in erupted volcanic clinopyroxene phenocrysts by proton–proton scattering and reproduce water contents measured by FTIR spectroscopy. We show that this technique provides significant advantages over other methods as it can provide a three-dimensional distribution of hydrogen within a crystal, making the identification of potential inclusions possible as well as elimination of surface contamination. The sample analysis is also independent of crystal structure and orientation and independent of matrix effects other than sample density. The results are used to validate the accuracy of wavenumber-dependent vs. mineral-specific molar absorption coefficients in FTIR spectroscopy. In addition, we present a new method for the sample preparation of very thin crystals suitable for proton–proton scattering analysis using relatively low accelerator potentials.

A decade’s worth of otolith PIXE analyses

Over the past 10 years, several thousand otoliths have been analyzed with PIXE (using 2.55 MeV protons) at LIBAF (Lund Ionbeam Analysis Facility, formerly LNMP Lund Nuclear Micro Probe). Over 40 elements have been identified in otoliths, many at levels suitable for PIXE analysis. Readily detectable elements in otoliths starting with Ca are: Ca (the matrix), Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Sr, Y, Zr, Mo, Cd, Sn (difficult), I, Ba (sometimes difficult), Pb (difficult). The detector system, used over this time period, is more sensitive than many other X-ray detector systems, since it consists of eight HPGE detector elements (100 mm2 each), in an annular formation around the beam entrance. Using a thick absorber allows us to use quite high beam current, typically 12 nA, but sometimes up to 20 nA. This permits us to have low detection limits within short analysis times. Additionally, light stable isotope research is widespread in the sciences including ecology. Stable isotopes of N provide information about trophic level (“who eats who”), providing the opportunity to map out the switching of diets from one food type to another. Oxygen isotopes are useful as “environmental thermometers”. Currently, most of such analyses require destruction of the otolith, and nitrogen isotope analysis may require dissolving entire otoliths, thus losing all temporal information. We present new techniques using new types of detectors, double side silicon strip detector (DSSSD). The detectors, electronics and the laboratory setup are described in detail; for our analysis, a MeV proton and a deuterium microbeam at LIBAF is used. The analysis is performed immediately after the PIXE analysis, without moving the sample.