Feg-EPMA and Nanosims Profiles of Zoned Crystals for Diffusion Chronometry

Abstract:

Diffusion chronometry is an alternative method that can be used to assess timescales of magmatic events. It relies on the chemical relaxation of chemical zoning within magmatic crystals. By studying a range of elements within a single crystal we can probe a range of magmatic processes and interrogate timescales of processes from days to millennia. Diffusion modifies the elemental concentrations of adjacent crystal zones. The timescales that can be investigated are limited not only by the diffusivity of an element and the available diffusion coefficients but also the resolution of the measured chemical profile and hence the analytical technique used to acquire these profiles. To obtained reliable diffusion timescales the analytical length scale must be shorter than the characteristic diffusion length. In addition sufficient analytical points must be present on the profile to ensure that the profile is ‘real’ and not a convolution artefact. Thus in some cases, sub-micron spatial resolution is required. Two such possible techniques that can achieve nanoscale resolution are field emission gun electron probe micro analyser (FEG-EPMA) and NanoSIMS. Plagioclase and pyroxene crystals were analysed by FEG-EPMA and NanoSIMS to investigate the achievable spatial resolution that could be attained. For quantitative analyses, analytical protocols for FEG-EPMA for plagioclase and pyroxene have been developed that can achieve spot analyses of down to 300 nm diameters with 300 nm spacing for major and trace elements. NanoSIMS can achieve a 200 nm spot diameter, but currently the chemical profiles are only qualitative. This increase in spatial resolution of analytical techniques has demonstrated that compositional boundaries within zoned crystals are relatively sharp (< 2 microns). Thus assuming step-profiles as an initial condition in simple 1D diffusion models is appropriate in many cases.