SIMS2015 Session GC-ThP: Geology, Geo- and Cosmochemistry Poster Session

Alteration layers form on the glass surface after corrosion. Glass corrosion is a complex process and Ion-exchange is one of the major corrosion mechanisms. For example, protons in aqueous solution can diffuse into glass and replace Na⁺ in glass. Sodium-hydrogen interface can be regarded as the corrosion front line. Such interface can be readily characterized using SIMS depth profiling. However, the alteration layers may not be a perfect layer-by-layer structure, but with a lot of nanoscale bumps and zig-zags. SIMS has been used to image the structure of the alteration layers and provides some very interesting results. More importantly, only 2-3 techniques can be used to image hydrogen with a nanoscale spatial resolution, and SIMS is one of them. ToF-SIMS and NanoSIMS are two major SIMS instruments that can be used for image sodium-hydrogen interface. In this presentation, we compare the performance of ToF-SIMS and NanoSIMS for imaging of sodium-hydrogen interface. Our data show that ToF-SIMS can perform better than NanoSIMS, and the major reason is that an oxygen sputtering beam can be used with interlaced mode during ToF-SIMS imaging, which can help to reduce background of hydrogen. Therefore, ToF-SIMS is a better choice for imaging of sodium-hydrogen interface in glass corrosion research.

Mg/Ca ratios in foraminifera are often used as a proxy for past ocean temperatures. However, over the last decade, it has become clear that Mg/Ca ratios in foraminifera are not constant throughout the shell. Instead, the Mg content of the foraminiferal calcite varies systematically between day and night by several fold, a phenomenon that has yet to be explained mechanistically. Determining whether elements other than magnesium also exhibit sub-micron banding is essential to properly interpret Me/Ca-based paleoproxies and to help understand the mechanism causing Me/Ca variability. Using Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS), an isotope mapping technique with a spatial resolution of roughly 300 nm, we discovered systematic Na/Ca banding in individuals of the planktic foraminifera O. universa that had been cultured at constant temperature. Furthermore, using stable-isotope time stamps we have been able to show that this Na/Ca banding exhibits three distinct patterns, depending on which part of the foram test was analyzed. For much of the test, Na/Ca varies inversely with Mg/Ca, with high Na/Ca during the day and low Na/Ca at night. In contrast, it appears that both Mg/Ca and Na/Ca are high at the location of the primary organic membrane. Additionally, Na/Ca is low in the slower-growing inner leaflet of the O. universa terminal sphere. Using a combination of analytical models and complementary instrumental techniques, we test whether these patterns can be explained by active exchange of 2Na+ for Mg2+ during biomineralization, by kinetic mineral growth effects, and/or by organic-templating processes.

The IMS 1280-HR is a large geometry magnetic sector ion microprobe delivering unequalled analytical performance for a wide range of applications: tracking geological processes using stable isotopes, dating minerals, determining the content of trace elements, screening and analyzing large numbers of particles…

High density cesium or oxygen primary ion beam bombardment combined with optimized transmission allow high precision stable isotope studies and analysis of trace elements at high sensitivity (e.g. mandatory for Pb analyses in Zircon). The multicollector system ensures ultimate reproducibility for stable isotope ratio measurements (H, C, O, S, Li, B, Mg…) and significantly increases the throughput of the instrument by reducing the total acquisition time. Thanks to its superior imaging capabilities (both microscope and microprobe), the IMS 1280-HR is capable of mapping the distribution of major, minor and trace elements or isotopes at sub-micron lateral resolution.

Tenth of permil isotopic ratio reproducibility can be routinely obtained on the IMS 1280-HR for multicollection analyses using multiple Faraday Cups ion detectors together with typically a few nA of primary current. Using FCs has nevertheless the drawback that it is not possible to achieve low count rate i.e. single ion counting. EM detectors (electron multipliers) work in a direct pulse counting mode, but when working with the high count rate required to obtain sufficient statistics in reasonable time, aging of the EMs is known to limit the isotopic ratio reproducibility.

We implemented an iterative real-time control of the EM high voltage (HV) for automatically re-adjusting the EM HV before each isotopic ratio measurement, thus minimizing the effect of EM aging on reproducibility.

We demonstrate that 0.2-0.3 permil isotopic reproducibility (1 sigma) is readily achievable on the IMS 1280-HR in automated mode, working with EM detectors and count rates up to the 1E6 counts/sec range.