ANALYTICAL TECHNIQUE DEVELOPMENT

A major limitation of applying experimental diffusion data from geologic materials – which commonly exhibit slow diffusion – to natural samples is that traditional analytical techniques often cannot resolve the short and low-concentration experimental profiles that would yield diffusion coefficients best approximating diffusion in natural settings. As a result, experiments are commonly run at relatively high temperatures (~1100-1500 °C) and the retrieved Arrhenius trends are extrapolated down to more geologically relevant temperatures, which introduces significant error into calculations utilising these data. Furthermore, slow-diffusing species in minerals that do not have high thermal stability limits (e.g., P in garnet or Pb in baddeleyite), are ostensibly impossible to study experimentally due to the prohibitively long experimental durations that would be required. Developing analytical techniques with both high sensitivity and high spatial resolution will also be a critical aspect of identifying and studying multi-species diffusion process, as described here.

In collaboration with Stephan Gerstl and the ScopeM analytical centre at ETH Zürich, I am currently working to develop analytical protocols for measuring ultra-short diffusion profiles – experimentally induced and natural – in minerals via LEAP tomography. The first in a series of manuscripts, in which we examined Ca diffusion in forsterite from 750 – 1300 °C at 1 atm. pressure, was recently published (Bloch et al., 2019). I have also recently begun collaborations with SIMS labs that host machines with ultra-shallow depth profiling capabilities (e.g., 7f and SC-Ultra), with the hopes of identifying the most advantageous analytical routines for measuring extremely short (<50 nm) diffusion profiles of both major and trace species.

In terms of field-based studies, I am currently exploring to what extent LEAP tomography can be used to quantify the absolute values of various isotopic ratios and their spatial variabilities on nanometer scales. For example, Michelle Foley (a Ph.D. student at UNIL) is currently measuring Zr isotopic ratios from both the Mud Tank and Duluth FC-1 zircon standards, in addition to her work looking at nm-scale reaction textures in igneous zircons from the Chon Aike silicic province in Patagonia. Mass-dependent fractionation of Zr isotopes, in both whole rocks and individual phases, has recently emerged as a powerful tool to track the thermal and chemical evolution of magmatic systems. The ability to measure absolute Zr isotope ratios at the nanometer scale via LEAP tomography would greatly improve our ability to assess and apply this emerging technique, as Zr phases such as zircon and baddeleyite have been shown to drive Zr isotopic fractionation in shallow magmatic systems.