The magmatic plumbing systems that feed volcanoes are often reconstructed based on diffusion modeling of chemical profiles in crystals. However, diffusion is not the only way to generate chemical profiles – crystal growth is equally capable of creating chemical zoning in crystals. An incorrect assumption about which process caused the observed zoning can result in erroneous conclusion. During my PhD, I demonstrated that in-situ Mg-Fe isotopic analyses could be used to identify the nature of chemical zoning in olivine. Negatively-coupled Mg-Fe isotopic profiles are associated with chemical diffusion (light isotopes diffuse faster than their heavier counterparts) whereas, in the case of crystal growth, no Mg-Fe isotope fractionation occurs. I obtained high precision Mg-Fe isotopic data via micro-drilling followed by solution multi-collector inductively coupled mass spectrometry (MC-ICPMS). To make this isotope approach to studying zoned minerals more versatile, I also used laser ablation MC-ICPMS and secondary ion mass spectrometry (SIMS) to demonstrate that the same analyses can be done using technique that provide higher spatial resolution.
To deconvolve the contribution of crystal growth vs. diffusion, the isotope effect for diffusion must be known. In other words, we need to know the isotope fractionation expected when diffusion is the sole mechanism that generates chemical zoning. To this end, I performed laboratory diffusion experiments and quantified the isotope effect for diffusion as a function of composition, temperature, and crystallographic direction. For natural samples, any isotope fractionation less than the maximum values achievable by diffusion may be attributed to crystal growth. Armed with these experimental data, I have begun collaborating with Dr. James Moore to create IsoSpeed: user-friendly software that can recover time-temperature histories of magmatic bodies by Monte Carlo inversion of chemical-isotopic profiles in crystals.
- Sio, C. K., Dauphas, N., Teng, F. Z., Chaussidon, M., Helz, R. T., & Roskosz, M. (2013). Discerning crystal growth from diffusion profiles in zoned olivine by in situ Mg–Fe isotopic analyses. Geochimica et Cosmochimica Acta, 123, 302-321.
- Sio, C. K., & Dauphas, N. (2017). Thermal and crystallization histories of magmatic bodies by Monte Carlo inversion of Mg-Fe isotopic profiles in olivine. Geology, 45(1), 67-70.
- Sio, C. K., Roskosz, M., Dauphas, N., Bennett, N. R., Mock, T., & Shahar, A. (2018). The isotope effect for Mg-Fe interdiffusion in olivine and its dependence on crystal orientation, composition and temperature. Geochimica et Cosmochimica Acta, 239, 463-480.