New x-eos for anhydrous alkaline to subalkaline magmatic systems

HPx-eos updates, MAGEMin

A post by Owen Weller and Eleanor Green

We’re delighted to be releasing a new suite of x-eos for calculations in anhydrous alkaline to subalkaline magmatic systems, as documented in Weller et al (2024): New thermodynamic models for anhydrous alkaline-silicate magmatic systems, Journal of Petrology 65 egae098.

The new x-eos include:

  • An anhydrous silicate melt model that is calibrated in both alkaline and subalkaline composition space.
  • X-eos for the alkaline phases nepheline, kalsilite, leucite and melilite.
  • A new x-eos for Mg-bearing ilmenite.
  • Minor updates to the garnet, orthopyroxene and clinopyroxene x-eos used in Holland et al (2018) and Tomlinson & Holland (2021).

Weller et al (2024) show how the x-eos were validated against seven experimental datasets encompassing a wide range of alkaline compositions. Based on these validation exercises, we suggest that the models are well calibrated in the range 640—1400 C and 0-22 kbar.

The new x-eos will soon (middle of this week?) be available for calculations in MAGEMin. You can find complete model descriptions on this website. However, there will be a short delay before they can be used in THERMOCALC, pending the upcoming release of THERMOCALC 3.51, which is currently expected in January 2025.

Finally, we are currently working on extending the Weller et al (2024) x-eos into H2O-bearing alkaline silicate systems. Watch this space for more news!

Buffering in THERMOCALC

Also included in the Weller et al. (2024) paper is documentation of the new ability in THERMOCALC to buffer oxygen fugacity relative a range of commonly used buffers, such as the fayalite-magnetite-quartz equilibria. As discussed in the paper, the new oxygen buffers are vital to use when comparing model results with experimental results that were acquired using buffered apparatus. We recommend that XFe3+ is tracked when using buffered pseudosections, as the implied bulk ‘O’ content varies across these diagrams in non-intuitive ways. See example figures in Weller et al. (2024) that compare XFe3+ and fO2 for both buffered and unbuffered scenarios. The new buffering feature will be available in THERMOCALC 3.51 and above.

Published