Author: Eleanor C R Green

Eleanor leads the development of the HPx-eos (compositionally-dependent equations of state based on the Holland & Powell dataset), and the associated phase equilibrium calculation software THERMOCALC. She runs the hpxeosandthermocalc site and discussion group. Find out more about Eleanor and the international team of geoscientists behind the HPx-eos at https://hpxeosandthermocalc.org/development-team/

MAGEMin

A guest post from Nicolas Riel, lead author of MAGEMin:

MAGEMin is an open-source parallel function written in C that minimizes the Gibbs free energy of multiphase and multicomponent systems. The main objective of MAGEMin is to provide a stable, consistent and as fast as possible phase equilibrium prediction routine.

The function receives bulk-rock composition, pressure and temperature to compute the most stable phase equilibrium. Presently, MAGEMin provides the thermodynamic data set for igneous systems (Holland et al., 2018; “from mantle to granite”). The data set used natively in THERMOCALC, is translated directly into C routines and implemented without transformation of variables or coordinate systems, thus eliminating inconsistencies. While MAGEMin has been primarily developed to predict phase equilibrium in magmatic systems, other database can/will be added to the function.

MAGEMin is easily installed and used on any operating system through either Julia or Matlab. The Julia interface (juila> ] add MAGEMin_C), offers a quick and efficient solution for point-wise minimization in serial or parallel. Moreover, the Julia interface allows for simplified integration of phase equilibrium modelling with geodynamic simulations (e.g., reactive magma flow) or petrological applications (e.g., computing liquid-line of descent).  The Matlab interface (PlotPseudosection.mlapp) includes automatic installation of MAGEMin binaries and fast calculation of pseudosections using the graphic user interface.

The detailed documentation, including tutorials on how to install and use MAGEMin is provided at:

https://computationalthermodynamics.github.io/MAGEMin/index.html

MAGEMin can be downloaded on our github at:

https://github.com/ComputationalThermodynamics/MAGEMin

We invite you to try it out! Please, do not hesitate to report issues and/or provide feedbacks 😊

MAGEMin is being actively developed at the department of geosciences of the University of Johannes Gutenberg (Mainz, Germany). The main contributors are N. Riel (nriel@uni-mainz.de), B. Kaus (kaus@uni-mainz.de), E. Green (eleanor.green@unimelb.edu.au) and N. Berlie. An exhaustive description of the algorithm is given in Riel et al., 2022.

Riel, N., Kaus, B. J. P., Green, E. C. R., & Berlie, N. (2022). MAGEMin, an efficient Gibbs energy minimizer: Application to igneous systems. Geochemistry, Geophysics, Geosystems, 23, e2022GC010427. https://doi.org/10.1029/2022GC010427

MAGEMin interface: Process of calculating a pseudosection for the MIX1G garnet pyroxenite composition of Hirschmann et al (2003).

Script round-up: calcsdnle

In THERMOCALC, versions up to and including the December 2020 release of TC 3.50, it is possible to print partial uncertainties (standard deviations) on calculated compositional variables and modes of phases. This is done using the script

calcsdnle yes

where the name of the script can be read as “calculate standard deviations on non-linear equations”.

When using this facility, it’s important to be aware that:

(1) The uncertainties that are printed are only those that stem from the dataset uncertainties. They do not include the uncertainties associated with the activity−composition relations, or with end-members that are not taken from the dataset. The uncertainties that are not captured by the calcsdnle script are generally larger, or much larger, than the dataset uncertainties.

(2) The uncertainties associated with the different compositional variables and modes are correlated, although the correlations are not given in the output. The correlations may mean that, with a small change in the thermodynamic input, two isopleths might intersect in a very different region of P-T space, come to intersect in a plausible P-T window when they did not previously, or cease to intersect in a plausible P-T window when they did previously (this is a starting point for understanding the upcoming avPT+ approach to uncertainty analysis).

A new x-eos for ternary feldspars

What’s new?

We have published new x-eos for plagioclase and alkali feldspars:

TJB Holland, ECR Green & R Powell (2021). A thermodynamic model for feldspars in KAlSi3O8-NaAlSi3O8-CaAl2Si2O8 for mineral equilibrium calculations. Journal of Metamorphic Geology, 1-14. DOI: 10.1111/jmg.12639

The preferred ternary feldspar x-eos in this paper is the 4TR model (the name is discussed below). This single x-eos replaces two previous x-eos: the Ibar1 and Cbar1 ternary feldspar x-eos of Holland & Powell (2003), Contributions to Mineralogy and Petrology, 145 492-501.

Additionally, we have introduced a binary x-eos to represent low albite with minor dissolved Ca. This can be used to model the peristerite gap in metabasites, where previously we used pure end-member albite.

Continue reading “A new x-eos for ternary feldspars” →

fO2 workshop/school this September

Please consider pre-registering for the following event!:

Understanding oxygen fugacity in Geoscience is a workshop/school running 5-9 September 2022, to highlight the state of the art, major debates and some case studies about redox processes and oxygen fugacity from the Earth’s interior to the surface. It will bring together experts from various disciplines and it is directed to students and scientists with background on chemistry and physics of the Earth and planetary interiors. The School is hosted by the Department of Mathematics and Geosciences, University of Trieste. We also hope to make the School available to online-only participants.

Eleanor Green and Katy Evans are among the speakers and practical leaders at this School, which will address thermodynamic modelling in addition to experimental, analytical and observational themes.

We thank Luca Ziberna for proposing this School, for patiently persisting with the idea throughout the disruption of the pandemic, and for his exceedingly hard work in leading the Organizing Committee!

Modelling deserpentinization

Serpentinite from the Thetford Mines Ophiolite Complex, Ordovician; Thetford Mines area, Quebec, Canada. Attribution: James St. John, https://creativecommons.org/licenses/by/2.0, via Wikimedia Commons

In a recent paper, Katy Evans and Ronald Frost discussed deserpentinization as a source of oxidation in arc magmas.

You can access their THERMOCALC thermo datafiles here. The a-x relations were developed by Roger Powell, for Rebay, Powell & Holland, in prep.