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Advanced Modeling Programs: MELTS

Karl Wirth, Macalester College and Rachel Teasdale, California State University, Chico

What is MELTS?

Increasing pressure (in kb and GPa) with depth (in km).

MELTS is a software package designed to model phase (mineral, rock and liquid) relations during melting and crystallization. MELTS can be used to model processes such as partial melting, equilibrium crystallization, fractional crystallization, and assimilation. Users can compute equilibrium phase relations for igneous systems over the temperature range 500-2000 °C and the pressure range 0-2 GPa (0-~65 km; 20 kb).

MELTS is based on the work of Ghiorso and Sack, 1995 and Asimow and Ghiorso, 1998.

Principles of MELTS

MELTS uses thermodynamic principles (variations in temperature, pressure, and volume in a system) to predict the chemical variation of a magmatic system. MELTS can predict the minerals that will crystallize from an evolving magma, the proportions of those crystals, as well as their composition at each stage of pressure-temperature conditions. In doing so, MELTS also tracks the proportion of remaining liquid (in melting or fractionation). MELTS is designed based on experiments and calculations so are considered predictive models rather than an explicit representation of the evolution of magmas.

MELTS models are based on (thermodynamics) which minimizes the Gibbs free energy (G), Enthalpy (H), entropy (S), pressure, and temperature. Examples of the relationship between lowest Gibbs free energy and phases present in the system at specific temperatures are shown with the Di-An phase diagrams below. MELTS models finds the minimum Gibbs free energy for a given set of P-T-X conditions. The relationship between minimum Gibbs free energy and the phases present are shown paired with phase diagrams below.

Gibbs free energy and phase diagram for the An-Di system at high temperature (1570°C). The liquid phase has the lowest G and is the stable phase for all compositions. Click on image to enlarge. Graphic from J. Brady.
Gibbs free energy and phase diagram for the An-Di system at intermediate temperature (1470°C) . The liquid phase and Anorthite have the lowest G and are the stable phases present. Click on image to enlarge. Graphic from J. Brady.
Gibbs free energy and phase diagram for the An-Di system at intermediate temperature (1375°C). The liquid phase, solid diopside, and solid anorthite are all predicted to be stable. Click on image to enlarge. Graphic from J. Brady.
Gibbs free energy and phase diagram for the An-Di system at low temperature (1220°C). The liquid two solid phases have the lowest G so no liquid is present. Click on image to enlarge. Graphic from J. Brady.

To use MELTS, users can enter the bulk composition data of a magma to observe the predicted path of evolution of the magma. Thermodynamic conditions such temperature (T), and pressure (P) can be varied for each MELTS model. Users also select the fO2 (oxygen fugacity) of the run conditions (click here for more information on fO2). The volatile content of magmas (e.g. H2O, CO2) can also be included in MELTS models. Results show the predicted evolution of the magma given constraints selected by the user. Output data can be compared with analyses of suites of samples to compare the evolution of samples with predictions from MELTS models.

Applications

Strengths and Limitations

The MELTS software is designed to investigate phase relations in magmatic systems that involve:

Appropriate ranges of P-T conditions for several MELTS model versions (xMELTS and mdMELTS are in development). Click on image to enlarge. Graphic from MELTS website.

The pMELTS software extends the MELTS model to investigations of melts of bulk mantle compositions under the following conditions:

Neither of the models should be applied to systems:

Users Guide

This users guide explains how to use the web applet version of MELTS .

Note that if the Java Applet version of MELTS does not run smoothly in one Internet browser, another browser may perform more successfully.

There is a step by step worked example for MELTS using the Java Applet here

The tabulated output of the Java Applet version is limited to the results of the final model calculation. This means that the model can run incrementally but tabulated outputs include only the final results. Graphs can be generated to show the incremental changes but are not preserved in table form (e.g. for use in an Excel spreadsheet).

The version of MELTS that runs from a Unix platform provides more advanced modeling capabilities and more comprehensive tabulated data files that can be easily imported into Excel spreadsheets.

Literature

The following list of publications provides a starting point for learning more about the MELTS and pMELTS models:

Related Links

For more information about MELTS and pMELTS follow the links below:

Teaching Activities

Examples of activities and exercises related to teaching using MELTS are provided below:


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