Initial Publication Date: August 10, 2007

Advanced Modeling Programs: MELTS

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

What is MELTS?

graph of lithostatic pressure at depth

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.

Diopside-Anorthite phase diagram Gibbs Free Energy Pressure Temperatur
Diopside-Anorthite phase diagram Gibbs Free Energy Pressure Temperature
Diopside-Anorthite phase diagram Gibbs Free Energy Pressure Temperature

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

  • MELTS calculations are based on low-pressure experiments and thermodynamic models so are best suited for phase relations at low pressure (<2 GPa or 20 kb).
  • MELTS can be used to model the crystallization of a liquid (e.g. crystallization of basalt shown below left) as well as the melting of a pre-existing rock (e.g. melting of peridotite shown below right). Pressure and temperature constraints allow the user to constrain the depth of each process.

    • Melting Fraction as a Function of Temperature During Peridotite Melting
    Liquid Composition as a Function of Melt Remaining During Crystallization
    MELTS Graph of Melt Fraction versus Phases

  • MELTS can model the composition and proportions of phases in closed systems (crystals and liquid remain in the system and are in equilibrium) as well as open systems (in which crystals can be removed from the system).
  • MELTS can find simple liquidus conditions to predict the temperature and first phase to crystallize (or melt) in the system
  • Calibration of MELTS is better for mafic compositions, particularly peridotite, MORBs and alkalic mafic magmas.

Strengths and Limitations

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

  • mafic compositions (it is especially well-suited for mid-ocean ridge and alkalic mafic magmas)
  • crystallization and melting by equilibrium or fractional processes (< 2 GPa; 500—2000 °C)
  • assimilation of wallrocks
  • a range of thermodynamic conditions (e.g., Gibbs energy minimization; constant enthalpy, entropy, or oxygen fugacity)
graph showing range of P T conditions for MELTS software

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

  • low degrees of melting (<30% melting)
  • upper mantle depths (1-3 GPa; approximately 33-100 kilometers depth)
  • moderate temperatures (1000—2500 °C)

Neither of the models should be applied to systems:

  • outside the calibrated ranges for temperature and pressure
  • of intermediate to felsic composition
  • involving mineral phases that include significant molecular water

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:

  • Asimow PD, Ghiorso MS (1998) Algorithmic Modifications Extending MELTS to Calculate Subsolidus Phase Relations. American Mineralogist 83, 1127-1131
  • Ghiorso, Mark S., 1997, Thermodynamic models of igneous processes. Annual Reviews of Earth and Planetary Sciences, v. 25 p. 221-241.
  • Ghiorso, Mark S., Hirschmann, Marc M., Reiners, Peter W., and Kress, Victor C. III (2002) The pMELTS: An revision of MELTS aimed at improving calculation of phase relations and major element partitioning involved in partial melting of the mantle at pressures up to 3 GPa. Geochemistry, Geophysics, Geosystems 3(5), 10.1029/2001GC000217
  • Ghiorso, Mark S., and Sack, Richard O. (1995) Chemical Mass Transfer in Magmatic Processes. IV. A Revised and Internally Consistent Thermodynamic Model for the Interpolation and Extrapolation of Liquid-Solid Equilibria in Magmatic Systems at Elevated Temperatures and Pressures. Contributions to Mineralogy and Petrology, 119, 197-212
  • Hirschmann, Mark, M., Ghiorso, M.S., Wasylenki, L.E., Asimow., P.D., and Stolper, E.M., 1998, Calculation of peridotite partial melting from thermodynamic models of minerals nd melts. I. Review of methods and comparison with experiments: Journal of Petrology, v. 39, no. 6, p. 1091-1115.

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:

  • Crystallization of the Kilauea Iki Lava Lake
  • Melting of mantle peridotite