Exploring Geochemistry in the Classroom Using MELTS Computational Tools

MELTS is a software package designed to facilitate thermodynamic modeling of phase equilibria in magmatic systems. Users can compute equilibrium phase relations for igneous systems over the temperature range 500-2000 °C and the pressure range 0-2 GPa.

Examining the structure of the solid Earth is important for understanding phenomena such as electromagnetic fields and plate tectonics. MELTS computational tools allow scientists to model igneous processes using data in a way that would not normally be accessible by direct inspection of the raw data. The data that scientists typically examine using melts include compositions of liquids and solids that are coexisting under known experimental conditions of temperature, pressure, and oxidation states. Scientists use MELTS tools to examine questions such as how the Earth melt along a pressure-temperature path defined by an adiabatic temperature gradient (where the total heat content of the system remains the same as the material moves up through the Earth's changing pressure). MELTS can be used to test diverse scenarios ranging from examining how much sulfur a volcanoes gives off, which has implications to climate change, to understanding and predicting how the Earth melts.

This tool can be used to teach the following topics and skills in petrology and geochemistry:

• Mineralogy
• Thermodynamics
• Mineral composition and properties of igneous rocks (including plutonic and volcanic rocks)
• Composition of materials and compositional variation
• Composition of the Earth's mantle and crust

• Interpreting phase diagrams
• Understanding how melting can drive compositional changes in the materials making up the Earth's crust and mantle
• Using forward modelling to examine crystal fractionation
• Using forward modelling to examine equilibrium crystallization
• Performing experiments on a computer using compositional data from scientific literature
• Using modelling to understand how temperature, pressure, and oxidation states of a system impact the crystallization of igneous rocks
• Understanding how subltle variations in mineral content, including presence of water, can impact modelling scenarios of material compositional changes (eg. the fractionation of basalt into rhiolite)

MELTS can be accessed as a web applet by choosing the 'run applet' option. Alternatively, Mac OS X and Unix versions can be downloaded for standalone use.

Several databases are available that provide rock and mineral composition data, including an experimental database at MELTS. Users can input these compositions into the MELTS program to perform computer experiments. Two other available databases are the Petrological Database of the Ocean Floor (PETDB) and EarthChem.

The MELTS Manual provides information about testing, graphs, output format, display elements, menus, and common problems. It also provides examples of MELTS experiments. This simple example is a good way to test newly downloaded MELTS software and begin to familiarize yourself with the program. For more advanced users, try using the Phase Properties Calculator Form or the Phase Properties Calculator Applet to compute thermodynamic properties of mineral-solid solutions that occur in nature with variable compositions. MELTS outputs files in comprehensive and tabular formats.

Data can be displayed as graphs, tables, or phase diagrams.

The MELTS program is based on a model that describes the geometry of the energy surface for a multi-dimensional system of up to 16 chemical components. The model predicts compositional changes by determining the minimal energy for the system under the user defined parameters (including initial composition, temperature, pressure, and oxidation state of the system).

MELTS is intended for modeling magmatic phase relations at low pressure (< 2 GPa). It is better calibrated in mafic systems and should work especially well for MORBs and alkalic mafic magmas. Phase equilibria involving hornblende and biotite are not modeled well by the MELTS package and consequently simulating the evolution of intermediate to silica-rich calc-alkaline systems is not recommended. MELTS results are only as good as the underlying experimental data source so make sure you are working from a reliable dataset.

Discusses the errors associated with forward modeling of mantle melting:

• Hirschmann, Marc M., Ghiorso, Mark S., Wasylenki, L. E., Asimow, P. D., Stolper, E.M. (1998) Calculation of peridotite partial melting from thermodynamic models of minerals and melts. I. Methods and comparison to experiments. Journal of Petrology 39, 1091-1115

Examples
Tutorial

• Ghiorso, Mark S. (1997) Thermodynamic Models of Igneous Processes. Annual Review of Earth and Planetary Sciences, 25, 221-241.

• The following references should be cited for results from using MELTS:
• 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.
• Asimow PD, Ghiorso MS (1998) Algorithmic Modifications Extending MELTS to Calculate Subsolidus Phase Relations. American Mineralogist 83, 1127-1131.
• The following references should be cited for results from using pMELTS:
• 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.