P-T-X paths tutorials (MAGEMinApp v0.8.6)

Here we provide a set of tutorials to generate various kind of pressure-temperature-composition paths: including batch melting, fractional melting and fractional crystallization.

1. Quick start - first P-T-X path

For the first P-T-X path, simply launch MAGEMinApp and navigate to the PTX path tab. Keep the default setting for the thermodynamic database (Igneous)(Green et al., 2025 after Holland et al., 2018) and default bulk-rock composition (KLB1 Peridotite anhydrous). In the Path definition panel click on Find solidus and Find liquidus and define the P-T points accordingly:

Note

• New points for the P-T-X path can be added by clicking on Add new point.

• To delete a point simply click on the cross icon on the left of point.

In the Path options panel, change the resolution to 32. This option defines the number of point-wise calculation between two defined points.

Hit Compute path and after a few seconds you should get the following result:

Then in the Composition panel, click liq which will display the evolution of the melt composition along the path:

Note

• Double-clicking on one ooxide will isolate it. For instance FeO:

• Double-clicking again on the same oxide, will bring back all the oxides.

TAS diagram

When liq is selected you can access the TAS diagram which displays the evolution of the melt composition (Total Alkali Silica):

Warning

• When computing a new PTX diagram, to refresh the TAS diagram, you need to unselect and reselect liq in the Composition panel.

2. Fractional melting

In this example, we are going to perform fractional melting using SM89 oxidized average MORB composition using the Metabasite thermodynamic database (Green et al., 2016). First make sure you select Aug in the clinopyroxene selection, then define the P-T points of the path as follow:

In the Path options panel, choose a resolution of 32, and select P-T-X mode = fractional melting, keep Assimiliation = false and Connectivity threshold [%] = 7:

Note

• The connectivity threshold is the value above which melt is extracted

• Presently, only the melt above this value is extracted to keep the melt fraction at the connectivity threshold.

• When computing a fractional melting path using a connectivity threshold, the displayed fraction of melt can be slightly above the threshold as the removed fraction of melt is only applied to the subsequent calculation step. This effect can however be minimized by increasing the resolution.

Process with the P-T-X path calculation, which should yield:

Note

• The black continuous line remaining % represents the remaining % with respect to the starting material.

• The black dashed line removed % is the mass % of material removed with respect to the starting material.

• remaining % + removed % = 100.0

3. Fractional crystallization

Let us the same database and bulk rock composition as for the fractional melting example. Simply change the path definition as follow:

In the Path options panel, choose a resolution of 32, and select P-T-X mode = fractional crystallization, keep Assimiliation = false and Remaining fraction [%] = 1:

Note

• Remaining fraction [%] can be thought as a small fraction of the solid rock carried by the fractionating melt.

Process with the P-T-X path calculation, which should yield:

and TAS diagram (Total Alkali Silica):

Note

• The size of the circle symbol in the TAS diagram scales with the remaining %. This gives an idea of the volume of generated magma along the fractional crystallization path.

4. Assimilation

In this example we are going to compute an equilibrium batch crystallization path of a wet basalt, and, progressive assimilation of tonalitic composition. Let's first select the Igneous database (Green et al., 2025, after Holland et al., 2018) and define the P-T path as follow:

Note

• Notice the new column in the P-T path definition Add [mol%]. Here you can define how much of the assimilated composition will be added for each P-T step.

In Path options, set Resolution = 32, P-T-X mode = Equilibrium and Assimilatiom = true. When Assimilatiom = true a second bulk-rock composition is available for selection in the Bulk-rock compositionleft panel. Choose Wet basalt for the left (starting) composition and Tonalite 101 for the right (assimilated) composition:

Performing the calculation of the P-T path gives:

and the following TAS diagram:

5. Variable buffer

To simulate a change in oxydation/reduction state of the system you can also provide variable buffer offsets. Let's start from previous assimilation example 4, and select Buffer = QFM and Variable buffer = true in the Configuration panel. A new column named Buffer is now available in the Path definition panel and you can modify the buffer offset to your liking. For instance:

Tip

Don't forget to oversaturate the O content of the bulk-rock compositions.

Performing the calculation of the P-T path gives:

and the following TAS diagram: