This example demonstrates how to do a simple isobaric, isothermal assimilation model using MELTS.
SiO2 48.21 TiO2 1.70 Al2O3 15.23 FeO(T) 10.00 MgO 8.72 CaO 11.51 Na2O 2.29 K2O 0.20 H2O 0.10This is done by clicking in the white box to the right of SiO2 (use the left mouse button to select the box) and typing the weight % value: . The delete key, arrow keys and mouse may be used to edit the typed value.
Typing a return advances the text entry cursor to the next oxide. Continue entering the composition until the liquid composition display panel looks like:
The starting and stopping temperature are set equal to each other since the model will be isothermal. The temperature increment is therefore zero and may be entered or left blank. The starting and stopping pressures are set equal to each other so that the model evolution path is isobaric. The pressure increment and dP/dT gradient may be left blank or set to zero.
For this example, a quartz, two-feldspar country rock will be used as an assimilant. To enter the phase proportions and compositions, do the following:
Note that the list entry liquid may be used to refer to the bulk composition of a rock or a residual glass that will be treated by the MELTS program as a liquid phase at the specified T and P of the assimilant.
In the current version of MELTS, only wt % values can be entered. In a future release, the units option menu will allow selection of wt% or vol %.
Note that a phase may be deleted from the assimilant by highlighting the phase in the above list and clicking on the Delete Selected button.
In this example, an oligioclase feldspar is entered by specifying the mole % concentrations of the three endmember components albite, anorthite and sanidine. In general, the contents of the Components in Selected Phase panel will refl ect whatever phase is selected in the List of Phases in Assimilant panel.
Note that the list now contains two feldspars. The only way to distinguish the two is to highlight one or the other and examine the composition of the phase displayed in the Components in Selected Phase panel.
Enter the composition of the second feldspar as follows:
This is an alkali feldspar. Keep in mind that MELTS does not require the phases in the assimilant be in thermodynamic equilibrium. Try selecting the other feldspar in the List of Potential Phases panel. The displayed composition shoul d revert to the oligioclase entered previously. Now re-select the alkali feldspar.
The mass entry is the total mass of the assimilant. In this case it is the total mass of the two-feldspar, quartz country rock that will be assimilated. As the model executes, this mass of assimilant will be added to original mass of the system in as many Increments as specified. The initial system mass is computed from the entered composition on the liquid composition panel. We entered the system composition above, when we input the bulk composition of the olivine tholeiite. The sum of the wt % quantities typed into the liquid composition panel is converted by MELTS to grams and this mass is taken to be the initial system mass. As the model executes, the mass of the system will increment in steps of mass/Inc - at each step results will be displayed and output to disk.
The temperature of the assimilant is entered in the text entry panel labeled T (C). The value is in degrees centigrade. The pressure on the assimilant is assumed to be identical to the system pressure. In isothermal assimilation models, the assimilant temperature is ignored. If isenthalpic constraints are specified, then the assimilant temperature is used to calculate the total enthalpy of the assimilant, which is in turn used to balance the enthalpy budget of the model and constrain the fin al temperature of the system.
This closes the dialog. The clear button will erase all information you have entered in this dialog. This option is sometimes useful with a complex multi-stage model.
To impose an oxygen fugacity equivalent to the Quartz-Fayalite-Magnetite buffer at the starting temperature and pressure, select the f O2 Constraint entry of the Intensive Variables menu and while depressing the left mouse button, slide the mouse to the right and down and release on the Q-Fa-Mt constraint buffer entry.
Next, go to the Composition menu and select the Compute Redox State entry. The Fe2O3 and FeO quantities displayed in the liquid composition panel should change to reflect a ferric/ferrous ratio appropriate to the QFM buffer for a hypothetical liquid of the bulk composition displayed at the starting T and P entered above.
Finally, turn off the f O2 constraint by selecting the absent option item of the f O2 Constraint entry of the Intensive Variables menu.
Enter a file name of your choice in the Selection box. To be readable by MELTS using the open... command, the file must have a .melts extension. You may specify a full directory path (as in this example) if you choose. Otherwise, the default directory identified in Filter will be assumed. The default directory will always be the one from which you launched the program. Click the OK button to close the dialog.
The model may be halted at any time by invoking the Execute/Halt entry of the Commands menu. The user may change bulk composition or model constraints and restart the calculation using the same menu entry.
A manual page describes many common numerical problems that may occur when the program is running.
is superliquidus. The extent of undersaturation may be gleaned from the affinity column of the Phase Composition display panel:
Since the model is isothermal and isobaric, MELTS plots a point for each mass increment of assimilant added to the system.