This example demonstrates how to do an isobaric, isenthalpic assimilation model using MELTS. We will use the same initial conditions and assimilant as in the isobaric, isothermal assimilation example and impose th e constraint of constant enthalpy.
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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 0.10
The starting temperature and pressure are set to the initial temperature and pressure of the system. These are the conditions at which the initial or reference enthalpy of the liquid+/-solid assemblage are calculated. The stopping temperature must be set to some value different from the starting temperature. As the program executes, the calculated temperature will be compared against the stopping temperature. If the calculated temperature falls below the stopping temperature, execution will halt. As temperature is a dependent variable in this model, the temperature increment is undefined. The stopping pressure is set equal to starting pressure because the evolution path is isobaric. The pressure increment may be left blank or set to zero. The dP/dT gradient is undefined.
In this model the system enthalpy is the sum of the reference enthalpy and the enthalpy of the added assimilant - The thermal heat flux is taken to be zero. Consequently, the increment in enthalpy is zero and the gradient dP/dH is undefined. Both values may be left blank. A non-zero value of the enthalpy increment would correspond to heat flow in (+) or out (-) of the system over and above that which accompanies the mass of assimilant added at each step of reaction progress.
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 final 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.
<-con H | con T->
Note that the process of assimilation has consumed the liquid phase in order to maintain thermal balance.
The evolution of liquid composition may be seen in the liquid composition graph: