Do’s and Don’ts

This section collects various recommendations and cautions for the use of MC_fit.

Do’s

  • Use the latest version of Perple_X and its data files. MC_fit is still in development and evolves rapidly.

  • Do an example before trying your own problem.

  • Use raw analytical data, it need not be normalized, compositional data may be converted to molar units. If you use mass units set the molar_composition_input option to F. This setting is global, all input must be in either molar or mass units. A bulk composition calculated from mineral compositions is not raw analytical data; the raw analytical data are the mineral modes and compositions (Appendix B).

  • Specify reasonable ranges for inversion parameters in the .imc file (my_project.imc). Excessive ranges reduce the probability of finding the best central model. Restrictive ranges exclude the best central model and, paradoxically, slow computations. To view the scatter of the central-model results in the parameter space, do an initial test with, say, 50 central-model tries (number_of_tries) and no perturbations (number_of_perturbations set to 0), then adjust the parameter ranges accordingly.

Don’ts

  • Don’t guestimate unmeasured components such as O2, H2O, CO2, or S. The guestimation scheme may assume site populations that are different from the thermodynamic solution model used in the inversion and this will create conflicts. MC_fit fits analytical data to the site populations implicitly in the thermodynamic solution model. If you don’t like those site populations, change the solution models.

  • Don’t include components in phase compositions that cannot be predicted by the solution model chosen for that phase. For example, don’t include MnO in an amphibole composition if the amphibole solution model cannot accommodate MnO. The MnO from the observed amphibole composition would need to be accommodated by some other phase that can incorporate MnO and make the composition of that phase excessively Mn-rich. Unpredictable components should be either simply eliminated or replaced by geochemical proxies (e.g., the amount of MnO can be added to FeO).

  • Don’t specify components in the problem definition file (my_project.dat) that cannot be accommodated by the observed phases. By definition, these cannot be predicted and will needlessly complicate the inversion.

  • Don’t use bulk compositional data together with modal data unless the two data sets are independently measured (Appendix B).