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Restart file use: phase metastability

Started by Paula, July 31, 2011, 03:36:02 PM

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Paula

As of July 22nd: I am in the process of documenting Adiabat_1ph 3 so that it may be released very soon. To that end I am posting some tips and examples etc.... This section from the old manual will eventually be updated and worked up into a more tutorial-style example.

When a text enrichment_file is read in within Source Mixer or during assimilation or flux melting calculations, the bulk composition is adjusted and a new equilibrium assemblage will be found from scratch. Similarly, in most cases when a binary restart_file is used for enrichment, for example during an isenthalpic assimilation calculation, weighted sums of the bulk composition and the relevant reference quantity (enthalpy in the example) are made and a new equilibrium assemblage calculated accordingly. The assumption is that the material being added is far from equilibrium with the current assemblage, due to bulk composition and / or temperature differences. The exception to this general rule of bulk-addition is in Source Mixer when a binary restart_file is read in, as in this case the individual phases are summed. This feature is intended only for adding small amounts of material that is close to equilibrium with the current bulk composition.

A suitable restart_file can be made by fractionating both solids and liquids (turning on both ADIABAT_FRACTIONATE_SOLIDS and ADIABAT_CONTINUOUS_MELTING) during a melting calculation. First find the appropriate melt fraction using trial-and-error (options 2 and 3; see 'Menu options' below). Then set ADIABAT_MINT to the final temperature and rerun so that when option 4 is called a single calculation is followed by fractionation of solids and liquid. It is best to increase ADIABAT_MASSIN, the amount of each solid phase retained, for this stage of the calculation (e.g. to 1 gram). Otherwise minor components, such as K and P, can go below the threshold at which the program considers them to have been exhausted, causing errors. Note that the solids are fractionated before the liquid so that setting ADIABAT_MINF to 0.2 in the above example would give 1 gram each of the liquid and the four solid phases. You will be given the opportunity to scale down the amounts later when reading the restart_file into Source Mixer so that the change in the bulk composition will be trivial.

In general, the program will reliably drop phases that are actually metastable. So, in this example, olivine is immediately dropped in the crystallization run due to its peritectic relationship with orthopyroxene and liquid. Both orthopyroxene and clinopyroxene stay in the assemblage once they have been added in the 'seed' composition. Naturally, it is worth double checking that phases are indeed dropped as appropriate and making sure that the 'seed' assemblage is not too far from equilibrium with bulk composition to which it is being added.