Fractionating water as a free phase

[giosp86]

Hello all!
I am trying to model the an isobaric fractional crystallization and equilibrium crystallization processes for sample of quartz-monzodiorite using alphamelts 2 (version 1.1.0). The problem is that I am not able to fractionate the water as a free phase, the melt is eternally enriching in water, reaching levels of 40% around the subliquidus. The procedure I did was to recalculate the composition and find the liquidus according to a pre-established fO2 buffer using easymelts. The output file is as follows:

Title: IGN56
Initial Composition: SiO2 58.1559
Initial Composition: TiO2 1.0996
Initial Composition: Al2O3 15.6219
Initial Composition: Fe2O3 1.1571
Initial Composition: FeO 4.8232
Initial Composition: MnO 0.1019
Initial Composition: MgO 2.5481
Initial Composition: CaO 4.5964
Initial Composition: Na2O 3.5870
Initial Composition: K2O 3.7634
Initial Composition: P2O5 0.6253
Initial Composition: H2O 3.9202
Initial Temperature: 1102.93
Final Temperature: 500.00
Initial Pressure: 5000.00
Final Pressure: 5000.00
Increment Temperature: -5.00
Suppress: leucite

I recalculated the superliquidus (function 3 - 3) and for the melt fraction 1.0, the temperature was around 1103.7ºC. From that I reset the Tinitial to 1105ºC

I would like to know if there is a specific function to fractionate the water because when I run the composition in MELTS-Excel, at about 750ºC the water appears as a free phase. I tried some functions in menu 9 (Adjust liquid or fluid settings (including melt extraction)) but it didn't work.

Regards,
Giovanna

[Paula]

Hi Giovanna,

I'm am sorry for the delay in replying. I had just got back from traveling and your post got buried in trying to catch up.

I'm assuming you mean you are using the rhyolite-MELTS model version 1.1.0. There's not much point using 1.1.0 without CO2. 1.1.0 uses the old water model, same as 1.0.2 (except that it in 1.0.2 it is called 'water' and 1.1.0 it is called 'fluid').

Which version of alphaMELTS 2 are you using? It should say at the top when you run it. The menu options, including those for solids and liquids have been gradually overhauled so it's possible that's why it's not working. Were you typing 'water' or 'fluid' at the menu?

You can also turn on fluid fractionation with a "Mode: fractionate fluids" line in the .melts file.

Best wishes,
Paula

[giosp86]

Dear Paula,

Thank you very much for your reply. Actually I'm using the version rhyolite- MELTS 1.1.0 in alphaMELTS 2. I didn't quite understand, but should I use version 1.1.0 only when I have the CO2 content data or can I run it only with the H2O content? Thanks for the clarification regarding the "Mode: fractionate fluids" variable.

Best regards,

Giovanna

[Paula]

See the MELTS decision tree:

https://melts.ofm-research.org/MELTS-decision-tree.html

Generally, you should run 1.2.0 if possible if you have H2O and/or CO2 present.

The exception is if you are trying to model the granite ternary minimum. For that specific case then you should run 1.1.0 if you have CO2. If you don't have CO2 then 1.1.0 and 1.0.2 amount to essentially the same thing.

[giosp86]

Paula, I thank you so much for the very enlightening explanation!!!

Best regards,

Giovanna