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#71
Website issues, and Miscellaneous / Correct application of fractio...
Last post by oliviahogg - November 21, 2022, 01:14:28 AM
Hello!
I'm running some fractional crystallisation simulations where I am interested in the cumulative mass of the fluid, solid and liquid at each step. I just wanted to check that selecting fractionate solids and fractionate fluids was the correct way to do this?
Thus far I have been using the Phase_mass_tbl output and calculating the cumulative masses of fluid/liquid(given anyway)/solids from there.
For FC of a basalt w 1 wt% H2O at 200 MPa through to a final mass of ~10g liquid (69 wt% SiO2), I am getting a total cumulative mass of fluid1 (i.e. adding up the mass in each step) =0.35g. To me this is quite low but it may be right hence why i wanted to double check with you first!

also the alphamelts manual refers to frac_water - is this used interchangeably with frac_fluids?

Thanks again,
Olivia
#72
The H2O model in rhyolite-MELTS 1.1.0 is the old one, same as rhyolite-MELTS 1.0.2 and the original MELTS model. How the thermodynamic properties of the H2O dissolved in the melt, including the Cp contribution, are calculated is described in the last paragraph of Ghiorso & Sack (1995):
Quote
Thermodynamic properties of the H20 component in molten silicate liquids are modeled following the method of Nicholls (1980; Ghiorso et al. 1983). We adopt his value for the reference state entropy (152.63 J/mol-K) and optimize a value of the reference enthalpy (-279.992 kJ/mol) to satisfy best the experimental data on water solubility discussed above. We modify Nicholls' (1980) function for the volume integral contribution to the molar Gibbs energy of dissolved H20 by subtracting the Gibbs energy of supercritical water tabulated by Robie et al. (1978) and adding the identical quantity from Berman (1988, see previous paragraphs). This insures the thermodynamic properties of the H20 component of Table AI are internally consistent with those of the supercritical fluid.

However, unless you are trying to model the granite ternary minimum you should probably be using rhyolite-MELTS 1.2.0, which includes the updated water solubility model (Ghiorso & Gualda, 2015). See the MELTS decision tree:

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

Paula
#73
Hello,

I am using alphaMELTS 2 with the rhyoliteMELTS 1.1.0 to make some numerical simulations. I wanted to know if (and how) the Cp of the melt phase is calculated, and how it takes into account the Cp of H2O dissolved. Checking on the papers I found that for the melt, the Cp is calculated by Lange & Navrosky (1992) model, but I don't know if this had being updated since to add the Cp of the water (for example, with the model of Bouhifd et al. (2006)).

Thank you very much!
#74
Thanks Paul for your reply! :)

When I see the change in Fe2O3 behaviour is indeed at the temperature of cpx precipitation in the residue...Thanks!
Sati
#75
Dear Sati,

What you are doing sounds correct. The reason the behavior you are seeing is unexpected is that the bulk system Fe2O3 is a combination of two effects: the varying Fe2O3/FeO of the melt (which is what Kress and Carmichael describe) and the varying melt fraction and mineralogy as you crystallize the system. The superposition of these two effects indeed gives a minimum in bulk system Fe2O3, I think at the temperature where clinopyroxene joins the residue.

-- Paul
#76
Hi everyone! I'm a new user and I'm trying to model the variation in Fe2O3 content as a function of fO2 at given P-T conditions, but I have few things that I'm unsure of...
First of all I'm using pMELT at 1GPa and 1600ÂșC, the process I'm using is to set an fO2, then computing redox state and then execute, and read directly the Fe2O3 content in the Bulk system window, is this the right approach?

What is the meaning of the bulk system composition with respect to the Fe2O3 content? does it indicate the overall Fe2O3 content of melt+solid system? by selecting "phase: liquid-00" this should give the fO2 of the melt produced in this system?

When I decrease the temperature at fixed fO2 and 1GPa, the bulk system Fe2O3 diminishes from 1600 to 1400C but then it rises from 1400 to 1200C, from my understanding there should be a direct relationship between T and Fe2O3 (e.g., Kress and Carmichael 91) which seems to not being followed here when the temperature goes below 1400 C (?)

Sorry for the very basic questions, I'm not yet familiar with the software  ::)
Thanks!
#77
alphaMELTS 2 text-menu interface / Re: Fractionating water as a f...
Last post by giosp86 - November 03, 2022, 11:17:56 AM
Paula, I thank you so much for the very enlightening explanation!!!

Best regards,

Giovanna
#78
alphaMELTS 2 text-menu interface / Re: Fractionating water as a f...
Last post by Paula - October 31, 2022, 01:56:45 PM
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.
#79
alphaMELTS 2 text-menu interface / Re: Fractionating water as a f...
Last post by giosp86 - October 31, 2022, 07:29:10 AM
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
#80
alphaMELTS 2 text-menu interface / Re: Fractionating water as a f...
Last post by Paula - October 25, 2022, 02:17:46 PM
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