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There is no easy answer to this question. It depends on how much information you have about the possible source rocks and tectonic setting, and on what you hope to learn from your modeling exercise.

You might start by testing whether the suite of sample compositions that you actually have can plausibly be related to one another by fractional crystallization along a liquid line of descent. For this, just choose the lowest SiO2 sample that you have, and use that as the starting composition. This is independent of what the primary liquid may have been, but if you can't pass this test then searching for a primary liquid won't mean very much. Add enough excess H2O to ensure vapor saturation. Then check various pressure and fO2 conditions, and see how the liquid evolves. Does it move along the trends defined by the sample suite? If not, what could that mean? It could mean you need to add some component of recharge. It could mean the rocks are not liquid compositions but contain a significant fraction of cumulate material. It could mean the rocks are not co-genetic.

Assuming you can find a set of conditions where your suite looks like a liquid line of descent (and I note that often you cannot, because granites are, in my experience, very rarely liquid compositions), you might then choose a parental melt that would be a typical mafic rock for the tectonic setting you suppose your granites were emplaced in. Fractionate that under the same conditions that allow your granitoid samples to be related to each other, and see if it comes close to your samples. If not, you can try manually varying the oxides to improve the fit. In alphaMELTS 1.9 you can use the amoeba to do this search automatically; I think this is not yet implemented in alphaMELTS 2, though.

If you cannot demonstrate to yourself that your samples are liquid compositions along a fractionation path, your other choice is simply to use the rhyoliteMELTS barometer (see Gualda and Ghiorso's paper) to try to locate the multiple saturation conditions where each sample might have formed, and not try to solve the (very difficult) parental melt problem.
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Hello everyone, I am confusing about the selection of parental melt composition. Because the SiO2 content of granite are higher than 67%. So the SiO2 data range will be small. And if there are some mafic magma charge,How can I model this process?

Maybe this question is stupid. But I hope people knows answer can give me.
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Dear Paula,
Thank you very much for your response.

Yes, I tried to decrease the fO2 buffer to FMQ = -1, and the initial jump down of the melting liquid disappear. I was wondering does that mean we decrease the fO2 buffers so that inhibit the crystallization of spinel that resolve the issue (liquid jump up)?

That makes sense now for step 10, I just import the melts output file to Matlab to create the plot. Recently, I just understood that MELTS has the MATLAB version so that I could incorporate melts in Matlab directly.

For the esseneite, yes, I believe that only small amount of esseneite appears in the system. For instance, normally, for batch melting, 0.01 GPa, anhydrous, we have 14 (gm), Al, Ti-rich diopside crystallized in the system, but the amount of esseneite is only approximately 0 -4 (gm) (the lower the temperature, the higher the esseneite we could obtain in the system). Perhaps, I could just ignore this esseneite in this case due to artefact? Thank you very much.

Sincerely,
Ri Cao   
4
In terms of mineral phases crystallizing/melting out, after checking the melts.out file, I believe that presumably, there is a type of spinel disappear from the system that causes the jump up of the liquid at the first step (please see detailed spinel information below).
OK. Might be worth trying different fO2 buffers (like you suggested before) and/or experiment with turning the buffer off after calculating the initial Fe2+/Fe3+ distribution.

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For the workshop exercise, yes, I just followed your suggestions (i.e., the second link you sent me in the previous message) and I could successfully run until step 15 (do 270 more calc steps at a T increment of -1 oC). My calculation finished at 918.04 oC, and that said the calculation fail to converge at that temperature. I was wondering probably I could decrease the calc steps a little bit in order to run through step 10, and 11?
The plot tab mentioned in step 10 is specific to easyMelts. None of the plots work in the graphical user interface anymore, e.g. see here. But you can use Combine_tbl (called Melts_Excel in the instructions) to plot results. Or you can try easyMelts!

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Finally, just out of curiosity, I run some Venus igneous system batch modelling recently, and it sounds like I got two clinopyroxenes, where the former one is Al,Ti rich diopside and the latter one is esseneite. since esseneite might not exist in the igneous system, I was wondering probably MELTS is considering the mass balance or chemical equilibrium of the whole system (i.e., there are some silica-poor Fe3+ left) that MELTS give us the output mineral esseneite? Any thoughts on this will be very helpful, too. Thank you very much.
Yes, it may just be an artifact as (I think) you are suggesting. How much of the essenite is MELTS predicting?
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Dear Paula,
Thank you very much for your response and clarification regarding my questions above.

In terms of mineral phases crystallizing/melting out, after checking the melts.out file, I believe that presumably, there is a type of spinel disappear from the system that causes the jump up of the liquid at the first step (please see detailed spinel information below).

spinel           mass = 2.81 (gm)  density = 3.88 (gm/cc)     (analysis in mole %)
                 Fe''0.31Mg0.72Fe'''0.34Al1.61Cr0.00Ti0.02O4
                 G = -39939.73 (J)  H = -32506.70 (J)  S = 6.02 (J/K)  V = 0.72 (cc)  Cp = 3.29 (J/K) 
           chromite     hercynite     magnetite        spinel    ulvospinel
               0.00          8.83         16.97         71.78          2.42


For the workshop exercise, yes, I just followed your suggestions (i.e., the second link you sent me in the previous message) and I could successfully run until step 15 (do 270 more calc steps at a T increment of -1 oC). My calculation finished at 918.04 oC, and that said the calculation fail to converge at that temperature. I was wondering probably I could decrease the calc steps a little bit in order to run through step 10, and 11?


Finally, just out of curiosity, I run some Venus igneous system batch modelling recently, and it sounds like I got two clinopyroxenes, where the former one is Al,Ti rich diopside and the latter one is esseneite. since esseneite might not exist in the igneous system, I was wondering probably MELTS is considering the mass balance or chemical equilibrium of the whole system (i.e., there are some silica-poor Fe3+ left) that MELTS give us the output mineral esseneite? Any thoughts on this will be very helpful, too. Thank you very much.

sincerely,
Ri Cao 
6
Yes, I tried to check the batch crystallization/melting and let the model go in each direction (i.e., decrease T and increase T), and the results looks almost the same. When I compare the Alkaline versus SiO2, or CaO versus SiO2 ...., the starting point and the endpoint looks exactly the same and just see a little bit of difference (within 1%) of Alkaline or CaO, TiO2 content in the middle of the calculation).
Have a look at what phases are crystallizing / melting out. What is produced or consumed when the amount of liquid jumps? It's possible that it's an artifact of some chemical components not having many solid phases to go into (in MELTS' world).
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I am not too sure which assembly is the stable one. I plotted the Gibbs free energy versus temperature for both batch melting and batch crystallization and the results look almost the same. Would you please provide some further insights on this?
Whichever has the more negative Gibbs Free Energy is the more stable one.
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I tried the exercise you mentioned below (workshop exercise), and I have some questions there:
(1) Regarding the fO2 buffer to QFM_M1, does that mean I should choose FMQ-Mt+1 in this case?
No, Q-Fa-Mt+1 will correspond to FMQ+1. You want Q-Fa-Mt-1.
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(2) Is there a ''isothermal'' button I should click in the options list?
No, that's an easyMelts setting. Calculations will be "isothermal" / "isobaric" by default in the graphical user interface. Just don't select Isenthalpic, Isentropic or Isochoric.
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(3) How shall I choose the Calc steps? (i.e., 5 Cals steps at -1oC temperature increment)? Shall I set up a specific stop temperature to constrain the Calc steps?
Yes, set the final temperature and temperature increment the same way as you did with the other calculations.
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PS: Are there any recordings for this recent workshop? or How shall I get the information or register if there are any future workshops regarding MELTS software in the near future? Thank you very much.
No recordings, sorry. We weren't allowed to record it. The next workshop is planned for Goldschmidt next summer. There will be announcements once the registration site goes live, which will be December / January time.

Best wishes,
Paula
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Dear Paula,
Thank you very much for your reply.

Yes, I tried to check the batch crystallization/melting and let the model go in each direction (i.e., decrease T and increase T), and the results looks almost the same. When I compare the Alkaline versus SiO2, or CaO versus SiO2 ...., the starting point and the endpoint looks exactly the same and just see a little bit of difference (within 1%) of Alkaline or CaO, TiO2 content in the middle of the calculation).

I am not too sure which assembly is the stable one. I plotted the Gibbs free energy versus temperature for both batch melting and batch crystallization and the results look almost the same. Would you please provide some further insights on this?


I tried the exercise you mentioned below (workshop exercise), and I have some questions there:

(1) Regarding the fO2 buffer to QFM_M1, does that mean I should choose FMQ-Mt+1 in this case?
(2) Is there a ''isothermal'' button I should click in the options list?
(3) How shall I choose the Calc steps? (i.e., 5 Cals steps at -1oC temperature increment)? Shall I set up a specific stop temperature to constrain the Calc steps?

Sorry for these repetitive questions and please take your time to respond. Any thoughts on this will be very helpful! Thank you very much.

PS: Are there any recordings for this recent workshop? or How shall I get the information or register if there are any future workshops regarding MELTS software in the near future? Thank you very much.

Sincerely,
Ri Cao
8
It is quite common to get some hysteresis near the solidus, and sometimes the calculated phase assemblage is actually metastable.

One thing you might check is whether the melt productivity is smooth going down and up temperature for batch crystallization / melting. Do you get the same assemblage in each direction? Which assemblage is the stable one (as judged by the total Gibbs free energy)?

There is a worked example for a comparable situation for fractional crystallization in the most recent workshop files (easyMelts is similar enough to the Rhyolite-MELTS graphical user interface that it should be possible to follow):

https://magmasource.caltech.edu/gitlist/Workshops.git/tree/main/workshop_files/RC12frac/easyMELTS/
https://magmasource.caltech.edu/gitlist/Workshops.git/tree/main/workshop_files/RC12frac_b/easyMELTS/
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Hi Paula,
Sorry for my interruption again but another question I have found is that:

After I started to run the fractional melting, something comes weird here: After finding the solidus temperature (960 oC), I started the fractional crystallization at 960.6 oC (only 0.72% melt), and I set up the temperature step to 1 or 2 oC. However, the first step of the fractional melting gave me a huge amount of the liquid in the system (approximately 9 gm), however, for the rest of the calculation, the maximum liquid mass for each step is <<1.

For instance, here are some proportions of data from melts-liquid.tbl file:
Index  Temperature Pressure      Liquid Mass
21      960.6           0.1      7.16E-01
22       962.6           0.1      9.99E+00
23      964.6           0.1      1.75E+00
24      966.6           0.1      1.89E+00
25      968.6           0.1      2.06E+00
26      970.6           0.1      2.29E+00
27      972.6           0.1      7.52E-01



I was wondering is there something wrong with the calculation for the first step at 962.6 oC temperature since we get a huge amount of the liquid? Thank you very much and I look forward to your response.

Sincerely,
Ri Cao
10
Hi Paula,
Thank you very much for your clarification regarding my questions above.

Following your suggestions, the minimum melt fraction I could get is approximately 8% (1 oC above the solidus temperature). On top of this, I started to increase the temperature to run the fractional melting with temperature step = 2 or 3 oC and that works now.

Regarding the redox state: Yes, I computed the redox state ahead of the calculation and also choose the FMQ = 0 for the calculation.

I was wondering that would be interesting to see the variation of FMQ (i.e., -1, or +1) could (1) potentially influence the calculation results or (2) could resolve the algorithm converge issue? Thank you very much for your suggestions.

Sincerely,
Ri Cao

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