Melting peridotite under continent with FMQ fO2; Cr in input file if possible

[elipes]

Hi there,
This may be a painfully novice problem within AlphaMELTS 2 but I am attempting to melt a peridotite composition close to that of Workman & Hart 2005 at mid continental conditions and impose a FMQ oxygen fugacity. I've worked through the Workman and Hart MORB melting example on the Magma Source workshop files successfully to equilibrate, impose an isentropic melting regime, set a starting pressure/depth, and execute. Here's my starting composition:

Title: Sample
Initial Composition: SiO2 45.2
Initial Composition: TiO2 0.16
Initial Composition: Al2O3 4.03
Initial Composition: Fe2O3 0.18
Initial Composition: Cr2O3 0.38
Initial Composition: FeO 8.17 (this is FeO(t) and I recalculated it to FeO: 8.008 & Fe2O3: 0.18 for the input file and replaced FeO(t))
Initial Composition: MgO 38.52
Initial Composition: CaO 3.36
Initial Composition: Na2O 0.29
Initial Temperature: 1500
Final Temperature: 1500
Initial Pressure: 1
Final Pressure: 1
Increment Temperature: 0.00
Increment Pressure: -25.00
log fo2 Path: None
mode: fractionate liquid
output: both

After I inputted my melt file into AlphaMELTS 2, I turned off the liquid (as per the tutorial using option 9), equilibrated the system, selected FQM for fO2, turned the liquid back on, set my initial P, ran a single calculation again (option 3), then option 4, suboption 2 to run the calculation. The calculation fails every time and I'm not sure where my error is. To be able to successfully put Cr in the melt, I had to increase the initial pressure to 40000 bars and the initial temperature past 1400 C; without Cr, melting worked about 30000 bars. (My advisor says even 30000 bars is way too high for a mid continental setting.) I'm also not sure if it's useful for me to recalculate FeO(t): I had understood by speaking with other people that when entering in an FeO(t), MELTS would recalculate it to FeO and Fe2O3. If you enter in the FeO and Fe2O3 totals manually, would this eliminate the need to impose an fO2 at all?

Still others have warned me adding Cr into the calculation could output a flawed result if not careful.

I'm also not running an isentropic melting option here since it looks like in the MELTS manual that specifying an fO2 and imposing isentropic melting will crash the calculation.

Fundamentally, what I am trying to accomplish is melt a composition similar to Workman & Hart with an FMQ fO2 underneath a continent instead of a mid ocean ridge. If I have to throw out Cr to accomplish this, so be it.

Thank you again for whoever can guide me in solving my problem! I am quite new at using MELTS and if I am overlooking something painfully obvious to veterans please let me know!  :D :D :D

Cheers, Liz

[asimow]

Hi Liz,

I ran your calculation and it worked fine, but I have a bunch of questions about what you're trying to do.

(a) Your input file seems to redundantly have the 0.18 g of Fe2O3 and in the total Fe as FeO, but maybe your text comment indicates that you actually put Initial Composition: FeO 8.008 in your real input file. So that's fine. It should not matter at all whether you put the Fe2O3 in initially or let the program calculate it when you impose the FMQ buffer ... except that your first calculation takes place before you turn the buffer on. Anyway, 0.18 g of Fe2O3 is only equivalent to FMQ at certain conditions, so if you want to stay on the buffer, you should not run closed system with Fe2O3 and FeO separately conserved.
So, for example, if I run with 0.18 g of Fe2O3, then 30000 bars and 1500 °C is above the solidus, but if I turn on FMQ at the same P, T then it is subsolidus.

(b) Why are you doing the business with turning liquid off and equilibrating at 1500 °C and 1 bar? That part of the tutorial is only to obtain a reference entropy for subsequent use in isentropic calculations. If you're not going to run isentropic, you don't need it.

(c) The calculation you describe is a strange one. It appears to be isothermal decompression fractional melting, starting at high pressure and 1500 °C and continuing down pressure in 25 bar steps at constant temperature. This path implies addition of entropy from somewhere ... both decompression and melting should cause the system to cool, and you are not letting it do so. So, what do you really want? Are you trying to find an equilibrium of the source composition at some particular P and T? Do you really mean to do fractional melting? Think about what you are asking the program to do and why.

(d) I don't know at what step you are crashing, so I can't really advise on that.

(e) I don't know what you mean by "to be able to successfully put Cr in the melt...". At 1500 °C, this composition, with Cr and buffered to FMQ, crosses the solidus at about 29550 bars (though it scrapes along at very, very low melt fraction until 28900 bars).

(f) If you include Cr, you will always have residual Cr-spinel. That may be what people meant when they warned you about Cr. You can't ever get a four-phase garnet peridotite, because MELTS doesn't know where to put Cr except in spinel and liquid. Apart from that, and hence a very wide range of garnet-spinel coexistence, the behavior with Cr is OK.

(g) You're right that simultaneously applying isentropic constraint and an fO2 buffer is complicated, but it can be done under some circumstances. The question is whether you need to do so.

So, if you could explain better just what you want ("melt a composition similar to Workman and Hart with an FMQ fO2 underneath a continent" is too vague), we can figure out how to do it.

-- Paul

[elipes]

Hi Paul,

Thanks for your feedback! I really appreciate your getting back to me.

a)   The composition I have explicitly reports only FeO total—I've attempted to recalculate this into FeO and Fe2O3. If AlphaMELTS 2 allows a user to report only the original FeO total in the MELTS file for the calculation, then impose an FMQ buffer and have the program redistribute and output the total Fe to both FeO and Fe2O3 quantities while melting proceeds, this is definitely preferable.

b)   I turned the melt on and off because I was following the steps in a tutorial (https://magmasource.caltech.edu/gitlist/Workshops.git/tree/main/workshop_files/isentropic_melting/alphaMELTS2/) for  melting a Workman & Hart depleted upper mantle composition, and attempting to adapt the steps toward my problem. Any unusual sounding decisions on my part are due to unfamiliarity with the program. If I could execute different calculations that follow different melting modes, i.e. melting the composition as a pure fractional melt, and then at a later time melt the composition as a pure batch melt, that would be great however I'm unsure how to accomplish that.

c)   I have some melt depleted natural samples (residues) reported in previous literature that recovered from a mid-continental zone, and I need to come up with a melting curve (I'm making a chart that plots Fe isotopic composition against Mg# where I measured the isotopes) that account for them. I'm not exactly sure of the P/T conditions needed to do this, but I have kept increased the temperature and pressure to cause a great enough degree of melting to produce residue with high Mg#s. The composition I want to melt starts with an Mg# of 89.4, and the most depleted literature sample I have has an Mg# of about 92. I want to melt my starting composition to the extent I could plausibly produce a highly depleted residue that looks like my depleted literature samples. Melting the starting composition isentropically at a very high P/T seems to do the trick but I don't think this matches conditions under the continent, my feeling is the P should be much lower.  And thus far the program has been unhappy when I've tried to impose an fO2 buffer.

d)   Would you mind telling me explicitly what steps you've used within your command/terminal window to execute this composition with the buffer, please? An example of this would be super helpful for me now and in the future.


Thank you again and please let me know if you need more details about what I'm trying to do here!
Cheers, Liz

[Paula]

Hi Liz,

Sorry for the late response. A couple of thoughts...

(a) If you think that the starting conditions are going to be above the solidus and you have a fO2 buffer turned on then putting separate FeO and Fe2O3 is redundant but will do no harm. If the starting conditions are subsolidus then you should definitely input separate FeO and Fe2O3, even if you have a buffer imposed (the distribution of FeO and Fe2O3 doesn't have to be all that close to the buffer but not as extreme as all Fe as FeO!) The reason is that the subsolidus start calculated using the norm routine may be far from equilibrium, and the subsolidus fO2 buffer can fail. Hence, the program may turn the buffer off temporarily to get closer to equilibrium, before trying to impose it properly.

(b) I'm not sure which MELTS manual you are looking at but it sounds like it might be out of date. You can run isentropic calculations with an fO2 buffer in more recent versions. In alphaMELTS 1.X you needed set an environment variable (ALPHAMELTS_IMPOSE_FO2) but the rhyolite-MELTS graphical user interface and alphaMELTS 2 both automatically switch between isentropic steps that equilibrate the system with no buffer and isothermal steps that reimpose the buffer. The main differences are that the GUI doesn't have a subsolidus start option, and that alphaMELTS 2 always ends with an equilibration step, rather than a buffering step (GUI is other way round).

(c) I agree with what Paul is implying that you should turn off fractional melting, at least for a start. Maybe explore the space just above the solidus with the fO2 buffer on and look at the resulting residue compositions.

(d) I don't know exactly what steps Paul used. My hunch is that if you avoid turning on the buffer and the liquid in the same step that it should work better. The equivalent of ALPHAMELTS_ALTERNATIVE_FO2 is also implemented inside alphaMELTS 2 - I just need to hook it up to the menu / code that reads the input file. I'll try to get that in the next release, which may help here too.

Cheers,
Paula