Ni partitioning in cpx (oxide vs trace element)

[BDreyer]

Hi All,
I'm fractionating mafic basalts and noticed that NiO increases as MgO decreases during cpx and plag fractional crystallization. Only when olivine appears does NiO decrease. It seems true then, that Ni isn't partitioned into cpx, and perhaps more realistic results would be obtained treating Ni as a trace element in alphaMELTS? I am aware that there are known issues with the partitioning of Cr2O3- what about CoO? Thanks for any insight.
-Brian

[asimow]

Hi Brian,

You are correct. As a major oxide component, NiO can ONLY go into olivine and liquid (though if you are reduced enough, Ni can go into Fe-Ni metal alloy, too). MELTS does not know how to put it in cpx. The same for CoO.

Cr2O3 is a similar but not identical situation -- it only goes into spinel and liquid, no other phases.

So, yes, you can treat Ni as a trace element. But the results will not be that accurate, because Ni partition coefficients are not very constant. Currently, D_Ni^(ol/liquid), D_Ni^(cpx/liquid) and D_Ni^(opx/liquid) ALL default to 9.4. Ni default to perfectly incompatible in every other phase. Currently, you can change these to new constant values (and make them different from one another), but you cannot implement anything better ... Ni is not one of the elements that can be treated by lattice strain theory. In the near future, we will be implementing extensions to alphaMELTS that will let you define general functions for such partition coefficients, but not yet. So, your best bet is to do your calculations of fractional crystallization without considering Ni, and then post-process a trace element treatment in outside software such as Excel or MATLAB, where you can implement something like the Beattie-Jones parameterization for D_Ni as a function of D_Mg.

[GIGN]

Hi Paul,
The software you have developed is excellent and has greatly assisted me in understanding magmatic processes. However, I have encountered some issues during the operation that require your help.

I am a student who has recently been using alphaMELTS to simulate the mobility of the Sc (Scandium) element in layered mafic–ultramafic intrusions. That is, the process of Sc enrichment from a deep magma chamber (5kbar) to near the surface (1 kbar) and ultimately into different minerals (e.g., Cpx, Ilm, Amp).

I have already attempted simulation in the deep magma chamber under isobaric conditions, with an initial composition of Sc at 37.41 ppm. At the end of the simulation, only Cpx showed enrichment of Sc (26 ppm), and other minerals (e.g. Ilm, Amp) did not exhibit enrichment of Sc.

I also tried changing the partition coefficient of Sc in clinopyroxene (D=0.51 to 4), modifying it directly from the default_trace_data.txt file, but it did not work. The exported results show the partition coefficient fluctuating between 0.49xxxx and 0.51xxxx, which is not the value I set.

I would like to ask:
(1) Can the simulation of Sc enrichment in other minerals be performed? (e.g., Ilm, Amp) I saw a post on the forum mentioning that "the system cannot simulate Co and Ni because the partition coefficients of these elements in Ol/Cpx/Opx are unstable............" Is Sc subject to the same issue?
(2) If I want to simulate both major and trace elements, especially the enrichment trends of some key metallic elements, do you have any suggestions?
(3) There are some teaching videos on YouTube from a few years ago, which are basically for macOS and Linux OS. Since my computer operates on Windows, are there any teaching videos suitable for Windows users?


Best wishes,
Jimmy Young

[asimow]

Hi Jimmy,

It should be possible to set a mineral/melt partition coefficient for Sc for each phase. The default Sc partition coefficients for many phases is 0 (the only defined values in the default file are: clinopyroxene 0.51, feldspar 0.02, garnet 2.27, olivine 0.16, orthopyroxene 0.33). So, let's say you want to change D_Sc_cpx/melt to 4 and, for good measure, D_Sc_ilm/melt to, say, 0.25.

You make a file with these values to override the defaults. Let's call it new_trace_data.txt:
Set D: clinopyroxene Sc 4
Set D: rhm-oxide Sc 0.25

Add as many lines as you like to activate partitioning of Sc into those phases.

Then in your setting file, such as alphamelts_default_env.txt, make sure you have these lines:
ALPHAMELTS_DO_TRACE true
ALPHAMELTS_TRACE_INPUT_FILE new_trace_data.txt

That last one is what tells the program to read in your new partition coefficients. Just editing default_trace_data.txt doesn't do this ... that file is for information purposes only, unless you tell alphaMELTS to read it using ALPHAMELTS_TRACE_INPUT_FILE

Then when you run, you should see values of bulk D above one when the solid assemblage is cox-rich.

Try it.

Meanwhile, as for Co and Ni:

[asimow]

Meanwhile, as for Co and Ni:
• These don't do anything as major element in pMELTS; they were left out of the calibration
• in MELTS and rhyoliteMELTS, CoO and NiO are handled by thermodynamic calculation, not as partition coefficients, at least for olivine-liquid partitioning, following the work of Hirschmann. Of course, you can also set partition coefficients for them and track them as trace elements in parallel with the thermodynamic modeling of them as oxide components of the system.
• Not sure what the previous post meant by "unstable". As major oxides, they may not be right, and they don't account for any solids other than olivine, but they shouldn't be unstable.

If you want to simulate major and trace elements together:
• It sounds like you are already doing that. That is what turning on trace elements in alphaMELTS 1.x does.

Windows:
• Most things now work the same on any operating system, once you get past installation. easyMELTS and alphaMELTS both run native on Windows now without emulators or Linux subsystem. To run alphaMELTS, you just have to have a perl installation. But it sounds like you are past those initial steps already. We don't have a lot of good teaching videos ... the conferences that have hosted our workshops recently have not allowed us to record them.