Negative Mole Fraction for Si02

[gwynr]

Hello,

I am working on a project investigating the crystallization of Mercury's magma ocean, and was hoping for some guidance on how to use MELTS to that end.  The issue that I am having is the extreme reducing conditions and odd composition of the magma. I have my starting composition based off of the Indarch meteorite (from Berthet et al. 2009) and was going to remove silicon from the composition at varying levels to simulate the Si being partitioned into the core. I am unable to run any calculations past finding the liquidus which is much lower temp than I would expect (around 678 C). When I attempt to execute I get a SiO2 has a negative mole fraction, and as I've looked through the forum I have not found a solution as of yet.

Any help would be greatly appreciated!

wt. %.

SiO2 - 38.67
MgO - 49.22
Al2O3 - 4.08
CaO - 2.68
Na2O - 2.84
K2O - 0.31
Cr203 - 1.32
MnO - 0.70
TiO2 - 0.17

Temp was 1800 C to 1400 C
Pressure was 2.5 GPa to 1.0 GPa
fO2 (IW) was -4.93 from computing the redox state.

The system crashes at any attempt to execute past finding the liquidus.

Thanks again for any help!

Cheers,
Richard Gwyn
gwynr@wwu.edu

[asimow]

Hi Richard,

First, this composition is illegal as a superliquidus state in MELTS, because it doesn't have enough SiO2 to plot between the liquid end members that have been adopted to describe the silicate liquid phase.

Let me explain: MELTS represents the liquid as a solution among mineral-like molecules. The molecules that were chosen to represent the 9 oxides you are using are Si4O8, TiO2, Al4O6, MgCr2O4, MnSi0.5O2, Mg2SiO4, NiSi0.5O2, CaSiO3, Na2SiO3, KAlSiO4. To make a legal liquid, the number of each of this molecules must be non-negative. But if you convert your weight percents to oxide mole fractions, you'll see right away that SiO2 < (MgO/2) + CaO ... that is, you don't have enough SiO2 to make all the forsterite molecules needed to take up the MgO and the wollastonite molecules needed to take up the CaO. So the error you're getting is entirely appropriate.

Second, did you mean to have absolutely no iron in the system, or did you leave it out? At IW-4.93, maybe there is no FeO left ... I'm just checking. In any case, imposing an fO2 has no meaning in MELTS is there is no Fe in the system ... the only way that it knows how to measure or buffer fO2 is by via the Fe3+/Fe2+ ratio.

In principle, there may be a workaround if you don't try to model the near-liquidus region. A subsolidus initial guess may exist from a state that contains forsterite plus periclase plus other minerals. Then you could work up from there. But the subsolidus initial routine built into alphaMELTS isn't general enough to handle that case. You'd have to use the MATLAB or Python interface to construct the initial guess.