Quadratic minimization algorithm convergence failure

[andyb]

Hi all

I am running rhyolite-MELTS v 1.0.2 attempting to plot the cooling path of a specific magma.
The composition is as follows:

Si02      51.760
Ti02        0.280
Al203     4.280
Fe2O3     13.530
MnO       0.240
Mg0       24.150
Ca0       5.000
Na20       0.660
K20        0.080
P2O5     0.030

The intensive variables I have tried are: Liq. T@ 1616.6 C to 900 C (incl. 5.0) , isobaric P at 10,000 bars

After executing, the quadratic minimization algorithm convergence failure comes up. Can you suggest what I can do to stop this error?
Thanks!

Andy

[Paula]

Hi Andy,

Do you have an oxygen buffer set? If not, try either setting one or putting some of the iron as FeO. In fact, it can help to distribute the iron between ferrous and ferric, even if you intend to turn the buffer. That way, you are starting a bit closer to equilibrium.

If that isn't the problem then you could experiment with taking out minor components, like K2O and P2O5.

If it is fractional crystallisation that you are modelling, you may find that a smaller temperature increment will help.

Cheers,
Paula

[andyb]

Thanks for the help, Paula. I have had some success with changing the parameters and the composition etc.

The model I'm running should need to include fractional crystallisation, could you explain the various options available?
I'm also trying to include the assimilation of TTG crust with the cooling magma. I have a composition in wt% oxide.. how would I go about adding this assimilant to the model?

Thanks in advance!

Andy

[Paula]

Hi Andy,

There is a MELTS manual on Mark Ghiorso's OFM website. It was written for the original GUI (pre-rhyolite-MELTS) but a lot of the information still applies. In particular look at the tutorials:

Getting Started: Testing and Examples (together with this forum post that describes some of the ways that the interface has changed since the original tutorial was written)
Sample Problem: Crystallizing theoretical MORB liquid, exploring the MELTS interface
Sample Problem: Assimilation in an isenthalpic model

That should get you started with AFC. Feel free to get back to me with any questions.

Paula