Modelling trace elements in peralkaline melts

[catherinemmj]

Hi there - I'm new to MELTS and have a couple of questions.

Firstly, I'm looking for advice as to which MELTS programme to use. I am working with unusual peralkaline rhyolite glass compositions, which have evolved in shallow magma chambers.
My aim is to model FC and AFC between magma compositions erupted through time. I have lots of trace element data - but am I right in thinking that rhyolite-MELTS only models major elements? If this is the case, should I use alpha-MELTS?

Specifically, I am interested in barium, which increases in concentration in eruptives from my study volcano through time! x-y plots of Ba vs incompatible elements show that Ba is behaving as a weakly compatible to mildly incompatible element in these melts, contrary to its usual compatibility in feldspars. I am interested in whether the magma has been held deeper prior to eruption (suppressing feldspar crystallisation?), whether country rock has been assimilated, and if the partition coefficient for Ba is changing with peralkalinity. Is it possible to tweak the default partition coefficients for trace elements in MELTS for peralkaline compositions?

I am not quite sure whether to begin - so if you have any advice I would be really appreciative :)

Thanks - Erin.

[Paula]

Firstly, I'm looking for advice as to which MELTS programme to use. I am working with unusual peralkaline rhyolite glass compositions, which have evolved in shallow magma chambers.
My aim is to model FC and AFC between magma compositions erupted through time. I have lots of trace element data - but am I right in thinking that rhyolite-MELTS only models major elements? If this is the case, should I use alpha-MELTS?

Yes, only alphaMELTS models trace elements at the same time as major elements, though you can take the output from rhyoliteMELTS and use it to calculate the trace element concentrations separately. You may need to do that as alphaMELTS does not, yet, have the rhyolite-MELTS + H2O-CO2 fluid models.

alphaMELTS 2 does have the rhyolite-MELTS + H2O-CO2 fluid models. A more usable beta version will be out in the next few weeks but it won't have trace elements. I'm not sure how long it will take to finish the trace element engine, but hope to get something out by the summer.

One last thing to note is that the way the assimilant is specified in AFC calculations is slightly different between the rhyolite-MELTS graphical user interface and alphaMELTS. alphaMELTS 2 will allow you to use either format.

Specifically, I am interested in barium, which increases in concentration in eruptives from my study volcano through time! x-y plots of Ba vs incompatible elements show that Ba is behaving as a weakly compatible to mildly incompatible element in these melts, contrary to its usual compatibility in feldspars. I am interested in whether the magma has been held deeper prior to eruption (suppressing feldspar crystallisation?), whether country rock has been assimilated, and if the partition coefficient for Ba is changing with peralkalinity. Is it possible to tweak the default partition coefficients for trace elements in MELTS for peralkaline compositions? 

alphaMELTS can calculate partition coefficients using the method of Wood & Blundy, though the allowed combinations of elements and phases is limited - see the "Trace elements and water partitioning" section of the documentation. For Ba there are D(P,T,X) for clinopyroxene and feldspar. So turning on that (via the trace_data_file or the ALPHAMELTS_TRACE_DEFAULT_DPTX environment variable) may give you the compositional dependence you are looking for in peralkaline compsoitions. If not, you will need to do a separate calculation using the rhyolite-MELTS or alphaMELTS output.

alphaMELTS 2 will have a much expanded variable partition coefficient module - both lattice strain model and, eventually, user-defined models that may be empirical parameterisations. Again, hope that this will be available for public testing in the coming months.

Cheers,
Paula