How to select parental melt composition？

[chengkaide]

Hello everyone, I am confusing about the selection of parental melt composition. Because the SiO2 content of granite are higher than 67%. So the SiO2 data range will be small. And if there are some mafic magma charge，How can I model this process？

Maybe this question is stupid. But I hope people knows answer can give me.

[asimow]

There is no easy answer to this question. It depends on how much information you have about the possible source rocks and tectonic setting, and on what you hope to learn from your modeling exercise.

You might start by testing whether the suite of sample compositions that you actually have can plausibly be related to one another by fractional crystallization along a liquid line of descent. For this, just choose the lowest SiO2 sample that you have, and use that as the starting composition. This is independent of what the primary liquid may have been, but if you can't pass this test then searching for a primary liquid won't mean very much. Add enough excess H2O to ensure vapor saturation. Then check various pressure and fO2 conditions, and see how the liquid evolves. Does it move along the trends defined by the sample suite? If not, what could that mean? It could mean you need to add some component of recharge. It could mean the rocks are not liquid compositions but contain a significant fraction of cumulate material. It could mean the rocks are not co-genetic.

Assuming you can find a set of conditions where your suite looks like a liquid line of descent (and I note that often you cannot, because granites are, in my experience, very rarely liquid compositions), you might then choose a parental melt that would be a typical mafic rock for the tectonic setting you suppose your granites were emplaced in. Fractionate that under the same conditions that allow your granitoid samples to be related to each other, and see if it comes close to your samples. If not, you can try manually varying the oxides to improve the fit. In alphaMELTS 1.9 you can use the amoeba to do this search automatically; I think this is not yet implemented in alphaMELTS 2, though.

If you cannot demonstrate to yourself that your samples are liquid compositions along a fractionation path, your other choice is simply to use the rhyoliteMELTS barometer (see Gualda and Ghiorso's paper) to try to locate the multiple saturation conditions where each sample might have formed, and not try to solve the (very difficult) parental melt problem.