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I am currently working on a set of geochemical data representing calc-alkaline igneous intrusions from central Canada. The samples were analyzed in ~2013-2014. These samples have been tested for LOI wt%, but not H2O, CO2...etc. Historical sampling from the same area representing the same suite of rocks (but different sample locations) have results for H2O and CO2 wt%. I need H2O wt% values for my samples in order to complete a calculation for viscosity of the intrusions, but I won't be doing any more testing. Am I able to estimate H2O in my samples using the historical data? And if so, how would I go about doing that? And how would I correct for (if possible) the difference in quality of the measurements due to more advanced and accurate/precise instrumentation?

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If you are sure that the samples for which you have H$_2$O data are from the same intrusion than yours, then I don't see a problem for using these values for viscosity modelling, as long as it is clearly mentioned in the paper. After all, a sample is supposed to representative of the rock unit, and science is done by standing on the shoulders of giants!

If you have several historical samples with different H$_2$O contents, this gives you a range of water contents. Do you intend to use a model like Giordano et al. (2008) ? If so, you also need a temperature estimate, which you may have by geothermometry. These temperature estimates have an uncertainty, see Putirka (2008) for a review. For example, if you use the two-feldspar thermometer, it gives you $T$ to $\pm$30 °C.

So, what you could do is to model two scenarios: one "cool and dry", with the lowest $T$ and H$_2$O content, which gives you the highest viscosity; and one "hot and wet", with the highest $T$ and H$_2$O content, which gives you the lowest viscosity. This way you can be quite confident that the viscosity was somewhere between these two values. Usually a $\pm$30 °C interval will change your result by half a log unit, and a $\pm$0.2 wt% H$_2$O interval will change it by one log unit, so you'd get something like $\log \eta = X \pm 0.75$ Pa s, which is not that bad.

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There are several problems with what you're trying to do.

First, you are looking at the H2O and CO2 contents of the rock as they are today. It is ignoring the fact that H2O and CO2 are lost during rising and cooling of the magma. They can also be gained during subsolidus alteration, which I suspect happened in your case because the solubility of CO2 in granitic magma is extremely low, and it should not contain any carbonate minerals. Therefore, late addition of alteration minerals such as calcite would change the values.

Second, the viscosity depends on the ratio between the crystal and melt load in the magma mush. Figuring out what was solid and what was liquid and at what stage, after it has all solidified, isn't easy. What if your H2O was all in wet amphibole phenocrysts? Or was it all in the melt? These are things you need to take into consideration.

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