<p>If you are sure that the samples for which you have H<sub>2</sub>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 <em>representative</em> of the rock unit, and science is done by standing on the shoulders of giants!</p>
<p>If you have several historical samples with different H<sub>2</sub>O contents, this gives you a range of water contents. Do you intend to use a model like <a href="https://doi.org/10.1016/j.epsl.2008.03.038" rel="nofollow noreferrer">Giordano et al. (2008)</a> ? If so, you also need a temperature estimate, which you may have by geothermometry. These temperature estimates have an uncertainty, see <a href="https://doi.org/10.2138/rmg.2008.69.3" rel="nofollow noreferrer">Putirka (2008)</a> for a review. For example, if you use the two-feldspar thermometer, it gives you <span class="math-container">$T$</span> to <span class="math-container">$\pm$</span>30 °C.</p>
<p>So, what you could do is to model two scenarios: one &quot;cool and dry&quot;, with the lowest <span class="math-container">$T$</span> and H<sub>2</sub>O content, which gives you the highest viscosity; and one &quot;hot and wet&quot;, with the highest <span class="math-container">$T$</span> and H<sub>2</sub>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 <span class="math-container">$\pm$</span>30 °C interval will change your result by half a log unit, and a <span class="math-container">$\pm$</span>0.2 wt% H<sub>2</sub>O interval will change it by one log unit, so you'd get something like <span class="math-container">$\log \eta = X \pm 0.75$</span> Pa s, which is not that bad.</p>

CC BY-SA 4.0
2021-02-01T13:01:04.297Z
2022-07-03T13:07:09.907Z
2022-07-03T13:07:09.907Z
Jean-Marie Prival
<p>There are several problems with what you're trying to do.</p>
<p>First, you are looking at the H<sub>2</sub>O and CO<sub>2</sub> contents of the rock as they are <em>today</em>. It is ignoring the fact that H<sub>2</sub>O and CO<sub>2</sub> <em>are</em> 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 CO<sub>2</sub> 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.</p>
<p>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 H<sub>2</sub>O was all in wet amphibole phenocrysts? Or was it all in the melt? These are things you need to take into consideration.</p>

CC BY-SA 4.0
2021-02-03T23:45:42.250Z
2021-02-03T23:45:42.250Z
Gimelist
<p>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 H<sub>2</sub>O, CO<sub>2</sub>...etc.  Historical sampling from the same area representing the same suite of rocks (but different sample locations) have results for H<sub>2</sub>O and CO<sub>2</sub> wt%.  I need H<sub>2</sub>O 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 H<sub>2</sub>O in my samples using the historical data, and if so, how would I go about doing that?  Also, how would I correct for (if possible) the difference in quality of the measurements due to more advanced and accurate/precise instrumentation?</p>

<geochemistry>
Calculating/estimating volatiles based on other representative samples
Nancy
# Calculating/estimating volatiles based on other representative samples

(Asked by: username_0 on 2021-01-30)

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? Also, how would I correct for (if possible) the difference in quality of the measurements due to more advanced and accurate/precise instrumentation?

**Tags:** geochemistry


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## 2 answers

### Answer by username_1 on 2021-02-01. Score: 3

If you are sure that the samples for which you have H2O 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 H2O contents, this gives you a range of water contents. Do you intend to use a model like [Giordano et al. (2008)](https://doi.org/10.1016/j.epsl.2008.03.038) ? If so, you also need a temperature estimate, which you may have by geothermometry. These temperature estimates have an uncertainty, see [Putirka (2008)](https://doi.org/10.2138/rmg.2008.69.3) 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 H2O content, which gives you the highest viscosity; and one "hot and wet", with the highest $T$ and H2O 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% H2O 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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### Answer by username_2 on 2021-02-03. Score: 2

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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