Tuesday, 27 April 2010

When you don’t understand something, look it up, even if it takes multiple sources

I mentioned recently that I had a particularly good conversation at the last conference I attended. In follow up emails exchanged with the famous scientist in question he inquired if I’d complied a list talc-garnet occurrences and natural partitioning data. I actually had to ask him what he meant by that (the part about making a list of all of the published instances where talc-garnet occur (preferably in equilibrium) together was straightforward (answer: no, but I think that I will do so, it could be very useful), but I didn’t really understand what he meant by “natural partitioning data”. His reply further specified that he “was thinking of Fe/Mg (and possibly Mn) partitioning data between garnet and talc, in order to better constrain the HP phase relations”.

Ok, the word “partitioning” is English, and I kind of understand what he is saying, but I still wasn’t completly clear. Therefore I grabbed the dictionary of geologic terms I purchased back when I was an undergrad and looked it up. Not listed. Then I checked the index of Spear’s 1995 book. Not listed there either.

My next step was to look in an on-line dictionary of geologic terms. This one does list "partitioning method" ("A resistivity method in which a special electrode configuration is used, consisting of five electrodes, instead of the usual number of four, to provide a check on the observations"), but this is clearly not what he meant. It also lists "partition curve" ("A curve indicating, for each specific gravity (or size) fraction, the percentage that is contained in one of the products of the separation; e.g., the reject. Syn: distribution curve"), which is also not what he meant. Neither were any of the several definitions in the OED of any use.
Clearly, though the term is in general enough use amongst the metamorphic petrology community that he expected me to understand it, it isn’t yet showing up in the dictionaries.
Therefore I did a Scopus search for "Fe/Mg partitioning", and downloaded an article*, which might shed some light on the concept. Stay tuned for updates once I think I really understand what I think he meant…

*Sakai, T., Ohtani, E., Terasaki, H., Miyahara, M., Nishijima, M., Hirao, N., Ohishi, Y. and Sata, N. (2009). "Fe-Mg partitioning between post-perovskite and ferropericlase in the lowermost mantle." Physics and Chemistry of Minerals (volume in-press) 1-10.


Anonymous said...

Partitioning is a standard term in metamorphic petrology, but it is interesting that it hasn't made any dictionaries.

For Fe/Mg the general concept is that the two elements are broadly equivalent in many minerals, hence the existence of minerals such as garnet where the ratio of Fe/Mg can vary across a single grain.

Partitioning is the idea that regardless of the *whole rock* proportions of Fe and Mg, the relative amounts in two mineral phases will be dependent on thermodynamic properties such as pressure and temperature (for Fe/Mg it tends to be temperature).
So to put it crudely, as the temperature increases, Fe is happier in one phase than in the other and is preferentially partitioned into it.

So, after making a heroic number of assumptions you can measure the amount of Fe/Mg in two phases (e.g. garnet and biotite), do some maths and then estimate the temperature at which the minerals grew. Generally it helps to have an estimate of pressure as well (to get a point in P/T space). The partitioning of Ca between Garnet and plagioclase feldspar is a classic for this.

I guess your colleague plans something similar involving talc....

A Life Long Scholar said...

I rather guessed that it was something like that--I made use of this fact often doing my geothermobarometric calculations for Tasmanian metamorphic rocks. Yet, somehow, managed to overlook the term.

Perhaps it is time to purchase a new geologic dictionary--it has been a long time since I was an undergrad (taking the better part of a decade off between one's master's a PhD does that), the newer ones might include such terms, even if the on-line one I checked didn't.