Archive for August, 2010

The Geo-Metamorphism listserv can be hit or miss.   Some months, there are relatively few posts and they don’t encourage large-scale discussions (e.g. June 2010).   But some posts lead to a deluge of responses, which is what occurred when Alexander Smith from Uni Potsdam asked:

I am looking for a, let´s say list, with “closure temperature” estimates of Mg, Fe, Mn and Ca in high-grade garnets. I know that the term closure temperature might be inappropriate in this case, yet there has to be some estimate at which point during cooling these elements stop to diffuse.

If you check the August archive, you’ll notice that this post generated 26 responses(!).   But it was also timely due to the 2010 GSA session entitled “Garnet and Its Use in Unraveling Metamorphic and Tectonic Processes,” for which abstracts were due on 10. August (Wednesday talks & posters).

Highlights of the discussion:

  • modeling garnet zoning vs. diffusion rates is still problematic due to presence / absence of a fluid or melt phase, attempting to extrapolate experimental results to geological timescales & conditions, and large variety of garnet compositions possible [several people pointed out that we have equations & data that seem to fit the natural data fairly well]
  • are the zoning patterns due to growth then diffusion or are they a result of the varying availability of elements during the growth history of garnet? (e.g. bell-shaped REE & Y profiles due to sequestration of those elements so they are simply not available to the garnet at the end of its growth) [the former seemed to win out without too much issue]
  • the type of diffusion also plays a role, e.g. is it entirely lattice diffusion or is the movement along microfractures (+/- fluid)
  • if an aqueous phase is present, the different solubilities of various elements will also play a role in how quickly diffusion will occur
  • preservation of garnet zoning is dependent on how long the rock was held at a high enough temperature to allow diffusion–but there is a question of what kind of timescales are required and how warm was the rock; the “how long” and “how warm” will also vary for each element
  • diffusion is driven by chemical gradients, but what if the very small amount of REEs within a garnet form only a small gradient between the core & rim of the grain?
  • the different garnet end-members are not volumetrically equal, which could lead to pressure gradients

A number of papers were suggested (I’ll list them at the end) and vary from articles published in the 70’s to just last month.

(For those not aware, the “Eric” referred to several times is Eric Essene, who passed away May, 2010.)

Papers (almost all behind a paywall):

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