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):
- Anczkiewicz, R., Szczepanski, J., Mazur, S., Storey, C., Crowley, Q., Villa, I.M., Thirlwall, M.F., and Jeffries, T.E., 2007, Lu-Hf geochronology and trace element distribution in garnet: Implications for uplift and exhumation of ultra-high pressure granulites in the Sudetes, SW Poland: Lithos, v. 95, 363-380.
- Bayly, Brian, 1993, Chemical change in deforming minerals:Oxford Monographs on Geology and Geophysics #21: New York, Oxford University Press, 256 p.
- Carlson, W.D., 2006, Rates of Fe, Mg, Mn, and Ca diffusion in garnet: American Mineralogist, v. 91, 1-11.
- Chakraborty, S., and Ganguly, J., 1991, Compositional zoning and cation diffusion in garnets, in Ganguly, J., ed., Diffusion, atomic ordering, and mass transport; selected topics in geochemistry: Advances in Physical Geochemistry, vol. 8: New York, NY, Springer, p. 120-175.
- Chakraborty, S., and Ganguly, J., 1992, Cation diffusion in aluminosilicate garnets: experimental determination in spessartine-almandine diffusion couples, evaluation of effective binary diffusion coefficients, and applications: Contributions to Mineralogy and Petrology, v. 111, 74-86.
- Ganguly, J., Cheng, W., and Chakraborty, S., 1998, Cation diffusion in aluminosilicate garnets: experimental determination in pyrope-almandine diffusion couples: Contributions to Mineralogy and Petrology, v. 131, 171-180.
- Ganguly, J., Tirone, M., and Hervig, R.L., 1998, Diffusion kinetics of samarium and neodymium in garnet, and a method for determining cooling rates of rocks: Science, v. 281, 805-807.
- Harley, S.L, and Kelly, N.M., 2007, The impact of zircon-garnet REE distribution data on the interpretation of zircon U-Pb ages in complex high-grade terrains: An example from the Rauer Islands, East Antarctica: Chemical Geology, v. 241, 62-87.
- Hermann, J. and Rubatto, D., 2003, Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust: Journal of Metamorphic Geology, v. 21, 833-852.
- Kelly, N.M., Clarke, G.L., and Harley, S.L., 2006, Monazite behaviour and age significance in poly-metamorphic high-grade terrains: A case study from the wester Musgrave Block, central Australia: Lithos, v. 88, 100-134.
- Skora, S., Baumgartner, L.P., Mahlen, N.J., Johnson, C.M., Pielt, S., and Hellebrand, E., 2006, Diffusion-limited REE uptake by eclogite garnets and its consequences for Lu-Hf and Sm-Nd geochronology: Contributions to Mineralogy and Petrology, v. 152, 703-720.
- Smit, M.A., Scherer, E.E., Broeker, M., and van Roermund, H.L.M., 2010, Timing of eclogite facies metamorphism in the southernmost Scandinavian Caledonides by Lu-Hf and Sm-Nd geochronology: Contributions to Mineralogy and Petrology, v. 159, 521-539.
- Tirone, M., Ganguly, J., Dohmen, R., Langenhorst, F., Hervig, R., and Becker, H.W., 2005, Rare earth diffusion kinetics in garnet: Experimental studies and applications: Geochimica et Cosmochimica Acta, v. 69, 2385-2398.
- Tracy, R.J., 1982, Compositional zoning and inclusions in metamorphic minerals, in Ferry, J.M., ed., Characterization of metamorphism through mineral equilibria: Reviews in Mineralogy, vol. 10, p. 355-397. (link is simply to table of contents, since an online copy is not linked)
- Van Orman, J.A., Grove, T.L., Shimizu, N., and Layne, G.D., 2002, Rare earth element diffusion in a natural pyrope single crystal at 2.8 GPa: Contributions to Mineralogy and Petrology, v. 142, 416-424. (Note: Carlson referenced this as a 2002 paper)
- Vielzeuf, D., Baronnet, A., Perchuk, A.L., Laporte, D., and Baker, M.B., 2007, Calcium diffusivity in alumino-silicate garnets: an experimental and ATEM study: Contributions to Mineralogy and Petrology, v. 154, 153-170.
- Vielzeuf, D., and Saul, A., 2010, Uphill diffusion, zero-flux planes and transient chemical solitary waves in garnet: Contributions to Mineralogy and Petrology, (currently Online First paper)
- Woodsworth, G.J., 1977, Homogenization of zoned garnets from pelitic schists: The Canadian Mineralogist, v. 15, 230-242.
- Yardley, B.W.D., 1977, An empirical study of diffusion in garnet: American Mineralogist, v. 62, 793-800. (link is simply to table of contents, since an online copy is not linked)
Elli Goeke
