Can we say that for a single sample, metamorphic zircon domains yield lower 176Lu/177Hf ratios but higher εHf (t) values than magmatic domains? Is it suitable for all kinds of rocks? I ask this question because Zheng et al.(2005) obtained this conclusion based on analyses on the eclogite-facies granitic gneiss and metabasite from Dabie, China. (Metamorphic effect on zircon Lu–Hf and U–Pb isotope systems in ultrahigh-pressure eclogite-facies metagranite and metabasite). Can anyone give some interpretations? Thank you.
I would say it might be true in some situations, but not in all rocks. Presumably it depends on the presence / absence of Garnet (or other relevant phases) and also, the composition of any fluid involved in the metamorphism. See for example:
http://www.sciencedirect.com/science/article/pii/S0016703710002942
Often metamorphic zircon will yield lower εHf (t) than the magmatic source. See for example:
http://www.sciencedirect.com.ezproxy.lib.monash.edu.au/science/article/pii/S0301926814001831
Dear Professor Armit, thank you for your help. However, I have no access to the link. Do you have any other ways to make it?Thank you again.
Hi Houxiang,
You can find the paper I mentioned in RG at:
https://www.researchgate.net/publication/262699366_U-Pb_and_Hf_isotopic_evidence_for_Neoarchean_and_Paleoproterozoic_basement_in_the_buried_northern_Gawler_Craton_South_Australia
Article U-Pb and Hf isotopic evidence for Neoarchean and Paleoproter...
In general, the Hf isotope composition of metamorphic zircon is dictated by the composition of source rocks and the P-T conditions of metamorphism. This issue is further address ed by a new paper in Lithos:
N.A. Zirakparvar, 2015. Cathodoluminescence guided zircon Hf isotope depth profiling: Mobilization of the Lu–Hf system during (U)HP rock exhumation in the Woodlark Rift, Papua New Guinea. Lithos 220–223, 81–96.
Dear Houxiang,
I wonder whether you want to distinguish between magmatic zircons received as detritus in a metasediment from zircons metamorphically grown within it. I think it will be necessary to take each lithology separately while discussing the isotopic composition of the zircons, since you can also have metamorphic zircons in magmatic protoliths that metamorphosed. Similarly, one can have a S-type granite which can have magmatic overgrowths on metamorphic zircon cores too. Thus, before any interpretation the evolution of a rock (and its zircons) has to be crystal clear in our minds.
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