I analysed10 samples of grantic gneisses taken along a transect. All the samples show negative EU anomaly but one of them shows negative Ce anomaly. Please what is the implication of negative Ce anomaly or could you please suggest any paper that discusses this anomaly? I have seen papers on Ce anomaly in weathering profiles and mafic igneous rocks but not on granitic gneisses. Thank you.
Dear Mr. Adeoti,
I studied these REE anomalies in fluorite deposits and in granite associated in time and space with pegmatites/aplites. Ce anomalies reflect the oxygen fugacity in the environment of formation. Under oxidizing conditions Ce (III) is transformed into the
Ce (IV) state. In this context, those minerals that can accept only trivalent REE over tetravalent REE display a strong Ce-depletion resulting in a negative Ce anomaly in the PAAS-normalized REE patterns. In host rock gneisses and in granites a slight decrease in the oxygen fugacity during fractionation left the Ce anomalies almost identical, attesting to a moderately oxygenated system . Granites and the associated pegmatites/aplites which lack any Ce anomaly or even display a positive one, formed
at considerable depth under reducing conditions.
H.G.Dill
Dear Mr Adeoti,
The negative Ce anomalies in the granitic gneisses were probably inherited from their source rocks, see work Zong-Yong Yang et al. (2018) for details.
Article Rare earth element tetrads and negative Ce anomalies of the ...
Regards, HG
You may check this one " Cause of large negative Eu anomaly in the highly evolved A-type granites with REE tetrad pattern"
As Harald Dill pointed it out, negative Ce anomalies are a direct indicator of oxidizing conditions. On the other hand, negative Eu anomalies indicate reducing conditions which, for that matter, are likely to prevail during the gneiss formation (lower oxidation state and larger-sized Eu2+ is fractionated in feldspar, mostly plagioclase). If oxidizing conditions are unlikely during the gneiss formation, the cause of the anomaly must be either older or younger. Hripsime suggested a Ce-depleted source, which is possible, but not very likely; as an alternative the anomaly could be secondary, related to alteration or even weathering. The most likely answer can be facilitated by complementary observations (mineralogy, structure, additional chemical data), especially by comparison with the "normal" samples from the lot.
Thank you Gavril Săbău . Your explanation is very elaborate and very helpful.
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