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Iron isotopic fractionation during eclogite anatexis and adakitic melt evolution: insights into garnet effect on Fe isotopic variations in high-silica igneous rocks
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Iron isotopic fractionation during eclogite anatexis and adakitic melt evolution: insights into garnet effect on Fe isotopic variations in high-silica igneous rocks
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Iron isotopic fractionation during eclogite anatexis and adakitic melt evolution: insights into garnet effect on Fe isotopic variations in high-silica igneous rocks
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Journal Article
Iron isotopic fractionation during eclogite anatexis and adakitic melt evolution: insights into garnet effect on Fe isotopic variations in high-silica igneous rocks
2022
Overview
How and what extent residual garnet in the source and garnet fractionation contribute to the heavy Fe isotopic compositions in high-silica igneous rocks remain unexplored. We here measured the Fe isotopic compositions of garnet-bearing leucosomes, garnet-free leucosomes, garnetites, garnet-amphibolites, eclogites and their constituent minerals in the North Qaidam ultrahigh pressure metamorphic belt. Garnet-amphibolites were derived from the retrogression of eclogites. Garnet-bearing leucosomes were formed by partial melting of eclogites with residual garnet in the source, while garnet-free leucosomes and garnetites originated from garnet-bearing leucosomes by garnet fractionation and accumulation, respectively. Iron isotopic measurements show that eclogites and garnet-amphibolites have similar Fe isotopic compositions with δ
56
Fe of ~ 0.05‰, indicating that fluids involved in retrograde metamorphism are internally buffered and thus did not modify the Fe elemental and isotopic budget of metamorphic rocks. Garnet-bearing leucosomes have heavier Fe isotopic compositions (δ
56
Fe ~ 0.09‰) than eclogites and garnet-amphibolites, consistent with residual isotopically light Fe-rich garnet in the source and the preferential extraction of isotopically heavy Fe-rich omphacite into melts during eclogite partial melting. The δ
56
Fe values of garnet-bearing and garnet-free leucosomes increase with the increase of SiO
2
, Fe
3+
/ΣFe and (Na + K)/Al, and with the decrease of FeO
T
, suggesting that isotopically light Fe-rich garnet fractionation, together with compositional changes of magmas, controls the observed Fe isotopic variations. Rayleigh-fractionation modeling estimates Fe isotopic fractionation factors between garnet and felsic melts to be
α
garnet-felsic melt
= 0.999958–0.999988. Such small Fe isotopic fractionation factors suggest that except for garnet fractionation, fractionation of other isotopically light Fe-rich minerals (e.g., ulvospinel–titanomagnetite, ilmenite, biotite) is also required to explain the extremely heavy Fe isotopic enrichments observed in high-silica igneous rocks worldwide.
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