Iron isotopic fractionation by magmatic-hydrothermal fluid-melt interaction in the Himalayan leucogranites

Abstract

Iron (Fe) isotopes have been used to trace magmatic evolution (e.g., mineral fractional crystallization, partial melting degree, and fluid exsolution), but the behavior of Fe isotopes in magmatic-hydrothermal processes remains poorly constrained. Here we report Fe isotope compositions of the Kampa leucogranites from the Himalayan orogenic belt to understand the fluid-melt interaction and rare metal enrichment in highly-evolved granites. The Nb/Ta ratio divides samples into two groups: group 1 with high Nb/Ta ratios while group 2 with low Nb/Ta ratios. The δ⁵⁶Fe of group 1 leucogranites range from 0.14‰ to 0.23‰, isotopically heavier than the average upper continental crust value, indicative of the residues from fractional crystallization. On the contrary, the group 2 leucogranites exhibit large range of δ⁵⁶Fe from −0.21‰ to 0.21‰. The correlations between fluid-mobile/fluid-immobile element ratios, and iron isotope, and tetrad effect were attributed to the effect of magmatic-hydrothermal fluid-melt interaction during magma evolution. A fluid-melt interaction model with an initial δ⁵⁶Fe of -0.4‰ for the fluid and a fractionation factor Δ⁵⁶Fe_fluid-melt of −0.35‰ successfully explains the observed Fe isotope data. It is likely that these magmatic-hydrothermal fluids are derived from exsolution from the underlying magma reservoirs in the middle crust, which can modify the Fe isotope compositions and bring abundant rare metal elements into the leucogranites. Therefore, Fe isotope data can reveal the properties of magmatic-hydrothermal fluids and serve as an indicator for ore-forming processes.