Rubidium isotope compositions of the average upper continental crust and the Himalayan leucogranites: Implications for magmatic-fluid interaction

Rubidium is a highly fluid-mobile and incompatible element. Rb isotopes have great potential to trace the migration and enrichment of Rb in magmatic and fluid-related processes. In order to decipher the behavior of Rb isotopes during magmatic-fluid interaction, this study presents an estimate of average δ87Rb of the upper continental crust (UCC) and Rb isotope data for highly evolved leucogranites from Kampa, Himalayan orogen. The samples used to estimate the average UCC include 21 granites, 42 loess, and 10 sediments, which show small variations from −0.27‰ to 0.02‰, −0.17‰ to −0.07‰, and −0.31‰ to 0.07‰, respectively. Based on the weighted average δ87Rb of these samples, the estimate of average Rb isotope composition of UCC is −0.14 ± 0.01‰ (2SD/√n, n = 73), which is consistent with the δ87Rb of the bulk BSE (−0.13 ± 0.01‰).A comparison of the Kampa leucogranites with the UCC indicates that the δ87Rb of the Kampa leucogranites (−0.29‰ to 1.20‰) is significantly higher than the average value of the UCC. Such heavy Rb isotope composition cannot be due to chemical weathering, source heterogeneity, fractional crystallization, or partial melting. Instead, because the Kampa leucogranites have been modified by magmatic fluids during magma evolution, the high δ87Rb most likely reflects the signature of magmatic fluids derived from deeper magma. Our study for the first time indicates that the heavy Rb isotope characteristics of highly fractionated leucogranites provide a novel tracer for processes involving fluid activity. This could be useful for understanding the enrichment and mineralization of rare metals related to magmatic fluids.