New insight into disturbance of U-Pb and trace-element systems in hydrothermally altered zircon via SHRIMP analyses of zircon from the Duluth Gabbro

摘要

Redistribution and retention behavior of major (Zr, Si, O, and Hf) and trace (Li, Al, P, K, Ca, Ti, Mn, Fe, rare earth elements (REE), Pb, U, and Th) elements in zircon during hydrothermal alteration were investigated by a sensitive high-resolution ion microprobe (SHRIMP). AS3 zircons collected from the Duluth Complex, U.S.A. were classified into three domains: Type-A comprise domains with a darker backscattered electron (BSE) and cathodoluminescence (CL) response; Type-B comprise domains with fine fractures, and; Type-C comprise domains where the dark response areas in CL are bright in BSE and vice versa, and are free of fractures. The Type-A domains are characterized by discordant U-Pb ages, light REE enrichment, depletion of Zr and Si, and higher contents of Ca, Mn, Fe, Al, Li, and K, which suggests that elemental redistribution occurred during hydrothermal alteration. Although the Type-B domains also show depletion of Zr and enrichments of Ca, Mn, Fe, Al, Li and K, the Zr contents and the contents of Ca, Mn, Fe, Al, Li, and K are higher and lower than those of the Type-A domains, respectively. This feature of the Type-B domains suggests existence of thin altered domains surrounding the fractures like "clads". Therefore, there is a possibility that the Type-B domains are mixtures of the altered clads and unaltered domains. The Ca contents show a correlation with the Zr contents, which indicates that the Ca content is a suitable criterion for identifying hydrothermally altered zircon. There is no obvious correlation between Ca content and disturbance of the U-Pb system, but enrichment of some trace elements and REE's are correlated to the degree of Ca enrichment. The monovalent elements (Li and K) and the trivalent elements (especially light REE) are enriched at Ca contents of 1 ppm and 10 ppm, respectively. Mobility of the tetravalent elements (Hf and Ti) is increased at Ca contents 100 ppm. In spite of the elemental redistribution during the hydro