Abstract
Zircons widely occur in magmatic rocks and often display internal zonation finely recording the magmatic history. Here, we presented in situ high-precision (2SD <0.15‰ for δ94Zr) and high-spatial-resolution (20 µm) stable Zr isotope compositions of magmatic zircons in a suite of calc-alkaline plutonic rocks from the juvenile part of the Gangdese arc, southern Tibet. These zircon grains are internally zoned with Zr isotopically light cores and increasingly heavier rims. Our data suggest the preferential incorporation of lighter Zr isotopes in zircon from the melt, which would drive the residual melt to heavier values. The Rayleigh distillation model can well explain the observed internal zoning in single zircon grains, and the best-fit models gave average zircon-melt fractionation factors for each sample ranging from 0.99955 to 0.99988. The average fractionation factors are positively correlated with the median Ti-in-zircon temperatures, indicating a strong temperature dependence of Zr isotopic fractionation. The results demonstrate that in situ Zr isotope analyses would be another powerful contribution to the geochemical toolbox related to zircon. The findings of this study solve the fundamental issue on how zircon fractionates Zr isotopes in calc-alkaline magmas, the major type of magmas that led to forming continental crust over time. The results also show the great potential of stable Zr isotopes in tracing magmatic thermal and chemical evolution and thus possibly continental crustal differentiation.
Highlights
Zircons widely occur in magmatic rocks and often display internal zonation finely recording the magmatic history
We present in situ Zr isotope compositions of magmatic zircon in a calc-alkaline plutonic suite from the juvenile part of the Gangdese arc, southern Tibet
The results show large variations ranging from −0.86‰ to 0.41‰, and most individual zircon grains exhibit internal zoning with low δ94Zr in the core and higher values toward the rim. This indicates that zircon favors light Zr isotopes from the melt, and its crystallization would drive melt to heavier Zr isotope compositions
Summary
Cathodoluminescence images of zircon are presented on the Left, showing the internal structures. The crystallization temperature of zircon was estimated using the Ti-in-zircon thermometer [33] (SI Appendix, Tables S2 and S5) This thermometer requires inputs of zircon Ti concentrations, bulk-rock SiO2 and TiO2 activities, and the pressure. The activities have a critical impact on the estimated temperatures [34], which were calculated using the thermodynamic software Perple_X [35] except for the biotite-rich enclave. This sample has quartz and rutile (inclusions in biotite), and the activities of SiO2 and TiO2 were both assigned to 1. (B–G) Histograms of δ94Zr values for the core, mantle, and rim of all zircon grains in each sample. The temperature differences in different samples are consistent with their lithologies and petrographic textures (SI Appendix)
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