Prolonged mantle residence of zircon xenocrysts from the western Eger rift

Abstract

Zircon is a common mineral in continental crustal rocks. As it is not easily altered in processes such as erosion or transport, this mineral is often used in the reconstruction of geological processes such as the formation and evolution of the continents. Zircon can also survive under conditions of the Earth’s mantle, and rare cases of zircons crystallizing in the mantle significantly before their entrainment into magma and eruption to the surface have been reported1,2,3. Here we analyse the isotopic and trace element compositions of large zircons of gem quality from the Eger rift, Bohemian massif, and find that they are derived from the mantle. (U–Th)/He analyses suggest that the zircons as well as their host basalts erupted between 29 and 24 million years ago, but fragments from the same xenocrysts reveal U–Pb ages between 51 and 83 million years. We note a lack of older volcanism and of fragments from the lower crust, which suggests that crustal residence time before eruption is negligible and that most rock fragments found in similar basalts from adjacent volcanic fields equilibrated under mantle conditions. We conclude that a specific chemical environment in this part of the Earth’s upper mantle allowed the zircons to remain intact for about 20–60 million years.

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Figure 1: Study area and collage of zircon photographs.
Figure 2: Cathodoluminescence images and U–Pb data of fragments from different zircon xenocrysts.
Figure 3: REE patterns measured on four different zircon xenocrysts.

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Acknowledgements

Y. Hong-Lin, Northwest University, Xi’an, and W. Fang, CAS Beijing, assisted with Lu–Hf measurements, E. Reitter, Tübingen University, with Sm–Nd analyses, N. Evans and B. McDonald, Curtin University of Technology, Perth, with U and Th analyses. Ion microprobe work at UCLA was supported by a grant from the Instrumentation and Facilities Program, NSF. We thank J. C. Harvey (Caltech Pasadena, USA) for revising the English and I. Williams (ANU Canberra, Australia) for his valuable comments on an earlier version of the manuscript.

Author information

S.M. and S.W. provided the zircons and valuable background information. Experiments were carried out by A.K.S. (oxygen, trace elements), F.C. (Lu–Hf), M.D. ((U–Th)/He), S.E. and W.S. (U–Pb, cathodoluminescence). W.S. designed the study. W.S. and A.K.S. interpreted the data and wrote the paper with substantial contribution made by M.D.

Correspondence to Wolfgang Siebel.

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Siebel, W., Schmitt, A., Danišík, M. et al. Prolonged mantle residence of zircon xenocrysts from the western Eger rift. Nature Geosci 2, 886–890 (2009). https://doi.org/10.1038/ngeo695

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