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Why allanite may swindle about its true age

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Abstract

Allanite from the Tertiary Rieserferner pluton (Austrian Alps) is texturally and chemically heterogeneous. Continuous covariation trends reflect coupled substitution of Ca+Al vs. Fe+REE+Th in allanite, whereas systematic variations in La/Nd demonstrate the increasingly stronger depletion of LREE in the melt during its crystallization. Allanite samples (corrected for 206Pbexcess) from two rocks scatter in the concordia diagram and define discordias from 31.8±0.4 Ma and 32.2±0.4 Ma to ca. 540 Ma. The apparent inheritance does not originate from the inclusion of older allanite or a high-μ phase, such as monazite, xenotime, or zircon, but from the incorporation of radiogenic Pb originating from a precursor. Since allanite requires a high enrichment of Th, (U), and LREE, it may form at the expense of a Th-LREE-rich precursor in metamorphic rocks or where such a phase had dissolved in melts. Likely precursors acquire with time radiogenic Pb isotopic compositions. This Pb, if incorporated in the product mineral, may give the illusion of inheritance. The allanite samples from the Rieserferner pluton show a tendency from high Th/Ucalc (30–50) in samples with an Alpine age to low Th/Ucalc (4–12) in samples with distinct inheritance. This relation between extent of inheritance and apparent Th/Ucalc indicates a lower Th/Ucalc for a possible precursor, falling into the Th/U range commonly encountered for monazite. Precursor monazite would have originated from assimilated Palaeozoic rocks and give rise to localized enrichments of Th and LREE in the melt, thus eventually enabling the growth of allanite.

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Acknowledgments

We thank Sten Littmann (GFZ Potsdam) for his work with the electron-microprobe, Andre Steenken (Göttingen) for the mineral separation, and Catrin Schulz (GFZ Potsdam) for help with the sample preparation in the clean laboratory. Fritz Finger (Salzburg) and Felix Oberli (ETH Zürich) provided detailed and constructive reviews. We gratefully appreciate their efforts that eventually resulted in a more focused presentation.

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Authors and Affiliations

  1. GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 , Potsdam, Germany

    Rolf L. Romer

  2. Institut für Geologie und Dynamik der Lithosphäre, Georg-August-Universität, Goldschmidtstr. 3, 37077 , Göttingen, Germany

    Siegfried Siegesmund

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  1. Rolf L. Romer
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  2. Siegfried Siegesmund
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Correspondence to Rolf L. Romer.

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Editorial responsibility: J. Hoefs

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Romer, R.L., Siegesmund, S. Why allanite may swindle about its true age. Contrib Mineral Petrol 146, 297–307 (2003). https://doi.org/10.1007/s00410-003-0494-6

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