Effect of metamorphic reaction history on the U-Pb dating of titanite
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Drill core samples of garnet-clinopyroxene granulite at Tirschheim and a reference sample at Waldheim (Saxon Granulite Massif, Germany) endured the same P-T conditions, but developed variable mineral assemblages due to differences in bulk chemistry, reaction progress, deformation and retrogression. Titanite formed during peak-metamorphic conditions of 22–24 kbar and 1020–1050 °C. Dating titanite from the various samples should yield the same age for all. The observed age variation, which exceeds the duration of the entire metamorphic cycle, originates from the contrasting preservation of isotopic inheritance during peak metamorphism and from post-peak re-equilibration. (1) Pb inheritance observed in some peak-metamorphic titanite demonstrates that geochronologically relevant elements are redistributed among remaining reactants and reaction products during prograde metamorphism and that the sequence of metamorphic reactions does not result in isotopic homogenization. Instead, metamorphic minerals inherit the radiogenic signatures of the precursor minerals and may in extreme cases approach the age of the precursor mineral. (2) Titanite that formed at peak-metamorphic conditions is characterized by high A1 contents and XF ≈ 0.8−1. Texturally comparable titanite that re-equilibrated during cooling (reduced Al contents and XF) yields too young U-Pb ages. The age of such re-equilibrated titanite does not correspond to the age of the event indicated by the texture.