Incorporation of hydrogen into zircon through multi-coupling mechanisms and its geochemical implications

Zircon is a versatile mineral for geochemical studies. The slow diffusion of water in zircon makes it reliable to record the original water content of magma during the zircon crystallization. However, the application of water-in-zircon in Earth Sciences is hampered by the ambiguity in the hydrogen incorporation mechanism. In this study, we measured the zircon water and trace element contents on ten natural megacrystic zircons from different magmatic settings (incl. Reference zircon Plešovice, KIM-G, D15395, and SA01) with secondary ion mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), in order to explore the hydrogen-trace elements coupling mechanism in zircon. Our results show that water content in zircon from various magmatic settings conspicuously surpasses the amount required to charge balancing with excess REY (REE and Y). This implies that the H-REY charge balance is not the sole mechanism for H incorporation into zircon as supposed previously. This conclusion is also supported by FTIR analysis, which shows multiple OH stretching bands, especially at ∼ 3100, ∼ 3200, ∼ 3385 cm−1 (E//a), and ∼ 3420 cm−1 (E//c). The 3100 cm−1 absorption band is most probably related to the H-compensated REY cations, as suggested by the corresponding water content with that required to the charge balance of excess REY. The hydrogrossular-type substitution at the Si or Zr vacancy [SiO4 or ZrO4 = (OH)4] is indicated by the pair of oriented OH absorption bands at 3420 cm−1 (E//c) and 3385 cm−1 (E//a) or 3200 and 3180 cm−1, respectively, which accounts for the large H excess (>62% H2O, unrelated to extrinsic cations). This indicates that vacancy substitution dominates the zircon water uptaking in our samples. Furthermore, we show that the zircon water content is correlated negatively with P/REY, likely implying that the magma water content may also affect the partitioning of P and REY between zircon and melt. Integrating various published zircon water content datasets from different geological backgrounds, we posit that the water content in igneous zircon can reflect that in its parental magma, similar to other nominally anhydrous minerals (e.g., olivine, pyroxene, and plagioclase) used for this purpose.