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Hydrogen incorporation in natural zircons occurs through charge balance substitutions

The concentration of rare earth elements in pristine zircons is strongly correlated to hydrogen, highlighting their role in the incorporation of hydrogen.

Nominally anhydrous minerals (NAMs) is the name given to minerals without water in their structural formulae. The concentration of H2O in these minerals can nevertheless be substantial, up to several thousand ppm, and has huge implications for the water storage capacity of magmas and the mantle.

A new study by De Hoog et al. sheds light on how hydrogen is incorporated into the structure of one such NAM: zircon. By conducting Secondary Ion Mass Spectrometry (SIMS) on a population of young (<14 Ma) pristine zircons, the authors were able to measure the abundance of H2O at various points in individual crystals. They found that the concentration of H2O was strongly correlated with the content of P and rare earth elements (REE), and concluded that hydrogen is incorporated by a charge-balance mechanism whereby H+ and REE3+ substitute for Zr4+ in the mineral lattice.

Zircon

Cathodoluminescence (CL) and IR imaging of a zircon analysed in the study

The findings from natural zircons were corroborated by analysis of experimental zircons grown in controlled conditions. The authors note that the substitution mechanism is modulated by the presence of P in the mineral lattice and the melt, and therefore precludes the use of zircon to perform a straightforward back-calculation of H2O concentration in a co-existing melt.

De Hoog JCM, Lissenberg CJ, Brooker RA, Hinton R, Trail D, & Hellebrand E (2014). Hydrogen incorporation and charge balance in natural zircon. Geochimica et Cosmochimica Acta, 141, 472-486.. http://dx.doi.org/10.1016/j.gca.2014.06.033

SCIENTIFIC ABSTRACT

The water and trace element contents of natural igneous zircons were determined to constrain the mechanism of hydrogen incorporation. The low radiation-damage zircons were derived from Fe–Ti oxide gabbros from the Vema Fracture Zone (11°N, Mid-Atlantic Ridge). They contain up to 1212 ppmw H2O, 1.9 wt.% Y2O3 and 0.6 wt.% P2O5 and are generally strongly zoned. REE + Y are partially charge-balanced by P (Y, REE3+ + P5+ = Zr4+ + Si4+), but a large REE excess is present. On an atomic basis, this excess is closely approximated by the amount of H present in the zircons. We therefore conclude that H is incorporated by a charge-balance mechanism (H+ + REE3+ = Zr4+). This interpretation is consistent with FTIR data of the Vema zircons, which shows a strongly polarised main absorption band at ca. 3100 cm−1, similar to experimentally grown Lu-doped hydrous zircon. The size of this 3100 cm−1 band scales with H and REE contents. Apart from a small overlapping band at 3200 cm−1, no other absorption bands are visible, indicating that a hydrogrossular-type exchange mechanism does not appear to be operating in these zircons. Because of charge-balanced uptake of H, P and REE in zircon, the partitioning of these elements into zircon is dependent on each of their concentrations. For instance, DREEzrc/melt increases with increasing H and P contents of the melt, whereas DHzrc/melt increases with increasing REE content but decreases with increasing P content. In addition, H–P–REE systematics of sector zoning indicate kinetic effects may play an important role. Hence, using H in zircon to determine the water content of melts is problematic, and REE partitioning studies need to take into account P and H2O contents of the melt.

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