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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

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