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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

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Experimental tracking of primitive magmas beneath Grenada, Lesser Antilles

High pressure experiments on a high-Mg basalt indicate parental magmas beneath Grenada are oxidised, and resolve the origin of two distinct lavas series

Experimental petrologists at the University of Bristol conducted experiments on lavas from Grenada using a range of experimental apparata to simulate to pressures and temperatures found beneath the island arc volcano. The redox conditions of the experimental runs were measured using the Diamond Light Source synchrotron, UK, and spanned a wide range of oxygen fugacities.  Synthetic replicas of natural rocks produced at moderately oxidising conditions were found to be comparable to the most primitive lavas erupted on Grenada.

Stamper and co-workers were able to use the composition of olivine crystals produced in experiments to calibrate a novel oxybarometer, which uses the partitioning of Fe and Mg between liquid and crystals to measure the oxygen fugacity of an olivine-bearing basalt.

Piston cylinder experiment from Stamper et al. 2014

A synthetic replica of a Grenadan magma produced during a high pressure experiment, as seen through a scanning electron microscope (gl: glass, ol: olivine, qu: quench, spl: splinel)

Experiments from this study also resolve the origin of the geochemically and petrographically distinct M- and C-series lavas, the latter type being unique to Grenada. At high pressures, experimental liquids are able to track the geochemical evolution of the highly magnesian M-series. In contrast, at lower pressures, clinopyroxene saturation is displaced to lower temperatures, relative to olivine, and so residual melts generated at these conditions become enriched in calcium, replicating the characteristic feature of the C-series.

Stamper CC, Melekhova E, Blundy JD, Arculus, RJ, Humphreys, MCS & Brooker, RA (2014) ‘Oxidised phase relations of a primitive basalt from Grenada, Lesser Antilles’, Contibutions to Mineralogy and Petrology, 167:954. http://dx.doi.org/10.1007/s00410-013-0954-6

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Raman spectroscopy offers new insights into the CO2 contents of magmas

A new calibration for micro-Raman spectroscopy paves the way for easy and accurate quantification of CO2 dissolved in volcanic glasses

CO2 is an important volcanic volatile. It is commonly the second most abundant dissolved gaseous species in a molten rock (after H2O) and it can have a dramatic effect on the phase relations and rheology of degassing magmas. The release of CO2 dissolved in magmas is also a vital part of the global carbon cycle. Thus, there has been considerable experimental effort dedicated to measuring CO2 solubility in silicate melts.

Raman laser

Laser path of the micro-Raman spectrometer in the School of Earth Sciences at the University of Bristol

Raman spectroscopy is a non-destructive spectroscopic technique that harnesses the scattering of light to provide information about the molecular structure of sample, e.g., CO2 content. Micro-Raman has advantages over other comparable techniques because it can analyse <10 μm spot sizes and it requires relatively minimal sample preparation; however, the analysis requires a compositionally dependent calibration.

To this end, Morizet and co-authors present a new calibration for the quantification of CO2 in geologically relevant glass compositions by micro-Raman. The study collected micro-Raman CO2 data for an extensive database of synthetic and natural samples, whose CO2 content had previously been quantified by bulk analysis, and found a relationship between the spectral features in the high-frequency region of aluminosilicate glasses and the spectral peak associated with dissolved carbonate. This new calibration is found to be accurate to better than ±0.4 wt% CO2.

Morizet Y, Brooker RA, Iacono-Marziano G, & Kjarsgaard BA (2013) Quantification of dissolved CO2 in silicate glasses using micro-Raman spectroscopy. American Mineralogist, 98(10), http://dx.doi.org/10.2138/am.2013.4516

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