Effect of iron and nanolites on Raman spectra of volcanic glasses: A reassessment of existing strategies to estimate the water content

Danilo Di Genova,Stefania Sicola,Claudia Romano,Alessandro Vona,Sara Fanara,Laura Spina

doi:10.1016/j.chemgeo.2017.10.035

Abstract

The effect of iron content and iron nanolites on Raman spectra of hydrous geologically-relevant glasses is presented. Current procedures to estimate the water content using Raman spectra were tested to explore potential effects of iron content, its oxidation state, and nanolites on models' reliability. A chemical interval spanning from basalt to rhyolite, including alkali- and iron-rich compositions, with water content up to 5.6wt% was investigated using two spectrometers. When considering nanolite-free samples, the area of the band at 3550cm−1 linearly correlates with the sample water content regardless of chemical composition. Using this approach, data were reproduced with a root-mean-square error (RMSE) of ~0.15wt%. Depending on the sample chemistry, water content, and acquisition conditions the laser-induced sample oxidation led to underestimating the water content up to ~90% with a long acquisition time (26min). Normalising the water band region to the silicate band region minimises such a limitation. The area ratio between these bands linearly correlates with the water content and the use of different baseline procedures does not remove the dependence of such a correlation by the iron content and its oxidation state. With this procedure, data were reproduced with a RMSE of ~0.16wt%. For both approaches, the presence of iron nanolites may result in underestimating the water content.

Highlights

Water is the most abundant volatile species dissolved in natural melts and greatly affects, even at low concentration, a variety of thermodynamic and physical properties, from phase equilibria, to reaction kinetics, element diffusivities, electrical conductivity, heat capacity, and partial melting (Behrens and Zhang, 2009; Giordano et al, 2015; Lange and Carmichael, 1990; Poe et al, 2012; Scaillet and Macdonald, 2001; Stebbins et al, 1995)
The area ratio between these bands linearly correlates with the water content and the use of different baseline procedures does not remove the dependence of such a correlation by the iron content and its oxidation state
By using Raman spectra, this study provides best procedures to both estimate water content of natural glasses and recognize iron nanolites

Summary

Introduction

Water is the most abundant volatile species dissolved in natural melts and greatly affects, even at low concentration, a variety of thermodynamic and physical properties, from phase equilibria, to reaction kinetics, element diffusivities, electrical conductivity, heat capacity, and partial melting (Behrens and Zhang, 2009; Giordano et al, 2015; Lange and Carmichael, 1990; Poe et al, 2012; Scaillet and Macdonald, 2001; Stebbins et al, 1995) Bulk properties such as viscosity and density of the melt can vary by several orders of magnitude depending on the dissolved water content (Bouhifd et al, 2015; Dingwell et al, 1996; Lange and Carmichael, 1990; Whittington et al, 2000). Investigations of run products from solubility, diffusion, decompression, crystallisation, and bubble nucleation experiments help to constrain the timescale of physical and chemical processes in hydrous systems (Blundy and Cashman, 2005; Fanara et al, 2015; Gardner et al, 2000; Gonnermann and Gardner, 2013; Hammer et al, 2000; Le Gall and Pichavant, 2016; Martel and IaconoMarziano, 2015; Shishkina et al, 2010)

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