The physical properties of silicate melts are of critical importance for understanding magmatic and volcanic processes on Earth and other planets. Most physical properties of melts are, ultimately, a consequence of the structural organization of the melt. Robust and fully generalizable strategies for the prediction of properties of naturally occurring melts as functions of composition, temperature, and pressure remain a challenging goal. Given the structural origin of macroscopic properties, Raman spectroscopy of glasses, which provides information on melt and glass structure, may provide a useful technique to understanding and quantify variations in macroscopic melt properties. Here, with the aim of providing a generalizable model for predicting the viscosity of silicate melts, we present the results of a Raman spectroscopy campaign performed on 30 anhydrous multicomponent silicate glasses resulting from quenching of remelted and homogenized volcanic rocks and synthetic equivalents. The sample suite comprises one of the largest databases of multicomponent melts for which (a) chemical compositions and (b) physical properties (i.e., viscosity, fragility, heat capacity, and glass transition temperature) are known. Raman spectra have been collected using green light sources at wavelengths of 532 nm. Spectra were collected on the same sample suite in four independent laboratories involving instruments from different manufacturers and, thus, using different spectrometers, detectors, and analytical conditions. Our results are also compared and integrated with published data on some of the same samples derived from two others setups using green light sources with 514.5 and 532 nm wavelegths. For the same sample, the Raman spectra acquired using different setups show different intensities and intensity ratios. However, a strategy based on the ratio between the low‐ and high‐wavenumber peaks (R) was developed to standardize the data to normalized Raman ratios (Rn) and thus to remove interlaboratory differences. Using these advances, we predict melt viscosity solely with the use of Raman spectral measurements of multicomponent silicate glasses, thus demonstrating the potential of the method in describing physical properties of silicate melts.

The interdiffusion of six major elements (Si, Ti, Fe, Mg, Ca, K) between natural shoshonite and a high-K calc-alkaline rhyolite (Vulcano island, Aeolian archipelago, Italy) has been experimentally measured by the diffusion couple technique at 1200°C, pressures from 50 to 500 MPa and water contents from 0.3 ('nominally dry') to 2 wt%. The experiments were carried out in an internally heated pressure vessel, and major element profiles were later acquired by electron probe microanalysis. The concentration-distance profiles are evaluated using a concentration-dependent diffusivity approach. Effective binary diffusion coefficients for four intermediate silica contents are obtained by the Sauer-Freise modified Boltzmann-Matano method.At the experimental temperature and pressures, the diffusivity of all studied elements notably increases with dissolved H 2 O content. Particularly, diffusion is up to 1.4 orders of magnitude faster in a melt containing 2 wt.% H 2 O than in nominally dry melts. This effect is slightly enhanced in the more mafic compositions. Uphill diffusion was observed for Al, while all other elements can be described by the concept of effective binary interdiffusion. Ti is the slowest diffusing element through all experimental conditions and compositions, followed by Si. Fe, Mg, Ca and K diffuse at similar rates but always more rapidly than Si and Ti. This trend suggests a strong coupling between melt components. Since effects of composition (including water content) are dominant, a pressure effect on diffusion cannot be clearly resolved in the experimental pressure range.

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The mixing of magmas is a fundamental process in the Earth system causing extreme compositional variations in igneous rocks. This process can develop with different intensities both in space and time, making the interpretation of compositional patterns in igneous rocks a petrological challenge. As a time-dependent process, magma mixing has been suggested to preserve information about the time elapsed between the injection of a new magma into sub-volcanic magma chambers and eruptions. This allowed the use of magma mixing as an additional volcanological tool to infer the mixing-to-eruption timescales. In spite of the potential of magma mixing processes to provide information about the timing of volcanic eruptions its statistical robustness is not yet established. This represents a prerequisite to apply reliably this conceptual model.Here, new chaotic magma mixing experiments were performed at different times using natural melts. The degree of reproducibility of experimental results was tested repeating one experiment at the same starting conditions and comparing the compositional variability. We further tested the robustness of the statistical analysis by randomly removing from the analysed dataset a progressively increasing number of samples.Results highlight the robustness of the method to derive empirical relationships linking the efficiency of chemical exchanges and mixing time. These empirical relationships remain valid by removing up to 80% of the analytical determinations. Experimental results were applied to constrain the homogenization time of chemical heterogeneities in natural magmatic system during mixing. The calculations show that, when the mixing dynamics generate millimetre thick filaments, homogenization timescales of the order of a few minutes are to be expected.3 Keywords: magma mixing, chemical exchanges, homogenization time, volcanic chronometer volumes of melts whose compositional variation is difficult to reconcile with classical geochemical models (e.g. Fourcade and Allegre, 1981).The use of numerical models and experimental petrology provided a powerful tool in the study of magma mixing, and several attempts have been made to capture the most relevant parameters involved during the interaction between magmas (Kouchi and Sunagawa, 1985; Laumonier et al., 2014a-b;Bergantz et al., 2015;Schleicher et al., 2016). These studies also highlighted an extreme complexity of the mixing process in space and time due to the interplay of the fluid dynamic regime and chemical exchanges between the interacting magmas.

The diffusive exchange of 30 trace elements (Cs, Nb) during the interaction of natural mafic and silicic alkaline melts was experimentally studied at conditions relevant to shallow magmatic systems. In detail, a set of 12 diffusion couple experiments have been performed between natural shoshonitic and rhyolitic melts from the Vulcano Island (Aeolian archipelago, Italy) at a temperature of 1200 °C, pressures from 50 to 500 MPa, and water contents ranging from nominally dry to ca. 2 wt. %. Concentration-distance profiles, measured by Laser Ablation ICP-MS, highlight different behaviours, and trace elements were divided into two groups: (1) elements with normal diffusion profiles (13 elements, mainly low field strength and transition elements), and (2) elements showing uphill diffusion (17 elements including Y, Zr, Nb, Pb and rare earth elements, except Eu). For the elements showing normal diffusion profiles, chemical diffusion coefficients were estimated using a concentrationdependent evaluation method, and values are given at four intermediate compositions (SiO2 equal to 58, 62, 66 and 70 wt. %, respectively). A general coupling of diffusion coefficients to silica diffusivity is observed, and variations in systematics are observed between mafic and silicic compositions. Results show that water plays a decisive role on diffusive rates in the studied conditions, producing an enhancement between 0.4 and 0.7 log units per 1 wt.% of added H2O. Particularly notable is the behaviour of the trivalent-only REEs (La to Nd and Gd to Lu), with strong uphill diffusion minima, diminishing from light to heavy REEs. Modelling of REE profiles by a modified effective binary diffusion model indicates that activity gradients induced by the SiO2 concentration contrast are responsible for their development, inducing a transient partitioning of REEs towards the shoshonitic melt. These results indicate thatdiffusive fractionation of trace elements is possible during magma mixing events, especially in the more silicic melts, and that the presence of water in such events can lead to enhanced Watson. Furthermore, diffusion studies using natural rock compositions, that are potentially more relevant for geological applications, are scarce in the literature (e.g. Baker, 1990; Lundstrom, 2006).Recently, González-García et al. (2017) reported results on the diffusive exchange of major elements in shoshonite-rhyolite couples. Here we use the same set of experiments to investigate the trace element diffusion in natural silicate melt compositions at physical conditions relevant for subvolcanic magmatic systems. Our primary objectives are: (1) to investigate the diffusive behaviours of trace elements in the presence of strong compositional gradients, linking the results to major element diffusivity; and (2) to assess the influence of water on trace element diffusivities. The implications of this study for element fractionation during mixing of magmas and timescale estimations of related geological processes are discussed.