Multiphase rheology of Italian alkaline magmas

Abstract

As a step towards the characterization of the rheological properties of natural magmatic mixtures a detailed experimental investigation on natural sample derived from Italian Volcanic systems ( Stromboli, Etna and Phlegrean Fields) has been performed. Viscosity measurements were conducted on basaltic melts from Stromboli and Etna during their crystallization. At the investigated subliquidus temperatures ( 1131°C < T < 1187°C), a strong increase in viscosity due the presence of crystals (ϕ=0.06-0.27) was observed. At T > 1175°C and ϕ up to 0.12-0.14 both studied compositions behave as Newtonian, while with decreasing temperature, the increase in crystal content leads to the onset of a weak shear thinning behavior (n=0.9). A strong departure from the well-known (strain-rate independent) Einstein-Roscoe equation and from the strain-rate dependent model developed by Costa et al. (2009) has been observed. The origin of this departure is related primarily to the effect of elongated plagioclase in increasing crystal interactions at low ϕ. Detailed textural analyses allowed to develop for the first time on natural liquids a parameterization of the multiphase rheology, which takes into account the crystal shape, crystal size distribution of the suspension and the deformation regime. The results were applied to a synthetic suspension HPG8Na20 containing a variable amount of almost spherical corundum particles with known textural features at T=1000-1100 °C. Multiphase rheology of natural vesicles- and crystal- bearing magmas from Monte Nuovo have been investigated by uniaxial deformation experiments in the T range 600°C-800°C and strain rates between 10-7 and 10-4 s-1. Brittle behavior of the magma has been observed at T<600°C and strain rates of 10-5 s-1 and at T=800°C for the higher applied strain rate (10-4 s-1). All other experiments were performed in the viscous regime and Monte Nuovo magma showed always a non-Newtonian shear thinning behavior. Effective viscosity in the range of 1010-1013 were calculated by the no slip model by Gent (1960). Fitting of experimental data using the Herschel-Bulkley model revealed that at the studied condition, no yield stress was present and yielded a parametrization of the flow behavior in terms of consistency K and flow index n. The relative viscosities indicated a dependence from experimental temperature and applied strain rate. Higher relative viscosities (up to 2.1 log10 units) were observed with increasing temperature and decreasing strain rates. Negative relative viscosities were observed at T=640°C, when the melt approaches the glass transition temperature (Giordano et al., 2005). To derive the net effect of vesicles on the rheology of Monte Nuovo magmas, the relative viscosity was further normalized to the relative viscosity obtained by Caricchi et al. (2008) on the same sample composition and no porosity. Normalization revealed that the presence of vesicles has a major influence on the rheology of magmas and can lead to a decrease of the viscosity of more than two orders of magnitude. Viscosity measurements have been performed on basaltic melts from Etna at low oxygen fugacity conditions and T between 1150°C-1300°C. Reducing conditions yield a slight decrease in viscosity compared to the expected viscosity versus temperature relationship at fO2 = air (Giordano and Dingwell 2003). Moreover, another effect of lowering the oxygen fugacity conditions is the decrease of liquidus temperature of the melt which leads to none or minor crystallization compared to experiments conducted at fO2=air. The results of two experiments conducted at two different oxidation state at T=1300°C (Fe3+/ΣFe = 0.18 and 0.76, respectively) indicated that the viscosity decrease observed as an inverse function of fO2, is smaller than what observed by many authors on simplified iron-bearing silicate melts. We suggest that this behavior is linked to the competition in natural systems between Fe3+ and Al3+ for the tetrahedric structural sites. Being Al favored in tetrahedrical coordination, part of Fe3+ is forced to behave as network-modifier also at high fugacity condition, thus the depolymerization of the melt when the reduction Fe3+- Fe2+ takes place is limited. Our data seem to be consistent with those obtained by Bouhifd et al. (2004) on basalts, confirming that the effect of oxidation state on viscosity is more pronounced at lower than at higher temperatures and for Al-free synthetic liquids rather than for natural melts. Rheological measurements conducted on basaltic melts from Stromboli and Etna at subliquidus conditions (see above) were also used to investigate the effect of the deformation rate (gammai=4.26 s-1 for Stromboli and gammai=0.53 s-1 for Etna) on the crystallization kinetics of plagioclase at different degree of undercooling (ET), varying from 24 to 45°C for Stromboli and from 21 to 72°C for Etna. Post-run plagioclase crystal contents are between ϕ=0.11-0.23 for Stromboli and between ϕ=0.06-0.18 for Etna with increasing undercooling. Different kinetics of the crystallization process has been observed. Higher deformation rates seem to play a fundamental role in favoring the achievement of phase equilibria in a short time-scale. Crystal Size Distribution (CSD) of the experimental plagioclase crystals are always characterized by a log-linear decrease of population density as a function of crystal size. With increasing degree of undercooling a systematic increase of the steepness of the curves has been observed for both datasets. The CSD parameter of all samples (slopes and intercept) are linearly correlated with the overall crystal content. However, given the difference of the crystal content vs. undercooling behavior for the two datasets, two different linear correlation have been found for Stromboli and Etna, relating the CSD parameters to the degree of undercooling. In the framework of the CSD theory average value of G and average J were obtained. Stromboli data always show higher values of both G and J with respect to Etna data. Stromboli growth rates G are quite stable with increasing undercooling, however they display a relative small variation ranging between 2.5-3 x10-8 m/s. A slight increase of G can be observed for Etna with decreasing temperature, from 0.6 x 10-8 to 1.3 x 10-8 m/s. Nucleation rates J always continuously increase with undercooling from 1.3 x 107 to 8.72 x 107 m.3 s-1 for Stromboli and for Etna (9.2 x 104 m.3 s-1 - 2.9 x 107 m.3 s-1). However, the lowest value of J for Etna is relative to a sample in which evidence of textural coarsening have been observed and hence it represents a minimum value. Data obtained for G and J in this study are comparable with those obtained in literature, both from natural samples and from experimental studies. Data from this study can be applied to natural volcanic system to take into account the effect of deformation rate on the variation of crystallization kinetics. G and J data obtained in this work can be applied to natural volcanic system to infer residence times in magma chambers and magma ascent velocities. This work underline also the need and the importance to consider the deformation rates in the variation of the crystallization kinetics.