The role of deformation on the early crystallization and rheology of basaltic liquids

Abstract

This study highlights the rheological variation of magmatic systems during the early crystallization stage, which undergoes very different shear stress. Etna basaltic glass, made from natural rock powder, was used as a starting material. Nine shear rate-controlled experiments were conducted at 1150 °C (below liquidus temperature and undercooling degree ΔT ∼ 40 °C) with shear rates (γ˙) of 0.1, 1 and 10 s−1 using wide-gap concentric cylinder viscometry. Three additional experiments were conducted without spinning the melts (no shear, γ˙= 0 s−1). Run-products were collected after 3, 6 and 9 h. The experiment with the highest shear rate (γ˙=10s−1) showed a brittle failure when viscosity reached the value of 2.90 log (Pa·s) and after ca. 1150s. The measured viscosity matches the shear stress at 7244 Pa, corresponding to the brittle failure in our partly crystallised system at these conditions. After 9 h, the response of the partly crystallised Etna basalt to different deformation rates results in decreasing viscosity from 4.89 to 3.83 and 2.90 (log Pa s) as the γ˙ increases from 0.1 to 1 and 10 s−1, respectively. The main outcome of this study relates to the nucleation and growth of minerals with shear deformation. The deformation-free (γ˙ = 0 s−1) runs show the presence of only two phases: glass and Fe-oxides (Fe-ox) with only a few vol% (1–3) of oxides crystals after 3, 6 and 9 h. The deformation-bearing (γ˙ = 0.1, 1 and 10 s−1) runs show different scenarios: after 3 h, we observed only Fe-ox for a γ˙ of 0.1 s−1 (similar to deformation-free ones). As the shear rate increases to 1 s−1 and 10 s−1, solid phases after 3 h experiments are Fe-ox, plagioclase and clinopyroxene. Crystal growth rate depends on the applied shear rate: the highest rate is 1.1 × 10−6 cm/s and was measured for plagioclase after a 3 h experiment for γ˙ = 10 s−1. At γ˙ = 0.1 s−1, the plagioclase growth rate decreases to 2.70 and 1.35 × 10−6 cm/s as experimental time increases from 6 to 9 h, respectively. The vast range of shear stress and the systematic data obtained in this study are fundamental to deciphering crystallization dynamics suffered by magmas in volcanic reservoirs, dikes, conduits and lavas.