Solidification and Turbulence (Non-laminar) during Magma Ascent: Insights from 2D and 3D Analyses of Bubbles and Minerals in an Etnean Dyke

Abstract

Solidification, emplacement and fluid dynamics of a sub-volcanic rock at Mt Etna have been investigated through two-dimensional (2D) and three-dimensional (3D) textural analyses of the hosted bubbles and minerals. This rock is a 4 3m thick aphyric dyke (DK) that solidified at a depth of 100–300 m, below the pristine surface level. Seven samples from the dyke rim (DK1) to core (DK7) have been analysed in two dimensions by using a high-resolution scanner, a transmission optical microscope and scanning electron microscopy imaging with back-scattered electrons, and in three dimensions by microfocus X-ray computed tomography. Field observations and mesoscopic polished rock surfaces show bubble-rich, -poor and -free patches even in rock pieces of a few cubic centimetres, with changes in sizes and shapes; even so, their shapes and spatial arrangement can never be attributed to high degrees of strain. In parallel, the amount of bubbles irregularly varies from dyke rim to core, whereas plagioclase (plg), clinopyroxene (cpx), titanomagnetite (timt), and olivine (ol) show only limited variations. The fabric of bubbles retrieved by 3D orientation of their maximum length (i.e. elongation) is invariably random in space for each DK sample. These bubble features have been attributed to transitional to turbulent flows; that is, non-laminar regimes (Reynolds number>1000), predicted for a long time from numerical models and that occurred before the crystallization of minerals. Water solubility, volume of bubbles, magma density and viscosity models indicate that, at pressure higher than 10 MPa, 1wt % H2O was dissolved in the original trachybasaltic magma, which, in turn, was close to its liquidus temperature. As the pressure decreased at very shallow levels, the magma significantly degassed and volatile exsolution induced marked crystallization (mostly plg followed by cpx). The viscosity of the system increased, decelerating and halting the magmatic suspension. The textures and fabrics of bubbles were suddenly frozen in, despite crystals continuing to grow under the effect of cooling rate variables from the inner (DK7) to outer (DK1) dyke portions. Fluid-dynamic computations suggest that the DK trachybasaltic magma ascended with a velocity of few metres per second in a transitional to turbulent regime, before the growth of minerals.