Coupled degassing and crystallization: experimental study at continuous pressure drop, with application to volcanic bombs

Abstract

Experiments on degassing of water-saturated granite melts with a pressure drop from 100 and 450 MPa to 40 and 120 MPa, respectively, at temperatures close to feldspar liquidus (750-700 degrees C), were carried out to determine the modality of water exsolution and vesicle formation at the liquidus temperature. Pressure-drop rates as small as approximately 100 bar/day were used. Uniform space distributions of bubbles of exsolved water were obtained with starting glass containing a small fraction (approximate to 0.5 vol.%) of trapped air bubbles. Volume crystallization of feldspar was observed in degassed melts supplied with seeds. Bubble size distributions (BSD) measured in granite glasses after degassing are presented. Data on vesicle characteristics (number, radius, area, elongation) were acquired on images digitized with standard software, while the reconstruction of size distributions was performed with the Schwartz-Saltikov "unfolding" procedure. Bubble size distributions of size classes in the range 5-1000 mu m were acquired with proper magnification and satisfactory statistical reliability of determined number densities. The BSDs of the experimental samples are compared with the results of measurements of rapidly degassed products of Mt. Etna and Vulcano Island. Many particular features of the bubble nucleation and growth can be distinguished in an individual BSD. However, the general BSD of the whole data set, including natural ones, can be relatively well described with linear regression in bilogarithmic coordinates. The slope of this regression is approximately 2.8 +/- 0.1. This dependence is in striking contrast with distributions theoretically predicted with classical nucleation models based on homogeneous nucleation of vesicles. The theoretical distribution requires the occurrence of strong maxima that are not observed in our experimental and natural samples, thus arguing for heterogeneous nucleation mechanisms

Structure and physical characteristics of pumice from the climactic eruption of Mount Mazama (Crater Lake), Oregon

“…Larger vesicles have power law distributions with an average fractal dimension D=3.3. We interpret this power law signature to reflect bubble coalescence (Gaonac'h et al 1996a) rather than continuous or sequential nucleation (Simakin et al 1999;Blower et al 2001) on the basis of both D>3 and abundant textural evidence for extensive coalescence. Additionally, some of the larger vesicles may have been inherited from the magma storage region, particularly those which deviate from simple power law distributions or where large bubbles have experienced different deformation histories from the matrix vesicles.…”

“…Number densities in clasts from pyroclastic flows of the Wineglass Welded Tuff (WT) and ring-vent phase (PF) are slightly higher on average, with N m V~1 -2×10 10 cm -3 . Vesicle size distributions obeying power law behavior have recently been described in both basaltic (Gaonac'h et al 1996a(Gaonac'h et al , 1996b and silicic (Simakin et al 1999;Blower et al 2001) systems. Figure 10 shows that vesicle populations in Mazama clasts also define power law trends (linear distributions on Fig.…”

“…Power law distributions can also be generated by continuous nucleation, as seen in slow decompression of silicic melts (Simakin et al 1999). Here successive nucleation events permit bubbles to nucleate only in spaces between existing bubbles and, ideally, produce an 'Apollonian' packing structure with D=2.45 .…”

“…Vesicle number densities (e.g., Toramaru 1990;Mangan et al 1993;Klug and Cashman 1994;Mangan and Cashman 1996) provide information on bubble nucleation, and both vesicle volume and power law distributions of vesicle numbers (e.g., Gaonac'h et al 1996aGaonac'h et al , 1996bSimakin et al 1999;Blower et al 2001) can be used to quantify the extent of bubble coalescence.…”

Mechanism of formation of volcanic bombs: insights from a pilot study of anisotropy of magnetic susceptibility and preliminary assessment of analytical models

“…With some variants, this mechanism is similar to that used to explain the formation of Pele's hair and Pele's tears (Walker and Croasdale 1972;Shimozuru 1994). In addition to those two general models, the association of key morphological features of bombs with specific eruption or transport conditions has been established, for very specific morphological types (Benage et al 2014;Carracedo Sánchez et al 2010;Carracedo Sánchez et al 2009Simakin et al 1999;Wright et al 2007). Nevertheless, from a general point of view, the mechanism of formation of volcanic bombs and the factors influencing the acquisition of their general shape remain relatively unexplored.…”