«DISS. ETH NO. 20698 PHYSICAL PROPERTIES OF CRYSTAL- AND BUBBLE-BEARING MAGMAS A dissertation submitted to ETH ZURICH for the degree of Doctor of ...»
DISS. ETH NO. 20698
PHYSICAL PROPERTIES OF
CRYSTAL- AND BUBBLE-BEARING MAGMAS
A dissertation submitted to
for the degree of
Doctor of Sciences
Master of Geodynamics, Geophysics, Volcanology
Università La Sapienza, Roma (Italy)
born on January 5, 1984
citizen of Italy
accepted on the recommendation of:
examiner Prof. Dr. Peter Ulmer ETH Zürich co-examiner Prof. Dr. Luca Caricchi University of Geneva co-examiner Prof. Dr. Jean-Pierre Burg ETH Zürich co-examiner Prof. Dr. Kelly Russel UBC Vancouver A Luigi e alle sue montagne Alla mamma, al papà e alla sorella Claudia Abstract/Riassunto ABSTRACT Laboratory experiments at high temperature and/or high pressure were conducted to determine the physical properties of magmas with focus on the rheological behavior of bubble- and crystal-bearing suspensions. Two experimental investigations have been performed: one focused on magma rheology; the other centered on studying the magma vesiculation in-situ.
The rheology of three-phase magmas, composed of carbon dioxide-bearing gas bubbles, quartz crystals and haplogranitic melt, was investigated by deformation experiments at high pressure (200-250 MPa) and temperature (673-1023 K). The three-phase samples for these experiments were first synthesized to produce specimens with different volumetric proportions of crystals (24-65 vol.%) and bubbles (9-12 vol.%). The rheological and microstructural results have been combined to explain specific physical processes, such as the generation of shear bands in magmas at high crystallinity and outgassing in magmas at low crystallinity. The same results have been also used to provide empirical equations describing the general rheological behavior of three-phase magmas. Such results compose a solid background to better understand the physical processes occurring within magmatic chambers and along volcanic conduits.
High-temperature vesiculation of silicate melts has been tested using in-situ synchrotron experiments, to directly observe and quantify the dynamics of such a process.
Water-bearing silicate glasses were heated up to 1297 K to promote bubble nucleation and growth, while acquiring ultrafast (13 seconds) three-dimensional tomographic scans. This novel technique permitted to quantify the gas overpressure within gas bubbles, the surface tension between bubbles and silicate melt and the effective viscosity of magmas vesiculating in real time. Particularly, the determination of gas overpressure in magmas results fundamental because this parameter represents a key factor that can trigger hazardous explosive eruptions. Although the limited spatial and temporal scale of the experiments, such an in-situ experimental technique is a new frontier for the investigation of the physical processes responsible for the rheological behavior of magmas within magmatic chambers and along volcanic conduits.
i Abstract/Riassunto RIASSUNTO Sono stati condotti degli esperimenti di laboratorio ad alta temperatura e/o alta pressione per determinare le proprietà fisiche dei magmi, con particolare attenzione al comportamento reologico dei magmi aventi cristalli e bolle di gas. Sono state compiute due indagini sperimentali: una focalizzata sulla reologia dei magmi; l’altra dedicata allo studio della vescicolazione dei magmi in-situ.
La reologia dei magmi aventi tre fasi, ossia contenenti simultaneamente bolle di gas di anidride carbonica, cristalli di quarzo e fuso silicatico di composizione aplogranitica, è stata indagata con esperimenti di deformazione ad alta pressione (200-250 MPa) e alta temperatura (673-1023 K). I campioni a tre fasi utilizzati per questi esperimenti sono stati sintetizzati secondo differenti proporzioni volumetriche di cristalli (24-65 vol.%) e bolle di gas (9-12 vol.%). I risultati reologici e microstrutturali sono stati utilizzati per spiegare determinati processi fisici, quali la formazione di zone di taglio in magmi ad alta cristallinità e il degassamento nei magmi a bassa cristallinità. Gli stessi risultati sono anche serviti per la costruzione di equazioni empiriche in grado di descrivere il generale comportamento reologico di magmi a tre fasi. Questi risultati rappresentano una solida base per meglio comprendere i processi fisici che accadono all’interno di camere magmatiche e condotti vulcanici.
La vescicolazione di fusi silicatici ad alta temperatura è stata testata con esperimenti in-situ al sincrotrone, volti all’osservazione e alla quantificazione diretta di tale processo.
Vetri silicatici idrati sono stati riscaldati fino a 1297 K per promuovere la nucleazione e crescita di bolle di gas, mentre le scansioni tomografiche tridimensionali venivano rapidissimamente acquisite (in 13 secondi). Questa nuova tecnica ha permesso di quantificare la sovrappressione di gas all’interno delle bolle, la superficie di tensione tra bolle e fuso silicatico e l’effettiva viscosità di magmi che vescicolano in tempo reale. In modo particolare, la determinazione della sovrappressione di gas nei magmi risulta fondamentale poiché quest’ultima rappresenta il fattore chiave che puó innescare le pericolose eruzioni esplosive. Nonostante le ridotte scale spaziali e temporali degli esperimenti, tale tecnica sperimentale in-situ rappresenta una nuova frontiera nella invesitgazione dei processi fisici responsabili del comportamento reologico dei magmi all’interno di camere magmatiche e condotti vulcanici.
1.1 Terminology At the outset, it is important to define clearly some extremely important definitions of the terms "melt", "liquid", "glass" and "magma", elegantly described by Dingwell (2006).
A Melt is a material in the molten state, which, at different temperature or stress conditions, can behave as a glass or a liquid.
A Liquid is a material characterized by Newtonian viscosity (i.e., no dependence of viscosity on the applied stress or deformation rate) and a nil shear modulus (i.e., no shear strength).
A Glass is an amorphous material that exhibits a solid-like response to stress. This state is characteristic of relatively low temperatures and short timescales for the application of physical or thermal perturbations. Liquid-like behavior is instead characteristic of relatively high temperatures and long timescales for the application of perturbations. The transition from liquid to glassy behavior is confined by the glass transition temperature (Tg).
Finally, Magma is a melt-bearing multiphase geo-material, generally containing crystals and bubbles in suspension on a silicate-melt-matrix. Some eruptions release virtually crystal-free magmas, which, however, contain bubbles (obsidians).
Melt is used in the text to indicate the molten state of the suspending silicate phase under high-temperature and high-pressure conditions; Glass is used for microstructural observations in post-mortem deformed samples.
Furthermore, we would like to point out the difference between "bubbles" and "vesicles". Following Cashman et al., (2000): "bubbles" are pockets of pressurized gas in a melt; "vesicles" are holes or cavities preserved in solidified volcanic pyroclasts or shallow level plutonic rocks ("bubble fossils"). However, in this thesis the globular voids (vesicles) in the post-mortem microstructures are called bubbles.
1.2 Summary of previous investigations on magma rheology Natural magmas are complex multiphase mixtures where suspended solid crystals and gas bubbles are commonly present and transported by the separate carrier melt phase.
Specifically, when hydrous, felsic, partially molten magmas at sub-liquidus temperature conditions ascend to shallow crustal levels (500-0.1 MPa), such as magmatic reservoirs (from 20 to 8 km) and volcanic conduits (from 8 km to surface), oversaturation in volatiles
Chapter 1 Introduction(mainly water and carbon dioxide) is attained and bubble nucleation and growth occur. The release of water triggers additional crystallization and magma becomes a multiphase suspension with complex flow dynamics. The contemporaneous presence of crystals and bubbles dramatically changes the physical transport of magma to the surface.
Viscosity of the magma represents the principal parameter that controls the behavior of magmas; it is affected by numerous factors, such as major element composition, volatile content (mainly water), temperature, pressure, thermal history and strain rate. If crystals are present, their volume fraction, size distribution, shape and orientation/anisotropy influences significantly magma rheology. The addition of gas bubbles introduces new variables (bubble volume fraction, size distribution, number density, shape and orientation/anisotropy), which also affect magma viscosity. Over the last three decades numerous studies in different fields of volcanology, petrology and rock physics (field work, experimental laboratory, numerical and analytical modeling) were conducted to explore the rheological properties of magmas.
Among these, the following experimental studies focused on the physical properties of twophase systems:
- crystal-bearing magmas (van der Molen and Paterson, 1979; Lejeune and Richet, 1995;
Rutter and Neumann, 1995; Smith, 1997; Deubener and Brückner, 1997; Bagdassarov and Dorfman, 1998a; 1998b; Paterson, 2001; Petford, 2003; Mecklenburgh and Rutter, 2003;
Rutter et al., 2006; Lavallée et al., 2007; 2008; Arbaret et al., 2007; Caricchi et al., 2007;
2008a; 2008b; Champallier et al., 2008; Cordonnier et al., 2008; Kohlstedt and Holtzman, 2009; Mueller et al., 2010; Vona et al., 2011; Forien et al., 2011);
- bubble-bearing magmas (Bagdassarov and Dingwell, 1992; 1993a; 1993b; Stein and Spera, 1992; 2002; Lejeune et al., 1999; Llewellin et al., 2002; Rust and Manga, 2002a;
2002b; Okumura et al., 2006; 2008; 2009; 2010; Kameda et al., 2008; Caricchi et al., 2011).
These studies aimed at constraining the rheological effects exerted by crystals or bubbles (only) on the flow of magma. The presence of crystals influences the viscosity of magmas with variations up to 9 orders of magnitudes from pure melt to crystal fractions of about 0.8 (summarized by Costa et al., 2009). Depending on the applied deformation rate and bubble relaxation time (timescale of the bubble to respond to its shear environment; Llewellin and Manga, 2005), bubbles can behave as rigid inclusions (capillary number = product of strain rate and bubble relaxation time, Ca 1) or inviscid objects (capillary number, Ca 1). In the first case, bubbles behave like solid crystals, thereby increasing the viscosity with increasing bubble volume; in the second case, bubbles induce a decrease of magma viscosity.
Chapter 1 IntroductionThe three-phase mixtures of melt, bubbles and crystals have not intensively been studied so far. Bagdassarov et al. (1994) performed an experimental investigation on a synthetic rhyolite containing both crystals and bubbles under atmospheric conditions. They deformed the samples in oscillatory configuration in the range 1023-1323 K. The results conform to the general theory: crystals increase viscosity, while bubbles lead to a decrease of the viscosity under conditions of high capillary number (Ca 1). The crystal and bubble contents did not vary independently in their study; in fact, samples with equal volumetric amounts of crystals and vapor bubbles (9, 16 and 40 vol.%) were used in the experiments. It is, therefore, difficult to evaluate the relative contributions of crystals and bubbles to the resulting viscosity of the three-phase suspension. Thies (2002) investigated porous (up 60 vol.% pores) and crystal-bearing (5-15 vol.% needle shaped crystals) melts separately and modeled the rheology of the inferred three-phase systems as a function of three-step shear thinning behavior (decrease of viscosity with increasing deformation rate), where each phase (melt, crystals and pores) contributes to shear thinning. Unfortunately, such a model is inappropriate if all phases (melt, crystals and real gas-pressurized bubbles) simultaneously interact. The combination of different rheologies in a unique model is inadequate, since the respective binary rheologies of crystal-bearing magmas and bubble-bearing systems do not account for the interactions between the different phases characterizing three-phase suspensions. To date, there is no viscosity model available that describes the physical behavior of three-phase magmatic systems appropriately.
1.3 Outline of the dissertation The present laboratory-based study aimed at the investigation of the physical properties of crystal and bubble-bearing magmas. This study is divided in two main parts
(1) rheological behavior of three-phase magmas (composed of solid crystals, gas bubbles and melt in different proportions) undergoing simple shear deformation;
(2) in-situ synchrotron study of bubble nucleation and growth at high temperature in crystalfree magmas.
(1) Chapters 3-7 focus on magma rheology concentrating on the Non-Newtonian behavior, characteristic of high stress, high strain rate experiments and relatively low temperatures. This work enabled us to access aspects of magma flow, outgassing and fragmentation. Additionally, we have focused the attention on the mutual interactions between crystals, bubbles and melt during deformation, through analyzing rheological