PREFACE

How and when a volcano will erupt, and the damage and disruption it may cause, is a question of great scientific and public concern. However, up to now we do not have the relevant knowledge to answer this fundamental question. While volcanic events can be catastrophic, they remain mostly unpredictable. More insights are needed into the key mechanisms involved in these dramatic natural events.

One of the areas requiring greater study is understanding the magmatic processes responsible for the chemical and textural signatures of volcanic products and igneous rocks. This includes investigating both equilibrium scenarios (e.g., phase stabilities, element partitioning) and kinetically controlled events such as the function of changing magma chemistry (replenishment), cooling, decompression (e.g., crystal and bubble nucleation and growth), devolatilization, and shear stress.

The coexistence of crystals, bubbles, and liquid phases needs to be considered as a dynamic environment in which temporal evolution depends on the system's characteristic features (e.g., bulk composition) and on its thermodynamic conditions. In this scenario, kinetics play a major role allowing possible liquid to solid phase transitions occurring at different times. Knowing the rate at which such transformations occur will provide fundamental information as it defines the eruption behavior of volcanoes. Kinetic processes are, indeed, responsible for abrupt changes in the rheological behavior of magmas, resulting in rapid and unexpected changes from low- to high-energy eruptions. It is then obvious that a better understanding of magma dynamics will improve our abilities to monitor volcanic activity and mitigate its potentially devastating impact.

The complexity of the physico-chemical processes involved during active volcanism and dynamic magmatism, which cover wide ranges of dimensions (from submicron to intercontinental) and timescales (from seconds during explosive eruptions to thousands of years for volcano-tectonic processes), demands multidisciplinary research approaches. Covering all scientific aspects related to this topic is far beyond the scope of this book, but this compilation of geochemical, petrological, physical, and thermodynamic studies provides a comprehensive overview about recent progress, often achieved using unconventional and novel techniques. The book can be a useful compendium for lecturers and students, as well as a reference for researchers developing new and innovative ideas and projects.

This book is divided into two sections. The first section (chapters 1-4) focuses on geochemical, petrological, and geophysical studies to understand dynamic magma evolution and the timescales involved, including processes such as magma recharge, mixing, and mingling. The second section (chapters 5-9) focuses on physical, thermodynamic, and theoretical approaches to investigating magmas according to the different geological scenarios that occur naturally.

Chapter 1 by Petrelli and Zellmer deals with rates and timescales of crustal magma transfer, storage, emplacement, and eruption. The authors review the most pertinent unresolved questions in this field, and highlight geochemical and geophysical methods that are available to address these questions. Long-storage magmatic timescales are discussed in detail as well as the influence of volatiles in volcanic environments and possible storage depths or ascent rates.

Chapter 2 by Longpré, Stix, and Shimizu, deals with melt inclusions as a unique material to retrieve information on the volatile budgets and compositional diversity of magmatic systems. Their results document a rare case of boundary-layer melt entrapment in natural magmas, indicating that melt inclusions hosted by fast-grown spinel are not a reliable record for melt evolution. The authors highlight the importance of boundary-layer melt entrapments as a tracer for dynamic magmatic processes.

Chapter 3 by Connors, Carley, and Fiege is dedicated to the properties of apatite as a tool for understanding the evolution of volatile and trace elements in magmatic systems. Torfajökull, a historically active Icelandic volcano erupting apatite-bearing magmas, is a key study site since it exhibits a unique compositional history, showing transitioning from more-evolved peralkaline rhyolites (Pleistocene) to less-evolved metaluminous rhyolites (Holocene). This study reveals the potential of apatite to elucidate magmatic and volcanic processes in Iceland.

Chapter 4 by Yamashita and Toramaru investigates the crystal size distribution (CSD) of plagioclase phenocrysts in four historical lavas from Sakurajima volcano, southern Kyushu, Japan. The results reveal a correlation between CSD and geological data (volumes and intervals between eruptions) for large historical eruptions, suggesting that the supply rate from the mantle controls the triggering of eruptions.

Chapter 5 by Giuliani, Iezzi and Mollo investigates the solidification of magmas occurring by cooling- (?T/?t) and/or degassing-induced decompression (?P/?t), as a function of solidus temperature, glass transition temperature, and melting temperature, respectively. The study evaluates the influence of bulk chemical composition of a silicate liquid, P, T, fO2, and H2O on magma solidification. The most relevant solidification conditions of magmas leading to possible crystallization paths and relative physical models and reconstruction of magmatic intensive variables using mineral composition variability, are discussed. Chemical attributes of minerals are reviewed in order to discriminate their equilibrium and disequilibrium formations and, finally, using chemical composition of solid phases the authors reconstruct magmatic intensive variables.

Chapter 6 by Nazzareni, Rossi, Petrelli, and Caricchi is dedicated to the Main Ethiopian Rift (MER). The MER represents a young continental rift with an associated large volume of magmatism that forms one of the major large igneous provinces. The authors used clinopyroxene crystals in order to record variations of P, T, and fO2 with the aim of reconstructing the geological history of host rocks. Clinopyroxene geobarometry was performed combining X-ray diffraction with mineral chemistry to highlight a complex polybaric plumbing system active since 7.5-3.7 Ma. The continuous polybaric MER clinopyroxene crystallization from the lower to middle crust is explained by a plumbing system composed of a dyke complex where magmas rise, stall, and, finally, crystallize.

In Chapter 7, Vetere and Holtz compare viscosity data from three different compositions: basalt, andesite, and a synthetic pyroxenite. In addition to the determination of melt viscosities, viscosity data were collected at subliquidus conditions in partially crystallized systems, under controlled shear rates of 0.1 and 1 s-1. Experimental data show that changes of shear rates from 0.1 and 1 s-1 may cause a viscosity difference of half to one order of magnitude, pointing to the so called "shear-thinning effect." The authors suggest that this effect should be taken into account when considering magmatic processes occurring in volcanic conduits as it could drastically change the dynamics of the magmatic system.

Chapter 8 by Zanatta, Petrillo, and Sacchetti propose the time-resolved neutron diffraction as a tool for the study of crystallization kinetics of glasses and supercooled liquids. In the situation of multiple crystallizing phases, the analysis of the Bragg peaks can provide an unambiguous identification of the crystal structures with proper lattice parameters and crystallization timescales for each phase. Results show that the isotopic sensitivity of neutrons could be exploited to highlight a single species with respect to the surrounding medium, thus facilitating data interpretation for complex systems such as ascending magmas. Neutron-based techniques are particularly suitable to measure bulk samples controlling environmental parameters such as T and P. This can be pivotal for geological studies aiming at in situ measurements during time-dependent processes such as crystallization in magmas. Moreover, an empirical model for the interpretation of the crystallization kinetics in supercooled liquids is presented.

Chapter 9 by Koepke and Zhang sheds new light on the complexity of magmatic and metamorphic processes ongoing within and at the roof of axial melt lenses (AMLs), with a focus on the petrological and geochemical record provided by fossilized AMLs. The International Ocean Discovery Program (IODP) Hole 1256D in the equatorial Pacific is the location, where for the first time, the transition between sheeted dikes and gabbros in an intact fast spreading crust was penetrated, providing a core with a continuous record of the upper part of an AML. This location can be regarded as Rosetta Stone to answer long-standing questions on the complex magmatic evolution within an AML, as well as on metamorphic and anatectic processes ongoing at the roof of a dynamic AML.

The topics presented in this book cover a wide range of observations and measurements on dynamic magmatic processes, merging insights from fieldwork and experimental results, theoretical approaches, and computational modeling.

The coexistence of crystals and melt must be seen as a dynamic process for which time evolution depends on various intrinsic (e.g., chemical compositions, temperature, pressure)...