An introduction to magma dynamics

A variety of methods have been employed to decipher magmatic systems, including geophysical, petrological, textural and geochemical approaches, and these elucidate a large variety of characteristics of different plumbing systems and magmatic differentiation processes. A common theme to the papers presented in this book is the observation of transport of small volume magma batches with a relatively high frequency, as opposed to less frequent transport of larger magma volumes that would require storage in large crustal reservoirs for long periods of time. The implications of this observation are discussed in the context of a possible tectonic control on crustal magma dynamics. This book addresses the rapidly developing fields of crustal magma transfer, storage and evolution. During both transfer trough and storage within the crust, magmas are subject to a series of processes that lead to their differentiation. Depths and mechanisms of differentiation, crustal contributions to magma generation through wall-rock assimilation, rates and timescales of magma generation, transfer and storage, and how these link to the thermal state of the crust, are subject to lively debate and controversy. This volume presents a collection of papers that provide a balanced overview of the diverse approaches available to elucidate these topics, and includes both theoretical models and case studies. By integrating petrological, geochemical and geophysical approaches, it provides the reader with new insights to the subject of magmatic processes operating within the Earth’s crust, and reveals important links between subsurface processes and volcanism. This volume is divided into four sections: ‘Magma transfer: from mantle to surface’ addresses the ascent and evolution of magmas from the zone of melt generation in the mantle to eruption at the surface, forming a backdrop for the detailed studies of distinct parts of magma plumbing systems addressed later. ‘Dynamics of magma transport’ focuses on theoretical and geophysical approaches to understanding magma movement through the crust. ‘Magma reservoir dynamics’ provides insights from petrographic and mineral chemical studies into the processes occurring in crustal magma chambers. Finally, ‘Processes of silicic melt generation’ concludes the book with a dedicated section on the long-standing question of where and how magma differentiation may take place. In nature, these issues are of course intimately related, and some of the papers in this volume address more than one of these aspects. Therefore, the reader may obtain additional insights to a particular theme by referring to the other sections of the book. With the exception of two contributions (Leeman et al.; Wright & Klein), all case studies presented in this volume deal with subduction zone magmatism. The inferences made here on the dynamics of magma ascent, storage and differentiation are therefore biased towards this tectonic setting. It may be argued that subduction-related magmatic systems are likely to have very different petrogenetic characteristics than ocean ridge and intraplate volcanism. Firstly, the primary magmas are produced at different depths within the mantle, and have different temperatures and compositions, particularly with regard to their volatile contents. Secondly, as a result of these differences, their petrogenetic evolution within crustal magma systems will differ significantly. For example, crystallization of volatile-rich arc magmas may be triggered by rapid decompression-induced degassing during magma ascent through the crust, a process that is not readily applicable for ocean ridge and intraplate magmatic systems. Thirdly, tectonic controls on the geometry of the plumbing systems differ considerably. Ocean ridges are in extension, resulting in rapid magma ascent through dykes and movement From: ANNEN, C. & ZELLMER, G. F. (eds) Dynamics of Crustal Magma Transfer, Storage and Differentiation. Geological Society, London, Special Publications, 304, 1–13. DOI: 10.1144/SP304.1 0305-8719/08/$15.00 # The Geological Society of London 2008. of newly created crust away from the heat source. In contrast, intraplate magmatism may favour magma storage and differentiation within the crust due to repeated sill intrusion and resulting progressive elevation of the geothermal gradient. Arcs, on the other hand, may be situated within extensional, transtensional or compressional regimes, potentially resulting in differences between the plumbing systems of different arcs (cf. Zellmer). Further work will be required to gain a balanced understanding of the dynamics of magma plumbing systems within different tectonic settings. Magma transfer: from mantle to surface Studies of igneous processes often focus on those parts of the crustal plumbing system that are best elucidated by the samples or methods available. It is rare that systems have been studied in sufficient detail to inform the entire process from magma generation in the mantle to eruption at the surface. This book starts with two studies where the amount of data is sufficient to provide such insights, on one hand using global volcanological, geophysical and geochemical datasets to present a broad overview of magma transfer (Zellmer), on the other hand focussing on the single edifice of Stromboli Volcano to a gain detailed understanding of the petrogenetic processes occurring along the entire mantle-crust section beneath this volcano (Cigolini et al.). These papers address the dynamics of igneous processes operating at sites of ongoing volcanic activity, and therefore form a backdrop for the detailed studies of the distinct parts of magmatic plumbing systems focused on in the later sections of this book. From global correlations between eruptive style, surface heat flux and convergence rates of different volcanic arcs, Zellmer infers that the rate of melt production in the mantle wedge ultimately controls the dynamics of magma transfer through the crust, and thereby the chemical and physical properties of magmas and eruption products. It is shown that a deep crustal hot zone (Annen et al. 2006) does not buffer the effects of subduction velocity on melt production, and that the rate of magma generated in and released from the hot zone is proportional to the magma advected to the hot zone from the mantle wedge. Crystal size distributions, bubble content and magma rheology, petrology and chemistry are a number of parameters that – when studied in combination – may offer a very detailed picture of the processes operating within magma plumbing systems, and can be used to quantify pressures, temperatures and the rates of preeruptive crystallization and gas exsolution. Such a multi-faceted approach is used by Cigolini et al. to elucidate the plumbing system of Stromboli volcano from upper mantle to surface. The data suggest that phenocrysts nucleate within a few days in a magma reservoir that extends vertically from 11 to 5.4 km below the summit of Stromboli. A new model is proposed where the magma chamber takes the shape of a vertically elongated ellipsoid that is penetrated by a feeder dyke sourced from over 30 km depth, i.e. in the mantle. According to this model, the instantaneous elastic rebound of the walls of the depressurizing subvolcanic reservoir explains the occurrence of intermittent paroxysmal eruptions at this volcano. Dynamics of magma transport