6.07 Magmatism, Magma, and Magma Chambers B. D. Marsh, Johns Hopkins University, Baltimore, MD, USA ª 2007 Elsevier B.V. All rights reserved. 6.07.1 Introduction 276 6.07.2 The Nature of Magma 277 6.07.2.1 Transport Characteristics 278 6.07.2.2 Phase Equilibria 279 6.07.2.3 Solidification Fronts 279 6.07.3 Crystals in Magma 281 6.07.3.1 Solidification Front Crystallization or Phenocryst-Free Magmas 281 6.07.3.2 Phenocryst-Bearing Magma 284 6.07.3.2.1 Kilauea Iki Lava Lake 285 6.07.3.3 Primitive versus Primary Magmas 288 6.07.3.4 Historical Note on Solidification Front Fractionation 289 6.07.4 Magma Chambers 289 6.07.4.1 The Problem: The Diversity of Igneous Rocks 289 6.07.4.2 George Becker’s Magma Chamber 290 6.07.5 Historical Setting 292 6.07.5.1 Life Time Lines 293 6.07.6 Initial Conditions of Magmatic Systems 295 6.07.6.1 Cooling from the Roof 296 6.07.6.2 Style of Crystal Nucleation and Growth 296 6.07.6.3 The Critical Connection between Space and Composition 297 6.07.6.4 The Sequence of Emplacement or Delivery of the Magma 300 6.07.6.4.1 Forms of magmatic bodies 300 6.07.6.4.2 Internal transport style 300 6.07.6.4.3 Eruptive timescales and fluxes 301 6.07.6.4.4 Filling times 301 6.07.6.4.5 Magmatic deliveries, episodes, periods, and repose times 302 6.07.6.5 Thermal Ascent Characteristics and The Role of Thermal Convection 302 6.07.6.5.1 Superheat 303 6.07.6.6 Summary of Magmatic Initial Conditions 305 6.07.7 End-Member Magmatic Systems 306 6.07.7.1 The Sudbury Igneous Complex (SIC) 306 6.07.7.2 Ferrar Dolerites, Antarctica 309 6.07.8 Lessons Learned from Sudbury and the Ferrar Dolerites 313 6.07.9 Ocean Ridge Magmatism 314 6.07.10 Island Arc Magmatism 316 6.07.10.1 Introductory 316 6.07.10.2 Arc Form 317 6.07.10.2.1 Spacing of the volcanic centers 317 6.07.10.2.2 Arc segmentation 318 6.07.10.3 Character of the Volcanic Centers 318 6.07.10.4 Magma Transport 320 6.07.10.5 Subduction Regime 320 6.07.10.6 Subducting Plate Internal State 321 6.07.10.6.1 Thermal regime 321 6.07.10.6.2 Hydrothermal flows 322 6.07.10.7 The Source of Arc Magma 322 275 276 Magmatism, Magma, and Magma Chambers 6.07.10.7.1 Slab quartz-eclogite 324 6.07.10.8 Diapirism, Rayleigh–Taylor Instability, and Volcano Spacing 325 6.07.10.9 Alkali Basalts Posterior to Arcs 326 6.07.10.10 The Arc Magmatic System 326 6.07.11 Solidification Front Differentiation Processes 326 6.07.11.1 Introduction 326 6.07.11.2 Solidification Front Instability 326 6.07.11.3 Silicic Segregations and Crust Reprocessing in Iceland 328 6.07.11.4 Sidewall Upflow 328 6.07.11.5 Fissure Flushing 329 6.07.12 Magmatic Systems 330 References 331 6.07.1 Introduction emplacement of higher-temperature basaltic magma in underplating (Bergantz, 1989) or prolonged hea- Magmatism is directly linked to tectonism. Where ting of wall rock on the conduit of a volcanic system. there is no tectonic activity, as in continental shield Meteorite impact is also an effective means of magma regions, there is no magmatism. Tectonic activity is a production. The Sudbury melt sheet of Ontario, clear sign of convection in the mantle and often the some 35 000 km3 of magma, was produced in 5 min crust, although the associated length scales may be by a 10–12 km bolide 1.85 Ga. Wholesale melting of much different. Relative to conduction of heat, con- continental crust produced a magma superheated to vection moves material at rapid rates. This manifests 1700C, which is never found in endogenetic mag- itself in the inability of rock to cool easily during matism. Only the prolific volcanism of Io, due to convection, which, coupled with the condition that viscous dissipation in tidal pumping by Jupiter, is much of the lower crust and mantle is already near similarly superheated. The most voluminous and melting, brings on melting as rock is adiabatically steady magmatisms of Earth, like those of the ocean convected through a solidus. Convection through a ridges and Hawaii, erupt magma at or near the liqui- phase boundary is the major process of producing dus, but never superheated. The low crystallinity of magma on Earth. This form of melting is progressive, these magmas reflects this high-temperature eruptive and instabilities associated with the field of affected state, whereas many island arc magmas, especially rock, which depend on the size of the field, the the andesitic ones, can be of high crystallinity; the degree of melting, and the nature of the melt itself, most dangerous ones flirt with the point of critical simultaneously arise to collect and transport the crystallinity at 55 vol.%. As the phenocryst content magma upward to Earth’s surface. Most magmas approaches maximum packing at critical crystallinity thus arise through partial melting of source or parent the magma becomes a dilatant solid, expanding upon rock. The collected magma may contribute to vol- shear. The volcano becomes, in effect, corked or canism or, perhaps more often, become stalled at plugged, and can only erupt explosively. High- depth in plutonism. This overall process is called temperature, low-crystallinity basaltic magmas are ‘magmatism’. This is the process that has given rise not generally explosive, but the exceeding mobility to the diversity of rocks on Earth’s surface and per- of the lavas is dangerous. Cinder cone volcanism, haps also to the very structure of Earth itself. Within commonly areally sporadic and associated with alkali any process of magmatism there are certain physical basalts, is just the reverse. The early phase of volcan- and chemical processes that are truly fundamental to ism is explosive, almost regardless of crystal content, shaping the behavior and outcome of the magma and but the associated lavas are sluggish and immobile. it is these that are considered herein. Explosiveness can thus reflect an enhanced volatile Volcanoes on a planet reflect the physical pro- content, high crystallinity, or high silica content. cesses at depth of magma production, collection, and The pattern and style of magmatism intimately transport. This is in contrast to magma produced reflect the nature of the causative process. Globally in situ through an externally applied heat source as widespread magmatism, as on Io, reflects planetwide in partial melting of granitic continental crust by melting as in tidal pumping or a heavy impact flux. Magmatism, Magma, and Magma Chambers 277 Linear arrays or strings of magmatism, as along ocean voluminous systems, like Hawaii, are valuable in ridges, island arcs, and some ocean islands, reflect inferring the deeper geometric form, extent, and melting associated with thermoconvective flows sense of connection between eruptions and deep tightly focused within the phase field of the source transport. Some styles of harmonic tremor, which rock. That is, either the thermal regime or the source sometimes occurs during eruptive stages, seem to material (or both) is spatially focused. Areally wide- indicate the pre-eruptive state of magma held in spread, small-volume magmatism as, for example, that vertically oriented cylindrical conduits. Ground of cinder cone fields, reflects a pervasive, marginally deformation associated with tumescence prior to focused convective flow, much like a broad low-pres- eruption consistently indicates through elastic mod- sure system in the atmosphere, where melting eling a local, near-surface staging region, which is instabilities are mainly due to local irregularities in commonly identified with the concept of a magma the source rock detailed composition. Typical convec- chamber. This is reinforced by the very nature and tive flows of this nature are the broad, gently pervasiveness of plutons. Overall, the most com- upwelling flows associated with the wedge flows dri- monly held conceptualization of a generic magmatic ven by subduction. Widespread, but voluminous plumping system has a deep source region linked to a magmatisms, as in the silicic volcanism of the western near-surface magma chamber through a poorly United States over the past 50 My, reflect convective discerned transport region. Aside from the gross stretching and destabilization of the continental litho- inferences from seismic studies, the intervening sphere. The asthenosphere thermal regime is, in effect, plumbing linking source to near surface, the ascent brought to the Moho where vast regions of continental path, has been difficult to ascertain. Although magma crust of irregular thickness are accessible to melting. perhaps spends most of its life in this part of the Volcanoes themselves, as opposed to impact melt system, it has, per force, been largely ignored physi- sheets, reflect deep-seated, endogenetic processes, cally and chemically due to lack of a clear and and in a simple way the volcanic edifice is a measure detailed conception of this important feature. This of the hydrostatic (i.e., magmastatic) head of the basic magmatic architecture, source, ascent path, system. The size of volcanoes also reflects the activity chamber, and pluton or volcano, is found in different or strength of the system, the mobility of the source forms in most magmatic systems (see Figure 1). relative to the surface plate, the strength (and thus Magma chamber is a particularly important con- age) of the local lithosphere, and the intensity of the cept. Volcanologists use the mechanics of magma gravitational field.