Magma reservoir architecture and dynamics

Santorini is one of the most explosive arc calderas in the world, having had over ten plinian eruptions, and at least four caldera collapses, over the last 350,000 years. It lies on continental crust about 25 km thick. This paper reviews what we have learned about the plumbing system of Santorini through a combination of sample characterisation, phase equilibria experimentation, melt inclusion volatile barometry, and multi-mineral diffusion chronometry. Mantle-derived basalt intrudes into the lower to middle crust, where it fractionates to andesitic, then silicic, melts in gabbroic to dioritic mush layers. Immediately prior to a plinian eruption, a large volume of evolved melt is extracted from melt lenses within these mushes and ascends to the top of the subcaldera pluton through high permeability channels. The bubbly melt (plus entrained antecrysts) then intrudes to form a large, sill-shaped reservoir that in turn feeds the plinian eruption less than a few centuries to decades later. These upper crustal reservoirs are transient holding chambers that possibly form where vertical gradients in crustal rheology cause the ascending melt to stall. During ascent from the middle to upper crust, the silicic melts grow phenocrysts by vapour-saturated decompression upon entrained, deeper-derived antecrysts. Melt extraction from depth is probably triggered by destabilisation of mush layers, due either to tectonic forces or to spontaneous breakdown of the solids framework once the melt fraction reaches a critical fraction. The smaller silicic eruptions of interplinian periods appear to be characterised by similar processes and timescales. Interplinian eruptions preceding and following plinian eruptions can tap mid-crustal mush layers different from those feeding the plinian eruptions themselves, as shown by trace element and isotopic signatures of the magmas. We favour a model in which the rheological behaviour of the subcaldera crustal column modulates magma fluxes and storage levels through the crust, and influences eruption styles.

Professor Tim Druitt, University Clermont-Auvergne, France

Professor Tim Druitt, University Clermont-Auvergne, France

Tim Druitt is a volcanologist in the Laboratory of Magmas and Volcanoes of Clermont-Auvergne University, Clermont-Ferrand in France. He obtained his PhD from Cambridge in 1984, and following positions at the U.S. Geological Survey, the University of Liverpool, and the University of Cardiff, moved in 1993 to France, where he is now a professor of volcanology. He headed the volcanology group in Clermont for ten years and is now scientific director of the multidisciplinary project ClerVolc, which links laboratories in geosciences, meteorology, mathematics, physics, computer science and psychology. He has been editor in chief of the two main international journals in volcanology. Tim’s research specialises in explosive volcanism, which he has studied using field, modelling and petrological techniques. His knowledge of magma reservoir architecture and dynamics derives particularly from his work at Crater Lake (USA) and Santorini (Greece) calderas.

Constraining magma reservoirs is important for understanding eruptive processes and the formation of continental crust. Geophysical imaging is a particularly powerful tool as it provides information on 3D structure, which most other methods cannot address. Geophysics however, is far from a silver bullet and has limitations arising mostly from finite resolution and non-uniqueness of geological interpretations. Recent advances in geophysical inversion and interpretation promise to revolutionize the amount of information and level of detail that they can gain from geophysical data. Michele will discuss the work that his group are doing at Imperial College to advance the state of the art in volcano tomography. The group focus on three aspects: 1) joint inversion of multiple geophysical observables (e.g. seismic and gravity data); 2) full-waveform inversion of seismic data; 3) quantitative interpretation. Seismic velocities are affected by many factors (composition, porosity, temperature, melt content), therefore interpretation of P-wave velocity models can rarely provide robust estimates on melt distribution in the crust. Joint inversion of independent geophysical datasets allows one to recover multiple parameters and start reducing the non-uniqueness of interpretations. Full-waveform inversion (FWI) exploits the information contained in the full record of natural or man-made seismic sources, resulting in 3-5 times better resolution than traditional seismic inversion techniques. FWI also allows the recovery of a wider range of elastic parameters including attenuation, anisotropy and S-wave velocities. Quantitative interpretation uses these multiple parameters to place stronger constraints on melt content and distribution and the composition of the host rock. These techniques are being used to image Santorini Volcano, Greece, using data from the NSF-funded PROTEUS experiment (Plumbing Reservoirs Of The Earth Under Santorini). The experiment was designed for high-density spatial sampling of the seismic wavefield to allow us to apply state-of-the-art 3D inversion methods, and includes 89 ocean bottom seismometers, 65 land stations and 13,300 airgun shots.

Dr Michele Paulatto, Imperial College London, UK

Dr Michele Paulatto, Imperial College London, UK

Michele’s research focuses on geophysical constraints on magma and fluids at active tectonic margins. After graduating in Physics of the Earth and Environment from the University of Trieste, Italy, he obtained a PhD in Marine Geophysics at the National Oceanography Centre Southampton, UK, with a thesis on seismic tomography of the magma plumbing system of the island of Montserrat. This was followed by three years as a postdoc at the University of Oxford working on a NERC-funded project to study seamount subduction. In 2014-2016 Michele was based at Géoazur, France, supported by an AXA Postdoctoral Fellowship, where he studied fluid transport and release in the Lesser Antilles subduction system. He is currently applying novel geophysical inversion techniques to rubustly constrain magma supply, storage and differentiation at Santorini volcano.

The central Andes is a key global location to study the architecture of transcrustal magmatic systems as it currently hosts the world's largest zone of silicic partial melt in the form of the Altiplano-Puna Magma or Mush Body (APMB).  Further, the APMB is dynamic, with a ground deformation pattern 150 km in diameter that has been ongoing for several decades, centered on Uturuncu volcano, Bolivia.  I will discuss results from the recently completed international PLUTONS project, which focused a suite of geophysical, petrological, and geomorphological techniques at Uturuncu with numerical modeling to infer the subsurface distribution, quantity, and movements of partial melts. We find geophysical anomalies extending from the base of the crust to the surface  indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different physical properties. The characteristics of the geophysical anomalies differ somewhat depending on the technique used - reflecting the different sensitivity of each method to subsurface melt (or fluid) of different compositions, temperature, connectivity, and volatile content, and highlighting the need for integrated, multi-disciplinary studies. We offer a new interpretation of the cause of ground deformation related to reorganization of the magma mush that causes no net ground deformation over hundred to thousands of years that is consistent with the lack of tilted shorelines around Uturuncu.

Professor Matt Pritchard, Cornell University, USA

Professor Matt Pritchard, Cornell University, USA

Matt Pritchard is a Professor of Geophysics and Director of the Institute for the Study of Continents at Cornell University in Ithaca, New York. He studies how the Earth's surface deforms in response to earthquakes, magma movements, glacier dynamics, and human activities through field observations, using radar and optical satellites, and numerical modeling. He was educated at the University of Chicago (BA, Physics) and the California Institute of Technology (MS and PhD, Geophysics), and was a Harry Hess Postdoctoral Scholar at Princeton University.  During 2016, Pritchard was a Visiting Professor at the University of Bristol and he hopes to return to the UK for seven months on sabbatical in 2019. He currently serves on the National Academy of Sciences Standing Committee on Seismology and Geodynamics, is a member of the Advisory Board of the Carl Sagan Institute, and is the US national correspondent to the International Association of Geodesy.

The orders of magnitude difference in viscosity between crystal-rich regions (mush) and melt-rich regions (chambers) controls the behaviour of vertically-extensive magmatic systems. High-resolution satellite geodesy provides a unique insight into the patterns of magma movement over short (weeks-decades) timescales, reflecting the geometric distribution of the mobile, eruptible component of the system. In this talk, Juliet will discuss the insights provided by 25 years of InSAR observations of deformation patterns at volcanoes around the world. To the first order, co-eruptive deflation and pre-eruptive inflation have been seen but typically the deformation is temporally variable, suggesting pulsed magma supply within viscoelastic rheology. In addition, there are numerous examples of co-eruptive uplift, typically attributed to endogenous growth of the edifice, and inter-eruptive subsidence, attributed to cooling, degassing or hydrothermal processes. The spatial pattern of deformation reveals that many systems are composed of multiple, interacting magma chambers, which can be tens of kilometers from the eruptive vent. The spatial patterns of subsurface magma chambers relates to surface alignments suggesting that pre-existing structures, such as faults, play an important role on the growth of magma reservoirs. This suggests a revised model of the development of shallow magma bodies based on growth and coalescence along fault structures.

Dr Juliet Biggs, University of Bristol, UK

Dr Juliet Biggs, University of Bristol, UK

Juliet’s research primarily uses satellite measurements of surface deformation to understand the physics of magmatic and tectonic processes. Juliet’s current areas of interest are the East African Rift and the volcanoes of Central America and the Northern Andes. She received BA and MSci degrees in Natural Sciences in 2003 from the University of Cambridge and her PhD from the University of Oxford in 2007 for her work on the earthquake cycle in Alaska. Juliet was awarded the 2012 Winton Capital Award of the Royal Astronomical Society, the 2014 Lloyds of London Science of Risk Prize, the Bullerwell Lecture of the British Geophysical Association in 2016 and the American Geophysical Union (AGU) Geodesy Section Award in 2017.

Distinguishing time dependent magma chamber pressurization from relaxation of a viscoelastic aureole surrounding the chamber using geodetic measurements has remained challenging.  Elastic models with mass inflow proportional to the pressure difference between the chamber and a deep reservoir predict exponential re-inflation.  For a spherical chamber surrounded by a Maxwell viscoelastic shell with pressure dependent recharge, the surface deformation is the sum of two exponentials, dependent on refilling and Maxwell times.  

GPS displacements following eruptions of Grímsvötn, Iceland in 2004 and 2011 exhibit rapid post-eruptive inflation, followed by inflation with a much longer time constant.  Markov Chain Monte Carlo inversion with the viscoelastic model shows the time series can be fit with viscosity of ~ 2e16 Pa-s, and a relatively incompressible magma. The latter appears to conflict with the ratio of erupted volume to geodetically inferred source volume change of 10, obtained for the best fitting Mogi source, which implies a very compressible magma.  

Re-examination of the co-eruptive GPS and tilt data with a more general ellipsoidal model reveals that the best fitting source are oblate and deeper, consistent with seismic tomography, with much larger volume changes, relative to spherical models, consistent with incompressible melt.  This talk will also show that non-Newtonian magma rheology can, at least qualitatively, explain the data.  

Professor Paul Segall, Stanford University, USA

Professor Paul Segall, Stanford University, USA

Paul Segall is a geophysicist with interests in earthquake and volcanic processes.  He is known for developing methods for utilizing deformations of the earth’s crust, determined by both space and ground based sensors, to reveal fault slip and magma chamber dilation at depth in the earth, and for developing physics-based models of faults and magmatic systems.  Current interests involve coupling physics-based models of eruptions with surface deformation and other observables, and modeling time-dependent magma chamber pressurization.  He worked at the U.S. Geological Survey, Office of Earthquake Studies from 1981 until 1993, at which time he joined the Geophysics faculty at Stanford.  He is a Fellow of the American Geophysical Union and the Geological Society of America, a member of the U.S. National Academy of Sciences, and was awarded the James B. Macelwane Medal (1990) and the Charles A. Whitten Medal (2014) of the American Geophysical Union.

The transfer of heat between a cooling magma body and its host rock induces metamorphic thermal aureoles. In the case of tabular intrusions, simple analytical models predict that the thickness of thermal aureoles scales linearly with the thickness of magma intrusions. However, some aureoles are much thicker or much thinner than predicted by analytical models. With numerical simulations, several factors affecting the size of thermal aureoles can be examined. A contrast in the rate of heat transfer between the magma and the country rock significantly impacts the maximum temperatures reached in the aureole. This contrast in heat transfer can be due to differences in conductivities or to the effect of convection, either in the magma or in the country rock. The emplacement dynamics of the magma also control the size of an aureole. Incremental emplacement, where magma injection is focused closed to one contact produces asymmetric aureoles. In comparison with a similar intrusion that is instantaneously emplaced, fast incremental emplacement induces a thicker aureole whereas slow incremental emplacement induces a thinner aureole. Inversion of aureole sizes can shed light on the associated intrusion dynamics provided that all other parameters affecting heat transfer are taken into account.

Dr Catherine Annen, Univeristy of Bristol, UK

Dr Catherine Annen, Univeristy of Bristol, UK

Catherine Annen is a computational Earth Scientist and currently an honorary senior research fellow at University of Bristol, UK. Her expertise is in the modeling of magmatic processes. She studied Earth Sciences at the University of Geneva, Switzerland, and holds a PhD in volcanology jointly awarded by the University Blaise Pascal of Clermont-Ferrand, France, and the University of Geneva. Her research interests include the generation of silicic melts, the condition of formation of magma chambers, the relationship between plutonism and volcanism, the pace of magma transfers and the formation of thermal aureoles.

Arc magmas typically represent a complex blend of heterogeneous material from diverse sources within the volcanic plumbing system. The complexity and provenance of this material can provide a wealth of clues to understand the crustal architecture of the magmatic system, the mechanisms of magma assembly, and the degree of chemical heterogeneity within individual volcanic systems. Foreign crystals, (xenocrysts) are straightforward to identify as they display a variety of strong disequilibrium textures. Glass-bearing crystal clusters or clots are interpreted to represent larger fragments disaggregated from either plutonic equivalents of the erupted magmas, or intrusive mafic magmas. Here I show that we can identify the petrological signatures of at least two further mechanisms of accumulating material. Firstly, fine-scale textures show suppression of crystal growth in the vicinity of impinging adjacent grains. These textures indicate slow growth, with the crystals representing partially solidified plutonic material from deeper in the system. Secondly, the trace element compositions of mafic minerals such as amphibole can be inverted to reflect the composition of their equilibrium melt source. The resulting variations in elemental ratios record interaction with (or crystallisation from) melts with a wide range of trace element geochemistry despite ‘normal’ major element composition. This shows a record of anomalous trace element compositions reflecting the deeper (and older) roots of the volcanic system. This talk will illustrate this using examples from arc magmas worldwide. 

Dr Madeleine Humphreys, Durham University, UK

Dr Madeleine Humphreys, Durham University, UK

Dr Madeleine Humphreys is an igneous petrologist and Royal Society University Research Fellow at Durham University. Her research interests are in geochemistry, petrology and volcanology, with an emphasis on understanding magma generation, migration and storage, particularly in subduction zone settings. A key focus is the role of volatiles in the generation, evolution and eruption of magmas. She uses the chemistry of the rock record, combined with textural observations, to answer fundamental questions relating to the formation and differentiation of the Earth's crust through volcanic processes.

Crystals have long been used to understand conditions and processes within magma reservoirs, yet making use of the archives contained within crystals requires integrating different kinds of information derived from the same crystals and comparing them to other types of studies of magmatic processes. Challenges include relating information at the crystal- or sub-crystal spatial scale (micrometers to millimeters) to the scale of a magma reservoir (tens of meters to tens of kilometers), and relating processes occurring over years to decades to processes occurring over millennia to hundreds of thousands of years. This talk will discuss successes and discrepancies in comparing different types of crystal records and also comparing crystal records with other methods of investigating magmatic processes. For example: 1) Contrasts between radiometric (absolute) ages of crystals and time scales of diffusion of trace elements between zones within crystals, 2) Contrasts between crystallization temperatures and inferred crystal storage temperatures, and the implications for heating and cooling within a reservoir, 3) Comparisons of thermal conditions within the reservoir from numerical modeling and crystal records, and 4) Comparison of numerical modeling of mush reactivation times with the reactivation and assembly times indicated by intra-crystalline diffusion.

Professor Kari Cooper, University of California, Davis, USA

Professor Kari Cooper, University of California, Davis, USA

Kari M Cooper is a Professor in the Department of Earth and Planetary Sciences at the University of California, Davis. Her research is aimed at understanding how magmas evolve and interact with each other and with their surroundings, with a particular focus on combining dating volcanic crystals with measurements of their compositions in order to understand the timescales and conditions of magma storage and mobilization prior to eruptions. Professor Cooper received a BA in Geology from Carleton College, an MS in Geology from the University of Washington, and a PhD in Geochemistry from the University of California, Los Angeles. She is a fellow of the Geological Society of America.

It is increasingly recognized that chemical differentiation in subduction zone settings occurs within crystal-dominated mushy systems that span the entire crust, rather than, discrete melt-dominated magma chambers in the traditional sense. Mushy systems involve the independent movement of buoyant melts and fluids through a deformable crystalline matrix, consistent with observations at active arc volcanoes. Percolating fluids and melts react with their solid matrix, undergoing chemical modification en route, as melts and fluids from deeper parts of the system encounter materials (solids, melts, fluids) in the shallower part of the systems. Thus, volcanoes are the final expression of processes that operate over a considerable time and depth range. Trans-crustal mush systems have implications not only for magma differentiation and crust formation, but also for hydrothermal ore deposition because they provide an effective means to focus the flow of melts and fluids sourced from a large magmatic system through a narrow conduit. The long-term stability of transcrustal mushes and their sensitivity to external or internal forces remains poorly understood. In the context of volcanic hazards a better understanding of mush dynamics is imperative for improved interpretation of monitoring data at restless volcanoes.

Professor Jon Blundy, University of Bristol, UK

Professor Jon Blundy, University of Bristol, UK

Jon is an igneous petrologist working on many different aspects of magmatism in the Earth, from magma generation in the mantle and crust to its eventual eruption at the surface. He uses a combination of geochemical analysis, fieldwork and experimental petrology to address these topics. Jon has current research projects at a number of active volcanoes, including those in the Lesser Antilles, Cascades, Kamchatka, Vanuatu and Mexican Volcanic Belt. Jon holds a BA in Geology from University of Oxford (1983) and a PhD from University of Cambridge (1989). He has been at University of Bristol ever since, first as a research fellow (NERC, Royal Society) and latterly as Professor of Petrology (since 2004). He was elected to Fellowship of the Royal Society in 2008 and Academia Europea in 2011.

In this talk, a series of idealised models will be presented to explore some of the fluid mechanical controls on the formation of mushy zones, including cumulate layers, as a magma cools and crystallises. These models will be used to inform the boundary conditions for the subsequent intrusion of more primitive and relatively hot magma from deeper in the crust. The interaction of this new magma with pre-existing, relatively cool, mushy zones, will be explored, assessing the potential for remobilisation of the mushy zone and any associated mixing between or overturn of the different layers of melt. The implications of these processes on the products of a subsequent eruption will also be explored, including their implications for the composition of mafic inclusions.

Professor Andrew Woods FRS, University of Cambridge, UK

Professor Andrew Woods FRS, University of Cambridge, UK

Magma degassing play a significant role in the thermo-mechanical evolution of shallow magma reservoirs. In this presentation, we focus on two questions related to outgassing (1) how does the migration of an exsolved fluid phase influence the eruptibility of a heterogeneous magma body? And (2), what are the physical processes and optimal conditions that allow efficient outgassing of crystal-rich magma as they solidify to form plutons? We use a pore-scale numerical approach to study the migration of an exsolved fluid phase in crystal-rich and crystal-poor magmas and show that the fluids migrates efficiently in crystal-rich parts of a magma reservoir and accumulate in crystal-poor regions. Moreover, we parameterized our pore-scale results for fluid migration and inserted them in a thermomechanical model to study outgassing under dynamical conditions where cooling controls the evolution of the magma reservoir. We find that buoyancy-driven outgassing allows for a maximum of 40–50% volatiles to leave the reservoir over the 0.4–0.7 crystal volume fractions, implying that a significant amount of outgassing must occur at high crystal content (>0.7), through veining and/or capillary fracturing.

Professor Olivier Bachmann, ETH Zurich, Switzerland

Professor Olivier Bachmann, ETH Zurich, Switzerland

Olivier Bachmann’s research focuses on better understanding the eruptive dynamics of volcanoes, and the generation of magmas, tied to the evolution of the continental crust. The main research areas of my group are (1) the causes and consequences of large, explosive, volcanic eruptions, (2) magmatic differentiation, (3) magma reservoir dynamics and (4) controls on eruptive styles. The Bachmann group use a variety of techniques, which involves field observations, geochemical analyses of natural and experimental samples, geophysical analyses and numerical modeling, to provide an integrated picture of the processes involved from magma generation to the eruption at the Earth’s surface.

There is controversy about the duration, dynamics and physical conditions of magmatic systems. Conflicting geological, geochemical and geophysical evidence suggest that magmas and migmatites are either short-lived or can persist in a crystal-rich state for up to a million years. In addition the mechanisms of melt migration from magma mushes is poorly understood as the processes of porosity reduction are not uniformly consistent with the commonly invoked crystal-plastic or viscous compaction. We will first review the expressions of crystal-rich processes as preserved in an intact deep arc crustal section from the Famatinian orogeny in Argentina, and describe the progressive scales of melt extraction and migration that culminate in the formation of canonical, compositionally stratified arc crust. This will motivate us to introduce and exemplify measures of microstructural contact, force and mineral shape fabric and their anisotropy using the respective tensors. We will introduce two dimensionless groups, the viscous and Sommerfeld numbers, which can be used to describe the state of the mush and the controls on mixing and modes of mechanical lock-up. We will close with a proposal of an integrated research program at the crystal-to-outcrop scale combing discrete numerical and Paterson rig experiments, with EBSD, color CL and other measure of microstructures, to aid progress in understanding the distinct character of magmatic processes and products.

Professor George Bergantz, University of Washington, USA

Professor George Bergantz, University of Washington, USA

George Bergantz is a Professor in the Department of Earth and Space Sciences at the University of Washington, and holds degrees in geological engineering, geophysics and a PhD Earth and Planetary Sciences from Johns Hopkins University. He heads the physical petrology group which has as its main emphasis the physics of magmas, hydrothermal systems, metamorphism and eruption processes. The specific objectives of the research program are to illuminate geologically constrained examples of the processes of melt generation and transport in the deep crust, the ascent and hybridization of magmas in the mid-crust and the assembly and life-cycles of volcanic systems. These observations are rationalized within the context of multiphase dynamics and flow, with a recent emphasis on Lagrangian-Eulerian numerical methods. The group has worked in Greece, Chile, Argentina, California, Oregon, Hawaii and Italy.