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Magma reservoir architecture and dynamics

Scientific meeting

Starts:

November
272017

09:00

Ends:

November
282017

17:00

Location

Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ

Overview

Theo Murphy international scientific meeting organised by Dr Marie Edmonds, Professor Kathy Cashman FRS, Professor Marian Holness and Professor Matthew Jackson.

Fernandina: ESA's Envisat satellite shows ground deformation around a volcano on Fernandina island in the Galápagos islands, following the eruption in April 2009. The caldera is outlined in white, and the lava flow produced by the eruption is also in white, stretching to the coast. Interferometric Synthetic Aperture Radar – or InSAR – is a technique where two or more satellite radar images over the same scene are combined to detect slight changes between them. Tiny changes on the ground cause changes in the radar signal and lead to rainbow-coloured patterns in the combined image. Credits: ESA/M. Bagnardi, University of Leeds.

A multi-disciplinary discussion meeting to discuss the state-of-the-art in our understanding of the form and behaviour of magmatic systems. Accumulating evidence suggests that magmatic systems are trans-crustal, complex, heterogeneous, assembled on long timescales and reorganised during recharge events before eruptions. This view has important implications for research including volcano monitoring, layered intrusions, crustal formation and ore deposit formation.

Information on the speakers and programme are available below. Recorded audio of the presentations are available below. Meeting papers will be published in a future volume of Philosophical Transactions A.

Enquiries: contact the Scientific Programmes team

Event organisers

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Schedule of talks

27 November

08:45-12:30

Session 1

7 talks Show detail Hide detail

Chairs

Dr Marie Edmonds, University of Cambridge, UK

09:00-09:30 Plumbing system architecture and dynamics beneath an island arc caldera (Santorini, Greece)

Professor Tim Druitt, University Clermont-Auvergne, France

Abstract

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.

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09:30-10:00 Beyond the blobs: advances in geophysical imaging of magmatic systems

Dr Michele Paulatto, Imperial College London, UK

Abstract

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.

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10:00-10:30 Architecture and dynamics of the Altiplano-Puna transcrustal magmatic system (central Andes) from the PLUTONS project

Professor Matt Pritchard, Cornell University, USA

Abstract

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.

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10:30-11:00 Coffee

11:00-11:30 The geometry of active magma systems

Dr Juliet Biggs, University of Bristol, UK

Abstract

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.

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11:30-12:00 Time dependence of volcano inflation: mass influx or viscoelastic relaxation?

Professor Paul Segall, Stanford University, USA

Abstract

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.  

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12:00-12:30 Why are thermal aureoles thinner or thicker than expected? The role of pluton emplacement dynamics

Dr Catherine Annen, Univeristy of Bristol, UK

Abstract

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.

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12:30-13:30

Lunch

13:30-17:00

Session 2

7 talks Show detail Hide detail

Chairs

Professor Kathy Cashman FRS, University of Bristol, UK

13:30-14:00 Understanding the recipe for arc magmas: identifying the ingredients

Dr Madeleine Humphreys, Durham University, UK

Abstract

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. 

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14:00-14:30 Making the most of crystal records to understand magmatic processes

Professor Kari Cooper, University of California, Davis, USA

Abstract

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.

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14:30-15:00 Transcrustal magmatic mushes

Professor Jon Blundy, University of Bristol, UK

Abstract

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.

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15:00-15:30 Tea

15:30-16:00 Some fluid mechanical controls on mushy zone formation and evolution

Professor Andrew Woods FRS, University of Cambridge, UK

Abstract

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.

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16:00-16:30 The mechanics of shallow magma reservoir outgassing

Professor Olivier Bachmann, ETH Zurich, Switzerland

Abstract

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.

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16:30-17:00 On the mechanics of crystal-rich systems

Professor George Bergantz, University of Washington, USA

Abstract

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.

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28 November

09:00-12:30

Session 3

7 talks Show detail Hide detail

Chairs

Professor Matthew Jackson, Imperial College London, UK

09:00-09:30 Deep crustal magma plumbing under Icelandic rift volcanoes

Professor Bob White, University of Cambridge, UK

Abstract

Bob White’s group has mapped magma transport paths from the deep (20 km) to the shallow (6 km) crust and in two cases to eventual surface eruption under several Icelandic rift volcanoes (Askja, Bardarbunga, Eyjafjallajökull, Upptyppingar). The group use microearthquakes caused by brittle fracture to map magma on the move and tomographic seismic studies of velocity perturbations beneath volcanoes to map the magma storage regions. High-frequency brittle failure earthquakes with magnitudes of typically 0-2 occur where melt is forcing its way through the country rock, or where previously frozen melt is repeatedly re-broken in conduits and dykes. The Icelandic crust on the rift zones where these earthquakes occur is normally ductile at depths greater than 8 km beneath the surface, so the occurrence of brittle failure seismicity at depths as great as 20 km is indicative of high strain rates, for which magma movement is the most likely explanation. White suggests that fractionation of magma and consequent high vapour pressures caused by the exsolution of carbon dioxide-rich fluids in the deep crust is driving magma movement through cracks and dykes connecting deep crustal sills. Moment tensor solutions of the seismic events show they are often caused by opening cracks. The group compare the depths of deep crustal sills mapped using microseismicity with petrological barometers of storage depths determined from the solidified products of Icelandic eruptions and find that both measures consistently show that melt resides at many different depths through the crust. Most of the melt eventually solidifies before it reaches the surface: probably only 10% is eventually erupted. Seismic velocity constraints from seismic refraction, tomography and receiver function studies can also be reproduced by modelling fractionation histories of primitive melts injected into the Icelandic crust.

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09:30-10:00 Bárðarbunga 2014-15 caldera collapse: implications for magma reservoir geometry

Professor Magnús Gudmundsson, University of Iceland, Iceland

Abstract

The geometry and internal architecture of magma bodies in the crust has been a subject of debate for a long time. Geophysical imaging has been elusive, providing only weak constraints on liquid/solid ratios within inferred magma bodies. Caldera collapses, however, provide better constraints, including minimum volumes of magma present at the start of collapse.  During the Bárðarbunga 2014-2015 caldera collapse in Iceland, a subsidence bowl (a down-sag caldera) formed, up to 65 meters deep and over 100 km2 in area during eruption of 1.4 km3 of lava and intrusion of ~0.5 km3 of magma.  Seismicity and modeling indicate a piston collapse at depth along a pre-existing ring fault and into a magma reservoir at ~10 km depth. The latter extended over 30-55 km2 with a minimum thickness and volume of respectively ~0.1 km and 1.8-2 km3.  The discharge of magma displayed an exponential decline. Magma chemistry did not change during the eruption and it had a crystal content of only a few percent. These observations are consistent with a single, liquid and convecting magma body. There are many similarities with the Laki eruption of 1783, often considered to have originated from a crustal magma reservoir under Grímsvötn.  In the light of the Bárðarbunga events a caldera subsidence of a few hundred meters is required to account for the volume erupted in 1783.  As with Bárðarbunga, published data on the geochemistry of Laki is consistent with a mostly liquid, convecting and interconnected magma body. Apparently, liquid magma bodies in the crust, having minimum volumes of 2-20 km3, are required to explain these large basaltic eruptions in Iceland.

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10:00-10:30 Stop-watch and Yard-stick: quantifying scales in basaltic systems

Dr John Maclennan, University of Cambridge, UK

Abstract

Iceland has long been a site of innovation in the development of models of magmatic systems. Concepts devised or refined through research on Iceland have often found general application in the study of mid-ocean ridge and ocean-island magmatism, accounting for two-thirds of the magmatic flux on Earth. The richness of the contextual information from geophysical, geological and petrological studies make Iceland uniquely well suited for the interpretation of microanalytical data. It is all very well to estimate a timescale from diffusion profiles, but quite another matter to understand what this means in terms of physical processes. Some ingenuity is required to match the volcanic record to geophysical observations of melt storage and transport processes in the plutonic realm. John will first describe how improving barometry in basaltic systems has helped to better define the vertical extent of the magmatic system, and the position of the switch from crustal accretion by cumulus processes to solidification without large-scale differentiation. He will then examine microanalytical constraints on the environment of evolution of melts in the lower oceanic crust. These indicate rapid cooling and crystallisation of incoming melts against a near-solidus cumulate host-rock, protracted storage of crystals in a thin mush layer, followed by rapid transport to the surface for eruption.

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10:30-11:00 Coffee

11:00-11:30 Magma reservoirs in the Ethiopian rift imaged using magnetotellurics

Professor Kathy Whaler, University of Edinburgh, UK

Abstract

Magnetotellurics (MT) is a good technique to use to image magma in the crust and upper mantle as magma and partial melt tends to be electrically conductive relative to host rock. Knowing the electrical properties of the melt, dependent on its chemistry, and how it is distributed, e.g. well connected or in isolated pockets, allows magma volumes to be estimated. However, beneath Aluto volcano in the central main Ethiopian rift (MER), MT images resistive crustal bodies where seismology and other techniques indicate the presence of magma. On the other hand, beneath the rift flanks to the north, there is a good conductor commensurate with large magma volumes, but less surface expression of volcanic eruption. The high resistivity of the bodies beneath Aluto strongly suggest that the magma there has a different composition from that beneath the rift flanks – the difference in electrical properties is too large to be simply caused by the melt arrangement. Hence the two bodies either have a different magma source, or the magma has evolved differently during its ascent and storage. Possibilities that would increase resistivity include loss of water or degassing. We incorporate new laboratory data on rhyolitic melt conductivity and its dependence on composition with information from other techniques and results of MT data inversion to place constraints on the magma system beneath and adjacent to Aluto, and compare with other volcanic centres in the MER and Afar region of Ethiopia.

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11:30-12:00 Rocks never lie: reinstating petrography in the igneous petrology toolbox

Professor Marian Holness, University of Cambidge, UK

Abstract

Since the pioneering work of Henry Clifton Sorby (1826-1908), it has been understood that rock history is recorded in the shape, size and orientation of their constituent grains. Although optical microscopy played a fundamental role in developing the discipline of igneous petrology, efforts to capitalise on new technological breakthroughs have resulted in the progressive relegation of petrography. Hence, models are developed based on bulk geochemical variations, or sophisticated numerical analysis, with no accompanying test based on close observation of the rocks themselves: it seems that few people ask “do the rocks record the hypothesised processes?”. This is illustrated by asking how large volumes of crystal-poor silicic liquids are extracted from crustal mushy zones. The mechanisms commonly proposed are hindered settling, micro-settling and compaction. The metallurgical community showed, several decades ago, that the concept of micro-settling is demonstrably wrong: the ease with which silicate minerals form sintered clusters and chains means that hindered settling can only operate at very low crystal concentrations: viscous compaction leaves a clear microstructural signature but few studies calling for melt segregation by compaction present any supporting evidence. The question of melt segregation can only be answered using detailed petrography in conjunction with geochemistry and field geology. 

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12:00-12:30 Consequences of a mush-dominated magma plumbing system: a mid-ocean ridge perspective

Dr Johan Lissenberg, Cardiff University, UK

Abstract

Mid-ocean ridges provide a unique opportunity to study the architecture and dynamics of volcanic systems: because the crust is continuously generated and hence zero age, observables relate directly to the ‘internal’ evolution of the magmatic system, without significant influence of older and/or compositionally distinct crust. Furthermore, geophysical data can be combined with sample suites from both the plutonic and volcanic sections, enabling a complete picture of the magmatic evolution to be reconstructed. Geophysical data indicate that, even at the most magmatically robust ridges, the magma plumbing system is typically comprised of crystal mush. The presence of a crystal mush-dominated magma plumbing system has a number of important consequences for the behaviour of the magmatic system. One of these is that melt migration by porous flow plays an important role. This leads to reactions between the mush and migrating melts, which may control the compositional evolution of both melt and mush. Another consequence is on magma evolution. Crystal mushes are inherently open systems, and melts are thus unlikely to evolve in chemical isolation. Furthermore, the mush may have a buffering effect on melt compositions. As a result, extracted melts are relatively homogeneous, and no longer reflect the source heterogeneity of their parental magmas. The presence of a mushy system also affects the nature of extracted melts. Melt extraction from a crystal mush enables mid-ocean ridge basalt to record a cryptic clinopyroxene fractionation signature, despite clinopyroxene rarely occurring as phenocrysts. It also leads to the entrainment of diverse populations of xenocrysts, which provide a useful tool reconstruct the dynamics of the magma plumbing system.

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12:30-13:30

Lunch

13:30-17:00

Session 4

7 talks Show detail Hide detail

Chairs

Professor Marian Holness, University of Cambidge, UK

13:30-14:00 Percolation of isotopically heterogeneous interstitial melts in the crystal mush of the Rum layered intrusion, NW Scotland

Dr Brian O'Driscoll, The University of Manchester, UK

Abstract

In order to shed further light on the importance of postcumulus processes in layered intrusions, and to demonstrate that crystal mushes behave as open-systems during the crystallisation of cumulates, mineral-scale textural and geochemical heterogeneity has been investigated in Unit 10 of the ~60 Ma Rum layered intrusion (NW Scotland). Numerous (~1 mm thick) Cr-spinel seams occur throughout the ~65 m Unit 10 peridotite stratigraphy. Unusually, intercumulus plagioclase and clinopyroxene crystals in the peridotite several centimetres above and below these seams exhibit complex optical and major element zoning. Sampling of individual intra-crystal zones in these phases was carried out using a New Wave Micromill, for analysis of their 87Sr/86Sr compositions to be measured on unspiked samples by TIMS. Both mineral phases reveal significant intra-crystalline isotopic heterogeneity. The new data demonstrate that multiple generations of isotopically distinct melts percolated through the Unit 10 crystal mush, suggesting solidification from cumulates that underwent repeated cycles of resorption and recrystallisation at the postcumulus stage. The cumulate products of layered intrusions may therefore form from magma addition within the crystal mush, and such a process might be especially relevant for precious metal enrichment, given the association between isotopic disequilibrium and the locations of Cr-spinel seams observed in the Rum intrusion.

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14:00-14:30 Crystal adhesion in magma

Professor Allen Glazner, The University of North Carolina at Chapel Hill, USA

Abstract

Magma chamber dynamics are commonly interpreted with analogy to fluvial and sedimentary processes such as gravity flows, turbidity currents, fluidization, and hydrodynamic sorting. An unstated but critical assumption in these analogies is that the crystals involved, like sand grains in water, are unreactive, passive particles carried by an inert fluid. However, unlike sand in water, crystals in magma were precipitated by the enclosing liquid and grow with each increment of heat released or water lost. Growing crystals adhere to one another, forming rigid interlocking piles whose behavior is quite different from unattached particles. In the lab, crystallization of salts from saturated solutions typically leads to rigid zero-porosity hard grounds that isolate liquid above from mush below. Many commonly applied hydrodynamic processes can be ruled out on Reynolds number arguments, and crystal adhesion further limits their applicability to magmas. The classic image of crystals homogeneously nucleating in a magma tank and then settling into piles on the bottom, where they slump and cascade in gravity currents, needs serious reexamination.

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14:30-15:00 The fluid mechanics of magmatic emplacement, cooling and crystallisation

Dr Jerome Neufeld, University of Cambridge, UK

Abstract

Magma is emplaced in the crust in a number of modes; for example in sills with a wide variety of aspect ratios in which the surfaces are relatively planar, or in large laccolith intrusions in which the upper sequence is highly deformed during emplacement. For many of these bodies, the physical processes acting at the front of the intrusion play a crucial role in the dynamics, and the final morphology of the emplacement. We show that when magma is input slowly the lateral growth of the intrusion may be limited by either viscous resistance to fluid flow, or by brittle deformation at the fracture front. In both cases a vapour tip, separating the fracture front from the fluid, is a physical requirement, as demonstrated in an analogue laboratory system. In contrast, when magma is rapidly input, the flow may be highly turbulent in which case the lateral growth no longer depends on physical processes at the front and is instead controlled turbulent stresses. While these differing mechanisms are reflected in the shape of intrusions, the distinction between laminar or turbulent emplacement, and the subsequent presence (or lack) of convection in the cooling body is potentially recorded in the crystal record. The talk will finish with some comments on the potential trends in crystal aspect ratios within intrusions as a means to distinguish between laminar/turbulent or stagnant/convecting intrusions.

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15:00-15:30 Tea

15:30-16:00 Impact of CO2-flushing on chemical and physical properties of magmas

Professor Luca Caricchi, University of Geneva, Switzerland

Abstract

The most abundant volatile species in magmas are H2O and CO2 and their solubility is primarily controlled by pressure. Water is significantly more soluble than CO2 and therefore, magma crystallisation at depth or its ascent to the surface leads to the exsolution of CO2-rich fluids. Thus, upper crustal reservoirs contain H2O-rich and CO2-poor magmas that interact with CO2-rich fluids released at depth (CO2-flushing). I will discuss how CO2-flushing impact the evolution of volume, pressure, compressibility, and chemistry of magmas stored in the upper crust. Understanding how this process controls the physical evolution of subvolcanic magma reservoirs may be of interest for the interpretation of geodetic signals recorded at active volcanic systems.

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16:00-16:30 Magma reservoirs

Professor Stephen Sparks CBE FRS, University of Bristol, UK

Abstract

The emerging concept of a magma reservoir is one in which regions containing melt extend from the source to the surface. The reservoir may contain both regions of partially molten rock (mush) and multiple magma chambers containing high enough melt fractions to be eruptible. The various parts of the system may be separated or connected and continuous. Magma reservoirs and their surroundings span a vast array of rheological properties, covering as much as 25 orders of magnitude from the high viscosity of the sub-solidus crustal rocks to magmatic fluids. Developing a comprehensive model of these systems is a major grand challenge that will require close collaboration between modellers, geophysicists, geochemists, geologists, volcanologists and petrologists.

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16:30-17:00 Discussion

Magma reservoir architecture and dynamics

Theo Murphy international scientific meeting organised by Dr Marie Edmonds, Professor Kathy Cashman FRS, Professor Marian Holness and Professor Matthew Jackson.

Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ
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