Mid-Ocean Ridges,

Magmatic Processes, Fluid Flow

& Textural Studies

(The Textural Analysis Research Team)

This web page is dedicated to the memory of Dr Bob Hunter. It was Bob who formed the TART and who is responsible for enthusing and educating many of the people listed on this page. His impact on us was immense & he is sadly missed.

The main objective of the ‘team’ is to better understand the processes involved in the generation, migration and emplacement of all melts & magmas. Our approach is multi-disciplinary, involving mathematical modeling, geochemistry, geophysics, laboratory observation (textures & microstructures), and fieldwork. Current field projects include ongoing study of the Rum layered Intrusion (NW Scotland), the Bushveldt layered Intrusion (S. Africa), the Stillwater Intrusion (Montana) and the South West Indian Ridge in the Indian Ocean.

Research Areas

Komatiites & Mantle Plumes

Layered Intrusions

Rock Textures

Fluid Flow & Physical Properties of Rocks

Origin of Granitic Magmas

Origin & Evolution of Sedimentary Basins

Processes at Slow Spreading Ridges

Future Research

People, Past & Present

Biographical Information & who to contact: Dr. M.J. Cheadle mailto:(cheadle@uwyo.edu)

Research Areas

Komatiites & Mantle Plumes

Komatiites, melting in mantle plumes; temperature constraints on the mantle through time

(Dr Mike Cheadle, Dr Kath Silva, Dr Dougal Jerram, with Prof. D. Sparks – Texas A & M University, Prof. N.T. Arndt –Universite Joseph Fourier Grenoble, Dr. Mary Gee & Prof. E.G. Nisbet-RHBNC).

Komatiites are the hottest lavas ever erupted on Earth (eruption temperatures 1400-1600^oC), and therefore they place important constraints on the temperature of the mantle especially during the Archaean, where they are most commonly found. However their origin remains enigmatic, mainly because of their age and restricted occurrence & because of their preservation. We have studied the petrology, geochemistry and origin of the freshest known komatiites from the Belingwe Greenstone Belt in Zimbabwe (2.7Ga). Dissertation work by Dr Kath Silva on the geochemistry on a complete komatiite sequence through the Belingwe Greenstone Belt has been used to place constraints on how this greenstone belt was formed.

Examples of the well preserved Belingwe komatiites. Spinifex texture from the upper part of the flows on the left and the texture of the cumulate zones from the lower part of the flows on the right.

The current ‘hot’ topic about komatiites is ‘do they represent hot dry magmas or do they represent less hot, wet, possibly subduction zone magmas’. Dr Mary Gee is currently measuring water contents in melt inclusions from the Belingwe Komatiites using FTIR methods. Initial results reveal very low water contents. If komatiites are indeed ‘dry’ magmas, they must have formed in mantle plumes….. but the implications for Archaean mantle temperatures are far ranging (+200-300^oC hotter than today).

Prof. Dave Sparks has developed 3-D numerical models of the temperature distribution within mantle plumes. We use these models, along with parameterisations of the temperature and pressure dependence of the chemistry of mantle melts, to predict the composition of melts produced by different mantle plumes. We’ve investigated the effects of varying mantle temperature, the variation in lithosphere thickness, and are now attempting to introduce the effect of water on melting. The work has been used to predict the mantle conditions required for the generation of komatiites, and has led to predictions of Archaean mantle temperatures.

Numerical model showing two isothermal surfaces (green & blue) which show the effects of dragging a lithospheric plate over an up-welling mantle plume. Box depth corresponds to the depth of the upper mantle.

Relevant Publications:

Arndt, N.T., Ginibre, C. Chauval, C., Albarede, F., Cheadle, M.J., Herzberg, C., Jenner, & Lahaye, Y., 1998. Were Komatiites Wet? Geology 26, 739-742.

Silva, K., Cheadle, M. J. & Nisbet E.G., 1997. The Origin of B-1 -zones in Komatiite Flows. Journal of Petrology, 38, 1565-1584.

Nisbet, E. G., Cheadle, M.J., Arndt, N.T., and Bickle, M.J. (1993). Constraining the potential temperature of the Archaean mantle: a review of the evidence from komatiites. Lithos. 30: 291-307.

Renner, R., Nisbet, E.G., Cheadle,M.J., Arndt, N.T., Bickle, M.,Cameron, W.E., 1993. Komatiite flows from the Reliance Formation, Belingwe Belt, Zimbabwe: 1- Petrography and Mineralogy. J. Petrology, 35, 361-400.

Nisbet, E.G., Arndt, N.T., Bickle, M.J., Cameron, W.E., Chauvel, C., Cheadle, M.J., Hegner, E., Kyser, T.K., Martin, A.,Renner, R., Roedder, E., 1987. Uniquely fresh 2.7 Ga old komatiites from the Belingwe greenstone belt, Zimbabwe. Geology, 15, 1147-1150.

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Layered Intrusions

The origin and sequence stratigraphy of layered intrusions using deep seismic reflection and geologic data

(Dr, Mike Cheadle, Dr Fiona Sargeant, Dr Lisa Worrell, Caroline LoRe, with Prof. N. Arndt - Universite Joseph Fourier Grenoble).

Layered igneous intrusions represent some of the most spectacular igneous rocks seen on Earth (The Bushvedt layered Intrusion of S. Africa may be 300 x 200 x 6 km in size!). They are assumed to be the frozen remains of magma chambers, which once fed volcanoes such as those found on Hawaii. Their importance is that they allow us to study the rocks & therefore the processes that occur within magma chambers below active volcanoes. Unfortunately the processes that occur within these chambers are still hotly disputed and layered intrusions are still remarkably poorly understood.

The four mountains (Barkeval, Hallival, Askival & Trollaval) of the Eastern Layered Series, Rum.

Our group has spent many years understanding and logging the rocks of the Tertiary Rum intrusion of N.W. Scotland. Rum has an ultramafic layered suite which is at least 1000 cubic kilometres in volume. We’ve used a multi-disciplinary approach (including mapping, logging, geophysical, geochemical, textural & microstructural studies) to better understand the origin of and the processes that go on within the intrusion. The incontrovertible evidence for magmatic sedimentological processes has led us to use sequence stratigraphy to describe and interpret the intrusion. As a test of our ideas, we are currently working on the much larger scale Bushveldt Intusion of S. Africa (Fiona Sargeant). Fiona has also investigated how the floor rocks of the intrusion deform and rise as the magma is intruded.

Our aim is to devise a sequence stratigraphic framework for layered intrusions based on deep seismic data from the Bushveldt layered intrusion (S. Africa), and careful geological observations on the Rum (Scotland) & Bushveldt layered intrusions. We believe the fundamental advances that sequence stratigraphy allowed in sedimentology can also be made in the study of layered intrusions.

Photograph of the typical igneous ‘layering’ seen in the Eastern Layered Series of the Rum Layered Suite. The photograph shows troctolites (allivalites) and subsidiary peridotites from Unit 14 on Hallival.

The schematic on the right shows a postulated mechanism for the formation of the layering. It attempts to show the importance of both in-situ crystal growth on the floor of the chamber AND deposition of crystals by sedimentary processes.

Rock Textures

The quantification of rock textures and application of textural studies to the understanding the origin of rocks

(Dr. Mike Cheadle, Caroline LoRe, Dr. Dougal Jerram, Dr. Lisa Worrell, Dr. John Wheeler (University of Liverpool, UK), Dr. Laurence Coogan (University of Leicester, UK)).

There is much to be learned from textural studies of rocks. This important avenue of research is often overlooked because it may require lengthy periods of time acquiring data by microscope study. New developments in computer aided image analysis and automated SEM study are now making textural analysis an exciting area of research. We are both developing new methods for analysing textures and are using existing methods to constrain the processes that occur during the formation of rocks.

Dr Dougal Jerram has devised innovative cluster analysis techniques to quantify the packing of grains in rocks. Packing is an often overlooked parameter (unlike grain-size, etc.) which yields important information about how much compaction a rock has undergone and how the grains in that rock accumulated. We hope to use cluster analysis studies to quantify the degree of compaction in rocks.

Illustration of serial sections produced from a 3-D reconstruction of randomly packed sphere data set from Finney (1970). The serial section make it possible to 'walk' through the texture and inspect the distribution of pore space and also 'random clusters of spheres'.

Dr. Lisa Worrell and Caroline LoRe are using electron backscatter microscopy to map the crystallographic preferred orientation (CPO) of crystals in rocks, to compare with the shape preferred orientation (SPO) of those crystals. These fabric measurements provide important information about how igneous cumulates form and the processes by which they compact and solidify. They are studying rocks from both the Rum & Bushveldt Intrusions and their results will help confirm whether processes like current flow and/or compaction played a role in the formation of the Rum cumulates.

We have recently obtained a state-of-the-art Electron BackScatter Diffraction (EBSD) system at the Department of Geology and Geophysics as part of our Materials Characterization Lab. We will use it to study the microstructure and crystallography of materials; both of which fundamentally control the physical properties and reflects both the crystallization and deformation history of the material. Consequently the ability to efficiently acquire such information from rocks and other materials is essential for geoscientists to fully understand how rocks crystallize and deform. Electron Backscatter Diffraction (EBSD) microscopy allows crystallographic data to be generated quickly and effectively within a microstructural framework. An EBSD system fitted to a conventional Scanning Electron Microscope (SEM) with an automated stage can measure crystal orientation directions in most types of polycrystalline materials to the sub-micron level at speeds of up to 0.2s per spot analysis. The equipment permits rapid determination of the absence or presence of crystallographic preferred orientations (CPOs) within a material, and provides statistical data describing the crystal misorientation distribution (MOD) of a material. It also permits 3-D crystallographic, grain-size or texture mapping and phase identification and indexing of the facets or cleavage planes of individual crystals.

The pictures to the left shows a small section of a pyroxene gabbro from the Stillwater intrusion

And to the right is a grain map collected totally automatically using the EBSD system.

Pole figures determined using the EBSD system for the plagioclase crystals in the rock.

The system allows the very efficient determination of crystal fabrics in rocks.

Magma generation, migration and crystallisation, including 3-D numerical models of crystallising media.

(Dr. Mike Cheadle, Dr. Mike Elliott).

The group has concentrated on developing numerical models to mimic the 3-D crystallisation of materials. We have previously developed 3-D models of texturally equilibrated materials and, recently, Dr Mike Elliott has developed 3-D numerical models of non-equilibrated crystallising media. Both single and multiphase systems consisting of different grain shapes and sizes can be modelled. The computer programs can generate 3-D images and 2-D slices through both texturally equilibrated and un-equilibrated materials. These models give insights into the way rocks crystallise and can be used to test our traditional models of crystallisation. They also permit calculation of the evolving permeability of a crystallising system and hence are fundamental to understanding magma generation and migration. One interesting result is that crystal shape has an extremely important effect on permeability at porosities of less than 10%.

The pictures above show the results of the 3-D crystallisation model. The figure on the left shows a completely crystallised cube of crystals and the lower picture shows the zoning within those crystals. The figure on the right shows the results of partially crystallising a cube. The upper figure shows the crystals and the lower figure the pore space between the crystals.

One of the applications of this work is to test the validity of dihedral angle measurements on 2-D sections. Here we show that measurements of the 2-D dihedral angle distribution of pores in a totally un-equilibrated rock (squares) are very similar to those measured for quartz + H₂0 & olivine and H₂0.

Relevant Publications:

Jerram, D.A. & Cheadle, M.J., 2000. On the cluster analysis of grains and crystals in rocks. American Mineralogist, 85, 47-67.

Prior, D.J., Boyle, A.P., Brenker, F., Cheadle, M.J., Day, A., Lopez, G., Potts, G.J., Reddy, S.M., Spiess, R., Trimby, P.W., Wheeler, J., & Zetterström, L., 1999. The application of Electron Backscatter Diffraction and Orientation Contrast Imaging in the SEM to textural problems in rocks. American Mineralogist 84, 1741-1759

Elliott, M.T., Cheadle, M.J., & Jerram D.A., 1997. On the Identification of Textural Equilibrium in Rocks using Dihedral Angle Measurements. Geology, 25, 355-358

Elliott, M.T. & Cheadle, M.J., 1997. On the Identification of Textural Equilibrium in Rocks using Dihedral Angle Measurements, Reply. Geology, 25, 1055.

Hunter, R.H., 1996, Texture Development in Cumulate Rocks In Layered Intrusions. Ed. R.G. Cawthorn, Elsevier, 77-101.

Bryon, D.N., Atherton, M.P., Cheadle, M.J. & Hunter, R.H., 1996. Melt movement and the occlusion of porosity in crystallising granitic systems, Mineralogical Magazine, 60, 163-171.

Jerram, D.A., Cheadle, M.J., Hunter, R.H. & Elliott, M.T., 1996. The Spatial Distribution of Grains & Crystals in rocks. Contributions to Mineralogy & Petrology, 125, 60-74.

Hunter, R.H., 1987, Textural equilibrium in layered igneous rocks: Dordrecht, Reidel, v. Origins of Igneous Layering.

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Fluid Flow & Physical Properties of Rocks

The physical properties of two-phase systems, and fluid flow in porous media

(Dr. Mike Cheadle, Dr. Mike Elliott, Heather Sheldon, Dr John Wheeler (University of Liverpool, UK).

The models mentioned above allow predictions of the evolving physical properties (seismic velocity, electrical conductivity and permeability) of fluid bearing media. Knowledge of these properties is essential to interpret the results of geophysical investigations of magma chambers (for example beneath mid-ocean-ridges). The result that crystal shape is important at low porosities has important implications for the volumes of magma present in magma chambers. Heather Sheldon is currently writing code to examine the effects of the physical and chemical processes that occur on the crystal scale during fluid flow. We intend to investigate the effect of the complicated feedback between precipitation and dissolution on permeability as a fluid in chemical dis-equilibrium with the solid phase moves through its host rock.

Pore space geometries of perfectly texturally equilibrated materials (the figure on the left has a dihedral angle of 1^o and a very low porosity, the one in the centre has a dihedral angle of 60^o and the one on the right has a dihedral angle of 180^o

Animation showing the occlusion of porosity.

Watch a portion of a rock crystallise!

The effect of the geometry of crystals on the electrical conductivity of rocks.

Relevant Publications:

Sheldon, H., Wheeler J., Worden R., Cheadle M.J., and Lind A. The importance of effective stress as the driving force for chemical compaction, 2003 Journal of Sedimentary Research

Elliott, M.T., Cheadle, M.J., & Jerram D.A., 1997. On the Identification of Textural Equilibrium in Rocks using Dihedral Angle Measurements. Geology, 25, 355-358.

Bryon, D.N., Atherton, M.P., Cheadle, M.J. & Hunter, R.H., 1996. Melt movement and the occlusion of porosity in crystallising granitic systems, Mineralogical Magazine, 60, 163-171.

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Origin of Granitic Magmas

The generation and migration of silicic magmas

(Dr. Mike Cheadle, Dr Matt Jackson, Heather Sheldon, Dr. Mike Atherton (University of Liverpool, UK)).

Dr Matt Jackson produced numerical models for the generation of silicic magmas by melting the lower crust. The important result is that the process of melt generation and migration through a thermal gradient naturally leads to the production of a large volume of a melt which is chemically the low melt fraction of the lower crust. This result implies that the tectonic ‘squeezing’ advocated by some, to explain the generation of a large volume of a small melt fraction, is not necessary. Heather Sheldon has devised a phase-diagram based numerical model, which permits investigation of the effects of spatial variations in the initial composition of the source rock. She shows that a compositionally layered source rock may develop local pockets of melt, wherever there is an upward change to a more refractory composition. This spatially complex distribution of melt and solid is eventually smoothed out by the passing of a larger porosity wave, if heating continues for long enough.

Relevant Publications:

Jackson, M.D., Cheadle, M.J. and Atherton M.P. 2003 Quantitative modeling of granitic melt generation and segregation in the continental crust, In Press: Journal of Geophysical Research

Jackson, M. & Cheadle, M.J., 1998. A Continuum model for the transport of heat, mass and momentum in a deformable, multi-component mush, undergoing solid-liquid phase change, International Journal of Heat & Mass Transfer, 41, 1035-1048.

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Origin & Evolution of Sedimentary Basins

The processes of crustal extension at continental margins.

Amy Heath (co-supervised by Dr. Pat Shannon (UCD)& Prof. N.J. Kusznir) is using seismic & bore-hole data to study the Rockall Trough (West of Ireland). This basin is a prime example of a rift basin that shows evidence of extensive crustal thinning, but little brittle faulting. Amy is hoping to determine whether the continental crust ‘broke’ beneath the Rockall Trough.

Relevant Publications:

Nadin, P.A., Kusznir, N.J., & Cheadle, M.J., 1997. Early Tertiary plume Uplift of the North Sea & Faeroe-Shetland Basins. E.P.S.L., 148, 109-127.

Cheadle, M.J., S. McGeary, M. Warner and D.H. Matthews, 1987. Extensional structures on the western UK continental shelf: a review of evidence from deep seismic profiling. (in) Coward, M.P., Dewey, J.F. and Hancock, P.L. (eds) Continental Extensional Tectonics, Geological Society Special Publication No. 28, 445-465.

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Processes at Ultra-Slow Spreading Ridges

Understanding anomalous uplift and magmatism at the South West Indian Ridge.

(Dr. Mike Cheadle, Graham Baines, Dr. Barbara John, Professor Nick Kusznir (University of Liverpool, UK)).

Graham Baines is investigating the processes and mechanisms which cause anomalous uplift at inside-corner highs and along transverse ridges adjacent to transform faults at the South West Indian Ridge. He is currently studying Atlantis Bank, which is an anomalously uplifted core complex that rises 3 km above sea-floor of the same age. Flexural uplift due to detachment faulting cannot fully account for this uplift. Multi-beam bathymetric data and direct observations have revealed large moderately dipping transform-parallel normal faults on the Eastern flank of Atlantis Bank. Flexural uplift following such faulting is consistent with gravity data and accounts for ~1-2 km of anomalous uplift. This faulting may be linked to the contiguous transverse ridge that extends to the South of Atlantis Bank. Similar transform-parallel faults are also observed on the most uplifted core-complexes on the Mid-Atlantic Ridge. The coincidence of anomalously elevated core-complexes and transverse ridges imply that the mechanism of uplift is the same in both cases.

The Shinkai6500 (JAMSTEC), which is the deepest going manned submersible.

We used the Shinkai6500 and the Kaiko ROV (Jamstec) on a previous cruise to dive to the bottom of the ocean and investigate the tectonic and magmatic processes occurring there.

A view of the bottom and an anemone from the window of the submarine.

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Future Research

Future research plans involve the continuation and development of the projects listed above. In particular:

i) The origin of Komatiites.

ii) Studies of the magmatism of the British margin of the North Atlantic Igneous Province (These will lead to better models of the shape & extent of the incipient Icelandic plume.)

iii) The origin of layered Intrusions, igneous cumulates and platinum deposits, with future work including the Stillwater Complex, the Bushveld Intrusion & the Rum Intrusion.

iv) Textural studies and better numerical modelling of the evolution between texturally equilibrated and texturally un-equilibrated rocks. (What is the real permeability of the mantle? How are melts extracted? And in different environments {M.O.R’s & Subduction zones}).

v) Continuing research into the generation, migration, and emplacement of granites.

vi) Application of seismic stratigraphy to understanding Igneous Intrusions.

vii) Understanding oceanic core complexes: structure & flexure.

viii) The origin of the Costa Rica arc.

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