Mid-Ocean Ridges,

Magmatic Processes, Fluid Flow

& Textural Studies

(The Textural Analysis Research Team)

bob

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. In particular, one of our main objectives is to better understand how oceanic crust is created. Our approach is multi-disciplinary, involving mathematical modeling, geochemistry, geochronology, geophysics, laboratory observation (textures & microstructures), structural geology and fieldwork. Current field projects include ongoing study of the East Pacific Rise, the Mid Cayman Rise, Mid Atlantic and South West Indian Ridges, the Dufek layered Intrusion (Antarctica); the Rum layered Intrusion (NW Scotland), the Bushveldt layered Intrusion (S. Africa), and the Stillwater Intrusion (Montana).

Research Areas

Processes at Slow Spreading Ridges

Komatiites & Mantle Plumes

Layered Intrusions

Rock Textures

Fluid Flow & Physical Properties of Rocks

Origin of Granitic Magmas

Origin & Evolution of Sedimentary Basins

Future Research

People, Past & Present

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

Research Areas

Processes at Slow and Ultra-Slow Spreading Ridges

The origin and construction of oceanic crust: tectonics vs. magmatism.

(Dr. Mike Cheadle, Dr Kay Achenbach, Scott Badham; Tyler Brown, Chris Christofferson, Dr Graham Baines, Lauren Colwell, Dr. Craig Grimes (Ohio University), Lars Hansen, Nicole Schoolmeesters, Dr Josh Schwartz, Dr. Barbara John, Dr Tim Schroeder (Bennington College), Dr Henry Dick (Wood Hole), Professor Nick Kusznir (University of Liverpool, UK), Dr Pete Reiners (University of Arizona), Dr Joe Wooden (Stanford University).

Slow spreading ridges are 'arguably' one of the last frontiers of plate tectonics, and there are lots of fascinating questions to investigate. Professor Barbara John and I have a group who use all techniques (structure, petrology, geochemistry & geophysics) to study these problems. Lars Hansen recently completed a Masters thesis looking at the high temperature deformation mechanisms of the Kane Oceanic Core Complex Detachment Fault in order to understand the conditions and depth at which the faults form. Scott Badham is looking at the role of Fe-Ti oxides in localizing faulting at Mid Ocean Ridges. Kay Achenbach has been studying the depth of strain localization in mantle peridotites. And Graham Baines and Craig Grimes have shown that the rate of slip of oceanic detachment faults is much greater than the symmetrical plate half spreading rate. This suggests that oceanic detachment faults are effectively the plate boundary at mid ocean ridges! and implies that detachment faulting leads to ridge migration.

Kane

The Kane Oceanic Core Complex; 23^oN, Mid Atlantic Ridge;

nearly 30km of exposed detachment fault footwall.

We are also applying geo-chronologic and thermochronologic methods to address how oceanic crust grows and how fast faults move. We are one of the first groups on the world to do this. Put simply, uranium bearing minerals, such as zircon are much more common in oceanic crust than conventionally thought (see pubs. below) and this is allowing us to apply all the techniques developed for continental crust to the study of cooling rates, accretion rates and faulting rates at Mid Oceanic Ridges. We have NSF funding for this research and much of the geochronologic work is done in conjunction with Joe Wooden at the Stanford/USGS SHRIMP facility. Graham Baines, Craig Grimes, Josh Schwartz and Nicole Schoolmeesters have all used the U-Pb and (U-Th)/He methods to address various aspects of the growth of oceanic crust. An important component of work is “going to sea” to collect samples and explore the seafloor; our students have enjoyed cruises to the Pacific, Atlantic and Indian Oceans. Current research areas: the Mid Cayman Rise, the South-West Indian Ridge, the Mid Atlantic Ridge and the Gakkel Ridge.

Mt dent

The Mount Dent Oceanic Core Complex (OCC); Mid Cayman Rise.

Comparison cross section

Comparison of the Mount Dent OCC to the Ruby Mtns continental core complex in the Basin & Range. Mount Everest also shown for comparison.

Critical questions include:

i) How is mantle deformed and exposed at these ridges?

ii) What is the origin of anomalous uplift seen at inside corner highs and along transverse ridges?

iii) How does magmatism occur at ultraslow spreading ridges?

iv) At what depths do gabbros crystallize?

v) And how do oceanic core complexes form?

We have just started a new initiative to look at how oceanic crust grows. Chris Christofferson just finished looking at the microstructures and fabrics of gabbros from oceanic core complexes and Tyler Brown is just beginning work on gabbros from the fast spreading East Pacific Rise, working with Laurence Coogan (University of Victoria) and Jeff Gee (Scripps). Next stop is IODP Leg 345 to Hess Deep to collect gabbros from the lowermost crust.

Zircons separated from oceanic crust.

Deciphering the construction of oceanic crust: Dating IODP Hole1309D (Grimes et al., 2008)

Tectonics vs. Magmatism at slow spreading ridges (from Schroeder et al., 2007)

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.

Relevant Publications:

Schoolmeesters, N., Cheadle, M.J., John, B.E., Reiners, P.W., Gee, J. and Grimes, C.B.,, The cooling history and the depth of detachment faulting at the Atlantis Massif oceanic core complex: Geochem. Geophys. Geosyst. (accepted),

Grimes, C. B., M. J. Cheadle, B. E. John, P. W. Reiners, and J. L. Wooden (2011), Cooling rates and the depth of detachment faulting at oceanic core complexes: Evidence from zircon Pb/U and (U-Th)/He ages, Geochem. Geophys. Geosyst., 12, Q0AG01, doi:10.1029/2010GC003391.

Achenbach, K.L.; Cheadle, M.J.; Faul, U.; Kelemen, P.; Swapp, S.; 2011, Lattice-preferred orientation and microstructure of peridotites from ODP Hole 1274A (15°39′N), Mid-Atlantic Ridge: Testing models of mantle upwelling and
tectonic exhumation. EPSL, 301, 199-212.

John, B.E., and Cheadle, M.J., 2010, Deformation and alteration associated with oceanic and continental detachment fault systems: are they similar?: in Rona, Devey, Dyment, and Murton, eds., Diversity of Hydrothermal Systems on Slow-spreading Ocean Ridges, AGU Monograph 188, p. 175-206.

Cheadle M.J. & Grimes, C.B., 2010. To Fault or Not to Fault, Nature Geosciences, News & Views, vol 3 454-456.

Schwartz, J.J. John, B.E., Cheadle, M.J., Wooden, J., Mazdab, F., Swapp, S, and Grimes, C.B., 2010, Dissolution-Reprecipitation of Igneous Zircon in Mid-Ocean Ridge Gabbro, Atlantis Bank, Southwest Indian Ridge: Chemical Geology, vol 274, p68-81.

Michael, P.J. & Cheadle, M.J., 2009, Making Crust. Science. (Perspectives) Vol. 323. no. 5917, pp. 1017 – 1018 DOI: 10.1126/science.1169556.

Schwartz, J.J., John, B.E., Cheadle, M.J., Reiners, P., & Baines, A.G., The cooling history of Atlantis Bank oceanic core complex: evidence for hydrothermal activity 2.6 Myr off-axis. Geochemistry, Geophysics, Geosystems (G3), 2009.

Baines, G., Cheadle, M.J., John, B.E., Grimes, C.B., and Wooden, J., Rapid accretion of gabbroic crust at Atlantis Bank on the ultra-slow-spreading SW Indian Ridge: EPSL, 2009 .

Grimes, C.B., John, B.E., Cheadle, M.J., Mazdab, F.K., Wooden, J., Swapp, S., and Schwartz, J., On the occurrence, trace element geochemistry, and crystallization history of zircon from in situ ocean lithosphere: Contributions to Mineralogy and Petrology. 2009

Baines, A.G., Cheadle, M.J., John, B.E., and Schwartz, J.J., 2008. Rate of detachment faulting at Atlantis Bank, South-west Indian Ridge: evidence for 100% asymmetry during the formation of oceanic core complexes. Earth and Planetary Science Letters, vol. 273, 105–114. doi:10.1016/j.epsl.2008.06.013.

Grimes, C.B., John, B.E., Cheadle, M.J., and Wooden, J.L., 2008. Evolution and timescales for accretion of slow-spreading oceanic crust: constraints from high resolution U-Pb zircon dating of a gabbroic crustal section at Atlantis Massif, 30º N, MAR: Geochemistry, Geophysics, Geosystems, vol. 9, no. 8, Q08012, doi:10.1029/2008GC002063.

Baines, A.G., Cheadle, M.J., Dick, H.J.B., Hosford-Scheirer, A., John, B.E., Kusznir, N.J., & Matsumoto. T., 2007,The evolution of the Southwest Indian Ridge and the implications of major changes in the ridge axis geometry since 25Ma. Geochemistry Geophysics Geosystems, vol. 8, doi:10.1029/2006GC001559.

Grimes, C.B., John, B.E., Kelemen, P.B., Mazdab, F., Wooden, J., Cheadle, M.J.,Hanghoi, K., and Schwartz, J.J., 2007, The trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance: Geology vol. 35, 643–646, doi:10.1130/G23603A.1.

John, B.E., and Cheadle, M.J., 2007, Slow-spreading mid-ocean ridges: McGraw-Hill Yearbook of Science and Technology, Tenth Edition.

Schroeder, T., Cheadle, M.J., Dick, H.J.B., Faul, U., Casey, J.F., and Kelemen, P.B., 2007, Non-volcanic seafloor spreading and corner-flow rotation accommodated by extensional faulting at 15°N on the Mid-Atlantic Ridge: A structural synthesis of ODP Leg 209: Geochemistry Geophysics Geosystems, vol. 8, doi:10.1029/2006GC001567.

Schwartz, J.J., John, B.E., Cheadle, M.J., Miranda^,E., Grimes^,C., Wooden, J., and Dick, H. 2005. Growth and Construction of Oceanic Crust at Slow-Spreading Ridges, Science, vol 310, p654-658.

John, B.E, Foster, D.A, Murphy, J.M., Cheadle, M.J., Fanning, C.M., Copeland, P. & Baines, A.G., 2004. Determining the cooling history of in-situ lower oceanic crust-Atlantis Bank, SW Indian Ridge. EPSL 222, 145-160.

Baines, A.G., Cheadle, M.J., Dick, H.J.B., Scheirer, A.H., John, B.E., Kusznir, N.J., and Matsumoto, T., 2003, Mechanism for generating the anomalous uplift of oceanic core complexes: Atlantis Bank, southwest Indian Ridge: Geology, v. 31, p. 1105-1108.

back to Contents

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 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.

komspin komcum

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.

plume

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:

Herzberg, C., Albarede, F., Arndt, N., Asimow, P.D., Lesher, M., Niu, Y., Fitton, J.G., Cheadle, M.J., and Saunders, A.D., 2007. Ultramafic Igneous Rocks: A Challenge for Alternatives to the Plume Hypothesis. Geochemistry Geophysics Geosystems. Volume 8, No. 2, Q02006, doi:10.1029/2006GC001390.

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.

back to Contents

Layered Intrusions

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

(Dr, Mike Cheadle, Matt Lusk, Dr Fiona Sargeant, Dr Lisa Worrell, Caroline LoRe, Dr Craig Grimes, Matt Lusk, Jake Carnes with Prof. N. Arndt - Universite Joseph Fourier Grenoble), Dr. Jeff Gee (Scripps); & Dr. Bill Meuer (Exxon).

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.

rum

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. Lisa Worrell and Caroline LoRe studied the chemistry and microstructures of the cumulates from the Rum felspathic peridotites in the Western Layered Series and the gabbros and troctolites of Unit 9 in the Eastern Layered Series. The incontrovertible evidence for magmatic sedimentological processes has led us to use sequence stratigraphy to describe and interpret the intrusion. 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. 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 “back-stripped” the seismic data to show how the floor rocks of the Bushveldt Intrusion deform and rise as the magma is intruded.

unit14 Rum

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, Dr Kay Achenbach, Caroline LoRe, Dr. Dougal Jerram, Dr. Lisa Worrell, Matt Lusk, Prof. John Wheeler (University of Liverpool, UK), Dr. Laurence Coogan (University of Victoria,)).

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.

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.

3Dsec Animat2

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'.

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 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. Lisa Worrell and Caroline LoRe used electron backscatter microscopy to map the crystallographic preferred orientation (CPO) of crystals in mafic rocks from layered intrusions, 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. Kay Achenbach has used our EBSD system to characterise the CPO’s of peridotites from four Mid Ocean Ridges in order to constrain mantle deformation mechanisms and to test 2-D vs 3-D upwelling beneath Mid Ocean Ridges. Kay has also looked into the uncertainties associated with quantifying the strength of CPO’s. How many crystals do you need to measure to be sure you have a statistically significant fabric?

Meurertrimmedpoor junk1-GrainMap

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.

PP046-15x15-plag-contour

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.

EBSD map of a “single” spinel from an abyssal peridotite (Achenbach, 2008).

The spinel aggregate is approximately 1cm long.

Pole figures for the spinel in the picture above (Achenbach, 2008)

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

(Dr. Mike Cheadle, Dr. Mike Elliott, Dr. Marion Holness (Cambridge University)).

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, 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%.

me1 Permelectalk

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.

mediang

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:

Holness, M., Cheadle, M.J., & McKenzie, D.P. 2005 On the use of changes in dihedral angle to decode late-stage textural evolution in cumulates. Journal of Petrology 46: 1565-1583; doi:10.1093/petrology/egi026.

Jerram, D.A., Cheadle, M.J. and Philpotts, A.R. 2003 Quantifying the building blocks of igneous rocks: Are clustered crystal frameworks the foundation? Journal of Petrology 44, 2033-2051 DOI: 10.1093/petrology/egg069.

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.

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.

back to Contents

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, Dr. 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 examined the effects of the physical and chemical processes that occur on the crystal scale during fluid flow. We are investigating 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.

da1 da60 da180

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

poranimat

Animation showing the occlusion of porosity.

Watch a portion of a rock crystallise!

elecond

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

Relevant Publications:

Cheadle, M.J., Elliott, M.T., and McKenzie D.P., 2004. The Percolation Threshold and Permeability of Crystallizing Igneous Rocks: The Importance of Textural Equilibrium. Geology 32, 757-760.

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.

back to Contents

Origin of Granitic Magmas

The generation and migration of silicic magmas

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

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.

Granitic Magma: Transition from Segregation to Ascent (after Matt Jackson)

Relevant Publications:

Jackson, M.D, Gallagher, K. Petford, N. & Cheadle M.J., 2005 Towards a coupled physical and chemical model for tonalite–trondhjemite–granodiorite magma formation: Lithos 79, 43-60.

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.

back to Contents

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) used 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:

Fletcher, R.; Kusznir, N.J., Cheadle, M.J., 2008 (in press). Melt initiation and mantle exhumation at the Iberian rifted margin: Comparison of pure-shear and upwelling-divergent flow models of continental breakup. (Geophysical Journal of the Royal Astronomical Society)

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.

McGeary ,S., M.J. Cheadle, M. Warner, and D.J. Blundell, 1987. Crustal structure of the continental shelf around Britain derived from BIRPS deep seismic profiling. (in), J. Brooks and K. Glennie (eds) Petroleum Geology of North West Europe, vol. 1, Graham & Trotman, London.

back to Contents

Future Research

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

i) Understanding the origin and generation of slow spreading oceanic crust using the novel application of geo- and thermo-chronologic techniques. Additionally understanding the origin of oceanic core complexes: structure & flexure.

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

iii) Application of seismic stratigraphy to understanding Igneous Intrusions.

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) The origin of Komatiites.

vi) Studies of the magmatism of the British margin of the North Atlantic Igneous Province.