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Contents

Preface

Sensitivity of teleseismic body waves to mineral texture and melt in the mantle beneath a mid-ocean ridge

1. Introduction

2. Mantle flow and melting at oceanic spreading centres

3. Modelling the effect of melt on seismic properties

4. Results

(a) Influence of melt on velocities

(b) Travel time predictions for melt-only models

(c) Travel time predictions for melt + texture models

References

5. Discussion

Evidence for accumulated melt beneath the slow-spreading Mid-Atlantic Ridge

1. Introduction

2. Tectonic setting

3. Experimental results

(a) The wide-angle seismic experiment

(b) The seismic reflection experiment

(c) The controlled-source electromagnetic experiment

(d) The magneto-telluric experiment

4. Conclusions

References

An analysis of variations in isentropic melt productivity

1. Introduction

2. Background and previous work

3. Isentropic melting in simple systems

(a) One-component systems

(i) Constant coefficients

(ii) Variable coefficients

(b) Multicomponent systems

(c) Two-component systems

(i) Isobaric melting

(ii) Isentropic melting

(d) Multicomponent systems

4. Model peridotite syst

5. Conclusions

Appendix A.

References

A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges

1. Introduction

(a) Residual peridotites are not in equilibrium with MORB

(b) MOST melt extraction is in chemically isolated conduits of focused flow

(c) Melt inclusions and magma conduits

(d) 'Near-fractional' melting models

(e) Not all melting is necessarily 'near-fractional'

(f) U/Th isotopic disequilibrium and reactive flow at the base of the melting regime

(g) Reactive flow also occurs in the shallow mantle

(h) Geophysical data indicate that average porosity is low

2. Melt migration features in the mantle section of ophiolites

(a) Mantle dunites

(i) Dunites are conduits for MORB transport in the asthenosphere

(ii) Dunite dimensions

(iii) Most mantle dunites are replacive features

(iv) How much dunite can be formed by reaction? < 5% of the melting region

(v) Is there a 'dunite signature' in MORB? Maybe

(vi) The reactive infiltration instability (RII)

(vii) Mechanical instabilities

(b) Dikes: lithospheric features, and - in Oman - generally not formed by MORB

(c) Are dunites associated with cracks? Some are. (Most are not?)

(d) Chromitites: focused flow, but not necessarily in asthenospheric fractures

(e) Summary

3. Constraints from physics

(a) Is porous flow fast enough to account for melt velocity estimates? Yes.

(b) Can hydrofracture occur in adiabatically ascending mantle? Maybe, but if so,then only in pre-existing conduits.

(c) Would a percolation threshold increase melt pressure? Probably not.

(d) Cracks from above?

(e) Closed, melt-filled conduits in porous media are not chemically isolated

(f) Pre-existing dunite zones around melt-filled conduits

4. Plate scale melt transport: how is melt flow focused to the ridge?

(a) Fracture is not an effective mechanism for focusing to the ridge

(b) Focused solid flow plus channels?

(c) Coalescing dissolution channels? Maybe.

(d) Other mechanisms for lateral focusing of porous flow

5. Conclusion

References

Rift-plume interaction in the North Atlantic

1. Introduction

2. Tectonomagmatic regimes

(a) Crust unbroken by fracture zones

(b) Orthogonal spreading crust with fracture zones

(c) Over-thickened oceanic crust

3. Mantle temperatures derived from residual basement heights and crustal thickness

(a) Residual heights along isochron profiles

(b) Residual heights along flowlines

(c) Mantle temperatures from crustal thickness

4. The evidence from geochemistry

5. Influence of mantle temperature on oceanic crustal formation

(a) Spreading axis unbroken by fracture zone offsets

(b) Oceanic crust broken by fracture zones

(c) Crust created directly above the mantle plume

6. History of mantle plume-ridge interaction

References

The ultrafast East Pacific Rise: instability of the plate boundary and implications for accretionary processes

1. Introduction

2. The present plate boundary

3. Kinematic evolution of the southern EPR since 7 Ma

4. Instability of the tectonic segmentation

5. Structure and rheology of the axial region

6. Ridge segmentation and variability of axial characteristics

7. Accretionary processes along the ultrafast EPR: two or three-dimensional?

(a) Arguments in favour of uniform accretion

(b) Arguments in favour of large scale along-axis magma transport

8. Discussion and conclusions

References

Seafloor eruptions and evolution of hydrothermal fluid chemistry

1. Introduction

2. Description of CoAxial site

3. Results

(a) General comments

(b) Flow site

(c) Floe site

(d) Source site

4. Discussion

References

Controls on the physics and chemistry of seafloor hydrothermal circulation

1. Introduction

2. Underlying thermal models

(a) Conductively cooled plate model

(i) Predictions of bathymetry and heat flow

(ii) Vertical extent of hydrothermal circulation

(b) Persistence of hydrothermal circulation in older lithosphere

(i) Lithospheric cooling models accommodating hydrothermal sinks and magmatic sources

(ii) Predicted hydrothermal heat loss per unit length ridge axis

(c) Magmatic budget

(d) Global hydrothermal water budget

(i) Temperature and pressure effects on water volume calculations

3. Diffuse flow is an intrinsic feature of high temperature flow

(a) A model of flow within a cracked permeable domain

4. Combined chemical and physical measurements of diffuse effluent

(a) Diffuse effluent chemistry

(i) Areal measurement of heat flux density and the longevity of the TAG system

5. Time series measurements at TAG

(a) Previous observations of tidal variability

(b) Tidal variability at TAG

(c) Mechanisms for tidal influence on hydrothermal flow and measurement

(i) Temporal variability in seafloor thermal boundary conditions

(ii) Bernoulli effects

(iii) Response of the seafloor to ocean tides and solid Earth deformations

(iv) Seafloor tidal deformation and modulation of permeability structure

(v) A model of lithospheric permeability

(vi) Continental analogues

References

Where are the large hydrothermal sulphide deposits in the oceans?

1. Introduction

2. Geological control on the location of major sulphide deposits in the oceans

(a) Slow spreading

(b) Fast spreading

3. Geological, physical and chemical factors controlling morphology and size of deposits

(a) Mixing

(6) Permeability

(c) Stability of hydrothermal system

(d) Boiling: water depth

(e) Geological trap/cap rocks

4. Conclusions and new perspectives for hydrothermal exploration

References

Sea water entrainment and fluid evolution within the TAG hydrothermal mound: evidencef rom analyses of anhydrite

1. Introduction

2. Sampling and methodology

3. Results

(a) Fluid inclusions

(b) Geochemistry

4. Discussion

(a) Fluid evolution

(i) Replenished system

(ii) Closed system

(b) Conductive heating

5. Conclusions

References

Thermocline penetration by buoyant plumes

1. Introduction

2. An exponential thermocline

3. A circular slab model of spreading

4. A Red Sea brine plume

5. Discussion

References

Crustal accretion and the hot vent ecosystem

1. Introduction

2. Distribution of vent habitat

3. Habitat stability

4. Spatial heterogeneity

5. Historical factors

6. Discussion

(a) Slow spreading: the Mid-Atlantic Ridge

(b) Intermediate spreading: Juan de Fuca area

(c) Lau/Fiji back-arc basins

(d) Fast spreading, the northern EPR

(e) Ultra-fast spreading, the southern EPR

7. Conclusion

References

Biocatalytic transformations of hydrothermal fluids

1. Introduction

2. Definitions

3. Aerobic chemosynthesis

4. Anaerobic chemosynthesis and high-temperature biocatalysis

5. Discussion

References

Index