Oceanic Basalts

I Structure.- 1 Introduction and the ophiolite model.- 1.1 Historical perspectives.- 1.2 Oceanic lithospheric processes.- 1.3 Concluding statements.- 2 Surveying and sampling the ocean floor.- 2.1 Introduction.- 2.2 Surveying the ocean floor.- 2.3 Acoustic systems.- 2.3.1 Multibeam swath bathymetric systems.- 2.3.2 Signal amplitude measurements: side-scan sonar.- 2.4 Deep-sea photography.- 2.5 Geological information from bathymetric mapping: Chile Triple Junction region.- 2.6 Selecting sampling targets.- 2.7 Sampling methods.- 2.7.1 Dredging.- 2.7.2 Coring.- 2.7.3 Drilling.- 2.7.4 Submersible sampling.- 2.8 Concluding statements.- 3 Structure of the oceanic crust from geophysical measurements.- 3.1 Introduction.- 3.2 Ocean basins.- 3.3 Normal oceanic crust.- 3.4 Spreading centres.- 3.5 Fracture zone structure.- 3.6 Oceanic islands and swells.- 3.7 Concluding statements.- 4 Structure of the oceanic crust as deduced from ophiolites.- 4.1 Introduction.- 4.2 Alpine-type peridotites: variants and nomenclature.- 4.3 Seismic comparisons between oceanic crust and mantle.- 4.4 Implications for magmatic processes occurring at oceanic ridges.- 4.5 Implications for metamorphic processes occurring in the oceanic crust.- 4.6 Concluding statements.- II Processes.- 5 Mineralogy and crystallization of oceanic basalts.- 5.1 Introduction.- 5.2 Quench textures: the consequences of supercooling.- 5.3 Primary mineralogical controls on fractionation pathways of abyssal tholeiites.- 5.3.1 General course of fractionation in abyssal tholeiites: influence of magma chamber mixing.- 5.3.2 Advanced differentiation.- 5.3.3 Role of oxygen fugacity and oxide minerals.- 5.3.4 Apatite and sulphides.- 5.4 Liquid immiscibility and the significance of melt densities.- 5.5 Mantle—crust environments controlling oxygen fugacity.- 5.6 Alkalic magmatic lineages on seamounts.- 5.7 Concluding statements.- 6 Experimental phase petrology of mid-ocean ridge basalts.- 6.1 Introduction.- 6.2 Experimental studies at 1 atm.- 6.2.1 Experimental techniques.- 6.2.2 Results from experimental studies of basalts at 1 atm.- 6.2.3 Poorly known aspects of low pressure crystallization.- 6.2.4 Extreme differentiation of MORBs at low pressure.- 6.3 Experimental studies at high pressure.- 6.3.1 Experimental techniques.- 6.3.2 Results from experimental studies at high pressure.- 6.3.3 Poorly known aspects of high pressure equilibria relevant to MORBs.- 6.4 Concluding statements.- 7 Magmatic processes in oceanic ridge and intraplate settings.- 7.1 Introduction.- 7.2 Compositional diversity of oceanic magmas.- 7.2.1 Mid-ocean ridge basalt.- 7.2.2 Oceanic island basalt.- 7.3 Phase equilibrium and fluid dynamic constraints.- 7.3.1 Phase equilibria.- 7.3.2 Fluid dynamics.- 7.4 Melt generation: active versus passive.- 7.4.1 Passive melting at ‘normal’ ridge systems.- 7.4.2 OIB shields: rising mantle jets versus shear melting.- 7.5 Melt transport and storage in the oceanic lithosphere.- 7.5.1 Fractionation mechanisms.- 7.5.2 Mid-ocean ridge fractionation models.- 7.5.3 Intraplate fractionation models.- 7.6 Concluding statements.- 8 Metamorphic and hydrothermal processes: basalt—seawater interactions.- 8.1 Introduction.- 8.1.1 Importance of seawater—rock interactions.- 8.1.2 Controls of seawater—rock interactions.- 8.1.3 Effects of seawater—rock interactions.- 8.2 Low temperature alteration.- 8.2.1 Dredged basalts.- 8.2.2 Drilled basalts.- 8.3 High temperature reactions.- 8.3.1 Dredged basalts.- 8.3.2 Experimental evidence.- 8.3.3 Ophiolite evidence.- 8.3.4 Drilled basalts.- 8.4 Concluding statements.- III Environments.- 9 Oceanic islands and seamounts.- 9.1 Introduction.- 9.2 Hypotheses of intraplate volcanism.- 9.2.1 Mantle plume model.- 9.2.2 Propagating fracture model.- 9.3 Seamount distribution and morphology.- 9.4 Internal structure and composition.- 9.5 Basalt types.- 9.6 Geochemical features.- 9.6.1 Incompatible element abundances.- 9.6.2 Rare earth elements.- 9.6.3 Highly incompatible element ratios.- 9.6.4 Radiogenic isotopes.- 9.6.5 Gaseous isotopes.- 9.7 The mantle and OIB.- 9.8 Chemical variation and tectonic setting.- 9.8.1 Linear island and seamount chains.- 9.8.2 Linear aseismic ridges.- 9.8.3 Island groups adjacent to spreading axes.- 9.8.4 Ridge flank young seamounts.- 9.9 Concluding statements.- 10 Back-arc basins.- 10.1 Introduction.- 10.2 Formation of marginal basins.- 10.3 Back-arc extension and magmatic activity: an overview.- 10.3.1 Mariana Trough.- 10.3.2 Sumisu Rift.- 10.3.3 Lau Basin.- 10.3.4 East Scotia Sea.- 10.3.5 Bransfield Strait.- 10.3.6 Japan Sea.- 10.3.7 Sulu, Banda and Celebes Seas.- 10.3.8 Gulf of California.- 10.3.9 Rocas verdes ophiolite complex, Chile.- 10.4 Compositional diversity of back-arc basin basalts.- 10.4.1 Textures and mineralogy.- 10.4.2 Major elements.- 10.4.3 Volatiles.- 10.4.4 Isotope data.- 10.4.5 Minor and trace elements.- 10.5 Processes.- 10.5.1 Origin and nature of the slab-derived component.- 10.5.2 Slab-melting or dehydration?.- 10.5.3 Mantle wedge and magma formation in back-arc regions.- 10.6 Concluding statements.- 11 Pacific ocean crust.- 11.1 Introduction.- 11.2 Active ridges.- 11.3 Inactive or failed ridge crests.- 11.4 Propagating rifts.- 11.5 Edge effects at ridge offsets.- 11.6 Older ridge-generated Pacific crust.- 11.7 Hot-spot volcanoes.- 11.8 Non-hot-spot seamounts.- 11.9 Oceanic plateaux.- 11.10 Concluding statements.- 12 Indian ocean crust.- 12.1 Introduction.- 12.2 Magmatic lineages of abyssal tholeiites in the Indian Ocean.- 12.3 Depths of partial melting.- 12.4 The mantle melting column.- 12.5 Mixing of parental magma stems.- 12.6 Mantle lithological heterogeneity and the melting column.- 12.7 Concluding statements.- 13 North Atlantic ocean crust and Iceland.- 13.1 Introduction.- 13.2 Morphology and structure of the Mid-Atlantic Ridge.- 13.3 Morphology and structure of Iceland.- 13.3.1 Present plate boundary configuration.- 13.3.2 The neovolcanic zone.- 13.3.3 Evolution of the Icelandic plate boundary.- 13.4 Mantle structure under the Atlantic and Iceland.- 13.4.1 Asthenospheric mantle flow.- 13.4.2 Lithospheric thickness.- 13.4.3 Existence of axial magma chambers.- 13.5 Petrographic series.- 13.5.1 Tholeiitic basalt series.- 13.5.2 Alkali basalt series.- 13.5.3 Relationship and origin of the different series.- 13.5.4 Clinopyroxene-phyric basalts.- 13.6 Geochemical variation.- 13.6.1 Basaltic chemical types and the plume model.- 13.6.2 Normal ridge segment: N-MORB tholeiites.- 13.6.3 Transitional ridge segments: T-MORB tholeiites.- 13.6.4 Enriched ridge segments: E-MORB tholeiites.- 13.6.5 Alkali basalts from Iceland and other Atlantic oceanic islands.- 13.7 Comparison of the North Atlantic and Iceland.- 13.8 Concluding statements.- IV Sources.- 14 Stable and noble gas isotopes.- 14.1 Introduction.- 14.2 Stable isotopes.- 14.2.1 Sampling and speciation.- 14.2.2 Nitrogen.- 14.2.3 Carbon.- 14.2.4 Hydrogen.- 14.2.5 Sulphur.- 14.2.6 Oxygen.- 14.3 Noble gases.- 14.4 Mantle models.- 14.5 Concluding statements.- 15 Oceanic peridotites.- 15.1 Introduction.- 15.2 Oceanic peridotites.- 15.2.1 Rift to passive margin transition.- 15.2.2 Mid-ocean ridge processes.- 15.2.3 Intraplate processes.- 15.2.4 Active margin processes.- 15.3 Petrogenetic models.- 15.3.1 Oceanic mantle under continents or vice versa?.- 15.3.2 Heterogeneous oceanic mantle?.- 15.4 Concluding statements.- Appendix A Maps showing the geographical location of features mentioned in the text.- Appendix B Abundance of the elements in the earth and mean carbonaceous chondrite, C1.- Appendix C Isotopic ratios commonly used in magmatic petrogenesis.- Appendix D Normalization factors for rare earth elements and incompatible elements used in the construction of multi-element diagrams.- References.