Oceanic Basalts

Basalt is the most voluminous of all the igneous rocks. Extensive field, experimental, petrographic and geochemical studies of basalt have provided us with a considerable understanding of igneous petrogenesis, plate tectonics, and crust-mantle interaction and exchange. One important aspect of geology that has developed over the last few decades is the study of oceanic basalts. The ocean basins cover about two thirds of the earth's surface and are floored by a basement of oceanic basalt that is continuously undergoing generation at spreading centres and destruction at subduction zones, a process which throughout geological time is recognized as the principal means of generating new crust. The study of oceanic basalts enables us to understand better the generation and recycling of crustal materials (including the continental crust), and the exchange between oceanic crust and seawater via hydrothermal activity. Compositional variations displayed by oceanic basalts provide windows into the mantle, and the identification of isotopically-distinct mantle reservoirs demonstrates that the source of oceanic basalts is heterogeneous and is controlled by convection and reservoir interactions within the mantle.

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.- References.