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Structure of the Ocean Lithosphere

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Structure of the Ocean Lithosphere

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    1. Structure of the Ocean Lithosphere How do we know the structure of the lithosphere? Geophysical data Seismic reflection and refraction, magnetics, gravity, heat flow Dredges and cores (fracture zones) Seafloor mapping Side scan sonar, Gloria (Geological Long Range Inclined Asdic), swath mapping, deep tow mapping, direct observation (Alvin) Ophiolites

    2. Structure of the Ocean Lithosphere

    3. Ocean Lithosphere Layer 1 Deep sea sediments, often include radiolarian cherts

    4. Ribbon Chert

    5. Structure of the Ocean Lithosphere

    6. Layer 2 2A, 2B and 2C- change in seismic velocity 2A = pillow lavas and sheet flows (extrusive) with voids 2B = pillow lavas and sheet flows (extrusive) with void filling clays and minerals 2C = sheeted dikes- injected into fractures in the ocean crust (intrusive), ~ 1 m wide Ocean Lithosphere

    7. Structure of the Ocean Lithosphere

    8. Layer 3 Massive gabbro Layer 4 Layered peridotite – ultramafic cumulates Massive peridotite Ocean Lithosphere

    9. Ocean Lithosphere- Seismic Velocities

    10. Ocean Lithosphere- The Moho

    11. Ocean Lithosphere Models

    12. Data indicate large magma chambers are short-lived or non existent Instead injection of crystal mush- starts to crystallize during ascent Fast-spreading- lens of magma over the xl mush Slow-spreading- narrower mush zone (less supply), no lens of magma Ocean Lithosphere

    13. Ocean Lithosphere- Geochemistry (abridged) MORB = tholeiitic basalts Produced by partial melt of depleted mantle material Depleted Mantle- subject to previous melt event which removed already removed incompatible elements

    14. Ocean Lithosphere- Geochemistry (abridged) Incompatible Elements (partitioned into melt) Elements that that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma HFS (High Field Strength)- high charge, highly insoluble in water dominated fluids, (Nb, Ta, Ti. Zr, Hf) LIL (Large Ion Lithophile)- 1+ and 2+ large ion elements that tend to be concentrated in silica melts, high radius/charge (K, Rb, Cs, Sr, Pb, Ba)

    15. Ocean Lithosphere-Geochemistry

    16. Ocean Lithosphere-Geochemistry Magma Series- evolution of mafic magma Tholeiites- lower Na, produced from reduced magmas. First xlz Mg-rich olivines and pyroxenes (Bowen’s Rxn Series), Mg/Fe decreases Rifting environments- partial melt of depleted mantle Calc-Alkaline- higher Na, produced from oxidized magmas. Fe oxidized ? magnetite , Mg/Fe more constant Subduction zones- partial melt of depleted mantle plus subducted material (hydrated)

    17. Ocean Lithosphere-Geochemistry Magma Series Alkaline- higher Na and K relative to SiO2. More enriched in incompatibles. Ocean island-hotspot environments- partial melt of more primitive mantle

    18. Ocean Lithosphere-Geochemistry

    19. What happens to the crust after it forms? Aging process Contracts, cools, deepens (density) Increase in seismic velocity of the upper crust Decrease in conductive and convective heat flow Decrease in remnant magnetism Ocean Lithosphere

    20. Age-Depth relationship Young, hot, buoyant ? older, cooler, more dense Depth to the ocean crust increases systematically with age Aging of Ocean Crust

    21. Aging of Ocean Crust

    22. Limitations Only applicable to ~80 Ma, after that lithosphere mostly cooled, nearing equilibrium Not all ridges start at 2500 m Aging of Ocean Crust

    23. Increased seismic velocity Rocks become more dense with age due to: Cooling and contracting Infilling of pore spaces and fractures (calcite and zeolite cements- usually related to fluid flow Aging of Ocean Crust

    24. Aging of Crust Seismic Velocity

    25. Heat Flow Lithosphere cools through Conduction- diffusion of heat from hot lithosphere to cold seawater or sediment interface Convection- transfer of heat by mass movement Heat flow decreases with age Aging of Ocean Crust

    26. Aging of Ocean Crust

    27. Decreased remnant magnetization Remnant = permanent magnetization induced by an applied field Low temperature alteration of the crust includes oxidation of titanomagnetites ? decreased magnetization Greatest change over the first 20 Ma Aging of Ocean Crust

    28. Where is ocean crust forming?

    29. Spreading Centers Mid ocean ridges (tholeiitic basalt) Hotspots tracks/Aseismic Ridges Hawaii, Line Islands Walvis Ridge, Rio Grande Rise, Ninety-east Ridge Large igneous provinces- voluminous outpourings of mafic material Ontong Java Plateau, Kerguelen, Deccan Formation of Ocean Crust

    30.

    33. Formation of Ocean Crust Today ~90% of new ocean crust is formed at the mid ocean ridge Crustal production rate ~1.8 x 106 km3/my During Cretaceous ~70% of new ocean crust formed at the mid ocean ridge, 30% formed at hot spots (large igneous provinces) Crustal production rate ~3.3 x 106 km3/my Hotspot activity may exceed ridge processes for short intervals

    34. Formation of Ocean Crust

    35. Formation of Ocean Crust Implications of LIPs Thickening of ocean crust Ontong Java ~40 km; Kerguelen ~25 km Reheat and uplift surrounding lithosphere Resist subduction? Nucleus for continent? Sea level Greenhouse gases

    36. Suggests 2 modes of heat and mass transfer from the mantle Prevalence of each mode varies through time Related to activity at the core/mantle boundary? Heat flux from plumes ~ cooling of the core Present day 60% of plume flux in the Pacific Formation of Ocean Crust

    37. Hot Spots What are their source depths? CMB (D”), 670 discontinuity, both… What is the link to climate? Greenhouse gases, sea level, circulation… What determines the location of a hotspot? Distance from ridge, random… What triggers hotspot activity? Continental breakup, plate reorganization, core processes…

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