The Mantle and Creation of the Oceanic Crust EAS 302 Lecture 28

The Mantle

 The Earth’s mantle is 84% of the volume of the Earth and 68% of its mass.  Convection in the mantle drives plate tectonics.  Melting and degassing of the mantle have created the crust and atmosphere.  Therefore, an understanding of the Earth’s evolution and behavior requires an understanding of the mantle.

1 Ways of Studying the Mantle

 Seismic waves and other geophysical approaches (gravity, electrical conductivity).  Direct samples:  “peridotite massifs”  Oceanic peridotites  Xenoliths (pieces of mantle carried to the surface in volcanic eruptions)  Indirect samples:  Basalts and other mantle-derived magmas

Seismic Structure of the Mantle

 Upper mantle seismic velocities consistent with densities of ultramafic rock: peridotite  Uppermost mantle:  olivine, orthopyroxene, and clinopyroxene + plagioclase, spinel, or garnet (depends on P)  Transition Zone: 400-670 km depth: series of phase changes  Lower mantle: Si in octahedral coordination in Mg-perovskite + ferripericlase ((Mg, Fe)O)

2 Mineralogy of the Upper Mantle

Silica Tetrahedron

Seismic Transition Zone

 At 300 km, pyroxenes dissolve in garnet, forming “majorite” garnet  At 400 km, olivine undergoes a structural change to “β” form.  At 500 km, olivine changes to “γ” or “spinel” structure.

3 Lower Mantle: Silicates Under High Pressure  Under extreme pressure, Si becomes octahedrally rather than tetrahedrally coordinated (i.e, surrounded by 6 oxygens rather than 4).  This transition accounts for jump in seismic velocity at 660 km.  Fewer cations can be accommodated in this structure, so some oxides phases form:  (Mg,Fe)2SiO4 → Structure of Mg-perovskite (Mg,Fe)SiO3+ (Mg,Fe)O

Convection and the Rayleigh Number

 Whether a fluid convects or not depends on whether buoyancy forces exceed viscous forces. The ratio of these forces is the Rayleigh Number: αΔTgρd 3 Ra = Kη  Where α is the coefficient of thermal expansion, ∆T is the temperature change over the height of the fluid, d is the height, g is the acceleration of gravity, ρ is density, K is thermal conductivity and η is viscosity.  Convection will occur if Ra exceeds 5000.

a Rayleigh Number and Convection in the Mantle  For the mantle, α = 3 x 10-5 K-1, ∆T = 3000 K, g = 10 m/s, ρ = 4660 kg/m3, K = 10-6 m2s-1, η = 1021 poise, and d = 2.9 x 106 m.  Therefore, Ra is ~ 108, so convection must occur.  Details of the convection pattern still unclear  Sources of heat  Radioactive decay (U, Th, K)  Accretional heat  Latent heat of crystallization of the inner core  Mantle is heated both from within and below.

Picturing the Mantle with Seismic Waves

Red = slow seismic velocity and high temperature

5 How tomography works

 Tomography begins with measuring seismic velocities based on travel times of earthquake waves.  From this, identify ray paths with anomalously slow or fast average velocities.  When ray paths intersect, it is possible to deduce where in the path the velocities are anomalous.

6 Geodynamics: Modeling Mantle Convection

 Problem of modeling is similar to modeling atmospheric circulation - dense (cold) areas sink and light (hot) areas rise.

What do the models tell us

 Whole mantle is probably involved in convection  The phase change at 660 km slows, but does not stop convecting material  Fully 3 dimensional, spherical model of mantle convection including plate tectonics is still not achievable with present computers.

7 Boundary Layers and Plate Tectonics

 Asthenosphere is the convecting part of the mantle.  Lithosphere is a thermal and mechanical boundary layer. It is a thermal boundary layer in that heat is conducted, not convected, through it; it is a mechanical boundary layer in that it responds to stress by brittle fracture rather than plastic flow.  Oceanic lithosphere is created as asthenosphere cools to be point that it becomes rigid. Oceanic lithosphere progressively thickens away from mid-ocean ridges. This lithosphere also contacts as it cools.

Plate Tectonics and Convection

 Plate Tectonics is the convection-driven motion of the boundary layer.  Plates are part of the convection system.  Volcanism at mid-ocean ridges releases heat to the surface.  Subduction of cool lithosphere cools the Earth’s interior.  Forces acting on plates  “Ridge push” (active upwelling pushing plates)  “Drag”: asthenosphere drags lithosphere above it.  “Slab pull”: Subducting lithosphere pulls plates away from ridges.  “Slab pull” is the most important force acting on plates.  Plates also influence motion of mantle at depth.

8 East Pacific Rise (EPR)

Melting and Volcanism

 In most places, melting occurs as a consequence of decompression of rising mantle.  In this way, volcanoes are much like clouds.  Warm air rises because of its lower density.  As it rises, it decompresses and cools.  Clouds form when air rises far enough that a phase boundary is crossed and water vapor condenses.  In a similar way, warm mantle rises because it is less dense. Eventually, it may cross a phase boundary, and partial melting occurs.

9 Decompression Melting

Mid-Ocean Ridge Basalt (MORB)

 Compositionally uniform tholeiitic basalts (tholeiites have more SiO2 than alkali basalt).  Larger extent of melting (~10%) and shallower melting than alkali basalt  Compositional variation can be related to axial depth and proximity to “hot spots”  Poor, i.e., depleted, in the incompatible elements (e.g., light rare earth depleted)  Isotopic compositions indicate long standing (~ Ga) depletion of the mantle source.  Mid-ocean ridges thus appear to sample that part of the mantle that is complimentary to the continental crust. This is presumably the upper mantle and is termed the “depleted mantle”.

10 Formation of the Oceanic Crust

 Melts of the mantle beneath mid-ocean ridges rise because they are less dense than the solid mantle.  These melts form the oceanic crust.

S-wave image of the East Pacific Rise

11 Magma Chambers of the EPR

Oceanic Crust

 Three main layers:  Layer 2A Lava flows (magma erupted on the surface)  Layer 2B Sheeted Dike Complex  (magma crystallized on way to surface)  Layer 3 Gabbro (magma that has crystallized in magma chamber).  Total thickness: ~6km.

12 Ridges and Rises

 Mid-Atlantic Ridge (MAR) has rift valley, EPR does not.  MAR has steep flanks, EPR does not.  EPR has permanent (“steady state”) magma chamber, MAR does not.  Why the difference?

Why are mid-ocean ridges ridges?

 Ridges stand above the surrounding seafloor by ~ 2 km.  The are not elevated because of a build-up of lava flows. The oceanic crust is typically 6 km thick everywhere (if anything, crust is thinner right at the axis).  Ridges and rises are elevated is because they are hot and thermally expanded.  A thought experiment:  Coefficient of thermal expansion, α, is only 10-5.  If the outer 100 km (lithospheric thickness) is 200°C hotter, then:  200˚C × 10-5 × 100 km = 2 km  After formation, the lithosphere slowly cools and thermally contracts. Consequently, the seafloor gets progressively deeper.  The cooling deepens only on time (decreases with the square root of age), not on spreading rate.

13 Ridges and Rises: the difference is spreading rate

 Graben forms on MAR because of tension  On EPR, volcanism is to frequent for a graben to develop  Difference in flank steepness is due to difference in spreading rate.  On the MAR, magma flux (and therefore heat flux) is not high enough to keep the magma chamber from freezing.

Spreading Rates and Sealevel Change

 Because faster spreading ridges remain higher further from the ridge axis, they occupy volume that would otherwise be occupied by seawater.  If global average seafloor spreading rates increase, sealevel will rise.  Seawater then spills over onto the continental margins.

14 Hydrothermal Processes

 Basalt fractures as it cools, allowing water to penetrate the young oceanic crust.  Water is heated and reacts with the oceanic crust.

15 Principle Hydrothermal Reactions  Precipitation of Anhydrite (CaSO4).  Removal of Mg from seawater, acidification: 2+  Mg + Mg2Si2O6 + 3H2O + → Mg3Si2O5(OH)4 + 2H  Reduction of sulfate: 2- 2–  SO4 + 8FeO → S + 4Fe2O3  Dissolution of Fe, Mn, Zn, Cu, etc.  Fe(solid) + 3H+ → Fe(diss) + 3H+(solid)  Precipitation of sulfides and hydroxides  Cu2+ + S2– → CuS -  Mn + 2OH → Mn(OH)2

16 Implications of Ridge Crest Hydrothermal Activity  “Buffers” composition of seawater (e.g., important ‘sink’ for Mg)  Responsible for many “base metal” (e.g., Cu, Zn, Pb) ores  Metamorphoses and “hydrates” oceanic crust  Sustains unique chemosynthetic communities.

Did life originate at hydrothermal vents?

 Energy source  Chemosynthesis is simpler than photosynthesis  Chemical raw materials  Variety of chemical raw materials  Also, variety of mineral surfaces to catalyze reactions.  Insulation from the hostile surface environment  Protection from UV radiation  Some protection meteorite, asteroid bombardment  Highly variable climate  Vent bacteria (Archaea) there are among the simplest, most primitive organisms known.