Subduction and Implications for Convection Flashcards
Upper mantle or whole mantle convection?
- Geochemical evidence for upper mantle
- Slab seismic tomography to the 660km discontinuity or beyond?
- Catastrophic overturn
EQ’s are limited to what depths?
Above 660km
Layered mantle hypothesis
- Convection cells are separated by the transition zone into upper and lower mantle
- Subducted slabs don’t pass through the 660km phase transition
Whole Mantle hypothesis
- Convection cells circulate through upper and lower mantle
- Subducted slabs may plunge down to the ‘D’-layer above the core-mantle boundary
Geochemical evidence for upper mantle circulation
- Incompatible elements (eg neodymium) don’t fit easily into crystal structure of mantle minerals (first to escape melts)
- Argon-40
- Seismic Tomopgraphy
Geochemical evidence, Incompatible elements
- Incompatible elements (eg neodymium) don’t fit easily into crystal structure of mantle minerals (first to escape melts)
- Upper mantle depleted in elements over time relative to lower primitive mantle
- Old continental crust enriched, measure enrichment, estimate how much mantle must be depleted
- Ex. Neodymium, depleted mantle volume is 25-50 percent of total, i.e. lower 50-75 percent of mantle is relatively pristine, therefore minimal mixing
Geochemical evidence, Argon-40
- Radioactive decay of K-40
- Calc K-40 content of Earth
- How much produced in Earth History (approx 150x10^18g)
- Current mass balance, approx. 50 percent of Earth’s Ar-40 in atmosphere/cont. crust, 50 percent must be in bulk of earth
- But MOR ridge basalts imply relatively low Ar-40 in upper mantle, remainder is likely in lower mantle (chemically isolated)
Seismic tomography
S-wave velocities
- Density increase, temp decrease, High velocity
- Convection driven by lateral differences in density and T (positive vs. Neg buoyancy)
- Greatest variations at top and bottom of mantle
Seismic tomography, <400km
- Related to surface tectonics
- Higher velocities beneath old, cold, continents
- Lower velocities beneath ocean ridges, rifts, backarc basins
Seismic tomography, Lower Mantle (greater than 1400km)
- Small variations, but ring of higher velocities around Pacific, especially large near base of mantle
Seismic tomography, X-section at equator through Earth from crust to CMB
- Low velocity in lowermost mantle beneath S. Africa and Central Pacific
- High velocity from crust to CMB beneath S. America and Indonesia
- Implicates high velocity cold regions associated w/ descending slabs, extend to near base of the mantle
Possible relationship of subduction zones, superswells, plumes and MOR to large-scale mantle convection
- High velocity from crust to CMB w/ Sub zones and superswells
- Low velocity under MOR’s
- Transition across Earth’s surface w/ alternating MOR, Subzone/Superswell
- Superswells on either side of planet opposite each other, same w/ sub zones and MOR’s
- Convection between these
Slab stagnation in mantle transition zone
- High velocity slab follows benioff-wadati zone
- Slab flattens sharply above 660km, extending greater than 100km
- Flattened portion thickens vertically downward
Implication of 660km discontinuity (seismic tomography)
- Partial slab barrier
- Stagnant material piles up at discontinuity
- Eventually sinks into lower mantle (due to gravitational forces
- Stronger for W. Pacific Arcs (Kuril, Japan, Izu-Bonin)
- Eg. Roll back of Izu-Bonin trench, horizontal elongated section approx. 2000km
What happens to the slab through 660km discontinuity?
- Severe distortion of slab
Rollback
- Slab elongated at 660km
- Slab held up at 660km
- Rollback stops after slab sinks into lower mantle, then upwelling
What happens when the slab mets boundary layer at 660km discontinuity?
- Buckles, thickens plastically, forms ‘megalith’
- Neutrally buoyant
- ‘floats’ in isostatic equilibrium
- More cold material accumulates and sinks due to greater density, negative buoyancy (slab avalanche)
- Therefore mature lithosphere may penetrate discontinuity but young may be stopped
Slab avalanche
- Accumulation of cool material causes negative buoyancy and greater density
- Causes sinking of mature lithosphere past 660km discontinuity
- Therefore young lithosphere may be stopped at discontinuity but mature may penetrate to exchange material btwn upper and lower mantle
Young vs. mature lithosphere at 660km discontinuity?
- Young may be stopped
- Mature may sink in slab avalanche due to density and negative buoyancy
MOMO
Mantle Overturn and Major Orogenies
- Mantle plumes and episodic crustal growth
- Episodic whole mantle convection
2-layer convection alternates with what?
- Episodes of whole-mantle convection
- MOMO
Episodic whole mantle convection
- MOMO
- Large mantle plume - flood basalts (e.g. cretaceous)
- Replenish incompatible elements in upper mantle
- Rapid growth of continental crust
- slab avalanche causes corresponding mantle plume?
4 steps of Normal Layered convection alternate w/ catastrophic mantle overturn
Cold blue fluids:
1- Accumulate until negative buoyancy overcomes 660km discontinuity
2- Catastrophically descend to CMB at >50cm/yr
3- Lower hot fluid moves upwards, whole mantle convection
4- After cold fluid descends (10Ma), spreads across base CMB, 2-layer convection begins again
