Subduction and Implications for Convection

Question 1

Upper mantle or whole mantle convection?

Answer 1

• Geochemical evidence for upper mantle
• Slab seismic tomography to the 660km discontinuity or beyond?
• Catastrophic overturn

Question 2

EQ’s are limited to what depths?

Answer 2

Above 660km

Question 3

Layered mantle hypothesis

Answer 3

• Convection cells are separated by the transition zone into upper and lower mantle
• Subducted slabs don’t pass through the 660km phase transition

Question 4

Whole Mantle hypothesis

Answer 4

• Convection cells circulate through upper and lower mantle

- Subducted slabs may plunge down to the ‘D’-layer above the core-mantle boundary

Question 5

Geochemical evidence for upper mantle circulation

Answer 5

• Incompatible elements (eg neodymium) don’t fit easily into crystal structure of mantle minerals (first to escape melts)
• Argon-40
• Seismic Tomopgraphy

Question 6

Geochemical evidence, Incompatible elements

Answer 6

• 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

Question 7

Geochemical evidence, Argon-40

Answer 7

• 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)

Question 8

Seismic tomography

Answer 8

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

Question 9

Seismic tomography, <400km

Answer 9

• Related to surface tectonics
• Higher velocities beneath old, cold, continents
• Lower velocities beneath ocean ridges, rifts, backarc basins

Question 10

Seismic tomography, Lower Mantle (greater than 1400km)

Answer 10

• Small variations, but ring of higher velocities around Pacific, especially large near base of mantle

Question 11

Seismic tomography, X-section at equator through Earth from crust to CMB

Answer 11

• 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

Question 12

Possible relationship of subduction zones, superswells, plumes and MOR to large-scale mantle convection

Answer 12

• 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

Question 13

Slab stagnation in mantle transition zone

Answer 13

• High velocity slab follows benioff-wadati zone
• Slab flattens sharply above 660km, extending greater than 100km
• Flattened portion thickens vertically downward

Question 14

Implication of 660km discontinuity (seismic tomography)

Answer 14

• 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

Question 15

What happens to the slab through 660km discontinuity?

Answer 15

• Severe distortion of slab

Question 16

Rollback

Answer 16

• Slab elongated at 660km
• Slab held up at 660km
• Rollback stops after slab sinks into lower mantle, then upwelling

Question 17

What happens when the slab mets boundary layer at 660km discontinuity?

Answer 17

• 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

Question 18

Slab avalanche

Answer 18

• 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

Question 19

Young vs. mature lithosphere at 660km discontinuity?

Answer 19

• Young may be stopped

- Mature may sink in slab avalanche due to density and negative buoyancy

Question 20

MOMO

Answer 20

Mantle Overturn and Major Orogenies

• Mantle plumes and episodic crustal growth
• Episodic whole mantle convection

Question 21

2-layer convection alternates with what?

Answer 21

• Episodes of whole-mantle convection

- MOMO

Question 22

Episodic whole mantle convection

Answer 22

• 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?

Question 23

4 steps of Normal Layered convection alternate w/ catastrophic mantle overturn

Answer 23

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