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Plate Tectonics > Mantle Dynamics > Flashcards

Mantle Dynamics Flashcards

1
Q

In what ways does the mantle interact with the surface? (5)

A
  1. Magmatism
  2. Generation of the atmosphere (primordial Earth)
  3. Atmospheric degassing (primordial Earth)
  4. Recycling of the lithosphere
  5. Plate tectonics
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2
Q

How many first order seismic discontinuities divide the Earth?

A
  1. Moho. 660km. CMB.
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3
Q

What defines the ‘upper mantle’?

A

The lithosphere + asthenospheric mantle
From the Moho (40km) —> 660km discontinuity
Stepped increases in density, and velocity

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4
Q

What extent of partial melt occurs at the LAB?

A

Around 1-5%

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5
Q

What defines the LAB?

A

The low seismic velocity zone (LVZ)
High electrical conductivity
Sheared boundary of peridotite xenoliths

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6
Q

How deep is the LAB?

A

100 - 300 km beneath cratons or continents
50 - 100km beneath oceans
Unknown if under PreCambrian shields/cratons

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7
Q

What is the LAB/LVZ?

A

A zone of partial melt (this is indicated by a change in S wave velocity). It separates the depleted lithosphere from the hydrated asthenosphere (which is more fertile).

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8
Q

What is the asthenosphere?

A

From the lithospheric base to 660km discontinuity
So around 350-600km thick
Part of the ‘convective’ mantle

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9
Q

How does the asthenosphere deform?

A

Creep. Creep is the tendency of solid material to move slowly or deform permanently. This occurs if a high level of stress has been applied for a very long time, but remains below the yield strength.

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10
Q

What does the 660km discontinuity separate?

A

The upper and lower mantle

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11
Q

How much seismic velocity increase occurs at the 660km boundary?

A

Around 5 - 7%

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12
Q

How much density change occurs at the 660km discontinuity?

A

Around 5%

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13
Q

What creates the 660km discontinuity?

A

A pressure induced phase change - from spinel to perovskite and magnesiowustite.

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14
Q

What is the lower mantle also called?

A

The mesosphere

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15
Q

How deep can the mesosphere go?

A

660 - 2900km

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16
Q

What are characteristics of the mesosphere?

A

A relatively constant increase in seismic velocity and density.

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17
Q

What is the dominant chemical phase at the lower mantle?

A

Mg-perovskite - around 50%

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18
Q

What is the D” layer?

A

A boundary layer around 200 - 250km above the CMB. Here the velocity and density gradients flatten/get very steep.

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19
Q

What does the D” layer separate?

A

The lower mantle (silicate) from the liquid outer core (iron).

20
Q

What is the estimated temp at the CMB?

A

Around 3570 K (+- 200K)

21
Q

What can cause temperature variations in the D” layer?

A
  • Cooling effective of subducting slabs

- Heat from the core

22
Q

What can cause compositional changes in the D” layer?

A

Leaking of molten Fe from core in high P silicate phases

23
Q

What can cause phase changes in the D” layer?

A

Breakdown of perovskite to Mg-wustite + Si

24
Q

What amount of partial melt occurs at the D” layer?

A

Around 15% partial melting

25
Q

Where does most of the partial melting occur in the D” layer?

A

Around 5 - 50km above the CMB.

26
Q

How do we see mantle composition at the surface? (3)

A

Ophiolites (show about 0 - 10km)
Xenoliths (igneous, and show around 200km depth).
Basalts, gabbros (melts)

27
Q

Where does basaltic magma come from?

A

Partial melting of lherzolite - shown by lherzolite residues in ophiolite suite.

28
Q

How long is the MORB system on Earth?

A

Around 65,000 km

29
Q

What is the typical composition of a MORB?

A

Olivine theoliitic with low K, and Ti. Does not usually contain more differentiated magmas like andesites or rhyolites.

30
Q

How do ocean island basalts differ from MORBs?

A

They are more enriched in HFSEs, more LREE
Also isotopically distinct - more Nd than Sm, and more Rb than Sr (remember isotopes aren’t affected by fractionation)
OIBs indicative of some sort of stratification of mantle composition.

31
Q

What would a MORB chemistry indicate?

A 
Widespread source (MOR)
Shallow, homogenous source reservoir in the convecting part of the mantle
32
Q

What do MORBs come from?

A

The Depleted Mantle (DM)

33
Q

Why is it called the Depleted Mantle?

A

It was ‘depleted’ during the formation of the continental crust.

34
Q

What does ‘depleted’ mean?

A

It has undergone one or more episodes of melt extraction

35
Q

What age might a HIMU mantle source indicate?

A

1.5 - 2Ga. These might indicate that they haven’t been mixed properly into the convecting mantle still.

36
Q

What is a HIMU mantle source?

A

High u (mu) - that is, it is enriched in radiogenic Pb.

37
Q

What would a HIMU source indicate?

A

Residual from a subducting slab - the rest of the Pb would be lost in the crust. High levels of Nb and Ta also indicate this (both are lithophiles that do not fractionate).

38
Q

What are characteristics of the Depleted Mantle?

A

Depleted isotopes like Sr, Pb
High Nd144
Low large ion lithophiles (depleted)

39
Q

What is Enriched Mantle (EM)?

A

Two different reservoir sources in the mantle that seem to contain more incompatible elements like Rb, Sm, U and Th.

40
Q

What is the EM1 reservoir thought to be sourced from?

A

EM1 has moderate Sr and low Pb. It is thought to be old oceanic or sub continental lithosphere, maybe old pelagic sediments.

41
Q

Where do EM2 sources originate from?

A

Subducted continental sediments. EM2 is much higher in incompatible elements than the crust.

42
Q

What is the Primitive Mantle?

A

The estimate average composition of the crust + mantle i.e. BSE. Similar composition to chondrites.

43
Q

Where is Primitive Mantle likely stored?

A

In the lower mantle

44
Q

What do convective upwellings form?

A

Ridges

45
Q

What do convective downwellings form?

A

Trenches

46
Q

What are the two models of convection?

A

Single and two layer

Plate Tectonics (10 decks)

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