Mantle Dynamics Flashcards
In what ways does the mantle interact with the surface? (5)
- Magmatism
- Generation of the atmosphere (primordial Earth)
- Atmospheric degassing (primordial Earth)
- Recycling of the lithosphere
- Plate tectonics
How many first order seismic discontinuities divide the Earth?
- Moho. 660km. CMB.
What defines the ‘upper mantle’?
The lithosphere + asthenospheric mantle
From the Moho (40km) —> 660km discontinuity
Stepped increases in density, and velocity
What extent of partial melt occurs at the LAB?
Around 1-5%
What defines the LAB?
The low seismic velocity zone (LVZ)
High electrical conductivity
Sheared boundary of peridotite xenoliths
How deep is the LAB?
100 - 300 km beneath cratons or continents
50 - 100km beneath oceans
Unknown if under PreCambrian shields/cratons
What is the LAB/LVZ?
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).
What is the asthenosphere?
From the lithospheric base to 660km discontinuity
So around 350-600km thick
Part of the ‘convective’ mantle
How does the asthenosphere deform?
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.
What does the 660km discontinuity separate?
The upper and lower mantle
How much seismic velocity increase occurs at the 660km boundary?
Around 5 - 7%
How much density change occurs at the 660km discontinuity?
Around 5%
What creates the 660km discontinuity?
A pressure induced phase change - from spinel to perovskite and magnesiowustite.
What is the lower mantle also called?
The mesosphere
How deep can the mesosphere go?
660 - 2900km
What are characteristics of the mesosphere?
A relatively constant increase in seismic velocity and density.
What is the dominant chemical phase at the lower mantle?
Mg-perovskite - around 50%
What is the D” layer?
A boundary layer around 200 - 250km above the CMB. Here the velocity and density gradients flatten/get very steep.
What does the D” layer separate?
The lower mantle (silicate) from the liquid outer core (iron).
What is the estimated temp at the CMB?
Around 3570 K (+- 200K)
What can cause temperature variations in the D” layer?
- Cooling effective of subducting slabs
- Heat from the core
What can cause compositional changes in the D” layer?
Leaking of molten Fe from core in high P silicate phases
What can cause phase changes in the D” layer?
Breakdown of perovskite to Mg-wustite + Si
What amount of partial melt occurs at the D” layer?
Around 15% partial melting
Where does most of the partial melting occur in the D” layer?
Around 5 - 50km above the CMB.
How do we see mantle composition at the surface? (3)
Ophiolites (show about 0 - 10km)
Xenoliths (igneous, and show around 200km depth).
Basalts, gabbros (melts)
Where does basaltic magma come from?
Partial melting of lherzolite - shown by lherzolite residues in ophiolite suite.
How long is the MORB system on Earth?
Around 65,000 km
What is the typical composition of a MORB?
Olivine theoliitic with low K, and Ti. Does not usually contain more differentiated magmas like andesites or rhyolites.
How do ocean island basalts differ from MORBs?
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.
What would a MORB chemistry indicate?
Widespread source (MOR) Shallow, homogenous source reservoir in the convecting part of the mantle
What do MORBs come from?
The Depleted Mantle (DM)
Why is it called the Depleted Mantle?
It was ‘depleted’ during the formation of the continental crust.
What does ‘depleted’ mean?
It has undergone one or more episodes of melt extraction
What age might a HIMU mantle source indicate?
1.5 - 2Ga. These might indicate that they haven’t been mixed properly into the convecting mantle still.
What is a HIMU mantle source?
High u (mu) - that is, it is enriched in radiogenic Pb.
What would a HIMU source indicate?
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).
What are characteristics of the Depleted Mantle?
Depleted isotopes like Sr, Pb
High Nd144
Low large ion lithophiles (depleted)
What is Enriched Mantle (EM)?
Two different reservoir sources in the mantle that seem to contain more incompatible elements like Rb, Sm, U and Th.
What is the EM1 reservoir thought to be sourced from?
EM1 has moderate Sr and low Pb. It is thought to be old oceanic or sub continental lithosphere, maybe old pelagic sediments.
Where do EM2 sources originate from?
Subducted continental sediments. EM2 is much higher in incompatible elements than the crust.
What is the Primitive Mantle?
The estimate average composition of the crust + mantle i.e. BSE. Similar composition to chondrites.
Where is Primitive Mantle likely stored?
In the lower mantle
What do convective upwellings form?
Ridges
What do convective downwellings form?
Trenches
What are the two models of convection?
Single and two layer
