Part 3- The Earth and its Reservoirs Flashcards
How are relative abundances of elements in the Sun measured?
Measuring the intensity of certain wavelengths of light emitted by elements in the photosphere and by looking at meteorites
How much of the mass of the solar system does the sun account for?
> 99%
What name is given to the group of meteorites that have relative elemental abundances almost identical to the photosphere?
Carbonaceous Chondrites
Particularly the most primitive- CI carbonaceous chondrites
What are CI carbonaceous chondrites used for?
To define the average composition of the solar system.
What abundance of O, Fe, Si and Mg does the earth have?
Chondritic relative abundances
Where does most of the Fe occur on earth?
In the core (in reduced form)
Where does most of the O, Si and Mg occur?
In the silicate mantle
What group of elements is the silicate portion of the earth highly depleted in?
Volatile elements
What does the volatile and refractory classification scheme describe?
At which temperatures elements condense to form dust grains at the conditions of the solar nebular.
What temperatures do refractory elements condense at?
Very high (>1400K)
Give an example of a common refractory element.
Fe, Mg, Si
What condensation temperatures do moderately volatile elements have?
1250 to 650 K
What condensation temperatures do highly volatile elements have?
<650K
Give an example of a volatile element.
Na, Zn, Ar, Ne
Why are the terrestrial planets of the inner solar system strongly depleted in volatile elements?
Too hot at time of formation.
What are the Earth’s 3 major geochemical reservoirs?
The core (1/3 of the mass of the earth), the mantle and the continental crust.
What does the Bulk Silicate Earth (BSE) describe?
BSE = Mantle + Continental Crust
What is the composition of the BSE identical to?
The primitive mantle (Earth’s mantle before the formation of the CC).
What is ~95% of the Earth’s core formed of? What is the other ~5%?
Iron and Nickel.
Unidentified light elements- probably O, Si, S or C.
Why does the composition of the BSE differ form the Solar System average?
- Volatile depletion in the solar nebular.
* Depletion of siderophile elements in the silicate Earth by core formation
Which groups have the largest depletions in the BSE?
Most volatile and most siderophile.
Which groups are enriched in the BSE?
Why?
Refractory and lithophile.
As a consequence of mass balance- depletion of others.
How does the continental crust differ from the mantle (2)?
CC is low in Mg.
CC is high in Ca, Al, Na and K.
Mantle is opposite.
Where else are the elements Ca, Al, Na and K enriched?
Why?
In basalts.
They partition preferentially into mantle melts.
What can the depleted (current) mantle be regarded as?
A residue of partial melting.
What rock does the CC have an average composition akin to?
Andesites
How do scientists know about the composition of the mantle (2)?
- Direct samples form the upper mantle- alpine peridotite massifs and xenoliths.
- Velocities of seismic waves through the mantle.
What is the relative composition of the mantle?
Relatively homogeneous ultramafic composition.
What is the structure of the mantle?
Exclusively solid with slow convection.
Whole mantle convection is accepted as opposed to layered convection.
