Bulk Earth, Core and Bulk Silicate Earth

Question 1

what is the main compositional difference between the bulk Earth and the solar system?

Answer 1

Earth’s depletion in volatile elements

Question 2

which four elements dominate bulk Earth composition?

Answer 2

O, Mg, Si and Fe

Question 3

why does the chemical composition of the bulk earth have a relatively large uncertainty of +-2%?

Answer 3

due to our limited understanding of the composition of the core which cannot be samples, whereas the silicate portion is relatively well defined.

Question 4

what are Earth’s main geochemical reservoirs and give approximate percentages

Answer 4

Core ~32% of mass
Mantle ~67% of mass
Continental crust ~0.4% of mass
Oceanic crust ~0.1% of mass

Question 5

why is the oceanic crust called a transient geochemical reservoir?

Answer 5

because the oceanic crust is recycled with an average life-span of about 50-100Myr. it is formed from the mantle by melting and then mixed back in, following subduction. hence, it is often ignored and included as part of the mantle.

Question 6

how do we know earths outer core is liquid?

Answer 6

it has low p-wave velocities and does not transmit s-waves

Question 7

is earths core pure Fe-Ni alloy? how do we know?

Answer 7

no. we know because of seismic wave speeds and earths moment of inertia - earth is not dense enough.

Question 8

what other elements are contained in earths core?

Answer 8

Fe-Ni and several unidentified light elements (H, C, O, Si, S)

Question 9

how are researchers trying to solve the light element question

Answer 9

by researching the partitioning of elements into Fe-Ni metal at high temperature and pressure

Question 10

Is the BSE almost compositionally identifical to the primitive mantle (PRIMA)

Answer 10

yes

Question 11

What does PRIMA stand for

Answer 11

Primitive mantle

Question 12

What are the two main reasons for the BSE differing from the solar system average composition?

Answer 12

1. condensation in a hot solar nebula and depletion of volatile elements (across all terrestrial planets)
2. core formation and associated depletion of siderophile elements

Question 13

which parts of the earth does the BSE comprise?

Answer 13

the modern mantle and continental crust

Question 14

what are some important features of the composition of the BSE?

Answer 14

1. enrichment if the refractory lithophile elements
2. depletion of the moderately siderophile elements
3. depletion of the highly siderophile elements (HSE)
4. depletion of the volatile elements

Question 15

metal silicate (mantle-core) segregation took place at high temperatures and pressures near the base of a 500-1000km deep magma ocean. how do we know?

Answer 15

from the good match between observed concentrations and Dmet/sil values determined at high pressure and temperature.

Question 16

what happened to initiate large scale melting necessary for the formation of a terrestrial magma chamber?

Answer 16

the moon-forming giant impact - a large and energetic collision dominating the later stages of earths protracted accretion

Question 17

the highly siderophile elements (HSE) include Re, Au and the platinum group elements which have very high but variable Dmet/sil values. therefore we would expect the HSE concentration in the mantle to be low and variable. but we dont - HSE concentrations in the mantle are relatively uniform and “high”. why?

Answer 17

the late veneer hypothesis - small amounts of material akin to chondritic meteorites added HSE to the mantle following cessation of metal segregation in the core.

Question 18

why is the BSE depleted in volatile elements?

Answer 18

due to the high temperatures in the inner regions of the solar nebula prior to and during the accretion of Earth. the high T prevented the condensation of volatile species such that they were either accreted into the Sun or blown off by the energetic early Sun.

Question 19

why are the highly volatile elements more strongly depleted in the BSE than the moderately volatile elements?

Answer 19

highly volatile elements have much lower condensation temperatures (<650C) than the moderately volatile. condensation is required for the material to remain in the inner solar system before it is slept away. depletion factor increases with decreasing condensation temperatures.

Question 20

the volatile element signature of the BSE argues for heterogenous accretion (as opposed to homogenous accretion). what does this mean?

Answer 20

earth had a major compositional change during the accretion process as suggested by the BSE volatile signature. where 10-15% of volatile rich CI material was added late to earth.

Question 21

which elements are