Bulk Earth, Core and Bulk Silicate Earth Flashcards
what is the main compositional difference between the bulk Earth and the solar system?
Earth’s depletion in volatile elements
which four elements dominate bulk Earth composition?
O, Mg, Si and Fe
why does the chemical composition of the bulk earth have a relatively large uncertainty of +-2%?
due to our limited understanding of the composition of the core which cannot be samples, whereas the silicate portion is relatively well defined.
what are Earth’s main geochemical reservoirs and give approximate percentages
Core ~32% of mass
Mantle ~67% of mass
Continental crust ~0.4% of mass
Oceanic crust ~0.1% of mass
why is the oceanic crust called a transient geochemical reservoir?
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.
how do we know earths outer core is liquid?
it has low p-wave velocities and does not transmit s-waves
is earths core pure Fe-Ni alloy? how do we know?
no. we know because of seismic wave speeds and earths moment of inertia - earth is not dense enough.
what other elements are contained in earths core?
Fe-Ni and several unidentified light elements (H, C, O, Si, S)
how are researchers trying to solve the light element question
by researching the partitioning of elements into Fe-Ni metal at high temperature and pressure
Is the BSE almost compositionally identifical to the primitive mantle (PRIMA)
yes
What does PRIMA stand for
Primitive mantle
What are the two main reasons for the BSE differing from the solar system average composition?
- condensation in a hot solar nebula and depletion of volatile elements (across all terrestrial planets)
- core formation and associated depletion of siderophile elements
which parts of the earth does the BSE comprise?
the modern mantle and continental crust
what are some important features of the composition of the BSE?
- enrichment if the refractory lithophile elements
- depletion of the moderately siderophile elements
- depletion of the highly siderophile elements (HSE)
- depletion of the volatile elements
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?
from the good match between observed concentrations and Dmet/sil values determined at high pressure and temperature.
what happened to initiate large scale melting necessary for the formation of a terrestrial magma chamber?
the moon-forming giant impact - a large and energetic collision dominating the later stages of earths protracted accretion
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?
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.
why is the BSE depleted in volatile elements?
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.
why are the highly volatile elements more strongly depleted in the BSE than the moderately volatile elements?
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.
the volatile element signature of the BSE argues for heterogenous accretion (as opposed to homogenous accretion). what does this mean?
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.
which elements are
