Cosmochemistry

Details

1. Origin of the elements and Earth

Big Bang (~15 Ga)

Solar system (~ 4.56 Ga)

The first 100,000 years

The next 10 million years

BOTTOM LINE: the Earth's composition is a product of its accretion history. However, as the process of chemical differentiation was not perfectly efficient, the Earth contains some of every stable element (not just those elements that were condensable at our distance from the sun). That said, only 7 elements comprise 97% of Earth: O (50.7%), Mg (15.3%), Fe (15.2%), Si (14.4%), S (3.0%), Al (1.4%), Ca (1.0%), consistent with solar abundances and condensates anticipated for Earth's position.

2. Differentiation of the Earth

Goldschmidt (1937) proposed that Earth's elements separate into different phases this concept gave rise to the terms:

lithophile ("stone-loving") elements form the light silicate phases

chalcophile ("copper-loving") elements form an intermediate sulfur phase

siderophile ("iron-loving") elements form a dense metallic phase

These three layers do not correspond to the three layers of the Earth. The core is siderophile, but chalcophile component likely dissolved in siderophile core and was never a separate phase.  Mantle is the lithophile phase; Earth's crust had not yet formed.

Most common lithophile components of early Earth (and mantle):

olivine (Mg,Fe)₂SiO₄

orthopyroxene (Mg, Fe)SiO₃

clinopyroxene Ca(Mg,Fe)Si₂O₆

3. How do we know this?

gravitational - use to calculate Earth's mass (average density), which is 5.52 g/cm³. Density of surface rocks rarely > 3, therefore the Earth must contain a large proportion of very dense material.

nebular composition -Together H & He comprise > 99% of all atoms in solar system. Decrease in abundance with increasing Z reflects increasing synthesis difficulty. Also evident is

(1) relatively low abundance of some elements - Li, Be, B, Sc – consequence of formation only by spallation by cosmic rays, supernova explosions and because of their consumption in subsequent fusion processes, and

(2) "sawtooth" pattern – "Oddo-Harkins rule", which says that atoms with even numbers are more stable because their nuclei are more tightly bound.

(3) Fe is particularly stable because its nucleus is tightly bound

Note abundance of Fe (plus Mg, Ni) in solar system relative to Earth's crust; used to infer that these components must constitute much of the Earth's core. Fe is also dense enough to satisfy density requirement.

seismic studies -velocities of P and S waves in various materials can be measured and compared with known seismic velocities. Reflection and refraction of seismic waves at discontinuities provides direct evidence for layered structure, while absence of shear wave transmission indicates the liquid nature of the outer core.

mantle rocks -ophiolites, xenoliths

3. Meteorites

Solid extraterrestrial objects that strike the Earth; many are likely fragments derived from collisions of larger bodies (particularly asteroid belt between Mars and Jupiter). Believed to represent early stages in development of the solar nebula, and thus provide information about state of early solar system.

Classification:

irons -composed primarily of Fe-Ni alloy (probably fragments of the core of some terrestrial planet; contain both siderophile - Fe-Ni alloy – and chalcophile – FeS troilite – phases)

stones -composed of silicates (can be difficult to tell from terrestrial rocks, but comprise about 94% of meteorites)

chondrites - contain chondrules (spherical silicate inclusions that appear to have formed as droplets of glass); considered to be "undifferentiated" meteorites – most primitive

achondrites -do not contain chondrules

stony-irons - contain subequal amounts of each

Chondrite Earth Model (C - average composition assumed to represent original composition of the Earth.EM) However, Earth is denser, and has a higher Fe/Si ratio, than provided by CEM.