Meteorites and the Compositon of the Earth and Solar System

Many found in Antarctica, in areas where evaporation exceeds snowfall
Source of meteorites: asteroid belt, Moon, and planets; asteroids with Earth-crossing orbits = Apollo asteroids
Orbits have large eccentricities (hence Earth crossing)
NASA: "poor man's space probe"
asteroids: primordial nebular "stuff," and protoplanets (some broken up)
Perhaps due to collisions in asteroid belt (lies between Mars & Jupiter)
Age: most date about 4.6 b.y., none significantly older
A few date about 1 b.y. May be ejecta from Moon or Mars
Tektites - a few m.y. - glass formed by meteorite impact
3 types relevant to Earth composition:
1. chondrites - similar to Sun; rich in water, carbon, other "volatiles"
2. achondrites (stones) - dark, dense "silicates", magnesium, iron, silicon, oxygen, and calcium; similar to Earth's mantle
3. irons - 90% metallic iron, 10% nickle; similar to Earth's core

Composition: Undifferentiated and Differentiated

Undifferentiated

Chondrites

More abundant than all other types combined)
Contain all major elements (except volatiles like H. He, Ar, N, C, Ne, +/-0)
Enriched in Li (relative to Sun) because Sun destroys Li. (slide)
Undifferentiated and therefore unmelted, since melting & freezing partitions elements into different phases
Volatiles lost because gaseous - temps too high & gravity too low to retain them
Texture-fine matrix of nebular dust with chondrules-glassy blobs resulting from rapid heating & cooling of isolated blobs of material
Classified according to degree of metamorphism
Fe occurs as taenite, with some exsolution to kamacite
Chondrules have composition close to matrix-Ca,Al,Mg,Ti,Si, etc.
Minerals are olivine, plagioclase, etc.
Chondrules are probably primary solar system condensate. They've been reheated, acquired magnetism, then aggregated. Hence, no coherent magnetization

Carbonaceous Chondrites

Slightly less metamorphosed than even chondrites.
In same abundances as Sun, and presumably the primordial nebula from which solar system formed.
Achondrites are depleted in volatile minor and trace elements to varying degrees.
Chondrite textures coarsen with increasing metamorphism.
Contain carbon, water, other volatile elements
Fe occurs as magnetite, Fe₃0₄
Carbonaceous chondrite meteorites are rare, but due to fragility: as many asteroids have reflective spectra similar to carbonaceous chondrites as other types

Differentiated Meteorites

Achondrites (stones)

Ultramafic igneous (silicate) rocks similar to mantle composition.
Depleted in volatiles, silica, iron and nickle

Irons

Fe-Ni mixture (90/10%)
Some sulfides, graphite, silicate inclusions.
Similar to that proposed for core

Stony-irons

Achondrites with Fe-Ni inclusions

Killer Meteorite!!

This fragment of the Nakhla, Egypt achondritic meteorite (an igneous rock), consists entirely of silicate minerals; no metal alloys are present. Nakhla is the only known meteorite to have struck and killed a living creature - a dog - when it fell.

Homogeneous Accretion Model

Material with carbonaceous chondrite composition is heated, melted, and fractionated.
Fe-Ni settles to core of body (gravity)
Volatiles are degassed
Si is concentrated in a "crust"
Leaving mantle-like achondrite region.

Heterogeneous Accretion Model

Fe-Ni and other refractory elements condense first.
If accretion times are rapid compared to condensation times, an Fe-Ni core accretes first, followed by silicate mantle.

Estimating Size of Parent Body for Differentiated Meteorites: Widmanstatten Structures (Patterns)

Iron meteorites are 90% Fe, 10% Ni.
As it cools, first forms alloy taenite (about 10% Ni)
With further cooling {about 690 ^oC) kamacite exsolves (about 5.5% Ni) and Ni content of taenite necessarily increases (phase diagram of taenite/kamacite)
Ni continues to diffuse from kamacite to taenite, but diffusivity of taenite decreases with temperature more than in kamacite, so Ni builds up on edges of taenite
Width of diffusion zone indicates slow cooling (150 to 6000 ^oC/m.y.) between 650 and 350 ^oC
Slow cooling indicates parent bodies of a few km radius at least

Conclusions Relative to Earth

Solar system formed from material with solar elemental composition (carbonaceous chondrite composition)
In the process of planetary formation, and during subsequent time (probably high temperature), Earth lost its volatiles (He, H, probably never incorporated to any large extent), and differentiated into core (Fe, Ni) and mantle (Fe, Mg rich silicates)
(Plate) tectonic processes have built Si, O, Na, K rich crust, further depleting mantle in those elements.

Origin of the Solar System: Nebular theory (Kant, LaPlace)

By the 18th century, astronomer/mathematician/physicist Pierre LaPlace could explain nature as a self-sufficient mechanism. He supposedly told Emperor Napolean that, as for God, "I have no need of that hypothesis!"
gas cloud: 80% H, 15% He (traces of Fe, Si, O, Al, ...) from previous star(s)
gravity caused contraction
rotation increased, gravity plus centrifugal force produced disk
temperature increased; hottest near center
planets accreted in local eddies and mass concentrations
- only Fe, Si, Al, Mg (O) have high enough boiling points to not vaporize near Sun
- small planets (small gravity) couldn't keep H, He
- away from sun, fewer elements vaporized (lower T); large planets, large gravity, keep H, He
Sun "turns on" (T-Tauri stage); fusion starts; gasses blown out of solar system
all this occurred in only a few millions (thousands) years, at 4.6 Ga
remaining debris produces many impacts for first few hundreds of millions of years

The Story of a Meteorite

Some History of Titius-Bode Law