Chapter 9 Meteorites a record of formation Flashcards
What is a Meteorite Fall?
This is when a meteorite is observed falling through the atmosphere and collected soon after.
What is a Meteorite Find?
These are meteorites that are discovered without having been observed fall. They are typically far older than finds.
What is a meteoroid?
This is typically a body in space smaller than 100m in diameter.
What is a meteor?
This is the name given to a small body as it passed through the Earths atmosphere.
What is a Meteorite?
This is what a meteor is called once it has made contact with the earths surface.
What happens to a meteor as it passes through the atmosphere?
The meteor is heated by the compressed air in front of it, which results in a glassy fusion crust. Molten material is stripped away from the surface by a process called ablation.
What are the 3 types of meteorites?
stones (or stony meteorites) are composed mostly of silicate materials.
irons (or iron meteorites) are predominantly composed of iron and nickel.
Stony-iron meteorites are mixtures of metals and silicates.
What are chondrites?
These are a subdivision of stones. they contain chondrules which are small spherules of silicate material (olivine, pyroxine, glass) which experienced melting before being incorporated into the meteorite.
What are ordinary chondrites?
These are the most common meteorites. They contain varying amounts of iron and nickel have have chondrules present.
How are chondrules formed?
They are suspected to be the result of rapid heating and cooling of material from impact events. Their spherical shape shows they cooled quickly in low gravity.
What are carbonaceous chondrites?
These are carbon rich meteorites that contain organic compounds such as amino acids. Overall they have a primitive chemical composition.
CI carbonaceous chondrites have composition very similar to that of the sun.
What are achondrites?
These are a sub-division of stony meteorites that do not contain any chondrules. They have generally been heated to their melting temperatures and formed by the crystallisation of magmas on their respective parent bodies.
What is chondritic composition?
This refers to the relative abundances of elements found in carbonaceous chondrites.
What are calcium-aluminium rich inclusions (CAIs)?
These are small inclusions in meteorites that contain some of the first minerals to form in the cooling of the solar nebula.
They may also be the evaporation residues (aggregations of materials that have lost all traces of volatile materials through evaporation).
How do you measure the age of a meteorite?
Radiometric dating techniques can be used to age meteorites. By looking for the abundance of elements that are the product of radioactive decay, you are able to gather a rough age of the subject.
Series of events to form meteorites
1- the accretion of calcium-aluminium-rich inclusions into small planetesimals.
2- onset of melting of planetesimals.
3- collisions of small planetesimals forming chondrules and releasing primitive melts.
4- accretion of 100 km - sized planetesimals.
5- melting of planetesimals, production of basaltic melts, differentiation of bodies.
(Pg 339)
What is differential isotope composition? (equation)
This is a method that is used to compare the ratio of isotopes in a sample to the ratio of isotopes in a standard.
δ (in ppmil) = ((Rs / Rr) - 1) x 1000
Where Rs is the isotope ratio in the sample and Rr is the isotope ratio in the standard.
(pg 342)
What is differential isotope composition? (equation)
This is a method that is used to compare the ratio of isotopes in a sample to the ratio of isotopes in a standard.
δ (in ppmil) = ((Rs / Rr) - 1) x 1000
Where Rs is the isotope ratio in the sample and Rr is the isotope ratio in the standard.
(pg 342)
What do iron meteorites represent?
Iron meteorites represent the cores of asteroids that underwent complete differentiation. This means at some stage they were entirely Magma.
Asteroid heating could have occurred due to the decay of short lived radioactive isotopes such as Al26, or due to being heated by the plasma from sola intense solar winds from T Tauri phase sun.
Also possibly through the electromagnetic induction heating through impact events.
What are Widmanstätten patterns?
When on iron-nickel meteorite is sawn open, polished and etched with a mild acid a distinctive pattern is seen. This is caused by the segregation of nickel rich and nickel poor alloys during extremely slow cooling.
Cooling rates can be calculated from different scales of patterns.
Where do meteorites come from?
They come from asteroids the moon or mars and small micrometeorites (dust particles) can come from comets.
Why are carbonaceous chondrites referred to as cosmic sediments?
This is because they represent a collection of materials formed in different thermal regimes, which settled out of the solar nebular and settled onto the surface of parent bodies.
What does terrestrial age mean and now is it measured?
Terrestrial age refers to how long a meteorite has been on earth.
If it is found in a geological structure it can be aged from the rocks around it. If it is more recent then its age can be estimated by measuring the abundance of specific short lived radionuclides. The nuclides in question are formed when the body is out in space via the interaction with high energy particles.
As soon as a body enters the atmosphere the radionuclides start to day and are no longer replenished. By knowing their halflives an age can be estimated.
What is chondrite normalized data?
When comparing the relative concentrations of elements between 2 samples, you can normalise the data by dividing the relative concentration of an element in one sample by the relative concentration of the some element in the second sample.
Conc in rock a compared to conc in rock b = (conc rock a) / (conc in rock b)
If the data is chondrite normalised, them the sample has been divided by the relative concentration of the element found in a chondrite (chondrite is rock b).
