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3-Observational evidence for relative abundances (meteorites) (Facts…
3-Observational evidence for relative abundances (meteorites)
Definitions
Metoeroid
before entering atmosphere
Meteor
Becomes incandescent in atmosphere (friction)
Meteorite
lands on earth
Facts
79000 tonnes on earth including dust
only 1 per 10^6 km^2 per year has mass >500g
Lots are found in antarctica
Allende meteorite
microscopic diamonds from before ss, may be from supernova
unequilibrated
chondrules
grains of minerals with different chemical histories
likely formed in high temp environment, no equilibrium with residual gas
isotopic anomalies potentially from nearby Sne
Family tree of meteorites
Iron
Hexahedrites
<6% Ni
Octahedrites
6-15% Ni
Ataxites
16-20% Ni
Also known as Siderites
Mainly Fe and Ni
<1% pop, 6% falls, 54% finds
Stony-Iron
Mesosiderites
Asteroid collision?
Pallasites
Asteroid core mantle boundary
<1% pop 2% falls 6% finds
Stony
5% pop 92% falls 40 % finds
Achondrites
Rarer
8% of falls
Mostly asteroidal, but some moon or mars
Made from rock crystalised from a molten stared
Lack chondrules
Chmeically similar to basalts
differentiated due to melted
silicate rich
Chondrites
85% of known falls
sources that have never undergone differentiation
almost all contain chondrules
glass spheroids of once molten silicates
with the exceptions of volotiles have composition closely matching that of the origional solar nebula
Carbonaceous contain carbon, evidence of h20 and sometimes volatiles
Cometary
Not meteorites
95% of meteor pop
frozen methane, ammonia, water etc
Why meteorites are important
all dates within a 16 Myr interval of the age of the ss
Evidence for water and amino acids
undifferentiated have elemental abundances that best represent start of ss
Radioactivity
Can be used to date material (e.g. carbon dating
3 types of radioactivity
decays with emission of nucleons
modes of beta decay
state changes of nucleus
random and unpredictable decay
Case study:meterorite
all chondrites have similar composition
unknown number of parent and daughter isotopes at time of meteorites formation
Normalise against stable isotope