Iron Meteorites
Introduction to the wonderful world
of Iron Meteorites
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What do we know?
Earth’s core is made of Fe
 So asteroids must have formed cores
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 Indirect
evidence: HED meteorites - no metal,
but…segregated?
 Meteorites that contain no metal but show
evidence of presence of metal
 Relationship between achondrites and iron
meteorites
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Differentiated asteroids also have achondrites
“-Philes”
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Various elements like to partition
themselves:
Siderophiles – affinity for metal
 Chalcophile – affinity for sulfides
 Lithophile – rock (silicate o oxide)
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I <3 metal
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Core formation scavenges siderophile
elements
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IE: Iron, nickel, platinum, cobalt etc
Achondritic meteorites derived by partial
melting must be depleted of the
siderophile elements in mantle
 Siderophile elements make up most of iron
meteorites
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Crystallization of Irons
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Liquid film as opposed to
molten beads sinking
Evidence: loss of metal
sulfides
Process:
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Forms kamacite or taenite
Depends on weight percent
and temperature
Widmanstatten pattern
Fe/Ni rich minerals
Widmanstatten pattern
Structural Classification
•Hexahedrites: Neumann lines, striations, all kamacite, mostly
featureless
•Octahedrites: 6-17% Ni, Kamacite and taenite, Widmanstatten
Om, Of
pattern, Largest y most diverse = Ogg, Og,
and Off groups
•Ataxites: microscopic Widmanstatten pattern, mostly teanite
•Anomalous: usually fine grained, cm sized crystals
Hexahedrites
Octahedrites
Ataxite and Anomalous
Chemical Classification
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Separated by groups
I-IV
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Subgroups
Belong to a limited
range of structural
classes
Correlation b/t band
width and chemical
grouping, etc
Chemical v. Structural
Classification
Solidification of Cores
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Variable because of
process of solidification to
form cores
Certain elements
increase while others
decrease
Trends can be used to ID
types of iron meteorites
From fractional
crystallization: solid
separates from liquid
Problem: localized
differentiation
Loose ends
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Variation of Widmanstatten pattern
widths : phase diagram
Silicate Inclusions – impacts o
incomplete melting?
Isochrones: Use radioactive elements to
date iron meteorites
Pallasites
The Prettiest of Them All
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Stony-irons
Composed of olivine crystals enclosed in metal
Attractive :-D
Olivine tends to be Mg rich
Extension if IIIAB
Eagle Station (KY) meteorite
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Iron-rich olivines, metals w/ composition unlike other
groups, differing oxygen isotope compositions
Pallasites
Mesosiderites
‘messys’ - accidents
 1:1 ratio of metals and silicates
 Mostly pyroxene and plagioclase
 No olivine
 Similar to HEDs
 Limited range of abundances
 From 2 differentiated asteroids colliding?
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Messys
The End
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For more coloured meteorites:
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http://www.meteorites.com.au/home.html