Meteorites I:
Chondrites &
Their Components
Lecture 40
Meteorites
• Meteorites, almost all of which are ~4.56 Ga old, provide the
best clues as to the formation of planets and the solar system.
• We can divide them into Falls and Finds.
o
Former are associated with an observed fireball and are most valuable because
they are least weathered and contaminated (although Antarctic and Sahara finds
are also little weathered).
• Three traditional classes
o
o
o
Stones
Irons
Stony-Irons
o
o
Primitive: Chondrites
Differentiated
• Achondrites
• Stony-Irons
• Irons.
Primitive meteorites are collections of nebular dust (metamorphosed or altered in an
asteroidal parent body).
Differentiated meteorites are pieces of asteroids that have melted and
differentiated (generally into silicate mantles and metal cores).
• A more useful classification is
o
o
Chondrites
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•
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Chondrites are samples of the
cloud of gas and dust from
which all bodies in the solar
system formed.
Chondrites are the most
common type of fall.
The can be subdivided into 3
main classes
o
o
o
Carbonaceous
• Rich in volatile elements, including C
and H
Ordinary
• by far the most common, generally
volatile depleted
Enstatite
• like ordinary, but highly reduced so
that most iron is in metal form. Since
the (Fe+Mg)/Si ratio is low, olivine,
the most common mineral in other
chondrites, is replaced by enstatite
(MgSiO3).
Leoville, a CV3 chondrite
Chondrite Components
• Chondrules: spherical objects
o can be 80% of some meteorites
• AOA’s: ameboidal olivine aggregates
• CAI’s: calcium-aluminum inclusions (also called
refractory inclusions)
• Matrix
Chondrules
• Spherical bodies,
generally of a few
mm in diameter
consisting (in
unequilibrated
meteorites) of glass
and quenched
crystals, most
commonly of
olivine.
• Once-molten
droplets formed in
brief (hours) high
temperature
events in the
nebula.
Ameboidal Olivine
Aggregates
• Very fine aggregated
olivine
• Most likely condensed
from high temperature
gas
Calcium-Aluminum
Inclusions
• Compositions consistent
with the first ~5% of material
to condense (or last 5% to
evaporate) from nebular
gas at high temperature.
• Most common minerals
include anorthite, melilite,
perovskite, aluminous spinel,
hibonite, calcic pyroxene,
and forsterite-rich olivine.
• Variety of types, formed in a
variety of ways.
• Most common in CM and
CV chondrites.
• These are the oldest objects
in the solar system.
o
Their age defines t = 0 in solar
system history.
Chondrite Classification
• C (carbonaceous)
o
o
o
o
o
CI: most primitive, oxidized, rare
CM
CO
CV
CK, CR, CH, CB: rare
• Ordinary
o H: High iron
o L: Low iron
o LL: Low iron, low metallic iron
• Enstatite: reduced
o EH
o EL
• Other: R, K (rare)
• Each class derived from a
different parent body.
CI chondrites
• Compositions of CI
chondrites matches
that of the Sun for
condensable elements
• CI chondrites contain
no chondrules,
consisting only of finegrained matrix (nearly
unprocessed nebular
dust).
• No metal (too
oxidized).
Petrologic Grade
• Meteorites also classes by the extent to which they have
been modified by processes in their parent bodies.
• Numerical scale 1-6
o Grade 3: least modified
o Grades 1 and 2 indicate aqueous, low temperature alteration
o Grades 4-6: increasing thermal metamorphism
• With increasing metamorphism, mineral compositions
become more uniform (hence they are said to be
equilibrated), glass devitrifies, chondrules become
indistinct
• With increasing aqueous alteration, water content
increases, oxidation increases
• Carbonaceous chondrites are always grades 1-3.
• Ordinary and enstatite chondrites are always grades 3-6.
Chondrites
Abee (EH4) note brecciation
Chelyabinsk (LL5)
Allende CV3
Ivuna (CI1)
Achondrites
•
Achondrites are
fundamentally igneous rocks
formed by crystallization of
melts on asteroidal parent
bodies.
o
o
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•
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both intrusive and extrusive types.
Like chondrites, can be brecciated.
Most common group is the
HED (Howardites, Eucrites,
Diogenites) meteorites, which
come from Vesta.
Also include the SNC
meteorites from Mars.
Primitive achondrites: bulk
compositions approximately
chondritic, but texturally
modified by partial melting or
metamorphic
recrystallization.
Eucrite in thin section
4 Vesta
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Dawn spacecraft orbited Vesta
for a year mapping the surface.
Spectral analysis confirmed its
surface composition matches
that of the HED meteorites, which
was long suspected.
Dawn is now on its way to 1
Ceres.
Vesta Spectral Map
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Blue shows eucrite (basalt).
Cyan areas show regions with eucrite and howardite (breccias).
Red areas: diogenite (intrusive cummulates).
Yellow areas: diogenite and howardite.
Irons
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Irons are mostly remnants of
the once molten metal
cores of disrupted asteroids.
Some irons (IAB’s)
crystallized from molten
metal that segregated from
silicate liquid in impact
melts.
Originally classified on basis
of texture (a function of
Fe/Ni ratio), they are now
classified by composition
(originally by Ga-Ge-Ni
concentrations).
Each class from a different
parent body. Chemical
variation within class reflects
fractional crystallization.
Stony-Irons
• Pallasites:
o a network of Fe-Ni metal with
nodules of olivine. They probably
formed at the interface between
molten metal and molten silicate
bodies, with olivine sinking to the
bottom of the silicate magma.
• Mesosiderites:
o The silicate portion is very similar
to diogenites – brecciated
pyroxene and plagioclase – and
a genetic relationship is
confirmed by oxygen isotopes.
The metal fraction seems closely
related to IIIAB irons. It is possible
they formed as the result of a
collision of two differentiated
asteroids, with the liquid core of
one asteroid mixing with the
regolith of the other.