Meteorites II:
Differentiated
Meteorites;
Ages
Lecture 41
Chondrites
Abee (EH4) note brecciation
Chelyabinsk (LL5)
Allende CV3
Ivuna (CI1)
Achondrites
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Achondrites are
fundamentally igneous rocks
formed by crystallization of
melts on asteroidal parent
bodies.
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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
Aubrite
Aubrites are related to enstatite chondrites. Brecciation
is common in both chondrites and achondrites.
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 cumulates).
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.
Time Zero
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As we noted, CAI’s are the oldest
known objects. The oldest is a CAI
from NWA2364 (CV3) with a Pb-Pb
age of 4568.67 ± 0.17 Ma.
The next oldest ages are 4567.59
± 0.11 Ma for a CAI from Allende
and 4567.11 ± 0.16 Ma for a CAI
from Efremovka; both also CV3’s.
These ages may need slight
revision due to variable 235U/238U
(possibly due to decay of 247Cm)
observed in meteorites.
Oldest age with measured
235U/238U is 4567.18 ± 0.50 Ma for an
Allende CAI.
This range of ages could reflect
aqueous alteration on parent
bodies.
Other Pb-Pb Ages
• Pb-Pb ages provide the
most accurate ages of
meteorites
o ages are on U-rich phases such
as CAI’s and phosphates.
• Range of high precision
ages is ~70 Ma, which
includes processing in
parent bodies.
o K-Ar ages range down to 4.4 Ga
and reflect impact events.
• Achondrites are
surprisingly old.
o Oldest high-precision Pb-Pb age
is 4564.42 ± 0.12 Ma for the
angrite D’Orbigny.
o Oldest HED meteorite is Ibitira, a
eucrite with an age of 4556 ± 6.
Extinct Radionuclides
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There is abundant and compelling evidence that certain short-lived
nuclides once existed in meteorites.
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This evidence consists of anomalously high abundances of the daughter nuclides in certain
meteorites, and fractions of meteorites that correlate with the abundance of the parent element.
The first of these to be discovered was the 129I–129Xe decay (Reynolds, 1960).
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Provides evidence of nucleosynthesis shortly before the solar system formed.
Provides a means of relative dating of events in the young solar system
Provides a source of energy to heat and differentiate early solar system bodies (26Al particularly).
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Significance is 3-fold:
Dating with Extinct Radionuclides
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Consider 53Cr/52Cr plotted as a
function of the 55Mn/52Cr. Provided
that
(1) all minerals formed at the same time, t=0,
(2) all remained closed to Mn and Cr since that time,
and
(3) 53Mn was present when they formed and has since
fully decayed,
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We can derive the following equation
from the fundamental equation of
radioactive decay:
æ 53Cr ö æ 53Cr ö æ 53 Mn ö æ 55 Mn ö
çè 52 Cr ÷ø = çè 52Cr ÷ø + çè 55 Mn ÷ø çè 52Cr ÷ø
0
0
where the subscript 0 denotes the
ratio at the initial time.
On the plot, the slope is (53Mn/55Mn)0.
(53Mn/55Mn), of course, decrease
through time: early formed objects
will have higher (53Mn/55Mn)0 than
later formed ones.
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Assuming the solar system began with some
uniform initial (53Mn/55M, we can assign objects
relative (but quantitative) ages because we know
the rate of decay of 53Mn.
26Al
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26Al
is particularly
significant because of its
short half-life (0.73 Ma)
and it’s abundance:
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o It allows a detailed chronology of
the earliest objects in the solar
system.
o It was abundant enough in the
early solar system to provide a
significant source of heat: partly
responsible for differentiated of
early-formed objects such as
Vesta.
26Al is produced in a type
of red-giant star (AGB
stars) - as is known both
from theory and (γ-ray)
spectral observation.
Solar System Chronology
• We can calibrate the
extinct radionuclide time
scale with Pb-Pb dating
to produce an absolute
chronology.
• Important points:
o CAI’s are the oldest objects
o Chondrule formation seems to
follow CAI formation by ~2 Ma.
o Parent bodies of achondrites,
such as Vesta formed, melted
and differentiated within 5 Ma of
time 0. They crystallized over an
additional ~5 Ma period (e.g.,
angrite parent body). ‘Cooling
ages’ of iron meteorites deduced
from diffusion profiles are
consistent - perhaps stretching
cooling times to 10 Ma.
Exposure Ages
• Exposure ages of stones
are even younger than
those of irons, telling us
meteorites were part of
larger bodies until quite
recently.
• Continual gravitational
disruption of asteroid
orbits produces collisions
and a flux of debris into
Earth-crossing orbits.
• We can classify asteroids
based on spectral
reflectance and make
potential identification of
these parent bodies.