Achondrites Achondrites are fundamentally igneous rocks formed
by crystallization of melts on asteroidal parent bodies. o both
intrusive and extrusive types. o 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
Slide 4
Aubrite Aubrites are related to enstatite chondrites.
Brecciation is common in both chondrites and achondrites.
Slide 5
4 Vesta 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.
Slide 6
Slide 7
Vesta Spectral Map Blue shows eucrite (basalt). Cyan areas show
regions with eucrite and howardite (breccias). Red areas: diogenite
(intrusive cumulates). Yellow areas: diogenite and howardite.
Slide 8
Irons Irons are mostly remnants of the once molten metal cores
of disrupted asteroids. Some irons (IABs) 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.
Slide 9
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.
Slide 10
Time Zero As we noted, CAIs 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 CV3s. These
ages may need slight revision due to variable 235 U/ 238 U
(possibly due to decay of 247 Cm) observed in meteorites. Oldest
age with measured 235 U/ 238 U is 4567.18 0.50 Ma for an Allende
CAI. This range of ages could reflect aqueous alteration on parent
bodies.
Slide 11
Other Pb-Pb Ages Pb-Pb ages provide the most accurate ages of
meteorites o ages are on U-rich phases such as CAIs 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
DOrbigny. o Oldest HED meteorite is Ibitira, a eucrite with an age
of 4556 6.
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Extinct Radionuclides There is abundant and compelling evidence
that certain short-lived nuclides once existed in meteorites. o
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.
o The first of these to be discovered was the 129 I 129 Xe decay
(Reynolds, 1960). Significance is 3-fold: o Provides evidence of
nucleosynthesis shortly before the solar system formed. o Provides
a means of relative dating of events in the young solar system o
Provides a source of energy to heat and differentiate early solar
system bodies ( 26 Al particularly).
Slide 13
Dating with Extinct Radionuclides Consider 53 Cr/ 52 Cr plotted
as a function of the 55 Mn/ 52 Cr. 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) 53 Mn was present when they formed and has
since fully decayed, We can derive the following equation from the
fundamental equation of radioactive decay: where the subscript 0
denotes the ratio at the initial time. On the plot, the slope is (
53 Mn/ 55 Mn) 0. ( 53 Mn/ 55 Mn), of course, decrease through time:
early formed objects will have higher ( 53 Mn/ 55 Mn) 0 than later
formed ones. o Assuming the solar system began with some uniform
initial ( 53 Mn/ 55 M, we can assign objects relative (but
quantitative) ages because we know the rate of decay of 53 Mn.
Slide 14
26 Al 26 Al is particularly significant because of its short
half-life (0.73 Ma) and its abundance: 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. 26 Al is produced in a type of
red-giant star (AGB stars) - as is known both from theory and
(-ray) spectral observation.
Slide 15
Solar System Chronology We can calibrate the extinct
radionuclide time scale with Pb-Pb dating to produce an absolute
chronology. Important points: o CAIs 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.
Slide 16
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.