Transcript Document

Definition of “fossil”
A fossil is defined as any remains,
trace or imprint of a plant or animal
that has been preserved by natural
processes in the Earth’s crust from
some distant geologic time and
provides a record of Earth’s history
from that time
Stromatolites, one of the oldest “fossils” of life on Earth
Meteorites as “fossils” of the earliest history of
our Solar System
Meteorites are also “fossils”, because they have
preserved within them a record of the most ancient
history of solar system material, a record not
preserved in any other rocks. For example, dating
of meteorites allows the determination of the time
of formation of solid materials during the dawn of
our Solar System, and of the mineralogic and
isotopic composition of “Star dust”, material that
formed in stars prior to the formation of our Solar
System.
Chondrites are meteorites from broken-up primitive,
undifferentiated asteroids that never melted. Thus, the properties
of their constituents [calcium-aluminum-rich inclusions (CAIs),
chondrules, matrix, and metallic Fe,Ni] are today as they were
when the Solar System formed
Chondrite NWA 5028
Radioactive parent isotopes, and stable daughter isotopes
(decay products of the parent) and their half-lifes of
common elements used in the dating of meteorites and
other rocks
A classical and one of the
earliest determinations of
the age of an ordinary
chondrite by Wasserburg
and co-workers, using the
Rb-Sr techniques. This
primitive chondrite is
4.56 billion years old and,
by inference, so is our
Solar System!
When did the Solar System form: Ages of formation of the first solid
materials in our Solar System
Sahara 192
Age of CAIs from CV chondrites: 4,567.4 ± 0.5 Ma
Age of chondrules from CR chondrites: 4,564.7 ± 0.6 Ma
(Amelin and Krot, 2002)
Thus, chondrules formed 2.7 Ma after the formation of CAIs
The discovery of oxygen nuclear isotopic anomalies in carbonaceous
chondrites
In 1977, Clayton and
co-workers discovered
isotopic anomalies in
the oxygen
compositions of
carbonaceous
chondrites. They
concluded that the
origin of these
anomalies can only be
explained by
nucleosynthesis, i.e.,
formation of the
carriers of the
anomalies in other
stars, such as a
supernova outside of,
and prior to, formation
of our Solar System
A grain of “stardust” a few microns in size of silicon carbide, SiC, whose
isotopic composition indicates that it must have formed outside of, and prior
to, the formation of the Solar System, most likely in a so-called AGB star
Raw materials for making stars and planets:
Presolar grains and stellar nucleosynthesis
•
Elements are made in the interiors of stars and
returned to interstellar space
•
Circumstellar (presolar) grains carry record of
nucleosynthesis in a star
•
Typical presolar grains are diamond, graphite, SiC,
corundum (Al2O3), spinel (MgAl2O4), TiC.
•
Sources: Supernova, AGB stars, novae, red giants,
etc.
•
We determine the structures and isotopic
compositions of circumstellar condensates
•
These data provide ground truth for testing
theoretical models of star formation
TiC
Graphite from Murchison CC
The center piece of the W.M. Keck Cosmochemistry Laboratory: The Cameca
1280 Secondary Ion Mass Spectrometer (SIMS; Ion Microprobe)
Cameca 1280 Ion Microprobe and Isotope Scanning Microscope
• A focussed primary beam of ~ 10 keV cesium or oxygen ions sputters secondary ions off of
the surface of a polished thin section, and the secondary ions are analyzed in a magnetic
sector mass spectrometer
• Most elements
and their
isotopes can be
measured in µsized objects
• The precision
of, for example,
O-isotope ratio
measurements
approaches that
achieved in a
conventional
gas source mass
spectrometer
• Measurements
preserve
petrographic
context
From gas to dust to planets: Formation of
our Solar System:
The chemical elements formed in the
interiors of stars. Dying stars ejected
material into interstellar space, and
presolar grains and amorphous material
condensed in stellar atmospheres of stars
such as Red Giants, AGB stars, and
supernovae. These materials survived the
long journey from the parent stars
through the interstellar medium into cold
molecular clouds whose cores collapsed to
form new stars (e.g., our Sun), the
planets, asteroids and comets. Asteroids
broke up due to collisions with other
asteroids, and the fragments (the
chondritic meteorites) fell on Earth ~ 4.57
Ga after the solar system formed,
bringing with them stardust from other
stars that formed before formation of the
solar system, as well as primitive solar
system materials (CAIs, chondrules) that
formed at the dawn of the solar system.
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