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

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

Sahara 192

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

Graphite from Murchison CC

TiC

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

• Most elements and their isotopes can be measured in µ-sized objects

• The precision of, for example, O-isotope ratio measurements approaches that achieved in a conventional gas source mass spectrometer

• Measurements preserve petrographic context

• 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

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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