1. Pt. II: Oxygen Isotopes in Meteorites
Stefan Schröder
February 14, 2006
Lecture Series
“Origin of Solar Systems”
by Dr. Klaus Jockers

2. Short summary on meteorite types Introduction, definitions
Outline
Short summary on meteorite types
Introduction, definitions
Three-isotope correlation diagram
Terrestrial Fractionation line
Carbonaceous Chondrite Anhydrous Mineral line
Selected examples:
Enstatite chondrite ↔ Earth similarity
Aqueous alteration of CM chondrites

3. Meteorite Summary
Chondrites contain chondrules, spherules of once molten silicates, and white lumps, called refractory inclusions (the earliest solid matter), in a matrix. Chondrite parent bodies have not undergone large scale melting, homogenization, and differentiation, and thus retain signatures of their early history
Achondrites have been melted (and in some cases homogenized, and differentiated), so that their pre-accretional internal isotopic variations are (usually) not preserved

4. Oxygen Isotopes
System to classify meteorites using oxygen isotopes is largely the work of Robert Clayton et al.
Three stable isotopes (Earth abundance):
16O (99.76%)
17O (0.039%)
18O (0.202%)
Definition: δ is variation (‰) from SMOW (Standard Mean Ocean Water):

5. Three-isotope correlation diagram
18O addition
17O addition
16O addition
fractionation
Terrestrial
Fractionation line
Carbonaceous Chondrite
Anhydrous Mineral line
Podosek (1987)

6. Fractionation Isotopic fractionation can occur through (examples):
evaporation of liquid (remaining liquid is enriched in heavier isotopes)
difference in chemical bonding in molecules (heavy isotopes are preferentially retained in sites with strongest binding)
Any process that leads to a change in δ17O will produce a change twice as large in δ18O, since the mass difference is twice as large → slope ½.

7. Terrestrial Fractionation line
Clayton & Mayeda (1996)
HED: howardites, eucrites, diogenites; single parent body suspected (Vesta?)

8. Allende’s calcium-aluminum-rich inclusions (CAI) define the Carbonaceous Chondrite Anhydrous Mineral line
Clayton (1993)
Spinel is the most refractory: represents the composition of the primary nebula
isotopic exchange with surrounding gas

9. Isotopic heterogeneity in solar nebula: Theories
Inheritance from the molecular cloud
dust and gas components had different proportions of supernova-produced 16O
gas component was depleted in 16O by photochemical processes in the molecular cloud (e.g. Van Dishoeck & Black 1988)
Locally generated heterogeneity within an initially homogenous nebula
gas-phase mass-independent fractionation reaction
isotopic self-shielding in the photolysis of CO during the accretion of the Sun (Clayton 2002)

10. Answers expected from the Genesis mission…

11. Enstatite chondrites may represent building block material
Chondrules of enstatite (MgSiO3) chondrites (× E) on TF line;
Chondrules of carbonaceous chondrites (○ C) border CCAM (CAI) line
Suggests enstatite chondrite material as building block for Earth (Javoy et al. 1986)
Clayton (1993)

12. CM chondrites show evidence for aqueous alteration
In carbonaceous chondrites (Mighei-type) high-temperature anhydrous silicates (e.g. olivine) co-exist with low-temperature phyllosilicates (clay minerals).
The latter are predominant in the matrix, and are thought to have been formed from the former by interaction at low temperature (0°C) with water, enriched in heavier isotopes.