Pt. II: Oxygen Isotopes in Meteorites

1
Pt. II Oxygen Isotopes in Meteorites

• Stefan Schröder
• February 14, 2006
• Lecture Series
• Origin of Solar Systems
• by Dr. Klaus Jockers

2
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 d is variation () from SMOW
  (Standard Mean Ocean Water)

5
Three-isotope correlation diagram
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 d17O will
  produce a change twice as large in d18O, since
  the mass difference is twice as large ? slope ½.

7
Terrestrial Fractionation line
HED howardites, eucrites, diogenites single
parent body suspected (Vesta?)
8
Allendes calcium-aluminum-rich inclusions (CAI)
define the Carbonaceous Chondrite Anhydrous
Mineral line
Clayton (1993)
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 representbuilding 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 (0C) with water, enriched in
heavier isotopes.