pt. ii: oxygen isotopes in meteorites stefan schröder february 14, 2006 lecture series “origin of...
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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
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Meteorite Summary
ChondritesChondrites 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
AchondritesAchondrites have been melted (and in some cases homogenized, and differentiated), so that their pre-accretional internal isotopic variations are (usually) not preserved
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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):
SMOW
16
18
0 O
O
R
sample
16
18
O
O
R ‰1000Oδ
0
018
R
RR
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TerrestrialTerrestrialFractionation Fractionation lineline Carbonaceous Carbonaceous
ChondriteChondriteAnhydrous Mineral Anhydrous Mineral lineline
Three-isotope correlation diagram
Podosek (1987)
18O addition
17O addition
16O addition
fractio
nation
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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.
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Terrestrial Fractionation line
Clayton & Mayeda (1996)
HED: howardites, eucrites, diogenites; single parent body suspected (Vesta?)
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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
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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)
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Answers expected from the Genesis mission…
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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)
Enstatite chondrites may represent
building block material
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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.
CM chondrites show evidence for aqueous
alteration
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