pt. ii: oxygen isotopes in meteorites stefan schröder february 14, 2006 lecture series “origin of...

Pt. II: Oxygen Isotopes in Meteorites

Stefan Schröder

February 14, 2006

Lecture Series“Origin of Solar Systems”

by Dr. Klaus Jockers

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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