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 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 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): – 16 O (99.76%) – 17 O (0.039%) – 18 O (0.202%) Definition: δ is variation (‰) from SMOW ( Standard Mean Ocean Water):

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

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 δ 17 O will produce a change twice as large in δ 18 O, 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 16 O –gas component was depleted in 16 O 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 Chondrules of enstatite (MgSiO 3 ) 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

12 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