Meteorites I: Chondrites & Their Components

1. Meteorites I:Chondrites & Their Components Lecture 40

2. Meteorites • Meteorites, almost all of which are ~4.56 Ga old, provide the best clues as to the formation of planets and the solar system. • We can divide them into Falls and Finds. • Former are associated with an observed fireball and are most valuable because they are least weathered and contaminated (although Antarctic and Sahara finds are also little weathered). • Three traditional classes • Stones • Irons • Stony-Irons • A more useful classification is • Primitive: Chondrites • Differentiated • Achondrites • Stony-Irons • Irons. • Primitive meteorites are collections of nebular dust (metamorphosed or altered in an asteroidal parent body). • Differentiated meteorites are pieces of asteroids that have melted and differentiated (generally into silicate mantles and metal cores).

4. Chondrites • Chondrites are samples of the cloud of gas and dust from which all bodies in the solar system formed. • Chondrites are the most common type of fall. • The can be subdivided into 3 main classes • Carbonaceous • Rich in volatile elements, including C and H • Ordinary • by far the most common, generally volatile depleted • Enstatite • like ordinary, but highly reduced so that most iron is in metal form. Since the (Fe+Mg)/Si ratio is low, olivine, the most common mineral in other chondrites, is replaced by enstatite (MgSiO3). Leoville, a CV3 chondrite

5. Chondrite Components • Chondrules: spherical objects • can be 80% of some meteorites • AOA’s: ameboidal olivine aggregates • CAI’s: calcium-aluminum inclusions (also called refractory inclusions) • Matrix

6. Chondrules • Spherical bodies, generally of a few mm in diameter consisting (in unequilibrated meteorites) of glass and quenched crystals, most commonly of olivine. • Once-molten droplets formed in brief (hours) high temperature events in the nebula.

7. Ameboidal Olivine Aggregates • Very fine aggregated olivine • Most likely condensed from high temperature gas

8. Calcium-Aluminum Inclusions • Compositions consistent with the first ~5% of material to condense (or last 5% to evaporate) from nebular gas at high temperature. • Most common minerals include anorthite, melilite, perovskite, aluminous spinel, hibonite, calcic pyroxene, and forsterite-rich olivine. • Variety of types, formed in a variety of ways. • Most common in CM and CV chondrites. • These are the oldest objects in the solar system. • Their age defines t = 0 in solar system history.

9. Chondrite Classification • C (carbonaceous) • CI: most primitive, oxidized, rare • CM • CO • CV • CK, CR, CH, CB: rare • Ordinary • H: High iron • L: Low iron • LL: Low iron, low metallic iron • Enstatite: reduced • EH • EL • Other: R, K (rare) • Each class derived from a different parent body.

10. CI chondrites • Compositions of CI chondrites matches that of the Sun for condensable elements • CI chondrites contain no chondrules, consisting only of fine-grained matrix (nearly unprocessed nebular dust). • No metal (too oxidized).

11. Petrologic Grade • Meteorites also classes by the extent to which they have been modified by processes in their parent bodies. • Numerical scale 1-6 • Grade 3: least modified • Grades 1 and 2 indicate aqueous, low temperature alteration • Grades 4-6: increasing thermal metamorphism • With increasing metamorphism, mineral compositions become more uniform (hence they are said to be equilibrated), glass devitrifies, chondrules become indistinct • With increasing aqueous alteration, water content increases, oxidation increases • Carbonaceous chondrites are always grades 1-3. • Ordinary and enstatite chondrites are always grades 3-6.

12. Chondrites Abee (EH4) note brecciation Allende CV3 Chelyabinsk (LL5) Ivuna (CI1)

13. Achondrites • Achondrites are fundamentally igneous rocks formed by crystallization of melts on asteroidal parent bodies. • both intrusive and extrusive types. • Like chondrites, can be brecciated. • Most common group is the HED (Howardites, Eucrites, Diogenites) meteorites, which come from Vesta. • Also include the SNC meteorites from Mars. • Primitive achondrites: bulk compositions approximately chondritic, but texturally modified by partial melting or metamorphic recrystallization. Eucrite in thin section

14. 4 Vesta • Dawn spacecraft orbited Vesta for a year mapping the surface. • Spectral analysis confirmed its surface composition matches that of the HED meteorites, which was long suspected. • Dawn is now on its way to 1 Ceres.

15. Vesta Spectral Map Blue shows eucrite (basalt). Cyan areas show regions with eucrite and howardite(breccias). Red areas: diogenite (intrusive cummulates). Yellow areas: diogenite and howardite.

16. Irons • Irons are mostly remnants of the once molten metal cores of disrupted asteroids. • Some irons (IAB’s) crystallized from molten metal that segregated from silicate liquid in impact melts. • Originally classified on basis of texture (a function of Fe/Ni ratio), they are now classified by composition (originally by Ga-Ge-Ni concentrations). • Each class from a different parent body. Chemical variation within class reflects fractional crystallization.

17. Stony-Irons • Pallasites: • a network of Fe-Ni metal with nodules of olivine. They probably formed at the interface between molten metal and molten silicate bodies, with olivine sinking to the bottom of the silicate magma. • Mesosiderites: • The silicate portion is very similar to diogenites – brecciated pyroxene and plagioclase – and a genetic relationship is confirmed by oxygen isotopes. The metal fraction seems closely related to IIIAB irons. It is possible they formed as the result of a collision of two differentiated asteroids, with the liquid core of one asteroid mixing with the regolith of the other.