Meteorites I: Chondrites & Their Components Lecture 40

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. o 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 o Stones o Irons o Stony-Irons A more useful classification is o Primitive: Chondrites o Differentiated Achondrites Stony-Irons Irons. o Primitive meteorites are collections of nebular dust (metamorphosed or altered in an asteroidal parent body). o Differentiated meteorites are pieces of asteroids that have melted and differentiated (generally into silicate mantles and metal cores).

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 o Carbonaceous Rich in volatile elements, including C and H o Ordinary by far the most common, generally volatile depleted o 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 (MgSiO 3 ). Leoville, a CV3 chondrite

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

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.

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

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. o Their age defines t = 0 in solar system history.

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

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).

Petrologic Grade Meteorites also classes by the extent to which they have been modified by processes in their parent bodies. Numerical scale 1-6 o Grade 3: least modified o Grades 1 and 2 indicate aqueous, low temperature alteration o 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.

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

Achondrites Achondrites are fundamentally igneous rocks formed by crystallization of melts on asteroidal parent bodies. o both intrusive and extrusive types. o 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

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.

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.

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.

Stony-Irons Pallasites: o 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: o 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.