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Iron Meteorites: Composition, Age, and Origin  

Edward R. D. Scott

Iron meteorites are thought to be samples of metallic cores and pools that formed in diverse small planetary bodies. Their great diversity offers remarkable insights into the formation of asteroids and the early history of the solar system. The chemical compositions of iron meteorites generally match those predicted from experimental and theoretical considerations of melting in small bodies. These bodies, called planetesimals, were composed of mixtures of grains of silicates, metallic iron-nickel, and iron sulfide with compositions and proportions like those in chondrite meteorites. Melting in planetesimals caused dense metal to sink through silicate so that metallic cores formed. A typical iron meteorite contains 5–10% nickel, ~0.5% cobalt, 0.1–0.5% phosphorus, 0.1–1% sulfur and over 20 other elements in trace amounts. A few percent of iron meteorites also contain silicate inclusions, which should have readily separated from molten metal because of their buoyancy. They provide important evidence for impacts between molten or partly molten planetesimals. The major heat source for melting planetesimals was the radioactive isotope 26Al, which has a half-life of 0.7 million years. However, a few iron meteorites probably formed by impact melting of chondritic material. Impact processes were also important in the creation of many iron meteorites when planetesimals were molten. Chemical analysis show that most iron meteorites can be divided into 14 groups: about 15% appear to come from another 50 or more poorly sampled parent bodies. Chemical variations within all but three groups are consistent with fractional crystallization of molten cores of planetesimals. The other three groups are richer in silicates and probably come from pools of molten metal in chondritic bodies. Isotopic analysis provides formation ages for iron meteorites and clues to their provenance. Isotopic dating suggests that the parent bodies of iron meteorites formed before those of chondrites, and some irons appear to be the oldest known meteorites. Their unexpected antiquity is consistent with 26Al heating of planetesimals. Bodies that accreted more than ~2 million years after the oldest known solids (refractory inclusions in chondrites) should not have contained enough 26Al to melt. Isotopic analysis also shows that iron meteorites, like other meteorite types, display small anomalies due to pre-solar grains that were not homogenized in the solar nebula (or protoplanetary disk). Although iron meteorites are derived from asteroids, their isotopic anomalies provide the best clues that some come from planetesimals that did not form in the asteroid belt. Some may have formed beyond Jupiter; others show isotopic similarities to Earth and may have formed in the neighborhood of the terrestrial planets. Iron meteorites therefore contain important clues to the formation of planetesimals that melted and they also provide constraints on theories for the formation of planets and asteroids.

Article

Meteorite Mineralogy  

Alan E. Rubin and Chi Ma

Meteorites are rocks from outer space that reach the Earth; more than 60,000 have been collected. They are derived mainly from asteroids; a few hundred each are from the Moon and Mars; some micrometeorites derive from comets. By mid 2020, about 470 minerals had been identified in meteorites. In addition to having characteristic petrologic and geochemical properties, each meteorite group has a distinctive set of pre-terrestrial minerals that reflect the myriad processes that the meteorites and their components experienced. These processes include condensation in gaseous envelopes around evolved stars, crystallization in chondrule melts, crystallization in metallic cores, parent-body aqueous alteration, and shock metamorphism. Chondrites are the most abundant meteorites; the major components within them include chondrules, refractory inclusions, opaque assemblages, and fine-grained silicate-rich matrix material. The least-metamorphosed chondrites preserve minerals inherited from the solar nebula such as olivine, enstatite, metallic Fe-Ni, and refractory phases. Other minerals in chondrites formed on their parent asteroids during thermal metamorphism (such as chromite, plagioclase and phosphate), aqueous alteration (such as magnetite and phyllosilicates) and shock metamorphism (such as ringwoodite and majorite). Differentiated meteorites contain minerals formed by crystallization from magmas; these phases include olivine, orthopyroxene, Ca-plagioclase, Ca-pyroxene, metallic Fe-Ni and sulfide. Meteorites also contain minerals formed during passage through the Earth’s atmosphere and via terrestrial weathering after reaching the surface. Whereas some minerals form only by a single process (e.g., by high-pressure shock metamorphism or terrestrial weathering of a primary phase), other meteoritic minerals can form by several different processes, including condensation, crystallization from melts, thermal metamorphism, and aqueous alteration.

Article

Solar Elemental Abundances  

Katharina Lodders

Solar elemental abundances, or solar system elemental abundances, refer to the complement of chemical elements in the entire Solar System. The Sun contains more than 99% of the mass in the solar system and therefore the composition of the Sun is a good proxy for the composition of the overall solar system. The solar system composition can be taken as the overall composition of the molecular cloud within the interstellar medium from which the solar system formed 4.567 billion years ago. Active research areas in astronomy and cosmochemistry model collapse of a molecular cloud of solar composition into a star with a planetary system and the physical and chemical fractionation of the elements during planetary formation and differentiation. The solar system composition is the initial composition from which all solar system objects (the Sun, terrestrial planets, gas giant planets, planetary satellites and moons, asteroids, Kuiper-belt objects, and comets) were derived. Other dwarf stars (with hydrostatic hydrogen-burning in their cores) like the Sun (type G2V dwarf star) within the solar neighborhood have compositions similar to the Sun and the solar system composition. In general, differential comparisons of stellar compositions provide insights about stellar evolution as functions of stellar mass and age and ongoing nucleosynthesis but also about galactic chemical evolution when elemental compositions of stellar populations across the Milky Way Galaxy is considered. Comparisons to solar composition can reveal element destruction (e.g., Li) in the Sun and in other dwarf stars. The comparisons also show element production of, for example, C, N, O, and the heavy elements made by the s-process in low to intermediate mass stars (3–7 solar masses) after these evolved from their dwarf-star stage into red giant stars (where hydrogen and helium burning can occur in shells around their cores). The solar system abundances are and have been a critical test composition for nucleosynthesis models and models of galactic chemical evolution, which aim ultimately to track the production of the elements heavier than hydrogen and helium in the generation of stars that came forth after the Big Bang 13.4 billion years ago.