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Complementary chemical and isotopic relationships between chondrules and matrix have the potential to distinguish between categories of chondrule forming mechanisms, e.g., exclude all mechanisms that require different reservoirs for chondrules and matrix. The complementarity argument is, however, often misunderstood. Complementarity requires different average compositions of an element or isotope ratio in each of the two major chondrite components chondrules and matrix, and a solar or CI chondritic bulk chondrite ratio of the considered elements or isotopes. For example, chondrules in carbonaceous chondrites typically have superchondritic Mg/Si ratios, while the matrix is subchondritic. Another example would be the Hf/W ratio, which is superchondritic in chondrules and subchondritic in matrix. We regard these ratios to be complementary in chondrules and matrix, because the bulk chondrite has solar Mg/Si and Hf/W ratios. In contrast, Al/Na ratios are also different in chondrules and matrix, but the bulk is not solar; therefore, Al/Na does not have a complementary relationship. A number of publications over the past decade have reported complementary relationships for many element pairs in different types of chondrites. Recently, isotopic complementarities have also been reported. A related, though different, argument can be made for volatile depletion patterns in chondrules and matrix, which can then also be considered as being complementary. The various models for chondrule formation require either that chondrules and matrix formed from a single (i.e., common) parental reservoir, or that chondrules and matrix formed in separate regions of the protoplanetary disk and were later mixed together. As chondrules and matrix have different compositions, mixing of these two components would result in a random bulk chondrite composition. The observation of complementary chondrule–matrix relationships together with a CI chondritic, element or isotope ratio is unlikely to be the result of a random mix of chondrules and matrix. It seems much more likely that chondrules and matrix formed in a single reservoir with initially CI chondritic element or isotope ratios. Incorporation of different minerals in chondrules and matrix together with volatile element depletion of the entire reservoir then resulted in chondrule-matrix complementarities and bulk chondrite volatile depletion. This excludes any chondrule formation mechanism that requires separate parental reservoirs for these components. Any chondrule forming mechanism must explain complementarity. Chondrules and matrix must have formed from a common reservoir.
Much of our knowledge about the formation of planets in the Solar System and in particular concepts and ideas about the origin of the Earth are derived from studies of extraterrestrial matter. Meteorites (Sears, 2004; Lauretta et al., 2006; Krot et al., 2006) were available for laboratory investigations long before space probes were sent out for in situ investigations of planetary surfaces, or Moon rocks were brought back to Earth. Meteorite studies provided such important parameters as the age of the Earth and the time of formation of the first solids in the Solar System (Chen and Wasserburg, 1981; Allegre et al., 1995; Amelin et al., 2002), as well as the average abundances of the elements in the Solar System (Anders and Grevesse, 1989; Palme and Jones, 2004). Traditionally, the study of rocky material requires techniques that fundamentally differ from astronomical techniques. While electromagnetic radiation from stars is analyzed by spectroscopy, the solid samples of aggregated cosmic dust and rocky matter from planetary surfaces require the use of laboratory instruments that allow the determination of their chemical and isotopic composition. Planetary surface materials are present in polymineralic assemblages. Formation conditions and thermal stability of individual minerals provide important boundary conditions for the genesis and history of the analyzed materials. Such studies require a thorough mineralogical background. The abundances and properties of the rock-forming elements, the focus of geo- and cosmochemical research, are, however, not necessarily of major concern to astrophysicists.
Our knowledge of the constitution and composition of the Earth's mantle has advanced enormously during the last 30 years… As a result of these developments many new and important boundary conditions for the origin of the Earth have emerged. I do not believe that the significance of these boundary conditions, mainly of a geochemical nature, [has] been adequately recognised in many recent discussions of the origins of terrestrial planets in general and of the Earth in particular.
— A. E. Ringwood (1979)
The formation of our solar system followed the collapse and fragmentation of a dense interstellar molecular cloud. As interstellar matter always has some angular momentum, the development of a central star by direct infall was not possible, and instead a rotating disk resulted. Material within the disk lost angular momentum through viscous dissipation or other processes, leading ultimately to the growth of a central star, our Sun. Only a tiny fraction of the mass of the solar system (∼0.1%) was left behind in the disk, eventually to form the planets and asteroids.
The duration of the initial collapse phase was short, less than 1 million years. After this phase, the remnants of the accretion disk may have persisted for as long as 10 million years before the planets were assembled. This history derives from astronomical observations and is consistent with isotopic evidence from meteorites (Podosek and Cassen, 1994). The mixture of gas and grains that made up the proto-solar accretion disk is known as the solar nebula.
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