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Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...1..431m&link_type=abstract
Treatise on Geochemistry, Volume 1. Editor: Andrew M. Davis. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp. 71
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Scientific paper
Chondritic Meteorites as Probes of Early Solar System EvolutionThe evolutionary sequence involved in the formation of relatively low-mass stars, such as the Sun, has been delineated in recent years through impressive advances in astronomical observations at a variety of wavelengths, combined with improved numerical and theoretical models of the physical processes thought to occur during each stage. From the models and the observational statistics, it is possible to infer in a general way how our solar system ought to have evolved through the various stages from gravitational collapse of a fragment of a molecular cloud to the accretion of planetary-sized bodies (e.g., Cameron, 1995; Alexander et al., 2001; Shu et al., 1987; André et al., 2000; see Chapters 1.04, 1.17, and 1.20). However, the details of these processes remain obscured, literally from an astronomical perspective, and the dependence of such models on various parameters requires data to constrain the specific case of our solar system's origin.Fortunately, the chondritic meteorites sample aspects of this evolution. The term "chondrite" (or chondritic) was originally applied to meteorites bearing chondrules, which are approximately millimeter-sized solidified melt droplets consisting largely of mafic silicate minerals and glass commonly with included metal or sulfide. However, the meaning of chondritic has been expanded to encompass all extraterrestrial materials that are "primitive," i.e., are undifferentiated samples having nearly solar elemental composition. Thus, the chondrites represent a type of cosmic sediment, and to a first approximation can be thought of as "hand samples" of the condensable portion of the solar nebula. The latter is a general term referring to the phase(s) of solar system evolution intermediate between molecular cloud collapse and planet formation. During the nebular phase, the still-forming Sun was an embedded young-stellar object (YSO) enshrouded by gas and dust, which was distributed first in an extended envelope which later evolved into an accretion disk that ultimately defined the ecliptic plane. The chondrites agglomerated within this accretion disk, most likely close to the position of the present asteroid belt from whence meteorites are currently derived. In addition to chondrules, an important component of some chondrites are inclusions containing refractory oxide and silicate minerals, so-called calcium- and aluminum-rich inclusions (CAIs) that also formed as free-floating objects within the solar nebula. These constituents are bound together by a "matrix" of chondrule fragments and fine-grained dust (which includes a tiny fraction of dust grains that predate the solar nebula; see Chapter 1.02). It is important to realize that, although these materials accreted together at a specific time in some planetesimal, the individual components of a given chondrite can, and probably do, sample different places and/or times during the nebular phase of solar system formation. Thus, each grain in one of these cosmic sedimentary rocks potentially has a story to tell regarding aspects of the early evolution of the solar system.Time is a crucial parameter in constructing any story. Understanding of relative ages allows placing events in their proper sequence, and measures of the duration of events are critical to developing an understanding of process. If disparate observations can be related temporally, then structure (at any one time) and evolution of the solar system can be better modeled; or, if a rapid succession of events can be inferred, it can dictate a cause and effect relationship. This chapter is concerned with understanding the timing of different physical and chemical processes that occurred in the solar nebula and possibly on early accreted planetesimals that existed during the nebula stage. These events are "remembered" by the components of chondrites and recorded in the chemical, and especially, isotopic compositions of the host mineral assemblages; the goal is to decide which events were witnessed by these ancient messengers and to decipher those memories recorded long ago.
Davis Aileen M.
McKeegan Kevin D.
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