Equilibrated chondrites

Several types of the early Solar System materials are available for laboratory analysis (see Chapter 1 and Table 1.1 and Fig. 1.1). Each material has unique characteristics and provides specific constraints on the chemistry of the solar nebula. Major components of this sample are meteorites, fragments of asteroids, that serve as an excellent archive of the early Solar System conditions. Primitive chondritic meteorites contain glassy spherical inclusions termed chondrules, some of the oldest solids in the Solar System. Most chondrites were modified by aqueous alteration or metamorphic processes in parent bodies but there are some chondrites that are minimally altered (un-equilibrated chondrites, UCs). They have yielded a wealth of information on the chemistry, physics, and evolution of the young Solar System. 

The study of chondrite matrices is both time-consuming and challenging, principally because of the fine-grained characteristics of the material. Transmission electron microscopy (TEM) has been the analytical technique of choice, because it provides textural and mineralogical information from the micron down to the nanoscale. Here, we briefly overview the main textural and mineralogical characteristics of matrices in very un-equilibrated chondrites, focusing principally on the carbonaceous chondrites. 

In summary, despite the caveats discussed above, we can state with confidence that the bulk (>95%) of material in highly un-equilibrated chondrite matrices has individual grain sizes (as defined earlier) less than 1 pm. We can, with equal confidence, conclude that a significant proportion of this dust had grain sizes that were < 100 nm in size, although exactly how much remains uncertain without more detailed studies. Importantly, a major part of the primitive dust preserved in chondrites has grain sizes that overlap with the range that is detectable by the spectroscopic techniques discussed above. 

Chondrites contain a small fraction of chondrules that may have formed on asteroids. One rare type of chondrule (—0.1%) in type 4-6 ordinary chondrite breccias is composed largely of plagioclase (or mesostasis of plagioclase composition) and chromite (Krot et al, 1993). Krot and Rubin (1993) suggest that these chondrules may have formed by impact melting as they find impact melts with similar compositions inside shocked ordinary chondrites and such chondrules are absent in type 3 chondrites. In addition, some chromite-rich chondrules contain chromite-rich aggregates, which appear to be fragments of equilibrated chondrites. Other possible impact products were described in LL chondrites, which are mostly... 

Experiments generally confirm the merrilite/apatite fractionations of actinides and lanthanides observed in equilibrated chondrites. Morilite concentrates Pu, Th and the REE, whereas apatite concentrates U. Consequently, Th/U and PuAJ ratios of merrilite are high and Th/U and PuAJ ratios of apatite are low [e.g., 62]. 

Gopel C, Manhes G, Allegre CJ (1994) U-Ph systematics of phosphates from equilibrated ordinary chondrites. Earth Planet Sci Lett 121 153-171... 

The peak temperatures that asteroids experienced can be estimated from chemical exchange reactions between minerals - so-called geothermometers. For example, the exchange of calcium between coexisting orthopyroxene and clinopyroxene in highly metamorphosed chondrites has been used to estimate their equilibration temperatures (Slater-Reynolds and McSween, 2005). For ordinary chondrites, these temperatures range up to -1175 K. The experimental conditions at which achondrites melt provide minimum temperatures for their parent bodies. Melting of achondrites typically requires temperatures of>1200 K. 

The classification of chondrites on the basis of their elemental composition is recent with respect to the more traditional classification based on textural and mineralogical differences 6). In the case of carbonaceous chondrites this classification was revised by Wasson in 19747). The petrological type (from 1 to 6 even if some authors also use 7) is intended to indicate the degree of equilibration and metamor-phical recrystallisation. So 1 indicates the least-equilibrated and 6 the most-... 

Therefore these data for the short-lived chronometer Hf-W provide a consistent picture of rapid accretion, equilibration, and planetesimal differentiation in the early solar system with only small (106-year) time differences resolvable between some events for the parent bodies of chondrites, basaltic achondrites, and iron meteorites. 

Figure 8.4 Chondrules dominate the volume and mass of primitive chondrites, such as in this un-equilibrated ordinary chondrite, Semarkona (LL3.0). Reprinted from A Color Atlas of Meteorites in Thin Section (D. S. Lauretta and M. Killgore, Golden Retriever Publications, 2005). 

For the siderophile elements, the metal particles contribute between 50 and 90% to the bulk composition of the soil samples. As carbonaceous chondrites do not contain metal, reduction and equilibration is required to explain the high contribution of the metal particles. 

Bourot-Denise M., Zanda B., and Javoy M. (2002) Tafassesset an equilibrated CR chondrite. In Lunar Planet. Sci. XXXin, 1611. The Lunar and Planetary Institute, Houston (CD-ROM). 

During metamorphism and recrystallization, oxygen isotopes are redistributed among mineral phases, according to the mass-dependent equilibrium fractionations corresponding to the peak metamorphic temperature. The measured mineral-pair fractionations (usually for major minerals olivine, pyroxene, and feldspar) can then be used for metamorphic thermometry, yielding temperatures of 600 °C for an L4 chondrite, and 850 50 °C for several type-5 and type-6 chondrites (Clayton et al., 1991). Isotopic equilibration, even in type-6 chondrites, involves oxygen atom transport only over distances of a few millimeters (Olsen et al., 1981). 

The chemical fractionations observed among chondrites and the compositions of many chondritic components are best understood in terms of quenched equilibrium between phases in a nebula of solar composition (Palme, 2001 Chapters 1.03 and 1.15). The equilibrium model assumes that minerals condensed from, or equilibrated with, a homogeneous solar nebula at diverse temperatures. Isotopic variations among chondrites and their components show that this assumption is not correct and detailed petrologic studies have identified relatively few chondritic components that resemble equilibrium nebular products. Nevertheless, the equilibrium model is invaluable for understanding the chemical composition of chondrites and their components as the solar nebular signature is etched deeply into the chemistry and mineralogy. 

Grains of metallic Fe,Ni in most unshocked type 3-6 chondrites provide a record of slow cooling at —1-1,000 K Myr through the temperature range —550-350 °C, when kamacite and taenite ceased to equilibrate (Wood, 1967). In most type 2 and 3.0-3.3 chondrites, metallic Fe,Ni grains typically contain concentrations of 0.1 -1 % chromium, silicon, and phosphorus, which are not found in type 4-6 chondrites, and reflect high-temperature processing prior to accretion. 

Folco L., Mellini M., and Pillinger C. T. (1997) Equilibrated ordinary chondrites constraints for thermal history from iron-magnesium ordering in orthopyroxene. Meteorit. Planet. Sci. 32, 567-575. 

Williams C. V., Keil K., Taylor G. J., and Scott E. R. D. (1999) Cooling rates of equilibrated clasts in ordinary chondrite regolith breccias implications for parent body histories. Chem. Erde 59, 287-305. 

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