Meteorites are samples of planetary material from our solar system that have been expelled from their parent bodies, traveled through space as meteoroids, survived passage through our atmosphere, and impacted with the Earth. If this impact is observed, meteorites that are subsequently gathered are called falls. A meteorite whose impact was not observed is called a find.
Figure 1. Processing of meteoritic material in the solar system.
While on and after its separation from its parent planet, a meteoroid can be subjected to a variety of chemical ans physical processes. Figure 1 illustrates the types of processes to which meteoritic material can be subjected. As the chemically and isotopically heterogeneous solar nebula cooled, dust and gas condensed and accreted to form a variety of primitive parent bodies. These primitive parent bodies, as they increased in size, might experience a variety of secondary processes such as aqueous alteration by liquid water, thermal metamorphism from the heat generated by decaying radionuclides and the post-shock residual temperature of impacts in the still accreting, young solar system. In some cases this thermal metamorphism caused partial or complete melting of the insulated interior of primitive parent bodies leading to separation of dense immiscible metal and less dense silicates. These processes might be repeated with varying degrees of melting, mixing, and fractional crystallization. Material on the surface of these evolving parent bodies was exposed to solar and galactic cosmic rays, collision with unaccreted molecules, dust, even other meteoroids and asteroids. These events collectively caused the process of brecciation. As collisions physically shocked the parent body, material could be excavated and accelerated so that it escaped the planet's gravitational potential well: such collisions could even cause disruption and break-up of the parent body. If a remnant of such a collision crossed Earth's orbit it might pass through Earth's atmosphere, causing atmospheric ablation, and impact with Earth. If the material is not recovered immediately, it will be exposed to terrestrial weathering.
Meteorites can, therefore, preserve information about both the chemical and physical environment prevailing during the early solar system. However, this primary chemical and isotopic information can be blurred or even destroyed by secondary and tertiary processes. These processes affect meteoritic material in decipherable ways and in some cases can be deconvoluted from the primary information.
METEORITE CLASSIFICATION
The process of meteorite classification is essential to interpretation of the information that meteorites possess. Perhaps one of the most useful classification schemes of meteorites is that of differentiated and undifferentiated meteorites. Differentiated meteorites are the products of planetary melting. Meteorites in this category are the achondrites, stony irons, and irons. Undifferentiated meteorites are also known as chondrites. Chondrites have not been subjected to planetary melting processes. Indeed these meteorites remain essentially unaltered since their formation 4.65±0.10 Gy ago and are the most primitive objects obtainable for study. Figure 2 illustrates the various major types of meteorites and their abundance indicated by the area shown. Chondrites are the most abundant type of meteorite. Of the 959 falls collected, 784 are chondrites (Graham et al., 1985).
Figure 2. Relative
abundances of recovered meteorite falls.
CHONDRITES: PRIMARY PROPERTIES
The undifferentiated meteorites, or chondrites, can be further classified on the basis of subtle differences in major, nonvolatile elemental composition into nine subgroups: the carbonaceous chondrites; CI, CM, CO, and CV; the enstatite chondrites; EH and EL; and the ordinary chondrites; H, L, and LL. The name chondrite is derived from the presence of the approximately mm-sized, previously partially or completely molten spheroidal silicate objects called chondrules. These objects are found in all classes of chondrites except CI (Wasson, 1985). Chondrites are essentially solar in composition with their volatile elements depleted to varying degrees. The composition of these nine distinct classes of chondrites was established in the solar nebula and thus reflects the chemical, isotopic, and physical conditions prevailing at the time of their formation.
Discrimination between the three main chondrite classes can be achieved with their nonvolatile lithophile elements. Figure 3 illustrates how the Ca/Si ratio resolves the various chondrite classes.
Figure 3.
Van Schmus Ca/Si plot.
Discrimination between the nine chondrite classes can be achieved by examining the oxidation state of their main elemental constituent, Fe (Figure 4). A plot of the amount of Fe0 versus Fe2+ not only demonstrates separation of carbonaceous, ordinary, and enstatite chondrites on the basis of the oxidation state of their Fe, but also resolves the ordinary and enstatite chondrites further into H, L, and LL ordinary chondrites and EH, EL, enstatite chondrites (Urey and Craig, 1953; Craig, 1964).
Figure 4. Urey Craig Fe oxidation state plot.
Relative abundances of the O isotopes provide an independent method of discrimination between chondrite classes. The power of this method came with the introduction of the three-isotope-procedure (Clayton et al., 1976). In this method the difference in the 17O/16O ratio between the meteorite and a standard (standard mean ocean water, SMOW) is plotted against the corresponding difference in 18O/ 16 O. These differences are expressed as per mil. Figure 5 illustrates this method's usefulness as a method of discrimination between chondrite classes. Carbonaceous chondrites plot in widely dispersed regions of this plot . The enstatite chondrites plot in similar regions of this plot with overlap between EH and EL chondrites. H, L, and LL chondrites plot in similar regions of this plot, with the H chondrites clearly resolved from the L and LL chondrites.
Figure 5. Clayton
oxygen isotope plot.
CHONDRITES: SECONDARY PROPERTIES
Secondary processes can perturb the primary (or nebular) chemical and isotopic record. Chondrites are subjected to secondary processes, though not as extreme as the secondary process that leads to differentiation. Such processes include aqueous alteration, thermal metamorphism, shock, and brecciation. In order to incorporate these processes into the classification scheme of chondrites, secondary classifications are used. Petrographic type, an integer from one to seven, is used to further classify the different classes of chondrites on the basis of the several parameters: homogeneity of olivine pyroxene compositions, structural state of low-Ca pyroxene, secondary feldspar development, igneous glass characteristics, metallic mineral characteristics, sulfide mineral characteristics, texture, C content, and H2O content (VanSchmus and Wood, 1967). This classification scheme relates primarily to aqueous alteration and thermal metamorphism.
Maximum shock pressure that material has been subjected to is related by further classification. Shock facies a-f represent increasing levels of shock as inferred from mineralogical barometers (Dodd and Jarosewich, 1979). Laboratory calibration experiments provide estimates of the corresponding pressures: a (< 5 GPa), b (5-20 GPa), c (20-22 GPa), d (22-35 GPa), e (35-57 GPa), and f ( > 57 GPa). More recently another shock classification scheme has been proposed by Stöffler et al. (1991). Information such as thermoluminescence sensitivity and trace element composition has also been found to be correlated with shock (Sears et al., 1984; Dennison and Lipschutz, 1987).
VOLATILE TRACE ELEMENTS
Volatile elements are extremely responsive to thermal processes, being either highly volatile during primary nebular condensation or extremely mobile during post-accretionary heating processes (Lipschutz and Woolum, 1988). The volatile elements Co, Rb, Ag, Se. Cs, Te, Zn, Cd, Bi, Tl and In span a wide range of physical and cosmochemical properties. This, along with their low concentration (part-per-million to part-per-trillion), allows small absolute differences in thermal history to be transformed into large relative variations in elemental concentrations. These elements thus provides unique information about the thermal history of a particular meteorite.