Chondrites chemical composition

Rietmeijer F. J. M. (1997) Interplanetary dust petrology, principal components analysis, chondrites, chemical composition, grain size, porosity, magnesium compounds, iron compounds, minerals. In Lunar Planet Set XXVIII, 1301. The Lunar and Planetary Institute, Houston (CD-ROM). 

Now, apart from the planets, many meteorites were formed, moving in quite different orbits and of quite different chemical composition. In particular, the so-called C-l meteorites composed of carbonaceous chondrites have a composition of elements much closer to that of the Sun. It is proposed (see for example Harder and also Robert in Further Reading) that many of these meteorites collided with very early Earth and became incorporated in it, so that eventually some 15% of Earth came from this material (see Section 1.11). Other planets such as Mars and the Moon could have had similar histories, but the remote planets and Venus are very different. 

The Murchison meteorite shown in Figure 6.7, like all meteorites, is named after the place from which it was recovered and in this case it is the town of Murchison, Victoria in Australia about 100 km north of Melbourne. The fall occurred in 1969 and was followed by an analysis of the chemical composition in some considerable detail. The Murchison meteorite is a carbonaceous chondrite containing about 2 per cent carbon, some as inorganic carbonates, and some as soluble compounds such as amino acids but the bulk as a macromolecular heterogeneous material referred to as kerogen. 

The Piton de la Fournaise volcano (Indian Ocean) erupts basalts with chemical compositions that change with time. The rare-earth elements have been measured on eight dated historic lavas (Table 3.9 and Figure 3.7, Albarede and Tamagnan, 1988), and chondrite-normalized (Ce/Yb)N ratios over the time interval 1948-1985 are given in Table 3.9. Calculate an annual interpolation of these results. 

A variety of techniques have been used to measure the bulk chemical compositions of Cl chondrites and other meteorites. 

Cosmochemistry is the study of the chemical compositions of various solar system materials. Chondrites are the most abundant primitive samples. They are essentially sedimentary rocks composed of mechanical mixtures of materials with different origins (chondrules, refractory inclusions, metal, sulfide, matrix), which we will call components. Chondrites formed by the accretion of solid particles within the solar nebula or onto the surfaces of growing planetesimals. They are very old (>4.5 billion years, as measured by radioactive chronometers) and contain some of the earliest formed objects in the solar system. Chondrites have bulk chemical compositions very similar to the solar photosphere, except... 

Matrix minerals are complex mixtures of silicates (especially olivine and pyroxene), oxides, sulfides, metal, phyllosilicates, and carbonates. The bulk chemical composition of matrix is broadly chondritic, and richer in volatile elements than the other chondrite components. Some chondrules have rims of adhering matrix that appear to have been accreted onto them prior to final assembly of the meteorite. Small lumps of matrix also occur in many chondrites. Presolar grains, described in Chapter 5, occur in the matrix. 

The systematic variations in oxygen isotopes provide an independent means of classifying chondrites that generates the same groups as the chemical compositions. The oxygen isotopes also work for classifying non-chondritic meteorites. Oxygen isotopic compositions are somewhat easier to obtain than detailed chemical data and so are often used to nail down a classification. 

Many asteroids are dry, as evidenced by meteorites in which water is virtually absent. These samples include many classes of chondrites, as well as melted chunks of the crusts, mantles, and cores of differentiated objects. Anhydrous bodies were important building blocks of the rocky terrestrial planets, and their chemical compositions reveal details of processes that occurred within our own planet on a larger scale. The distributions of these asteroids within the solar system also provide insights into their formation and evolution. 

The bulk chemical composition of the dust, obtained by averaging the compositions of particles in numerous tracks (Fig. 12.11a) and impact crater residues (Fig. 12.11b), is chondritic for iron, silicon, titanium, chromium, manganese, nickel, germanium, and selenium, within the 2o confidence level (Flynn el al., 2006). Copper, zinc, and gallium are... 

The average chemical compositions of Cl and CM chondrites are given in Table 12.2. We will illustrate some compositional differences among different groups using diagrams analogous to those used to describe the anhydrous chondrites in Chapter 11. 

Table 12.2 Average chemical compositions of hydrated carbonaceous chondrite groups...

A perplexing observation is that aqueous alteration appears to have been largely isochemical. The Cl chondrites, which provide the closest match to solar abundances, show the most extensive alteration. Likewise, the chemical compositions of CM chondrites are nearly uniform, despite significant differences in their degrees of aqueous alteration. Aqueous fluids can dissolve significant amounts of soluble materials during reactions, but there is little evidence in bulk Cl and CM chondrites that the dissolved material was transported anywhere by the fluids. Why solidified mud should have retained its cosmic composition is a mystery. 

Chondrites can be described as conglomerate rocks characterised by a overall chemical composition similar to the composition of the sun (with a depletion of hydrogen, helium and some other highly volatile elements). This latter characteristic is an easy way to distinguish clearly between stony meteorites and terrestrial stones. 

There is evidence from chondrites that the solar nebula was well mixed between 0.1 and 10 AU during its first several million years of the evolution, as shown by the homogeneity in concentrations of many isotopes of refractory elements (Boss 2004 Chapter 9). This is likely caused by the evaporation and recondensation of solids in the very hot inner nebula, followed by outward transport due to turbulent diffusion and angular momentum removal. Materials out of which terrestrial planets and asteroids are built have been heated to temperatures above 1300 K and are thus depleted in volatile elements. The inner solar nebula, with some exceptions, does not retain memories of the pristine interstellar medium (ISM) chemical composition (Palme 2001 Trieloff Palme 2006). 

The Apollo 11 rocks contain large amounts of ilmenite, as can be seen from Tables 2 and 3 (high titanium content). We have plotted the chemical composition of rock sample 12018 in Fig. 3a vs. that of the carbonaceous chondrites (the most primitive of all meteorites), in Fig. 3b vs. the basaltic achondrite (eucrite) Juvinas (a class of meteorites which have undergone magmatic differentiation) and in Fig. 4 vs. the average composition of the Earth s... 

Conventionally, a chondrite group is defined as having a minimum of five unpaired chondrites of similar mineralogy, petrography, bulk isotopic properties, and bulk chemical compositions in major, nonvolatile elements. According to this definition, K chondrites could be considered as a grouplet there are only two known K chondrites (see K (Kakangari-like) chondrites). 

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