Sodium chondrites

The largest class of meteorite finds is stony meteorites, made principally of stone. The general stony classification is divided into three subclasses called chondrites, carbonaceous chondrites and achondrites, and it is at this level of distinction at which we will stop. Before looking at their mineral and isotopic structure in more detail, it is useful to hold the composition of the Earth s crust in mind here for comparison. The Earth s crust is 49 per cent oxygen, 26 per cent silicon, 7.5 per cent aluminium, 4.7 per cent iron, 3.4 per cent calcium, 2.6 per cent sodium, 2.4 per cent potassium and 1.9 per cent magnesium, which must have formed from the common origin of the solar system. 

The aubrites are the most reduced achondrites (Keil et al., 1989). Their silicates are essentially free of iron, and they contain minor metallic iron. A variety of unusual sulfides of calcium, chromium, manganese, titanium, and sodium - all usually lithophile elements -occur in aubrites. These unusual sulfides also characterize the highly reduced enstatite chondrites, which may have been precursors for these rocks. 

In Figure 3, sodium, zinc, and sulfur are representative of the abundances of moderately volatile elements (Figure 2 and Table 2). Abundance variations reach a factor of 5 for sulfur and 10 for zinc. All three elements show excellent agreement of solar with Cl abundances, in contrast to other groups of chondritic meteorites, except for the enstatite chondrites, which reach the level of Cl abundances. However, enstatite chondrites... 

The bulk compositions of chondrites closely match the compositions of the solar photosphere, with the exception of a few highly volatile elements (hydrogen, carbon, nitrogen, helium) and lithium (see Chapter 1.03). The chondrite groups also show different levels of depletions in moderately volatile elements (e.g., manganese, sodium, potassium, gallium, antimony, selenium. 

Figure 5 Concentrations of soluble elements in CM chondrites, illustrating the restricted compositional range of these elements in chondrites with variable degrees of alteration, (a) Sodium versus potassium and (b) manganese versus calcium (sources Kallemeyn and Wasson, 1981 Jarosewich, 1990).

Figure 8 Bulk sodium versus iron contents of oxidized and reduced CV chondrites and dark inclusions. The enrichment in sodium and iron of the oxidized group appears to be the result of metasomatic processes (sources Kallemeyn and Wasson, 1981 Kracher et ai, 1985 Bischoff et al., 1988 Palme et al., 1989 ...

Jones R. H. (1997) Alteration of plagioclase-rich chondmles in C03 chondrites evidence for late-stage sodium and iron metasomatism in a nebular environment. In Workshop on Parent-body and Nebular Modification of Chondritic Materials (eds. M. E. Zolensky, A. N. Krot, and E. R. D. Scott). Lunar and Planetary Institute, Houston, pp. 30-31. 

Itqiy is distinct from chondritic meteorites in bulk composition. Aluminum, FREE, europium, sodium, potassium, vanadium, chromium, and manganese are aU depleted. Itqiy has La/Yb of 0. lOxCI, and Eu/Sm of 0.16 X Cl. Refractory siderophile elements are enriched —2-3 X Cl, while moderately volatile siderophile elements are at roughly Cl abundances. The bulk rock Mg/Si and Fe/Si ratios are greater than those of EH or EL chondrites. 

Tests. If Mercury is this refractory, its surface will have high concentrations of aluminum, calcium, magnesium, no FeO, sodium, or potassium, and chondritic abundances of thorium and uranium. The surface mineral assemblage would be like that of CAls. (Jeanloz et al. (1995) note that there is no spectral evidence for perovskite on Mercury. This observation, combined with the presence of at least some FeO, dampens enthusiasm for this type of extreme model.)... 

In addition to making comparisons with chondrites, the bulk composition of the Earth also has been defined in terms of a model mixture of highly reduced, refractory material combined with a much smaller proportion of a more oxidized volatile-rich component (Wanke, 1981). These models follow on from the ideas behind earlier heterogeneous accretion models. According to these models, the Earth was formed from two components. Component A was highly reduced and free of all elements with equal or higher volatility than sodium. All other elements were in Cl relative abundance. The iron and siderophile elements were in metallic form, as was part of the silicon. Component B was oxidized and contained all elements, including those more volatile than sodium in Cl relative abundance. Iron and all siderophile and lithophile elements were mainly in the form of oxides. 

The concentrations of four typical moderately volatile elements—manganese, sodium, selenium, and zinc—in the various classes of chondritic meteorites are shown in Figure 12, where elements are normalized to magnesium and CI-chondrites. Again there is excellent agreement between solar abundances and Cl-meteorites. A characteristic feature of the chemistry of carbonaceous chondrites is the simultaneous depletion of sodium and manganese in all types of carbonaceous chondrites, except Cl. Ordinary and enstatite chondrites are not or only slightly... 

In Figure 13 the Earth s mantle seems to extend the trend of the moderately volatile elements to lower abundances, at least for sodium, manganese, and zinc (zinc behaves as a lithophile element in the Earth s mantle (see Dreibus and Palme, 1996)). The elements lithium, potassium, and rubidium which are not plotted here, show similar trends. The carbonaceous chondrite trend of iron is not extended to the Earth, as most of the iron of the Earth is in the core. The magnesium abundance of the Earth shows a slightly different trend. If the core had 5% silicon (previous section) and if that would be added to the bulk Earth silicon, then the bulk Mg/Si ratio of the Earth would be the same as that of carbonaceous chondrites (Eigure 10) and the silicon abundance of the Earth s mantle in Figure 13 would coincide with the magnesium abundance. 

Figures 12 and 16). The Mn/Na ratio of the Earth s mantle is also chondritic (Figures 12 and 16). Heating of meteorite samples to temperatures above 1,000 °C for a period of days will inevitably lead to significant losses of sodium and potassium but will not affect manganese... 

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