Earth chondrite model

Sanloup, C., Jambon, A. and Gillet, P. (1999) A simple chondritic model of Mars. Earth and Planetary Science Letters, 112, 43-54. 

The issue of niobium in the core is of particular interest for the chondritic model of the bulk Earth. Niobium has always been thought to be refractory and hthophile, yet it is depleted in the upper mantle relative to other refractory and lithophile elements. Failure to locate hidden niobium-rich reservoirs in the lower mantle or the core would lead to serious problems for the well-established chondritic model. Recently, Wade and Wood (2001) studied partitioning of niobium between hquid metal and liquid sihcate under high pressure and temperature. They found that niobium becomes more siderophile with increasing pressure, hence opening up the possibility of storing niobium in the core. 

Data for the content of lithophile elements in the Earth plus knowledge of the iron content of the mantle and core together establish a bulk Earth compositional model (McDonough, 2001). This model assumes chondritic proportions of Fe/Ni in the Earth, given limited Fe/Ni variation in chondritic meteorites (see below). This approach yields... 

Javoy M. (1995) The integral enstatite chondrite model of the Earth. Geophys. Res. Lett. 22, 2219-2222. 

A chondritic model for the Earth s mantle is usually based upon the composition of Cl chondrites, adjusted for the loss of volatile elements and for the separation of the siderophile elements into the core. This leads to a mantle composition which is enriched in refractory lithophile elements by about 1.5 times the Cl chondrite value. There are however difficulties with the chondritic model because Cl chondrites and the Earth have different Mg/Si ratios (Fig. 3.9), as was discussed in Chapter 2 (Section 2.4.4). Some authors believe that this difference is original and dates from processes within the solar nebula indicating different evolutionary histories between the two. If this is true then Cl chondrites are not an appropriate starting composition for the composition of the bulk Earth and alternative models have to be considered. 

FIGURE 3.9 The geochemical/ cosmochemical fractionation diagram for the Earth showing the various estimates of primitive mantle composition discussed in the text. The elliptical shaded area is the field for Cl chondrites and the analyses in Table 3.1 are plotted as weight ratios, for the chondritic model (C), the pyrolite model (P) and the peridotite-chondrite model (PC). 

As an aside, it is also worth noting that a close look at the isotopic evolution of the Earth s mantle challenges the chondritic model for the Earth. This is particularly clear for the Lu-Hf and Re-Os isotope systems, where there is uncertainty about which meteorite type is the most appropriate starting point for the isotopic evolution of the Earth. Recent studies of chondritic Sm-Nd ratios by Boyet and Carlson (2005) further support the nonchondritic view of the bulk Earth. 

The slope of the best-fit line proportional to an age of 3.54 0.03 Ga the intercept of this line on the Nd/ Nd a3ds where Sm/ Nd - 0 is the initial Nd/ Nd ratio and is 0.50809 0.00004. The epsilon value (e j +L16) is a measure of the difference between the initial ratio and a chondritic model for the Earth s mantle at 3.54 Ca expressed in parts per 10 000 (see Section 6.3.4 and Box 6.2). The positive value for epsilon indicates thar the volcanic ro< were derived from a (slightly) depleted mantle source at 3.54 Ga, although there are large errors on this estimate. 

TrCHUR model ages The CHUR model assumes that the Earth s primitive mantle had the same isotopic composition as the average chondritic meteorite at the formation of the Earth, which in this case is taken to be 4.6 Ga. For neodymium isotopes CHUR is synonymous with the composition of the bulk Earth. A model age calculated reladve to therefore is the time in the past at which the... 

Thus compositional differences are best considered relative to a normalizing parameter which takes into account the age of the sample. One such reference point is the chondritic model of Earth composition (CHUR) and its evolution through time. Thus crustal samples may be plotted on a graph of isotopic composition vs time relative to a reference line such as the CHUR reservoir (see Section 6.3.3). For... 

CHondridc Uniform Reservoir — the chondritic model for the composition of the bulk earth Chemical Index of Alteration — a measure of the degree of chemical weathering... 

The U-Pb system has been a chronometer of choice for the Earth s age since the pioneering study of Clair Patterson in 1956, as discussed in Chapter 8. Virtually all U-Pb model ages of the Earth (reviewed by Allegre et al., 1995) are younger than the ages of chondritic and achondritic meteorites. The oldest Pb-Pb age, based on ancient terrestrial... 

The origin of the components that were accreted to make up the planets is the subject of intense discussion. Chondrite-mixing models attempt to build the planets using known chondritic materials. These models are constrained by the mean densities, moments of inertia, and, to the extent that they are known, the bulk chemical and isotopic compositions of the planets. Mars and 4 Vesta can be modeled reasonably well by known types of chondritic material (Righter et al., 2006). However, the Earth seems to have formed, at least in part, from materials that are not represented in our collections of chondritic meteorites (see below). 

The W isotopic compositions of various terrestrial samples, chondrites, iron meteorites, basaltic achondrites, lunar samples, and Martian meteorites are expressed as deviations in parts per 104 from the value for the silicate earth (such as the W in a drill bit or chisel), which are the same as those of average solar system materials, represented by carbonaceous chondrites. These values are summarized in Fig. 8.9, from which it can be seen that early segregated metals such as the iron meteorites and metals from ordinary chondrites have only unradiogenic W because they formed early with low Hf/W. The time differences between metal objects segregated from parents with chondritic Hf/W are revealed by the differences in W isotopic compositions between each of the metal objects and chondrites. The Hf-W model ages of all these metals indicate that all of their parent bodies formed within a few million years, implying rapid accretion in the early history of the solar system. 

Chondrites and the Composition of the Disk from Which Earth Accreted Chondritic Component Models Simple Theoretical Components... 

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. 

Ringwood (1979) first proposed these models but the concept was more fully developed by Wanke (1981). In Wanke s model, the Earth accretes by heterogeneous accretion with a mixing ratio A B — 85 15. Most of component B would be added after the Earth had reached about two thirds of its present mass. The oxidized volatile-rich component would be equivalent to Cl carbonaceous chondrites. However, the reduced refractory rich component is hypothetical and never has been identified in terms of meteorite components. 

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