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The Chemical Composition of the Earth EPSL ELSEVIER Earth and Planetary Science Letters 134 (1995) 515-526 The chemical composition of the Earth Claude J. Allkgre a, Jean-Paul Poirier b, Eric Humler a, Albrecht W. Hofmann apt a Laboratoire de G6ochitnie et Cosmochimie, Institut de Physique du Globe de Paris et lJniversit6 Paris VII, 4 place Jussieu, 75252 Paris Cedex 05, France b Laboratoire des G6omati?iaux, Institut de Physique du Globe de Paris, 4 place Jussiey 75252 Paris Cedex 05, France ’ Max-Planck-Institutfur Chemie, Postfach 3060, 55020 Ma&, Germany Received 18 July 1994; accepted 6 July 1995 Abstract The bulk composition of the Earth and the composition of the mantle and core are calculated using the ratios of major and trace elements. The ratios of elements which do not enter the core (lithophile) are the same in the bulk Earth as in the mantle. Bulk earth ratios involving an element that does enter the core (siderophile) are therefore determined from meteorite correlation diagrams of siderophile-lithophile ratios vs. lithophile-lithophile ratios and from primitive mantle composition in elements which do not enter the core (e.g., Al). The composition of the core is determined by difference, without resorting to assumptions about core formation processes. It is found that the core contains about 7.3 wt% silicon and 2.3 wt% sulphur. To account for the seismologically determined density deficit of the core, about 4 wt% oxygen must be added. The present results are compatible with the idea that the core material equilibrated at low pressure, in reducing conditions. Furthermore, we propose that the Earth is closer to CM rather than to Cl for non-volatile element ratios. 1. Introduction decrease its density below that of pure iron [I]. Knowledge of the nature and proportion of these The determination of the chemical composition of elements would constrain models of core formation. the Earth and its principal units, the mantle and core, Pioneering work on these questions was done by is one of the major goals in the Earth Sciences. Goldschmidt [2] and, later, by Ringwood [3-81, to Knowledge of this composition will constrain many whom we dedicate this paper. geophysical and geochemical problems. From a The classical contributions to this problem were chemical point of view, a knowledge of the bulk based on guesses which identified the Earth with a earth chemical composition will constrain models of specific type of chondrite or as a mixture of different continental crust differentiation and the structure of types of chondrites. A different approach, based on mantle convection through isotopic and chemical data from terrestrial as well as meteoritic materials, mass balance considerations. Geophysical evidence has been used in more recent years to compute the points to the core containing light elements that composition of the Earth. This combines the use of Elsevier Science B.V. SSDI 0012-821X(95)00123-9 516 C.J. AllSgre et al. /Earth and Planetary Science Letters 134 (1995) 515-526 the major elements with that of trace elements. In a crustal rocks, and these uniform ratios do conform to first attempt in this direction, Loubet et al. [9] used chondritic ratios [12]. This ‘coincidence’ constitutes the fact that REE patterns are similar for all types of substantial independent confirmation of the chon- meteorites and looked for peridotites with similar dritic hypothesis. The assumption of an essentially primitive compositions. Subsequently, Jagoutz et al. chondritic Earth has been implicitly or explicitly [lo] achieved a major breakthrough by using the made by all authors who have estimated the chemi- intersection of a chondritic trend with a mantle trend cal composition of the Earth. in Mg-Si-Al space. This approach has since been We will follow the common approach but with a slightly refined by Hart and Zindler [ 111. slightly different philosophy. We do not a priori Our approach differs from that of Jagoutz et al. identify the Earth with any specific class of mete- [lo] in that we assume that the intersection of the orites (such as CI chondrites). We only assume that meteoritic trend with the mantle trend in Mg-Si-Al the composition of the Earth follows the meteoritic space does not represent a priori ‘true’ primitive chemical trends in a manner similar to the postulates mantle, because Si may enter the core, so that the of Jagoutz et al. [lo] and Hart and Zindler [ll]. It silicon content of the mantle is reduced by core must be noted that for ratios of highly refractory formation. Our present calculations specifically take non-siderophile elements the chondritic trend in- this possibility into account, whereas the method of cludes both carbonaceous and ordinary chondrites Jagoutz et al. [lo] eliminated it a priori. but that for other elements there is one trend for the carbonaceous chondrites and another for the ordinary chondrites. This is particularly clear for ratios involv- 2. Basic assumptions ing Fe or Ni, which clearly separate L, LL and EL chondrites from other meteorites, as well as for ratios We use the following assumptions, which are also involving sulphur and the Nd/Ca ratio [ll]. implicit in most previous treatments of the subject, The idea that the mantle is homogeneous in terms although they have usually not been specifically of major elements is based on two simple considera- stated. tions. The first of these is straightforward. The conti- (1) For concentration ratios of elements that are nental crust, which is clearly different from the not highly volatile, we assume that the Earth follows mantle, constitutes such a small mass fraction of the the compositional trends of chondrites, or, as may total silicate portion of the Earth (about 0.5%) that happen, carbonaceous chondrites. extraction of the crust does not significantly alter the (2) For major elements, we assume that the man- concentration of the major elements of the residual tle is chemically homogeneous (i.e., that the lower mantle, even if this residual reservoir is restricted in mantle has the same composition as the upper man- volume to the upper mantle. This is a consequence of tle). The estimate of the composition of the primitive simple mass balance considerations which need not upper mantle is therefore valid for the bulk mantle. be elaborated upon here. The Earth’s mantle was (3) We accept the seismological and laboratory undoubtedly well mixed in the beginning, as core evidence which indicates that the Earth’s core is differentiation implies a large turnover. It is difficult made of 80-90% Fe-Ni alloy and of light elements to imagine a later process that could have altered this that lower its density by lo-15% with respect to that early homogeneization of major elements. Recent of pure iron in the same conditions. seismological observations [ 131 and geochemical Each of these assumptions can be challenged, but modelling [14] suggest that a fair amount of ex- not with an infinite degree of freedom. The hypothe- change takes place between the upper and lower sis that different classes of chondrites represent the mantle, which also favours homogeneization. complete set of primitive planetary materials existing The second consideration is more controversial, in the early solar system from which the Earth was although in our opinion unduly so. Claims that the made has never been proven and probably never will lower mantle is enriched in iron or silicon with be. Nevertheless, certain concentration ratios are ap- respect to the upper mantle, based on comparison of proximately uniform in most mantle-derived and density or seismic velocity profiles with profiles Cf. AUgre et al. /Earth and Planetary Science Letters 134 (I 995) 515-526 517 derived from mineral physics experiments, are cer- tive’ mantle) to determine the chemical composition tainly not universally accepted (e.g., 1151). The most of the core by difference. recent efforts to determine the pressure-tempera- ture-density relationships of mantle mineral assem- blages are consistent with a compositionally uniform 3. Chemical composition of the primitive mantle pyrolite model for the entire mantle 1161. Chemical (PRIMA) heterogeneity of the mantle, although not impossible, is clearly not required by seismological and mineral The chemical composition of what is called physics data. The strongest geochemical constraints ‘primitive’ mantle, i.e., the average mantle before against a major element chemical stratification of the extraction of the continental crust, is obtained from a mantle resulting from the crystallization of a magma set of samples from the upper mantle, even though ocean have been provided by the combined weight these samples have already been processed by the of analytical data on upper mantle peridotites and complex chemical machine of plate tectonics, which their basaltic derivatives and the partitioning experi- involves extraction and re-injection of partially ments of Kato et al. [17]. Although the literature is melted material as well as recycling of oceanic and still somewhat divided on this issue, it appears that continental crust. the defenders of the chemically stratified mantle for We use the same approach as employed by Lou- major elements must resort to ever more elaborate bet et al. [9], Jagoutz et al. [lo] and Hart and Zindler models in order to evade the obvious conclusion, [ll]: we consider the intersection of the chemical namely that this is an idea whose time has come and trend of peridotitic materials with the meteorite trend. gone [18]. The major-and fundamental-difference compared There is a third assumption that is not very con- to the methods of Jagoutz et al. [lo] and Hart and troversial. The meteorite model for the Earth virtu- Zindler [ll] is that we eliminate in this construction ally requires a major terrestrial repository for iron all elements which are suspected of entering the core and nickel, and the core is the only place on Earth (namely Fe and Ni) but also the light element ‘core where such quantities of these metals could be stored.
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