Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.239j&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 239
Mathematics
Logic
Scientific paper
The "primitive materials," i.e., carbonaceous chondrites (CV,CI,CM) [1] and interplanetary dust particles [2,3] have Mg/Si ratios similar to those of the solar photosphere [1]. The bulk composition of the Earth and that of the terrestrial planets are believed to have higher than "primitive" Mg/Si ratios [4], whereas ordinary and enstatite chondrites have lower than primitive Mg/Si ratios [5]. The separation of Mg from Si may result from reduction of silicon into a metallic phase through the reaction of silicates with C or H2(6) or through temperature-induced reduction of silicates (oxygen volatilization) [7,8]. Reduction leads to the formation of a residuum with higher than original Mg/Si ratios. High temperatures may also cause the vaporization of silicon and also lead to higher Mg/Si ratios in the residual phase [4]. Lower Mg/Si ratios could result from partial melting of a primitive material in which Si preferentially enters the melt, leaving a high Mg/Si residuum. The recrystallization of silicon- bearing metal in the presence of silicates may also provide a mechanism to decrease Mg/Si ratios. This process is documented by the presence of silica/silicates in the meteoritic metals (CV3 Leoville and Efremovka and other chondrites, e.g., Renazzo, ALH85085 [9]). Since Mg/Si fractionation may result from the extreme temperatures, the other siderophile and volatile element abundances must also be considered. The high contents of volatile elements in enstatite chondrites exclude the possibility of such high-temperature processes, and condensation under highly reducing conditions best explains the observed abundances. The aubrites, i.e., enstatite achondrites, however, all have attributes of high-T, low f[O(sub)2] processing and may have formed through the recrystallization of superheated highly fractionated (planetary processed?) material. An explanation of the Earth's higher than primitive Mg/Si was sought in the differences of volatility of Si and Mg, and hence in the thermal regime during the formation of the early solar system [4] and in the siderophile behavior of Si under reducing conditions [10]. Since Palme and Nickel [5] have shown that the Mg number (Mg/Si ratio) in the upper mantle of the Earth relates to the degree of fractionation of the mantle, e.g., to Ca/Al ratios. The explanation of different Mg/Si ratios between the primitive chondrites and Earth's mantle could be found in mantle fractionation, i.e., the formation of the Earth's crust. We argue for the chondritic composition of the Earth in respect of Mg/Si and that the ancient upper mantle was significantly fractionated prior to 3.9 AE [11,12,13]. The early fractionation is reflected in the high Mg/Si of the present peridotitic mantle. The upper mantle has been fractionated throughout geologic time due to the formation of the basaltic melts that contribute to the growth of the Earth's crust. These melts are rich in Si relative to Mg, whereas the residuum is rich in Mg relative to Si. The early melts were either erupted on the surface or retained within the mantle. The erupted portion may be returned to the mantle in the form of eclogite in the same way that recent oceanic "basaltic" crust (with eclogite mineralogy) is recycled to the mantle [14]. Since the eclogites do not completely homogenize with the upper mantle and retain their petrological and geochemical identity [15], the formation of basalts (eclogites) strongly influences the Mg/Si ratio of the noneclogitic, i.e., peridotite mantle (residual mantle has higher Mg/Si whereas basalt has lower Mg/Si than the original parent rock). The "lost" crustal component [11] (3.9 AE old) may be present in the upper mantle (transition) zone in its eclogitic (or griquaitic) form. We suggest therefore that sampling of the upper mantle through the analysis of pristine peridotites does not represent the bulk mantle but represents only one major mantle component. Mantle sampling is biased toward the rocks we interpret as primitive. The Earth's upper mantle may well be chondritic in respect to Mg/Si, if the eclogites present are added back into the estimates of mantle composition. References. [1] Anders E. and Grevesse N. (1989), Geochim. Cosmochim. Acta, 53, 197-214. [2] Rietmeijer F.J.M. (1987), Abs. LPSC 18, 832-833. [3] Dikov P.Yu. et al. (1990), Geokhimiya, 789-795. [4] Ringwood A.E. (1989), Earth Planet. Sci. Lett., 95, 1-7. [5] Palme H. and Nickel K.G. (1985), Geochim. Cosmochim. Acta, 49, 2123-2132. [6] Baedecker P.A and Wasson J.T. (1975), Geochim. Cosmochim. Acta, 39, 735-765. [7] Jakes P. et al., (1992), Abs. LPSC 23, 599-600. [8] Dickinson T.L. et al., (1992), Abs. LPSC 23, 309-310. [9] Zanda B. (1992), Abs. LPSC 23, 1569-1570. [10] Wanke H.(1981), Phil. Trans. R. Soc. Lond., A 303, 287-302. [11] Harper C.L. and Jacobsen S.B. (1992), EOS, 323. [12] Collerson K.D. et al. (1991), Nature, 349, 209-214. [13] Galer S.G.G and Goldstein S.L. (1991), Geochim. Cosmochim. Acta, 55, 227-239. [14] Ringwood A.E. [15] Hatton C.J. and Gurney J.J. (1987), in Mantle xenoliths P.H. Nixon ed., John Wiley & Sons.
Jakes Peter
Reid Michael A.
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