Carbon in mantle

Canil D. (1990) Experimental study bearing on the absence of carbonate in mantle-derived xenoliths. Geology 18, 1011-1013. 

Ionov D. A. and Harmer R. E. (2002) Trace element distribution in calcite-dolomite carbonatites from Spitskop inferences for differentiation of carbonatite magmas and the origin of carbonates in mantle xenoliths. Earth Planet. Sci. Lett. 198, 495-510. 

Despite the paucity of evidence from natural samples for carbonate being stable in the mantle, there has been much experimental work since the 1970s devoted to understanding the behavior of oxidized carbon (as CO2 and carbonate) at pressure and temperature conditions of the Earth s mantle (see reviews by Wyllie, 1995 Luth, 1999 Wyllie and Ryabchikov, 2000). As outlined by Eggler and Baker (1982) and by Luth (1993), understanding the stability of oxidized carbon provides fundamental constraints on the stability of reduced carbon in mantle mineral assemblages. 

A first-order minimum estimate of the mass of carbon in the mantle can be made by assuming that the MORB-source is typical of the mantle as whole, and that it contains 20 ppm C, in keeping with the new results on low carbon in the MORB source and the low content of carbon in mantle minerals. In this case the mantle contains 8.6 x 1022 g (86 X 106 Gt) of carbon. This estimate is smaller than, but of the same order of magnitude as, that of Zhang and Zindler (1993) who used C/3He ratios to estimate the carbon content of the degassed mantle reservoir as 22 x 1022g (1.8 x 1022 mol). 

Carbonatites have been defined as rare-metal-bearing rocks composed mainly of carbonate of mantle origin, formed in close association with alkaline rocks (Woolley Kempe 1989). However, there... 

Ives et al. (79) tended to reject our hypothesis that brown colours of mixed oxides (and in particular less pure NdaOs) are due to traces of praseodymium. However, these authors noted the interesting effect that such dark colours (also of Pro,oaTho.9802) bleach in the reflection spectrum at higher T. It was noted that mantles of NdaOa alone rapidly hydrate to a pinkish powder (carbonate ) in humid air. It is weU-known that -type sesquioxides are far more reactive, and for instance dissolve almost instantaneously in aqueous acid, than cubic C-type samples. Ives et al. 19) also studied the broad continuous spectrum of the orange light emitted from Thi- 11 0 2+2/ where the oxidation state of uranium is rather uncertain. 

The first applications of the rare earth elements / as already mentioned, were in the optical field, namely the Auer incandescent mantles and the arc light carbons. In 1964/65 as a result of the work of Levine and Palilla the use of the truly rare and therefore expensive europium together with yttrium made a major leap forward for the rare earth industry as red phosfdiors in color TV screens. Due to the strong and sharp emission line of europium at 610 A, without a yellow component, viiich is... 

From available, though approximate, estimates, about 1023 g of carbon-containing gases are concentrated in the rocks of the Earth s crust and mantle (lithosphere) (Korstenshtein, 1984 Sokolov, 1971). This mass of carbon exceeds by approximately 104 times the amount present today in the biosphere (over the Earth surface). Between the biosphere and lithosphere there is a constant, very intensive exchange of carbon that is self-regulatory. From the data of Barenbaum (2000, 2002), due to the Le Chatelier principle (Krapivin et al., 1982), the content of mobile carbon in the system tries to attain a stable relationship ... 

Figure 30 Primitive mantle normalized multielement patterns for carbonates from mantle xenoliths, modified from Ionov and Harmer (2002). Data shown as points are from Mongolia peridotite xenoliths, compared to carbonates in xenoliths from Kerguelen, Tanzania, and Patagonia. See Ionov (1998) and Ionov and Harmer (2002) for data sources.

The presence of diamond and graphite in mantle-derived samples such as kimberlites and the xenoliths they host is prima facie evidence that neutral carbon is stable in the Earth s mantle. Outstanding questions remain, however, concerning the stability of neutral carbon in areas other than those beneath continental cratons, and concerning the mechanism by which diamond forms. To a large extent, the latter question revolves around the unresolved problem of the oxidation state of the mantle, and how—and if— the oxidation state is controlled. 

Carbonate is rarely found in mantle-derived xenoliths. It has been found in mantle-derived garnets (McGetchin and Besangon, 1975 Smith, 1987), clinopyroxenes (Hervig and Smith, 1981) and in rare xenoliths (e.g., Ionov et al., 1993a, 1996 Lee et al., 2000 Laurora et al., 2001). 

Subducted carbon as carbonate can survive into the deep mantle either as carbonate or as neutral carbon, depending on the oxidation state, and indeed on what controls the oxidation state. Outstanding questions include the relative importance—and abundance—of carbonate versus elemental carbon in the mantle, with the attendant unresolved question of what controls the oxidation state of the mantle. 

Luth R. W. (1999) Carbon and carbonates in the mantle. In Mantle Petrology Field Observations and High Pressure Experimentation A Tribute to Francis R. (Joe) Boyd, Geochemical Society Special Pubhcation No. 6 (eds. Y. Fei, C. M. Bertka, and B. O. Mysen). Geochemical Society, Houston, pp. 297-322. 

Mathez E. A., Blade J. D., Beery J., Maggiore C., and Hollander M. (1984) Carbon abundances in mantle minerals determined by nuclear reaction analysis. Geophys. Res. Lett. 11, 947-950. 

McGetchin T. R. and Besanfon J. R. (1975) Carbonate inclusions in mantle-derived pyropes. Earth Planet. Sci. Lett. 18, 408-410. 

Schiano P., Clocchiatti R., Shimizu N., Weis D., and Mattielh N. (1994) Cogenetic sihca-rich and carbonate-rich melts trapped in mantle minerals in Kerguelen ultramafic xenoliths implications for metasomatism in the oceanic upper mantle. Earth Planet. Sci. Lett. 123, 167—178. 

Smith D. (1979) Hydrous minerals and carbonates in peridotite inclusions from the Green Knobs and Buell Park kimberlitic diatremes on the Colorado Plateau. In The Mantle Sample Inclusions in Kimberlites and other Volcanics (eds. E. R. Boyd and H. O. A. Meyer). American Geophysical Union, Washington, DC, pp. 345-356. 

---