Mantle carbon isotope composition

CO2 is the second most abundant gas species in magmatic systems. In a survey of CO2 emanations from tectonically active areas worldwide, Barnes et al. (1978) attributed 8 C-values between -8 and -4%c to a mantle source. This is, however, problematic, because average crustal and mantle isotope compositions are more or less identical and surflcial processes that can modify the carbon isotope composition are numerous. A more promising approach may be to analyze the C-content of CO2 collected directly from magmas at high temperatures. 

Nearly all lower-mantle diamonds have such low nitrogen that they are classified type II (McCammon, 2001). These features are very distinctive from upper-mantle diamonds, which are chiefly type la and have widely variable percentages of A, A/B, and B centers. The carbon isotopic composition of ultradeep, lower-mantle diamonds is surprisingly homogeneous, most samples having typical upper-mantle values of... 

Figure 64 Comparison of the carbon isotopic composition of diamonds containing inclusions ascribed to transition zone and sublithospheric depths (TZ-UM clear) to diamonds with inclusions derived from lower mantle depths (LM gray). The number of specimens is given for each group (n = 132). Note that the —4 to —6 bar for lower mantle diamonds is off-scale at a number of 68. Transition zone and upper mantle diamonds are from these kimberlites Jagersfontein (6), Juina (1), KanKan (5), Orapa (1), Premier (1), and Sao Luiz (10). Lower mantle diamonds (and the number analyzed from each) are from these kimberlites Dokolwayo (1), DO-27 (5), Juina (30), KanKan (36), Koffiefontein (3), Letseng-la-Terai (1), and Sao Luiz (33) (sources Daniels and Gurney, 1999 Davies et al, 1999 Deines etal, 1989,1991a, 1993 Hutchison eta/., 1999 Kaminsky etal, 2001 McDade and Harris, 1999 ...

Deines P., Harris J. W., and Gurney J. J. (1987) Carbon isotopic composition, nitrogen content and inclusion composition of diamonds from the Roberts Victor Kimberlite, South Africa evidence for C depletion in the mantle. Geochim. Cosmochim. Acta 51, 1227-1243. 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

Bonifacie M, Jendrzejewski N, Agiinier P, Humler E, Coleman M, Javoy M (2008) The chlorine isotope composition of the Earth,s mantle. Science 319 1518-1520 Borthwick J, Harmon RS (1982) A note regarding CIF3 as an alternative to BrFs for oxygen isotope analysis. Geochim Cosmochim Acta 46 1665-1668 Bottcher ME (1996) and C/ C fractionation during the reaction of carbonates with... 

Carbon. Galimov (1968) pointed out two lines of evolution of carbon. The first is traced from gaseous CO2 and CH4 coming from the mantle and endogenetic minerals into the carbon of the carbonaceous matter of the mantle and later into the carbonaceous matter of meteorites (Fig. 27). By far the largest part of the carbon characterized by a heavy isotopic composition (5 C from —4 to —7%) arrived in the Earth s crust from the mantle by that route. The second line is related to carbide matter in the mantle and meteorites, which does not enter into the formation of gaseous compounds. Carbon of the first line of evolution predominates in the Earth s crust. 

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