Chapter 1
Major Parameters of Diamond-Bearing and Affine Rocks
The bed-rocks of diamond-bearing rocks are, as is well known, kimberlites and lamproites. Those are depth magmatic rocks usually encountered only on the ancient continental platforms and forming subvolcanic bodies - blowpipes (diatremes) or magma-bringing dykes. Affine but somewhat less depth rocks are carbonatites and alkali-ultramafic rocks of a quite wide composition range. However, their common feature is low silica contents and relatively elevated concentration of magnesium. This enables attributing all these formations with ultramafic rocks. As opposed to the classical ultramafic rocks of the peridotite series mantle origin, kimberlites, lamproites and alkali-ultramafic rocks are enriched in the titanium, alkalis (first of all, potassium), phosphorus, rare lithophilic and volatile elements including water and carbon dioxide (especially carbonatites).
The chemistry and geochemistry of diamond-bearing kimberlites and lamproites is described in many articles and monographs [1-4]. There are also numerous descriptions of the alkali-ultramafic rocks [5-7] and carbonatites [8-10]. For this reason, there is no need to analyze here in detail the chemical composition of these exotic rocks. Attention must be paid only to the geochemical specifics of kimberlites as most typical representatives of this rock class.
Analyzing specifics of kimberlites' chemical composition, I.P. Ilunin with colleagues [3] noted that the SiO2/MgO and MgO/FeO ratios in kimberlites correspond with the dunite and peridotite values whereas the Al2O3 and Na2O concentrations are notably lower than in basalts. On the other hand, in contents of some rare elements the kimberlites are close to alkaline basalts. Nevertheless, it is emphasized that no mixing of the peridotites with alkali-basaltoid matter allows to come with the kimberlitic composition. That is because any notable addition of a basaltoid matter to the peridotite will unavoidably result in an increase of the silica, aluminum and sodium concentrations, and at insignificant addition will not occur contents of rare elements typical for the kimberlites.
Also important is that compared to the mantle ultramafic rocks (peridotite and lherzolite), kimberlites are substantially enriched in titanium, potassium and phosphorus. At that, usually enrichment of the kimberlites with rare elements correlates with elevated phosphorus concentrations. To an even greater extent such correlation shows up in carbonatites [3].
The kimberlite geochemistry specifics could have been visually manifested at their comparison with samples of undepleted mantle matter. However, to our great regret, we are never dealing with fresh samples of the mantle rocks wherein the rare elements' contents would have been preserved undistorted. Instability of the dispersed elements' direct determinations in the mantle rocks brought in to Earth's surface is caused by the fact that in the process they practically always experienced a very strong influence from metamorphogenic factors which substantially distorts their primary composition in the rare element domain. Thus, if we use ultramafic rock samples from ophiolite nappes, we should take into account that their matter was at least twice subjected to hydrothermal actions. The first time, at the time of the oceanic crust formation due to its hydration by the oceanic water saturated with alkalis and other easily dissolvable elements. The second time, in the process of this crust obduction (the ophiolite nappe) on continental margins due to the action of overheated and mineralized water coming from plate subduction zones. No less distorted turns out the primary composition of dispersed elements in ultramafic xenoliths within the kimberlites themselves. It is caused by two reasons. First, it is quite likely that these xenoliths are fragments of the ancient oceanic crust pulled in the past geological epochs under the continent plates. Second, due to the fact that over the extended time of a close contact with kimberlite melts in these samples could have occurred (and have occurred) substantial metamorphic alterations [11-13]. In most cases, such alterations should have been boiled down to ultramafic rocks contamination with dispersed elements coming from mineralized hydrotherms or from the kimberlite magmas saturated with volatile components.
Nevertheless, comparisons of kimberlites and lherzolite xenoliths average chemical composition [14] are quite demonstrative. These comparisons show that kimberlites are somewhat impoverished in such major petrogenic elements as Si, Mg, Na, Cr, and Ni whereas their contents of Al, Fe, Mn and some ore elements (Co, Zn) almost correspond with their concentrations in lherzolites. But the most typical feature of kimberlite rocks is their clear enrichment with dispersed elements. This is concerning especially of lithophilic and rare earth elements. Ya. Muramatsu's determinations showed that kimberlites are enriched with carbon 150-fold, phosphorus, 25-fold, alkali (K, Rb, Sc), 24-68-fold, light rare earth elements (La-Eu), 30 to 200-fold and radioactive elements Th and U, respectively, 80- and 60-fold.
It is difficult to verify these estimates for all trace elements by independent determinations but for some of them it was possible to accomplish. Accepting the quoted Muramatsu's concentrations of K, U and Th in the mantle as the genuine ones, it is easy to calculate that the total depth heat flow generated by them and coming from the mantle had to reach 4.8·1020 erg/s. Added to this mantle flow must be heat generated by radioactive elements concentrated in the continental crust, which amounts approximately is 0.9·1020 erg/s. In this case, total radiogenic heat generation in Earth must have reached 5.7·1020 erg/s. However, another exceptionally powerful source of the heat energy is operating within the present-day Earth. This is the process of the mantle matter gravity differentiation resulting in separation within the planet's central parts of a high-density oxide-ferric core and in the initiation of convection currents in the mantle. Inclusion of this energy source (around 3·1020 erg/s) would have made total Earth's heat loss in the considered case equal to 8.7·1020 erg/s. However, actual heat loss by Earth is only half of this and is equal approximately to 4.2-4.3·1020 erg/s [15-19].
The energy estimates quoted above indicate that accepted by Muramatsu [14] concentrations of a part of trace elements (K, U, and Th) in the mantle matter are clearly overvalued whereas the enrichment factor of kimberlite rocks with the same elements, substantially underestimated. More correct analysis of Earth's energy balance accounting for the energy of tidal interaction of Earth with Moon dispersed in mantle (close to 0.1·1020 erg/s), for the potassium content and K/U and K/Th ratios in the continental crust and in lunar rocks enabled us to determine that Earth's mantle currently contains no greater than 0.012% K, 2.6·10-7% U and 7·10-7% Th [19]. Assuming these concentration values, we come up with the kimberlite enrichment with potassium reaches not 24 but 87-fold. For uranium and thorium, the values are even greater: respectively, 1,200 and 2,300-fold (instead of 62- and 80-fold).
The above example of independent radioactive elements' mantle concentrations estimates is begging for a general conclusion that for some other trace elements the extent of kimberlite rocks enrichment, compared with their Clarke contents in the mantle matter may turn out substantially greater than determined by Muramatsu [14].
Our estimates (see below) give the mantle content of about 110 g/t of carbon dioxide and no greater than 0.05% water. According to J. Dawson [1], the kimberlites contain around 3.3-7.1% CO2 and 5.9-18.7% H2O. Therefore, the kimberlites are enriched in these volatile compounds respectively 300-650 and 120-370-fold.
It is, however, noteworthy that in kimberlite minerals [1] and even within the diamond crystals [20, 21] are often encountered inclusions of gaseous and liquid hydrocarbons and even alcohols and more complex organic compounds.
Accounting for all these factors is making even more acute the problem of the kimberlites' origin and of determining the mechanism of so great enrichment of these rocks with lithophilic elements with simultaneously of the silica contents in them. At that, a question needs to be answered of where hydrocarbons in inclusions, with specific for them negative isotopic shifts for the carbon, are coming from.
By definition [4], lamproites are a community of high-magnesium potassium alkaline rocks saturated or slightly undersaturated with the silica and with low contents of aluminum and calcium. The lamproite composition, compared with the kimberlites, is distinct in much broader variability. However, always typical for them are the highest concentrations of potassium (up to 7-10% K2O), rubidium (up to 300-500 g/t) and barium (up to 5,000-10,000 g/t), elevated content of strontium (up to 1,000-4,000 g/t) and light rare earth's elements (up to 300-600 g/t of La, up to 600-1,000 g/t of Ce, up to 250-500 g/t of Nd and up to 15-30 g/t of Sm). In the magnesium content (between 8 and 24% MgO), lamproites occupy an intermediate position between the mantle matter and basalts. On the other hand, they are enriched with the uranium (1 to 10 g/t) and thorium (12 to 150 g/t) respectively 400- to...
