by Oleg SOROKHTIN, Dr. Sc. (Phys. & Math.), P. P. Shirshov Institute of Oceanology, Russian Academy of Sciences;

Nikolai SOROKHTIN, Dr. Sc. (Geol. & Mineral.), Geological Institute of the Kola Research Center, Russian Academy of Sciences

The diamond is the hardest and the most expensive mineral on earth. For a long time it remained the most enigmatic stone in the world. Small wonder that its name originates from the Greek word adamas, which means insuperable, indomitable, inaccessible. Acad. Alexander Fersman (1883 - 1945), a great expert in diamonds, noted that the inaccessibility of diamond "runs through the entire history of this mineral species, as it has always and everywhere defied man, neither yielding to a polisher's hand, nor to the strongest reagents of a chemist, nor to the inquisitive mind of a scientist". Because of its exceptional hardness, it could be processed only by means of another diamond or diamond powder. But the most astonishing fact is that the chemical composition of this hardest mineral and of very soft graphite and of loose soot is alike: these are modifications of the same element-carbon. This connection was first noted by Antoine Lavoisier, a French chemist, in 1772.

The origination of diamonds and diamond rocks remained a sealed book for a long time. The hypotheses were many, including the plant origin. However, after the discovery of diamond deposits in the 1870s in subvolcanic formations (volcanic pipes) in the Kimberly province of South Africa, it became clear that the minerals so interesting to us are formed in highly specific, and, what is most important, in deep-seated (plutonic) mantle matter*.

These bedrocks include kimberlites and lamproites of magma origin, usually observed on ancient continental platforms and lying there in the form of the above-mentioned subvolcanic bodies: volcanic pipes (diatremes) or magma conduction dikes. Carbonatites and alkaline ultrabasic rocks of wide-spectrum composition, though related to them, are not diamond-bearing. Their common feature is a low content of silica and rather high concentration of magnesium, which makes it possible to assign all these formations to ultrabasic ones. Kimberlites, lamproites, and alkaline ultrabasic rocks differ from them by the high content of titanium, alkali (primarily potassium), phosphorus, rare lithophilic and volatile elements, including water and carbon dioxide (particularly carbonatites). The formative conditions of these rather specific rocks remained unknown for a long time.

* See: A. Portnov, "Diamonds From the Netherworld Vents", Science in Russia, No. 3, 2004; F. Kaminskii, S. Sablukov, Nontraditional Diamond Deposits", Science in Russia, No. 1, 2002. -Ed.

Articles in this rubric reflect the authors' opinion. -Ed.

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Giant quarry, where diamonds are extracted. Town of Mirny (Yakutiya).

Due to progress in experimental mineralogy, it was found that diamond (and all associate minerals) were definitely characterized by plutonic and magmatic origin. Hence the hypothesis on diamond formation due to the melting of the corresponding regions of the deep mantle matter enriched by "volatile" compounds, particularly by water and carbon dioxide, and many lithophilic elements (crust rock components).

Although these approaches are much popular, they have many fundamental flaws. According to modern petrological data on the conditions of kimberlite magma formation, the formative temperature is rather low, just about 1,000 - 1,250°C at a pressure of 45 - 70 kbar, while the melting temperature of the mantle matter is 1,570 - 1,720°C. It is obvious therefore that the kimberlite and related magmas could not appear due to the melting (be it even partial) of the mantle matter.

Besides, Yasuuki Muramatsu, a well-known Japanese geochemist, noted that the concentration of many elements, particularly lithophilic and mobile ones, is tens and even hundreds of times higher in kimberlites than in the parental rocks of the mantle. For example, the content of carbon is 150 times, of phosphorus 25, of alkali (K, Rb, Sc) 30 to 200, and of radioactive elements (Th and U) 80- and 60 times higher in kimberlites. According to our estimates, their enrichment by disperse elements is even higher: the concentration of potassium is increased as much as 90-fold, while for thorium and uranium it is 1,200 and 2,300-fold, respectively.

The parental rocks of the mantle contain about 0.01 percent carbon dioxide and no more than 0.05 percent water. Hence, their concentration in kimberlites reaches 3.3 - 7.1 and 5.9 - 18.7 percent, respectively, as they are enriched by these volatile compounds 300 - 650 and 120 - 370 fold, respectively. A characteristic feature of kimberlites is high (up to 500 - 800 g/t) content of rare-earth elements, typical of only carbonatites and sienites (originally related to kimberlites) and of phosphorites and iron and manganese ore-bearing silts of oceanic sediments, though their content in the ultrabasic mantle rocks ranges from 2 - 3 to 20 - 30 g/t.

Kimberlite minerals and even diamond crystals often contain gaseous liquid impregnations, enclosing the fluids (liquid solutions) from which they once had been crystallized. On the other hand, they usually contain water, carbon dioxide, carbon monoxide, nitrogen, methane, hydrogen, and even alcohols. Apatite sienites of the Khibini mountains contain rather high concentrations (up to 150 cm³/kg rock) gaseous hydrocarbons, heavy hydrocarbons, and more complex organic compounds, absolutely unstable at high temperatures and pressures of the earth mantle.

If the kimberlite melt emerged due to the partial melting of the mantle substance of lherzolite composition (lherzolites are ultrabasic rocks of the mantle), the resultant matter would not be so much enriched by lithophilic and volatile elements, and this enrichment would not be associated with so drastic a drop in silica concentrations, which is so typical of kimberlites and carbonatites in particular.

However, there is no proof of the existence of any appreciable chemical abnormalities in the mantle, to say nothing about such radical and outright "fulminant" ones, which could explain the geochemistry of kimberlites and related rocks. Moreover, in the course of the geological evolution of the earth the convective currents in the mantle, responsible for the continental drift, mixed the mantle substance so well that now we can speak even about the minor abnormalities of its composition only with great caution. Judging by the composition of basalts

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Formative process of deep mantle melts of alkaline-ultrabasic, lamproite, and kimberlite compositions: A) end of the early Proterozoic; B) at the turn of the early and middle Proterozoic; C) Riphean or Phanerozoic periods (the moment of escape ofplutonic magmas to the surface): 1 - lithosphere; 2 - asthenosphere; 3 - early Proterozoic oceanic crust with overlapping heavy ferrous sediments; 4 - continental crust; 5 - deep-seated melts.

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released in the oceanic rift zones encircling the earth by a single ridge of about 60 thousand kilometers long, the mantle contains about 0.01 percent carbon dioxide and no more than 0.05 percent water.

The study of carbon isotopes in diamonds, carried out in the 1980s by Vladimir and Nikolai Sobolev, Felix Kaminsky (Institute of Geology, Geophysics, and Mineralogy, Siberian Branch of the Russian Academy of Sciences), Acad. Eric Galimov, and other scientists showed an appreciable increase in the concentration of ¹²C (light isotope), this, in turn, indicating that they can contain organic carbon which traveled to lower depths from the surface of the planet. All these data do not agree with the hypothesis about the origination of diamond-containing and related rocks as a result of mantle matter melting and indicates that they might have formed due to remelting of oceanic deposits which sank into the earth's interior.

That is why, studying the problem of the origin of kimberlites and related rocks from the viewpoint of the plate tectonics theory, we have come to the conclusion that kimberlite, lamproite, carbonatite, and alkaline ultrabasic magmas appeared as a result of the remelting of the oceanic bottom sediments drawn into the zones of the underthrust of lithospheric plates (in subduction zones) to great depths. We cannot explain the carbon isotope composition of diamonds outside the crustal substance. A similar situation is observed in high-temperature depth carbonatite and kimberlite association rocks: the isotope composition of carbon and oxygen indicates that the carbon dioxide of primary deposition was involved in their formation. Dr. Elridge and colleagues (USA), analyzing the isotope shifts in sulfur and the lead isotope ratios in the diamond sulfide impregnations, came to similar conclusions, thus confirming the ancient age of these minerals (about 2 billion years), which we previously predicted by the mechanism of lithospheric plate tectonics.

However, it still remained to be proved that oceanic sediments could sink to great depths under the continents. In this connection in the middle of the 1970s, in cooperation with Dr. Leopold Lobkovsky from the RAS Institute of Oceanology, we postulated the mechanism of deep-sea deposits (formed on the continental slopes and oceanic bed) subsiding into the zones of the lithospheric plates underthrust under island arcs and continents. It was found that the relatively light from among recent ones sank no deeper than 25 - 30 km, as the magmas formed in the subduction zones during plate melting because of friction are squeezed out and incorporated into the earth crust or even brought to its surface. With these conclusions in mind, together with Acad. A. Monin we solved the problem of subduction of heavy (ferrous) deposits capable of sinking to the foot of the continental lithospheric plates as deep as 200 - 250 km.

However, the mass deposition of ferrous sediments in the evolutionary history of the earth took place only at the end of the Archean (about 2.8 - 2.6 billion years ago) and during the second half of the early Proterozoic (about 2.2 - 1.9 billion years ago). Yet in the former case the thickness of the continental plates and the crust was no more than 80 km, which was obviously not enough for the formation of diamonds. In addition, no subduction occurred during the Archean; its function was performed by the zones of crowding of thin oceanic plates and their thrust over the continental outskirts. Only during the early Proterozoic the thickness of the continental plates increased to 250 km, and the mechanism of oceanic lithospheric plates subduction under the continental ones came into action.

FORMATION OF DIAMOND-CONTAINING ROCKS

It is impossible to determine the regularities of diamond crystallization without analyzing the origination of diamond-containing kimberlites and related rocks. Their formation is discussed in much detail in our monographs about the origin of diamonds, written together with Felix Mitrofanov from the Geological Institute of the Kola Research Center of the Russian Academy of Sciences. According to our model, these and related rocks appeared due to the subtraction to a depth of 200 - 250 km of oceanic crust rocks and heavy (ferrous) crustal deposits of the early Proterozoic era. Because of high density, they were to "sink" into plate underthrust zones all by themselves, serving as a kind of "lubricants". Presumably for this reason these deposits at the end of the early Proterozoic (during Svecofennian orogeny, or 1.8 billion years ago) were mainly amagmatic, without calciferous alkaline (andesite) volcanism characteristic of the island arcs and active continental margins.

In accordance with this theory (and contrary to the current opinion) we believe that diamond-containing kimberlites, Iamproites, and carbonatites cannot be classified as mantle rocks but rather, as pseudomantle ones. As for the plutonic fragments in them, called eclogites, virtually all of them are metamorphous fragments of the basalt layer of the former oceanic crust, carried onto the surface. This conclusion can be made from the similar chemical composition of eclogites and mid-oceanic ridge basalts, melted under the near-surface conditions of the oceanic rift zones. In contrast to these, the garnet peridotite inclusions (ejected plutonic rocks) can be of different nature: their greater part are fragments of the third (serpentinite) layer of the former oceanic crust, while the rest are fragments of the subcrustal continental lithosphere ruptured from the volcanic vent walls. In fact only the former type of these ultrabasic xenoliths can be diamond-bearing, but never the latter one.

In this model the formative time of plutonic melts of kimberlite and lamproite composition is strictly limited by the latter half of the early Proterozoic. Importantly, this period coincides with the time of deposition of the jaspilite type heavy ferrous deposits, known by the iron ore deposits of the Kursk Magnetic Anomaly in Russia,

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Conditions of subtraction of heavy (ferrous) sediments in the early Proterozoic plate underthrust zones to great depths (to 250 km) under Archean continents and zones of formation ofplutonic rock melts.

Krivoi Rog in Ukraine, Hammersly in Western Australia, and the Lake Superior region in Canada and USA.

Hence from the standpoint of the plate tectonics theory the diamond-containing rocks could be formed only with the subsidence of the oceanic crust and overlapping heavy (ferrous) pelagic sediments under continental plates to a depth of 150 to 200 - 220 km.

During the deposition of iron ore sediments the atmosphere of the earth persisted almost oxygen-free, as nearly all oxygen produced by the then existing phytoplankton was spent for oxidation of bivalent iron dissolved in water to trivalent iron. In addition to overall accumulation of such sediments, this circumstance was to lead to hydrosulfide stagnation of the early Proterozoic oceans, and in addition to common sediments and ferrous hydroxide, the bottom was gradually covered by abundant ferrous sulfides, siderite, and organic matter. Besides, the depth of the oceans during the early Proterozoic did not exceed 1.5 km, reaching 4 - 5 km in just narrow deep trenches, and thus being everywhere less than the threshold depth of carbonate deposition. Hence, during that distant epoch carbonate silt saturated with organic substances and abiogenic methane could be deposited (in the tropical zone anyway) on the ocean floor simultaneously with ferric compounds.

During subtraction of heavy sediments into the zones of the early Proterozoic plate underthrust they were heated mainly by the abyssal thermal flow. Hence, the temperature of the deposits in the gap between the plates corresponded to the geotherm (continental plate temperature). Heavy sediments (deposits) started melting only at low depths, where the geotherm exceeded the corresponding temperature of water-saturated sediments. For the majority of silicates it dropped abruptly to 600 - 700C with an increase of pressure to 5 - 10 kbar; water-saturated carbonates and many other compounds behaved similarly. At still higher pressures the rock melting temperature smoothly increased with depth, the temperature minimum depending on the ratio of water and carbon dioxide concentrations, being usually close to 5 - 10 kbar.

Considering these regularities, we can expect that purely silicate water-saturated sediments melted already at a depth of 50 - 70 km, carbonate sediments at a depth of about 80 km. In time the temperature gradients under the Archean continents gradually decreased, and that is why by the present time the melted silicates in ancient sub-

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duction zones are preserved at depths below 70 - 80, carbonatite and kimberlite magmas-below 100 and 150 km.

The degree of melting sharply Increased below the critical level of the continental geotherm crossing the melting curve for the sedimented matter. The resultant melts were to be differentiated by density at greater depths. Heavy ferrous and sulfide fractions sank down and eventually submerged into the convecting mantle, gradually mixing with its substance. Light fractions, consisting of separated fluids, carbonate and silicate melts, could not come up because of the compression characteristic of the plate underthrust process and accumulated (and were conserved) in the lower layers of the ancient continental plates, gradually forming there foci of alkaline ultrabasic, carbonatite, and lamproite/kimberlite magmas.

The subduction process often ends in continental collisions. This situation was particularly typical of the Karelian diastrophism (about 1.9 - 1.8 billion years ago), when Megagea emerged from the fragments of the ancient Monogea supercontinent (formed at the turn of the Archean and Proterozoic). The melts formed in the numerous plate underthrust zones of that period were "sealed" (conserved) there. Due to the plutonic thermal flows rising from the mantle and sustaining at a stable level the regimen of the ancient continental plates, they were conserved in the lithospheric "traps" for hundreds of millions and even billions years actually without cooling.

Discussing the problem of the origin of diamond-containing rocks, we must point to many similar features of theirs. The main difference between lamproites and kimberlites is a small volume (or complete absence) of the carbonate phase, and high concentration of potassium in the former (lamproites). It seems that the initial composition of the underlying deposits had a significant impact on the composition of both types of rocks. It is most likely that kimberlites were formed during the subtraction of the carbonate-rich ferrous deposits of tropical zones into subduction zones, while lamproites were formed by the melting of carbonate-free sediments of the ferrosiliceous and clay deposition from the boreal or even the polar climatic zones of the earth. Interestingly, silicoferrous sediments started to deposit in the classical region of diamond-containing lamproites (West Australia) about 2.2 billion years ago; deep metamorphoses in these sediments gave rise to extensive deposits of jaspilites (ferrous quartzites). Besides, fluvioglacial deposit, the direct evidence of the cold climate during the early Proterozoic, likewise occur in this region.

THE ORIGIN OF DIAMONDS

This problem has been discussed for more than a hundred years. Most of the contemporary hypotheses suggest that these are the upper mantle minerals, formed at high pressure and temperature. At the end of the 1970s the most popular hypothesis proceeded from the assumption that diamonds were products of mantle rock disintegration. This hypothesis still lives on. The fact that the composition of inclusions in diamonds is usually similar to the minerals from fragments of plutonic rocks (eclogites and peridotites) is cited as proof positive. Thus diamonds are regarded as xenocrystals of mantle origin, with the kimberlite magma being just a transporter taking them (together with fragments of deep mantle matter) to the surface.

But according to our model, diamonds are formed by carbon reduction in carbon dioxide and monoxide reactions with methane or other organic carbohydrates drawn into the subduction zones together with sediments to great depths. Significantly, reducing conditions most likely predominated in the benthic water of the early Proterozoic oceans. Hence the concentration of organic substance in the oceanic sediments of that time had to be rather high. In the plate underthrust zones this substance rapidly went through all the stages of conversion into carbohydrates, nitrates, and ammonium compounds. Part of these mobile compounds together with the pore waters was no doubt squeezed from the plate underthrust zones at the uppermost horizons, while the rest progressed into the depth of the mantle.

It is known that the stability of all, without exception, carbohydrates decreases significantly with temperature and pressure increase, because of carbon bonds rupture in the long chains of supermolecules. As a result, their concentration gradually decreased with depth, while the concentrations of simple carbohydrates, vice versa, increased. Methane is characterized by the highest stability, heat-resistant as it is to 1,200°C (at normal pressure). That is why all of organic matter eventually transforms into methane, hydrogen, and free carbon. However, the thermal destruction of carbohydrates is an endothermic process that cannot give rise to carbon crystal phases. For this carbon should be liberated in an exothermic reaction causing a heat release and decrease in the inner energy of the system. Carbohydrates combined with carbon monoxide and carbon dioxide in reactions proceeding with energy release. Carbon dioxide was then liberated through thermal degradation of carbonates at hot sites of the plate underthrust zones in endothermic reactions, while carbon monoxide was presumably generated in an exothermic reaction, for example, through conversion of bivalent ferric oxide to magnetite. Due to water dissociation on bivalent (silicate) iron with the formation of a magnetite molecule, free hydrogen was released. Its combination with carbon oxides could generate abiogenic methane as well.

Drs C. Melton and R. Giardiani (USA) studied the gaseous liquid inclusions in diamonds in the 1970s and found that these inclusions contained 10 - 60 percent water, 2 - 50 percent hydrogen, 1 - 12 percent methane, 2 - 20 percent carbon dioxide, 0 - 45 percent carbon monoxide, 2 - 38 percent nitrogen, and about 0.5 - 1.2 percent radiogenic argon. In addition, these inclusions can contain ethylene (about 0.5 percent) and even ethanol (0.05 - 3 percent), but no free oxygen, which confirms once

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again the highly reducing conditions of diamond formation. We consider that this specific set of gases is certainly indicative of the exogenous (not juvenile) origination of the fluid phase from which diamonds had crystallized.

Consequently, considering diamond formation according to our model, we can only wonder not so much at the possibility of such formation from the exogenous carbon, but at the fact that this exceptionally rare accessory (associate) mineral did not become rock-forming with the excess of the initial carbon-containing compounds (CO₂ and CH₄). Two explanations are possible. First: diamond in the overheated and, presumably, exclusively liquid kimberlite melt should have sunk as a heavy fraction (density about 4 g/cm³) to sublithospheric levels and carried thence by convective currents all over the mantle (we should not rule out this possibility of diamonds getting into the ultrabasic rocks of obviously mantle origin). Second, it is probable that the amount of carbon monoxide in the system is limited. The same can be true of the extremely slow process of diamond formation-just small diamond crystals could be formed in the lifetime of kimberlite magma foci (1 - 2 billion years).

Besides, the above mechanism of diamond-containing rock formation explains in practical terms the specific isotope ratios in diamonds and kimberlites, and the neodymium-samarium and strontium ratios in kimberlites and lamproites.

LOCATION OF DIAMOND-CONTAINING MAGMATISM VENTS

Thus, we have clarified the cause-and-effect relationships in the process of interest to us: kimberlites and related alkaline-ultrabasic rocks are, no doubt, plutonic rocks, though they have emerged from exogenous (sedimented) substance. Lamproites and kimberlites are similar in origin, too.

The discussed formative mechanism of diamond-containing and related alkaline ultrabasic and carbonatite rocks and the time of iron ore accumulation allow us to outline predictive criteria for respective deposits on the earth surface. These deposits built up mainly on the Archean continental crust, though in some cases could intrude into the Proterozoic crust, but could not occur on the young (Phanerozoic) platforms, let alone the ocean floor. "Explosion pipes" corresponding to them and the related carbonatite and alkaline ultrabasic intrusions are located under the zones of plate underthrusts of the Svecofennian age about 2.0 - 1.8 billion years ago. Alkaline ultrabasic intrusions and sodium carbonatites are located closest (at a distance of 100 - 300 km) to the front of their former zones according to the plutonic formation of this series of rocks and the steep dip of plate underthrust zones. The zone of calcium carbonatites and melilitites and occasionally of non-diamond-containing kimberlites is located deeper (200 - 400 km). Diamond-containing kimberlite and lamproite diatremes are situated deeper than other similar formations (300 - 650 km).

The presence of early Proterozoic iron ore formation in the seam zone contacting the Archean crust block and, particularly, the submersion of these formations under the block can serve as an extra prognostic criterion for locating such subvolcanic complexes. Since the heavy iron ore deposits getting into the gap between the plates performed as lubricants, the early Proterozoic zones, into which such deposits were drawn, in practical terms always remained amagmatic, without shows of calcareous alkaline (andesite) volcanism.

These criteria determine just the essential possibility of the presence of kimberlite, alkaline ultrabasic, or carbonatite complexes in a certain area. The actual detection of these complexes depends on the occurrence of lithospheric extension in subsequent geological epochs. If independent geological data indicate that a region under study experienced stretching deformations after the early Proterozoic collision of plates, the probability of formation of the above series of rocks will be much higher. We can see that in the example of the richest diamond-containing provinces of Canada, South Africa, Brazil, India, Australia, and Siberia, as well as the northern part of the Baltic shield*. They really all started to form under the Archean crust and near the early Proterozoic zones of the ancient craton collisions taking place in the epoch of Megagea formation during the Svecofennian epoch of tectonic-magmatic activation. The final stage of the formation of these provinces, with the deduction of plutonic melts to the surface, occurred during the periods of breaks and stretching deformations of the ancient supercontinents Megagea, Mesogea, and Pangea. However, the main impulse of kimberlite and alkaline ultrabasic magmatism was associated with the breakdown of the last Mesozoic supercontinent, Pangea.

In addition to the detected diamond-containing provinces, which in fact conform to the above pattern for the formation of diamond-containing rocks, there are just as good prospects for the Baltic shield, the Russian platform with the Voronezh shield, and the Ukrainian shield. All that agrees with the postulated mechanism relevant to the formation of plutonic melts and origination of these rocks.

Illustrations supplied by the authors.

* See: S. Sablukov. "Diamond Treasures in Russia's North", Science in Russia, No. 1.2001. -Ed.