Genesis of diamonds-the metamorphic point of view Norman Fischer Hans-Otto-Str.2/409 04279 Leipzig Abstract. Metamorphic diamonds come up to our knowledge only in the last two decades. For the first time, a complex investigation was made in the Kokchetav Massiv in nothern Kazachstan. Basing on these data, three models for diamond genesis have been carried out.

1. The premetamorphic model 2. The metastability model 3. The UHPM model

Introduction

What is Diamond?

Diamonds is carbon in its most concentrated form. Except for trace impurities like boron and nitrogen, diamond is composed solely of carbon, the chemical element that is fundamental to all life. Diamond, is the hardest natural substance in the world. Each carbon atom is surrounded by four neighboring carbon atoms in a tetrahedral coordination that is the result of a covalent bond and a face-centered arrangement in the cubic unit cell. Genesis of the diamonds

Fig.1.this model shows genesis of diamonds ( American Museum of Natural History) The plot of pressure and temperature shows the conditions at which either diamond or graphit exist.The general conditions present in the earth are described by curved lines called geotherms. Note that there are two geotherms : - because the continental crust is old and thick, conditions are somewhat colder

-than beneath the much younger ocean basins.

Diamonds can from at depths as shallow as 150 kilometers and a pressure as 45000 kbar beneath the continental crust, while beneath oceans they need depths of least 200 kilometers as 63000 kbar, as shown by the diamond/graphite boundary in the diagramm. 2. History of discovery

The first river-bed (alluvial) diamonds were probably discovered in India, in around 800 B.C. The volcanic source of these diamonds was never discovered, but the alluvial deposits were rich enough to supply most of the world's diamonds until the eighteenth century, when dwindling Indian supplies probably spurred the exploration that led to the discovery of diamonds in Brazil, which became the next important diamond source. Beginning in l866, South Africa's massive diamond deposits were discovered, and a world-wide diamond rush was on. The South African diamond ouput was unrivaled until major deposits were found in Siberian permafrost in l954. And currently Western Canada is the site of the world's newest diamond rush.

Throughout much of history, diamonds were mined from the sand and gravel surrounding rivers. But in South Africa in 1870 diamond was found in the earth far from a river source, and the pratice of dry-digging for diamonds was born. More sophisticated mining techniques allowed deeper subterranean digging, as well as more efficient river (and, most recently, marine) mining, than ever before.

3. Occurrences of diamonds Diamonds have been discovered in many different countries around the world. In several countries in Africa and North America , as well as China, Venezuela, Australia, India, and Siberia , diamonds have been found in situ. The primary sources of economic concentrations of diamond are volcanic diatremes formed from kimbelitic and lamproitic magmas. The deposits are said to be "secondary", having resulted from the erosion of bedrock or "primary" sources 3.1. Placer in cratonic areas The oldest parts of continents are called cratons, and can be divided into two terranes: Archean-age archons, which are older than 2,500 million years, and Proterozoic-age protons, which are 1,600 -- 2,500 million years old. The distribution of these terranes is shown on the map. Kimberlite pipes occur in many parts of the continental crust, but most diamond-rich ones are found in archons. This fact suggests that most diamonds were formed and stored deep below the cratons, in the area shown in the lower figure, and were later transported to the

surface by kimberlite and lamproite magmas that extracted them and other samples from the mantle.

Fig.2. shows both the major deposits of diamonds and the ancient bedrocks (American Museum of Natural History) 3.2. Xenocrysts in kimberlite magmas Diamonds in kimberlites occur as sparse xenocrysts and within diamondiferous xenoliths hosted by intrusives emplaced as subvertical pipes or resedimented volcaniclastic and pyroclastic rocks deposited in craters. Kimberlites are volatile-rich, potassic ultrabasic rocks with macrocrysts (and sometimes megacrysts and xenoliths) set in a fine grained matrix. Kimberlite magmas carry foreign rocks -- xenoliths -- from Earth's mantle to the surface. Xenoliths are geologists' only samples from the deep Earth, and carry information about diamond growth conditions. The 2 most common types of xenoliths are peridotites and eclogites. Peridotite is the main constituent of the mantle beneath the crust and consists primarily of olivine -- the gem variety is peridot. Eclogite, consisting primarily of garnet and a green pyroxene, is formed by plate tectonics when basalt of the ocean crust founders into the mantle. Certain kinds of xenoliths contain diamonds.

Fig.3. Modified from Kirkley, M. B. et. al. (1991) An idealized model of a kimberlite pipe can be subdivided into three zones: the root, diatreme, and crater. The root zone is characterized by crystallized kimberlite magma with typical intrusive textures and containing xenoliths (fragments) and xenocrysts (crystals). The root zone extends into a feeder dike or fractures along which the magmatic fluid passed through. 3.3. UHP terranes The occurrences of ultrahigh-pressure (UHP) rocks have been increasingly recognized and extensively described (Ernst and Liou, 2000). Thus far, more than a dozen Eurasian UHP terranes have been documented, as shown in Figure 4. Most of these UHP terranes lie within

major continental plate collision belts in Eurasia (one is present in Africa). Each complex extends

several hundred km or more. They share common structural and lithological characteristics(Liou et al., 1998).

Fig.4. Global distribution of recognized diamond- and coesite-bearing UHP terranes ( modified after Liou et al. 2000b). The ages of UHP metamorphism are shown with numbers (in Ma) for each terrane. (1) Scattered UHP rocks are preserved mainly in eclogites and garnet peridotites

enclosed as pods and slabs within gneissic units. Some of these rocks contain minute inclusions of coesite and microdiamonds in zircon, garnet, and clinopyroxene in both eclogitic pods and enclosing metasedimentary country rocks.

(2) Lithologies are continental ± oceanic in chemical compositions. (3) Exhumed UHP units are now present in the upper continental crust as thin

subhorizontal slabs, bounded by normal faults above, and reverse faults below, and sandwiched among HP or lower grade metamorphic units.

(4) Coeval island-arc volcanic and plutonic rocks do not occur, whereas post-collisional or late-stage granitic plutons are common in some occurrences.

Approximately 10 years ago, diamonds (< 1 millimeter) in the metamorphic rock of crustal affinity in the Massif before Kokchetav of NordKazakhstan were recognized by (Sobolev and Shatsky 1990). However, their arrangement requires many higher conditions of the pressure temperature (pint) than those, which are expected within the crust of the Kokchetav Massif. Three fundamentally different models have been suggested, to explain the occurrence of diamonds in metamorphic rocks.

3.3.1. The premetamorphic relictmodel

Premetamorphic relict modell demands that the diamonds have a kimberlitic origin (Marakushev et al. 1995). Diamond of mantle origin were gotten by kimberlites or lamproites to the surface of the Kokchetav Massif. During weathering processes, the magmatic diamonds formed placers in supra-crustal sediments, which have been metamorphosed later. The idea is that diamonds would have survived the metamorphism , because they became a mantle of refractory minerals, which grew early in metamorphic history.

Fig.5. plate tectonic (American Museum of Natural History) When the ocean floor succeeded by subduction process into the Earth's mantle, basalt to eklogit and organic carbon changed themselves to diamond

3.3.2. The metastability model

The metastability modell maintains diamond genesis by condensation of reduced mantle fluids at temperatures of 600-1050 °C and pressure, which are below the equilibrium pressure in the local shear zones. Diamond formed at the crustal conditions of the metamorphism and it give no requirement for ultrahigh pressure (UHP), which is characteristic from the upper mantle (e.g. Nadejdina and Posukhova 1990; Dobrzhinetskaya et al. 1994; F. A. Letnikov, unpubl..-facts, 1995; Simakov 1995; Lavrova et al. 1997).

Fig.6. Model for the emergence of diamond-prominent lithosphere under old continental ranges (after Stachel) Now diamonds from orogenic belts provide a unique opportunity to direct study of fluid compositions occurring in a subduction zone at a depth of >120 km. It was observed that diamond-bearing multiphase pockets in garnets and zircons are frequently accompanied by hydrous phases. Molecular water and carbonate radicals are detected in Kokchetav diamonds by FTIR (De Corte et al.,1998), also nanometric inclusions of oxides of Si, Fe, Ti, Th, Cr, and cavities of former fluid phases were discovered in diamonds (Dobrzhinetskaya et al., 2000, 2003). On the basis of those observations two concepts were suggested for the explanation of the origin of such diamonds: (1) crystallization from a supercritical COH fluid; (2) crystallization from fluid-bearing alkaline-carbonate melt. Both concepts have been successfully confirmed by experimental synthesis of diamonds at high P & T

(Akaishi et al., 2002, Dobrzhinetskaya et al., 2002, Pol�anov et al., 2001). Diamond crystallization from graphite in the presence of H2O at high P & T

is the most realistic explanation of diamond formations within UHPM terranes because it is in agreement with observations on the natural rocks. Although concept #2 is also well verified by experiments. 3.3.3. The UHPM model Crustal rock subducted to the depths of over 150 kilometers metamorphosed and went back to the surface of the Kokchetav Massif. Diamonds crystallized with pressure higher than > 4 GPa and temperatures more than 900-1000°C.

Fig.7. The diagramm above illustratet the formation of a UHP terrane that can yield diamonds. At top, the down-going subducted ocean crust (green) has a thin covering of sediment (gray) that is sheared off and driven upward (inset), apparently caused by the continental collision (middle) that squeezes the diamond-bearing metamorphic rocks back into the crust (bottom). (American Museum of Natural History) 4. Host minerals Diamonds are found today as inclusions in garnet and clinopyroxene (and their secondary minerals), zircon, kyanite, zoisite, biotite and quartz as well as in garnet. Because the diamonds are included in these minerals, the diamonds crystallized before or synchronously with the host minerals. In this section we will explore some constraints on the conditions of diamond formation provided by the host minerals.

GARNET

Experimental work by Poli and Schmidt (1995) demonstrates that the grossular component in garnet decreases with decreasing pressure. Approximately 80% of the matrix garnets of diamond-bearing rocks are characterized by a Ca-rich core compared to their rim. Therefore, the garnet must have formed at or near the peak of UHPM and continued its growth during retrograde metamorphism. Hermann and Green (1999), support garnet growth during retrograde metamorphic conditions based on piston cylinder experiments in the range 700-1100°C and 2.0-3.5 GPa using gneissic to pelitic compositions.

ZIRCON

Zircon is extremely stable and resistant over a wide temperature and pressure interval (Chopin and Sobolev 1995) and, therefore, is considered to be the best UHP mineral container. Garnet, clinopyroxene, phengite, diamond, coesite, graphite, chlorite, quartz, plagioclase, K-feldspar, kyanite and amphibole have been identified as inclusions in zircon.

ZOISITE

Diamond inclusions in zoisite occur in core samples of zoisite-bearing gneiss from the Barchi locality. Diamonds occur more commonly in symplectitic zoisite-quartz intergrowths, but again only in zoisite gneisses from Barchi. The morphology of the diamonds from these rocks seems to be related to the presence of symplectitic zoisite. Diamonds with an octahedral habit are only observed in zoisite-bearing gneisses that are devoid of symplectitic zoisite, while zoisite-bearing gneisses with symplectitic zoisite are dominated by diamonds of cuboidal morphology ( De Corte et al. 1999)

MICA

Biotite containing cubo-octahedral diamonds has been found as an inclusion in garnet from a biotite gneiss at Kumdy-Kol. The biotite occurs in close relationship regarding locality with quartz. In only one sample were diamonds identified as inclusions in matrix biotite of biotite gneiss. The origin of the biotite is not yet fully understood.

Fig.8. A model of zircon growth history with mineral inclusions during the

various stages of prograde and retrograde event for Kokchetav UHP rocks (after Katayama et al. 2000). The P-T path is based on that of Parkinson (2000). 5. The Kokchetav Massif, Nordkazakhstan Very small �microdiamonds� averaging only 12 mikrometers across, were discoverd during diamond exploration in a region called the Kokchetav Massif, in nothern Kazakhstan, in large slices of metamorphic rock that must have been pushed at least 120 kilometers in into Earth and returned to the surface. Discovery of these process, termed ultra high pressure (UHP) metamorphism, has revolutionized ideas about and interest in what can happen to Earth`s crust. 5.1. Three unique petrotectonic characteristics. (Parkinson et al., 2000) (1) Neoblastic diamonds, identified microscopically and by micro-Raman spectroscopy, are abundant as inclusions in garnet and zircon within biotite gneiss and pyroxene-bearing marble (e.g., Sobolev and Shatsky, 1990; Copin and Sobolev, 1995; Zhang et al., 1997; Osagawara et al., Katayama et al., 2000a). Reported diamond occurrences in other UHP terranes such as the Dabie Mountains and the Western Gneiss Region of Norway are based on chemical dissolution of rocks and lack the unambiguous confirmation of microdiamond in situ. (2) UHP metamorphism of the Kokchetav Massif occurred at ca 540-530 Ma, and probably reflects the apparent change in P/T conditions of subduction-zone metamorphism at the Precambrian-Cambrian boundary (Maruyama and Liou, 1998). (3) some Kokchetav UHP rocks may have been recrystallized at the highest P conditions ever recorded for crustal rocks (P > 6 GPa), based on K-in-Cpx geobarometry (e. g., Okamoto and Maruyama, 1998; Okamoto et al., 2000) . 5.2. The Kokchetav Massif, deepest UHP terrane in the world

The Kokchetav Massif is a large, fault-bounded metamorphic complex of Proterozoic protolith age, surrounded by Caledonian rocks of the Ural-

Mongolian foldbelt. The complex consists of a number of discrete fault-bounded UHP and HP metamorphic units (the Zerenda Series), structurally underlain by the Daulet Suite of low P/T metapelites and associated rocks, and overlain by a feebly metamorphosed sequence of quartzites and carbonates. These contrasting lithologic and metamorphic units have been interpreted as a tectonic megamelange( e. g. Dobretsov et al., 1995, 1998; Shatsky et al., 1995; Theunissen et al., 2000) composed of boudins of orthogneiss, eclogite, and quartzite in a matrix of pelitic schist and paragneiss. The largest eclogite block, more than 1 km in length, forms the core of Sulu-Tjube Mountain (see Figure 7. for location), and exhibits at least two stages of recrystallization (eclogite- and garnet amphibolite-facies). Some units are coherent fault-bounded sheets consisting of recrystallized, gabbro-norite-diorite sills intruded into garnet + mica schist and kyanite/sillimanite-bearing aluminous gneisses. Pods of talc-kyanite-garnet whiteschist occur; their mineral parageneses have only been documented recently (Zhang et al., 1997; Parkinson 2000). Other pre-Ordovician metamorphic units include slightly metamorphosed platform sedimentary strata - chiefly quartzite, feebly recrystallized basaltic rocks, and undifferentiated late Proterozoic ortho- and paragneisses. Recent detailed mapping by a Japanese team (Kaneco et al., 2000; Maruyama and Parkinson, 2000; Parkinson et al., 2002) yielded several fault-bounded subhorizontal HP/UHP units as depicted in Figure 9.

Fig.9. simplified map showing various petrotectonic units of the Kokchetav Massif ( after Figure

3 of Maruyama and Parkinson, 2000). Unit 1 consists mainly of gneissose amphibolite and acidic gneisses with subordinate pelitic schists and orthogneisses; geothermobarometry of HP amphibolites from the Barchi-kol and Saldat-kol regions yielded P = 0.7-1.4 GPa and T = 570-680oC. Unit II occurs as the structural core of the massif, and is composed mainly of pelitic-psammitic gneiss and whiteschist surrounding discontinuous eclogite boudins, blocks and lenses. Minor amounts of dolomitic marble, garnet pyroxenite, Ti clinohumite-bearing garnet peridotite and orthogneiss are also present locally. Most constituents of Unit II suffered UHP metamorphism, as evidenced by scattered diamond and coesite inclusions and other compositional, mineralogic and textural indicators. Petrologic and mineralogic features of Unit II rocks will be described later. Unit III consists mainly of interlayered orthogneiss, migmatite and amphibolite, with small lenses of eclogite and garnet amphibolite. Similar to Unit I, the Unit III garnet amphibolites were recrystallized at P-T conditions of 730-750oC and 1.1-1.4 GPa. Unit IV is the structurally highest level of the massif, and consists mainly of quartzite and siliceous schist with minor metamafic intercalations. P-T estimates for Unit IV amphibolites are 0.7-0.9 GPa and 530oC; this unit is tectonically overlain by a feebly metamorphosed sequence of quartzites and carbonates of Unit V. Diamond-grade UHP rocks of Unit II extend westward from Kumdy-kol to Barchi-kol and occur within an area no less than 80-100 km2 (Fig. 9). Metamorphism of diamond-bearing paragneiss and schist and associated tectonic units took place in Early Cambrian time, as indicated by Sm-Nd and U-Pb ages between 530 and 540 Ma (Claoue-Long et al., 1991), K-Ar ages of 535 ± 3 Ma (Hacker et al., 2000) and SHRIMP U-Pb dates of 537 ± 9 Ma (Katayama et al., 2001). Detailed parageneses and compositions of minerals both as inclusions in zircons and matrix of a variety of HP-UHP rocks have been investigated. Those in metabasites and metapelites from diamond-eclogite (DEC), coesite-eclogite (CEC), quartz-eclogite (QEC) and high-pressure amphibolite (HAM) facies are illustrated in Figure 10.

Fig.10. Paragenesis of minerals in metabasites and as inclusions in zircon separates from metabasites and metapelites from the Kokchetav Massif. DEC: diamond eclogite facies, CEC: coesite eclogite facies, QEC: quartz eclogite facies, HAM: high-P amphibolite facies ( after Katayama et al. 2000a) 6. OCCURRENCE OF DIAMOND IN UHP TERRANES The Kokchetav Massif is the type locality of diamond of UHP origin (Sobolev and Shatsky, 1990; Zhang et al., 1997). Diamonds have been found only in metasediments including garnet-biotite gneisses and schists, garnet-phengite-kyanite schists, garnet-pyroxene rocks and dolomitic marbles; mafic eclogitic rocks do not contain diamonds. On the other hand, diamonds were reported as inclusions in garnets from eclogite, garnet-pyroxenite and jadeitite in the Dabie Mountains (Xu et al., 1992) and in residues separated from large eclogite and

garnet peridotite samples from the Sulu terrane (Xu et al., 1998). In addition, diamonds from these two UHP terranes show distinct difference in

grain sizes and in abundance; abundant micro-size diamonds in the Kokchetav whereas rare coarse-grained diamonds up to one cm occur in the Dabie-Sulu. Abundant diamond inclusions have been found in zircon, garnet, kyanite and clinopyroxene from metasediments of Unit II from the Kumdy-kol region; some have also been recently reported in Barchi-kol (Korsakov et al., 1998). Diamond occurs in a few dolomitic marbles and gneissic rocks; due to poor exposure, the contact relations between diamond-bearing and diamond-free rocks are not clear. Trenching, drilling and tunneling of the Kumdykol area suggests that diamond-bearing rocks occur along certain stratigraphic bands; such linear occurrence has been used to claim that the diamond formation is due to tectonic overpressure along shearing zones (e.g., Dobrzhinetskaya et al., 1994). Diamonds have been found in gneiss and marble samples, and are locally abundant. For example, Ishida and (Ogasawara 2000) reported 1021 diamond grains in 8 thin sections of dolomitic marbles; they occur mainly as inclusions in garnet and zircon and occasionally in diopside crystals. Many of these diamond grains coexist with phengite mica. Diamond-bearing marbles contain the peak assemblage dolomite + diopside + garnet + diamond, whereas the associated diamond-free marbles have the peak assemblage Mg-calcite, dolomite, forsterite, diopside, Ti-clinohumite and garnet pseudomorphed by diopside, spinel and Mg-calcite. (Ogasawara et al. 2000) attributed the restricted diamond occurrence in dolomitic marbles as precipitation from higher XCO2 fluid, whereas the associated carbonates with a lower XCO2 fluid have higher oxidized conditions, diamond becomes unstable and garnet is entirely replaced. Diamond-bearing gneissic rocks contain variable amounts of garnet, biotite, zoisite/ clinozoisite, coesite/quartz, K-feldspar, plagioclase, chlorite, tourmaline, calcite, and amphibole, along with minor apatite, rutile, and zircon. Abundant diamond inclusions were enclosed and associated with fine-grained clinopyroxene, phengite ± apatite; these fine-grained aggregates are in turn included in garnet . Some microdiamond grains have cores of graphite (Fig. 11); others are partly or completely replaced by graphite. Most diamonds exhibit discrete cubo-octahedral grains, and average 12 ∝m in diameter. Others, particularly those in marbles, show star-shaped grains consisting of cores (10 to 15 microns) and surrounding subhedral to euhedral grains of 2 to 5 microns. Several diamond inclusions in zircon are shown in Figure 11. The Kokchetav diamonds are characterized by (i) low 13C values, -10 to -19 0/00, suggestive of crustal biogenic carbon, (ii) a very high concentration of nitrogen impurities (~1470 ppm), and (iii) unusual cuboid-rich crystallographic habits (Sobolev and Shatsky, 1990; De Corte et al., 2000; Dobrzhinetskaya et al., 2001).

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