Download Genesis of diamonds-the metamorphic point of view

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.
2.
3.
The premetamorphic model
The metastability model
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 volatilerich, 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 postcollisional 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 diamondbearing 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 2.03.5 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 UralMongolian 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 amphibolitefacies). 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 orthoand 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 1.11.4 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-phengitekyanite 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 diamondbearing 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).
Fig.11. diamond inclusions in zircons from gneissic rock from Kudmy-Kol of the
Kokchetav Massif ( Katayama et al. 2000a).
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