.New Frontiers in Integrated Solid Earth Sciences Phần 10 pps

Ultradeep Rocks and Diamonds in the Light of Advanced Scientific Technologies 377

eclogite took place. Their pioneering work has trig￾gered further discoveries of ultradeep xenoliths con￾taining majoritic garnet from potassic ultramafic mag￾mas at the Ontong Java Plateau of Malaita, south￾west Pacific (Collerson et al., 2000). Within one

of these xenoliths, Collerson et al. (2000) have

described majoritic garnet in association with Ca- and

Mg-perovskite, Al-silicate phase with undetermined

structure, and microdiamond. The conventional geo￾barometry based on the chemistry of majoritic gar￾net suggested pressure of ∼22 GPa, whereas the Al￾silicate phase was assumed to be crystallized at 27 GPa

according to experiments. Taking in account the calcu￾lations and assumption above, Collerson et al. (2000)

have suggested the depth of the Malaita xenolith for￾mation at ∼600–670 km. However such a deep origin

was later questioned by Neal et al. (2001).

Because evaluation of depths from which man￾tle peridotites originate is always a subject of strong

discussions, the majoritic garnet – or its product of

decompression presented by pyrope with exsolution

lamella of pyroxenes – remains one of the best indi￾cators of the very high-pressure environments. It was

verified by many experiments conducted in different

laboratories that the majoritc garnet is stable at P > 5

GPa (e.g., Akaogi and Akimoto, 1977; Irifune, 1987).

The composition of majorite is represented by the com￾plex solid solution:

M3

VIII (Al2−2nMnSin)

VI Si3O12,

where M = Mg2+, Fe2+, and Ca2+, 0 ≤ n ≤ 1,

and superscripts indicate cations oxygen coordina￾tion. When pressure rises above 5 GPa, the garnet￾precursor is transformed into majorite (supercilisic

garnet) with Si (SiIV + SiVI) > 3 cations per formula

unit; the silica content also increases because the

Al3+ and Cr3+ are replaced by M and Si4+ cations

(e.g., Smith and Mason, 1970). Therefore, because

the Si content in the octahedral site of majoritic gar￾net increases with increasing pressures (Akaogi and

Akimoto, 1977; Irifune, 1997), and because the vol￾ume of the majoritic component dissolved in garnet

is calculated from experimental data (Gasparik, 2003),

the pyroxene exsolution lamellae in garnet can be used

as the “pressure indicator.”

Experiments on decompression of the majoritic gar￾net simulating the exhumation path of mantle peri￾dotites shows that at high-T (1,400◦C) decompression

from 14 to 12GPa, exsolutions of interstitial blebs of

diopside and Mg2SiO4- wadsleyite lamellae from a

parental majoritic garnet take place (Dobrzhinetskaya

et al., 2004, 2005a). These extend our interpreta￾tion of natural rocks, and allow reconstruction of the

former majoritic garnet in peridotites based on the

presence of the blebs of pyroxenes clustered around

the decompressed garnet containing exsolution lamel￾lae of olivine (former wadsleyite). Similar clusters

of clinopyroxenes around pyropic garnet containing

clinopyroxene lamellae exsolutions from the >300-

deep African xenolith were reported by Haggerty

and Sautter (1990), and Spengler et al. (2006) from

the >600-km-deep garnet peridotite from the West￾ern Gneiss region of Norway, an ultrahigh-pressure

terrane.

Diamonds from Kimberlitic Source

Diamond is the oldest (∼4,200 Ma) geological mate￾rial (Menneken et al., 2007) although in general,

diamond-beraing kimberlite/lamproite falling in range

from Earlier Archean to Eocene contain diamonds

which age is different than age of magmas formation

(e.g. Heaman et al., 2004). Diamond due to its chem￾ical inertness plays the role of a specific “container”

delivering solid and fluid inclusions unchanged from

Earth’s interior to its surface. Due to strong covalent

bonding of sp3 between carbon atoms, its structure

is stable at a wide range of pressures (4–>100 GPa)

and temperatures (1,000–3,500◦C) (Bundy, 1989).

Because diamond is stable through geologic time in

different geological environments (unless it is not oxi￾dized and is transformed back to graphite) it remains an

important material providing direct information about

pressure, temperature, and chemical conditions that

allow reconstruction of mantle mineralogy.

Diamonds from kimberlitic sources are the best

natural samples for evaluating the composition of

the mantle because they contain comparatively large

(from several hundred nm- to mm-size) inclusions

of different minerals. These inclusions are tradition￾ally used for establishing P & T conditions and the

depth of the diamond location during its growth. With

progress in high-resolution TEM and FIB technolo￾gies the research on large diamonds has revealed new

information that there is a continuum in inclusions

378 L.F. Dobrzhinetskaya and R. Wirth

size from those that are resolvable with electron

microprobe (EPM) down to those that are sub￾μm in size, including those, for instance, composed

of only a few water molecules that have dimen￾sions measured in angstroms. With advancement in

nanobeam technologies and synchrotron-assisted spec￾troscopic applications, the existing gap in knowledge

related to nanoscale inclusions in kimberlitic dia￾monds has recently began to be resolved (see Sec￾tion “Submicrometre- and Nanoscale-Size Inclusions

in Kimberlitic Diamonds”).

The range of estimated depth from which kim￾berlitic diamonds originate is as wide as ∼80

to >1,700 km (e.g., Stachel et al., 2005). Here, we

limit our discussion to diamonds with an exceptional

suite of mineral inclusions that suggest an origin from

the deep upper mantle transition zone, at a depth of

∼300–660 km (very deep diamonds) and to those that

are believed to originate from a lower mantle depth

of >660 km (superdeep diamonds).

The first diamonds containing ferripericlase (iron￾magnesium oxide) inclusions indicative of their very￾deep origin were found in Orroroo, South Australia,

and Koffeifontain, South Africa (Scott-Smith et al.,

1984). The authors have suggested the uppermost

lower mantle origin of these diamonds because fer￾ripericlase requires a minimum pressure of ∼25.5 GPa.

Such a pressure is expected below the 660 km seis￾mic discontinuity. Later, ferripericlase was found in

many other kimberlitic diamonds in Western and East￾ern Africa, North and South America, and Siberia

(e.g., Kaminsky et al., 2001). Numerous other lower￾mantle minerals, such as Mg- and Ca-perovskites

(high-pressure polymorphs of MgSiO3 and CaSiO3,

respectively) and tetragonal polymorph of pyropic gar￾net (TAPP) existing at pressures >22 GPa (e.g., Harris

et al., 1997; Harte et al., 1999) are known to exist in

kimberlitic diamonds.

The alluvial diamond deposits of the São Luiz

River, Juina Province, Brazil are known in the lit￾erature as a unique placer containing both very￾deep and superdeep diamond groups (e.g., Harte and

Harris, 1994; Harte et al., 1999; Kaminsky et al., 2001;

Hayman et al., 2005; Harte and Cayzer, 2007). Numer￾ous studies indicate that these diamonds are products

of erosion of the Cretaceous kimberlitic pipes (e.g.,

Costa et al., 2003). Very-deep diamonds from this area

contain abundant inclusions of garnet and clynopyrox￾ene with a wide variety of textural geometries, which

provides evidence that such diamonds must have come

from a depth of ∼450 km – and probably deeper (Harte

and Crayzer, 2007).

Harte and Cayzer’s (2007) paper presents a case

showing that, even though clinopyroxene and gar￾net inclusions in Juina diamonds do not exhibit typ￾ical “exsolution lamellae” geometry, the clynopyrox￾ene grains scattered inside the garnet and at its outer

zone are, indeed the result of the decompression of the

former majoritic garnet. Their electron back-scattered

diffraction (EBSD) studies show that each inclusion

of garnet is characterized by a constant crystallo￾graphic orientation, whereas the clynopyroxene grains

have orientations consistent with the {110} and <111>

directions of the garnet. The EBSD studies, along with

calculations of the integrated bulk chemistry of a gar￾net precursor, therefore confirm that that Cpx and Grt

inclusions were originally single-phase majoritic gar￾nets and that they preserve various states of decom￾pression during transport of the host diamonds from

depths of ≥450 km to the Earth’s surface (Harte

and Cayzer, 2007). Many of the inclusions of gar￾net and clynopyroxene in the Juina diamonds studied

by Harte and Cayzer (2007) now show compositions

that reflect their re-equilibration at lower pressures

corresponding to depths of ∼180–210 km. Because

the compositions of these re-equilibrated garnets and

clynopyroxenes are similar to those from eclogite

xenoliths that occurred in kimberlitic pipes, Harte and

Cayser hypothesize that such eclogitic xenoliths might

have originated from much greater depth within the

mantle.

The superdeep diamonds originating from the lower

mantle depth of 600∼1,700 km occur in the Juina area

in Mato Grosso, Brazil (e.g., Kaminsky et al., 2001;

Hayman et al., 2005). The Mato Grosso diamonds

contain inclusions of Fe-rich periclase (ferropericlase)

with Mg# = 0.65 (Hayman et al., 2005). Although fer￾ropericlase is the dominant inclusion in ultradeep dia￾monds, its presence alone does not signify their lower￾mantle origin because ferropericlase is also stable in

upper-mantle conditions. The coexistence of ferroper￾iclase with CaSiO3-perovskite or MgSiO3-perovskite

should be taken as strong confirmation of the lower￾mantle conditions (e.g., Stachel et al., 2005). So far,

in addition to ferripericlase, superdeep diamonds from

Juina contain CaSiO3-perovskite, Cr-Ti spinel, decom￾pressed “olivine,” Mn-Ilmenite, tetragonal alman￾dine pyrope phase (TAPP), and native Ni. Many

Ultradeep Rocks and Diamonds in the Light of Advanced Scientific Technologies 379

similar ultrahigh-pressure minerals or their assem￾blages are found as inclusions in other superdeep kim￾berlitic diamonds (e.g., Davies et al., 1999; Satchel

et al., 2000; Huttchison et al.,2001; Kaminsky et al.,

2001; Brenker et al., 2007). Corundum inclusions were

also found also in superdeep diamonds in association

with MgSiO3 perovskite and ferropericlase, suggest￾ing that a free Al phase can exist in the lower man￾tle (Huttchison et al., 2001). By now, the superdeep

diamonds are found at more than a dozen geographic

localities on eight cratons (e.g., McCammon, 2001).

Submicrometre- and Nanoscale-Size

Inclusions in Kimberlitic Diamonds

Using FIB-TEM techniques, R. Wirth has initiated

studies of sub-μm-size inclusions in alluvial diamonds

from Minas Gerais, Brazil, and from Ukraine and

Siberia, as well as diamonds from kimberlites of Slave

Craton in Northern Canada, South Africa, and the

Siberian pipes Udachnaya, Mir, Internationalnaya, and

Yubileynaya (Wirth, 2005; Kvasnytsya et al., 2005;

Klein-BenDavid et al., 2006; Kvasnytsya et al., 2006;

Logvinova et al., 2006; Wirth, 2006; Wirth et al.,

2007; Wirth, 2007; Klein-BenDavid et al., 2007). Until

recently, such tiny inclusions were not in the scope of

the researchers because only large inclusions (up to

several mm) were used for conventional EMP analy￾ses in the field of diamond studies, and methods (e.g.,

like FIB) for studies of nanoscale inclusions was not

available. The results show that sub-μm-size inclu￾sions in diamonds from different locations in the world

exhibit a surprising similarity. Usually, they consist of

a fluid or melt associated with solid phases that are

represented by silicates (e.g., phlogopite), carbonates,

phosphate (apatite with fluorine and/or chlorine), chlo￾rides (NaCl, KCl), sulphides (occasionally), ilmenite,

and rare magnetite. Carbonates are usually represented

by calcite with low strontium concentration, dolomite,

and/or pure BaCO3. Klein-BenDavid et al. (2006)

studied diamonds from Slave Craton (Diavik Mine)

and Siberia (Yubileinaya), and they have suggested

that micro-inclusions consisting of a dense supercrit￾ical fluid were trapped by diamonds during their crys￾tallisation in a media containing fluid phase. Later, dur￾ing cooling, secondary mineral phases grew up from

the trapped fluid.

Micrometre-sized olivine inclusions were found in

an alluvial diamond from the Macaubas River (Minas

Gerais) (de Souza Martins, 2006). The diamond crys￾tal was mechanically polished in such a way that

the olivine inclusions remained approximately 5 μm

below the surface to keep them closed. A TEM foil was

cut across the diamond-olivine interface using the FIB

technique. The high-resolution TEM imaging showed

that the interface consists of an approximately 50-nm￾thick amorphous layer; EELS analyses revealed the

presence of fluorine in this layer. Any contamination

with fluorine can be excluded because the diamond

was never exposed to HF and the olivine inclusion was

“sealed” inside of the diamond prior to the FIB milling.

The fluorine has been detected frequently with simi￾lar techniques in sub-μm inclusions in diamonds from

other localities – the Koffiefontein mine, South Africa

(Klein-BenDavid et al., 2007) and Siberia (Kvasnytsya

et al., 2006; Logvinova et al., 2006). In addition to

olivine, the KCl solid nano-inclusions co-existing with

a fluid phase were observed in the same diamond; in

this case, the KCl inclusion is surrounded by a corona

of a brine.

Several FIB foils were prepared from superdeep dia￾monds from the Juina area of Brazil where we expected

to find also some sub-micrometric fluid-solid inclu￾sion pockets. To our great surprise, no fluid-rich nano￾inclusions similar to those described above in dia￾monds from Minas Gerais, Brazil, or any other dia￾monds from the upper mantle, were observed in the

prepared foils. Only carbonate nano-inclusions were

found to be common for the Juina superdeep dia￾monds. Such contrasting observations may suggest

that the superdeep diamonds have been grown in an

anhydrous mantle environment that is different from

the one that exists in the shallower levels of the

Earth’s mantle above the ∼660-km seismic disconti￾nuity zone. Understanding such differences in the pres￾ence/absence of nano-inclusions may lead to the con￾clusion that superdeep diamond environments repre￾sent a “dry” media, which could result in a different

mechanism of diamond nucleation and growth. At the

current stage of our ongoing research, more observa￾tions need to be collected to support or deny this work￾ing hypothesis.

One other interesting research avenue is emerg￾ing from the presence of carbonate nano-inclusions

in superdeep diamonds. The FIB foil of one of

the superdeep Juina diamonds containing nanometric