Sergei Simakov - Academia.edu (original) (raw)
Papers by Sergei Simakov
P-T- Oxygen fugacity (fO2) conditions and fluid compositions were estimated for the formation con... more P-T- Oxygen fugacity (fO2) conditions and fluid compositions were estimated for the formation conditions of pyrope garnet inclusions in diamonds and xenocrysts from diamond-bearing and diamond-free kimberlites using their total chemical analyses and single oxythermobarometry. Our data indicate that optimal conditions for diamond growth and preservation occur in the presumed water-rich mantle fluids containing the lowest abundance of free atomic carbon. The majority of the calculated C-H-O fluid compositions for diamond formation in peridotite xenoliths from high diamond grade kimberlites correspond to a high hydrogen and low carbon and oxygen atomic fluid percents, while those from the majority of peridotite xenoliths in the low grade diamond kimberlites corresponds to the low hydrogen, high carbon and oxygen atomic percent fluids. This new approach defines the conditions of diamond formation for kimberlitic deposits. It better characterizes diamond grades in kimberlites in comparis...
Geoscience Frontiers, 2020
Rock fragments from the deepest parts of a buried hydrothermal system belonging to the Mesozoic T... more Rock fragments from the deepest parts of a buried hydrothermal system belonging to the Mesozoic Tethys Ocean entered as xenoliths in a Miocenic diatreme, hence brought to the surface, in the Hyblean Plateau (Sicily). Some xenoliths consist of strongly serpentinized ultramafic rocks bearing blebs of abiotic organic matter, where clusters of amorphous carbon nanoparticles, including nanodiamonds, are immersed. Such an occurrence conjures up established hypotheses that diamond surfaces are suitable catalytic platforms stimulating the assemblage of complex bio-organic molecules relevant to the emergence of life on Earth. The appearance of bio-organic molecules under primitive Earth conditions is one of the major unsolved questions on the origin of life. Here we report new micro-Raman spectra on blebs of abiotic organic matter from a selected xenolith. Diamond bands were related to hydrogenated nanocrystalline diamonds, with size of nearly 1-1.6 nm, formed from organics at low pressures and temperatures. In particular, diamond surfaces can give rise to crystalline interfacial water layers that may have played a fundamental role in the early biosphere evolution as a good medium for rapidly transporting positive charges in the form of hydrated protons. Nowadays, proton gradients in alkaline hydrothermal vents along oceanic ridges are generally viewed as key pre-biotic factors. In general, serpentinites span the entire geological record, including prebiotic times. These hydrous ultramafic rocks often display evidence of abiotic carbon species, both organic and inorganic, including nanodiamonds, being also capable to give rise to chemiosmotic processes and proton gradients necessary to the organisms, such as the "Last Universal Common Ancestor" (LUCA), in the prebiotic Earth.
Gondwana Research, 2019
Comment on "Comparison of enigmatic diamonds from the Tolbachik arc volcano (Kamchatka) and Tibet... more Comment on "Comparison of enigmatic diamonds from the Tolbachik arc volcano (Kamchatka) and Tibetan ophiolites: Assessing the role of contamination by synthetic materials" by Litasov et al. 2019, Gondwana Research,
Origins of Life and Evolution of Biospheres, 2019
Pyrite and organic matter closely coexist in some hydrothermally-altered gabbroic xenoliths from ... more Pyrite and organic matter closely coexist in some hydrothermally-altered gabbroic xenoliths from the Hyblean Plateau, Sicily. The representative sample consists of plagioclase, Fe-oxides, clinopyroxene, pyrite and minor amounts of many other minerals. Plagioclase displays incipient albitization, clinopyroxene is deeply corroded. Pyrite grains are widely replaced by spongy-textured magnetite, which locally hosts Ca-(and Fe-)sulfate micrograins and blebs of condensed organic matter. Whole-rock trace element distribution evidences that incompatible elements, particularly the fluid-mobile Ba, U and Pb, are significantly enriched with respect to N-MORB values. The mineralogical and geochemical characteristics of the sample, and its U-Pb zircon age of 216.9 ± 6.7 MA, conform to the xenolith-based viewpoint that the unexposed Hyblean basement is a relict of the Ionian Tethys lithospheric domain, mostly consisting of abyssal-type serpentinized peridotites with small gabbroic intrusions. Circulating hydrothermal fluids there favored the formation of hydrocarbons trough Fischer-Tropsch-type organic synthesis, giving also rise to sulfidization episodes. Subsequent variations in temperature and redox conditions of the system induced partial de-sulfidization, Fe-oxides precipitation and sulfate-forming reactions, also promoting poly-condensation and aromatization of the already-formed hydrocarbons. Here we show organic matter adhering to a crystal face of a microscopic pyrite grain. Pyrite surfaces, as abiotic analogues of enzymes, can adsorb and concentrate organic molecules, also acting as catalysts for a broad range of proto-biochemical reactions. The present data therefore may support established abiogenesis models suggesting that pyrite surfaces carried out primitive metabolic cycles in suitable environments of the early Earth, such as endolithic recesses in mafic rocks permeated by hydrothermal fluids.
Ore Geology Reviews, 2018
The processes of formation of some diamond types still raise contentious issues, mainly on the or... more The processes of formation of some diamond types still raise contentious issues, mainly on the origin of the largest diamond crystals recovered from kimberlites. These diamonds constitute less than 2% of worldwide resources and correspond to rare type IIa. They possess some peculiar features: (i) silicate and oxide inclusions are extremely rare, (ii) their δ 13 C ranges from-17 to-21 0 /00. The detailed estimation of the Premier pressuretemperature-oxygen fugacity parameters and the physic-chemical modeling of diamond growth-dissolution processes suggest that extra-large diamonds have multiple origins. Their formation may occur from lower mantle to crustal depths. Their main building-up takes place from fluids in the pegmatitic veins solidified along the contacts of kimberlite magma at a crustal depth. The model explains the main features of the largest kimberlitic diamonds, i.e. their great sizes, light δ 13 C signatures, low nitrogen contents, high degree of resorption, absence of mantle-derived mineral inclusions and their occurrence in the form of rare isolated crystals in the host kimberlite.
Eclogite is one of the major xenolith types in kimberlite. The investigation of these xenoliths p... more Eclogite is one of the major xenolith types in kimberlite. The investigation of these xenoliths provides important constraints on P, T distribution within the upper mantle and its chemical composition. A garnet-clinopyroxene barometer is proposed for mantle eclogites on the basis of the CaTs solubility in clinopyroxene coexisting with garnet on the basis of the reaction (Simakov and Taylor, 2000
Geochim Cosmochim Acta, 1998
Mafic boudins found at Viared in the central Eastern Segment display mineralogical and textural f... more Mafic boudins found at Viared in the central Eastern Segment display mineralogical and textural features which show that these rocks have been eclogites. This discovery together with occurrences of relict eclogites along the Mylonite Zone suggests that the whole of the Eastern Segment was subjected to eclogite facies. The mafic boudins found are hornblende-plagioclase amphibolite, but the interiors show evidence
Nanosized diamond particles in the interstellar space and in the Earth rocks related with water p... more Nanosized diamond particles in the interstellar space and in the Earth rocks related with water presence. In the paper proposed the model of the nanosized diamond particle formation from oxidized water-carbon dioxide gaseous mixtures.
DESCRIPTION Garnet–clinopyroxene barometer is proposed for mantle eclogites on the basis of CaTs ... more DESCRIPTION Garnet–clinopyroxene barometer is proposed for mantle eclogites on the basis of CaTs solubility in clinopyroxene coexisting with garnet. Such barometers permit estimation of P–T conditions for eclogite garnet–clinopyroxene assemblages for the ranges 1150≤ T≤1700 °C and 3 ≤ P≤10 GPa. The barometer were used to determine formation conditions of eclogitic inclusions in diamonds, diamondbearing and diamond-free eclogites from kimberlite pipes.
DESCRIPTION The composition of the C–O–H fluid is estimated on the basis of the composition of il... more DESCRIPTION The composition of the C–O–H fluid is estimated on the basis of the composition of ilmenite xenocrysts. The P–T parameters and oxygen fugacity were assessed for peridotitic ilmenites from the diamond and diamond�free kimberlites of Africa and Yakutia. The composition of the equilibrium C–O–H fluid for these conditions was calculated. The diamond and diamond�free pipes of Angola and Yakutia are characterized by the H2O-rich and CO2-rich fluids, respectively. The results indicate that estimation of the composition of the C–O–H fluid in equilibrium with picroilmenite fits the necessary petrological criteria and may be applied for assessment of the diamond potential of mantle objects.
Doklady Earth Sciences, 2015
Nature Precedings, 2010
The perspectives of nanosized diamond particles formation at metastable P-T parameters from organ... more The perspectives of nanosized diamond particles formation at metastable P-T parameters from organic matter at hydrothermal conditions is discussed. From the known data it's possible to conclude that we could provide the hydrothermal syntheses of nanodiamonds from the water-oil or water-alcohol-oil solutions.
Scientific reports, 2015
Most of Earth's diamonds are connected with deep-seated mantle rocks; however, in recent year... more Most of Earth's diamonds are connected with deep-seated mantle rocks; however, in recent years, μm-sized diamonds have been found in shallower metamorphic rocks, and the process of shallow-seated diamond formation has become a hotly debated topic. Nanodiamonds occur mainly in chondrite meteorites associated with organic matter and water. They can be synthesized in the stability field of graphite from organic compounds under hydrothermal conditions. Similar physicochemical conditions occur in serpentinite-hosted hydrothermal systems. Herein, we report the first finding of nanodiamonds, primarily of 6 and 10 nm, in Hyblean asphaltene-bearing serpentinite xenoliths (Sicily, Italy). The discovery was made by electron microscopy observations coupled with Raman spectroscopy analyses. The finding reveals new aspects of carbon speciation and diamond formation in shallow crustal settings. Nanodiamonds can grow during the hydrothermal alteration of ultramafic rocks, as well as during the ...
Nature Precedings, 2010
The model of nanosized diamond particles formation at metastable P-T parameters from fluid is pre... more The model of nanosized diamond particles formation at metastable P-T parameters from fluid is presented. It explains the specific of CVD diamond synthesis gases mixtures and hydrothermal growth of diamond at low P-T parameters as well as it explains the geneses of metamorphic and magmatic nano-and microdiamond in the shallow depth Earth rocks and the genesis of interstellar nanodiamond formations in the space. The optimal gases system compositions for metastable diamond formations have been long debated in many publications during the long period. Badziag et al. 1 came to the conclusion that nanometer-sized diamonds could be more stable than graphite when formed from hydrocarbons with a H/C ratio of more than 0.24. Simakov 2 has shown that the addition of oxygen to the hydrocarbon gases can stabilize the diamond nucleus formation in the P-T field of graphite stability. The question of whether diamond could be grown under hydrothermal conditions mimicking those under which it is formed in the Earth has been also long debated 3,4 . Roy et al. 5 have made the detailed study of the hydrothermal growth of diamond in the C-H-O and C-H-O-halogen systems. Shimansky et al. 6 have claimed hydrothermal growth of diamond but no details were given on the composition of the liquid and the characterization of the phases. Zhao et al. 7 provided the diamond hydrothermal synthesis from the mixture of the glassy carbon, powdered nickel, diamond seeds and water at 800 o C and 1.4 kbar. From another hand, Bachmann et al. 8 have analysed gaseous compositions of the different 78 CVD diamond syntheses and shown that their compositions lie in the corridor extended range from H to CO compositions of the C-O-H system. The triangularly shaped "diamond domain" is adjusted along the CO line of the diagram and is limited by the lines X CO2 0.57 on its carbon-rich side ≈ 1
Journal of Materials Research, 2010
The model of nanosized diamond particles formation at metastable P-T parameters from a C-H-O flui... more The model of nanosized diamond particles formation at metastable P-T parameters from a C-H-O fluid system is presented. It explains the hydrothermal formation and growth of diamond and the specifics of chemical vapor deposition (CVD) diamond synthesis gas mixtures at low P-T parameters. Further, the model explains the genesis of interstellar nanodiamond formations in space and the genesis of metamorphic microdiamonds in shallow depth Earth rocks. In contrast to models where many possible reactions are considered, the present model makes the simplest possible assumptions about the key processes, and is then able to account for various tendencies seen in experimental data.
Geochimica et Cosmochimica Acta, 1998
Doklady Earth Sciences, 2007
. Temperatures and pressures calculated for garnetclinopyroxene assemblages from eclogite xenolit... more . Temperatures and pressures calculated for garnetclinopyroxene assemblages from eclogite xenoliths of the Obnazhennaya (1) and Udachnaya (2) pipes and Venezuela diamonds (3) relative to the graphite-diamond equilibrium line (analyses were taken from ).
P-T- Oxygen fugacity (fO2) conditions and fluid compositions were estimated for the formation con... more P-T- Oxygen fugacity (fO2) conditions and fluid compositions were estimated for the formation conditions of pyrope garnet inclusions in diamonds and xenocrysts from diamond-bearing and diamond-free kimberlites using their total chemical analyses and single oxythermobarometry. Our data indicate that optimal conditions for diamond growth and preservation occur in the presumed water-rich mantle fluids containing the lowest abundance of free atomic carbon. The majority of the calculated C-H-O fluid compositions for diamond formation in peridotite xenoliths from high diamond grade kimberlites correspond to a high hydrogen and low carbon and oxygen atomic fluid percents, while those from the majority of peridotite xenoliths in the low grade diamond kimberlites corresponds to the low hydrogen, high carbon and oxygen atomic percent fluids. This new approach defines the conditions of diamond formation for kimberlitic deposits. It better characterizes diamond grades in kimberlites in comparis...
Geoscience Frontiers, 2020
Rock fragments from the deepest parts of a buried hydrothermal system belonging to the Mesozoic T... more Rock fragments from the deepest parts of a buried hydrothermal system belonging to the Mesozoic Tethys Ocean entered as xenoliths in a Miocenic diatreme, hence brought to the surface, in the Hyblean Plateau (Sicily). Some xenoliths consist of strongly serpentinized ultramafic rocks bearing blebs of abiotic organic matter, where clusters of amorphous carbon nanoparticles, including nanodiamonds, are immersed. Such an occurrence conjures up established hypotheses that diamond surfaces are suitable catalytic platforms stimulating the assemblage of complex bio-organic molecules relevant to the emergence of life on Earth. The appearance of bio-organic molecules under primitive Earth conditions is one of the major unsolved questions on the origin of life. Here we report new micro-Raman spectra on blebs of abiotic organic matter from a selected xenolith. Diamond bands were related to hydrogenated nanocrystalline diamonds, with size of nearly 1-1.6 nm, formed from organics at low pressures and temperatures. In particular, diamond surfaces can give rise to crystalline interfacial water layers that may have played a fundamental role in the early biosphere evolution as a good medium for rapidly transporting positive charges in the form of hydrated protons. Nowadays, proton gradients in alkaline hydrothermal vents along oceanic ridges are generally viewed as key pre-biotic factors. In general, serpentinites span the entire geological record, including prebiotic times. These hydrous ultramafic rocks often display evidence of abiotic carbon species, both organic and inorganic, including nanodiamonds, being also capable to give rise to chemiosmotic processes and proton gradients necessary to the organisms, such as the "Last Universal Common Ancestor" (LUCA), in the prebiotic Earth.
Gondwana Research, 2019
Comment on "Comparison of enigmatic diamonds from the Tolbachik arc volcano (Kamchatka) and Tibet... more Comment on "Comparison of enigmatic diamonds from the Tolbachik arc volcano (Kamchatka) and Tibetan ophiolites: Assessing the role of contamination by synthetic materials" by Litasov et al. 2019, Gondwana Research,
Origins of Life and Evolution of Biospheres, 2019
Pyrite and organic matter closely coexist in some hydrothermally-altered gabbroic xenoliths from ... more Pyrite and organic matter closely coexist in some hydrothermally-altered gabbroic xenoliths from the Hyblean Plateau, Sicily. The representative sample consists of plagioclase, Fe-oxides, clinopyroxene, pyrite and minor amounts of many other minerals. Plagioclase displays incipient albitization, clinopyroxene is deeply corroded. Pyrite grains are widely replaced by spongy-textured magnetite, which locally hosts Ca-(and Fe-)sulfate micrograins and blebs of condensed organic matter. Whole-rock trace element distribution evidences that incompatible elements, particularly the fluid-mobile Ba, U and Pb, are significantly enriched with respect to N-MORB values. The mineralogical and geochemical characteristics of the sample, and its U-Pb zircon age of 216.9 ± 6.7 MA, conform to the xenolith-based viewpoint that the unexposed Hyblean basement is a relict of the Ionian Tethys lithospheric domain, mostly consisting of abyssal-type serpentinized peridotites with small gabbroic intrusions. Circulating hydrothermal fluids there favored the formation of hydrocarbons trough Fischer-Tropsch-type organic synthesis, giving also rise to sulfidization episodes. Subsequent variations in temperature and redox conditions of the system induced partial de-sulfidization, Fe-oxides precipitation and sulfate-forming reactions, also promoting poly-condensation and aromatization of the already-formed hydrocarbons. Here we show organic matter adhering to a crystal face of a microscopic pyrite grain. Pyrite surfaces, as abiotic analogues of enzymes, can adsorb and concentrate organic molecules, also acting as catalysts for a broad range of proto-biochemical reactions. The present data therefore may support established abiogenesis models suggesting that pyrite surfaces carried out primitive metabolic cycles in suitable environments of the early Earth, such as endolithic recesses in mafic rocks permeated by hydrothermal fluids.
Ore Geology Reviews, 2018
The processes of formation of some diamond types still raise contentious issues, mainly on the or... more The processes of formation of some diamond types still raise contentious issues, mainly on the origin of the largest diamond crystals recovered from kimberlites. These diamonds constitute less than 2% of worldwide resources and correspond to rare type IIa. They possess some peculiar features: (i) silicate and oxide inclusions are extremely rare, (ii) their δ 13 C ranges from-17 to-21 0 /00. The detailed estimation of the Premier pressuretemperature-oxygen fugacity parameters and the physic-chemical modeling of diamond growth-dissolution processes suggest that extra-large diamonds have multiple origins. Their formation may occur from lower mantle to crustal depths. Their main building-up takes place from fluids in the pegmatitic veins solidified along the contacts of kimberlite magma at a crustal depth. The model explains the main features of the largest kimberlitic diamonds, i.e. their great sizes, light δ 13 C signatures, low nitrogen contents, high degree of resorption, absence of mantle-derived mineral inclusions and their occurrence in the form of rare isolated crystals in the host kimberlite.
Eclogite is one of the major xenolith types in kimberlite. The investigation of these xenoliths p... more Eclogite is one of the major xenolith types in kimberlite. The investigation of these xenoliths provides important constraints on P, T distribution within the upper mantle and its chemical composition. A garnet-clinopyroxene barometer is proposed for mantle eclogites on the basis of the CaTs solubility in clinopyroxene coexisting with garnet on the basis of the reaction (Simakov and Taylor, 2000
Geochim Cosmochim Acta, 1998
Mafic boudins found at Viared in the central Eastern Segment display mineralogical and textural f... more Mafic boudins found at Viared in the central Eastern Segment display mineralogical and textural features which show that these rocks have been eclogites. This discovery together with occurrences of relict eclogites along the Mylonite Zone suggests that the whole of the Eastern Segment was subjected to eclogite facies. The mafic boudins found are hornblende-plagioclase amphibolite, but the interiors show evidence
Nanosized diamond particles in the interstellar space and in the Earth rocks related with water p... more Nanosized diamond particles in the interstellar space and in the Earth rocks related with water presence. In the paper proposed the model of the nanosized diamond particle formation from oxidized water-carbon dioxide gaseous mixtures.
DESCRIPTION Garnet–clinopyroxene barometer is proposed for mantle eclogites on the basis of CaTs ... more DESCRIPTION Garnet–clinopyroxene barometer is proposed for mantle eclogites on the basis of CaTs solubility in clinopyroxene coexisting with garnet. Such barometers permit estimation of P–T conditions for eclogite garnet–clinopyroxene assemblages for the ranges 1150≤ T≤1700 °C and 3 ≤ P≤10 GPa. The barometer were used to determine formation conditions of eclogitic inclusions in diamonds, diamondbearing and diamond-free eclogites from kimberlite pipes.
DESCRIPTION The composition of the C–O–H fluid is estimated on the basis of the composition of il... more DESCRIPTION The composition of the C–O–H fluid is estimated on the basis of the composition of ilmenite xenocrysts. The P–T parameters and oxygen fugacity were assessed for peridotitic ilmenites from the diamond and diamond�free kimberlites of Africa and Yakutia. The composition of the equilibrium C–O–H fluid for these conditions was calculated. The diamond and diamond�free pipes of Angola and Yakutia are characterized by the H2O-rich and CO2-rich fluids, respectively. The results indicate that estimation of the composition of the C–O–H fluid in equilibrium with picroilmenite fits the necessary petrological criteria and may be applied for assessment of the diamond potential of mantle objects.
Doklady Earth Sciences, 2015
Nature Precedings, 2010
The perspectives of nanosized diamond particles formation at metastable P-T parameters from organ... more The perspectives of nanosized diamond particles formation at metastable P-T parameters from organic matter at hydrothermal conditions is discussed. From the known data it's possible to conclude that we could provide the hydrothermal syntheses of nanodiamonds from the water-oil or water-alcohol-oil solutions.
Scientific reports, 2015
Most of Earth's diamonds are connected with deep-seated mantle rocks; however, in recent year... more Most of Earth's diamonds are connected with deep-seated mantle rocks; however, in recent years, μm-sized diamonds have been found in shallower metamorphic rocks, and the process of shallow-seated diamond formation has become a hotly debated topic. Nanodiamonds occur mainly in chondrite meteorites associated with organic matter and water. They can be synthesized in the stability field of graphite from organic compounds under hydrothermal conditions. Similar physicochemical conditions occur in serpentinite-hosted hydrothermal systems. Herein, we report the first finding of nanodiamonds, primarily of 6 and 10 nm, in Hyblean asphaltene-bearing serpentinite xenoliths (Sicily, Italy). The discovery was made by electron microscopy observations coupled with Raman spectroscopy analyses. The finding reveals new aspects of carbon speciation and diamond formation in shallow crustal settings. Nanodiamonds can grow during the hydrothermal alteration of ultramafic rocks, as well as during the ...
Nature Precedings, 2010
The model of nanosized diamond particles formation at metastable P-T parameters from fluid is pre... more The model of nanosized diamond particles formation at metastable P-T parameters from fluid is presented. It explains the specific of CVD diamond synthesis gases mixtures and hydrothermal growth of diamond at low P-T parameters as well as it explains the geneses of metamorphic and magmatic nano-and microdiamond in the shallow depth Earth rocks and the genesis of interstellar nanodiamond formations in the space. The optimal gases system compositions for metastable diamond formations have been long debated in many publications during the long period. Badziag et al. 1 came to the conclusion that nanometer-sized diamonds could be more stable than graphite when formed from hydrocarbons with a H/C ratio of more than 0.24. Simakov 2 has shown that the addition of oxygen to the hydrocarbon gases can stabilize the diamond nucleus formation in the P-T field of graphite stability. The question of whether diamond could be grown under hydrothermal conditions mimicking those under which it is formed in the Earth has been also long debated 3,4 . Roy et al. 5 have made the detailed study of the hydrothermal growth of diamond in the C-H-O and C-H-O-halogen systems. Shimansky et al. 6 have claimed hydrothermal growth of diamond but no details were given on the composition of the liquid and the characterization of the phases. Zhao et al. 7 provided the diamond hydrothermal synthesis from the mixture of the glassy carbon, powdered nickel, diamond seeds and water at 800 o C and 1.4 kbar. From another hand, Bachmann et al. 8 have analysed gaseous compositions of the different 78 CVD diamond syntheses and shown that their compositions lie in the corridor extended range from H to CO compositions of the C-O-H system. The triangularly shaped "diamond domain" is adjusted along the CO line of the diagram and is limited by the lines X CO2 0.57 on its carbon-rich side ≈ 1
Journal of Materials Research, 2010
The model of nanosized diamond particles formation at metastable P-T parameters from a C-H-O flui... more The model of nanosized diamond particles formation at metastable P-T parameters from a C-H-O fluid system is presented. It explains the hydrothermal formation and growth of diamond and the specifics of chemical vapor deposition (CVD) diamond synthesis gas mixtures at low P-T parameters. Further, the model explains the genesis of interstellar nanodiamond formations in space and the genesis of metamorphic microdiamonds in shallow depth Earth rocks. In contrast to models where many possible reactions are considered, the present model makes the simplest possible assumptions about the key processes, and is then able to account for various tendencies seen in experimental data.
Geochimica et Cosmochimica Acta, 1998
Doklady Earth Sciences, 2007
. Temperatures and pressures calculated for garnetclinopyroxene assemblages from eclogite xenolit... more . Temperatures and pressures calculated for garnetclinopyroxene assemblages from eclogite xenoliths of the Obnazhennaya (1) and Udachnaya (2) pipes and Venezuela diamonds (3) relative to the graphite-diamond equilibrium line (analyses were taken from ).