Eclogites from the Sunndal-Grubse Ultramafic Mass, Almklovdalen, Norway and the T-P History of the Almklovdalen Masses     

LAPPIN  M. A. 《Journal of Petrology》1974,15(3):567-601

An orthopyroxene eclogite from the Sunndal-Grubse mass containsclinopyroxenes with lamellae of garnet and orthopyroxene. Differencesin mineral and bulk chemistry distinguish this, and adjacenteclogites, from eclogites in other Almklovdalen ultramafic masses.Analyses are used in conjunction with experimental data to assignthree T-P points in the complex T-P path of the ultramafic masses.Other T-P points are assigned on a basis of petrographic andstructural arguments. The inferred T-P path offers support toearlier suggestions that the masses are tectonically controlledintroductions within the surrounding gneisses. The survivalof eclogite facies assemblages within the ultramafic massesis attributed to a gneissic metamorphism where P_H2O < P_Total.The T-P path also suggests an extended history of subsolidusre-equilibration under mantle conditions possibly complicatedby open system reactions. The eclogites of the Sunndal-Grubsemass are thought to be cumulates and differentiates of relativelyhigh pressure (25–28 kb) melting in ultramafic rocks.The complex T-P path can be explained in terms of convectioncell mechanisms.  相似文献   

Diamonds from the Buffalo Head Hills,Alberta: Formation in a non-conventional setting     

《Lithos》2007,93(1-2):199-213

Kimberlite pipes K11, K91 and K252 in the Buffalo Head Hills, northern Alberta show an unusually large abundance (20%) of Type II (no detectable nitrogen) diamonds. Type I diamonds range in nitrogen content from 6 ppm to 3300 ppm and in aggregation states from low (IaA) to complete (IaB). The Type IaB diamonds extend to the lowest nitrogen concentrations yet observed at such high aggregation states, implying that mantle residence occurred at temperatures well above normal lithospheric conditions. Syngenetic mineral inclusions indicate lherzolitic, harzburgitic, wehrlitic and eclogitic sources. Pyropic garnet and forsteritic olivine characterize the peridotitic paragenesis from these pipes. One lherzolitic garnet inclusion has a moderately majoritic composition indicating a formation depth of ∼ 400 km. A wehrlitic paragenesis is documented by a Ca-rich, high-chromium garnet and very CaO-rich (0.11–0.14 wt.%) olivine. Omphacitic pyroxene and almandine-rich garnet are characteristic of the eclogitic paragenesis. A bimodal δ¹³C distribution with peaks at − 5‰ and − 17‰ is observed for diamonds from all three kimberlite pipes. A large proportion (∼ 40%) of isotopically light diamonds (δ¹³C < −10‰) indicates a predominantly eclogitic paragenesis.The Buffalo Head Terrane is of Lower Proterozoic metamorphic age (2.3–2.0 Ga) and hence an unconventional setting for diamond exploration. Buffalo Hills diamonds formed during multiple events in an atypical mantle setting. The presence of majorite and abundance of Type II and Type IaB diamonds suggests formation under sublithospheric conditions, possibly in a subducting slab and resulting megalith. Type IaA to IaAB diamonds indicate formation and storage under lower temperature in normal lithospheric conditions.  相似文献   

Associated chemical and carbon isotopic composition variations in diamonds from Finsch and Premier kimberlite,South Africa     

Peter Deines  J.J Gurney  J.W Harris 《Geochimica et cosmochimica acta》1984,48(2):325-342

The carbon isotopic composition of 66 inclusion-containing diamonds from the Premier kimberlite, South Africa, 93 inclusion-containing diamonds and four diamonds of two diamond-bearing peridotite xenoliths from the Finsch kimberlite, South Africa was measured. The data suggest a relationship between the carbon isotopic composition of the diamonds and the chemical composition of the associated silicates. For both kimberlites similar trends are noted for diamonds containing peridotite-suite inclusions (P-type) and for diamonds containing eclogite-suite inclusions (E-type): Higher δ¹³C P-type diamonds tend to have inclusions lower in SiO₂ (ol), Al₂O₃ (opx, gt), Cr₂O₃, MgO, $\text{Mg}\text{(Mg + Fe)}$ (ol, opx, gt) and higher in FeO (ol, opx, gt) and CaO (gt). Higher δ¹³C E-type diamonds tend to have inclusions lower in SiO₂, Al₂O₃ (gt, cpx), MgO, $\text{Mg}\text{(Mg + Fe)}$ (gt), Na₂O, K₂O, TiO₂ (cpx) and higher in CaO, $\text{Ca}\text{(Ca + Mg)}$ (gt, cpx).Consideration of a number of different models that have been proposed for the genesis of kimberlites, their xenoliths and diamonds shows that they are all consistent with the conclusion that in the mantle, regions exist that are characterized by different mean carbon isotopic compositions.  相似文献   

Diamonds: time capsules from the Siberian Mantle   总被引：1，自引：0，他引：1  

Lawrence A. Taylor  Mahesh Anand 《Chemie der Erde / Geochemistry》2004,64(1):1-74

Diamonds are thought to be “time capsules” from the Earth's mantle. However, by themselves, consisting of nearly pure carbon, diamonds provide little geochemical information about their conditions of formation and the nature of their mantle hosts. This obstacle to studying the origin of diamonds and their hosts can be overcome by using two main approaches that focus on studying: (1) the rocks that contain diamonds, i.e., diamondiferous xenoliths; and (2) mineral inclusions within the diamonds, the time capsule's little treasures, if you will. Diamondiferous xenoliths, their diamonds, and mineral inclusions within the diamonds are the subject of this review, focusing on studies of samples from the Yakutian kimberlites in the Siberian Platform.Studies of diamondiferous eclogite xenoliths significantly enhance our understanding of the complex petrogenesis of this important group of rocks and their diamonds. Such studies involve various geochemical and petrological investigations of these eclogites, including major and trace-element, radiogenic as well as stable isotopic analyses of whole rocks and minerals. The results from these studies have clearly established that the Group A-C eclogites originate from subduction of ancient oceanic crust. This theory is probably applicable worldwide.Within the last several years, our research group at Tennessee has undertaken the systematic dissection (pull apart) of diamondiferous eclogites from Siberia, consisting of the following steps: (1) high-resolution computed X-ray tomography of the xenoliths to produce 3D images that relate the minerals of the xenoliths to their diamonds; (2) detailed dissection of the entire xenolith to reveal the diamonds inside, followed by characterization of the setting of the diamonds within their enclosing minerals; and (3) extraction of diamonds from the xenolith for further investigation of the diamonds and their inclusions. In this last step, it is important that the nature and relative positions of the diamond inclusions are carefully noted in order to maximize the number of inclusions that can be exposed simultaneously on one polished surface. In this modus operandi, cathodoluminescence imaging, plus FTIR/N aggregation and C/N isotopic analyses are performed on polished diamond surfaces to reveal their internal growth zones and the spatial relationship of the mineral inclusions to these zones.Knowledge gained by such detailed, albeit work-intensive, studies continues to add immensely to the constantly evolving models of the origin of diamonds and their host rocks in the Earth's mantle, as well as to lithospheric stability models in cratonic areas. Multiple lines of evidence indicate the ultimate crustal origin for the majority of mantle eclogites. Similar pieces of evidence, particularly from δ¹³C in P-type diamonds and δ¹⁸O in peridotitic garnets lead to the suggestion that at least some of the mantle peridotites, including diamondiferous ones, as well as inclusions in P-type diamonds, may have had a crustal protolith as well.  相似文献   

Homogenization of carbonate-bearing microinclusions in diamond at <Emphasis Type="Italic">P</Emphasis>–<Emphasis Type="Italic">T</Emphasis> parameters of the upper mantle     

A. L. Ragozin  Yu. N. Palyanov  D. A. Zedgenizov  A. A. Kalinin  V. S. Shatsky 《Doklady Earth Sciences》2016,470(2):1042-1045

The staged high-pressure annealing of natural cubic diamonds with numerous melt microinclusions from the Internatsional’naya kimberlite pipe was studied experimentally. The results mainly show that the carbonate phases, the daughter phases in partially crystallized microinclusions in diamonds, may undergo phase transformations under the mantle P–T conditions. Most likely, partial melting and further dissolution of dolomite in the carbonate–silicate melt (homogenization of inclusions) occur in inclusions. The experimental data on the staged high-pressure annealing of diamonds with melt microinclusions allow us to estimate the temperature of their homogenization as 1400–1500°C. Thus, cubic diamonds from the Internatsional’naya pipe could have been formed under quite high temperatures corresponding to the lithosphere/asthenosphere boundary. However, it should be noted that the effect of selective capture of inclusions with partial loss of volatiles in relation to the composition of the crystallization medium is not excluded during the growth. This may increase the temperature of their homogenization significantly between 1400 and 1500°C.  相似文献   

First African diamonds discovered in Algeria by the ancient Arabo-Berbers: History and insight into the source rocks     

《Comptes Rendus Geoscience》2014,346(7-8):179-189

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Physicochemical formation conditions of natural diamond deduced from experimental study of the eclogite-carbonatite-sulfide-diamond system     

Yu. A. Litvin 《Geology of Ore Deposits》2012,54(6):443-457

A diagram of the syngenesis of diamond, silicate, carbonate, and sulfide minerals and melts is compiled based on experimental data on phase relations in the heterogeneous eclogite-carbonate-sulfidediamond system at P = 7 GPa. Evidence is provided that silicate and carbonate minerals are paragenetic, whereas sulfides are xenogenic with respect to diamond. Diamond and paragenetic phases are formed in completely miscible carbonate-silicate growth melts with dissolved elemental carbon. Coherent data of physicochemical experiment and mineralogy of primary inclusions in natural diamonds allows us to prove the mantle-carbonatite theory of diamond origin. The genetic classification of primary inclusions in natural diamonds is based on this theory. The phase diagrams of syngenesis are applicable to interpretation of diamond and syngenetic minerals formation in natural magma sources. They ascertain physicochemical mechanism of natural diamond formation and conditions of entrapment of paragenetic and xenogenic mineral phases by growing diamonds.  相似文献   

Fe–S melt as a likely solvent of diamond under mantle conditions     

E. I. Zhimulev  V. M. Sonin  V. P. Afanasiev  A. I. Chepurov  N. P. Pokhilenko 《Doklady Earth Sciences》2016,471(2):1277-1279

The first results of experimental study of diamond dissolution in a S-bearing Fe melt at high P–T parameters are reported and the morphology of partially dissolved crystals is compared with that of natural diamonds. Our results show that under the experimental conditions (4 GPa, 1400°C), flat-faced octahedral diamond crystals are transformed into curve-faced octahedroids with morphological features similar to those of natural diamonds.  相似文献   

Platelet development in cuboid diamonds: insights from micro-FTIR mapping   总被引：1，自引：1，他引：0  

D. Howell  C. J. O’Neill  K. J. Grant  W. L. Griffin  S. Y. O’Reilly  N. J. Pearson  R. A. Stern  T. Stachel 《Contributions to Mineralogy and Petrology》2012,164(6):1011-1025

Fourier transform infrared mapping of diamonds can reveal detailed information on impurities, with a spatial context. We apply this technique, combined with in situ isotopic analysis of carbon, to the study of cuboid diamond growth in a sample that exhibits some mixed-habit growth. While there has been some uncertainty in the literature regarding sectoral differences in nitrogen aggregation and subsequent platelet development, the data from this study appear far more conclusive. We show that despite nitrogen being concentrated in octahedral sectors, there is no detectable difference in the concentration-adjusted rate of nitrogen aggregation within octahedral and cuboid sectors. However, the resultant platelet development is significantly reduced in cuboid sectors compared to contemporaneously formed octahedral sectors. This finding has significant implications for the classification of diamonds using the relationship between their platelet intensity and the absorption caused by B centres. It means that cuboid diamonds naturally fall below the linear relationship that has been termed regular, which would lead to them being incorrectly interpreted as having experienced heating or deformation. The data also support earlier suggestions that large hydrogen concentrations in the diamond-forming fluid may be required for cuboid growth. We further suggest that high nitrogen and hydrogen concentrations are required for mixed-habit diamond growth, which might be the product of specific fluid chemistries that occur in reducing mantle environments.  相似文献   

The carbon isotopic composition of diamonds: relationship to diamond shape,color, occurrence and vapor composition   总被引：3，自引：0，他引：3  

Peter Deines 《Geochimica et cosmochimica acta》1980,44(7):943-961

Three hundred and thirty new ¹³C analyses of diamonds are presented, indicating, in conjunction with earlier published work, a range of about 30%. in the carbon isotopic composition of diamonds. The frequency distribution of diamond δ¹³C analyses shows a very pronounced mode at ?5 to ?6%.vs PDB, a large negative skewness, and a sharp boundary at about ?1%.. Analyses of diamonds from the Premier and Dan Carl mines, South Africa, demonstrate that: (1) differences in ¹³C content that can be related to diamond color and shape are smaller than 1%.; (2) the mean ¹³C content of kimberlite carbonates is 1–2%. lower than that of associated diamonds; (3) significant differences in ¹³C content exist between the mean isotopic compositions of diamonds from these two pipes; (4) the variability in δ¹³C differs from one mine to the other.Computations were carried out evaluating the effect on the ¹³C content of diamonds of: (i) various precipitation processes; (ii) the abundance of the species H₂, H₂O, CH₄, CO, CO₂ and O₂ in the vapor; (iii) the initial isotopic composition variability of the source carbon; (iv) variations of the carbon isotope effects resulting from changes in pressure and temperature and (v) reservoir effects (Rayleigh fractionation). Fifty-eight genetic models were investigated for compatibility with the ¹³C distribution in diamonds and associated carbonate. The modeling does not permit an unambiguous answer to the question whether or not a vapor participated in diamond formation, although the presence of methane during diamond formation is compatible with the carbon isotopic composition data, possible oxygen fugacities in the mantle and with the composition of gases liberated from diamonds. In all probability carbon isotope effects in the diamond formation process were small, and the very large range in δ¹³C observed was inherited from the source carbon.  相似文献   

Apatite inclusions in natural diamond coat     

A. R. Lang  J. C. Walmsley 《Physics and Chemistry of Minerals》1983,9(1):6-8

Electron microscopic techniques have been applied to investigate the composition and structure of the non-diamond particles, of submicrometre dimensions, that profusely populate the coat of natural coated diamonds. Energy dispersive X-ray spectroscopic analysis (EDS) has shown phosphorus to be commonly present in this included matter, and in the case of some individual crystallites calcium and phosphorus have been the only elements detected with atomic number greater than 11 (the minimum atomic number detectable in the experiments). The latter inclusions exhibit hexagonal prismatic facets and their electron diffraction pattern corresponds to the apatite structure. Approximate lattice parameter values found were c/a=0.73 with a=9.5 Å.  相似文献   

A simple predictive model of quartz solubility in water-salt-CO₂ systems at temperatures up to 1000 °C and pressures up to 1000 MPa     

Nikolay N. Akinfiev 《Geochimica et cosmochimica acta》2009,73(6):1597-1608

Knowledge of the solubility of quartz over a broad spectrum of aqueous fluid compositions and T-P conditions is essential to our understanding of water-rock interaction in the Earth’s crust. We propose an equation to compute the molality of aqueous silica, m_SiO2(aq), mol·(kg H₂O)⁻¹, in equilibrium with quartz and water-salt-CO₂ fluids, as follows: