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1.
Multiple inclusions of minerals in diamonds from the Snap Lake/King Lake kimberlites of the southeastern Slave craton in Canada have been analyzed for trace elements to elucidate the petrogenetic history of these inclusions, and of their host diamonds. As observed worldwide, the harzburgitic-garnet diamond inclusions (DIs) possess sinusoidal REE patterns that indicate an early depletion event, followed by metasomatism by LREE-enriched, HREE-depleted fluids. Furthermore, these fluids appear to contain appreciable concentrations of LILE and HFSE, based on the increasing abundances of these elements in the olivine inclusion that occurs at the outer portion of a diamond compared to that near the core. The compositions of these fluids are probably a mixture of hydrous-silicic melt, carbonatitic melt, and brine, similar to the compositions of micro-inclusions in diamonds reported by Navon et al. (2003). Comparison between the compositions of majoritic and normal harzburgitic garnets shows that the former are more depleted in terms of major/minor elements (higher Cr#) but significantly more enriched in the REE (up to 10×). This characteristic may indicate the higher susceptibility for metasomatic enrichment of previously more depleted garnets. Garnets of eclogitic paragenesis show strong LREE-depleted patterns, whereas the coexisting omphacite inclusion has relatively flat light- and middle-REE but depleted HREE. Whole-rock reconstruction from coexisting garnet and omphacite inclusions indicates that the protolith of these inclusions was probably the extrusive section of an oceanic crust, subducted beneath the Slave craton.  相似文献   

2.
Diamonds and their mineral inclusions are valuable for studying the genesis of diamonds, the characteristics and processes of ancient lithospheric mantle and deeper mantle. This has been paid lots of attentions by geologists both at home and abroad. Most diamonds come from lithospheric mantle. According to their formation preceded, accompanied or followed crystallization of their host diamonds, mineral inclusions in diamonds are divided into three groups: protogenetic, syngenetic and epigenetic. To determine which group the mineral inclusions belong to is very important because it is vital for understanding the data’s meaning. According to the type of mantle source rocks, mineral inclusions in diamonds are usually divided into peridotitic (or ultramafic) suite and eclogitic suite. The mineral species of each suite are described and mineralogical characteristics of most common inclusions in diamonds, such as olivine, clinopyroxene, orthopyroxene, garnet, chromite and sulfide are reviewed in detail. In this paper, the main research fields and findings of diamonds and their inclusions were described: ①getting knowledge of mineralogical and petrologic characteristics of diamond source areas, characteristics of mantle fluids and mantle dynamics processes by studying the major element and trace element compositions of mineral inclusions; ②discussing deep carbon cycle by studying carbon isotopic composition of diamonds; ③determining forming temperature and pressure of diamonds by using appropriate assemblages of mineral inclusions or single mineral inclusion as geothermobarometry, by using the abundance and aggregation of nitrogen impurities in diamonds and by measuring the residual stress that an inclusion remains under within a diamond ; ④estimating the crystallization ages of diamonds by using the aggregation of nitrogen impurities in diamonds and by determine the radiometric ages of syngenetic mineral inclusions in diamonds. Genetic model of craton lithospheric diamonds and their mineral inclusion were also introduced. In the end, the research progress on diamonds and their inclusions in China and the gap between domestic and international research are discussed.  相似文献   

3.
Trace element compositions of submicroscopic inclusions in both the core and the coat of five coated diamonds from the Democratic Republic of Congo (DRC, formerly Zaire) have been analyzed by Laser Ablation Inductively Coupled Mass Plasma Spectrometry (LA-ICP-MS). Both the diamond core and coat inclusions show a general 2-4-fold enrichment in incompatible elements relative to major elements. This level of enrichment is unlikely to be explained by the entrapment of silicate mantle minerals (olivine, garnet, clinopyroxene, phlogopite) alone and thus submicroscopic fluid or glass inclusions are inferred in both the diamond coat and in the gem quality diamond core. The diamond core fluids have elevated High Field Strength Element (Ti, Ta, Zr, Nb) concentrations and are enriched in U relative to inclusions in the diamond coats and relative to chondrite. The core fluids are also moderately enriched in LILE (Ba, Sr, K). Therefore, we suggest that the diamond cores contain inclusions of silicate melt. However, the Ni content and Ni/Fe ratio of the trapped fluid are very high for a silicate melt in equilibrium with mantle minerals; high Ni and Co concentrations in the diamond cores are attributed to the presence of a sulfide phase coexisting with silicate melt in the diamond core inclusions. Inclusions in the diamond coat are enriched in LILE (U, Ba, Sr, K) and La over the diamond core fluids and to chondrite. The coats have incompatible element ratios similar to natural carbonatite (coat fluid: Na/Ba ≈0.66, La/Ta≈130). The coat fluid is also moderately enriched in HFSE (Ta, Nb, Zr) when normalized to chondritic Al. LILE and La enrichment is related to the presence of a carbonatitic fluid in the diamond coat inclusions, which is mixed with a HFSE-rich hydrous silicate fluid similar to that in the core. The composition of the coat fluid is consistent with a genetic link to group 1 kimberlite.  相似文献   

4.
We studied diamonds from a 2.697–2.700 Ga Wawa metaconglomerate (Southern Superior craton) and identified mineral inclusions of high-Cr, low-Ca pyrope garnet, low-Ti Mg-chromite, olivine (Fo93), and orthopyroxene (En94). The diamonds have δ13C of ?2.5 to ?4.0 ‰ and derive from the spinel-garnet and garnet facia of harzburgite. Geothermobarometry on non-touching, coexisting garnet-olivine and garnet-orthopyroxene pairs constrains the maximum geothermal gradient of 41 mW/m2 for the Neoarchean and a minimum lithosphere thickness of 190 km. The depleted harzburgitic paragenesis equilibrated at a relatively cold geotherm suggests the presence of a pre-2.7 Ga diamondiferous cratonic root beneath the northern Wawa terrane or the Opatica terrane of the Southern Superior craton, i.e., beneath terranes identified as sources for the metaconglomerate diamonds. Geophysical surveys, geothermal data, and petrology of mantle xenoliths emplaced in the Proterozoic-Mesozoic trace evolution of the mantle thermal regime and composition from the Archean to present. The root was thinned down to 150 km by the Jurassic, when the mantle was heated to 41–42 mW/m2. The diamondiferous root destruction was accompanied by more significant heating and was complete by 1.1 Ga in areas adjacent to the Midcontinent Rift. The geometry of the current high-velocity root and spatial correlations with boundaries of crustal terranes that docked to the nuclei of the Superior protocraton in the Neoarchean suggest that the root destruction in the Southern Superior may have been associated with tectonic erosion, craton amalgamation, and ensuing ingress of asthenospheric fluids.  相似文献   

5.
Integrated models of diamond formation and craton evolution   总被引：4，自引：0，他引：4  
Two decades of diamond research in southern Africa allow the age, average N content and carbon composition of diamonds, and the dominant paragenesis of their syngenetic silicate and sulfide inclusions to be integrated on a cratonwide scale with a model of craton formation. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the mid-Proterozoic and display little correspondence with the prominent variations in the P-wave velocity (±1%) that the mantle lithosphere shows at depths within the diamond stability field (150–225 km). Silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane show a regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity relative to the craton average correlates with a greater proportion of eclogitic vs. peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds. The oldest formation ages of diamonds support a model whereby mantle that became part of the continental keel of cratonic nuclei first was created by middle Archean (3.2–3.3 Ga or older) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of eclogitic sulfide inclusions in the 2.9 Ga age population links late Archean (2.9 Ga) subduction–accretion events to craton stabilization. These events resulted in a widely distributed, late Archean generation of eclogitic diamonds in an amalgamated craton. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite. Similar age/paragenesis systematics are seen for the more limited data sets from the Slave and Siberian cratons.  相似文献   

6.
With an age of ca. 2.7 Ga, greenschist facies volcaniclastic rocks and lamprophyre dikes in the Wawa area (Superior Craton) host the only diamonds emplaced in the Archean available for study today. Nitrogen aggregation in Wawa diamonds ranges from Type IaA to IaB, suggesting mantle residence times of tens to hundreds of millions of years. The carbon isotopic composition (δ13C) of cube diamonds is similar to the accepted mantle value (− 5.0‰). Octahedral diamonds show a slight shift (by + 1.5‰) to isotopically less negative values suggesting a subduction-derived, isotopically heavy component in the diamond-forming fluids. Syngenetic inclusions in Wawa diamonds are exclusively peridotitic and, similar to many diamond occurrences worldwide, are dominated by the harzburgitic paragenesis. Compositionally they provide a perfect match to inclusions from diamonds with isotopically dated Paleo- to Mesoarchean crystallization ages. Several high-Cr harzburgitic garnet inclusions contain a small majorite component suggesting crystallization at depth of up to 300 km. Combining diamond and inclusion data indicates that Wawa diamonds formed and resided in a very thick package of chemically depleted lithospheric mantle that predates stabilization of the Superior Craton. If late granite blooms are interpreted as final stages of cratonization then a similar disconnect between Paleo- to Mesoarchean diamondiferous mantle lithosphere and Neoarchean cratonization is also apparent in other areas (e.g., the Lac de Gras area of the Slave Craton) and may suggest that early continental nuclei formed and retained their own diamondiferous roots.  相似文献   

7.
The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.  相似文献   

8.
The results of integrated studies of inclusion-containing diamonds from kimberlites of the Snap Lake dike complex (Canada) are presented. Features of the morphology, defect–impurity composition, and internal structure of the diamonds were determined by optic and scanning microscopy. The chemical composition of crystalline inclusions (olivine, garnet, and pyroxene) in diamonds was studied using a microanalyzer with an electronic probe. The inclusions of ultramafic paragenesis in the diamond (87%) are predominant. Carbonates, sulfide and hydrated silicate phases were found only in multiphase microinclusions. The large phlogopite inclusion studied was similar in composition to earlier studied nanosize inclusions of high-silica mica in diamonds from Snap Lake kimberlites. Revealed features of studied diamonds and presence of high-silica mica suggest that diamonds from Snap Lake have formed as the result of interaction between enriched in volatile and titanium high-potassium carbonate–silicate melts and peridotitic substrate at the base of thick lithospheric mantle.  相似文献   

9.
Several thousand clinopyroxene, garnet, and phlogopite inclusions of mantle rocks from Jurassic and Triassic kimberlites in the northeastern Siberian craton have been analyzed and compared with their counterparts from Paleozoic kimberlites, including those rich in diamond. The new and published mineral chemistry data make a basis for an updated classification of kimberlite-hosted clinopyroxenes according to peridotitic and mafic (eclogite and pyroxenite) parageneses. The obtained results place constraints on the stability field of high-Na lherzolitic clinopyroxenes, which affect the coexisting garnet and decrease its Ca contents. As follows from analyses of the mantle minerals from Mesozoic kimberlites, the cratonic lithosphere contained more pyroxenite and eclogite in the Mesozoic than in the Paleozoic. It virtually lacked ultradepleted harzburgite-dunite lithologies and contained scarce eclogitic diamonds. On the other hand, both inclusions in diamond and individual eclogitic minerals from Mesozoic kimberlites differ from eclogitic inclusions in diamond from Triassic sediments in the northeastern Siberian craton. Xenocrystic phlogopites from the D’yanga pipe have 40Ar/39Ar ages of 384.6, 432.4, and 563.4 Ma, which record several stages of metasomatic impact on the lithosphere. These phlogopites are younger than most of Paleozoic phlogopites from the central part of the craton (Udachnaya kimberlite). Therefore, hydrous mantle metasomatism acted much later on the craton periphery than in the center. Monomineral clinopyroxene thermobarometry shows that Jurassic kimberlites from the northeastern craton part trapped lithospheric material from different maximum depths (170 km in the D’yanga pipe and mostly < 130 km in other pipes). The inferred thermal thickness of cratonic lithosphere decreased progressively from ~ 260 km in the Devonian-Carboniferous to ~ 225 km in the Triassic and to ~ 200 km in the Jurassic, while the heat flux (Hasterok-Chapman model) was 34.9, 36.7, and 39.0 mW/m2, respectively. Dissimilar PT patterns of samples from closely spaced coeval kimberlites suggest different emplacement scenarios, which influenced both the PT variations across the lithosphere and the diamond potential of kimberlites.  相似文献   

10.
D. Phillips  J.W. Harris  K.S. Viljoen 《Lithos》2004,77(1-4):155-179
Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (<5.8 wt.%) and Cr2O3 contents (3.0–15.0 wt.%), although the majority are harzburgitic with very low calcium concentrations (<2 wt.% CaO). Eclogitic garnet inclusions are characterised by a wide range in CaO (3.3–21.1 wt.%) with low Cr2O3 (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr2O3 and <0.5 wt.% TiO2. Olivine and orthopyroxene inclusions are magnesium-rich with Mg-numbers of 93–97. Olivine inclusions in chromite exhibit the highest Mg-numbers and also contain elevated Cr2O3 contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K2O. Eclogitic and websteritic clinopyroxene inclusions exhibit overlapping compositions with a wide range in Mg-numbers (66–86).

Calculated temperatures for non-touching inclusion pairs from individual diamonds range from 1082 to 1320 °C (average=1197 °C), whereas pressures vary from 4.6 to 7.7 GPa (average=6.3 GPa). Touching inclusion assemblages are characterised by equilibration temperatures of 995 to 1182 °C (average=1079 °C) and pressures of 4.2–6.8 GPa (average=5.4 GPa). Provided that the non-touching inclusions represent equilibrium assemblages, it is suggested that these inclusions record the conditions at the time of diamond crystallisation (1200 °C; 3.0 Ga). The lower average temperatures for touching inclusions are attributed to re-equilibration in a cooling mantle (1050 °C) prior to kimberlite eruption at 85 Ma. Pressure estimates for touching garnet–orthopyroxene inclusions are also skewed towards lower values than most non-touching inclusions. This apparent difference may be an artefact of the Al-exchange geobarometer and/or the result of sampling bias, due to limited numbers of non-touching garnet–orthopyroxene inclusions. Alternatively pressure differences could be caused by differential uplift in the mantle or possibly variations in thermal compressibility between diamond and silicate inclusions. However, thermodynamic modelling suggests that thermal compressibility differences would cause only minor changes in internal inclusion pressures (<0.2 GPa/100 °C).  相似文献   


11.
A mineral inclusion, carbon isotope, nitrogen content, nitrogen aggregation state and morphological study of 576 microdiamonds from the DO27, A154, A21, A418, DO18, DD17 and Ranch Lake kimberlites at Lac de Gras, Slave Craton, was conducted. Mineral inclusion data show the diamonds are largely eclogitic (64%), followed by peridotitic (25%) and ultradeep (11%). The paragenetic abundances are similar to macrodiamonds from the DO27 kimberlite (Davies, R.M., Griffin, W.L., O'Reilly, S.Y., 1999. Diamonds from the deep: pipe DO27, Slave craton, Canada. In: Gurney, J.J., Gurney, J.L., Pascoe, M.D., Richardson, S.H. (Eds.), The J. B. Dawson Vol., Proc. 7th Internat. Kimberlite Conf., Red Roof Designs, Cape Town, pp. 148–155) but differ to diamonds from nearby kimberlites at Ekati (e.g., Lithos (2004); Tappert, R., Stachel, T., Harris, J.W., Brey, G.P., 2004. Mineral Inclusions in Diamonds from the Panda Kimberlite, S. P., Canada. 8th International Kimberlite Conference, extended abstracts) and Snap Lake to the south (Dokl. Earth Sci. 380 (7) (2001) 806), that are dominated by peridotitic stones.

Eclogitic diamonds with variable inclusion compositions and temperatures of formation (1040–1300 °C) crystallised at variable lithospheric depths sometimes in changing chemical environments. A large range to very 13C-depleted C-isotope compositions (δ13C=−35.8‰ to −2.2‰) and an NMORB bulk composition, calculated from trace elements in garnet and clinopyroxene inclusions, are consistent with an origin from subducted oceanic crust and sediments. Carbon isotopes in the peridotitic diamonds have mantle compositions (δ13C mode −4.0‰). Mineral inclusion compositions are largely harzburgitic. Variable temperatures of formation (garnet TNi=800–1300 °C) suggest the peridotitic diamonds originate from the shallow ultra-depleted and deeper less depleted layers of the central Slave lithosphere. Carbon isotopes (δ13C av.=−5.1‰) and mineral inclusions in the ultradeep diamonds suggest they formed in peridotitic mantle (670 km). The diamonds may have been entrained in a plume and subcreted to the base of the central Slave lithosphere.

Poorly aggregated nitrogen (IaA without platelets) in a large number of eclogitic (67%) and peridotitic (32%) diamonds, with similar nitrogen contents, indicates the diamonds were stored in the mantle at low temperatures (1060–<1100 °C) following crystallisation in the Archean. Type IaA diamonds have largely cubo-octahedral growth forms, and Type II and Type IaAB diamonds, with higher nitrogen aggregation states, mostly have octahedral morphologies. However, no correlation between these groups and their mineral inclusion compositions, C-isotopes, and N-contents rules out the possibility of unique source origins and suggests eclogitic and peridotitic diamonds experienced variable mantle thermal states. Variation in mineral inclusion chemistries in single diamonds, possible overgrowths of 13C-depleted eclogitic diamond on diamonds with peridotitic and ultradeep inclusions, and Type I ultradeep diamond with low N-aggregation is consistent with diamond growth over time in changing chemical environments.  相似文献   


12.

Olivine in kimberlites can provide unique insights into magma petrogenesis, because it is the most abundant xenocrystic phase and a stable magmatic product over most of the liquid line of descent. In this study we examined the petrography and chemistry of olivine in kimberlites from different tectonic settings, including the Slave craton, Canada (Ekati: Grizzly, Koala), the Brasilia mobile belt (Limpeza-18, Tres Ranchos-04), and the Kaapvaal craton, South Africa (Kaalvallei: Samada, New Robinson). Olivine cores display a wide range of compositions (e.g., Mg# = 78–95). The similarity in olivine composition, resorption of core zones and inclusions of mantle-derived phases, indicates that most olivine cores originated from the disaggregation of mantle peridotites, including kimberlite-metasomatised lithologies (i.e. sheared lherzolites and megacrysts). Olivine rims typically show a restricted range of Mg#, with decreasing Ni and increasing Mn and Ca contents, a characteristic of kimberlitic olivine worldwide. The rims host inclusions of groundmass minerals, which implies crystallisation just before and/or during emplacement. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. There are no differences in olivine composition between explosive (Grizzly) and hypabyssal (Koala) kimberlites. Olivine in kimberlites also displays transitional zones and less common internal zones, between cores and rims. The diffuse transitional zones exhibit intermediate compositions between cores and rims, attributed to partial re-equilibration of xenocrystic cores with the ascending kimberlite melt. In contrast, internal zones form discrete layers with resorbed margins and restricted Mg# values, but variable Ni, Mn and Ca concentrations, which indicates a discrete crystallization event from precursor kimberlite melts at mantle depths. Overall, olivine exhibits broadly analogous zoning in kimberlites worldwide. Variable compositions for individual zones relate to different parental melt compositions rather than variations in tectonic setting or emplacement mechanism.

  相似文献   

13.
从金刚石成因研究,流体包裹体、表征流体存在的固态包裹体及晶格原子、流体成分组合的制约因素等方面,论述了深部地幔流体的存在。华北地台原生金刚石中与流体包裹体共存的钛铁矿+铬铁矿、金红石+金云母+橄榄石、锆石+碳酸盐+橄榄石、磷灰石+金云母+橄榄石组合。相平衡原理揭示地幔流体存在着复杂的自催化反应。华北地台由金刚石流体包裹体所反映的深部地幔组成C、H、O、S和N分别为16.87%、47.47%、30.90%、3.02%和5.38%(皆为原子百分比),与现代大气圈、水圈相比明显富H贫O。  相似文献   

14.
A comparison of the diamond productions from Panda (Ekati Mine) and Snap Lake with those from southern Africa shows significant differences: diamonds from the Slave typically are un-resorbed octahedrals or macles, often with opaque coats, and yellow colours are very rare. Diamonds from the Kaapvaal are dominated by resorbed, dodecahedral shapes, coats are absent and yellow colours are common. The first two features suggest exposure to oxidizing fluids/melts during mantle storage and/or transport to the Earth's surface, for the Kaapvaal diamond population.

Comparing peridotitic inclusions in diamonds from the central and southern Slave (Panda, DO27 and Snap Lake kimberlites) and the Kaapvaal indicates that the diamondiferous mantle lithosphere beneath the Slave is chemically less depleted. Most notable are the almost complete absence of garnet inclusions derived from low-Ca harzburgites and a generally lower Mg-number of Slave inclusions.

Geothermobarometric calculations suggest that Slave diamonds originally formed at very similar thermal conditions as observed beneath the Kaapvaal (geothermal gradients corresponding to 40–42 mW/m2 surface heat flow), but the diamond source regions subsequently cooled by about 100–150 °C to fall on a 37–38 mW/m2 (surface heat flow) conductive geotherm, as is evidenced from touching (re-equilibrated) inclusions in diamonds, and from xenocrysts and xenoliths. In the Kaapvaal, a similar thermal evolution has previously been recognized for diamonds from the De Beers Pool kimberlites. In part very low aggregation levels of nitrogen impurities in Slave diamonds imply that cooling occurred soon after diamond formation. This may relate elevated temperatures during diamond formation to short-lived magmatic perturbations.

Generally high Cr-contents of pyrope garnets (inside and outside of diamonds) indicate that the mantle lithosphere beneath the Slave originally formed as a residue of melt extraction at relatively low pressures (within the stability field of spinelperidotites), possibly during the extraction of oceanic crust. After emplacement of this depleted, oceanic mantle lithosphere into the Slave lithosphere during a subduction event, secondary metasomatic enrichment occurred leading to strong re-enrichment of the deeper (>140 km) lithosphere. Because of the extent of this event and the occurrence of lower mantle diamonds, this may be related to an upwelling plume, but it may equally just reflect a long term evolution with lower mantle diamonds being transported upwards in the course of “normal” mantle convection.  相似文献   


15.
Trace element concentrations in the four principal peridotitic silicate phases (garnet, olivine, orthopyroxene, clinopyroxene) included in diamonds from Akwatia (Birim Field, Ghana) were determined using SIMS. Incompatible trace elements are hosted in garnet and clinopyroxene except for Sr which is equally distributed between orthopyroxene and garnet in harzburgitic paragenesis diamonds. The separation between lherzolitic and harzburgitic inclusion parageneses, which is commonly made using compositional fields for garnets in a CaO versus Cr2O3 diagram, is also apparent from the Ti and Sr contents in both olivine and garnet. Titanium is much higher in the lherzolitic and Sr in the harzburgitic inclusions. Chondrite normalised REE patterns of lherzolitic garnets are enriched (10–20 times chondrite) in HREE (LaN/YbN = 0.02–0.06) while harzburgitic garnets have sinusoidal REEN patterns, with the highest concentrations for Ce and Nd (2–8 times chondritic) and a minimum at Ho (0.2–0.7 times chondritic). Clinopyroxene inclusions show negative slopes with La enrichment 10–100 times chondritic and low Lu (0.1–1 times chondritic). Both a lherzolitic and a harzburgitic garnet with very high knorringite contents (14 and 21 wt% Cr2O3 respectively) could be readily distinguished from other garnets of their parageneses by much higher levels of LREE enrichment. The REE patterns for calculated melt compositions from lherzolitic garnet inclusions fall into the compositional field for kimberlitic-lamproitic and carbonatitic melts. Much more strongly fractionated REE patterns calculated from harzburgitic garnets, and low concentrations in Ti, Y, Zr, and Hf, differ significantly from known alkaline and carbonatitic melts and require a different agent. Equilibration temperatures for harzburgitic inclusions are generally below the C-H-O solidus of their paragenesis, those of lherzolitic inclusions are above. Crystallisation of harzburgitic diamonds from CO2-bearing melts or fluids may thus be excluded. Diamond inclusion chemistry and mineralogy also is inconsistent with known examples of metasomatism by H2O-rich melts. We therefore favour diamond precipitation by oxidation of CH4-rich fluids with highly fractionated trace element patterns which are possibly due to “chromatographic” fractionation processes. Received: 27 January 1996 / Accepted: 5 May 1997  相似文献   

16.
Omphacite is a typomorphic mineral of eclogites, which is inappropriate to mineral assemblages of peridotites. Nevertheless, findings of this mineral in inclusions in peridotitic diamonds can be considered as indirect evidence for the existence of this paradoxical mineral assemblage.In this paper we present experimental results on the interaction between carbonate-bearing amphibolite and olivine that model processes operated at the crust–mantle boundary in subduction zones. The experiments demonstrate growth of omphacite at the interface between acid melt and peridotite media at 2.9 GPa and 850–900 °C; the omphacite coexists either with garnet and orthopyroxene or with phlogopite. The synthetic omphacite is exclusively of reactive-magmatic origin and does not form in metasomatic way. Findings of omphacite inclusions in peridotitic diamonds and in some pyroxenites from kimberlites are discussed in scope of the obtained experimental data.  相似文献   

17.
The Orapa and Jwaneng kimberlites are located along the western margin of the Kalahari Craton and the prevalence of eclogitic over peridotitic diamonds in both mines has recently been linked to lower P-wave velocities in the deep mantle lithosphere (relative to the bulk of the craton) to suggest a diamond formation event prompted by mid-Proterozoic growth and modification of preexisting Archean lithosphere (Shirey et al. 2002). Here we study peridotitic diamonds from both mines, with an emphasis on the style of metasomatic source enrichment, to evaluate their relationship with this major eclogitic diamond formation event. In their major element chemistry, the peridotitic inclusions compare well with a world-wide database but reveal differences to diamond sources located in the interior of the Western Terrane of the Kaapvaal block, where the classical mines in the Kimberley region are located. The most striking difference is the relative paucity of low-Ca (<2 wt% CaO in garnet) harzburgites and a low ratio of harzburgitic to lherzolitic garnets (2:1). This suggests that lithospheric mantle accreted to the rim of the Zimbabwe and Kaapvaal blocks was overall chemically less depleted. Alternatively, this more fertile signature may be assigned to stronger metasomatic re-enrichment but the trace element signature of garnet inclusions is not in favor of strong enrichment in major elements. For both mines the majority of lherzolitic and harzburgitic garnet inclusions are characterized by moderately sinusoidal REEN patterns and low Ti, Zr and Y contents, indicative of a metasomatic agent with very high LREE/HREE and low HFSE. This is consistent with metasomatism by a CHO-fluid or, as modeled by Burgess and Harte (2003), a highly fractionated, low-volume silicate melt from the MORB-source. In both cases, changes in the major element chemistry of the affected rocks will be limited. In a few garnets from Orapa preferential MREE enrichment is observed, suggesting that the percolating fluid/melt fractionated a LREE-phyllic phase (such as crichtonite). The overall moderate degree of metasomatism reflected by the inclusion chemistry is in stark contrast to lithospheric sections for Orapa and Jwaneng based on mantle xenocrysts and xenoliths, revealing extensive mantle metasomatism (Griffin et al. 2003). This suggests that the formation of peridotitic diamonds predates the intensive modification of the subcratonic lithosphere during Proterozoic rifting and compression, implying that diamonds may survive major tectonothermal events.Editorial responsibility: J. Hoefs  相似文献   

18.
Here, we compare nitrogen aggregation characteristics and carbon isotopic compositions in diamonds from Mesoproterozoic (T1) and Jurassic (U2) kimberlites in the Attawapiskat area—the first diamond-producing area on the Superior craton. The T1 kimberlite sampled diamonds from the lithospheric mantle at 1.1 Ga, at the same time as the major Midcontinent Rift event. These diamonds have a narrow range in δ13C (mode of ?3.4 ‰), with compositions that overlap other diamond localities on the Superior craton. Some diamond destruction must have occurred during the Mesoproterozoic in response to the thermal impact of the Midcontinent Rift—the associated elevated geotherm caused a narrow diamond window (<30 km) close to the base of the lithosphere, compared to a wide diamond window of ~85 km following thermal relaxation (sampled by Jurassic kimberlites, such as U2). T1 diamonds have highly aggregated nitrogen, possibly due to the thermal effect of the rift. Diamond-favourable conditions were re-established in the lithospheric mantle after the thermal impact of the Midcontinent Rift dissipated. The poorly aggregated nature of nitrogen in U2 diamonds—compared to highly aggregated nitrogen in diamonds from T1—indicates that renewed diamond formation must have occurred only after the thermal impact of the Midcontinent Rift at 1.1 Ga had subsided and that these newly formed diamonds were subsequently sampled by Jurassic kimberlites. The overall δ13C distribution for U2 diamonds is distinct to T1 and other Superior diamonds, further suggesting that U2 diamonds are not related to the older pre-rift diamonds.  相似文献   

19.
 Multianvil experiments were carried out at 10–15 GPa and 1600–1700 °C to match the compositions of majoritic garnet inclusions from diamonds, and to determine the compositions of other phases potentially coexisting with these inclusions in the source. Most experiments produced coexisting majoritic garnet, diopsidic clinopyroxene, one or more (Mg,Fe)2SiO4 polymorphs, and quenched carbonatic melt. The experimental garnets had relatively high Ca and Fe contents similar to the observed Ca and Fe contents of the inclusions. The resulting Si contents confirmed that the depth of origin of the inclusion with the highest Si content did not exceed 410 km, thus none of the majoritic garnet inclusions found so far originated in the transition zone (410–660 km). The evidence from inclusions and experiments is consistent with the presence of an eclogite layer occurring globally between 200 and 410 km. Compositional variations observed among more than 100 majoritic garnet inclusions with their Si content, which is a measure of pressure and depth, are consistent with the origin of the eclogite layer by crystal fractionation in a magma ocean. The compositions of olivine coexisting with majoritic garnet in the experimental products had the average Fe/(Fe + Mg) ratios between 0.16 and 0.28. Inclusions with such high Fe contents have not been found; the Fe/(Fe + Mg) ratio of the olivine inclusions in diamonds usually varies between 0.05 and 0.09. Hence, the mantle between 200 and 410 km may not contain olivine. In the absence of olivine, the discontinuity at 410 km is most likely a chemical boundary between the 200-km-thick eclogite layer and a more mafic transition zone. Received: 15 March 2001 / Accepted: 14 September 2001  相似文献   

20.
We present the first data on the petrology of the mantle lithosphereof the Southeastern (SE) Slave craton, Canada. These are basedon petrographic, mineralogical and geochemical studies of mantlexenoliths in Pipe 5034 of the Cambrian Gahcho Kué kimberlitecluster. Major types of mantle xenoliths include altered eclogite,coarse garnet or spinel peridotite, and deformed garnet peridotite.The peridotites belong to the low-temperature suite and formedat T=600–1300°C and P= 25–80 kbar in a thick(at least 220–250 km), cool lithosphere. The SE Slavemantle is cooler than the mantle of other Archaean cratons andthat below other terranes of the Slave craton. The thick lithosphereand the relatively cool thermal regime provide favourable conditionsfor formation and preservation of diamonds beneath the SE Slaveterrane. Similar to average Archaean mantle worldwide, the SESlave peridotite is depleted in magmaphile major elements andcontains olivine with forsterite content of 91–93·5.With respect to olivine composition and mode, all terranes ofthe Slave mantle show broadly similar compositions and are relativelyorthopyroxene-poor compared with those of the Kaapvaal and Siberiancratons. The SE Slave spinel peridotite is poorer in Al, Caand Fe, and richer in Mg than deeper garnet peridotite. Thegreater chemical depletion of the shallow upper mantle is typicalof all terranes of the Slave craton and may be common for thesubcontinental lithospheric peridotitic mantle in general. Peridotiticxenoliths of the SE Slave craton were impregnated by kimberliticfluids that caused late-stage recrystallization of primary clinopyroxene,spinel, olivine and spinel-facies orthopyroxene, and formationof interstitial clinopyroxene. This kimberlite-related recrystallizationdepleted primary pyroxenes and spinel in Al. The kimberliticfluid was oxidizing, Ti-, Fe- and K-rich, and Na-poor, and introducedserpentine, chlorite, phlogopite and spinel into peridotitesat P < 35 kbar. KEY WORDS: kimberlite xenolith; lithosphere; mantle terrane; chemical zoning; thermobarometry; Slave craton  相似文献   

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