共查询到20条相似文献，搜索用时 46 毫秒
1.
Analyses of mineral inclusions, carbon isotopes, nitrogen contents and nitrogen aggregation states in 29 diamonds from two Buffalo Hills kimberlites in northern Alberta, Canada were conducted. From 25 inclusion bearing diamonds, the following paragenetic abundances were found: peridotitic (48%), eclogitic (32%), eclogitic/websteritic (8%), websteritic (4%), ultradeep? (4%) and unknown (4%). Diamonds containing mineral inclusions of ferropericlase, and mixed eclogitic-asthenospheric-websteritic and eclogitic-websteritic mineral associations suggests the possibility of diamond growth over a range of depths and in a variety of mantle environments (lithosphere, asthenosphere and possibly lower mantle). Eclogitic diamonds have a broad range of C-isotopic composition (δ13C=−21‰ to −5‰). Peridotitic, websteritic and ultradeep diamonds have typical mantle C-isotope values (δ13C=−4.9‰ av.), except for two 13C-depleted peridotitic (δ13C=−11.8‰, −14.6‰) and one 13C-depleted websteritic diamond (δ13C=−11.9‰). Infrared spectra from 29 diamonds identified two diamond groups: 75% are nitrogen-free (Type II) or have fully aggregated nitrogen defects (Type IaB) with platelet degradation and low to moderate nitrogen contents (av. 330 ppm-N); 25% have lower nitrogen aggregation states and higher nitrogen contents (30% IaB; <1600 ppm-N). The combined evidence suggests two generations of diamond growth. Type II and Type IaB diamonds with ultradeep, peridotitic, eclogitic and websteritic inclusions crystallised from eclogitic and peridotitic rocks while moving in a dynamic environment from the asthenosphere and possibly the lower mantle to the base of the lithosphere. Mechanisms for diamond movement through the mantle could be by mantle convection, or an ascending plume. The interaction of partial melts with eclogitic and peridotitic lithologies may have produced the intermediate websteritic inclusion compositions, and can explain diamonds of mixed parageneses, and the overlap in C-isotope values between parageneses. Strong deformation and extremely high nitrogen aggregation states in some diamonds may indicate high mantle storage temperatures and strain in the diamond growth environment. A second diamond group, with Type IaA–IaB nitrogen aggregation and peridotitic inclusions, crystallised at the base of the cratonic lithosphere. All diamonds were subsequently sampled by kimberlites and transported to the Earth's surface. 相似文献
2.
The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. [Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ13C-values range from −20.3‰ to −0.5‰ ( n=48) for peridotitic diamonds and from −38.5‰ to −1.6‰ ( n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ13C from −8.5‰ to −2.7‰ ( n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ15N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from −10‰ to +13‰ without correlations with either N-content or δ13C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies. 相似文献
3.
Mineral inclusions recovered from 100 diamonds from the A154 South kimberlite (Diavik Diamond Mines, Central Slave Craton, Canada) indicate largely peridotitic diamond sources (83%), with a minor (12%) eclogitic component. Inclusions of ferropericlase (4%) and diamond in diamond (1%) represent “undetermined” parageneses. Compared to inclusions in diamonds from the Kaapvaal Craton, overall higher CaO contents (2.6 to 6.0 wt.%) of harzburgitic garnets and lower Mg-numbers (90.6 to 93.6) of olivines indicate diamond formation in a chemically less depleted environment. Peridotitic diamonds at A154 South formed in an exceptionally Zn-rich environment, with olivine inclusions containing more than twice the value (of 52 ppm) established for normal mantle olivine. Harzburgitic garnet inclusions generally have sinusoidal rare earth element (REEN) patterns, enriched in LREE and depleted in HREE. A single analyzed lherzolitic garnet is re-enriched in middle to heavy REE resulting in a “normal” REEN pattern. Two of the harzburgitic garnets have “transitional” REEN patterns, broadly similar to that of the lherzolitic garnet. Eclogitic garnet inclusions have normal REEN patterns similar to eclogitic garnets worldwide but at lower REE concentrations. Carbon isotopic values (δ13C) range from − 10.5‰ to + 0.7‰, with 94% of diamonds falling between − 6.3‰ and − 4.0‰. Nitrogen concentrations range from below detection (< 10 ppm) to 3800 ppm and aggregation states cover the entire spectrum from poorly aggregated (Type IaA) to fully aggregated (Type IaB). Diamonds without evidence of previous plastic deformation (which may have accelerated nitrogen aggregation) typically have < 25% of their nitrogen in the fully aggregated B-centres. Assuming diamond formation beneath the Central Slave to have occurred in the Archean [Westerlund, K.J., Shirey, S.B., Richardson, S.H., Gurney, J.J., Harris, J.W., 2003b. Re–Os systematics of diamond inclusion sulfides from the Panda kimberlite, Slave craton. VIIIth International Kimberlite Conference, Victoria, Canada, Extended Abstracts, 5p.], such low aggregation states indicate mantle residence at fairly low temperatures (< 1100 °C). Geothermometry based on non-touching inclusion pairs, however, indicates diamond formation at temperatures around 1200 °C. To reconcile inclusion and nitrogen based temperature estimates, cooling by about 100–200 °C shortly after diamond formation is required. 相似文献
4.
Carbon isotope measurements on diamonds from the Letlhakane kimberlite, and the analyses of their inclusions, permit the examination of km-scale mantle-composition variations by comparing the results with those for the nearby Orapa kimberlite. Diamonds from Letlhakane have a wide range in carbon isotopic composition (−3‰ to −21‰); however, the relative abundance of diamonds depleted in 13C is significantly lower than in the Orapa kimberlite. Most of the 13C-depleted diamonds belong to the eclogictic or websteritic paragenesis. The relative abundance of inclusions in diamonds and their composition indicate that there are significant differences in petrology in the mantle below the two locations. At Letlhakane, peridotitic compositions are more prevalent than at Orapa and the protolith of P-Type inclusions in diamonds may have experienced a higher degree of partial melting at Letlhakane compared to Orapa. P/ T estimates for both W- and E-Type diamonds indicate that a region of 13C-depletion may exist beneath the two kimberlites. The relationships between carbon isotopic composition of the host diamond and the Al 2O 3/Cr 2O 3 ratios of their websteritic and eclogitic garnet inclusions indicate that the low δ13C regions may represent a primary mantle feature, unrelated to a crustal component. 相似文献
5.
U-type paragenesis inclusions predominate (94.7%) among the crystalline inclusion suite of 115 diamonds (−4+2 mm) obtained from the recently discovered Snap Lake/King Lake (SKL) kimberlite dyke system, Southern Slave, Canada. The most common inclusions are olivine (90) and enstatite (22). Sulfide, Cr-pyrope, chromite and Cr-diopside inclusion are less abundant (15, 10, 5 and 1, respectively). Results of the inclusion composition study demonstrate the following. (a) The relatively enriched character of the mantle parent rocks of the U-type diamonds. The average Mg# of olivine inclusions is 92.1, and of enstatite inclusions average 93.3. CaO content in Cr-pyrope inclusions is relatively high (3.73–5.75 wt.%). (b) Four of ten U-type Cr-rich pyrope inclusions contain a majoritic component up to 16.8 mol.% which requires pressures of 110 kbar. Carbon isotopes compositions for 34 diamonds with U-type inclusions have a δ13C range from −3.2‰ to −9‰ with a strong peak around −3.5‰. This is much heavier than the ratios of U-type diamonds from Siberia and South Africa (4.5‰). Diamonds with olivine inclusions can be divided into two groups based on their δ13C values as well as the Mg# and Ni/Fe ratio in the olivines. Most show a narrow range of δ13C values from −3.2‰ to −4.8‰ (average −3.72‰) and have olivine inclusions with Mg# less than 92.3 and relatively high Fe/Ni ratios. A second group is characterized by a much wider variation of C isotope composition ( δ13C varies from −3.8‰ to −9.0‰, average −5.97‰), and the olivine inclusions having a higher Mg# (up to 93.6) and relatively low Fe/Ni ratios. This difference in the C isotope composition may have several explanations: (a) peculiarities of asthenosphere degassing coupled with an abnormal thickness of lithosphere; (b) the abnormal thickness and enriched character of lithospheric mantle; (c) involvement of subducted C of crustal origin in the processes of the diamond formation. The presence of subcalcic Cr-rich majorite (up to 17 mol.%) pyropes of low-Ca harzburgite paragenesis among the crystalline inclusion suite of SKL diamonds is strong evidence for the existence of diamondiferous depleted peridotite in lithospheric mantle at depth near 300 km beneath Southern Slave area and is postulated to be one of the main reasons for the much heavier C isotope composition of SKL U-type diamonds in comparison with those from Siberian and South African kimberlites. 相似文献
6.
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
7.
Diamonds from the Kankan area in Guinea formed over a large depth profile beginning within the cratonic mantle lithosphere and extending through the asthenosphere and transition zone into the lower mantle. The carbon isotopic composition, the concentration of nitrogen impurities and the nitrogen aggregation level of diamonds representing this entire depth range have been determined. Peridotitic and eclogitic diamonds of lithospheric origin from Kankan have carbon isotopic compositions (' 13C: peridotitic -5.4 to -2.2; eclogitic -19.7 to -0.7) and nitrogen characteristics (N: peridotitic 17-648 atomic ppm; eclogitic 0-1,313 atomic ppm; aggregation from IaA to IaB) which are generally typical for diamonds of these two suites worldwide. Geothermobarometry of peridotitic and eclogitic inclusion parageneses (worldwide sources) indicates that both suites formed under very similar conditions within the cratonic lithosphere, which is not consistent with a derivation of diamonds with light carbon isotopic composition from subducted organic matter within subducting oceanic slabs. Diamonds containing majorite garnet inclusions fall to the isotopically heavy side (' 13C: -3.1 to +0.9) of the worldwide diamond population. Nitrogen contents are low (0-126 atomic ppm) and one of the two nitrogen-bearing diamonds shows such a low level of nitrogen aggregation (30% B-centre) that it cannot have been exposed to ambient temperatures of the transition zone (̿,400 °C) for more than 0.2 Ma. This suggests rapid upward transport and formation of some Kankan diamonds pene-contemporaneous to Cretaceous kimberlite activity. Similar to these diamonds from the asthenosphere and the transition zone, lower mantle diamonds show a small shift towards isotopic heavy compositions (-6.6 to -0.5, mode at -3.5). As already observed for other mines, the nitrogen contents of lower mantle diamonds were below detection (using FTIRS). The mutual shift of sublithospheric diamonds towards isotopic heavier compositions suggests a common carbon source, which may have inherited an isotopic heavy composition from a component consisting of subducted carbonates. 相似文献
8.
Cathodoluminescence (CL) imaging of polished sections of a diamond from the Guaniamo region of Venezuela suggests a history of the diamond involving two periods of growth separated by a period of resorption and possibly brittle deformation. In situ electron probe analysis of multiple eclogitic garnet inclusions reveals a correlation between garnet composition and location in the stone. An early-formed garnet in the diamond core has higher Ca/(Ca+Mg) and lower Mg/(Mg+Fe) values than later garnets associated with the second period of diamond growth. This variation conforms to an extensive trend of variation in the suite of eclogitic garnets extracted from Venezuelan diamonds. The diamond is zoned in carbon isotope composition (in situ secondary ion mass spectrometry, SIMS, data). The core compositions ( δ13C PDB), corresponding to the first stage of growth, average −17.7‰. The second period of growth is apparently in two sub-sets of CL zones with mean values of −13.0‰ and −7.9‰. Nitrogen contents of diamond are low (30–300 atomic ppm) and do not correlate with carbon isotope composition. Oxygen isotope ratios of the garnet inclusions are elevated substantially above those expected for “common mantle”; δ18O VSMOW of early garnet is approximately +10.5‰ and two late garnets average +8.8‰. The evolutionary trend of magnesium enrichment in garnet is unlikely to represent igneous fractionation. The stable isotope data are consistent with diamond formation in subducted meta-basic rocks that had interacted with sea water at low temperatures at or near the sea floor and contained a substantial biogenic carbon component. During or following subduction, diamonds continued to form in an evolving system that was progressively modified by interaction with mantle material. 相似文献
9.
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
10.
The diamond population from the Jagersfontein kimberlite is characterized by a high abundance of eclogitic, besides peridotitic
and a small group of websteritic diamonds. The majority of inclusions indicate that the diamonds are formed in the subcratonic
lithospheric mantle. Inclusions of the eclogitic paragenesis, which generally have a wide compositional range, include two
groups of eclogitic garnets (high and low Ca) which are also distinct in their rare earth element composition. Within the
eclogitic and websteritic suite, diamonds with inclusions of majoritic garnets were found, which provide evidence for their
formation within the asthenosphere and transition zone. Unlike the lithospheric garnets all majoritic garnet inclusions show
negative Eu-anomalies. A narrow range of isotopically light carbon compositions (δ 13C −17 to −24 ‰) of the host diamonds suggests that diamond formation in the sublithospheric mantle is principally different
to that in the lithosphere. Direct conversion from graphite in a subducting slab appears to be the main mechanism responsible
for diamond formation in this part of the Earth’s mantle beneath the Kaapvaal Craton. The peridotitic inclusion suite at Jagersfontein
is similar to other diamond deposits on the Kaapvaal Craton and characterized by harzburgitic to low-Ca harzburgitic compositions. 相似文献
11.
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
12.
Microinclusions analyzed in a coated diamond from the Diavik mine in Canada comprise peridotitic minerals and fluids. The fluids span a wide compositional range between a carbonatitic melt and brine. The diamond is concentrically zoned. The brine microinclusions reside in an inner growth zone and their endmember composition is K 19Na 25Ca 5Mg 8Fe 3Ba 2Si 4Cl 32 (mol%). The carbonatitic melt is found in an outer layer and its endmember composition is K 11Na 21Ca 11Mg 26Fe 7Ba 2Si 10Al 3P 2Cl 5. The transition in inclusion chemistry is accompanied by a change in the carbon isotopic composition of the diamond from −8.5‰ in the inner zone to −12.1‰ in the outer zone. We suggest that this transition reflects mixing between already evolved brine and a freshly introduced carbonatitic melt of different isotopic composition. The compositional range found in diamond ON-DVK-294 is the widest ever recorded in a single diamond. It closes the gap between brine found in cloudy octahedral diamonds from South Africa and carbonatitic melt analyzed in cubic diamonds from Zaire and Botswana. Thus, all microinclusions analyzed to date fall along two arrays connecting the carbonatitic melt composition to either a hydrous-silicic endmember or to a brine endmember. This connection suggests that many diamonds are formed from fluids derived form a mantle source not significantly influenced by local heterogeneities. 相似文献
13.
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
14.
The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/ Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO 2 content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P- T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m 2 conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle. 相似文献
15.
The Attepe district consists of Precambrian, Lower–Middle Cambrian, Upper Cambrian–Lower Ordovician and Mesozoic formations. It contains several iron deposits and occurrences. Three types of iron-mineralizations can be distinguished in the area; (i) Sedimentary Fe-sulfide in Precambrian bituminous metapelitic rocks, and Fe-oxides in Precambrian metasandstones (SISO), (ii) vein-type Fe-carbonate and oxides composed of mainly siderite, ankerite and hematite including barite in Lower–Middle Cambrian metacarbonates of the Çaltepe Formation (HICO), (iii) karstic Fe-oxides and hydroxides essentially in the Lower–Middle Cambrian metacarbonates and the unweathered Fe-carbonates (KIO). The latter type is more widespread and located at the upper parts of the most important mineable iron deposits like Attepe deposit. Oxygen-, carbon-, sulfur- and strontium-isotope studies have been performed on siderites and barites in the vein-type ores, and on calcites in the recrystallized Çaltepe Limestones to investigate the sources and formation mechanism of primary ore-forming constituents. The δ13C values of siderites and calcites in limestones of the Çaltepe Formation range from −10.10‰ to −8.20‰, and from −0.8‰ to 2.30‰. Both carbonate minerals show δ18O values between 17.50–18.30‰ and 16.20–23.00‰, respectively. The δ13C and δ18O isotopic variations do not indicate any direct or linear relations between siderites and limestones. However, it is possible that the carbon and oxygen isotopic compositions of carbonate minerals could be changed to some extent, when limestones were subjected to hydrothermal processes or thermal alterations during metamorphism. The isotopic values of barites display 32.40–38.30‰ for δ34S and 12.20–14.70‰ for δ18O. The strontium isotope ratios (0.717169–0.718601) of barites and the sulfur isotope compositions of barites and pyrites suggest that there are no direct linkages of ore-forming compounds neither with a magmatic source nor with sedimentary pyrite formations in the Precambrian bituminous shales of the Attepe formation. According to the field observations and the stable isotope data, siderites and ankerites should be formed by interaction between iron-rich hydrothermal fluids and Çaltepe limestones, whereas isotope ratios of barites indicate that they were formed by mixing of sulfur-rich meteoric waters and deeply circulated hydrothermal solutions. 相似文献
16.
Previous studies of diamonds from Finsch have shown that eclogitic inclusions are rare at Finsch and that the eclogitic garnet and clinopyroxenes are iron and manganese-rich. In order to expand the current database of information, 93 eclogitic diamonds were selected for this study. Eight diamonds were polished into plates for cathodoluminescence studies and infrared examination of diamond growth and 31 diamonds were cracked to retrieve inclusions. The eclogitic garnets analysed in this study are enriched in Fe and are relatively depleted in Ca and Mg relative to worldwide data. FeO contents for garnet range from 15 to 27 wt.% and MnO contents reach a maximum value of 1.6 wt.%. The eclogitic clinopyroxenes have relatively high FeO contents, up to 14.8 wt.% and K 2O contents are low (<0.4 wt.%). Three non-touching garnet–clinopyroxene mineral pairs produce equilibration temperatures of 1138–1179 °C at an assumed pressure of 50 kb. No Type II diamonds were found during this study, all diamonds are of Type IaAB. Total nitrogen contents of Type IaAB diamonds range from 11 to 1520 ppm, with variable aggregation states (up to 84% nitrogen aggregated as B-defects). Distinct infrared characteristics suggest that the Finsch kimberlite sampled either more than one mantle source region of similar age but differing temperature, or two different populations of diamonds with different ages. The diamonds provide evidence of changing mantle conditions during crystallisation. Continuous diamond growth is illustrated by the presence of regular octahedral growth zones, although in some diamonds cubic growth is noted. One diamond shows evidence of platelet degradation, suggesting exposure to high temperatures and/or shearing stresses. 相似文献
17.
博茨瓦纳是世界上金刚石资源最为丰富的国家之一。奥拉帕金刚石矿床是该国最大的金刚石矿,矿床的金伯利岩为Ⅰ型,其中的包体可以分为2种:橄榄岩型和榴辉岩型;金刚石可以分为3类:橄榄岩型、榴辉岩型及两者的过渡类型-二辉岩型。其中,橄榄岩型和部分榴辉岩型金刚石来自于地幔结晶堆晶体,而榴辉岩型则与板块俯冲的构造-热事件有关。金伯利岩的形成时代主要为白垩纪,而金刚石则主要形成于元古宙和太古宙,金伯利岩和金刚石为不同时期的产物,金刚石为金伯利岩侵位期间捕获的上地幔物理破碎产物。其中年龄为900~1000Ma的金刚石为板块构造-热事件的产物,并对早期金刚石进行了改造破坏。 相似文献
18.
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 Cr 2O 3 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 Cr 2O 3 (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr 2O 3 and <0.5 wt.% TiO 2. 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 Cr 2O 3 contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K 2O. 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
19.
This paper reports on the petrology and geochemistry of a diamondiferous peridotite xenolith from the Premier diamond mine in South Africa. The xenolith is altered with pervasive serpentinisation of olivine and orthopyroxene. Garnets are in an advanced state of kelyphitisation but partly fresh. Electron microprobe analyses of the garnets are consistent with a lherzolitic paragenesis (8.5 wt.% Cr2O3 and 6.6 wt.% CaO). The garnets show limited variation in trace element composition, with generally low concentrations of most trace elements, e.g. Y (<11 ppm), Zr (<18 ppm) and Sr (<0.5 ppm). Garnet rare earth element concentrations, when normalised against the C1 chondrite of McDonough and Sun (Chem. Geol. 120 (1995) 223), are characterised by a rare earth element pattern similar to garnet from fertile lherzolite. All diamonds recovered are colourless. Most crystals are sharp-edged octahedra, some with minor development of the dodecahedral form. A number of crystals are twinned octahedral macles, while aggregates of two or more octahedra are also common. Mineral inclusions are rare. Where present they are predominantly small black rosettes believed to consist of sulfide. In one instance a polymineralic (presumably lherzolitic) assemblage of reddish garnet, green clinopyroxene and a colourless mineral is recognised. Infrared analysis of the xenolith diamonds show nitrogen contents generally lower than 500 ppm and variable nitrogen aggregation state, from 20% to 80% of the ‘B’ form. When plotted on a nitrogen aggregation diagram a well defined trend of increasing nitrogen aggregation state with increasing nitrogen content is observed. Carbon isotopic compositions range from −3.6 ‰ to −1.3 ‰. These are broadly correlated with diamond nitrogen content as determined by infrared spectroscopy, with the most negative C-isotopic compositions correlating with the lowest nitrogen contents. Xenolith mantle equilibration temperatures, calculated from nitrogen aggregation systematics as well as the Ni in garnet thermometer are on the order of 1100 to 1200 °C. It is concluded that the xenolith is a fertile lherzolite, and that the lherzolitic character may have resulted from the total metasomatic overprinting of pre-existing harzburgite. Metasomatism occurred prior to, or accompanied, diamond growth. 相似文献
20.
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
|
