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1.
Graphite-bearing peridotites, pyroxenites and eclogite xenoliths from the Kaapvaal craton of southern Africa and the Siberian craton, Russia, have been studied with the aim of: 1) better characterising the abundance and distribution of elemental carbon in the shallow continental lithospheric mantle; (2) determining the isotopic composition of the graphite; (3) testing for significant metastability of graphite in mantle rocks using mineral thermobarometry. Graphite crystals in peridotie, pyroxenite and eclogite xenoliths have X-ray diffraction patterns and Raman spectra characteristic of highly crystalline graphite of high-temperature origin and are interpreted to have crystallised within the mantle. Thermobarometry on the graphite-peridotite assemblages using a variety of element partitions and formulations yield estimated equilibration conditions that plot at lower temperatures and pressures than diamondiferous assemblages. Moreover, estimated pressures and temperatures for the graphite-peridotites fall almost exclusively within the experimentally determined graphite stability field and thus we find no evidence for substantial graphite metastability. The carbon isotopic composition of graphite in peridotites from this and other studies varies from δ13 CPDB = ? 12.3 to ? ?3.8%o with a mean of-6.7‰, σ=2.1 (n=22) and a mode between-7 and-6‰. This mean is within one standard deviation of the-4‰ mean displayed by diamonds from peridotite xenoliths, and is identical to that of diamonds containing peridotite-suite inclusions. The carbon isotope range of graphite and diamonds in peridotites is more restricted than that observed for either phase in eclogites or pyroxenites. The isotopic range displayed by peridotite-suite graphite and diamond encompasses the carbon isotope range observed in mid-ocean-ridge-basalt (MORB) glasses and ocean-island basalts (OIB). Similarity between the isotopic compositions of carbon associated with cratonic peridotites and the carbon (as CO2) in oceanic magmas (MORB/OIB) indicates that the source of the fluids that deposited carbon, as graphite or diamond, in catonic peridotites lies within the convecting mantle, below the lithosphere. Textural observations provide evidence that some of graphite in cratonic peridotites is of sub-solidus metasomatic origin, probably deposited from a cooling C-H-O fluid phase permeating the lithosphere along fractures. Macrocrystalline graphite of primary appearance has not been found in mantle xenoliths from kimberlitic or basaltic rocks erupted away from cratonic areas. Hence, graphite in mantle-derived xenoliths appears to be restricted to Archaean cratons and occurs exclusively in low-temperature, coarse peridotites thought to be characteristic of the lithospheric mantle. The tectonic association of graphite within the mantle is very similar to that of diamond. It is unlikely that this restricted occurrence is due solely to unique conditions of oxygen fugacity in the cratonic lithospheric mantle because some peridotite xenoliths from off-craton localities are as reduced as those from within cratons. Radiogenic isotope systematics of peridotite-suite diamond inclusions suggest that diamond crystallisation was not directly related to the melting events that formed lithospheric peridotites. However, some diamond (and graphite?) crystallisation in southern Africa occurred within the time span associated with the stabilisation of the lithospheric mantle (Pearson et al. 1993). The nature of the process causing localisation of carbon in cratonic mantle roots is not yet clearly understood.  相似文献   

2.
The first results of study of minerals and diamonds of diamond-bearing eclogites from kimberlites of the Yubileinaya pipe with a variable percent amount of clinopyroxene and garnet are presented. Samples with a garnet content from 30 to 90% of the xenolith volume are dominant among the round to oval xenoliths with diamonds. Five eclogite samples contain grains of accessory rutile, as well as corundum and kyanite. Some samples host two or more diamond crystals.  相似文献   

3.
贵州镇远是中国金刚石原生矿找矿的重点区域之一。镇远地区马坪D1号岩体是1965年中国首次发现的含原生金刚石金伯利岩。该岩体岩石具典型的金伯利岩结构和组成特征,其中的锆石捕虏晶U–Pb年代学和Hf同位素分析结果表明,该地区存在未暴露的太古宙基底物质残余。基于壳幔耦合性规律,可能对应有古老的岩石圈地幔,这种古老的克拉通属性是金刚石形成的有利因素。但另一方面,马坪金伯利岩普遍含有伴生矿物含铬镁铝榴石,其CaO含量较高,多数属于G9(二辉橄榄岩)类型,不是全球富含金刚石的方辉橄榄岩原岩类型(G10),暗示当时的岩石圈发生了部分改造而可能不利于高品质金刚石的形成。需要注意的是,在金刚石找矿过程中,应该以详细的野外工作与岩石学对比研究为基础,同时依赖于金伯利岩及其相关的岩浆活动所携带的捕虏体/捕虏晶的研究,配合以岩浆成分来反演地幔源区特征,才能较全面地揭示古老大陆岩石圈的形成年龄与演化历史、物质组成与精细结构,以及大陆岩石圈根的厚度、热状态、氧逸度、流体作用等,进而为寻找金刚石提供重要的依据。  相似文献   

4.
Diamonds: time capsules from the Siberian Mantle   总被引：1，自引：0，他引：1  
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   

5.
金刚石母岩可以是榴辉岩、辉石岩、橄榄岩等多种岩石,它们与金刚石都是在地幔深处形成的,并上侵最终固结于地壳中。母岩中的金刚石等矿物在地壳中又发生准稳定生长。地壳中金伯利岩和钾镁煌斑岩岩浆作用对金刚石母岩侵入体进行改造,使原生金刚石发生破碎、溶解、再生长等一系列变化,并形成巨晶、劣质金刚石和黑金刚石等新类型,这些现象不是在金刚石母岩形成之初发生的。  相似文献   

6.
We have performed dissections of two diamondiferous eclogites (UX-1 and U33/1) from the Udachnaya kimberlite, Yakutia in order to understand the nature of diamond formation and the relationship between the diamonds, their mineral inclusions, and host eclogite minerals. Diamonds were carefully recovered from each xenolith, based upon high-resolution X-ray tomography images and three-dimensional models. The nature and physical properties of minerals, in direct contact with diamonds, were investigated at the time of diamond extraction. Polished sections of the eclogites were made, containing the mould areas of the diamonds, to further investigate the chemical compositions of the host minerals and the phases that were in contact with diamonds. Major- and minor-element compositions of silicate and sulfide mineral inclusions in diamonds show variations among each other, and from those in the host eclogites. Oxygen isotope compositions of one garnet and five clinopyroxene inclusions in diamonds from another Udachnaya eclogite (U51) span the entire range recorded for eclogite xenoliths from Udachnaya. In addition, the reported compositions of almost all clinopyroxene inclusions in U51 diamonds exhibit positive Eu anomaly. This feature, together with the oxygen isotopic characteristics, is consistent with the well-established hypothesis of subduction origin for Udachnaya eclogite xenoliths. It is intuitive to expect that all eclogite xenoliths in a particular kimberlite should have common heritage, at least with respect to their included diamonds. However, the variation in the composition of multiple inclusions within diamonds, and among diamonds, from the same eclogite indicates the involvement of complex processes in diamond genesis, at least in the eclogite xenoliths from Yakutia that we have studied.  相似文献   

7.
Eclogitic (E-type) and related parageneses of natural diamonds are represented by suites of diamond inclusions and xenoliths of diamondiferous eclogites. Major-element data are presented for 32 coexisting minerals forming 19 bimineralic and trimineralic inclusions from diamonds, including omphacite-orthopyroxene (1 sample), garnet-omphacite (5 samples), garnet-coesite (5 samples), omphacite-coesite (2 samples), garnet-picroilmenite (2 samples), garnet-kyanite (1 sample), omphacite-phlogopite (2 samples), and garnel-omphacite-phlogopite (1 sample). Major-element variations of coexisting minerals are typical of corresponding eclogites. Omphacite with 5.02 wt% Na2O, inter-grown with orthopyroxene with Mg# 83.7, represents the first example of a diamondiferous websterite paragenesis including Na-clinopyroxene. This indicates a broader range in mineral compositions of E-type-related websteritepyroxenite-associated diamonds than known previously. This unique websterite-pyroxenitic mineral assemblage represents a transitional paragenesis between peridotitic or ultramafic (U-type) and E-type parageneses.

Bimineralic eclogites, ilmenite eclogites, coesite + corundum + kyanite eclogites, and grospydites occur not only as sets of inclusions in diamonds but, with a few exceptions (ilmenite and coesite eclogites), also as diamondiferous eclogite xenoliths. The coesite eclogite paragenesis is a significant inclusion suite in diamonds, and was detected in about 15 diamond occurrences worldwide. It represents from 15% to 22% of all E-type diamonds in several occurrences, and thus should not be considered as rare.  相似文献   

8.
We present petrography and mineral chemistry for both phlogopite,from mantle-derived xenoliths(garnet peridotite,eclogite and clinopyroxene-phlogopite rocks)and for megacryst,macrocryst and groundmass flakes from the Grib kimberlite in the Arkhangelsk diamond province of Russia to provide new insights into multi-stage metasomatism in the subcratonic lithospheric mantle(SCLM)and the origin of phlogopite in kimberlite.Based on the analysed xenoliths,phlogopite is characterized by several generations.The first generation(Phil)occurs as coarse,discrete grains within garnet peridotite and eclogite xenoliths and as a rock-forming mineral within clinopyroxene-phlogopite xenoliths.The second phlogopite generation(Phl2)occurs as rims and outer zones that surround the Phil grains and as fine flakes within kimberlite-related veinlets filled with carbonate,serpentine,chlorite and spinel.In garnet peridotite xenoliths,phlogopite occurs as overgrowths surrounding garnet porphyroblasts,within which phlogopite is associated with Cr-spinel and minor carbonate.In eclogite xenoliths,phlogopite occasionally associates with carbonate bearing veinlet networks.Phlogopite,from the kimberlite,occurs as megacrysts,macrocrysts,microcrysts and fine flakes in the groundmass and matrix of kimberlitic pyroclasts.Most phlogopite grains within the kimberlite are characterised by signs of deformation and form partly fragmented grains,which indicates that they are the disintegrated fragments of previously larger grains.Phil,within the garnet peridotite and clinopyroxene-phlogopite xenoliths,is characterised by low Ti and Cr contents(TiO_21 wt.%,Cr_2 O_31 wt.% and Mg# = 100 × Mg/(Mg+ Fe)92)typical of primary peridotite phlogopite in mantle peridotite xenoliths from global kimberlite occurrences.They formed during SCLM metasomatism that led to a transformation from garnet peridotite to clinopyroxene-phlogopite rocks and the crystallisation of phlogopite and high-Cr clinopyroxene megacrysts before the generation of host-kimberlite magmas.One of the possible processes to generate low-Ti-Cr phlogopite is via the replacement of garnet during its interaction with a metasomatic agent enriched in K and H_2O.Rb-Sr isotopic data indicates that the metasomatic agent had a contribution of more radiogenic source than the host-kimberlite magma.Compared with peridotite xenoliths,eclogite xenoliths feature low-Ti phlogopites that are depleted in Cr_2O_3 despite a wider range of TiO_2 concentrations.The presence of phlogopite in eclogite xenoliths indicates that metasomatic processes affected peridotite as well as eclogite within the SCLM beneath the Grib kimberlite.Phl2 has high Ti and Cr concentrations(TiO_22 wt.%,Cr_2O_31 wt.% and Mg# = 100× Mg/(Mg + Fe)92)and compositionally overlaps with phlogopite from polymict brecc:ia xenoliths that occur in global kimberlite formations.These phlogopites are the product of kimberlitic magma and mantle rock interaction at mantle depths where Phl2 overgrew Phil grains or crystallized directly from stalled batches of kimberlitic magmas.Megacrysts,most macrocrysts and microcrysts are disintegrated phlogopite fragments from metasomatised peridotite and eclogite xenoliths.Fine phlogopite flakes within kimberlite groundmass represent mixing of high-Ti-Cr phlogopite antecrysts and high-Ti and low-Cr kimberlitic phlogopite with high Al and Ba contents that may have formed individual grains or overgrown antecrysts.Based on the results of this study,we propose a schematic model of SCLM metasomatism involving phlogopite crystallization,megacryst formation,and genesis of kimberlite magmas as recorded by the Grib pipe.  相似文献   

9.
Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ13C values. While diamonds from high-MgO eclogites have low nitrogen contents (5-82 ppm) and extremely low δ13C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and δ13C values from −3.5‰ to −5.3‰.Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ13C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ13C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ13C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ13C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0 Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction.SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ13C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ13C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ13C-N yielded a partition coefficient (KN) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ13C values of diamond cores (−4‰) dictate the growth medium had higher δ13C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates.  相似文献   

10.
金刚石及其寄主岩石是人类认识地球深部物质组成和性质、壳幔和核幔物质循环重要研究对象。本文总结了中国不同金刚石类型的分布,着重对比了博茨瓦纳和中国含金刚石金伯利岩的地质特征,取得如下认识:(1)博茨瓦纳含矿原生岩石仅为金伯利岩,而中国含矿岩石成分复杂,金伯利岩主要出露在华北克拉通,展布于郯庐、华北中央和华北北缘金伯利岩带,具有工业价值的蒙阴和瓦房店矿床分布于郯庐金伯利岩带中;钾镁煌斑岩主要出露在华南克拉通,重点分布在江南和华南北缘钾镁煌斑岩带中;(2)钙钛矿原位U-Pb年龄和Sr、Nd同位素显示,86～97 Ma奥拉帕金伯利岩群和456～470 Ma蒙阴和瓦房店金伯利岩均具有低87Sr/86Sr(0.703～0.705)和中等εNd(t)(-0.09～+5)特征,指示金伯利岩浆源自弱亏损地幔或初始地幔源区;(3)博茨瓦纳金伯利岩体绝大多数以岩筒产出,而中国以脉状为主岩筒次之;博茨瓦纳岩筒绝大部分为火山口相,中国均为根部相,岩筒地表面积普遍小于前者;(4)奥拉帕A/K1和朱瓦能金伯利岩体是世界上为数不多的主要产出榴辉岩捕虏体和E型金刚石的岩筒之一,而同位于奥拉帕岩群的莱特拉卡内、丹姆沙和卡罗韦岩体与我国郯庐带的金伯利岩体类似,均主要产出地幔橄榄岩捕虏体以及P型和E型金刚石;(5)寻找含矿金伯利岩重点注意以下几点:克拉通内部和周缘深大断裂带是重要的控岩构造;镁铝榴石、镁钛铁矿、铬透辉石、铬尖晶石和铬金红石等是寻找含金刚石金伯利岩重要的指示矿物;航磁等地球物理测量需与土壤取样找矿方法相结合才能取得更好效果;(6)郯庐金伯利岩带、江南钾镁煌斑岩带和塔里木地块是中国重要含矿岩石的找矿靶区,冲积型金刚石成矿潜力巨大。  相似文献   

11.

The first studies of diamonds in eclogitic xenoliths from the Komsomolskaya kimberlite pipe are described. Among round and oval-shaped xenoliths with diamond ingrowths, samples with a garnet content of 40–90% of the xenolith volume dominate. Two eclogite samples contain grains of accessory rutile; a kyanite sample is also revealed. Certain samples contain two or more crystals of diamonds. Diamonds with an octahedral habit and crystals with transitional habits, which belong to an octahedral-rhombic dodecahedral row, dominate in eclogites; there are many variety VIII aggregates. A high concentration of structural nitrogen, commonly in the A form, was registered in most of the crystals. Diamonds with a small content of nitrogen impurities, 40–67% in the B1 form, are present in a number of xenoliths. The calculated temperatures of the formation of eclogitic xenoliths is 1100–1300°C. Diversity in the impurity compositions of diamonds in the same xenolith shows that these diamonds were formed at various times and in different settings. The diamond position in xenoliths, the various level of nitrogen aggregation in the diamonds, and a number of other factors point to the later formation of the diamonds, as compared to minerals of eclogites, from fluid or fluid-melts in the process of metasomatosis.

  相似文献   

12.
This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr2O3 contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190 km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130 km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155 km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155 km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF.  相似文献   

13.
张宏福 《地球科学》2006,31(1):31-37
地幔和／或下地壳中存在2类“复杂”橄榄岩捕虏体:一类为“混杂”,即“polymict”橄榄岩捕虏体;另一类为相对常见的“复合”,即“composite”橄榄岩捕虏体．复合橄榄岩捕虏体通常由橄榄岩和辉石岩／麻粒岩或由橄榄岩夹辉石岩脉2部分或多部分组成．岩相学上易于辨认,有时手标本上即可辨认．而混杂橄榄岩捕虏体为各种橄榄岩和／或辉石岩来源的矿物和熔体混杂在一起组成的构造岩,岩相学上很难辨认,只有通过矿物组成分析来判断．混杂橄榄岩捕虏体仅见于南非Kaapvaal 克拉通金伯利岩岩管中,该类捕虏体来源于地幔塑性剪切带,记录了流／熔体参与下的地幔变形和交代作用的全过程．复合橄榄岩捕虏体较常见,出现在全球各类玄武岩和金伯利岩中．复合橄榄岩捕虏体有多种成因,如地幔变形、壳幔过渡带样品、交代作用和堆积成因等等．这些复合橄榄岩捕虏体同样记录了岩石圈地幔和／或下地壳交代作用／改造过程的中间状态．因此,这些罕见而重要的复杂橄榄岩捕虏体为大陆岩石圈动力学演化过程提供了最直接的样品．  相似文献   

14.
The objects of study are Triassic hypabyssal diamondiferous kimberlites with an age of 220-245 Ma, containing macrocrysts of unaltered olivine. The latter are close in the time of formation to the main stage of intrusion of the Siberian Trap Province (252 Ma), which lasted less than 1 Myr. A comparative high-precision analytical study of the Ti, Ca, Cr, and Al impurity patterns in about 1000 olivine macrocryst samples with a forsterite content Fo = (100Mg/(Mg + Fe)) of 78 to 93 has demonstrated the effect of traps on the lithospheric composition. A comprehensive comparative study of diamonds from northern placers and Triassic kimberlites, including determination of their carbon isotope composition, was performed. Chromatography-mass-spectroscopic analysis of submicron fluid inclusions in diamonds from northern placers and kimberlites has shown predominant hydrocarbons of a wide range of compositions and subordinate contents of N2, H2O, and CO2. These findings, together with the results of previous studies of subcalcic Cr-pyropes and diamonds found in the Lower Carboniferous gritstones of the Kyutyungde graben, lead to the conclusion that the Toluopka kimberlite field is promising for Paleozoic kimberlites. The results of comprehensive studies of diamonds and indicator minerals and U/Pb isotope dating of numerous detrital zircon samples from the basal horizon of the Carnian Stage (Upper Triassic) of the Bulkur site in the lower reaches of the Lena River suggest the presence of diamondiferous kimberlites within the northeastern Siberian Platform. The age of the probable primary diamond sources in the study area can be evaluated by an integrated U/Pb isotope dating of zircons, perovskites, and rutiles from the developed diamond placers and the basal horizon of the Carnian Stage.  相似文献   

15.
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   

16.
The diamond-bearing mantle keels underlying Archean cratons are a unique phenomenon of Early Precambrian geology. The common stable assemblage of the Archean TTG early continental crust and underlying subcontinental lithospheric mantle clearly shows their coupled tectogenesis, which was not repeated in younger geological epochs. One of the least studied aspects of this phenomenon is concerned with the eclogitic xenoliths carried up by kimberlite pipes together with mantle-derived nodules. The eclogitic xenoliths reveal evidence for their subduction-related origin, but the Archean crustal counterparts of such xenoliths remained unknown for a long time, and the question of their crustal source and relationships to the formation of early continental crust remained open. The Archean crustal eclogites recently found in the Belomorian Belt of the Baltic Shield are compared in this paper with eclogitic xenoliths from kimberlites in the context of the formation of both Archean subcontinental lithospheric mantle (SCLM) and early continental crust. The crustal eclogites from the Belomorian Belt are identical in mineral and chemical compositions to the eclogite nodules (group B), including their diamond-bearing varieties. The eclogite protoliths are comparable in composition with the primary melts of the Meso- and Neoarchean oceanic crust, which was formed at a potential temperature of the upper mantle which exceeded its present-day temperature by 150–250 K. The reconstructed pathways of the Archean oceanic crust plunging in the upper mantle suggest that the Archean mantle was hotter than in the modern convergence settings. The proposed geodynamic model assumes coupled formation of the Archean diamond-bearing SCLM and growth of early continental crust as a phenomenon related to the specific geodynamics of that time controlled by a higher terrestrial heat flow.  相似文献   

17.
Three-dimensional neutron and X-ray tomography reveals the textural and spatial relationship of diamonds and associated minerals in situ, in a unique suite of 17 diamondiferous eclogites. We emphasize the reporting of X-ray imaging on mantle xenoliths, which in combination with neutron imaging enables the clear identification of diamonds and interstitial metasomatic secondary minerals. In particular, neutrons are highly sensitive to hydrogen (H), allowing for the identification of OH- and H2O-bearing metasomatic minerals. The identification of metasomatic minerals allows for the delineation of distinct metasomatic pathways through the eclogite xenoliths. Diamonds are readily identified as the darkest greyscales due to their low attenuation, and are typically surrounded by secondary minerals, never in contact with primary minerals, and always confined within metasomatic pathways. The ubiquitous occurrence of diamonds in association with pathways suggests a potential genetic link. Both octahedral and dodecahedral diamonds are observed within individual xenoliths, suggesting multiple heterogeneous growth and dissolution processes at small scales. The distinct age dichotomy between eclogite xenoliths and metasomatic mineral assemblages implies that the observed textural relationship of diamonds and late-stage metasomatic pathways for this suite of 17 eclogites casts doubt on the theory that eclogitic diamonds formed billions of years ago. Diamonds are interpreted to have formed from multiple growth episodes, with the last of these episodes represented by the metasomatic assemblages observed in this study. This further indicates that eclogitic diamond inclusions may span large time scales from ancient ages (>2 Ga) all the way to the last growth event, perhaps even close to the time of kimberlite emplacement (~360 Ma), which has significant implications for age-dating of diamonds and the study of diamonds as a whole.  相似文献   

18.
Basedontheconsiderationswhetherornottherearedif-ferencesincompositionsofthemantlematerial,betweenPale-ozoicandCenozoic,andthedifferencesinrocksamp1esfrommantleliewithinandoutsideoftheTancheng-Lujiang(Tanlu)faultzoneinCenozoic,themineralinclusionsindia-mQndandxenolithsfromMengyinandFuxiankimberliteswerechosenforconstrainingthenatureofPaleozoicSCLM,andthexenolithsfromtheShanwangandQixiabasaltswererepresentativelychosenforconstrainingthenatureofCeno-zoicSCLMinsideandoutsideoftheTanlufault…  相似文献   

19.
The Origins of Yakutian Eclogite Xenoliths   总被引：2，自引：2，他引：2       下载免费PDF全文
Owing to the association with diamonds, eclogite xenoliths havereceived disproportionate attention given their low abundancein kimberlites. Several hypotheses have been advanced for theorigin of eclogite xenoliths, from the subduction and high-pressuremelting of oceanic crust, to cumulates and liquids derived fromthe upper mantle. We have amassed a comprehensive data set,including major- and trace-element mineral chemistry, carbonisotopes in diamonds, and Rb–Sr, Sm–Nd, Re–Os,and oxygen isotopes in ultrapure mineral and whole-rock splitsfrom eclogites of the Udachnaya kimberlite pipe, Yakutia, Russia.Furthermore, eclogites from two other Yakutian kimberlite pipes,Mir and Obnazhennaya, have been studied in detail and offercontrasting images of eclogite protoliths. Relative to eclogitesfrom southern Africa and other Yakutian localities, Udachnayaeclogites are notable in the absence of chemical zoning in mineralgrains, as well as the degree of light rare earth element (LREE)depletion and unradiogenic Sr; lack of significant oxygen, sulfur,and carbon isotopic variation relative to the mantle; and intermineralradiogenic isotopic equilibration. Several of these eclogitescould be derived from ancient, recycled, oceanic crust, butmany others exhibit no evidence for an oceanic crustal protolith.The apparent lack of stable-isotope variation in the Udachnayaeclogites could be due to the antiquity of the samples and consequentlack of deep oceanic and biogenically diverse environments atthat time. Those eclogites that are interpreted to be non-recycledhave compositions characteristic of Group A eclogites from otherlocalities that also have been interpreted as being directlyfrom the mantle. At least two separate and diverse isotopicreservoirs are suggested by Nd isotopic whole-rock reconstructions.Most samples were derived from typical depleted mantle. However,two groups of three samples each indicate both enriched mantleand possible ultra-depleted mantle present beneath Yakutia duringthe late Archean and early Proterozoic. The vast majority ofeclogites studied from the Obnazhennaya pipe also exhibit characteristicsof Group A eclogites and are probably derived directly fromthe mantle. However, the eclogites from the Mir kimberlite aremore typical of other eclogites world-wide and show convincingevidence of a recycled, oceanic crustal affinity. We concurwith the late Ted Ringwood that eclogites can be formed in avariety of ways, both within the mantle and from oceanic crustalresidues. KEY WORDS: diamonds; eclogite xenoliths; isotopic composition; REE; Yakutia  相似文献   

20.
对已经发表的数十篇关于澳大利亚金刚石的英文文献进行了梳理,从其金刚石的品质、颜色类型、形态及表面特征、生长结构及微量元素、包裹体、C同位素等方面探索了澳大利亚不同区域金刚石可能存在的产地来源特征.研究显示,澳大利亚金刚石可分为岩石圈地幔成因、超深地幔成因和俯冲环境来源等成因类型;大部分澳大利亚金刚石都因经历过强烈的晶格变形或熔蚀作用而晶体圆化,但不同产地来源的金刚石在颜色组合、橄榄岩型和榴辉岩型金刚石比例、C同位素组成特征等方面存在一定差异.上述结果表明,总体上,澳大利亚不同区域金刚石具有一定的产地来源个性,但无法简单确认澳大利亚金刚石“整体”的产地来源特征;只有结合成因来源进行分析,才能够较深入地理解不同区域金刚石的特征组合及其意义,从而为理解其产地来源的特殊性提供帮助.  相似文献   

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