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1.
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   

2.
Trace element characteristics of seven coesite-bearing eclogitic xenoliths from the Roberts Victor kimberlite demonstrate that this suite of eclogites originated as gabbroic cumulates in oceanic crust that was subsequently subducted. All but one of the garnets show positive Eu anomalies, accompanied by a flat heavy rare earth pattern, which is atypical of garnet, but characteristic of plagioclase, arguing for a considerable amount of plagioclase in the protoliths. Forward modelling of the accumulation of liquidus minerals from primitive komatiitic, picritic, and basaltic liquids suggests that at least some of the eclogite protoliths were not derived from basaltic parental liquids, whereas derivation from either komatiitic or picritic liquids is possible. The reconstructed eclogite bulk rocks compare favourably with oceanic gabbros from ODP hole 735B (SW Indian Ridge), even to the extent that oxygen isotopic systematics show signs of low-temperature seawater alteration. However, the oxygen isotope trends are the reverse of what is expected for cumulates in the lower section of the oceanic crust. These new findings show that δ18O values in eclogitic xenoliths, despite being sound indicators for their interaction with hydrothermal fluids at low pressure, do not necessarily bear a simple relationship with the inferred oceanic crustal stratigraphy of the protoliths.  相似文献   

3.
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   

4.
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   

5.
New trace-element data of rutile in kimberlite-borne ~1.85 Ga eclogite and pyroxenite xenoliths from the central Slave craton, as well as ~110 Ma MARID xenoliths from the Kaapvaal craton, provide constraints on the origins of lithospheric and sublithospheric mantle variability in high field strength element ratios. Rutiles in eclogites and pyroxenites have Zr/Hf ranging from 20 to 62 and Nb/Ta ranging from 10 to 40. Rutiles in MARID xenoliths have Zr/Hf from 24 to 33 and Nb/Ta from 10 to 41. Calculated whole-rock Zr/Hf is suprachondritic for eclogites with suggested gabbroic protoliths and subchondritic for boninite-like eclogites; the latter is consistent with cpx-controlled depletion in the protolith source. Within each eclogite type, positive correlations of Zr/Hf with La/Lu and negative correlations with Lu/Hf likely reflect fractionation of cpx and/or plagioclase during crystallisation of the protoliths. Zr/Hf–Nb/Ta relationships of some MARID-type rocks, which are products of lithospheric mantle metasomatism, and eclogite xenoliths plot on a silicate differentiation trend, whereas other samples have higher Nb/Ta at a given Zr/Hf. Fractionation of a few percent rutile from an HFSE-rich mafic melt can generate a trend towards strongly increased Nb/Ta at minimally changed Zr/Hf in the residual melt. Superposition of rutile fractionation on the effects of silicate differentiation, which fractionates Zr/Hf more strongly than Nb/Ta, can explain the Zr/Hf–Nb/Ta relationships of most eclogites from the central Slave craton as well as those of MARID rocks, metasomatised peridotites and group II kimberlites. By contrast, Zr/Hf–Nb/Ta relationships suggest that Group I kimberlites are mixtures between depleted peridotite and carbonatite. Thus, high Nb/Ta is a signature of lithospheric processes and may not be important in deeply subducted eclogites that bypass extended residence in the lithosphere. Conversely, considerable primary Zr/Hf variability was inherited by the eclogites, which is indicative of the compositional diversity of ancient subducted oceanic crust, which is expected to have generated substantial heterogeneity in sublithospheric basalt sources.  相似文献   

6.
Major-element and REE compositions of 14 diamondiferous eclogites from the Udachnaya kimberlite in Yakutia, Siberia have been determined by electron microprobe and secondary ion mass spectrometer (SIMS). Based on previous clinopyroxene classification schemes (e.g., Taylor and Neal 1989), all of these eclogite xenoliths belong to Group B/C, although some of the garnet compositions and mineral REE abundances are inconsistent with the indicated groups. This demonstrates the inadequacy of the classification scheme based on African eclogites for application to Siberian samples. Because of the coarse grain size of the Udachnaya nodules, meaningful modal abundances could not be obtained. However, reconstructed REE compositions using various garnet: clinopyroxene ratios demonstrate relative insensitivity to changes in mode for common eclogitic assemblages. Many of these reconstructed REE compositions show LREE depletions. Some depletions are consistent with an origin (either directly or through partial melting) as normal or Type-I ocean floor basalt. Others, however, require material of eclogitic or pyroxenitic affinities to undergo partial melting; this facilitates the depletion of LREE while leaving the HREE at nearly original levels. Many of the eclogites of South Africa are consistent with a protolith of anomalous or Type II ocean floor basalt. This fundamental difference between the two regions is the likely cause of the inconsistencies with the chemicallybased classification.  相似文献   

7.
Major- and trace-element and Sr–Nd–Hf isotopic compositionsof garnet and clinopyroxene in kimberlite-borne eclogite andpyroxenite xenoliths were used to establish their origins andevolution in the subcontinental lithospheric mantle beneaththe central Slave Craton, Canada. The majority of eclogitescan be assigned to three groups (high-Mg, high-Ca or low-Mgeclogites) that have distinct trace-element patterns. Althoughpost-formation metasomatism involving high field strength element(HFSE) and light rare earth element (LREE) addition has partiallyobscured the primary compositional features of the high-Mg andhigh-Ca eclogites, trace-element features, such as unfractionatedmiddle REE (MREE) to heavy REE (HREE) patterns suggestive ofgarnet-free residues and low Zr/Sm consistent with plagioclaseaccumulation, could indicate a subduction origin from a broadlygabbroic protolith. In this scenario, the low REE and smallpositive Eu anomalies of the high-Mg eclogites suggest moreprimitive, plagioclase-rich protoliths, whereas the high-Caeclogites are proposed to have more evolved protoliths withhigher (normative) clinopyroxene/plagioclase ratios plus trappedmelt, consistent with their lower Mg-numbers, higher REE andabsence of Eu anomalies. In contrast, the subchondritic Zr/Hfand positive slope in the HREE of the low-Mg eclogites are similarto Archaean second-stage melts and point to a previously depletedsource for their precursors. Low ratios of fluid-mobile to lessfluid-mobile elements and of LREE to HREE are consistent withdehydration and partial melt loss for some eclogites. The trace-elementcharacteristics of the different eclogite types translate intolower Nd for high-Mg eclogites than for low-Mg eclogites. Withinthe low-Mg group, samples that show evidence for metasomaticenrichment in LREE and HFSE have lower Nd and Hf than a samplethat was apparently not enriched, pointing to long-term evolutionat their respective parent–daughter ratios. Garnet andclinopyroxene in pyroxenites show different major-element relationshipsfrom those in eclogites, such as an opposite CaO–Na2Otrend and the presence of a CaO–Cr2O3 trend, independentof whether or not opx is part of the assemblage. Therefore,these two rock types are probably not related by fractionationprocesses. The presence of opx in about half of the samplesprecludes direct crystallization from eclogite-derived melts.They probably formed from hybridized melts that reacted withthe peridotitic mantle. KEY WORDS: eclogites; pyroxenite xenoliths; mantle xenoliths; eclogite trace elements; eclogite Sr isotopes; eclogite Hf isotopes; eclogite Nd isotopes  相似文献   

8.
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   

9.
The chemical compositions of garnets from 58 eclogite, 72 peridotite and 4 pyroxenite xenoliths in kimberlites have been estimated from their unit cell edge length and refractive indices. The samples studied were obtained from 17 kimberlite occurrences and include all those of known source which remain in the famous Williams (1932) collection which is stored at the University of Cape Town. Every suitable sample available to the authors has been examined.A gap in the range of garnet volume percentages occurs in the samples studied between approximately 15 and 30%. Garnet peridotites characteristically have <15% garnet and eclogites >30% garnet. Very rare exceptions occur. Our collection contains no eclogites with olivine and only one with orthopyroxene. All but two of the peridotite-pyroxenite group contain orthopyroxene. The garnets from the peridotites and pyroxenites plot on a pyrope-almandine-uvarovite triangle in a narrow band with a remarkably constant almandine/uvarovite ratio. Garnets from the eclogites are plotted on a pyrope-almandine-grossularite triangle and have a wide spread of compositions. These fall into 4 groups viz. eclogite I, eclogite II, kyanite eclogite and corundum eclogite.The reasons for the differences in garnet chemistry are considered and a tentative evolutionary scheme suggested by partial melting of the garnet peridotite which is assumed to occur in the upper mantle. Recent models of upper mantle composition and the genesis of garnet-bearing xenoliths in kimberlite are briefly and critically examined.S.A. UMP Publication No. 9.  相似文献   

10.

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   

11.
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   

12.
Mantle xenoliths from the Obnazhennaya kimberlite pipe, Yakutia, possess a large range of mineralogical and chemical compositions, from both group A and B eclogites. Major-element contents of the group A eclogites exhibit transitional features between the group B eclogites and peridotite. The Mg# of clinopyroxenes is 0.86–0.94, with 0.60–0.84 for garnets. Differences in concentration of LREEs exist between the Obnazhennaya group A and the well-studied group B eclogites from the Udachnaya kimberlite pipe. In general, garnets in the group A eclogites contain lower LREEs than those from the group B eclogites; however, the trend for clinopyroxene is reversed. High d 18O (5.46–7.81) values, and the positive Eu anomalies in the garnets and clinopyroxenes (Eu/Eu* 1.2–1.4) demonstrate the involvement of an oceanic crustal component in the formation of the group A eclogites. The group A eclogites formed between 21.0 and 37.6 kbar, and 711 and 923 °C, in a time interval of 1,071–1,237 Ma. An innovative model is proposed to explain the formation of the group A eclogites and websterites. It involves the reaction of a depleted mantle peridotite with TTG and carbonatite melts closely related to the subduction of oceanic crust.  相似文献   

13.
Nature and origin of eclogite xenoliths from kimberlites   总被引：16，自引：0，他引：16  
D.E. Jacob   《Lithos》2004,77(1-4):295-316
Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.  相似文献   

14.
Mantle xenoliths brought to the surface by kimberlite magmas along the south-western margin of the Kaapvaal craton in South Africa can be subdivided into eclogites sensu stricto, kyanite eclogites and orthopyroxene eclogites, all containing omphacite, and garnet clinopyroxenites and garnet websterites characterised by diopside. Texturally, chemically (major elements) and thermally, we observe an evolution from garnet websterites (TEG = 742–781 °C) towards garnet clinopyroxenites (TEG = 715–830 °C) and to eclogites (TEG = 707–1056 °C, mean value of 913 °C). Pressures calculated for orthopyroxene-bearing samples suggest upper mantle conditions of equilibration (P = 16–33 kb for the garnet websterites, 18 kb for a garnet clinopyroxenite and 23 kb for an opx-bearing eclogite). The overall geochemical similarity between the two groups of xenoliths (omphacite-bearing and diopside-bearing) as well as the similar trace element patterns of clinopyroxenes and garnet suggest a common origin for these rocks. Recently acquired oxygen isotope data on garnet (δ18Ognt = 5.25–6.78 ‰ for eclogites, δ18Ognt = 5.24–7.03 ‰ for garnet clinopyroxenites) yield values ranging from typical mantle values to other interpreted as resulting from low-temperature alteration or precursors sea-floor basalts and associated rocks. These rocks could then represent former magmatic oceanic rocks that crystallised from a same parental magma as plagioclase free diopside-bearing and plagioclase-bearing crustal rocks. During subduction, these oceanic rock protoliths equilibrated at mantle depth, with the plagioclase-bearing rocks converting to omphacite and garnet-bearing lithologies (eclogites sensu largo), whereas the plagioclase-free diopside-bearing rocks converted to diopside and garnet-bearing lithologies (garnet websterites and garnet clinopyroxenites).  相似文献   

15.
A suite of 14 diamond-bearing and 3 diamond-free eclogite xenoliths from the Newlands kimberlite, South Africa, have been studied using the Re–Os isotopic system to provide constraints on the age and possible protoliths of eclogites and diamonds. Re concentrations in diamond-bearing eclogites are variable (0.03–1.34 ppb), while Os concentrations show a much more limited range (0.26–0.59 ppb). The three diamond-free eclogites have Re and Os concentrations that are at the extremes of the range of their diamond-bearing counterparts. 187Os/188Os ranges from 0.1579 to 1.4877, while 187Re/188Os varies from 0.54 to 26.2 in the diamond-bearing eclogites. The highly radiogenic Os in the diamond-bearing eclogites (γOs=23–1056) is consistent with their high 187Re/188Os and requires long-term isolation from the convecting mantle. Re–Os model ages for 9 out of 14 diamond-bearing samples lie between 3.08 and 4.54 Ga, in agreement with FTIR spectra of Newlands diamonds that show nitrogen aggregation states consistent with diamond formation in the Archean. Re–Os isochron systematics for the Newlands samples do not define a precise isochron relationship, but lines drawn between subsets of the data provide ages ranging from 2.9 to 4.1 Ga, all of which are suggestive of formation in the Archean. The Re–Os systematics combined with mineral chemistry and stable isotopic composition of the diamond-bearing eclogites are consistent with a protolith that has interacted with surficial environments. Therefore, the favored model for the origin of the Newlands diamond-bearing eclogites is via subduction. The most likely precursors for the Kaapvaal eclogites include komatiitic ocean ridge products or primitive portions of oceanic plateaus or ocean islands.  相似文献   

16.
Eclogites are often the only tangible high-pressure evidence we have from a paleosubduction zone, and they potentially preserve important geochemical information from the descending slab. Selected Group B/C eclogites and metapelites from the Trescolmen locality in the Adula nappe in the central Swiss Alps were chosen for a detailed investigation to determine oxygen isotope ratios and major- and trace-element compositions of the main rock-forming minerals. Complex major-element zonation patterns in garnet porphyroblasts indicate a pre-Alpine, medium-pressure growth history coupled with a high-pressure modification during the Alpine orogeny. Garnet REE patterns are notably HREE depleted in rim regions, with high overall REE content, particularly in the cores of grains. Omphacites are relatively homogenous in major elements, and show LREE- and HREE-depleted patterns, but with overall abundances of REEs lower than in garnets. These patterns are best explained by partitioning of the HREEs into garnet and the LREEs into zoisite. Oxygen-isotope systematics indicate limited fluid flow in eclogites and surrounding metapelites. δ18O values of quartz and garnet at the interface between eclogites and metapelites are indistinguishable and point to fluid exchange. Oxygen equilibrium conditions among rock-forming minerals, particularly between quartz and garnet in eclogites and metapelites, were reached, and fractionation indicates temperatures of ～600°C. The δ18O of unaltered eclogites (5.5 to 7.5 ‰) suggests a basaltic, MORB-type protolith.  相似文献   

17.
P. Peltonen  K. A. Kinnunen  H. Huhma 《Lithos》2002,63(3-4):151-164
Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.  相似文献   

18.
A comprehensive study of 26 mafic mantle xenoliths from the Udachnaya kimberlite pipe was carried out. The contents of major and trace elements, equilibrium temperature parameters, and water content in the rock-forming minerals were determined. The temperatures of formation of the studied rocks are estimated at 800–1300 °C. According to IR spectroscopy data, the water content in clinopyroxenes from the studied eclogites varies from values below the detection limit to 99 ppm. The IR spectra of garnets lack bands of water. The water content in clinopyroxene and orthopyroxene from garnet websterite is 72 and 8 ppm, respectively. The water content in the average rock, calculated from the ratio of the rock-forming minerals, varies from a few to 55 ppm. No relationship among the water content, equilibrium temperatures, and rock composition is established. The low water contents in the eclogites are close to the earlier determined water contents in peridotites from the same pipe and are, most likely, due to the re-equilibration of the eclogites with the rocks of the peridotitic lithospheric mantle. The dehydration of the protolith during its subduction and the partial melting of eclogites before their removal by kimberlitic magma to the surface might be an additional cause of the low water contents in the mantle eclogite xenoliths.  相似文献   

19.
 Diamond-bearing eclogites are an important component of the xenoliths that occur in the Mir kimberlite, Siberian platform, Russia. We have studied 16 of these eclogite xenoliths, which are characterized by coarse-grained, equigranular garnet and omphacite. On the basis of compositional variations in garnet and clinopyroxene, this suite of eclogites can be divided into at least two groups: a high-Ca group and a low-Ca group. The high-Ca group consists of high-Ca garnets in equilibrium with pyroxenes that have high Ca-ratios [Ca/(Ca+Fe+Mg)] and high jadeite contents. These high-Ca group samples have high modal% garnet, and garnet grains often are zoned. Garnet patches along rims and along amphibole- and phlogopite-filled veins have higher Mg and lower Ca contents compared to homogeneous cores. The low-Ca group consists of eclogites with low-Ca garnets in equilibrium with pyroxenes with a low Ca-ratio, but variable jadeite contents. These low-Ca group samples typically have low modal% of garnet, and garnets are rarely compositionally zoned. Three samples have mineralogic compositions and modes transitional to the high- and low-Ca groups. We have arbitrarily designated these samples as the intermediate-Ca group. The rare-earth-element (REE) contents of garnet and clinopyroxene have been determined by ion microprobe. Garnets from the low-Ca group have low LREE contents and typically have [Dy/Yb]n < 1. The high-Ca group garnets have higher LREE contents and typically have [Dy/Yb]n > 1. Garnets from the intermediate-Ca group have REE contents between the high- and low-Ca groups. Clinopyroxenes from the low-Ca group have convex-upward REE patterns with relatively high REE contents (ten times chondrite), whereas those from the high-Ca group have similar convex-upward shapes, but lower REE contents, approximately chondritic. Reconstructed bulk-rock REE patterns for the low-Ca group eclogites are relatively flat at approximately ten times chondrite. In contrast, the high-Ca group samples typically have LREE-depleted patterns and lower REE contents. The δ18O values measured for garnet separates range from 7.2 to 3.1‰. Although there is a broad overlap of δ18O between the low-Ca and high-Ca groups, the low-Ca group samples range from mantle-like to high δ18O values (4.9 to 7.2‰), and the high-Ca group garnets range from mantle-like to low δ18O values (5.3 to 3.1‰). The oxygen isotopic compositions of two of the five high-Ca group samples and four of the eight low-Ca group eclogites are consistent with seawater alteration of basaltic crust, with the low-Ca group eclogites representative of low-temperature alteration, and the high-Ca group samples representative of high-temperature hydrothermal seawater alteration. We interpret the differences between the low- and high-Ca group samples to be primarily a result of differences in the protoliths of these samples. The high-Ca group eclogites are interpreted to have protoliths similar to the mid to lower sections of an ophiolite complex. This section of oceanic crust would be dominated by rocks which have a significant cumulate component and would have experienced high-temperature seawater alteration. Such cumulate rocks probably would be LREE-depleted, and can be Ca-rich because of plagioclase or clinopyroxene accumulation. The protoliths of the low-Ca group eclogites are interpreted to be the upper section of an ophiolite complex. This section of oceanic crust would consist mainly of extrusive basalts that would have been altered by seawater at low temperatures. These basaltic lavas would probably have relatively flat REE patterns, as seen for the low-Ca group eclogites. Received: 10 July 1995 / Accepted: 17 May 1996  相似文献   

20.
Petrographic and geochemical features of a suite of eclogite xenoliths from the Rietfontein kimberlite that erupted through probable Proterozoic crust west of the Kaapvaal Craton in the far Northern Cape region of South Africa, are described. Group II eclogites dominate the suite both texturally and chemically, but can be subdivided into bimineralic, opx-bearing and kyanite-bearing groups. Temperature estimates from different geothermometers range from 700 to 1,000°C, indicating derivation from relatively shallow mantle depths. Orthopyroxene-bearing eclogites are inferred to originate from depths of 85 to 115 km and lie close to the average cratonic thermal profile for southern Africa. These uppermost mantle temperatures during the late Cretaceous provide evidence for equilibration of the off-craton lithosphere to craton-like thermal conditions following Namaqua-Natal orogenesis. The kyanite eclogites are distinct from the remaining eclogites in terms of both major and trace element compositions and their lesser degree of alteration. Garnets are richer in Ca, and are Cr-depleted relative to garnets from the bimineralic and opx-bearing eclogites, which tend to be more magnesian. Clinopyroxenes from the kyanite eclogites are more sodic, with higher Al2O3 and lower MgO contents than the bimineralic and opx-bearing eclogites. LREE-depletion, positive Sr and Eu anomalies, and the Al-rich, Si-poor bulk composition suggest a plagioclase-rich, probably troctolitic protolith for the kyanite eclogites. In contrast, the major and trace element bulk compositions of the high-MgO bimineralic and orthopyroxene-bearing eclogites are consistent with gabbroic or pyroxenitic precursors, or high-pressure cumulates, rather than mafic to ultramafic lavas. δ18O values for garnets do not deviate significantly from typical mantle values. The observations reported do not discriminate unambiguously between continental and oceanic origins for the various eclogite components in the mantle lithosphere of this region.  相似文献   

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