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1.
Kimberlites with different diamond grades from the Zolotitsa, Verkhotina, and Kepina occurrences of the Zimny Bereg field (Arkangel’sk oblast) have been compared in order to ascertain geochemical criteria of their diamond resource potential. A new collection of 21 core samples taken within a depth interval of 207–940 m from nine boreholes drilled in the central and western portions of the high-grade diamond-bearing Grib kimberlite pipe was subjected to comprehensive petrographic and geochemical examination, including Sr, Nd, and Pb isotopes and trace elements determined with ICP-MS. The compositional variations in kimberlites are controlled by the structural types of rocks. Porphyritic kimberlite (PK) distinctly differs from autolithic kimberlite breccia (AKB). Autoliths (Av) and PK are enriched in Th, U, Nb, Ta, La, Ce, Pr, P, Nd, Sm, Eu, Ti, LREE, and MREE, whereas HREE contents are rather uniform in all types of kimberlites. No lateral zoning was observed in pipes pertaining to the same structural type. The composition of kimberlites in the Zimny Bereg field and their diamond resource potential are variable. In the series of the Zolotitsa, Verkhotina, and Kepina occurrences, the Ti content increases, the La/Yb ratio grows from 18–44 to 70–130, and the diamond grade diminishes in the Kepina occurrence. The variations in kimberlite compositions are considered in terms of the degree of partial melting in the mantle, the role of volatiles, etc. As follows from the variation in the Ce/Y ratio, kimberlites from the Zolotitsa occurrence were formed at a lower degree of partial melting in comparison with the Kepina occurrence. Products of different degrees of partial melting are recognized within the Grib pipe; Av were likely formed at a somewhat higher degree of melting than AKB. An appreciable isotopic heterogeneity of the mantle is recorded in variable Nd and Sr isotopic compositions of kimberlites. The Kepina kimberlites were derived from a source slightly depleted relative to CHUR (?Nd(t) reaches +4) and are close to kimberlites of group I in South Africa. Kimberlites from the Grib pipe with transitional Nd isotopic composition plotted near the Bulk Silicate Earth (BSE) value in the ?Nd(t)-?Sr(t) diagram adjoin the first group. The source of kimberlites of the Zolotitsa occurrence falls in the field of enriched mantle and is considered to be a product of interaction of an asthenospheric plume with the ancient enriched lithospheric mantle. Kimberlites depleted in Ti, Zr, and Th are related to a source formed as a result of a multistage process that included mantle metasomatism with participation of fluids. Devonian kimberlites derived from sources that involve crustal material (a shift of 206Pb/204Pb, minimums of Th, U, Nb, and Ta contents) are diamond-bearing both in the East European Platform (the Zolotitsa and Verkhotina occurrences) and in the Siberian Craton (the Nakyn field).  相似文献   

2.
Minor magmatic intrusions of kimberlite, melilitite and cpx-melilitite occur in the southern part of the Kola Peninsula, Russia, on the Terskii Coast and near the town of Kandalaksha. They yield K-Ar ages of 382 ± 14 Ma and 365 ± 16 Ma, similar to the magmatic rocks from the Kola Alkaline Province. The Terskii Coast kimberlites have mineralogical and geochemical affinities with group 1 kimberlites, whereas the Kandalaksha monticellite kimberlite more closely resembles calcite kimberlites. The lower Al2O3 content in the Kola kimberlites indicates a strongly depleted harzburgitic source, while higher Al2O3 in the melilitites suggests a lherzolitic source. The Terskii Coast kimberlites are anomalously potassic and significantly enriched in P and Ba compared to other group 1 kimberlites. In contrast, the melilitites are sodic and are anomalously depleted in P compared to worldwide melilitites. Trace element patterns of the Kola kimberlites and melilitites indicate the presence of K- and P-rich phases in the mantle source. To account for the K-troughs shown by both magma types, a K-rich phase such as phlogopite is thought to be residual in their sources; however, the anomalous K-enrichment in the Terskii Coast kimberlites may indicate that an additional metasomatic K-rich phase (e.g. K-richterite and/or a complex K-Ba-phosphate) existed in the kimberlite source. The P-depletion in the melilitites may suggest that a phosphate phase such as apatite remained residual in the melilititic source. However, anomalous P-enrichment in the kimberlites cannot be explained by complete melting of the same phase because the kimberlites are a smaller degree melt; thus, it is most likely that another metasomatic phosphate mineral existed in the source of the kimberlites. The Kola kimberlites and melilitites are all strongly LREE-enriched but the kimberlites have a steeper REE pattern and are significantly more depleted in HREE, indicating a higher proportion of garnet in their source. Higher Nb/Y ratios and lower SiO2 values in the kimberlites indicate that they were a smaller degree partial melt than the melilitites. The presence of diamonds in the Terskii Coast kimberlites indicates a relatively deep origin, while the melilitites originated from shallower depth. The non-diamondiferous Kandalaksha monticellite kimberlite has lower abundances of all incompatible trace elements, suggesting a higher degree of partial melting and/or a less enriched and shallower source than the Terskii Coast kimberlites. The 87Sr/86Sri, 143Nd/144Ndi and Pb isotope compositions confirm that the Terskii Coast kimberlites have close affinities with group 1 kimberlites and were derived from an asthenospheric mantle source, while the Kandalaksha monticellite kimberlite and Terskii Coast melilitites were derived from lithospheric mantle. Impact of a Devonian asthenospheric mantle plume on the base of the Archaean-Proterozoic lithosphere of the Kola Peninsula caused widespread emplacement of kimberlites, melilitites, ultramafic lamprophyres and other more fractionated alkaline magmas. The nature of the mantle affected by metasomatism associated with the plume and, in particular, the depth of melting and the stability of the metasomatic phases, gave rise to the observed differences between kimberlites and the related melilitites and other magmas. Received: 3 March 1997 / Accepted: 7 October 1997  相似文献   

3.
The Late Cretaceous (ca. 100 Ma) diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan (Sask) craton, Canada, is one of the largest known kimberlite fields on Earth comprising essentially pyroclastic kimberlites. Despite its discovery more than two decades ago, petrological, geochemical and petrogenetic aspects of the kimberlites in this field are largely unknown. We present here the first detailed petrological and geochemical data combined with reconnaissance Nd isotope data on drill-hole samples of five major kimberlite bodies. Petrography of the studied samples reveals that they are loosely packed, clast-supported and variably sorted, and characterised by the presence of juvenile lapilli, crystals of olivine, xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite. Interclast material is made of serpentine, phlogopite, spinel, carbonate, perovskite and rutile. The mineral compositions, whole-rock geochemistry and Nd isotopic composition (Nd: + 0.62 to − 0.37) are indistinguishable from those known from archetypal hypabyssal kimberlites. Appreciably lower bulk-rock CaO (mostly < 5 wt%) and higher La/Sm ratios (12–15; resembling those of orangeites) are a characteristic feature of these rocks. Their geochemical composition excludes any effects of significant crustal and mantle contamination/assimilation. The fractionation trends displayed suggest a primary kimberlite melt composition indistinguishable from global estimates of primary kimberlite melt, and highlight the dominance of a kimberlite magma component in the pyroclastic variants. The lack of Nb-Ta-Ti anomalies precludes any significant role of subduction-related melts/fluids in the metasomatism of the FALC kimberlite mantle source region. Their incompatible trace elements (e.g., Nb/U) have OIB-type affinities whereas the Nd isotope composition indicates a near-chondritic to slightly depleted Nd isotope composition. The Neoproterozoic (~ 0.6–0.7 Ga) depleted mantle (TDM) Nd model ages coincide with the emplacement age (ca. 673 Ma) of the Amon kimberlite sills (Baffin Island, Rae craton, Canada) and have been related to upwelling protokimberlite melts during the break-up of the Rodinia supercontinent and its separation from Laurentia (North American cratonic shield). REE inversion modelling for the FALC kimberlites as well as for the Jericho (ca. 173 Ma) and Snap Lake (ca. 537 Ma) kimberlites from the neighbouring Slave craton, Canada, indicate all of their source regions to have been extensively depleted (~ 24%) before being subjected to metasomatic enrichment (1.3–2.2%) and subsequent small-degree partial melting. These findings are similar to those previously obtained on Mesozoic kimberlites (Kaapvaal craton, southern Africa) and Mesoproterozoic kimberlites (Dharwar craton, southern India). The striking similarity in the genesis of kimberlites emplaced over broad geological time and across different supercontinents of Laurentia, Gondwanaland and Rodinia, highlights the dominant petrogenetic role of the sub-continental lithosphere. The emplacement of the FALC kimberlites can be explained both by the extensive subduction system in western North America that was established at ca. 150 Ma as well as by far-field effects of the opening of the North Atlantic ocean during the Late Cretaceous.  相似文献   

4.
The Jurassic to Early Cretaceous magmatic arc of the Andes in northern Chile was a site of major additions of juvenile magmas from the subarc mantle to the continental crust. The combined effect of extension and a near stationary position of the Jurassic to lower Cretaceous arc favoured the emplacement and preservation of juvenile magmatic rocks on a large vertical and horizontal scale. Chemical and Sr, Nd, and Pb isotopic compositions of mainly mafic to intermediate volcanic and intrusive rock units coherently indicate the generation of the magmas in a subduction regime and the dominance of a depleted subarc mantle source over contributions of the ambient Palaeozoic crust. The isotopic composition of the Jurassic (206Pb/204Pb: ∼ 18.2; 207Pb/204Pb: ∼ 15.55; 143Nd/144Nd: ∼ 0.51277; 87Sr/86Sr: ∼ 0.703–0.704) and Present (206Pb/204Pb: ∼ 18.5; 207Pb/204Pb: ∼ 15.57; 143Nd/144Nd: ∼ 0.51288; 87Sr/86Sr: ∼ 0.703–0.704) depleted subarc mantle beneath the Central and Southern Andes (18°–40°S) was likely uniform over the entire region. Small differences of isotope ratios between Jurassic and Cenozoic to Recent of subarc mantle-derived could be explained by radiogenic growth in a still uniform mantle source.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .  相似文献   

5.
The REE and Pb, Sr, Nd isotopes in three xenoliths from limburgite and scoria-breccias, including spinel-lherzolite, spinel-garnet-lherzolite and phlogopite-gamet-lherzolite, were analysed. The REE contents of the xenoliths are 1.3 to 3.3 times those of the chondrites with their REE patterns characterized by weak LREE depletion. The143Nd/144Nd values of whole rocks and minerals range from 0.51306 to 0.51345 with εNd=+ 8.2− +15.8,206Pb/204 Pb < 18.673, and207Pb/204Pb < 15.574. All this goes to show that the upper mantle in Mingxi at the depth of 67–82 km is a depleted mantle of MORB type, with87Sr/86 Sr ratios 0.70237–0.70390. In Nd-Sr diagram the data points of whole rocks are all out of the mantle array, implying that the xenoliths from Mingxi have more radiogenic Sr isotopes than those of the mantle array.  相似文献   

6.
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   

7.
High-K mafic alkalic lavas (5.4 to 3.2 wt% K2O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; 87Sr/86Sr ratios decrease and ɛNd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; 87Sr/86Sr ratios range from 0.7121 to 0.7105 and ɛNd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, 206Pb/204Pb ∼17.2, 207Pb/204Pb ∼15.5, and 208Pb/204Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛNd values and unradiogenic 206Pb/204Pb ratios have relatively low 87Sr/86Sr ratios (the EM I array), whereas basalts with low ɛNd values and high 87Sr/86Sr ratios have radiogenic 206Pb/204Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high 87Sr/86Sr and 208Pb/204Pb ratios and low ɛNd values and 206Pb/204Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998  相似文献   

8.

Here we present new data from a systematic Sr, Nd, O, C isotope and geochemical study of kimberlites of Devonian age Mirny field that are located in the southernmost part of the Siberian diamondiferous province. Major and trace element compositions of the Mirny field kimberlites show a significant compositional variability both between pipes and within one diatreme. They are enriched in incompatible trace elements with La/Yb ratios in the range of (65–300). Initial Nd isotope ratios calculated back to the time of the Mirny field kimberlite emplacement (t = 360 ma) are depleted relative to the chondritic uniform reservoir (CHUR) model being 4 up to 6 ɛNd(t) units, suggesting an asthenospheric source for incompatible elements in kimberlites. Initial Sr isotope ratios are significantly variable, being in the range 0.70387–0.70845, indicating a complex source history and a strong influence of post-magmatic alteration. Four samples have almost identical initial Nd and Sr isotope compositions that are similar to the prevalent mantle (PREMA) reservoir. We propose that the source of the proto-kimberlite melt of the Mirny field kimberlites is the same as that for the majority of ocean island basalts (OIB). The source of the Mirny field kimberlites must possess three main features: It should be enriched with incompatible elements, be depleted in the major elements (Si, Al, Fe and Ti) and heavy rare earth elements (REE) and it should retain the asthenospheric Nd isotope composition. A two-stage model of kimberlite melt formation can fulfil those requirements. The intrusion of small bodies of this proto-kimberlite melt into lithospheric mantle forms a veined heterogeneously enriched source through fractional crystallization and metasomatism of adjacent peridotites. Re-melting of this source shortly after it was metasomatically enriched produced the kimberlite melt. The chemistry, mineralogy and diamond grade of each particular kimberlite are strongly dependent on the character of the heterogeneous source part from which they melted and ascended.

  相似文献   

9.
In the Mediterranean area, lamproitic provinces in Spain, Italy, Serbia and Macedonia have uniform geological, geochemical and petrographic characteristics. Mediterranean lamproites are SiO2-rich lamproites, characterized by relatively low CaO, Al2O3 and Na2O, and high K2O/Al2O3 and Mg-number. They are enriched in LILE relative to HFSE and in Pb, and show depletion in Ti, Nb and Ta. Mediterranean lamproites show huge regional variation of Sr, Nd and 207Pb/204Pb isotopic values, with 87Sr/86Sr range of 0.707-0.722, εNd range from −13 to −3, and 207Pb/204Pb range of 15.62-15.79.Lamproitic rocks are derived from melts with three components involved in their origin, characterized by contrasting geochemical features which appear in 206Pb/204Pb, 87Sr/86Sr and 143Nd/144Nd space: (i) a mantle source contaminated by crustal material, giving rise to crust-like trace element patterns and radiogenic isotope systematics, (ii) an extremely depleted mantle characterized by very low whole-rock CaO and Al2O3, high-Fo olivine and Cr-rich spinel, which isotopically resembles European peridotitic massifs and lithospheric mantle; (iii) a component originating from the convecting mantle, characterized by unradiogenic 87Sr/86Sr and radiogenic 143Nd/144Nd and 206Pb/204Pb. These components demand multistage preconditioning of the lamproite-mantle source, involving an episode of extreme depletion, followed by involvement of terrigenous sediments, and finally interaction with melts originating from the convecting mantle, some of which are probably carbonatitic.We use our data on Mediterranean lamproites to characterize the mantle composition under the whole Alpine-Himalaya belt. Lamproites are an integral part of postcollisional volcanism, and are the most extreme melting products from a mantle which is ubiquitously crustally metasomatized. Enriched isotope signatures in Himalayan volcanics can also be explained by the involvement of subducted sediments instead of by proterozoic mantle lithosphere.  相似文献   

10.
Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton, have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich (~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). 87Sr/86Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and 176Hf/177Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich (850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched in incompatible elements (La/Ybn ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1) and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during the Proterozoic by calc-alkaline fluids/melts.  相似文献   

11.
Using the ICP-MS method we have studied the isotope systematics of Sr and Nd as well as trace element composition of a representative collection of kimberlites and related rocks from the Siberian Platform. The summarized literature and our own data suggest that the kimberlites developed within the platform can be divided into several petrochemical and geochemical types, whose origin is related to different mantle sources. The petrochemical classification of kimberlites is based on persistent differences of their composition in mg# and in contents of indicator oxides such as FeOtot, TiO2, and K2O. The recognized geochemical types of kimberlites differ from one another in the level of concentration of incompatible elements as well as in their ratios.Most of isotope characteristics of kimberlites and related rocks of the Siberian Platform correspond to the earlier studied Type 1 basaltoid kimberlites from different provinces of the world: Points of isotopic compositions are in the field of primitive and weakly depleted mantle. An exception is one sample of the rocks from veins of the Ingashi field (Sayan area), which is characterized by the Sr and Nd isotopic composition corresponding to Type 2 micaceous kimberlites (orangeites).The most important feature of distribution of isotopic and trace-element compositions (incompatible elements) is their independence of the chemical rock composition. It is shown that the kimberlite formation is connected with, at least, two independent sources, fluid and melt, responsible for the trace-element and chemical compositions of the rock. It is supposed that, when rising through the heterogeneous lithosphere of the mantle, a powerful flow of an asthenosphere-derived fluid provoked the formation of local kimberlite chambers there. Thus, the partial melting of the lithosphere mantle led to the formation of contrasting petrochemical types of kimberlites, while the geochemical specialization of kimberlites is due to the mantle fluid of asthenosphere origin, which drastically dominated in the rare-metal balance of a hybrid magma of the chamber.  相似文献   

12.
The Emeishan flood basalts can be divided into high-Ti (HT) basalt (Ti/Y>500) and low-Ti (LT) basalt (Ti/Y<500). Sr, Nd isotopic characteristics of the lavas indicate that the LT- and the HT-type magmas originated from distinct mantle sources and parental magmas. The LT-type magma was derived from a shallower lithospheric mantle, whereas the HT-type magma was derived from a deeper mantle source that may be possibly a mantle plume. However, few studies on the Emeishan flood basalts involved their Pb isotopes, especially the Ertan basalts. In this paper, the authors investigated basalt samples from the Ertan area in terms of Pb isotopes, in order to constrain the source of the Emeishan flood basalts. The ratios of 206Pb/204Pb (18.31–18.41), 207Pb/204Pb (15.55–15.56) and 208Pb/204Pb (38.81–38.94) are significantly higher than those of the depleted mantle, just lying between EM I and EM II. This indicates that the Emeishan HT basalts (in the Ertan area) are the result of mixing of EMI end-member and EMII end-member.  相似文献   

13.
Petrological and geochemical data obtained on Neogene magmatism restricted to a deep fault in Andree Land at Spitsbergen Island, which was related to the overall restyling of the Arctic territory at 25–20 Ma, indicate that the derivation of the Neogene magmas was significantly affected by the continental pyroxenite mantle. The Neogene basalts are noted for a radiogenic isotopic composition of Pb (207Pb/204Pb= 15.5–15.55, 206Pb/204Pb = 18.4–18.6, 208Pb/204 Pb = 38.4–38.6) and Sr(87Sr/86Sr = 0.7038–0.7048) at low 143Nd/144Nd = 0.5129. Melts of this type are the extremely enriched end member of the isotopic mixing of a depleted and enriched sources and determine a geochemical trend that passes through the compositions of alkaline magmas from Quaternary volcanoes at Spitsbergen and weakly enriched tholeiites of the Knipovich Ridge, which started to develop simultaneously with the onset of Neogene magmatism in the western part of Spitsbergen. The composition of the liquidus olivine (which is rich in NiO) indicates that melting occurred in the olivine-free mantle. Our data thus testify that a significant role in the genesis of the Neogene magmas was played by continental pyroxenite mantle.  相似文献   

14.
Mica kimberlite and alkali picrite were identified in the northwestern Urik-Iya Graben of the eastern Sayan region. Typomorphism of Cr-diopside and high-Cr (up to 55.22 wt % Cr2O3) spinel from kimberlite of the Bushkanai dike indicate that the melt was generated in the mantle, composed of spinel peridotite. The high content of Cr-spinel (45–55 wt % Cr2O3) microlites in the groundmass of kimberlite and small amounts of ulvospinel and titanomagnetite in the absence of perovskite testifies to the diamond potential of this kimberlite. Picroilmenite, manganoilmenite with an anomalously high MnO content (11.37–17.78 wt %), and barium titanate with (wt %) 62.21 TiO2, 0.61 Cr2O3, 15.89 FeO, 4.05 MnO, 1.71 CaO, and 11.13 BaO close in composition to a new mineral species from the Murun pluton were identified in the groundmass for the first time. Kimberlite from the Bushkanai dike belongs to the Zolotitsa low-Ti geochemical type of kimberlites derived from the slightly enriched lithospheric mantle EM1. The distribution of trace elements, including REE, in picrite from the same dike corresponds to the slightly depleted asthenospheric mantle. Different mantle sources of kimberlite and picrite from the same dike indicate that these rocks are related to independent melts rather than to products of fractionation of a common parental alkaline ultramafic magma.  相似文献   

15.
During serpentinization and subsequent alteration in the absence of brucite, kimberlites accumulate uncompensated silica. Its amount can be calculated from the average compositions of the rock-forming minerals (olivine, calcite, phlogopite) and the chemical compositions of the rocks. The contents of rock-forming oxides and REE were determined in 12 kimberlite pipes of the Yakutian kimberlite province, in 413 samples from secondary-alteration zones and of unaltered kimberlites. Columns of successive kimberlite alterations were constructed for each pipe on the basis of secondary-quartz data; here, the behavior of rock-forming oxides and REE was assessed. All the studied rocks had experienced different degrees of postmagmatic hydrothermal metasomatism at different depths in all the pipes. The changes were reflected in the supply/loss of rock-forming oxides and REE. The supply of REE during the hydrothermal metasomatism enriched the kimberlites with TiO2, P2O5, and CaO. During the removal of REE, most of the rock-forming oxides were partially lost. The maximum REE supply was 67% in the Udachnaya-Vostochnaya pipe and 59% in the Nyurbinskaya pipe as compared with the unaltered kimberlites. The maximum REE loss was 87% in the Aikhal pipe and 81% in the Internatsional’naya pipe as compared with the unaltered kimberlites. The initial REE contents of the postmagmatically altered kimberlites changed considerably in all the studied cases. This conclusion was drawn owing to the use of normative-quartz content as a criterion for secondary alteration.  相似文献   

16.
We report the first estimates of primary kimberlite melt compositionfrom the Slave craton, based on samples of aphanitic kimberlitefrom the Jericho kimberlite pipe, N.W.T., Canada. Three samplesderive from the margins of dykes where kimberlite chilled againstwall rock (JD51, JD69 and JD82) and are shown to be texturallyconsistent with crystallization from a melt. Samples JD69 andJD82 have geochemical characteristics of primitive melts: theyhave high MgO (20–25 wt %), high mg-numbers (86–88),and high Cr (1300–1900 ppm) and Ni (800–1400 ppm)contents. They also have high contents of CO2 (10–17 wt%). Relative to bulk macrocrystal kimberlite, they have lowermg-numbers and lower MgO but are enriched in incompatible elements(e.g. Zr, Nb and Y), because the bulk kimberlite compositionsare strongly controlled by accumulation of mantle olivine andother macrocrysts. The compositions of aphanitic kimberlitefrom Jericho are similar to melts produced experimentally bypartial melting of a carbonate-bearing garnet lherzolite. Onthe basis of these experimental data, we show that the primarymagmas from the Jericho kimberlite could represent 0·7–0·9%melting of a carbonated lherzolitic mantle source at pressuresand temperatures found in the uppermost asthenosphere to theSlave craton. The measured CO2 contents for samples JD69 andJD82 are only slightly lower than the CO2 contents of the correspondingexperimental melts; this suggests that the earliest hypabyssalphase of the Jericho kimberlite retained most of its originalvolatile content. As such these samples provide a minimum CO2content for the primary kimberlite magmas from the Slave craton. KEY WORDS: kimberlite; melt; primitive; primary magma; Slave craton  相似文献   

17.
K. J. Fraser  C. J. Hawkesworth   《Lithos》1992,28(3-6):327-345
Major, trace element and radiogenic isotope results are presented for a suite of hypabyssal kimberlites from a single pipe, at the Finsch Mine, South Africa. These are Group 2 kimberlites characterised by abundant phlogopite ± serpentine ± diopside; they are ultrabasic (SiO2 < 42 wt.%%) and ultrapotassic (K2O/Na2O > 6.9) igneous rocks, they exhibit a wide range in major element chemistry with SiO2 = 27.6−41.9 wt. % and MgO = 10.4−33.4 wt. %. (87Sr/86Sr)i=0.7089 to 0.7106, εNd is −6.2 to −9.7 and they have unradiogenic (207Pb/204Pb)i contents which ensure that they plot below the Pb-ore growth curve. They have high incompatible and compatible element contents, a striking positive array between Y and Nb which indicates that garnet was not involved in the within suite differentiation processes, and a negative trend between K/Nb and Nb contents which suggests that phlogopite was involved. In addition, some elements exhibit an unexpected order of relative incompatibility for different trace elements which suggests that the intra-kimberlite variations are not primarily due to variations in the degree of partial melting. The effects of fractional crystallization are difficult to establish because for the most part they have been masked by the entrainment of 50–60% mantle peridotite. Thus, the Finsch kimberlites are interpreted as mixtures of a melt component and entrained garnet peridotite, with no evidence for significant contamination with crustal material. The melt component was characterised by high incompatible element contents, which require both very small degrees of partial melting, and source regions with higher incompatible element contents than depleted or primitive mantle. Since the melt component was the principal source of incompatible elements in the kimberlite magma, the enriched Nd, Sr and Pb isotope ratios of the kimberlite are characteristic of the melt source region. The melt fractions were therefore derived from ancient, trace elements enriched portions of the upper mantle, most probably situated within the sub-continental mantle lithosphere, and different from the low 87Sr/86Sr garnet peridotite xenoliths found at Finsch. Within the sub-continental mantle lithosphere old, incompatible element enriched source regions for the kimberlite melt fraction are inferred to have been overlain by depleted mantle material which became entrained in the kimberlite magma.  相似文献   

18.
Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos Plate were also compiled. In the MVB the isotopic ratios range as follows:87Sr/86Sr 0.703003-0.70841;143Nd/144Nd 0.512496-0.513098;206Pb/204Pb 18.567-19.580;207Pb/204Pb 15.466-15.647;208Pb/204Pb 38.065-38.632. The SMO shows a large variation in87Sr/86Sr ranging from ∼0.7033 to 0.71387.143Nd/144Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows:206Pb/204Pb 18.060-18.860;207Pb/204Pb 15.558-15.636;208Pb/204Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261 for Nd). Pb isotopes for PacOc show the following range:206Pb/204Pb 18.049-19.910;207Pb/2047Pb 15.425-15.734;208Pb/204Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc. Most samples with reported Sr and Nd isotopic data are spread within and around the “mantle array”. The SMO seems to have been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data in all Mexican magmatic provinces and of Nd isotopes in BC, AP, SMS, and SMOr.  相似文献   

19.
The Denizli region of the Western Anatolia Extensional Province (WAEP) includes a typical example of intra-plate potassic magmatism. Lamproite-like K-rich to shoshonitic alkaline rocks erupted in the Upper Miocene-Pliocene in a tensional tectonic setting. The absence of Nb and Ta depletion, low Th/Zr and high Nb/Zr ratios and distinct isotopic values (i.e. low 87Sr/86Sr, 0.703523–0.703757; high 143Nd/144Nd, 0.512708–0.512784; high 206Pb/204Pb, 19.079–19.227, 207Pb/204Pb, 15.635–15.682, 208Pb/204Pb, 39.144–39.302) mark an anorogenic geochemical signature of the Denizli volcanics. All of the lavas are strongly enriched in large-ion-lithophile elements (e.g. Ba 1,100–2,200 ppm; Sr 1,900–3,100 ppm; Rb 91–295 ppm) and light rare-earth elements (e.g. LaN?=?319–464), with a geochemical affinity to ocean-island basalts and lack of a recognizable subduction signature or any evidence for crustal contamination. The restricted range of isotopic (Sr, Nd, Pb) ratios in both near-primitive (Mg# 66.7–77.2) and more evolved (Mg# 64.6–68.7) members of the Denizli volcanics signify their evolution from an isotopically equilibrated parental mantle source. Their high Dy/Yb and Rb/Sr values also suggest that garnet and phlogopite were present in the mantle source. Their strong EM-II signature, very low Nd model ages (0.44–049 Ga) and isotopic (Sr-Nd-Pb) values analogous to those of the Nyiragongo potassic basanites and kimberlites from the African stable continental settings, suggest that the parental melts that produced the Denizli volcanics are associated with very young and enriched mantle sources, which include both sublithospheric and enriched subcontinental lithospheric mantle melts. Mantle-lithosphere delamination probably played a significant role in the generation of these melts, and could be related to roll-back of the Aegean arc, lithospheric extension and asthenospheric mantle upwelling.  相似文献   

20.
The Qinling Mountains in Central China mark a gigantic composite orogenic belt with a complex tectonic evolution involving multiple phases of rifting and convergence. This belt separates the North China and South China Blocks and consists of the South and North Qinling terranes separated by the Shangdan suture. The suture is marked by the Grenvillian Songshugou ophiolite along the southern margin of the North Qinling terrane, which is key to understanding the Proterozoic tectonic evolution of the belt. The ophiolite consists of highly metamorphosed ultramafic and mafic rocks. Three groups of meta-basalts are present: group 1 rocks are LREE depleted and have a MORB compositional affinity. Their low Ta/Yb ratios (<0.1) are consistent with high degrees of partial melting of a depleted asthenospheric mantle. Rocks of group 2 have higher TiO2 (1.63–2.08 wt%) and Ta/Yb ratios (>0.12), and display slight enrichment of LREE, suggesting that the original magmas were derived from a depleted mantle source mixed with some enriched material. Samples from group 3 are enriched in LREE and other incompatible elements (Ti, Zr, Ta, Nb), suggesting derivation from an enriched mantle source, possibly a plume. All the basalts have high εNd(t) (+4.2 to +6.9), variable εSr(t) and high 207Pb/204Pb and 208Pb/204Pb ratios for given 206Pb/204Pb ratios. These characteristics are compatible with formation at a mid-ocean ridge system above an anomalous Dupal mantle region. The mafic rocks have a Sm–Nd whole-rock isochron age of 1030 ± 46 Ma.The Songshugou ophiolite was emplaced onto the southern margin of the North Qinling terrane, an active continental margin from the Meso-Proterozoic to Neo-Proterozoic.  相似文献   

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