Isotope geochemistry of xenoliths from East Africa: Implications for development of mantle reservoirs and their interaction

Pb, Nd and Sr isotope analyses together with U, Pb, Sm, Nd, Rb and Sr concentrations have been obtained for separated phases of lherzolite and bulk rock mafic granulite xenoliths in Recent volcanics from Tanzania. A garnet lherzolite from the Lashaine vent has yielded the least radiogenicPb(²⁰⁶Pb/²⁰⁴Pb= 15.55) and Nd(¹⁴³Nd/¹⁴⁴Nd= 0.51127; ?_Nd⁰ = ?26.7) isotope compositions recorded so far for an ultramafic xenolith, and ⁸⁷Sr/⁸⁶Sr= 0.83604. The Pb isotope compositions of the mafic granulites are variable 15.77<²⁰⁶Pb/²⁰⁴Pb<17.50 and some show evidence for depletion of U relative to Pb up to 2.0 Ga ago. Overall the isotope results suggest that the mantle part of the continental lithosphere beneath Tanzania has components that have undergone a complex history that includes major chemical fractionations ca. 2.0 Ga ago. A phlogopite-amphibole vein from the Pello Hill sample has Sr, Nd and Pb isotope compositions similar to those of mid-ocean ridge basalts, indicating both a young emplacement age for the vein material and a source which had an isotopic signature characteristic of depleted mantle.The Sr, Nd and Pb isotope systematics of ultramafic xenoliths do not conform with those of MORB, particularly in terms of their PbSr, and NdPb relationships. In this regard they are similar to some ocean islands and could be a viable source material for some ocean island basalts at least. The mantle part of the continental lithosphere is as likely to contain recycled components derived from the continental crust as are other regions of mantle. If the mantle part of continental lithosphere is invoked as a source for ocean islands, it does not negate the possibility that substantial recycled components are involved.     

Co-existing fluid and silicate inclusions in mantle diamond

We document the compositions of co-existing silicate macro-inclusions and fluid micro-inclusions in the fibrous coats of eight coated diamonds from the Panda kimberlite (Canada). The mineral inclusions in the diamond coats come from either the peridotite suite (Cr-pyrope, orthopyroxene, olivine and Cr-diopside) or the eclogite suite (omphacite). Therefore, fibrous diamonds grow in the same paragenetic environments as octahedral diamonds. The inclusions document a more fertile source composition (lower Mg# and higher CaO) than for equivalent phases in octahedral diamonds from Panda and worldwide. However, moderate to high Cr₂O₃ contents in garnet and clinopyroxene inclusions suggest that this apparent fertility is due to a secondary process. Geothermometry of the silicate inclusions yields low equilibration temperatures of 930 to 1010 °C. The co-existing fluid micro-inclusions are dominated by H₂O, carbonate and KCl. Fluid inclusions in both the peridotitic and eclogitic samples fall along linear arrays between Fe–Ca–Mg carbonate and KCl. Inclusions in the one eclogitic sample also contain quartz. We suggest that the diamonds have trapped both metasomatised minerals and the metasomatic fluid, and so provide a snap shot of a metasomatic event in the mantle.     

Deformation and seismic anisotropy of the lithospheric mantle in the southeastern Carpathians inferred from the study of mantle xenoliths

Peridotite xenoliths with a broad range of textures provides evidence for consistent microstructural evolution in a vertical transect of the shallow lithospheric mantle (35–55 km depth) beneath the Persani Mountains, SE Carpathians, Romania, due to ongoing plate convergence in the Carpathian Arc nearby. The recrystallized grain size, crystal preferred orientations strength, and resulting seismic anisotropy vary continuously and display a strong correlation to equilibrium temperatures, suggesting a continuous change in deformation conditions with depth. The shallowmost xenoliths have microstructures typical of high stress deformation, marked by strong recrystallization to fine grain sizes, which results in weak crystal preferred orientations and anisotropy. The deepest xenoliths have coarse-grained porphyroclastic microstructures and strong crystal preferred orientations. Replacive orthopyroxene structures, consuming olivine, and high H₂O concentrations in the pyroxenes are observed in some xenoliths indicating limited percolation of fluids or volatile-rich melts. Despite the high stress deformation and high H₂O contents in some of the studied xenoliths, analysis of olivine crystallographic orientations indicates that [100] slip systems, rather than “wet” [001] accommodate most of the deformation in all samples. Seismic anisotropy estimated from the measured olivine and pyroxene crystal preferred orientations suggests that the strike-parallel fast SKS polarization directions and ~ 1 s delay times measured in the SE Carpathians are likely the consequence of convergence-driven belt-parallel flow in the lithospheric mantle.     

Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths

We examine in this paper the use of helium isotope ratios for the study of hotspot volcanism along age-progressive island volcanic chains. The Hawaiian Islands are the original “high ³He” hotspot, with ³He/⁴He ratios as high as 32 × the atmospheric ratio; in the Pacific they stand out against the surrounding sea of MORB (rather uniformly 8 × atmospheric) which fills the entire Pacific with the exception of the Macdonald-Mehetia-Samoa axis in the South Pacific. The recent availability of a variety of alkalic and tholeiitic glasses from the U.S. Geological Survey and our own dredge hauls has prompted us to look first at isotopic variability within a single fresh and new volcano which is probably sitting directly atop a mantle plume. Thus we have looked in some detail at the total helium in glass pillow rims, at He in the enclosed vesicles, and at He in the glass itself, in both tholeiitic and alkalic lavas, and also at helium in associated phenocrysts and xenoliths. The measured ³He/⁴He ratios range from atmospheric to 30 × atmospheric, but we see clear evidence that the highly vesiculated lavas suffer exchange of He between the thin glass walls of vesicles and ambient seawater, so that we observe a post-eruptive isotopic disequilibrium between glass and gas phases. The primary effect is the very large loss of initial He content during eruptive vesiculation, which results in quite large isotopic effects from small additions of ambient He (of the order of 0.02 μcc He per gram of basalt; corresponding to a “water/rock ratio” of 0.5). Phenocrystic He in olivines verifies that the gas-phase He is not affected by vesicularities up to about 5%. Alkali basalt He appears to be independent of vesicularity up to values as high as 35%; this He is somewhat lower in ³He/⁴He ratio, but matches precisely the associated xenolithic He. However, from the present data we cannot exclude the possibility that diffusive exchange with seawater has affected the He ratio in alkalic vesicles.On the large scale, along the 10% of the Hawaiian chain available for subaerial sampling, we find high ³He/⁴He ratios (24 × atmospheric) in 5.5 × 10⁶-year-old lavas on Kauai. Maximum values of the ratio so far observed are in the pre-erosional Kula basalts on Maui, confirming the previous results of Kaneoka and Takaoka. Hawaii, where these high values were first observed is now seen to range from MORB ratios at Mauna Loa to only 15 × R_A at Kilauea fumaroles. Most xenolithic He so far measured is MORB He, but Loihi xenoliths have high values and are quite different in this respect. Finally, we discuss also the hydrogen and carbon isotope results on Loihi lavas, and show that these elements resemble MORB and appear not to show a distinctive plume signature.     

Geothermal gradient of the upper mantle beneath Jeju Island, Korea: Evidence from mantle xenoliths

Abstract Ultramafic xenoliths found in alkali basalts from Jeju Island, Korea are mostly spinel lherzolites accompanied by subordinate amount of spinel harzburgites and pyroxenites. The combination of results from a two-pyroxene geothermometer and Ca-in-olivine geobarometer yields temperature–pressure (T–P) estimates for spinel peridotites that fall in experimentally determined spinel lherzolite field in CaO-Fe-MgO-Al₂O₃-SiO₂-Cr₂O₃ (CFMASCr) system. These T–P data sets have been used to construct the Quaternary Jeju Island geotherm, which defines a locus from about 13 kbar at 880°C to 26 kbar at 1040°C. The geothermal gradient of Jeju Island is greater than that of the conventional conductive models, and may be as a result of a thermal perturbation by the heat input into the lithospheric mantle via the passage and emplacement of magma. Spinel–lherzolite is the main constituent rock-type of the lithospheric mantle beneath Jeju Island. Pyroxenites may be intercalated in peridotites at similar depth and temperature as re-equilibrated veins or lenses.     

Intergranular trace elements in mantle xenoliths from Russian Far East: Example for mantle metasomatism by hydrous melt

Based on both major and trace element chemistry, the occurrence of the intergranular component in mantle-derived xenoliths from far eastern Russia has been constrained. Whole-rock trace element measurements of one xenolith show apparent negative anomalies in Ce, Th, and high field strength elements on normalized trace element patterns. The trace element pattern of the whole rock differs from those of constituent minerals, indicating that the anomalies in the whole rock are attributable to the presence of an intergranular component. That assumption was confirmed using in situ analysis of trace elements in the intergranular substance and melt inclusion using laser ablation inductively coupled plasma–mass spectrometry. Both the intergranular component and the melt inclusions have identical trace element patterns, which mean that these materials are a cognate metasomatizing agent. The anomalies are regarded as mantle metasomatism related to an aqueous fluid. Hydrous minerals were observed on the wall of the melt inclusions using micro-Raman spectroscopy, indicating that the melt inclusions contained a large amount of water. Thus, this study reveals a trace element composition of a hydrous metasomatizing agent in the mantle.     

Fractionation of platinum group elements in upper mantle: Evidence from peridotite xenoliths from Wangqing

The peridotite xenoliths from Wangqing display a downward-concave PGE pattern with Pt at maximum due to Pt-Pd fractionation, which is different from the flat or negatively sloped patterns commonly observed for worldwide peridotite xenoliths and massifs. Taking into account the difference in partition coefficients of Pt and Pd between alloy and sulfide melt, it is suggested that some of the Wangqing peridotites must have been equilibrated with a melt in which alloy and sulfide coexisted. Segregation of Pt-rich alloy from Pd-sulfide melts due to density contrast and extraction of residual melts without sulfide saturation resulted in the fractionation of Pt from Pd. Project supported by the Ministry of Human Affairs and a part of an international collaboration program between the Chinese Academy of Sciences and Centre National de Recherche Scientifique (CNRS, France), and Australian Institute of Nuclear Sciences and Engineering (AINSE) grant to SJR.     

Hot-spots,mantle plumes and a model for the origin of ultramafic xenoliths in alkali basalts

The Wilson-Morgan hypothesis of hot-spots, characterized by high heat flow, positive gravity anomaly and alkalic volcanism, assumes that such hot-spots are surface expressions of mantle plumes rising by thermal convection. Possible evidence of this mantle upwelling is shown here from textural, structural and chemical aspects of ultramafic xenoliths in alkalic basalts. The xenolith-bearing basalts are constanly associated with Wilson-Morgan hot-spots in the ocean basins and with their continental counterparts in the rift valleys which show extensional tectonics. Most of the xenoliths are considered to be accidental fragments of the lithosphere in the host basalts. One remarkable aspect of xenoliths from all parts of the world is their ubiquitous tectonite fabric. The microstructures of these xenoliths are due to plastic deformation. Some of the xenoliths from Baja California show characteristic deformational features which are also found in the marginal parts of diapirically intruded high-temperature peridotite massifs. A model is proposed for the origin of xenoliths in alkalic basalts by mantle upwelling in which the plastic deformation of the xenoliths reflects this dynamic uprise.     

Helium isotope studies of the mantle xenoliths and megacrysts from the Cenozoic basalts in the eastern China

Helium isotope compositions of the mantle xenoliths and megacrysts in the Cenozoic basalts in the eastern China were measured. The samples were collected from Ludao of Heilongjiang, Huinan and Jiaohe of Jilin, Kuandian of Liaoning, Hannuoba of Hebei, Nüshan of Anhui, Dingan of Hainan. The³He/⁴He ratios of the mantle xenoliths and megacrysts from the most areas were about 1 × 10^-5, and were similar to those of the MORB, thus reflecting the characteristics of the MORB-typed depleted mantle. The³He/⁴He ratios of the mantle xenoliths from Jiaohe were 4.8×10^-6 and the³He/⁴He ratios of xenoliths from Hannuoba vary from 0.15× 10^-6 to 7.4 ×10^-6, obviously lower than those of the MORB, and even lower than the atmospheric helium isotope ratios, indicating that the continental mantle was strongly replaced in Jiaohe and Hannuoba areas. The helium isotope compositions of the mantle xenoliths and megacrysts in the same region vary in a very wide range. It is inferred that the mantle xenoliths and megacrysts were from different parts of the continental mantle. There were not necessary origin relations between the mantle xenoliths, megacrysts and their host basalts. An extremely high³He/⁴He ratio of garnet megacryst from Hannuoba, Hebei was found.     

Characteristics of subcontinental lithospheric mantle beneath Baegryeong Island, Korea: Spinel peridotite xenoliths

Abstract   Geochemical data on Baegryeong Island spinel peridotites found in Miocene alkali basalt provide the information for lithosphere composition, chemical processes, equilibrium pressure and temperature conditions. Spinel peridotite xenoliths, showing transitional textures between protogranular and porpyroclastic textures, were accidentally trapped by the ascending alkali basalt magma. The xenoliths originate at depths from 50 to 70 km with a temperature range from 800 to 1100°C. The variations of modal and mineral compositions of the spinel peridotite xenoliths indicate that the xenoliths have undergone 1–10% fractional melting. The spinel peridotites from Baegryeong Island have undergone cryptic mantle metasomatism subsequent to melt extraction. Metasomatic agent of enriched spinel peridotite xenoliths was carbonatite melt.     

综合物探在鄂东南地区大理岩捕虏体勘探中的应用研究

勘查区内主要研究隐伏大理岩捕虏体的分布规律和物化探异常特征,对有成矿地质背景条件的物化探异常进行验证,寻找发现赋存于石英闪长岩中的隐伏大理岩残留体,并对其含矿性进行评价.在对勘查区内重力原始数据进行处理的基础上,对重力场进行了包括区域场与剩余场的分离、小波多尺度分解、延拓等方面的处理,共圈定局部重力异常13个,重点选取两处异常布设了5条重力、磁法、激电中梯和激电测深精测剖面,以查明异常在纵向上的延展情况及性质.在ZP01、ZP02、ZP03三条剖面上均发现较好的激电异常,异常特征呈现低阻高极化和高阻高极化两种,经钻孔验证低阻高极化异常是由矿化蚀变叠加岩性破碎引起.高阻高极化异常推测为不同期次的石英闪长岩叠加矽卡岩化大理岩俘虏体所引起.     

上地幔顶部折射波走时层析成像研究综述

简要介绍了上地幔顶部折射波走时层析成像研究的发展历程，详细介绍了上地幔项部折射波走时层析成像研究的各种方法并分析了其优缺点，介绍了国内外有关上地幔顶部折射波的研究工作和成果，阐述了上地幔顶部速度结构与构造活动的关系及其地球动力学意义。     

Geodynamic significance of ultramafic xenoliths from Eastern Serbia: Relics of sub-arc oceanic mantle?

A suite of highly depleted peridotite xenoliths in East Serbian Palaeogene basanites represents the lithospheric mantle beneath the Balkan Peninsula. The xenoliths are harzburgites, clinopyroxene-poor lherzolites and rare dunites. They contain mostly <5 vol.% of modal clinopyroxene and are characterized by high Mg# in silicates (>91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al₂O₃ contents in orthopyroxene (mostly 1–2 wt.%). They have experienced some mantle metasomatism which has slightly obscured their original composition. Nevertheless, the general characteristics of the xenoliths imply a composition which is significantly more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peridotites. Geological and compositional evidence suggests that the xenoliths do not represent Archean mantle. The existence of Proterozoic mantle cannot be entirely excluded, although it is in disagreement with geological evidence. On the other hand, the studied xenoliths are compositionally very similar to peridotites of modern oceanic sub-arc settings. The existence of such a depleted lithospheric mantle segment is also inferred from the presence of rare orthopyroxene-rich xenoliths in the same suite. These are interpreted to have originated as lithospheric precipitates of high-Mg, SiO₂-saturated magmas that require a highly depleted mantle source. Such source is typically required by boninitic-like magmas of intraoceanic suprasubduction settings. A proposed geodynamic model to explain these observations involves accretion or underplating of the lower parts of the Tethyan oceanic lithosphere during the Upper Jurassic closure of the eastern branch of the Vardar ocean.     

Depleted sublithospheric mantle beneath the Bohemian Massif and Pannonian Basin: New data on the composition and helium isotopes of the mantle xenoliths and enclosed basalts and the problem of existence of mantle plumes in Central Europe

New data are presented on the chemical composition and helium isotopes for the mantle xenoliths and enclosed Cenozoic basalts of the Pannonian Basin and Bohemian Massif. New data on the helium isotopes in the basalts and mantle xenoliths of the Pannonian Basin and Bohemian Massif, which have been obtained for the first time, show very low R/Ra ratios close to or slightly above the typical values of the atmosphere. This indicates that the mantle beneath the considered areas is strongly degassed. The fact that, according to seismic tomography, the mantle plumes beneath the regions of recent volcanism in Central Europe are not traced below 200 km points to the extensive spreading of the plume material beneath the lithosphere of Europe, due to which the plume tail is too thin to be detectable by the existing methods.     

我国深部探测技术与实验研究进展综述

深部探测技术与实验研究专项(SinoProbe,2008-2012)是我国历史上实施的规模最大的地球深部探测计划.专项开展了全国4°×4°、华北和青藏高原1°×1°的大地电磁阵列观测,建立了全国地球化学基准网(含78种元素),完成了青藏高原、华南-中央造山带、华北和东北等四条超长深地震反射剖面,部署了罗布莎、金川、腾冲、南岭、庐枞和铜陵等大陆科学钻探实验,开展了青藏高原东南缘和华北地区地应力监测;在我国东部长江中下游和南岭成矿带开展的矿集区立体探测卓有成效.同时,专项还开展了岩石圈三维结构与地球动力学数值模拟、大陆地壳结构与演化的综合研究.专项全面实施以来,已经完成约6000 km的深地震反射剖面,成功研究、实验了地壳与地幔深部探测的一系列技术方法,积累了丰富经验,极大地加快了我国深部探测的进度,在国内外产生了强烈的反响.专项实现了技术组合创新、技术进步与重大科学发现的并举,适应我国地质地貌条件和地壳/岩石圈结构特征,初步形成了具有不同层次、不同尺度、不同精度探测空间组合的深部探测技术方法体系,建立了若干各具地质特色的探测试验基地.专项实验已经取得了一系列重大突破与重要成果,深部探测关键仪器装备自主研发获得重大突破,为全面开展地壳探测工程的组织实施奠定了必要的技术基础.     

Major and trace element studies on garnets from Palaeozoic kimberlite-borne mantle xenoliths and megacrysts from the North China craton

Palaeozoic kimberlites from Mengyin, Shandong and Fuxian, Liaoning, eastern China, contain plenty of mantle xenoliths (peridotites, eclogites) and megacrystic minerals. In-situ electron and ion microprobe analyses on garnets from these xenoliths and megacrysts as well as relevant theoretical modeling reveal that these garnets were more or less affected by kimberlitic silicate melts prior to the encapsulation, in which eclogitic garnet from Fuxian, Liaoning Province, was little affected by mantle metasomatism, representing the primitive depleted mantle composition. In contrast, garnet from Mengyin, Shandong Province, and all megacrystic garnets were completely modified by metasomatic melts/fluids and reached perfectly chemical equilibrium, thus reflecting the characteristics of the enriched mantle. It is inferred that old lithospheric mantle beneath the North China craton was fairly strongly modified by metasomatism before Palaeozoic kimberlite emplacement.     

上地幔电导率-温度关系及在大地电磁测深法中应用综述

上地幔热结构的研究对探索地球内部物质状态和岩石圈形成演化过程, 评估自然资源的存储状况, 监控火山区岩浆活动从而降低自然灾害风险等有着重要的意义.本文概述了基于不同岩石物理学高温高压实验得到的上地幔矿物电导率-温度关系.通过分析并对比前人的实验结果, 讨论了不同实验得到的同一矿物的电导率-温度经验关系中参数差异产生的原因.大地电磁测深法(MT)以其探测深度大, 受浅部高阻体影响较小且对低阻体分辨率较高等特点, 在研究壳幔电性结构、热状态和地球动力学机制中得到了广泛的应用.以MT得到的上地幔电阻率模型为基础, 结合岩石物理学实验标定的矿物电导率-温度经验关系, 建立上地幔的温度和熔融百分比模型, 这项工作在研究上地幔热状态中起着不可或缺的作用.笔者总结并讨论了前人以MT方法获得电性结构为基础, 利用电导率-温度经验关系评价上地幔热状态的应用实例, 并对未来的研究工作做了展望, 同时对其可行性做了评估.

     

Structure and development of the lower crust and upper mantle of Southwestern Japan: Evidence from petrology of deep-seated xenoliths

Abstract Basic and ultrabasic xenoliths included in Cenozoic alkali basalts from the Kibi and Sera plateaus, Southwest Japan, can be classified into five groups on the basis of mineral association and texture. Their equilibration P-T conditions estimated from paragenesis and mineral chemistry indicate that the dominant rock type from the lower crust to upper mantle changes with increasing depth as follows: (i) pyroxene granulite (Group V) and meta-sediments; (ii) garnet gabbro (Group 111) and corundum anorthosite (Group IV); (iii) spinel pyroxenite (Group 11); and (iv) spinel peridotite and pyroxenite (Group I). Groups I1 and I11 show a lower degree of recrystallization than Groups I and V, and have similarities in composition and mineral chemistry to host basalts. Based on these facts along with the P-T conditions of equilibration, Groups I1 and I11 are interpreted as formed from basaltic magma that intruded beneath the crust-mantle boundary at an early stage of the magmatism of the alkali basalts, where the lower crust and uppermost mantle had consisted of Group V and metasediments, and Group I, respectively. It follows that the crust has grown downward due to underplating of basaltic magma beneath the bottom of pre-existing crust. Group IV has commonly the same mineral assemblage, corundum + calcic plagioclase + aluminous spinel, and shows locally, nearby kyanite crystals, almost the same texture as fine-grained aggregates in a quartzite xenolith. The aggregates appear to have been formed by reaction between kyanite and host basalt, and accordingly Group IV is interpreted as formed by reaction between metasediments and basaltic magma at the time of the underplating. The Kibi, Sera and Tsuyama areas are distinguished from the areas nearby the Sea of Japan by the occurrence of the garnet gabbro and corundum anorthosite xenoliths, by the absence of the association of olivine + plagioclase in basic and ultrabasic xenoliths, and by the lower temperature of equilibration of basic xenoliths. From these facts it is stressed that in general the crust becomes thinner and geothermal gradient becomes higher towards the back-arc side. Such a regional variation in crustal structure must reflect the tectonic situation of Southwest Japan at the time of the magmatism of the alkali basalts, namely rifting and shallow-level magmatism at the back-arc side.     

The mineral chemistry of pyroxenite xenoliths in the volcanic rocks of Hoh Xil and their significance

The pyroxenite xenoliths in the volcanic rocks of Hoh Xil consist of clinopyroxenes and orthopyroxenes. The mineral composition of these pyroxenes is similar to that of mantle xenoliths including peridotite and pyroxenite from China and abroad, and different from that of granulites. The pyroxenes formed at 1101–1400°C (averaging 1250°C) and under 30–60 kb (averaging 46 kb). We deduced that the magma was derived from the mantle at a depth of more than 150 km, which fits in with the geophysical conclusion that the low-velocity layer existed in the mantle under 150 km.

     

Lateral variation in upper mantle viscosity: role of water

Differences in the viscosity of the earth's upper mantle beneath the western US (∼10¹⁸-10¹⁹ Pa s) and global average values based on glacial isostatic adjustment and other data (∼10²⁰-10²¹ Pa s) are generally ascribed to differences in temperature. We compile geochemical data on the water contents of western US lavas and mantle xenoliths, compare these data to water solubility in olivine, and calculate the corresponding effective viscosity of olivine, the major constituent of the upper mantle, using a power law creep rheological model. These data and calculations suggest that the low viscosities of the western US upper mantle reflect the combined effect of high water concentration and elevated temperature. The high water content of the western US upper mantle may reflect the long history of Farallon plate subduction, including flat slab subduction, which effectively advected water as far inland as the Colorado Plateau, hydrating and weakening the upper mantle.