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1.
An integrated study based on incompatible trace elements and Sr–Nd–Pb isotopes is presented in order to assess the mantle sources involved in the genesis of the Paraná Magmatic Province (PMP) tholeiites. Particular emphasis is given to 33 new Pb isotope and concentration data obtained in representative samples of low-TiO2 (LTiB) and high-TiO2 (HTiB) flood basalts that occur in the province. Results show important differences with respect to type and location of these rocks. The LTiB and HTiB from northern PMP exhibit very similar initial Pb isotope ratios (average LTiB vs HTiB: 206Pb/204Pbi=17.78±0.03 vs 17.65±0.02; 207Pb/204Pbi=15.53±0.01 vs 15.52±0.01; 208Pb/204Pbi=38.12±0.03 vs 38.05±0.04). The LTiB from southern PMP, with initial 87Sr/86Sri≤0.7060, show small variation in initial Pb isotope compositions (average 206Pb/204Pbi=18.20±0.07; 207Pb/204Pbi=15.61±0.01; 208Pb/204Pbi=38.32±0.10), which are highly enriched in radiogenic Pb in comparison to the northern PMP analogues. The HTiB from southern PMP have initial Pb isotope ratios (average 206Pb/204Pbi=17.45±0.09; 207Pb/204Pbi=15.50±0.01; 208Pb/204Pbi=37.89±0.03) slightly less radiogenic compared with the HTiB from northern PMP. The data cover a large range of isotope compositions, which are accompanied by systematic changes in incompatible trace element ratios and Sr–Nd isotopes, indicating contributions from different mantle sources. The remarkable chemical and isotope differences between PMP basalts, N-MORB and Tristan da Cunha least evolved volcanics indicate that these asthenospheric sources did not play a significant role in the basalt genesis, suggesting generation from the melting of heterogeneous lithospheric mantle sources. The close similarity between the radiogenic isotopes of the Cretaceous carbonatites that surround the PMP and those of the HTiB rock-types and the LTiB from the northern Paraná suggests the involvement of the same mantle components in their genesis: a dominant EMI end member and a radiogenic isotope enriched component of EMII-type, as some phlogopite-peridotite mantle xenoliths (Japecanga) from the Alto Paranaíba Igneous Province. The latter component seems also to have an important role in the origin of the LTiB from the southern Paraná, where the other end member is highly depleted in radiogenic lead similar to DMM.  相似文献   

2.
Re–Os isotope compositions of syngenetic sulphide inclusions in both eclogite suite (E-type) and peridotite suite (P-type) parageneses in diamonds from the Koffiefontein mine, South Africa have been analysed using a technique capable of analysing single inclusion grains, or, in some cases multiple inclusions from the same diamonds. Sulphide inclusion Ni contents broadly correlate with Os abundances in that low-Ni (6.8–8.7% Ni), E-type sulphides have 4.7 to 189 ppb Os whereas the two high-Ni (25%), P-type sulphides have 5986 and 6097 ppb Os. Two P-type sulphides from the same diamond define the first mineral isochron obtained for a single diamond which has an age of 69±30 Ma with chondritic initial 187Os/188Os. This indicates that the sulphides, and hence the host diamond, crystallised close to the time of kimberlite emplacement (90 Ma), in the Mesozoic. This is supported by Pb isotopic measurements of a fragment from one of the sulphides, together with the absence of significant Type IaB nitrogen aggregation in the host diamond lattice. E-type sulphide inclusions have radiogenic Os isotopic compositions, 187Os/188Os 0.346 to 2.28, and Re–Os model ages from 1.1 to 2.9 Ga. They define an array on a Re–Os isochron diagram that may be interpreted as defining a single period of E-type sulphide growth at 1.05±0.12 Ga, with an elevated initial 187Os/188Os. Alternatively, two episodes of sulphide crystallisation, from a chondritic reservoir, may be invoked in the Archaean and in the Proterozoic. The results for both P- and E-type diamonds point to a spectrum of diamond crystallisation ages. High contents of both Re and Os, and the similarity of Re/Os ratios of sulphide inclusions in diamonds to whole rock eclogite and peridotite xenoliths indicate that small amounts of sulphides can dominate the mantle budget of both these elements during melting. Recent addition to the lithospheric mantle of high-Os material similar to that from which the P-type sulphides crystallised may explain the variable, sometimes young Os model ages seen in whole rock xenolith Re–Os data.  相似文献   

3.
The pre‐Cenozoic history of the South China Sea (SCS) region is the object of continued debate. To trace the evolution of the SCS, a better understanding of the petrogenesis and tectonic affiliation of the granitic rocks that comprise the microblocks within the region is necessary. In this study, we analyzed the whole‐rock oxygen and lead isotope ratios of granitic samples dredged from two locations in the Nansha microblock, one of the microblocks in the SCS. Oxygen isotope data combined with previously published Sr isotope data show that group I rocks (δ18O = 6.00–7.20‰; average = 6.64‰) originated from a mantle source contaminated by material and/or fluid input from a Mesozoic subduction zone in the southeastern side of the microblock. Group II rocks (δ18O = 6.86‰–9.13‰; average = 7.75‰) also came from the same source, but they were additionally affected by crustal contamination. The Nansha microblock has high radiogenic lead ratios (206Pb/204Pbi = 18.602–18.756, 207Pb/204Pbi = 15.660–15.713, 208Pb/204Pbi = 38.693–38.893), which indicate that the Nansha microblock is tectonically affiliated with the Nanling–Hainan or South China block. This notion is consistent with the results of a previous Nd isotope study. As a whole, results of our study suggest that some of the other microblocks dispersed in the SCS are also possible fragments of South China block, and thus further studies are needed to better constrain the pre‐Cenozoic evolutionary history of the SCS region.  相似文献   

4.
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.  相似文献   

5.
Abstract We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (87Sr/86Srinitial = 0.7041 ~ 0.7050, ?Nd(t) = 2.3 ~ 4.0, 206Pb/204Pbfeldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (87Sr/86Srinitial = 0.7047 ~ 0.7065, ?Nd(t) = 3.9 ~ 2.8, 206Pb/204Pbfeldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.  相似文献   

6.
New trace element and Hf, Nd, and Pb isotope data are reported for 22 basalts collected between 22°N and 35°N on the Mid-Atlantic Ridge. (La / Sm)N ratios identify the presence of enriched (E)-MORB in the northernmost part of this area and normal (N)-MORB elsewhere. A negative correlation is observed when 143Nd / 144Nd is plotted against 206Pb / 204Pb, 207Pb / 204Pb, and 208Pb / 204Pb, whereas 176Hf / 177Hf appears not to correlate with any of the other isotopic ratios. The E-MORB samples are characterized by high 206Pb / 204Pb, 207Pb / 204Pb, 208Pb / 204Pb, and low 143Nd / 144Nd. Principal Component Analysis (PCA) of Pb isotopes alone identifies three, and only three, significant geochemical end-members (‘components’). Including Nd and Hf isotopic data in the PCA produces spurious components, partly because of curved mixing relationships, and partly because of fractionation during melting. Our preferred interpretation of why 176Hf / 177Hf is decoupled from the other isotopic ratios is, as inferred from recent experimental data, that the Hf isotopic compositions of the melt and the residue fail to equilibrate during melting. A strong correlation between (Sr / Nd)N and (Eu / Eu*)N indicates that plagioclase is a residual phase of N-MORB, but not of E-MORB melting. The three end-members identified in this study are the depleted mantle, a common-type component, and an enriched plume-type end-member. The common, or ‘C’-type, end-member is characteristic of E-MORB and may itself be a mixture containing recycled oceanic crust (the MORB suite, terrigenous sediments, and/or oceanic plateaus). The plume-type end-member is likely to represent the lower mantle and may involve some primordial material. It is shown that mantle isochrons in general and the Pb–Pb isochron in particular do not characterize a specific geodynamic process acting to create mantle heterogeneities.  相似文献   

7.
Lead (Pb) concentrations and isotope ratios of two different geochemical archives are compared; lake sediment cores and lichens (Hypogymnia physodes, naturally growing and transplanted) from a ca. 80 km-long transect centred on the Cu smelter and former mining town of Karabash, Ural Mountains, Russia. Lead concentrations in sediment cores from 10 lakes were generally low near their base and show an abrupt increase in their upper portions interpreted to coincide with the onset of large-scale smelting operations in 1910. Lead isotope ratios derived from 204Pb, 206Pb, 207Pb, 208Pb of the bottom layers differed significantly from those of the top. The top sediments have isotope ratios that show distinct end members, one of which was the stack dust from the Karabash smelter, which is similar to the Pb derived from ores from Sibay, a major mine in the Urals. The composition of the bottom sediment layers generally fall slightly off a mixing line between the top sediments and average Earth’s upper crust. Lichens transplanted from a reference site, as well as naturally growing lichens, sampled from southwest of the smelter have isotope ratios similar to those of the stack dust. Lichens to the northeast contained Pb from the smelter, but are increasingly influenced by other sources probably leaded petrol and local soils, and a signature derived from a source enriched in 207Pb. Vegetables collected from local kitchen gardens contained Pb from an additional atmospheric source, possibly coal. Our work confirms that: (1) Pb isotopes in lake sediments provide a long-term record of inputs and allows the characterisation of natural and anthropogenic sources; (2) Pb isotopes in lichens provide a short-term record of local and long-range atmospheric deposition at high spatial resolution and short time scales as they replace their Pb content within a few months; (3) determination of all four stable Pb isotopes is necessary for the identification of the sources of Pb and is extremely sensitive for discerning minor source signatures, even in an area with a dominant source such as a smelter. Particularly significant for the Karabash area is that ore-smelter-derived airborne Pb is a major component in the lake sediments and lichens but its contribution reaches insignificant levels ca. 40 km from the smelter.  相似文献   

8.
Quan-Ru  Geng  Zhi-Ming  Sun  Gui-Tang  Pan  Di-Cheng  Zhu  Li-Quan  Wang 《Island Arc》2009,18(3):467-487
The well‐studied Mesozoic and Cenozoic volcanic rocks of the Gangdise Terrane, southern Tibet, are widely interpreted to have resulted from subduction of the Neotethys; however, Late Paleozoic volcanic rocks and their tectonic setting remain poorly studied. Based on new geological data, we carried out stratigraphical and geochemical analyses of Permian volcano‐stratigraphic sequences within an east–west‐trending, fault‐bounded zone of uplift in the central Gangdise Terrane. Sedimentary rocks in this area consist of platform carbonates and terrigenous clastic rocks that represent widespread shallow‐marine sedimentary basins developed around northern Gondwana. A regression or tectonic uplift event is recorded in Permian sedimentary rocks that show the local development of fluvial environments. The sedimentary succession contains evidence of two volcanic stages: a period of basaltic extrusions and younger explosive felsic magmatism. The first volcanic stage is Early and Middle Permian in age. Tholeiitic basaltic lavas are exposed around Maizhokunggar (Tangjia) and Lhunzhub in central Gangdise. The Lower Permian basalts are relatively enriched in MgO (4.58–12.19%), whereas the Middle Permian basalts are characterized by high Al2O3 contents (11.75–21.22%). Rocks of both ages are enriched in large‐ion lithophile elements (LILE) and light rare earth elements (LREE), and show pronounced negative Nb and Ta anomalies. Total REE contents and light (LREE)/heavy (HREE) ratios increased from the Early to Middle Permian. Observed variations in initial Sr, Nd, and Pb isotopes (87Sr/86Sri = 0.7013–0.7066, 207Pb/204Pbi = 15.53–15.63, and 208Pb/204Pbi = 38.04–38.64 for a given 206Pb/204Pbi; εNd = +0.69 to ?11.55) can be explained by crustal interaction with mantle sources, as is characteristic of metasomatism by slab‐derived fluids or assimilation and fractional crystallization (AFC) processes during magmatic evolution. The observed geochemical signatures, coupled with stratigraphic constraints, support the hypothesis that an initial arc formed during the Permian due to southward subduction of the Paleotethys, predating the well‐known Mesozoic arc preserved in the Gangdise Terrane.  相似文献   

9.
The chronology and isotope geochemistry of a selection of Proterozoic Scourie dykes has been investigated in order to specify both their time of emplacement within the thermal history of the Archaean crust of N.W. Scotland, and to attempt to characterise the evolution of continental lithosphere. SmNd, RbSr and UPb isotope analyses are presented. Primary, major igneous minerals separated from four well preserved dykes yield SmNd ages of 2.031 ± 0.062Ga, 2.015 ± 0.042Ga, 1.982 ± 0.044Ga and 2.101 ± 0.078Ga, which are interpreted as crystallisation ages.The initial Nd isotope compositions in the dykes at their emplacement age of 2.0 Ga, range from +3.4 to −6.8, indicating the presence of an older lithospheric component. SmNd whole-rock isotope data for fifteen dykes, if interpreted to have age significance, yield an “age” of 3.05 ± 0.27 Ga. SmNd crustal residence ages for the same dykes average 2.95 Ga, which is interpreted as the time that small melts were added to the Lewisian lithosphere. The possibility that correlated147Sm/144Nd and143Nd/144Nd ratios are an artifact of mixing between depleted mantle melts generated at 2.0 Ga, and an older enriched lithospheric component is not eliminated by the data, but the relationship between 1/Nd and143Nd/144Nd ratios rules out any simple mixing. UPb isotope data for plagioclase feldspars and whole-rock samples of dykes provide useful estimates of initial Pb-isotope composition of the dykes at the time of their emplacement. Initial206Pb/204Pb and207Pb/204Pb ratios vary considerably and range from 13.98 to 15.78, and 14.72 to 15.56 respectively, and suggest that the UPb fractionation responsible must have occurred at least 2.5 Ga ago.The Scourie dykes have inherited a trace element enriched component from the Lewisian lithosphere, which has resided there since ca. 3 Ga ago. Whether the dykes inherited this material from the crust or the mantle portions of the lithosphere or both, it seems likely that small basaltic melts derived from asthenospheric mantle were ultimately responsible for the enrichment. The simplest view is that these small melt fractions had been resident in the mantle part of the Lewisian lithosphere. In this case the Archaean trace-element enrichment and element fractionation in the Lewisian lithospheric mantle sampled by the dykes was closely associated in time with the generation of the 2.9 Ga old crustal portion of the lithosphere [36,37].  相似文献   

10.
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11.
Late Cretaceous (66.2 ± 0.5 Ma amphibole and 66.7 ± 0.2 Ma phlogopite 40Ar/39Ar ages) nephelinitic volcanic rocks from Godzilla Seamount in the eastern North Atlantic (34°N latitude) have trace element and Sr–Nd–Pb–Hf-isotope compositions similar to the Enriched Mantle I (EM-I) endmember, except for their low 207Pb/204Pb relative to 206Pb/204Pb ratios (206Pb/204Pbin = 17.7, 207Pb/204Pbin = 15.34) plotting below the Northern Hemisphere Reference Line on the uranogenic Pb isotope diagram. O isotope data on amphibole separates are mantle-like (δ18O = 5.6–5.8‰). Age and location of the isolated Godzilla Seamount, however, preclude it from being derived from the Madeira or Canary hotspots, making a lower-mantle origin unlikely. Therefore we propose derivation from a shallow (lithospheric/asthenospheric) melting anomaly. As observed in mid-ocean-ridge and ocean-island basalts, there is a systematic decrease of 207Pb/204Pb ratios (and Δ7/4) in the individual EM-I endmember type localities towards northern latitudes with Godzilla lying on the extension of this trend. This trend is mirrored in ultra-potassic volcanic rocks such as lamproites and kimberlites, which reflect the composition of enriched subcontinental lithospheric mantle. Therefore, a global pattern in 207Pb/204Pb ratios and Δ7/4 is suggested. The geochemical composition of EM-I endmember type localities, including Godzilla lavas, and the enriched (DUPAL) anomaly in the southern hemisphere could reflect derivation from ancient, metasomatized subcontinental lithospheric mantle. We propose a two-stage model to explain the trace element and isotopic composition of the EM-I mantle endmember localities worldwide: 1) during the early history of the Earth, subcontinental lithosphere was metasomatized by melts from subducted slabs along convergent margins generating high μ (238U/204Pb) sources, and 2) as the Earth cooled, hydrous fluids replaced hydrous melts as the main slab component metasomatizing the subcontinental lithospheric mantle (generating EM-I sources with lower μ). In accordance with this model, the global variations in 207Pb/204Pb ratios and Δ7/4 could reflect geographic differences in μ and/or the age at which the transition from stages 1 to 2 took place in the Archaean lithosphere. The model would require a re-definition of the EM-I endmember to low 206Pb/204Pb, high 208Pb/204Pb (positive Δ8/4) but variable 207Pb/204Pb (positive and negative Δ7/4).  相似文献   

12.
The sulphur isotope composition of 16 pyrite and chalcopyrite samples from recent sulphide deposits (“Cyana”—project RITA) and active sulphide mineralisation (“Alvin”—project RISE) associated with hydrothermal sources at 380±30°C on the East Pacific Rise at latitude 21°N have been measured. The34S/32S ratios are relatively uniform and essentially identical for both sites: δ34S=+1.4to3.0%. (CDT), mean +2.1‰. The sulphides were analysed after the majority of the very numerous micro-inclusions of anhydrite had been removed.Two independent physico-chemical analyses of the data demonstrate that about 90% of the sulphur was leached from the basaltic host rocks by the circulating seawater-hydrothermal fluids.  相似文献   

13.
Increasing use is being made of sulphide minerals in isotope geothermometry. Sulphur isotope fractionation factors for34S exchange between sphalerite (ZnS) and galena (PbS) have been calculated for various temperatures between 0 and 1000°C. The reduced partition function ratios have been calculated using a “shell” model for the forces derived from inelastic neutron scattering studies of the lattice dynamics of sphalerite and galena. A new formalism of the current theory has been developed which enables the accuracy of the calculation to be determined.The Zn34S—Pb34S equilibrium constant obtained by the shell model calculations is 1.0060 to 100°C, 1.0031 at 250°C and 1.0005 at 1000°C, in agreement with experimental determinations.  相似文献   

14.
By modelling the observed distribution of210Pb and210Po in surface waters of the Pacific, residence times relative to particulate removal are determined. For the center of the North Pacific gyre these are τPo = 0.6years andτPb = 1.7years. The surface ocean τPb is determined by particulate transport rather than plankton settling. The fact that it is about two orders of magnitude smaller than τPb for the deep ocean implies a sharp change in the adsorptive quality of particles during descent through the water column.  相似文献   

15.
The fundamental assumption of 210Pb sediment dating is the stable flux of 210Pbex, which was derived from atmosphere and then transferred into sediments via lake water. When the sedimentation rate is relatively constant, the 210Pbex activity in sediments will be exponentially reduced with sedimentation age. 210Pbex in lake water is incorporated into sediments mainly via organic particulates. If the sedimentation flux of organic matter in lake water is suddenly increased, 210Pbex will be significantly deposited and then transferred into sediments. On the one hand such sudden purification effect is obviously unfit for the fundamental assumption of 210Pb dating; on the other hand, the sudden enhancement of 210Pbex flux would be indicative of the conspicuous variation of primary productivity of lake water. This problem will be discussed in accordance with the variation trend of 210Pbex in the vertical profile of recent sediments of Lake Chenghai, Yunnan Province. The sediment core was collected from the deep-water area of Lake Chenghai in June 1997. The vertical profile of 137Cs activity is characterized by a tree-peak pattern. This profile gave reliable ages, and also showed the stability of sediment accumulation in the recent ten years. The vertical profile of 210Pbex activity displays a specific distribution of peaks, and is similar to the vertical profile of Corg. This phenomenon seems to be related to the mechanism of constraining the transfer of 210Pbex into lake sediments. The average atomic ratios of Horg/Corg and Corg/Norg in Lake Chenghai sediments are 5.51 and 7.04, respectively, indicating that the organic matter was predominantly derived from the remains of endogenic algae. In terms of the three-stage evolutionary characteristics of organic matter in sediments, i.e., “deposition-de-composition-accumulation”, the sedimentation fluxes (F(Corg)) of organic carbon (Corg) since 1970 were calculated by modeling. The sedimentation fluxes of 210Pbex (F(210Pbex)) in different years display good synchronous relations with the sedimentation flux of organic carbon (F(Corg)), especially in the years of 1972–1974 and 1986–1989. The variation of F(Corg) led to the variation of F(210Pbex); the variation of F(210Pbex) reflects, to some extent, the historical variation of lake productivity.  相似文献   

16.
Pb, Hf, Nd and Sr isotopes of basaltic lavas from the two Réunion Island volcanoes are reported in order to examine the origin of the sources feeding these volcanoes and to detect possible changes through time. Samples, chosen to cover the whole lifetime of the two volcanoes (from 2 Ma to present), yield a chemically restricted (compared to OIB lavas) but complex distribution. Réunion plume isotopic characteristics have been defined on the basis of the composition of uncontaminated shield-building lavas from the Piton de la Fournaise volcano. The average ?Nd, ?Hf, 87Sr/86Sr and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb isotope ratios calculated for this component are + 4.4, + 9.1, 0.70411, 18.97, 15.59 and 39.03, respectively. In Pb–Pb isotope space, each volcano defines a distinct linear trend but slight variations are also detected within the various volcanic sequences. The Piton des Neiges volcano yields a distinct and significantly more scattered isotopic distribution than Piton de la Fournaise for both Pb, Hf and Nd isotope tracers. A principal component analysis of the Pb isotope data from Piton de la Fournaise reveals a major contribution of the C and EM-1 components (with a clear Dupal flavor) as main components for the modern Réunion plume. The same components have been identified for Piton des Neiges but with a stronger participation of a depleted mantle component and a weaker EM-1 contribution. The compositional change of the lavas erupted by the Piton des Neiges and Piton de la Fournaise volcanoes is attributed to the impingement of two small-scale blobs of plume material at the base of the Réunion lithosphere. Compared to other hot-spots worldwide, in particular Hawaii and Kerguelen, magmas beneath Réunion are generated from a considerably more homogeneous, compositionally more primitive plume higher in 206Pb. Although shallow-level contamination processes have been locally detected they did not alter significantly the composition of the plume magmas. This is tentatively attributed to mantle dynamics producing small, high-velocity blobs that ascend rapidly through the lithosphere, and to the lack of a well-developed magma chamber at depth in the lithosphere.  相似文献   

17.
Selective chemical dissolution has been used to study the distribution of Pb and Tl in an ultrafine ?20-μm matrix separate of Allende. The matrix was exposed to high-purity reagents ranging from H2O, then HCl of increasing concentration and finally HF-HCl mixtures. A total of 17 extractions were obtained, each for a minimum period of 10 days. The isotopic compositions of the Pb released during the slow dissolution of the matrix fall into four distinct groups. The first, consisting of four extractions, released a component of terrestrial Pb isotopic composition with a total abundance of about 1 ppb. The next six extractions, which contained the bulk of the indigenous Pb and Tl corresponding to 96% and 94%, respectively, of the total matrix abundance, was of a reasonably homogeneous Pb isotopic composition with mean ratios of206Pb204Pb= 10.00and207Pb204Pb= 10.74. In the final seven extractions, the released Pb falls into two higher isotopic groupings and probably results from the dissolution of debris from chondrules and inclusions. The apparent age of the internal matrix isochron is4562 ± 14 My. The release of Pb and Tl shows a reasonable correlation with the matrix dissolution. This indicates that the Pb and Tl reside predominantly within the matrix phases rather than as a localised phase. The Tl isotopic composition of two matrix fractions and whole meteorite were measured and found to be indistinguishable from the terrestrial205Tl/203Tl ratio. Measurement of a terrestrial reagent standard in the range 1–10 ng Tl gave, for 20 analyses, a mean205Tl/203Tl ratio of2.38907 ± 0.00102 (2σ).The estimate of terrestrial Pb contamination is considerably lower than the 6–300 ppb assumed in some recent studies in order to explain the phenomenon of apparent excess radiogenic Pb in chondrites. The problem of terrestrial Pb pollution and the evidence which argues against a relatively severe and homogeneous Pb contamination of meteorites, is briefly considered. The apparent initial isotopic composition of the bulk of the indigenous Pb in the Allende matrix was found to be206Pb204Pb= 9.57and207Pb204Pb= 10.47. This is of a higher composition than the Pb in the Can?on Diablo troilite phase and further indicates that the phenomenon of apparent excess radiogenic Pb in chondrites is real.  相似文献   

18.
Concentrations of heavy metals and Pb isotopes were measured in the 1-M HCl leaching fraction of core sediments spanning the last 400 years. This sedimentary record of pollution history in metal concentrations shows a good correlation with the increases in industrialization, urbanization, and energy consumption since 1901s. Notably, the Pb concentration and the 207Pb/206Pb and 208Pb/206Pb ratios were constant before the 1910s (16.7 μg/g, 0.844, and 2.098, respectively), whereas they increased steadily up to 21.9 μg/g, 0.848, and 2.101 after the 1910s. The correlations between Pb isotope ratios (206Pb, 207Pb, and 208Pb) showed different linear regression trends for core sediments before and after the 1910s, indicating differences in Pb sources. Our interpretation suggests that the source of anthropogenic Pb in Korean coastal region and the Yellow Sea shelf was presumed to be Chinese coals or ores, which have also played a major role as sources of atmospheric particulate Pb.  相似文献   

19.
This study presents major-, trace-element, and rhenium–osmium (Re–Os) isotope and elemental data for basalts and gabbros from the Zermatt-Saas ophiolite, metamorphosed to eclogite-facies conditions during the Alpine orogeny. Igneous crystallisation of the gabbros occurred at 163.5 ± 1.8 Ma and both gabbro and basalt were subject to ‘peak’ pressure–temperature (P–T) conditions of > 2.0 GPa and ~ 600 °C at about 40.6 ± 2.6 Ma.Despite such extreme P–T conditions, Re–Os isotope and abundance data for gabbroic rocks suggest that there has been no significant loss of either of these elements during eclogite-facies metamorphism. Indeed, 187Re–187Os isotope data for both unaltered gabbros and gabbroic eclogites lie on the same best-fit line corresponding to an errorchron age of 160 ± 6 Ma, indistinguishable from the age of igneous crystallisation. In contrast, metamorphosed basalts do not yield age information; rather most possess 187Re/188Os ratios that cannot account for the measured 187Os/188Os ratios, given the time since igneous crystallisation. Taken with their low Re contents these data indicate that the basalts have experienced significant Re loss (∼ 50–60%), probably during high-pressure metamorphism. Barium, Rb and K are depleted in both gabbroic and basaltic eclogites. In contrast, there is no evident depletion of U in either lithology.Many ocean-island basalts (OIB) possess radiogenic Os and Pb isotope compositions that have been attributed to the presence of recycled oceanic crust in the mantle source. Published Re–Os data for high-P metabasaltic rocks alone (consistent with this study) have been taken to suggest that excessive amounts of oceanic crust are required to generate such signatures. However, this study shows that gabbro may exert a strong influence on the composition of recycled oceanic crust. Using both gabbro and basalt (i.e. a complete section of oceanic crust) calculations suggest that the presence of ≥ 40% of 2 Ga oceanic crust can generate the radiogenic Os compositions seen in some OIB. Furthermore, lower U/Pb ratios in gabbro (compared to basalt) serve to limit the 206Pb/204Pb ratios generated, while having a minimal effect on Os ratios. These results suggest that the incorporation of gabbro into recycling models provides a means of producing a range of OIB compositions having lower (and variable) 206Pb/204Pb ratios, but still preserving 187Os/188Os compositions comparable to HIMU-type OIB.  相似文献   

20.
We report new Nd, Hf, Sr, and high-precision Pb isotopic data for 44 lava and tephra samples from Erebus volcano. The samples cover the entire compositional range from basanite to phonolite and trachyte, and represent all three phases of the volcanic evolution from 1.3 Ma to the present. Isotopic analyses of 7 samples from Mt. Morning and the Dry Valley Drilling Project (DVDP) are given for comparison. The Erebus volcano samples have radiogenic 206Pb/204Pb, unradiogenic 87Sr/86Sr, and intermediate 143Nd/144Nd and 176Hf/177Hf, and lie along a mixing trajectory between the two end-member mantle components DMM and HIMU. The Erebus time series data show a marked distinction between the early-phase basanites and phonotephrites, whose Nd, Hf, Sr, and Pb isotope compositions are variable (particularly Pb), and the current ‘phase-three’ evolved phonolitic lavas and bombs, whose Nd, Hf, Sr, and Pb isotope compositions are essentially invariant. Magma mixing is inferred to play a fundamental role in establishing the isotopic and compositional uniformity in the evolved phase-three phonolites. In-situ analyses of Pb isotopes in melt inclusions hosted in an anorthoclase crystal from a 1984 Erebus phonolite bomb and in an olivine from a DVDP basanite are uniform and identical to the host lavas within analytical uncertainties. We suggest that, in both cases, the magma was well mixed at the time melt inclusions were incorporated into the different mineral phases.  相似文献   

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