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1.
207Pb/204Pb-206Pb/204Pb whole meteorite isochrons for Richardton (H5) and Farmington (L5) are presented and give Pb-Pb ages of 4.545 ± 0.010 and 4.620 ± 0.010 Ga respectively (errors ± 2σ). The Pb-Pb isochron for Farmington passes below the Can?on Diablo troilite composition, which may therefore not be the initial Pb composition for this meteorite.All samples show an apparent excess radiogenic lead for single-stage (closed-system) evolution when Can?on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess Pb cannot be explained by terrestrial contamination. There is no unique isotopic composition for initial lead that yields concordant ages at 4.55 Ga for all samples for either meteorite. The data likewise cannot be reconciled to Can?on Diablo initial lead through any of the conventional two- and three-stage evolution models.The apparent excess Pb, with respect to a Can?on Diablo troilite composition and a single-stage closed-system history, and the apparent inhomogeneous initial Pb isotopic compositions, appear to be real. This may be an indication that the U-Pb systems in these meteorites are disturbed, but this disturbance cannot be described consistently by any of the conventional episodic evolution models.  相似文献   

2.
New lead isotope data for calc-alkaline volcanic rocks from New Zealand and the Lesser Antilles, combined with published data for Japan and the Andes, show that the spread of isotopic composition in each volcanic arc region is small (2–4% range in Pb206/Pb204) compared to the range of values observed (8%). Pb207 and Pb206 increase systematically from Japan to the Andes to New Zealand to the Caribbean. Likewise Pb208 and Pb206 are positively correlated, but there is evidence of long term (108 m.y.) differences of Th/U between the regions studied. The apparent U/Pb ratios of Peruvian, New Zealand and Caribbean calc-alkaline volcanics do not differ greatly from the apparent ratio for the single stage growth curve for stratiform Pb ores. In contrast the apparent U/Pb ratios for Japanese calc-alkaline volcanics are distinctly lower. Although the Japanese Pb has model ages near zero, the other volcanic arcs have negative (future) model ages, the Caribbean samples being most extreme in this respect. Published oceanic volcanic and sediment lead isotopic composition data and the new results are consistent with a model of volcanic arc evolution in which oceanic sediments are dragged into the mantle, mixed to some degree with mantle material, and partially melted to form calc-alkaline magmas. Either constant continental volume or continental growth are compatible with this process. The mixing of two separate « frequently mixed » leads is the minimum complexity required to explain volcanic are leads. Mathematically there are probably no single-stage leads but isotopic homogenization during earth history has caused lead isotopes to closely approximate a single stage growth. The use of lead isotopic composition as a « tracer » suggests that mantle — crust geochemical evolution involves an exchange of material and is not simply a one-way process. The Pb isotopic composition of the Auckland, New Zealand alkali basalts is apparently the result of incomplete mixing of two leads to give a linear array of Pb207/Pb204-Pb206/Pb204 data with negative slope.  相似文献   

3.
As one of the important deposit-clustered regionsof China, the Liaodong (eastern Liaoning Province)rift zone contains many large lead, zinc, gold, boron,magnesite and talc ore deposits with a long mininghistory. Recently, large-scale Xiaotongjiapuzi gold andGaojiapuzi silver deposits were discovered in the re-gion. Much work on the origin of these deposits hasbeen carried out, however, the discrepancy on the ageand the metal source of the deposits still rem- discu…  相似文献   

4.
Concentrations of lead, uranium and thorium and isotopic compositions of lead are reported for twelve Cretaceous kimberlites and five Cretaceous nucleated autoliths. The samples are from Lesotho and from the area around Kimberley (Cape Province, South Africa). In the case of the autoliths potassium, rubidium and strontium concentrations and87Sr/86Sr ratios were also measured.Work reported on clinopyroxenes from mantle-derived xenoliths in kimberlites includes lead isotopes for twelve samples and strontium isotopes for nine of these.The autoliths have initial87Sr/86Sr ratios between 0.7035 and 0.7095. A large spread in initial lead isotope ratios (206Pb/204Pb: 17.6–20;208Pb/204Pb: 37.7–39.5) was found in the matrix kimberlites and autoliths. In the207Pb/204Pb vs.206Pb/204Pb plot, the initial lead isotope ratios of the kimberlite and autolith samples roughly define a slope of 0.10, corresponding to an age of 1575 m.y. With respect to the spread of initial ratios as well as with respect to this slope, the kimberlite and autolith lead isotopic pattern is comparable to patterns obtained from carbonatites and ocean island volcanics.The xenoliths studied include coarse-granular and porphyroclastic material from the Kimberley area and coarse-granular samples from Lesotho. Their87Sr/86Sr ratios are generally between 0.704 and 0.706 but a value of 0.713 was found in one sample. They show a surprisingly large spread in lead isotope ratios (206Pb/204Pb: 17.5–20;208Pb/204Pb: 37.3–39.4).The isotopic patterns of the xenolithic material and of the kimberlites and autoliths are considered to provide a strong indication that the upper mantle beneath Southern Africa is isotopically heterogeneous on a regional scale.  相似文献   

5.
New isotopic analyses are presented for 3 plagioclase-rich fractions and one mafic fraction from ferroan anorthosite 60025. The observed206Pb/204Pb ratios vary between 52.5 and 60.5, all much higher than the ratio for terrestrial contamination. In a207Pb/206Pb204Pb/206Pb correlation diagram, the plagioclase data define a model PbPb age of 4.520 ± 0.007 AE using meteoritic primordial lead for the non-radiogenic component. In the concordia diagram the plagioclase data yield intersections at 4.503 ± 0.007 and 0.28 AE. The meaning of the lower intercept is obscure. The earlier 60025 analysis of Tera and Wasserburg [1], with an observed206Pb/204Pb of 23.0, agrees closely with the new plagioclase data in the isotope correlation and concordia diagrams. Since the apparent age does not correlate with the206Pb/204Pb ratios and U contents of the samples, it does not appear to be controlled by terrestrial lead contamination. The time-averaged μ values for the plagioclase leads are exceptionally low, 16–55, and agree within factors of 2 with the observed μ values in the samples. These are much lower than the values observed for mafic rocks or their sources, showing that the anorthosite lead has never been associated for a substantial length of time with any high μ source. In this way the 60025 data differ substantially from UPb data for two other lunar anorthosites, 15415 and 60015. The results suggest that the averaged model Pb ages of 4.51 ± 0.01 AE closely approximate the crystallization age for the plagioclase fraction of the anorthosite, and that it dates back to an early phase in lunar history. One sample from the mafic fraction of 60025 yields a younger model Pb age of 4.42 AE. The age may have been lowered by post-crystallization disturbances or perhaps this fraction is not coeval with the plagioclase fraction.  相似文献   

6.
The Tadhak alkaline ring-complex of Permian age provides two whole rock UPb isochrons giving concordant ages in agreement within relative errors with the RbSr isochron age:235U207Pb isochron: 271 ± 32Ma(MSWD= 0.3);238U206Pb isochron: 254 ± 18Ma(MSWD= 7.8), both on 8 whole-rock samples. The existence of these isochrons indicates that in favorable conditions U (and Pb) can be immobile. This can be due either to the lack of hard oxidizing conditions and/or to the location of U, in very low concentrations, in weathering-resistant minerals. The initial ratios (206Pb/204Pb = 18.714 ± 70and207Pb/204Pb = 15.589 ± 16), corrected for their Permian age, lie in the range observed for oceanic island basalts or continental alkali basalts and indicate an origin in a similar mantle, without any significant crustal contamination. This was also suggested by the initial87Sr/86Sr ratio of 0.70457 ± 4. Moreover, these Sr and Pb isotopic characteristics belong to the field of the so-called “Dupal” anomaly and indicate that it existed already 270 Ma ago. This study shows the potential interest of isotopic investigations of within-plate alkaline ring-complexes to characterize subcontinental mantle compositions, particularly in the past.  相似文献   

7.
Lead isotopic measurements were made on Andean igneous rocks of Jurassic to Recent age in Moquegua and Tacna Departments, southernmost Peru, to clarify the petrogenesis of the rocks and, in particular, to investigate the effect of crustal thickness on rock composition. This location in the Cordillera Occidental is ideal for such a study because the ca. 2 Ga Precambrian basement rocks (Arequipa massif) have a distinct Pb isotopic signature which is an excellent tracer of crustal interaction, and because geomorphological research has shown that the continental crust was here thickened drastically in the later Tertiary.Seven samples of quartz diorites and granodiorites from the Ilo and Toquepala intrusive complexes, and seven samples of Toquepala Group subaerial volcanics were analyzed for Pb isotopic compositions. The plutonic rocks range in age from Jurassic to Eocene; the volcanic rocks are all Late Cretaceous to Eocene. With one exception, the Pb isotopic ratios are in the ranges 206Pb/204Pb= 18.52–18.75, 207Pb/204Pb= 15.58–15.65, and 208Pb/204Pb= 38.53–38.74. The data reflect very little or no interaction with old continental material of the Arequipa massif type.Lead from four Miocene Huaylillas Formation ash-flow tuffs, two Pliocene Capillune Formation andesites and five Quaternary Barroso Group andesites has lower 206Pb/204Pb than that in the pre-Miocene rocks, but relatively high 207Pb/204Pb and 208Pb/204Pb (206Pb/204Pb= 18.16–18.30, 207Pb/204Pb= 15.55–15.63, 208Pb/204Pb= 38.45–38.90). Tilton and Barreiro [9] have shown that contamination by Arequipa massif granulites can explain the isotopic composition of the Barosso Group lavas, and the new data demonstrate that this effect is evident, to varying degrees, in all the analysed Neogene volcanic rocks. The initial incorporation of such basement material into the magma coincided with the Early Miocene uplift of this segment of the Cordillera Occidental [32], and thus with the creation of a thick crustal root. The data strongly imply a relationship between crustal thickness and degree of crustal contamination of magmas in this area, but a rigorous relationship is not yet established.  相似文献   

8.
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   

9.
Lead (Pb) isotope ratios were measured in the marine macroalga Iridaea cordata collected from four locations in the Windmill Islands, East Antarctica. Based on the masses of thalli collected, samples analysed in this study were likely to be a mixture of one and two year old thalli. For a sample of thalli of various ages (<12 months to 2 years old) from the same site there was no apparent variation in Pb concentration or Pb isotope ratio with thallus mass/age, indicating that contaminant sources had been constant over the lifetime of the thalli sampled. I.cordata samples close to the Thala Valley waste disposal site (Brown Bay Inner) near the Australian Station, Casey, displayed isotopic signatures (208Pb/204Pb 35.99; 206Pb/207Pb 1.066; n = 3; average values shown) trending towards that possessed by major Australian Pb sources (Broken Hill and Mt Isa, 208Pb/204Pb 35.60; 206Pb/207Pb 1.041) suggesting that these samples had been exposed to anthropogenic Pb originating from the Thala Valley waste disposal site. Material collected hundreds of metres from the tip location at Brown Bay Outer had isotopic values (208Pb/204Pb 36.32; 206Pb/207Pb 1.088; n = 10) intermediate between Brown Bay Inner and sites further from the contaminant source at Sparkes Bay and Wilkes (208Pb/204Pb 36.46; 206Pb/207Pb 1.094; n = 4) showing that contaminant transport was predominantly restricted to Brown Bay Inner. This study demonstrates that the isotope ratios of Pb in marine macroalgae can provide valuable information as to the origin and extent of heavy metal flux in a marine environment.  相似文献   

10.
In an earlier study of Mesozoic and Cenozoic mineralization in Central America and the Caribbean region, we found that lead isotopic compositions of deposits in northern Central America, which is underlain by a pre-Mesozoic craton, ranged to higher206Pb/204Pb and207Pb/204Pb compositions than did deposits from elsewhere in the region, where the basement is Mesozoic oceanic material. Using 16 analyses for 12 new deposits, as well as new analyses for 11 of the samples studied previously, we have found that lead isotopic compositions correlate closely with crustal type but show little or no correlation with depth to the M-discontinuity. The deposits are divisible into three main groups including (in order of increasing207Pb/204Pb and208Pb/204Pb ratio): (1) deposits in southern Central America and all deposits in the Greater Antilles except Cuba; (2) all deposits in northern Central America; and (3) the Cuban deposits. Southern Central American and Caribbean lead is higher in207Pb/204Pb and208Pb/204Pb than most mid-ocean ridge basalts but could have been derived directly or indirectly from undepleted mantle. Northern Central America can be divided into the Maya block, which belongs to the Americas plate, and the Chortis block, which belongs to the Caribbean plate. Maya block deposits fall along a linear array whereas those of the Chortis block (except the Monte Cristo deposit) form a cluster. These results suggest that the Maya block is underlain by crust or mantle with a large range of U/Pb and Th/U ratios, whereas the Chortis block basement is more homogeneous. Two-stage model calculations indicate an age of about 2280±310 m.y. for the Maya block basement, although no such rocks are known in the region. Comparison of the Chortis block data to our recently published lead isotopic analyses of Mexican deposits shows considerable similarities suggesting that the Chortis block could have been derived from Mexico.  相似文献   

11.
Lead isotope data of sulfides and host volcanic rocks from the Bukit Botol and Bukit Ketaya deposits, the two representative deposits of the Tasik Chini volcanic‐hosted massive sulfide (VHMS) deposit, Central Belt of Peninsular Malaysia, are reported. Lead isotope compositions of the associated sulfide minerals and volcanic rocks from the Bukit Botol deposit exhibit homogeneous and less radiogenic values (206Pb/204Pb showing a range of composition from 18.14 to 18.20, 207Pb/204Pb between 15.52 and 15.59 and 208Pb/204Pb from 37.96 to 38.35). Similarly, the Pb isotopic compositions of the host volcanic rocks from the Bukit Ketaya deposit yielded a narrow range to those of the sulfide samples (206Pb/204Pb from 18.04 to 18.20, 207Pb/204Pb between 15.43 and 15.57 and 208Pb/204Pb of 37.96 to 38.30). The uniform Pb‐isotope compositions of the sulfides in the ore horizon and the host volcanic rocks from both deposits suggest a derivation from a similar source reservoir and mineralization processes. In the framework of the tectonic model for the Central Belt of Peninsular Malaysia, both deposits display a range of lead isotopic compositions originated from mixing of bulk crust/juvenile arc and minor mantle sources, which are typical for VHMS deposits in an island arc–back arc setting.  相似文献   

12.
Summary Lead-zinc mineralization occurs in Miocene sediments along the Egyptian Red Sea Coast in several localities. Galena samples were collected from some of these localities for isotopic and trace elements studies. The isotopic ratios206Pb/204Pb,207Pb/204Pb,209Pb/204Pb for these samples indicate that they are contaminated with radiogenic lead (J-type anomalous lead) with different degrees. The variation in the degree of contamination is significant and indicates a non-uniform source of uranium and thorium from which radiogenic lead was deriven. The highst contaminations were observed in samples collected from sediments overlying or close to acidic basement rocks. The trace element content indicates that galenas were formed at low temperature from solutions that circulated the underlying basement picking several trace elements in addition to radiogenic lead.  相似文献   

13.
Tholeiitic basalts and sulfide deposits from the “Cyana” and “Alvin” diving programs (RITA project) on the East Pacific Rise were analyzed for Pb and Sr isotopes. The basalt data plot within the field defined previously by other East Pacific Rise basalts (206Pb/204Pb: 18.35–18.58;207Pb/204Pb: 15.48–15.53;208Pb/204Pb: 37.8–38.1;87Sr/86Sr: 0.7022–0.7025). Pb, U and Sr contents (～0.5, ～0.05 and ～110 ppm, respectively) and μ values (～6) are typical of MORB, whereas Th/U ratios (～3.5) are significantly higher.The Pb isotopic ratios of the sulfide samples are very homogeneous (206Pb/204Pb～18.47207Pb/204Pb～15.49208Pb/204Pb～37.90), and plot in the middle of the basalt field. This indicates that (1) the sulfide Pb was derived from the basaltic crust without any significant contribution from either seawater or hemipelagic sediments, and (2) the solutions from which the sulfides were deposited had uniform Pb isotopic composition. The Pb contents of three sulfide samples is relatively high (170–1310 ppm).The Sr contents of five sulfide samples are widely scattered from 12 to 210 ppm, with87Sr/86Sr ratios intermediate between basaltic and seawater values (0.70554±0.00005 to0.70795±0.00011). Leaching experiments show that both basalt-derived Sr and seawater Sr were present in the solutions which deposited the sulfides. In some cases, Sr was also adsorbed from seawater onto the sulfides following their deposition. Basalt-derived Sr and seawater Sr are also present in associated non-sulfide phases.  相似文献   

14.
Determinations of40Ar/39Ar and U-Th-Pb are reported for three clasts from the Abee (E4) enstatite chondrite, which has been the object of extensive consortium investigations. The clasts give40Ar/39Ar plateau ages and/or maximum ages of 4.5 Gy, whereas two of the clasts give average ages of 4.4 Gy. Within the range of 4.4–4.5 Gy these data do not resolve any possible age differences among the three clasts.206Pb measured in these clasts is only ～1.5–2.5% radiogenic, which leads to relatively large uncertainties in the Pb isochron age and in the207Pb/206Pb model ages. The Pb data indicate that the initial207Pb/206Pb was no more than 0.08±0.07% higher than this ratio in Can?on Diablo troilite. The U-Th-Pb data are consistent with the interpretation that initial formation of these clasts occurred 4.58 Gy ago and that the clasts have since remained closed systems, but are contaminated with terrestrial Pb. The40Ar/39Ar ages could be gas retention ages after clast formation or impact degassing ages. The thermal history of Abee deduced from Ar data appears consistent with that deduced from magnetic data, and suggests that various Abee components experienced separate histories until brecciation no later than 4.4 Gy ago, and experienced no appreciable subsequent heating.  相似文献   

15.
Five nodules of eclogite, one nodule of garnet peridotite and one sample of kimberlite from the Roberts Victor mine were analyzed for concentrations of U, Th, Pb, Rb and Sr and isotopic compositions of Pb and Sr. In the eclogites, U content ranges from 0.09 to 0.26 ppm, Th from 0.35 to 1.1 ppm, Pb from 0.79 to 5.5 ppm, Rb from 2.1 to 28 ppm and Sr from 133 to 346 ppm;206Pb/204Pb ratios range from 14.8 to 18.5,207Pb/204Pb from 14.9 to 15.7,208Pb/204Pb from 35.2 to 38.5. The garnet peridotite contains 0.22 ppm U, 0.97 ppm Th, 1.05 ppm Pb, 6.9 ppm Rb and 108 ppm Sr and the kimberlite contains 2.5 ppm U, 30 ppm Th, 37 ppm Pb, 113 ppm Rb and 2040 ppm Sr. The lead in the eclogites has two components, a lead pyroextractable at 1100–1200° and a non-pyroextractable residual lead. In three of the eclogites, which are to some extent altered, a proportion of the pyroextractable lead may be contaminating lead from the kimberlite, but an altered kyanite eclogite does not appear to be contaminated by this same kimberlite. The pyroextractable lead from a less altered eclogite contains a much larger proportion of206Pb. Compositions calculated for the residual leads vary greatly. In many of the pyroextraction runs the primary eclogitic phases disappeared and the new phases plagioclase, clinopyroxene and a magnetic iron compound were formed. Why part of the lead should have been retained by these new phases is not understood.  相似文献   

16.
Leads in basaltic suites from seven oceanic islands form linear arrays on206Pb/204Pb versus207Pb/204Pb diagrams. These arrays are more reasonably interpreted as secondary isochrons than as mixing lines, because of their systematic relationship. Separate two-stage histories calculated for the leads from each island indicate that the source materials for the magmas were derived from a single primary reservoir with present238U/204Pb of 7.91 ± 0.04 by secondary enrichment in U/Pb at different times from 2.5 to 1 Ga ago. This is confirmed by a plot of isochron slope versus intercept, on which the points describing each island's Pb-Pb array all lie very near a single straight line. The isochrons for the Canary Islands and Hawaii, at least, are significantly different. The208Pb/204Pb versus206Pb/204Pb relationships are less coherent. The lead isotopic characteristics are consistent with a model in which lead in the oceanic island magmas is derived from ancient subducted oceanic crust. In particular, this explains the close relationship between lead in mid-ocean ridge and oceanic island basalts without invoking mixing.  相似文献   

17.
The Philippine islands are situated between two oppositely dipping zones of seismicity. With the exception of a few areas, such as in the west central Philippines where the North Palawan continental terrane (NPCT) has collided with the archipelago, these seismic zones are well defined to depths of 200 km. Active volcanic chains overlay segments in each of these zones, suggesting that subduction is presently taking place both east and west of the islands. Lavas we have studied are thus divided between what has been termed the West Philippine arc and the East Philippine arc.West Philippine arc volcanic rocks which were extruded before the Philippine archipelago collided with the NPCT, or which are younger than the collision but crop out hundreds of kilometers from the collision zone, and all but one of the rocks from the East Philippine arc fall in the MORB field on207Pb/204Pb versus206Pb/204Pb covariation diagrams. This is surprising considering the frequency with which arc materials have207Pb/204Pb ratios higher than those of MORB, the highBa/REE and Sr/REE ratios in the lavas and the possibility of sediment subduction given the small accretionary prisms. All of these rocks have high208Pb/204Pb ratios with respect to Pacific and Atlantic Ocean MORB, but are similar to Indian Ocean MORB and IOB. Thus the Philippines consist of island arcs with the peculiar Dupal isotopic anomaly documented between 0° and 60°S in the southern hemisphere and particularly in the Indian Ocean region. This demonstrates that the Dupal isotopic anomaly is not restricted to the southern hemisphere, or to MORB and OIB.Post-collision rocks cropping out near the NPCT, in the West Philippine arc, have elevated208Pb/204Pb and 207Pb/204Pb ratios that could be attributed to assimilation of the newly introduced continental crust (NPCT) by mantle-derived magmas or to the addition of a sedimentary component to mantle-derived magmas.  相似文献   

18.
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   

19.
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20.
We report Sr, Nd and Pb isotope ratios and parent and daughter element concentrations in 34 volcanic rocks from Samoa. The highly undersaturated post-erosional volcanics, which have erupted in Recent to Historic time along a 250-km-long fissure, have isotopic compositions that define fields distinct from those of the tholeiitic to alkalic lavas of the older Samoan shield volcanoes. Most shield lavas have206Pb/204Pb of 18.9–19.4,87Sr/86Sr of 0.7045–0.7055 and87Sr/86Sr (to 0.7075). In general, isotopic compositions of the shield lavas are similar to those of the Marquesas and Society Islands. Post-erosional samples have lower206Pb/204Pb and143Nd/144Nd and higher87Sr/86Sr than most shield lavas. The most striking feature of the post-erosional data is a negative correlation between207Pb/204Pb and206Pb/204Pb. This suggests that post-erosional lavas are derived from mixtures of the shield source and a high-207Pb/204Pb,87Sr/86Sr, low-206Pb/204Pb and143Nd/144Nd post-erosional source which may contain recycled ancient sediment. This enriched mantle domain may also underlie the Ontong-Java and Manihiki Plateaus to the north and west. Although both the Samoan shield and post-erosional lavas show chemical characteristics often associated with mantle plumes, only the shield volcanism can plausibly be related to a plume. The post-erosional eruptions appear to be the result of flexure and rifting due to plate bending at the northern termination of the Tonga Trench.  相似文献   

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