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1.
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   

2.
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   

3.
Historical volcanic rocks of the Aeolian islands range in composition from shoshonitic basalts to rhyolites, which might reflect fractional crystallization of a shoshonitic parent magma. However Sr and Pb isotopic data indicate a more complex history. The shoshonitic basalts at present erupted at Stromboli, although chemically similar to the postulated parent magma, are genetically unrelated to the other studied rocks. Sr isotopes indicate that Vulcano, Vulcanello and Lipari had independent magma sources. It is proposed that crustal contamination raised the Sr isotopic composition of the Lipari rhyolites. The rocks of these island are related by a common very steep trend of207Pb/204Pbvs. 206Pb/204Pb. Such a trend is a common feature of orogenic magmas and shows that Pb was derived by mixing of at least two components. Presently it is impossible to constrain precisely either the timing or the physical meaning of the Pb end members. The Pb isotopic trend in the Eolian island is very distinct from those recorded in volcanic rocks both from behind the arc (Etna, Iblean Mts.) and from Central and Southern Italy.  相似文献   

4.
Igneous rocks from the Philippine tectonic plate recovered on Deep Sea Drilling Project Legs 31, 58 and 59 have been analyzed for Sr, Nd and Pb isotope ratios. Samples include rocks from the West Philippine Basin, Daito Basin and Benham Rise (40–60 m.y.), the Palau-Kyushu Ridge (29–44 m.y.) and the Parece Vela and Shikoku basins (17–30 m.y.). Samples from the West Philippine, Parece Vela and Shikoku basins are MORB (mid-ocean ridge basalt)-like with 87Sr/86Sr= 0.7026−0.7032, 143Nd/144Nd= 0.51300−0.51315, and 206Pb/204Pb= 17.8−18.1. Samples from the Daito Basin and Benham Rise are OIB (oceanic island basalt)-like with 87Sr/86Sr= 0.7038−0.7040, 143Nd/144Nd= 0.51285−0.51291 and 206Pb/204Pb= 18.8−19.2. All of these rocks have elevated 207Pb/204Pb and 208Pb/204Pb compared to the Northern Hemisphere Regression Line (NHRL) and have δ207Pb values of 0 to +6 and δ208Pb values of +32 to +65. Lavas from the Palau-Kyushu Ridge, a remnant island arc, have 87Sr/86Sr= 7032−0.7035, 143Nd/144Nd= 0.51308−0.51310 and 206Pb/204Pb= 18.4−18.5. Unlike the basin magmas erupted before and after them, these lavas plot along the NHRL and have Pb-isotope ratios similar to modern Pacific plate MORB's. This characteristic is shared by other Palau-Kyushu Arc volcanic rocks that have been sampled from submerged and subaerial portions of the Mariana fore-arc.At least four geochemically distinct magma sources are required for these Philippine plate magmas. The basin magmas tap Source 1, a MORB-mantle source that was contaminated by EMI (enriched mantle component 1 [31]) and Source 2, an OIB-like mantle source with some characteristics of EMII (enriched mantle component 2 [31]). The arc lavas are derived from Source 3, a MORB-source or residue mantle including Sr and Pb from the subducted oceanic crust, and Source 4, MORB-source or residue mantle including a component with characteristics of HIMU (mantle component with high U/Pb [31]). These same sources can account for many of the isotopic characteristics of recent Philippine plate arc and basin lavas. The enriched components in these sources which are associated with the DUPAL anomaly were probably introduced into the asthenosphere from the deep mantle when the Philippine plate was located in the Southern Hemisphere 60 m.y.b.p.  相似文献   

5.
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   

6.
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   

7.
Major- and rare-earth-element (REE) concentrations and UThPb, SmNd, and RbSr isotope systematics are reported for Cenozoic volcanic rocks from northeastern and eastern China. These volcanic rocks, characteristically lacking the calc-alkaline suite of orogenic belts, were emplaced in a rift system which formed in response to the subduction of the western Pacific plate beneath the eastern Asiatic continental margin. The rocks sampled range from basanite and alkali olivine basalt, through olivine tholeiite and quartz tholeiite, to potassic basalts, alkali trachytes, pantellerite, and limburgite. These rock suites represent the volcanic centers of Datong, Hanobar, Kuandian, Changbaishan and Wudalianchi in northeastern China, and Mingxi in the Fujian Province of eastern China.The major-element and REE geochemistry is characteristic of each volcanic suite broadly evolving through cogenetic magmatic processes. Some of the outstanding features of the isotopic correlation arrays are as follows: (1) NdSr shows an anticorrelation within the field of ocean island basalts, extending from the MORB end-member to an enriched, time-averaged high Rb/Sr and Nd/Sr end-member (EM1), (2) SrPb also shows an anticorrelation, similar to that of Hawaiian and walvis Ridge basalts, (3) NdPb shows a positive correlation, and (4) the 207Pb/204Pb vs 206Pb/204Pb plot shows linear arrays parallel to the general trend (NHRL) for MORB on both sides of the geochron, although in the 208Pb/204Pb vs 206Pb/204Pb plot the linear array is significantly displaced above the NHRL in a pattern similar to that of the oceanic island basalts that show the Dupal signatures. In all isotope correlation patterns, the data arrays define two different mantle components—a MORB-like component and an enriched mantle component. The isotopic data presented here clearly demonstrate the existence of Dupal compositions in the sources of the continental volcanic rocks of eastern China. We suggest that the subcontinental mantle beneath eastern China served as the reservoir for the EMI component, and that the MORB component was either introduced by subduction of the Kula-Pacific Ridge beneath the Asiatic plate in the Late Cretaceous, as proposed by Uyeda and Miyashiro, or by upwellings in the subcontinental asthenosphere due to subduction.  相似文献   

8.
Geochemical analyses of dikes, sills, and volcanic rocks of the Mesozoic Appalachian Tholeiite (MAT) Province of the easternmost United States provide evidence that continental tholeiites are derived from continental lithospheric mantle sources that are genetically and geochronologically related to the overlying continental crust. Nineteen olivine tholeiites and sixteen quartz tholeiites from the length of this province, associated in space and time with the last opening of the Atlantic, display significant isotopic heterogeneity: initial εNd = +3.8 to −5.7; initial 87Sr/86Sr= 0.7044−0.7072; 206Pb/204Pb= 17.49−19.14; 207Pb/204Pb= 15.55−15.65; 208Pb/204Pb= 37.24−39.11. In PbPb space, the MAT define a linear array displaced above the field for MORB and thus resemble oceanic basalts with DUPAL Pb isotopic traits. A regression of this array yields a secondary PbPb isochron age of ≈ 1000 Ma (μ1 = 8.26), similar to Sm/Nd isochrons from the southern half of the province and to the radiometric age of the Grenville crust underlying easternmost North America. The MAT exhibit significant trace element ratio heterogeneity (e.g., Sm/Nd= 0.226−0.327) and have trace element traits similar to convergent margin magmas [e.g., depletions of Nb and Ti relative to the rare earth elements on normalized trace element incompatibility diagrams, Ba/Nb ratios (19–75) that are significantly greater than those of MORB, and low TiO2 (0.39–0.69%)].Geochemical and geological considerations very strongly suggest that the MAT were not significantly contaminated during ascent through the continental crust. Further, isotope and trace element variations are not consistent with the involvement of contemporaneous MORB or OIB components. Rather, the materials that control the MAT incompatible element chemistry were derived from subcontinental lithospheric mantle. Thus: (1) the MAT/arc magma trace element similarities; (2) the PbPb and Sm/Nd isochron ages; and (3) the need for a method of introducing an ancient (> 2−3 Ga) Pb component into subcontinental mantle that cannot be much older than 1 Ga leads to a model whereby the MAT were generated by the melting of sediment-contaminated arc mantle that was incorporated into the continental lithosphere during arc activity preceding the Grenville Orogeny (≈ 1000 Ma).  相似文献   

9.
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   

10.
There are wide spread Cenozoic volcanic rocks in Tengchong (CVRT), Yunnan province, SW China. These rocks comprise three rock types: basalt, andesite (dominant type) and dacite. Most samples are sub‐alkaline, and among the sub‐alkaline rocks, most are high‐K calc‐alkaline. These rocks have a SiO2 range of 49.1 wt.% to 66.9 wt.%. TiO2 contents are not high and have a variation of 0.7 wt.%–1.6 wt.%. Trace element concentrations and element ratios (such as Nb/U, Ce/Pb, Nb/La, etc.) of these rocks have a large variation. 87Sr/86Sr values fall in the range of 0.7057–0.7093 and 143Nd/144Nd values change from 0.5120 to 0.5125. 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios are in the range of 17.936–19.039, 15.614–15.810, and 38.894–39.735, respectively. These geochemical characteristics of CVRT make them resemble island‐arc volcanic rocks. We suggest that the magmas were generated in the lithospheric mantle that had already been metasomatized by previous subduction processes. By the study of the uplift history of the Tibetan Plateau, we found that the beginning of the geotectonic processes to the eruption of CVRT was coeval with one uplift event. Therefore, we propose that the uplift of the Tibetan Plateau caused collapse of the collisional orogeny in Tengchong, which further triggered the generation and eruption of the CVRT magmas.  相似文献   

11.
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   

12.
The Oligocene–Pleistocene Tethyan Realm Lamproites, from the Mediterranean to Himalayas, have a clear subduction-related signature and represent one of the most exotic and rare ultrapotassic mantle-derived magmas. They share the major element and mineralogical characteristics of the lamproite clan rocks, but clearly define a distinct subgroup with respect to within-plate lamproites on the basis of a number of key trace element ratios and radiogenic isotopes. The most striking characteristic of the Tethyan Realm Lamproites is the positive correlation between Th/La (up to > 1.5) and Sm/La (up to > 0.3), which is opposite to what observed in subduction-related magmas worldwide and cannot be reconciled with typical slab recycling processes. The geochemical conundrum of the Tethyan Realm Lamproites requires a component with high Sm/La and Th/La (hence named SALATHO), in addition to a normal K2O–rich sediment melt component (with low Th/La and Sm/La). The Pb isotope composition of the Tethyan Realm Lamproites also displays a mixing array from a normal sediment melt component to a high 208Pb/206Pb and low 206Pb/204Pb end-member that can be reconciled with the SALATHO component. This end-member requires a history of high time-integrated κ? (232Th/238U) and low time-integrated μ (238U/204Pb) and ought to be older than some hundreds of million years.We propose a multi-stage process for the formation of the mantle sources of the Tethyan Realm Lamproites related to the tectonic mélange domains (i.e. chaotic mixture of depleted peridotite, basalt, and sediment) accreted to the Eurasia plate during the collisional events of the northward drifting continental slivers from Gondwana. In a first stage, the mélange domains experienced high P and low T metamorphism with segregation and stabilisation of lawsonite and zoisite/epidote veins, which potentially match the geochemical characteristics required by the SALATHO component. Successively, the subduction of the Neotethys and Alpine Tethys oceanic plates produced normal K2O-rich sediment melts that migrated through the mantle wedge and metasomatised the depleted lithospheric mantle blocks within the chaotic mélange, forming a clinopyroxene–phlogopite vein network. Eventually, the Tertiary orogenic belt collapses triggered the onset of low-degree melting of the low-solidus fractions within the mélange domains, producing lamproitic magmas.  相似文献   

13.
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   

14.
The isotopic compositions of Sr, Nd and Pb together with the abundances of Rb, Sr, U and Pb have been determined for mafic and felsic potassic alkaline rocks from the young Virunga volcanic field in the western branch of the East African rift system.87Sr/86Sr varies from 0.7055 to 0.7082 in the mafic rocks and from 0.7073 to 0.7103 in the felsic rocks. The latter all come from one volcano, Sabinyo. Sabinyo rocks have negative εNdvalues ofεNd = ?10. Nd and Sr isotopic variations in the basic potassic rocks are correlated and plot between Sabinyo and previously reported [1] compositions (εNd = +2.5;87Sr/86Sr≈ 0.7047) for Nyiragongo nephelinites. The Pb isotopic compositions for Sabinyo rocks are nearly uniform and average206Pb/204Pb≈ 19.4,207Pb/204Pb= 15.79–15.84,208Pb/204Pb≈ 41.2. The basic potassic rocks have similar206Pb/204Pb values but range in207Pb/204Pb and208Pb/204Pb from the Sabinyo values to less radiogenic compositions.Excellent correlations of87Sr/86Sr with Rb/Sr, 1/Sr and207Pb/206Pb for Sabinyo rocks suggest these to be members of a hybrid magma series. However, the nearly uniform Pb compositions for this series points to radiogenic growth of87Sr in the magma source region following an event which homogenized the isotopic compositions but not Rb/Sr. The Rb-Sr age derived from the erupted Sabinyo isochron-mixing line is consistent with the ～500 Myr Pb-Pb age from Nyiragongo [1], which suggests that this event affected all Virunga magma sources. The event can again be traced in the Pb-Pb, Pb-Sr and Nd-Sr isotopic correlations for all Virunga rocks, including Nyiragongo, when allowances are made for radiogenic growth subsequent to this mixing or incomplete homogenization event. Inferred parent/daughter element fractionations point to a metasomatic event during which a mantle fluid invaded two lithospheric reservoirs: a +εNd reservoir sampled by the Nyiragongo nephelinites and suggested to be the subcontinental mantle and a ?εNd reservoir sampled by the mafic and felsic potasssic volcanism. Whether this ?εNd reservoir is the crust, continental crustal material in the mantle or anomalous mantle cannot be decided from the data. The simplest answer, that this reservoir is the continental crust, seems to be at variance with experimental evidence suggesting a subcrustal origin for basic potassic magmas. Partial melting of the ancient metasomatised lithospheric domains and ensuing volcanism seems to be entirely a response to decompression and rising geotherms during rifting and thinning of the lithosphere.  相似文献   

15.
The isotopic composition of lead was determined for two carbonaceous, two H, and two L chondrites. All are falls. The206Pb/204Pb ratios cover a range from 9.45 to 37.33; the207Pb/204Pb ratios range from 10.39 to 26.10. The isotopic data define a207Pb/206Pb age of 4.635 AE. Uranium and lead concentration data indicate that the isotopic lead ages for the Bruderheim chondrite are concordant within approximately 20%. This contrasts with lead data in the literature for chondrites, which consistently indicate discordant isotopic lead ages due to large excesses of radiogenic lead by factors of two or more. The isotopic lead ages for Pultusk may be concordant; those for a sample of Richardton are not. The lack of concordance for Richardton is not due to the analytical procedures, rather it is likely a result of the handling history of the chondrite prior to analysis.The L-3 chondrite, Mezo-Madaras contains such a high concentration of lead - 5.27 PPM - that accurate ratios for primordial lead can be obtained. These are 206Pb/204Pb= 9.310;207Pb/204Pb= 10.296, values which are in close agreement with a recently reported measurement on lead in troilite from the Canyon Diablo iron meteorite.  相似文献   

16.
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   

17.
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18.
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   

19.
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   

20.
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   

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