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1.
We report the first precise U–Pb isotope data on cassiterite from the large Xianghualing tin-polymetallic deposit in the central Nanling district, South China. The results show that four separates from sample XF-51 have a relatively narrow range of 206Pb/238U apparent ages, varying from 152 to 157 Ma, and the three 206Pb/238U apparent ages yield a weighted average value of 156 ± 4 Ma (MSWD = 0.32). Separates from two other cassiterite samples do not have sufficient radiogenic Pb to generate a reliable 206Pb/238U age. Seven separates from the above three cassiterite samples define a well-constrained 238U–206Pb isochron corresponding to an age of 157 ± 6 Ma (MSWD = 34). A comparison of the U–Pb cassiterite ages with published Ar–Ar dates on muscovite from this deposit and K–Ar age data on biotite from the pluton genetically related to the tin mineralization in this area demonstrates that the U–Pb isotope system of cassiterite is a potential geochronometer. Combined with the Ar–Ar dates of muscovite from this deposit, we can constrain the absolute age of tin-polymetallic mineralization in Xianghualing at 154–157 Ma. The dates obtained in this study, consistent with the published geochronological results from other important deposits in this region, reveal that the large-scale tungsten–tin mineralization in the central Nanling region was predominantly emplaced during 150–161 Ma.  相似文献   

2.
In this paper, we evaluate the factors that influence the accuracy of lead (Pb)-isotopic ages of meteorites, and may possibly be responsible for inconsistencies between Pb-isotopic and extinct nuclide timescales of the early Solar System: instrumental mass fractionation and other possible analytical sources of error, presence of more than one component of non-radiogenic Pb, migration of ancient radiogenic Pb by diffusion and other mechanisms, possible heterogeneity of the isotopic composition of uranium (U), uncertainties in the decay constants of uranium isotopes, possible presence of “freshly synthesized” actinides with short half-life (e.g. 234U) in the early Solar System, possible initial disequilibrium in the uranium decay chains, and potential fractionation of radiogenic Pb isotopes and U isotopes caused by alpha-recoil and subsequent laboratory treatment. We review the use of 232Th/238U values to assist in making accurate interpretations of the U-Pb ages of meteorite components. We discuss recently published U-Pb dates of calcium-aluminum-rich inclusions (CAIs), and their apparent disagreement with the extinct nuclide dates, in the context of capability and common pitfalls in modern meteorite chronology. Finally, we discuss the requirements of meteorites that are intended to be used as the reference points in building a consistent time scale of the early Solar System, based on the combined use of the U-Pb system and extinct nuclide chronometers.  相似文献   

3.
This study considers the potential of using the U-Pb dating of garnet for determining quantitative P-T-t paths for the late Archean metamorphism in the Pikwitonei granulite domain. Garnets for U-Pb dating were selected mainly from samples that also provide information on pressure and temperature. The garnets used for dating were clear and free of any visible inclusions. Pb concentrations range from 63 ppb to 966 ppb and U from 136 ppb to 1143 ppb. The measured 206Pb/204Pb ratios range from 52.8 to 529.4. The ages are generally discordant with U/Pb ages that may lie above or below concordia. The discordance is caused by a recent disturbance of the U/Pb ratio in the garnets as indicated by replicate analyses on the same garnet separates that reproduce 207Pb/206Pb ages well within analytical uncertainty and in most cases within ±1.5 Ma at 2600–2750 Ma. High grade metamorphism continued over a period of at least one hundred million years, but the garnet-K-feldspar Pb-Pb ages suggest that, during this time, garnet growth has been favored during three distinct periods in the Cauchon Lake area: 2700–2687 Ma 2660–2637 Ma 2605–2591 Ma The ca. 2695 Ma garnet ages from Cauchon Lake date the time of melting and staurolite breakdown during prograde metamorphism, the ca. 2640 Ma ages date the time of extensive migmatization and the last period of metamorphic garnet growth, the ca. 2600 Ma ages date the time of crystallization of igneous garnet in late granitic intrusions. Peak metamorphism occurred around 2640 Ma followed by the intrusions of pegmatites starting at 2629 Ma. The Pb-Pb ages for garnet are similar to the U-Pb ages for zircon that date a leucocratic mobilizate (2695 Ma), a plagioclaseamphibole mobilizate (2637 Ma) and pegmatite (2598 Ma) (Heaman et al. 1986 a; Krogh et al. 1986; this study). Xenocrysts of garnet from 2600 Ma old graphic granites give minimum ages of 2984 Ma and 2741 Ma which are minima for the times of garnet growth in the source of the granites. The agreement of the zircon and garnet ages suggests that the metamorphism may have been punctuated by events that led to the development of melts or encouraged mineral growth at specific times. If so, the prograde and retrograde paths of metamorphism in the area may have contained minor excursions in pressure, temperature or fluid fugacities. In the Natawahunan Lake area some 50 km northwest of Cauchon Lake, garnet growth associated with the prograde breakdown of staurolite occurred at ca. 2744–2734 Ma. This suggests that a similar style of metamorphism may have occurred earlier in the Natawahunan Lake area than at Cauchon Lake area, or higher grades of metamorphism were reached earlier and were of longer duration associated with the somewhat greater depths in the Natawahunan Lake area. These results indicate the these garnets, which are 0.1–1 cm in diameter, have maintained closed system behavior for U and Pb at peak metamorphic conditions, i.e. temperatures up to 800° C and pressures of 7.5 kb.  相似文献   

4.
We have investigated the potential of hübnerite for U-Pb dating. Hübnerite forms typically at medium to low-temperatures in a wide range of pneumatolytic-hydrothermal mineral deposits, particularly porphyry molybdenum and Sn-specialized granites. Hübnerite from the Sweet Home Mine (Alma, Colorado) formed in a Pb-rich, U-poor environment, but still developed relatively radiogenic Pb isotopic compositions. The low Pbcommon contents in hübnerite (0.075 to 0.155 ppm) demonstrate that Pb is efficiently excluded from the crystal lattice. In contrast, U may substitute for Mn. The U-Pb data of hübnerite scatter. Most of the scatter originates from samples with 206Pb/204Pb values below 50, where Pbblank contributes up to 30% to Pbtotal. Using the least radiogenic galena Pb, samples with 206Pb/204Pb values above 70 have overlapping 206Pb∗/238U and 207Pb∗/235U values and yield a 206Pb/238U age of 25.7 ± 0.3 Ma (2σ). Late stage apatite from the Sweet Home Mine yields a 206Pb/204Pb-238U/204Pb isochron corresponding to an age of 24.8 ± 0.5 Ma (2σ). A comparison of the U-Pb hübnerite ages with literature 40Ar/39Ar ages on earlier sericite and the U-Pb age on later apatite suggests that (i) hübnerite yields accurate U-Pb ages and (ii) the evolution of the Sweet Home mineralization from greisen-type mineralization to medium-temperature hydrothermal vein mineralization took place in a few hundred thousand years at most. Aqueous low-N2-bearing and aqueous inclusions in the dated hübnerite have homogenization temperatures between 325 and 356 °C and moderate salinity (up to 6.7 wt% NaCl equiv.). Thus, hübnerite represents one of the rare examples of a mineral that can be dated accurately and carries petrological information.  相似文献   

5.
Monazite is accepted widely as an important U-Pb geochronometer in metamorphic terranes because it potentially preserves prograde crystallization ages. However, recent studies have shown that the U-Pb isotopic system in monazite can be influenced by a variety of processes that partially obscure the early growth history. In this paper, we attempt to interpret complex monazite and xenotime U-Pb data from three Paleoproterozoic granite dikes exposed in the Grand Canyon. Single-crystal monazite analyses from an unfoliated granite dike spread out along concordia from the crystallization age of the dike (defined by U-Pb zircon data to be 1685 ± 1 Ma) to 1659 ± 2 Ma, a span of 26 million years. Back-scattered electron (BSE) imaging reveals that magmatic domains within most crystals from this sample are truncated by secondary domains associated with prominent embayments at the grain margin. Fragments of a single crystal yield contrasting, concordant dates and fragments from the edges and tips of crystals yield the youngest dates. Based on these observations we suggest that the secondary domains formed at least 26 million years after the crystal formed. Monazite and xenotime dates from the second sample, a sheared dike that cross-cuts the previous dike, spread out along concordia over 16 million years and range up to 2.4% normally discordant. Again, BSE imaging reveals secondary domains that truncate both magmatic zoning and xenocrystic cores. Fragments sliced from specific domains of a previously imaged monazite crystal demonstrate that the secondary domain is 13 million years younger than the core domain. Textures revealed in BSE images suggest that the secondary domains formed by fluid-mineral interaction. Normal discordance appears to result from both radiation damage accumulated at temperatures below 300 °C and water-mineral interaction. Monazite data from the third sample exhibit dispersion in both the 207Pb/206Pb dates (1677–1690 Ma) and discordance (+ 1.6% to − 3.1%). Reverse discordance in these monazites cannot be explained by incomplete dissolution or excess (thorogenic) 206Pb. Sliced fragments from several crystals reveal dramatic intragrain U-Pb disequilibrium that does not correlate with either Th or U concentration or position within the crystal. We suggest that reverse discordance resulted from mechanisms that involve exchange or fractionation of elemental U or elemental Pb, and that neither the U-Pb dates nor the 207Pb/206Pb dates are reliable indicators of the rock's crystallization age. Given the large number of processes proposed in the recent literature to explain monazite U-Pb systematics from rocks of all ages, our results can be viewed as another cautionary note for single-crystal and multi-crystal monazite geochronometry. However, we suggest that because individual crystals can preserve a temporal record of primary and secondary monazite growth, micro-sampling of individual monazite crystals may provide precise absolute ages on a variety of processes that operate during the prograde, peak and/or retrograde history of metamorphic terranes. Received: 9 June 1996 / Accepted: 18 October 1996  相似文献   

6.
Garnets from different migmatites and granites from the Damara orogen (Namibia) were dated with the U-Pb technique after bulk dissolution of the material. Measured 206Pb/204Pb ratios are highly variable and range from ca. 21 to 613. Variations in isotope (208Pb/204Pb, 206Pb/204Pb) and trace element (Th/U, U/Nd, Sm/Nd) ratios of the different garnets show that some garnets contain significant amounts of monazite and zircon inclusions. Due to their very low 206Pb/204Pb ratios, garnets from pelitic migmatites from the Khan area yield Pb-Pb ages with large errors precluding a detailed evaluation. However, the 207Pb/206Pb ages (ca. 550–500 Ma) appear to be similar to or older than U-Pb monazite ages (530±1–517±1 Ma) and Sm-Nd garnet ages (523±4–512±3 Ma) from the same sample. It is reasonable to assume that the Pb-Pb garnet ages define growth ages because previous studies are consistent with a higher closure temperature for the U-Pb system in garnet relative to the U-Pb system in monazite and the Sm-Nd system in garnet. For igneous migmatites from Oetmoed, Pb-Pb garnet ages (483±15–492±16 Ma) and one Sm-Nd garnet whole rock age (487±8 Ma) are similar whereas the monazite from the same sample is ca. 30–40 Ma older (528±1 Ma). These monazite ages are, however, similar to monazite ages from nearby unmigmatized granite samples and constrain precisely the intrusion of the precursor granite in this area. Although there is a notable difference in closure temperature for the U-Pb and Sm-Nd system in garnet, the similarity of both ages indicate that both garnet ages record garnet growth in a migmatitic environment. Restitic garnet from an unmigmatized granite from Omaruru yields similar U-Pb (493±30–506±30 Ma) and Sm-Nd (493±6–488±7 Ma) garnet ages whereas the monazite from this rock is ca. 15–25 Ma older (516±1–514±1 Ma). Whereas the monazite ages define probably the peak of regional metamorphism in the source of the granite, the garnet ages may indicate the time of melt extraction. For igneous garnets from granites at Oetmoed, the similarity between Pb-Pb (483±34–474±17 Ma) and Sm-Nd (492±5–484±13 Ma) garnet ages is consistent with fast cooling rates of granitic dykes in the lower crust. Differences between garnet and monazite U-Pb ages can be explained by different reactions that produced these minerals at different times and by the empirical observation that monazite seems resistant to later thermal re-equilibration in the temperature range between 750 and 900 °C (e.g. Braun et al. 1998). For garnet analyses that have low 206Pb/204Pb ratios, the influence of high- inclusions is small. However, the relatively large errors preclude a detailed evaluation of the relationship between the different chronometers. For garnet with higher 206Pb/204Pb ratios, the overall similarity between the Pb-Pb and Sm-Nd garnet ages implies that the inclusions are not significantly older than the garnet and therefore do not induce a premetamorphic Pb signature upon the garnet. The results presented here show that garnet with low 238U/204Pb ratios together with Sm-Nd garnet data and U-Pb monazite ages from the same rock can be used to extract geologically meaningful ages that can help to better understand tectonometamorphic processes in high-grade terranes.Editorial responsibility: J. Hoefs  相似文献   

7.
Disturbance of the zircon U-Pb isotopic system has been investigated extensively, but mostly in lab, in the last decades. Here, we reported a field-based study on intensive sericitization, K-feldsparthization and the impacts of mylonitization on zircons from the Fangcheng syenites. The Fangcheng syenites occur in the eastern part of the Qinling orogen and consist mainly of aegirine-augite syenite, aegirine nepheline syenite, biotite syenite and hornblende nepheline syenite. Zircons from the slightly sericitized aegirine augite syenite are colorless, transparent crystals and exhibit well-developed oscillatory and sector zoning on the cathodoluminescence (CL) images which are typical of magmatic zircons from alkaline rocks. Zircon U-Pb determinations by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) showed that the syenite was formed in Neoproterozoic time, the weighted average of 206Pb/238U ages is 844.3±1.6 Ma (MSWD=0.86). In contrast, the hydrothermally altered zircons (hydrothermal zircon) from the intensively sericitized, K-feldsparthized, and weakly mylonitized aegirine augite syenite are conglomerates, yellowish to brown in color, generally translucent and internally textureless. The CL and backscatter electron (BSE) images of hydrothermal zircons exhibit fractured, textureless or mosaic textures, and occasionally show “sponge texture” with the veinlets and inclusions of K-feldspar; however, relicts of magmatic oscillatory zoning can still be discerned locally in individual grains. LA-ICPMS analyses of the hydrothermal zircons demonstrated that the zircons are chemically inhomogeneous, with enhanced and widely varied Pb, U, and Th contents. The U and Th contents of the hydrothermal zircons are estimated to be 32×10−6−1550×10−6 and 188×10−6−4059×10−6, respectively, with Th/U ratios within the range of 0.7–44.9. 206Pb/238U apparent ages of the hydrothermal zircons are negatively correlated with the contents of U, and radiogenic and common Pb. As the U and Th concentrations of the magmatic zircons are rather low, the α-decay doses (3.65×1014−2.04×1015 α-decay events/mg) are much lower than those at the first percolation point (3.5× 1015 a-decay events/mg), thus, Pb mobility resultant from diffusion could be safely neglected. Disturbance of the U-Pb isotopic system of zircons is most likely to be attributed to the intensive sericitization and K-feldsparthization coupled with mylonitization, the hydrothermal fluids reacted with zircons along the rims and fractures of the distorted zircon crystals, giving rise to the chemically inhomogeneous hydrothermal zircons through a series of complicated mechanisms such as dissolution-reprecipitation. The significance of the U-Pb age (the lower intercept age on the discordia U-Pb plot) of hydrothermal zircons is uncertain. We argued that interpretations of the hydrothermal zircon data as the age of hydrothermal events or hydrothermal ore-forming processes are questionable and hence cautions must be taken.  相似文献   

8.
Isotopic and geochemical data of the Zerenda series metamorphic rocks from the Kokchetav massif are reported. Some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism (P > 40 kbar, T = 900–1000 °C). The diamond-bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks. The REE patterns range from LREE depleted to slightly LREE enriched [chondrite normalized (La/Yb)N– 0.1–5.4] with a negative Eu anomaly. They are depleted in incompatible elements (e.g. Sr, Ba, U, Th) with respect to the upper crust. In contrast non-diamondiferous rocks of the Zerenda series exhibit normal crustal geochemistry. All rocks of the Zerenda series have very radiogenic lead isotopes. The measured μ values (238U/204Pb) compared with those calculated for the interval between crust formation and ultrahigh pressure (UHP) metamorphism suggest a decrease by factors of up to 200 during the UHP metamorphism. The Sm-Nd mineral isochrons from the diamond-bearing rocks and other rock types of the Zerenda series give a Middle Cambrian (524–535 Ma) age of metamorphism. The Nd model ages show that crust formation occurred about 2.3 Ga ago. Significant fractionation of Sm and Nd and loss of incompatible elements may be due to partial melting of the protoliths. The Ar-Ar age determinations of secondary biotite and muscovite from the diamond-bearing rocks yield an age of 517 ± 5 Ma. This cooling age requires a short time interval between UHP metamorphism and uplift to a crustal level. Ultrahigh pressure metamorphism might be a significant source of Pb for the mantle. We propose that the radiogenic Pb of the oceanic array is the contamination traces of numerous UHP events. Beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using Nd isotopes. We compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the Sm-Nd system during very high grade metamorphic events. Received: 14 August 1998 / Accepted: 1 June 1999  相似文献   

9.
An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1 Ga, although there is no record of rocks older than 1.6 Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have 207Pb/206Pb ages between 1367 and 1939 Ma, with frequency maxima in the range 1.85 to 1.70 Ga and 1.60 to 1.50 Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core 207Pb/206Pb age of 2383 Ma. Two discrete, unzoned zircons have 207Pb/206Pb ages of 1122 and 1133 Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent “inner cores” which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant 206Pb/238U versus 207Pb/235U ratios and maximum 207Pb/206Pb ages of 1438 Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0 Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward younging trend of the Precambrian crust, inferred from the southeastern part of the Baltic Shield. Received: 8 April 1997 / Accepted: 14 July 1997  相似文献   

10.
U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The 206Pb/204Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206Pb/204Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207Pb/204Pb vs 204Pb/204Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed 87Sr/86Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970).The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.  相似文献   

11.
长江铀矿田位于诸广山复式岩体中南部,是典型的花岗岩型铀矿田.前人采用锆石U-Pb定年方法对赋矿花岗岩进行了年代学研究,但由于全岩和锆石铀含量较高,锆石往往发生了蜕晶化,可能导致锆石U-Pb定年数据散乱,影响锆石U-Pb年龄的可靠性.独居石是花岗岩中广泛存在的含铀副矿物,铀和钍含量均较高,可达10000×10-6,普通铅...  相似文献   

12.
Lead isotope analyses of galena from five ore deposits and six prospects in the Aysén region of southern Chile are reported. Most of the deposits are either low sulfidation epithermal gold–silver veins or skarn and manto deposits; the majority are either suspected to be, or dated as, Late Jurassic to mid-Cretaceous. Galena lead isotope data for most of the deposits from southern Chile cluster near the “orogene” within a “plumbotectonic” model framework. Average values (206Pb/204Pb=18.53, 207Pb/204Pb=15.63, and 208Pb/204Pb=38.50) are near Jurassic to Cretaceous model ages on the “orogene” curve of Zartman and Doe (1981) and the second-stage curve of Stacey and Kramers (1975) on a 206Pb/204Pb versus 207Pb/204Pb plot. These model ages are compatible with absolute ages as currently known. The elongate trends in the general cluster indicate mainly an orogenic model fit, suggesting variable mixing of lead from different sources, mainly model upper crust and lesser model mantle and lower crust reservoirs. Galena lead associated with one deposit (El Faldeo) is relatively radiogenic, and lies near a Jurassic age on the “upper crustal” curve of Zartman and Doe (1981), which is compatible with the Ar/Ar age of the deposit. Galena lead isotope clusters define three main groups of deposits. These three groups appear to be related to three mineralizing events, dated by K–Ar and Ar/Ar, in the Late Jurassic (group 3), and in the Early and mid-Cretaceous (groups 1 and 2 respectively). Averages for group 1, the northern group including El Toqui and Katerfeld, are 206Pb/204Pb=18.51, 207Pb/204Pb=15.62, 208Pb/204Pb=38.48. Averages for group 2, the southern group with Fachinal and Mina Silva, are 206Pb/204Pb = 18.56, 207Pb/204Pb=15.63, 208Pb/204Pb=38.52. Averages for group 3, the southernmost group with the El Faldeo, Lago Chacabuco and Lago Cochrane prospects, are 206Pb/204Pb=18.83, 207Pb/204Pb=15.65, 208Pb/204Pb=38.63. The Cretaceous deposits (groups 1 and 2) contain orogene-type lead that becomes increasingly radiogenic southward. Lead from the Late Jurassic deposits (group 3) appears to reflect mixing of orogene lead with highly radiogenic lead. The observed linear array of lead in group 3 probably reflects mixing of orogene lead with highly radiogenic lead, which was likely extracted by selective leaching of mineralizing hydrothermal solutions from the metamorphic basement. Received: 10 July 1999 / Accepted: 15 July 2000  相似文献   

13.
Uranium and lead concentrations and the isotopic compositions of lead were determined on samples of total rock, matrix, white inclusion, pink inclusion, white aggregate and four chondrules from the Allende carbonaceous chondrite. Observed 206Pb/204Pb ratios varied from 10.004 to 107.29; 207Pb/204Pb ratios from 10.695 to 69.07; 206Pb/204Pb ratios from 30.062 to 207.96. In a 207Pb/204Pb-206Pb/204Pb diagram a regression line fitted to all of data has a slope of 0.6240 ± 0.0015, corresponding to a single stage model age of 4.565 ± 0.004 AE. The regression line also includes the ratios for primordial lead as determined in previous investigations from Canyon Diablo troilite and the Mezö-Madaras chondrite.Although the lead in the matrix is not very radiogenic, the 207Pb/206Pb ages of four samples average 4.505 AE, a value 0.06 AE younger than that of the chondrules and inclusions. The matrix age agrees closely with a total rock Pb/Pb model age previously reported for Allende by Tatsumoto, Knight and Allegre. The matrix Pb/Pb model age is also 0.06 AE younger than the Pb/Pb isochron ages determined by previous investigators on total samples of H and L chondrites. The H and L chondrite and Allende chondrule and inclusion Pb/Pb ages are indistinguishable. The lead isotope systematics require either that the matrix is ca. 0.06 younger than the silicate inclusions and chondrules (or that radiogenic lead was inherited from a younger external source) or that the initial lead in the matrix differed from primordial lead. The lead data cannot be reconciled to a model in which the bulk material of Allende first crystallized 4.57 AE ago, followed by transfer of radiogenic lead between phases since that time.In a concordia diagram four chondrules and three inclusions plot along a chord intersecting concordia at 4.57 and 0.28 ± 0.07 AE. This indicates disturbance of the U-Pb systems relatively recently, perhaps around 0.3 AE ago. The time of disturbance is not readily understood and needs further confirmation. It correlates most closely with a possible cut-off in K-Ar and U, Th-He ages of chondrites.Although the Th/U ratios of the bulk samples and matrix are around the normal value of 3.8, much higher values are observed in some of the inclusions, the highest being 9.0.  相似文献   

14.
 The Lengenbach Pb-Zn-As-Tl-Ba mineralisation is located in Triassic dolostones of the Penninic zone in the Swiss Alps where Alpine metamorphism reached upper greenschist to lower amphibolite grade. Geochemical data are used to constrain the origin of this unique occurrence. Two metamorphic redox environments are present: the As(III)-rich zone is controlled by barite-pyrite while the reduced zone contains graphite or pyrrhotite-pyrite and formally zerovalent As. The As(III)-rich zone is characterised by a mineral assemblage consistent with fO2 in the stability field of barite+pyrite. An As-(Pb, Tl)-rich sulphide melt coexisted with a hydrothermal fluid at >kk300 °C in this zone. Mineralised dolostones are anomalous in As, Pb, Ag, Tl, Hg, Zn, Ba, Cd, Fe, Cu, Mo, U, V, B, Ga, Cr and possibly Sn and Au (in order of decreasing enrichment). As, Pb and Zn are present in the 0.1 to 1% range, Tl and Ag reach several hundred ppm. Uraninite is concentrated in silicate-rich bands and yields a late Alpine U-Pb age of 18.5±0.5 Ma. Pb- and S isotopic variations are interpreted by metamorphic overprinting and re-equilibration within an isochemically metamorphosed mineralisation. Hydrothermal sulphides are more strongly affected by uranogenic Pb than massive Pb-As-sulphides representing a former sulphide melt. The least overprinted mineralisation is characterised by 206Pb/204Pb U003U=18.44−18.56, 207Pb/204Pb=15.60−15.75, 208Pb/204Pb =38.44−38.84 and δ34S (sulphide)=−25±2‰. S isotopic variations are largely a result of sulphide-sulphate re-equilibration yielding temperatures of 450± 30 °C. 87Sr/86Sr ratios of mineralised samples are lower than or equal to host dolostones, precluding major infiltration of basement-derived fluids during Alpine metamorphism. The Sr source (87Sr/86Sr close to 0.708) probably was seawater with a radiogenic, detrital mineral component. The genesis of the unique Lengenbach mineralisation is interpreted as the result of isochemical metamorphic overprinting of a carbonate hosted stratiform sulphide mineralisation. Well-crystallised sulphide minerals in fissures and druses formed during retrograde cooling of a sulphide melt in equilibrium with a hydrothermal fluid. The primary mineralisation was probably formed at or close below the sea floor and fed by sulphide-poor hydrothermal fluids. Sulphide was largely derived from seawater by open system bacterial sulphate reduction. U, V and Mo may be seawater-derived. Received: 1 February 1995/Accepted: 10 January 1996  相似文献   

15.
Base metal–Ag mineralisation at Dikulushi and in other deposits on the Kundelungu Plateau (Democratic Republic of Congo) developed during two episodes. Subeconomic Cu–Pb–Zn–Fe polysulphide ores were generated during the Lufilian Orogeny (c. 520 Ma ago) in a set of E–W- and NE–SW-oriented faults. Their lead has a relatively unradiogenic and internally inhomogeneous isotopic composition (206Pb/204Pb = 18.07–18.49), most likely generated by mixing of Pb from isotopically heterogeneous clastic sources. These sulphides were remobilised and enriched after the Lufilian Orogeny, along reactivated and newly formed NE–SW-oriented faults into a chalcocite-dominated Cu–Ag mineralisation of high economic interest. The chalcocite samples contain only trace amounts of lead and show mostly radiogenic Pb isotope signatures that fall along a linear trend in the 207Pb/204Pb vs. 206Pb/204Pb diagram (206Pb/204Pb = 18.66–23.65; 207Pb/204Pb = 15.72–16.02). These anomalous characteristics reflect a two-stage evolution involving admixture of both radiogenic lead and uranium during a young fluid event possibly c. 100 Ma ago. The Pb isotope systematics of local host rocks to mineralisation also indicate some comparable young disturbance of their U–Th–Pb systems, related to the same event. They could have provided Pb with sufficiently radiogenic compositions that was added to less radiogenic Pb remobilised from precursor Cu–Pb–Zn–Fe polysulphides, whereas the U most likely originated from external sources. Local metal sources are also suggested by the 208Pb/204Pb–206Pb/204Pb systematics of combined ore and rock lead, which indicate a pronounced and diversified lithological control of the immediate host rocks on the chalcocite-dominated Cu–Ag ores. The Pb isotope systematics of polysulphide mineralisation on the Kundelungu Plateau clearly record a diachronous evolution.  相似文献   

16.
We present the results of a comparative study of several geochronometer minerals (baddeleyite, zircon, apatite, phlogopite and tetraferriphlogopite) and isotopic systems (U-Pb, Th-Pb and Rb-Sr) from phoscorites (magnetite-forsterite-apatite-calcite rocks) and carbonatites of the Kovdor ultramafic-alkaline-carbonatite massif, Kola Peninsula, Russia. Uranium, thorium and their decay products are extremely fractionated by minerals that crystallise from carbonatite and phoscorite magma. We obtain high-precision ages from different chronometers, compare their accuracy, and evaluate the role of geochronological pitfalls of initial radioactive disequilibrium, differential migration of radiogenic isotopes, and inaccurate decay constants.Apatite yielded concordant U-Th-Pb ages between 376 and 380 Ma. The accuracy of the apatite 238U-206Pb ages is, however, compromised by uncertainty in the amount of radiogenic 206Pb produced from initial excess 230Th. The 235U-207Pb ages are relatively imprecise due to large common Pb correction and the uncertainty in the initial Pb isotopic composition. The Th-Pb system yields a more precise age of 376.4 ± 0.6 Ma.Zircon from two carbonatite samples is characterised by moderate to low U contents, high Th contents, and very high Th/U ratios up to 9000. The 206Pb*/238U systems in the zircon are strongly affected by the presence of excess 206Pb*, produced by decay of initial 230Th. The 208Pb*/232Th ages of zircon from both carbonatite samples are uniform and yield a weighted average of 377.52 ± 0.94 Ma.Baddeleyite U-Pb analyses are 3 to 6% normally discordant and have variable 207Pb*/206Pb* apparent ages. Eleven alteration-free baddeleyite fractions from three samples with no evidence for Pb loss yield uniform 206Pb*/238U ages with a weighted average of 378.54±0.23 Ma (378.64 Ma after correction for initial 230Th deficiency), which we consider the best estimate for age of the phoscorite-carbonatite body of the Kovdor massif. The 206Pb*/238U ages of baddeleyite fractions from five other samples spread between 378.5 and 373 Ma, indicating a variable lead loss up to 1.5%. The anomalously old 207Pb/235U and 207Pb/206Pb ages are consistent with the presence of excess radiogenic 207Pb* in the baddeleyite. We interpret this as a result of preferential partitioning of 231Pa to baddeleyite.Fifteen phlogopite and tetraferriphlogopite fractions from five carbonatite and phoscorite samples yielded precise Rb-Sr isochron age of 372.2 ± 1.5 Ma, which is 5 to 7 m.y. younger than our best estimate based on U-Th-Pb age values. This difference is unlikely to be a result of the disturbance or late closure of Rb-Sr system in phlogopite, but rather suggests that the accepted decay constant of 87Rb is too high.Comparative study of multiple geochronometer minerals from the Kovdor massif has revealed an exceptional complexity of isotopic systems. Reliable ages can be understood through systematic analysis of possible sources of distortion. No single geochronometer is sufficiently reliable in these rocks. Th-Pb and Rb-Sr can be a very useful supplement to U-Pb geochronometry, but the routine use of these geochronometers together will require more precise and accurate determination of decay constants for 232Th and 87Rb.  相似文献   

17.
Granites and primary tin mineralization in the Erzgebirge were dated using (1) conventional U–Pb dating of uraninite inclusions in mica, (2) Rb–Sr dating of inclusions in quartz that represent highly evolved melts, (3) Re–Os dating of magmatic–hydrothermal molybdenite, and (4) chemical Th–U–Pb dating of uraninite. Conventional isotope dilution and thermal ion mass spectrometry and chemical Th–U–Pb dating of uraninite in granites from the Ehrenfriedersdorf mining district provide ages of 323.9 ± 3.5 Ma (2σ; Greifenstein granite) and 320.6 ± 1.9 and 319.7 ± 3.4 Ma (2σ, both Sauberg mine), in agreement with U–Pb apatite ages of 323.9 ± 2.9 and 317.3 ± 1.6 Ms (2σ, both Sauberg mine). Rb–Sr analysis of melt inclusions from Zinnwald gives highly radiogenic Sr isotopic compositions that, with an assumed initial Sr isotopic composition, permit calculation of precise ages from single inclusions. The scatter of the data indicates that some quartz-hosted melt inclusions have been affected by partial loss of fluid exsolved from the melt inclusion. Re–Os dating of two molybdenite samples from Altenberg provides ages of 323.9 ± 2.5 and 317.9 ± 2.4 Ma (2σ). Together with age data from the literature, our new ages demonstrate that primary tin mineralization and the emplacement of the large Sn-specialized granites in the Erzgebirge fall in a narrow range between 318 and 323 Ma. Primary Sn mineralization occurred within a short interval during post-collisional collapse of the Variscan orogen and was essentially synchronous over the entire Erzgebirge. In contrast to earlier claims, no systematic age difference between granites of the eastern and western Erzgebirge was established. Furthermore, our data do not support a large age range for Late-Variscan granites of the Erzgebirge (330–290 Ma), as has been previously suggested.  相似文献   

18.
Ion microprobe U-Th-Pb isotopic analyses of zircons from a granodioritic orthogneiss from the Napier Complex, Mount Sones, Enderby Land, Antarctica, have identified an unambiguous example of unsupported radiogenic Pb in a 3,950 Ma-old crystal. At one 40 m spot on the crystal an unusually large heterogeneity in Pb content was found, the concentration of radiogenic Pb ranging from 5 to 50 percent higher than could have been generated in 3,950 Ma by radioactive decay of the co-existing U and Th. This relative excess of radiogenic Pb is attributed to Pb gain rather than to U and Th loss because first, the Pb content varied by more than the U or Th contents and secondly, changes in the Pb/U, Pb/Th and Pb isotopic composition correlated directly with changes in the Pb concentration. The individual 207Pb/206Pb apparent ages ranged from 4,000 Ma to 4,145 Ma, all greater than the inferred age of the crystal. A correlation between 207Pb/206Pb and Pb/U shows that the Pb excess has not resulted from recent Pb movement. The spot apparently gained radiogenic Pb about 2,500 Ma ago, at the same time as the majority of the other zircons in the rock suffered substantial Pb loss. The Pb movement occurred in response to a discrete geologic event. Reverse discordance is a phenomenon that must be considered when interpreting zircon U-Pb ages, especially 207Pb/206Pb ages of single crystals or portions of crystals. Decay constants. Ages in this paper are calculated using the decay constant convention recommended by the Subcommission on Geochronology (Steiger and Jaeger 1977).  相似文献   

19.
The Austroalpine basement to the south of the Tauern Window once was part of the northern margin of Gondwana. It includes the “Altkristallin” and the phyllitic Thurntaler Complex. In the Altkristallin (AMU, MPU), suites of arc-related metamafic sequences occur together with calc-alkaline metagranite. SHRIMP U–Pb dating of zircon from calc-alkaline metagranite associated with an eclogitic amphibolite give an age of 470 ± 3 Ma interpreted as the age of protolith emplacement. In the Thurntaler Complex, metaporphyroids occur together with tholeiitic as well as alkaline within-plate basalt-type metabasite. The metaryholites of this association give a crystallization SHRIMP age of 477 ± 4 Ma, which suggests contemporaneity of arc-related and extensional settings in the Austroalpine basement units. The age data demonstrate widespread magmatic activity associated with the Early-Ordovician amalgamation at the end of the 550–470 Ma subduction–accretion–collision cycle. The Pb–Pb and U–Pb systematics of step-wise leached staurolite and kyanite from the peak-metamorphic assemblage of the Altkristallin indicate that (1) step-wise leaching of staurolite and kyanite yields the age of inclusions rather than the host; (2) zircon inclusions in staurolite suggest an Ordovician or older age for the precursor of the staurolite-schists; (3) the weighted average of the 206Pb/238U data of the various leaching steps yields a Variscan age for the inclusions (ilmenite, biotite, and andesine). Since these inclusions are part of the metamorphic mineral assemblage, this age provides a minimum estimate for staurolite growth, i.e., metamorphism. Thus, the Pb–Pb and U–Pb systematics of staurolite provide evidence for a Variscan metamorphism of the Austroalpine basement, e.g., MPU, AMU and Thurntaler Complex, to the south of the Tauern Window.  相似文献   

20.
Iron oxide–apatite (IOA) deposits, often referred to as Kiruna-type iron ore deposits, are known to have formed from the Proterozoic to the Tertiary. They are commonly associated with calc–alkaline volcanic rocks and regional- to deposit-scale metasomatic alteration. In the Bafq District in east Central Iran, economic iron oxide–apatite deposits occur within felsic volcanic tuffs and volcanosedimentary sequences of Early Cambrian age. In order to constrain the age of formation of these ores and their relationship with the Early Cambrian magmatic event, we have determined the U–Pb apatite age for five occurrences in the Bafq District. In a 206Pb/238U vs. 207Pb/235U diagram, apatite free of or poor in inclusions of other minerals plots along the Concordia between 539 and 527 Ma with four out of five samples from one deposit clustering at the upper end of this range. For this deposit, we interpret this cluster to represent the age of apatite formation, whereas the spread towards younger ages may reflect either minor Pb loss or several events of IOA formation. Apatite with inclusions of monazite (±xenotime) yields disturbed systems with inclusions having developed after formation of the iron ore–apatite deposits, possibly as late as 130–140 Ma ago. Obtained apatite ages confirms that (IOA) and the apatite-rich rocks (apatites) of the Bafq district formed coevally with the Early Cambrian magmatic (-metasomatic) events.  相似文献   

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