Concordia Diagram Research Articles

U-Pb systematics of detrital zircons from low-grade metamorphic sandstones of the Trinity Peninsula Group (Antarctica)

U-Pb isotope analyses of detrital euhedral zircons from two Triassic metasandstones of the Trinity Peninsula Formation demonstrate that: ( i) zircons less than 125 μm in size are cogenetic; and ( ii) in a Concordia diagram, the different zircon populations define a fan-shaped area with an origin on the concordia of approximately 320 Ma. This Carboniferous age is interpreted as the final event affecting a heterogeneous population of zircons before they returned into the exogenous cycle. The U-Pb systematics of rare pink, well rounded zircons define a zircon suite with an older, Cambrian to Precambrian age information. The morphology of the dominant euhedral zircons, which reflects a granitic environment of formation, and their U-Pb data suggest that the Triassic sedimentary basin was surrounded by granitoid to migmatic rocks of Carboniferous age.

60025: relict of primitive lunar crust?

New isotopic analyses are presented for 3 plagioclase-rich fractions and one mafic fraction from ferroan anorthosite 60025. The observed 206Pb/ 204Pb ratios vary between 52.5 and 60.5, all much higher than the ratio for terrestrial contamination. In a 207Pb/ 206Pb 204Pb/ 206Pb correlation diagram, the plagioclase data define a model Pb Pb 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 observed 206Pb/ 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 the 206Pb/ 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 U Pb 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.

Evidence for old Precambrian relics shown by UPd zircon dating of eclogites and associated rocks in the Hercynian belt of South Brittany, France

UPb dating of zircon from an eclogite of the Champtoceaux Complex, south Brittany, has yielded an upper intercept age of 2208 ± 39 Ma. This age is interpreted as representing contamination processes or the emplacement of an eclogite protolith having geochemical affinities with intermediate rocks of talc-alkaline magmatic series. The lower intercept on the concordia diagram is interpreted as the age of the high-pressure-high-temperature metamorphic event at 413 ± 16 Ma. Results obtained on a serpentinite from the Baie d'Audierne Complex indicate an ultrabasic protolith crystallization age older than 840 Ma. Taken in conjunction with published upper intercept ages of 1300 Ma for the eclogite protoliths from the Vendée and the Baie d'Audierne, the present data would suggest the existence of relics of a Precambrian basement in south Brittany. The geological history of this basement can be summarized as follows: 1. (1) a magma of intermediate composition emplaced during the early Proterozoic, around 2.2 Ga ago, in an active margin-type tectonic environment. This date is in agreement with the possible existence of continental crust as old as Archaean in south Brittany, as shown by the presence of inherited zircons in Palaeozoic formations with upper intercept ages around 2.5 Ga; 2. (2) eclogite protoliths were preserved in a continental crustal environment as far as 440-380 Ma; 3. (3) around 400 Ma eclogite-facies metamorphism affected the ancient basement rocks of south Brittany, probably as a result of continental collision during the early stage of the Hercynian orogeny. The regional zonation of the Siluro-Devonian high-pressure event (436-413-384 Ma) throughout the south Brittany area could be due to a succession of small collisions or related to the convergence of plates with irregular outlines.

Early planetary metamorphism in chondritic meteorites

Lead isotope relations were studied in whole rock and separated phases of Mezö-Madaras (L3) and Sharps (H3) chondrites in order to study the record of early events in the solar system and to seek further information on the isotopic composition of primordial lead. The internal 207Pb/ 206Pb ages are4.480 ± 0.011AE (1AE= 10 9 years) for Mezö-Madaras and4.472 ± 0.005AE for Sharps. The ages are not significantly changed when Canyon Diablo troilite lead is included in the data sets, suggesting that the initial Pb isotopic composition in both meteorites was the same as that in the troilite. U Pb data from both meteorites plot along chords in concordia diagrams that indicate recent disturbances inU/Pb ratios. The chords are poorly defined owing to the relatively non-radiogenic character of the lead isotopes. Rb Sr measurements on Sharps likewise fail to yield an isochron, in agreement with the U Pb data. Data from the literature indicate a similar disturbance in the Rb Sr system for Mezö-Madaras. The 4.48 AE ages could be caused by pre-analysis contamination with terrestrial lead, however statistical comparison of isotope correlations between the acid-washes of analyzed samples and the residual washed samples suggests that the ages are real and not due to terrestrial contamination. The 4.48 AE age, which is distinctly younger than the well-established ages of 4.54–4.56 AE for the Allende chondrite and Angra dos Reis achondrite, appears to date an early metamorphic event rather than the formation of the chondrites. Rb Sr, Sm Nd and K Ar ages in support of the 4.48 AE metamorphic event are reviewed. Such a metamorphic age is not necessarily in conflict with 129I/ 129Xe data which indicate that the parent material of most chondrites, including those of type 3, cooled through temperatures sufficient to retain radiogenic Xe within a time interval of ca. 0.02 AE.

Mineralogical, chemical and isotopic (U-Pb method) studies of Hercynian uraniferous mineralizations (Margnac and Fanay mines, Limousin, France)

Mineralogical and chemical studies of pitchblendes, pyrites and coffinites from the Margnac and Fanay mines (Limousin, France) indicate that chemical exchanges between uraniferous solutions and vein wall-rocks are reflected in the chemical compositions of pitchblendes. An age of 276 Ma is given for the primary pitchblende—pyrite association by the U-Pb method. Isotopic analysis of Pb in galena, crystallized during coffinitization of pitchblende, gives an interval of 6 Ma between pitchblende precipitation and its remobilization as coffinite. Analyses by electron microprobe indicate that this hydrothermal transformation affects more or less all the pitchblende spherulites, even if the volume of coffinite is low. This leads to Si-enriched pitchblende and explains the systematic age of 270 Ma obtained by Pb-Pb and concordia diagrams. A mineralogically undetected remobilization stage is also found in some samples at 170–175 Ma. Finally, this work specifies the life time (20–30 Ma) of the geothermal system which is considered to be responsible for the episyenitization of the two-mica granite, the precipitation of pitchblende and its remobilization as coffinite.

Rb–Sr and U–Pb dating of bentonites

A 6 in. (15 cm) bentonite in the Z coal (Cretaceous–Tertiary boundary) in eastern Montana was sampled at four different places, and biotite, sanidine, and zircon were separated from the clay. U–Pb analyses of purified zircons yielded small systematic variations from concordant U–Pb dates. Plotting the data on a concordia diagram, a short linear discordia line intersects the concordia at [Formula: see text] with an MSWD of 1.16. The systematic variation of the four sets of zircon U–Pb data on the concordia plot may be an artifact of the sampling and purification procedure, or could result from natural sample variation from minor contamination. Biotite fractions of varying specific gravity were obtained for each of the four Z coal bentonite samples and (together with the matching purified sanidine fraction) were analysed for Rb–Sr dating. Excluding those lighter biotite fractions found to have lost 30% or more of their original Rb, an isochron was obtained giving an age of 63.7 ± 0.3 Ma with an initial 87Sr/86Sr ratio of 0.7061 ± 1 and an MSWD of 1.07.To investigate further the Rb–Sr variations in altered bentonite biotite, a large biotite sample from the Ordovician Kinnekulle A1 bentonite of southwestern Sweden was separated into 11 fractions of decreasing specific gravity. Rb–Sr analysis of these fractions also showed a departure from a linear isochron when more than about 30% of the original Rb had been lost. Chemical analysis and X-ray diffraction revealed the biotite-alteration process to be vermiculitization, but gave no definite reason why biotites that retain more than 70% of their original Rb give usable Rb–Sr data. Though some of the alteration may have taken place when the bentonite was deposited as an ash, most of the alteration probably occurred in recent times. The Kinnekulle A1 bentonite Rb–Sr isochron for biotite and sanidine gives an age of 447 ± 1 Ma with an initial 87Sr/86Sr ratio of 0.7094 ± 0.0003 and an MSWD of 3.7.

U–Pb geochronology of the Bokan Mountain peralkaline granite, southeastern Alaska

The Bokan Mountain arfvedsonite–aegirine granite is the only peralkaline acid intrusion actually known on the margin of the Canadian Cordillera. It is the end product of a peralkaline magmatic evolution and is characterized by genetically associated concentrations of U and Th. The U–Pb method on zircons has been used to date two samples: one from a barren peralkaline granite and one from a low-grade U–Th mineralized albitite. A typological study has revealed two genetically different populations of zircons in the barren granite. The upper intercept age of 171 ± 5 Ma obtained on a concordia diagram dates the emplacement of the peralkaline granite. Acid wash experiments on zircons have allowed us to remove important quantities of common lead; this point is discussed as well as the abnormally low Pb content of the feldspar extracted from the barren granite.A Jurassic period of magmatic activity must be integrated into the geological history of Prince of Wales Island. Peralkaline magmatism may have occurred when the Alexander Terrane rifted away from its more southerly area of origin.

Concordant 230Th and 231Pa ages of marine phosphorites

The reliability of uranium-series age determinations of marine phosphorites has been tested by simultaneous determination of 230 Th/ 234 U and 231 Pa/ 235 U. The measured 230 Th/ 234 U and 231 Pa/ 235 U activity ratios in representative phosphorite samples from the continental margins of Eastern Australia and Peru are consistent with the theoretically expected development with time of these isotope ratios on a concordia diagram and hence support the assumption that the initial 230 Th and 231 Pa contents are insignificant in comparison with the uranium contents, and that the phosphorites have remained a chemically closed system with respect to uranium, thorium and protactinium. Alpha-recoil-induced leakage of 234 U from the phosphorites appears to be too slow to cause any serious errors in the 230 Th ages, and hence can be neglected. These results demonstrate that reliable uranium-series age determinations can be obtained for marine phosphorites on the basis of a simple, closed system model.

U-Pb investigations on zircons from pre-Variscan gneisses—I. A study from the Schwarzwald, West Germany

Field observations in the Schwarzwald indicate that pre-Variscan rocks were formed by the following events: 1. (1) Intrusion of granitic to granodioritic magma into sediments. 2. (2) A sequence of two anatectic events with an intervening regional metamorphism and development of a gneissic schistosity. The absolute chronology of these older gneisses and migmatites has not yet been satisfactorily resolved. We collected several samples of a coarse-grained, homogeneous orthogneiss of granodioritic composition and one sample of a metasedimentary source rock related to this orthogneiss body in the Einetwald forest north of Steinach (Kinzig valley). The metasedimentary rock corresponds to the biotitepoor paragneisses in the Schwarzwald. The zircons in the latter sample are detrital and only very small specimens (⩽ 45μm) are euhedral. The zircons from the orthogneiss samples are euhedral. Some are zoned and some have dark inclusions. Analyses of the three coarser fractions of the paragneiss form a linear array on a concordia diagram with upper and lower intercepts of about 2000 Myr and 524 ± 20 Myr, respectively. The 2000 Myr value is interpreted as the average age of the sedimentary source rocks for this material. The 524 Myr intercept defines an event responsible for most of the lead-loss in the zircons as well as new zircon growth. Analyses of the zircons from the orthogneiss samples do not show a linear array on the concordia diagram because of inclusions of older inherited zircons. Stepwise leaching of two zircon fractions shows newly grown U-rich zircon rims around old cores. A 207Pb 206Pb - age of 517 ± 15 Myr was found for the first dissolution steps (about 50% of the zircons dissolved). In subsequent steps with progressive dissolution of the residue. 207Pb 206Pb - age increased to 950 Myr. The U-Pb zircon data for the paragneiss and the orthogneiss establish an age of 520 ± 15 Myr for the magma intrusion.

U-Pb Studies of Zircon Cores and Overgrowths, and Monazite: Implications for Age and Petrogenesis of the Northeastern Idaho Batholith

U-Pb isotopic studies of zircons, many containing xenocrystic cores with euhedral overgrowths, and monazite from igneous rocks and metasedimentary inclusions of the northeastern Idaho batholith yield linear arrays on concordia diagrams. We interpret these as mixing lines between an old component (cores) and a young component (overgrowths and zircons without cores). The lower intercept of such arrays with concordia may yield the minimum age of the rocks if the overgrowths and zircons without cores are discordant, or the crystallization age if they are concordant. Monazites yield apparently concordant ages either equal or less than the lower intercept zircon ages. The samples studied yield lower intercept ages ranging from $73.5 \pm 6 m.y.$ (foliated quartz diorite) to $46.5 \pm 1 m.y.$ (feldspar megacryst granite); ages obtained are consistent with crosscutting relations observed in the field. Upper intercepts yield ages of 1700 to 2349 m.y. These are interpreted to indicate the mean age of xenocrystic zir...

Uranium-lead age of the Bruderheim L6 chondrite and the 500-Ma shock event in the L-group parent body

Measured Pb-Pb whole meteorite data for the Bruderheim L6 chondrite scatter slightly about a line passing above Can˜on Diablo lead and yielding an age of 4.482 + 0.017 Ga, using the terrestrial 238U/ 235U ratio of 137.88. The measured U isotopic composition for Bruderheim, using the dissolution procedures employed for these U-Pb studies, is near the terrestrial composition. In the concordia diagram the U-Pb data chiefly plot above the concordia curve and define a line which intersects the concordia curve at 4.536 + 0.006 Ga and 0.495 Ga, but the data for Bruderheim cannot be understood at all in terms of the more usual two- or three-stage episodic U-Pb models involving a fixed μ-value in the first stage. Most samples show an apparent excess of 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 radiogenic lead cannot be explained by terrestrial contamination alone. A different U-Pb model is presented which describes qualitatively and quantitatively most features of the U-Pb data for Bruderheim. If this model correctly describes the U-Pb evolution of Bruderheim then the “formation” age is given as 4.536 Ga by both U-Pb and Pb-Pb data, the meteorite U-Pb system was disturbed by a later (shock?) event at about 500 Ma ago, and the data are consistent with (though do not require) a Can˜on Diablo initial lead composition. This interpretation suggests that the classical phenomenon of apparent excess radiogenic lead reflects the application of a single-stage model to a meteorite that has evidently experienced at least a two-stage history. The explanation of the observation that in the concordia diagram most meteorite samples (corrected for Can˜on Diablo lead) plot in the lead excess region remains obscure, though this may be due to the wrong choice of initial lead for Bruderheim.

U-Pb zircon, monazite and Rb-Sr whole rock systematics of granitic gneiss and psammitic to semi-pelitic host gneiss from Glenfinnan, Northwestern Scotland

New U-Pb zircon data from a segregation pegmatite in the granitic gneiss at Glenfinnan yield discordant points which appear to be aligned along a chord on a concordia diagram with upper and lower intersection ages of 1,517±30 Ma and 556±8 Ma, respectively. The results are similar to published U-Pb zircon data from the granitic gneiss but the lower intersection age does not correspond to concordant ages of 455±3 Ma obtained for monazites from the segregation pegmatite and from paragneiss which hosts the granitic gneiss. The apparent U-Pb zircon chord also gives no indication of a 1,030±50 Ma (large sample) Rb-Sr whole rock isochron age for the granitic gneiss (Brook et al. 1976). A traverse of adjacent 5–8 cm thick slabs in the paragneiss yields a Rb-Sr ‘errochron’ of 455±60 Ma which also does not agree with the U-Pb zircon lower intersection age. The scale of this Sr whole rock diffusion (ca. 10 cm) is not at variance with existing thermal, temporal and experimental constraints.

Age of zircon from volcanic rocks of the central North Carolina Piedmont and tectonic implications for the Carolina volcanic slate belt

U-Pb analyses of four fractions of zircon from felsic volcanic rocks in the upper part of the Uwharrie Formation plot on a concordia diagram along a chord between 586 and 340 m.y. These data indicate an Early Cambrian or late Precambrian age of 586 ± 10 m.y. for deposition of the upper part of the Uwharrie. The lower concordia intercept at 340 m.y. might represent a time of episodic lead loss, perhaps indicating Acadian metamorphism in the area. However, all data points cluster near the upper concordia intercept, and the lower intercept is not accurately defined. A continuous lead-loss model cannot be ruled out. The zircon ages from the Uwharrie show that in central North Carolina the conformable volcanic and sedimentary sequence ranges from rocks older than 586 m.y. up to beds about 552 m.y. old containing probable Middle Cambrian trilobites. This contrasts with relations reported from the Roxboro-Durham area to the northeast, where a similar volcanic and sedimentary sequence with a 620 ± 20–m.y. zircon age was folded and faulted by the Virgilina deformation before emplacement of a 575 ± 20–m.y.-old pluton. The events of the Roxboro-Durham area can be correlated with those of central North Carolina in three possible ways that are consistent with the radiometric and fossil data: (1) The stratigraphic sequences of the two areas are partly correlative. The Virgilina deformation was synchronous with deposition of the upper part of the central North Carolina sequence, but the deformation did not extend into the central North Carolina area. (2) The stratigraphic sequences of the two areas are correlative, and the Virgilina deformation was younger than the central North Carolina sequence but was weak or absent in that area. (3) The central North Carolina sequence is entirely younger than the Virgilina deformation, and the volcanic rocks may represent an extrusive phase of the plutonism of the Roxboro-Durham area. Interpretation 1 is preferred because it harmonizes best with both lithologic correlations and ages. Interpretation 2 offers the poorest agreements as to age, and interpretation 3 conflicts with the suggested lithologic correlation of the two stratigraphic sequences.

U-Th-Pb geochronology of the Massabesic Gneiss and the granite near Milford, South-Central New Hampshire: New evidence for avalonian basement and taconic and alleghenian disturbances in Eastern New England

U-Th-Pb systematics for zircon and monazite from Massabesic Gneiss (paragneiss and orthogneiss) and the granite near Milford, New Hampshire, were determined. Zircon morphology suggests that the paragneiss may be volcaniclastic (igneous) in origin, and thus the age data probably record the date (minimum of 646 m.y.) at which the rock was extruded. A two-stage lead-loss model is proposed to explain the present array of data points on a concordia diagram. Orthogneiss ages range only narrowly and are clustered around 475 m.y. Data for the granite of Milford, New Hampshire, are scattered, but may be interpreted in terms of inheritance and modern lead loss, yielding a crystallization age of 275 m.y. This is the only known occurrence of Avalonian-type basement in New Hampshire and as such provides evidence for the location of the paleo-Africa-paleo- North America suture. The geochronology also further documents the occurrence of disturbances during the Ordovician and Permian.

U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

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.

Isotopic lead investigations on the Allende carbonaceous chondrite

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.

Uranium and lead isotopic dating with grain-by-grain zircon analysis: A study of complex geological history with a single rock

A charnockitic paragneiss from the In'Ouzzal (Ahaggar, Algeria) Formation was studied using the U-Th-Pb zircon technique. Analysis with 1-mg samples gives an upper intercept of 2.17 b.y. on the U-Pb concordia diagram. A grain-by-grain study allows one to distinguish two zircon populations: yellow, euhedral zircons give an age of 2.05 b.y., and brown, corroded zircons give an age of 2.9–3.3 b.y. in agreement with Rb-Sr whole rock data. Some consequences of these results are discussed, including: (1) U-Th-Pb evolution of inherited zircons, with multistages models, and (2) comparison of U-Pb and Rb-Sr data in granulite facies environment.

U–Pb systematics in heterogeneous zircon populations from the Precambrian basement of the Maryland Piedmont

The aim of the study was — besides the dating of metamorphic events — to evaluate the effects of multi-stage crystal growth, episodic and continuous Pb loss, and U gain on the discordant age patterns found for zircon populations of the polymetamorphic Baltimore Gneiss, the Precambrian basement in the Maryland Piedmont. Eight gneiss and migmatite samples were collected at two localities in the Phoenix and Towson dome, respectively. Their zircon populations were separated into twenty-three fractions of different size and optical appearance. A low-contamination method ( T.E. Krogh, 1973) was used for the U-Pb analyses. Microscopy and electron-microprobe studies revealed internal heterogeneities of the zircon crystals: at least half of the grains of each population reflect more than one stage of crystal growth, with the last stage consisting of U-poor overgrowths (U: below 400 ppm, mostly below 200 ppm). Evidence for episodic U gain and overgrown material other than zircon has not been found. On a concordia diagram the “ages” obtained by upward extrapolations (1080 and 1180 m.y.) and downward extrapolations (421 and 455 m.y.) of the best-fit lines to the data points are in fair agreement with the geochronologic data found by other investigators and the probable times of metamorphic periods of Precambrian (Grenville) and early Paleozoic (Taconic) orogenies. Models of Pb loss by continuous diffusion cannot adequately explain the discordant age patterns: these are essentially the result of superposition of episodic Pb loss and zircon overgrowth during the Taconic (and Acadian?) metamorphisms. The zircon overgrowth appears to be present in all fractions, but its influence on the U-Pb systematics is generally not perceptible because it is overridden by the effect of episodic Pb loss. For the fractions showing the most discordant ages, the contribution of Pb loss to the discordancy was found to be at least 85 %. From the microscopic picture and the isotopic data, it appears that the bulk of the zircon substance crystallized during one or several high-grade metamorphisms accompanied by migmatization and granitization of the rocks in the course of the Grenville orogeny. Under consideration of zircon ages of Baltimore Gneiss rocks of Pennsylvania, the results point to a complex Grenville metamorphic history in the Maryland and Pennsylvania Piedmont, that lasted from at least 1200 m.y. until about 980 m.y. The granulite-facies metamorphism in the West Chester Prong, Pennsylvania, may be 50–200 m.y. younger than the metamorphic events in the gneiss domes of the Baltimore area. Although it seems that real differences exist with respect to the Precambrian ages of major zircon-forming events between the Phoenix and the Towson dome, the apparent difference of about 100 m.y. should be interpreted with caution, because it is impossible, so far, to evaluate quantitatively the influence of possibly much older inherited zircon components.

Episodic uranium‐lead models and the interpretation of variations in the isotopic composition of lead in rocks

Controversy over the use of rectilinear plots for the interpretation of isotopic rock lead studies has prompted the following analysis of discrete two‐ and three‐stage episodic models, which is partly in the nature of a review, but which also introduces some new results. An investigation is made of the restrictive conditions under which these models lead to rectilinear plots in the isochron, lead‐lead, and modified concordia diagrams; the question of the solubility of these models and the possibility of discriminating between them are discussed. It is shown that there are two special three‐stage models that lead to rectilinear plots in all diagrams and that one of these models cannot (from uranium‐lead data alone) be distinguished from the two‐stage model. The alternative special rectilinear three‐stage model, which can be distinguished both from the two‐stage and from other three‐stage models on the basis of uranium‐lead data alone, cannot be fully solved on the basis of these data alone. The necessity of using data other than the uranium‐lead data (such as the ‘age of the earth’ or the duration of the last stage of lead evolution) either to solve or to distinguish the models makes it clear that a fully independent age of the earth cannot be derived from uranium‐lead data alone. A discussion is also given of the solubility of general three‐ and four‐stage models by use of ‘age‐corrected’ isotope ratios to correct back one stage. It is shown that careless application of a particular multistage model without proper investigation of its consistency with the data can lead to false interpretations. Recent controversy over the use of multistage models is discussed; much of the controversy is shown to arise out of a confusion of the models with the particular diagram used to plot the data. In general no one diagram is to be preferred to another, with the exceptions that if µ1 is variable the modified concordia diagram must be used, whereas the discussion of age‐corrected isotope ratios necessitates use of the lead‐lead diagram. A brief discussion is given of the interpretation of oceanic lead data, for which the analytical precision at present may often be insufficient for the proper application of consistency tests; individual sets of oceanic lead therefore are not susceptible of unambiguous interpretation at this time. It is emphasized that this real lack of information cannot be overcome (as has been implied in some papers) by the use of alternative presentations of the data. The new Tera‐Wasserburg diagram is examined, as is the recently introduced idea that the initial 207Pb/206Pb ratio may serve as an important chronometer for early planetary system evolution. Both ideas are shown to contribute no information that cannot be derived from the modified concordia diagram. The application of the Tera‐Wasserburg diagram in the attempt to derive from lunar basalt U‐Pb data an independent age of the moon is shown to rest on precisely the same principles and to be subject to the same restrictions and uncertainties as earlier attempts to derive an age of the earth from terrestrial oceanic basalts. A discussion is also given of the question of homogenization and of the application of the systematics to the special case of zircons.

Age and origin of detrital zircons from the pre-Permian basements of the Bohemian Massif and the Alps

U-Pb isotopic analyses were made on detrital zircon populations from sandstones and quartzites of the pre-Permian basement in an attempt to shed light on the presedimentary history of the zircons and the age of their primary source rocks. Eight rock samples were collected from the Saxothuringian and Moldanubian parts of the Bohemian Massif, the western part of the Upper Austroalpine Nappes, and the Southern Alps. The heterogeneous populations were separated into fractions of different size, magnetic susceptibility, color, and shape. Because of their typically pitted surface all zircon grains from the sandstones and quartzites appear to be detrital. Only in three samples from the Alps—one from a contact metamorphic aureole—the zircons show surface recrystallization and minor new growth. With the exception of some euhedral crystals in the Saxothuringian quartzites all zircon fractions have highly discordant U-Pb ages. On a concordia diagram their data points scatter slightly around best-fit lines with upper intersections between 2000 and 2300 m.y. From this pattern the following conclusions are reached: The lower intersections of the best-fit lines with the concordia curve cannot be clearly correlated with an episodic disturbance of the U-Pb systems during weathering and sedimentation and/or during regional metamorphism. For the zircons of the Bohemian Massif a disturbing event, about 550 to 600 m.y. ago, is likely. Clear, euhedral, but nevertheless detrital zircons found among the zircon populations of two Saxothuringian quartzites (“Plattenquarzit” of the pre-Ordovician “Arzberger Serie” and Lower Ordovician “FrauenbachQuarzit”) crystallized most probably during the Upper Proterozoic and/or the Assyntian petrogenesis. The highly discordant age pattern of the detrital zircons from the Alps is likely to be the result of the Caledonian and/or Hercynian (=Variscan) metamorphism. Differences in concentration levels of common lead in detrital zircons and the problem of red zircons as indicators of Precambrian origin are discussed.