Radiogenic Pb Component Research Articles

Testing for Pb isotopic differences between minerals in the Kiglapait layered intrusion by LA-ICP-MS

Mineral-scale Pb isotopic variations in cumulates of the Kiglapait intrusion, the largest troctolitic intrusion of the Proterozoic Nain Plutonic Suite in Labrador (Canada), are evaluated directly in thin section by single spot LA-ICP-MS analysis of minerals with low Pb concentrations (e.g., plagioclase = 0.31–3.3 ppm, clinopyroxene = 0.06–1.1 ppm) at high spatial resolution (<100 μm). Analysis of reference materials (BCR-2G, NIST 612, KL2-G, MASS-1) demonstrates that the results are accurate despite the very low amounts of Pb measured (0.04–18 pg) to preserve high spatial resolution. The Pb isotopic ratios of all plagioclase spot analyses overlap within uncertainty and with co-existing clinopyroxene. There are no significant differences between the cores and rims of plagioclase or clinopyroxene, between interstitial or cumulus augite, and with the analyses of interstitial sulfide, biotite, and symplectites (late-stage reactive microstructures). The LA-ICP-MS results support closed-system crystallization of the constituent phases (primocrysts, interstitial minerals) in the Kiglapait cumulates. The results contrast with published Pb isotopic ratios of mineral separates (unleached, leached, leachates) from the Kiglapait intrusion analyzed in solution by MC-ICP-MS where only the relatively unradiogenic leached plagioclase, taken as representative of the initial isotopic composition at 1307 Ma, has the same isotopic composition as that defined by the LA-ICP-MS analyses. A radiogenic Pb component, volumetrically minor and not detected by the LA-ICP-MS analyses (n = 483), appears to have been introduced by a cryptic alteration event from high-temperature fluids during the earliest stages of cooling of the Kiglapait intrusion. Determination of the Pb isotope compositions of common rock-forming minerals, characterized by low Pb concentrations (a few ppm), in mafic layered intrusions can be achieved rapidly by LA-ICP-MS and has the potential to yield significant new insights related to the petrogenesis of plutonic rocks, and phenocryst-bearing volcanic rocks, in the Earth's crust.

The promise and potential pitfalls of acid leaching for Pb[sbnd]Pb chronology

Acid leaching has been widely used in Pb–Pb chronology especially on meteorite samples, as it enhances the precision and accuracy of the chronology by separating radiogenic Pb from non-radiogenic Pb in the samples. Yet, the detailed mechanism of the radiogenic Pb separation remains elusive. Here we report scanning electron microscopy observations of acid-leached minerals from three achondrites that were previously analyzed for pyroxene PbPb dating (D'Orbigny, Ibitira and Northwest Africa 6704), and one stony‑iron meteorite (Brenham). Moreover, we present chemical analysis of acid leachates and residues of pyroxene and whole-rock fractions from the three achondrites using inductively coupled plasma mass spectrometry. The combined results reveal the resistance of minerals to acid treatments with HNO3, HCl, and HF and, in turn, allow us to explore the prospects and limitations of acid leaching for PbPb chronology. In particular, we show that (i) washing with dilute acids can efficiently liberate contaminant terrestrial Pb adsorbed on the mineral surface; (ii) hot and more concentrated (~6 M) HNO3 and HCl can separate pyroxenes from sulfides and anorthitic plagioclase that are highly enriched in initial non-radiogenic Pb; (iii) albitic plagioclase and pyroxenes show limited dissolution during the HNO3 and HCl treatments, but are progressively leached by hot 1 M HF, making it difficult to separate them from each other; and (iv) within single pyroxene grains having exsolved lamellar textures, high-Ca lamellae are more efficiently leached by the HF treatment than low-Ca ones. In the context of a two-stage Pb isotopic evolution model for pyroxene lamellae, we demonstrate that heterogeneous and incomplete dissolution of pyroxene lamellae with dilute HF can result in inaccurate PbPb age estimates. This may account for previously observed scattered Pb isotopic data for meteorite samples that were taken as evidence for isotope fractionation in a radiogenic Pb component during HF treatments.

Petrogenesis of Quebrada de la Mina and Altar North porphyries (Cordillera of San Juan, Argentina): Crustal assimilation and metallogenic implications

We investigate the geology of Altar North (Cu–Au) and Quebrada de la Mina (Au) porphyry deposits located in San Juan Province (Argentina), close to the large Altar porphyry copper deposit (995 Mt, 0.35% Cu, 0.083 g/t Au), to present constraints on the magmatic processes that occurred in the parental magma chambers of these magmatic-hydrothermal systems. Altar North deposit comprises a plagioclase-amphibole-phyric dacite intrusion (Altar North barren porphyry) and a plagioclase-amphibole-biotite-phyric dacite stock (Altar North mineralized porphyry, 11.98 ± 0.19 Ma). In Quebrada de la Mina, a plagioclase-amphibole-biotite-quartz-phyric dacite stock (QDM porphyry, 11.91 ± 0.33 Ma) crops out. High Sr/Y ratios (92–142) and amphibole compositions of Altar North barren and QDM porphyries reflect high magmatic oxidation states (fO2 = NNO +1.1 to +1.6) and high fH2O conditions in their magmas. Zones and rims enriched in anorthite (An37–48), SrO (0.22–0.33 wt.%) and FeO (0.21–0.37 wt.%) in plagioclase phenocrysts are evidences of magmatic recharge processes in the magma chambers. Altar North and Quebrada de la Mina intrusions have relatively homogeneous isotopic compositions (87Sr/86Sr(t) = 0.70450–0.70466, εNd(t) = +0.2 to +1.2) consistent with mixed mantle and crust contributions in their magmas. Higher Pb isotopes ratios (207Pb/204Pb = 15.6276–15.6294) of these intrusions compared to other porphyries of the district, reflect an increase in the assimilation of high radiogenic Pb components in the magmas. Ages of zircon xenocrysts (297, 210, 204, 69 Ma) revealed that the magmas have experienced assimilation of Miocene, Cretaceous, Triassic and Carboniferous crustal rocks.Fluids that precipitated sulfides in the Altar deposit may have remobilized Pb from the host rocks, as indicated by the ore minerals being more radiogenic (207Pb/204Pb = 15.6243–15.6269) than their host intrusions. Au/Cu ratio in Altar porphyries (average Au/Cu ratio of 0.14 × 10−4 by weight in Altar Central) is higher than in the giant Miocene porphyry deposits located to the south: Los Pelambres, Río Blanco and Los Bronces (Chile) and Pachón (Argentina). We suggest that the increase in Au content in the porphyries of this region could be linked to the assimilation of high radiogenic Pb components in the magmas within these long-lived maturation systems.

Heterogeneous Pb isotope composition in the Archean lower crust of the North China Craton induced by Cenozoic basaltic magma underplating

Granulite xenoliths entrained in Cenozoic basaltic rocks in several regions of the North China Craton (NCC) provide important constraints to evaluate the evolution of lower crust. Here we employ the Pb isotopic composition of plagioclase in granulite xenoliths from Yangyuan obtained through fLA (femtosecond laser ablation)-MC-ICPMS analyses to probe the Phanerozoic lower crustal evolution in the NCC. The newly-discovered granulite xenoliths from Yangyuan can be classified into felsic granulites, garnet-bearing pyroxene granulites and garnet-free pyroxene granulites. Our results show a large variation in plagioclase Pb isotopes (206Pb/204Pb=15.3–18.8; 207Pb/204Pb=14.9–15.9; 208Pb/204Pb=35.0–39.9) among and within samples. Common Pb in plagioclase and the single stage Pb evolution model indicate that the mantle from which the lower crust of the NCC was extracted had a 238U/204Pb ratio of 9.3 at 2.65±0.14Ga (all uncertainties are 2σ). The isotopic heterogeneity reflects the diverse formation and evolution history of the xenoliths. The felsic granulites show the least radiogenic Pb isotopes (206Pb/204Pb=15.3–15.8; 207Pb/204Pb=14.9–15.2; 208Pb/204Pb=35.0–35.9; 238U/204Pb=11.5 and 232Th/238U=4) among the entire xenolith suite with their Pb isotopes lying close to those of the exposed Archean granulite terranes of the NCC, suggesting that the granulites might have inherited the Pb isotope composition of ancient lower crust. The plagioclase from garnet-bearing pyroxene granulites shows the most radiogenic Pb isotopes (206Pb/204Pb=16.5–18.8; 207Pb/204Pb=15.3–15.9; 208Pb/204Pb=36.5–39.9; 238U/204Pb=18.2; 232Th/238U=4.3) in comparison to those of their garnet-free counterparts (206Pb/204Pb=15.9–17.3; 207Pb/204Pb=15.1–15.5; 208Pb/204Pb=36.1–37.5). The Pb isotopes for the former fall within or even above the fields of the I-MORB (Indian Mid-Ocean Ridge Basalt) and OIB (Ocean Island basalt) in 206Pb/204Pb vs. 207Pb/204Pb or 208Pb/204Pb, whereas the latter fall in the fields between the felsic and garnet-bearing pyroxene granulites. We consider that the linear correlation reflects the mixing of the ancient crust with radiogenic Pb in the melt rather than representing an isochron. The garnet-bearing pyroxene granulites might represent the lower crust which was likely modified by the underplating of basaltic magma during Cenozoic. Such extensive magma underplating might have significantly modified the Pb isotopic composition of the ancient lower crust. Individual plagioclase crystals from the garnet-bearing pyroxene granulites show variation in isotopic composition from less radiogenic Pb in the core to more radiogenic in the rim. Together with electron microprobe data, a decompressional melting of the lower crust is inferred in response to the lithosphere extension, with input of radiogenic Pb component.

Behavior of fluid-mobile elements in serpentines from abyssal to subduction environments: Examples from Cuba and Dominican Republic

Serpentinites from subduction environments represent an important sink for fluid-mobile elements. In order to constrain geochemical behavior of fluid-mobile elements hosted by serpentine phases during subduction processes, we carried out a geochemical study (trace elements and Pb isotopes) of a series of serpentinites and cumulates from the accretionary wedge of Greater Caribbean (Cuba and Dominican Republic). The trace element compositions of the primary and alteration-related phases were analyzed in situ using LA–HR-ICP-MS techniques. The studied samples represent parts of the subducted proto-Atlantic oceanic lithosphere, which has experienced low to high grade metamorphism (greenschist to eclogite facies), before being exhumed; a subset of these samples were derived from the mantle wedge. This sampling provides the opportunity to trace the chemical mobility of fluid-mobile elements during prograde metamorphism along a cold geotherm in an oceanic subduction setting.Serpentinites display strong enrichment in fluid-mobile elements indicating extensive fluid–rock interaction. In situ analyses allow distinction of three types of serpentines related to the nature of primary minerals (olivine, ortho- or clinopyroxene). Compositions of subducted samples, especially in fluid-mobile elements, are relatively close to those of abyssal peridotites without noticeable evidence of mobility for trace elements during subduction-related prograde metamorphism, with the exception of B. This confirms that the observed enrichment results from seawater/peridotite interactions during residence time in the ocean. It also suggests that most mobile elements stored in serpentine minerals are immobile during subduction processes. A major consequence of this observation is that serpentine minerals are a good sink for mobile elements in subduction zones, until their dehydration. Additionally, Pb isotopes and over-enrichment in As–Sb in high-grade subducted serpentines (antigorite) suggest the contribution of a sedimentary component during a secondary hydration taking place at the lizardite/antigorite transition. We propose that this new serpentinization event, taking place at greater depth, results from mixing between sediments and serpentinites in the subduction channel.Mantle wedge serpentinites present imprints of hydrothermal fluids: they are B-rich but without strong enrichment in As and Sb, and show evidence for moderate contributions of a radiogenic Pb-component. This suggests that the fluids that produced the mantle wedge serpentinites derived from the dehydration of the oceanic crust, with moderate to no contribution of sediments. We posit that mantle wedge serpentinization took place around 20–25km depth: at such depth and temperature conditions (T>200°C), the subducted sediments still released their B-rich pore fluids while their structural water incorporated in hydrous minerals (phengite, lawsonite) remained stable. The existence of various potential reservoirs for fluid-mobile elements in subduction zone environments (subducted serpentinites, mantle wedge serpentinites, as well as subducted sediments and altered oceanic crust) that potentially release their fluids at different depths has strong implications for arc lava formation.

The Solar System primordial lead

Knowledge of the primordial isotope composition of Pb in the Solar System is critical to the understanding of the early evolution of Earth and other planetary bodies. Here we present new Pb isotopic data on troilite (FeS) nodules from a number of different iron meteorites: Canyon Diablo, Mundrabilla, Nantan, Seeläsgen, Toluca (IAB–IIICD), Cape York (IIIA), Mt Edith (IIIB), and Seymchan (pallasite). Lead abundances and isotopic compositions typically vary from one troilite inclusion to another, even within the same meteorite. The most primitive Pb was found in three leach fractions of two exceptionally Pb-rich Nantan troilite nodules. Its 204Pb/ 206Pb is identical to that of Canyon Diablo troilite as measured by Tatsumoto et al. [M. Tatsumoto, R.J. Knight, C.J. Allègre, Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206, Science 180(1973) 1279–1283]. However, our measurements of 207Pb/ 206Pb and 208Pb/ 206Pb are significantly higher than theirs, as well as other older literature data obtained by TIMS, while consistent with the recent data of Connelly et al. [J.N. Connelly, M. Bizzarro, K. Thrane, J.A. Baker, The Pb–Pb age of Angrite SAH99555 revisited, Geochim. Cosmochim. Acta 72(2008) 4813–4824], a result we ascribe to instrumental mass fractionation having biased the older data. Our current best estimate of the Solar System primordial Pb is that of Nantan troilite, which has the following isotopic composition: 204Pb/ 206Pb = 0.107459(16), 207Pb/ 206Pb = 1.10759(10), and 208Pb/ 206Pb = 3.17347(28). This is slightly less radiogenic than the intercept of the bundle of isotopic arrays formed in 207Pb/ 206Pb– 204Pb/ 206Pb space by our measurements of Canyon Diablo, Nantan, Seeläsgen, Cape York, and Mundrabilla, as well as literature data, which, in spite of rather large uncertainties, suggests a common primordial Pb component for all of these meteorites. The radiogenic Pb present in most of these irons is dominantly asteroidal and indicates evolution in a high-U/Pb environment. The apparent age of the radiogenic Pb component is consistent with the 39Ar– 40Ar ages of silicate inclusions found in the same meteorites. We propose that the radiogenic Pb was introduced more recently into troilite, from the surface rubble of the parent asteroid, possibly during the impacts that generated the IAB iron meteorites. The excellent correlation between 208Pb/ 206Pb and 204Pb/ 206Pb translates into a Th/U ratio of 3.876 ± 0.016 for the asteroid, which is the most precise estimate for the solar nebula to date.

Pb isotope evidence for contributions from different Iceland mantle components to Palaeogene East Greenland flood basalts

We present new Pb isotope data on 21 samples of break-up-related flood basalts (56–54 Ma) from the Blosseville Kyst region of East Greenland. These samples show a considerable range in isotopic composition ( 206Pb/ 204Pb 17.6 to 19.3) that broadly correlates with compositional type. The ‘low-Ti’ type is characterised by the lowest 206Pb/ 204Pb (17.6 to 18.2) and highest εNd 55Ma (+8 to +11), whereas the ‘very high-Ti’ type has the opposite characteristics ( 206Pb/ 204Pb 18.9 to 19.3: εNd 55Ma +5 to +6), with the ‘high-Ti’ type lying in between ( 206Pb/ 204Pb 18.3 to 19.0: εNd 55Ma +6 to +8). Crustal contamination in this region produces unradiogenic 206Pb/ 204Pb and εNd values, as is clearly seen in one sample ( 206Pb/ 204Pb 16.2; εNd 55Ma +2). Thus, the negative correlation between 206Pb/ 204Pb and εNd shown by most of the samples confirms inferences from trace element data that crustal interaction has been minimal in the selected samples. Uncontaminated Palaeogene East Greenland flood basalts appear to have sampled the same broad range in mantle compositions seen in Recent Iceland basalts. In contrast to the peripheral lava suites from the British Isles and Southeast Greenland, where the inferred uncontaminated magmas have a restricted and relatively unradiogenic 206Pb/ 204Pb composition (‘North Atlantic End-Member’—NAEM: [J. Petrol. 41 (2000) 919]), the uncontaminated lavas and dykes of East Greenland and the Faroes from close to the plume track show a wide compositional range and include magmas with high 206Pb/ 204Pb, similar to the most radiogenic values found in recent Icelandic basalts. Furthermore, the main volume of lavas in East Greenland is displaced away from the NAEM towards this radiogenic Pb component. Thus, this ‘Iceland radiogenic Pb end-member’ component was a significant contributor to the break-up-related magmatism, at least close to the inferred plume track, suggesting that it has been an inherent component of the Iceland plume upwelling for the last 56 Myr or so, although perhaps absent earlier.

Single Zircon Evaporation Thermal Ionisation Mass Spectrometry: Method and Procedures†

Zircon evaporation thermal ionisation mass spectrometry (TIMS) is used in geochronology to determine absolute 207Pb*/206Pb* ages and Th/U ratios of single zircon crystals. The process involves the breakdown of zircon (ZrSiO4) to porous baddeleyite (ZrO2) along a reaction front which progresses into the interior of the crystal. Evaporation of high quality zircons thus allows us to distinguish between crystal rim (overgrowth) and core, providing precise information about the time of magmatic crystal growth, partial dissolution, and/or metamorphic overgrowth. Derived Th/U ratios complement age data interpretation and provide valuable petrogenetic implications. A double Re-filament ion source is used. The zircon is encased in the evaporation filament and heated step-wise to 1200–1300 °C to strip off unsupported common and radiogenic Pb components. After cleaning, evaporation proceeds in temperature steps of ∼20 °C. The evaporate (SiO2, Pb, REEs, and U from the zircon and Re from the evaporation filament) of each step is deposited for 45 min on the cold ionisation filament and subsequently analysed. Lead isotopic composition is determined using a dynamic secondary electron multiplier ion counter or static Faraday cup data acquisition schemes. Lead ratios are corrected for fractionation using correction factors derived from standard measurements of a 1 ng NBS SRM 982 sample. The precision on 207Pb/206Pb ratios is <1%. Only high temperature steps (>1300 °C) with 206Pb/204Pb > 5000 are used for age calculations. The ages reported (single temperature step, multi-temperature step means) are weighted means calculated from at least 20 measured 207Pb*/206Pb* ratios with 2 standard errors of the mean. Precision of ages is strongly dependent on age range and varies between 0.1 and 10%.

U-Pb and Rb-Sr isotopic systematics of fluids associated with mineralization of the Dartmoor granite, southwest England

Quartz veins from early hydrothermal mineralization within the 280 Ma Dartmoor granite contain abundant fluid inclusions, which were sampled using crush-leach techniques and analyzed for Rb, Sr, U, and Pb content and isotopic ratios. The present-day lead isotopic signature of some of the crushleach fluids is similar to that of granitic alkali feldspars from Dartmoor and other localities in southwest England, although some fluid samples contain a more radiogenic Pb component. The more radiogenic fluids fell into, or near, the granitic K-feldspar field upon correction of the present-day fluid Pb ratios for in situ U decay over ca. 280 Ma, but the lead isotope ratios of the less-radiogenic fluids became significantly lower than that of the granitic K-feldspars. The apparent overcorrection may be real, or may reflect either fluid mixing due to leaching of secondary fluid inclusions, differential leaching of U and Pb, the presence of an undetermined U-rich included phase (or fracture-filling) within the quartz, or a combination of the above. The variation of fluid U/Pb ratios also suggests some complications regarding either the selective retention of U and Pb on quartz surfaces, or the presence of submicroscopic U-rich inclusions. Therefore, we suggest that in situ decay corrections to lead isotopic data obtained using crush-leach techniques be applied with great caution. The fluid 87Rb/ 86Sr values vary little (1.40–1.74), as do their present-day strontium isotope signatures ( 87Rb/ 86Sr = 0.71814–0.71968). When corrected for in situ 87Rb decay over 280 Ma, the fluids contain significant excess radiogenic Sr (Sr i = 0.7118–0.7141), relative to the Dartmoor granite S i (=0.7101). If the Sr in the fluids is derived exclusively from the granite, or some granite-related source, and closedsystem evolution of the Rb-Sr isotopic system is assumed, the amount of excess 87Sr is a function of the time difference between Sr closure in the granite and Sr closure in the vein. Age estimates from the intersection of granite and fluid inclusion Sr evolution curves are in agreement with published 40Ar- 39Ar ages of late dikes and sericitic alteration at Birch Tor-Vitifer. Our results further suggest that the fluids related to quartz-tourmaline-cassiterite mineralization within the Dartmoor granite are themselves related to granite-derived, late-stage aplitic melts.

Inherited Pb components in magmatic titanite and their consequence for the interpretation of UPb ages

The occurrence of inherited radiogenic Pb components in 35 Ma old magmatic titanite is reported here for the first time. The about 810 Ma age of the inherited components is identical to that contained in zircon from the same amphibole-bearing syenite sample. In both minerals the Precambrian components were not totally reset during magmatic new growth at 35 Ma. This age pattern substantiates that the UPb chronometer in titanite is resistant to temperatures in excess of 712°C, as is already known for zircon and monazite. In consequence, UPb ages from crystalline titanites have to be interpreted as the age of mineral formation or re-crystallization, rather than the time of UPb system closure during cooling of the rock.

Isotope systematics in vein gold from Brusson, Val d'Ayas (NW Italy), 1. Pb/Pb evidence for a Piemonte metaophiolite Au source

The Monte Rosa Gold District (NW Italian Alps) comprises hundreds of late Alpine, epigenetic, gold-bearing quartz veins. We report a lead isotope study on an important one of these veins, Fenilia (Brusson, Val d'Ayas), which reveals a complex isotope geochemistry of petrographically cogenetic vein minerals. Petrographic observations allow to distinguish a quartz-dominated main-stage and a quartz-poor final-stage mineral assemblage. Most of the free gold crystallized in the beginning of the final stage, whereas sulphides were deposited during its waning. Three distinct lead isotope fields result for the paragenetic sequence of vein minerals. The variable lead isotope signature (15.517 < 207Pb/ 204Pb < 15.623) trapped in final-stage free gold, and minor quartz can be interpreted as a variable mixture between a primitive MORB-type and a radiogenic Pb component. It differs distinctly from the radiogenic lead signature of the main-stage mineral assemblage ( 207Pb/ 204Pb > 15.706), as well as from the uniform radiogenic lead (15.649 < 207Pb/ 204Pb < 15.691) characteristic of the waning final stage. We favour a model where the deeply circulating final-stage fluid, the ultimate source of which remains obscure, acquired lead and thus possibly its gold in Piemonte metaophiolitic rocks beneath the Monte Rosa nappe. During ascent, the primitive gold-bearing fluid was variably contaminated by an evolved crustal lead (15.706 < 207Pb/ 204Pb < 15.743) compatible with derivation from Monte Rosa nappe rocks. During deposition of final-stage minerals the Pb isotope composition of the fluid changed to a constant signature becoming again dominated by a radiogenic lead source. Thus within an assemblage defined as cogenetic on petrographic grounds the Pb isotopic signature is not necessarily uniform and source modelling based on lead isotopes of associated minerals may be inappropriate. It remains open at present, whether the inferred Piemonte metaophiolite gold source is confined to the Fenilia vein in Val d'Ayas only, or whether it might be more widespread in the Monte Rosa Gold District.

Th [sbnd]Pb versus U [sbnd]Pb isotope systematics in allanite from co-genetic rhyolite and granodiorite: Implications for geochronology

U Th Pb isotopic analyses were performed on single allanite crystals from a rhyolite of the Atesina volcanic complex (AVC) and a granodiorite of the Cima d'Asta pluton (CAP) which form the largest calc-alkaline volcano-plutonic association of Late Hercynian age in the South Alpine domain (northern Italy). The best approximation to the time of emplacement of the AVC and CAP is given by essentially coeval 232Th 208Pb ages of 276.3 ± 2.2 Ma (95% confidence limit; n = 2) and 275.5 ± 1.5 Ma ( n = 4) for AVC rhyolite and CAP granodiorite, respectively. Although the AVC/CAP magmatites have been overprinted by a Triassic hydrothermal event, the Th Pb isotopic system in allanite remained virtually undisturbed, whereas the U Pb systems display variable degrees of open-system behaviour. The allanite Th Pb ages are neither affected by initial radioactive disequilibrium nor by the inheritance of radiogenic Pb components, which underlines the importance of the Th Pb isotopic system for accurate dating of Th-enriched minerals. The isotopic results demonstrate that even hydrothermally altered magmatites can successfully be dated by this technique.

The effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis: A model

The initial Pb isotopic composition of anatectic granites is modelled as a mixture of Pb derived from major minerals that have characteristically low 238U 204Pb and 232Th 204Pb ratios (i.e., feldspars, biopyroboles, etc.) with Pb contributed from dissolved accessory minerals with characteristically high 238U 204Pb and 232Th 204Pb ratios (i.e., zircon, monazite, etc.). The initial Pb isotopic composition of anatectic granites is a function of (a) the age, type, modal distribution and heterogeneity in the initial U and Th content of the accessory minerals, (b) variation in melt composition and temperature during anatexis (c) the fraction of the source melted, and (d) the extent to which the melt was homogenized prior to crystallization. The differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the Pb isotopes from other chemical and isotopic systems in granite rocks. Granitic plutons derived by anatexis of the same source material are not required to have the same initial Pb isotopic composition. Low temperature, peraluminous granites are more likely to exhibit homogeneous initial Pb isotopic compositions dominated by the Pb contained within the major mineral assemblage of the source material. In contrast, the initial Pb isotopic composition of higher temperature, metaluminous or peralkaline anatectic granites is more likely to be heterogeneous, unless the melts are homogenized during emplacement, and will record a larger fraction of the radiogenic Pb component contained in the accessory mineral assemblage of the source material. Heterogeneity in the initial Pb isotopic composition of individual granites or granitic suites can preserve the weighted average age of the radiogenic Pb present in the accessory mineral assemblage of the source region even if these minerals are entirely dissolved during anatexis. The use of isotopic tracer studies in terrane analysis will be optimized by the isotopic characterization of granitic suites.

Metallogenesis of the Abitibi greenstone belt of Canada: a contribution from the analysis of trace lead in sulfide minerals

Pb-isotopic compositions are reported for 12 ore localities within the late Archean Abitibi greenstone belt. The studied samples carry massive or disseminated sulfides hosted in a variety of materials, including mafic–ultramafic igneous rocks, felsic lavas, porphyries, and sedimentary iron formations. Repeated leaching experiments on these sulfides frequently revealed the presence of a radiogenic Pb component, which is attributed to in situ decay of U and Th. The leaching experiments make it possible, in some cases, to separate the radiogenic Pb from the initial Pb included in the minerals. Six Pb–Pb isochrons formed by the analyses on leachates and residues show little evidence of secondary perturbations and yield ages that are, within error, similar to those determined for the supracrustal assemblage. This implies that the ores were concentrated synchronously with the main phases of magmatic activity, close to 2.7 Ga.The initial isotopic compositions of the sulfide specimens point to the existence of two different sources of metals: (i) juvenile, mantle-derived igneous rocks and (ii) older recycled supracrustal series. Ore formation frequently involves mixing of metals from these two sources in variable proportions. The initial isotopic composition of these two reservoirs is best evaluated by examining the composition of sulfides associated with komatiitic lava flows and with sedimentary iron formations, respectively.

Single-zircon dating by stepwise Pb-evaporation constrains the Archean history of detrital zircons from the Jack Hills, Western Australia

Pb isotope analyses have been carried out on 42 zircon grains from a Western Australian metaconglomerate using stepwise Pb-evaporation directly in the ion source of a thermal ionization mass spectrometer. The metaconglomerate is from the Archean Jack Hills Metasedimentary Belt, and is known from ion microprobe (“SHRIMP”) analyses to contain a complex zircon population with ages between 4.2 Ga and 3.1 Ga. The same complex pattern of ages is found by the Pb evaporation studies. Five grains yielded minimum crystallization ages from 4.17 Ga to 4.07 Ga. The main population appears significantly younger, having been generated at about 3.55-3.3 Ga. The agreement between the two analytical approaches confirms the SHRIMP results and demonstrates the value of the stepwise-evaporation technique in determining the age patterns of mixed zircon populations. In many of the evaporative Pb isotope records the 207/206 ratios remained constant for all evaporation steps, which we interpret as evaporation from concordant zircon phases. However, for the majority of zircons 207/206 ratios increased with increasing evaporation temperature, and usually approached constant values during evaporation at the highest temperatures. This can be attributed to mixing of different radiogenic Pb components from either crystalline zircon phases of different ages or from domains of isotopically disturbed metamict zircon. Present results confirm > 4 Ga zircon ages in the metaconglomerate from the Jack Hills and substantiate formation of crust at a very early stage in the evolution of the earth. Results also confirm a major crust-forming event 3.55-3.3 Ga ago.

Petrogenesis of Hercynian leucogranites from the southern Armorican Massif: contribution of REE and isotopic (Sr, Nd, Pb and O) geochemical data to the study of source rock characteristics and ages

REE geochemistry and isotopic (Pb, Sr, Nd, O) compositions have been studied in up to nine separate leucogranite massifs from three structurally adjacent zones in South Brittany. Ages decrease from 340 Ma in the north to 300 Ma in the south whereas initial 87Sr/ 86Sr ratios increase from 0.706 to 0.710–0.717. REE abundances are relatively low (La N ∼ 20–80 × chondrites) with a strong enrichment of LREE relative to HREE (La N/Lu N= 13.4–26.6). Pb isotope ratios are homogeneous for a given massif but heterogeneous among massifs and plot very close to a normal terrestrial lead growth curve at 300 Ma ago. 18O/ 16O ratios are high and relatively uniform at 10.7 to 12.8, mean 11.9. U Pb studies on zircon indicate the presence of an inherited radiogenic Pb component and that the sediment-dominated source region had been metamorphosed during the Siluro-Devonian (Ligerian). Negativeε Nd(T) values imply that the source region had been enriched in LREE prior to the melting episode. Based on the combined data, large-scale partial melting of crustal material, dominantly of sedimentary origin, is proposed. The model age of the source rocks is not very old (600–700 to 1200 Ma). The Pb and Sr isotopic variations observed among different massifs is probably the result of age differences in the source materials of the different leucogranites.

UThPb systematics of the eucrite “Juvinas”: Precise age determination and evidence for exotic lead

The U-Th-Pb isotope systematics of the eucrite “Juvinas” have been studied in whole rock fragments as well as in plagioclases and pyroxenes. The results show that this monomict breccia crystallized with a very high U Pb initial ratio at T = 4.539 ± 0.004 AE ago. There is evidence for a less radiogenic Pb component ( 206Pb 204Pb = 13.0 ; 207Pb 204Pb = 13.5 ; 208Pb 204Pb = 32.71 ) interpreted as “exotic lead” induced by a meteoritical impact at the surface of the Juvinas parent body, 1.92 ± 0.06 AE ago.

Lead-isotopic signatures of porphyry copper deposits in oceanic and continental settings, Colombian Andes

Three discrete sub-belts of porphyry copper-type mineralization are recognized in the Colombian Andes: a western Eocene sub-belt, an eastern Jurassic to early Cretaceous sub-belt and, between them, a central Miocene sub-belt. The western sub-belt is part of an oceanic domain constituted by oceanic crust and overlying immature island-arc rocks, the eastern sub-belt is within a continental domain underlain by the leading edge of the Guayana shield, and the central sub-belt spans the faulted boundary between them. The thicker continental crust includes important granulitic rocks which crop out locally, as in the vicinity of the Mocoa porphyry copper deposit. Pb-isotopic ratios were determined for pyrite samples collected from 6 porphyry copper centers, 3 in the western sub-belt, 2 in the eastern sub-belt, and one in the central sub-belt. Ratios fall into 3 discrete populations: the most radiogenic values represent the western sub-belt, the least radiogenic represent the eastern sub-belt, and an intermediate value corresponds to the central sub-belt. Ratios therefore become progressively less radiogenic from the western oceanic domain to the eastern cratonic domain. Comparison of the Pb-isotopic ratios with those given in the literature for possible source materials for Colombian porphyry copper leads enables the subcontinental mantle wedge, subducted oceanic crust and subducted metalliferous sediments to be discounted as principal sources. The relatively radiogenic signatures of 5 of the porphyry copper centers appear to be broadly compatible with either a subducted pelagic sediment source or an upper continental crust source, whereas the sixth center, Mocoa, is characterized by a distinctly less radiogenic 206Pb 204Pb ratio. An admixture of a relatively small percentage of non-radiogenic Pb from granulitic material in the upper crust with the more radiogenic Pb typical of the western sub-belt centers could account for the Mocoa data. Because much of the upper crustal section in the region of Mocoa is thought to be dominated by granulitic material, the radiogenic Pb component could be best derived from subducted pelagic sediments. If this is the case, then a subducted pelagic sediment source could dominate the Pb-isotopic signatures of all Colombian porphyry copper mineralization, with some of the Pb-isotopic differences being produced by selective contamination with upper crustal leads of variable character. If this conclusion is correct, then the fundamental metal budgets of Colombian porphyry copper systems are probably largely independent of upper crustal influences.