Th Pb Research Articles

Survival of whole-rock Sm–Nd isotope system from REE redistribution and mineral-scale isotopic resetting amid hydrothermal alteration in REE-rich Fe-Cu deposit

The 147Sm–143Nd isotope system has long been used as a powerful tracer and geochronometer for a wide range of geological process, yet the robustness of Sm–Nd system has been questioned in the presence of multistage hydrothermal alteration. Here we provide robust evidence for the survival of whole-rock Sm–Nd isotopes from REE redistribution and mineral-scale isotopic resetting during multistage fluid infiltrations in the Lala Fe-Cu-REE deposit in Kangdian region, southwestern China.The Lala deposit is hosted in the ∼1.72–1.68 Ga rift-related Hekou Group and contains variable proportions of REE-rich apatite and fluorite in the ores. Both of the apatite and fluorite grains show textures of extensive metasomatic alteration, during which large amounts of REE, especially LREE, were leached out and mostly re-incorporated into newly-formed monazite, bastnäsite, and xenotime. Quantifying REE distribution budget at thin-section scale confirms that the amounts of REE (1583–5403 ppm) leached from primary apatite and fluorite are compatible with those (1636–5526 ppm) incorporated in the newly crystallized REE minerals, indicating that REE are internally redistributed within investigated samples. The whole ores yield an 18-point 147Sm–143Nd errorchon date of ∼1.71 Ga, which is in accordance with the other analogous Fe-Cu deposits in Kangdian region, and supports a ∼1.71 Ga primary mineralization at Lala. By contrast, SHRIMP 232Th–208Pb dating of texturally constrained monazite yield a crystallization age of ∼0.92 Ga, which corresponds well with the combined monazite-bastnäsite-apatite Sm–Nd isochron ages of representative samples, demonstrating a major REE redistribution and Sm–Nd isotopic resetting event at this time. The results suggest that Sm–Nd mobility was limited to within the whole-rock scale or even within the thin-section scale in the vast majority of cases, such that the whole-ore Sm–Nd isotopic system largely remained closed, despite complete resetting of the major REE minerals during younger fluid infiltration events. Our study exemplifies the apparent “decoupling” of Sm–Nd isotopic systems between macro- and microscales, which can advance our understanding of the complex history of hydrothermal systems. Our findings of a major transition (80–96%) in REE repositories also highlight the economic significance of hydrothermal alteration for modifying ore mineral assemblages, which resembles a natural REE refinement process and helps create beneficial REE resources to meet the society increasing demands.

A new approach for constraining the magnitude of initial disequilibrium in Quaternary zircons by coupled uranium and thorium decay series dating

Abstract We have measured 238U–206Pb, 235U–207Pb, and 232Th–208Pb ages on Quaternary zircons by laser ablation, single-collector, magnetic sector inductively coupled plasma mass spectrometry (LA-ICP-MS). To obtain reliable ages for Quaternary zircons, corrections for initial disequilibrium associated with deficits and excesses of both 230Th and 231Pa relative to secular equilibrium resulting from differential partitioning during zircon crystallization or source melting must be made. In contrast, the 232Th–208Pb decay system is clearly advantageous for samples affected by disequilibrium because the 232Th decay system lacks long-lived intermediate daughter isotopes. Conventionally, the initial disequilibrium for the 238U and 235U decay series has been determined by the distribution ratio between the melt and zircon (i.e., ƒTh/U = (Th/U)Zircon/(Th/U)Melt and ƒPa/U = (Pa/U)Zircon/(Pa/U)Melt). In our study, these correction factors were determined from comparison of the measured 238U–206Pb and 235U–207Pb ages with 232Th–208Pb ages obtained for three zircons of known eruption and, in some cases, zircon crystallization ages (Kirigamine Rhyolite, Bishop Tuff, and Toga Pumice). The resulting correction factors are ƒTh/U = 0.19 ± 0.14 and ƒPa/U = 3.66 ± 0.89 (Kirigamine Rhyolite), ƒTh/U = 0.24 ± 0.20 and ƒPa/U = 3.1 ± 1.2 (Bishop Tuff), and ƒTh/U = 0.28 ± 0.17 and ƒPa/U = 3.04 ± 0.99 (Toga Pumice). Although the uncertainties of these f values are relatively large, our results support the adequacy of the conventional approach for correction of initial disequilibrium. A recent study published results that apparently show zircon crystallization ages are younger than the eruption age of Bishop Tuff. It seems to be difficult to eliminate these discrepancies, even if the Th/U partitioning and disequilibrium generated during partial melting are taken into account for recalculation of its zircon age. However, magma chamber process and history of Bishop Tuff are too complex to obtain accurate zircon ages by U–Pb method. To overcome this, therefore, the Th–Pb zircon dating method is a key technique for understanding complex, pre-eruptive magma processes, and further efforts to improve its precision and accuracy are desirable.

Syn‐metamorphic folding in the Tauern Window, Austria dated by Th–Pb ages from individual allanite porphyroblasts

AbstractHigh‐precision 232Th–208Pb dates have been obtained from allanite porphyroblasts that show unambiguous microstructural relationships to fabrics in a major syn‐metamorphic fold in the SE Tauern Window, Austria. Three porphyroblasts were analysed from a single garnet mica schist from the Peripheral Schieferhülle in the core of the Ankogel Synform, one of a series of folds which developed shortly before the thermal peak of Alpine epidote–amphibolite facies metamorphism: allanite grain 1 provided two analyses with a combined age of 27.7 ± 0.7 Ma; grain 2, which was slightly bent and fractured during crenulation, provided two analyses with a combined age of 27.7 ± 0.4 Ma; a single analysis from grain 3, which overgrew an already crenulated fabric, gave an age of 28.0 ± 1.4 Ma. The five 232Th–208Pb ages agree within error and define an isochron with an age of 27.71 ± 0.36 Ma (95% confidence level; MSWD = 0.46). The results imply that the crenulation event was in progress in a short interval (<1 Ma) c. 28 Ma, and that the Ankogel Synform was forming at this time. The thermal peak of regional metamorphism in the SE Tauern Window was probably attained shortly after 28 Ma, only c. 5 Ma after eclogite facies metamorphism in the central Tauern Window. Metasediment may contain allanite porphyroblasts with clear‐cut microstructural relationships to fabric development and metamorphic crystallization; for such rocks, 232Th–208Pb dating on microsamples offers a powerful geochronological tool.

Zircon from the East Orebody of the Bayan Obo Fe–Nb–REE deposit, China, and SHRIMP ages for carbonatite-related magmatism and REE mineralization events

Extremely U-depleted (<1 ppm) zircons from H8 banded ores in the East Orebody of the Bayan Obo REE–Nb–Fe deposit are presented, with mineral compositions, textures, 232Th–208Pb SHRIMP ages and petrological context. Cores of East Orebody zircon contain up to 7 wt% HfO2 and are zoned, depicting bipyramidal crystal forms. A distinct generation of patchy, epitaxial rim zircon, similarly depleted in U, is intergrown with rare earth ore minerals (bastnasite, parisite, monazite). Overprinting aegirine textures indicate paragenetically late, reactive Na-rich fluids. Chondrite-normalized REE patterns without Eu anomalies match closely with those from the Mud Tank and Kovdor carbonatitic zircons. Increased HREE in rims ((Lu/Gd)N 43–112) relative to cores ((Lu/Gd)N 6–7.5) and the localized presence of xenotime are attributable to reactive, mineralizing fluid compositions enriched in Y, REE and P. Cathodoluminescence further reveals HREE fractionation in rims, evidenced by a narrow-band Er3+ emission at 405 nm. The extreme depletion of U in core and rim zircon is characteristic for this mineral deposit and is indicative of a persistent common source. U depletion is also a characteristic for zircons from carbonatitic or kimberlitic systems. 232Th–208Pb (SHRIMP II) geochronological data reveal the age of zircon cores as 1,325 ± 60 Ma and a rim-alteration event as 455.6 ± 28.27 Ma. The combined findings are consistent with a protolithic igneous origin for zircon cores, from a period of intrusive, alkaline–carbonatitic magmatism. Fluid processes responsible for the REE–Nb mineralizations affected zircon rim growth and degradation during the widely reported Caledonian events, providing a new example in a localized context of HREE enrichment processes.

Mantle convection and evolution with growing continents

We present a three‐dimensional spherical shell numerical model for chemical differentiation and redistribution of incompatible elements in a convective Earth's mantle heated mostly from within by U, Th, and K and slightly from below. The evolution‐model equations guarantee conservation of mass, momentum, energy, angular momentum, and four sums of the number of atoms of the pairs 238U–206Pb, 235U–207Pb, 232Th–208Pb, and 40K–40Ar. The pressure‐ and temperature‐dependent viscosity is supplemented by a viscoplastic yield stress, σy. The lithospheric viscosity is partly imposed to mimic its increase by dehydration of oceanic lithosphere and other effects. Also, the asthenosphere is generated not only by the distribution of temperature and melting temperature, but essentially by the profiles of water solubility and water abundance. Therefore we introduced a radial viscosity profile factor describing that behavior. However, the focus of this paper is the episodic growth of continents and oceanic plateaus. As a complement, the differentiation generates the depleted MORB mantle (DMM) which predominates immediately beneath the lithosphere. Our continents are not artificially imposed on the surface of the spherical shell, but instead they evolve by the interplay between chemical differentiation and convection/mixing. No restrictions are imposed regarding number, size, form, and distribution of continents. However, oceanic plateaus that impinge upon a continent have to be united with it. This mimics the accretion of terranes. The numerical results show an episodic growth of the total mass of the continents and display a plausible time history for the laterally averaged surface heat flow, qob, and the Rayleigh number, Ra. We use our model to explore a moderate region of Ra–σy parameter space. We find Earth‐like continent distributions in a central part of the Ra–σy space we explored. We identified a Ra–σy region where the calculated total continental volume is very close to the observed value; another Ra–σy region where the Urey number, Ur, is close to the accepted value; a third Ra–σy area where surface heat flow is very close to the present‐day observed mean global heat flow using typical abundances of the heat‐producing elements. It is remarkable that these different acceptable Ra–σy regions share a common overlap area, where Earth‐like behavior is simultaneously fulfilled.Although the convective flow patterns and the chemical differentiation of oceanic plateaus are coupled, the evolution of time‐dependent Rayleigh number, Rat, is relatively well predictable and the stochastic parts of the Rat(t) curves are small. Regarding the time distribution of juvenile growth rates of the total mass of the continents, predictions are possible only in the first epoch of the evolution, presumed that the initial conditions are given. Later on, the distribution of the continental growth episodes is increasingly stochastic. Independent of the varying individual runs, our model shows that the total mass of the present‐day continents is not generated in a single process at the beginning of the thermal evolution of the Earth but in episodically distributed processes. This is in accord with observation.

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.

Combined high-precision chronometry and geochemical tracing using accessory minerals: applied to the Central-Alpine Bergell intrusion (central Europe)

The behaviour of Nd, U, Th and Pb isotopes in granitic rocks is usually controlled by accessory minerals. In this study an approach is adopted which uses the initial 143 Nd 144 Nd as a geochemical tracer together with high-precision UThPb geochronology for accessory phases and their respective whole rock. This combination of techniques is applied to zircon, allanite, sphene, apatite and thorite from both a tonalite and a granodiorite from the Tertiary Alpine Bergell intrusion. Inherited isotopic signatures have been identified in bulk-zircon fractions by inverse discordance in the UPb system. In the case of the granodiorite, the initial ϵ Nd in the zircon is 1.77 units lower than the bulk rock. This result indicates that Nd isotopes in zircon may serve as a useful tool for tracing crustal precursors in silicic rocks. All other accessories yield an initial ϵ Nd equal to their respective whole rock and UThPb ages equal to or lower than the respective intrusion ages. This indicates either the absence of inherited cores, or that such cores have exchanged their Nd and Pb (in the case of apatite also Sr) by diffusion. Where available, experimental mineral melt equilibria and diffusion models are consistent with this behaviour. UPb geochronology of carefully selected, core-free zircons yields a precise intrusion age of 31.88 ± 0.09 Ma for the tonalite. In contrast, granodioritic zircons are partially reset due to radiation damage caused by high U concentrations. However, an alternative means of dating young intrusions is to use 232Th 208Pb ages of allanite. In the case of allanite the presence of a large excess of 206Pb derived from 230Th indicates that no total Pb loss has occurred since crystallization, and the Nd isotopes indicate the absence of inheritance. Using both the allanite age and a 238U 206Pb age for sphene an intrusion age of 30.13 ± 0.17 Ma is indicated for the granodiorite. Taken together with an allanite ThPb age for the tonalite, these results strongly support the high closure temperatures for the allanites studied here (⩾ 700–800°C at high post-magmatic cooling rates). All other accessory mineral ages are reset to values similar to hornblende 39Ar 40Ar ages, and reflect the effect of post-magmatic cooling superimposed on post-metamorphic cooling.