Auswahl der wissenschaftlichen Literatur zum Thema „Carbonate U-Pb dating“

Abstract. In situ carbonate U–Pb dating studies have proliferated dramatically in recent years. Almost all these studies have targeted relatively young terrestrial calcite up to Carboniferous in age. To assess the robustness of the carbonate U–Pb chronometer in deep time, we carried out in situ U–Pb analyses in magnesite–ankerite–calcite carbonates in the martian meteorite Allan Hills (ALH) 84001. Carbonates in ALH 84001 formed at ca. 3.94 Ga, and there is little evidence that much happened to this rock since then, making it an ideal sample to test the robustness of the U–Pb system in old carbonates. We obtained a concordant date of 3941 ± 49/110 Ma (n=14, MSWD = 2.0), which is identical to the step-leaching Rb/Sr date determined previously. These results thus confirm that old carbonates are amenable to U–Pb dating in samples that have had a relatively simple history post-carbonate formation.

Abstract. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb geochronology of carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. The high spatial resolution of LA-ICP-MS U–Pb carbonate geochronology has benefits over traditional isotope dilution methods, particularly for diagenetic and hydrothermal calcite, because uranium and lead are heterogeneously distributed on the sub-millimetre scale. At the same time, this can provide limitations to the method, as locating zones of radiogenic lead can be time-consuming and “hit or miss”. Here, we present strategies for dating carbonates with in situ techniques, through imaging and petrographic techniques to data interpretation; our examples are drawn from the dating of fracture-filling calcite, but our discussion is relevant to all carbonate applications. We review several limitations to the method, including open-system behaviour, variable initial-lead compositions, and U–daughter disequilibrium. We also discuss two approaches to data collection: traditional spot analyses guided by petrographic and elemental imaging and image-based dating that utilises LA-ICP-MS elemental and isotopic map data.

Abstract. Recent developments in analytical capabilities in the field of in situ laser ablation mass spectrometry (LA-ICPMS) have expanded the applications of U–Pb geochronometers in low-U minerals such as carbonates or garnets. The rapid evolution of the technique relies on well-characterized matrix-matched reference materials. In this article, we explore the suitability of using carbonate as an “almost-matrix-matched reference material” for in situ U–Pb dating of sulfates. For such purpose, we have used the astrochronologically dated gypsum and anhydrite samples deposited during the Messinian Salinity Crisis (5.97–5.33 Ma) and compared these dates with the U–Pb ages obtained by LA-ICPMS. Although the majority of the samples failed due to the elevated common Pb content and low 238U/204Pb ratios, five of the samples showed a higher dispersion on U/Pb ratios. The obtained dates in four of these samples are comparable with the expected ages, while another gave an unexpected younger age, each of them with 6 %–11 % of uncertainty. The pit depth of the spots showed that the sulfates ablate similar to carbonates, so the offset due to the crater geometry mismatch or downhole fractionation can be assumed to be negligible. To sum up, the bias between the U–Pb and expected cyclostratigraphic ages, if any, is included in the uncertainty, and thus the results obtained here suggest that carbonate reference material is currently the best option for standardization of in situ U–Pb sulfate analyses.

Abstract. Laser ablation U–Pb analyses of carbonate (LAcarb) samples has greatly expanded the potential for U–Pb dating to a variety of carbonate-producing settings. Carbonates that were previously considered impossible to date using isotope dilution methods may preserve radiogenic domains that can be dated using spatially resolved laser ablation geochronology techniques. Work is ongoing to identify reference materials and to consider best practices for LAcarb. In this study we apply standard and emerging characterization tool sets on three natural samples with the dual goal of enhancing the study of carbonates and establishing a new set of well-characterized natural reference materials for LAcarb studies. We start with the existing carbonate reference material WC-1 from the Permian Reef Complex of Texas, building on the published description to offer a deeper look at U and associated trace elements. We consider a tufa sample from the Miocene Barstow Formation of the Mojave Block, California, as a possible secondary calcite reference material due to its well-behaved U–Pb systematics. There are currently no natural dolomite standards. We present an unusual dolomite sample with very well-behaved U–Pb systematics from the Miocene of the Turkana Basin of Kenya as a possible dolomite reference material for LAcarb dating. In addition to using X-ray fluorescence (XRF) mapping and spectroscopy to better understand U in these natural samples, we have analyzed multiple aliquots of each of them for 87Sr/86Sr by thermal ionization mass spectrometry (TIMS). The Sr isotope compositions are analytically homogeneous within petrographically homogeneous regions of all three samples, and thus these materials could be used as Sr isotope standards as well. While not part of the current contribution, this combination could streamline simultaneous LA analyses of 87Sr/86Sr and U–Pb geochronology.

The results of geochemical, isotope-geochemical (Sr, C, O) surveys of the Kinterep Formation carbonate deposits of Northwestern Salair and U-Pb dating (LA-ICP-MS) of zircons were given. Carbonate rocks represented by pure limestones are characterized by normal values of δ18OSMOW from 19.8 to 23.8 ‰ and δ13 CPDB from –0.7 to +0.9, the isotopic composition of Sr (87Sr/86Sr ratio) varies in a narrow range from 0.708486 to 0.708582.Using Sr and C isotope chemostratigraphy and U-Pb dating of zircons, age limits (525-510 Ma) were determined for the formation time of carbonate deposits of the Kinterep Formation. The first data of isotope chemostratigraphy in combination with the results of geochronological studies made it possible to compare the limestones of the Kinterep Formation of Northwestern Salair with carbonate sections of Siberia and Central Asia.

Abstract. The recent development of methods for in situ U–Pb age determination in carbonates has found widespread application, but the benefits and limitations of the method over bulk analysis (isotope dilution – ID) approaches have yet to be fully explored. Here we use speleothems – cave carbonates such as stalagmites and flowstones – to investigate the utility of in situ dating methodologies for “challenging” matrices with typically low U and Pb contents and predominantly late Cenozoic ages. Using samples for which ID data have already been published, we show that accurate ages can be obtained for many speleothem types by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Consideration of our own and literature data suggest that most carbonates with >1 ppm uranium and a few hundred parts per billion of Pb should be good targets for in situ methodologies, regardless of age. In situ analysis often provides a larger spread in U∕Pb ratios, which can be advantageous for isochron construction, but isochron ages rarely achieve the ultimate precision of ID analyses conducted on the same samples simply because signal sizes are dramatically reduced. LA analysis is faster than ID and thus will play a significant role in reconnaissance studies. The major advantage of the in situ methodology appears to be the potential for successful dating outcomes in sample types requiring high spatial-resolution analysis or those with a high common-Pb component where LA approaches may facilitate identification of the most radiogenic regions for analysis.

At the boundary of the Meso- and Neoproterozoic, Grenville collision events were widely manifested on the margins of ancient continental blocks. Most of the sedimentary sequences that had accumulated by that time had undergone significant thermal–metamorphic changes. In many ways, this is the main reason for the lack of isotopic data for carbonate deposits in the interval of 1200–900 Ma in world practice. Sr–isotopic composition in carbonate rocks with an age of 980–920 Ma was determined in the only section of the World – the Huainan Formation of the North China Platform with accumulation time determined by U–Pb dating of detrital zircon grains. In the upper part of the Nizhny Tunguska suite of the Turukhansk uplift, among carbonate deposits, we established the presence of altered volcanic rocks, as well as weathering crust products along them – poor bauxite and chamosite ores. Based on U–Pb zircon isotope dating, the age of the volcanic rocks is 964 Ma. For the least altered limestones of the upper part of the Nizhny Tungusska Formation, received 87Sr/86Sr values are 0.70532–0.70578, which are close enough to those found in the rocks of the Huainan Formation. Geochronological age of the studied limestones gives us more correct data. These data can be used to refine the previously proposed configuration of the Sr–isotopic composition variation curve in the Early Neoproterozoic. The Nizhny Tunguska Formation is the only carbonate section of the Early Neoproterozoic in the world with correctly geochronologically substantiated dating (based on zircon from subsynchronous volcanic rocks). This compares favorably with the Huainan formation.

Shallow-water carbonate rocks constitute a crucial component of large guyots, arising in distinct environments and harboring valuable insights into the evolutionary stages of seamount islands as well as the tectonic conditions of the underlying oceanic plate. Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) was used to conduct in situ U–Pb isotope dating of carbonate minerals with low uranium content collected from Weijia Guyot. This dating approach yielded crucial evidence for the vertical development of the seamount. Our study indicates that shallow-water carbonate rocks in Weijia Guyot had a temporal range between 91 My and 137 My. The carbonate rocks underwent two growth phases, Hauterivian to Barremian and Cenomanian to Turonian, with a hiatus of approximately 20 My. Since the Hauterivian age, the shield volcano of Weijia Guyot is essentially complete, with its seamount top exposed at or near sea level and receiving its first stage of shallow-water carbonate sedimentation. Based on the dating of both shallow-water carbonate rocks and hawaiite within the Weijia Guyot, it is inferred that approximately 10 My elapsed from shield-building volcanism to late alkalic volcanism. During the Turonian age, the main reason for the second phase of shallow-water carbonate rocks in the seamounts was the regional tectonic uplift triggered by the drift of the Weijia Guyot along with the Pacific Plate toward the Society hotspot.

Les dépôts lacustres font l'objet d'études approfondies car ils sont propices à l’enregistrement des modifications de nombreux facteurs environnementaux (changement climatique, modification du couvert végétal…), et également car ils renferment des ressources en matière première de toute première importance. Cependant, les reconstructions chronostratigraphiques dans ces contextes sont généralement entravées par la rareté des données permettant d’établir l'âge des dépôts de manière continue. L'objectif principal de ce projet de thèse est d'examiner les avantages et les limites de la datation au carbonate U-Pb (LA-ICPMS) (méthode LAcarb) comme un nouvel outil chronostratigraphique pour les dépôts lacustres. Le deuxième objectif est d'étudier plus précisément le potentiel de datation de la méthode en analysant sa capacité à établir un âge avec résolution temporelle pertinente en fonction des différents types de carbonates et de minéralogies. Pour atteindre ces objectifs, plusieurs phases carbonatées (microbialites, ooides, oncoides) et ciments diagenétiques ont été collectés dans deux exemples lacustres fossiles ayant un âge absolu relativement bien établit: le Miocène du bassin du cratère Ries (SW Allemagne) et la formation Yacoraite (Crétacé-Paléogène) du bassin de Salta (NW de l'Argentine). Une étude sédimentologique et diagenétique s’appuyant sur des analyses pétrographiques et des isotopes stables de l’oxygène (O) de du carbone (C), a permis de sélectionner les phases carbonatées les plus précoces. Ce travail a été réalisé afin de maximiser les chances d’avoir préservé la composition géochimique originelle en U-Pb des fluides en présence lors du dépôt. Dans le bassin du cratère Ries, la méthode LAcarb a fourni des âges précis qui ont permis des corrélations chronostratigraphiques à une résolution temporelle de l’ordre de la séquence stratigraphique du 3e ordre (0,5 à 5 Ma). Dans le cadre de la formation Yacoraite, deux modèles d'âge ont été obtenus le long d'une coupe stratigraphique de référence. L'un est dérivé de la géochronologie des zircons extraits des dépôts de cendre volcaniques intercalés dans la formation Yacoraite (modèle d'âge maximum des dépôts; MDA). Le second est dérivé de la méthode LAcarb (modèle d'âge minimum des dépôts; MIDA). Les deux modèles se superposent remarquablement et permettent de décrire une même dynamique des taux de sédimentation avec une résolution temporelle d'environ 0,9 à 2% (2σ). L'âge des dépôts révisé pour la formation Yacoraite a ensuite été intégré dans un modèle chronostratigraphique combinant les données biostratigraphiques, hemiostratigraphiques et magnétostratigraphiques issues de la littérature. La localisation de la limite Crétacé-Paléogène (KPg) a bien été confortée dans la formation Yacoraite. De plus, la révision des âges de la partie sommitale de la formation Yacoraite a également permis de valider la localisation des deux phases hyperthermiques du Paléocène-Éocène (Paleocene-Eocene Thermal Maximum: PETM, et l’optimum climatique de l’Eocène; EECO) dans les deux formations sus-jacentes. Une analyse statistique a ensuite été réalisée sur la base des âges de 80 phases carbonatées de la formation Yacoraite. Les microbialites ont fourni les plus faibles succès en potentiel de datation (41 % d'âges cohérents avec le modèle MDA) ainsi que les plus basses précisions concernant l'âge (2σ <10 %). A l’inverse, les ciments lacustres ont été couronnés des meilleurs potentiels de datation (64 % d'âges cohérents avec le modèle MDA) avec des âges de meilleure précision. De plus, les phases dominées par la calcite ont généralement fourni des âges plus précis que les phases dolomitiques. Les avancées concernant les potentialités et les limites associées à la méthode LAcarb ont ainsi permis de proposer au travers de cette thèse une méthode complète pour construire un modèle d'âge robuste des dépôts en milieu lacustre. Les perspectives de ces travaux sont multiples, avec par exemple [...]
Lacustrine deposits are extensively investigated because they play a pivotal role as environmental recorders and host valuable economic resources. However, chronostratigraphic reconstructions in these settings are usually hampered by the scarcity of data required to establish the depositional age of the system. The prime objective of this PhD project was examining benefits and limitations of carbonate U-Pb (LA-ICPMS) dating (LAcarb) as new chronostratigraphic tool for lacustrine deposits. The second objective was to investigate the dating potential, in terms of dating success and time resolution, of various carbonate types and mineralogies with the aim to better organize future LAcarb based research. To achieve these goals, depositional (microbialites, ooids, oncoids) and early diagenetic carbonate phases of known absolute age were collected from two lacustrine settings: the Ries Crater basin (Miocene, SW Germany) and the Yacoraite formation (Fm.) from the Salta rift basin (Cretaceous-Paleogene, NW Argentina). A sedimentologic and diagenetic study based on petrography and Oxygen (O) and Carbon (C) stable isotope analysis allowed to select carbonate phases that most possibly preserved the pristine U-Pb geochemical composition and consequently inform on the timing of deposition. In the Ries Crater basin, LAcarb provided accurate ages that allowed chronostratigraphic correlations at a time resolution of the 3rd order stratigraphic sequence (0.5–5Ma). In the framework of the Yacoraite Fm., two depositional age depth model were obtained along a stratigraphic section. One derived from zircon (ash layer) geochronology (maximum depositional age depth model; MDA depth model) and the other from LAcarb (minimum depositional age depth model; MIDA depth model). The two models remarkably overlap and describe the same sedimentation rate dynamic with a time resolution between 0.9 and 2% (2σ). The revised depositional age of the Yacoraite Fm. was then integrated in a chronostratigraphic model merging biostratigraphic, chemostratigraphy and magnetostratigraphy data from literature. Accordingly, the KPg limit was interpreted to be located in the Yacoraite Fm. whereas the two Paleocene-Eocene hyperthermals (Paleocene-Eocene Thermal Maximum, PETM; early Eocene Climate Optimum, EECO) were identified in the two overlying formations. A statistical analysis was achieved based on the ages of 80 carbonate phases from the Yacoraite Fm. Microbialites provided the lowest dating success (41% ages consistent with the MDA depth model) and age precision (2σ < 10%) in contrast with lacustrine cements that yielded the highest dating success (64% ages consistent with MDA depth model) and age precision (2σ < 3%). Furthermore, calcitic phases usually provided more precise ages than dolomitic phases. The better comprehension of potentialities and limitations of LAcarb acquired in this PhD allowed to propose a workflow to build a robust depositional age depth model in lacustrine settings. Three possible perspective scenarios were introduced: 1) the study of the Ries Crater basin as analogue of paleolakes on Mars; 2) the use of LAcarb to select pristine carbonates for chronostratigraphic studies; and 3) basin scale chronostratigraphic correlations in the Yacoraite Fm

More than 500 occurrences of carbonate-hosted Pb-Zn ores are documented in the Eastern and Southern Alps. They are invariably hosted by shallow lagoonal and reef carbonates of Middle and more frequently Upper Triassic (Anisian and Carnian) age and are collectively termed “Alpine-type” deposits. The local palaeogeography and synsedimentary structures influenced ore mineralization. Although they occur in a wide region, they share common features such as a simple mineralogical composition, complex ore textures, light sulphur isotopic compositions, Late Palaeozoic Pb model ages, and similar trace element compositions in sphalerite, galena and pyrite. Sphalerite records a low-temperature (60-140°C) precipitation. It is low in Fe, Mn, Co, Ag and In, but commonly contains elevated Cd, Ge, As, Tl and Pb. Galena is Ag-poor, although both sphalerite and galena tend to higher Ag concentrations towards the northern Districts. Minor Fe sulphides are low in Co and Ni but carry considerable As and Tl. Rb-Sr dating of sphalerite from the type locality Bleiberg reveals an age of ≈229 Ma for ores of the Raibl Group, and ages of ≈207 and ≈201 Ma for trace element-rich breccia ore in the western part of the Bleiberg deposit. The older age corresponds to U-Pb ages obtained on calcite associated with the Pb-Zn mineralization at the Gorno deposit (Southalpine). The younger age suggests fluid flow within the carbonate sequence in an extensional tectonic regime due to fracturing of the carbonate platform during initial rifting of the Penninic Ocean.