From precise date to process rate: a high-precision zircon U-Pb geochronology perspective on the timing, rates and durations of geological processes

This thesis comprises a series of papers addressing the potential and limitations of accurate and precise uranium-lead geochronology of accessory minerals for quantifying the time-scales of a variety of geological processes. The first part consists of three papers focusing on quantifying the time-scales of magmatic processes in some well characterised magmatic systems. By integrating high precision U-Pb geochronology with trace element, oxygen and hafnium isotopic data, we place quantitative constraints on the durations and rates of some important processes in these magmatic systems. The second part comprises two chapters addressing the potential and limitations of high-precision zircon U-Pb geochronology as a tool for calibrating sedimentary records of Earth history. These papers document the present state of the art, various applications and future directions of high-precision U-Pb geochronology by isotope dilution thermal ionization mass spectrometry

High-precision zircon U–Pb geochronology of astronomically dated volcanic ash beds from the Mediterranean Miocene

Several orbitally tuned Miocene sedimentary sequences around the Mediterranean contain abundant intercalated volcanic ash beds. These sequences provide the rare opportunity to directly compare radioisotopic dating methods with independent and accurate deposition ages derived from astrochronology. We present a large data set (N=16N=16, n=166n=166) of zircon U–Pb dates obtained by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) techniques for ash beds from an almost continuous orbitally tuned Messinian to Langhian (6.2–15.4 Ma) sedimentary sequence exposed along the Adriatic coast south of Ancona, Italy. We use this unique data set to evaluate (1) the accuracy of zircon U–Pb dates, (2) the significance of initial intermediate daughter product disequilibria for zircon U–Pb geochronology of young rocks, (3) the effect of prolonged pre-eruption zircon crystallization and zircon recycling on U–Pb derived ash bed deposition ages, and (4) discuss the implications for the intercalibration of radioisotope geochronometers and the calibration of the Geologic Time Scale

Hf isotope analysis of small zircon and baddeleyite grains by conventional Multi Collector-Inductively Coupled Plasma-Mass Spectrometry

Protocols for solution nebulization (SN) multi collector (MC) ICP-MS analysis of the Hf isotopic composition of accessory minerals (zircon and baddeleyite) selected for high-precision ID-TIMS U\Pb geochronology have improved significantly during the last two decades. Precise crystallization age and Hf isotopic composition from the exact same volume of mineral can be retrieved, provided that time-consuming sample preparation and purification is possible. The present work focuses on the analytical capabilities of SN-MC-ICP-MS for Hf isotope analysis on non-purified reference materials. Analytical issues arise for (i) igneous and metamorphic zircons, often enriched in trace elements and REEs, which increase the amount of isobaric interferences. (ii) Small minerals that challenge the limits of quantification of the apparatus. JMC475 Hf standard, Plešovice and Temora-2 zircons and Phalaborwa baddeleyite were used to characterize the sources of analytical bias and to set up an optimized protocol for Hf isotope analysis of small accessory minerals with accurate correction of the isobaric interferences and accounting for the matrix effects. Doping of JMC475 with major (Zr, Si), trace (U, Th, Y, Ti) and rare earth elements (Ce, Nd, Gd, Dy, Ho, Er) shows that oxide species produced in the ICP induce an offset of the measured 176Hf/177Hf ratios that is corrected by normalization to the reference value. The 176Yb isobaric interference is efficiently removed for samples with Yb/Hf ratios b0.6 using in-run determined 176Yb/173Yb reference ratios from Yb doped reference material. This isotopic ratio varies (i) with time and (ii) sample matrix. Plasma loading by Zr is not the source of these variations. We propose that Yb oxidation in the plasma torch is responsible for variations in the 176Yb/173Yb ratio. Our dataset shows that matrix matched standards should be preferred for data reduction, and that Plešovice is a good zircon standard because of its availability, quantifiable REEs composition and potential forwide range of Yb doping. Consequently, lowHf concentration solutions fromTemora-2 zirconswere accurately measured in a small sample volume (b120 μl), using a short acquisition protocol (50 × 1 s cycles), and sample-standard bracketing for mass bias coefficient determination in a [Hf]~10 ng/g Plešovice solution measured through 80 × 4 s cycles. The repeatability of the measurement (2SD) is improved by factors of up to ~4.4 and the limit of quantification is brought down to b2 ng/g Hf, corresponding to zircon minerals of b24 ng

Assessing the age of the Late Cretaceous Danek Bonebed with U–Pb geochronology

An ash-rich volcaniclastic sandstone immediately underlying dinosaur-rich material from the Danek Bonebed in the Horseshoe Canyon Formation (HCF), Edmonton, Alberta, Canada, contains accessory zircon, which have been dated employing U–Pb geochronology. Both laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and chemical abrasion isotope dilution thermal ionization mass spectrometry (ID-TIMS) U–Pb analyses have been conducted. The zircon age distributions are complex with U–Pb dates ranging from Precambrian to Cretaceous. We consider the youngest ID-TIMS 206Pb/238U date of 71.923 ± 0.068 Ma as the maximum deposition age of the ash-rich sandstone, placing the overlying Danek bonebed in the early Maastrichtian. This age is compatible with the paleontological assemblage from the Danek Bonebed and the regional stratigraphy. The zircon age distribution also implies that the HCF had a complex provenance of the detritus with some Archean and Proterozoic zircons, a group of Mesozoic, and a large compliment of Cretaceous grains. The results highlight the importance of high precision geochronology in constraining the age of important fossil deposits such as the Danek Bonebed

Zircon Petrochronology of the Meghri-Ordubad Pluton, Lesser Caucasus: Fingerprinting Igneous Processes and Implications for the Exploration of Porphyry Cu-Mo Deposits

The trace element composition of zircon, especially in tandem with U-Pb geochronology, has become a powerful tool for tracing magmatic processes associated with the formation of porphyry copper deposits. However, the use of the redox-sensitive Eu and Ce anomalies as a potential mineral exploration proxy is controversial. This study presents a comprehensive, temporally constrained data set of zircon trace element compositions (n = 645) for three compositionally distinct magmatic series identified in the Meghri-Ordubad pluton, southernmost Lesser Caucasus. The 30 million years of Cenozoic magmatism in the Meghri-Ordubad pluton are associated with several ore-forming pulses leading to the formation of porphyry copper deposits and epithermal-style mineralization. Our zircon geochemical data constrain the thermal and chemical evolution of this complex intrusive suite and allow an evaluation of the usefulness of zircon as a mineral exploration proxy for porphyry copper deposits. Our results combined with Rayleigh fractionation modeling indicate that the trace element composition of zircon (Th/U, Hf, Ti, YbN/DyN, Eu anomalies) is influenced by the composition and the water concentration of the parental magma, as well as by co-crystallizing titanite and apatite. In contrast, the variations of Ce anomalies remain difficult to explain by magmatic processes and could rather be ascribed to relative fluctuations of the redox conditions. In the Meghri-Ordubad pluton, we do not observe any systematic patterns between the trace element composition in zircons and the different ore-forming pulses. This questions the reliability of using the trace element composition in zircon as an exploration mineral proxy, and it rather emphasizes that a good knowledge of the entire magmatic evolution of a metallogenic province is required

Mass Spectrometry in Earth Sciences: The Precise and Accurate Measurement of Time

Precise determinations of the isotopic compositions of a variety of elements is a widely applied tool in Earth sciences. Isotope ratios are used to quantify rates of geological processes that occurred during the previous 4.5 billion years, and also at the present time. An outstanding application is geochronology, which utilizes the production of radiogenic daughter isotopes by the radioactive decay of parent isotopes. Geochronological tools, involving isotopic analysis of selected elements from smallest volumes of minerals by thermal ionization mass spectrometry, provide precise and accurate measurements of time throughout the geological history of our planet over nine orders of magnitude, from the accretion of the proto-planetary disk, to the timing of the last glaciation. This article summarizes the recent efforts of the Isotope Geochemistry, Geochronology and Thermochronology research group at the University of Geneva to advance the U-Pb geochronological tool to achieve unprecedented precision and accuracy, and presents two examples of its application to two significant open questions in Earth sciences: what are the triggers and timescales of volcanic supereruptions, and what were the causes of mass extinctions in the geological past, driven by global climatic and environmental deterioration

How Accurately Can We Date the Duration of Magmatic-Hydrothermal Events in Porphyry Systems?

Determining the absolute duration of magmatic-hydrothermal events leading to the formation of porphyry systems (i.e., including porphyry copper, skarn, and epithermal deposits) is one of the key questions in ore geology. This is so because the duration of magmatic-hydrothermal events in porphyry systems is instrumental to the development of genetic models necessary to explore a category of mineral deposits that provide most of the copper and significant amounts of base and precious metals to our economy. The problem of determining the absolute duration of magmatic-hydrothermal events in porphyry systems has been addressed through ther- mal modeling of cooling intrusions and time needed to precipitate specified metal amounts from active hydro- thermal systems with known metal concentrations and fluid fluxes. Both these methods have shown that the likely duration of hydrothermal systems is on the order of a few tens of kilo-annum (ka). Isotopic dating in con- trast is the only possible way to determine the life span of magmatic-hydrothermal events in fossil porphyry sys- tems. Analytical and methodological developments during the last decade in the fields of the most robust iso- topic systems commonly used for absolute dating (U-Pb, 40Ar/39Ar, Re-Os) allow us to date minerals with internal precisions <0.2% (2σ). For a 10-Ma-old mineral this corresponds to a <20-ka uncertainty, which is mar- ginally sufficient to discriminate the duration of hydrothermal systems at the tens of kilo-annum scale. How- ever, many geochronological studies on fossil porphyry systems have shown that these are most often formed through repeated cycles of several intrusion events, which extend the overall life of the porphyry systems to a few 0.X and up to ~2 Ma in some cases. Internal precisions of the above mentioned dating methods allow us, in theory, to comfortably discriminate events at the 0.X scale and the combination of U-Pb, 40Ar/39Ar, and Re- Os geochronology is a tool widely used by ore geologists to bracket the duration of cyclic magmatic-hydro- thermal events in porphyry systems. In this review we discuss some fundamental problems that are systematically overlooked in most geochrono- logical studies trying to bracket the life span of porphyry systems. We show that if these problems are not adequately taken into account and tackled the result will be that fundamentally wrong life spans of porphyry sys- tems will be estimated. We also provide basic guidelines to follow when trying to resolve the duration of mag- matic-hydrothermal events in porphyry systems with the highest accuracy and precision currently achievable

Zircon petrochronology reveals the temporal link between porphyry systems and the magmatic evolution of their hidden plutonic roots (The Eocene Coroccohuayco deposit, Peru)

We present zircon geochronologic (LA-ICPMS and ID-TIMS), trace element and Hf isotopic evidence for a complex evolution of the plutonic roots of the Eocene Coroccohuayco porphyry system, southern Peru. LA-ICPMS U-Pb dating has initially been carried out to optimize grain selection for subsequent high-precision ID-TIMS dating and to characterize crustal assimilation (xenocrystic cores). This combined in-situ and whole-grain U-Pb dating of the same grains has been be further exploited to derive a robust temporal interpretation of the complex magmatic system associated with the Coroccohuayco porphyry-skarn deposit. Our data reveal that a heterogeneous gabbrodioritic complex was emplaced at ca. 40.4 Ma and was followed by a nearly 5 Ma-long magmatic lull until the emplacement of dacitic porphyry stocks and dykes associated with the mineralizing event at ca. 35.6 Ma. However, at the sample scale, zircons from the porphyries provide insight into a 2 Ma-long lived “hidden” magmatism (probably at 4-9 km paleodepth) prior to porphyry intrusion and mineralization for which no other evidence can be found on the surface today. These dates together with zircon trace element analysis and Hf isotopes argue for the development of a long-lived magmatic system dominated by amphibole fractionation with an increasing amount of crustal assimilation and the development of a large and sustained thermal anomaly. The system was probably rejuvenated at an increasing rate from 37.5 to 35.6 Ma with injection of fresh and oxidized magma from the lower crust, which caused cannibalism and remelting of proto-plutons. The porphyry intrusions at Coroccohuayco were emplaced at the peak thermal conditions of this upper crustal magma chamber, which subsequently cooled and expelled ore fluids. Zircon xenocrysts and Hf isotopes in the porphyritic rocks suggest that this large upper crustal system evolved at stratigraphic levels corresponding to Triassic sediments similar to the Mitu group that may be present below the district. Using the zircon Ce anomaly as a proxy for oxidation state of the magma through time, we show that the high oxidation state of the porphyries is not the result of upper-crustal processes but is rather controlled by magmatic processes occurring at deeper levels. Comparison of our data with available high-precision geochronologic data at other porphyry systems suggest that such deposits may form when injection rate, volume and heat of their long-lived upper crustal magmatic system reach their peaks. Theses features might be diagnostic of a productive deposit

Jurassic to Palaeogene tectono-magmatic evolution of northern Chile and adjacent Bolivia from detrital zircon U-Pb geochronology and heavy mineral provenance

Heavy mineral provenance data presented in this article reinforce evidence for significant late Palaeogene deformation and relief formation along the western margin of the Central Andean Plateau. Late Eocene–Oligocene onset of molasse-type sedimentation records initial range uplift. Strikingly different basement sources of sediments deposited to the east and west of the Late Palaeogene range indicate that initial relief development was governed by a bivergent thrust system. Significantly higher sediment accumulation rates to the east of the range compared to the west suggest that the generated relief acted already as an effective orographic barrier at that time. Higher precipitation and denudation along the eastern slope facilitated deeper erosion of the trust bel