Studying the Stability of the K/Ar Isotopic System of Phlogopites in Conditions of High T, P: 40Ar/39Ar Dating, Laboratory Experiment, Numerical Simulation

Typically, 40Ar/39Ar dating of phlogopites from deep-seated xenoliths of kimberlite pipes produces estimates that suggest much older ages than those when these pipes were intruded. High-pressure (3 GPa) laboratory experiments enabled the authors to explore the behaviour of argon in the phlogopite structure under the conditions that correspond to the mantle, at the temperatures (from 700 to 1000 °С), far exceeding closure temperature of the K/Ar isotopic system. “Volume diffusion” remains foremost for describing the mobility of argon in phlogopite at high pressures. The mantle material age can be estimated through the dating of the phlogopites from deep-seated xenoliths of kimberlites, employing the 40Ar/39Ar method, subject to correction for a partial loss of radiogenic 40Ar when xenolith moves upwards to the Earth’s surface. The obtained data served as the basis for proposing the behaviour model of the K/Ar isotopic system of minerals in conditions of great depths (lower crust, mantle), and when transporting xenoliths in the kimberlite melt.

40Ar-39Ar geochronology of the Karoo flood basalts: tracking disturbance in the isotopic system.

<p>High precision dating of Large Igneous Provinces (LIP) is not only useful to understand their link to environmental changes and mass extinctions (Courtillot and Renne, 2003), but they also provide insights into the geodynamic setting in which they form (Encarnación et al., 1996). The Drakensberg continental flood basalts of South Africa and Lesotho are part of the Karoo LIP, which is presumably responsible for a phase of global climate change and disturbance of the oceanic ecosystems (the so-called Toarcian oceanic anoxic event T-OAE; Pálfy and Smith (2000)). However, the paucity of zircon or baddeleyite in most continental flood basalts renders is difficult to match the sub-permil age precision and accuracy that is typical for high-precision U/Pb CA-ID-TIMS age determination. Previous attempts to date the Karoo lavas using the <sup>40</sup>Ar-<sup>39</sup>Ar method failed to yield sufficient precision and accuracy for resolving the sequential stacking of the different basalt units. For example, <sup>40</sup>Ar-<sup>39</sup>Ar  analyses of carefully selected plagioclase separates yielded dates that are inverted relative to their stratigraphic position, with uncertainties that encompass the entire duration of volcanism in the area (Jourdan et al., 2007; Moulin et al., 2017). Here we test the hypothesis that previous, inconsistent <sup>40</sup>Ar-<sup>39</sup>Ar dates of plagioclase were a consequence of degassing of primary, metasomatic and alteration phases (mainly zeolites with subordinate sericite and carbonate) within single or multiple crystals. The lavas are mainly tholeiitic basalts that display two distinct sizes of plagioclase, which can be dated separately. Petrological characterization of these two size fractions shows that the larger plagioclase crystals (100-400 μm) are more altered and fractured than the smaller grains and are therefore more likely affected by post-crystallization disturbance of the Ar isotopic system. We present preliminary <sup>40</sup>Ar-<sup>39</sup>Ar data from i) untreated plagioclase that hosts visible alteration phases, ii) untreated plagioclase that is devoid of visible alteration phases (2 grain size aliquots), and iii) leached plagioclase that is devoid of visible alteration phases (2 grain size aliquots). The results of this study may enhance the effectiveness of the <sup>40</sup>Ar-<sup>39</sup>Ar dating technique to accurately constrain the crystallisation ages of altered mafic lavas, which form the majority of the exposed Karoo LIP flood basalts. Ar isotope data were collected using a multi-collector Argus VI mass spectrometer, and irradiated in an un-shielded reactor position to optimize the formation of <sup>38</sup>Ar from Cl to permit identification of different gas reservoirs in the sample through isochemical dating, based on Ca, K and Cl in-situ concentration (EPMA) and Ar isotopic ratios.</p>

The Metallogenic Setting of the Jiangjiatun Mo Deposit, North China: Constraints from a Combined Zircon U–Pb and Molybdenite Re–Os Isotopic Study

The Jiangjiatun Mo deposit is a recently discovered molybdenum deposit in the easternmost area of the Yan-Liao metallogenic belt, North China Craton. Quartz vein-type Mo mineralization at Jiangjiatun is associated with the granitic porphyry stock. In this study, we performed a combined zircon U–Pb and molybdenite Re-Os dating study on the Jiangjiatun Mo deposit to constrain its mineralization age and metallogenic setting. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) zircon U–Pb analyses suggest that the granitic porphyry was formed during the Late Jurassic, with a weighted mean 206Pb/238U age of 154 ± 1 Ma (2σ). Seven molybdenite samples from the Jiangjiatun deposit yield a 187Re–187Os isochron age of 157.5 ± 0.5 Ma (2σ). The discrepancy between the U–Pb and Re–Os ages may be explained (1) by the “2 sigma” measurement uncertainty, or (2) by the different closure temperature of the Re–Os isotopic system of molybdenite and the U–Pb isotopic system of zircon. Even though there is a small difference between the zircon U–Pb and molybdenite Re–Os ages, we can clearly identify a Late Jurassic Mo mineralization event at Jiangjiatun in the easternmost area of the Yan-Liao metallogenic belt. The moderate Re concentrations (13 to 73 ppm) in molybdenite from the Jiangjiatun Mo deposit are indicative of the involvement of the mantle materials into the Mo mineralization. The Jiangjiatun Mo deposit is likely the result of the subduction of the paleo-Pacific plate beneath the North China Craton during the Late Jurassic. Combined with the available published regional robust geochronological data, we proposed that the Mo mineralization in the Yan-Liao belt is in good agreement with the tectonic transition from Late Triassic post-collision extensional setting due to the closure of the paleo-Asian ocean to the Yanshanian (J–K1) continental arc setting in response to the subduction of the paleo-Pacific Plate. The study highlights that regional mineralization may provide an excellent constraint on tectonic change.

New data refer to palaeoproterozoic age of the Elabuga deformation zone metamorphism of the Volgo-Uralia, East European Craton

This paper presents age data of the Elabuga zone rocks metamorphism and of the detrital zircon of the Elabuga zone paragneisses and their correlation with the age of Middle-Volga megablock rocks. The Elabuga zone rocks and the Middle-Volga megablock rocks have similar geochemical characteristics and degree of metamorphism. Difference is as follows: Paleoproterozoic metamorphism is occurred at the studied Elabuga zone rocks and such metamorphism age is not presented at the Middle-Volga megablock rocks. The study of U-Pb isotopic system in external zones in the Elabuga deformation zone zircon crystals suggests that rocks were underwent by metamorphism of two stages 1.99 and 1.95 Ga ago.