Electron microprobe dating of monazite

Abstract Because monazite is extremely rich in U and Th, radiogenic Pb ( * Pb) accumulates very quickly, and reaches, in about 100 Ma, a level where it is possible to analyse it with the electron probe. Assuming that common Pb is negligible, and that partial loss of Pb has not occurred, the simultaneous measurement of U, Th, and Pb allows to obtain a geologically meaningful age from a single electron probe analysis. Here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method. A specific statistical method to deal with the large number of data which can be obtained on a single sample is described, and several guidelines, illustrated by examples, are suggested to optimize the method. Electron probe measurements carried out on samples of known age, from 200 Ma to 3.1 Ga, yield ages that always fall inside the confidence interval of the isotopically determined age, demonstrating that this method is reliable. The younger age limit is approximately 100 Ma, although it can be younger in some favourable cases. In old monazites, extremely high *Pb contents have been found (up to 5 wt%) indicating that monazite can tolerate high radiation doses without experiencing lead loss. The final precision on the age, for a 'normal' monazite, is + 30-50 Ma, for a total counting time of 600 s. A complete dating procedure can be completed in less than 1 h. First results indicate that old ages can be preserved in monazite, either in small relict cores in crystals, or by the coexistence of several generations of monazites in a sample. This method has all the advantages of the electron probe: it is non-destructive, has an excellent spatial resolution (monazites as small as 5 I~m can be dated), and because it is possible to work on normal polished thin-sections, the petrographical position of the dated crystal is known. This method offers a large number of geologists access to an in-situ dating technique at moderate cost

Etude expérimentale et théorique du mûrissement d'Ostwald dans les systèmes magmatiques (Implications pétrologiques)

CLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocFONTAINEBLEAU-MINES ParisTech (771862302) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Transcrystalline melt migration in clinopyroxene

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Simulation expérimentale de l'ascension et de la vésiculation des magmas rhyolitiques (application à la cinétique de nucléation des bulles et implications volcanologiques)

L'étude du processus de nucléation des bulles dans les magmas rhyolitiques a été abordée au cours de cette thèse. Des expériences de décompression isothermes en autoclave à chauffage externe et trempe rapide ont été réalisées afin de mieux comprendre : (1) les effets de différentes populations cristallines sur la cinétique de nucléation des bulles d'eau, et (2) les effets du CO2 sur la nucléation homogène des bulles. L'objectif ultime de nos travaux était d'identifier les paramètres texturaux qui pourraient constituer des marqueurs robustes de la dynamique d'ascension des magmas rhyolitiques. Le résultat majeur de cette étude est la démonstration que la relation très forte entre [dP/dt] et n3D tient aussi dans le cas de la nucléation hétérogène. La principale implication volcanologique est que l'étude texturale des ponces naturelles pourrait servir à des applications vélocimétriques et fournir des renseignements sur la dynamique d'ascension des magmas dans les conduits volcaniquesCLERMONT FD-BCIU Sci.et Tech. (630142101) / SudocFONTAINEBLEAU-MINES ParisTech (771862302) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF

Croissance des volcans: l'effet de la mise en place des systèmes intrusifs

International audienceThe contribution of intrusive systems to the growth and shape of volcanoes has never been studied in a quantitative way. However, it is recognized, from eroded edifices, that intrusions contribute significantly to their volumes at depth. Moreover, permanent deformations are recorded on active volcanoes, during intrusive events. Here, we describe a preliminary approach, applicable to basaltic shield volcanoes, to evaluate the significance of intrusions on edifice shape and height

P – V – T – X evolution of olivine-hosted melt inclusions during high-temperature homogenization treatment

International audienceDuring low–high temperature (T) cycles imposed on olivine-hosted melt inclusions (MIs) we observe a systematic increase in homogenization temperature (Th) with time, regardless of their initial major-element and H2O contents. Bubble persistence at high T suggests that inclusion internal pressure (Pint) is lower than its original, trapping pressure. We explore how reversible and irreversible processes modify the composition (X), volume (V) and Pint of heated MIs, and compare the results of theoretical modeling with experimental observations of MIs from FAMOUS Zone (FZ, Mid-Atlantic Ridge) and La Sommata (SOM, Vulcano, Aeolian Islands) basaltic samples. Due to olivine dissolution at inclusion walls and thermoelastic deformation, Pint–V–X conditions change significantly upon heating. Olivine dissolution induces changes in major-element composition (i.e., enrichment in Fe and Mg), morphology and volume (up to +25% at 1500 °C). We provide equations for the thermoelastic deformation of olivine bearing a two-phase, liquid–gas inclusion for the end-member cases of chemical equilibrium and no exchange between gas and liquid. These equations allow Pint–V evolution to be related to variations in bubble volume fraction. Upon heating, both Pint and V variations are smaller in the presence of a gas bubble than for a homogeneous liquid inclusion, at the same T. Dissolution–reprecipitation and thermoelastic deformation of the olivine host are reversible processes, so initial Pint–V–X conditions are restored upon cooling. On the contrary, water loss from MIs and plastic deformation of the olivine host are processes that irreversibly lower Pint, and account for the systematic increase of Th with time. Our theoretical and experimental investigations suggest that the increase of Th in volatile-rich SOM MIs is mainly related to progressive release of water. Compared to larger MIs located at a similar distance from the olivine rim, smaller MIs show a faster increase in Th with time, consistent with the effects of diffusive water loss. Nonetheless, we cannot exclude the combined effect of incipient plastic deformation, which would enhance water loss by diffusion along dislocations. The increase in Th in volatile-poor FZ MIs is driven mainly by elasto-plastic deformation of the olivine host, which becomes more marked with increasing T and decreased distance from MI wall to olivine rim. Occurrence of plastic deformation in FZ olivines is testified by dislocation patterns observed around inclusions. In general, conducting homogenization experiments at 1 atm prevents MI homogenization happening at a Th equal to entrapment T. This is due to a drop in Pint caused by the elastic deformation that affects olivine phenocrysts bearing pressurized MIs during magma ascent. Predicted increase in Th ranges from a few degrees to tens of degrees depending on entrapment conditions, melt composition and volatile contents

Dissolution and precipitation of forsterite in a thermal gradient: implications for cellular growth of olivine phenocrysts in basalt and melt inclusion formation

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