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

Transcrystalline melt migration in clinopyroxene

International audienc

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

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

Magma ascent rate and initial water concentration inferred from diffusive water loss from olivine-hosted melt inclusions

International audienceAs the water concentration in magma decreases during magma ascent, olivine-hosted melt inclusions will reequilibrate with the host magma through hydrogen diffusion in olivine. Previous models showed that for a single spherical melt inclusion in the center of a spherical olivine, the rate of diffusive reequilibration depends on the partition coefficient and diffusivity of hydrogen in olivine, the radius of the melt inclusion, and the radius of the olivine. This process occurs within a few hours and must be considered when interpreting water concentration in olivine-hosted melt inclusions. A correlation is expected between water concentration and melt inclusion radius, because small melt inclusions are more rapidly reequilibrated than large ones when the other conditions are the same. This study investigates the effect of diffusive water loss in natural samples by exploring such a correlation between water concentration and melt inclusion radius, and shows that the correlation can be used to infer the initial water concentration and magma ascent rate. Raman and Fourier transform infrared spectroscopy measurements show that 31 melt inclusions (3.6-63.9 μm in radius) in six olivines from la Sommata, Vulcano Island, Aeolian Islands, have 0.93-5.28 wt% water, and the host glass has 0.17 wt% water. The water concentration in the melt inclusions shows larger variation than the data in previous studies (1.8-4.52 wt%). It correlates positively with the melt inclusion radius, but does not correlate with the major element concentrations in the melt inclusions, which is consistent with the hypothesis that the water concentration has been affected by diffusive water loss. In a simplified hypothetical scenario of magma ascent, the initial water concentration and magma ascent rate are inferred by numerical modeling of the diffusive water loss process. The melt inclusions in each olivine are assumed to have the same initial water concentration and magma ascent rate. The melt inclusions are assumed to be quenched after eruption (i.e., the diffusive water loss after eruption is not considered). The model results show that the melt inclusions initially had 3.9-5.9 wt% water and ascended at 0.002-0.021 MPa/s before eruption. The overall range of ascent rate is close to the lower limit of previous estimates on the ascent rate of basalts

Transcrystalline melt migration driven by natural thermal gradients.

International audienc

The rate of water loss from olivine-hosted melt inclusions

International audienc

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

International audienc

Kinetics of heterogeneous bubble nucleation in rhyolitic melts: implications for the number density of bubbles in volcanic conduits and for pumice textures

International audienceWe performed decompression experiments to simulate the ascent of a phenocryst-bearing rhyolitic magma in a volcanic conduit. The starting materials were bubble-free rhyolites water-saturated at 200 MPa–800°C under oxidizing conditions: they contained 6.0 wt% dissolved H2O and a dense population of hematite crystals (8.7 ± 2 9 105 mm-3). Pressure was decreased from the saturation value to a final value ranging from 99 to 20 MPa, at constant temperature (800C); the rate of decompression was either 1,000 or 27.8 kPa/s. In all experiments, we observed a single event of heterogeneous bubble nucleation beginning at a pressure PN equal to 63 ± 3 MPa in the 1,000 kPa/s series, and to 69 ± 1 MPa in the 27.8 kPa/s series. Below PN, the degree of water supersaturation in the liquid rapidly decreased to a few 0.1 wt%, the nucleation rate dropped, and the bubble number density (BND) stabilized to a value strongly sensitive to decompression rate: 80 mm-3 at 27.8 kPa/s vs. 5,900 mm-3 at 1,000 kPa/s. This behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite-H2O system and in numerical simulations of vesiculation in ascending magmas. Similar degrees of water supersaturation were measured at 27.8 and 1,000 kPa/s, implying that a faster decompression rate does not result in a larger departure from equilibrium. Our experimental results imply that BNDs in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate jdP=dtj and the number density of phenocrysts, especially the number density of magnetite microphenocrysts (1–100 mm-3), which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation. Very low BNDs (&1 mm-3) are predicted in the case of effusive eruptions (jdP=dtj & 0.1 kPa/s). High BNDs (up to 107 mm-3) and bimodal bubble size distributions are expected in the case of explosive eruptions: (1) a relatively small number density of bubbles (1–100 mm-3) will first nucleate in the lower part of the conduit (jdP=dtj & 10 kPa/s), either at high pressure on magnetite or at lower pressure on quartz and feldspar (or by homogeneous nucleation in the liquid) and (2) then, extreme decompression rates near the fragmentation level (jdP=dtj & 103 kPa/s) will trigger a major nucleation event leading to the multitude of small bubbles, typically a few micrometers to a few tens of micrometers in diameter, which characterizes most silicic pumices

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