Clustering and interfacial segregation of radiogenic Pb in a mineral host – inclusion system Tracing two-stage Pb and trace element mobility in monazite inclusions in rutile

Accessory minerals like zircon, rutile and monazite are routinely studied to inform about the timing and nature of geological processes. These studies are underpinned by our understanding of the transfer processes of trace elements and the assumption that the isotopic systems remain undisturbed. However, the presence of microstructures or Pb-bearing phases in minerals can lead to the alteration of the Pb isotopic composition. To gain insight into the relationship between Pb isotopic alterations from inclusions and microstructures, this study focused on inclusions from an ultra-high temperature metamorphic rutile.The studied inclusions are submicron monazites, a common mineral rich in Pb but normally not present in rutile. The sample is sourced from Mt. Hardy, Napier Complex, East Antarctica, an ultra-high temperature (UHT) metamorphic terrane. By applying correlative analytical techniques including electron backscatter diffraction mapping, transmission electron microscopy (TEM), and atom probe tomography, it is shown monazite inclusions are often in contact with low-angle boundaries and yield no preferred orientation. TEM shows the monazite core has a mottled texture due to the presence of radiation damage and nanoclusters associated with the radiation damage defects that are rich in U, Pb, and Ca. Some monazites exhibit a core-rim structure. The rim yields clusters composed of Ca- and Li-phosphate that enclose Pb nanoclusters that are only present in small amounts compared to the core, with Pb likely diffused into the rutile-monazite interface. These textures are the result of two-stages of Pb mobility. Initial Pb segregation was driven by volume diffusion during UHT metamorphism (2500 Ma). The second stage is a stress-induced recrystallization during exhumation, leading to recrystallization of the monazite rim and trace element transport. The isotopic signature of Pb trapped within the rutile-monazite interface constrains the timing of Pb mobility to c. 550 Ma

Volcanic SiO2-cristobalite: A natural product of chemical vapor deposition

© 2020 Walter de Gruyter. Cristobalite is a low-pressure, high-temperature SiO2 polymorph that occurs as a metastable phase in many geologic settings, including as crystals deposited from vapor within the pores of volcanic rocks. Such vapor-phase cristobalite (VPC) has been inferred to result from silica redistribution by acidic volcanic gases but a precise mechanism for its formation has not been established. We address this by investigating the composition and structure of VPC deposited on plagioclase substrates within a rhyolite lava flow, at the micrometer to nanometer scale. The VPC contains impurities of the form [AlO4/Na+]0 - coupled substitution of Al3+ charge-balanced by interstitial Na+ - which are typical of cristobalite. However, new electron probe microanalysis (EPMA) element maps show individual crystals to have impurity concentrations that systematically decline from crystal cores-to-rims, and atom probe tomography reveals localized segregation of impurities to dislocations. Impurity concentrations are inversely correlated with degrees of crystallinity [observed by electron backscatter diffraction (EBSD), hyperspectral cathodoluminescence, laser Raman, and transmission electron microscopy (TEM)], such that crystal cores are poorly crystalline and rims are highly ordered tetragonal α-cristobalite. The VPC-plagioclase interfaces show evidence that dissolution-reprecipitation reactions between acidic gases and plagioclase crystals yield precursory amorphous SiO2 coatings that are suitable substrates for initial deposition of impure cristobalite. Successive layers of cubic β-cristobalite are deposited with impurity concentrations that decline as Al-bearing gases rapidly become unstable in the vapor cooling within pores. Final cooling to ambient temperature causes a displacive transformation from β→α cristobalite, but with locally expanded unit cells where impurities are abundant. We interpret this mechanism of VPC deposition to be a natural proxy for dopant-modulated Chemical Vapor Deposition, where halogen-rich acidic gases uptake silica, react with plagioclase surfaces to form suitable substrates and then deposit SiO2 as impure cristobalite. Our results have implications for volcanic hazards, as it has been established that the toxicity of crystalline silica is positively correlated with its purity. Furthermore, we note that VPC commonly goes unreported, but has been observed in silicic lavas of virtually all compositions and eruptive settings. We therefore suggest that despite being metastable at Earth's surface, cristobalite may be the most widely occurring SiO2 polymorph in extrusive volcanic rocks and a useful indicator of gas-solid reaction having occurred in cooling magma bodies

Mechanical twinning of monazite expels radiogenic lead

International audienceMechanical twins form by the simple shear of the crystal lattice during deformation. In order to test the potential of narrow twins in monazite to record the timing of their formation, we investigated a ca. 1700 Ma monazite grain (from the Sandmata Complex, Rajasthan, India) deformed at ca. 980 Ma, by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT). APT 208 Pb/ 232 Th ages indicate that the twin was entirely reset by radiogenic Pb loss during its formation at conditions far below the monazite closure temperature. The results are consistent with a model where Pb is liberated during rupture of rare earth element-oxygen (REE-O) bonds in the large [REE]O 9 polyhedra during twinning. Liberated Pb likely migrated along fast diffusion pathways such as crystal defects. The combination of a quantitative microstructural investigation and nanogeochronology provides a new approach for understanding the history of accessory phases

Life on the edge: Microbial biomineralization in an arsenic- and lead-rich deep-sea hydrothermal vent

© 2019 Unravelling complex microbial activity in modern hydrothermal vents can provide crucial insights into the evolution of ancient life on Earth. It is well established that microorganisms in hydrothermal vents have a significant impact on the cycling of metals and mineral formation. However, the detailed roles played by microorganisms in driving sulfide deposition and cycling of toxic metals, like arsenic (As) and lead (Pb), in high-temperature deep-sea hydrothermal vents remain unknown. The understanding of these mechanisms in extreme environments is of particular importance as As has been postulated as a driver of microbial activities on the early Earth. Here, we present the first geologic evidence of Pb[sbnd]As rich microbial filamentous clusters observed in a modern high-temperature black smoker from the Manus back-arc basin, Papua New Guinea. The clusters occur as net-like structures on the surface of barite and sulfides and are composed of multiple filaments and fine-grained Pb[sbnd]As sulfosalt. Each of the filaments includes an As-Pb-rich sulfosalt core and organic-rich shell structure with elevated carbon, nitrogen and phosphorus. Further synchrotron X-ray absorption near edge structure analysis shows that the clusters contain a mixture of As (II) and As (III). Additionally, those filaments show a close association with realgar (As4S4), by penetrating and dissolving this As-sulfide mineral. We interpret the filaments to be a result of As-related microbial activity in As- and Pb-enriched hydrothermal environments. The findings show possible processes through which extremophiles live in Pb and As-rich environments during chimney growth. In addition, as hydrothermal vents are regarded as modern analogs of ancient volcanogenic massive sulfide deposits, the observed biominerals present the potential to be used as proxies to trace the signatures of early life in ancient geological systems

Le suivi de l'état de santé des récifs coralliens de Polynésie française et leur récente évolution

La Polynésie française, 118 îles au coeur du Pacifi que, possède une surface de plus de 15 000 km2 de récifs et lagons gérés par le gouvernement polynésien. Le tourisme et la perliculture représentent les deux ressources économiques majeures du Pays. Les formations récifales très diversifi ées sont parmi les mieux connues. Plusieurs suivis d'exploitation des ressources sont opérationnels depuis des décennies : granulats coralliens, pêche pour l'alimentation, collecte et exportation de mollusques nacriers, production de perles, poissons d'ornement. À l'échelle du Pays de très nombreux programmes de surveillance de l'état des récifs et des perturbations qu'ils subissent, naturelles et anthropiques, ont été mis en place : perturbations cycloniques et sismiques, qualité des eaux, état de santé des peuplements benthiques et ichtyologiques, pathologie des nacres, radiobiologie. Toutes ces données recueillies au fi l des décennies ont permis d'établir l'importance relative des dégradations naturelles et anthropiques sur les récifs et lagons polynésiens et d'expliquer leur état de santé actuel en considérant différentes échelles spatiales. Les périodes cycloniques dévastatrices pour les récifs sont rares (1903-1906, 1982-1983 et épisodiquement) mais les cyclones ont parfois anéanti les communautés coralliennes de pentes externes dans certaines îles. Les blanchissements suivis de mortalités importantes à des échelles spatiales diverses, ont été surtout ceux de 1991, 1994 et 2003. Les explosions démographiques d'Acanthaster ont détruit de nombreux récifs (lagons et pentes externes) en 1978-1982 et une nouvelle pullulation s'amplifi e depuis 2006 dans plusieurs îles de la Société. Les crises dystrophiques n'ont perturbé qu'épisodiquement certains lagons. Si les événements naturels précédents dégradent les récifs à l'échelle de plusieurs îles, d'archipel ou du Pays, les dégradations anthropiques sont limitées à quelques îles peuplées de la Société, plus exceptionnellement dans les atolls et encore moins dans un tiers d'entre eux qui sont inhabités. Les remblais en zone frangeante, les extractions de matériaux coralliens, la surpêche, l'absence de réseaux d'assainissement des eaux usées urbaines et le développement d'activités de loisir et du tourisme sont les causes essentielles de la dégradation des communautés coralliennes du lagon dans certains secteurs de Tahiti et de Moorea. Ainsi apparaît-il clairement que les dégradations majeures des récifs en Polynésie sont occasionnées par des phénomènes naturels compte tenu de leur étendue géographique. En revanche les dégradations anthropiques sont géographiquement plus localisées. Malheureusement la synergie des deux causes de dégradation ne facilite pas la récupération des récifs. Il est établi qu'une pente externe avec un recouvrement corallien de 50-60 % est à son optimum. Une dégradation majeure (cyclone, blanchissement, Acanthaster) réduit ce recouvrement à moins de 10 %. La communauté met une douzaine d'années pour revenir au recouvrement optimum si aucune autre perturbation importante ne survient. La très large majorité des 15 000 km2 de récifs et lagons de Polynésie française sont en bonne santé. Avec leurs voisins du Pacifi que Est et Central, ces formations coralliennes sont considérées comme les moins dégradées au monde et à faible risque de dégradation dans les prochaines décennies. Toutefois les inquiétudes sont grandissantes sur l'avenir des récifs dans le monde entier si l'on se réfère aux prédictions de changement climatique où les impacts majeurs tiendraient à l'augmentation des températures océaniques, à un renforcement des cyclones et à l'acidifi cation des eaux perturbant le métabolisme de calcifi cation des coraux