Search results for "Silicic"

The role of melt composition on aqueous fluid vs. silicate melt partitioning of bromine in magmas

2018

International audience; Volcanogenic halogens, in particular bromine, potentially play an important role in the ozone depletion of the atmosphere. Understanding bromine behaviour in magmas is therefore crucial to properly evaluate the contribution of volcanic eruptions to atmospheric chemistry and their environmental impact. To date, bromine partitioning between silicate melts and the gas phase is very poorly constrained, with the only relevant experimental studies limited to investigation of synthetic melt with silicic compositions. In this study, fluid/melt partitioning experiments were performed using natural silicate glasses with mafic, intermediate and silicic compositions. For each co…

Middle Triassic conodonts from northeastern Spain: biostratigraphic implications

2000

The facies development of the Spanish Triassic corresponds to the typical three-fold subdivision of the Germanic Facies: Buntsandstein, Muschelkalk and Keuper. Two intervals interpreted as epeiric carbonate platforms: lower Muschelkalk (Anisian) and upper Muschelkalk (Ladinian) are recognized during the Middle Triassic of northeastern Spain. These carbonate intervals are separated by one siliciclastic/evaporitic interval interpreted as sabbkha and saline deposits: middle Muschelkalk facies (Lower Ladinian). In northeastern Spain (Catalonian Coastal Ranges), two Middle Triassic sections comprising lower Muschelkalk facies have yielded the following conodont taxa: Paragondolella bulgarica, P.…

Poly(silicate)-metabolizing silicatein in siliceous spicules and silicasomes of demosponges comprises dual enzymatic activities (silica polymerase an…

2007

Siliceous sponges can synthesize poly(silicate) for their spicules enzymatically using silicatein. We found that silicatein exists in silica-filled cell organelles (silicasomes) that transport the enzyme to the spicules. We show for the first time that recombinant silicatein acts as a silica polymerase and also as a silica esterase. The enzymatic polymerization/polycondensation of silicic acid follows a distinct course. In addition, we show that silicatein cleaves the ester-like bond in bis(p-aminophenoxy)-dimethylsilane. Enzymatic parameters for silica esterase activity are given. The reaction is completely blocked by sodium hexafluorosilicate and E-64. We consider that the dual function o…

Cover Picture: Pleiotropic Role of Recombinant Silaffin-Like Cationic Polypeptide P5S3: Peptide-Induced Silicic Acid Stabilization, Silica Formation …

2016

Pleiotropic Role of Recombinant Silaffin-Like Cationic Polypeptide P5S3: Peptide-Induced Silicic Acid Stabilization, Silica Formation and Inhibition …

2016

Silica-mineralizing organisms such as diatoms manage several aspects of silica chemistry when polymerizing monomeric silicic acid into amorphous silica. Silicic acid is undersaturated in the diatoms’ habitats and mechanisms of enrichment and prevention of uncontrolled mineralization are not well understood. Diatom-biosilica is associated with organic compounds, including polycationic, post-translationally modified peptides termed silaffins, which induce the condensation of silicic acid under supersaturated conditions. Here, we report the pleiotropic action of the designed silaffin-like peptide P5S3, which (i) stabilizes 4–8x silicic acid (in supersaturated conditions of 8.3 mm), (ii) decele…

2016

The mechanisms of hazardous silicic eruptions are controlled by complex, poorly-understood conduit processes. Observations of recent Chilean rhyolite eruptions have revealed the importance of hybrid activity, involving simultaneous explosive and effusive emissions from a common vent. Such behaviour hinges upon the ability of gas to decouple from magma in the shallow conduit. Tuffisite veins are increasingly suspected to be a key facilitator of outgassing, as they repeatedly provide a transient permeable escape route for volcanic gases. Intersection of foam domains by tuffisite veins appears critical to efficient outgassing. However, knowledge is currently lacking into textural heterogeneiti…

Rhyolitic dykes of Paros Island, Cyclades

2005

Abstract The discovery of rhyolitic dykes from the NE part of the island of Paros is described here for the first time. The dykes that can be mapped for a length of ca. 1.1 km are striking about 6° and 38° NE. The width reaches up to 11m maximum. The rhyolitic rocks are porphyric with a fine-grained matrix of mainly feldspar, quartz, some biotites and opaques. K-feldspar, biotite and plagioclase occur as phenocrysts, and mafic-intermediate enclaves are common. At the chilled margin in contact to the gneissic country rocks the dykes are vitric. There, the enclaves have been preserved from contact reaction with the melt and alteration effects and show primary igneous minerals such as clinopyr…

Sm–Nd, Rb–Sr and Pb–Pb dating of silicic carbonates from the early Archaean Barberton Greenstone Belt, South Africa

1998

Abstract Two silicic carbonate sequences occurring in the Onverwacht Group (OG) and the overlying Fig Tree Group (FTG) of the early Archaean Barberton Greenstone Belt, South Africa, contain silicified dolomites with primary structures and textures. They occur with volcanic rocks dated by single zircons. The associated Onverwacht felsic volcanics have an age of ∼3440–3416 Ma, whereas the Fig Tree volcanics are ∼3225 Ma old. Therefore, the depositional age of both sequences is between 3440 and 3225 Ma. Pb-isotopic data for different parts of a single OG whole rock sample form a linear array corresponding to a Pb–Pb age of 2632±99 Ma. Sm–Nd and Rb–Sr isotopes were analysed in four whole rock s…

Exceptional mobility of an advancing rhyolitic obsidian flow at Cordón Caulle volcano in Chile

2013

The emplacement mechanisms of rhyolitic lava flows are enigmatic and, despite high lava viscosities and low inferred effusion rates, can result in remarkably, laterally extensive (30 km) flow fields. Here we present the first observations of an active, extensive rhyolitic lava flow field from the 2011-2012 eruption at Cordón Caulle, Chile. We combine high-resolution four-dimensional flow front models, created using automated photo reconstruction techniques, with sequential satellite imagery. Late-stage evolution greatly extended the compound lava flow field, with localized extrusion from stalled, ~35 m-thick flow margins creating80 breakout lobes. In January 2013, flow front advance continu…

The Devonian Stratigraphic Succession and Evolution of the Baltic Sedimentary Basin

2014

Facies analysis, biostratigraphic and taphonomic studies, a re-evaluation of signatures of worldwide events, and a new mathematical model of the Baltic sedimentary basin (BSB) have together enabled a better understanding to be gained of the development of the Devonian basins in the Baltic area. We have established four stages of basin evolution: (a) remnant basin stage (Lochkovian); (b) shallow epeiric basin stage, with mainly siliciclastic sedimentation (Pragian–early Frasnian); (c) shallow epeiric basin stage, with mainly carbonate sedimentation (Frasnian); and (d) infilling stage of the progressively narrowing shallow epeiric basin, with mixed sedimentation (Famennian).