Search results for "Silicic"
showing 10 items of 95 documents
A mechanism forming silicic segregations from basaltic magma discovered in igneous rocks of Western Sicily
1983
Summary. Mechanisms forming silicic segregations from basaltic magmas are considered of primary importance when dealing with magmatic problems. However, the processes which give rise to silicic segregations from basaltic magmas are so far obscure. Fortunately, the discovery of spheroidal felsic masses in some basic rocks of Western Sicily throws light on this subject. To clarify the relationships between felsic and basic fractions particular attention has been paid to the interactions which occurred at their contact. Textural evidence indicates that the accretion mechanism of the Sicilian felsic segregations tends to obliterate the silicate liquid immiscibility effects and suggests that the…
Crystal size distribution in Jurassic Ferrar flows and sills (Victoria Land, Antarctica): evidence for processes of cooling, nucleation, and crystall…
1996
Jurassic Ferrar rocks in Victoria Land (Antarctica) occur predominantly as basaltic or andesitic flows and sills. Both show characteristic petrographical and chemical variations, which can be related to in-situ differentiation processes. Such characteristics have been investigated at one flow (“Colonnade flow”) and one sill (“Thumb Point sill”) in the Prince Albert Mountains (Central Victoria Land) based on a statistical grain size analysis and the application of the crystal size distribution theory. A third magma body (“HiTi-unit”), which in previous literature was described as a flow, does not show clear similarities to either the flow or sill. Sill and flow are in-situ differentiated wit…
Fracture stratigraphy and oil first migration in Triassic shales, Favignana Island, western Sicily, Italy
2021
This study aims at evaluating the control exerted by fracture stratigraphy and diagenetic processes on oil first migration through an outcropping, mixed carbonate-siliciclastic succession. The present work included results of sedimentological, paleontological, mineralogical, petrographic, structural, and microstructural analyses carried out on organic-rich shales exposed at the Favignana Island of Sicily, Italy. The analyses focus on Upper Triassic yellowish siltstones and greyish laminated dolomitic limestones, which form a 10's of m-thick succession exposed along the westernmost portion of the Sicilian fold-and-thrust belt. The studied succession deposited in a coastal lagoon associated t…
Bulk carbonate isotope stratigraphy from CRP-3 core (Victoria Land Basin, Antarctica): evidence for Eocene–Oligocene palaeoclimatic evolution
2005
Bulk carbonate isotope compositions and carbonate petrography from upper Eocene and lower Oligocene siliciclastic sediments of the CRP-3 sediment core (Victoria Land Basin, Antarctica) have been investigated with the aim of contributing to reconstruction of the Antarctic Cenozoic palaeoclimate. Most of the carbonate is calcite cement occurring as patches, nodules and horizons and consisting of equant and/or drusy sparry calcite, pervasive blocky calcite and rare poikilotopic calcite spar. 18O-depleted values (from −17 to −8 δ‰) of the CRP-3 carbonates suggest that the precipitating fluids were a mixing between marine and meteoric waters from melting glaciers. The δ18O record exhibits a numb…
Understanding Degassing Pathways Along the 1886 Tarawera (New Zealand) Volcanic Fissure by Combining Soil and Lake CO2 Fluxes
2019
CO2 flux measurements are often used to monitor volcanic systems, understand the cause of volcanic unrest, and map sub-surface structures. Currently, such measurements are incomplete at Tarawera (New Zealand), which erupted with little warning in 1886 and produced a ∼17 km long fissure. We combine new soil CO2 flux and C isotope measurements of Tarawera with previous data from Rotomahana and Waimangu (regions also along the 1886 fissure) to fingerprint the CO2 source, understand the current pathways for degassing, quantify the CO2 released along the entire fissure, and provide a baseline survey. The total CO2 emissions from the fissure are 1227 t⋅d–1 (742–3398 t⋅d–1 90 % confidence interval…
Diagénesis, análisis de procedencia y ambiente tectónico de rocas siliciclásticas. Un caso de estudio en el Devónico Superior de la Cordillera Ibéric…
2022
This paper describes the petrography and infers the provenance of siliciclastic rocks from the Upper Devonian of the Iberian Chains (Tabuenca, NE Spain), and outlines the tectonic setting associated with the Ebro Massif. These Devonian deposits are constituted by four different siliciclastic units: the Rodanas, Bolloncillos, Hoya and Huechaseca Formations. The provenance and diagenesis of over 400 sedimentary rocks samples are studied with a combination of petrographic polarizing microscope, scanning electron microscopy, atomic absorption spectroscopy, X-ray fluorescence and X-ray diffraction. In this sense, AAS and XRF analysis were used to determine the content of Ca, Mg, Fe, Mn, Na, K an…
Arginine kinase in the demosponge Suberites domuncula:regulation of its expression and catalytic activity by silicic acid
2005
SUMMARY In Demospongiae (phylum Porifera) the formation of the siliceous skeleton,composed of spicules, is an energetically expensive reaction. The present study demonstrates that primmorphs from the demosponge Suberites domuncula express the gene for arginine kinase after exposure to exogenous silicic acid. The deduced sponge arginine kinase sequence displays the two characteristic domains of the ATP:guanido phosphotransferases; it can be grouped to the `usual' mono-domain 40 kDa guanidino kinases (arginine kinases). Phylogenetic studies indicate that the metazoan guanidino kinases evolved from this ancestral sponge enzyme; among them are also the `unusual'two-domain 80 kDa guanidino kinas…
The role of melt-fracture degassing in defusing explosive rhyolite eruptions at volcán Chaitén
2012
Explosive volcanic eruptions of silicic magma often evolve towards non-explosive emissions of lava. The mechanisms underlying this transition remain unclear, however, a widely cited idea holds that shear-induced magma fragmentation plays a critical role by fostering volatile loss from fragmentary magma and through ash-filled cracks termed tuffisite. We test this hypothesis by measuring H2O concentrations within glassy tuffisite from the 2008–2011 rhyolitic eruption at volcan Chaiten, Chile. We show that while H2O concentrations decrease next to tuffisite veins and at the margins of obsidian fragments, the depletions cannot account for the disparity in H2O between explosively and effusively …
Explosive origin of silicic lava: Textural andδD–H 2 O evidence for pyroclastic degassing during rhyolite effusion
2014
A long-standing challenge in volcanology is to explain why explosive eruptions of silicic magma give way to lava. A widely cited idea is that the explosive-to-effusive transition manifests a two-stage degassing history whereby lava is the product of non-explosive, open-system gas release following initial explosive, closed-system degassing. Direct observations of rhyolite eruptions indicate that effusive rhyolites are in fact highly explosive, as they erupt simultaneously with violent volcanic blasts and pyroclastic fountains for months from a common vent. This explosive and effusive overlap suggests that pyroclastic processes play a key role in rendering silicic magma sufficiently degassed…
Evolution of depositional settings in the Torrey area during the Smithian (Early Triassic, Utah, USA) and their significance for the biotic recovery
2015
This work focuses on well-exposed Lower Triassic sedimentary rocks in the area of Torrey (south-central Utah, USA). The studied Smithian deposits record a large-scale third-order sea-level cycle, which permits a detailed reconstruction of the evolution of depositional settings. During the middle Smithian, peritidal microbial limestones associated with a rather low-diversity benthic fauna were deposited seaward of the tidal flat siliciclastic red beds. Associated with siliceous sponges, microbial limestones formed small m-scale patch reefs. During the late middle to late Smithian interval, the sedimentary system is characterized by tidal flat dolostones of an interior platform, ooid-bioclast…