Search results for "chemistry [Arsenates]"
showing 10 items of 2372 documents
Resurgent uplift at large calderas and relationship to caldera-forming faults and the magma reservoir: New insights from the Neapolitan Yellow Tuff c…
2021
Abstract Resurgence uplift is the rising of the caldera floor, mainly due to pressure or volume changes in the magma reservoir. Identifying resurgence structures and understanding their relationship to the magmatic reservoir is challenging. We investigate the resurgence structures of the Neapolitan Yellow Tuff caldera (Italy) by integrating bathymetric data, high-resolution seismic profiles and Differential Synthetic-Aperture Radar Interferometry data. Our results show that the resurgent area is manifested as 1) a central dome constituted by two main blocks bounded by NNE-SSW trending faults, 2) an apical graben developed on top of the most uplifted block, 3) a peripheral zone including sev…
Geochemistry of gases and waters discharged by the mud volcanoes at Paternò, Mt. Etna (Italy)
1996
Approximately 20 km south of Mt. Etna craters, at the contact between volcanic and sedimentary formations, three mud volcanoes discharge CO2-rich gases and Na–Cl brines. The compositions of gas and liquid phases indicate that they are fed by a hydrothermal system for which temperatures of 100–150 °C were estimated by means of both gas and solute geothermometry. The hydrothermal system may be associated with CO2-rich groundwaters over a large area extending from the central part of Etna to the mud volcanoes. Numerous data on the He, CH4, CO2 composition of the gases of the three manifestations, sampled over the past 5 years, indicate clearly that variations are due to separation processes of…
Phase petrology reveals shallow magma storage prior to large explosive silicic eruptions at Hekla volcano, Iceland
2017
Abstract Understanding the conditions that culminate in explosive eruptions of silicic magma is of great importance for volcanic hazard assessment and crisis mitigation. However, geological records of active volcanoes typically show a wide range of eruptive behavior and magnitude, which can vary dramatically for individual eruptive centers. In order to evaluate possible future scenarios of eruption precursors, magmatic system variables for different eruption types need to be constrained. Here we use petrological experiments and microanalysis of crystals to clarify the P – T – x state under which rhyodacitic melts accumulated prior to the H3 eruption; the largest Holocene Plinian eruption of…
The percolation threshold and permeability evolution of ascending magmas
2017
Abstract The development of gas permeability in magmas is a complex phenomenon that directly influences the style of a volcanic eruption. The emergence of permeability is linked to the concept of percolation threshold, which is the point beyond which gas bubbles are connected in a continuous network that allows gas escape. Measurements of the percolation threshold, however, range from ∼30 to 78 vol%. No known combination of parameters can explain such a wide range of threshold values, which affects our understanding of the relationship between percolation and permeability. We present permeability calculations on bubble-bearing rhyolitic melts that underwent experimental decompression. Sampl…
Using 81Kr and Noble Gases to Characterize and Date Groundwater and Brines in the Baltic Artesian Basin on the One-Million-Year Timescale
2017
Analyses for $^{81}$Kr and noble gases on groundwater from the deepest aquifer system of the Baltic Artesian Basin (BAB) were performed to determine groundwater ages and uncover the flow dynamics of the system on a timescale of several hundred thousand years. We find that the system is controlled by mixing of three distinct water masses: Interglacial or recent meteoric water $(\delta^{18}\text{O} \approx -10.4\unicode{x2030})$ with a poorly evolved chemical and noble gas signature, glacial meltwater $(\delta^{18}\text{O} \leq -18\unicode{x2030})$ with elevated noble gas concentrations, and an old, high-salinity brine component $(\delta^{18}\text{O} \geq -4.5\unicode{x2030}, \geq 90 \text{g …
Stratigraphy of the Neoproterozoic Damara Sequence in northwest Namibia: Slope to basin sub-marine mass-transport deposits and olistolith fields
2016
Abstract The Neoproterozoic Damara Sequence (>1000 m thick) is composed of siliciclastic and carbonate rocks that crop out in the Damara Belt, Namibia. In Damaraland (including the Vrede, Bethanis, Austerlitz and Toekoms farms), these rocks were deformed and metamorphosed under greenschist facies (biotite zone) conditions during the Damara Orogeny. The stratigraphy and paleoenvironments of the Damara Sequence rocks are debated by the scientific community. We use field data, including detailed 1:25,000 geological mapping, elaboration of stratigraphic profiles and observation of preserved primary structures, textures and composition, to identify lithofacies and lithofacies associations, and t…
The analcime problem and its impact on the geochemistry of ultrapotassic rocks from Serbia
2004
AbstractTertiary ultrapotassic volcanic rocks from Serbia occasionally display low levels of K2O and K2O/ Na2O. In these rocks, analcime regularly appears as pseudomorphs after pre-existing leucite microphenocrysts. The process ofleucite transformation in Serbian ultrapotassic rocks is very thorough: fresh leucite survives only in ugandites from the Koritnik lava flows as well as in rare inclusions in Cpx. This paper focuses on the impact of ‘analcimization’ on the mineralogy and geochemistry ofthe Serbian ultrapotassic rocks, using the samples where leucite survived as a monitor for the process.Analcimization has had a great impact on the geochemistry of the rocks, but affects only a restr…
Metamorphic P–T paths for the Archean Caozhuang supracrustal sequence, eastern Hebei Province, North China Craton: Implications for a sagduction regi…
2020
Abstract Archean supracrustal rocks of amphibolite-facies occur as enclaves within granitoids gneiss domes and belts between domes, representing collision or sagduction regimes. In order to distinguish between tectonic regimes using metamorphic patterns, systematic data on metamorphic evolution and zircon age dating are presented for the Caozhuang supracrustal sequence of eastern Hebei Province, China, which occurs as enclaves in gneisses. A garnet biotite gneiss records a P–T path involving pre-peak isobaric heating to peak conditions at 780–800 °C and 10–11 kbar (medium-P/T type), followed by decompression to 5–6 kbar. Two Mg/Al-rich schists from the same locality show low P/T conditions …
Reply to comment by Ngako and Njonfang on “The Adamawa-Yade domain, a piece of Archaean crust in the Neoproterozoic Central African Orogenic belt (Ba…
2018
Abstract V. Ngako and E. Njonfang addressed in their comment an interesting and debated issue about the collision geodynamics of the Pan-African Central African Orogenic Belt (CAOB) in Cameroon. However, the reason for this comment is still not clear to us since it mainly deals with issues far away from the scope of our study, namely, the pre-collisional set up of the belt. The comment raises two main points – the reworking process during plate collision and the Pan-African strain pattern – not discussed in our paper for the simple reason that it was not our goal and because our data set does not even allow such discussion. Having said that, the comment gives us the opportunity to clarify o…
Three-dimensional kernel-based coda attenuation imaging of caldera structures controlling the 1982-84 Campi Flegrei unrest
2019
Abstract Coda-wave attenuation imaging has risen as a state-of-the-art technique to depict volcanic structures using their dispersion effects. The 1982–84 seismic and deformation unrest at Campi Flegrei caldera (Italy) is a unique example of non-eruptive volcanic activity in a structured caldera. Here, we propose the first application of 3D coda-attenuation kernels to image caldera structures at multiple frequencies during unrest. Using sensitivity kernels is necessary to assess the effective resolution of coda imaging in highly heterogeneous volcanoes. The technique relies on the solution of Paasschens' equations in the framework of radiative transfer theory. The results map coda attenuati…