Search results for "Mineral redox buffer"
showing 10 items of 11 documents
Crystallization of late-stage MORB under varying water activities and redox conditions: Implications for the formation of highly evolved lavas and ox…
2018
Abstract In order to understand late magmatic processes that occur in the deep oceanic crust, we performed a phase-equilibria study in a representative late-stage system at a pressure of 200 MPa with a special focus on the role of water and oxygen fugacity. The starting composition for the experiments was evaluated based on a statistical approach using evolved fresh MORB glasses from the database PETDB highest in FeO and TiO2 (in average 17.92 wt% and 3.73 wt%, respectively), assumed to represent frozen liquids erupted at the seafloor generated by extensive differentiation of MORB. We conducted crystallization experiments in a range of temperatures from 850 to 1050 °C with water activities …
The compositional variability of eudialyte-group minerals
2011
AbstractEudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mössbauer spectroscopy and LA-ICP-MS study and EGM which crystallized in various fractionation stages from different parental melts and mineral assemblages in silica over- and undersaturated systems are compared. Compositional variability is closely related to texture, allowing for reconstruction of locally acting magmatic to hydrothermal processes. Early-magmatic EGM are invariably dominated by Fe whereas hydrothermal EGM can be virtually Fe-free and form pure Mn end-members. Hence the Mn/Fe ratio is the most suitable fractionation …
Experimental calibration and implications of olivine-melt vanadium oxybarometry for hydrous basaltic arc magmas
2018
The strong dependence of vanadium partitioning between olivine and silicate melt (DVOl-M) on redox conditions (fO2) can be used as sensitive oxybarometer in magmatic systems. Here we extend the experimental database on DVOl-M, obtained so far at high temperatures (mainly above 1250 °C), to lower temperatures that are typical for island-arc basalts. Crystallization experiments were performed using a composition from Mutnovsky volcano (Kamchatka), and the investigated temperature, pressure, and oxygen fugacity ranges were 1025–1150 °C, 0.1 and 0.3 GPa, and ΔQFM of –0.5 to +3.2, respectively. The water content in melts ranged from 0.6 to ∼6.5 wt% H2O. The data demonstrate a strong negative cor…
Magmatic signature in acid rain at Masaya volcano, Nicaragua: Inferences on element volatility during lava lake degassing
2021
Abstract Major, minor and trace element concentrations of single rainfall events were investigated at Masaya volcano (Nicaragua) in order to determine the relative contributions of volcanogenic elements. Most of the samples were collected in the summit area of the volcano around the Santiago crater, and two samples, representative of the local background, were collected at a 4.3 km upwind site. Samples from the summit are very acidic with pH down to 2.14 and contain large amounts of volcanogenic elements that can be clearly distinguished from the local background. These elements are released into the atmosphere from the continuously degassing lava lake of the Santiago crater, Masaya volcano…
First determination of magma-derived gas emissions from Bromo volcano, eastern Java (Indonesia)
2015
The composition and fluxes of volcanic gases released by persistent open-vent degassing at Bromo Volcano, east Java (Indonesia), were characterised in September 2014 from both in-situ Multi-GAS analysis and remote spectroscopic (dual UV camera) measurements of volcanic plume emissions. Our results demonstrate that Bromo volcanic gas is water-rich (H2O/SO2 ratios of 56-160) and has CO2/SO2 (4.1 +/- 0.7) and CO2/S-tot (3.2 +/- 0.7) ratios within the compositional range of other high-temperature magma-derived gases in Indonesia. H-2/H2O and H2S/SO2 ratios constrain a magmatic gas source with minimal temperature of 700 degrees C and oxygen fugacity of 10(-17)-10(-18) bars. UV camera sensing on …
Melt inclusions track melt evolution and degassing of Etnean magmas in the last 15 ka
2019
We present major elements compositions and volatiles contents of olivine-hosted melt inclusions from Etna volcano (Italy), which extend the existing database with the aim of interpreting the chemical variability of Etnean magmas over the last 15 ka. Olivine phenocrysts were selected from the most primitive Fall Stratified (FS) eruptive products of picritic composition (Mg# = 67–70, Fo 89–91 ), the Mt. Spagnolo eccentric lavas (Mg# = 52–64, Fo 82–88 ) and among the more recent 2002–2013 eruptive products (Mg# = 33–53, Fo 68–83 ). Crystal fractionation and degassing processes were modeled at temperatures of 1050–1300 °C, pressures <500 MPa, and oxygen fugacity between 1 and 2 log units abo…
The oxidation state of iron in silicic melt at 500 MPa water pressure
2002
Abstract The dependence of the ferric–ferrous ratio in silicate melts on oxygen fugacity was studied in the system SiO2(Qz)–NaAlSi3O8(Ab)–CaAl2Si2O8(An)–H2O using Mossbauer spectroscopy. Experiments were performed under water-saturated conditions at 500 MPa, and at temperatures of 850 and 950 °C, covering a range typical for magmatic processes. The oxygen fugacity was varied in the fO2 range from Cu–Cu2O buffer to slightly more reducing conditions than the wustite–magnetite buffer. The iron redox ratio was determined by analyzing the Mossbauer parameter distribution that was modeled based on experimental spectra collected at room temperature on the quenched samples. The obtained iron redox …
A Reappraisal of Redox Melting in the Earth’s Mantle as a Function of Tectonic Setting and Time
2010
Redox melting refers to any process by which melt is generated by the contact of a rock with a fluid or melt with a contrasting oxidation state. It was originally applied to melting owing to the oxidation of reduced CH4and H2-bearing fluids in contact with more oxidized blocks in the mantle, particularly recycled crustal blocks.This oxidation mechanism causes an increase in the activity of H2O by the reaction of CH4 with O2, and the increased aH2O causes a rapid drop in the solidus temperature, and is here termed hydrous redox melting (HRM). Recently, a second redox melting mechanism (carbonate redox melting; CRM) has been discovered that operates in more oxidized conditions, and may post-d…
Genesis and evolution of the fumaroles of vulcano (Aeolian Islands, Italy): a geochemical model
1981
A geochemical model explaining the presence of fumaroles having different gas composition and temperature at the top of the crater and along the northeastern coast of Vulcano island is proposed. A pressurized biphase (liquid-vapor) reservoir at the depth of about 2 km is hypothesized. Energy and mass balance sheets controlP-T conditions in the system.P-T must vary along a boiling curve of brine as liquid is present. The CO2 content in the steam is governed by the thermodynamic properties of the fluids in the H2-NaCl-CO2 system. On the assumption that oxygen fugacity in the system is between the HM-FMQ oxygen buffers, observed SO2/H2S, CO2/CO, CO/CH4 ratios in the fumarolic gases at the Foss…
Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient
2021
AbstractThe occurrence of phlogopite and amphibole in mantle ultramafic rocks is widely accepted as the modal effect of metasomatism in the upper mantle. However, their simultaneous formation during metasomatic events and the related sub-solidus equilibrium with the peridotite has not been extensively studied. In this work, we discuss the geochemical conditions at which the pargasite-phlogopite assemblage becomes stable, through the investigation of two mantle xenoliths from Mount Leura (Victoria State, Australia) that bear phlogopite and the phlogopite + amphibole (pargasite) pair disseminated in a harzburgite matrix. Combining a mineralogical study and thermodynamic modelling, we predict …