Detailed studies of melt inclusion and matrix glass suggest multi-stage melting beneath the South Atlantic Ridge axis. Primary glass, with an approximate composition of MgO=12,5wt % Ti02 = 0.4 wt % and Mg# = 75, may have formed by a first stage of melting, and Mg-quartz tholeiite be a second stage of melting. Most of the matrix glass from basalts, erupted along the ridge, lies along the evolved part of the liquid line of descent of olivine and plagioclase fractionation, which suggests presence of a magma chamber beneath the South Atlantic Ridge. More primitive liquid were recorded in melt inclusions trapped in xenocrysts of plagioclase in almost every dredge hole along the Ridge. Independent of the offset size, some fracture zone, such as Ascension F.Z., 9.75°S F.Z. and Bode Verde F.Z. in the South Atlantic correspond to a boundary for chemical elements. Therefore, it seems that a magma chamber are a cornman feature beneath the South Atlantic Ridge.
The Mount St. Helens, May 18 pumice is a dacite containing 60% glass by weight and phenocrysts of plagioclase, orthopyroxene, amphibole, titaniferous magnetite, and ilmenite. The glass is uniform in composition, a rhyodacite with 73 wt % SiO 2 ; the phenocrysts are also uniform in composition except for the plagioclase, which has cores averaging An 57 and rims averaging An 49 . Analyses of seven pairs of coexisting Fe‐Ti oxides in a representative sample of the light pumice were recast using various mineral calculation procedures; they yielded temperatures ranging from 920° to 940°C and a ‐log ƒ O 2 of 10.3–10.1. Electron microprobe analyses of 57 glass inclusions trapped in plagioclase phenocrysts in the light pumice showed little deviation from an average rhyodacitic composition (69.90±0.87 wt % SiO 2 ) when special care was taken to account for Na loss during the analysis. The difference between the average total of these glass inclusion analyses and 100% is 4.6±1 wt %, which is interpreted to be volatiles dissolved in the glass. On an anhydrous basis the average glass inclusion composition is identical to the matrix glass, indicating that neither underwent significant fractionation after melt was trapped by the plagioclase. Experimentally determined phase relations for the representative dacite sample place limits on conditions in the May 18 Mount St. Helens magma chamber, assuming that the dissolved volatiles were 4.6±1 wt % and the temperature was 920°–940°C. Hydrothermal experiments over a range of P, T, and ƒ O 2 indicate that at no pressure is the observed phase assemblage and residual melt chemistry produced when P H 2 O = P Total . Experiments using CO 2 ‐H 2 O fluids to achieve P H 2 's less than P Fluid did reproduce the observed residual melt chemistry and an An 50 plagioclase at a specific set of conditions, i.e., at ƒ O 2 's between the NNO and MNO buffers, at a P Fluid of 220 MPa (2.2 kb), and at a P H 2 = 110 MPa (all at 920°–940°C). Amphibole was not stable under these conditions but possibly would be if the P H 2 / P Fluid ratio was raised to 0.7 or if fluorine were added to the experimental system. It is concluded that just prior to eruption, the upper part of the Mount St. Helens magma chamber was at a pressure of 220±30 MPa corresponding to a depth of 7.2±1 km, P H 2 was 0.5 to 0.7 P Total , and the temperature was 930°±10°C.
Major volcanic eruptions disperse large quantities of sulfur compound throughout the Earth's atmosphere. The sulfuric acid aerosols resulting from such eruptions are scavenged by snow within the polar regions and appear in polar ice cores as elevated acidity layers. Glacio-chemical studies of ice cores can, thus, provide a record of past volcanism, as well as the means for understanding the fate of volcanic sulfur in the atmosphere. The primary objectives of this project are to study the chemistry and physical properties of volcanic fallout in a Greenland Ice Core in order to evaluate the impact of the volcanic gases on the atmospheric chemistry and the total atmospheric mass of volcanic aerosols emitted by major volcanic eruptions. The authors propose to compare the ice core record to other atmospheric records performed during the last 10 years to investigate transport and deposition of volcanic materials.
Global climatic effects brought about by volcanism are related to the impact of volcanic gases and their derivative aerosols on the atmosphere, rather than the effects of volcanic ash. Evidence from both historic eruptions and polar ice cores indicate that volcanic sulfur gases are the dominant aerosol-forming component, resulting in produciton of a sulfuric acid-rich stratosphere aerosol that can have profound effects on the earth radiation budget over periods of a few years. Due to highly variable sulfur content of different magma types, the climatic effects do not relate simply to total erupted mass. There is a close relationship between volcanic sulfur yield to the atmospheric and hemispheric surface temperature decrease following an eruption, with up to 1 C surface temperature decrease indicated following a major volcanic event such as the 1815 Tambora eruption. While the erupted mass of HCl and HF is equal to or greater than that of sulfur gases in some volcanic events, the halogens do not form known aerosols nor are they abundant in ice core acidity layers. The early removal of halogens from eruption columns occurs by rain flushing and adsorption onto tephra particles, but the fate of halogens in the atmosphere following very large explosive eruptions is unknown. The CO2 flux to the atmosphere from volcanic eruptions is volumetrically one of the most important of the gas species, but owing to the huge size of the atmospheric reservoir of this gas, the volcanic contribution is likely to have negligible effects.
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