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Abstract

The majority of the Mio-Pleistocene monogenetic volcanoes in Western Hungary had, at least in their initial eruptive phase, phreatomagmatic eruptions that produced pyroclastic deposits rich in volcanic glass shards. Electron microprobe studies on fresh samples of volcanic glass from the pyroclastic deposits revealed a primarily tephritic composition. A shape analysis of the volcanic glass shards indicated that the fine-ash fractions of the phreatomagmatic material fragmented in a brittle fashion. In general, the glass shards are blocky in shape, low in vesicularity, and have a low-to-moderate microlite content. The glass-shape analysis was supplemented by fractal dimension calculations of the glassy pyroclasts. The fractal dimensions of the glass shards range from 1.06802 to 1.50088, with an average value of 1.237072876, based on fractal dimension tests of 157 individual glass shards. The average and mean fractal-dimension values are similar to the theoretical Koch-flake (snowflake) value of 1.262, suggesting that the majority of the glass shards are bulky with complex boundaries. Light-microscopy and backscattered-electron-microscopy images confirm that the glass shards are typically bulky with fractured and complex particle outlines and low vesicularity; features that are observed in glass shards generated in either a laboratory setting or naturally through the interaction of hot melt and external water. Textural features identified in fine- and coarse-ash particles suggest that they were formed by brittle fragmentation both at the hot melt-water interface (forming active particles) as well as in the vicinity of the interaction interface. Brittle fragmentation may have occurred when hot melt rapidly penetrated abundant water-rich zones causing the melt to cool rapidly and rupture explosively.

Abstract

The Pannonian Basin (Central Europe) hosts numerous alkali basaltic volcanic fields in an area similar to 200 000 km2. These volcanic fields were formed in an approximate time span of 8 million years producing smallvolume volcanoes typically considered to be monogenetic. Polycyclic monogenetic volcanic complexes are also common in each field however. The original morphology of volcanic landforms, especially phreatomagmatic volcanoes, is commonly modified. by erosion, commonly aided by tectonic uplift. The phreatomagmatic volcanoes eroded to the level of their sub-surface architecture expose crater to conduit filling as well as diatreme facies of pyroclastic rock assemblages. Uncertainties due to the strong erosion influenced by tectonic uplifts, fast and broad climatic changes, vegetation cover variations, and rapidly changing fluvio-lacustrine events in the past 8 million years in the Pannonian Basin have created a need to reconstruct and visualise the paleoenvironment into which the monogenetic volcanoes erupted. Here phreatomagmatic volcanic fields of the Miocene to Pleistocene western Hungarian alkali basaltic province have been selected and compared with modern phreatomagmatic fields. It has been concluded that the Auckland Volcanic Field (AVF) in New Zealand could be viewed as a prime modern analogue for the western Hungarian phreatomagmatic fields by sharing similarities in their pyroclastic successions textures such as pyroclast morphology, type, juvenile particle ratio to accidental lithics. Beside the AVF two other, morphologically more modified volcanic fields (Pali Aike, Argentina and Jeju, Korea) show similar features to the western Hungarian examples, highlighting issues such as preservation potential of pyroclastic successions of phreatomagmatic volcanoes.

basalt), sideromelane (glassy basalt), and sideromelane fragments by the quenching and fragmentation of lava (Figs. 11a–11c). The interparticle pores are filled with yellow smectite clays. The sequence of glass alteration is similar to that in JJ1B: interparticle smectites 1 and 2, hemispherical oscillatory zoning of zone I, intraparticle pores in zone II, and a lack of zones III and IV (Fig. 7d). Zone I shows a successive concentric growth of hemispheres, convex toward the glass (Figs. 11e–11f). The hemispheres consist of dark and bright bands in BSE images

FORMATION IN LOW-SILICA TEPHRA DEPOSITS Alkaline, low-silica tuffs only 10-20 m thick may be zoned zeolitically to the same extent as silicic tuffs 500-1,000 m thick. Zonation at shallow depth reflects the highly reactive nature of alkalic, low-silica glass. The formation of zeolites from mafic glasses in open hydrologie systems is somewhat more complex than it is from silicic glasses. Mafic glass (or sideromelane) reacts to form palagonite and zeolites, in an appropriate chemical environment which is generally alkaline. Pala- gonite can be viewed as a hydrous, iron

. Mafic glass (or sideromelane) reacts to form 268 Sheppard & Hay palagonite and zeolites in appropriate alkaline chemical environments. Palagonite can be viewed as a hydrous, iron-rich gel fonned from mafic glass. It contains somewhat less Al, Si, Ca, Na, and K than the original glass. These components provide the raw materials for the crystallization of zeolites in pore spaces. Phillipsite and chabazite are probably the most common zeolites in low-silica tuffs, but natrolite, gomiardite, analcime and other low-silica zeolites are commonly recognized. In Hawaii

alterations occur, they can be distinguished from biotic alterations due to differences in banding and appearance. For example, altered basaltic glass from Sites 504B and 896A were described as being abiotically altered from sideromelane to palagonite when zoned alteration bands of regular thickness were seen, compared to the bulbous protusions observed during biotic alteration [114]. Additionally, nucleic acid stains were colocalized with alteration zones at Site 896A, showing the presence of DNA and also the observation of bacteria and a few archaea at alteration fronts

alterations occur, they can be distinguished from biotic alterations due to differences in banding and appearance. For example, altered basaltic glass from Sites 504B and 896A were described as being abiotically altered from sideromelane to palagonite when zoned alteration bands of regular thickness were seen, compared to the bulbous protusions observed during biotic alteration [114]. Additionally, nucleic acid stains were colocalized with alteration zones at Site 896A, showing the presence of DNA and also the observation of bacteria and a few archaea at alteration fronts

asphaltenes and model compounds. J Am Chem Soc 111:3182-3186 Gurenko AA, Schmincke HU (1998) Petrology, geochemistry, S, CI and F abundances, and S oxidation state of sideromelane glass shards from Pleistocene ash layers north and south of Gran Canaria (ODP Leg 157). Contrib Mineral Petrol 131:95-110 Gurenko AA, Schmincke HU (2000) S concentrations and its speciation in Miocene basaltic magmas north and south of Gran Canaria (Canary Islands): Constraints from glass inclusions in olivine and clinopyroxene. Geochim Cosmochim Acta 64:2321-2337 Hansen MR, Brorson M