Volcanic glass textures, shape characteristics and compositions from phreatomagmatic rock units of the western Hungarian monogenetic volcanic fields and their implication to magma fragmentation

Mio-Pliocene (~8 – 2.3 My) monogenetic volcanic fields in western Hungary (Bakony-Balaton Highland and Little Hungarian Plain Volcanic Fields) consist of eroded maar, tuff ring and scoria cones. Erosion advanced in many cases, and today the crater and volcanic conduit filling pyroclastic assemblages are preserved. The majority of the volcanoes had at least in their initial eruptive phase phreatomagmatic eruptions that produced pyroclastic beds deposited mainly from base surges and subordinate pyrolcastic falls. These phreatomagmatic rock units are rich in well-preserved volcanic glass shards. Electron microprobe studies on fresh volcanic glass reviled that they are primarily tephritic in composition. Textural analysis of the shape parameters of the glass shards were carried out with an aim to determine the magma fragmentation style was responsible for their formation. The shape analysis indicated that the majority of the magma was fragmented in a brittle fashion. Not only the fine ash fraction but the coarse ash fraction of the phreatomagmatic pyroclastic rocks suggested brittle fragmentation style of the magma due to thermohydraulic magma and external water interaction triggered eruptions. The glass shards are primarily blocky in shape, low in vesicularity and have low to moderate microlite content. The glass shape analysis was supplemented by fractal dimensions calculation 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 and a mean value of 1.24521 based on fractal dimension test on 157 individual glass shards. The average and mean fractal dimension values are similar to the theoretical Koch-flake (snowflake) value of 1.262 suggesting complex boundaries but bulky shape of the majority of the glass shards inferred to be typical for pyrolasts formed by the brittle fragmentation of hot melt through explosive magma and water interaction. Light microscopy and backscattered electron microscopy images show well the bulky, fractured and complex particle outline of the individual glass shards. Abundant and complex micro-fractures, low vesicularity and the complex, moss-like particle boundary of the studied glass shards are characteristic features of both laboratory generated and natural glass shards as a result of hot melt and external water interaction. The similar textural features identified in fine and coarse ash particles, suggest that the particles were formed by processes that triggered brittle fragmentation of the melt in the hot melt and water interface (active particles) as well as in the vicinity of the interaction interface (non-interactive particles). Such scenario can be envisioned where hot melt rapidly penetrate abundant water-rich zones such as a) watersaturated soft-substrate, b) surface water body, or c) quickly recharging fracture-filled ground-water and the melt quickly cooled down to a temperature where it has been fragmented in brittle fashion and dispersed quickly from the explosion locus by the kinetic energy released in the magma – water interface. The variety of moss-like, blocky, bulky and heavily fractured complex glass particles all attest the phreatomagmatic fragmentation formed the pyroclastic deposits from where the studied volcanic glass particles were collected.