Factors controlling the formation of a very large maar volcano and the fragmentation process in phreatomagmatic eruptions: Lake Purrumbete Maar, southeastern Australia
2017-02-28T23:39:05Z (GMT) by
The Pleistocene Lake Purrumbete Maar is one of the largest maar volcanoes world wide, with a complex pyroclastic sequence that reflects variations in the eruption style, ranging from magmatic influenced dry phreatomagmatic to wet phreatomagmatic. The country rock xenoliths within the pyroclastic deposits originate from very shallow stratigraphic layers indicating a very shallow bowl shaped maar diatreme, that is difficult to explain by the traditional model of downwards growth of a singular explosion point. The detailed field studies, presented here, including mapping, stratigraphic logging, measurements of base surge transport directions and ballistic impact structures, led to the reconstruction of at least three major vent locations, suggesting that the large size is the result of multiple coalesced craters. Vent migration occurred along basement structures and was probably caused by the collapse of the crater walls and clogging of the vent. In addition, a complex eruption history was determined with three eruption phases and at least one significant volcanic hiatus. This is supported by the geochemical data of the Lake Purrumbete pyroclastic sequence, which shows distinct trace element trends that correlate with the stratigraphic height and the three determined eruption phases, indicating that the maar activity was controlled by deep processes within the feeding system. The trace element trends cannot be explained by crystal fractionation or crustal contamination and are best explained by melt differentiation caused by melt transport through highly porous mantle channels. Tapping these melt channels may have controlled the release of single magma pulses and therefore the activity of the Lake Purrumbete Maar. Analysis of the bubble number densities and the bubble size distribution of vesicles preserved in ash sized juvenile clasts of the pyroclastic sequence show that vesicle nucleation appeared in different phases within the conduit, with an early phase deeper in the conduit and a late phase of vesicle nucleation near the fragmentation level, caused by the exsolution of H2O and acceleration of the magma. Strain within the magma caused by the acceleration of the melt is also indicated by micro-textures, including stretched vesicles and aligned microlites, whereby the difference in the microlite abundances and the microlite shape, observed in ash particles are difficult to explain with the traditional models of decompression and cooling induced microlite formation. The results of fragmentation experiments conducted in this project show that microlites can form on extremely small time scales. Furthermore the exclusive occurrence of microlites in particles that experienced mechanical stress during brittle fragmentation, suggests that microlite formation may be the result of mechanical stress. The differences in the microlite populations observed in the Lake Purrumbete ash particles are therefore best explained by modification of the microlite population due to stress induced microlite formation caused by the strain of the magma during acceleration near the fragmentation level. In addition, high strain rates may also cause deformation of the magma and the formation of fractures. The fragmentation experiments have also shown that vesiculated magma can react with water in the way of molten fuel coolant interaction, if the magma/water interface is enhanced by fractures, suggesting that the phreatomagmatic eruption style at Lake Purrumbete was supported by fracturing of the magma due to acceleration near the fragmentation level. Furthermore, the results show that pre-stress of the magma, that is most likely caused by strain, has a major influence on the shape of the produced pyroclasts. This research project shows that the formation of a very large maar system is controlled by many factors starting at the source of the melt and magma ascent dynamics to subsurface processes, whereby the formation of a specific volcano form is predominantly controlled by subsurface and surface processes including the availability of water and the surrounding stratigraphy. The results, presented here show that the combination of hard and soft rock substrate together with fractures within the country rock and the location on fault structures led to the formation of this very large maar structure. Aquifer sourced water supply, mass flow of the Black Rock Sandstone and water, together with changing magma ascent rates influenced the explosive eruption dynamics. This research projected has touched on many different aspects of the eruption of very large maar systems and raised new questions which should be addressed in future work. In addition, it has shown the importance of detailed stratigraphic work on this volcano type as the basis of many other studies, such as geochemistry and pyroclast textures. The geochemical variations detected within the Lake Purrumbete pyroclastic sequence suggest, that this volcano type is perfect for studies of mantle processes, as the magma rises very fast to the surface leaving no time for crustal contamination and magma differentiation. Furthermore, highly porous melt channels within the mantle may be the explanation and reason for the activity of intra-plate volcanic provinces and should be further investigated in the future. In addition, strain within the magma seems to have a major influence on the eruption type and the resulting pyroclast shape, the results presented here suggesting that the strain may be linked to the acceleration of the melt near the fragmentation level and to the exsolution of H2O. This connection should be further addressed in future work.