Vesicle microtexture analysis and eruption dynamics of selected high fountaining episodes at Pu`u `Ō`ō, Kīlauea volcano, Hawai`i between 1985-1986.

Abstract:

The early phases of the ongoing eruption of Pu`u `Ō`ō in the East Rift Zone (ERZ) of Kīlauea on Hawai`i provide a unique opportunity to study the vesicle microtexture of tephra from five high (≥200m) Hawaiian fountaining events, from a single vent, over a prolonged period of time. The high Hawaiian fountains erupted at Pu`u `Ō`ō varied in height from 200 m up to a maximum of 467 m, during which the shallow conduit at Pu`u `Ō`ō remained stable. We conducted microtextural analysis of pyroclasts from five high (264 to 391 m) Hawaiian fountaining episodes at Kīlauea, Episodes 32, 37, 40, 44 and 45, erupted from the Pu`u `Ō`ō vent between 1985 and 1986 in order to constrain the parameters that lead to large variations in fountain height of Hawaiian fountains at Pu`u `Ō`ō. Our results show that pyroclasts from a single episode can vary greatly in texture (from bubbly to foamy) and have vesicle volume densities (N^m_v) that vary by an order of magnitude. This range in vesicle texture and population is due to extensive growth and coalescence of vesicles within the eruption jet post-fragmentation, resulting in the observed vesicle texture not being wholly indicative of the syn-fragmentation vesicle population. Only four pyroclasts were found to have textures that are interpreted to be indicative of the vesicle population at the moment of fragmentation, all of which have bubbly texture, high density, high N^m_v, and low vesicle-to-melt ratio (V_G/V_L). Due to the paucity of pyroclasts representative of syn-eruption vesiculation processes, comparison of shallow conduit dynamics across episodes can only be qualitative observations, which suggest the ascending melt is thermally and mechanically heterogeneous on a small scale during Hawaiian-style fountaining. This highlights the importance for detailed micro-scale qualitative textural observations on pyroclasts with end-member densities, as well as modal densities, when carrying out vesicle microtexture analysis. This will ensure that a sufficient number of pyroclasts with textures representative of the syn-fragmentation vesicle population are identified, in order to carry out quantitative comparisons across episodes.