The thermal history of a spatter-fed lava flow: the 8-ka pantellerite flow of Mayor Island, New Zealand

Abstract

The thermal history of the 8-ka spatter-fed lava flow of Mayor Island, New Zealand, has been investigated via relaxation geospeedometry. Cooling rates extant during the vitrification of the clastogenic pantellerite flow have been quantified. Cooling rates modelled for two obsidian layers at the top and the base of the flow, vary over four orders of magnitude. The flow base defined by a lower obsidian layer shows a gradual increase in cooling rates from 0.00072 K/min adjacent to the crystalline core of the flow to 0.017 K/min at the contact to a basal crumble breccia. Cooling rates of flow ridges formed by an upper obsidian layer vary between 0.001 and 6 K/min. Based on field observation and thermal modelling, cooling rates of Kelvins to tens of Kelvins per day for both upper and lower obsidians are difficult to interpret as simple cooling processes. Instead, we propose a thermal history involving thermal annealing as an alternative explanation for these slow effective cooling rates. Such thermal annealing appears to have facilitated welding and rheomorphism of the spatter deposit leading to the development of a coherent lava sheet. Based on the results obtained here, we present a quantitative scenario of the thermal history during the final stages of flow emplacement as a contribution to understanding the rheology of rheomorphic spatter deposits associated with explosive peralkaline rhyolitic volcanism and briefly address the hazard potential of such deposits.

Influence of composition and thermal history of volcanic glasses on water content as determined by micro-Raman spectrometry

“…During volcanic eruptions, annealing can play an important role during welding of clastogenic lava flows (Gottsmann and Dingwell, 2002) or at conduit walls heated by magma shearing (Polacci et al, 2001, Rosi et al, 2004and Vedeneeva et al, 2005. Absence of correlation between dissolved water content and distribution of microlites in banded obsidians from Milos (Greece) suggests that degassing is not the only process that can produce the observed heterogeneities.…”

“…Furthermore, previous researches have preferentially characterized hydrous synthetic glasses of simple composition (Mysen et al, 1980;Mysen and Virgo, 1986) or silica-rich glasses (quartz, rhyolites) (McMillan and Remmele, 1986, Thomas, 2000and Chabiron et al, 2004. Finally, melt inclusions in crystals and groundmass melts can experience complex thermal paths in magma reservoir (Bachmann and Bergantz, 2003), during ascent in the conduit (Rosi et al, 2004) and during emplacement (Gottsmann and Dingwell, 2002) or simply during sample preparation (e.g. heating for homogenization of glass inclusions or heating stages during the viscosity measurements).…”

Proposing new approaches for dating young volcanic eruptions by luminescence methods

“…Flow surface cooling rates for a 20-70 m thick rhyolitic flow of Mayor Island (NZ) are given between 0.001 and 6°C min -1 with the majority of the data around 0.001°C min -1 (Gottsmann and Dingwell, 2002). For basaltic compositions, Flynn and Mouginis-Mark (1992) report cooling of the thin surface crust of a very small volume (2 m wide, 3 m long, 30 cm thick) flow from the Pu'u O'o fissure, Kilauea, Hawaii, from 768 to 390°C over a period of 59 minutes after emplacement.…”

Paleointensities on 8 ka obsidian from Mayor Island, New Zealand

“…Samples of both LOBS and UOBS are very pristine with no indication of secondary hydration or alteration processes. Both layers show very similar chemical composition with an average mol Al 2 O 3 /mol[(Na 2 O+K 2 O)] of 1.54 (Gottsmann and Dingwell, 2002), which does not change with layer depth or horizontal sample location within the sites (Gottsmann and Dingwell, 2002). …”

“…The deposit has been interpreted by Stevenson et al (1993) as a fountain-fed flow that deformed after emplacement. All samples for this study were originally obtained by Gottsmann and Dingwell (2002) without orientation for their study on the thermal history of the 8 ka flow. A 70 cm thick vertical profile of LOBS (Fig.…”