Cooling dynamics of spatter-fed phonolite obsidian flows on Tenerife,Canary Islands

Relaxation geospeedometry has been applied to two series of clastogenic obsidian flows on Tenerife to determine their thermal history across the glass transition. The phonolite flows investigated were both generated by lava fountaining activity followed by rheomorphism of the deposits. The detailed sampling resolution within the two series enabled an accurate quantification of their thermal history. Cooling rates within the investigated spatter-fed flows vary over more than two orders of magnitude. The highest cooling rates of 0.39 K/min were modeled for the central vesiculated part of one flow. The dense basal obsidian layers of both flows were cooled at substantially lower rates of 0.0042 and 0.0028 K/min, respectively. There appears to be an influence of in-situ vesiculation processes on the thermal budget of the investigated flows. In addition, the slow cooling rates for the basal portions of both flows seem to be associated with a stage of thermal buffering. Continual advective heat transport of hot material along a basal shear plane may sustain elevated temperatures associated with (quasi-) isothermal annealing within this “décollement”. Numerical simulations based on conductive heat loss concepts fail to resolve the cooling history quantified through relaxation geospeedometry for the investigated flows. The effects of vesiculation and thermal annealing on the cooling behavior of the clastogenic flows across the glass transition are discussed in the light of these new data. In addition, viscometric data on these phonolites are used to correlate the known cooling rates to viscosities at the glass transition.