Constraints on cooling rates and permeabilities of pumice in an explosive eruption jet from colour and magnetic mineralogy

In explosive volcanic eruptions, vesicular magma droplets, produced by frag mentation, are propelled into the atmosphere where they are chilled to form pumices. The thermal history of droplets and the permeability of their int ernal bubble networks determine how much they are deformed in the eruption jet, and hence what information pumices record about the state of the magma at fragmentation. We study these aspects of the 'Minoan' plinian eruption of Santorini Volcano by quantifying the rate of oxidation reactions that to ok place when air entered the hot magma fragments. In our experiments white Minoan pumices were heated for minutes to hours between 600 and 850 degree s C, either in air, or in an atmosphere with an oxygen fugacity at the Ni-N iO buffer. Pumices were unchanged by heating at Ni-NiO. Those heated in air often became pink to dark pink, depending on heating time, and their Curie temperatures, as determined by magnetic susceptibility measurements, incre ased. We use oxidation rates deduced from these experiments, in conjunction with calculations of the rate of conductive cooling and of the rate at whi ch air can enter a pumice, to constrain the conditions experienced by pumic es during the eruption. Natural Minoan pumices less than about 5 cm in radi us are white, whereas larger ones often have white rims and pink interiors with Curie temperatures higher than those of white material. We infer that small pumices were cooled before being oxidized, and that oxidation of the interiors of large clasts mostly took place during flight, at temperatures within a few tens of degrees of magmatic values. White rims of large pumice s, despite being permeable, were cooled before oxidation could occur. Perme ability developed in the liquid state, but did not develop early enough, wi th respect to cooling, or was not large enough to allow extreme oxidation. We give measurements of pumice permeabilities that should be close to magma tic values. (C) 1998 Elsevier Science B.V. All rights reserved.