Crystallization and oxidation of Kilauea basalt glass: Processes during reheating experiments

Crystallization and oxidation are usually considered as a consequence of cooling and emplacement of a magma, whereas little is known about crystallization upon reheating. However, the common eruption of basalt on the Earth and other rocky planetary bodies results in reheating at the interface. To explore the range of possible processes of crystallization and oxidation, reheating experiments were carried out on basalt glass from Kilauea in air and in argon atmosphere. In air, interface-controlled crystallization starts with the formation of pyroxene dendrites and Fe-Ti oxides at their apices below 840 degreesC. Above 920 degreesC, crystallization of the bulk glass occurs and includes in addition growth of plagioclase. In argon, the growth of only pyroxene is observed between 850 and 936 degreesC. Crystallization is interface controlled below 890 degreesC, and occurs within the bulk above 890 degreesC. On the basis of T-dependent studies of the formation of Fe3+ (detected by Mossbauer spectroscopy), DeltaG of oxidation is estimated to -105 and -67 kJ/mol (+/- 7 kJ/mol) for glass chunks and powder, respectively. The activation energy is 100 kJ/mol for glass chunks, and 32 and 20 kJ/mol for powders, below and above the glass transition temperature, respectively. However, oxidation does not involve an exchange with the environmental atmosphere, as shown by the constant delta O-18 (4.6-5.8 +/- 0.2 parts per thousand) for any reheating below the liquidus. Significant cation diffusion of mainly Ca2+ to the sample-air interface forming lime (rather than anion diffusion of O2- into the sample), may account for the charge balance and explain at least the high rate of surface oxidation. Although the extent of diffusion processes to the sample's interior cannot be observed (but is at least 80 mum) because of overlapping effects by crystallizaiton, an increase in cationic mobility, dependent on the oxygen gradient between sample and environmental atmosphere, might be the reason for an increasing degree of crystallization observed.