Rheological properties of natrocarbonatite lavas from Oldoinyo Lengai, Tanzania
Norton, Gill; Pinkerton, Harry
European Journal of Mineralogy Volume 9 Number 2 (1997), p. 351 - 364
published: Jun 26, 1997
manuscript accepted: Dec 4, 1996
manuscript received: May 14, 1996
ArtNo. ESP147050902012, Price: 29.00 €
Abstract A systematic series of measurements of the rheological properties of natrocarbonatites erupted on Oldoinyo Lengai (Tanzania) during November 1988 has shown how these properties are influenced by changes in composition, temperature, crystallinity and vesicularity. Our field and laboratory measurements reveal that aphyric natrocarbonatites are newtonian with eruptive viscosities over an order-of-magnitude lower than the most fluid basaltic melt. At a typical eruption temperature on Oldoinyo Lengai (590°C), apparent-viscosities of the 1988 natrocarbonatites measured in the laboratory using a rotational viscometer ranged from 0.15 Pa s for degassed, phenocryst-poor lava to 85 Pa s for a degassed lava with a high phenocryst content. Field measurements on similar lavas vary from 1 to 5 Pa s for gas-poor, low-crystallinity lavas to 120 Pa s for a highly vesicular high-crystallinity lava. The higher apparent-viscosities measured in the field result mainly from the higher vesicularities of lavas in the field, and the loss of up to 3.6 wt.% of volatiles during melting in the laboratory. The lowest viscosities measured in the laboratory were for a residual melt that had been filter-pressed from the front of a stationary lava flow. This aphyric sample, which was enriched in halogens, has an extrapolated apparent-viscosity of 0.018 Pa s at 590°C Our laboratory measurements show that a three-fold increase in halogen content in carbonatites is accompanied by a three-order-of-magnitude decrease in apparent-viscosity. The effects of crystals on rheological properties of natrocarbonatites were clearly demonstrated during one set of measurements when measured apparent- viscosities dropped from 64 Pa s to 1.5 Pa s over a melting interval of 19.2°C. This reduction in apparentviscosity was accompanied by a change from highly non-newtonian to newtonian flow behaviour. All lavas, apart from the residual melt, behaved as highly visco-elastic fluids at temperatures below 535°C. An extreme example of the rheological importance of crystals on Oldoinyo Lengai was noted after the eruption, in June 1993, of a poorly vesicular, crystal-rich, 6 m thick flow that was morphologically similar to many rhyolites. Our calculations suggest that this flow had an apparent-viscosity, during flow, of 3.107 Pa s to 7.108 Pa s.