RHEOLOGICAL PROPERTIES OF NATROCARBONATITE LAVAS FROM OLDOINYO-LENGAI, TANZANIA

A systematic series of measurements of the rheological properties of n atrocarbonatites erupted on Oldoinyo Lengai (Tanzania) during November 1988 has shown how these properties are influenced by changes in comp osition, temperature, crystallinity and vesicularity. Our field and la boratory measurements reveal that aphyric natrocarbonatites are newton ian with eruptive viscosities over an order-of-magnitude lower than th e most fluid basaltic melt. At a typical eruption temperature on Oldoi nyo Lengai (590 degrees C), apparent-viscosities of the 1988 natrocarb onatites measured in the laboratory using a rotational viscometer rang ed 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 si milar lavas vary from 1 to 5 Pa s for gas-poor, low-crystallinity lava s to 120 Pa a for a highly vesicular high-crystallinity lava. The high er apparent-viscosities measured in the field result mainly from the h igher vesicularities of lavas in the field, and the loss of up to 3.6 wt.% of volatiles during melting in the laboratory. The lowest viscosi ties 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 degrees 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-viscosi ty. The effects of crystals on rheological properties of natrocarbonat ites were clearly demonstrated during one set of measurements when mea sured apparent-viscosities dropped from 64 Pa s to 1.5 Pa s over a mel ting interval of 19.2 degrees C. This reduction in apparent-viscosity was accompanied by a change from highly non-newtonian to newtonian flo w behaviour. All lavas, apart from the residual melt, behaved as highl y visco-elastic fluids at temperatures below 535 degrees 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, cry stal-rich, 6 m thick flow that was morphologically similar to many rhy olites. Our calculations suggest that this flow had an apparent-viscos ity, during flow, of 3.10(7) Pa s to 7.10(8) Pa s.