Rheological properties of natrocarbonatite lavas from Oldoinyo Lengai, Tanzania

A series of measurements of the rheological properties of natrocarbonatites erupted on Oldoinyo Lengai in November 1988 has shown that these properties are influenced by changes in composition, T, crystallinity and vesicularity. At a typical eruption T (590 degrees C), apparent-viscosities of the 1988 natrocarbonatites 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 varied from 1-5 to 120 Pa s; the higher field values being attributed to the higher vesicularities of the lavas and the loss of up to 3.6 wt.% of volatiles during melting in the laboratory. Laboratory measurements also show that a three-fold increase in halogen content in carbonatites is accompanied by a three-order-of-magnitude decrease in apparent-viscosity. The rheological importance of crystals on Oldoinyo Lengai was noted after the eruption (June 1993) of a poorly vesicular, crystal-rich, 6 m thick flow morphologically similar to many rhyolites; calculations suggest that this flow had an apprarent-viscosity, during flow, of 3 X 10 (super 7) to 7 X 10 (super 8) Pa s.