SUBLIQUIDUS RHEOLOGY OF ARC MAGMAS: A COMPARISON OF PACAYA, GUATEMALA, AND STROMBOLI, ITALY

The viscosity of basaltic magma varies by approximately nine orders of magnitude between eruption and quenching or emplacement. Viscosity is a major control on eruption style, increasing due to decreasing temperature, decreasing volatile content and/or increasing crystal fraction. We studied basalts erupted from Pacaya, Guatemala, and Stromboli, Italy, which exhibit various styles of effusive activity with strombolian eruptions and lava flows common at both locations. We remelted samples of a lava flow from Pacaya and a bomb from Stromboli, which have similar bulk compositions. By combining both high temperature and low temperature results from remelts of Pacaya and Stromboli samples, best fit TVF curves were obtained to calculate melt viscosity (η) as a function of temperature (T).

For Pacaya, the equation is: log η (Pa s) = -3.54+5010.9/[T(K) -325.45]

For Stromboli, the equation is: log η (Pa s) = -4.65 +6653.0/[T(K) -531.14]

Remelted liquids range in viscosity from 40 Pa s (Pacaya) or 50 Pa s (Stromboli) at 1300˚C, to >4х10¹⁰ Pa s (Pacaya) or > 2х10¹² Pa s (Stromboli) at 680˚C.

The lava flow sample from Pacaya was~25% vesicular, with a ~75% crystalline matrix, and has an effective viscosity >10¹² Pa s at ~1000˚C. The bomb sample from Stromboli was ~20% vesicular, with a ~60% crystalline matrix, and has an effective viscosity ~10¹⁰ Pa s at ~1000˚C. The remelted sample from Stromboli was subjected to varying degrees of crystallization in the parallel-plate viscometer. Four different subliquidus thermal histories produced different crystal fractions up to 60 volume percent, which is the same as in the natural sample, although the textures are very different. At 1000˚C, the viscosity of ~60% crystalline magma is ~4 х 10¹⁰Pa s, ~6 orders of magnitude higher than that of the crystal-free melt and close to the obtained value for the bulk rock measured at the same temperature. Sub-liquidus viscosity measurements are in progress for the Pacaya lava flow. We conclude that the basaltic lavas at Pacaya and Stromboli are effectively rigid at temperatures as high as 1000˚C .The results from this project will be used to quantify the effects of crystallization on viscosity, and therefore help to determine how rheology changes during a single eruption, and between eruptions.