Timescales of degassing and conduit dynamics inferred from 210Pb-226Ra disequilibria in Volcán de Colima 1998-2010 andesitic magmas

Abstract

Determining the timescales of magma degassing is essential for understanding the mechanisms controlling the eruption style and the dynamics of magmatic systems. Towards this end, we measured 210Pb-226Ra disequilibria in andesite lavas erupted from Volcán de Colima between 1998 and 2010. (210Pb/226Ra)0 activity ratios range from 0.86 to 1.09, and are best explained in terms of 222Rn degassing and accumulation. The range in 210Pb deficits indicates that the timescales of 222Rn degassing did not exceed 11 years. 210Pb excesses are rare and small (<10%), which signifies that 222Rn degassing is more effective than 210Pb accumulation in this intermediate system despite the relatively low gas output at the surface. The absence of significant 210Pb excesses strongly suggests that the volcanic activity results from episodic ascent of small magma batches through the vapour-saturated section of the magmatic system. Overall, the degassing models based on 210Pb-226Ra disequilibrium suggest an open and complex subvolcanic magmatic system comprising several conduits in which multiple magma batches reside for up to 10 years. Shifts from effusive to explosive Vulcanian eruptive phases are not related to changes in degassing mode on timescales resolvable using 210Pb-226Ra disequilibria.