Declining Gas Accumulation Under Hekla Volcano, Iceland, From 210Pb-226Ra Disequilibrium

Abstract

Magmatic volatiles exceeding the solubility limits will form a gas phase that accumulates at the top of a magma chamber. If that magma chamber is hermetic, the exsolved gas can lead to overpressure causing an eruption. Such gas accumulation is hard to detect but the decay of ²²⁶Ra, in the ²³⁸U decay chain, produces ²²²Rn that will accumulate together with the major gas species, and rapidly decay to ²¹⁰Pb. Such radon accumulation and decay thus will lead to an excess of ²¹⁰Pb over ²²⁶Ra in the magma accumulating the gas phase. Eruptions at Hekla volcano, Iceland, start explosively producing tephra before an effusive lava forming phase. The tephra of the five latest eruptions of Hekla (1947–2000) has either an excess of ²¹⁰Pb over ²²⁶Ra, a consequence of the radon accumulation, or are in radioactive equilibrium as all the lava. Tephra is taken to represent the mass of magma accumulating gas and since 1947 the tephra volume has declined regularly. A simple model shows that the mass of the degassing deep magma has remained twice that of the gas-accumulating magma. Consequently, the diminishing mass of degassing basalt at depth is likely to result in decreasing gas flux into the shallower basaltic andesite magma chamber beneath Hekla. The negligible mass of tephra from the last eruption in the year 2000 therefore suggests that renewal of the basalt source seems to be needed for the next eruption at Hekla volcano.