Discussing the timescales of pre-eruptive gas accumulation beneath Hekla volcano, Iceland, from 210Pb-226Ra systematics.

The last five eruptions at Mt Hekla, Iceland (1947, 1970, 1980, 1991 and, 2000) occurred at a frequency ranging from several decades to every ten years since 1970. The eruptions start with an explosive sub-Plinian phase during which most tephra is emitted. A decrease in eruptive intensity cause basalt-andesite lava emission (SiO₂~54 wt%). Hautemann et al. (2017) linked the regular dynamic of Hekla to the accumulation of gas in a deep plumbing system (< 10 km) during periods of quiescence between eruptions. In this study we determine whether the proposed gas accumulation affected ²¹⁰Pb-²²⁶Ra radioactive disequilibrium.

Radon being a noble gas has great affinity for the gas phase, but due to its low concentrations in magma is not able to form a mobile bubble. However, if a major gas phase such as CO₂ is released, Radon can diffuse rapidly into the gas-bubble and therefore appears to be a good tracer for tracking a potential gas accumulation. One of its radioactive isotopes present in the ²³⁸U series is ²²²Rn (half-live 3.82 days). It is produced by the decay of ²²⁶Ra (half-live 1600 yrs) and quickly decays to form ²¹⁰Pb. Because ²¹⁰Pb has a half-life of 22.3 years and higher affinity for the melt phase than Rn, it can be measured in the erupted products. If gas accumulates at the top of the basalt andesite magma chamber, ²²²Rn carried by the major gas phase should accumulate in the upper part of the reservoir and disintegrate rapidly to ²¹⁰Pb The first erupted products could thus register an excess of ²¹⁰Pb over its parent isotopes, ²²⁶Ra.

The alpha and gamma spectrometry analysis of the initial tephra from the five most recent eruptions at Hekla volcano reveals a (²¹⁰Pb/²²⁶Ra) of 1.057±0.014 (2σ), (²¹⁰Pb/²²⁶Ra)=1.093±0.020 (2σ) for the 1947 and 1970 eruptions, respectively, while a radioactive equilibrium is observed for the 1980, 1991 and 2000 eruptions. Recalculated to the time of eruption, the ratios amount to (²¹⁰Pb/²²⁶Ra)₀=1.58±0.14 (2σ), (²¹⁰Pb/²²⁶Ra)₀=1.47±0.09 (2σ) and equilibrium, respectively. These results are consistent with an accumulation of Radon at the top of the magma chamber reflecting CO₂ accumulation. At first glance, the rate of gas accumulation seems to be linked to the quiescent time between eruptions. However, other factors, including decrease of the mass of the degassing deep magma may affect this accumulation phenomenon. Evidence of gas accumulation is restricted to the tephra whereas (²¹⁰Pb/²²⁶Ra)₀ is equal to unity in the lava, which magma neither accumulated nor degassed Radon. Therefore, a deeper magma (basalt) contributed the CO2 and Radon to the Hekla plumbing system. The decrease in the (²¹⁰Pb/²²⁶Ra)₀ in the tephra over the past 70 years coincides with the decline in tephra volume, strongly suggest diminishing mass of deep degassing basalt. The decrease in gas supply and accumulation may explain the current dormancy of Hekla volcano.

References:

Hautmann, Stefanie, I. Selwyn Sacks, Alan T. Linde, and Matthew J. Roberts. 2017. « Magma Buoyancy and Volatile Ascent Driving Autocyclic Eruptivity at Hekla Volcano (Iceland) ». Geochemistry, Geophysics, Geosystems 18 (9): 3517‑29. https://doi.org/10.1002/2017GC007061.