Primary versus secondary degassing during basaltic eruptions

Volcanic gases are a major concern, especially when eruptions take place in inhabited or touristic areas. Several studies have revealed that during basaltic eruptions, toxic metals such as Pb, Cd, As and Zn are efficiently outgassed, carried by the major gas species, mainly sulfur and halogens. However, part of the degassing occurs after the eruption, while the lava flow is solidifying, and the composition of this secondary gas is virtually unknown.

After the primary (syn-eruptive) degassing, the lava is depleted in sulfur, leading to relative enrichment in halogens in secondary (post-eruptive) gas emission. This change in major species concentration could impact the volatility of metals and thus the toxicity of the gas emitted.

To investigate this subject, we collected, using filter packs, gas samples of both the primary and the secondary gas phases of the Geldingadalir and Meradalir eruptions. The filters were then leached in diluted acid and the resulting solution analyzed for trace element composition.

Results show syn-eruptive gas samples with very homogeneous trace volatile element composition and distinct from all the post-eruptive gas. Conversely, the secondary gas is more diverse, with distinct composition in samples collected around the main Geldingadalir crater and those collected on the lava flow.

To compare our gas samples (having different air dilution factors), we normalized each element to Cu (well measured and moderately volatile). Overall, the lava flow post-eruptive gas appears enriched in Zn, Sb and Pb with respect to syn-eruptive (10-100 times higher normalized enrichment factor). These elements are known to form chloride species and could thus have an enhanced volatility due to higher Cl concentration in the secondary gas phase. The Sulfur-loving (chalcophile) element Te has, on the other hand, a 10 times lower normalized enrichment factor in the lava flow gas, which is consistent with a sulfur depletion.

It thus seems that volcanic gas emission changes radically between primary and secondary degassing. Increase volatility of some metals such as Lead or Zinc might lead to higher toxicity, with important hazard for the local population and environment.