Reconstructing the deep CO₂ degassing behaviour of large basaltic fissure eruptions

Highlights

• CO₂ in shrinkage bubbles in melt inclusions can be measured by Raman spectroscopy.

• >90% of the total melt inclusion CO₂ may be contained in the bubble.

• The deep degassing path of the Laki magma is reconstructed using CO₂/Nb systematics.

• Laki melt inclusions preserve a record of concurrent crystallisation and degassing.

• The total mass of CO₂ exsolved from the Laki magma is ∼304 Mt.

Abstract

The mass of volatiles degassed from volcanic eruptions is often estimated by comparing the volatile concentrations in undegassed glassy melt inclusions with the volatile concentrations in the degassed matrix glass. However, melt inclusions are prone to post-entrapment modification, including diffusive H⁺ loss through the host olivine crystal lattice which lowers the H₂O content of the inclusion, and the degassing of CO₂ into a bubble in response to cooling and crystallisation on the inclusion walls. Such bubbles are very common in olivine-hosted melt inclusions from the AD 1783–1784 Laki eruption, south–east Iceland. We have determined the CO₂ content of these bubbles using micro-Raman spectroscopy, and the CO₂ concentration in the glass by SIMS. Our results show that >90% of the total inclusion CO₂ may be sequestered into the bubble, which demonstrates the importance of measuring the compositions of both vapour bubbles and the glass phase in melt inclusions. We reconstruct the deep degassing path of the Laki magma by using Nb as proxy for the undegassed CO₂ content of the melt inclusions. The substantial CO₂/Nb variation in the Laki melt inclusions (3.8–364) can be explained by concurrent crystallisation and CO₂ degassing in the Laki magmatic system. We calculate the amount of CO₂ lost from individual melt inclusions, assuming CO₂/Nb ≈ 435 for enriched Icelandic mantle and CO₂/Nb ≈ 171 for depleted mantle. Melt inclusions with the greatest saturation pressures have lost the least CO₂ prior to inclusion trapping. At any given saturation pressure, the most enriched melt inclusions have lost the most CO₂, while the most depleted inclusions have lost very little CO₂. Enriched primary melts with high initial CO₂ concentrations are therefore useful for investigating deep degassing behaviour in magmatic systems because a range of melt inclusion saturation pressures are recorded during crystallisation and degassing. Depleted melt inclusions with low initial CO₂ concentrations remain vapour-undersaturated to shallow levels and cannot be used to constrain deep degassing behaviour. The cumulative CO₂ mass release from the Laki magma is determined as a function of pressure and extent of crystallisation. Using an updated petrologic method that takes into account the diversity of primary melts and CO₂ sequestration into vapour bubbles, we calculate the total mass of CO₂ exsolved from the Laki magma to be 304 Mt.