Timescales of Magma Rise and CO2 Transfer in Basaltic Systems: Integrating FENS Diffusion Chronometry, Melt Inclusions and Petrological Barometry for Icelandic Eruptions

Iceland is an excellent target for the development of diffusion chronometry: a number of recently active volcanic systems have been the target of extensive geological, geophysical and petrological study. It is therefore possible to understand the physical significance of timescales extracted from diffusion profiles in crystals and to link them to processes that may have relevance for volcano monitoring efforts.

We have performed focused micro-analytical studies of a set of 6 Icelandic basaltic eruptions, including both large fissure eruptions (e.g. Laki) and smaller monogenetic events (e.g. Borgarhraun). Detailed petrography and textural analysis of compositional maps permits well-constrained thermobarometry for these eruptions, and we have recovered storage depths from near-Moho ( 20 km) through to shallow crust (<5 km). The CO₂ content of melts at these depths was constrained by SIMS analyses of melt inclusion CO₂and trace element from each eruption.

We developed FENS, a method for the robust extraction of magmatic timescales from chemical profiles. This uses a 3D Finite Element solver (FEniCS) for the diffusion equation and Bayesian Nested Sampling algorithm (PyMultiNest) for optimization. This powerful method allows for comprehensive evaluation of the uncertainty structure of the recovered timescales: a crucial advance for making links between petrology and volcano monitoring data.

The recovered timescales for magma rise from the final storage depth to eruption are uniformly short ( 10⁰-10²days) and can be combined with the CO₂ budget to estimate the rate of pre-eruptive transfer of CO₂ from the silicate melt to a free vapour phase. These results indicate that CO₂ transfer rates in the range 10¹-10³ kt/day for hours to days before eruption. In addition, Laki preserves a multi-decadal record of magmatic gas formation during assembly of the magma body. Understanding the fate of CO₂ after its transfer from melt to magmatic gas is now crucial for making the link to volcano monitoring.