Longevity and crystal mush stability of the IDDP-1 magma body at Krafla

    The 2009 intersection of molten rhyolitic magma at ~2.1 km depth by the IDDP-1 well revealed an active shallow magmatic system - shallower than initially believed - beneath the Krafla caldera (NE Iceland), challenging the prevailing assumption that shallow bodies should rapidly solidify. Close to aphyric molten rhyolite was found directly below nearly completely solidified felsite, while a crystal mush was not found, contradicting current views.

    We have developed a thermomechanical numerical model to investigate the dynamics, thermal evolution, and longevity of this magma pocket. The model couples partially compressible Stokes flow with heat transport, explicitly accounting for latent heat during crystallization. Magma is treated as a multicomponent mixture of melt, crystals, and volatiles, with thermophysical properties evolving self-consistently with temperature, pressure and composition. Conductive heat exchange with the host rocks leads to cooling of the magma chamber. We have implemented this new framework in the finite element code GALES (Garg and Papale, Frontiers in Earth Sciences 2022).

    Numerical simulations, constrained by IDDP-1 observations (initial T ≈ 900 °C, P ≈ 45 MPa), show an initial rapid cooling phase followed, after ~50 years, by a quasi-stationary cooling characterized by thermal oscillations. This behavior is driven by latent heat release during crystallization and it delays solidification, maintaining a largely molten state for several hundred years. In addition, cooling and crystallization -induced convection redistributes heat and remobilizes marginal crystal accumulations, preventing the formation of a stable crystal mush at the chamber roof and walls.

    These processes provide an explanation for the direct encounter with molten magma in the IDDP-1 borehole and indicate that small, shallow rhyolitic magma bodies can persist in a molten state over much longer timescales than commonly tought. The results have direct implications for future drilling-to-magma initiatives such as KMT (Krafla Magma Testbed).