Follow the CO 2 – drilling into an actively degassing intraplate volcano underlain by a silicate–carbonatite intrusion

The Eifel region of Germany hosts hundreds of distributed volcanoes of Quaternary age in an intracontinental setting. This includes many maar volcanoes, for which the Eifel is the type locality. Laacher See volcano in the eastern part of the region stands out as a sizable (erupted volume of 6.3 km 3 dense rock equivalent or a volcanic explosivity index (VEI) of 6) and dormant but actively deforming and degassing system. Plutonic ejecta clasts in pyroclastic deposits of the Laacher See volcano provide evidence that it is underlain by one of the youngest silicate–carbonatite subvolcanic intrusive complexes worldwide. It has long been appreciated that the Laacher See region has potential to significantly enhance our knowledge on distributed volcanic fields and their specific hazards resulting from high CO 2 fluxes from the mantle to the surface, causing active deformation, as well as diffuse and sometimes punctuated explosive degassing. This is largely due to the Eifel boasting an extensive record of past research, easy access, and excellent infrastructure that uniquely permits implementation of cutting-edge scientific methods. Recently, three workshops were held to sharpen scientific questions of global scope and relevance to study this type of distributed volcanism. Workshop participants discussed opportunities and challenges associated with drilling in the Laacher See region, identified promising sites, and explored the potential of novel drilling techniques. The clear conclusion of these workshops is that Laacher See would be an ideal test bed to evaluate the physical and chemical properties of a shallow (∼ 4–6 km depth at its top, thus making it accessible to drilling) silicate–carbonatite intrusive complex formed by volatile-rich melts and associated with active degassing. Only drilling can provide answers to key problems related to the geodynamics, geohazards, and resource potential of such magma systems. Questions include how and at what rates translithospheric transport of magma and fluids occurs in continental intraplate settings. It is also puzzling why shallow fluids above a residual magma system after an eruption that occurred only 13 000 years ago are seemingly cold, although the system still appears to be highly dynamic based on ongoing CO 2 degassing, seismic activity, and exceptionally high uplift rates on spatial scales of hundreds of kilometres. Critically, drilling enables linking deep and shallow observables related to melt and fluid migration and provides access to samples of CO 2 -rich fluids and their host rocks at depth. Such samples are tangible evidence required to properly balance CO 2 fluxes from degassing magma with CO 2 sequestration in carbonatites or fluid-precipitated carbonates. Core samples from a maar structure proximally to the Laacher See volcano can establish an unprecedented geological record of precursor events prior to its cataclysmic eruption. Importantly, drilling also permits improved geophysical monitoring from instrumented wells that can reveal high-resolution details on maar diatreme architecture and deflation-related faulting resulting from the evacuation of the Laacher See magma reservoir. Lastly, silicate–carbonatite intrusions are globally recognised as major hosts for critical metal deposits, which, at Laacher See, could be topics of investigation in the making. Ultimately, this project can provide fundamental insights into processes of fluid-mediated element transport and sequestration not achievable in inactive carbonatites. Overall, these goals are best achieved in two phases that encompass drilling (1) four holes (300–2000 m) to enable detailed studies of the pre-eruptive evolution of the Laacher See volcano, its subvolcanic structures, and ongoing fluid or magma transport and (2) a subsequent deep hole (3000–4000 m) to penetrate and core a syenitic–carbonatitic intrusive carapace and its hydrothermal aureole.