An integrated geological-geophysical approach to subsurface interface reconstruction of muon tomography measurements in high alpine regions

Muon tomography is an imaging technique that emerged in the last decades. The principal concept is similar to X-ray tomography, where one determines the spatial distribution of material densities by means of penetrating photons. It differs from this well-known technology only by the type of particle. Muons are continuously produced in the Earth’s atmosphere when primary cosmic rays (mostly protons) interact with the atmosphere’s molecules. Depending on their energies these muons can penetrate materials up to several hundreds of metres (or even kilometres). Consequently, they have been used for the imaging of larger objects, including large geological objects such as volcanoes, caves and fault systems. This research project aimed at applying this technology to an alpine glacier in Central Switzerland to determine its bedrock geometry, and if possible, to gain information on the bedrock erosion mechanism. To this end, two major experimental studies have been conducted with the aim to reconstruct bedrock geometries of two different glaciers. Given this framework, I present in this thesis my contribution to the project in which I worked for 5 years. Most of the technological know-how of muon tomography still lies within physics institutes who were the key drivers in the development of this method. As the geophysical/geological community is nowadays an important user of this technology, it is important that also non-physicists familiarise themselves with the theory and concepts behind muon tomography. This can be seen as an effective method to bring more geoscientists to utilize this new technology for their own research. The first part of this thesis is designed to tackle this problem with a review article on the principles of muon tomography and a guide to best practice. A second important aspect is the reconstruction of the bedrock topography given muon flux measurements at various locations. Many to-date reconstruction algorithms include supplementary geological information such as density and/or compositional measurements only on the side. A probabilistic framework was successfully set up that allows for such additional data to be included into the inversion. This may be used to better constrain the bedrock geometry. Moreover, this flexible framework allows also for the inclusion of modelling errors in the physical models which may result in a more reliable estimate of the mean and standard deviation of the bedrock position. The third article is concerned with the determination of the effect of rock composition on the muon flux measurements. Researchers in the community use a made-up rock, called “standard-rock” in their calculations. Hitherto, it was unclear in which geological settings this is a valid assumption and in which the induced error becomes too large. Simulations that use this fantasy rock are performed and compared to simulations that use a more realistic rock model. It was found that for felsic rocks the standard-rock approximation is valid over all thickness ranges, while for mafic rocks and limestones this can lead to a serious bias if the rock is thicker than 300m

Thermochemical Storage Conditions of Caldera Forming Magmatic Systems Revealed by Diffusion Chronometry

Large silicic magmatic systems are responsible for producing the largest explosive volcanic eruptions on earth. These phenomena, although infrequent (i.e., 1 per 100,000 years), impact the global climate, deposit ash over continent sized regions, and significantly alter landscapes. Silicic magmatism also plays important roles in the formation and ongoing evolution of continental crust. This makes understanding the processes that generate, transport, store, and erupt large volumes of silicic magma extremely important. In this dissertation, I present two case studies that explore the long-term thermal evolution of two caldera forming magmatic systems: Cerro Galán in NW Argentina and Toba in Sumatra. In them, I quantify the amount of time that the system experiences thermochemical conditions (i.e., temperatures > 750°C) sufficient to produce and store large volumes of eruptible magma (i.e., < ~50 % crystals and at a viscosity below the magma extrusion limit of 106 - 108 Pa·s). This eruptible time window, or thermal history, is recorded by the minerals (i.e., plagioclase) that crystallize within the reservoir and can be quantified by fitting forward models of trace element diffusion (i.e., Sr in plagioclase, Mg in plagioclase, Sr in hornblende) to the observed trace element profiles within the grain. We find that both the Cerro Galán and Toba magmatic systems experience relatively short time intervals (e.g., decades to centuries) during their overall history in which the thermal state of the magmatic system is sufficient to produce and store the large volumes of eruptible magma that constitute their eruptive products. This implies that these systems were dominantly stored in relatively cool, crystal-rich conditions and were only remobilized shortly before eruption. In addition, all the other processes that occur in the formation and evolution of large bodies of silicic magma – such as melting, recharge etc. also all need to fall within this “thermal budget”. This is also consistent with the dearth of geophysically resolved liquid-rich magma bodies under active volcanoes. A large part of the work described above uses the chemical variations within minerals to study magmatic processes, and by necessity much of this is done on 2D sections of minerals. However, the 3D distribution of elements in minerals is also important to evaluate, and thus the second part of this study investigates chemical zoning in sanidine, a common volcanic mineral, in 3D using micro computed tomography (microCT). We show that X-ray attenuation is largely a function of Ba concentration in sanidine and from this develop a method to quantify Ba zoning in 3D. We also show that this method can help mitigate problems in diffusion modeling caused by sectioning effects (i.e., apparent diffusion widths) by allowing for the true chemical gradient to be extracted from a 3D volume rather than interpreted from a random 2D section. Furthermore, we show that true zoning geometries can be observed in 3D, allowing for the interpretations that come from studying them to be more completely understood