Research within the frame of Experimental Mineralogy

 

Magmatic and volcanic systems at high pressure and temperature are not directly accessible to study their physicochemical properties and related kinetic processes. The Experimental Mineralogy provides tools to investigate these systems at controlled lab conditions, e.g. determination of phase relations, solubility and diffusion of volatile gas components, and degassing processes. The experimental results are crucial for the understanding of magma properties and dynamics in the Earth’s interior, during volcanic eruptions at the interface of the geosphere, hydrosphere and atmosphere up to the interaction of molten volcanic ash with turbine blades in jet engines.

 

Properties and dynamics of ascending magmatic systems

Degassing of H2O saturated silicate melt significantly affects the dynamics of magma ascent and volcanic eruptions. The experimental investigation of ascending H2O-rich magma during controlled decompression is essential for the understanding of H2O fluid-silicate melt phase separation, degassing and at the end for the interpretation of textures and volatile contents recorded in rocks and ashes of high risk volcanic systems like Vesuvius and Campi Flegrei. These investigations provide useful tools for volcanic hazard assessment.

 

Physicochemical properties of volatile-bearing magma

The significant compositional variations of gases released during active volcanic phases are poorly understood. Consequently, the modelling of such processes are not well constrained. The Mount Etna is an ideal reference system for experimental studies because the petrology and the released gas components are already investigated and monitored in detail. The experimental determination of volatile partitioning between basaltic melts and complex H-O-S-Cl – fluids as function of pressure and temperature are a prerequisite for the geochemical modelling of basaltic melt degassing. Other major focuses are the determination of diffusivity and diffusion mechanisms of volatiles and the spectroscopic investigations of incorporation mechanisms of volatile components in silicate melts at high temperature and pressure.

 

Phase relations in magmatic systems of large igneous provinces and layered intrusions

The formation of large igneous provinces in Earth’s history is often considered synchronous with crises in global climate and mass extinctions due to massive volatile release into the atmosphere and hydrosphere. Besides the effect of bulk chemical composition, dissolved volatiles and oxygen fugacity play a fundamental role on phase relations and thus on the magmatic differentiation and degassing trends. The determination of pre-eruptive conditions of the magma chambers of large igneous province basalts by a comparison of the chemistry of natural rocks with results from volatile solubility and phase equilibria experiments at high pressure and temperature is essential to estimate the volatile budget and the degassing potential of large igneous provinces.

Highly fractionated peralkaline iron rich mantle derived melts of the Ilímaussaq magmatic complex (South Greenland) show an unusually high crystallization interval of more than 500 °C. To follow up the liquid line of descent, fractional crystallization is simulated experimentally at highly reduced conditions using multi-step experiments in graphite capsules to reproduce the different stages of magmatic phase assemblages. These data are important to model the evolution of melt composition and the enrichment of rare Earth elements during the formation of complex magmatic layered intrusions.

 

Transition metals in silicate melts and crystalline phases

Transition metals are important compatible trace elements in magmatic systems. The mineral melt partitioning is crucial for understanding major Earth-forming processes such as the evolution of the mantle and the crust. Spectroscopic studies reveal that the most abundant volatile H2O influences the coordination environment of transition elements. The formation of hydration shells around transition elements significantly affect partitioning of these elements between silicate melts, minerals and fluids by two orders of magnitude. This would be sufficient to convert transition metals from compatible into slightly incompatible elements during magmatic fractionation.

The high pressure behavior of chromous orthosilicate (Cr2SiO4) has been studied by means of electronic absorption spectroscopy and X-ray diffraction in a diamond anvil cell. The experimental data indicate that at high pressure the chromium cations form dimers with a weak metal-metal bond in the silicate structure. It is highly probable that multiple metal-metal bonding and dimerization stabilize transition metal species in silicate crystal- and melt-structures at high pressures and temperatures. This surprising result provides important geochemical constraints for geochemical fractionation of transition metals during the early evolution of the Earth.

 

Volcanic ash deposition in gas turbines and implications for jet engine safety

The protracted grounding of commercial aircraft throughout Europe due to the Eyjafjallajökull volcanic eruption in Iceland April 2010, has alerted the public to the potential dangers of aircraft encounters with ash clouds. One of the most serious issues is the failure of jet turbines due to the deposition of molten silicate ash particles on hot turbine components. In an experimental study, we highlight the influence of volcanic ash composition, crystal/glass ratio and resulting bulk viscosity on the interaction of ash particles with hot turbine blades and vanes. A range of volcanic materials are used to simulate ash melting during transport through the combustor and deposition on a turbine blade of nickel superalloy material commonly used for the hot components in jet engines. The results show how ‘on-blade’ accumulation of molten particles can lead to efficient adhesion (wetting) and subsequent rapid accumulation of further molten material in some circumstances. In other cases particles form a cinder-like layer or entirely bounce off the blade. Any deposits will disrupt the air flow in the turbine, clog the cooling system and eventually cause the engine to stall. However, the cinder deposits can be removed in our experiments (as well as ‘in-flight’ for a real engine) by shutting off the heat source, allowing the deposit to quench and dislodge by thermal stress cracking. However, this currently recommended airplane safety procedure will not work for more basaltic melts which wet the blade surface more efficiently. Our experiments demonstrate how the nature of the incoming ash particle strongly influences the type of deposit formed, the important parameters being bulk ash composition, crystal proportion and particle size.

 

Improvement of high-pressure high-temperature experimental techniques

A bunch of experimental high-pressure high-temperature techniques at the limit of technical and material feasibility are necessary to investigate a wide spectrum of mineralogical and geological questions, ranging from magmatic differentiation, magma ascent and degassing, explosive volcanic eruptions and carbon-dioxide sequestration in potential host rocks at the interlocking of biosphere, hydrosphere and geosphere. Thus, the successful development of experimental high-pressure high-temperature apparatuses are a prerequisite to simulate extreme pressure and temperature conditions, ranging from the development of a sapphire anvil cell for in situ spectroscopic studies of hydrous silicate melt to improvements of pressure vessels with rapid quench facilities, control of oxygen fugacity in pressure vessels, very high temperature gas pressure vessel furnaces up to 1500 °C and a high-pressure low-flow metering valve for controlled pressure release. An olivine single crystal capsule technique was successfully established to run experiments with sulfur saturated basaltic melts at temperatures > 1050 °C and high pressure. Furthermore, an autoclave system was constructed for investigating carbon isotope fractionation between CO2 gas and aqueous fluid at elevated pressures and temperatures. Gas- and fluid-phase can be sampled during the experiments for carbon isotope analysis. A flow through autoclave was constructed and run successfully for the investigation of CO2-fluid-rock interaction at elevated pressures and temperatures.

Related publications are listed here: uni-tuebingen.de/fakultaeten/mathematisch-naturwissenschaftliche-fakultaet/fachbereiche/geowissenschaften/arbeitsgruppen-kontakte/mineralogie-geodynamik/forschungsbereich/experimentelle-mineralogie/arbeitsgruppe/publikationen/

 

Some Research Perspectives

The fundamental goals of the working group “Experimental Mineralogy” are to gain detailed insights into dynamic processes of the Earth’s interior that have led to the present shape of the Earth and that still have effects on our habitat. This research provides several powerful tools for investigating composition, structure and dynamic processes of the Earth’s interior at high temperatures and pressures. The research is motivated to understand the interaction of fluids and volatiles (e.g. H2O, CO2, SO2/H2S, Cl, noble gases) with the geosphere to reconstruct eruptive processes from volatile distribution in volcanic glasses. These investigations are not only of local interest, e. g. estimation of endangering potential of population living nearby active volcanoes, they are also of global importance, e. g. to understand the climatic impacts of large igneous provinces and super-eruptions in Earth’s history.

A precondition for the understanding and modeling of processes relevant for magma ascent and volcanic eruptions is a combined knowledge of the kinetic and thermodynamic properties of magma at elevated pressures and temperatures. Degassing and crystallization of magma significantly influence rheological properties and thus fragmentation and eruption style. The piezoactor driven high-pressure low-flow metering valve coupled to a gas pressure vessel constructed by our group facilitates controlled continuous decompression experiments to investigate degassing and crystallization processes comparable to nature.

 

Fluid-melt-rock interactions: from micron- to global-scale

Fluid-rock interactions on a micron-scale trigger macroscopic metamorphic processes and, at sufficiently high temperatures, induce intense magma formation in the Earth’s crust and upper mantle. Fluid-mineral-melt interactions control magma ascent and volcanic eruptions to a great extent.

For given pressure, temperature and composition conditions, thermodynamics predict the macroscopic state of an equilibrated system. However, regardless of scale (e. g. regional or microscopic), fluid-melt-rock interactions are controlled by a number of dynamic nano- and micron-scale processes, including (1) solution/precipitation, (2) recrystallization, (3) surface and grain boundary diffusion and (4) volume diffusion. Time and geometry have to be considered to investigate the reaction kinetics of fluid-melt-rock interaction processes. Chemical and isotopic heterogeneities are a powerful source of information on the conditions and temporal history of fluid-melt-rock interaction, providing crucial evidence of processes and rates that impact mass transfer up to the global scale. However, to properly interpret this information, we have to understand the complex interactions among the different processes, mechanisms and rates.

Despite significant effort during the last decades, the understanding of the interaction among fluids, melts and minerals on a micron-scale is still challenging. The basic parameters which control reaction of minerals, de-volatilization, melt formation, fluid dissolution and volatile degassing in dynamic systems like subduction zones or large igneous provinces with sources deep in the Earth’s mantle, which are important parts of geochemical cycles, are still rare. Thus, it would be challenging to link experimentally determined fluid-mineral-melt interactions and geochemical analytical methods on the micron-scale to large scale geologic processes.

The research perspectives outlined above show that Experimental Mineralogy is an important interface to many research areas like petrology, geochemistry, geophysics, applied geology, crystallography, and materials sciences.