Thesis: Physical and chemical processes during the dehydration of subducting oceanic lithosphere: Reactive fluid flow under high-pressure conditions.
Thesis: Subduction and continental collision in the Lufilian Arc - Zambezi Belt orogen: A petrological, geochemical, and geochronological study of eclogites and whiteschists (Zambia).
Thesis: Metamorphic and geodynamic evolution of the southern Ashanti Belt (SW Ghana), in German; Mapping: Geological mapping of the Palaeozoic / Cenozoic units of the north-western Gulf of Suez (Egypt) in German.
Physical and chemical processes during the dehydration of subducting oceanic lithosphere (reactive fluid flow under high-pressure / low-temperature conditions).
During subduction, downgoing plates heat up and their hydrous minerals become progressively unstable and break down to produce water. It is now evident that the released fluids can reach the mantle wedge and cause melting. The most prominent metamorphic reaction occurring in subduction zones is the eclogitization of oceanic crust. It is commonly assumed that the density increase of this reaction makes plate subduction self-sustained. Seismic studies indicate that this transformation happens under non-equilibrium conditions and field evidence points to fluids as a trigger. The details of intra-slab fluid flow are, however, not yet understood. Field evidence points to not yet quantified relationships between reactions, fluid flow, seismicity and mobilization of both major and trace elements. I use a combined field and laboratory based approach to explore the interrelationships of these processes. I investigate how fluids migrate inside slabs (i.e. if fluid migration occurs mainly by distributed porous flow or by channelized fluid flow) and furthermore the interrelation between fluid flow and seismic slip.
Reactive fluid flow and transport of so-called fluid-immobile elements.
The presence of fluids in Earth's crust results in processes that enable significant element mobilization. Metasomatic changes in rocks due to open system reactive fluid flow often cause surprisingly high solubilities and transport rates of so-called fluid-immobile elements, such as HFSE (high field strength elements, e.g., Th, Nb, Ta, Ti) and the REE (rare earth elements). During metasomatism new minerals form which are more stable under the changing physical and chemical conditions. The associated element transport and mineral growth may finally result in economic ore mineralization. We are currently investigating how titanium, tantalum, and niobium (Ti-Nb-Ta) are mobilized and transported and how Nb-Ta are fractionated, even though they are regarded as geochemical identical twins. Metasomatic processes are also interesting with respect to thorium (Th), because this rather immobile element becomes redistributed and concentrated into accessory minerals, while the host rocks experience infiltration of highly oxidizing fluids.
Global chlorine-cycle: halogen concentrations and stable isotopes as tracer for fluid related processes.
The large recycling machines, such as subduction zones and spreading centres, are important for element exchange and transport on Earth. Even though the volatile element chlorine represents an important component of the fluids stored in subducting oceanic lithosphere, the behaviour of chlorine within subduction zones and during the formation of magmas in island arc, in plumes or at spreading ridges is not understood yet. This part of my research is mainly emphasised to establish a conservative trace for subduction zone fluids, i.e. halogen ratios and stable chlorine isotopes (delta37Cl), to identify different fluid sources and to quantify fluid-rock interaction and fluid fluxes. Therefore two main topics are currently in investigation, one is to define the Cl-isotopical signal of the main fluid sources of the deeper fluid cycle, i.e. serpentinite and blueschist dehydration; the other is to investigate how conservative the Cl-isotopes behave during fluid flow and fluid-rock in teraction. I will also try to find "fluid-composition-indicative" minerals to use those as a fluid-probe.
Evolution of craton margins during continent break up and continental collisions (reconstruction of supercontinents and the break up of Gondwana).
Geodynamic processes during orogenesis cause changes in pressure and temperature conditions (P-T conditions), which affect the rocks within an orogen. The reconstruction of the P-T evolution of a metamorphic rock allows to determine the geodynamic causes of metamorphism (e.g., subduction, tectonic thickening, extension during orogenic collapse or rifting, erosion, magmatic underplating). To be able to distinguish between the possible causes for metamorphism and to get a more complete understanding of geodynamic processes the knowledge of the age and the duration of metamorphic events, i.e. the P-T-t path, is essential. My current research is mainly focused on the geodynamic reconstruction of the Neoproterozoic to Cambrian orogens of south central Africa, which are related to the formation of the supercontinent Gondwana. I started to investigate the geodynamic evolution of the Tianshan melange zone (NW China) as a prime example for a cold subduction zone and the continetal margin of Ghana as a prime example for rifting processes along oceanic and continental transform margins.
2006 and before