Structure of the Southern Tyrrhenian subduction system: insights from seismological analysis of anisotropy and attenuation

The Southern Tyrrhenian subduction system shows a complex interaction among asthenospheric flow, subducting slab and overriding plate. To shed light on the deformations and mechanical properties of the slab and surrounding mantle, I investigated seismic anisotropy and attenuation properties through the subduction region. I used both teleseisms and slab earthquakes, analyzing shear-wave splitting on SKS and S phases, respectively. The fast polarization directions φ, and the delay time, δt, were retrieved using the method of Silver and Chan [1991. SKS and S φ reveal a complex anisotropy pattern across the subduction zone. SKS-rays sample primarily the sub-slab region showing rotation of fast directions following the curved shape of the slab and very strong anisotropy. S-rays sample mainly the slab, showing variable φ and a smaller δt. SKS and S splitting reveals a well developed toroidal flow at SW edge of the slab, while at its NE edge the pattern is not very clear. This suggests that the anisotropy is controlled by the slab rollback, responsible for about 100 km slab parallel φ in the sub-slab mantle. The slab is weakly anisotropic, suggesting the asthenosphere as main source of anisotropy. To investigate the physical properties of the slab and surrounding regions, I analyzed the seismic P and S wave attenuation. By inverting high-quality S-waves t* from slab earthquakes, 3D attenuation models down to 300 km were obtained. Attenuation results image the slab as low-attenuation body, but with heterogeneous QS and QP structure showing spot of high attenuation , between 100-200 km depth, which could be due dehydration associated to the slab metamorphism. A low QS anomaly is present in the mantle wedge beneath the Aeolian volcanic arc and could indicate mantle melting and slab dehydration.

Theoretical studies of microfluidics: "walking" drops and "melting" snowflakes.

The present PhD thesis summarizes two examples of research in microfluidics. Both times water was the subject of interest, once in the liquid state (droplets adsorbed on chemically functionalized surfaces), the other time in the solid state (ice snowflakes and their fractal behaviour). The first problem deals with a slipping nano-droplet of water adsorbed on a surface with photo-switchable wettability characteristics. Main focus was on identifying the underlying driving forces and mechanical principles at the molecular level of detail. Molecular Dynamics simulation was employed as investigative tool owing to its record of successfully describing the microscopic behaviour of liquids at interfaces. To reproduce the specialized surface on which a water droplet can effectively “walk”, a new implicit surface potential was developed. Applying this new method the experimentally observed droplet slippage could be reproduced successfully. Next the movement of the droplet was analyzed at various conditions emphasizing on the behaviour of the water molecules in contact with the surface. The main objective was to identify driving forces and molecular mechanisms underlying the slippage process. The second part of this thesis is concerned with theoretical studies of snowflake melting. In the present work snowflakes are represented by filled von Koch-like fractals of mesoscopic beads. A new algorithm has been developed from scratch to simulate the thermal collapse of fractal structures based on Monte Carlo and Random Walk Simulations (MCRWS). The developed method was applied and compared to Molecular Dynamics simulations regarding the melting of ice snowflake crystals and new parameters were derived from this comparison. Bigger snow-fractals were then studied looking at the time evolution at different temperatures again making use of the developed MCRWS method. This was accompanied by an in-depth analysis of fractal properties (border length and gyration radius) in order to shed light on the dynamics of the melting process.

Ophiolites of the eastern Vardar Zone, N. Greece

In the present thesis, the geochemistry, petrology and geochronology of ophiolite complexes from central northern Greece were studied in detail in order to gain insights on the petrogenetic pathways and geodynamic processes that lead to their formation and evolution. The major- and trace-element content of minerals and whole rocks from all four ophiolite complexes was determined using high-precision analytical equipment. These results were then coupled with Nd and Sr isotopic measurements. In order to precisely place the evolution of these ophiolites in time, U-Pb geochronology on zircons was conducted using a SHRIMP-II. The data obtained suggest that the ophiolites studied invariably show typical characteristics of subduction-zone magmatism (e.g. negative Nb anomalies, Th enrichment). In N-MORB-normalised multielement profiles the high field-strength elements display patterns that vary from depleted to N-MORB-like. Chondrite-normalised rare-earth element (REE) profiles show flat heavy-REE patterns suggesting a shallow regime of source melting for all the ophiolites, well within the stability field of spinel lherzolite. The majority of the samples have light-REE depleted patterns. 87Sr/86Sr isotopic ratios range from 0.703184 to 0.715853 and are in cases influenced by alteration. The εNd values are positive (the majority of the mafic samples is typically 7.1-3.1) but lower than N-MORB and depleted mantle. With the exception of the Thessaloniki ophiolite that has uniform island-arc tholeiitic chemical characteristics, the rest of the ophiolites show dual chemistry consisting of rocks with minor subduction-zone characteristics that resemble chemically back-arc basin basalts (BABB) and rocks with more pronounced subduction-zone characteristics. Tectonomagmatic discrimination schemes classify the samples as island-arc tholeiites and back-arc basin basalts or N-MORB. Melting modelling carried out to evaluate source properties and degree of melting verifies the dual nature of the ophiolites. The samples that resemble back-arc basin basalts require very small degrees of melting (<10%) of fertile sources, whereas the rest of the samples require higher degrees (25-15%) of melting. As deduced from the present geochemical and petrological investigation, the ophiolites from Guevguely, Oraeokastro, Thessaloniki, and Chalkidiki represent relics of supra-subduction zone crust that formed in succeeding stages of island-arc rifting and back-arc spreading as well as in a fore arc setting. The geochronological results have provided precise determination of the timing of formation of these complexes. The age of the Guevguely ophiolite has been determined as 167±1.2 Ma, that of Thessaloniki as 169±1.4 Ma, that of Kassandra as 167±2.2 Ma and that of Sithonia as 160±1.2 Ma.

Partial melting within the lower crust – Constraints from bulk rock geochemical data and trace element mineral analyses of granulites and migmatites from Central Finland

This PhD thesis concerns geochemical constraints on recycling and partial melting of Archean continental crust. A natural example of such processes was found in the Iisalmi area of Central Finland. The rocks from this area are Middle to Late Archean in age and experienced metamorphism and partial melting between 2.7-2.63 Ga. The work is based on extensive field work. It is furthermore founded on bulk rock geochemical data as well as in-situ analyses of minerals. All geochemical data were obtained at the Institute of Geosciences, University of Mainz using X-ray fluorescence, solution ICP-MS and laser ablation-ICP-MS for bulk rock geochemical analyses. Mineral analyses were accomplished by electron microprobe and laser ablation ICP-MS. Fluid inclusions were studied by microscope on a heating-freezing-stage at the Geoscience Center, University Göttingen. Part I focuses on the development of a new analytical method for bulk rock trace element determination by laser ablation-ICP-MS using homogeneous glasses fused from rock powder on an Iridium strip heater. This method is applicable for mafic rock samples whose melts have low viscosities and homogenize quickly at temperatures of ~1200°C. Highly viscous melts of felsic samples prevent melting and homogenization at comparable temperatures. Fusion of felsic samples can be enabled by addition of MgO to the rock powder and adjustment of melting temperature and melting duration to the rock composition. Advantages of the fusion method are low detection limits compared to XRF analyses and avoidance of wet-chemical processing and use of strong acids as in solution ICP-MS as well as smaller sample volumes compared to the other methods. Part II of the thesis uses bulk rock geochemical data and results from fluid inclusion studies for discrimination of melting processes observed in different rock types. Fluid inclusion studies demonstrate a major change in fluid composition from CO2-dominated fluids in granulites to aqueous fluids in TTG gneisses and amphibolites. Partial melts were generated in the dry, CO2-rich environment by dehydration melting reactions of amphibole which in addition to tonalitic melts produced the anhydrous mineral assemblages of granulites (grt + cpx + pl ± amph or opx + cpx + pl + amph). Trace element modeling showed that mafic granulites are residues of 10-30 % melt extraction from amphibolitic precursor rocks. The maximum degree of melting in intermediate granulites was ~10 % as inferred from modal abundances of amphibole, clinopyroxene and orthopyroxene. Carbonic inclusions are absent in upper-amphibolite facies migmatites whereas aqueous inclusion with up to 20 wt% NaCl are abundant. This suggests that melting within TTG gneisses and amphibolites took place in the presence of an aqueous fluid phase that enabled melting at the wet solidus at temperatures of 700-750°C. The strong disruption of pre-metamorphic structures in some outcrops suggests that the maximum amount of melt in TTG gneisses was ~25 vol%. The presence of leucosomes in all rock types is taken as the principle evidence for melt formation. However, mineralogical appearance as well as major and trace element composition of many leucosomes imply that leucosomes seldom represent frozen in-situ melts. They are better considered as remnants of the melt channel network, e.g. ways on which melts escaped from the system. Part III of the thesis describes how analyses of minerals from a specific rock type (granulite) can be used to determine partition coefficients between different minerals and between minerals and melt suitable for lower crustal conditions. The trace element analyses by laser ablation-ICP-MS show coherent distribution among the principal mineral phases independent of rock composition. REE contents in amphibole are about 3 times higher than REE contents in clinopyroxene from the same sample. This consistency has to be taken into consideration in models of lower crustal melting where amphibole is replaced by clinopyroxene in the course of melting. A lack of equilibrium is observed between matrix clinopyroxene / amphibole and garnet porphyroblasts which suggests a late stage growth of garnet and slow diffusion and equilibration of the REE during metamorphism. The data provide a first set of distribution coefficients of the transition metals (Sc, V, Cr, Ni) in the lower crust. In addition, analyses of ilmenite and apatite demonstrate the strong influence of accessory phases on trace element distribution. Apatite contains high amounts of REE and Sr while ilmenite incorporates about 20-30 times higher amounts of Nb and Ta than amphibole. Furthermore, trace element mineral analyses provide evidence for magmatic processes such as melt depletion, melt segregation, accumulation and fractionation as well as metasomatism having operated in this high-grade anatectic area.

Deciphering petrological signatures of reactive melt stagnation and cooling in the oceanic mantle underneath ultraslow-spreading ridges

The global mid-ocean ridge system creates oceanic crust and lithosphere that covers more than two-thirds of the Earth. Basalts are volumetrically the most important rock type sampled at mid-ocean ridges. For this reason, our present understanding of upper mantle dynamics and the chemical evolution of the earth is strongly influenced by the study of mid-ocean ridge basalts (MORB). However, MORB are aggregates of polybarically generated small melt increments that can undergo a variety of physical and chemical processes during their ascent and consequently affect their derivative geochemical composition. Therefore, MORB do not represent “direct” windows to the underlying upper mantle. Abyssal peridotites, upper mantle rocks recovered from the ocean floor, are the residual complement to MORB melting and provide essential information on melt extraction from the upper mantle. In this study, abyssal peridotites are examined to address these overarching questions posed by previous studies of MORB: How are basaltic melts formed in the mantle, how are they extracted from the mantle and what physical and chemical processes control mantle melting? The number of studies on abyssal peridotites is small compared to those on basalts, in part because seafloor exposures of abyssal peridotites are relatively rare. For this reason, abyssal peridotite characteristics need to be considered in the context of subaerially exposed peridotites associated with ophiolites, orogenic peridotite bodies and basalt-hosted xenoliths. However, orogenic peridotite bodies are mainly associated with passive continental margins, most ophiolites are formed in supra-subduction zone settings, and peridotite xenoliths are often contaminated by their host magma. Therefore, studies of abyssal peridotites are essential to understanding the primary characteristics of the oceanic upper mantle free from the influence of continental rifting, subduction and tectonic emplacement processes. Nevertheless, numerous processes such as melt stagnation and cooling-induced, inter-mineral exchange can affect residual abyssal peridotite compositions after the cessation of melting. The aim of this study is to address these post-melting modifications of abyssal peridotites from a petrological-geochemical perspective. The samples in this study were dredged along the axis of the ultraslow-spreading Gakkel Ridge in the Arctic Ocean within the “Sparsely Magmatic Zone”, a 100 km ridge section where only mantle rocks are exposed. During two expeditions (ARK XVII-2 in 2001 and ARK XX-2 in 2004), exceptionally fresh peridotites were recovered. The boulders and cobbles collected cover a range of mantle rock compositions, with most characterized as plagioclase-free spinel peridotites or plagioclase- spinel peridotites. This thesis investigates melt stagnation and cooling processes in the upper mantle and is divided into two parts. The first part focuses on processes in the stability field of spinel peridotites (>10 kb) such as melt refertilization and cooling related trace element exchange, while the second part investigates processes in the stability field of plagioclase peridotites (< 10 kb) such as reactive melt migration and melt stagnation. The dissertation chapters are organized to follow the theoretical ascent of a mantle parcel upwelling beneath the location where the samples were collected.

UHP metamorphic rocks of the Eastern Rhodope Massif, NE Greece: new constraints from petrology, geochemistry and zircon ages

In this PhD thesis, a multidisciplinary study has been carried out on metagranitoids and paragneisses from the Eastern Rhodope Massif, northern Greece, to decipher the pre-Alpine magmatic and geodynamic evolution of the Rhodope Massif and to correlate the eastern part with the western/central parts of the orogen. The Rhodope Massif, which occupies the major part of NE Greece and S Bulgaria, represents the easternmost part of the Internal Hellenides. It is regarded as a nappe stack of high-grade units, which is classically subdivided into an upper unit and a lower unit, separated by a SSE-NNW trending thrust plane, the Nestos thrust. Recent research in the central Greek Rhodope Massif revealed that the two units correspond to two distinct terranes of different age, the Permo-Carboniferous Thracia Terrane, which was overthrusted by the Late Jurassic/Early Cretaceous Rhodope Terrane. These terranes are separated by the Nestos suture, a composite zone comprising metapelites, metabasites, metagranitoids and marbles, which record high-pressure and even ultrahigh-pressure metamorphism in places. Similar characteristic rock associations were investigated during this study along several well-constrained cross sections in vincity to the Ada, Sidiro and Kimi villages in the Greek Eastern Rhodope Massif. Field evidence revealed that the contact zone of the two terranes in the Eastern Rhodope Massif is characterized by a mélange of metapelites, migmatitic amphibolites/eclogites, strongly sheared orthogneisses and marbles. The systematical occurrence of this characteristic rock association between the terranes implies that the Nestos suture is a continuous belt throughout the Greek Rhodope Massif. In this study, a new UHP locality could be established and for the first time in the Greek Rhodope, metamorphic microdiamonds were identified in situ in their host zircons using Laser-Raman spectroscopy. The presence of the diamonds as well as element distribution patterns of the zircons, obtained by TOF-SIMS, indicate metamorphic conditions of T > 1000 °C and P > 4 GPa. The high-pressure and ultrahigh-pressure rocks of the mélange zone are considered to have formed during the subduction of the Nestos Ocean in Jurassic times at ~150 Ma. Melting of metapelitic rocks at UHP conditions facilitated the exhumation to lower crustal levels. To identify major crust forming events, basement granitoids were dated by LA-SF-ICPMS and SHRIMP-II U-Pb analyses of zircons. The geochronological results revealed that the Eastern Rhodope Massif consists of two crustal units, a structurally lower Permo-Carboniferous unit corresponding to the Thracia Terrane and a structurally upper Late Jurassic/Early Cretaceous unit corresponding to the Rhodope Terrane, like it was documented for the Central Rhodope Massif. Inherited zircons in the orthogneisses from the Thracia Terrane of the Eastern Rhodope Massif indicate the presence of a pre-existing Neoproterozoic and Ordovician-Silurian basement in this region. Triassic magmatism is witnessed by the zircons of few orthogneisses from the easternmost Rhodope Massif and is interpreted to be related to rifting processes. Whole-rock major and trace element analyses indicate that the metagranitoids from both terranes originated in a subduction-related magmatic-arc environment. The Sr-Nd isotope data for both terranes of the Eastern and Central Rhodope Massif suggest a mixed crust-mantle source with variable contributions of older crustal material as already indicated by the presence of inherited zircons. Geochemical and isotopic similarity of the basement of the Thracia Terrane and the Pelagonian Zone implies that the Thracia Terrane is a fragment of a formerly unique Permo-Carboniferous basement, separated by rifting and opening of the Meliata-Maliac ocean system in Triassic times. A branch of the Meliata-Maliac ocean system, the Nestos Ocean, subducted northwards in Late Jurassic times leading to the formation of the Late Jurassic/Early Cretaceous Rhodope magmatic arc on remnants of the Thracia Terrane as suggested by inherited Permo-Carboniferous zircons. The ~150 Ma zircon ages of the orthogneisses from the Rhodope Terrane indicate that subduction-related magmatism and HP/UHP metamorphism occurred during the same subduction phase. Subduction ceased due to the closure of the Nestos Ocean in the Late Jurassic/Early Cretaceous. The post-Jurassic evolution of the Rhodope Massif is characterized by the exhumation of the Rhodope core complex in the course of extensional tectonics associated with late granite intrusions in Eocene to Miocene times.

Cut-first branch-and-price-second for the capacitated arc-routing problem

This paper presents the first full-fledged branch-and-price (bap) algorithm for the capacitated arc-routing problem (CARP). Prior exact solution techniques either rely on cutting planes or the transformation of the CARP into a node-routing problem. The drawbacks are either models with inherent symmetry, dense underlying networks, or a formulation where edge flows in a potential solution do not allow the reconstruction of unique CARP tours. The proposed algorithm circumvents all these drawbacks by taking the beneficial ingredients from existing CARP methods and combining them in a new way. The first step is the solution of the one-index formulation of the CARP in order to produce strong cuts and an excellent lower bound. It is known that this bound is typically stronger than relaxations of a pure set-partitioning CARP model.rnSuch a set-partitioning master program results from a Dantzig-Wolfe decomposition. In the second phase, the master program is initialized with the strong cuts, CARP tours are iteratively generated by a pricing procedure, and branching is required to produce integer solutions. This is a cut-first bap-second algorithm and its main function is, in fact, the splitting of edge flows into unique CARP tours.

Granulite facies metamorphism and partial melting processes in the Ivrea Zone, Northern Italy

The Ivrea Zone in northern Italy has been the focus of numerous petrological, geochemical and structural studies. It is commonly inferred to represent an almost complete section through the mid to lower continental crust, in which metamorphism and partial melting of the abundant metapelites was the result of magmatic underplating by a large volume of mantle-derived magma. This study concerns amphibolite and granulite facies metamorphism in the Ivrea Zone with focus on metapelites and metapsammites/metagreywackes from Val Strona di Omegna and metapelites from Val Sesia and Val Strona di Postua, with the aim to better constrain their metamorphic evolution as well as their pressure and temperature conditions via phase equilibria modelling.rnrnIn Val Strona di Omegna, the metapelites show a structural and mineralogical change from mica-schists with the common assemblage bi-mu-sill-pl-q-ilm ± liq at the lowest grades, through metatexitic migmatites (g-sill-bi-ksp-pl-q-ilm-liq) at intermediate grades, to complex diatexitic migmatites (g-sill-ru-bi-ksp-pl-q-ilm-liq) at the highest grades. Within this section several mappable isograds occur, including the first appearance of K-feldspar in the metapelites, the first appearance of orthopyroxene in the metabasites and the disappearance of prograde biotite from the metapelites. The inferred onset of partial melting in the metapelites occurs around Massiola. The prograde suprasolidus evolution of the metapelites is consistent with melting via the breakdown of first muscovite then biotite. Maximum modelled melt fractions of 30–40 % are predicted at the highest grade. The regional metamorphic field gradient in Val Strona di Omegna is constrained to range from conditions of 3.5–6.5 kbar at T = 650–730 °C to P &gt; 9 kbar at T &gt; 900 °C. The peak P–T estimates, particularly for granulite facies conditions, are significantly higher (around 100 °C) than those of most previous studies. In Val Sesia and Val Strona di Postua to the south the exposure is more restricted. P–T estimates for the metapelites are 750–850 °C and 5–6.5 kbar in Val Sesia and approximately 800–900 °C and 5.5–7 kbar in Val Strona di Postua. These results show similar temperatures but lower pressure than metapelites in Val Strona di Omegna. Metapelites in Val Sesia in contact with the Mafic Complex exhibit a metatexitic structure, while in Val Strona di Postua diatexitic structures occur. Further, metapelites at the contact with the Mafic Complex contain cordierite (± spinel) that overprint the regional metamorphic assemblages and are interpreted to have formed during contact metamorphism related to intrusion of the Mafic Complex. The lower pressures in the high-grade rocks in Val Sesia and Val Strona di Postua are consistent with some decompression from the regional metamorphic peak prior to the intrusion of the Mafic Complex, suggesting the rocks followed a clockwise P–T path. In contrast, the metapelites in Val Strona di Omegna, especially in the granulite facies, do not contain any cordierite or any evidence for a contact metamorphic overprint. The extrapolated granulite facies mineral isograds are cut by the rocks of the Mafic Complex to the south. Therefore, the Mafic Complex cannot have caused the regional metamorphism and it is unlikely that the Mafic Complex occurs in Val Strona di Omegna.

Modelling, diagnostics and experimental analysis of plasma assisted processes for material treatment

This work presents results from experimental investigations of several different atmospheric pressure plasmas applications, such as Metal Inert Gas (MIG) welding and Plasma Arc Cutting (PAC) and Welding (PAW) sources, as well as Inductively Coupled Plasma (ICP) torches. The main diagnostic tool that has been used is High Speed Imaging (HSI), often assisted by Schlieren imaging to analyse non-visible phenomena. Furthermore, starting from thermo-fluid-dynamic models developed by the University of Bologna group, such plasma processes have been studied also with new advanced models, focusing for instance on the interaction between a melting metal wire and a plasma, or considering non-equilibrium phenomena for diagnostics of plasma arcs. Additionally, the experimental diagnostic tools that have been developed for industrial thermal plasmas have been used also for the characterization of innovative low temperature atmospheric pressure non equilibrium plasmas, such as dielectric barrier discharges (DBD) and Plasma Jets. These sources are controlled by few kV voltage pulses with pulse rise time of few nanoseconds to avoid the formation of a plasma arc, with interesting applications in surface functionalization of thermosensitive materials. In order to investigate also bio-medical applications of thermal plasma, a self-developed quenching device has been connected to an ICP torch. Such device has allowed inactivation of several kinds of bacteria spread on petri dishes, by keeping the substrate temperature lower than 40 degrees, which is a strict requirement in order to allow the treatment of living tissues.

On arc-routing problems

Das Basisproblem von Arc-Routing Problemen mit mehreren Fahrzeugen ist das Capacitated Arc-Routing Problem (CARP). Praktische Anwendungen des CARP sind z.B. in den Bereichen Müllabfuhr und Briefzustellung zu finden. Das Ziel ist es, einen kostenminimalen Tourenplan zu berechnen, bei dem alle erforderlichen Kanten bedient werden und gleichzeitig die Fahrzeugkapazität eingehalten wird. In der vorliegenden Arbeit wird ein Cut-First Branch-and-Price Second Verfahren entwickelt. In der ersten Phase werden Schnittebenen generiert, die dem Master Problem in der zweiten Phase hinzugefügt werden. Das Subproblem ist ein kürzeste Wege Problem mit Ressourcen und wird gelöst um neue Spalten für das Master Problem zu liefern. Ganzzahlige CARP Lösungen werden durch ein neues hierarchisches Branching-Schema garantiert. Umfassende Rechenstudien zeigen die Effektivität dieses Algorithmus. Kombinierte Standort- und Arc-Routing Probleme ermöglichen eine realistischere Modellierung von Zustellvarianten bei der Briefzustellung. In dieser Arbeit werden jeweils zwei mathematische Modelle für Park and Loop und Park and Loop with Curbline vorgestellt. Die Modelle für das jeweilige Problem unterscheiden sich darin, wie zulässige Transfer Routen modelliert werden. Während der erste Modelltyp Subtour-Eliminationsbedingungen verwendet, werden bei dem zweiten Modelltyp Flussvariablen und Flusserhaltungsbedingungen eingesetzt. Die Rechenstudie zeigt, dass ein MIP-Solver den zweiten Modelltyp oft in kürzerer Rechenzeit lösen kann oder bei Erreichen des Zeitlimits bessere Zielfunktionswerte liefert.