Forschung

GMG: Mineralogie - Petrologie

Diffusion of Si and O in silicate minerals and glasses (SFB E1)

Silicon and oxygen build up the basic structural framework of silicates - in the crystalline as well as the molten and amorphous states. As a result, the response of these materials to a deforming force i.e. the rheological behavior depend to a large extent on how the strong silicon-oxygen bonds are broken and re-built, mediated by defect structures. One approach to study these processes is through the study of diffusion rates of Si and O in silicates as a function of various intensive variables. We have developed the technique of production of nanoscale thin films of complex silicates using pulsed laser deposition to address aspects of this problem that were inaccessible before.

We have been able to characterize the rheological behavior of high pressure phases that occur in the transition zone in the mantle (wadsleyite, ringwoodite), the effect of water on the rheological behavior of mantle minerals such as olivine and the flow behavior of melts at high pressures using this approach.
Collaborators: R. Dohmen, S. Chakraborty (Bochum), Hans-Werner Becker, S. Borinski (Bochum), P. Sengupta (Mumbai), A. Shimojuku and T. Kubo (Fukuoka),T. Katsura (Misasa), J. Craven (Edinburgh).

Diffusion of Cations in minerals for applications in Geospeedometry

We use the "diffusion clock" to determine the duration of a variety of terrestrial and extra terrestrial processes. The application of these clocks requires the knowledge of diffusion coefficients of different elements in various minerals and melts as a function of intensive variables such as temperature, pressure, composition as well as oxygen and water fugacities. Miniaturization to nanoscales (tools: pulsed laser deposition of thin films, Rutherford Backscattering Spectroscopy) now allows these measurements to be carried out at conditions at which the relevant processes occur in nature. This makes extrapolations, a major source of error in the past, unnecessary. Our current studies involve the minerals garnet, pyroxene, olivines and plagioclases.
Collaborators: Hans-Werner Becker, S. Chakraborty, Thomas Müller, Patrick Remmert, Sascha Borinski, Ralf Dohmen (Bochum), Stefanie Duchene (Nancy), J.Ganguly (Arizona)

Reaction mechanism of minerals

Novel experiments using a double capsule technique, in combination with theoretical tools developed by us, allow us to understand the mechanism of element and isotopic exchange reactions in mineral solid solution systems. Such reactions form the basis of various forms of geothermometry. We have shown earlier that mineral reactions can occur by many mechanisms other than simple diffusion- or surface controlled processes. In our most recent experiments we demonstrate that slight changes in the solid solution composition of the reacting mineral can shift the reaction mechanism from a diffusion controlled one to a dissolution – precipitation controlled one.
Collaborators: S. Chakraborty, Thomas Müller and R. Dohmen (Bochum).

Diffusion mechanism in Glasses and melts

Diffusion rates of cations in silicate melts and glasses are related to other physical properties such as viscosity and electrical conductivity. However, the quantitative relationship varies with melt compositions. It is necessary to understand these in order to be able to predict transport rates at a wide range of compositions and conditions (e.g. for different natural magmas) as well as to gain a better grasp on Materials Science applications related to glass transition and relaxation behavior. We are carrying out experiments at controlled conditions to address these issues.
Collaborator: P. Sengupta (Mumbai), S. Chakraborty and S. Fanara (Bochum)

Development of diffusion barriers for Superalloys

Alloys of Ni and other metals have many properties that make them attractive for a variety of applications at extreme conditions. However, lifetimes of metallic furnaces used to contain corrosive melts (silicates, borates) at high temperatures are shortened by reaction with the melts. Similar problems arise in a variety of applications (e.g. nuclear waste disposal), making it desirable to develop diffusion barriers that may protect the metals from corrosion. We are developing novel thin films of ceramic materials, bonded onto the metal surface by pulsed laser deposition, to produce such barriers.
Collaborator: P. Sengupta (Mumbai) and S. Chakraborty (Bochum)

Phase-relations in the system MgO-Al₂O₃-SiO₂-H₂O (MASH) at high pressures

The well-known high-pressure rocks of the Dora Maira Massif (Italy, Western Alps) serve as an analogue for comparing the phase assemblages found in natural rocks with those from experiments done in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). In this project we want to determine the stability fields of the ternary phases of this system up to pressures of 10 GPa in the presence of water. The results contribute toward an understanding of the phase relations in this simple system and those related to it.
Collaborator: T. Fockenberg (Bochum)
Picture: Scanning-electron microscope image of synthetic chlorite crystals. Their shape is platy showing a pseudo-hexagonal habit. Run conditions: 25 kbar, 600°C, 15 days; composition of the starting gel was ideal clinochlore.

Pelites at ultra-high-pressure conditions: An experimental approach

The chemical system K2O-MgO-Al2O3-SiO2-H2O (KMASH) serves as the simplest system to study pelitic rocks. During high-pressure metamorphism the stable phases may be so dense that they become even denser than rocks of basaltic compositions. This may cause the break-off of subducted Si-Al-rich rocks into the Earth´s mantle during the collision of continental plates. Experiments are conducted up to 10 GPa using synthetic materials of different composition to clarify the phase relations at those unusual conditions. The results are expected to serve as a basis for computational simulations.
Collaborators:M.Janitschke and T. Fockenberg (Bochum)

Development of codes and models for diffusive exchange and transport in multiphase materials (e.g. rocks) at high pressures and temperatures (e.g. igneous and metamorphic processes on the Earth and other planetary objects).

Currently, a number of projects are in progress to theoretically analyze kinetic processes involving minerals and to develop numerical codes for various applications. Some of these include

• Development of a reaction mechanism map for reactions that involve simultaneous net transfer and element / isotope exchange.


• Development of an user friendly code for carrying out geospeedometry using garnets. However, instead of a constant cooling rate, more complex thermal histories can be retrieved by modelling zoning profiles using this code.


• Calculation of disequilibrium partitioning of trace elements during partial melting / crystallization events.


• Multicomponent mixing of magmas

Collaborators: Sascha Borinski, Ulrich Hoppe (Bochum), Ed Bolton (Yale), S. Chakraborty, R. Dohmen (Bochum)

Himalaya

The well known tectonic framework (collision of India and Asia) and recent age (lack of overprinting events) of the Himalayan mountain chain makes it an ideal natural laboratory to study a variety of processes. At the same time, understanding processes that occur at depth is crucial for proper planning and natural resource management in this densely populated, active tectonic region. We have been pursuing research projects in two segments of the Indian Himalaya in the states of Sikkim and Himachal Pradesh (in particular the Sutlej valley section):

• Higher Himalayan Crystallines – timing and duration of melting:

We are studying the nature of partial melting and the conditions at which these occur. Using this information as basis, we are trying to constrain the timing and duration of the melting events and rates of exhumation of the partially molten rocks.

Methods: Thin section petrography, conventional thermobarometry, pseudosection calculations using minimization of Gibbs Free Energy, geospeedometry using diffusion modeling of trace and major elements, in situ dating of monazite and zircon using SHRIMP, spatially resolved dating of garnets using Sm-Nd and Lu-Hf.

The results are important for answering a number of different questions:

(i) How were these High Himalayan rocks exhumed? What role, if any, did the process of melting play in the exhumation?

(ii) To what extent and how does channel flow tectonics operate in the Himalaya?

(iii) What is the mechanics of partial melting and melt segregation in a tectonically well constrained crustal section?

Collaborations: Prof. S. Dasgupta, Prof. D. Mukhopadhyay, Nilanjana Sarkar (Kolkata and Roorkee, India); Dr. D. Rubatto (Canberra, Australia), Dr. R.Anzkiewicz (Krakow, Poland), T. Gerya, M. Faccenda (Zürich).

• Lesser Himlayan Sequence – nature and rate of inverted metamorphism:

One of the most intriguing, but persistent, features observed all along the Himalayan range is the so called inverted metamorphism – rocks metamorphosed to higher grades occurring at structurally higher levels. The cause of this inversion remains debated. However, the persistence of this feature indicates that understanding the origin of this feature is key to the understanding of the mountain belt itself. Such inverted sequences have been reported from other tectonic regions as well, so that this is not a peculiarity of the Himalayan range. We are trying to constrain the pressure –temperature histories followed by the rocks in the sequence as well as the timing and duration of metamorphism to address this problem.

At the same time, rocks of the inverted sequence represent some of the most complete Barrovian sequences known, so that studies of this sequence can help us to understand fundamental aspects of metamorphism such as nucleation and growth of minerals, the role of fluids etc.
Collaborations: Prof. S. Dasgupta, Prof. D. Mukhopadhyay, Nilanjana Sarkar (Kolkata and Roorkee, India); Dr. D. Rubatto (Canberra, Australia), Dr. R.Anzkiewicz (Krakow, Poland), T. Gerya, M. Faccenda (Zürich), Dr. F. Gaidies (Ottawa, Canada), S. Chakraborty (Bochum).

Genese der Jadeitite und Ihrer Rahmengesteine, Rio San Juan Komplex, Dominikanische Republik

Der Begriff Jade beschreibt dichte, schleifwürdige, fast monomineralische Gesteine, die entweder aus überwiegend Amphibol (Nephrit) oder Pyroxen (Jadeitit) bestehen. Jade wird seit 7000 Jahren als „Kulturstein“ benutzt und ist deshalb auch von großem kulturhistorischem Interesse, sowohl in der Alten als auch in der Neuen Welt (z.B. Harlow & Sorensen, 2005). Jadeitite sind relativ selten und z.Z. nur von ca. 15 Lokalitäten weltweit bekannt. Dieses Projekt befasst sich ausschließlich mit Jade aus dem Mineral Jadeit, also mit Jadeititen, die in der Dominikanischen Republik in einer Serpentinitmélange vorkommen, dort aber auch in Kontakt zu Lawsonit-Blauschiefern als direktem Nebengestein.
Die Entstehung von Jadeititen wird von manchen Autoren (z.B. Bleeck, 1907; Chhibber, 1934; Dobretsov & Ponomareva, 1865; Compagnoni & Rolfo, 2003, Compagnoni et al., 2007; Shigeno et al., 2005) auf eine Metamorphose von Albit-reichem magmatischem Protolith zurückgeführt, wie z.B. metasomatisch beeinflusste Gänge von Albit-Granit, Trondhjemit, Plagiogranit, "Albitit" und ähnlichen Granitoiden oder auch Leukogabbro. Harlow & Sorensen (2005) fassen Argumente zusammen, die auf eine direkte Kristallisation des Jadeits aus Na-Al-Si-reichen, wässrigen Fluiden unter HP/LT-metamorphen Bedingungen schließen lassen.
Im Rio San Juan Komplex (RSJC) gibt es Jadeit-führende Gesteine unterschiedlicher mineralogischer Zusammensetzungen (möglicherweise auch unterschiedlicher Entstehungsgeschichte). Manche sind sicher auf Metasomatose eines geeigneten Protoliths zurückzuführen, andere können als Resultat zirkulierender wässriger Fluide im "subduction channel" einer Subduktionszone angesehen werden. Somit kommt den Jadeititen eine besondere Bedeutung als Zeugen der Zusammensetzung von Subduktionsfluiden sowie Fluid-Gesteins-Wechselwirkungen bei hohen Drücken zu.

Mit den geplanten Arbeiten beabsichtigen wir, einerseits über die lokalen Vorkommen der Jadeitite der Dominikanischen Republik wesentliche Neuerkenntnisse zu gewinnen und andererseits Schlüsselfragen über die noch nicht verstandenen Bildungsmechanismen von Jadeititen in Serpentiniten generell zu beantworten:

• Bei welchen Druck-Temperaturbedingungen kam es zur Bildung der Jadeitite?
• Wie korrelieren die P-T-Pfade der Jadeitite mit den bereits sehr gut bekannten P-T-t-Pfaden andere Blöcke im Subduktionskanal des RSJC (Krebs et al., 2007).
• Gibt es Hinweise auf eine direkte Entstehung der Jadeitite aus wässrigen Fluiden (z.B. Fluideinschlüsse)? Falls ja, wo ist der Ursprung dieser wässrigen Fluide zu suchen?
• Stellen die Jadeititgänge in den Lawsonit-Blauschiefern direkte Ausfällungen aus einem wässrigen Fluid in Bruchsystemen dieser Gesteine dar, oder handelt es sich um eine lokale Verdrängung des Blauschiefers? Zur Beanwortung dieser Frage sind u.a. Untersuchungen von Spurenelement- und Sauerstoffisotopenmuster der Zirkone in den Jadeititen und in den angrenzenden Nebengesteinen geplant.
• Gibt es auch Jadeitite, die im direkten Kontakt mit der Serpentinitmatrix vorkommen bzw. kann ein solcher direkter Kontakt durch „blackwall“-Bildung nachvollzogen werden?

Zusammenarbeit: H.-P. Schertl, W.V. Maresch, A. Hertwig (Bochum), W.C. McClelland (Iowa), K. Stanek (Freiberg)

Diamant-führende Metamorphite des Kokchetav-Massivs, Kazakhstan

Die Entdeckung von metamorphem Diamant in Krustengesteinen (Sobolev und Shatsky, 1990) revolutionierte das geodynamische Denken, belegen doch Metamorphosebedingungen im Stabilitätsfeld von Diamant (P > 45 kbar) Subduktion und Exhumierung solcher Gesteine von Tiefen größer als 120 km. Seit den 1990er Jahren waren die unterschiedlichen Lithologien der Ultrahochdruck- (UHP) Metamorphite des Kokchetav Massivs Gegenstand vielfältiger Untersuchungen (Petrologie, Geochemie, Geochronologie, Isotopenuntersuchungen, etc.). Diamant-führende Gesteine umfassen i.W. Biotit-Gneise, Kalksilikat-Gesteine und Granat-Pyroxen-Gesteine (mit oder ohne Karbonatminerale). Die Gehalte an Diamant sind z.T. erheblich, ihre Korngröße kann in seltenen Fällen 200 μm erreichen. In Kalksilikaten wurden mittels Kathodolumineszenz-Mikroskopie interessante Entdeckungen gemacht (Schertl et al., 2004), u.a. lumineszierende Granate und Klinopyroxene, deren chemische Zusammensetzungen z.T. deutlich variieren (Shatsky et al. 2006a, b; Sobolev et al. 2001; 2006; 2007). So gibt es z.B. lumineszierende Granate (ältere Relikte?), die von Adern nicht lumineszierender Granate durchsetzt sind. Die Anteile solcher lumineszierender Bereiche in Granat variieren zwischen 5 und 70%, in derselben Probe zeigen über 80% der vorhandenen Granate keine Lumineszenz.

Gegenstand momentaner Untersuchen sind:

• Allgemeine Kathodolumineszenzuntersuchungen,
• Anfertigung und Auswertung von Farbspektren,
• Mineralanalytische Untersuchungen mit der Elektronenstrahlmikrosonde,
• TEM-Untersuchungen von Mikrogefügen,
• Sauerstoffisotopieanalysen von Pyroxen und Granat.Dabei kommen ortsaufgelöste Methoden wie die Ionensonde zum Einsatz, aber auch Arbeiten an Mineral-Konzentraten mittels „Laser Fluorination“.

Zusammenarbeit: H.-P. Schertl, R. Neuser (Bochum), N.V. Sobolev, V. Shatsky (Novosibirsk), J.W. Valley (Wisconsin, Madison), S.-L. Hwang, H.-T. Chu, T.-F. Yui, P. Shen (Taiwan), J.G. Liou (Stanford)

Coesit-führende UHP-Metamorphite, Dora Maira Massiv, Italienische Westalpen

Bis zur Mitte der 1980 er Jahre waren klassische metamorphe Faziesdiagramme auf obere Drücke von ca. 10-12 kbar beschränkt, was die „normale“ Mächtigkeit kontinentaler Kruste reflektiert. Die Entdeckung von metamorphem Coesit in den Ultrahochdruck-(UHP) Gesteinen des Dora Maira Massivs vor mehr als 25 Jahren (Chopin, 1984) bewies, dass Krustenmaterial bis in Erdtiefen von mehr als 100 km subduziert und anschließend wieder -durch tektonische Prozesse- herausgehoben werden kann. Einen besonderen Stellenwert nehmen die Pyrop-Quarzite des Dora Maira Massivs ein, die weltweit einzigartig sind und chemisch durch hohe MgO- sowie sehr niedrige FeO-, CaO- und Na2O-Gehalte charakterisiert sind. Im Laufe der vergangenen Jahre wurden die Pyrop Quarzite aber auch andere Lithologien intensiv petrologisch, geochemisch und geochronologisch untersucht (z.B. Chopin, 1984; Paquette et al., 1989; Tilton et al., 1991; Chopin et al., 1991; Schertl et al., 1991; Hirajima & Compagnoni, 1993; Philippot, 1993; Gebauer et al., 1997; Simon et al., 1997; Hermann, 2003; Groppo et al., 2007; Castelli et al., 2007; Schertl & Schreyer, 2008; Ferrando et al., 2009). Die z.T. ungewöhlichen Gesteine und darin enthaltene neu entdeckten Minerale, Mikrogefüge, Entmischungen etc. geben immer wieder Anlaß zu weiteren Untersuchungen. Im Moment liegen die Schwerpunkte der Untersuchungen auf Kathodolumineszenzmikrokopie, damit verbundener „High Resolution Specroscopy“-Analytik und EBSD-Studien.
Zusammenarbeit: H.-P. Schertl, R. Neuser (Bochum), C. Chopin (Paris)

Mt. Etna (SFB B7)

This project is a part of the SFB 526 in Bochum on the Rheology of the Crust. It is a collaborative effort with members of the theoretical mechanics department. Petrological studies are used to constrain the nature of the plumbing system responsible for the recent (1991 – current) eruptions of Mt. Etna. Nature of magma mixing and the time scales on which these occur are constrained by diffusion modelling, the temperature of the magmas and the depths at which such mixing occur are constrained by thermodynamics. These results are used as inputs in a mechanical model that uses damage theory to understand how the thermal pulses caused by magma input affects the rheological (brittle as well as ductile) behavior of the crust. All of these results are tested and correlated against the continuous monitoring data of various kinds (seismic, microgravity, gas flux) available at Mt. Etna. This allows signals observed at the surface to be related to actual physical processes occurring at depth.
Collaborators: Maren Kahl (Geoscience, Bochum), S. Chakraborty (Bochum), Bojan Dmitrevic, Klaus Hackl (Mechanics, Bochum), M. Pompilio (INGV, Pisa), M. Liuzzo (INGV – Palermo), Fidel Costa (Singapore)

Origin of oceanic crust at Fast spreading ridges (Pito Deep / Hess Deep)

Although oceanic crust is being produced continually, the origin of the oceanic crust remains controversial. One reason is the difficulty of actually imaging a magma chamber, if they exist, using geophysical methods. The two currently popular models for the origin of the crust - the Gabbro glacier model and the Sheeted sill model, differ in what they predict the cooling rates of rocks as a function of depth in the crust should be. We are using drill core samples from the Pito Deep and the Hess Deep in the Pacific Ocean to calculate cooling rates as a function of depth. Diffusion modelling of compositional gradients in plagioclases is the tool being used. The results should help us to identify which of the two existing models describe the observations the best.
Collaborators: S. Chakraborty, Kathrin Faak (Bochum), L. Coogan (Victoria, Canada)

Meteorites (e.g. Acapulcoite / Lodranite)

Acapulcoite and Lodranites are interesting classes of meteorites that have an undifferentiated (chondritic) chemistry but a basaltic texture. The trace element distribution in the minerals of this meteorite appear to be out of equilibrium with each other. We are carrying out a disequilibrium melting model to constrain the time scale of melting and cooling of these unusual meteorites. The results should help us to understand the time scales of early evolution of the solar nebula.
Collaborator: H. Palme (Frankfurt) and S. Chakraborty (Bochum)