41 research outputs found
Neutron time-of-flight texture measurements in Dubna: Status and developments
The multidetector texture diffractometer
SKAT (from Russian: Spektrometer
Kolitshestvennovo Analiza Tekstury) at
the pulsed reactor IBR-2 in Dubna,
Russia, started operation in March
1997 and is open for users from all
countries. Application of time-of-flight
(TOF) diffraction to texture measurements
offers the opportunity to record
complete diffraction patterns, i.e., to
measure several pole figures simultaneously. To allow high
spectral resolution for measurements on polyphase geological samples with many
diffraction patterns, the instrument was
placed at the end of an over 100m long
flight path. In this paper, we will summarize
advantages and disadvantages of
the SKAT, as well as intended improvements.conferenc
Causes and consequences of the great strength variability among soft Nankai accretionary prism sediments from offshore SW-Japan
Geophysical Research AbstractsVol. 16, EGU2014-10666, 2014EGU General Assembly 2014© Author(s) 2014. CC Attribution 3.0 License.Causes and consequences of the great strength variability among softNankai accretionary prism sediments from offshore SW-JapanMichael Stipp (1), Kai Schumann (1), Bernd Leiss (2), and Klaus Ullemeyer (3)(1) GEOMAR, Marine Geodynamics, Kiel, Germany ([email protected], [email protected]), (2)Geowissenschaftliches Zentrum, UniversitĂ€t Göttingen, Germany ([email protected]), (3) Institut fĂŒr Geowissenschaften,UniversitĂ€t Kiel, Germany ([email protected])The Nankai Trough Seismogenic Zone Experiment of the International Ocean Discovery Program (IODP) isthe very first attempt to drill into the seismogenic part of a subduction zone. Offshore SW-Japan the oceanicPhilippine sea plate is subducted beneath the continental Eurasian plate causing earthquakes of magnitude 8.0 to8.5 and related tsunamis with a recurrence rate of 80-100 years. For the tsunamigenic potential of the forearc slopeand accreted sediments their mechanical strength, composition and fabrics have been investigated. 19 drill coresamples of IODP Expeditions 315, 316 and 333 were experimentally deformed in a triaxial cell under consolidatedand undrained conditions at confining pressures of 400-1000 kPa, room temperature, axial shortening rates of0.01-9.0 mm/min, and up to an axial strain of64% (Stipp et al., 2013). With respect to the mechanical behavior,two distinct sample groups could be distinguished. Weak samples from the upper and middle forearc slope ofthe accretionary prism show a deviatoric peak stress after only a few percent strain (< 10%) and a continuousstress decrease after a maximum combined with a continuous increase in pore pressure. Strong samples fromthe accretionary prism toe display a constant residual stress at maximum level or even a continuous stressincrease together with a decrease in pore pressure towards high strain (Stipp et al., 2013). Synchrotron textureand composition analysis of the experimentally deformed and undeformed samples using the Rietveld refinementprogram MAUD indicates an increasing strength of the illite and kaolinite textures with increasing depth downto 523 m below sea floor corresponding to a preferred mineral alignment due to compaction. Experimentallydeformed samples have generally stronger textures than related undeformed core samples and they show alsoincreasing strength of the illite and kaolinite textures with increasing axial strain. Mechanically weak samples havea bulk clay plus calcite content of 31-65 vol.-% and most of their illite, kaolinite, smectite and calcite [001]-polefigures have maxima >1.5 mrd. Strong samples which were deformed to approximately the same amount of strain(up to 40%) have no calcite and a bulk clay content of 24-36 vol.-%. Illite, kaolinite and smectite [001]-pole figuremaxima are mostly <1.5 mrd, except for one sample which was deformed to a considerably higher strain (64%).The higher clay and calcite content and the stronger textures of the mechanically weak samples can be related toa collapsing pore space of the originally flocculated clay aggregates. This process is insignificant in the strongsamples from the prism toe, for which deformation would tend to involve large rock volumes and lead to straindissipation. The weak samples from the forearc slope which become even weaker with increasing strain mayprovoke mechanical runaway situations allowing for earthquake rupture, surface breakage and tsunami generation.Stipp, M., Rolfs, M., Kitamura, Y., Behrmann, J.H., Schumann, K., Schulte-Kortnack, D. and Feeser, V.2013. G-Cubed 14/11, doi: 10.1002/ggge.20290
Neue Perspektiven der Texturanalytik von Gesteinen mit konventioneller Röntgenbeugung
Technische Entwicklungen und verbesserte
Messmethoden haben in den vergangenen
15 Jahren in der Gesteinstexturanalyse
zu einer zunehmenden Nutzung
vor allem der Elektronen- und
Neutronenbeugung gefĂŒhrt. Dabei ist
die Anwendung der Röntgentexturanalyse
in der Geologie in den Hintergrund
getreten. Neue technische Optionen
dieser Methode, wie sie zum Beispiel
fĂŒr die QualitĂ€tskontrolle in der Siliziumchipherstellung
entwickelt worden
sind, haben noch keinen Eingang in die
Gesteinstexturanalyse gefunden. Steht
die volumenbezogene Gesamttexturanalyse
im Vordergrund, so hat die Röntgenbeugung
spezifische Vorteile. GegenĂŒber
der Elektronenbeugung am Rasterelektronenmikroskop
(Backscatter Electron
Diffraction) ist keine aufwendige
ProbenprÀparation notwendig und es
kann ein wesentlich gröĂeres Probenvolumen
erfasst werden. GegenĂŒber der
Neutronenbeugung ist die Röntgenbeugung
wesentlich kostengĂŒnstiger und die
zur VerfĂŒgung stehende Messzeit ist im
Prinzip unbeschrÀnkt.
Nachteile der Röntgentexturanalyse
sind die notwendige Messdatenkorrektur
aufgrund der Defokussierung des
Messstrahls im Verlauf der Messung
(Ullemeyer & Weber 1994), die unter
UmstĂ€nden schlechte Auflösung bezĂŒglich
Gitterabstand d, und das gegenĂŒber
der Neutronenbeugung wesentlich geringere messbare Probenvolumen. Die
Anwendung der Röntgentexturanalyse
war daher klassischerweise auf
monophase und feinkörnige Gesteine
beschrÀnkt.
Um diese Nachteile der Röntgentexturanalyse
kostengĂŒnstig zu minimieren,
wurde fĂŒr die Abteilung Strukturgeologie
und Geodynamik des Geowissenschaftlichen
Zentrums der UniversitÀt
Göttingen ein neues Röntgentexturgoniometer
auf der Basis von neu
entwickelten standardisierten Bauteilen
fĂŒr die Gesteinstexturanalyse individuell
konfiguriert und die Messdatenauswertestrategie
angepasst.conferenc
Crystallographic preferred orientation and elastic anisotropy of high-pressure rocks from the Eclogite Zone of the TauernWindow, Austria
POWTEX Neutron Diffractometer at FRM II - new perspectives for in-situ rock deformation analysis
EGU2012-13521
In Geoscience quantitative texture analysis here defined as the quantitative analysis of the crystallographic preferred orientation (CPO), is a common tool for the investigation of fabric development in mono- and polyphase rocks, their deformation histories and kinematics. Bulk texture measurements also allow the quantitative characterisation of the anisotropic physical properties of rock materials. A routine tool to measure bulk sample volumes is neutron texture diffraction, as neutrons have large penetration capabilities of several cm in geological sample materials.
The new POWTEX (POWder and TEXture) Diffractometer at the neutron research reactor FRM II in Garching, Germany is designed as a high-intensity diffractometer by groups from the RWTH Aachen, Forschungszentrum JĂŒlich and the University of Göttingen. Complementary to existing neutron diffractometers (SKAT at Dubna, Russia; GEM at ISIS, UK; HIPPO at Los Alamos, USA; D20 at ILL, France; and the local STRESS-SPEC and SPODI at FRM II) the layout of POWTEX is focused on fast time-resolved experiments and the measurement of larger sample series as necessary for the study of large scale geological structures. POWTEX is a dedicated beam line for geoscientific research.
Effective texture measurements without sample tilting and rotation are possible firstly by utilizing a range of neutron wavelengths simultaneously (Time-of-Flight technique) and secondly by the high detector coverage (9.8 sr) and a high flux (ïżœ~1x10 7 n/cm2s) at the sample. Furthermore the instrument and the angular detector resolution is designed also for strong recrystallisation textures as well as for weak textures of polyphase rocks. These instrument characteristics allow in-situ time-resolved texture measurements during deformation experiments on rocksalt, ice and other materials as large sample environments will be implemented at POWTEX.
The in-situ deformation apparatus is operated by a uniaxial spindle drive with a maximum axial load of 250 kN, which will be redesigned to minimize shadowing effects inside the cylindrical detector. The HT deformatione experiments will be carried out in uniaxial compression or extension and an upgrade to triaxial deformation conditions is envisaged. The load frame can alternatively be used for ice deformation by inserting a cryostat cell for temperatures down to 77 K with a triaxial apparatus allowing also simple shear experiments on ice. Strain rates range between 10-8 and 10-3 s-1 reaching to at least 50% axial strain. The deformation apparatus is designed for continuous long-term deformation experiments and can be exchanged between in-situ and ex-situ placements during continuous operation inside and outside the neutron detector
Bulk rock elastic moduli at high pressures, derived from the mineral textures and from extrapolated laboratory data
The elastic anisotropy of bulk rock depends on the mineral textures, the crack fabric and external parameters like, e.g., confining pressure. The texture-related contribution to elastic anisotropy can be predicted from the mineral textures, the largely sample-dependent contribution of the other parameters must be determined experimentally. Laboratory measurements of the elastic wave velocities are mostly limited to pressures of the intermediate crust. We describe a method, how the elastic wave velocity trends and, by this means, the elastic constants can be extrapolated to the pressure conditions of the lower crust. The extrapolated elastic constants are compared to the texture-derived ones. Pronounced elastic anisotropy is evident for phyllosilicate minerals, hence, the approach is demonstrated for two phyllosilicate-rich gneisses with approximately identical volume fractions of the phyllosilicates but different texture types
Crystallographic preferred orientations of exhumed subduction channel rocks from the Eclogite Zone of the Tauern Window (Eastern Alps, Austria), and implications on rock elastic anisotropies at great depths
Highlights
âą Analysis of crystallographic preferred orientations of high-pressure polymineralic rocks by time-of-flight neutron diffraction
âą Elastic properties of a complete set of subduction channel rocks calculated from their crystallographic preferred orientation
âą Vp/Vs ratio and P-wave anisotropy of eclogites and metasediments
âą Influence of eclogite retrogression during exhumation on their elastic properties
âą Evaluation of the seismic signature of both clastic and carbonate sediments in subduction channels
Abstract
Crystallographic preferred orientations (CPO) of rocks from an exhumed subduction channel of the Alpine orogen were determined using time-of-flight neutron diffraction. This method allows the investigation of large polymineralic samples and, more importantly, the application of full pattern fit methods to constrain CPOs of mineralogically complex rocks. Samples studied include intensely deformed fresh and retrogressed eclogites, as well as metasediments, which are interleaved with the eclogites in the subduction channel. From the CPO, seismic properties of the samples were calculated. P- wave anisotropies of the eclogite samples are fairly low, with an average of about 1.5%, and mainly constrained by pronounced omphacite CPO. Growth and deformation of retrograde amphibole in the eclogites also led to a pronounced CPO, which has a large impact on seismic anisotropies by raising them to up to 3.7% and changing the orientations of velocity maxima. Elastic anisotropies of the subducted metasediments are higher (up to 7.4%) and constrained by quartz and mica CPO in clastics and by calcite CPO in marble. VP/VS ratios may help to distinguish fresh eclogites from retrogressed ones, and both rock types from mantle peridotites of downgoing lithospheric slabs in seismic imaging. Our data also indicate that subducted terrigenous sediments are not only strongly anisotropic, but also have low VP/VS ratios. This way there may be potential to image them by seismic tomography at depth in active subduction channels
Texturanalysen von Halitmyloniten aus den Salzstrukturen Gorleben, Morsleben und Teutschenthal
Das VerstÀndnis der Mechanismen und
der Prozesse der GefĂŒgeentwicklung und
die damit verbundene Charakterisierung
der anisotropen physikalischen Eigenschaften
von natĂŒrlich deformiertem
Steinsalz sind von grundlegender Bedeutung.
So lassen sich damit u.a. Aussagen
zur Entwicklung von Salzstrukturen
vom mikroskopischen bis zum regionalen
MaĂstab machen, aber auch wichtige
Parameter u.a. fĂŒr den Kavernenbau
oder die Endlagerung toxischer Stoffe in
Salzstrukturen gewinnen. Ein wichtiger
GefĂŒgeparameter ist dabei die kristallographische
Vorzugsorientierung (Textur).
In der Literatur gibt es im Gegensatz
zu Deformationsexperimenten und
numerischen Simulationen relativ wenige
Untersuchungen natĂŒrlicher deformierter
Steinsalze (fĂŒr einen Ăberblick
siehe ScheffzĂŒk 1999). Die meisten der
bislang untersuchten Proben sind Einzelproben
und sind nicht nach mylonitischen
oder rekristallisierten Steinsalzgesteinen
unterschieden (u.a. Schwerdtner
1966, 1968, Goemann & Schumann
1977, Ertel 1987). Aussagen sind daher
nicht zwingend reprÀsentativ und eine
Charakterisierung der gesamten Salinarstruktur
nicht möglich. Ziel unserer Arbeiten ist daher eine strukturbezogene (Falten, Scherzonen etc.) GefĂŒgecharakterisierung durch Korrelation von Kornformanalysen,
Texturen, makroskopischen Strukturen
im dm- bis 10er Meter Bereich und
der gesamten Salzstruktur...conferenc
Evaluation of intrinsic velocity-pressure trends from low-pressure P-wave velocity measurements in rocks containing microcracks
Dependent on the âintrinsicâ effects on the crystal lattice of the rock constituents and the diminishing âextrinsicâ effects of pores and microcracks, elastic wave velocity versus pressure trends in cracked rocks are characterized by non-linear velocity increase at low pressure. At high pressure the âextrinsicâ influence vanishes and the velocity increase becomes approximately linear. Usually, the transition between non-linear and linear behaviour, the âcrack closure pressureâ, is not accessible in an experiment, because actual equipment is limited to lower pressure. For this reason, several model functions for describing velocityâpressure trends were proposed in the literature to extrapolate low-pressure P-wave velocity measurements to high pressures and, in part, to evaluate the âintrinsicâ velocityâpressure trend from low-pressure data. Knowing the âintrinsicâ velocity trend is of particular importance for the quantification of the crack influence at low pressure, at high pressure, the âintrinsicâ trend describes the velocity trend as a whole sufficiently well. Checking frequently used model functions for suitability led to the conclusion that all relations are unsuitable for the extrapolation and, if applicable, the estimation of the âintrinsicâ velocity trend. However, it can be shown that the âintrinsicâ parameters determined by means of a suitable model function, the zero pressure velocity and the pressure gradient depend on maximum experimental pressure in a non-linear way. Our approach intends to obtain better estimates of particular parameters from observed non-linear behaviour. A converging exponential function is used to approximate particular trends, assuming that the point of convergence of the function represents a better estimate of the zero pressure velocity and the pressure gradient, respectively. Whether the refined âintrinsicâ velocity trend meets the âtrue intrinsicâ velocity trend within acceptable errors cannot be proven directly due to missing experimental data at very high pressure. We, therefore, conclude that our approach cannot ensure absolutely certain âintrinsicâ velocity trends, however, it can be shown that the optimized trends approximate the âtrue intrinsicâ velocity trend better as all the other relations do