Rock Analogue Research Articles

Structural and textural specific features of rocks of Trap Rock Association in the Norilsk Trough (NW of Siberian platform)

This paper presents a detailed analysis of the structure of the Trap Rock Association in the Norilsk Trough and presents data from the petrographic examination of the volcanics. We recorded new data that allow us to renew and supplement the available information on the trap association. We used the drill core from a drill hole 1100 m deep (the hole penetrated 1039 m of volcanics) as a study target. We were able to reveal the thicknesses of the formations, their flows, the petrographic composition of volcanics, and their structural and textural specific features. We examined rock variability up the section in analogous rocks belonging to different rock associations.

Composition and origin of metamorphosed ferromanganese rocks in metavolcanics of Eastern Kamchatka

This paper presents detailed mineralogical and geochemical data on the metamorphosed ferromanganese rocks observed as rare lenticular bodies among the metavolcanics of the Khavyven Highland and Ganal Ridge in Eastern Kamchatka. They are specific porphyry-type banded Mn-garnet and amphibole rocks with large (up to 1–5 mm) magnetite porphyroblasts. In the Khavyven Highland, ilmenohematite is associated with magnetite in the metamorphosed ferromanganese rocks of the greenschist facies. In the Ganal Ridge, magnetite in metamorphosed ferromanganese rocks of the amphibolite facies shows decay structures and contains Mn-ilmenite lamellae surrounded by tiny crystals of ferroaluminous Zn-spinel. Comparison of Fe-Ti oxides in the ferromanganese rocks of the Khavyven Highland and Ganal Ridge shows that metamorphism of these structures differed not only in temperature and pressure levels but also in the oxygen fugacity. The rocks of the Khavyven Highland were metamorphosed under greater oxidizing conditions relative to the analogous rocks of the Ganal Ridge.

Earthquakes Promote Bacterial Genetic Exchange in Serpentinite Crevices

We report the results of our efforts to study the effects of seismic shaking on simulated biofilms within serpentinite fissures. A colloidal solution consisting of recipient bacterial cells (Pseudomonas sp. or Bacillus subtilis), donor plasmid DNA encoded for antibiotic resistance, and chrysotile (an acicular clay mineral that forms in crevices of serpentinite layers) were placed onto an elastic body made from gellan gum, which acted as the biofilm matrix. Silica beads, as rock analogues (i.e., chemically inert mechanical serpentinite), were placed on the gellan surface, which was coated with the colloidal solution. A rolling vibration similar to vibrations generated by earthquakes was applied, and the silica beads moved randomly across the surface of the gellan. This resulted in the recipient cells' acquiring plasmid DNA and thus becoming genetically transformed to demonstrate marked antibiotic resistance. Neither Pseudomonas sp. nor B. subtilis were transformed by plasmid DNA when chrysotile was substituted for by kaolinite or bentonite in the colloidal solution. Tough gellan (1.0%) promoted the introduction of plasmid DNA into Pseudomonas sp., but soft gellan (0.3%) had no such effect. Genetic transformation of bacteria on the surface of gellan by exposure to exogenous plasmid DNA required seismic shaking and exposure to the acicular clay mineral chrysotile. These experimental results suggest that bacterial genetic exchange readily occurs when biofilms that form in crevices of serpentinite are exposed to seismic shaking. Seismic activity may be a key factor in bacterial evolution along with the formation of biofilms within crevices of serpentinite.

Gelatins as rock analogs: A systematic study of their rheological and physical properties

In a laboratory model the behavior of the selected analog material must be properly scaled to nature. Here, we systematically investigate both rheological and physical properties of a wide range of gelatins as functions of temperature, composition, concentration, ageing and applied strain rate. Results show that gelatins' behavior changes gradually from purely elastic to visco-elasto-brittle to purely viscous (nonlinear) rheology going from the gel- to the sol-state. The rheological variability of gelatins appears promising for the potential use of these analog materials to simulate crustal and lithospheric rheological behavior. In particular, we found that pig skin 2.5 wt.% at 10 °C has the required rheological properties for a suitable experimental set-up to model crustal deformation.

A rocky road to Mercury

Researchers prepare for a 2014 ESA mission to Mercury by collecting X-ray fluorescence data on planetary analog rock samples.

Measuring and Interpreting X-ray Fluorescence from Planetary Surfaces

As part of a comprehensive study of X-ray emission from planetary surfaces and in particular the planet Mercury, we have measured fluorescent radiation from a number of planetary analog rock samples using monochromatized synchrotron radiation provided by the BESSY II electron storage ring. The experiments were carried out using a purpose built X-ray fluorescence (XRF) spectrometer chamber developed by the Physikalisch-Technische Bundesanstalt, Germany's national metrology institute. The XRF instrumentation is absolutely calibrated and allows for reference-free quantitation of rock sample composition, taking into account secondary photon- and electron-induced enhancement effects. The fluorescence data, in turn, have been used to validate a planetary fluorescence simulation tool based on the GEANT4 transport code. This simulation can be used as a mission analysis tool to predict the time-dependent orbital XRF spectral distributions from planetary surfaces throughout the mapping phase.

RODINGITE FORMATION IN ULTRAMAFIC ROCKS FROM THE KOZIAKAS OPHIOLITE, WESTERN THESSALY, GREECE: CONDITIONS OF METASOMATIC ALTERATION, GEOCHEMICAL EXCHANGES AND T-X(CO2) EVOLUTIONARY PATH

Rodingite dikes occur in serpentinized harzburgites in the Koziakas ophiolite complex, western Thessaly, in Greece. On the basis of petrographic and geochemical data, a gabbroic protolith is inferred. The interaction of a H 2 O-rich fluid phase must have played a significant role during the metasomatic event to form the rodingites. An isocon analysis was used to investigate the chemical exchanges. They occurred under relatively oxidizing conditions and required considerable gain in Ca and losses in Si, alkalis, Mg and most of the trace elements. Titanium and the rare-earth elements were essentially immobile, pointing to the involvement of a CO 2 -poor solution. The geochemical characteristics of the rodingites studied are similar to analogous rocks elsewhere. A suggested T– X (CO 2 ) path involves an aqueous fluid that produced mainly hydrogrossular, diopside and vesuvianite, whereas a late-stage externally derived influx of CO 2 is required for the formation of tremolite at the expense of diopside. The reaction series occurred at temperatures considerably below 670°C.

Healing of simulated fault gouges aided by pressure solution: Results from rock analogue experiments

Slide‐hold‐slide friction experiments are reported on fault gouges of salt and salt‐muscovite mixtures to investigate the effects of fluids and phyllosilicates on strength gain. Healing rates of salt gouges in the presence of saturated brine are an order of magnitude higher than dry salt and water‐saturated quartz at 65°C. Fault gouges consisting of salt‐muscovite (80:20) mixtures show healing rates half that of 100 wt % salt; this is consistent with the effects of lower porosity and reduced dilation resulting from lower friction associated with muscovite. Half of the strength gain can be attributed to dilational work. The remainder of the strength gain can be explained by a microphysical model of compaction via pressure solution. Our model predicts the rate of contact area growth and of frictional restrengthening. The model predicts the observed rate of restrengthening for long hold periods for wet salt but underestimates the values for shorter hold periods. The short time response is attributed to strengthening of the grain boundary, elevating the resistance to frictional sliding on its interface, which is likely to be operative at longer hold periods as well but is masked by the strength gain owing to the increase in contact area. Our observations are consistent with an increased resistance to sliding of the contact at short term, the growth of the contact beyond this, and dilational hardening at all hold durations. To predict the magnitude and rates of healing in natural fault gouges under hydrothermal conditions, knowledge of the “state” of the fault gouge is required.

Dykes, cups, saucers and sills: Analogue experiments on magma intrusion into brittle rocks

Magma is transported in the crust by blade-like intrusions such as dykes, sills, saucers, and also collects in thicker laccoliths, lopoliths and plutons. Recently, the importance and great number of shallow (< 5 km) saucer-shaped intrusions has been recognized. Lopoliths and cup-shaped intrusions have also been reported in many geological contexts. Our field observations indicate that many intrusions, especially those emplaced into breccias or fractured rocks, have bulging, lobate margins and have shear faults at their bulbous terminations. Such features suggest that magma can propagate along a self-induced shear fault rather than a hydraulic tension-fracture. To investigate this we use analogue models to explore intrusion propagation in a brittle country rock. The models consist of the injection of analogue magma (honey or Golden syrup) in a granular material (sand or sieved ignimbrite) that is a good analogue for brittle or brecciated rocks. These models have the advantage (over other models that use gelatin) to well represent the properties of brittle materials by allowing both shear-faults and tension fractures to be produced at suitable stresses. In our experiments we mainly obtain vertical dykes and inverted-cone like structures that we call cup-shaped intrusions. Dykes bifurcate into cup-shaped intrusions at depths depending on their viscosity. All cup-shaped intrusions uplift a central block. By injecting against a vertical glass plate we obtain detailed observations of the intrusion propagation style. We observe that dykes commonly split and produce cup-shaped intrusions near the surface and that shear zone-related intrusions develop at the dyke tip. We conclude that many dykes propagate as a viscous indenter resulting from shear failure of host rock rather than tensional hydraulic fracturing of host rocks. The shear propagation model provides an explanation for the shape and formation of cup-shaped intrusions, saucer-sills and lopoliths.

Microbial Rock Inhabitants Survive Hypervelocity Impacts on Mars-Like Host Planets: First Phase of Lithopanspermia Experimentally Tested

The scenario of lithopanspermia describes the viable transport of microorganisms via meteorites. To test the first step of lithopanspermia, i.e., the impact ejection from a planet, systematic shock recovery experiments within a pressure range observed in martian meteorites (5-50 GPa) were performed with dry layers of microorganisms (spores of Bacillus subtilis, cells of the endolithic cyanobacterium Chroococcidiopsis, and thalli and ascocarps of the lichen Xanthoria elegans) sandwiched between gabbro discs (martian analogue rock). Actual shock pressures were determined by refractive index measurements and Raman spectroscopy, and shock temperature profiles were calculated. Pressure-effect curves were constructed for survival of B. subtilis spores and Chroococcidiopsis cells from the number of colony-forming units, and for vitality of the photobiont and mycobiont of Xanthoria elegans from confocal laser scanning microscopy after live/dead staining (FUN-I). A vital launch window for the transport of rock-colonizing microorganisms from a Mars-like planet was inferred, which encompasses shock pressures in the range of 5 to about 40 GPa for the bacterial endospores and the lichens, and a more limited shock pressure range for the cyanobacterium (from 5-10 GPa). The results support concepts of viable impact ejections from Mars-like planets and the possibility of reseeding early Earth after asteroid cataclysms.

Plagioclase compositions derived from thermal emission spectra of compositionally complex mixtures: Implications for Martian feldspar mineralogy

The compositions of plagioclase, the most abundant mineral in the Martian crust, reflect changing conditions during magmatic evolution. Plagioclase contains spectral features at thermal infrared wavelengths that permit its detection by thermal emission spectrometers (Thermal Emission Spectrometer (TES), Thermal Emission Imaging System (THEMIS), and Miniature TES (Mini‐TES)) on Mars spacecraft. Previous studies have determined the accuracy with which average plagioclase compositions can be modeled in simple two‐component sand mixtures and terrestrial volcanic rocks. Studies of terrestrial rock analogs suffer from difficulties in accurately determining the average plagioclase composition for comparison with the spectrally modeled composition. Sand mixtures, however, provide a means of controlling plagioclase compositions for direct comparison to those modeled by linear deconvolution. This has allowed us to address how compositional complexity may affect our ability to derive average plagioclase compositions from thermal emission data. In this study, we examine the accuracy with which average plagioclase compositions can be modeled from emission spectra of complex mixtures of three, four, and five compositions of coarse (500–850 μm) plagioclase sand. Additionally, we examine multiphase mixtures of plagioclase with pyroxene, olivine, magnetite, and ilmenite that are analogous to selected Martian surface materials. Increasing the number of plagioclase compositions or introducing additional mineral phases does not affect the accuracy previously reported for modeling average plagioclase compositions. Plagioclase can be modeled to within 6 An of measured compositions at laboratory, TES, THEMIS, and Mini‐TES resolution (within 1σ standard deviation).

Hydrocarbons and other volatile components in alkaline rocks from the Ukrainian shield and Kola Peninsula

Gas chromatography and other analytical techniques (EMR, PMR, and IR spectroscopy) were used to examine volatile components (CH4, C2-C3, CO2, CO, H2, H2O, and others) in alkaline rocks and minerals from the Ukrainian Shield (eight massifs and dikes of grorudites) and from the Khibina and Lovozero massifs in the Baltic Shield. The alkaline rocks from the Ukrainian Shield are mostly of Proterozoic (1.7–2.1 Ga) age. The alkaline rocks from the Kola Peninsula were confirmed to be rich in methane (21 ± 14 μl/g on average) and other hydrocarbons, whereas the analogous rocks from the Ukrainian Shield are poor in methane (2.1 ± 1.6 μl/g on average at a maximum of 14 μl/g). The latter rocks are richer in CO2, which is one of the major volatile components of alkaline rocks, including agpaitic nepheline syenites from the Kola Peninsula. The rocks from the Ukrainian Shield often have elevated contents of nitrogen (up to 20 μl/g). The reasons for the differences in the composition of volatile components of rocks from the Kola Peninsula and Ukrainian Shield are as follows: the agpaitic crystallization trends of large massifs in the Kola Peninsula and much less clearly pronounced agpaitic trends in the small massifs in the Ukrainian Shield, the affiliation of these rocks with different complexes, the deeper erosion levels of the Ukrainian alkaline massifs, different ages of these rocks, etc.

Experimental evidence for the potential impact ejection of viable microorganisms from Mars and Mars-like planets

Bacterial spores (Bacillus subtilis), cyanobacteria (Chroococcidiopsis sp.), and lichen (Xanthoria elegans) embedded in martian analogue rock (gabbro) were exposed to shock pressures between 5 and 50 GPa which is the range of pressures observed in martian meteorites. The survival of Bacillus subtilis and Xanthoria elegans up to 45 GPa and of Chroococcidiopsis sp. up to 10 GPa supports the possibility of transfer of life inside meteoroids between Mars and Earth and it implies the potential for the transfer of life from any Mars-like planet to other habitable planets in the same stellar system.

PETROLEUM POTENTIAL, THERMAL MATURITY AND THE OIL WINDOW OF OIL SHALES AND COALS IN CENOZOIC RIFT BASINS, CENTRAL AND NORTHERN THAILAND

Oil shales and coals occur in Cenozoic rift basins in central and northern Thailand. Thermally immature outcrops of these rocks may constitute analogues for source rocks which have generated oil in several of these rift basins. A total of 56 oil shale and coal samples were collected from eight different basins and analysed in detail in this study. The samples were analysed for their content of total organic carbon (TOC) and elemental composition. Source rock quality was determined by Rock‐Eval pyrolysis. Reflected light microscopy was used to analyse the organic matter (maceral) composition of the rocks, and the thermal maturity was determined by vitrinite reflectance (VR) measurements. In addition to the 56 samples, VR measurements were carried out in three wells from two oil‐producing basins and VR gradients were constructed. Rock‐Eval screening data from one of the wells is also presented. The oil shales were deposited in freshwater (to brackish) lakes with a high preservation potential (TOC contents up to 44.18 wt%). They contain abundant lamalginite and principally algal‐derived fluorescing amorphous organic matter followed by liptodetrinite and telalginite (Botryococcus‐type). Huminite may be present in subordinate amounts. The coals are completely dominated by huminite and were formed in freshwater mires. VR values from 0.38 to 0.47%Ro show that the exposed coals are thermally immature. VR values from the associated oil shales are suppressed by 0.11 to 0.28%Ro. The oil shales have H/C ratios &gt;1.43, and Hydrogen Index (HI) values are generally &gt;400 mg HC/g TOC and may reach 704 mg HC/ gTOC. In general, the coals have H/C ratios between about 0.80 and 0.90, and the HI values vary considerably from approximately 50 to 300 mg HC/gTOC. The HImax of the coals, which represent the true source rock potential, range from ∼160 to 310 mg HC/g TOC indicating a potential for oil/gas and oil generation. The steep VR curves from the oil‐producing basins reflect high geothermal gradients of ∼62°C/km and ∼92°C/km. The depth to the top oil window for the oil shales at a VR of ∼0.70%Ro is determined to be between ∼1100 m and 1800 m depending on the geothermal gradient. The kerogen composition of the oil shales and the high geothermal gradients result in narrow oil windows, possibly spanning only ∼300 to 400 m in the warmest basins. The effective oil window of the coals is estimated to start from ∼0.82 to 0.98%Ro and burial depths of ∼1300 to 1400 m (∼92°C/km) and ∼2100 to 2300 m (∼62°C/km) are necessary for efficient oil expulsion to occur.

Mechanical behaviour of granular materials used in analogue modelling: insights from grain characterisation, ring-shear tests and analogue experiments

The mechanical behaviour of several dry granular materials is investigated through ring-shear tests, grain characterisation, and simple analogue experiments analysed by X-ray computed tomography. An improved knowledge of granular materials is essential to determine their suitability as analogues for upper crustal rocks in experimental models and to compare analogue and numerical experiments. The ring-shear tests show that the granular materials have an elastic/frictional plastic behaviour with strain-hardening preceding failure at peak strength, followed by strain softening until a dynamic-stable value is reached. This is similar to the behaviour exhibited by experimentally deformed rocks. The physical characteristics of the grains determine the amount of diffuse deformation before failure, the percentage of strain softening and act on the thickness of the shear zones before broadening. Initial shear zone width in extensional and contractional experiments is between 11 and 16 times the mean grain size. The angle of internal friction defining one of the mechanical properties of granular materials and thus fault dip is not only related to physical characteristics of the grains and to the handling technique used (e.g. sieving or pouring), but also to the overburden and to the experimental setup used.

Coeval folding and boudinage under plane strain with the axis of no change perpendicular to the layer

Plane-strain coaxial deformation of a competent plasticine layer embedded in an incompetent plasticine matrix was carried out to improve our understanding about the evolution of folds and boudins if the layer is oriented perpendicular to the Y-axis of the finite strain ellipsoid. The rock analogues used were Beck’s green plasticine (matrix) and Beck’s black plasticine (competent layer), both of which are strain-rate softening modelling materials with a stress exponent n=ca. 8. The effective viscosity η of the matrix plasticine was changed by adding different amounts of oil to the original plasticine. At a strain rate $$\dot e$$ of 10−3 s−1 and a finite strain e of 10%, the effective viscosity of the matrix ranges from 1.2×106 to 7.2×106 Pa s. The effective viscosity of the competent layer has been determined as 4.2×107 Pa s. If the viscosity ratio is large (ca. 20) and the initial thickness of the competent layer is small, both folds and boudins develop simultaneously. Although the growth rate of the folds seems to be higher than the growth rate of the boudins, the wavelength of both structures is approximately the same as is suggested by analytical solutions. A further unexpected, but characteristic, aspect of the deformed competent layer is a significant increase in thickness, which can be used to distinguish plane-strain folds and boudins from constrictional folds and boudins.

Deformation‐induced grain boundary wetting and its effects on the acoustic and rheological properties of partially molten rock analogue

An experimental study was performed to investigate the effects of deviatoric stress on the microstructure of partially molten polycrystalline aggregates. Borneol (organic crystal) + melt system having an eutectic temperature of 43°C and a moderate dihedral angle was used as a partially molten rock analogue. Large samples (70 mm cube) having melt fractions of 0.089–0.22 were deformed ductilely under a uniform pure shear stress (shear strain rate of ∼0.8–7×10−7 s−1), while monitoring the sample microstructure in situ using ultrasonic shear waves. Each sample was deformed repeatedly by changing the principal stress direction, resulting in the microstructural changes well detectable under the microscope. The most remarkable features of the stress‐ and/or deformation‐induced microstructural changes are enhanced grain boundary wetting, enhanced grain coarsening, and formation of large (approximately sample scale) melt sheet parallel to the shear plane, which is an assembly of completely wetted two‐grain boundaries at the microscopic scale. Significant changes in the elastic, anelastic, and viscous properties associated with these microstructural changes were observed. The velocity monitoring provided a detailed picture of growth and healing of grain boundary melt films in response to loading cycles. The microstructural changes observed in this study have many similarities with the “dynamic wetting” reported for the partially molten peridotite.

The Managua Graben and Las Sierras-Masaya volcanic complex (Nicaragua); pull-apart localization by an intrusive complex: results from analogue modeling

There is a well-documented association between pull-apart basins in strike-slip zones and large volcanic caldera complexes. Las Sierras-Masaya volcanic complex, Nicaragua, is a large basaltic lava and ignimbrite shield with nested calderas. The widest caldera is partly underlain by a large and dense intrusive complex, evidenced by a positive gravity anomaly. The Las Sierras Caldera, relating to this cumulate complex, is less than 30,000 years old and is probably still hot and ductile. The inner Masaya Caldera hosts an active magmatic system. The volcanic complex is in a dextral transtensional tectonic context of the Nicaraguan Depression. The highly active Managua Graben is on the northern part of the volcano. We speculate that the graben and volcano are linked tectonically, with the graben initiating in response of a regional stress field modified around the dense, ductile intrusive complex. Previous field work and reappraisal of structures with digital elevation model morphological analysis show that the volcano is surrounded by a rhombic fault pattern that may form a nascent pull-apart basin. We have done scaled analogue models to test the effect of intrusion density and rheology anomalies on strike-slip fault geometries. The models show that intrusion density variations alone do not significantly change fault patterns. In contrast, ductile rocks (silicone in the models as analogue for hot mafic intrusive rocks) markedly alter strike-slip fault patterns. In transtension the presence of a ductile intrusion causes the formation of a pull-apart, while in pure strike-slip or transpression, uplift and thrusting is generated. Pull-apart and uplift structures are rhomb-shaped even when the ductile inclusion is circular. We conclude that a pull-apart is developing at Las Sierras-Masaya volcanic complex in response to the transtensive regional deformation regime and the dense, ductile intrusive complex. We suggest that the volcano and graben are one dynamic system and should be monitored as one entity.

Influence of phyllosilicates on fault strength in the brittle-ductile transition: insights from rock analogue experiments

Abstract Despite the fact that phyllosilicates are ubiquitous in mature fault and shear zones, little is known about the strength of phyllosilicate-bearing fault rocks under brittle-ductile transitional conditions where cataclasis and solution-transfer processes are active. In this study we explored steady-state strength behaviour of a simulated fault rock, consisting of muscovite and halite, using brine as pore fluid. Samples were deformed in a rotary shear apparatus under conditions where cataclasis and solution transfer are known to dominate the deformation behaviour of the halite. It was found that the steady-state strength of these mixtures is dependent on normal stress and sliding velocity. At low velocities (&lt;0.5 µm s −1 ) the strength increases with velocity and normal stress, and a strong foliation develops. Comparison with previous microphysical models shows that this is a result of the serial operation of pressure solution in the halite grains accommodating frictional sliding over the phyllosilicate foliation. At high velocities (&gt;1 µm s −1 ), velocity-weakening frictional behaviour occurs along with the development of a structureless cataclastic microstructure. Revision of previous models for the low-velocity behaviour results in a physically realistic description that fits our data well. This is extended to include the possibility of plastic flow in the phyllosilicates and applied to predict steady-state strength profiles for continental fault zones containing foliated quartz-mica fault rocks. The results predict a significant reduction of strength at mid-crustal depths and may have important implications for crustal dynamics and seismogenesis.

Fault zone weakening and character of slip along low-angle normal faults: insights from the Zuccale fault, Elba, Italy

A seismically active low-angle normal fault is recognized at depth in the Northern Apennines, Italy, where recent exhumation has also exposed ancient examples at the surface, notably the Zuccale fault on Elba. Field-based and microstructural studies of the Zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability, promoting influx of CO 2 -rich hydrous fluids. This triggered low-grade alteration and the onset of stress-induced dissolution–precipitation processes (e.g. pressure solution) as the dominant grain-scale deformation process in the pre-existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine-grained hydrous mineral phases. These rocks exhibit ductile deformation textures very similar to those formed during pressure-solution-accommodated ‘frictional–viscous’ creep in experimental fault rock analogues. The presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core, which significantly enhanced the sealing capacity of the fault zone. A slip model for low-angle normal faults in the Apennines is proposed in which aseismic frictional–viscous creep occurs on a weak, slow-moving (slip rate &lt;1 mm a −1 ) fault, interspersed with small seismic ruptures caused by cyclic hydrofracturing events. Our findings are potentially applicable to other examples of low-angle normal faults in many tectonic settings.