Feldspar Grains Research Articles

OSL chronology of socio-ecological systems during the mid-Holocene in the eastern coast of the Sultanate of Oman (Arabian Peninsula)

The eastern coast of the Arabian Peninsula is a key region to explore past climatic and environmental changes and their impact on the coastal ecosystems and Neolithic communities during the mid-Holocene (~8000–5000 years ago). Indeed, it is located at the frontier zone between dry and wet tropical systems, where the monsoon has reduced its activity and gradually shifted southward from the sixth millennium BC. Past human adaptation to new ecological conditions (more arid) and resources is reflected in the archaeological record, as a response to important climatic and eustatic fluctuations.In order to correlate human occupation during the Neolithic with the local climatic and palaeoenvironmental records, sediments were sampled at two archaeological sites and surrounding sabkhas, located in the Ja’alan region, on the eastern coast of the Sultanate of Oman. Optically and infrared stimulated luminescence dating were applied to quartz and feldspar grains extracted from sediments coming from both archaeological (Ruways 1 and Suwayh 1 shell middens) and palaeoenvironmental (current sabkhas) contexts in order to provide, together with radiocarbon dating, a chronology for this area. The results obtained allowed constraining human occupation during the Neolithic between 8340 ± 480 and 5390 ± 290 years (~6800–3100 years BC) at Ruways 1, and between 6180 ± 310 and 5480 ± 270 years BP (~4500–3200 years BC) at Suwayh 1. These occupations coincide with the development of a lagoon/mangrove, recorded in sediments from cores drilled under the current sabkha, whose last stage of development is dated to 5320 ± 590 (C2) and 5220 ± 570 (C1) years (3900–2600 years BC) at Khor al Jaramah.

Role of fluid in strain softening within the Main Central thrust in Sikkim: The origin of quartz-rich mylonites

The presence of fluids promotes strain softening, and profoundly affects the evolution of shear zones. The Main Central thrust (MCT) is a major shear zone that accommodated at least 90 km of shortening, and played a significant role in Cenozoic evolution of the Himalaya. Surprisingly, no information exists on the role of fluid in evolution of the MCT. This study integrates mineralogical, and geochemical analyses with mass-balance calculations to explore the role of fluids in evolution of the MCT shear zone from four exposures, which span ~40 km distance along the transport direction in Sikkim, eastern India. Our analyses reveal significant fluid-induced strain softening in the investigated exposures, attested by retrogression of biotite to chlorite, and feldspar to muscovite, and quartz. The retrogressed feldspar grains are surrounded by recrystallized quartz, and the muscovite, and chlorite produced due to retrogression define the mylonite foliations, which indicate fluid activity was syn-tectonic. We propose a three-stage conceptual model to analyze the role of fluid in the evolution of the MCT in Sikkim: First, the shear zone nucleated along pre-existing structurally weak, and compositionally distinct zones. Following which, the proto shear zone developed in closed-system conditions, and deformation localized by dynamic recrystallization, and dislocation creep resulting in formation of ~200–1000 m-thick protomylonites. Then, fluids infiltrated the shear zone, and induced retrogression, and element mobility. In the hinterland-most exposures, significant fluid-induced retrogression transformed the calc-silicate and paragneiss protoliths to mylonites resembling micaceous quartzites. In the frontal-most, and structurally intermediate exposures, the muscovite and chlorite grains underwent further dissolution, leaving behind a quartz-rich residue; thus, transforming tonalite orthogneiss, and paragneiss protoliths to mylonites resembling monomineralic quartzites. The deformation front narrowed through time, and became progressively localized within the ~120-410 m-thick strain-softened mylonites at the shear zone core, whereas, deformation ceased or continued slowly in the ~100-880 m-thick protomylonites at the margins. The study improves our understanding of the strain softening mechanisms operative within the MCT, and the origin of quartz-rich mylonites.

OSL dating of landslide-dammed-lake deposits in the North of Tehran, Iran: 958 Ray-Taleghan/Ruyan earthquake

One large landslide (Soleghan) in the north of the megacity, Tehran, has been surveyed for dating with OSL methods. Landslide dammed-lake deposits of the Soleghan landslide was used to obtain the age of the landslide event using the SAR protocol for both quartz and feldspar grains. The age of the event was 1.09 ± 0.09 ka for quartz, and 1.03 ± 0.08 ka and 1.10 ± 0.13 ka for feldspars (stimulation at 50 °C and 225 °C, respectively). These results suggest that the Soleghan mass movement was an earthquake-induced landslide triggered by the 23rd February 958 7.7 M Ray-Taleghan/Ruyan earthquake. Based on this work, thorough review studies of landslides using OSL dating methods can be used to determine the age of other mass movements in Iran. Multiple landslides with identical ages, which may be attributed to a seismic event, could be created by a known earthquake or an unknown earthquake. Subsequently, finding an unknown earthquake will complete the earthquake catalogue resulting in a better hazard assessment.

Mineral-specific heterogeneous neptunium sorption onto geological repository rocks in oxic and anoxic conditions and various temperatures

Neptunium poses challenges for radioactive wastes management due to its long half-life and ingrowth from americium decay, as well as its high mobility in water in its oxidized forms. Np(V)-bearing aqueous solutions were contacted with polished and crushed gneiss and dolerite from the Yeniseisky site, Russia, which is under consideration as a location for an underground repository, both in oxic and anoxic conditions at 20 and 90 °C. Neptunium sorption on crushed gneiss and dolerite in oxic conditions at 20 °C was quite low, but increased substantially in anoxic conditions. Autoradiography on samples treated at 20 °C revealed that sorbed Np was associated mostly with dark colored iron-bearing minerals, and solvent extraction indicated partial reduction of Np(V) to Np(IV). Sorption of Np from aqueous solutions contacted with dolerite and gneiss at 90 °C was in all cases higher than at the lower temperature. Autoradiography of polished rock samples treated in anoxic conditions at 90 °C revealed that Fe-bearing minerals had 3–4 orders of magnitude more Np sorbed on them than the quartz and feldspar grains. X-ray photoelectron spectroscopy indicated this sorbed Np was present as NpO2. This study supports the concept that the geologic conditions present at the planned repository site will retard Np migration.

Preferred orientation distribution of shock-induced planar microstructures in quartz and feldspar.

Shocked quartz and feldspar grains commonly exhibit planar microstructures, such as planar fractures, planar deformation features, and possibly microtwins, which are considered to have formed by shock metamorphism. Their orientation and frequency are typically reported to be randomly distributed across a sample. The goal of this study is to investigate whether such microstructures are completely random within a given sample, or whether their orientation might also retain information on the direction of the local shock wave propagation. For this work, we selected samples of shatter cones, which were cut normal to the striated surface and the striation direction, from three impact structures (Keurusselkä, Finland, and Charlevoix and Manicouagan, Canada). These samples show different stages of pre‐impact tectonic deformation. Additionally, we investigated several shocked granite samples, selected at different depths along the drill core recovered during the joint IODP‐ICDP Chicxulub Expedition 364 (Mexico). In this case, thin sections were cut along two orthogonal directions, one parallel and one normal to the drill core axis. All the results refer to optical microscopy and universal‐stage analyses performed on petrographic thin sections. Our results show that such shock‐related microstructures do have a preferred orientation, but also that relating their orientation with the possible shock wave propagation is quite challenging and potentially impossible. This is largely due to the lack of dedicated experiments to provide a key to interpret the observed preferred orientation and to the lack of information on postimpact orientation modifications, especially in the case of the drill core samples.

The effect of residual signal on dose measurements using MET-pIRIR signals from K-feldspar

Overestimation in dose recovery results was observed at high doses (>1000 Gy) when a single-aliquot regenerative-dose (SAR) procedure is applied to the multiple-elevated-temperature post-IR IRSL (MET-pIRIR) signals from potassium-rich feldspar (K-feldspar) grains from the Nihewan Basin. Such overestimation results from the non-bleachable (residual) signals that were not appropriately corrected for. A signal-subtraction method was proposed to solve this problem, which involves building growth curves for the bleachable and non-bleachable signals. Here we modelled the effect of residual signal on equivalent dose (De) and found that even a small amount of residual signal may give rise to large overestimation at high doses. Our results highlight that residual signals may not only affect young samples but may lead to erroneous results for old samples. Our study also suggests that although residual signals may result in significant error in the dose recovery, they may have a very small effect on natural samples because their residual signals seem to be relatively small.

Single grain infrared photoluminescence (IRPL) measurements of feldspars for dating

Existing infrared photoluminescence (IRPL) systems have used pulsed infrared stimulation (~830 nm) and measured IRPL emission (at 880 or 955 nm) using time resolved data collection with photomultipliers. Breakthrough of the infrared stimulation light overwhelms the IRPL, but the delayed emission during the laser-off period has been used instead.This paper describes a system for measurement of the IRPL signal from single sand-sized grains of feldspar. The attachment uses an electron-multiplying charge-coupled device (EMCCD) imaging system, and has two innovations that make it possible to use such a detector to obtain IRPL data. First, the optical detection system has been designed to minimise stray light and maximise the efficiency with which filters reject the stimulation light. This acts to reduce, but not eliminate, the breakthrough. Second, by placing the sample to be measured in a clearly defined sample grid, the spatial resolution provided by the EMCCD has been used to differentiate between regions of the image where IRPL is emitted and adjacent regions where only breakthrough is expected. This allows quantification of the breakthrough and effective subtraction to isolate the IRPL signal from the grains of interest.The attachment has been used to measure IRPL from single sand-sized grains of feldspar from an aeolian dune from New Zealand. A 1W UV LED (365 nm) is also added to the system and this is effective at resetting the IRPL signal, permitting a single aliquot regenerative dose (SAR) protocol to be used to measure equivalent dose (De). Measurement of a known laboratory dose (104 Gy) demonstrates the reproducibility of the attachment, with no overdispersion observed in the resulting single grain De values. The recovered dose is within 10% of the given dose. The natural IRPL signal yields De values from single grains with low overdispersion (22%) and giving a weighted mean value (103 ± 5.8 Gy) that is consistent with that obtained using post-IR IRSL measurements (105 ± 3.8 Gy). The attachment described here provides IRPL measurements on single grains suitable for exploring the potential of this novel and exciting signal for dating geological sediments.

Mineralogical and geochemical analysis of sodium bentonites in continental settings: The Uspallata Group (Triassic) of the Cuyana Basin, Mendoza province, Argentina

The Triassic Uspallata Group of the Cuyana rift Basin is characterized by continental deposits including alluvial, fluvial and lacustrine systems, mainly composed of volcaniclastic (reworked) and pyroclastic sediments. The present work focuses on the ash fall tuffs levels (bentonites) in order to understand the origin of their sodium composition since classic worldwide natural sodium bentonites link with marine environments as a source of sodium. The mineralogical attributes and relationship with diagenetic alterations have been explored by petrography, X-ray diffraction, and geochemical analysis. The tuff levels, recorded in the Cerro de las Cabras, Potrerillos and Cacheuta formations, provide the most reliable representation of the composition of the original material and diagenetic alterations due to these levels are devoid of detrital input. Accordingly, they represent key elements to explore the origin of huge bentonite deposits in the basin. In the study units, Na-montmorillonite and analcime, found at concentrations of up to or more than 50 vol%, grew at the expense of vitric components and represent authigenic minerals developed under early diagenesis. Following sedimentological and paleogeographic evidence the alteration of volcanic glass occurred in hydrologically open lacustrine systems that reached saline-alkaline conditions. The changes in the (Na + K)/H+ ratio influenced the formation of Na-montmorillonite and analcime through the succession. In addition, kaolinite is an important alteration mineral that fills cavities, replaces the feldspar grains, and probably the early diagenetic minerals. The origin of kaolinite is due to the combined effects of decomposition of organic matter and incursion of meteoric freshwater during late-stage telogenesis which generated the acidification of pore waters. Based on geochemical proxies, geochronological data and the geodynamic context the input of high volumes of pyroclastic material may thus be the result of explosive volcanism from an arc-subduction complex located to the west at these latitudes. The present work suggests that the composition of Na-rich pyroclastic ashes accumulated in hydrologically-open lake systems which conditioned the formation of sodium bentonites in continental settings.

A Study of Shock-Metamorphic Features of Feldspars from the Xiuyan Impact Crater

Feldspar is the most abundant mineral in the Earth’s crust and is widely distributed in rocks. It is also one of the most common minerals in meteorites. Shock-metamorphic features in feldspar are widely used to calibrate the temperature and pressure of shock events and can also provide clues for searching for impact craters on Earth. In this study, shocked alkali feldspars in the lithic breccia and suevite from Xiuyan Impact Crater were investigated using polarizing optical microscopes, Raman spectroscopy and electron microprobes to better constrain the shock history of this crater. For this study, feldspar grains occurring in gneiss clasts in the impact breccia and four shock stages were identified, e.g., weakly shocked feldspar, moderately shocked feldspar, strongly shocked feldspar, and whole rock melting. According to the shock classification system for alkali feldspar and felsic rocks, we estimated the shock pressure (SP) and post-shock temperature (PST) histories of these gneiss clasts. Weakly shocked feldspars display irregular fractures and undulatory extinction, and their shock stage is F-S2, which indicates that SP and PST are from ~5 to ~14 GPa and ~100 °C, respectively. Moderately shocked feldspars show planar deformation features and are partially transformed into diaplectic glass, which indicates that the F-S5 shock stage of SP and PST is from ~32 to ~45 GPa and 300–900 °C. Strongly shocked feldspars that occur as vesicular glass indicate a shock stage of F-S6, and the SP and PST are 45–60 GPa and 900–1500 °C, respectively. The whole felsic rock melting occurs as mixed melt glass clast and belongs to the F-S7 stage, and SP and PST are >60 GPa and >1500 °C, respectively.

Exotic Cu-mineralization in Triassic red beds from Navas de San Juan (Jaén, Spain)

The Navas de San Juan copper deposit is located at the southeast edge of the Iberian massif (Spain). It occurs within Triassic continental red beds that represent an undeformed, horizontal to sub-horizontal sequence, overlying unconformably folded Paleozoic metasedimentary rocks and, locally, granitic rocks. The red beds consist essentially of siliciclastic detrital materials including sandstone, silt, and clay, with some thin layers of micritic limestones and edaphic carbonates. This sequence was formed in a sedimentary framework typical of a low sinuosity fluvial system of braided type. The copper mineralization is hosted by a fine-grained sandstone (arkose) with abundant plant debris and detrital grains cemented by dolomite, which was deposited in a secondary fluvial channel.The copper mineralization includes chrysocolla, azurite and malachite with a stratabound morphology. Chrysocolla occurs unevenly in the sandy levels of the mineralized horizon replacing feldspar grains, coating detrital grains and dolomitic cement, and filling pores and cavities. Copper carbonates appear essentially inside or near the organic matter-rich zones filling cavities and veinlets. They also replace more or less limonitized plant debris preserving the original cellular structure of organic material. Azurite and malachite occur in different layers or zones without any evidence of reaction between them.The δ13C-values of azurite and malachite are indicative of a carbon source derived from the decomposition of C3 plant debris. The δ 18O values of the solutions in equilibrium with copper carbonates are consistent with solutions of meteoric origin. The differences between the values for azurite and malachite would reflect variations of the climatic conditions, probably related to seasonal changes in the hydrological parameters and arid degree of the system.Chrysocolla formation was due to the neutralization of copper solutions during the hydrolysis and replacement of the feldspars, with additional precipitation of chrysocolla in interstitial spaces, cavities and microfractures. Copper carbonates derived from the interaction between the neutralized copper solutions and the CO2 released by the decomposition of organic matter under oxidizing conditions. Based on geological, mineralogical and petrographic criteria, in conjunction with isotopic data, the Navas de San Juan deposit is interpreted to represent an exotic copper mineralization. The Cu-bearing vein mineralizations located in the Variscan basement are inferred as the possible copper source.

Low–pressure granulite–facies metamorphism in the Southern Chinese Altay orogenic belt, NW China: P–T estimates, U–Pb ages and tectonic implications

The Chinese Altay orogenic belt formed in the Paleozoic is an important part of the Central Asian Orogenic Belt (CAOB), accompanying with remarkable metamorphism. Its tectono–metamorphic evolution history is still a controversial topic. In this contribution, we investigate the petrography, metamorphic peak P–T conditions and zircon geochronology of the newly discovered garnet–absent metapelitic and felsic gneisses at Wuqiagou area in the southern Chinese Altay. They are characterized by an assemblage of orthopyroxene + biotite + plagioclase + quartz + Fe–Ti oxides ± cordierite ± K–feldspar, in which most K–feldspar grains have transformed into perthites with thin albite lamellae. The Wuqiagou garnet–absent gneisses experienced pervasive partial melting, as testified by microstructures such as mineral pseudomorphs after melt films or pockets. Phase diagram modelling constrains the peak P–T conditions to 3.5–5.5 kbar and 800–900 °C, with possible geothermal gradients of 45–75 °C/km, indicative of a prominent low–pressure granulite–facies metamorphic event in the southern Chinese Altay. SIMS zircon U–Pb dating results show a weighted mean age of 255.8 ± 1.8 Ma. This age is interpreted to represent the timing of this low–pressure granulite facies metamorphism, which is highly coeval with the timing of their hosted mafic granulite lenses (∼255 Ma) and nearby mantle–derived mafic intrusions (∼257 Ma). Based on available petrological, geochemical and chronological data, we propose that the Permian low–pressure granulite–facies metamorphic event in the southern Chinese Altay was likely associated with the intrusions of deep–derived mafic magma at a relatively shallow crustal level (12–18 km) in a post–orogenic extensional setting, with a possible link with the Tarim mantle plume activity.

Mechanical and seismic anisotropy of rocks from the ONKALO underground rock characterization facility

Olkiluoto is an island that was selected as the site for a final nuclear waste disposal facility in Finland. ONKALO underground research facility was constructed for the purpose of rock characterization. A hybrid stress path, incorporating the uncertainties of the near-field stress state estimated for the different Posiva Olkiluoto Spalling Experiment (POSE) phases, was developed as the basis for boundary conditions for a series of true-triaxial tests. Evolution of each principal stress component as a result of excavations and heating of the experimental holes to understand the phenomenon of excavation damage zone (EDZ) development was evaluated. The analysis of the temporal and spatial acoustic emission (AE) activity, construction of 3D damage fracture network images from X-ray CT, and thin section studies performed on tested specimens are presented here. AE events are used to evaluate the chronological occurrence of the damage fracture networks within the induced fractures preferably initiating and propagating within the foliation zones characterized by mica grains, and predominantly avoiding quartz and feldspar grains. Veined gneiss (VGN) samples with a 30° foliation angle relative to the maximum loading direction, and the minimum loading direction perpendicular to the strike of foliation, exhibited the lowest strength, showing shear type fracture initiating parallel to the foliation planes at 55 MPa of axial stress. This value corresponds to the stress state at the POSE experiment hole wall when the first shear type damage was observed to be parallel to the foliation planes. Comparison of the evolution of AE events and seismic velocity measured at the field site for granitic pegmatite (PGR) with its counterpart laboratory specimen was found to be in good agreement. PGR is the strongest member of the rock mass and shows significant AE activity for specimens subjected to the simulated in situ loading without development of macroscale fractures.

The effects of mafic-felsic magma interaction on magma diversity: insights from an early Paleozoic hornblendite-quartz monzonite suite in the South China block

Understanding the mechanisms responsible for the interplay between mafic and felsic magmas is the key to retrieving information on their sources, and characterizing the exchange of mass between them. In order to characterize compositional and mineralogical changes in the mafic end-member during mafic-felsic magma interaction and to better understand the nature of early Paleozoic intracontinental magmatism in the South China Block (SCB), a detailed study was conducted on an early Paleozoic hornblendite-quartz monzonite suite in the SCB. The amphibole phenocrysts in the hornblendite are zoned with respect to major and trace elements. From the brown core to the light-green rim, these amphibole phenocrysts display significant increases in Si, Mg, and Mn, coupled with abrupt decreases in Al, Ti, Na, K, and most of the trace elements, but only minor variations in Ca, Fe, Co, and Ni. The light-green matrix amphiboles in the hornblendite have similar compositions to the outer rim of amphibole phenocrysts (except Na). It is important to note that the amphibole grains in the quartz monzonite have significantly higher rare-earth element (REE) contents than the amphibole grains in the hornblendite. There is convincing evidence to support a significant transfer of incompatible elements (e.g., K, Na, LILE, LREE, U, and Th) from the felsic magma to the mafic magma, such as (1) the absence of high-Ca plagioclase in hornblendite, with the majority of feldspar grains being albite (Ab96–97) and orthoclase (Or94–96), and (2) uniform Sr-Nd-Hf isotope compositions (initial 87Sr/86Sr = 0.7081–0.7098; eNd(t) = −6.8 to −6.3; weighted mean zircon eHf(t) = −8.0 to −7.4) for the hornblendite and quartz monzonite samples. It is, therefore, suggested that during mafic-felsic magma interaction, water was transferred from the quartz monzonite magma to the coeval hornblendite magma and promoted the formation of the amphibole crystals in the latter. The incompatible elements transferred from the quartz monzonite magma to the hornblendite magma were mainly incorporated into the late-crystallized anhedral phases in the hornblendite (e.g., orthoclase, sodic plagioclase, quartz, zircon, and apatite). This study suggests that water, which behaves as a supercritical fluid in most mafic-felsic magmas, may play a key role in the exchange of mass between mafic and felsic magmas, and this may be extended to the petrogenesis of biotite-amphibole aggregations in intermediate-felsic magmas.

Effects of Water and scCO2 Injection on the Mechanical Properties of Granite at High Temperatures

Geothermal energy is an important resource to substitute for traditional fossil fuels. The mechanical properties of reservoir rock under the conditions of water and scCO2 injection at different temperatures are crucial for the safety of Enhanced Geothermal Systems. However, the effects of working fluid on the mechanical properties of granite at in situ temperatures are still rarely reported. To reveal the impact mechanisms, conventional triaxial compression experiments were conducted on granite specimens with different confining pressures (2–20 MPa), different pore fluid (10 MPa water or CO2), and different temperatures (25–150°C) in the present study. SEM analyses were applied to the specimens to determine failure surface morphologies after the experiments. The experimental results show that the effective confining pressure, pore fluid, and temperature have significant effects on the strength of granite specimens. The strength of granite increases with the increase of effective confining pressure, with similar granite strength under the same effective confining pressure (dry, water, and CO2). Temperature strengthening of granite is limited by high confining pressure (∼15 MPa). Under the effective confining pressure of 5 MPa, temperature weakening occurs on granite specimens when temperature is higher than 90°C. There is fluid diffusion in the specimens during compression. The higher viscosity of water may cause a temporary decrease in effective confining pressure, which may increase the strength of granite. The growth or formation of cracks is mainly observed in quartz and feldspar grains without short‐term chemical effects. More visible cracks are observed on the specimens and more volume of fluid is injected under CO2 injection conditions, which may be beneficial to increase the permeability of geothermal reservoir.

Bond-slip characterization of the interfacial transition zone of cement concrete and granite rock using push-out test

Push out tests of the rock (in the forms of rods) embedded in the cement mortar have been conducted to measure the bond strength of the interfacial transition zone (ITZ) in Mode-II. Locally available granite rock which contains the medium size of Quartz, Feldspar grains have been collected from the near vicinity of Mandi (HP, India) town. Cylindrical specimens of size 18 mm in diameter have been obtained using drill machine. A modified push out test setup developed for the experiments. It observed in the trial tests that due to the roughness of the rock specimens, the contribution of the frictional stresses is very high at the ITZ. This frictional contribution has made the crack propagation difficult in Mode-II. Hence the failure is extended through the mortar phase. Different combinations have been tried and finally, an optimized push out test setup geometry has been developed. Next, the effect of the water-cement (w/c) ratios (i.e.0.25, 0.30 and 0.35) on the Mode-II strength of ITZ for the age of 7 days has been studied. It is observed that the Mode-II strength of ITZ is sensitive to the water-cement ratio. The strength has increased by 37% when the w/c ratio is reduced from 0.35 to 0.25.

ArcGIS<sup>TM</sup> Proximity and Cluster Analysis of Electron Probe Micro-Maps of Round Top Critical Mineral Deposit

Critical and rare earth elements are in high demand for their increasing incorporation in modern technological devices for applications in the military, industrial, commercial, and consumer sectors. Round Top Mountain, a rhyolite laccolith in Sierra Blanca, west Texas, U.S.A. is a unique mineral deposit that offers opportunity for development of rare earth elements, especially the heavy rare earths, as well as associated critical elements. The main objective here is to evaluate the distances between accessory minerals of potential economic value (yttrofluorite, cryolite, uraninite, thorite, cassiterite, and columbite), and to major (potassium feldspar, albite, and quartz) and minor minerals (annite mi-ca, magnetite, and zircon). In this study we explore the proximity and clustering of these minor and accessory minerals, at the mi-cron-to-millimeter scale, from mineral maps constructed in a previous application of ArcGISTM tools to electron probe microanal-ysis (EPMA) element maps. Our goal is to determine whether specific minerals cluster spatially and, if so, at what distances. We noted that the high-value target yttrofluorite grains often neighbor potassium feldspar and quartz grains, but less commonly magnetite and mica grains. With regard to cluster analysis, most minor and accessory minerals were found to group together at small scales (low micrometer) and were dis-persed or random at larger (up to 1 mm) distances.

Granulometric properties of particles in Upper Miocene sandstones from thin sections, Szolnok Formation, Hungary

Particle size and shape are among the most important properties of sedimentary deposits. Objective and robust determination of granulometric features of sediments is a challenging problem, and has been standingin the focal point of sedimentary studies for many decades. In this study, we provide an overview of a new analytical approach to characterize particles from thin sections of sandstones by using 2D automated optical static image analysis. The analysed samples are originated from the turbiditic Lower Pannonian (Upper Miocene) sediments of Szolnok Formation. Sandstone samples were analysed from 1,500 to 2,250 m depth range. According to the previous studies: the detrital components are quartz, muscovite, dolomite, K-feldspar and plagioclase. Diagenetic minerals are mostly carbonates (calcite, Fe-dolomite, ankerite, siderite), clay minerals (illite, kaolinite), ankerite, siderite and kaolinite. As the discussed Szolnok Formation is considered as a potential CO2 storage system (to reduce atmospheric CO2 concentration), special attention has to be paid on grain size and shape alteration evaluation, since pore water-rock interactions affected by CO2 injection may cause changes in particle properties. The primarily aim of the present study was to develop a method for effective characterization of the particle size and shape of sandstones from thin sections. We have applied a Malvern Morphologi G3SE-ID automated optical static image analyser device, what is completed with a Raman spectrometer. Via the combined analysis of granulometry and chemical characterization, it was obvious that there were specific relationships among various grain shape parameters (e.g., circularity values correlate to width and length ratios, as well as to convexity) and the results indicated that based simply on particle shapes, muscovites can be effectively separated from other minerals. Quartz and feldspar grains showed the highest variability in shapes as these are detrital ones, and sometimes arrived as lithic fragments from which other parts were dissolved The size and shape of carbonate minerals depends highly on the original pore size and shape because these minerals are mainly diagenetic. The shape of detrital dolomites depends on diagenetic ankerite, as it replaces the rim of dolomites.

Fine-grained volcanic-hydrothermal sedimentary rocks in Permian Lucaogou Formation, Santanghu Basin, NW China: Implications on hydrocarbon source rocks and accumulation in lacustrine rift basins

Abstract Subaqueous volcanic-hydrothermal field is a complex hybrid geological system, where sediments originated from volcanic, hydrothermal, lacustrine, terrestrial, and biotic sources are mixed. Recent petrological studies on such sediments have indicated a potential relationship between volcanic-hydrothermal activities and oil generation and accumulation. Here, we explore the mechanisms of sedimentation and oil accumulation in well-laminated organic-rich fine-grained sedimentary rocks in Permian Lucaogou Formation deposited in the lacustrine Santanghu rift basin, NW China. We conducted petrographic, carbon and oxygen isotopic, and organic geochemical analyses of fifty-four core samples from three wells (forty-eight samples from Well W1, two from W2, four from W3) designed for tight oil exploration. Four main observations and interpretations are: 1) The fine-grained sediments are a mixture of volcanic-hydrothermal detrital feldspar grains, authigenic quartz crystals, and primary lacustrine dolomite, forming interbedded tuffaceous shale and dolostone; 2) authigenic quartz crystals were precipitated from acidic hydrothermal fluids, whereas fine-grained angular alkaline feldspars were derived from subaqueous volcanic eruption; 3) the TOC content and oil generation potential increase with increased mixing of feldspars and carbonate minerals. Hence, the dolomite-bearing tuffaceous shale and tuffaceous dolostone are the best source rocks; and 4) the fine-grained sedimentary rocks are interpreted as a subaqueous volcanic-hydrothermal deposit that controls the source potential. The sedimentary processes of an ancient sediment-hosted geothermal system and its implications on hydrocarbon source rocks and accumulation were first identified. Algal bloom induced by subaqueous eruptions is the main cause for episodic deposition of organic matter and extreme heterogeneity in both lithofacies and source-rock distribution. The results suggest that cone-shaped buildups on seismic sections, which were formed by volcanic-hydrothermal deposits and contain highly-mixed feldspar and carbonate sediments, are the best hydrocarbon reservoirs, and demonstrate a strong relationship between volcanic-hydrothermal activities and hydrocarbon accumulation in an extensional tectonic setting.

Influence of the weathering rate on the response of granite to nanosecond UV laser irradiation

Laser ablation is an accepted cleaning technology for cultural heritage stonework. Optimized laser ablation may, however, have different side effects depending on the mineralogical composition of the stone. In the case of granitic rocks, observed side effects – colour changes, fracturing or/and mineral melting – have been attributed to laser parameter interactions with the patina to be cleaned from this complex rock with a grained texture and polymineral composition. We describe the influence of the weathering degree of a granite on its response to laser irradiation, confirming that the existence of secondary minerals in the rock influences the intensity and typology of side effects. Knowledge of this influence is of special relevance to the conservation of heritage built with Western European Variscan granites, which are already slightly weathered in quarry.Two specimens of the same Variscan granite taken from the same quarry but with different colours (suggesting different weathering rates) were subjected to nanosecond (ns) Nd:YVO4 laser irradiation, working at 355 nm (UV irradiation) under different fluences. The specimens responded very differently to laser irradiation in terms of colour changes and resistance of the main minerals to laser radiation. The existence on one of the specimens of kaolinite deposits covering feldspar grains and Fe oxyhydroxides filling fissures seemed to be the reason for the different response to laser radiation. Our findings would suggest the need, during laser interventions, to take into account – in addition to texture and porosity – the degree of weathering of this particular kind of granite, widely used in the architectural and archaeological heritage of Western Europe.

Grain-scale deformation mechanisms and evolution of porosity in experimentally deformed Boom Clay

Boom Clay is a soft, slightly overconsolidated, uncemented claystone considered as potential host material for a radioactive waste repository in Belgium. We studied the evolution of microfabrics in samples which were shortened to 20% bulk strain in consolidated-undrained (CU) triaxial experiments at effective confining pressures of 0.375, 0.750 and 1.5 MPa, respectively.Results show a geomechanical behavior in agreement with previous studies, with total strain partly localized in shear zones and partly diffuse outside the shear zones. The diffuse strain is accommodated by pore compaction without any discernible microstructural changes compared to the starting material. In the shear zones, pore collapse reduces SEM-visible porosity and further deformation mechanisms within the shear zones are particulate flow (grain boundary sliding), particle rotation and the formation of microcracks. There is no evidence for comminution of quartz and feldspar grains. Strain localization on macro- and mesoscale is governed by viscosity contrasts between harder clasts (e.g. quartz and feldspar) embedded in a soft, porous, phyllosilicate-rich matrix.Our microstructures are comparable to those observed in Boom Clay deformed naturally and in the excavation damaged zone of the underground research facility. This suggests that our results are representative of Boom Clay's geomechanical behavior at the microscale.