Common Rock Types Research Articles

Fracture propagation characteristics and energy evolution law of deep coal seam containing gangue based on discrete lattice method

Hydraulic fracturing is a key technology for the development of deep coal-rock reservoirs. Gangue, a common rock type within coal-bearing strata, significantly influences fracture morphology after fracturing. Understanding fracture propagation in coal-rock containing gangue and effectively controlling artificial fractures remain critical challenges in fracturing operations. To address this, the study investigates the coal-bearing strata of the Benxi Formation in the Daning-Jixian area of the Ordos Basin, characterized by the presence of gangue. A three-dimensional discrete lattice model was developed to analyze the effects of gangue-related factors, including the number, thickness, dip angle, and type, on hydraulic fracturing. The fracture propagation patterns and stimulation effects under these factors were systematically compared. Using node path tracking and post-processing techniques, the study identified the fracturing characteristics and fracture propagation modes at different stages. The findings indicate that gangue characteristics significantly impact fracture morphology and propagation paths. Specifically, an increase in the number of gangue layers enhances fracture complexity but reduces the effective stimulation ratio. Gangue thickness positively correlates with fracture tortuosity and inversely correlates with the effective stimulation ratio. The dip angle of gangue determines the direction of fracture propagation but has minimal influence on the stimulation area. The gangue type also affects fracture morphology; for instance, sandstone gangue leads to narrower fractures and more interface fractures at the sandstone–coal boundary compared to mudstone gangue. Fracture propagation, characterized by energy changes, can be divided into four distinct stages: Stage I (initial injection phase), Stage II (onset of fracture propagation), Stage III (fluid energy storage within gangue), and Stage IV (fracture penetration through gangue). These stages are marked by variations in fluid injection energy, fluctuation energy, surface energy, and strain energy as the fracture penetrates the layer, deflects, and forms horizontal fractures. The simulation of fracture network evolution in coal-bearing strata containing gangue provides significant theoretical guidance for understanding, predicting, and controlling fracture network morphology in coal reservoirs. These findings are instrumental for the efficient development of deep coalbed methane.

Petrogenesis of ultramafic rocks from the Neoproterozoic Bou Azzer Ophiolite (Morocco): new insights into a long-standing geodynamic question

ABSTRACT The Neoproterozoic Bou Azzer Ophiolite, located in the Pan-African orogen of the Anti-Atlas, Morocco, exposes a well-preserved mantle section of the ophiolitic sequence. Although ultramafic rocks are the most common rock types in the sequence, they have received less attention in previous studies. In this work, we investigate the petrogenesis of these ultramafic rocks in detail, including their petrology, mineralogy, and whole rock geochemistry. The mineral assemblages are mainly formed by antigorite with minor lizardite, chlorite, and accessory Cr-spinel. The only preserved primary mantle mineral is Cr-spinel, which shows unaltered cores with Cr# between 0.61 and 0.71 and alteration rims to ferrian chromite and magnetite. Whole rock analyses indicate almost total serpentinization and, locally, strong carbonation (mass loss of ignition between 11.94 and 22.20 wt%). The non-carbonated samples preserve protolith signatures, with decreased MgO/SiO2 suggesting metasomatic processes related to serpentinization. Immobile trace elements compare well with fore-arc peridotites, while melting modelling indicates intense and polyphasic melting events. We propose polyphase melting in a subduction-initiation setting, with a first stage of 14–23% anhydrous melting, forming fore-arc basalts, and a second stage of 15–25% melting with 1 wt% H2O, forming boninites. The Bou Azzer ultramafic rocks represent the fore-arc region of a supra-subduction zone, challenging previous interpretations that suggested a back-arc position.

Influence of different cooling methods on the creep characteristics and damage model of layered limestone after thermal treatment

In the actual underground energy extraction projects, the long-term stability of the well wall surrounding rock through the layered rock body often faces the influence of complex factors such as joints, thermal and cooling. The laminated limestone as a common rock type with obvious laminated structure and petroleum reservoirs, under high temperature environment can directly relate to the stability and safety of underground engineering structures. This paper investigates the creep characteristics of laminated limestone under high-temperature natural cooling and water cooling, analyzing the correlation between creep deformation, creep rate, long-term strength, and the angle of laminations, temperature, cooling mode. It then introduces a non-constant fractional-order cohesive body to construct an improved model based on these findings. The results indicate that under the same temperature and cooling method, as the lamination angle increases, the long-term strength shows a V-shaped trend, while the creep rate shows an inverted V-shaped trend. Specifically, the water cooling conditions at 500 °C (0°, 30°, 45°, 60°, and 90°) are 85.75 MPa, 71.24 MPa, 42.30 MPa, 61.52 MPa, and 87.40 MPa, respectively. For specimens with the same lamination angle, the creep mechanical properties of specimens cooled by water are more deteriorated than those cooled naturally. Taking the 100 °C action stratified limestone, long-term strength (0°, 30°, 45°, 60°, 90°) corresponding to natural cooling are 120.15 MPa, 106.12 MPa, 72.71 MPa, 91.43 MPa, and 123.32 MPa, respectively, while the long-term strengths corresponding to water cooling are 116.96 MPa, 100.97 MPa, 70.65 MPa, 85.34 MPa and 119.63 MPa, respectively. Finally, by comparing with the experimental results of nodular limestone under different cooling methods, it is found that the creep damage model proposed in this paper is not much different from the experimental curves, and it can effectively express the creep deformation and mechanical behavior of laminated limestone after high temperature. The results and findings are important for optimizing the design and prevention of potential underground energy extraction project disasters, and also provide important data support.

Prediction of Rock Unloading Strength Based on PSO-XGBoost Hybrid Models.

Rock excavation is essentially an unloading behavior, and its mechanical properties are significantly different from those under loading conditions. In response to the current deficiencies in the peak strength prediction of rocks under unloading conditions, this study proposes a hybrid learning model for the intelligent prediction of the unloading strength of rocks using simple parameters in rock unloading tests. The XGBoost technique was used to construct a model, and the PSO-XGBoost hybrid model was developed by employing particle swarm optimization (PSO) to refine the XGBoost parameters for better prediction. In order to verify the validity and accuracy of the proposed hybrid model, 134 rock sample sets containing various common rock types in rock excavation were collected from international and Chinese publications for the purpose of modeling, and the rock unloading strength prediction results were compared with those obtained by the Random Forest (RF) model, the Support Vector Machine (SVM) model, the XGBoost (XGBoost) model, and the Grid Search Method-based XGBoost (GS-XGBoost) model. Meanwhile, five statistical indicators, including the coefficient of determination (R2), mean absolute error (MAE), mean absolute percentage error (MAPE), mean square error (MSE), and root mean square error (RMSE), were calculated to check the acceptability of these models from a quantitative perspective. A review of the comparison results revealed that the proposed PSO-XGBoost hybrid model provides a better performance than the others in predicting rock unloading strength. Finally, the importance of the effect of each input feature on the generalization performance of the hybrid model was assessed. The insights garnered from this research offer a substantial reference for tunnel excavation design and other representative projects.

Mineral- and Rock Type Localities in Romania and Their Potential Geoheritage Value

A complete inventory of 16 mineral type localities and 1 rock type locality in Romania, as well as their assessed geoheritage characteristics and values are presented for the first time. Thirty-nine mineral species, one chemical element and one common rock type were discovered, described and named in Romania. Most of these are closely linked to traditional mining areas: the Baia Mare region, the Apuseni Mts. and the Banat region. Three mineral type localities are outstandingly rich: Săcărâmb/Nagyág in the southern Apuseni Mts. (8 new minerals), Baia Sprie/Felsőbánya in the Baia Mare region (6 new minerals), and Băița Bihor/Rézbánya in the Northern Apuseni Mts. (6 new minerals). The scientific, educational and touristic value and significance of the type localities regarded as „geosites” and „geodiversity sites” are discussed. Based on the quantitative assessment of the scientific, educational and touristic values, the type localities were hierarchically classified, Baia Sprie, Săcărâmb and Măgura Uroiului being the highest ranked. From the geoconservation perspective, site protection and valuation strategies at local and/or regional scales are envisaged.

Experimental Research on Anisotropy Characteristics of Shale under Triaxial Incremental Cyclic Loading and Unloading

Shale is a common rock type that is associated with underground engineering projects, and several important factors, such as bedding structure, confining pressure, and the loading and unloading path, significantly influence the anisotropy of shale. Triaxial monotonic loading tests and triaxial incremental cyclic loading and unloading tests of shale under three kinds of confining pressures and five types of bedding inclination angles (θ) were thus performed to investigate the anisotropy of shale in terms of mechanical behavior, acoustic emission (AE), and energy evolution, and reveal the mechanism by which shale anisotropy is weakened. The results show that (1) the compressive strength and elastic modulus of shale decrease and then increase as the θ increases, and that both σ3 and incremental cyclic loading and unloading reduce the anisotropy in terms of the compressive strength and elastic modulus of shale, with the ratio of plastic strain to total strain reaching its maximum at a θ of 60° during each loading and unloading cycle. (2) The failure modes of shale with θ of 0°, 30°, and 90° under triaxial monotonic loading are similar to the counterparts under triaxial incremental cyclic loading and unloading, while the failure modes of shale with θ of 45° and 60° differ significantly under the two loading conditions, and interestingly, the degree to which the bedding plane participates in shale crack evolution under incremental cyclic loading and unloading is considerably lower than that under triaxial monotonic loading. (3) The cumulative AE count and AE b-value of shale first decrease and then increase as the θ increases, while the Felicity ratio decreases as the number of cycles increases. (4) As the θ increases, the total energy density U0 and the parameter m, which reflects the accumulation rate of elastic energy, first decrease and then increase, with both reaching a minimum at a θ of 60°. (5) The mode by which cyclic loading and unloading leads to failure in shale with a θ of 60° is similar to that at a θ of 0° and is the main mechanism by which shale anisotropy weakening occurs as a result of cyclic loading and unloading. The results provide experimental support and a theoretical basis for safer and more efficient underground engineering projects that involve shale.

Seismic Anisotropy of Mafic Blueschists: EBSD‐Based Constraints From the Exhumed Rock Record

AbstractSeismic anisotropy constitutes a useful tool for imaging the structure along the plate interface in subduction zones, but the seismic properties of mafic blueschists, a common rock type in subduction zones, remain poorly constrained. We applied the technique of electron backscatter diffraction (EBSD) based petrofabric analysis to calculate the seismic anisotropies of 14 naturally deformed mafic blueschists at dry, ambient conditions. The ductilely deformed blueschists were collected from terranes with inferred peak P‐T conditions applicable to subducting slabs at or near the plate interface in active subduction zones. Epidote blueschists display the greatest P wave anisotropy range (AVp ∼7%–20%), while lawsonite blueschist AVp ranges from ∼2% to 10%. S wave anisotropies generate shear wave splitting delay times up to ∼0.1 s over a thickness of 5 km. AVp magnitude increases with glaucophane abundance (from areal EBSD measurements), decreases with increasing epidote or lawsonite abundance, and is enhanced by glaucophane crystallographic preferred orientation (CPO) strength. Two‐phase rock recipe models provide further evidence of the primary role of glaucophane, epidote, and lawsonite in generating blueschist seismic anisotropy. The symmetry of P wave velocity patterns reflects the deformation‐induced CPO type in glaucophane—an effect previously observed for hornblende on amphibolite P wave anisotropy. The distinctive seismic properties that distinguish blueschist from other subduction zone rock types and the strong correlation between anisotropy magnitude/symmetry and glaucophane CPO suggest that seismic anisotropy may be a useful tool in mapping the extent and deformation of blueschists along the interface, and the blueschist‐eclogite transition in active subduction zones.

Experimental study of carbonate samples dissolution using X-ray microcomputer-based tomography

Background: The study of the interaction of hydrochloric acid with carbonate materialsis important in the oil and gas industry. Carbonate rocks are common rock types, and half of all petroleum reserves worldwide are found in carbonate deposits. Understanding the mechanisms and characteristics of dissolution of carbonate rocks is of great practical importance in the production of hydrocarbons and the injection of carbon dioxide into formations.
 Aim: The purpose of this article is to study the dissolution processes of carbonate samples in laboratory conditions using X-ray microcomputer-based tomography.
 Materials and methods: The study used 5 cylindrical carbonate samples, which were tested during the injection of hydrochloric acid solutions. Additional experimental and digital data from 8 samples are also used. The three-dimensional pore space of the samples was obtained using specialized software based on tomographic images.
 Results: The results obtained demonstrate the significance of the use of X-ray computed tomography for a deeper understanding of dissolution processes in geological and engineering studies. The study highlighted the complexity of the rock dissolution process, which depends on many factors. The created three-dimensional models of the samples allowed us to visualize wormholes, including branched and dominant wormholes. 3D imaging provided valuable information about changes in the pore structure of the samples before and after acid exposure.
 Conclusion: The results of this study highlight the importance of considering physical and structural properties when analyzing dissolution processes in carbonate samples. These data can have practical applications in the oil and gas industry, contributing to a more accurate understanding and optimization of the processes of interaction of acid solutions with carbonate samples.

Delineating of Groundwater Aquifer Potential Using Vertical Electrical Sounding (VES) Methods in Giriloyo, Wukirsari Village, Imogiri District, Bantul Regency, Special Region of Yogyakarta

The geophysical investigation in Giriloyo, Wukirsari Village, has defined the groundwater potential zones. The research region underwent a geophysical examination utilizing the electrical resistance method, which comprised the vertical electrical sounding (VES) technique and the Schlumberger array system. The study area is surrounded by common rock types such as lava, tuff, agglomerate, and alluvial. In the study region, five lines were explored. Data for subsurface resistivity were gathered with an Oyo McOhm type 2115. IPI2win, a piece of computer software that analyzes data and automatically interprets apparent resistivity, was used to examine the data. The VES data showed the varied nature of the subsurface geological sequence. The geoelectrical cross-sections along the profile of (VES 2-3-4-1) show an aquifer, which stretches from the southeast towards the northwest part of the basin. Tuff is seen to have formed shallow aquifers due to the top weathered part at VES-2 and 3, while Quartz chlorite calcite (VES-4) and Plagioclases (VES-1) are devoid of shallow aquifers. The higher-elevation lithological areas recharge the low-elevation aquifer zones. At VES 2 and 3, relatively low resistivity values (< about 100 Ωm) have been observed. A field observation at these locations reveals that these lithologies are highly fractured with a weathering profile of up to 10 m. Thus, these are the sites where groundwater potential zones can be marked (figure 4). The high resistivity (about 3162 Ωm regions is observed extending at the VES-4; this may be due to the least weathered Quartz chlorite calcite at high elevation. VES-1 is also located on the plagioclase and shows a slight resistivity zone, which could be considered a groundwater recharge zone. This profile shows the high possibility of groundwater potential due to the zone of lineaments.

Mineralogical Study of Respirable Dust in Artisanal SmallScale Quarries of Misisi Compound, Lusaka, Zambia

Misisi compound in Lusaka is characterized by artisanal small scale quarrying activities which are often dusty in nature and prone to hazards. This study reports respirable dust concentration and the mineralogical composition of rocks in the workplace. Dust sampling was carried out with the help of gravimetric samplers at five quarries. Respirable dust samples were collected during digging, chiselling, crushing, and loading sites. The results of the sampled dust indicated high levels of respirable dust at all five quarry crushing sites. For example, based on Time Weighted Average (TWA), the respirable dust concentration at the crushing site was recorded as 12.75 mg/m3 at quarry#1 compared to the Maximum Allowable Concentration (MAC) of 5 mg/m3 of Occupational Safety and Health Administration (OSHA). The results of mineralogical testing and analyses revealed that marble was the common rock type that was being mined by most of the ASMs in the studied area. Based on the point count data analysis, marble contained 98% CaCO3 and 2% SiO2.

Elemental and boron isotopic variations in tourmaline in two-mica granite from the Cuona area, Tibet: Insights into the evolution of leucogranitic melt

Two-mica granite is the most common magmatic rock type in the Himalayan leucogranite belt, which has close relationship with rare metal mineralization. Its genesis is generally attributed to magmatic differentiation. In recent years, the mineral geochemical compositions are increasingly used to study magmatic differentiation, which are significant for deciphering the melt evolution and element migration processes. In this study, in-situ major and trace element and boron isotope compositions for tourmalines from two-mica granites in the Cuona and Cuonadong leucogranites in the Cuona area are conducted to determine microscopic changes in mineral assemblages and geochemical compositions. Analytical results show that the tourmalines in the Cuonadong leucogranite were crystallized earlier relative to the tourmalines in the Cuona leucogranite during magmatic differentiation. The volatile contents have a genetic relationship with incompatible elements in tourmaline, which is possibly responsible for the formation of tourmaline zonation and the enrichment of Sr, Zn, and Pb during magmatic differentiation. The B isotopic composition of tourmaline in the Cuona area suggests that the granitic magma was dominantly derived from the partial melting of the metasedimentary source rocks. Their B isotope variations likely resulted from fluid exsolution during B-rich melt evolution. High rare metal contents in tourmalines indicate that the two-mica granites in the Cuona area may have great mineralization potential.

Investigation of the microstructural characteristics of heated sandstone by micro-computed tomography technique

Heritage buildings always pose challenges due to experiencing high temperatures and pressure over time. Sandstone is one of the common sedimentary rock types used for these buildings. Therefore, it is very important to understand the microstructural variations of rocks associated with these constructions along with the mechanical variations. In this study, the microstructural and mechanical alteration of selected types of sandstones is investigated after it is heated from room temperature to 800 °C. Micro X-ray computed tomography (µXCT), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry (TG) and derivative thermogravimetry (DTG) techniques were used to identify the physical, chemical, mineralogical and microstructural changes of sandstone after different heat treatments. The mechanical alteration of the heated rock specimens was also studied using the point load index (PLI). The main changes in microstructure were observed when the sandstone’s temperature was greater than 400 °C. The total porosity measured by µXCT of sandstone increased by more than 70% at 800 °C compared to its porosity at room temperature. When the temperature increases, the open porosity increases while the closed porosity decreases. Noticeable changes in rock mineralogy were identified at temperatures exceeding 400 °C, which can be attributed to the phase transition of quartz, decomposition of feldspar and dehydroxylation of kaolinite. The TG and DTG analyses and point load index were in good agreement concerning these changes in rock mineralogy. Microstructural variation is one of the main reasons for the discrepancy in threshold values between different sandstones.

Geochemical Characteristics and the Sedimentary Environment of Lower Cambrian Argillaceous Rocks on the Kongquehe Slope, Tarim Basin, China

The deposition of the Lower Cambrian argillaceous rocks on the Kongquehe slope provides a good opportunity to reconstruct the paleoenvironmental conditions of the Tarim Basin. To explore the paleoredox conditions, paleoclimate, and provenance of this deposit, 21 samples were collected from Well A, and the concentrations of major and trace elements were analyzed. The V/(V + Ni), V/Cr, Ni/Co, U/Th, Uau, and V/Sc ratios indicated that the sediments in the water body from the Lower Cambrian were in a reducing environment, and the degree of reduction weakened from the bottom to the top. The Sr/Cu, Mg/Ca, and Fe/Mn ratios in the sediments revealed that the Lower Cambrian was mainly characterized by a warm and humid paleoclimate, and there may be a paleoclimatic transition toward drought conditions as recorded by the sediments of Xidashan Formation. The chemical alteration index, compositional variation index, plagioclase alteration index values, and Th/U and K/Rb ratios of the Lower Cambrian mudstone all reflected that the parent rock in the study area was mainly subject to low- to medium-degree chemical weathering. Longitudinally, the Lower Cambrian gradually increased and then weakened from Xishanbulake Formation to Xidashan Formation. The ratios of trace elements and rare earth elements as well as Th–Sc–Zr/10, Th–Co–Zr/10, and ∑REE–La/Yb diagrams all showed that the mudstones of Xishanbulake and Xidashan Formations of the Lower Cambrian in the study area have a common parent rock type, mainly sedimentary rocks rich in felsic minerals. Additionally, the structural setting discrimination diagrams of SiO2–K2O/Na2O, Th–Sc–Zr/10, Th–Co–Zr/10, and La–Th–Sc showed that the structural setting of the Lower Cambrian mudstone deposition is mainly a continental island arc environment and has the characteristics of an active continental margin.

Defining the contact zone between Archean-Proterozoic terranes with distinct geologic histories: The scenario from eastern Botswana

Understanding when Archean and Proterozoic terranes of distinct geologic histories became proximal to each other is an important topic to investigate. This study aims to define the contact zone between the southern Motloutse Complex, an Archean medium-grade terrane with prominent Neoarchean-Mesoarchean rock units, and the northern Mahalapye Complex, a Paleoproterozoic high-grade granitoid-gneiss terrane, in eastern Botswana. The contact zone between the two terranes is obscured by cover rocks. Field reconnaissance aided with remote sensing and aeromagnetic imaging delineated the NW-SE trending Radisele-Mogome corridor beneath the cover rocks. Tonalite gneiss with amphibolite and layered ultramafic rock xenoliths dominate the NE part, and are common rock types in the southern Motloutse Complex. Granodiorite with diorite gneiss xenoliths dominate the SW part, and are extensions of the NW-SE elongate c.2.04 Ga Mokgware pluton in northern Mahalapye Complex. The c.2.02 Ga Mogome granite intrudes across the contact zone, and contains xenoliths of rocks from both complexes. Shear deformation and metasomatism variably overprint the rocks along the contact zone. U-Pb zircon and 40Ar/39Ar amphibole–biotite geochronology were carried out on representative rock types to understand the chronology of events along the contact zone. The 2056 ± 40 Ma (MSWD = 5.2; n = 56) diorite gneiss from the NE margin of the contact zone contains c.2.6 Ga and c.2.9 Ga xenocrystic zircons. No Archean zircons were found in the 2041 ± 88 Ma (MSWD = 8.4; n = 13) diorite gneiss from the SW margin of the contact zone. The 2036 ± 12 Ma (40Ar/39Ar amphibole) and 2039 ± 11 Ma (40Ar/39Ar biotite) ages, respectively from the NE and SW margins, argue for the c.2.04 Ga age to represent the first common event along the contact zone. Shear deformation and metasomatism are respectively dated at 1987 ± 7 Ma and 1951 ± 11 Ma (40Ar/39Ar biotite), and account for the Pb-loss in dated zircons. It is argued that the Archean Motloutse Complex and Paleoproterozoic Mahalapye Complex terranes came together at c.2.04 Ga, with the emplacement of syntectonic Mokgware granodiorite. The results highlight the factors that are important to define the contact zone between Archean-Proterozoic terranes with distinct geologic histories.

DIGITAL ROCK ANALYSIS: AN ALTERNATIVE METHOD TO PREDICT PETROPHYSICAL PROPERTIES, CASE STUDY FROM MISHRIF FORMATION

The digital core analysis of petrophysical properties replace the use of conventional core analysis by reducing the required time for investigation. Also, the ability to capture pore geometries and fluid behavior at the pore-scale improves the understanding of complex reservoir structures. In this work, 53 samples of 2D thin section petrographic images were used for analyses from the core plugs taken from the Buzurgan oil field. Each sample was impregnated with blue-dyed epoxy, thin sectioned and then was stained for discrimination of carbonate minerals. Each thin section has been described in detail and illustrated by photomicrographs. The studied samples include a variety of rock types. Packstone is the most common rock type observed followed by grainstone and packstone – wackestone. Floatstone and dolostone are noted rarely in the studied interval. However, the samples of thin section images are processed and digitized, utilizing MATLAB programming and image analysis software. The entire workflow of digital core analysis from image segmentation to petrophysical rock properties determination was performed. A focused has been made on determining effective and total porosity, absolute permeability, and irreducible water saturation. Absolute permeability is estimated with the Kozeny-Carman permeability correlation model and Timur-Coates permeability correlation model. Irreducible water saturation simply is derived from total and effective porosity. Also, some pore void characteristics, such as area and perimeter, were calculated. The results of Digital 2D image analysis have been compared to laboratory core measurements to investigate the reliability and restrictions of the digital image interpretation techniques.

Seismic Properties of a Unique Olivine-Rich Eclogite in the Western Gneiss Region, Norway

Investigating the seismic properties of natural eclogite is crucial for identifying the composition, density, and mechanical structure of the Earth’s deep crust and mantle. For this purpose, numerous studies have addressed the seismic properties of various types of eclogite, except for a rare eclogite type that contains abundant olivine and orthopyroxene. In this contribution, we calculated the ambient-condition seismic velocities and seismic anisotropies of this eclogite type using an olivine-rich eclogite from northwestern Flemsøya in the Nordøyane ultrahigh-pressure (UHP) domain of the Western Gneiss Region in Norway. Detailed analyses of the seismic properties data suggest that patterns of seismic anisotropy of the Flem eclogite were largely controlled by the strength of the crystal-preferred orientation (CPO) and characterized by significant destructive effects of the CPO interactions, which together, resulted in very weak bulk rock seismic anisotropies (AVp = 1.0–2.5%, max. AVs = 0.6–2.0%). The magnitudes of the seismic anisotropies of the Flem eclogite were similar to those of dry eclogite but much lower than those of gabbro, peridotite, hydrous-phase-bearing eclogite, and blueschist. Furthermore, we found that amphibole CPOs were the main contributors to the higher seismic anisotropies in some amphibole-rich samples. The average seismic velocities of Flem eclogite were greatly affected by the relative volume proportions of omphacite and amphibole. The Vp (8.00–8.33 km/s) and Vs (4.55–4.72 km/s) were remarkably larger than the hydrous-phase-bearing eclogite, blueschist, and gabbro, but lower than dry eclogite and peridotite. The Vp/Vs ratio was almost constant (avg. ≈ 1.765) among Flem eclogite, slightly larger than olivine-free dry eclogite, but similar to peridotite, indicating that an abundance of olivine is the source of their high Vp/Vs ratios. The Vp/Vs ratios of Flem eclogite were also higher than other (non-)retrograded eclogite and significantly lower than those of gabbro. The seismic features derived from the Flem eclogite can thus be used to distinguish olivine-rich eclogite from other common rock types (especially gabbro) in the deep continental crust or subduction channel when high-resolution seismic wave data are available.

Rock Classification from Field Image Patches Analyzed Using a Deep Convolutional Neural Network

The automatic identification of rock type in the field would aid geological surveying, education, and automatic mapping. Deep learning is receiving significant research attention for pattern recognition and machine learning. Its application here has effectively identified rock types from images captured in the field. This paper proposes an accurate approach for identifying rock types in the field based on image analysis using deep convolutional neural networks. The proposed approach can identify six common rock types with an overall classification accuracy of 97.96%, thus outperforming other established deep-learning models and a linear model. The results show that the proposed approach based on deep learning represents an improvement in intelligent rock-type identification and solves several difficulties facing the automated identification of rock types in the field.

A topological model for defects and interfaces in complex crystal structures

Abstract A topological model (TM) is presented for the complex crystal structures characteristic of some minerals. We introduce a tractable method for applying the TM to characterize defects in these complex materials. Specifically, we illustrate how structural groups, each with a motif containing multiple atoms, provide lattices and structures that are useful in describing dislocations and disconnections in interfaces. Simplified methods for determining the shuffles that accompany disconnection motion are also described. We illustrate the model for twinning in albite owing to its potential application for constraining the rheological properties of the crust at conditions near the brittle-plastic transition, where plagioclase is a major constituent of common rock types. While deformation twins in plagioclase are often observed in crustal rocks, the interpretation of the stress states at which they form has not advanced. The concept of structural groups makes an analysis of the twinning process easier in complex minerals and explicitly predicts the interface structure of the deformation twins.

The Upper Cretaceous intrusive rocks with extensive crustal contribution in Hacımahmutuşağı Area (Aksaray/Turkey)

Abstract The Hacımahmutuşağı area (Aksaray/Turkey) is located in the western part of the Central Anatolian Crystalline Complex (CACC). Gneiss and marble compose the basement units, while intrusive rocks are gabbros and granitoids. The pegmatitic hornblende gabbros contain pegmatitic to fine-grained hornblendes, plagioclase, clinopyroxene, and accessory opaque minerals. The fine-grained gabbros, on the other hand, are composed of plagioclase, hornblende, and biotite as major components whereas the apatite and opaque minerals are present in accessory content. Granitic– granodioritic rocks are the common intrusive rock types in the area, and constitute quartz, orthoclase, plagioclase and biotite, and accessory zircon and opaque minerals. Leucogranites, comprising quartz, orthoclase, plagioclase with minor biotite, hornblende, and with accessory apatite and opaque minerals, are found as dykes intruding the marble and the granitic–granodioritic rocks. Strontium–neodymium isotope data of gabbros and granitoids have high 87Sr/86Sr(i) ratios (0.7076 to 0.7117) and low ɛNd(i) values (−5.0 to −9.8) point out enriched source and pronounced crustal contribution in their genesis. In the Hacımahmutuşağı area, it is plausible that the heat increase caused by the hot zone, which was generated by underplating mafic magma along with the hydrous mafic sills in the lower crust, might have resulted in partial melts from crystallized mafic sills and older crustal rocks. It can be suggested that these hybrid melts adiabatically rose to the shallow crust, ponded and crystallized there and formed the magma source of the intrusive rocks within the Hacımahmutuşağı area and the other hybrid granitic rocks with crustal signatures in the CACC. Geochemical data indicate that granitoids and gabbros are collision to post-collision related sub-alkaline rocks derived from an enriched source with extensive crustal inputs.

Shipboard Characterization of Tuvalu, Samoa, and Lau Dredge Samples Using Laser-Induced Breakdown Spectroscopy (LIBS)

Chemical analysis using laser-induced breakdown spectroscopy (LIBS) is well suited for field applications and was applied here for shipboard characterization of a large sample set during the RR1310 rock dredging expedition to the Tuvalu Seamounts. Although recently the most common data treatment for LIBS has consisted of a partial least squares approach to define sample groupings, we show that quantitative data of useful quality can be obtained with a univariate approach. Here, our analysis goal was a quantitative comparison of the total alkali (Na2O + K2O) versus silica (SiO2) contents of 586 representative dredge samples with known ranges in common rock types. Out of those samples, >400 form a single large group of alkalic basalts with minor basanites/tephrites (SiO2: 43-48 wt%, Na2O + K2O: 3-5 wt%), similar to known shield-stage compositions of the Rurutu and Samoa hotspots in the sampling area. In contrast, several dredge hauls contain samples with compositions that do not overlap with the majority of samples. This includes three dredges performed on the northern boundary of the Lau Basin that contain similar SiO2 compositions, but slightly higher total alkali (Na2O+K2O) content. Despite this difference, they classify as basanite/tephrite, similar to a subset of the main group. More importantly, similar compositions were previously reported from the same tectonic boundary, ascribed to hotspot mantle source material mixed into the Lau Basin back-arc. Although the quality of the compositional data suffices to enable sample selection for time-intensive analyses, higher precision is required for more in-depth petrogenetic interpretation. Error analysis based on repeat standard measurements suggests averaging 100 spectra per sample is optimal here, while use of a higher resolution spectrometer, together with better laser control, would improve results and interpretations.