Modal Grain Research Articles

Using the traditional microscope for mineral grain orientation determination: A prototype image analysis pipeline for optic-axis mapping (POAM).

This paper reports on the development of an open-source image analysis software 'pipeline' dedicated to petrographic microscopy. Using conventional rock thin sections and images from a standard polarising microscope, the pipeline can classify minerals and subgrains into objects and obtain information about optic-axis orientation. Five metamorphic rocks were chosen to test and illustrate the method. Thin sections were imaged using reflected and cross- and plane-polarised transmitted light. Images were taken at different angles of the polariser and analyser (360° with 10° steps), both with and without the full-lambda plate. The resulting image stacks were analysed with a modular pipeline for optic-axis mapping (POAM). POAM consists of external and internal software packages that register, segment, classify, and interpret the visible light spectra using object-based image analysis (OBIAS). The mapped fields-of-view and grain orientation stereonets of interest are presented in the context of whole-slide images. Two innovations are reported. First, we used hierarchical tree region merging on blended multimodal images to classify individual grains of rock-forming minerals into objects. Second, we assembled a new optical mineralogy algorithm chain that identifies the mineral slow axis orientation. The c-axis orientation results were verified with scanning electron microscopy electron backscattered diffraction (SEM-EBSD) data. For quartz (uniaxial) in a granite mylonite the test yielded excellent correspondence of c-axis azimuth and good agreement for inclination. For orthorhombic orthopyroxene in a deformed garnet harzburgite, POAM produced acceptable results for slow axis azimuth. In addition, the method identified slight anisotropy in garnet that would not be appreciated by traditional microscopy. We propose that our method is ideally suited for two commonly performed tasks in mineralogy. First, for mineral grain classification of entire thin sections scans on blended images to provide automated modal abundance estimates and grain size distribution. Second, for prospective fields of view of interest, POAM can rapidly generate slow axis crystal orientation maps from multiangle image stacks on conventionally prepared thin sections for targeting detailed SEM-EBSD studies.

Seasonal ice-covered lake surface likely caused the spatial heterogeneity of aeolian sediment grain-size in the source region of Yellow River, northeastern Tibetan Plateau, China

The area of lakes in Tibetan Plateau (TP) is 36522 km2, accounting for nearly half (49.1%) of the total lake surface area in China, and the lakes in TP are seasonally ice-covered for 4–5 months per year. In such a high-cold Third Pole with extensive lakes, how does aeolian sediment transport on ice cover and to what extent can seasonal ice-covered lake cause sediment redistribution by providing pathways for sediment migration is rarely studied. The source region of Yellow River (SRYR) is located in the northeastern TP with an altitude above 4000 m, is home to large area of seasonal frozen lakes. Nine sections of aeolian sediments were collected from SRYR for grain-size study. The end-member modeling analysis (EMMA) provides a greater chance of resolving aeolian sediment sources since it can quantitatively separate the particle size components of various sedimentary dynamic processes and sources in the sediment. The result shows great spatial difference of the mean grain sizes (mainly varying between 70 and 230 μm). Parametric EMMA is applied to study the provenance tracing of aeolian sediment, combining with remote sensing images and wind data. Aeolian processes were analyzed by separating and extracting the grain size end-members of nine sections, and four statistical end-members (modal grain size is 8.9, 79.5, 141.6, and 251.8 μm, respectively) were classified from the grain size distribution. It shows that the sedimentary sequences in sections 7 and 8 have high EM2 and EM3 fractions and very low EM4 content at all depths. Based on comprehensive analysis of aeolian sediment grain-size, phenology of ice lake, wind regime and remote sensing images, it revealed that the fine aeolian sediments (sections 7 and 8) on the downwind shore of Ngoring Lake likely transported from the upwind shore, which were blown across the ice-covered lake surface by prevailing west wind in winter and spring, but the coarse sediments could be trapped by ice cracks. Therefore, it’s concluded that the aeolian sediment transport on seasonal ice-covered lakes may lead to the spatial heterogeneity of aeolian sediment grain-size in the SRYR.

Investigating Potential Links Between Fine-Grained Components in Loess and Westerly Airflow: Evidence From East and Central Asia

Loess is a ubiquitous, silty aeolian sediment common across the semiarid to subhumid regions in the Northern Hemisphere. As such, the physical characteristics of loess sediment, such as modal grain size and quartz crystallinity index, have the potential to inform us about dust transport pathways and corresponding atmospheric circulation responsible for entrainment, transport, and deposition. In this article, we combine detailed granularity analyses and non-parametric end-member modeling with investigations of the quartz crystallinity index of three loess deposits, extending from East to Central Asia, with the aim of identifying dust transport processes and the climate subsystems responsible for them. We find marked differences in the grain size characteristics of Eastern and Central Asian loess. Given potential linkages of fine particles to the westerly jet stream, here we focused on the fine-grained end-members in the loess. The fine-grained components of the Central Asian Bishkek (BSK) and Chashmanigar (CMG) loess may be derived from individual particles carried at high altitude by major climate subsystems such as the westerlies. However, similar quartz crystallinity indexes of fine- and coarse-grained components in the Central Asian loess indicated that surface-level wind strength might influence the releases from proximal-sourced regions and abundances in the upper atmosphere for the fine particles. By contrast, the fine-grained component of the East Asian [Fanshan18 (FS18)] loess seems to yield physical similarities to the Tertiary Red Clays underlying many loess stratigraphies, and has been interpreted to relate to transport associated with the East Asian winter monsoon. It was further supported by quartz crystallinity indexes of the <16 μm fractions in the FS18 loess, basically consistent with those of the deserts in North China, highlighting the near-surface northwesterly winter monsoon as the dominant transport dynamics of fine-grained dust. In addition, comparisons of quartz crystallinity indexes of fine-grained components between the East and Central Asian loess also suggest that variations in fine particles within the loess might not be related to transport by the higher-level westerlies, which is in disagreement with the previous model. Therefore, our results indicate that variations in fine-grained end-members in loess are difficult to reliably document the variabilities of higher-level westerlies.

Analysis of the size–dependency of relevant mineralogical and textural characteristics to particles strength

In developing a geometallurgical characterisation for comminution, the geological information, usually captured from drill core with a resolution of down to micrometre scale, is compared with the results of comminution tests. These tests require several kilograms of material and are empirical tests that do not focus on understanding the effect of ore characteristics separately from the machine functioning.The resultant particles from the comminution process depend upon the operating conditions of the comminution devices (i.e., ball size and load, rotational speed, mill specifications) and the heterogeneity of the rock characteristics (i.e., texture and mineralogy) that influence the mechanical properties of particles which are being reduced in scale from metres to micrometres. A newer approach for comminution modelling and characterisation considers the measurement of the particle strength and minimum required energy to break them at different sizes and its application as a parameter to characterise the breakage behaviour of the ores. However, the relationship between particle strength and minimum energy with the ore characteristics that may link this information to ore' comminution potential has not been explored yet.This study focuses on selecting variables that have been previously identified as relevant for strength at the core scale (i.e., mineralogy, mineral association, grain size, grain shape and porosity) and quantifying their variation as the particle size decreases from 30 to 3 mm. The results show that for a set of ten different rock types, the median value of ore characteristics such as modal mineralogy, mineral association and grain shape does not change with particle size. However, as the particle size is reduced over that size range, these characteristics do become more variable. This behaviour is similar to the variation observed in the strength values of samples, which shows size independence within the studied particle size range, but it is not clear which characteristic has more relevance to the strength results.Detailed geological characterisation information is rarely used to predict how the geological variability may affect the plant's operation. The results from this work indicate that mineralogical and textural features might be controlling the strength of particles formed during comminution. By understanding the physics of the fracture process, these characteristics may be identified. So, a novel link between the detailed mineralogical and textural characterisation at different particle scales and the strength of particles could be developed, which would be helpful in the connection of geological data with the newest developments in comminution research.

Sea level change and Kuroshio intrusion dominated Taiwan sediment source-to-sink processes in the northeastern South China Sea over the past 244 kyrs

The terrigenous sediment source-to-sink processes in continental margins are determined by complex interactions among climate, sediment discharge, sea level and oceanic circulations on various temporal and spatial scales. The northeastern South China Sea (NSCS) margin is a natural laboratory to catch a glimpse of these processes due to large amounts of terrigenous sediment inputs and continuous sedimentations during the late Quaternary. Based on a 35 m-long sedimentary sequence (core MD12-3429) retrieved from the NSCS upper continental slope, we distinguished sediment sources based on geochemical compositions, and then linked sedimentary responses to sea level change and Kuroshio intrusion over the past 244 kyrs via various sedimentary records including mass accumulation rates (MAR), foraminifera data, grain-size end-member modeling (EMM) and spectral analysis. Geochemical proxies including major element ratios and fractionation parameters of rare earth elements suggest that the detrital sediment in the core was derived primarily from the Taiwan. The MAR of terrigenous sediment yields an average 9.6 g/cm2/kyr, and high MAR values are both observed in glacial and interglacial intervals. The results of EMM reveal three end-members with dominant modal grain sizes of 6.6 μm (EM1), 26.3 μm (EM2) and 49 μm (EM3), respectively. The temporal variations of the finest end-member (EM1) and the coarsest end-member (EM3) demonstrate clearly glacial-interglacial cyclicity, and the spectral analysis indicates the dominance of 100-kyr eccentricity, which suggests that sea level changes are the first-order control for the NSCS continental margin sedimentation. In addition, the temporal variations in hydrodynamic sensitive component (EM2) and its MAR display gradually increasing trends with weakening East Asian monsoon. Combing with modern observation and modeling results, we suggest that enhanced Kuroshio intrusion might account for the transport of Taiwan-derived sediment during the late Quaternary. As indicated by a 31 kyr periodicity in EM2, we infer that the development of El Niño-like condition strengthened the intrusion of oligotrophic Kuroshio from the Luzon Strait, resulting in the obvious increase of Taiwan terrigenous MAR but decreases of the primary productivity in the northeastern South China Sea. This study provides deep insight into the complex terrigenous sediment source-to-sink processes in a fast-changing marginal sea environment during the late Quaternary.

Reconstruction of Aeolian Activity Variability in the Central Tibetan Plateau From Grain-Size Distributions: End-Member Modeling and Source Attribution of Sediments From Lake Selin Co

The grain size analysis and end-member modeling of clastic fractions in the Selin Co basin sedimentary sequence have been revealing variations of aeolian activity in the central Tibetan Plateau since 3,900 cal yr BP. The grain-size distributions measured in this study were classified into three statistical end-members that may be connected to clastic material inputs from a variety of sources and transport processes. Among these, the EM3 fraction (modal grain size of ∼138 μm) was suggested to indicate the aeolian activity history in the central Tibetan Plateau. The results indicate that the aeolian activity in this region occurred under dry and cold conditions since 3,900 cal yr BP, with three strong aeolian activity intervals at 3,200–2,900 cal yr BP, 2,400–1,400 cal yr BP, and after 130 cal yr BP. Comparison with other records showed that solar insolation, weakening Indian summer monsoon, and the strengthening westerlies have a direct relationship with the aeolian activity in the central Tibetan Plateau. We therefore suggest that the atmospheric circulation patterns and effective humidity may have a strong impact on the water balance and weathering intensity in the arid and semiarid areas, hence affecting dust emissions. Also, the coarse detrital end-member fraction of lake sediments is suggested as a potential aeolian activity proxy.

Variation of the winter mid-latitude Westerlies in the Northern Hemisphere during the Holocene revealed by aeolian deposits in the southern Tibetan Plateau

AbstractThe mid-latitude Westerlies (MLW) are one of the most important atmospheric circulation systems in the Northern Hemisphere, exerting a huge influence on the climate of the region downwind, and thus on vegetation, water resources, and human wellbeing. However, the seasonal variation of the MLW during the Holocene is not yet been fully understood, especially when its contribution is the most important. Here, we used end-member (EM) modeling analysis of the grain-size distributions of a high-altitude aeolian sedimentary sequence (4452 m a.s.l.) from the Yarlung Zangbo River valley in the southern Tibetan Plateau to reveal variations in the winter MLW during the Holocene. Analysis of seasonal differences in modern atmospheric circulation suggests that the southern Tibetan Plateau was heavily influenced by the mid-latitude Westerlies at the 400, 500, and 600 hPa levels in winter, while it was seldom influenced at these levels in summer. Four grain-size end-members are identified, representing distinct aerodynamic environments, of which EM1 (modal grain size 8.1 μm) can be used as a proxy of the winter MLW. A reconstruction of the variation of the winter MLW during the Holocene based on EM1 revealed that a weaker winter MLW occurred during the Early to Middle Holocene, and a stronger winter MLW during the Middle to Late Holocene. Overall, we suggest that this change in the winter MLW was closely related to the insolation/temperature/pressure gradient between low and high latitudes in the Northern Hemisphere.

Origin of Fe–Ni–Cu (Co) sulfide and Fe–Ti oxide minerals in the c. 1.77 Ga dolerite dyke, Singhbhum Craton (eastern India)

Abstract Dolerite dyke swarms are widespread within the Singhbhum Craton (eastern India) that emplaced from the Neoarchean to Paleoproterozoic era just after the stabilization of crust before c. 3 Ga. These dyke swarms are oriented in NE–SW to NNE–SSW, NW–SE to WNW–ESE, east–west and north–south directions. The WNW–ESE-trending c. 1.77 Ga Pipilia dyke swarm is sampled from the Satkosia area of Orissa state. The dyke shows a noticeable disparity in terms of the modal proportion and grain size of pyroxenes, plagioclase, Fe–Ti oxide minerals and texture across the trend. In places the primary silicates are altered to secondary hydrated mineral assemblages of amphibole, chlorite and sericite. The primary silicates are clinopyroxene (augite – Mg# = 65.7–82.6; En 37–48 Fs 11–17 Wo 36–41 ), orthopyroxene (clinoenstatite – Mg# = 68.5–78; En 63–70 Fs 20–29 Wo 4–5 ) and plagioclase (An 11–39 Ab 44–82 Or 1–7 ) and the Fe–Ti oxides are titanomagnetite (FeO, 34.38–39.50 wt%; Fe 2 O 3 , 48.26–56.21 wt%; TiO 2 , 5.05–9.60 wt%) and ilmenite (FeO, 40.75–43.79 wt%; Fe 2 O 3 , 3.54–10.03 wt%; TiO 2 , 47.82–50.87 wt%). Application of two-pyroxene thermometry yields an equilibration temperature range of 1065–978°C, and coexisting titanomagnetite–ilmenite pairs reveal 731.39–573.37°C under the oxygen fugacity ( f O 2 ) condition NNO + 0.3 to FMQ − 1.03. The dyke contains disseminated sulfides at the interstices of Fe–Ti oxides and silicates. Major sulfide minerals are pyrite, chalcopyrite and vaesite; pyrite–vaesite assemblages occur in association with secondary silicate minerals. Pyrite grains contain variable concentrations of Co (0.01–5.70 wt%) and Ni (0.02–1.95 wt%). Coexisting vaesite contains Co (2.42–10.44 wt%), Ni (26.40–47.88 wt%) and Fe (7.32–26.55 wt%). Texture, sulfide–silicate assemblage and the presence of low metal/S sulfides such as the pyrite–vaesite assemblage indicate that primary Fe–Ni sulfides (pyrrhotite–pentlandite) that segregated from immiscible sulfide liquid at high temperature are modified by late magmatic/hydrothermal fluid activities. Numerous sulfide-bearing deposits hosted in ultramafic–mafic intrusions of Paleoproterozoic age have been recorded globally and the occurrence of Fe–Ni sulfides in the c. 1.77 Ga Pipilia dyke swarm in the Singhbhum Craton enhances the exploration potential of this craton in eastern India.

Nebular thermal processing of accretionary fine-grained rims in the Paris CM chondrite

Fine-grained rims (FGRs) are ubiquitous in chondrites. They consist of unequilibrated mineral assemblages that surround chondrules and refractory inclusions. As such, they carry information about the material that was accreted onto chondrules. To decipher the nature and the formation mechanism of FGRs and compare them to adjacent matrix material, we investigated their composition, mineralogy, density and texture in the pristine Paris CM chondrite. We coupled a new method at the SEM scale (ACADEMY) that allows high-resolution quantitative petrology and an analytical TEM study.Significant differences in modal abundance, grain size and porosity are observed between the FGRs and their adjacent matrix. Amorphous silicates domains embedding nanosulfides are indicative of a high preservation degree. They are less abundant in the matrix than in the rims. In contrast, secondary alteration phases (phyllosilicates, carbonates and tochilinites) are more abundant in the matrix and associated with larger and fewer sulfides grains. The similar composition of the amorphous silicate in the rims and the matrix attests for a close relationship between the two reservoirs. However, matrix underwent more aqueous alteration. We interpret it as the result of the accretion of material with a higher water/rock ratio in the matrix, leading to a more aqueously altered microenvironment. We also find that coarse-grained anhydrous silicates (olivine and pyroxene) are present in the matrix but not in the FGRs, likely as a result of a chondrule fragmentation episode that occurred after FGR but before matrix accretion.Most of the time, FGRs display distinct inner and outer layers. The inner part is compact and displays larger sulfide grains than the outer part, which is more porous (porosity ∼ 45%) and altogether more pristine. These mineral and textural differences are not easily explained by differential aqueous alteration. Instead, a pre-accretion thermal process that preferentially affected the inner rim could have induced loss of porosity, compaction of the amorphous silicate domains as well as sulfides growth. We therefore suggest that FGRs acquired their characteristics in the nebula before matrix accretion and discuss possible mechanisms such as dust heating in the chondrule formation environment or secondary heating episode of the previously rimmed chondrule.

A novel protocol for mapping the spatial distribution of storm derived sediment in lakes

A novel geomatics methodology is presented for locating optimal lake coring sites to potentially capture evidence of paleo-storms. One hundred sediment–water interface samples collected from Harvey Lake, NB, Canada (45° 43′ 45″ N; 67° 00′ 25″ W) were analyzed using: end member mixing analysis (EMMA), which can be used to recognize modal grain size distributions derived from sediment resuspension during major storms; and Itrax X-ray fluorescence core scanning-derived Ti, an indicator of catchment runoff, which is enhanced during major storm events. Simple geospatial models based on lake bathymetric and historical wind speed data (Fredericton INTL A climatological station; 1953–2015) were used to determine lake bottom areas susceptible to wave base sediment resuspension. EMMA End Member (EM) 02 (mode = 40 μm) was widely distributed in areas > 4.4 m water depth, which have been unimpacted by wave base remobilization since 1953. Deposition of EM 02 in deeper water areas was interpreted to be of major storm derivation, the result of fallout of resuspended sediments from the water column. This EM was most concentrated in the central part of the lake at >6 m water depth, as well as at the z-max (~ 11 m), and in Herbert’s Cove (3–6 m). The main source of run-off derived Ti into the lake was through Sucker Brook, with the highest concentrations in Herbert’s Cove and the central part of the lake, including the lake z-max. This assessment indicates that the best undisturbed sedimentary record of paleo-storms is mostly likely in the central part of the lake north of the z-max at water depths of > 6 m, as well as deeper water areas of Herbert’s Cove.

Provenance and tectonic setting of the Ordovician sedimentary succession at the southeastern margin of the Yangtze Block, South China: Implications for paleotopographic evolution of northestern Gondwana

Abstract Ordovician sedimentary strata at the southeastern margin of the Yangtze Block consist of, from bottom to top, fine-grained siliciclastic rocks, cherts with interlayers of siliceous/carbonaceous slate, and coarse-grained siliciclastic rocks, which are important for understanding the tectono-sedimentary evolution in South China. A combined analysis of petrography, whole-rock geochemistry, and zircon U–Pb dating was carried out on these strata. The cherts with radiolarian fossils have high Al/(Al + Fe + Mn) values (0.38–0.94) and no obvious Ce anomalies (1.09 ± 0.05), suggesting a biogenic origin on a continental margin. The geochemical features of the siliciclastic rocks, including ratios of elements, discriminant functions, and ternary diagram positions, suggest a passive margin setting that received detritus mostly derived from intermediate to felsic igneous rocks and recycled quartzose sedimentary rocks. The source areas of the Ordovician siliciclastic rocks existed in semiarid–arid paleoclimate and moderate weathering conditions. The sandstone modal grain compositions and detrital zircon cumulative curves indicate a collisional setting of the source areas. Zircon age populations have been time-varying during the Early Paleozoic. Based on the above-mentioned evidences, the following basin filling and paleotopographic evolution process has been proposed in research area. In the Cambrian, a bilateral filling pattern reflected a flat and broad basin. In the Early Ordovician, longer transportation of SE-sourced sediments reflected the uplift of Gondwana. In the Late Ordovician, uplift of the Wuyi–Yunkai Orogen blocked some Gondwanan detritus (500–700 Ma) and released recycled (900–1100 Ma) and syn-orogenic detritus. Meanwhile, synchronous uplift of the Jiangnan Orogen led to a steeper and narrower basin, and sediments could reach farther distances in a shorter time. In the Silurian, a unidirectional filling pattern with overwhelming SE-sourced detritus indicated intense uplift of Gondwana and the Wuyi–Yunkai Orogen.

Cover Image, Volume 103, Issue 4

Cover Photograph: Optical micrograph of a polycrystalline alumina surface containing a Vickers indentation performed in air: image in reflected light; alumina modal grain size about 15 μm; indentation by 50 N load to generate impression width about 70 μm. Radial cracks at impression corners are significantly influenced by material microstructure, subsurface lateral cracks apparent as bright areas. Details in feature article (alumina equivalent to material A6). Credit: Robert F. Cook. DOI:10.1111/jace.16925. image

A source of loess-like deposits and their attendant palaeoenvironment – Orava Basin, Western Carpathian Mountains, S Poland

Abstract Loess-like deposits in the Orava Basin are aeolian sediments that were laid down during Pleistocene three cold periods: MIS 2-5d, MIS 6-10, and MIS 12-19. These deposits lie at the upper portion of glaciofluvial fans associated with the Czarny Dunajec River in the Orava Basin and are the youngest lithostratigraphic division that is present there. Results of granulometric analyses place their modal grain size in the coarse silt and fine sand fractions (sediment type 1.a). Geomorphic interactions between local topography, aeolian movements of sand and distribution of dust played the leading role in formation of these deposits. Two series of sediments have been distinguished: Series A which represents proximal depositional provenance, and Series B – distal depositional provenance. The source of the loess-like deposit material must have been local as the Orava Basin is isolated from foreland by highly elevated mountain ranges. Glaciofluvial sediments were deflated then laid down on fans at the bottom of the Orava Basin by rivers draining the western Tatra Mountains. Also ‘cannibalized’ were top portions of loess-like deposits that had accumulated during earlier Pleistocene cold periods. Fine sediment was also deflated from regolith mantle that formed on the slopes of surrounding hills. This conclusion is based on the results of analyses of microtextures of sand- and silt-sized grains. Grains with surfaces that were primarily shaped by chemical weathering (coated) and by mechanical weathering (fresh fractured) predominate both fractions. These sand grains were transported over a short distance; they were not subject to repeated redeposition.

Stratigraphic analysis of a sediment pond within the New England Mud Patch: New constraints from high-resolution chirp acoustic reflection data

The New England Mud Patch (NEMP) is an anomaly on the Atlantic coast of North America. This ~13,000 km2 area, located south of Cape Cod between the ~60 m and 160 m isobaths, is a region of active fine-grained deposition on a shelf that is predominantly non-depositional or erosional. Prior studies theorized that NEMP sediments are derived from fines winnowed from Georges Bank, transported westward by coastal currents, and then settled in more quiescent conditions. A chirp seismic reflection (2015) and coring (2016) survey of part of the NEMP was conducted in support of a planned acoustic experiment for the Office of Naval Research. The survey focused on a ~30 km (E-W) by ~8 km (N-S) region between the 70 m and 85 m isobaths, encompassing a sediment “pond” >12 m thick. The dense (250 m) survey lines allow a pseudo-3D stratigraphic interpretation. The sediment pond occupies an embayment eroded into pre-LGM substrate sediments, perhaps by glacial outwash. Marine sands beneath the mud are organized into oblique sand ridge morphology. The sense of obliquity, morphologic asymmetry, and internal dipping reflectors indicate that the sand ridges formed under east-directed transport. However, as evidenced by westward-prograding depocenters and onlap, NEMP deposition occurred under west-directed transport, consistent with modern shelf conditions. The onset of fine-grained deposition was therefore contemporaneous with a significant shift in the hydrologic regime. Muds deposited immediately above the sand ridges include a significant sand component, whose modal grain size is identical to that of the sand ridges. This admixture may indicate that, during the early stages of mud deposition, the tops of the sand ridges remained unburied and exposed to reworking. Episodic, storm-driven transport of sand to the muddy deposits may account for some of the sand component of NEMP sediments.

The effect of mineralogical parameters on the mechanical properties of granitic rocks

The mechanical properties of a given rock volume are dependent on its geological composition. Being able to predict these mechanical properties based on mineralogical parameters could prove useful in engineering and construction settings. The aim of this study is to propose relationships between mineralogical parameters and mechanical properties of granitic rocks. Furthermore, relevant correlations proposed previously in literature were analyzed within the context of a larger dataset.An extensive literature search was performed to compile a database of laboratory compression testing data for granitic rock. Mineralogical parameters and mechanical properties of 58 distinct granitic rock types from 12 countries were analyzed in order to obtain a more complete view of their interrelationships. The following geological properties were compiled: grain sizes (overall and mineral-specific), modal percentages of minerals (quartz, alkali feldspar, plagioclase, and micas), density, and average Mohs Hardness. The following mechanical properties were assessed: Unconfined Compressive Strength (UCS), Brazilian Tensile Strength (BTS), crack initiation stress (CI), crack damage stress (CD), Young's Modulus (E), Poisson's ratio (ν), and the Hoek-Brown constant, mi. The modal percentage of mica minerals was found to have a strong negative correlation with crack initiation stress, crack damage stress, and tensile strength. Additionally, minimum overall grain size is strongly correlated with the CD/CI ratio. A multivariate regression analysis utilized various combinations of five independent variables (minimum overall grain size, average overall grain size, standard deviation of overall grain size, quartz modal percentage, and mica modal percentage) to predict CI, CD, and tensile strength. Multivariate analysis showed that the use of two geological predictor variables typically improved models for CI, CD, and BTS; however, the addition of further predictor variables produced minimal additional improvement. The most important finding of this analysis was that joint consideration of mica modal percentage and grain size parameters allows for improved predictions of CI, CD, and tensile strength to be made for granitic rock.Additionally, it was found that correlations proposed previously in literature appear to be spurious when considered in the context of the larger data set compiled in this research. This occurrence is most likely a consequence of unknown additional variables (confounding variables) that is correlated to both the independent and dependent variables used in forming the original correlations.

Temporal variations of bed load transport rate and the grain size distribution of non-uniform size sediment during a constant flow rates

Bedload transport rates at a point using both laboratory and field measurement vary with time, from almost zero to several folds of the mean rate. The need to carry out point measurements over a sufficiently long sampling period is very important to cover inconsistency in the transport rate during a short period of observation. In this experiment the bedload was collected at a slot type sediment trap with ten-minute intervals throughout the tests. The aim was to examine the behaviour of mixed grain size sediment transportation during constant flowrates. Two hydrographs were applied to quantify the transport mode changes caused by an increase in time length as the mixtures were continuously exposed to the flow. It shows that intense bed degradation process occurred within three hours of the tests as indicated by almost identical rates of bedload transport during this period and the diminishing rate in the remaining hours. This reflects that a stable bed was achieved after three hours in which the armouring process was believed to take place. The grain size distribution suggests that the proportion of fine mode was relatively stable and the modal grain size of this mode experienced a similar pattern throughout the tests whilst the coarse mode was inconsistent and varied periodically.

Compositional variations in sands of the Bagnold Dunes, Gale crater, Mars, from visible‐shortwave infrared spectroscopy and comparison with ground truth from the Curiosity rover

AbstractDuring its ascent up Mount Sharp, the Mars Science Laboratory Curiosity rover traversed the Bagnold Dune Field. We model sand modal mineralogy and grain size at four locations near the rover traverse, using orbital shortwave infrared single‐scattering albedo spectra and a Markov chain Monte Carlo implementation of Hapke's radiative transfer theory to fully constrain uncertainties and permitted solutions. These predictions, evaluated against in situ measurements at one site from the Curiosity rover, show that X‐ray diffraction‐measured mineralogy of the basaltic sands is within the 95% confidence interval of model predictions. However, predictions are relatively insensitive to grain size and are nonunique, especially when modeling the composition of minerals with solid solutions. We find an overall basaltic mineralogy and show subtle spatial variations in composition in and around the Bagnold Dunes, consistent with a mafic enrichment of sands with cumulative aeolian‐transport distance by sorting of olivine, pyroxene, and plagioclase grains. Furthermore, the large variations in Fe and Mg abundances (~20 wt %) at the Bagnold Dunes suggest that compositional variability may be enhanced by local mixing of well‐sorted sand with proximal sand sources. Our estimates demonstrate a method for orbital quantification of composition with rigorous uncertainty determination and provide key constraints for interpreting in situ measurements of compositional variability within Martian aeolian sandstones.

The paleoenvironmental significance of spatial distributions of grain size in groundwater‐recharged lakes: A case study in the hinterland of the Badain Jaran Desert, northwest China

AbstractFor lakes in desert hinterlands that are not recharged by river runoff, sediment input solely comes from wind transport. While the processes of sediment transport and deposition in these lakes differ significantly from those with river discharge, the spatial distribution of sediment grain size in these groundwater‐recharged lakes remains largely unknown. Moreover, whether the grain size distribution in these lake sediments can be used as a proxy in the study of past climatic change and environmental evolution studies is unclear. In this study, five lakes with a range of surface areas that had no runoff recharge were selected from the hinterland of the Badain Jaran Desert of north‐western China, and a total of 108 samples of lake surface sediments were collected to examine the spatial distribution of grain size. Moreover, an end‐member‐modeling algorithm was used to calculate end members from all grain size measurements. Our results showed that both the median and mean grain sizes in the lake sediments decreased from the nearshore to the offshore, deep‐water zone. However, the lowest median and mean grain sizes were not found in the center of the lakes, in contrast to lakes recharged by surface runoff. The median grain size of sediment in the lake center was negatively correlated with lake level, and thus could help reveal lake evolution at low resolutions. Moreover, EM1 and EM2 were interpreted as wind transported sediment, and sediment perturbed by lake waves after wind transport, respectively. The modal grain size of EM1 varied slightly between lakes, while changes in the modal grain size of EM2 were related to lake area. Given the positive relationship found between EM2 content and lake level, changes in the EM2 content (%) can serve as a rough indicator of lake level fluctuations at low temporal resolutions. Copyright © 2017 John Wiley & Sons, Ltd.

An aeolian sediment reconstruction of regional wind intensity and links to larger scale climate variability since the last deglaciation from the east coast of southern Africa

Few long-term environmental records are available for southern Africa where shifts in atmospheric circulation and changes in sea surface temperatures interact to influence regional climate dynamics. We present downcore grain size and inorganic geochemistry data covering the last ~23,000years from a peatland on the east coast of South Africa and examine links between shifts in regional wind activity and palaeoclimatic variability. Our record documents substantial variations in aeolian flux associated with changes in regional climate and wind patterns that reflect larger scale atmospheric circulation patterns. Substantially higher fluxes observed during the Last Glacial Maximum (LGM) are linked to widespread aridification and an expansion in local source areas brought about by a clear shift to dry and cool conditions. Variations in grain size distribution reveal that the aeolian record from Mfabeni comprises two dominant end-members; locally-derived coarse-grained material and a more fine-grained dust component. Marked changes in composition and modal grain size suggest that hydrological shifts in the region during the LGM were accompanied by an increase in storm frequency and wind strength that we link to a northward displacement in the westerly wind belt and a strengthening in wind intensity. Coupling between a rapid increase in sea surface temperature (SST) and an approximate three-fold decrease in aeolian activity after 15kcalyrBP suggests that changes in SST and its effect on the position and intensity of the westerlies in the Southern Ocean was the dominant climatic driver in the region during deglaciation. Substantially lower aeolian activities through the early Holocene indicate a warming in regional climate and the establishment of more humid conditions under the influence of enhanced tropical easterly flow. Our record also documents more subtle changes in climate over the mid to late Holocene and provides support for an arid phase in southern African climate 6–4kcalyrBP, as well as an increase in climate variability associated with a strengthening in El Niño–Southern Oscillation (ENSO) activity ~2kcalyrBP. The study contributes to current knowledge of atmospheric circulation patterns in the Southern Hemisphere and provides new insight into links between aeolian activity, regional wind patterns and climatic variability over glacial-interglacial timescales for a region where existing palaeoclimate records are scarce.

Effects of intensification of the Indian Summer Monsoon on northern Andaman Sea sediments during the past 700 years

Recent studies have revealed an intensification of the Indian Summer Monsoon (ISM) over the past 700 years, but its influence on the sediments in the northern Andaman Sea is unclear. To examine this influence, we used radiocarbon-dated sediment core StMY6, acquired 100 km offshore in the northern Andaman Sea from the Ayeyarwady River mouth, and obtained a 700-year-long record of the grain-size distribution and geochemistry of the sedimentary sequence. The ISM influences precipitation surrounding the Ayeyarwady River, and thus the river water and sediment discharges to the northern Andaman Sea near the river mouth in the Ayeyarwady River and the weathering intensity in the river catchment. Based on the system, we propose that higher sedimentation rates and larger modal grain size (the most abundant size in the grain-size distribution of sediments) and lower carbonate concentrations after about AD 1600 can be attributed to higher ISM intensity. Profiles of total organic carbon (TOC) content and carbon/nitrogen (C/N) ratios, however, did not show any trends after about AD 1600, which suggests that the ISM has had little influence on these proxies at this site.