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Heavy‐Mineral Grain Counting: Counting Techniques, Error Estimation, and the Number of Grains to be Counted
No full textHeavy‐mineral assemblages of sediments and sedimentary rocks record information regarding provenance, including the source rocks involved, tectonic setting, climatic conditions, and modifications from source to sink. Drawing conclusions on provenance and provenance changes requires robust quantification of individual heavy‐mineral species contents, including error estimates. Nevertheless, it is common practice to count sub‐quantities of grains from aliquots and not considering the bias introduced by (a) counting similar numbers of grains from aliquots containing different total numbers of grains, and (b) using variable counting methods. Consequently, reported heavy‐mineral contents estimated from counting sub‐quantities are affected by errors of unknown extent, making it infeasible to determine whether intra‐ or intersample variations are statistically significant. Based on 65 heavy‐mineral aliquots of variable grain size, mineral species contents, total number of grains, and known composition determined by counting all grains (
n
= 80,393), here >31 million countings of heavy‐mineral sub‐quantities are simulated using (a) ribbon counting with varying ribbon size, ribbon position, ribbon orientation, total number of counts, and ways of aggregating counts from multiple ribbons, and (b) a newly proposed counting technique called cluster counting. I show that (a) error estimation for a specific aliquot requires a finite population correction; (b) compared to adjacent ribbons, aggregating counts of spatially distant ribbons reduces the error; (c) cluster counting further reduces the error, showing the best fit with theory; and (d) the Wilson score interval enables error calculation as well as the number of grains to be counted to achieve an operator‐specific aim.Plain Language Summary
Rocks exposed to rain, ice, and wind will disintegrate, get transported, and end‐up as sediment or sedimentary rock like sand or sandstone, respectively. Such sediments are mainly composed of light mineral grains including quartz and feldspar, but additionally contain heavy minerals with a density >2.85 g/cm
3
. While being volumetrically subordinate, the types and proportions of different heavy‐mineral species are particularly diagnostic for the original rocks eroded to form the sediment under investigation. Commonly, heavy minerals are separated by standardized lab routines, and finally a portion of heavy minerals is counted under the microscope to estimate their content in the sediment. Unfortunately, there is no standardized routine of (a) how to count the grains, (b) how many grains to count, and (c) how to estimate the counting error. In this work, I computationally simulated countings and their errors using the most common technique (ribbon counting) with varying parameters as well as a newly proposed technique called cluster counting. I show that cluster counting not only reduces the counting error but also, more importantly, improves the predictability of the counting error. This provides scientists with a new tool to argue whether differences between different samples are statistically significant.Key Points
Counting errors and their variations are related to the capability of the applied counting technique to counteract spatial heterogeneity
Area counting is inappropriate to counteract spatial heterogeneity, but spacing out ribbons evenly gives reasonable results
A newly proposed counting technique (cluster counting) reduces the bias, enabling error calculations and the number of grains to be counte
Comment on: “Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the neoproterozoic Brasília Orogen, west Gondwana margin”
No full textCorrection to: Garnet major-element composition as an indicator of host-rock type: a machine learning approach using the random forest classifier
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The sedimentary record of ultrahigh-pressure metamorphism: a perspective review
No full textGerman Research Foundatio
Life-cycle analysis of coesite-bearing garnet
Full text linkDetrital coesite-bearing garnet is the final product of a complex geological cycle including coesite entrapment at ultra-high-pressure conditions, exhumation to Earth’s surface, erosion and sedimentary transport. In contrast to the usual enrichment of high-grade metamorphic garnet in medium- to coarse-sand fractions, coesite-bearing grains are often enriched in the very-fine-sand fraction. To understand this imbalance, we analyse the role of source-rock lithology, inclusion size, inclusion frequency and fluid infiltration on the grain-size heterogeneity of coesite-bearing garnet based on a dataset of 2100 inclusion-bearing grains, of which 93 contain coesite, from the Saxonian Erzgebirge, Germany. By combining inclusion assemblages and garnet chemistry, we show that (1) mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and (2) felsic garnet contains variable amounts of coesite inclusions per grain, whereby coesite-poor grains are enriched in the coarse fraction and coesite-rich grains extensively disintegrated into smaller fragments resulting in an enrichment in the fine fraction. Raman images reveal that: small coesite inclusions of dimension < 9 µm are primarily monomineralic, whereas larger inclusions partially transformed to quartz; and garnet fracturing, fluid infiltration and the coesite-to-quartz transformation is a late process during exhumation taking place at c. 330°C. A model for the disintegration of coesite-bearing garnet enables the heterogeneous grain-size distribution to be explained by inclusion frequency. High abundances of coesite inclusions cause a high degree of fracturing and fracture connections to smaller inclusions, allowing fluid infiltration and the transformation to quartz, which in turn further promotes garnet disintegration
Tracing ultrahigh-pressure metamorphism at the catchment scale
No full textAbstractFinding traces of ultrahigh-pressure (UHP) metamorphism in the geological record has huge implications for unravelling Earth’s geodynamic evolution, such as the onset of deep subduction. Usually, UHP rocks are identified by specific mineral inclusions like coesite and characteristic petrographic features resulting from its (partial) transformation to the lower-pressure polymorph quartz in thin sections of crystalline rocks. This approach relies on very small sample size and is thus limited to a few points within large regions. Here we present the first findings of coesite inclusions in detrital mineral grains. The intact monomineralic inclusions were detected in garnets from a modern sand sample from the Western Gneiss Region, SW Norway. They represent the first known intact monomineralic coesite inclusions in the Western Gneiss Region, and their presence is suggested to indicate the erosion of UHP rocks in the sampled catchment area. The novel approach introduced here allows for tracing UHP metamorphic rocks and their erosional products at the catchment scale instead of being limited to outcrops of crystalline rocks. It opens new avenues for the prospective exploration of UHP metamorphism in Earth’s geological record.</jats:p
Intra‐Seasonal Variability in Sediment Provenance and Transport Processes in the Brahmaputra Basin
No full textAbstractSediment composition in modern fluvial settings is commonly assessed regarding spatial but rarely temporal variability, potentially leading to a bias of unknown extent. Here, we present the grain‐size distribution, bulk chemical and mineralogical composition of a time‐series set of 36 suspended sediment samples from the Brahmaputra river, as well as clay and heavy mineral analysis of selected samples. Sampling covers the June–November 2021 period, which included two major flooding events. We show that the two flooding events are characterized by contrasting grain size, with the first event characterized by a grain‐size minimum and the second by a grain‐size maximum. Although grain sizes of the first flood and the period after the second are similar, their compositions differ significantly, highlighted by a factor‐two decrease of biotite largely compensated by an increase in quartz. By contrast, the content of garnet, clinopyroxene, sillimanite, and rutile increased compared to epidote and amphibole during the second flood event. By relating the results to spatio‐temporal rainfall and discharge patterns and basin morphology, we conclude that the first flooding primarily mobilized hydraulically pre‐sorted sediments from the exposed sandbars of the floodplains, while those sandbars are already submerged during the second flooding in a single‐channel system, resulting in higher sediment contributions from highland tributaries draining igneous and high‐grade metamorphic rocks. Such temporal variations pose constraints on the interpretation of compositional differences between individual samples regarding sediment provenance and dispersal and should be considered in studies of modern drainage basins as well as ancient sediment routing systems.Plain Language Summary: Sediment provenance, which refers to where the sediment in a river comes from, is important to understand because it can tell us about the geology of an area, various earth‐surface processes and how the landscape is changing over time. However, sediment provenance is typically studied at a spatial scale in present day river basins, and temporal variability is rarely considered. This study examines the physical, chemical and mineralogical properties of sediment in the Brahmaputra river during two major flooding events that occurred in the same season. The results show that the sediment composition varies between the events, indicating a change in the relative proportions of distinct sources. This emphasizes the importance of considering temporal variations in sediment composition when interpreting sediment provenance signals.Key Points:
Time‐series analysis of sediment composition during two major flooding events of a single monsoon season is presented
The two flooding events show contrasting grain‐size, chemical and mineralogical composition
Temporal variations in sediment composition pose constraints on the interpretation of provenance and dispersal based on individual samples
DAADGerman Ministry of Education and Researchhttps://doi.org/10.5281/zenodo.7588054http://flood.umd.edu
Early Jurassic initiation of the modern drainage pattern of the Dabie orogen (East China) revealed by a multi‐proxy provenance approach
No full textAbstract The timing of the initiation of the present‐day tectonic architecture and drainage systems in eastern China remains debated. This study presents a comprehensive provenance study of the Early Jurassic peripheral basins surrounding the Dabie orogen including framework petrography, heavy‐mineral analysis, single‐grain chronology and chemistry. Clasts of high‐grade schist, muscovite grains, rare gneissic fragments, abundant metamorphic garnet and phengite (Si > 3.3 pfu), combined with a main 216–256 Ma rutile U–Pb population found in these Early Jurassic sandstones, indicate a source from the Triassic (U)HP belt in the Dabie orogen. Sedimentary lithics and ultra‐stable heavy‐mineral assemblages indicate an additional source of recycled sedimentary rocks. Combined with the continuous shift of the youngest detrital rutile age population toward younger ages toward the north that mimics the pattern of metamorphic bedrock ages in the Dabie orogen, we infer that the present surface tectonic architecture and paleodrainage patterns of the Dabie orogen were established in the Early Jurassic. Thus, the Early Jurassic exhumation of the Dabie orogen marked the development of the watershed between Northern and Southern China, namely the Huai River and several principal tributary systems of the middle‐lower Yangtze River.National Natural Science Foundation of China https://doi.org/10.13039/50110000180
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