Exploring the pore fluid origin and methane-derived authigenic carbonate properties in response to changes in the methane flux at the southern Ulleung Basin, South Korea

We investigated the geochemistry of gas, pore fluid, and methane-derived authigenic carbonate (MDAC) from four sites in the southern Ulleung Basin, South Korea. In contrast to Sites 16GH-P1 and 16GH-P5, Sites 16GH-P3, and 16GH-P4 are characterized by acoustic chimney structures associated with gas flux. The composition of gas and isotopic signatures of methane (CH4) (C1/C2+ > 300, δ13CCH4 < -60‰, δDCH4 ≤ -190‰) indicate microbial source CH4 at all sites. The upward migration of CH4 can affect the chemical and isotopic properties of pore fluid and gas-related byproducts (e.g., gas hydrate (GH) and MDAC) within the shallow sediments including the current sulfate-methane transition (SMT) (< 5 meters below seafloor). Although no GH was found, elevated Cl- concentrations (maximum = 609 mM) with low δD and δ18O values in Site 16GH-P4 pore fluids delineate the influence of massive GH formation in deeper sediment. In contrast, relatively constant Cl-, δD, and δ18O values in fluids from Sites 16GH-P1, 16GH-P3, and 16GH-P5 indicate a predominant origin from seawater. Pore fluids also exhibit higher concentrations of H4SiO4, B, Mg2+, and K+, along with increasing alkalinity compared to seawater. These observations suggest that marine silicate weathering alters fluid chemistry within the sediment, affecting element and carbon cycles. High alkalinity (up to 60 mM) and Mg2+/Ca2+ ratios (> 6) alongside decreasing Ca2+ and Sr2+ concentrations imply carbonate precipitation. MDACs with diverse morphologies, mainly composed of aragonite and magnesian calcite, and characterized by low carbon isotopic values (δ13CMDAC < -31.3‰), were found at Sites 16GH-P3 and 16GH-P4. Interestingly, δ13CMDAC values at Site 16GH-P3 are clearly differentiated above and below the current SMT. High δ13CMDAC values above the SMT (> -34.3‰) suggest the combined influence of seawater and CH4 migrating upward on MDAC precipitation, whereas low δ13CMDAC values below it (< -41.6‰) indicate a predominant impact of CH4 on MDAC formation. Additionally, the vertical variation of δ18OMDAC values at Site 16GH-P4, compared to the theoretical values, reflects an association with GH dissociation and formation. Our findings improve the understanding of fluid, gas, and MDAC geochemistry in continental margin cold seeps, providing insights into global carbon and element cycles

Impact of High Methane Flux on the Properties of Pore Fluid and Methane-Derived Authigenic Carbonate in the ARAON Mounds, Chukchi Sea

We investigated the pore fluid and methane-derived authigenic carbonate (MDAC) chemistry from the ARAON Mounds in the Chukchi Sea to reveal how methane (CH4) seepage impacts their compositional and isotopic properties. During the ARA07C and ARA09C Expeditions, many in situ gas hydrates (GHs) and MDACs were found near the seafloor. The fluid chemistry has been considerably modified in association with the high CH4 flux and its related byproducts (GHs and MDACs). Compared to Site ARA09C-St 08 (reference site), which displays a linear SO42- downcore profile, the other sites (e.g., ARA07C-St 13, ARA07C-St 14, ARA09C-St 04, ARA09C-St 07, and ARA09C-St 12) that are found byproducts exhibit concave-up and/or kink type SO42- profiles. The physical properties and fluid pathways in sediment columns have been altered by these byproducts, which prevents the steady state condition of the dissolved species through them. Consequently, chemical zones are separated between bearing and non-bearing byproducts intervals under non-steady state condition from the seafloor to the sulfate-methane transition (SMT). GH dissociation also significantly impacts pore fluid properties (e.g., low Cl-, enriched delta D and delta O-18). The upward CH4 with depleted delta C-13 from the thermogenic origin affects the chemical signatures of MDACs. The enriched delta O-18 fluid from GH dissociation also influences the properties of MDACs. Thus, in the ARAON Mounds, the chemistry of the fluid and MDAC has significantly changed, most likely responding to the CH4 flux and GH dissociation through geological time. Overall, our findings will improve the understanding and prediction of the pore fluid and MDAC chemistry in the Arctic Ocean related to CH4 seepage by global climate change

Editorial for Special Issue “Microtexture Characterization of Rocks and Minerals”

Microtextures, the physical or structural aspects of minerals and rocks, may include the interrelationships of minerals, the preferred orientation of grains, the internal textures of minerals, etc [...

Illite-Age-Analysis (IAA) for the Dating of Shallow Faults: Prerequisites and Procedures for Improvement

Fault age determination using the illite-age-analysis (IAA) method for fault gouges has played a key role in providing absolute age information in tectonic evolution studies for the last 20 years. The accuracy and precision of the IAA method depend on (1) how to reasonably quantify the relative content of 1M/1Md illite generated from fault activity compared to detrital 2M1 illite in the size fractions of the fault gouge, and (2) how to minimize the error factors in K-Ar or Ar-Ar dating analysis. XRD-based quantitative analysis of illite polytype has made great progress in accuracy by generating a simulated XRD pattern of 1M/1Md polytype using WILDFIRE© and full-pattern-fitting it with the XRD pattern measured from size fractions of the fault gauge. Nevertheless, the results of quantitative analysis of illite polytypes may vary depending on the sample state of the size fractions for XRD analysis, especially the preferred orientation due to the layered crystal structure of illite. In addition, the radiometric dating results may be distorted depending on the error factor of the dating method itself and on the mineral composition of the size fractions, that is, the presence of K-containing minerals such as biotite and K-feldspar other than illite. In this study, we reviewed previous studies that determined fault activity ages by applying IAA to fault gouges. From this, the prerequisites and recommendations for each of the five steps (particle size separation process, XRD analysis process, polytype quantification, radiometric dating, IAA plot) for improving the IAA method are summarized and presented. The continuous application of the improved IAA is expected to greatly contribute to the study of tectonic evolution through geological time

Illite-Age-Analysis (IAA) for the Dating of Shallow Faults: Prerequisites and Procedures for Improvement

Fault age determination using the illite-age-analysis (IAA) method for fault gouges has played a key role in providing absolute age information in tectonic evolution studies for the last 20 years. The accuracy and precision of the IAA method depend on (1) how to reasonably quantify the relative content of 1M/1Md illite generated from fault activity compared to detrital 2M1 illite in the size fractions of the fault gouge, and (2) how to minimize the error factors in K-Ar or Ar-Ar dating analysis. XRD-based quantitative analysis of illite polytype has made great progress in accuracy by generating a simulated XRD pattern of 1M/1Md polytype using WILDFIRE© and full-pattern-fitting it with the XRD pattern measured from size fractions of the fault gauge. Nevertheless, the results of quantitative analysis of illite polytypes may vary depending on the sample state of the size fractions for XRD analysis, especially the preferred orientation due to the layered crystal structure of illite. In addition, the radiometric dating results may be distorted depending on the error factor of the dating method itself and on the mineral composition of the size fractions, that is, the presence of K-containing minerals such as biotite and K-feldspar other than illite. In this study, we reviewed previous studies that determined fault activity ages by applying IAA to fault gouges. From this, the prerequisites and recommendations for each of the five steps (particle size separation process, XRD analysis process, polytype quantification, radiometric dating, IAA plot) for improving the IAA method are summarized and presented. The continuous application of the improved IAA is expected to greatly contribute to the study of tectonic evolution through geological time

Sequential Scheelite Mineralization of Quartz–Scheelite Veins at the Sangdong W-Deposit: Microtextural and Geochemical Approach

The Sangdong W (tungsten)-deposit is known as one of the world’s largest W-deposits, a magmatic–hydrothermal ore deposit including both skarn and hydrothermal alteration zones. The strata-bound characteristic of the deposit resulted in three major orebodies (hanging wall, main, footwall). The main ore mineral is a scheelite (CaWO4)–powellite (CaMoO4) solid solution. We examined the fluid evolution and scheelite formation process of the quartz–scheelite veins of the ore deposit, based on the microtextures and geochemical characteristics of the scheelite. After the initial magmatic–hydrothermal fluid release from the granitic body, prograde skarn is formed. In the later prograde stage, secondary fluid rises and precipitates stage I scheelite. Well-developed oscillatory zoning with the highest Mo content indicates continuous fluid infiltration under an open system. Pressure rises as mineralization occurs, generating the pressure release of the retrograde fluid. Fluid migrates downward by the gravitational backflow mechanism, forming stage II to IV scheelites. Dented oscillatory zoning of stage II scheelite is strong evidence of this pressure release. Stage III and IV scheelite do not show specific internal structures with pure scheelite composition. Retrograde scheelites are formed by fractional crystallization under a closed system. The observation of systematical fractional crystallization in the quartz–scheelite vein system is a meaningful result of our research. The geochemical characteristics and microtextural evidence imprinted in scheelites from each stage provide crucial evidence for the understanding of sequential scheelite mineralization of the quartz–scheelite vein system of the Sangdong W-deposit

Measurement of 3D-Shape Preferred Orientation (SPO) Using Synchrotron μ-CT: Applications for Estimation of Fault Motion Sense in a Fault Gouge

We propose a 3D-shape preferred orientation (SPO) measurement method of rigid grains using synchrotron micro-computational tomography (μ-CT). The method includes oriented sampling, 3D μ-CT imaging, image filtering, ellipsoid fitting, and SPO measurement. After CT imaging, all processes are computerized, and the directions of thousands of rigid grains in 3D-space can be automatically measured. This method is optimized for estimating the orientation of the silt-sized rigid grains in fault gouge, which indicates P-shear direction in a fault system. This allows us to successfully deduce fault motion sense and quantify fault movement. Because this method requires a small amount of sample, it can be applied as an alternative to study fault systems, where the shear sense indicators are not distinct in the outcrop and the fault gouge is poorly developed. We applied the newly developed 3D-SPO method for a fault system in the Yangsan fault, one of the major faults in the southeastern Korean Peninsula, and observed the P-shear direction successfully

In-situ δ18O and 87Sr/86Sr proxies in an unconformable clastic unit at the Ordovician–Silurian transition

Abstract Clastic successions found in the carbonate platform of continental margin during the Ordovician–Silurian Transition (OST) period are archives for interpreting paleo-depositional systems. Here, we report in-situ δ18Oquartz and 87Sr/86Srcarbonate isotope chemo-stratigraphy for an unconformable clastic unit from the Cathaysia terrane that rifted off the Gondwana Supercontinent in the Early Paleozoic Era. Our results suggest a depositional proxy and model for geological events attributed to rapid changes in the sedimentary environment during the OST period. Importantly, these results present crucial clues that infer the influence of Paleo-Tethys Sea opening, global eustatic regression, and rapid sedimentary provenance change. Our study provides insight into paleo-tracer that could be a key method for interpreting depositional system of carbonate platform based on in-situ mineral isotope chemo-stratigraphy that preserves the original value of provenance and geochemical condition