Dehydration Of Clay Minerals Research Articles

Formation of bedding-parallel, fibrous calcite veins in laminated source rocks of the Eocene Dongying Depression: A growth model based on petrographic observations

Bedding-parallel, fibrous calcite veins (commonly referred to as “beefs”) are widely developed within Eocene, lacustrine, laminated organic-rich source rocks in the Dongying Depression of Bohai Bay Basin, East China. Understanding the formation of this type vein is essential to elucidate the diagenetic evolution and fluid migration in muddy source rocks. This study investigates the mechanism(s) of growth of bedding-parallel veins in hydrocarbon source rocks of the Es4 upper and the Es3 lower units in the Dongying Depression, based on systematic observations of macro- and micro-scale features both in veins and their wall-rocks, combined with X-ray diffraction and analyses of geochemical indicators.The source rocks are heterogeneous in both composition and structure. The beefs mainly occur in laminated rocks consisting of organic-rich laminae, which occur together with horizontally-continuous banded lamalginites and rare carbonate-rich laminae, overall corresponding to high TOC, low carbonate contents, and low maturity (average Ro = 0.57%). Most beefs are embedded in the neighboring lamalginites of organic-rich laminae with low clay-mineral contents. The beefs are products of diagenesis accompanying burial, and both the initial fracture opening and subsequent dilation represent early stages of hydrocarbon generation and expulsion. The presence of bedding-parallel bituminite veins and the median plane (or “parting”) is evidence for horizontally oriented hydrofracturing due to fluid overpressure. In turn, fluid overpressure reflects the generation and primary migration of hydrocarbon-bearing fluids. Hydrocarbon generation (organic matter evolution) and clay-mineral dehydration (illitization of smectite) were responsible for the fluid overpressure. Crystal growth within the beefs was a continuous process with an antitaxial growth direction, and there was no growth competition between adjacent fibres. These observations indicate that the crystal growth and fracture dilation were synchronous and growth space was limited. The force exerted by crystallizing vein fill contributed to the high fluid pressure, suggesting that fibrous vein minerals actively promoted further dilation as the veins grew. Vein dilation accompanied the migration of hydrocarbon-bearing fluids, evinced by the occurrence of primary hydrocarbon inclusions found in the fibrous calcites. Therefore, the hydrocarbon-bearing fluids were present at the time of horizontal fluid flow, from the start of fracture opening to the end of the dilation.

Genesis of mud volcano fluids in the Gulf of Cadiz using a novel basin-scale model approach

Mud Volcanism and fluid seepage are widespread phenomena in the Gulf of Cadiz (SW Iberian Margin). In this seismically active region located at the boundary between the African and Eurasian plates, fluid flow is typically focused on deeply rooted active strike-slip faults. The geochemical signature of emanating fluids from various mud volcanoes (MVs) has been interpreted as being largely affected by clay mineral dehydration and recrystallization of Upper Jurassic carbonates. Here we present the results of a novel, fully-coupled 1D basin-scale reactive-transport model capable of simulating major fluid forming processes and related geochemical signatures by considering the growth of the sediment column over time, compaction of sediments, diffusion and advection of fluids, as well as convective and conductive heat flow. The outcome of the model is a realistic approximation to the development of the sediment pore water system over geological time scales in the Gulf of Cadiz. Combined with a geochemical reaction transport model for clay mineral dehydration and calcium carbonate recrystallization, we were able to reproduce measured concentrations of Cl, strontium and 87Sr/86Sr of emanating mud volcano fluids. These results support previously made qualitative interpretations and add further constraints on fluid forming processes, reaction rates and source depths. The geochemical signature at Porto MV posed a specific problem, because of insufficient constraints on non-radiogenic 87Sr/86Sr sources at this location. We favour a scenario of basement-derived fluid injection into basal Upper Jurassic carbonate deposits (Hensen et al., 2015). Although the mechanism behind such basement-derived flow, e.g. along permeable faults, remains speculative at this stage, it provides an additional source of low 87Sr/86Sr fluids and offers an idea on how formation water from the deepest sedimentary strata above the basement can be mobilized and eventually initiate the advection of fluids feeding MVs at the seafloor.The dynamic reactive-transport model presented in this study provides a new tool addressing the combined simulation of complex physical-geochemical processes in sedimentary systems. The model can easily be extended and applied to similar geological settings, and thus help us to provide a fundamental understanding of fluid dynamics and element recycling in sedimentary basins.

Influence of igneous intrusions on thermal maturity and optical texture: Comparison between a bituminous marl and a coal seam of the same maturity

The impact of a 120 cm thick dike on dispersed organic matter in the Triassic Marry Harbour (MH) bituminous marl in the Raanes Peninsula, Ellesmere Island, Arctic Canada, was studied using reflected light microscopy and Rock-Eval pyrolysis. Results were compared to a coal seam of similar maturity that was thermally altered by an igneous intrusion. The MH bituminous marl is mature and within the oil window (Ro = 0.94%), whereas the coal is high-volatile B bituminous rank (Ro = 0.80%). Thermal alteration impacted differently the organic-rich marl than the coal seam. Most of the heat generated by the intrusive emplacement in contact with coal is spent transforming coal to coke in the plastic layer/zone, similar to the process that coal undergoes in industrial coking operations. In contrast, the thermal alteration of sedimentary rocks occurs over longer distances as a result of the absence of a plastic layer/zone. The impact zone of the 120 cm thick igneous intrusion was greater in the MH bituminous marl, which became thermally altered (Ro = 2.34%) at 190 cm from the contact with the dike. On the contrary, the coal became thermally altered (%Ro = 5.0%) by a 240 cm alkali-basalt intrusion (twice the thickness of the MH dike) and its impact zone was only 65 cm. Heat and hot volatile matter produced by the thermal decomposition of organic matter and minerals (carbonate, dehydration of clay minerals) in contact with the igneous intrusion body is transferred through the sedimentary rock matrix by convection through fractures. This transformation is evident by the occurrence of the pyrolytic carbon at various distances in the fractured MH marl from contact. The hydrogen index (HI) shows a similar trend to that of the remaining source potential (S2). Both parameters decrease systematically toward the dike, and go through a minimum at 20 cm from the dike contact, as the indigenous organic matter is converted to hydrocarbons or devolatilized. At the dike contact, both S2 and HI have higher values. These trends are similar to those of the volatile elements H and Cl in the intruded coal seam. The free hydrocarbons (S1) showed a slightly different trend than S2 and HI. It increased initially from 300 cm and reached a maximum at about 190 cm from the dike contact before decreasing continually and reaching a minimum at 20 cm from the contact. At the dike contact, the S1 increased slightly, similar to the HI and S2. The presence of oil droplets trapped in carbonate concretions, in combination with the highest value of oil expelled from the source rock (based on S1/TOC) at distance of 190 cm from dike, is an indication of the influence of contact metamorphism on oil generation locally. Furthermore, it indicates that intrusion of igneous dikes into the bituminous MH marl resulted in an increase of organic matter maturation locally and in hydrocarbon generation.

Hydrogen and oxygen isotopic anomalies in pore waters suggesting clay mineral dehydration at gas hydrate-bearing Kedr mud volcano, southern Lake Baikal, Russia

A multibeam echosounder survey was conducted (deeper than ca. 300 m water depth, total area: ca. 1.8 × 103 km2) in the southern basin of Lake Baikal, Russia, in June 2015, 2016, and 2017. Characteristic morphology of the lake floor was mapped on the ancient Tankhoy stratum, covered with present sediment, by high-resolution bathymetry. Sediment core sampling operations were conducted in August 2015 and August 2016 at a characteristic mound-like landform named Kedr (after the Kedrovaya River). Sub-surface gas hydrates (GH), containing not only microbial but also thermogenic gases (Hachikubo et al. 2016), were retrieved. Core lithology and sub-bottom profiler survey suggest that Kedr is a mud volcano (MV). Hydrogen and oxygen isotopic anomalies were observed in the sediment pore waters. This suggests the water results from clay mineral dehydration, which is the first observation of this process in Lake Baikal sediment pore water. The thermogenic gases, mud breccia, and water from clay mineral dehydration suggest potential ascending gas as well as water from greater depths, presumably from the ancient Tankhoy stratum under the Kedr MV.

Origin of low-chloride fluid in sediments from the eastern continental margin of India, results from the National Gas Hydrate Program Expedition 02

Abstract National Gas Hydrate Program Expedition 02 was conducted in 2015 by D/V Chikyu in the western Bay of Bengal, India. Analyses of interstitial water in sediment from four drilling sites along the eastern and western transect in the Krishna–Godavari Basin show that dissolved chloride (Cl−) concentrations decreased with depth from seawater values (>550 mM) near the sediment surface to 300–400 mM at 350–400 m below seafloor (mbsf), suggesting the upward advection of low-Cl– fluid. Excursions of Cl− concentrations toward low values in gas hydrate zones were attributed to the presence of fresh water released by dissociation of gas hydrate during core recovery. At Sites in the western transect, Cl− concentrations below the gas hydrate reservoir (around 280 mbsf) were ca. 60 mM and ca. 45 mM lower than those from above the reservoir, and δ18O and δD values were 1–2‰ and 0.7–1.3‰ higher than those from above the reservoir, respectively. These results suggest that a fraction of migrating low-Cl– fluid is trapped in the gas hydrate zone, causing decreases of δ18O and δD in the residual water due to isotopic fractionation during gas hydrate formation. From mass-balance calculations, we estimated that 10–20% of the advected fluid is trapped at the gas hydrate zone. At all sites, the δ18O and δD values below the gas hydrate zone, representing relatively unaltered low-Cl– fluid, were 0.5–1‰ higher and ca. 13–15‰ lower than those of seawater, respectively. This trend is consistent with water derived from the dehydration of clay minerals, which generally occurs in deeply buried sediments. The contribution of clay mineral dehydration is relatively large at the western transect compared to that in the eastern transect. Our results imply that fluid advection contributes to the accumulation of gas hydrate in the study area.

Low-Temperature Clay Mineral Dehydration Contributes to Porewater Dilution in Bering Sea Slope Subseafloor

Widespread diagenesis of clay minerals occurs in deeply buried marine sediments under high-temperature and high-pressure conditions. For example, the smectite-to-illite (S-I) transformation has been often observed in sediments at in situ temperatures above ~60°C. However, it remains largely unknown whether such diagenetic processes naturally occur in relatively shallow and low-temperature sediments and, if so, what the consequences are of any related chemical reactions to the geochemical characteristics in the deep biosphere. We evaluated the possibility of naturally occurring S-I transformation at temperatures below 40°C in continental slope sediments of the Bering Sea by examining porewater chemistry, clay mineralogy, and chemical composition of clay minerals measured to ~800 m beneath the seafloor (mbsf) in core samples acquired during Integrated Ocean Drilling Program Expedition 323. In porewater from these cores, chloride concentrations decreased with increasing depth from 560 mM near the seafloor to 500 mM at ~800 mbsf; δ18O increased from 0‰ to 1.5‰; and δD decreased from –1‰ to –9‰. These trends are consistent with the addition of water derived from S-I transformation. The discrete low Cl– spikes observed between ~200 mbsf and ~450 mbsf could be attributed to the dissociation of methane hydrate. X-ray diffraction analysis of the clay-size fraction (<2 µm) showed an increase of illite content in the I/S mixed layer with increasing depth to 150 mbsf. This increase may imply the occurrence of S-I transformation. The decrease of Fe3+/Fe2+ in the clay-size fraction with increasing depth strongly suggests microbial reduction of Fe(III) in clay minerals with burial, which also has the potential to promote the S-I transformation. Our results imply the significant ecological roles on the diagenesis of siliciclastic clay minerals underlying the high-productivity surface seawater at continental margins.

The role of mud volcanism and deep‐seated dewatering processes in the Nankai Trough accretionary prism and Kumano Basin, Japan

AbstractCirculation of water at moderate depths in subduction zones is dominantly driven by clay mineral dehydration over distinct pressure and temperature gradients. The signature of these dehydration reactions is found in mud volcano pore waters, however, it is largely unknown, how much of the deep‐seated fluids are emitted at mud volcanoes. To unravel this relation for the region off the Kii Peninsula, Japan, we calculated the water volume that is subducted in the Nankai Trough using input data from IODP holes C0011 and C0012 and the correspondent water volume released from the subducted plate under the Kumano Basin, in an area where 13 mud volcanoes are located. According to our model, water released at depth in the mud volcano area is derived almost entirely from basaltic saponite and sedimentary smectite transformation [up to 96%]. Nonetheless, the mud volcanoes themselves expel ≪1% of the total volume. To test the contribution of the accreted strata and the Kumano Basin fill to the water budget, we run a second model. Water loss due to compaction of sediments and smectite‐illite transition below the basin floor have been calculated. The results were compared with salinity measurements on background cores scattered in the study area to extrapolate the volume of water loss at depth. The comparison of the two methods yielded similar results and led us to conclude that the bulk part of the deep‐seated fluid reenters the hydrosphere via the basin floor, a mechanism rarely taken into account in fluid budgets in the literature.

The geochemistry and origin of the hydrothermal water erupted at Lusi, Indonesia

The spectacular Lusi mud-eruption started in northeast Java the 29th of May 2006. Despite extensive research, the origin of the erupted water remains elusive and poorly constrained. Here we present a comprehensive study of the geochemistry of Lusi waters compared with those collected from surrounding areas, all collected between 2006 and 2013, including data from mud volcanoes and volcano-hosted hydrothermal springs. Within this broad context, the geochemical characteristics of the fluids expelled in the Lusi region suggest that we can classify the waters in three groups: 1) meteoric waters expelled in cold springs and artesian wells, 2) hydrothermal waters typically mixed with meteoric waters, and 3) formation water from marine sediments altered by diagenetic processes such as clay-mineral dehydration. Samples collected from the Lusi crater are Cl and Na dominated (up to 527 mM and 471.7 mM, respectively) similar to seawater indicating that altered sedimentary formation waters are predominant in this system. In addition they are enriched in Sr (up to 808.4 μM) and other elements commonly associated with hydrothermal systems, such as Li (up to 877.6 μM compared to 26 μM in seawater). Some of these elements are up to ten times enriched compared to seawater values. High-temperature fluid mineral interactions in the subsurface appear to have facilitated the transfer of Li and other mobile elements into the fluids. High temperature fluid-mineral interaction reactions are also supported by Si concentrations significantly higher compared to other sampled mud volcanoes in the island. Crater samples also show the highest δ18O values (+5‰ after correction for evaporation compared to +1‰ at the MV localities). 87Sr/86Sr ratios vary between of 0.7077 and 0.7083 and seem to reflect a general mixture of fluids from clay-mineral dehydration, carbonate recrystallization, alteration of volcanic rocks and hydrothermal imprint. Eight years of geochemical monitoring indicate that the composition of the deep-sourced Lusi fluids remain fairly constant through time. Thus our findings show that the Lusi crater waters represent a regional geochemical anomaly, and we suggest that a combination of high temperatures in the source region, and fluid-rock interactions with silicates and, possibly, carbonate-rich lithologies can explain the data. This is consistent with a model where the emitted gases migrate from a deep-seated (>4 km) source region, likely associated with the presence of hot igneous intrusions and/or high T reactions related to the presence of neighbouring active volcanoes.

Geochemical characterization of two distinctive systems with evidence of chemosynthetic activity, explored at the SE Pacific margin off Chile (46°S and 33°S)

This study presents the geochemical composition of superficial sediment under oxic and suboxic bottom water conditions along the Chilean continental margin (SE Pacific), where evidence for benthic chemosynthetic activity associated with diffuse seeping of chemically reduced fluids has been reported. The exploration was carried out at: (1) the Chilean Triple Junction (CTJ), at a water depth of ∼2900m, with the additional indication of hydrothermal activity near a methane-rich cold-seep area (46°S) (German et al., 2010); and (2) the El Quisco methane seep site (EQSS), at ∼340mwater depth (33°S) (Melo et al., 2007; Krylova et al., 2014). While the deeper CTJ is located within an oxic environment (dissolved oxygen in the bottom waters: 164μM), the shallower EQSS lies within a suboxic environment (dissolved oxygen in bottom water: 23μM), located within the lower limit of the SE Pacific oxygen minimum zone (OMZ).Pore water from short cores was analyzed for dissolved major, minor, and trace elements (Cl, Na, Mg, K, Ca, Sr, Si, B, P, Ba, Pb, Mn, Fe, Cd, U, and Mo), δ13DIC, sulfide, sulfate, and methane. The solid sediment fraction was likewise analyzed for total organic carbon (TOC), metals, and redox potential. Elevated sediment temperatures were found in superficial sediments (5–13°C) at the CTJ site, which could be due to warm fluids associated with the proximity of the ridge, where hydrothermal vents may occur. Reduced fluids were also present here, indicated by higher Mn fluxes toward the water column even in oxidized sediments (RPD>8cm), which contrasted with the lower fluxes in reduced sediments of the EQSS site (RPD∼2cm). 13C-depleted DIC, anomalously low pore water Cl (∼15ppb), and low concentrations of other major elements may be the result of dilution by fluid seeping and precipitation of major elements, producing authigenic enrichment (Ca, Mg, Sr). The fluid could also: (a) be diluted by pure water produced during methane hydrate dissociation, as observed in other cold-seep areas; and (b) correspond to clay mineral dehydration, as reported in plate subduction systems. The reducing conditions established at the CTJ conduct the Cd enrichment at a similar magnitude of that seen at the shallower suboxic site (EQSS). Evidence of chimney or vent fauna was not observed. At the EQSS, higher TOC and total sulfide contents were consistent with enhanced deposition of organic matter and reducing conditions developed in the OMZ, favoring the authigenic enrichment of Cd, U and Pb. The geochemical evidence, based only on methane concentrations and δ13DIC, is insufficient to establish the presence of methane seeps, as previously reported.

Manifestation of carbonate-barite mineralization around methane seeps in the Sea of Okhotsk (the western slope of the Kuril Basin)

First data are reported on a new manifestation of carbonate-barite mineralization found at a site of methane emanations on the western slope of the Kuril Basin, Sea of Okhotsk. Morphological types of barite, aragonite, and low-magnesian calcite are considered in detail; the results of carbon and oxygen isotope study of carbonate concretions and crusts are presented. It is shown that the barite was formed in sediments owing to the diffusion infiltration of the barium-rich fluids through sedimentary succession. The component and isotope compositions of gases are determined and the relatively elevated content of heavy hydrocarbons is revealed. It was assumed that the relatively heavy isotope composition of carbonates is caused by the influence of fluid released from deep sedimentary horizons owing to the dehydration of clay minerals during post- sedimentation transformations. Obtained data show that the origin of carbonate-barite mineralization is related to the migration of hydrocarbons (mainly methane) and barium-bearing cold gas-fluid flows, which were derived not only from near-surface reservoirs but also from deeper-seated sources.

Strike-slip faults mediate the rise of crustal-derived fluids and mud volcanism in the deep sea

We report on newly discovered mud volcanoes located at ~4500 m water depth ~90 km west of the deformation front of the accretionary wedge of the Gulf of Cadiz, and thus outside of their typical geotectonic environment. Seismic data suggest that fluid flow is mediated by a >400-km-long strike-slip fault marking the transcurrent plate boundary between Africa and Eurasia. Geochemical data (Cl, B, Sr, 87 Sr/ 86 Sr, d 18 O, dD) reveal that fluids originate in oceanic crust older than 140 Ma. On their rise to the surface, these fluids receive strong geochemical signals from recrystallization of Upper Jurassic carbonates and clay-mineral dehydration in younger terrigeneous units. At present, reports of mud volcanoes in similar deep-sea settings are rare, but given that the large area of transform-type plate boundaries has been barely investigated, such pathways of fluid discharge may provide an important, yet unappreciated link between the deeply buried oceanic crust and the deep ocean.

Origin and transport of pore fluids in the Nankai accretionary prism inferred from chemical and isotopic compositions of pore water at cold seep sites off Kumano

We used push corers during manned submersible dives to obtain sediment samples of up to 30 cm from the subseafloor at the Oomine Ridge. The concentrations of B in pore water extracted from the sediment samples from cold seep sites were higher than could be explained by organic matter decomposition, suggesting that the seepage fluid at the site was influenced by B derived from smectite-illite alteration, which occurs between 50°C and 160°C. Although the negative δ18OH2O and δDH2O values of the pore fluids cannot be explained by freshwater derived from clay mineral dehydration (CMD), we considered the contribution of pore fluids in the shallow sediments of the accretionary prism, which showed negative δ18OH2O and δDH2O values according to the results obtained during Integrated Ocean Drilling Program (IODP) Expeditions 315 and 316. We calculated the mixing ratios based on a four-end-member mixing model including freshwater derived from CMD, pore fluids in the shallow (SPF) accretionary prism sediment, seawater (SW), and freshwater derived from methane hydrate (MH) dissociation. However, the Oomine seep fluids were unable to be explained without four end members, suggesting that deep-sourced fluids in the accretionary prism influenced the seeping fluids from this area. This finding presents the first evidence of deep-sourced fluids at cold seep sites in the Oomine Ridge, indicating that a megasplay fault is a potential pathway for the deep-sourced fluids.

Characteristics of Deep Abnormal High Pressure and its Controlling on Hydrocarbon Accumulation in Zhanhua Sag

Study on abnormal pressure of sedimentary basin is of great significance. According to the ultrasonic time difference method and measured practical data, some sectional and planar pressure distribution of Zhanhua Sag are obtained while most belong to overpressure, and the pressure distribution and the formation mechanism of high pressure are also analyzed. It is showed that the pore fluid pressure of Es3 and the upper part of Es4 members are higher in Bonan subsag and Gubei subsag and the distribution of abnormal high pressure reached a maximum at the early Es3. The disequilibrium compaction, hydrocarbon generation and some clay mineral dehydration are suggested to contribute to the formation of normal high pressure, including the rapid deposition as one of the most key factor. The lower part of Es3 and Es4 themselves within the deep high pressure system, as good lithologic reservoirs, can transport oil to the upper formation by episodic release to form allochthonous atmospheric hydrocarbon reservoirs.

Evolution of quartz cementation and burial history of the Eau Claire Formation based on in situ oxygen isotope analysis of quartz overgrowths

Individual quartz overgrowths in siltstone of the late Cambrian Eau Claire Formation (Fm.) are systematically zoned in oxygen isotope ratio (δ18O). In situ analysis of δ18O was performed with 3 and 15μm beam spots by secondary ion mass spectrometer (SIMS) on detrital quartz grains and quartz overgrowths. These results from thin lenses within impermeable mudstones reflect samples that were sealed from basin-wide fluid flow and compliment previous studies of more permeable sandstones. Individual grains of detrital quartz (DQ) are homogeneous in δ18O. The average δ18O values in fine-grained detrital quartz in mudstones and siltstones and in coarser-grained quartz in the Eau Claire Fm., Mt. Simon and St. Peter Sandstones (Ss.) are essentially identical at δ18O=10‰ VSMOW, suggesting that detrital quartz is dominantly igneous in origin. The δ18O values of overgrowth quartz (OQ) of buried samples from the Illinois Basin are higher and quartz overgrowths are systematically zoned outward from the detrital cores. These gradients are similar to those from the underlying Mt. Simon Ss., and are best explained by increasing temperatures during burial. Pressure solution is evident in thin section and may have supplied significant silica for overgrowths. In contrast to the deeply buried samples from the Illinois Basin, quartz overgrowths in samples from the Wisconsin Arch are homogeneous and higher in δ18O. Those overgrowths are interpreted as quartz cements formed in a near-surface environment (<40°C), which is consistent with geological evidence that these rocks were only shallowly buried (<500m).Based on these δ18O(OQ) results and the modeled thermal history during burial of the basin, the earliest-formed quartz overgrowths were produced at low temperature from low δ18O(water) around 450Ma. The δ18O values in traverses of single overgrowths decrease by up to 9.1‰, showing continued cementation with increased burial, pressure solution, and heating until ~250Ma. In traverses of the outermost zone of some overgrowths, oxygen isotope values become constant or increase slightly, possibly due to clay mineral dehydration reactions or later fluid infiltration. We present a new cementation and basin evolution model, in which the δ18O of cement correlates to the age of formation and the late overgrowths formed between 270 and 250Ma, during and/or after the migration of brines that formed the Pb–Zn deposits of the Upper Mississippi Valley District (270Ma). Cementation around 270Ma would have reduced permeability, possibly ending the flow of ore forming brines.

Gas Hydrate Occurrence Inferred from Dissolved Cl− Concentrations and δ18O Values of Pore Water and Dissolved Sulfate in the Shallow Sediments of the Pockmark Field in Southwestern Xisha Uplift, Northern South China Sea

Deep-water pockmarks are frequently accompanied by the occurrence of massive gas hydrates in shallow sediments. A decline in pore-water Cl− concentration and rise in δ18O value provide compelling evidence for the gas hydrate dissociation. Mega-pockmarks are widely scattered in the southwestern Xisha Uplift, northern South China Sea (SCS). Pore water collected from a gravity-core inside of a mega-pockmark exhibits a downward Cl− concentration decrease concomitant with an increase in δ18O value at the interval of 5.7–6.7 mbsf. Concentrations of Cl−, Na+, and K+ mainly cluster along the seawater freshening line without distinct Na+ enrichment and K+ depletion. Thus, we infer that the pore water anomalies of Cl− concentrations and δ18O values are attributed to gas hydrate dissociation instead of clay mineral dehydration. Moreover, the lower δ18O values of sulfate in the target core (C14) than those in the reference core (C9) may be associated with the equilibrium oxygen fractionation during sulfate reduction between sulfate and the relatively 18O-depleted ambient water resulting from gas hydrate formation. The gas hydrate contents are estimated to be 6%–10% and 7%–15%, respectively, according to the offset of Cl− concentrations and δ18O values from the baselines. This pockmark field in southwestern Xisha Uplift is likely to be a good prospective area for the occurrence of gas hydrate in shallow sediments.

Fluid sources and chemical processes inferred from geochemistry of pore fluids and sediments of mud volcanoes in the southern margin of the Junggar Basin, Xinjiang, northwestern China

Understanding fluid sources, water–rock interactions and the biogeochemical processes involved in terrestrial mud volcanoes is necessary in order to predict the chemical processes most responsible for methane emissions to the atmosphere. Mud sediments ejected from the Dushanzi and Sikeshu mud volcanoes, located along the southern margin of the Junggar Basin, northwestern China, were collected by hand core sampling in order to explore whether surface and subsurface geochemical processes occur in their fluids. The ionic compositions of the pore fluids, minerals and major elements of the ejected sediments and surface sediments were analyzed. The pore fluids were mainly derived from ancient deeper sedimentary fluids which had mixed with meteoric surface water, but altered by diagenesis processes. Relative to seawater, the mud volcano pore fluids have higher ratios of Na/Cl and Li/Cl and lower ratios of K/Cl and Mg/Cl. The mud sediments are also enriched in illite, chlorite and calcite, but depleted in smectite. In addition, they are enriched in Ca and Mn, followed by Fe, Mg and P, and depleted in Si relative to the wall rocks. These chemical and mineralogical changes in the mud sediments and pore fluids are related to diagenesis processes. Clay mineral dehydration (mainly involving the conversion of smectite to illite) released large amounts of water. Ion exchange among clay minerals increased Na+ in the pore fluid. Water–rock interaction increased Fe and Mn, but decreased Si in the mud sediments. Carbonate precipitation decreased Ca2+ and Mg2+ concentrations of the pore fluid but increased Fe, Mg and P in the mud sediments. These results indicate that the mud volcanoes system is continuously recharged from deeper sedimentary sources. The difference in fluid and sediment geochemistry of the mud volcanoes can be ascribed to the different depths of the fluid and mud sources and the different diagenesis processes during the rising of fluid and mud.

Influence of Fluidized Fly Ash on the Selected Physical-Mechanical Properties of the Autoclaved Aerated Concrete

Fluidized fly ash has different physical and chemical properties compared to fly ash emerging from classic combustion. It contains amorphous phases resulting from a dehydration of clay minerals as well as unreacted sorbent of CaCO3, free CaO and anhydrite (CaSO4). Work targets the possibilities of production of an autoclaved aerated concrete (AAC) from fluidized fly ash, and its influence on particular physical-mechanical properties of autoclaved aerated concrete.

Fluid transport properties in sediments and their role in large slip near the surface of the plate boundary fault in the Japan Trench

Fluid transport properties such as permeability and porosity are significant parameters that affect earthquake generation. We measured the transport properties of shallow sediments sampled around the plate boundary near the Japan Trench during IODP Expedition 343 at confining pressures up to 40 MPa. The permeabilities of samples from the shallow plate boundary fault were very low at 10−20 m2, equivalent to a hydraulic diffusivity of 10−10 m2s−1. Permeability and porosity in the core of the fault zone at the plate boundary were lower than those in the immediately overlying sediments and the surrounding intact sediment, suggesting that the plate boundary fault can act as a barrier for fluid flow. Low permeability and high pore compressibility in the shallow plate boundary fault create a strong potential for dynamic fault weakening due to fluid pressurization with frictional heating, even when the initial shear stress is low. Our investigation supports the hypothesis that thermal pressurization on the fault plane helped facilitate the extremely large slip in the shallow part of the subduction zone during the Tohoku earthquake. As the fault zone has a lower permeability than the surrounding sediments and a higher clay content, pore pressure generation at depth by dehydration of clay minerals can explain formation of the shallow strong patch on the fault more reasonably than continuous fluid influx from the subducting oceanic crust, which does not affect pore pressure at depth in the fault zone. Although there are many possible mechanisms of fault weakening, thermal pressurization can act relatively efficiently as slip begins, even at shallow depths. Therefore our results support the role of thermal pressurization in shallow slip during the Tohoku earthquake.

Tracing the evolution of seep fluids from authigenic carbonates: Green Canyon, northern Gulf of Mexico

Authigenic carbonates from hydrocarbon seeps are unique long-term archives of past fluid flow. The studied samples were collected from Green Canyon block 140 at a water depth of 260 m in the Gulf of Mexico. Petrography, X-ray diffraction, stable isotopes and 14C dating were applied to assess the evolution of seep activity and potential driving forces. The carbonates are dominated by high-Mg calcite (HMC) and aragonite, with a minor amount of low-Mg calcite (LMC) and dolomite. Petrographically, peloids, clotted microfabric, acicular aragonite and a variable content of bioclasts were observed. Three types of carbonates are recognized. Structure I carbonates, with 14C ages from 46.5 ka to 25.8 ka BP, are characterized by δ13C values from −23.2‰ to 5.1‰, suggesting multiple carbon sources that include thermogenic methane, biodegraded crude oil, seawater and residual CO2 from methanogenesis at greater depth. In contrast, Structure II carbonates formed between 17.6 ka and 11.7 ka BP and have δ13C values varying from −22.2‰ to −8.8‰, suggesting carbon sources similar to those of Structure I carbonates but with a negligible influence of residual CO2 from methanogenesis. In addition, the presence of LMC in this type of carbonate may be associated with brine seepage. Structure III carbonates among the youngest of the samples analyzed with 14C ages of 1.2 ka BP. These carbonates have the most negative δ13C values ranging from −36.1‰ to −26.8‰, suggesting that thermogenic methane is the primary carbon source. The majority carbonates of both Structure I and II are slightly 18O-enriched, which is most likely related to the incorporation of water from dehydration of clay minerals. The considerable range of mineralogical and isotopic variations of the studied carbonates highlights the local control of the seepage flux. It is proposed that factors affecting the activity of hydrocarbon seeps are sea level changes and salt movement. The combination of petrography, stable isotopes, and dating approach used here, highlights that these are valuable tools to assess the variability of past fluid flow at hydrocarbon seeps.

Origin of gases and water in mud volcanoes of Andaman accretionary prism: implications for fluid migration in forearcs

Extensive mud volcanism on the Andaman accretionary prism occurs above a complex network of faults and is caused by the convergence of the Indian plate and the Burmese microplate. Mud volcanoes of the Andaman forearc have received little attention in spite of the fact that they are one of the important features of this tectonic setting, located within an ocean basin that has one of the highest sedimentation rates in the world, and that the materials emitted by them present a unique opportunity to study the chemistry of the detachable parts of the subducting slab. In this study we present mineralogical, chemical and isotopic data for argillaceous matter (mud matrix), gases and water emitted by these mud volcanoes and attempt to understand the variations in terms of their sources and processes within the forearc. The mud matrix is composed of smectite–illite–kaolinite–chlorite–plagioclase–quartz–calcite assemblage derived both from sediments and altered oceanic crust and originates from a deep-burial diagenetic environment. The modes of δ13C distributions for methane (>−42‰), ethane (>−27‰) and CO2 (<−3‰) indicate thermogenic origin for these gases, with TOC and N of associated mud suggesting marine organic matter as the source. The water ejected at these mud volcanoes is much fresher (Cl−=45 to 135mM) than seawater and its δ18O (−0.2 to 2.6‰) and δD (−24 to −14‰) isotopic compositions fall well below the global meteoric water line. From their trace element contents and stable isotopic compositions we infer that the mud water is a mixture of sediment pore water (ancient seawater) and water released from dehydration of clay minerals. The 87Sr/86Sr of mud water (~0.7071) confirms the above inference and points out that altered oceanic crust plays a significant role in controlling the chemistry of water. The formation temperatures of mud ejecta, derived from mineralogical (smectite/illite), chemical (K+/Na+) and isotopic (δD/δ18O) geothermometers, lie in the range of 50°C–120°C — which corresponds to a depth zone of 2 to 6km within the Andaman forearc. Based on all the evidences we conclude that the sampled mud volcano ejecta originate at the plate-boundary décollement zone, from the sediments and altered oceanic crust of the subducting Indian plate.