Seawater-like Fluids Research Articles

An Evaluation of Secondary Mineral Formation/Dissolution and Phase Separation Based on Mg Isotopic Fractionation: The Shallow-Water Hydrothermal System in Milos, Greece

This study investigates Mg isotopes (δ26Mg) in vent fluids from Milos, Aegean Sea, to evaluate phase separation and secondary mineral formation. The δ26Mg vary significantly in Milos, exceeding 0.66‰, allowing for the classification of the fluids into three sub-groups based on chemical characteristics: seawater-like, cave fluids, and submarine-brines. The seawater-like fluids exhibit large δ26Mg variation, −0.64 to −1.18‰, and mostly follow a Rayleigh fractionation trend, with a fractionation factor α = 1.00020 ± 0.00011. The cave fluids are highly acidic, have low Cl, are vapor-rich, and display heavy δ26Mg compositions (−0.52 to −0.63‰). The submarine-brines are characterized by high Cl, high non-volatile metals, and light δ26Mg (−0.65 to −1.00‰). The latter two fluid types represent vapors and brines, respectively, which underwent phase separation at depth in Milos. These δ26Mg values were combined with major/trace elements, as well as Li and B isotopes, to explore possible controlling mechanisms. We report for the first time a shallow submarine hydrothermal system that has a vapor component enriched in heavy δ26Mg, but with no detectable isotopic changes in the brines. It is evident that δ26Mg in vent fluids is unique for separating effects of water/rock interaction and secondary mineral and phase separation at shallow-water systems.

A dolomite-based record of seawater calcium isotope composition over the Neogene

The calcium isotope composition of seawater (δ44/40Casw) records important information on the evolution of the calcium and carbon cycles over geologic time. Over the past two decades, many attempts have been made to reconstruct the Neogene δ44/40Casw variation using various calcium isotope records of seawater or seawater-like pore fluids. Nonetheless, large discrepancies still exist among these Neogene δ44/40Casw records. Here we report δ44/40Ca values in a suite of Neogene island dolomites (20.8–3.4 Ma, n = 18) from the South China Sea. Combining multiple geochemical proxies (δ44/40Ca, δ13C, and δ18O, this study; Sr/Ca, ∑REY, δ7Li, δ26Mg, 87Sr/86Sr, and Δ47, previous studies) and evidence from petrography, fluid inclusions, and magnetostratigraphy, we suggest that the dolomitization for these dolomites was buffered by seawater-like fluids in the shallow marine burial domain or even marine diagenetic domain (<180 m) over a considerable period of time (<2 Myr). Then, we apply the equilibrium calcium isotope fractionation factor between dolomite and seawater of −0.52 ± 0.20 ‰ (2SD, n = 13) estimated from the literature to these fluid-buffered dolomites and reconstruct a new Neogene δ44/40Casw record. After using the Locally Weighted Regression (LOWESS) curve fitting method, our dolomite-based δ44/40Casw record is consistent with the reconciled seawater record from other calcium isotope archives, including shark teeth, corals, foraminifera, brachiopods, barites, phosphates, and bulk pelagic carbonates. Our results support the use of fabric-retentive early marine diagenetic dolomites to trace the evolution of δ44/40Casw through Earth’s history.

Unraveling overprinted formation mechanisms of massive dolostone in the Lower Triassic sequence of an isolated carbonate platform in Nanpanjiang Basin, south China

Massive dolomitization is common in carbonate platforms but determining the causes of dolomitization remains challenging. A particular difficulty lies in identifying cases where petrographic and geochemical attributes of dolostone related to one mechanism could be obscured by a later, different one. To better understand whether traditional approaches are sufficient to unravel the origins of dolostone resulting from successive, different mechanisms, this study investigates the formation mechanism(s) of dolostone along a platform-to-basin transect of a Permian-Triassic isolated platform in the Nanpanjiang Basin. The dolostone in the Lower Triassic succession comprises three dolomite phases that can be distinguished through field relationships, petrography, 87Sr/86Sr ratios, and microthermometry. Dolomite type 1 formed due to the reflux of platform-top evaporated seawater that flowed through the platform interior in the Early Triassic. Dolomite types 2 and 3 are interpreted to have formed at elevated temperatures during or after Late Triassic platform burial and to have played a secondary role in forming the dolostone. The dolomitizing fluids that resulted in the formation of dolomite types 2 and 3 were derived from Early Triassic seawater-like fluid that was expelled from the Lower Triassic basinal carbonate sediments and moved updip to the platform interior. Dolomitized clasts in partially or non-dolomitized slope breccias demonstrate pre-burial timing of dolomite type 1, and distinguish the earlier dolomitization from later, post-burial dolomitization represented by dolomite types 2 and 3. Dolomite type 1 retains its Early Triassic seawater δ13C and 87Sr/86Sr signatures, whereas overlapping geochemical fields of the three types of dolomite (trace element concentration, δ18O) imply that burial dolomitizing fluids locally reset the geochemistry of dolomite type 1. This finding suggests that the same dolomite archive may retain well-preserved or altered data depending on the specific geochemical proxy and that identifying individual dolomitization mechanisms using geochemical proxies is possible only in some cases.

Evolution of fluid flow and carbonate recrystallization rates in deep-sea sediments of the Equatorial Pacific

Fluid flow and carbonate recrystallization rates of deep-sea sediments from eight locations in the Equatorial Eastern Pacific were determined by using δ44/40Ca values of pore water and corresponding sediments. The studied drill sites of IODP Exp. 320/321 are located along a transect of decreasing crustal age and reveal different characteristic pore water depth profiles. The younger sites show an overall isotopic equilibration with the sediment in the upper part of the sedimentary column. In the lower part, the δ44/40Ca of the pore water increases back to seawater-like values at the sediment/basalt interface, forming a bulge-shaped pore water profile. The magnitude of the δ44/40Ca pore water bulge decreases with increasing age of the oceanic crust and sediment cover, resulting in seawater-like δ44/40Ca values throughout the sedimentary column in the oldest Sites U1331 and U1332. These findings indicate a seawater-like fluid input from the underlying crust into the sediment. Thus, after sedimentation, carbonate recrystallization processes start to enrich the pore water in 40Ca, and after a time of carbonate recrystallization and cooling of oceanic crust, a flow of seawater-like fluid starts to move upwards through the sedimentary column, enriching the pore water with 44Ca. We established a carbonate recrystallization and fluid flow model to quantify these processes. Our determined carbonate recrystallization rates between 0.000013e(−t/15.5) and 0.00038e(−t/100.5) and fluid flow rates in the range of 0.42–19 m*Myr−1 indicate that the fluid flow within the investigated sites of IODP Exp. 320/321 depends on the sedimentary composition and location of the specific site, especially the proximity to a recharge or discharge site of a hydrothermal convection cell.

Synthetic Fluid Inclusions XXIV. In situ Monitoring of the Carbonation of Olivine Under Conditions Relevant to Carbon Capture and Storage Using Synthetic Fluid Inclusion Micro-Reactors: Determination of Reaction Rates

Ultramafic and mafic rocks are possible targets for CO2 sequestration via mineral carbonation. The determination of reaction kinetics and the factors that control mineralization are important in order to understand and predict how fast injected CO2 will react with host rocks to permanently isolate and store the carbon. Here we present experimental results of olivine carbonation experiments using synthetic fluid inclusions (SFI) as micro-reactors. The micro-reactor technique coupled with non-destructive Raman spectroscopy allows us to monitor the reaction progress in situ and in real time at elevated temperatures (50–200°C) and pressures (several 10's to a few hundred bars), and quantify the amount of CO2 consumed in the reaction using the Raman CO2 densimeter and mass-balance calculations. Results show a measurable decrease of CO2 density in the fluid inclusions as a result of the reaction between the CO2-bearing seawater-like aqueous solution and olivine. Magnesite formation was observed within hours at ≥100°C, while at 50°C magnesite nucleation and precipitation was only observed after a few weeks. Raman mapping and FIB-SEM analysis confirmed the formation of a non-continuous Si-rich layer on the inclusion wall and the presence of ferroan magnesite as a reaction product. Reaction rates [log J (mol/m−2 s−1)] obtained for olivine carbonation range between ~-8.4 at 50°C and −4.7 at 200°C, which is sufficiently rapid to be suitable for commercial CO2 injection projects. Reaction rates involving a seawater-like fluid were similar to rates published for high salinity solutions containing NaHCO3, and were faster compared to rates involving solutions with low salinity. Thus, CO2 injection into submarine environments might offer some advantages over CO2 storage in onshore basalts where the pores are likely to be filled with low salinity meteoric water. The application of the synthetic fluid inclusion technique, combined with non-destructive analytical techniques, is a promising tool to monitor rates of fluid-rock reactions in situ and in real time. Here, we have documented its application to experimentally study carbonation reactions in the olivine-H2O-CO2-NaCl-MgCl2 system.

Magnesium partitioning into vaterite and its potential role as a precursor phase in foraminiferal Mg/Ca thermometer

The foraminifera shell Mg/Ca thermometer has been extensively used in reconstructing temperature evolution of the past oceans. An important, yet poorly understood observation is the 1-2 orders of magnitude lower Mg/Ca in foraminifera calcite shells than expected from inorganically precipitated calcite. Transient unstable carbonate was long hypothesized as a precursor of foraminiferal calcification, but its potential role in explaining apparent ‘vital effects’ of trace element partitioning is not well understood. In particular, a recent study has identified vaterite as a likely precursor phase during biomineralization of calcite foraminifera shells (Jacob et al., 2017). Here, we explore the possibility of vaterite as a precursor phase in affecting Mg incorporation into foraminifera calcite shells by experimentally calibrating the temperature dependent Mg partition coefficients between carbonate solution and vaterite under different solution Mg concentrations. The experimentally determined sensitivity of Mg incorporation into vaterite as a function of temperature (T in °C) is expressed as: ln⁡(λvateriteMg)=0.022±0.002⋅T−3.65±0.03. It is demonstrated that Mg partitioning is mainly controlled by the strain energy originated from cation substitution in the discontinuous lattices of vaterite particles, without distinct kinetic-controlled effects. We have developed a two-step partition model applying the above experimental results, with the assumption that vaterite is directly precipitated from a seawater-like fluid and later converts to calcite through dissolution-reprecipitation mediated by a second fluid in a localized environment. Within the constrained range of model parameters during phase conversion, the two-step partition model could directly reproduce foraminiferal calcite Mg/Ca temperature dependencies in both magnitude and sensitivity. Our model provides a theoretical framework that could be broadly applied to understand the impact of precursor phase on trace metal partition in biominerals. Overall, our study implies that the energy-consuming biological pumping of Mg outside foraminifera calcifying fluid, as widely believed, might not be a precondition for foraminifera shell mineralization.

Equilibrium calcite-fluid Sr/Ca partition coefficient from marine sediment and pore fluids

The equilibrium partition coefficient of strontium (KSreq) between aqueous solutions and calcite is still poorly known, even though it is a valuable parameter for studies involving the use of calcite trace element geochemistry for reconstructing paleoenvironments and fluid chemistry. In this paper we use pore fluid data from deep sea carbonate sediments to constrain KSreq at low temperature (5–17 °C) and show that the derived values are consistent with laboratory calcite precipitation experiments at 25 °C when the latter are corrected for kinetic effects. Using these low-temperature values, and experimental data available at higher temperature based on replacement reactions, we show that all of the data can be accounted for by a single formulation based on the thermodynamics of the relevant components of seawater-like fluids and the SrCO3–CaCO3 solid solution. The value of pore fluid data is that the fluids and carbonate sediment have been in contact for millions of years so local equilibrium is approached, the overall system can be treated with one-dimensional models, and the temperature is constrained. These natural systems provide an opportunity to investigate low-temperature equilibrium in the carbonate system that is difficult to probe in the laboratory because of sluggish exchange kinetics. We estimate values of KSreq using measured Sr and Ca concentrations in pore fluid and sediment solid calcite and a numerical model of sediment deposition, reaction and transport. The model is used to fit the Sr, Ca, and sulfate concentrations observed in pore fluids of several calcite-dominated sites that we believe are optimal for understanding Sr partitioning. Combining the pore fluid results with experimental measurements at higher temperature results in the following expression which applies for the temperature range 0–200 °C:KSreqT=0.025expΔGr,0R1298.15-1Twhere ΔGr,0 is the free energy change associated with the exchange reaction and temperature is in Kelvin. The uncertainty is approximately ±20%. Recently summarized thermodynamic data yield ΔGr,0 = 1.2 kcal/mol (5.0 kJ/mol) which fits well the lower limit of the high temperature data. The corresponding activity coefficient for the SrCO3 component in the calcite crystal structure is 5.4 for the lower value of ΔGr,0, and 3.17 for the higher value. The derived low-temperature values of KSreq of 0.020 to 0.025 (for 0 °C to 25 °C) have implications for models of marine carbonate diagenesis, and the interpretation of vein carbonate Sr/Ca in oceanic crust. Published data showing much higher KSr values in Mg-bearing solutions are not representative of equilibrium values for either Mg or Sr.

New insight on Li and B isotope fractionation during serpentinization derived from batch reaction investigations

Multiple batch experiments (100°C, 200°C; 40MPa) were conducted, using Dickson-type reactors, to investigate Li and B partitioning and isotope fractionation between rock and water during serpentinization. We reacted fresh olivine (5g; Fo90; [B]=<0.02μg/g; δ11BOlivine −14‰; [Li] = 1.7μg/g; δ7LiOlivine = +5.3‰) with seawater-like fluids (75ml, 3.2wt.% NaCl) adjusted with respect to their Li (0.2, 0.5µg/ml; and δ7LiFluid +55‰) and B (∼10µg/ml and δ11BFluid −0.3‰) characteristics. At 200°C a reaction turnover of about 70% and a serpentinization mineral assemblage matching equilibrium thermodynamic computational results (EQ3/6) developed after 224days runtime. Characterization of concomitant fluid samples indicated a distinct B incorporation into solid phases ([B]final_200°C=55.61µg/g; DS/FB200°C=13.42) and a preferential uptake of the lighter 10B isotope (Δ11BS-F=−3.46‰). Despite a low reaction turnover at 100°C (<12%), considerable amounts of B were again incorporated into solid phases ([B]final_100°C=25.33µg/g; DS/FB100°C=24.2) with even a larger isotope fractionation factor (Δ11BS-F=−9.97‰). While magnitude of isotope fraction appears anti-correlated with temperature, we argue for an overall attenuation of the isotopic effect through changes in B speciation in saline solutions (NaB(OH)4(aq) and B(OH)3Cl−) as well as variable B fixation and fractionation for different serpentinization product minerals (brucite, chrysotile). Breakdown of the Li-rich olivine and limited Li incorporation into product mineral phases resulted in an overall lower Li content of the final solid phase assemblage at 200°C ([Li]final_200°C=0.77µg/g; DS/FLi200°C=1.58). First order changes in Li isotopic compositions were defined by mixing of two isotopically distinct sources i.e. the fresh olivine and the fluid rather than by equilibrium isotope fraction. At 200°C primary olivine is dissolved, releasing its Li budget into the fluid which shifts towards a lower δ7LiF of +38.62‰. Newly formed serpentine minerals (δ7LiS = +30.58‰) incorporate fluid derived Li with a minor preference of the 6Li isotope. At 100°C Li enrichment of secondary phases exceeded Li release by olivine breakdown ([Li]final_100°C=2.10µg/g; DS/FLi100°C=11.3) and it was accompanied by preferential incorporation of heavier 7Li isotope that might be due to incorporation of a 7Li enriched fluid fraction into chrysotile nanotubes.

Origin of dolomite in the third member of Feixianguan Formation (Lower Triassic) in the Jiannan area, Sichuan Basin, China

The third member of Feixianguan Formation, Jiannan area consists of marine carbonates, parts of which were dolomitized to form high-quality reservoirs. Based on petrographical features, three types of dolomite phases are distinguished, including very finely–finely crystalline dolomite (type-D1), medium crystalline dolomite (type-D2), and coarse crystalline saddle dolomite (type-D3).Type-D1 dolomite has characteristics of micritic to fine crystals (<5–60 μm) with dull to no cathodoluminescence (CL). It commonly preferentially replaced the matrix components between grains in partially dolomitized limestone. These features suggest that type-D1 had an early, near-surface origin. However, the δ18O values imply partly diagenetic stabilitization during burial. Type-D2 dolomite has characteristics of mainly medium (100–250 μm) subhedral crystals with zoned to uniform dull red CL. The estimates from δ18O values suggest precipitation at depths of 760–1100 m. Values of δ13C and 87Sr/86Sr of type-D2 are similar to values of host limestone, suggesting that dolomitization was mediated by T1f3 seawater-like fluids. Type-D3 is strictly associated with type-D2. High homogenization temperatures and limited distributions indicate that type-D3 formed in a deep, closed environment.Dolostone made of type-D1 dolomite is volumetrically minor and is characterized by micropores and low permeability, while type-D3 as a void-filling cement slightly reduces the reservoir porosity. High-quality reservoirs are only intimately associated with dolostone made of type-D2. A burial-compaction model is proposed for type-D2. Mg2+ was derived from seawater-derived, compaction fluid that was expelled from the trough strata. The potential high-quality reservoirs are likely to occur in opposing margin shoals.

Origin of “island dolostones”: A case study from the Cayman Formation (Miocene), Cayman Brac, British West Indies

Cayman Brac, the easternmost of the Cayman Islands, is 19 km long, 1.5 to 3 km wide, and rises ~ 40 m above sea level at its eastern end. Geographically isolated by the deep oceanic waters of the Caribbean Sea, this island, with its thick succession of pervasively dolomitized Tertiary rocks, is a natural laboratory for assessing the origin of “island dolostones”. Dolostones in the Miocene Cayman Formation (~ 100 m thick), with most crystals < 20 μm long, are formed of low-Ca calcian dolomite (LCD — < 55 mol% CaCO 3) and lesser amounts of high-Ca Calcian dolomite (HCD — > 55 mol% CaCO 3). Geochemically, they are characterised by δ 13C values of 1.6 to 3.5‰, δ 18O values of 2.3–4.0‰, 80–279 ppm Sr, 52–340 ppm Fe, and 9–82 ppm Mn. The inverse correlation between δ 18O and mol% CaCO 3 and positive correlation between Sr and mol% CaCO 3 largely reflect kinetic effects on oxygen isotope fractionation and the Sr partition behaviour between dolomite and water. Dolomite stoichiometry indicates formation from waters with a high Mg/Ca ratio and δ 18O values, corrected for biases associated with dolomite stoichiometry and phosphoric acid fractionation, indicate formation from normal seawater-like fluids under near surface conditions. The 87Sr/ 86Sr ratios point to two phases of dolomitization. Phase I, in the Late Miocene (6–8 Ma), caused partial dolomitization of the basal part of the Cayman Formation. Phase II, during the Pliocene to Early Pleistocene (1–5 Ma), completed dolomitization of the Cayman Formation. Both phases of dolomitization were mediated by similar fluids. Although available information suggests that dolomitization was probably linked to sea level fluctuations, the timing of dolomitization relative to the transgressive–regressive cycle remains open to debate.

Two-cells phase separation in shallow submarine hydrothermal system at Milos Island, Greece: Boron isotopic evidence

[1] Three types of hydrothermal vent fluids, herein referred to as cave, submarine-brine and seawater-like, were recovered from a shallow submerged system at Milos in the Aegean Sea, Greece, for detailed chemical and isotopic analyses. The cave fluids discharge through rock fissures near sea-level and have low pH, chlorinity, and B concentrations relative to seawater. The submarine-brine fluids are characterized by high Cl and contain >10 times seawater B concentrations. A scenario involving a two-cells circulation is proposed; one occurs at 1–2 km and another at shallower depth. The deeper saline reservoir has experienced subcritical phase separation, partitioning 0.42 mM B in vapor and 6.8 mM in brine with no detectable isotopic fractionation. The reaction temperature in the saline reservoir is 313°C calculated from the Na-K-Ca geothermometry. The vapors rise directly to form the cave vents, whereas the saline fluids transport in different pathways and are influenced by seawater mixing to form the variable submarine-brine fluids. The seawater-like fluids circulate at shallower depths, where calculated temperature is 248°C and show slightly diluted B (0.36–0.41 mM) and seawater δ11B. These fluids probably resulted from heating of down-flow seawater and may have experienced groundwater discharge and partial Mg removal. This study represents the first two-cells circulation occurring at Milos and emphasizes the important role of phase separation in shallow submarine hydrothermal system.

Tidally oscillating bisulfide fluxes and fluid flow rates observed with in situ chemical sensors at a warm spring in Monterey Bay, California

An In Situ Ultraviolet Spectrophotometer (ISUS) was coupled to a benthic chamber to characterize the bisulfide flux emanating from a warm spring at the Extrovert Cliff locality within Monterey Bay, California. The chamber was periodically flushed with bottom seawater to reset chemical concentrations, which enabled deployments over multiple days. Data from several deployments, each lasting at least 10 days, were used to calculate flow rates, fluid concentrations, and fluxes over time. The bisulfide concentration of the fluid entering the chamber varied from 75 to 4500 μmol l −1. Positive temperature anomalies up to 3.5° were associated with these elevated concentrations. Linear flow rates ranged from 2 to >17 m day −1, while the bisulfide fluxes varied from 0.2 to 80 mol m −2 day −1. The bisulfide originated at depth and was not produced in the surface sediments via an anaerobic oxidation of methane coupled to sulfate reduction. Tides modulated the flow as well as the composition of the fluid entering the chamber. It appeared that a deep sourced fluid, which supplied the bisulfide, was mixed with a second, ambient seawater-like fluid before entering the flux chamber. At low tides, flow rates were at their highest and the contribution of the deep sourced fluid to the fluid entering the chamber was at a maximum.

IODP Expedition 322 Drills Two Sites to Document Inputs to The Nankai Trough Subduction Zone

The primary goals during Expedition 322 of the Integrated Ocean Drilling Program were to sample and log the incoming sedimentary strata and uppermost igneous basement of the Shikoku Basin, seaward of the Nankai Trough (southwestern Japan). Characterization of these subduction inputs is one piece of the overall science plan for the Nankai Trough Seismogenic Zone Experiment. Before we can assess how various material properties evolve down the dip of the plate interface, and potentially change the fault’s behavior from stable sliding to seismogenic slip, we must determine the initial pre-subduction conditions. Two sites were drilled seaward of the trench to demonstrate how facies character and sedimentation rates responded to bathymetric architecture. Site C0011 is located on the northwest flank of a prominent basement high (Kashinosaki Knoll), and Site C0012 is located near the crest of the seamount. Even though significant gaps remain in the coring record, and attempts to recover wireline logs at Site C0012 failed, correlations can be made between stratigraphic units at the two sites. Sedimentation rates slowed down throughout the condensed section above the basement high, but the seafloor relief was never high enough during the basin’s evolution to prevent the accumulation of sandy turbidites near the crest of the seamount. We discovered a new stratigraphic unit, the middle Shikoku Basin facies, which is typified by late Miocene volcaniclastic turbidites. The sediment-basalt contact was recovered intact at Site C0012, giving a minimum basement age of 18.9 Ma. Samples of interstitial water show a familiar freshening trend with depth at Site C0011, but chlorinity values at Site C0012 increase above the values for seawater toward the basement contact. The geochemical trends at Site C0012 are probably a response to hydration reactions in the volcaniclastic sediment and diffusional exchange with seawater-like fluid in the upper igneous basement. These data are important because they finally establish an authentic geochemical reference site for Nankai Trough, unaffected by dehydration reactions, and they provide evidence for active fluid flow within the upper igneous crust. Having two sets of geochemical profiles also shows a lack of hydrogeological connectivity between the flank and the crest of the Kashinosaki Knoll. doi:10.2204/iodp.sd.10.02.2010

IODP Expedition 322 Drills Two Sites to Document Inputs to The Nankai Trough Subduction Zone

The primary goals during Expedition 322 of the Integrated Ocean Drilling Program were to sample and log the incoming sedimentary strata and uppermost igneous basement of the Shikoku Basin, seaward of the Nankai Trough (southwestern Japan). Characterization of these subduction inputs is one piece of the overall science plan for the Nankai Trough Seismogenic Zone Experiment. Before we can assess how various material properties evolve down the dip of the plate interface, and potentially change the fault’s behavior from stable sliding to seismogenic slip, we must determine the initial pre-subduction conditions. Two sites were drilled seaward of the trench to demonstrate how facies character and sedimentation rates responded to bathymetric architecture. Site C0011 is located on the northwest flank of a prominent basement high (Kashinosaki Knoll), and Site C0012 is located near the crest of the seamount. Even though significant gaps remain in the coring record, and attempts to recover wireline logs at Site C0012 failed, correlations can be made between stratigraphic units at the two sites. Sedimentation rates slowed down throughout the condensed section above the basement high, but the seafloor relief was never high enough during the basin’s evolution to prevent the accumulation of sandy turbidites near the crest of the seamount. We discovered a new stratigraphic unit, the middle Shikoku Basin facies, which is typified by late Miocene volcaniclastic turbidites. The sediment-basalt contact was recovered intact at Site C0012, giving a minimum basement age of 18.9 Ma. Samples of interstitial water show a familiar freshening trend with depth at Site C0011, but chlorinity values at Site C0012 increase above the values for seawater toward the basement contact. The geochemical trends at Site C0012 are probably a response to hydration reactions in the volcaniclastic sediment and diffusional exchange with seawater-like fluid in the upper igneous basement. These data are important because they finally establish an authentic geochemical reference site for Nankai Trough, unaffected by dehydration reactions, and they provide evidence for active fluid flow within the upper igneous crust. Having two sets of geochemical profiles also shows a lack of hydrogeological connectivity between the flank and the crest of the Kashinosaki Knoll. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.10.02.2010" target="_blank"&gt;10.2204/iodp.sd.10.02.2010&lt;/a&gt;

Complex flow patterns through Hydrate Ridge and their impact on seep biota

Our recent measurements of long‐term fluid flux rates and output fluid chemistry at seep sites on Hydrate Ridge, Cascadia, suggest that the mechanisms driving fluid flow in gas hydrate bearing forearc settings may be significantly more complex than previously thought. Temporal changes in the polarity and magnitude of flow and in seep fluid chemistry were observed. The nature of flow appears to generate a strong response in the chemosynthetic seep community ecology. We document here: 1) the frequency and range of temporal variability in flow rates, 2) the relatively rapid changes in outflow composition, 3) strong outflow of seawater‐like fluids, 4) the association of Calyptogena (sp) clams with oscillatory and inflow settings, and 5) microbial mat communities associated with strong advection of altered fluids. These results suggest that a reappraisal may be required of the nature of the hydrologic system and its effect on related processes such as benthic seep community ecology and biogeochemical cycles.

Processes of brine generation and circulation in the oceanic crust: Fluid inclusion evidence from the Troodos Ophiolite, Cyprus

Detailed temporal, thermal, and compositional data on aqueous fluid inclusions from a suite of plutonic and diabase samples from the Troodos ophiolite, Cyprus provide the first documentation that generation of high‐temperature brines may be common at depth in the oceanic crust. Anastomosing arrays of fluid inclusions in rocks of the upper intrusive sequence record episodic fracturing events. The earliest fracturing event, at temperatures &gt;450–600°C resulted in entrapment of brine‐rich aqueous fluids with salinities of 36–61 wt % NaCl equivalent. Homogenization of the brine inclusions by haute dissolution, the virtual absence of vaporrich fluid inclusions throughout the upper level plutonic sequence, and the restriction of brine inclusions to the most evolved plutonic rocks suggests that exsolution of brines off of the late stage gabbro and plagiogranite melts played a significant role in generating the quartz‐hosted, high‐salinity inclusions. Cooling of the fluids during pulses of fluid migration associated with episodic fracturing events, resulted in entrapment of the brines in the deep‐seated, high‐temperature portion of the hydrothermal system. In localized areas, the high‐temperature brines (NaCl±KCl±CaCl2) caused extreme alteration of the plagiogranite bodies and in the formation of podiform epidosites. Arrays of low‐temperature, low‐salinity fluid inclusions, which in some samples crosscut fractures dominated by brine inclusions, indicate downward propagation of a cracking front subsequent to collapse of the high‐temperature magmatic system, resulting in penetration of seawaterlike fluids into the plutonic sequence at temperatures &gt;200–400°C. Hydration reactions under greenschist facies conditions, or limited mixing with brine‐rich fluids, may have resulted in salinity variations from 70% below to 200% above seawater concentrations. Temperatures and compositions of the low‐salinity inclusions are similar to those found in stockwork systems beneath Troodos ore deposits and to those of fluids exiting active submarine hydrothermal vents at mid‐ocean ridge spreading centers. The low‐temperature fracture networks may represent an extensive deep‐seated feeder system which coalesced to form zones of concentrated hydrothermal upflow.