V21D-0749
Variable Basement Compostion and Magma Degassing Affecting Hydrothermal Systems in the Eastern Manus Basin
In August 2006, hydrothermal activity within the eastern Manus Basin was investigated using a combination of mapping and sampling (using AUV ABE and ROV Jason2). Objectives included identifying geological settings, examining interactions of seawater with felsic rocks, and determining the extent of volatile magmatic inputs into these systems. Hydrothermal systems in the eastern Manus Basin show a range in (1) fluid composition, (2) massive sulfide composition, and (3) rock alteration style that all point to input of magmatic volatiles into the hydrothermal system that varied in space and time. Basement compostions range from fresh arc basalt to rhyolite, with compositions following a classic calc-alkaline trend. The most felsic rocks display a wide range in volatiles dissolved in glass. Rhyolitic lavas form the Roman Ruins Vent Field in the PACMANUS area have high chlorine contents that plot along a conservative trend. Rhyolites from North Su volcano, in contrast, show more variable but generally much lower Cl contents, indicating more pronounced degassing in that area. Vent fluids exhibit great compositional variability between and within the vent fields. Fluids are variably enriched in carbonic and sulfuric acid, indicative of degassing of CO2 and SO2. Evidence for present and past SO2 degassing is preserved in the form of native sulfur and advanced argillic rock alteration, predominantly in parts of PACMNUS, at SuSu Knolls and at Desmos.
V21D-0750
Distribution and Sources of Trace Metals in Volcaniclastic Sediments of the SuSu Knolls Hydrothermal Field, Eastern Manus Basin, Papua New Guinea
Thirty-one sediment cores from the Suzette sulfide mound (renamed Solwara 1 by Nautilus Minerals Inc) in the SuSu Knolls hydrothermal field, eastern Manus back-arc basin, were studied in order to outline anomalies in metal concentrations within the mound and to explain the sources of the anomalies. The sediment cores were collected during expeditions of Nautilus Minerals Inc in 2006 and 2007. The work complements our previous study of metalliferous sediments of the SuSu Knolls and aims to provide guidelines for exploration for seafloor massive sulfide deposits in both modern and ancient back-arc environments. In contrast to mid-ocean ridges, the sedimentation in back-arc basins is more complex and involves deposition of large amount of volcaniclastic material that may mask the hydrothermal signal. The SuSu Knolls are covered by an apron of laminated dark gray volcanic sandy silts and silty sands composed of various amounts of volcanic rock fragments, volcanic glass, Ca plagioclase, pyroxene, cristobalite, Si-rich amorphous material, alunite, pyrite, barite and magnetite. In many cases the gray volcaniclastic sediments exhibit patches and layers having a black or greenish-brown color that contain fecal pellets. On the western slope of Suzette (Solwara 1), dark gray volcaniclastic sediments overlie greenish, greenish-brown and greenish-black volcaniclastic sediments containing up to 10 wt % clay-size component that comprises alteration products of volcanic glass such as smectite, chlorite and X-ray amorphous material. In most cases black and greenish-brown colored sediments contain fecal pellets at different stages of preservation. The distributions of Au (19 ppb to 2 ppm), Cu (159 ppm to 1 wt %), Zn (35 ppm to 1333 ppm), Pb (7 ppm to 977 ppm) and Ba (0.05 wt % to 2.8 wt %) outline patchy anomalies throughout the sediments of the mound. The study showed that some volcaniclastic sediments as deep as 25 cm below seafloor that are proximal to chimneys and chimney fragments do not exhibit clear metal anomalies. Local strong anomalies in metal concentrations caused by dispersal of chimney sulfides in the volcaniclastic sediments were found on the rim of the mound. More widespread anomalies were detected down to 80 cm depth in greenish-brown and greenish- black volcaniclastic sediments from the western slope of the mound. Metal anomalies in these sediments may be a result of dispersal of fine-grained particles of secondary minerals, such as atacamite and Fe-oxyhydroxides, derived from oxidation of sulfide chimneys. Another possible source of the metals is hydrothermal particles that were deposited in the sediments within fecal pellets. With the exception of local anomalies in surface sediments around active chimneys, the current particulate plume emanating from black smokers does not leave a clear signal in the sediments covering the mound.
V21D-0751
Removal of trace elements in hydrothermal plume at submarine volcanic arc hydrothermal systems
On the study of geochemical fluxes of trace elements from the hydrothermal system, it is necessary to collect not only samples by the hydro-cast from surface ship and fluid samples using a submersible but also temporally and spatially continuous samples ranging from a fluid to a hydrothermal plume. For that purpose, the sampling method along the diluting and rising plume just after erupting from a hydrothermal vent is effective. The mini CTDT-RMS was installed onto the submersible. The hydrothermal plume samples were collected with monitoring the anomalies of temperature and turbidity by taking the distance from the hydrothermal vent gradually. Unfiltered sample for total (particulate + dissolved) trace element concentration and filtered sample for dissolved trace element concentration were analyzed on land. In V, Ni, Cu, Mo, Cd, Pb and Zn, particulate form was predominant in the fluid. The elements that are easy to form a sulfide such as Cu, Cd and Pb were removed as a sulfide precipitate from the fluid before erupting to the deep ocean. Therefore, the concentration of these trace elements in the hydrothermal plume showed superiority of a dissolved form, and was slightly high or same concentration in the deep ocean. The concentration of Fe in the fluid was extremely higher (500 - 100,000 times) than that in the deep ocean, and showed a fifty-fifty partition between dissolved form and particulate form. In the hydrothermal plume, Fe formed hydroxide mainly and was removed gradually from the plume as a particulate form in dilution and diffusion process of the plume. These hydroxides may play a role of the precipitant that coprecipitate with absorbing the other trace elements. Because Mn is hard to deposit as a sulfide, dissolved form was predominant in the fluid and Mn showed extreme high concentration same as Fe. Mn was discharged to the deep ocean as a dissolved form and removed from the plume as an oxide with increasing the particulate form gradually in dilution and diffusion process of the plume. In addition, existence of an organic trace element as one of a chemical species of dissolved form was confirmed in the hydrothermal plume, and the relationship between the hydrothermal ecosystem and the organic trace elements is very interesting.
V21D-0752
Spreading Rate Dependence of Hydrothermal Plume Buoyancy Flux at Mid-ocean Ridges
We have compiled the plume characteristics of 57 known focused hydrothermal vents along mid-ocean ridges of full spreading rates from 10 to 150 mm/yr. The calculated vent fluxes show a strong dependence on seafloor spreading rate. Hydrothermal vents at the fast-spreading northern and southern East Pacific Rise, with full spreading rates greater than 80 mm/yr, have a relatively narrow range of seafloor depths (2,500 to 2,700 m) and calculated buoyancy fluxes B0 (10-4 to 10-3 m4/s3). In contrast, at ridges with full spreading rate less than 80 mm/yr, the seafloor depth of hydrothermal vents varies by a factor of two (<2,000 m to >4,000 m), while the calculated B0 varies over three orders of magnitude (10-3 to 10-1 m4/s3). The relatively small values of the calculated buoyancy flux and shallow seafloor depth of individual vents at fast-spreading ridges, together with their relatively narrow range of variations, imply relatively uniform along-axis distribution of hydrothermal heat release along fast-spreading ridges. This is, in turn, is consistent with a relatively small variation in both the magma supply and permeability structure at ridge axis along these fast spreading ridges. In contrast, the much greater ranges in the calculated fluxes and vent depth at slow and ultraslow spreading ridges reflect significantly greater spatial and temporal variability in heat sources beneath the ridge axis and more complex tectonic and permeability structure of the oceanic lithosphere.
V21D-0753
A New Tool for Detecting Hydrothermal Plumes: an ORP Sensor for the PMEL MAPR
Mapping hydrothermal plumes in the water column above mid-ocean ridges, submarine arc volcanoes, and back- arc spreading centers has lead to the discovery of many seafloor hydrothermal systems. Hydrothermal plumes can be identified by strong optical backscattering anomalies, which indicate particulates injected into the water above the seafloor by the vents, and by chemical anomalies coincident with the particulate signal. The chemical anomalies are often characterized by increased concentrations of dissolved reduced species such as iron, manganese, and sulfur that decrease the oxidation-reduction potential (ORP) of the plume relative to ambient seawater. Some of these reduced chemical species are short-lived relative to the particulates, so having a method for measuring the ORP along with optical backscattering has improved efforts to locate source vent fields. Greater ORP anomalies indicate "younger" or "fresher" parts of the plume, which are closer to the source. Additionally, some low temperature vent fields often have minimal particle signatures, yet their above-bottom plumes can be detected by changes in ORP. The PMEL Miniature Autonomous Plume Recorder (MAPR), an instrument with pressure, temperature and optical backscatter sensors, has been used extensively in explorations for hydrothermal systems. We have now enhanced MAPRs to include a newly-designed platinum electrode paired with a silver/silver chloride reference electrode and high impedance circuit for the measurement of ORP in seawater. Results from the laboratory and field demonstrate this new sensor system effectively detects ORP anomalies associated with hydrothermal plumes. Laboratory experiments with Fe(II) and sulfide (dissolved Na2S) show that changes in OPR are rapid when the sensors are exposed to reduced species in solution, and the magnitude of the change is proportional to the concentration of the reduced species. In the field, intercomparison of several individual OPR-capable MAPRs deployed simultaneously confirms their response is rapid when plumes are encountered and that all sensors had comparable magnitudes of response. One MAPR was recently deployed on the ABE autonomous vehicle for detailed mapping missions at Brothers Volcano, Kermadec Arc. Response of the MAPR ORP sensor mirrored that of the Japanese (AIST) ORP sensor routinely mounted on ABE. Both sensors provided high- resolution maps of near-bottom vent effluent over known vent fields on Brothers.
V21D-0754
Quantitative Effect of Variable Entrainment on the Hydrothermal Heat Outputs Deduced From the Heights Reached by Submarine Buoyant Plumes
In the absence of direct measurements of the heat flux released by hydrothermal vent activity, the large submarine plumes associated provide a key information to estimate thermal outputs. Physical models of turbulent plumes have shown that the maximum height reached by a plume in a stratified environment can be related only to its source buoyancy flux, the stratification of the surrounding fluid and the efficiency of the mixing between both fluids. If one assumes that the mixing is constant along the distance from the source and if the ambient stratification is known, the maximum height reached by the plume provides directly the source heat flux. These models have been adapted to predict the rise of buoyant hydrothermal plumes as a function of the source heat flux in the ambient seawater of the Atlantic and the Pacific Oceans. Five plumes heights detected above the TAG vent field in the Mid-Atlantic Ridge (26° N) have been used to calculate the heat and vent fluxes emitted from this hydrothermal system. We will show that these estimations are erroneous by a factor 3 because of the simple assumption that the mixing is constant with depth. We present a new model of turbulent entrainment highlighting the fundamental effect of the plume buoyancy on the mixing. The stratification of the ambient seawater induces a negative buoyancy close to the maximum height which can considerably reduce the turbulent entrainment. We re- evaluate the thermal ouput of the TAG hydrothermal field to 2850 MW. This generic approach can be applied to any submarine hydrothermal system for which the density stratification in the ambient seawater is known.
V21D-0755
Emission of CO2 from seafioor hydrothermal systems at Mariana Trough
Hydrothermal vent fluids are highly enriched in CO2 and the CO2 rich fluids are released into the ocean as a hydrothermal plume. Especially, the emission of hydrothermal-related liquid CO2 from the seafloor (about 1500m) was discovered at the Okinawa Trough and Mariana Trough. At these areas, it is considered that the liquid CO2 rises up to shallow depth as a CO2 droplet and that the rising CO2 droplet dissolves gradually in ambient seawater. The observation of the hydrothermal-related CO2 would provide the opportunity for understanding the physic-chemical behavior and diffusion process of liquid CO2 in the ocean. Newly developed in-situ pH/pCO2 sensor can detect precisely and rapidly the changes of pH and pCO2 derived from high CO2 concentration. At southern Mariana Trough, the pH/pCO2 sensor was installed onto the manned submersible and in-situ pH and pCO2 data were measured every 10 seconds during the operation on the hydrothermal active site. Mapping survey of low pH and pCO2 distribution was performed on the hydrothermal active site by the grid navigation of the manned submersible that installed the pH/pCO2 sensor. The results of pH mapping survey showed only localized pH depression at the hydrothermal active site. At NW Eifuku submarine volcano, hydrothermal-related liquid CO2 dispersion was observed by using a towing multi-layer monitoring system. This system can observe the dispersion behavior of CO2 by towing several in-situ pH/pCO2 sensors and SSBL transponders in the high CO2 plume. Low pH plume of 100m high and 200m wide was detected above the summit of NW Eifuku submarine volcano.
V21D-0756
Acive shallow-water submarine hydrothermal venting and occurence of chimney-like mineral deposits from Northern Kagoshima Bay, South Kyushu, Japan
At least three active volcanoes are located in and around Kagoshima Bay, South Kyushu, Japan. The bay has been considered a tectonic graben formed under extensional stress associated with subduction of the Philippine-sea plate sinking below the Eurasia plate and the axis of graben is extended along the volcanic front. In addition, the bay head area is a part of the Aira caldera, it is considered to form ca. 25,000 yeas ago. The Wakamiko crate (31°40'N, 130°46'E) is an active crater of the caldera and the depression of crater is buried by a thick (up to 80m) unconsolidate deposits. Therefore, the crater floor shows quite flat feature and the depth of floor is ca. 200m. In the crater hydrothermal activity has been recognized, however, active vent of hydrothermal fluid is not found except two gentle shimmering from fissure and small collapse developed on muddy floor. During the dive study of ROV/Hyper-Dolphin of JAMSTEC (R/V Natsushima, NT07-09 Cruise) in June 2007, active hydrothermal venting (T > 186°C) was found on the crater floor, and the venting as ca. 10- cm-wide jet of hot water without gas was observed atop of mineralized chimney-like structure up to 1.5 m in height. The chimney was composed mainly native sulfur, sulfate minerals, silica, and small amount of sulfides such as pyrite, stibnite, realgar and cinnabar. Chemistry of the venting fluid was almost comparable with one of the shimmering fluids obtained as pore water, which was characterized by low salinity (Cl = 300mM) and higher K concentration than seawater. Analyses of detailed chemistry and mineralogy of the fluid and chimney is under way.
V21D-0757
Tonga - Kermadec Arc Calderas as Sites of Hydrothermal Venting: A Review of Caldera Morphology and Structure
Within the 870 km long sector of the Tonga Kermadec arc (between 35°S and 21°S), submarine hydrothermal venting is dominantly hosted within calderas associated with silicic eruptives. Over 50 individual >1 km wide calderas have been identified from five multibeam mapping surveys from 2002 - 2007. Caldera distribution is mostly, though not exclusively, along the central - northern Kermadec and southern Tonga arc sectors. In contrast, metrics of caldera morphology are mostly consistent along the arc; average caldera floor water-depth is 1050 m (range 1860 - 260 m), average caldera long axis is 4.9 km (range 11.6 - 1 km), and average caldera rim water-depth is 800 m (range 1500 - 160 m). As in the onshore extension of the arc (Taupo Volcanic Zone), regional arc and backarc extension strongly controls caldera structure; caldera elongation (average length:width ratio of 1.26) is orientated predominantly orthogonal to the basement rift fabric. Various styles of caldera collapse (including trap-door and funnel structures) are recognised from both multibeam and multi-channel seismic reflection data. More than half of the calderas show evidence of repetitive and nested collapse. Caldera breaching and rim collapse are common, and can be associated with mass-gravity pyroclastic debris flows, imaged as large-scale sediment bed-forms in multibeam and multi-channel seismic data. Similarly, post-collapse volcanism (but not structural resurgence) is common with both intra-caldera dome construction, and formation of cone vents and explosion pits around the caldera rim. Such post-collapse volcanism is variable in composition, however, intra-caldera dome and rim eruptives are commonly rhyolitic and basaltic, respectively.
V21D-0758
Lead isotopic compositions of hydrothermal sediments on the slope of Eolo seamount (Aeolian Arc): implications for anthropogenic lead impact on recent hydrothermal processes
Eolo Seamount is a submarine volcano located in the eastern part of the Aeolian volcanic arc. During an R/V Bannock cruise in 1977, a 327.5 cm long core was taken from an area of extensive hydrothermal mineralization. We investigated the Pb isotopic compositions of the hydrothermal sediments of this core in order to provide insight into Pb behavior in low-temperature hydrothermal settings. Eolo hydrothermal sediments exhibit a wide range of Pb isotopic ratios with 206Pb/204Pb ranging from 17.446 to 18.916, 207Pb/204Pb from 15.570 to 15.683, and 208Pb/204Pb from 37.420 to 38.975. The isotopic compositions at the low-radiogenic end are unusual and cannot be derived from the possible natural sources in the area. Mixing considerations indicate that up to 80 percent of the lead in these sediments is anthropogenic in origin. The highest proportion of anthropogenic Pb is found in a hydrothermal nontronite layer, formed by precipitating hydrothermal Fe-Si oxyhydroxides onto bacterial walls. The Pb isotopes indicate that a significant part of the lead in the hydrothermal precipitates was derived from leaded gasoline, which constrains the hydrothermal activity to a time period between the 1950s and the 1970s. The anthropogenic Pb was probably scavenged from the Mediterranean seawater by sinking hydrothermal plume particles that then became incorporated in the Eolo hydrothermal sediments. The lead isotopic systematics indicate that the anthropogenic Pb has swamped the natural Pb signal in the Mediterranean seawater at the time of the hydrothermal activity. Furthermore, these results imply that the hydrothermal plume particles did not carry a significant amount of Pb extracted from the underlying rocks, suggesting that the Pb budget in low-temperature systems is controlled by seawater composition, not by leaching from underlying rocks.
V21D-0759
Hydrothermal fountains imaged by high resolution side-scan sonar equipped on a cruising AUV, URASHIMA
Mapping of an area and intensities of activity at a particular hydrothermal field has required huge effort so far, typically several tens of dives of manned submersibles and/or ROVs to obtain detailed locality map with needed resolutions. Thus, appropriate remote sensing techniques have been desired since the discovery of seafloor hydrothermal field. A series of successful trials has been performed by ABE of WHOI equipped with a Eh-sensor (Yoerger et al., Oceanography, 2007). A 100kHz side-scan sonar (SSS) equipped on a cruising AUV, URASHIMA, caught detailed structural image of hydrothermal fountains rooting active chimneys during YK07-07 Cruise off Okinawa Isl. (May 6-18, 2007). The URASHIMA AUV is a 10-m-length cylindrical-shaped one that originally optimized to long distance cruise. In the expedition, she cruised near the sea floor with 50-100 m altitude, at the area of 1000-1500 m in WD. She has currently basic oceanographic/CTD sensors, a 400kHz echo-sounder and sonars of 100\/400 kHz side-scan sonar and up to 6 kHz sub-bottom profiler. In this operation, pH and ORP sensors (CRIEPI) were also attached in front of AUV. On the pre-processing image of SSS, numbers of filament-shape echoes were recorded within water column zone. The reason why they should be the echo from hydrothermal plumes are as folows; 1) the echoes in the water column were limitedly recorded above the active hydrothermal field; 2) CTD and pH sensors show temperature and pH anomaly corresponding to the record of echoes; 3) some of the root of the filament-shape echoes correspond to the hydrothermal mound recognized in the detailed bathymetry obtained with SeaBat7125 MNBES. This-like technique should revolute the mapping work prior to the sampling at the particular hydrothermal site.
V21D-0760
Volcanic, tectonic, and hydrothermal features identified in the Lau Basin with near-bottom multibeam sonar data
We present high-resolution bathymetric maps of six hydrothermal vent fields located along the Eastern Lau Spreading Center and Valu Fa Ridge in the Lau backarc basin. Generated with near-bottom SM2000 multibeam sonar data collected with ROV Jason 2, these maps provide sufficient detail to quantify tectonic, volcanic, and hydrothermal features that reveal important differences in these hydrothermal systems, their geologic settings, and the nature of their volcanic substrate. The Kilo Moana and Tow Cam vent fields, hosted in basaltic substrate, have bathymetric characteristics that distinguish them from the Mariner and Vai Lili fields that are hosted in andesitic substrate. The basalt-hosted vent fields are located on relatively flat seafloor crosscut by extensive faults and fissures. At these sites, the high-resolution bathymetry indicates that volcanic activity pre-dates tectonic activity. Pillow and lobate flows at these sites can be discerned as sub-meter roughness, with amplitudes of 0.1- 0.4 m over length scales of 0.5-1 m. Vent structures (5-15 m tall) are clearly identifiable. In contrast, the andesite- hosted vent fields have more relief, and roughness is evident as finger-like flow fronts related to more viscous andesitic flows. Volcanic domes (15-30 m diameter), some with collapse craters (10-20 m diameter), are also evident at these vent fields. Hydrothermal structures at Mariner are as tall as 25m, while at Vai Lili vents are smaller and difficult to distinguish from the rough seafloor that surrounds them. The detailed bathymetry at these vent fields is consistent with the tectonic activity pre-dating volcanic activity. The bathymetric features documented at the ABE and Tui Malila vent fields are transitional to the contrasting features observed to the north (at Kilo Moana and Tow Cam) and south (Mariner and Vai Lili). Bathymetric maps from ABE and Tui Malila reveal pillow and lobate flows, faults that post-date volcanic activity, finger-like flow fronts, and volcanic domes (Tui Malila). Hydrothermal structures at these sites are <5m tall.
V21D-0761
Archean hydrothermal sea-floor surface environment: Australia VS South Africa
Archean greenstone belt typically contains chemical sedimentary sequences such as cherts and BIFs and underlying thick volcanics. These sequences may be used to extract information on hydrothermal activity and potential biological activity on early Earth. We compare the geological and geochemical characteristics of such chemical sediments in the 3.2 Ga Cleaverville Group and 3.4 Ga Warrawoona Group (Marble Bar Chert) in the Pilbara district and coeval Onverwacht Group (Masauri River Chert) and Komati River Formation in the Barberton Greenstone Belt. The Cleaverville Group formed by bimodal volcanism in an immature island arc. The Dixon Island Formation of the Cleaverville Group is one of the best-preserved sequences of Archean hydrothermal activity. The Dixon Island Formation is composed of Komatiite-Rhyolite Tuff, Black Chert, and Varicolored Chert Members to the top. The Komatiite-Rhyolite Tuff Member contains highly altered volcanic rocks with black chert veins. The thick Black Chert Member is characterized by low organic carbon contents (<0.5 percent) and low carbon isotope compositions (- 28 to -40 per mil). It contains fossil-like carbonaceous materials. The Masauri River Chert is also a hydrothermal sequence that is quite similar to the Dixon Island Formation. It contains highly altered volcanic rocks with numerous quartz swarms and black chert dikes, which are covered by volcanic tuff and well-stratified black/white cherts. Its upper sequence contains Fe- rich black/white cherts. Black chert of the Masauri River Chert is also characterized by low organic carbon content (<0.5 percent) and relatively high carbon isotope compositions (-20 to -30 per mil). Similarity in the petrographical and geochemical characteristics of black chert veins and massive black chert beds in the Australian and South African sections suggest that the depositional environments for those rocks were also similar. These evidences strong suggest that Archean hydrothermal system contains organic matter which is blowout from the see-floor surface. Early life exuberate (such as bacteria) on these organic rich see-floor along the vent system.
V21D-0762
The Lateral Change of Archean Hydrothermal System, The Dixon Island Formation in the Coastal Pilbara Terrane, Australia
The 3.2 Ga Dixon Island Formation in the coastal Pilbara terrane, Western Australia, is one of the most complete and best-preserved examples of Mesoarchean submarine hydrothermal system. Its continuous section is well exposed along the northern coastline of the Dixon Island. Based on our detailed geological mapping, the 6km- wide outcrop could be subdivided into six blocks, namely DX-A to DX-F, which were defined by dextral strike-slip faults. The 400m-thick Dixon Island Formation is composed of, in an ascending order, Komatiite-Rhyolite tuff (KR), Black Chert (BC), and Varicolored Chert (VC) Members. To explore lateral variations of potential biological activity in submarine hydrothermal system and depositional environments, we focus on the organic carbon (Corg) bearing BC member and perform petrographic investigation of thin sections under microscope and measured the Corg contents, Corg isotope compositions, and rare earth elements (REEs) abundance. The massive and laminated black cherts in DX-B are thicker than those in DX-F. The amount of massive dark green chert laterally increases from DX-B to DX-F. The shale-normalized REE+Y patterns of both massive and laminated cherts similarly show overall enrichment of heavy REEs and positive Eu anomalies. The Corg contents are generally very low (<0.05 wt.%) throughout the area. Those of massive black chert in the BC Member slightly decrease laterally from west to east (DX-B to DX-F), ranging from 0.08 wt.% to 0.02 wt.%. Their Corg isotopic compositions decrease from the lower -25 to -30‰ to middle part -30 to -35‰, and increase to the upper part -20 to -30‰. Some thin layers in the BC Member have slightly high Corg contents (0.10%) with very low Corg isotopic compositions -45 to -40‰. However, the average Corg isotopic compositions are similar in DX-A through DX-E -33 ‰ for DX-A and DX-B, 31‰ for DX-C through E. DX-F is an exception; it has less negative 26.6‰ values. Based on the field observation and obtained geochemical data set, we suggest that the chert units with similar Corg isotopic compositions have carbonaceous matters of similar origin. We also suggest that the thickest unit DX-B with positive Eu anomaly represents sediments that were most substantially influenced by submarine hydrothermal activity. Such hydrothermal activity most likely influenced biological activity, which resulted in systematically different Corg contents and Corg isotopic compositions both in lateral and vertical extent.
V21D-0763
Numerical Simulation of Magma-hydrothermal System at Iwodake Volcano, Satsuma- Iwojima, Japan
The thermal activity of Iwodake volcano, Satsuma-Iwojima, south of Kyusyu, Japan, is characterized by the predominant volcanic gas emission at the summit crater and the heat discharge from high ground temperature area, which is distributed widely from the summit crater to hillside of the mountain. The volcanic gas at the summit crater is of magmatic origin and its temperature is observed to be 880 °C in maximum. The total amount of the volcanic gas discharged from the summit area is estimated to be 200kg /sec from the SO2 measurement and the chemical composition of the volcanic gas. The heat discharge rate from the high ground temperature area at the hillside is estimated to be 80MW from the surface temperature measurement. These thermal activities of Iwodake volcano are thought to be continued for more than 800 years. The continuous active degassing causes the hydrothermal system within the volcano because the volcanic gas, ascending the conduit, is diffused to the surrounding formation. The development of such hydrothermal system is studied using the mathematical simulation. The simulator accounts for mass and heat convection of liquid water and/or vapor within a porous media. In this calculation, the degassing is considered as the source at the top of the columnar magma with constant high temperature. The results show that the overall thermal activity of Iwodake volcano such as the volcanic gas ejection at the summit crater, widely distributed ground temperature anomaly at hillside can be caused by the volcanic gas flow which is diffused from the vent. The important factors in order to induce the wide-ranging hydrothermal system are permeability of the volcanic edifice and the depth of the degassing. The simulation indicates that the permeability of 0.1 darcy and the degassing above sea level are necessary condition for the Iwodake thermal activity.
V21D-0764
Geology of the Early Archean Mid-Ocean Ridge Hydrothermal System in the North Pole Dome, Pilbara Craton, Western Australia
An Archean hydrothermal system in the North Pole Dome, Pilbara Craton is associated with extensive fluid circulation driven by numerous extensional fracture systems and the underlying heat source. The fracture system is now occupied by abundant fine-grained quartz aggregate, hence we call this as silica dikes. Some of the fracture system extends deeper structural levels as listric normal faults down to 1000 m depth in the MORB crust. Barite-bearing fine-grained quartz predominant mineralogy indicates the extensive development of fracturing and quenching in a short time. Accompanying the fluid circulation, the extensive metasomatism proceeded to form the four different chemical courses, (1) silicification, (2) carbonation, (3) potassium-enrichment, and (4) Fe- enrichment. Silicification occurs along the silica dikes, carbonated greenstones are distributed relatively shallower level. Potassium-enriched (mica-rich) greenstones occur at the top of the greenstone sequence, and Fe-enriched (chlorite-rich) greenstones are distributed at lower part of the basaltic greenstones. The down going fluid precipitated carbonate-rich layer at shallow levels, whereas depleted in SiO2. Then, the fluid went down to more deeper level, and was dissolved SiO2 at high temperature (~350°C) and chlorite-rich greenstone was formed by water-rock interaction. The upwelling fluid precipitated dominantly SiO2 and formed silica dikes. Silica dikes cement the fractures formed by extensional faulting at earliest stage of development of oceanic crust. Therefore, the hydrothermal system must have related to normal fault system simultaneously with MORB volcanism. Particularly the greenish breccia with cherty matrix (oregano chert) was formed at positions by upwelling near ridge axis. After the horizontal removal of MORB crust from the ridge-axis with time, the propagating fracture into deeper levels, transports hydrothermal fluids into 500-1000 m depth range where metasomatic element exchange between fluid and country rocks proceed and transport rock components to the surface through the fractures.