The 1995 Northern Niigata Earthquake with special attention to a geopressured hydrothermal system

The 1995 Northern Niigata Earthquake of magnitude (M) 5.5 occurred at the eastern margin of the Niigata seismic gap and might have been a precursor of a large destructive earthquake. The anomaly areas in temperature, electrical conductivity and Cl^- concentration of groundwater were approximately coincident with the area of the seismic intensity 6 on the Japan Meteorological Agency scale, and convincingly demonstrated the presence of a buried active fault beneath the epicentral area, as was suggested by a linear distribution of seismic intensity 6. These anomalies of groundwater were created by the expulsion of geopressured hydrothermal water along an active fault. Anomalies in local groundwater and hotspring systems associated with the earthquake and the proximity of the earthquake to the Niitsu oil field led to an interpretation that the earthquake might have been triggered by activity within the geopressured hydrothermal system. The accumulation of geopressured hydrothermal water in combination with high rock temperature might reduce fracture strength of the rock, and trigger earthquake occurrence.     

Expulsion of geopressured hydrothermal system along active faults and its relation to the occurrence of earthquakes in the Shinanogawa seismic belt,Japan

The 1995 Northern Niigata Earthquake (M 6.0) occurred at a shallow depth in the Niigata seismic gap. The anomaly areas in temperature, electrical conductivity and Cl- concentration of groundwater trend northeast as linear distribution in the epicentral area and are approximately coincident with the area of the seismic intensity 6 (JMA scale). The distributions of seismic intensity 6 and groundwater anomalies convincingly imaged the presence of a buried active fault beneath the epicentral area. The occurrence of this earthquake and the anomalies of groundwater were related to the expulsion of geopressured hydrothermal system (GHS). All epicenters of the destructive earthquakes along the Shinanogawa seismic belt are actually located in the buried active fault zones characterized by the areas of temperature and geochemical anomalies of groundwater. These earthquakes might have been triggered by the activity of GHS. The expulsion of GHS along an active fault in combination with the thermal softening of fault     

Expulsion of a geopressured hydrothermal system associated with destructive earthquakes and buried active faults in the Shinanogawa Seismic Belt, Japan

Abstract   The Shinanogawa Seismic Belt in the Northern Fossa Magna, Honshu Island, Japan, extends along the Shinano River, bounding the Eurasian Plate and the Okhotsk Plate. The geopressured hydrothermal system occurs widely in the Northern Fossa Magna region. Many destructive earthquakes are related to the activity of this system in the Shinanogawa Seismic Belt. Expulsion of a geopressured hydrothermal system and rising from depth along an active fault triggers the occurrence of an earthquake and opens the fault as a pathway. Anomalous areas in temperature, electrical conductivity and Cl^– concentration of groundwater trend north–east in a linear distribution, and convincingly demonstrate the presence of a buried active fault at the epicentral area of the destructive earthquake in the Shinanogawa Seismic Belt. The distribution of the major axis of the anomalous area in groundwater temperature shows a strong positive relationship with earthquake magnitude, which means that the distribution of this area may indicate the scale of earthquake fault. The linearly anomalous areas in groundwater temperature, resulting from the percolation of a geopressured hydrothermal system, that have no record of previous destructive earthquake are predicted to be areas where destructive earthquakes could occur in the future. Four potential earthquake areas are proposed and discussed in this paper, based on re-examination of active faults and seismicity in the Shinanogawa Seismic Belt.     

1995年日本新泻北部M6.0地震及其由地震烈度和地下水地热异常分布所揭示的隐伏地震断层

1995年日本新泻北部M6.0地震发生在新泻地震空区的东部边缘.由于此次地震震源较浅(10 km),造成了55栋房屋倒塌、165栋房屋半倒.通过计算倾倒墓碑的地震矩,对本区地震烈度进行了分析,发现烈度6度区(据日本JMA度)为一覆盖面积6.1×1 km2,呈NNE-SSW向分布的条带,表明震中区的冲积平原下存在一条隐伏断层.震中区地下水的温度、电导率和Cl-浓度等异常区与呈线性分布的6度烈度区大致吻合,也有力地证明了震中区下面存在一条隐伏断层.这次地震可能是由高压型热水系沿隐伏活动断层喷溢引起.带着高温的高压热水降低了岩石的断裂强度,从而触发了地震.     

Temporal changes in the chemistry of lake water within Yugama Crater,Kusatsu-Shirane Volcano,Japan: Implications for the evolution of the magmatic hydrothermal system

The chemical composition and temperature of lake water within Yugama Crater, Kusatsu-Shirane Volcano, Japan, have fluctuated markedly over the period 1966 to 2005. From 1966 to 1981, water temperature was relatively low, and Cl⁻ and SO₄²⁻ concentrations gradually decreased. These trends are explained by the growth of a sealing zone that formed between the region of hot rock surrounding the degassing magma and cool crust saturated with groundwater. The sealing zone acted to restrict the emission of magmatic volatiles.     

Occurrence conditions of hyperpycnal flows, and their significance for organic-matter sedimentation in a Holocene estuary, Niigata Plain, Central Japan

River floods influence sedimentary environments and ecosystems from the terrestrial to the deep-marine. This study documents the occurrence conditions of hyperpycnal flows generated by river floods and related organic-matter sedimentation for Holocene sediments of the Niigata Plain, Central Japan, based on detailed sedimentary facies, total sulfur and total organic carbon content, diatom assemblages and organic-matter composition. Holocene sediments of the Niigata Plain consist of sand, mud and gravel that were deposited in estuarine and fluvial systems during a sea-level rise (15 000–6800 years BP) and stillstand (after 6800 years BP) following the Last Glacial Maximum. Hyperpycnites are present in the upper part of the estuarine lagoon sediments. The depositional age is considered to be about 5000 years BP. The hyperpycnites comprise two successions of a top fining-up unit and a basal coarsening-up unit, and include abundant terrigenous organic matter and freshwater diatoms. A large volume of freshwater is inferred to have flowed into the lagoon during deposition of the upper part of the lagoon sediments. In consequence, hyperpycnal flows may have readily formed in the lagoon, because the halocline was weak. The hyperpycnal flows also produced a layer of concentrated terrigenous organic matter in the uppermost part of the hyperpycnites. The abundant organic matter on the estuarine floor is inferred to have produced anoxic bottom conditions owing to oxidative decomposition by benthic bacteria.     

Sr isotope diversity of hot spring and volcanic lake waters from Zao volcano,Japan

The ratio of ⁸⁷Sr/⁸⁶Sr was measured from different water samples of thermal/mineral (hot spring as well as crater lake) and meteoric origins, in order to specify the location and to verify the detailed model of a volcano-hydrothermal system beneath Zao volcano. The ratio showed a trimodal distribution for the case of thermal/mineral water: 0.7052–0.7053 (Type A, Zao hot spring), 0.7039–0.7043 (Type B, Okama crater lake and Shin-funkiko hot spring), and 0.7070–0.7073 (Type C, Gaga, Aone, and Togatta hot springs), respectively. However, in comparison, the ratio was found to be higher for meteoric waters (0.7077–0.7079). The water from the central volcanic edifice (Type B) was found to be similar to that of nearby volcanic rocks in their Sr isotopic ratio. This indicates that the Sr in water was derived from shallow volcanic rocks. The ⁸⁷Sr/⁸⁶Sr ratio for water from the Zao hot spring (Type A) was intermediate between those of the pre-Tertiary granitic and the Quaternary volcanic rocks, thus suggesting that the water had reacted with both volcanic and granitic rocks. The location of the vapor–liquid separation was determined as the boundary of the pre-Tertiary granitic and the Quaternary volcanic rocks by comparing the results of this strontium isotopic study with those of Kiyosu and Kurahashi [Kiyosu, Y., Kurahashi, M., 1984. Isotopic geochemistry of acid thermal waters and volcanic gases from Zao volcano in Japan. J. Volcanol. Geotherm. Res. 21, 313–331.].     

Geochemistry of the magmatic–hydrothermal system of Kawah Ijen volcano, East Java, Indonesia

Samples from Kawah Ijen crater lake, spring and fumarole discharges were collected between 1990 and 1996 for chemical and isotopic analysis. An extremely low pH (<0.3) lake contains SO₄–Cl waters produced during absorption of magmatic volatiles into shallow ground water. The acidic waters dissolve the rock isochemically to produce “immature” solutions. The strong D and ¹⁸O enrichment of the lake is mainly due to enhanced evaporation at elevated temperature, but involvement of a magmatic component with heavy isotopic ratios also modifies the lake D and ¹⁸O content. The large Δ_SO4–S⁰ (23.8–26.4‰) measured in the lake suggest that dissolved SO₄ forms during disproportionation of magmatic SO₂ in the hydrothermal conduit at temperatures of 250280°C. The lake δ¹⁸O_SO4 and δ¹⁸O_H2O values may reflect equilibration during subsurface circulation of the water at temperatures near 150°C. Significant variations in the lake's bulk composition from 1990 to 1996 were not detected. However, we interpret a change in the distribution and concentration of polythionate species in 1996 as a result of increased SO₂-rich gas input to the lake system.Thermal springs at Kawah Ijen consist of acidic SO₄–Cl waters on the lakeshore and neutral pH HCO₃–SO₄–Cl–Na waters in Blawan village, 17 km from the crater. The cation contents of these discharges are diluted compared to the crater lake but still do not represent equilibrium with the rock. The SO₄/Cl ratios and water and sulfur isotopic compositions support the idea that these springs are mixtures of summit acidic SO₄–Cl water and ground water.The lakeshore fumarole discharges (T=170245°C) have both a magmatic and a hydrothermal component and are supersaturated with respect to elemental sulfur. The apparent equilibrium temperature of the gas is 260°C. The proportions of the oxidized, SO₂-dominated magmatic vapor and of the reduced, H₂S-dominated hydrothermal vapor in the fumaroles varied between 1979 and 1996. This may be the result of interaction of SO₂-bearing magmatic vapors with the summit acidic hydrothermal reservoir. This idea is supported by the lower H₂S/SO₂ ratio deduced for the gas producing the SO₄–Cl reservoir feeding the lake compared with that observed in the subaerial gas discharges. The condensing gas may have equilibrated in a liquid–vapor zone at about 350°C.Elemental sulfur occurs in the crater lake environment as banded sediments exposed on the lakeshore and as a subaqueous molten body on the crater floor. The sediments were precipitated in the past during inorganic oxidation of H₂S in the lake water. This process was not continuous, but was interrupted by periods of massive silica (poorly crystallized) precipitation, similar to the present-day lake conditions. We suggest that the factor controlling the type of deposition is related to whether H₂S- or silica-rich volcanic discharges enter the lake. This could depend on the efficiency with which the lake water circulates in the hydrothermal cell beneath the crater. Quenched liquid sulfur products show δ³⁴S values similar to those found in the banded deposits, suggesting that the subaqueous molten body simply consists of melted sediments previously accumulated at the lake bottom.     

Geochemical processes and origin of boron isotopes in geothermal water in the Yunnan-Tibet geothermal zone

Boron concentrations and isotope compositions have been measured for 93 water samples from the hot springs and drill-holes in the geothermal system in the Yunnan-Tibet Geothermal Belt(YTGB),China.Boron concentrations range from 0.036–472.4ppm,and theδ11B values range from -16.0‰to 13.1‰,indicating the non-marine origin for each geothermal system.We observed a clear binary mixing relationship between the B concentrations and B isotope compositions in Tibet geothermal area.This relationship can be well explained by two sources,i.e.,marine carbonate rocks and magmatic rocks,for the Tibet geothermal water.No evidence supports a mantle contribution to B.In addition,we found that the precipitation only plays a dilution role for B of geothermal waters.δ11B values for the precipitation across the southern Tibetan Plateau area range from -6.0‰ to -6.8‰at least.Due to data scarcity in Yunnan geothermal area,we observed possible different boron sources from the Tibet geothermal system.Comparing it with other geothermal systems in the world,we found that the samples from YTGB have the lowestδ11B values and the largest range of B concentration,which might be related to their special geological background.On the whole,the world geothermalδ11B-Cl/B relation suggests a mixing process between marine and non-marine sources.Additionally,we suggest that B source of B-enriched geothermal waters is mainly from B-enriched crustal country-rocks,instead of mantle.     

Petroleum geological dynamics of Lower Paleozoic in the Ordos Basin, northwest China

Taking the hydrocarbon source rocks of Ordovician, Lower Paleozoic in the Ordos Basin as the main research object, the characteristics of petroleum geological dynamics about geohistory, geothermal history, hydrocarbon generation history and hydrocarbon expulsion history were studied by using the methods of basin numerical modeling dynamically and pool-forming dynamics. It is shown that the Ordovician strata had entirely undergone five stages of initial deposition, uplift and erosion, rapid subsidence, alternating uplift and subsidence, and differential uplift and erosion; that under the background of lower heat flow on the whole, the paleoheat flow of Ordovician strata in the basin could be divided into two large stages of relatively high heat flow values period before Cretaceous deposition and relatively low ones after it; that the thermal evolution of organic matters in the hydrocarbon source rocks of Ordovician had entered into high mature-postmature stage on the whole and the intensity of gas generated was greater than that of oil generated, the hydrocarbon being mainly natural gas; and that the curves of the intensity of oil and gas expulsion at each time unit showed the feature of multi-peak-type, the accumulative intensity of gas expulsion was greater than that of oil expulsion. Thus natural gas exploration potential is good.     

Using environmental isotopes to investigate the groundwater recharge mechanisms and dynamics in the North-eastern Basin,Palestine

ABSTRACT

This study aims to differentiate the potential recharge areas and flow mechanisms in the North-eastern Basin, Palestine. The results differentiate the recharge into three main groups. The first is related to springs and some of the deep wells close to the Anabta Anticline, through the Upper Aquifer (Turonian) formation, with depleted δ¹⁸O and δ²H. The second is through the Upper Cenomanian formation surrounding the Rujeib Monocline in the southeast, where the lineament of the Faria Fault plays an important role, with relatively enriched δ¹³C_DIC values of about ?4‰ (VPDB). The third is the Jenin Sub-series, which shows higher δ¹³C_DIC values, with enriched δ¹⁸O and δ²H and more saline content. The deep wells from the Nablus area in the south of the basin indicate low δ¹³C_DIC values due to their proximity to freshwater infiltrating faults. The deep wells located to the northwest of the basin have δ¹³C_DIC values from ?8 to ?9‰ (VPDB), with enriched δ¹⁸O signatures, indicating slow recharge through thick soil.     

Distribution and significance of hydrothermal alteration minerals in the Tuzla hydrothermal system, Çanakkale, Turkey

Hydrothermal alteration zones have been investigated by X-ray diffraction, mineralogical–petrographical techniques, and geochemical analysis. Examination of cores and cuttings from two drill sites, obtained from a depth of about 814–1020 m, show that the hydrothermal minerals occuring in the rock include: K-feldspar, albite, chlorite, alunite, kaolinite, smectite, illite, and opaque minerals.In the studied area, silicified, smectite, illite, alunite, and opal zones have been recognized. These alteration mineral assemblages indicate that there are geothermal fluids, which have temperatures of 150–220°C in the reservoir.The distribution of the hydrothermal minerals shows changes in the chemical composition of the hydrothermal fluid, which are probably due not only to interaction with host rock, but also to dilution of the Na–K–Cl-rich hydrothermal fluid of the deep reservoir by cold sea water at shallow levels. Geochemical analyses of the solid and liquid phases indicate that the hydrothermal fluids of the Tuzla geothermal system are in equilibrium with alteration products.The tectonic structure of the studied area is caused by NW–SE and NE–SW directional forces. The volcanic rocks where hydrothermal zones are observed in the studied area are of Lower–Middle Miocene age comprise latite, andesite, dacite, rhyolite-type lavas, tuff, and ignimbrites.     

Implication of the temporal sulphur isotopic variation during the 2000 eruption of Miyakejima Volcano, Japan

Abstract   The variation of sulphur isotopic composition during the 2000 eruption of the Miyakejima Volcano was examined in order to monitor the temporal change of the volcanic activity. The δ³⁴S values of water-soluble sulphate leached from volcanic ash effused during intermittent eruptions from July to September 2005 range from +5 to +11‰ with a fluctuation of ca 3‰ within a single eruption. The δ³⁴S value of sulphuric acid mist collected with 'Cu-metal trap' placed on the flank of the volcano from December 2000 to January 2001 is +6.2‰. These sulphur isotopic compositions of sulphate, which were isotopically equilibrated in the subvolcanic hydrothermal system, indicate that the temperature of the hydrothermal system beneath the caldera increased after the period of intermittent phreatic and phreatomagmatic eruptions. Then, the δ³⁴S value of sulphuric acid trapped from January to March 2001 was +9.0‰ and the δ³⁴S value of water-soluble sulphate on volcanic ash emitted with minor eruption in May 2001 was +11.0‰, suggesting a decrease in temperature of the subvolcanic hydrothermal system.     

Geochemical characteristics of organic matter-rich strata of lower Cambrian in Tarim Basin and its origin

The organic matter-rich strata of the Yurtusi Formation of Lower Cambrian are wildly spread and steady in the Tarim Basin, in which cherts are developed, companied with phosphorite. Al/(Al+Fe+Mn) ratio and Si/(Si+Al+Fe) ratio of cherts range from 0.0023 to 0.0046 and 0.965 to 0.98, respectively, suggesting that cherts are formed in submarine hydrothermal activity and far away from terrestrial. Trace elements, such as As, Hg, Pb, Zn, Cu, Co, P, V, Ba, etc., are markedly rich in organic matter-rich sedimentary rocks of which Ba/Sr ratios are between 21.2 and 158.1. Compared with modern hydrothermal sediments, their Ba/Sr ratios are similar, it indicates that the organic matter-rich strata have the geochemical feature of submarine hydrothermal sediments. The total content of rare earth elements (ЕREE) in cherts is from 8.81 to 56.682 μg/g, on average 31.41 μg/g and the ΕREE of cherts is between continental margin chert's and abyssal chert's. The LREE/HREE ratio of cherts varies from 1.01 to 3.56, which reveals the characteristics of hydrothermal sediments. There is positive correlation between the total organic content (TOC) and Ba/Sr ratio which is geochemical index of submarine hydrothermal activity, which indicates that the submarine hydrothermal activity obviously influences the richness of organic matter in the Yurtusi Formation.     

The hydrothermal karstification and its effect on Ordovician carbonate reservoir in Tazhong uplift of Tarim Basin, Northwest China

With a detailed study on petrology, mineralogy and geochemistry of some important Ordovician carbonate well core samples in Tazhong uplift of Tarim Basin, the distinguishing symbols of hydrothermal karstification are first put forward as the phenomena of rock hot depigmentation, hot cataclasm and the appearance of typical hydrothermal minerals such as fluorite, barite, pyrite, quartz and sphalerite. The main homogenization temperatures of primary fluid inclusions in fluorite are from 260 to 310℃, indicating the temperature of hydrothermal fluid. The fluid affected the dissolved rocks and showed typical geochemistry features with low contents of Na and Mg, and high contents of Fe, Mn and Si. The ratio of 3He/4He is 0.02Ra, indicating the fluid from the typical continental crust. The hydrothermal fluid karstification pattern may be described as follows the hot fluid is from the Permian magma, containing dissolving ingredients of CO2 and H2S, and shifts along fault, ruptures and unconformity, and dissolves the surrounding carbonates while it flows. The mechanism of hydrothermal karstification is that the mixture of two or more fluids, which have different ion intensity and pH values, becomes a new unsaturated fluid to carbonates. The hydrothermal karstification is an important process to form hypo-dissolved pinholes in Ordovician carbonates of Tazhong uplift of Tarim Basin, and the forming of hydrothermal minerals also has favorable influence on carbonate reservoirs.     

腾冲火山岩同位素地球化学研究

研究了腾冲火山岩的Ｐｂ、Ｓｒ、Ｎｄ同位素地球化学特征;该区火山岩具高的＾２０６Ｐｂ／＾２０４ｐｂ、＾２０７ｐｂ／＾２０４ｐｂ、＾２０８ｐｂ／＾２０４ｐｂ、＾８７Ｓｒ／＾８７Ｓｒ和低的＾１４３Ｎｄ／＾１４４Ｎｄ值,所反映的岩浆源区为ＥＭⅡ。模拟计算表明：地幔源中下地壳组分（含少量大洋沉积物）占２０％,地幔源相当于榴辉岩组成。     

乌兹别克斯坦阿姆河流域水体中多环芳烃的分布、来源及风险评估

为研究乌兹别克斯坦境内阿姆河地区水体中多环芳烃（PAHs）污染特征、来源并进行风险评估,采用高效液相色谱二极管阵列检测器串联荧光检测器法,对研究区域50个采样点中16种优先控制的多环芳烃进行了检测分析.结果表明,阿姆河地区水体中多环芳烃总浓度范围为3.19~779 ng/L,平均值为98.4 ng/L,中位值为40.1 ng/L,单体浓度范围为0~333 ng/L,检出浓度最高的单体为苊烯,5种单体芴、蒽、荧蒽、芘和的检出率为100%,单体苯并[b]荧蒽的检出总量最高,水样中总浓度为786 ng/L,平均值为15.7 ng/L,中值为2.79 ng/L.不同水体含中低环多环芳烃（2~4环）与高环多环芳烃（5~6环）总浓度相近,但不同采样点间浓度差异较大.浓度较高的采样点主要集中在阿姆河三角洲的城市、农业灌溉区及近咸海区域.与世界不同研究区域相比,阿姆河流域多环芳烃浓度处于中等水平.采用相对丰度法、同分异构体比值法及正定矩阵分解法相结合进行源解析,表明研究区域水体中多环芳烃多为混合来源,其中阿姆河下游河段水体多环芳烃主要来源于生物质燃烧,而阿姆河三角洲区域主要来源于生物质燃烧、石油、天燃气燃烧及汽车尾气排放.生态风险评估结果显示,研究区水体单体多环芳烃中萘、苊、菲和蒽的生态风险较低,其余单体处于中等风险等级,其中苯并[b]荧蒽的污染程度较为严重;总体上阿姆河流域ΣPAHs风险等级相对较低,但仍有12和8个点位分别处于中等风险2和高风险等级,且主要集中在阿姆河三角洲地区,需采取相应措施加以控制.     

Carbon isotope composition and its implications of Lower Cretaceous Aptian-Albian shallow water carbonates in the Cuoqin Basin, North Tibet

Since Schlanger and Jenkyns (1976)[1] first ad-vanced the ?oceanic anoxic events ?(OAEs) model to explain the origin of the worldwide distributed Early Cretaceous Aptian-Albian (115—103 MaBP) and Late Cretaceous Cenomanian-Turonian (95—88 MaBP) black shales, the OAEs and responses to them have been one of the 揾ot spots?in geological circles. Up till now, researchers at home and abroad have exten-sively studied the sedimentary and geochemical re-sponses to the two anoxic events, espe…     

Geothermal constraints on the hydrological regime of the TAG active hydrothermal mound, inferred from long-term monitoring

During August 1994 to March 1995, a period that included ODP Leg 158 drilling, bottom-water and sub-bottom temperatures were continuously logged by a long-term temperature monitoring system ‘Daibutsu’ at the base of the central black-smoker complex (CBC) and within the low heat flow zone at the TAG hydrothermal mound on the Mid-Atlantic Ridge. The temperature of hydrothermal fluid at CBC was also measured with a small high-temperature probe ‘Hobo’. Bottom-water temperature variations measured with Daibutsu at both sites have predominant semi-diurnal periods, causing the sub-bottom temperatures to fluctuate at these periods with reduced amplitudes and phase delays at sub-bottom depths. Seawater entrainment into the mound has been previously suggested at the low heat flow zone. We quantitatively evaluate the seawater entrainment rate at both sites from a one-dimensional numerical model, combined with a heat conduction model for the semi-diurnal variations. The entrainment rate of seawater at the base of CBC is estimated as 1.3±0.5×10⁻⁵ m/s, at least from August 17 to 30, 1994. On the other hand, the seawater entrainment rate at the low heat flow zone was undetected by long-term temperature monitoring at shallow sub-bottom depth. Nevertheless an increase in heat flow observed at the low heat flow zone during ODP drilling can be interpreted as a decrease in the entrainment rate of seawater. Before ODP Leg 158, Daibutsu measured three sub-bottom temperature anomalies at the base of CBC not derived from bottom-water temperature variations and Hobo also detected a CBC fluid temperature anomaly, indicating some natural changes in fluid flow within the mound. Daibutsu and Hobo also measured temperature anomalies during and after drilling at the ODP TAG-1 area. The Hobo temperature anomalies are inferred to have occurred when the cold fluid entrained through the drill holes at TAG-1 site reached or cooled the main fluid path to CBC. The entrained seawater through the drill holes appears to have contributed to dissolution and precipitation of anhydrite within the mound and perhaps affected the local permeability structure inside the mound. The temperature anomalies measured with Daibutsu at the base of CBC may have been induced by the change in the fluid flow pattern as a result of such permeability changes within the mound.     

He, Ne and Ar isotope compositions of fluid inclusions in hydrothermal sulfides from the TAG hydrothermal field Mid-Atlantic Ridge

Helium, neon and argon isotope compositions of fluid inclusions have been measured in hydrothermal sulfide samples from the TAG hydrothermal field at the Mid-Atlantic Ridge. Fluid-inclusion³He/⁴He ratios are 2.2—13.3 times the air value (Ra), and with a mean of 7.2 Ra. Comparison with the local vent fluids (³He/⁴He=7.5—8.2 Ra) and mid-ocean ridge basalt values (³He/⁴He=6—11 Ra) shows that the variation range of³He/⁴He ratios from sulfide-hosted fluid inclusions is significantly large. Values for²⁰Ne/²²Ne are from 10.2 to 11.4, which are significantly higher than the atmospheric ratio (9.8). And fluid-inclusion⁴⁰Ar/³⁶Ar ratios range from 287 to 359, which are close to the atmospheric values (295.5). These results indicate that the noble gases of fluid inclusions in hydrothermal sulfides are a mixture of mantle- and seawater-derived noble gases; the partial mantle-derived components of trapped hydrothermal fluids may be from the lower mantle; the helium of fluid inclusions is mainly from upper mantle; and the Ne and Ar components are mainly from seawater.