Low-pH waters discharging from submarine vents at Panarea Island (Aeolian Islands,southern Italy) after the 2002 gas blast: Origin of hydrothermal fluids and implications for volcanic surveillance

A geochemical survey of thermal waters collected from submarine vents at Panarea Island (Aeolian Islands, southern Italy) was carried out from December 2002 to March 2007, in order to investigate (i) the geochemical processes controlling the chemical composition of the hydrothermal fluids and (ii) the possible relations between the chemical features of the hydrothermal reservoir and the activity of the magmatic system. Compositional data of the thermal water samples were integrated in a hydrological conceptual model, which describes the formation of the vent fluid by mixing of seawater, seawater concentrated by boiling, and a deep, highly-saline end-member, whose composition is regulated by water-rock interactions at relatively high temperature and shows clear clues of magmatic-related inputs. The chemical composition of concentrated seawater was assumed to be represented by that of the water sample having the highest Mg content. The composition of the deep end-member was instead calculated by extrapolation assuming a zero-Mg end-member. The Na–K–Ca geothermometer, when applied to the thermal end-member composition, indicated an equilibrium temperature of approximately 300 °C, a temperature in agreement with the results obtained by gas-geothermometry.     

常见稀有气体采样容器研究进展及意义

稀有气体是地学研究的重要手段之一,在研究成矿流体来源、壳幔相互作用过程中具有重要的研究意义,其组成及同位素比值是研究天体和地质体来源、成岩机理及各种地质和物理化学过程的关键,可作为地球化学示踪剂。如何有效地在大气圈、水圈和岩石圈进行稀有气体同位素样品的提取,是气体同位素研究急需解决的基础科学问题,所以气体采样容器和取样技术显得尤为重要。本文在文献调研的基础上,对常见的采样容器的优缺点进行对比,并总结了不同采样容器对稀有气体取样的优劣性。常见的气体采集容器包括不锈钢瓶、铜管、玻璃瓶、气体采样袋、注射器等,而稀有气体的采集容器常为不锈钢瓶和铜管等;通过对比表明不锈钢瓶具有耐高温、耐高压、抗强腐蚀、不易燃、不易爆等特点和优越性,在气体样品采集和运输过程中稳定性最好,实验效果也最好;铜管采样效果和密封性好,但操作较为复杂;玻璃采样容器效果次之;石英玻璃瓶虽然操作简便,但是运输保存不便;气体采样袋和注射器的采集和运输储存效果较差。因此建议稀有气体样品采集使用不锈钢瓶和铜管以及钠钙材质的玻璃瓶,不建议将石英材质玻璃容器以及注射器和气袋作为稀有气体的采样工具。该工作可为气体地球化学的研究提供新的参考。     

Proterozoic seawater — felsic volcanics interaction W. Bergslagen,Sweden. Evidence for high REE mobility and implications for 1.8 Ga seawater compositions

Mg enrichment in diffusely bounded, schistose alteration zones occurring in Proterozoic submarine felsic volcanics NE of Hjulsjö, W. Bergslagen is shown to result from felsic rock-seawater interaction in a sub-seafloor hydrothermal system. The alteration is apparently in two stages, feldspar being altered first to a kaolinite-type clay which is then replaced by Mg-chlorite. Major and trace element data are presented for the schist and sheridanite (chlorite). Relative to the least altered felsic volcanics, and for negligible volume change, the schist shows strong addition of Mg and to a lesser extent Si; K, Al and Rb are apparently conserved, while most other elements are strongly depleted, including the REEs which are removed for ～75%, indicating a high degree of mobility. The chlorite, with its HREE enriched pattern, is considered to have equilibrated with the hydrothermal fluid, which consisted predominantly of seawater. Comparison with modern seawater leads to the tentative conclusion that the Proterozoic seawater HREE composition was not drastically different from that of modern seawater.     

Fluid–Sediment Interaction in a Marine Shallow-Water Hydrothermal System in the Wakamiko Submarine Crater, South Kyushu, Japan

The Wakamiko submarine crater is a small depression located in Kagoshima Bay, southwest Japan. Marine shallow‐water hydrothermal activity associated with fumarolic gas emissions at the crater sea floor (water depth 200 m) is considered to be related with magmatic activity of the Aira Caldera. During the NT05‐13 dive expedition conducted in August 2005 using remotely operated vehicle Hyper‐Dolphine (Japan Agency for Marine‐Earth Science and Technology), an active shimmering site was discovered (tentatively named the North site) at approximately 1 km from the previously known site (tentatively named the South site). Surface sediment (up to 30 cm) was cored from six localities including these active sites, and the alteration minerals and pore fluid chemistry were studied. The pore fluids of these sites showed a drastic change in chemical profile from that of seawater, even at 30 cm below the surface, which is attributed to mixing of the ascending hydrothermal component and seawater. The hydrothermal component of the North site is estimated to be derived from a hydrothermal aquifer at 230°C based on the hydrothermal end‐member composition. Occurrence of illite/smectite interstratified minerals in the North site sediment is attributed to in situ fluid–sediment interaction at a temperature around 150°C, which is in accordance with the pore fluid chemistry. In contrast, montmorillonite was identified as the dominant alteration mineral in the South site sediment. Together with the significant low potassium concentration of the hydrothermal end‐member, the abundant occurrence of low‐temperature alteration mineral suggests that the hydrothermal aquifer in the South site is not as high as 200°C. Moreover, the montmorillonite is likely to be unstable with the present pore fluid chemistry at the measured temperature (117°C). This disagreement implies unstable hydrothermal activity at the South site, in contrast to the equilibrium between the pore fluid and alteration minerals in the North site sediment. This difference may reflect the thermal and/or hydrological structure of the Wakamiko Crater hydrothermal system.     

Rare earth element and stable isotope (O, S) geochemistry of barite from the Bijgan deposit, Markazi Province, Iran

The Bijgan barite deposit, which is located northeast of Delijan in Markazi Province of Iran, occurs as a small lenticular body at the uppermost part of an Eocene volcano-sedimentary rock unit. The presence of fossiliferous and carbonaceous strata suggests that the host rocks were deposited in a quiet marine sedimentary environment. Barite, calcite, iron oxides and carbonaceous clay materials are found as massive patches as well as thin layers in the deposit. Barite is marked by very low concentrations of Sr (1–2%) and total amounts of rare earth elements (REEs) (6.25–17.39?ppm). Chondrite-normalized REE patterns of barite indicate a fractionation of light REEs (LREEs) from La to Sm, similar to those for barite of different origins from elsewhere. The La_CN/Lu_CN ratios and chondrite-normalized REE patterns reveal that barite in the Bijgan deposit is enriched in LREE relative to heavy rare earth elements (HREEs). The similarity between the Ce/La ratios in the barite samples and those found in deep-sea barite supports a marine origin for barite. Lanthanum and Gd exhibit positive anomalies, which are common features of marine chemical sediments. Cerium shows a negative anomaly in most samples that was inherited from the negative Ce anomaly of hydrothermal fluid that mixed with seawater at the time of barite precipitation. The δ¹⁸O values of barites show a narrow range of 9.1–11.4‰, which is close to or slightly lower than that of contemporaneous seawater at the end of the Eocene. This suggests a contribution of oxygen from seawater in the barite-forming solution. The δ³⁴S values of barites (9.5–15.3‰) are lower than that of contemporaneous seawater, which suggests a contribution of magmatic sulfur to the ore-forming solution. The oxygen and sulfur isotope ratios indicate that submarine hydrothermal vent fluids are a good analog for solutions that precipitated barite, due to similarities in the isotopic composition of the sulfates. The available data including tectonic setting, host rock characteristics, REE geochemistry, and oxygen and sulfur isotopic compositions support a submarine hydrothermal origin for the Bijgan barite deposit. At the seafloor, barite deposition occurred where ascending Ba-bearing hydrothermal fluids encountered seawater. Sulfate was derived from the sulfate-bearing marine waters, and, to a lesser extent, by oxidized H₂S, which was derived from magmatic hydrothermal fluids.     

Microtexture and Distribution of Minerals in Hydrothermal Barite-Silica Chimney from the Franklin Seamount, SW Pacific: Constraints on Mode of Formation

An extinct hydrothermal barite-silica chimney from the Franklin Seamount of the Woodlark Basin, in the southwestern Pacific Ocean, was investigated for mineral distribution and geochemical composition. Six layers on either side of the orifice of a chimney show significant disparity in color, mineral assemblage and major element composition. Electron microscope(SEM) images reveal that the peripheral wall of the chimney is composed of colloform silica, suggesting that incipient precipitation of silica-saturated hydrothermal fluid initiated the development of the chimney wall. Intermediate layers, between the exterior wall and the inner fluid-orifice, dominate with barite and sulfides. Low Sr-to-Ba ratios(SrO/BaO = 0.015–0.017) indicate restricted fluid-seawater mixing, which causes relatively high-temperature formation of the intermediate layers. Whereas the innermost layer bordering the chimney orifice is characterized by more silica and a higher Sr-to-Ba ratio(SrO/BaO = 0.023), could have formed due to a paragenetic shift from a high-temperature active phase to a cooler waning stage of formation. A paragenetic shift is also probably responsible for the change in mineral formation mechanism that resulted in the textural variation of barite and colloform silica developed during different growth phases of this barite-silica chimney.     

Highly alkaline,high-temperature hydrothermal fluids in the early Archean ocean

Based on the petrology of hydrothermally altered Archean basaltic greenstones, thermodynamic calculations of phase equilibria were conducted to estimate the composition of a high-temperature (∼350 °C) hydrothermal fluid in an Archean subseafloor basalt-hosted hydrothermal system. The results indicate that the hydrothermal fluid was highly alkaline attributed to the presence of calcite in the alteration minerals under a high-CO₂ condition, and predict a generation of SiO₂-rich, Fe-poor hydrothermal fluids in the Archean subseafloor hydrothermal system. The chemically reactive mixing zones between alkaline hydrothermal fluids and slightly acidic-neutral seawater are characterized by inverse pH and chemical polarity to modern hydrothermal systems, leading to extensive precipitation of silica and iron oxyhydroxides on/under the seafloor. Such processes can be responsible for the abiotic formation of voluminous chert and subseafloor silica dike, the mechanism of silicification, and the pH-controlled generation of banded iron formation that has been arising mainly from the redox chemistry in the Archean ocean. Such high-temperature alkaline fluids could have had a significant role not only in the early ocean geochemical processes but also in the early evolution of life.     

大西洋洋中脊TAG热液区中块状硫化物的Os同位素研究

新测得TAG热液区中5件海底块状硫化物样品的锇含量及其同位素组成,187Os/186Os比值在2.305～7.879之间,均值为5.986,介于现代海水和上部洋壳岩石的锇同位素组成之间,表明该区海底块状硫化物中锇是海水和上部洋壳来源锇混合的产物.在海底热液循环过程中,海水的混入对该区热液流体的Os浓度及其同位素组成产生了明显的影响。     

Submarine silica-rich hydrothermal activity during the earliest Cambrian in the Tarim Basin,Northwest China

Siliceous rocks were widely deposited in many continents during the Ediacaran–Cambrian (E–C) transition. Based on detailed field investigations in the Aksu area of the Tarim Basin in Northwest China, this study presents evidence of a submarine silica-rich hydrothermal system preserved in the E–C boundary successions. This system consists of a lower stockwork silica-dominant vein swarm zone in the karstified dolostone of the uppermost Ediacaran Qigebulake Formation, which terminates directly under the overlying bedded chert and black shale succession of the lowermost Cambrian Yurtus Formation. The stockwork vein swarms were filled dominantly by a wide spectrum of silica precipitates (amorphous silica, chalcedony, spherulite, fine to coarse quartz) with subordinate pyrite, Fe-(oxyhydr)oxide, and barite. The host dolostones that were dissected by the vein swarms also suffered extensive silicification and recrystallization. The vertical stacking relationship of silica-dominant vein swarms and overlying bedded chert suggests they were formed by an identical low-temperature, silica-rich diffusive submarine hydrothermal system in the earliest Cambrian. This suggestion is further supported by fluid inclusion microthermometry (T_h 40–200°C) of the quartz-barite vein fills. In this case, silica-rich hydrothermal fluids were channelled and precipitated partially along the stockwork veins in the antecedent karstified dolostone and vented mostly into seawater, promoting widespread deposition of bedded chert on the seafloor of Tarim Basin in the earliest Cambrian. This study provides a useful clue and analogue to understand the widespread silica deposition and coeval vast oceanic and geochemical changes during the E–C transition in the Tarim Basin and elsewhere.     

Thermodynamic model of ore-forming processes in a submarine island-arc hydrothermal system

A thermodynamic model suggested for ore-forming processes in a hydrothermal system (HS) in an island arc is based on the technique suggested earlier in [1] for simulating ore-forming hydrothermal systems in mid-oceanic ridges. This technique make use of the principle of flow-through multistep reactor and encompasses (a) the region where hydrothermal solutions are generated when seawater interacts with rocks (descending convection branch); (b) the region where material is transported with the solution at decreasing pressure (feeder channel); and (c) the region where the ore material is deposited (orebody). Hydrothermal systems in island arcs exhibit the following distinctive features taken into account in the model: (1) the composition of the host crustal rocks (rocks of mafic-acid composition instead of basalt and serpentinite) and (2) possible significant involvement of magmatic gases in the feeding of the hydrothermal system. The naturally occurring prototype of the simulated system is the hydrothermal system in the caldera of a submarine volcano in an island arc. The model is simulated in a number of variants in which the hydrothermal fluid is exogenic (heated seawater convecting through hot volcanic rocks), magmatic, or mixed (magmatic plus exogenic) is involved. The simulations were carried out using the HCh version 4.3 [2] program package for the multisystem H-O-K-Na-Ca-Mg-Fe-Al-Si-C-S-Cl-Cu-Zn-Pb-As-Sb-Ag-Au at temperatures of 25?C370°C and pressures of 10?C500 bar. The multisystem included 88 possible solid phases and aqueous solution with 95 species. The thermodynamic properties of compounds were calculated using the UNITHERM databank. The model is underlain by the principle of multiwave flow-through multistep reactor (MFTMR) with a starting rock/water (R/W) ratio of 1: 1. As progressively more solution portions passed through the rocks, the participation of fresh rock in the interaction accordingly diminished because the rock material was gradually exhausted in the system. The magmatic fluid had a composition selected based on data on fumaroles at Kudryavyi volcano [3] with a correction for the degassing pressure. The evolution of ore deposition was simulated in compliance with the scheme described in [4], which was implemented using the technology of ??openness from above?? [3]. The model was simulated with various compositions of the host rocks (basalts, andesites, dacites, and rhyolites) and the origin of the fluid (magmatic fluid alone, seawater alone, and variable proportions of both). Our simulation results indicate that the metallogeny (relative enrichment in Pb, As, Sb, or Ag) of island-arc ore deposits is controlled by the abundances of metals in the host rocks predominant in the hydrothermal system. The mineralogy and geochemistry of ores generated in arc hydrothermal systems are predetermined by the effective transport of metalloids (S, As, and Sb) that have a high migration capacity in these systems. Magmatic gases introduced in the hydrothermal systems play dualistic roles in the ore-forming processes. If the hydrothermal fluid in a hydrothermal system is dominated by magmatic components, deposits of native sulfur are formed, and the precipitation of base metal is thereby suppressed because of the high acidity of the generated hydrothermal solutions. The involvement of magmatic gases in an amount of a few percent in a hydrothermal system enhances the overall oregenerating potential of the system in terms of sulfide ores.     

全球海水剖面Fe同位素组成的不均一性及其影响因素

全球海水剖面Fe同位素组成存在显著不均一性.对大西洋洋中脊、大西洋近海岸带、东太平洋和西太平洋弧后扩张中心多个站位的海水剖面溶解Fe浓度和Fe同位素组成进行了综合分析,得出以下主要认识:（1）不同区域的海水剖面溶解Fe浓度和Fe同位素组成呈现不同的变化特征,海水Fe同位素的变化趋势与海水溶解氧浓度变化一致,而与海水溶解Fe浓度呈镜像变化关系;（2）不同深度的海水溶解Fe浓度和Fe同位素组成特征的主要控制因素不同.表层海水受到大气降尘、生物作用影响呈现富重Fe同位素特征,受河流的影响Fe同位素组成偏轻;深层海水主要受到深海沉积和海底热液活动的影响,其中沉积物中的非还原溶解Fe导致海水富集重Fe同位素,而受洋中脊热液流体影响的深部海水显著富集轻Fe同位素;（3）将目前已知海底热液溶解Fe通量最小值（0.5 Gmol/a）作为全球大洋的热液溶解Fe通量,利用不同来源的溶解Fe同位素与其通量间的关系估算海底热液对海洋的Fe循环的贡献为~5.5%.由于海底热液流体的Fe通量可能远大于0.5 Gmol/a,因此,海底热液活动对海洋溶解Fe的贡献可能远超过前人的估算结果（6.0%）.      

Epitaxial intergrowths—an indicator of thermodynamic conditions of diamond formation

Hydrothermal activity at Bahía Concepción, on the western coast of the Gulf of California, is not linked to present volcanic activity. This site is a potential energy source; however, geothermal modelling of the system is needed in order to determine the processes that generated this activity. Two processes might lead to the formation of the secondary fluids that were sampled in this submarine hydrothermal system: the thermal endmember might be undergoing either boiling or mixing with a fluid trapped within the sedimentary strata.

However, boiling was ruled out as this process would not produce a highly saline fluid within the temperature range determined for the Bahía Concepción system (～87°C). We modelled the mixing of a highly saline fluid with a geothermal fluid of meteoric origin. Our results suggest that the composition of the thermal water representing the hot endmember at Bahía Concepción consists of 20–30% highly saline fluid and 70–80% aqueous fluid of meteoric origin. The computed fluid has a chemical composition similar to that determined for the submarine vents. Its derived contents of calcium, chloride, and silica are similar to those we measured for the vent discharge. Our hypothesis of saline water addition is supported by the mineralizing fluid characteristics determined from fluid inclusion microthermometric study of a fossil intertidal system that discharges in the vicinity of the vent areas. This intertidal spring fluid at ～40°C, according to the geochemical model, would be supersaturated in opaline silica and calcite, consistent with the mineralogy of the stromatolitic sinter encircling the discharges.     

Hydrothermal Fe-Si-Mn oxide deposits from the Central and South Valu Fa Ridge, Lau Basin

A series of samples from the Hine Hina hydrothermal field (HHF) and the Mariner hydrothermal field (MHF) in the Central and Southern Valu Fa Ridge (VFR), Lau Basin were examined to explain the source origin and formation of the hydrothermal Fe-Si-Mn oxide deposits. The mineralogy was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), Mössbauer spectroscopy, and energy-dispersive spectroscopy (EDS). For the Fe-Mn oxide crusts in the HHF, varying amounts of volcanic fragments and some seawater contributions were recognized, along with higher concentrations of Mn, Al, Co, Ni, Zn, Sr, Mo, elevated ∑REE and negative Ce anomalies. In contrast, the Si-rich oxide samples of the MHF were enriched in Cu, Pb and Ba, indicative of proximity to a hydrothermal jet. Moreover, conductive cooling of hydrothermal fluid evoked the Si-rich deposit formation in the MHF. The Sr, Nd and Pb isotope data provided further constraints regarding the source and formation of the Fe-Si-Mn deposits in the VFR by showing that the samples of the HHF are a mixture of three components, namely, hydrothermal fluid, seawater and volcanic materials, whereas the samples of the MHF were dominated by hydrothermal fluids. The seawater had a minor influence on the Nd isotope data, and the Pb isotope data exhibited a close association with the substrate rock and preformed volcaniclastic layers in this area. The occurrence of relatively high Mn/Fe ratios in the hydrothermal deposits of this area may be a good indicator of the propagating activities of the VFR over geological time.     

Fe Isotope Compositions of Cyprus Umbers

Submarine metalliferous sedimentary rocks are chemical precipitates resulted from hydrothermal exhalation near mid‐ocean ridge or faults. They record the submarine hydrothermal activity between lithosphere and hydrosphere and are critical for understanding Fe cycling in marine environment. Fe was expelled from the hydrothermal vent systems and was oxidized and precipitated in the ambient seawater, where the precipitation of hydrothermal Fe is largely controlled by oxidation state of seawater and is potentially revealed by its Fe isotope compositions. This hydrothermal process in modern hydrothermal vent systems have been well observed, but that for the ancient ones are still not well known. Umbers, or ferromanganoan sediments, overlying Troodos ophiolite in Cyprus of Mid‐Cretaceous age thus provides an excellent example for understanding the Fe cycles in ancient submarine hydrothermal process. Samples were collected from Margi village in Troodos and are mostly amorphous Fe‐Mn oxy‐hydroxides with very minor quartz, goethite, smectite and silicates such as clinopyroxene derived from the volcanic rocks. There is no terrestrial, detrital component. Samples were analyzed for their whole‐rock element and Fe isotope compositions. The results show that samples are composed mainly of SiO₂ (13～80 wt%), Fe₂O₃ (9～54 wt%) and MnO (1.5～10.4 wt%), with minor Al₂O₃ (0.7～4.3 wt%). PAAS‐normalized REE patterns are near flat with significantly negative Ce anomalies (Ce/Ce* is from 0.2 to 0.5) and slightly positive Eu anomalies (Eu/Eu* is around 1.1), indicating a source from the oxidized seawater and the high‐temperature hydrothermal fluids. δ⁵⁶FeIRMM‐014 values of samples are ‐0.32‰ to ‐0.15‰, with an average of ‐0.20‰, which are consistent with those of the hydrothermal fluids previously reported. The narrow Fe isotope compositions of Cyprus umbers that are close to those of submarine hydrothermal fluids indicates near complete oxidation of hydrothermal Fe²⁺ during its expulsion from the hydrothermal vent.     

新疆预须开普台铁铜矿床成因探讨

根据矿床地质地球化学等方面的研究，认为该矿床成矿物质来源于深部地壳或上地幔；通过海底火山同流作用，在酸性相对还原的热学沉积盆地中沉积成矿。成矿流体来源岩浆水，并有海水混入。红碧玉、重晶石等具有喷流岩的特征。矿床成因属于海底火山喷流一热水沉积矿床。     

Insights to magmatic-hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

Microchemical analyses of rare earth element (REE) concentrations and Sr and S isotope ratios of anhydrite are used to identify sub-seafloor processes governing the formation of hydrothermal fluids in the convergent margin Manus Basin, Papua New Guinea. Samples comprise drill-core vein anhydrite and seafloor massive anhydrite from the PACMANUS (Roman Ruins, Snowcap and Fenway) and SuSu Knolls (North Su) active hydrothermal fields. Chondrite-normalized REE patterns in anhydrite show remarkable heterogeneity on the scale of individual grains, different from the near uniform REE_N patterns measured in anhydrite from mid-ocean ridge deposits. The REE_N patterns in anhydrite are correlated with REE distributions measured in hydrothermal fluids venting at the seafloor at these vent fields and are interpreted to record episodes of hydrothermal fluid formation affected by magmatic volatile degassing. ⁸⁷Sr/⁸⁶Sr ratios vary dramatically within individual grains between that of contemporary seawater and that of endmember hydrothermal fluid. Anhydrite was precipitated from a highly variable mixture of the two. The intra-grain heterogeneity implies that anhydrite preserves periods of contrasting hydrothermal versus seawater dominant near-seafloor fluid circulation. Most sulfate δ³⁴S values of anhydrite cluster around that of contemporary seawater, consistent with anhydrite precipitating from hydrothermal fluid mixed with locally entrained seawater. Sulfate δ³⁴S isotope ratios in some anhydrites are, however, lighter than that of seawater, which are interpreted as recording a source of sulfate derived from magmatic SO₂ degassed from underlying felsic magmas in the Manus Basin. The range of elemental and isotopic signatures observed in anhydrite records a range of sub-seafloor processes including high-temperature hydrothermal fluid circulation, varying extents of magmatic volatile degassing, seawater entrainment and fluid mixing. The chemical and isotopic heterogeneity recorded in anhydrite at the inter- and intra-grain scale captures the dynamics of hydrothermal fluid formation and sub-seafloor circulation that is highly variable both spatially and temporally on timescales over which hydrothermal deposits are formed. Microchemical analysis of hydrothermal minerals can provide information about the temporal history of submarine hydrothermal systems that are variable over time and cannot necessarily be inferred only from the study of vent fluids.     

南秦岭地区下寒武统水沟口组黑色岩系成因及其沉积环境

早寒武世全球广泛发育的黑色岩系记录了重要海洋环境信息.为了解决该时期黑色岩系成因模式和沉积环境问题,本文选取南秦岭淅川—内乡地区下寒武统水沟口组黑色岩系为研究对象,通过剖面详测、薄片鉴定、电子探针和主微量元素等岩石学-地球化学分析方法,对其物质来源、形成机制和沉积环境进行分析.研究表明:水沟口组黑色岩系普遍具有海相硅质...     

Silicon isotope and trace element constraints on the origin of ∼3.5 Ga cherts: Implications for Early Archaean marine environments

Silicon (Si) isotope variability in Precambrian chert deposits is significant, but proposed explanations for the observed heterogeneity are incomplete in terms of silica provenance and fractionation mechanisms involved. To address these issues we investigated Si isotope systematics, in conjunction with geochemical and mineralogical data, in three well-characterised and approximately contemporaneous, ∼3.5 Ga chert units from the Pilbara greenstone terrane (Western Australia).We show that Si isotope variation in these cherts is large (−2.4‰ to +1.3‰) and was induced by near-surface processes that were controlled by ambient conditions. Cherts that formed by chemical precipitation of silica show the largest spread in δ³⁰Si (−2.4‰ to +0.6‰) and are characterised by positive Eu, La and Y anomalies and overall depletions in lithophile trace elements. Silicon isotope systematics in these orthochemical deposits are explained by (1) mixing between hydrothermal fluids and seawater, and/or (2) fractionation of hydrothermal fluids by subsurface losses of silica due to conductive cooling. Rayleigh-type fractionation of hydrothermal fluids was largely controlled by temperature differences between these fluids and seawater. Lamina-scale Si isotope heterogeneity within individual chemical chert samples up to 2.2‰ is considered to reflect the dynamic nature of hydrothermal activity. Silicified volcanogenic sediments lack diagnostic REE+Y anomalies, are enriched in lithophile elements, and exhibit a much more restricted range of positive δ³⁰Si (+0.1‰ to +1.1‰), which points to seawater as the dominant source of silica.The proposed model for Si isotope variability in the Early Archaean implies that chemical cherts with the most negative δ³⁰Si formed from pristine hydrothermal fluids, whereas silicified or chemical sediments with positive δ³⁰Si are closest to pure seawater deposits. Taking the most positive value found in this study (+1.3‰), and assuming that the Si isotope composition of seawater is governed by input of fractionated hydrothermal fluids, we infer that the temperature of ∼3.5 Ga seawater was below ∼55 °C.     

广西大厂锡矿长坡—铜坑矿区赋矿硅质岩研究

通过对大厂锡矿长坡―铜坑矿区主要赋矿层位硅质岩岩石学、岩石化学的研究,认为矿床赋矿硅质岩以及其中各类纹层、条带状含电气石、长石及硫化物的沉积物是海底热液循环系统中喷出海底的热水溶液以化学沉积方式形成的,生物可能参与了部分硅质岩的形成;另外,在某些热水沉积作用减弱时期,可能有少量陆源碎屑物质的加入,形成了硅质岩建造中少量绢云母泥质纹层条带;并且得出形成硅质岩的硅质来源,主要来自海底热卤水循环系统,部分来自海底中基性火山岩海解或脱玻的产物,硅质岩的形成同大厂锡矿床的形成在成因上有着一致性关系。     

海底热液沉积物稀土元素组成及其意义

总结了海底热液及其沉积物的稀土元素组成特征和控制因素。热水沉积物作为热液与海水混合的产物,其稀土元素组成及配分模式随海水混入的比例增加而发生一系列变化,反映了二端元流体不同混合程度的地球化学特征。Ｅｕ异常、Ｃｅ异常以及ｗ（Ｙ）／ｗ（Ｈｏ）比值是用来示踪古流体形成的物理化学环境及流体组成的重要参数。