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1.
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 LaCN/LuCN 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.  相似文献   

2.
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.  相似文献   

3.
Several hydrothermal sulfide structures were sampled using Mir manned submersibles in the relatively shallow Lucky Strike vent field, Mid-Atlantic Ridge; the bathymetric position of these structures varies by approximately 100 m. The investigation of the chemical and mineral compositions of hydrothermal ore occurrences led to the conclusion that the initial high-temperature ore-bearing solution ascending toward the surface became unstable and experienced phase separation beneath the ocean floor. The phase separation was responsible for the bathymetric control of hydrothermal ore formation in the ocean.  相似文献   

4.
Rare earth element (REE) concentrations are reported for a large suite of seafloor vent fluids from four hydrothermal systems in the Manus back-arc basin (Vienna Woods, PACMANUS, DESMOS and SuSu Knolls vent areas). Sampled vent fluids show a wide range of absolute REE concentrations and chondrite-normalized (REEN) distribution patterns (LaN/SmN ∼ 0.6-11; LaN/YbN ∼ 0.6 - 71; ). REEN distribution patterns in different vent fluids range from light-REE enriched, to mid- and heavy-REE enriched, to flat, and have a range of positive Eu-anomalies. This heterogeneity contrasts markedly with relatively uniform REEN distribution patterns of mid-ocean ridge hydrothermal fluids. In Manus Basin fluids, aqueous REE compositions do not inherit directly or show a clear relationship with the REE compositions of primary crustal rocks with which hydrothermal fluids interact. These results suggest that the REEs are less sensitive indicators of primary crustal rock composition despite crustal rocks being the dominant source of REEs in submarine hydrothermal fluids. In contrast, differences in aqueous REE compositions are consistently correlated with differences in fluid pH and ligand (chloride, fluoride and sulfate) concentrations. Our results suggest that the REEs can be used as an indicator of the type of magmatic acid volatile (i.e., presence of HF, SO2) degassing in submarine hydrothermal systems. Additional fluid data suggest that near-seafloor mixing between high-temperature hydrothermal fluid and locally entrained seawater at many vent areas in the Manus Basin causes anhydrite precipitation. Anhydrite effectively incorporates REE and likely affects measured fluid REE concentrations, but does not affect their relative distributions.  相似文献   

5.
A large variety of barites collected from marine and continental environments was analyzed by neutron activation for the rare-earth elements (REE) La, Ce, Sm, Eu and Dy. Relative to chondrites, all barites show a decrease of the lighter REE from La toward Eu. The abundance and distribution of rare earths in barites show a distinction of barite types. Deep-sea barites have large REE concentrations as do other authigenic deep-sea minerals and display the chondrite normalized Eu minimum, but not the negative Ce anomaly, of sea water. Other barites, mostly on land, some hydrothermal, and others of shallow marine origin, display lower total Ree concentrations. Chondrite normalized positive Eu anomalies are displayed by those varieties of reducing sedimentary and metamorphic origin.Distribution of REE in barite can be attributed both to crystallographic constraints of substitution, and to solution complexing of REE in the precipitating medium. Plots of rare earth partitioning versus effective ion size suggest that the decreasing enrichment toward Eu for all barite types is caused by crystallographic constraints due to contraction of the substituting REE ion sizes relative to the size of the host Ba ion. Solution effects on REE substitution in barite can be evaluated by writing solid solution distribution equations based on mass action of REE and Ba sulfates and the lanthanide (Ln) solution species Ln (CO3)?54), LnSO+4, LnCl+2 and LnF+2. Under normal sea water conditions, solution complexing plays a minor role. However, increased alkalinities of reducing sediments and increased brine chlorinities could cause significant complexing and deplete REE heavier than Eu. Besides Dy in barites, this could be true for aqueous precipitation of REE in general.  相似文献   

6.
The Chilpi Group (2050–1850 Ma) in the Bastar Craton contains Banded Iron Formation (BIF) deposited immediately after the Great Oxidation Event but before the “Proterozoic iron ore gap”. Baseline geochemical data generated from this crucial period of Earth's history representing the transition from an initial high productivity period followed by productivity collapse, thereby delaying the evolution of biota, are interpreted in terms of the redox state of the ocean and atmospheric oxygen content. Presence of chamosite, greenalite and siderite in the ironstones of the Chilpi Group, identified during present analysis, provide valuable information regarding the redox state of the shallow sea. Geochemical analyses reveal high concentrations of Fe2O3total and SiO2 (average ~ 87.85 wt%) in iron-rich bands. Trace elements commonly enriched in detrital phases (e.g. Sc, Hf, Nb, Th and Zr) show good correlation with total REE concentration, but these have concentration below the cut-off limit defined for detrital sediments (except 3 samples). Higher concentration of these elements in greenalite/chamosite-rich samples indicates accumulation of these elements in greenalite/chamosite during primary precipitation. Most of the samples have low concentrations of Al2O3 (<5 wt%) and TiO2 (<0.5 wt%), but some chamosite-bearing samples show enrichments of Al2O3 (up to ~20 wt%). Post-Archean Australian Shale normalised rare earth elements with Y (REEY) patterns, showing a superchondritic Y/Ho ratio (average 32.15) and positive La, Gd and Y anomalies, indicate the preservation of seawater like signatures. Though a low positive Eu-anomaly, and Al/(Al + Fe + Mn) versus Fe/Ti plot suggest seawater signature with possible mixing of hydrothermal fluids and/or <10 % detrital components, preservation of seawater signatures and no involvement of high-temperature hydrothermal fluids are deduced from Eu/Sm versus Sm/Yb and Eu/Sm versus Y/Ho patterns. The Eu/Sm and Y/Ho ratios lower than the seawater and a mixing trend away from oceanic hydrothermal solution indicate possible mixing of freshwater with seawater for the observed REEY patterns. Redox-sensitive trace element ratios indicate a dysoxic to suboxic-anoxic condition in the depositional basin. Presence of oolitic textures and occurrence of chamosite support the shallow water (<60 m water depth) and anoxic condition in the depositional basin. The oxygen content of 10−3–10−5 times the present atmospheric level in the atmosphere is inferred for the period during the deposition of the BIF.  相似文献   

7.
《China Geology》2018,1(2):225-235
For the first time, we present the rare earth element (REE) and sulfur isotopic composition of hydrothermal precipitates recovered from the Tangyin hydrothermal field (THF), Okinawa Trough at a water depth of 1206 m. The natural sulfur samples exhibit the lowest ΣREE concentrations (ΣREE= 0.65×10–6–4.580×10–6) followed by metal sulfides (ΣREE=1.71×10–6–11.63×10–6). By contrast, the natural sulfur-sediment samples have maximum ΣREE concentrations (ΣREE=11.54×10–6–33.06×10–6), significantly lower than those of the volcanic and sediment samples. Nevertheless, the δEu, δCe, (La/Yb)N, La/Sm, (Gd/Yb)N and normalized patterns of the natural sulfur and metal sulfide show the most similarity to the sediment. Most hydrothermal precipitate samples are characterized by enrichments of LREE (LREE/HREE=10.09–24.53) and slightly negative Eu anomalies or no anomaly (δEu=0.48–0.99), which are different from the hydrothermal fluid from sediment-free mid-oceanic ridges and back-arc basins, but identical to the sulfides from the Jade hydrothermal field. The lower temperature and more oxidizing conditions produced by the mixing between seawater and hydrothermal fluids further attenuate the leaching ability of hydrothermal fluid, inducing lower REE concentrations for natural sulfur compared with metal sulfide; meanwhile, the negative Eu anomaly is also weakened or almost absent. The sulfur isotopic compositions of the natural sulfur (δ34S=3.20‰–5.01‰, mean 4.23‰) and metal sulfide samples (δ34S=0.82‰–0.89‰, mean 0.85‰) reveal that the sulfur of the chimney is sourced from magmatic degassing.  相似文献   

8.
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 REEN patterns measured in anhydrite from mid-ocean ridge deposits. The REEN 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.  相似文献   

9.
Farsesh barite in the central part of Iranian Sanandaj-Sirjan zone is a sample of epigenetic hydrothermal mineralization in dolomitized limestone, which provides appropriate chemicophysical conditions making the passage of mineral-bearing fluids possible. Barite veins may range from a few centimeters to 2 m in thickness that increases downward. The microthermometry measurements obtained from more than 30 fluid inclusions show relative homogenization temperatures ranging from 125 to 200 °C with an average of 110 °C for Farsesh barite deposits. The mean salinity measured proves 16 times as much as weight percentage of NaCl for barite. Coexistence of liquid- and vapor-rich fluid inclusions in barite minerals may provide an evidence of boiling in ore veins. Moreover, occurrence of bladed calcite, high-grade ore zones, and presence of hydrothermal breccia are all consistent with boiling. Thermometric studies indicate that homogenization temperatures (Th) for primary and pseudosecondary fluid inclusions in barite range from 125 to 200 °C with an average of 1,100 °C. The δ34S values of barite also lie between 8.88 and 16.6 %. The relatively narrow spread in δ34S values may suggest uniform environmental conditions throughout the mineralization field. Thus, δ34S values are lower than those of contemporaneous seawater, which indicates a contribution of magmatic sulfur to the ore-forming solution. Barite is marked by total amounts of rare Earth elements (REEs) (6.25–17.39 ppm). Moreover, chondrite-normalized REE patterns of barite indicate a fractionation of light REEs (i.e., LREEs) from La to Sm, similar to those for barite from different origins. The LaCN/LuCN ratios and chondrite-normalized REE patterns reveal that barite in Farsesh deposit is enriched in LREEs compared with heavy rare Earth elements (HREEs). Similarity between Ce/La ratios in barite samples and those found in deep-sea barite supports its marine origin. Lanthanum and Gd exhibit positive anomalies, which are common features of chemical marine sediments. Cerium shows a negative anomaly in most samples inherited from the negative Ce anomaly of hydrothermal fluid that is mixed with seawater at barite precipitation. The available data including tectonic setting, host rock characteristics, REE geochemistry, and sulfur isotopic compositions may support a hydrothermal submarine origin for Farsesh barite deposit.  相似文献   

10.
详细报道了冀东、五台和吕梁地区条带状铁矿全岩样品的稀土元素分析结果。结果表明,研究区BIF具有非常相似的特征：稀土总量均较低;经页岩标准化的稀土元素配分模式均呈现轻稀土亏损、重稀土富集的特征;Y/Ho比值较高;具有明显的Eu、Y、La的正异常,且这些特征表明研究区BIF的稀土元素来源于火山热液和海水的混合溶液。虽然BIF均显示Eu正异常,但不同类型、不同沉积年龄BIF的铕异常程度不同：与吕梁地区Superior型铁矿相比,冀东和五台地区的Algoma型铁矿显示了更大的Eu正异常;并且自中太古代-新太古代-古元古代,BIF的铕正异常逐渐减小,这可能反映了随着BIF沉积年龄的减小,进入到该地区海水中的高温热液流体逐渐减少;同时,研究区BIF缺乏明显的Ce负异常,可能暗示在BIF沉积时海水的氧化还原状态为缺氧环境。  相似文献   

11.
Rare earth element (REE) abundances were measured by neutron activation analysis in anhydrite (CaSO4), barite (BaSO4), siderite (FeCO3) and galena (PbS). A simple crystal-chemical model qualitatively describes the relative affinities for REE substitution in anhydrite, barite, and siderite. When normalized to ‘crustal’ abundances (as an approximation to the hydrothermal fluid REE pattern), log REE abundance is a surprisingly linear function of (ionic radius of major cation—ionic radius of REE)2 for the three hydrothermal minerals, individually and collectively. An important exception, however, is Eu, which is anomalously enriched in barite and depleted in siderite relative to REE of neighboring atomic number and trivalent ionic radius. In principle, REE analyses of suitable pairs of co-existing hydrothermal minerals, combined with appropriate experimental data, could yield both the REE content and the temperature of the parental hydrothermal fluid.The REE have only very weak chalcophilic tendencies, and this is reflected by the very low abundances in galena—La, 0.6 ppb; Sm, 0.06 ppb; the remainder are below detection limits.  相似文献   

12.
BIF微量稀土元素分析方法及其在冀东司家营铁矿中的应用   总被引：3，自引：2，他引：1  
李文君  靳新娣  崔敏利  王长乐 《岩石学报》2012,28(11):3670-3678
以硅铁条带交替出现为特征的条带状铁建造(BIF)是世界上最重要的铁矿资源类型,精确分析磁铁矿的化学组成具有重要意义。本文开展了磁铁矿样品不同溶样方法分析结果的比对,并详细分析和讨论了冀东司家营铁矿磁铁矿与燧石单条带的微量及REE元素分析的地球化学特征。分析结果表明,对于磁铁矿样品,常规HF+HNO3溶样法与HBr+HNO3组合溶样法具有一致的溶样效果;司家营BIF的Zr,Sc,Th含量极低,表明未受陆源碎屑的污染;铁质与硅质具有低LREE、高HREE、La和Y正异常的海水REE特征,同时具有Eu正异常的热液REE特征;Ce负异常的缺乏,说明当时的古海洋是一个缺氧的环境。研究发现富铁条带的稀土总量大于富硅条带的稀土总量,这可能与硅、铁沉积物的地球化学习性相关,铁质沉积物更易吸收稀土元素。富矿和普通矿石具有原生的热液与海水的混合来源,部分富矿受到后期流体的强烈扰动,甚至表现出热液流体的特征。  相似文献   

13.
Two submarine hydrothermal vent fields at 5°S, Mid-Atlantic Ridge (MAR) - Turtle Pits and Comfortless Cove - emanate vapor-phase fluids at conditions close to the critical point of seawater (407 °C, 298 bars). In this study, the concentration and distribution of rare earth element (REE) and yttrium (Y) has been investigated. Independent of the major element composition, the fluids display a strong temporal variability of their REE + Y concentrations and relative distributions at different time scales of minutes to years. Chondrite-normalized distributions range from common fluid patterns with light REE enrichment relative to the heavy REE, accompanied by positive Eu anomalies (type I), to strongly REE + Y enriched patterns with a concave-downward distribution with a maximum enrichment of Sm and weakly positive or even negative Eu anomalies (type II). The larger the sum of REE, the smaller CeCN/YbCN and Eu/Eu∗. We also observed a strong variability in fluid flow and changing fluid temperatures, correlating with the compositional variability.As evident by the positive correlation of total REE, Ca, and Sr concentrations in Turtle Pits and Comfortless Cove fluids, precipitation/dissolution of hydrothermal anhydrite controls the variability in REE concentrations and distributions in these fluids and the transformation of one fluid type to the other. The variable distribution of REE can be explained by the accumulation of particulate anhydrite (with concave-downward REE distribution and negative Eu anomaly) into a fluid with common REE distribution (type I), followed by the modification of the REE fluid signature due to dissolution of incorporated anhydrite. A second model, in which the type II fluids represent a primary REE reaction zone fluid pattern, which is variably modified by precipitation of anhydrite, can also explain the observed correlations of total REE, fractionation of LREE/HREE and size of Eu anomaly as well as Ca, Sr. The emanation of such a fluid may be favored in a young hydrothermal system in its high-activity phase with short migration paths and limited exchange with secondary minerals. However, this model is not as well constrained as the other and requires further investigations.The strongly variable REE fluid signature is restricted to the very hot, actively phase-separating hydrothermal systems Turtle Pits and Comfortless Cove at 5°S and has not been observed at the neighboring Red Lion vent field, which continuously emanates 350 °C hot fluid and displays a stable REE distribution (type I).  相似文献   

14.
A geochemical investigation has been conducted of a suite of four sediment cores collected from directly beneath the hydrothermal plume at distances of 2 to 25 km from the Rainbow hydrothermal field. As well as a large biogenic component (>80% CaCO3) these sediments record clear enrichments of the elements Fe, Cu, Mn, V, P, and As from hydrothermal plume fallout but only minor detrital background material. Systematic variations in the abundances of “hydrothermal” elements are observed at increasing distance from the vent site, consistent with chemical evolution of the dispersing plume. Further, pronounced Ni and Cr enrichments at specific levels within each of the two cores collected from closest to the vent site are indicative of discrete episodes of additional input of ultrabasic material at these two near-field locations. Radiocarbon dating reveals mean Holocene accumulation rates for all four cores of 2.7 to 3.7 cm.kyr−1, with surface mixed layers 7 to 10+ cm thick, from which a history of deposition from the Rainbow hydrothermal plume can be deduced. Deposition from the plume supplies elements to the underlying sediments that are either directly hydrothermally sourced (e.g., Fe, Mn, Cu) or scavenged from seawater via the hydrothermal plume (e.g., V, P, As). Holocene fluxes into to the cores’ surface mixed layers are presented which, typically, are an order of magnitude greater than “background” authigenic fluxes from the open North Atlantic. One core, collected closest to the vent site, indicates that both the concentration and flux of hydrothermally derived material increased significantly at some point between 8 and 12 14C kyr ago; the preferred explanation is that this variation reflects the initiation/intensification of hydrothermal venting at the Rainbow hydrothermal field at this time—perhaps linked to some specific tectonic event in this fault-controlled hydrothermal setting.  相似文献   

15.
Talc, kerolite–smectite, smectite, chlorite–smectite and chlorite samples from sediments, chimneys and massive sulfides from six seafloor hydrothermal areas have been analyzed for mineralogy, chemistry and oxygen isotopes. Samples are from both peridotite- and basalt-hosted hydrothermal systems, and basaltic systems include sediment-free and sediment-covered sites. Mg-phyllosilicates at seafloor hydrothermal sites have previously been described as talc, stevensite or saponite. In contrast, new data show tri-octahedral Mg-phyllosilicates ranging from pure talc and Fe-rich talc, through kerolite-rich kerolite–smectite to smectite-rich kerolite–smectite and tri-octahedral smectite. The most common occurrence is mixed-layer kerolite–smectite, which shows an almost complete interstratification series with 5 to 85% smectitic layers. The smectite interstratified with kerolite is mostly tri-octahedral. The degree of crystal perfection of the clay sequence decreases generally from talc to kerolite–smectite with lower crystalline perfection as the proportion of smectite layers in kerolite–smectite increases.Our studies do not support any dependence of the precipitated minerals on the type/subtype of hydrothermal system. Oxygen isotope geothermometry demonstrates that talc and kerolite–smectite precipitated in chimneys, massive sulfide mounds, at the sediment surface and in open cracks in the sediment near seafloor are high-temperature (> 250 °C) phases that are most probably the result of focused fluid discharge. The other end-member of this tri-octahedral Mg-phyllosilicate sequence, smectite, is a moderate-temperature (200–250 °C) phase forming deep within the sediment (～ 0.8 m). Chlorite and chlorite–smectite, which constitute the alteration sediment matrix around the hydrothermal mounds, are lower-temperature (150–200 °C) phases produced by diffuse fluid discharge through the sediment around the hydrothermal conduits. In addition to temperature, other two controls on the precipitation of this sequence are the silica activity and Mg/Al ratio (i.e. the degree of mixing of seawater with hydrothermal fluid). Higher silica activity favors the formation of talc relative to tri-octahedral smectite. Vent structures and sedimentary cover preclude complete mixing of hydrothermal fluid and ambient seawater, resulting in lower Mg/Al ratios in the interior parts of the chimneys and deeper in the sediment which leads to the precipitation of phyllosilicates with lower Mg contents. Talc and kerolite–smectite have very low trace- and rare earth element contents. Some exhibit a negative or flat Eu anomaly, which suggests Eu depletion in the original hydrothermal fluid. Such Eu depletion could be caused by precipitation of anhydrite or barite (sinks for Eu2+) deeper in the system. REE abundances and distribution patterns indicate that chlorite and chlorite–smectite are hydrothermal alteration products of the background turbiditic sediment.  相似文献   

16.
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17.
湖南沃溪金-锑-钨矿床成因的稀土元素地球化学证据   总被引：11，自引：0，他引：11  
沃溪金-锑-钨矿床的稀土元素地球化学组成良好地反映了成矿作用的条件和过程,并为示踪矿床成因提供了有用的信息.以流体包裹体为代表的成矿溶液,以较高的稀土总量、显著的轻稀土富集和缺乏明显的铕异常为特征,代表了一种通过在碎屑沉积物柱中循环而萃取矿质的演化的海水热液.矿石相对于成矿流体(母液)富集重稀土而轻微亏损铕,反映了矿石沉淀过程中来自于海水的稀土元素掺合.同一矿层内由下往上,重稀土相对富集的程度逐渐增大而稀土总量则逐渐降低,表明随着热液化学沉淀作用的进行,海水掺合的影响逐渐增强.矿石的稀土元素组成,无论在分布模式还是在轻重稀土之间的分馏程度上,均与其他许多 Sedex型多金属矿床十分相似,暗示了这些矿床具有相似的成因机制.稀土元素地球化学特征支持矿床同生沉积成因的观点.  相似文献   

18.
Ocean Drilling Program (ODP) leg 193 successfully drilled four deep holes (126 to 386 m) into basement underlying the active dacite-hosted Pacmanus hydrothermal field in the eastern Manus Basin. Anhydrite is abundant in the drill core material, filling veins and vesicles, cementing breccias, and occasionally replacing igneous material. We report rare-earth element (REE) contents of anhydrite from a site of diffuse venting (Site 1188) which show extreme variability, in terms of both absolute concentrations (e.g., 0.08–28.3 ppm Nd) and pattern shape (LaN/SmN=0.08–3.78, SmN/YbN=0.48–23.1, Eu/Eu*=0.59–6.1). The range of REE patterns in anhydrite includes enrichments in the middle and heavy REEs and variable Eu anomalies. The patterns differ markedly from those of anhydrite recovered during ODP Leg 158 from the TAG hydrothermal system at the Mid-Atlantic Ridge which display uniform LREE-enriched patterns with positive Eu anomalies, very similar to TAG vent fluid patterns. As the system is active, the host-rock composition is uniform, and the anhydrite veins appear to relate to the same hydrothermal stage, we can rule out predominant host-rock and transport control. Instead, we propose that the variation in REE content reflects waxing and waning input of magmatic volatiles (HF, SO2) and variable complexation of REEs in the fluids. REE speciation calculations suggest that increased fluoride and possibly sulfate concentrations at Pacmanus may affect REE complexation in fluids, whereas at TAG only chloride and hydroxide complexes play a significant role. The majority of the anhydrites do not show positive Eu anomalies, suggesting that the fluids were more oxidizing than in typical mid-ocean ridge hydrothermal systems. We use other hydrothermal fluids from the Manus Basin (Vienna Woods and Desmos), which bracket the Pacmanus fluids in terms of acidity and ligand concentrations, to examine the dependence of REE complexation on fluid composition. Geochemical modeling reveals that under the prevailing conditions at Pacmanus (pH~3.5, T=250–300 °C), Eu oxidation state and the relative importance of fluoride versus chloride complexing are very sensitive to small variations in oxygen fugacity, temperature, and pH. Patterns with extreme mid-REE enrichment may reflect speciation effects (free-ion abundance) coupled with crystal chemical control. We conclude that the great variability in REE concentrations and pattern shape is likely due to variable fluid composition and REE complexation in the fluids. Editorial handling: L. Meinert  相似文献   

19.
The legendary cruise of H.M.S. Challenger (1872-1876) around the globe must always occupy an eminent place in the annals of oceanography, as being the first systematic attempt made on a global scale to explore the ocean. This expedition made fundamental discoveries in biology and geology which have not been surpassed by any later scientific cruise. Sediment with high content of metals (later called “metalliferous”) was among the enigmatic findings taken onboard. Although the nature of metalliferous sediments is well known today, the very first sampled sediments of this type have not been studied to date. Motivated by the historical value of Challenger’s metalliferous sediment collection we undertook an investigation addressing two questions: (1) the composition of sediments from seafloor for which we have very limited data; (2) Sr-Nd-Pb-Fe-Zn-isotope signature of these sediments collected before the substantial human impact on the ocean during the 20th century.The SE Pacific metalliferous sediments sampled by the Challenger’s explorers are of 2 types: (1) metalliferous oozes blanketing ridge crests and flanks down to the calcite compensation depth (CCD); and (2) stripped of CaCO3 metalliferous sediments located beneath the CCD in the deeps near the mid-ocean ridges. The abiogenic part of these sediments is composed mainly of poorly-crystalline to X-ray amorphous Fe-Mn-oxyhydroxides, and an amorphous silicate phase. These sediments have geochemical features similar to those of all the other metalliferous sediments: very high Fe and Mn content (on abiogenic basis), very low Al/(Al + Fe + Mn), and high content (on abiogenic basis) of As, Ba, Be, Bi, Cd, Co, Cu, Mo, Ni, Pb, Sb, Th, Tl, U, V, W, Y, Zn and Zr. Their REE distribution patterns are similar to that of deep seawater and show weak signs of hydrothermal imprint (weak positive or no Eu anomaly).Seawater and/or terrigenous input from South America control the Sr-Nd-Pb-isotope signature of the Challenger metalliferous sediments and have almost completely obliterated any original MORB-derived hydrothermal signal. Zn isotopes are mainly contributed from seawater although other Zn sources (hydrothermal fluid and detrital aluminosilicates, barite and volcanic glass) are necessary to fully explain Zn-isotope ratios. Fe isotopes indicate relatively slow Fe2+ to Fe3+ oxidation in the non-buoyant plume, thus producing relatively lighter Fe-isotope signature of the FeOOH particles that formed the studied metalliferous sediments.  相似文献   

20.
The playas (saline lakes) situated in the Thar Desert, north-west India, provide prominent examples of alkaline brine and varying assemblages of detrital and evaporite mineralogy. The eastern margin of the desert is relatively semi-arid, whereas the central to western region is arid to hyper-arid in nature. Rare earth elements (REEs) systematics in the sediments of nine different playas of the Thar Desert were studied to understand the provenance of the sediments and the intensity of chemical weathering in the region. Based on the REE patterns, fractionation of light REE (LREE) (La/Sm)N and heavy REE (HREE) (Gd/Yb)N, and Eu anomaly (Eu/Eu*), the upper continental crust normalised playa sediments are divided into two different groups. The eastern margin playa sediments show homogeneous REE contents, relatively positive Eu anomaly and depleted HREE values, whereas the western arid core playa sediments have highly variable REE contents, relatively negative Eu anomaly and similarly fractioned LREE and HREE patterns. The dissimilarity in the degree of HREE fractionations both in the eastern and western playa sediments is attributed to the differential distribution of minerals, depending upon their resistance to chemical weathering. It is believed that the relatively higher abundance of REE bearing heavy minerals and the presence of higher amounts of evaporites influence the large variation of REE distribution and enriched HREE in the western playa sediments. Apart from the relatively higher abundance of heavy minerals, the presence of rock fragments of variable petrographic character and roundness mirror the lower rock–water interaction in the arid western region. The presence of well-rounded metamorphic rock fragments and minerals, sourced from the eastern margin Aravalli mountains, indicates that the playas of the entire desert get the detrital and dissolved material mainly from the Aravalli mountains. Additionally, the western playas receive sediments from their surrounding Proterozoic and Mesozoic formations. This interpretation is supported by the presence of angular rock fragments of basalt, rhyolite and limestone in the western playas.  相似文献   

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