川西呷村黑矿型多金属矿床热液体系稀土元素组成特征

本文首次测定了呷村黑矿矿石中流体包裹体的ＲＥＥ组成,计算了与含矿流纹岩系熔体平衡的岩浆热液ＲＥＥ含量。主成矿期流体具有轻稀土富集、Ｅｕ明显正异常的特点,但岩浆热液却具有明显的Ｅｕ负异常特征,结合热液体系氧同位素及稀土元素交换反应模拟,表明岩浆热液不是直接的成矿热液。蚀变围岩具显著的Ｅｕ正异常,其它稀土元素出现亏损。蚀变反应水／岩比值较大,蚀变岩中的ＲＥＥ组成反映了成矿热液ＲＥＥ的特点,且ＲＥＥ亏损     

A New Report of Serpentinites from Northern Central Indian Ridge (at 6oS) — An Implication for Hydrothermal Activity

Serpentinites from the inside corner high （6°38.5′S/68°19.34′E） from the Northern Central Indian Ridge （NCIR） are comprised mainly of high Mg-rich lizardite and chrysotile pseudomorphs with varying morphologies. ＇Mesh rim＇, ＇window＇, ＇hourglass＇ and ＇bastite＇ are the most common textures displayed by both chrysotile and lizardite. Numerous chrysotile veins in association with cross cutting magnetite veins indicate an advanced stage of serpentinisation. The relatively high abundance of chrysotile and lizardite suggest their close association and formation at a temperature below 250℃. Abundant ＇mesh rim＇ and ＇bastite＇ texture and variegated matrix reveal that the present serpentinites might have formed due to the interaction of harzburgite and seawater. Positive Eu anomaly （Eu/Eu^＊ up to ＋3.38）, higher La/Sm （up to 4.40） and Nb/La （up to 6.34） ratios suggest substantial hydrothermal influence during the formation of the serpentinites.     

川西呷村超大黑矿型矿床成矿流体烯土元素组成

本文用ＩＣＰ－ＭＳ首次测定了呷村银多金属黑矿型矿床矿石流体包裹体中的稀土元素含量,研究表明,主成矿期流体稀土元素配分模式均为轻稀土富集,Ｅｕ具明显正异常,通过初步对比,本区主成矿期流体与东太平洋脊、大西洋脊等现代高温酸性地热系统热液具有相似的稀土模式,反映了它们物化条件的相似性;但前者∑ＰＥＥ高于后者,且两者Ｅｕ／Ｅｕ＾＊值不同,经过分析,本区成矿流体Ｅｕ正异常主要为Ｔ、ｐＨ、ｆｏ２控制,另外,围     

ON THE REACTION OF OLIGOCLASE WITH WATER UNDER CONDITIONS OF HIGH TEMPERATURE AND PRESSURE

Feldspar group minerals reacting with water at depth determine, by their reaction with environment, the character of newly formed minerals. Although plagioclase is the most abundant member of this group experimental data are very limited on the nature of its reactions under high pressure and temperature conditions. The author experimented on acid plagioclase with analytical data and physical constants corresponding to those of oligoclase. The reaction between oligoclase and water was studied under various pressures and temperatures, using a diffusion autoclave (previously described by the author [1]). Silicon, sodium, and aluminum in significant amounts went into solution; and, smaller amounts of calcium and potassium were observed. With increased temperature, larger amounts of material constituting oligoclase went into solution: The maximum occurred at 350 to 400° C; and, a marked decrease, at 500 to 600° C. Quantitative ratios of material in the condensate were found to correspond to albite indices. Increased pressure does not alter this behavior, there is only increased passage of separate components of the mineral into solution. At 350°, newly formed material was not observed microscopically; at higher temperatures, kaolinite and chalcedony appeared at sucessively higher temperatures. Experimentation by Morey and Chen on albite indicate that transfer of the albite component of oligoclase into solution is predominant. By comparison of its stability with that of two other minerals, albite is the most reactive with water. At 350° Centigrade and under pressure of 350 kilograms per square centimeter, the transfer into solution (in milligrams per liter) is for albite, 318; for microcline, 268; and, for oligoclase, 220. Labradorite was studied under analogous conditions; in that the transfer of labradorite predominates over that of the anorthite molecule, its behavior corresponds to albite. It was found that basic plagioclase shows less stability than is evident from microscopic examination of plagioclase in rocks. Data from these and previous experiments indicate possible types of solutions that can form within intragranular spaces in rocks by reaction between acid plagioclase and water. It is evident from the data that leaching of the albite component predominates by variation in composition of the solution throughout a wide range of temperature and pressure conditions. --D. D.Fisher     

Mineral chemistry and geochemical aspects of Gebel Filat granites, South Eastern Desert, Egypt

Gebel Filat granites form one of Egyptian younger granite intrusions in Wadi Allaqi region, South Eastern Desert of Egypt. They are perthitic monzogranites composed mainly of K-feldspars, plagioclase, and quartz with minor biotite. Plagioclase feldspars are Na-rich and have low anorthite content (An_2–3). Potash feldspars are mainly perthitic microcline and have chemical formula as (Or_96–96.6 Ab_3.4–4 An₀). Biotite is Mg-rich and seems to be derived from calc-alkaline magma. Chlorite is pycnochlorite with high Mg content, revealing its secondary derivation from biotite. The estimated formation temperatures of biotite and chlorite are (689–711°C) and (602–622°C), respectively. Gebel Filat monzogranites are metaluminous, high-K calc-alkaline, I-type granites. They are late orogenic granites related to subduction-related volcanic arc magmatism. They are enriched in LILE and depleted in HSFE indicating highly differentiation character. The REE patterns display an enrichment in LREE due to presence of zircon and allanite as accessories and depletion in HREE with slight negative Eu anomaly $ \left( {{\text{Eu}}/{\text{Eu}} * = 0.51 - 0.97} \right) $ . The parent magma of Gebel Filat monzogranites were emplaced at moderate depths (20–30 km) under moderate conditions of water-vapor pressure (1–5 kbar) and crystallization temperature [700–750°C]. The source magma of these granites seems to be derived from partial melting of lower crust material rather than upper mantle. The geochemical characteristics of pegmatites revealed that they are related to post orogenic within plate magmatism and not genetically related to the parent magma of Gebel Filat monzogranites. Distribution of radioactive elements (U and Th) in the studied rocks indicates normal U–Th contents for Filat monzogranites and U–Th bearing pegmatites. The positive correlations of each of Zr and Y versus U and Th are attributed to presence of zircon and allanite as accessories which incorporate U and Th in their crystal lattice.     

重庆高燕锰矿床含锰碳酸盐岩稀土元素特征

重庆城口高燕锰矿属于大型碳酸盐锰矿床。文章在系统野外地质调查及样品采集基础上,结合前人研究工作,采用等离子质谱仪(ICP-MS)分析了高燕矿区师家沟、板相沟及750平硐样品的稀土元素含量。分析结果显示,其稀土元素总量范围49.79×10~(-6)~281.24×10~(-6),平均含量125.92×10~(-6);轻重稀土比值范围1.79~3.58,显示轻稀土元素富集,且轻稀土元素之间的分馏程度高于重稀土元素;w(La)/w(Ce)=0.36~0.93,δEu=1.13~2.96,呈现弱正铕异常;δCe=2.09~5.84,有强的正铈异常。认为含锰碳酸盐岩形成于强还原的碱性环境沉积过程中,且有热水参与,成岩成矿的温度小于250℃,锰富集阶段处于滨岸的高能环境中。CeS10在本区具有示矿作用。     

Kinetics of Hydrothermal Reactions of Minerals in Near-critical and Supercritical Water

This work presents new experimental results on the kinetics of mineral dissolution in near-critical and supercritical water in a temperature range (T) from 25 to 400℃ and a constant pressure of 23 MPa. Kinetic experiments were carried out by using a flow reactor (packed bed reactor) of an open system. The dissolution rates of albite and magnetite were measured under these experimental conditions. Na, Al and Si release rates for albite dissolution in water were measured as a function of the temperature and flow velocity in the reaction system. The maximum release rates of Na, AI and Si of albite dissolution in the hydrothermal flow systems under different flow velocities were always obtained at 300℃, that is to say, the maximum albite dissolution rates in the flow systems, regardless of different flow rates, were repeatedly measured at 300℃. Results indicate a wide fluctuation in albite dissolution rates occurring close to the critical point of water. The dissolution rates increased when the temperatu     

Hydrothermal Alteration Zoning and Kinetic Process of Mineral-Water Interactions

This study reports the kinetic experimental results of albite in water and in KCI solution at 22 MPa in the temperature range of 25 to 400℃. Kinetic experiments have been carried out in an open flow-through reaction system (packed bed reactor). Albite dissolution is always incongruent in water at most temperatures, but becomes congruent at 300℃ (close to the critical point 374℃). At temperatures from 25 to 300℃, the incongruent dissolution of albite is reflected by the fact that sodium and aluminum are easily dissolved into water; from 300 to 400℃ it is reflected by silicon being more easily dissolved in water than Al and Na. Maximum albite dissolution rates in the flow hydrothermal systems have been repeatedly observed at 300℃, independent of flow rates.The kinetic experiments of albite dissolution in a KCl aqueous solution (0.1 mol KCl) indicate that the dissolution rate of albite increases with increasing temperature. Maximum silicon release rates of albite have been observed at 400℃, while ma     

Petrology of the Late Jurassic ultramafic-mafic Veselki massif,southeastern framing of the Siberian craton

The Veselki peridotite-websterite-gabbronorite massif was dated by U-Pb zircon method at 154 ± 1 Ma. This is the first Late Jurassic date obtained for ultramafic-mafic massifs in the eastern part of the Selenga-Stanovoy superterrane bounding the southeastern margin of the Siberian craton. The mineralogical specifics of the massif is expressed in the presence of three-pyroxene assemblage [bronzite-pigeonite-augite (diopside)] and exsolution lamellae of Cr-spinel and Cr-magnetite in Fe-Cr picotite and suggests unstable crystallization at shallow depths. Geochemical similarity between the Upper and Lower series attests to their genetic relation through intrachamber differentiation. The massif was generated from a highly evolved melt, as is seen from the LREE enrichment (La/Yb)_N = 3.89–30. Plagioclase varieties display a weak positive Eu anomaly (Eu/Eu* = 1.1–1.25), whereas other rocks have an insignificant negative Eu anomaly (Eu/Eu* = 0.85–0.97). Model calculations show that parental melt was close to subalkaline picrite, which evolved along two fractionation trends into dunites and subalkaline gabbroids and monzodiorites.     

Rare-earth element mobility during ore-forming hydrothermal alteration: A case study of Dongping gold deposit Hebei Province,China

REE mobility during hydrothermal ore-forming processes has been extensively investigated in recent years and the potential of REE to provide information about ore forming processes has commonly been recognized.The Dongping gold deposit,which is located in northwestern Hebei Province,China,occurring in the inner contact zone of the Shuiquangou syenite complex,is spatially,and probably genetically,related to the syenite,the deposit was formed under the moderate to high temperature(220℃ to 320℃),weakly acidic to weakly alkaline,rather high fo2(lgfo2=-30～-34)environment.The REE study of the host rocks,altered wall rocks,ores and gangue minerals from the deposit suggests that the REEs have been mobilized and differentiated during K-feldspathization and silicification.The extremely altered syenite enveloping auriferous quartz vein shows positive Ce anomaly and larger LREE/HREE ratio than that of the unaltered syenite.The REE concentrations and patterns of the ores are determined by the ore types and mineral assemblages,LREE/HREE ratios in the gangue quartz and hydrothermal Kfeldspars are relatively low.The most significant observation is that the gangue quartz shows significant positive Eu anomaly,whereas the hydrothermal K-feldspars show less significant or no positive Eu anomaly at all relative to the primary feldspar in the unaltered syenite. It is evident that the REEs are mobile during K-feldspathization and silicification in the ore forming process.Weak to moderate K-feldspathization caused REE mobility without apparent differentiation with the exception of extreme K-feldspathization and silicification which resulted in significant depletion of HREE and Eu and relative enrichment of Ce.The REE,Y,U,Th and Au contents of the syenite decrease as the degrees of K-feldspathization and silicification of the rocks increase towards the auriferous quartz veins.As the ores were deposited under a rather oxidized environment,Ce^4 predominated over Ce^3 .The precipitation of the former in the form of CeO2 or absorpted onto the secondary mineral assemblage resulted in the inconsistent removal of the REE and the relative Ce enrichment in the strongly altered rocks.in contrast,Eu was present mainly in a low valence state (Eu^2 ).The geochemical differences from the other REE^3 and much less sites in the secondary minerals to accommodate the Eu released form the original minerals resulted in the enrichment of Eu in the fluids.The mobility and differentiation of REE and the coherent mobilities of Y,U,Th and Au also support the argument that the syenite is one of the source rocks for gold mineralization.The REE contents and patterns of the altered rocks enveloping the auriferous quartz vein could be used as a guide for locating ore veins in mineral exploration.     

REE Geochemistry of Precambrian Metamorphic Rocks in Wutaishan Region

The metasedimentary-volcanic series of the Wutai and Hutuo groups experienced regional metamorphism and thus turned into moderate-to low-grade metamorphic rocks.REE abundances and REE distribution patterns in the Shizui and Taihuai Subgroup metasedimentary-volcanic rocks are typical of the Archean,whereas the Gaofan Subgroup and the Hutuo Group show post-Archean REE geochemical char-acteristics.Five types of REE distribution pattern are distinguished:(1)rightward inclined smooth curves with little REE anomaly(Eu/Eu*=0.73-0.95) and heavy REE depletion (e.g.the Late Archean metasedimentary rocks);(2)rightward inclined V-shaped curves with sharp Eu anoma-ly (Eu/Eu*=0.48-0.76) and slightly higher ∑REE (e.g.the post-Archean metasedimentary rocks);(3) rightward inclined steep curves with negative Eu anomaly(Eu/Eu*=0.73-0.76) and the lowest ∑REE (e.g.the post-Archean dolomites);(4)rightward inclined,nearly smooth curves with both positive Eu anomaly and unremarkable positive Eu anomaly(Eu/Eu*=0.95-1.25)(e.g.the meta-basic volcanic rocks);and (5) rightward inclined curves with Eu anomaly(Eu/Eu*=1.09-1.19)and heavy REE depletion(e.g.the meta-acid volcanic rocks).Strata of the two groups are considered to have been formed in an island-arc belt-an instable continental petrogenetic environment.     

Sillimanite—disordering enthalpy by calorimetry

Heat of solution measurements in an oxide-melt were performed on samples of natural sillimanite heat-treated in the range 1200–1700°C at pressures of 16–23 kbar. A distinct enthalpy of solution decrement relative to the unheated sillimanite of about 1.3 kc/mole is shown by samples run at 1400–1550°C. Pressure variations in the range 16–23 kbar cause little change in the heat of solution in this temperature range. This pressure-independent ‘plateau’ in heat of solution is interpreted to be due to Al-Si disordering on tetrahedral sites in the sillimanite structure. A simple temperature-dependent disordering model developed by navrotsky and Kleppa (1967) for spinels leads to an Al-Si interchange enthalpy, Δ_Hnt, of16 ± 1 kcal/mole, in good agreement with the value derived by Holdaway(1971) on entirely different grounds.Above 1550°C, larger heat of solution decrements were observed. Microprobe analyses of quenched samples indicate that the sillimanite has not deviated significantly from the ideal formula. Some unknown profound disordering process may account for the heat effects in the very high temperature range.Unit cell volumes of quenched samples also describe a ‘plateau’ region in the temperature range 1400–1550°C. This plateau consists of an increase of the 6 crystallographic axis beginning at 1350°C without much change in the other axes in the range 1350–1550°C. A sudden expansion of the α-axis occurs between 1550 and 1630°C.We conclude that Al-Si disorder of the type postulated by Beger et al. (1970), and Holdaway (1971) has been confirmed calorimetrically for samples heated under pressure in the temperature range 1400–1550°C.     

河北汤泉地热田地温场分布及其控制因素研究

汤泉位于河北省遵化市西北部,为山前丘陵地貌,地热资源丰富。本文对汤泉地热田内分布的诸多基岩热水井进行了钻孔测温,利用测温结果对该地热田地温场分布特征及控制因素进行了研究。研究发现:钻孔温度明显受地下水流的影响,绝大部分测温井表现为对流传热特征,个别表现为传导为主的传热特征;地温异常区域位于汤泉福泉宫和疗养院一带,异常中心呈两极分布,地热异常中心50 m埋深水温为51~54℃,100 m埋深水温可达60~67℃;该地热系统中地热水系地下水在深循环过程中,在正常的大地热流背景下被围岩逐渐加热所致;由花岗岩隔水底板构造形态所形成的隐伏盆地,构成了福泉宫至疗养院一带的蓄水构造;由于断裂构造切割花岗岩体,造成深部的热流沿断裂上升,混合并加热赋存于福泉宫至疗养院一带蓄水构造中的片麻岩裂隙水,形成该地段的地热异常现象;福泉宫地区和疗养院地区片麻岩含水层裂隙发育,使得深部热量能够快速到达浅部地层,并在浅部出现局部异常高温;汤泉地热田片麻岩热储层地热流体属于含岩盐地层溶滤的陆相沉积水,主要来源于大气降水。     

腾冲地热区高温热泉水中稀土元素特征

腾冲地热区位于印度-欧亚板块碰撞带东北缘,构造和岩浆活动频繁,地热作用明显,热泉广泛分布,是现代热泉研究的天然实验室。热泉水中的稀土元素特征是指示浅层水岩反应的重要指标。由于热泉水中稀土元素含量较低且变化范围大,腾冲地区热泉水稀土元素组成的报道比较罕见。本文尝试研究了腾冲高温热泉水中稀土元素组成特征,并与美国内华达州贝奥沃韦地热田和加利福尼亚希伯地热田热泉水的稀土元素特征进行了对比。本文选取腾冲地区的高温热泉,应用ICP-MS,测试了热泉水中稀土元素。分析结果表明热泉水中稀土元素含量相对较低且变化范围较大,其变化范围为球粒陨石的10-5～10-2倍。稀土元素球粒陨石标准化配分模式整体为轻微右倾型或平坦型,显示轻重稀土分异不明显;δEu既有正异常也有负异常;各热泉点热泉水稀土配分模式之间存在差异。虽然腾冲地热区热泉水的REE含量比美国贝奥沃韦地热田和希伯地热田热泉水的高10倍,两者之间的稀土元素分布特征具有一致性。热泉水中的δEu负异常为深循环的大气降水与具有负铕异常火山岩水岩反应的结果,正铕异常可能是由快速上返的大气降水与蒸发岩中的石膏反应导致。     

Bottom temperature monitoring in Lake Baikal

We report results of bottom temperature monitoring of 2003–2004 in the deepwater South Baikal basin (Lake Baikal) near active gas-fluid methane vents at lake depths of 1020 and 1350 m. Sediments and water temperatures were measured using an autonomous temperature recorder designed at the Institute of Geophysics (Novosibirsk). Experiments implied short-duration recording and pioneering continuous 350 day-long monitoring near the Staryi vent. Measurements within a 1 m thick layer above and below the bottom showed notable variations in water (up to 0.07 °C) and sediment temperatures and in geothermal gradient. The long temperature records include a relatively steady period (mid-June 2003-early February 2004) with smooth temperature variations (especially in sediments) and two transient unsteady periods. The steady season is the best time for heat flow studies in the South Baikal basin. The 0.04–0.05 °C drop in bottom water temperature during the unsteady periods may result from intrusion of cold surface water. A positive temperature anomaly of ∼0.04 °C recorded in April 2003 may be caused, among other reasons, by active gas venting.     

铕异常在地质学中的应用

Eu是地学界最受关注的变价稀土元素(Eu、Eu2+和Eu3+)。长期以来, "Eu异常"(δEu)作为重要的地球化学参数而被广泛应用于刻画精细地质过程。一般认为, Eu2+的存在及其在自然环境中的稳定性是造成"Eu异常"的主要原因, 但查阅近期有关文献显示, 很多关于"Eu异常"的成因解释存在模棱两可甚至相互矛盾的情形。本文在梳理"Eu异常"的定义以及计算公式的基础上, 总结了不同地质过程中Eu异常的应用现状及相关的实验地球化学成果, 包括岩浆演化、热液作用、沉积作用、变质作用、风化作用等。综合研究表明, 造成不同地质过程中"Eu异常"差异的主要因素有: (1)母岩浆(熔/流体)的Eu含量; (2)不同矿物相的晶体化学效应(离子半径); (3)温度-压力条件; (4)氧化-还原条件(氧、硫逸度); (5)水-岩作用; (6)流体酸碱度(pH值); (7)络合作用; (8)吸附作用; (9)风化作用; 以及(10)星云冷凝与挥发等。其中, "Eu异常"的定量表述和应用均涉及到岩浆、熔/流体或矿物中Eu2+和Eu3+的精确测定。但纵观当前应用于地球科学的众多测试手段, 除X射线吸收光谱(XAS)可以原位测定Eu2+、Eu3+的相对含量外, 尚缺乏精确测定研究对象中Eu2+、Eu3+含量的可行方法。由此提出, 实验地球化学将是解决"Eu异常问题"的新途径, 利用已知Eu2+、Eu3+含量的熔/流体开展重要地质过程的模拟, 可有效揭示"Eu异常"变化的关键制约因素。     

Fluid Inclusion Studies and Geochemistry of Rare Earth Elements of Hydrothermal Fluorites from Pöhrenk,Kirşehir,Central Turkey

Fluorite mineralization occurs along fractures and cracks of Middle Eocene and Pliocene limestones and marls in the north and northeast of the Pöhrenk region (Çiçekdagi, Kirsehir). Tb/Ca – Tb/La and Y/Ho ratios were obtained from REE contents of fluorites which have revealed that mineralization is of hydrothermal type. Negative Ce anomalies and positive Eu anomalies reflect that hydrothermal solutions once had high oxygen fugacity. Fluid inclusion studies indicate that homogenization temperatures of mineralization varied between 90°C and 200°C, and hydrothermal solutions are composed of NaCl + KCl + MgCl₂ + H₂O. In addition, salinity measurements show that hydrothermal solutions were mixed with meteoric or rock formation water. Geologic setting, REE geochemistry and fluid inclusion studies suggest that mineralization was deposited from a solution generated by mixing of magmatic and meteoric water under epithermal conditions.     

Rare—Earth Element Geochemistry of Elogites from the Ultra—High Pressure Metamorphic Belt in Central China

Based on their REE contents and REE patterns,eclogites from the ultra-high pressure metamorphic belt in central China may be roughly divided into xis types including LREE-rich.LREE-rich positive Eu anomaly,LREE-rich negative Eu anomaly,REE pattern-smooth,MREE-rich and HREE-rich.The LREE_rich,LREE-rich positive Eu anomaly and LREE-rich negative Eu anomaly types of eclogites are dominant .REE types of eclogites in different areas can be compared and the REE feactures of the same REE type of eclogites in different areas are similar.The results of reconstruction of the primary rocks show that the primary rocks of eclogites possibly are dominated by continental tholeiites which are the product of partial melting of relatively fertile mantle and the rocks of tholeiite crystallization-differentiation.There is perfect evolution relationship among the primary rocks of the LREE-rich, LREE-rich positive Eu anomaly and LREE-rich negative Eu anomaly types of eclogites and among those of the REE pattern smooth and MREE-rich types of eclogites,the former three types were deried from continental settings and the latter two from nearly oceanic settings.Meanwhile,it is concluded that the mantle sources of primary rocks of the eclogites are inhomogeneous and the primary rocks of eclogites in this area appear to have undergone varying degree of crustal contamination.     

Negative Ce Anomaly in the Indian Banded Iron Formations: Evidence for the Emergence of Oxygenated Deep-Sea at 2.9-2.7 Ga

Abstract: Major and rare earth element contents are reported for Late Archean banded iron formations (BIFs) in the Bababudan Group of the Dharwar Craton, South India. The BIFs are mostly composed of SiO₂ (average1ρ = 54.88.1 wt%) and Fe₂O₃* (44.38.2 wt%). The Al₂O₃ and TiO₂ contents are remarkably low, suggesting that detrital components were starved during the BIF deposition. The BIFs have a LREE-enriched pattern with a relatively high (La/Yb)_N (6.644.07). Total REE concentrations (RE) vary from 5.2 to 65.3 ppm. The REE patterns are characterized by the presence of a very large negative Ce anomaly (Ce/Ce*: 0.13-0.83) and a positive Eu anomaly (Eu/Eu*: 0.96-2.45). The Eu/Eu* decreases and (La/Yb)_N increases with a increase of RE. These correlations of REE indices are similar to those of modern hydrothermal iron-rich sediments near a mid-ocean ridge (MOR). Greenstones associated with the BIFs have MORB-like geochemical features. These geochemical and geological lines of evidence indicate that the depositional site of the BIFs was remote from a continent and/or island arc and that the BIFs were in situ hydrothermal sediments near a MOR. A striking negative Ce anomaly in the BIFs indicates that oxygenated deep-sea environments emerged at 2.9-2.7 Ga. The existence of contemporaneous Mn deposits in the Dharwar Craton supports this assertion. Our scenario of oxygen in the Earth's surface of the Late Archean is different from long-held notion that the atmosphere and ocean were persistently anoxic throughout the Archean.     

A New Report of Serpentinites from Northern Central Indian Ridge (at 6°S)—An Implication for Hydrothermal Activity

Serpentinites from the inside corner high (6°38.5'S/ 68°19.34'E) from the Northern Central Indian Ridge (NCIR) are comprised mainly of high Mg-rich lizardite and chrysotile pseudomorphs with varying morphologies.'Mesh rim','window', 'hourglass'and'bastite'are the most common textures displayed by both chrysotile and lizardite.Numerous chrysotile veins in association with cross cutting magnetite veins indicate an advanced stage of serpentinisation.The relatively high abundance of chrysotile and lizardite suggest their close association and formation at a temperature below 250°C. Abundant 'mesh rim' and 'bastite'texture and variegated matrix reveal that the present Serpentinites might have formed due to the interaction of harzburgite and seawater.Positive Eu anomaly (Eu/Eu*up to 3.38), higher La/Sin (up to 4.40) and Nb/La (up to 6.34) ratios suggest substantial hydrothermal influence during the formation of the Serpentinites.