Electrical properties of slow-spreading ridge gabbros from ODP Site 735, Southwest Indian Ridge

ODP hole 735B (ODP Legs 118 and 176) samples a block of igneous crust which was accreted at the ultraslow-spreading Southwest Indian Ridge, and was uplifted to seafloor by progressive unroofing along a north-dipping low-angle detachment fault. Physical properties of a set of gabbroic samples from ODP Hole 735B have been measured in the laboratory, with a particular emphasis on the analysis of electrical properties. The electrical formation factor (F) and surface conductivity (C_s) are calculated using the model of Revil and Glover [Geophys. Res. Lett., 25 (1998) 691], from measurements at room pressure, and different salinities of the saturating fluid. The acoustic compressional velocities are in the same range as those previously measured on ODP Leg 118 samples [Proc. ODP, Sci. Results, 118 (1991) 227]. The porosity (φ) is low (<1%) in most, fresh samples. The analysis of the porosity structure, characterised by the electrical tortuosity (τ) and the electrical cementation factor (m), reveals that the cored gabbro section is segmented in two parts. The upper part (approximately the upper half) has a nearly constant τ of 15, independent of the degree of alteration of the sample, indicating that the porous network is controlled by primary microstructures, such as grain boundaries, and by extension plastic foliations. Modifications with time and alteration are restricted to increases of φ and m, i.e. a higher variability of the channel thicknesses. In the lower part of the hole, fresh rocks predominate, and the porosity structure is different, with variable τ (3–10), very low m (1.4±SEM) and low φ (0.8±SEM%). This indicates a simpler porous network compared to that at shallower depths, probably dominated by well aligned cracks. The change in porosity structure downhole may be explained several ways, and may integrate the whole history of the crustal section cored at ODP site 735 since accretion, including plastic deformation related to unroofing of the lower crust gabbros along a low-angle detachment fault, cooling, and alteration due to hydrothermal flow at various temperatures. The electrical conduction via electronic processes in metallic minerals is not taken into account in our analysis of the electrical properties; we propose a new constitutive law for electrical conduction, that would account for the conductive mineral matrix.     

Isotopic Compositions of Helium and Argon in Hydrothermal Sulfides from Southwest Indian Ridge and their Geological Inferences

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中印度洋脊Edmond热液区黄铁矿的标型特征及其对海底成矿作用环境的指示

对中印度洋脊Edmond热液区的丘顶和丘坡进行电视抓斗取样,获得了大量的金属硫化物等热液作用产物样品。在进行细致的矿相学分析基础上,对硫化物中黄铁矿的标型特征及其演化进行了详细研究。结果表明,Edmond热液区的黄铁矿可分3种类型,其形态标型和成分标型分别具有以下特征:Ⅰ型:呈自形立方体状,S/Fe原子数比接近于2.00,微量元素具有高Cu,低Zn、As、Pb、Ni和Ag的特点,Cu/Zn值高,平均可达4.26;Ⅱ型:呈半自形立方体状,S/Fe原子数比平均2.03,具有高Zn、Co、低Cu的特点,Cu/Zn值(平均0.04)低;Ⅲ型:主要呈胶状、莓球状,S/Fe原子数比平均2.04,具有Ag、Pb、Zn、As元素富集的特点,Cu/Zn值介于前两者之间(平均1.09)。根据硫化物中矿物共生组合关系,认为这3类黄铁矿分别对应高温、中高温和低温成矿作用,黄铁矿的形态和成分能很好地指示热液区的成矿环境和成矿条件的演变。     

高镁安山岩及其地球动力学意义

高镁安山岩是当前国际地质研究的一个热点之一。本文重点介绍了高镁安山岩的分类、成因、成矿和地球动力学意义。高镁安山岩主要包括高镁埃达克岩、巴哈岩、赞岐岩和玻安岩等四类。高镁安山岩的成分复杂,可以包含地幔源区组分,以及俯冲大洋板片(玄武质洋壳或洋壳沉积物)或俯冲陆壳沉积物熔体或流体组分,有时也可能包含拆沉下地壳熔体组分。高镁安山岩可以通过地幔橄榄岩的直接熔融形成,也可通过熔体与地幔的相互作用形成。高镁安山岩的形成环境特殊：除青藏高原中部的一些新生代高镁安山岩外,几乎所有其它新生代的高镁安山岩都形成在会聚板块边界,且大都与年轻的、热的洋壳或洋脊的俯冲有关。高镁安山岩对揭示大陆地壳的形成以及金属矿化方面都有非常重要的意义。最后,对当前高镁安山岩的研究中存在的问题进行了探讨。     

Hydrothermal Activity Events at Kairei Field,Central Indian Ridge 25°S

The Kairei hydrothermal field is located on the Central Indian Ridge at 70°02′E, 25°19'S, which was discovered in 2000. Eight representative hydrothermal product samples including massive sulfide, sulfide chimney and mineralized rock breccia samples collected from this field were age‐dated using the ²³⁰Th/²³⁴U and ²¹⁰Pb/Pb methods. Four episodes of hydrothermal activity were determined: 94.5 to 96.3 ka (event I), 56.6 to 61.2 ka (event II), 8.4 to 10.6 ka (event III), and <180 a to present (event IV). Among them, event I is the oldest and is characterized by the low‐temperature mineralization of rock breccias which probably represents for the onset of the hydrothermal activity of the field. Events II to IV represent the episodic high‐temperature hydrothermal activities characterized by Cu and Zn‐rich sulfides. The recent hydrothermal activity has lasted for at least 180 a.     

Glass and Mineral Chemistry of Northern Central Indian Ridge Basalts:Compositional Diversity and Petrogenetic Significance

Abstract: The glass and mineral chemistry of basalts examined from the northern central Indian ridge (NCIR) provides an insight into magma genesis around the vicinity of two transform faults: Vityaz (VT) and Vema (VM). The studied mid-ocean ridge basalts (MORBs) from the outer ridge flank (VT area) and a near-ridge seamount (VM area) reveal that they are moderately phyric plagioclase basalts composed of plagioclase (phenocryst [An60–90] and groundmass [An35–79]), olivine (Fo81–88), diopside (Wo45–51, En25–37, Fs14–24), and titanomagnetite (FeOt ~63.75 wt% and TiO2 ~22.69 wt%). The whole-rock composition of these basalts has similar Mg# [mole Mg/mole(Mg+Fe2+)] (VT basalt: ~0.56–0.58; VM basalt: ~0.57), but differ in their total alkali content (VT basalt: ~2.65; VM basalt: ~3.24). The bulk composition of the magma was gradually depleted in MgO and enriched in FeOt, TiO2, P2O5, and Na2O with progressive fractionation, the basalts were gradually enriched in Y and Zr and depleted in Ni and Cr. In addition, the SREE of magma also increased with fractionation, without any change in the (La/Yb)N value. Glass from the VM seamount shows more fractionated characters (Mg#: 0.56–0.57) compared to the outer ridge flank lava of the VT area (Mg#: 0.63–0.65). This study concludes that present basalts experienced low-pressure crystallization at a relatively shallow depth. The geochemical changes in the NCIR magmas resulted from fractional crystallization at a shallow depth. As a consequence, spinel was the first mineral to crystallize at a pressure >10 kbar, followed by Fe-rich olivine at <10 kbar pressure.     

PGE and Os Isotopes as Tracers in Polymetallic Massive Sulfide Deposits from the Edmond and Kairei Hydrothermal Fields, Central Indian Ridge

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中印度洋脊Edmond区热液硫化物的形成——来自铅和硫同位素的约束

位于中印度洋脊23°52’S的Edmond热液区发现于2000年,属于典型的以玄武岩为宿主的活动热液区。首次测得了Edmond热液区9件硫化物的铅同位素和6件样品的硫同位素组成,结果表明:硫化物矿石的206Pb/204Pb为17.879～17.970,207Pb/204Pb为15.433～15.550,208Pb/204Pb为37.743～38.130。Pb-Pb图解表明,Edmond热液区硫化物的铅同位素数据与中印度洋脊玄武岩的铅同位素组成较一致,与印度洋沉积物和锰结壳相比具较低放射性成因铅的特征,说明硫化物中的铅主要来源于地幔(玄武岩),海水的贡献微弱。硫化物的δ34S为5.7‰～7.2‰,明显高于玄武岩的硫同位素组成(δ34S≈0‰),认为Edmond热液区硫化物中的硫除地幔的贡献外,海水中硫酸盐还原作用产生的硫的贡献可能超过30%。中印度洋脊Edmond热液区存在非常活跃的浅循环系统,可能是造成硫化物中硫同位素组成偏重的主要原因。     

Helium and Argon Isotopic Compositions of Hydrothermal Sulfides from Southwest Indian Ridge and Their Geological Inferences

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西南印度洋脊龙旂热液区富铝蚀变黏土矿物类型和地球化学特征研究

产出于不同地质背景下的热液成因黏土矿物组成、晶体结构及化学成分等信息,可指示与海底热液作用有关的水-岩反应过程和流体的物理化学条件变化。但目前对于以西南印度洋脊为代表的超慢速扩张脊热液区的黏土矿物研究程度较低,尚未了解其经历的热液蚀变作用及形成过程。本文综合应用SEM-EDS、XRD、FT-IR、EPMA和LA-ICP-MS等多种分析测试手段对采自龙旂热液区矿化蚀变角砾的形貌结构、矿物组成及其化学成分进行系统表征。研究表明:该蚀变角砾中的共生矿物相主要由具二八面体结构、富Al端元的蒙皂石族矿物贝得石与蛋白石组成,角砾中可见呈细粒浸染状的TiO_2。蚀变黏土矿物的化学成分较为单一,具有富Al、贫Mg和贫Fe的特征;其稀土元素总量普遍不高(2. 43～43. 45μg/g),配分模式呈负Eu异常(0. 31～0. 53)而未显示Ce异常(1. 09～1. 16)。推断产出于硫化物堆积丘体边部的矿化角砾长期受酸性、相对还原的、低温热液流体持续叠加和淋滤改造,除Al和Ti以外大部分元素被活化迁移,形成矿物组成简单的富铝黏土矿物相。本研究查明了龙旂热液区新的蚀变黏土矿物类型及其元素地球化学特征,反映该区广泛发育低温热液蚀变作用,为进一步探讨西南印度洋超慢速扩张脊热液成矿系统的水-岩反应过程提供了一定依据。     

Cumulate gabbros from the Southwest Indian Ridge, 54°S-7° 16′ E: implications for magmatic processes at a slow spreading ridge

A diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the Southwest Indian Ridge (54°S, 7°16 E) between the Islas Orcadas and Shaka Fracture Zones. The cumulus nature of the gabbroic rocks in the suite is indicated by phase, modal and cryptic layering, igneous lamination, and low incompatible element abundances. We present a mass-balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk-rock major element compositions. The model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid. Calculated proportions of trapped liquid range from 3%–15%; values that are characteristic of adcumulates to mesocumulates. Models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids, thus explaining the high TiO₂ contents observed in magnesian clinopyroxenes. Cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers, but disequilibrium plagioclase compositions suggest some crystallization at greater depth. Furthermore, basalt compositions projected onto the olivine-clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures (from less than 10 to greater than 20 kb) as well as polybaric fractional crystallization. We suggest that the Southwest Indian Ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros. The adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large, but ephemeral magma chambers below segments of the Southwest Indian Ridge.     

Mineralogy and Chemistry of Sulfides from the Longqi and Duanqiao Hydrothermal Fields in the Southwest Indian Ridge

Recent investigations found that hydrothermal activity and sulfide mineralization occurs along the Southwest Indian Ridge (SWIR). The Longqi and Duanqiao hydrothermal fields between 49° E and 53° E of the SWIR are two prospective mineralization areas discovered by Chinese scientists. With the aim to determine the mineralogical and chemical characteristics of sulfide minerals, we have conducted detailed studies for samples from the two areas using an optical microscope, X-ray diffractometer, scanning electron microscope, and electron microprobe. The mineralization processes in the Longqi area are divided into three main stages: (1) the low-medium-temperature stage: colloform pyrite (Py I) + marcasite → euhedral pyrite (Py II), (2) the high-temperature stage: isocubanite (±exsolved chalcopyrite) + pyrrhotite → coarse-grained chalcopyrite (Ccp I), and (3) the medium–low-temperature stage: sphalerite + fine-grained chalcopyrite inclusions (Ccp II) → aggregates of anhedral pyrite (Py III) ± marcasite → Fe-oxide (-hydroxide) + amorphous silica. The mineralization processes in the Duanqiao area are divided into two main stages: (1) the medium–high-temperature stage: subhedral and euhedral pyrite (Py I′) → coarse-grained chalcopyrite (Ccp I′) and (2) the medium–low-temperature stage: sphalerite → fine-grained chalcopyrite (Ccp II′) + chalcopyrite inclusions (Ccp II′) → silica-cemented pyrite (Py II′) + marcasite → Fe-oxide + amorphous silica. We suggest that the fine-grained chalcopyrite inclusions in sphalerite from Longqi and Duanqiao were formed by co-precipitation and replacement mechanisms, respectively. Primary sphalerites from both fields are enriched in Fe (avg. 5.84 wt% for the Longqi field vs. avg. 3.69 wt% for the Duanqiao field), Co (avg. 185.56 ppm for the Longqi field vs. 160.53 ppm for the Duanqiao field), and Cd (avg. 1950 ppm for the Longqi field vs. avg. 525.26 ppm for the Duanqiao field). Cu contents in pyrite from the Duanqiao field (Py I′: avg. 849.23 ppm and Py II′: avg. 1191.11 ppm) tend to be higher than those from the Longqi field (Py I: avg. 26.67 ppm, Py II: avg. 445 ppm, and Py III: avg. 179.29 ppm). Chalcopyrite from both fields is enriched in Zn (Ccp I: avg. 3226.67 ppm, Ccp II: avg. 9280 ppm, Ccp I′: avg. 848 ppm, Ccp II′ (inclusions): avg. 1098 ppm, and Ccp II′ (fine-grained): avg. 1795 ppm). The varying contents of Zn in the different pyrite and chalcopyrite generations may result from the zone refining process. An integrated study of the mineralogy and mineralogical chemistry suggests that the hydrothermal fluids of the Longqi area are likely conditioned with higher temperatures and relatively lower fO2 and fS2 than those of the Duanqiao area, but in contrast to the former, the latter is much affected by the compositions of the surrounding rocks.     

Seismic moment release along selected major transforms on the Central Indian Ridge,Western Indian Ocean,and its tectonic implications

A detailed seismicity map of the Central Indian Ridge for the period 1912–1993 is presented, and the earthquakes pertaining to four major transforms offsetting the ridge are utilized to study the moment release pattern. The scalar moment release for the period 1912–1993, and the summed moment rate tensors for both short period (1977–1993) and long period (1912–1993) bring out a unified picture of moment release pattern. The fraction of seismic slip calculated based on depths of 100°C and 400°C limiting temperatures suggests that the Marie-Celeste transform requires a slip almost to a depth of 400°C isotherm to account for the observed moment, and the Argo transform requires depth of faulting much above the 400°C isotherm. A very small fraction of slip is accounted seismically for Vema (53%) and 12° 12′S (23%) even to depths of 100°C isotherm, suggesting a very low order of moment release along these transforms. The horizontal plate velocities and the corresponding strain rates obtained from moment tensor summation of long period data (82 years) give rise to (V _y ^y ; V _y ^x mm. yr⁻¹) of 6.0 and 6.1 along Marie-Celeste, 1.3 and 0.50 along Argo, 0.06 and 0.06 along 12° 12′S, 1.6 and 0.25 along Vema transforms. The corresponding strain rates (ε _y ^y ;ε _y ^x × 10⁻¹⁵ S⁻¹) are 12.7 and 6.8 along MarieCeleste, 6.9 and 1.4 along Argo, 0.27 and 0.14 along 12° 12′S, 7.3 and 0.58 along Vema transforms. These results suggest that the strain rates were highest and almost all predicted motion is taken up seismically along the Marie-Celeste transform. The strain rates are lower along Argo transform and the observed moment release require shallower depth of faulting in order to slip to be accounted seismically. The Vema and 12° 12′S transforms are characterized by low strain rates and less than 15 per cent of motion is accommodated seismically within the seismogenic layer. It is proposed that the deficiency of moment release along the Vema and 12° 12′S multiple transform system may be due to most of the plate motion occurring aseismically.     

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.     

Garnet-field Melting and Late-stage Refertilization in 'Residual' Abyssal Peridotites from the Central Indian Ridge

The role of residual garnet during melting beneath mid-oceanridges has been the subject of many recent investigations. Toaddress this issue from the perspective of melting residues,we obtained major and trace element mineral chemistry of residualabyssal peridotites from the Central Indian Ridge. Many clinopyroxeneshave ratios of middle to heavy rare earth elements (MREE/HREE)that are too low to be explained by melting in the stabilityfield of spinel peridotite alone. Several percent of meltingmust have occurred at higher pressures in the garnet peridotitestability field. Application of new trace element partitioningmodels, which predict that HREE are compatible in high-pressureclinopyroxene, cannot fully explain the fractionation of theMREE from the HREE. Further, many samples show textural andchemical evidence for refertilization, such as relative enrichmentsof highly incompatible trace elements with respect to moderatelyincompatible trace elements. Therefore, highly incompatibleelements, which are decoupled from major and moderately incompatibletrace elements, are useful to assess late-stage processes, suchas melt entrapment, melt–rock reaction and veining. Moderatelyincompatible trace elements are less affected by such late-stageprocesses and thus useful to infer the melting history of abyssalperidotites. KEY WORDS: abyssal peridotites; mantle melting; garnet     

Magnetic rock properties of the gabbros from the ODP Drill Hole 1105 A of the Atlantis Bank, Southwest Indian Ridge

Laboratory studies of 30 samples from 158 m long drill core of the Hole 1105 A (ODP Leg 179) of the Atlantis Bank, Southwest Indian Ridge have revealed magnetic properties of the gabbros, olivine gabbros, oxide gabbros and olivine oxide gabbros down the core. Comparison of modal proportions of the oxides, grain sizes and magnetization parameters of the rocks has confirmed that most coarse-grained oxide mineral bearing rocks record low Koenigsberger ratio (2 to 5) and median destructive fields (5 to 7 mT). Average natural remanent magnetization (J_nrm) and stable remanent magnetization (J_st) of the core samples are 5.8 A/m and 1.9 A/m, respectively. Their mean stable magnetic inclination is 66‡ ± 4‡, about 14‡ steeper than the expected dipole inclination of the area similar to the one reported at Hole 735 B. The excess inclination perhaps marks a tectonic block rotation of the reversely magnetized rocks of the bank. We interpret that gabbros and serpentinites devoid of basaltic carapace significantly contribute to seafloor spreading anomalies of the bank.     

西南印度洋洋中脊表层沉积物地球化学特征及其热液活动指示意义

洋中脊热液硫化物勘探技术的滞后,严重制约了对海底热液硫化物资源的勘探开发。以我国西南印度洋硫化物勘探合同区域采集的25个表层沉积物样品为研究对象,基于主量、微量和稀土元素检测数据,采用元素含量特征、相关性分析、元素对比值、特征元素三角图解,以及稀土元素分馏特征值等手段,开展沉积物热液信息研究。结果表明:样品所代表的大部分研究区域内主要为钙质生物沉积,部分样品元素地球化学特征受沉积物中玄武岩风化碎屑的影响,龙旂热液区的部分样品中表现出一定的热液迹象,稀土元素分馏特征和配分模式、（Al+K）?Mg?（Fe+Mn）三角图解可较好地指示热液活动。研究不仅为我国在西南印度洋的硫化物勘探提供基础数据参考,同时也是对海底热液硫化物勘探的沉积物地球化学找矿理论和方法的初步探索。     

Geochronological (Rb-Sr and Sm-Nd) studies on intrusive gabbros and dolerite dykes from parts of Northern and Central Indian cratons: Implications for the age of onset of sedimentation in Bijawar and Chattisgarh basins and uranium mineralisation

The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1–1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (T_DM), obtained, for Dargawan gabbros, is c. 2876–3145 Ma. High initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ɛ_Ndi (at 1.9 Ga) value of −1.5 to − 4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7098 and ɛ_Ndi value of −3.5 to −3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (T_DM) than Dargawan gabbros as c. 2462–2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial ⁸⁷Sr/⁸⁶Sr and low initial ¹⁴³Nd/¹⁴⁴ ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar-Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.     

洋中脊富钛铁氧化物辉长岩成因的一个新思路:以西南印度洋脊ODP 735B岩心为例

西南印度洋脊的ODP 735B岩心中拥有大量富含钛铁氧化物的含氧化物辉长岩,此现象在岩性单元Ⅳ中表现尤为显著.同时,岩性单元1中主要硅酸盐矿物亦具有独特的成分变化趋势.以前研究将造成岩性单元I中氧化物辉长岩成因和主要造岩矿物成分变化趋势归因于具浮力的、高度分异的富钛铁熔体的作用,这些熔体应形成于735B钻孔之下或其附近...