Multi-Stage Modification of the Northern Slave Mantle Lithosphere: Evidence from Zircon- and Diamond-Bearing Eclogite Xenoliths Entrained in Jericho Kimberlite, Canada

The Jericho kimberlites are part of a small Jurassic kimberlitecluster in the northern Slave craton, Canada. A variety of datingtechniques were applied to constrain the nature and age of twoJericho kimberlites, JD-1 (170·2 ± 4·3Ma Rb–Sr phlogopite megacrysts, 172·8 ±0·7 Ma U–Pb eclogite rutile, 178 ± 5 MaU–Pb eclogite zircon lower intercept) and JD-3 (173 ±2 Ma Rb–Sr phlogopite megacryst; 176·6 ±3·2 Ma U–Pb perovskite), and all yielded identicalresults within analytical uncertainty. As there is no discernibledifference in the radiometric ages obtained for these two pipes,the composite Rb–Sr phlogopite megacryst date of 173·1± 1·3 Ma is interpreted as the best estimate forthe emplacement age of both Jericho pipes. The initial Sr isotopecomposition of 0·7053 ± 0·0003 derivedfrom phlogopite megacrysts overlaps the range (0·7043–0·7084)previously reported for Jericho whole-rocks. These strontiumisotope data, combined with the radiogenic initial ²⁰⁶Pb/²⁰⁴Pbratio of 18·99 ± 0·33 obtained in thisstudy, indicate that the Jericho kimberlites are isotopicallysimilar to Group 1 kimberlites as defined in southern Africa.The Jericho kimberlites are an important new source of mantlexenoliths that hold clues to the nature of the Slave cratonsubcontinental mantle. A high proportion (30%) of the Jerichomantle xenolith population consists of various eclogite typesincluding a small number (2–3%) of apatite-, diamond-,kyanite- and zircon-bearing eclogites. The most striking aspectof the Jericho zircon-bearing eclogite xenoliths is their peculiargeochemistry. Reconstructed whole-rock compositions indicatethat they were derived from protoliths with high FeO, Al₂O₃and Na₂O contents, reflected in the high-FeO (22·6–27·5wt %) nature of garnet and the high-Na₂O (8·47–9·44wt %) and high-Al₂O₃ (13·12–14·33 wt %)character of the clinopyroxene. These eclogite whole-rock compositionsare highly enriched in high field strength elements (HFSE) suchas Nb (133–1134 ppm), Ta (5–28 ppm), Zr (1779–4934ppm) and Hf (23–64 ppm). This HFSE enrichment is linkedto growth of large (up to 2 mm) zircon and niobian rutile crystals(up to 3 modal %) near the time of eclogite metamorphism. Thediamond-bearing eclogites on the other hand are characterizedby high-MgO (19·6–21·3 wt %) garnet andultralow-Na₂O (0·44–1·50 wt %) clinopyroxene.Paleotemperature estimates indicate that both the zircon- anddiamond-bearing eclogites have similar equilibration temperaturesof 950–1020°C and 990–1030°C, respectively,corresponding to mantle depths of 150–180 km. Integrationof petrographic, whole-rock and mineral geochemistry, geochronologyand isotope tracer techniques indicates that the Jericho zircon-bearingeclogite xenoliths have had a complex history involving Paleoproterozoicmetamorphism, thermal perturbations, and two or more episodesof Precambrian mantle metasomatism. The oldest metasomatic event(Type 1) occurred near the time of Paleoproterozoic metamorphism(1·8 Ga) and is responsible for the extreme HFSE enrichmentand growth of zircon and high-niobian rutile. A second thermalperturbation and concomitant carbonatite metasomatism (Type2) is responsible for significant apatite growth in some xenolithsand profound light rare earth element enrichment. Type 2 metasomatismoccurred in the period 1·0–1·3 Ga and isrecorded by relatively consistent whole-rock eclogite modelNd ages and secondary U–Pb zircon upper intercept dates.These eclogite xenoliths were derived from a variety of protoliths,some of which could represent metasomatized pieces of oceaniccrust, possibly linked to east-dipping subduction beneath theSlave craton during construction of the 1·88–1·84Ga Great Bear continental arc. Others, including the diamond-bearingeclogites, could be cumulates from mafic or ultramafic sillcomplexes that intruded the Slave lithospheric mantle at depthsof about 150–180 km. KEY WORDS: zircon- and diamond-bearing eclogites; Jericho kimberlite, geochronology; Precambrian metasomatism, northern Slave Craton     

2008年初贵州低温雨雪凝冻灾害天气成因分析

利用NECP 2.5°×2.5°的日平均再分析资料和常规观测资料,对2008年1月12日-2月13日期间贵州省历史罕见的持续低温雨雪凝冻天气过程的环流特征及物理量进行了分析,并对中东部凝冻严重的前期和西部凝冻严重的后期做了对比.揭示了此次灾害天气的几个主要天气特征:(1)地面静止锋的位置关系着我省低温雨雪凝冻灾害的区域.(2)500 hPa环流前期和后期影响系统明显不同.过程期间欧亚大陆500 hPa高度场距平呈"北高南低"分布,亚洲区极涡南掉,面积偏大.(3)过程前期850 hPa冷舌明显,等温线密集;后期700 hPa 温降低,有冷舌活动,850 hPa冷舌明显减弱,低温维持.中东部和西部地区冷暖平流垂直方向上的配制不同.强盛的700 hPa暖湿气流是此次过程的主要水汽来源.     

脱气温度和样品粒径对致密砂岩低温氮气吸附实验结果的影响

基于静态体积法的低温氮气吸附实验已广泛用于分析致密储集层的孔隙结构。本文探索了脱气温度和样品粒径对松辽盆地高台子致密砂岩低温氮气吸附实验结果的影响。结果显示,110℃的脱气条件并不能清除束缚水而使得孔隙体积、比表面积均偏低,300℃的脱气条件容易破坏样品中黏土矿物的结构令孔隙体积、比表面积减小,200℃是比较合适的脱气温度,既能去除束缚水又不破坏黏土矿物结构。样品粒径从5～10目减小至10～30目,氮气探测的孔隙数量增多使比表面积、孔体积增大。从10～30目减小至180～200目,黏土矿物相对含量降低令比表面积、孔体积显著减小;小于200目的样品中减少的黏土矿物主要集中在大于200目的岩样中,因此大于200目岩样的测定结果最高。因此,10～30目是利用低温氮气吸附实验寻求分析致密砂岩储集层特征的最佳粒径范围。     

大别造山带中镁铁质——超镁铁质岩石和榴辉岩有关问题的讨论

根据近年来积累的文献资料及研究讨论指出：（１）大别山造带镁铁质－－超镁铁质岩石可分成两大类;一类以任家湾、童家冲、祝家铺道士冲、青山、沙河等辉石辉长岩类为代表,它们具有相同的牲和成岩年龄,可能与华北、华南地块聚敛碰产生的岛弧或大陆岩浆岩有关;另一类以铙拔寨,大化坪、碧溪岭和毛屋岩体为代表,虽然它们的成岩年龄均在前寒武纪,但却有着不同的成岩时代和成岩环境。（２）大别造山带榴辉岩等多属异地来源,具有不     

Hydrogen and oxygen isotope studies of metamorphic fluid-rock interactions in the Dabie Mountains

Hydrogen and oxygen isotope studies were carried out on mineral separates from high to ultrahigh pressure metamorphic rocks at Huangzhen and Shuanghe in the eastern Dabie Mountains, East China. The δ¹⁸O values of eclogites cover a wide range of −5‰ to+9‰, but the δD values of micas fall within a narrow range of −85% to −70‰. Both equilibrium and disequilibrium oxygen isotope fractionations were observed between quartz and the other minerals, with reversed fractionations between omphacite and garnet in some eclogite samples. The δ¹⁸ O values of −5‰ to −1‰ for some of the eclogites represent the oxygen isotope compositions of their protoliths which underwent meteoric water-rock interaction prior to plate subduction. The preservation of oxygen isotope heterogeneity in the eclogites implies a channelized flow of fluids during progressive metamorphism caused by rapid subduction. Retrograde metamorphism has caused oxygen and hydrogen isotope disequilibria between some of the minerals, but the fluid for retrograde reactions was internally buffered in the stable isotope compositions. Project supported by the Chinese Ministry of Science and Technology (Grant No. 95-Pre-39), the National Natural Science Foundation of China (Grant Nos. 49794042, 49473173 and 49453003) and the Chinese Academy of Sciences (Grant No. KZ951-A1-401-5)     

松辽盆地东南部油气、煤层气后生蚀变硫同位素特征

对松辽盆地东南部上白垩统泉头组、姚家组和嫩江组岩石中黄铁矿及硫同位素特征研究表明,泉头组、姚家组中后生还原褪色作用形成的灰色、灰绿色砂岩中的黄铁矿及硫同位素均显示出了深部来源的油气、煤层气有机流体作用成分特征。该区原生沉积成岩岩石及黄铁矿中硫同位素的δ^34SV-CDT为正值,油气、煤层气有机流体还原作用岩石及黄铁矿中δ^34SV-CDT为负值。油气、煤层气还原作用不仅使杂色、紫红色原生氧化砂岩还原为灰色、灰绿色还原性砂岩,还使岩石还原容量大大增强,而且油气、煤层气在有机流体与岩石的相互作用过程中有铀的叠加富集,岩石铀含量普遍增加,局部可形成铀异常、铀矿化。首次在门达地区姚家组含矿砂岩中发现网脉状水云母化,显示铀成矿具有低温热液蚀变特征,铀矿化与低温热流体作用有关。     

滇西双江县勐库地区退变质榴辉岩中闪石类矿物的成因研究

勐库地区新近发现的退变质榴辉岩中晚期退变质作用形成的闪石类矿物十分丰富,其携带了大量晚期退变质过程中的岩石成因信息。本文通过系统的矿物学研究,鉴定出了3期10种闪石类矿物,其中以镁钙闪石、镁角闪石、铁镁钙闪石、铁韭闪石、透闪石、阳起石等钙质闪石类为主,有少量冻蓝闪石、蓝透闪石等钠-钙闪石类。研究发现,随着退变质过程中时间的推移,角闪石的形成环境逐渐由还原环境向弱氧化环境、氧化环境演变。根据角闪石电子探针成分测试结果计算了相应的温度、压力条件。结合岩相学的资料,将退变质过程分为3期:(1)第1期早阶段为近等温降压过程(p=0.56~0.75 GPa,t=642~709℃),主要形成铁韭闪石、铁镁钙闪石、镁钙闪石;晚阶段为等温降压过程(p=0.39~0.56 GPa,t=619~642℃),主要形成镁钙闪石及少量镁绿钙闪石、铁韭闪石;(2)第2期为降温降压过程(p=0.23~0.42 GPa,t=460~610℃),主要形成镁角闪石、冻蓝闪石及铁角闪石;(3)第3期为近等温降压过程(p=0.09~0.31 GPa,t=350~420℃),主要形成透闪石、阳起石、蓝透闪石等。闪石类的成分变化反映了退变质榴辉岩从地壳底部的角闪石榴辉岩-高压麻粒岩相温压环境向中上地壳角闪岩相、绿片岩相温压环境的渐次退变质作用过程。这与昌宁-孟连构造带上印支期的碰撞造山作用、燕山期的区域性地壳伸展、喜马拉雅期的陆内造山运动具有较好的对应关系。     

西南大面积低温成矿域：研究意义、历史及新进展

我国西南大面积低温成矿域是目前世界上公认的2个大面积低温成矿域之一,其面积之大（约100万km^2）、包含的矿种之多（Au、Hg、sb、As、P、Pb—Zn、Ag以及萤石、冰洲石、水晶、分散元素等）、矿床组成和组合之复杂全球罕见,是我国重要的Au、Hg、sb、As、Pb—zn、Ag以及多种分散元素生产基地。国内外学者对该成矿域进行了近30年的研究工作,取得了大量高水平研究成果,同时也存在许多有待深人研究的科学问题。本文回顾了我国西南大面积低温成矿域的研究历史,总结了各研究阶段的主要研究内容、主要研究成果和存在的主要科学问题,同时介绍了近年来我国西南大面积低温成矿域研究新进展。     

一株低温降解菌的分离鉴定及其降解特性研究

从刺参(Apostichopus japonicus)养殖池塘环境中驯化分离筛选到1株低温有机物和氨氮降解菌株DB11。根据其形态特征、生理生化特性以及16S rDNA序列分析,将该菌株鉴定为马氏副球菌(Paracoccus marcusii)。经驯化,该菌株以刺参饵料为唯一营养源,低温(15℃)、低接种量(<5×10-3)条件下能同时高效降解饵料中的有机物和氨态氮,5 d时间内对富集培养基中COD和NH4+-N的降解率分别达50%和98%。进一步研究其降解特性表明,菌株生长适温15~30℃,生长适宜pH值为7~10,降解刺参饵料中COD和NH4+-N的最适温度条件为15~20℃、最适pH条件为8.0~8.5;在最适降解条件下、接种量为5×10-3时,对10~20 g/L高质量浓度的刺参饵料液中COD降解效果显著,3 d时间去除率达56.9%~65.7%,对1~20 g/L质量浓度的刺参饵料液中NH4+-N 3 d时间的去除率达91.7%~99.9%。     

The causes of low-temperature demagnetization of remanence in multidomain magnetite

In this paper, the mechanisms of low-temperature demagnetization of remanence in multidomain magnetite are considered. New experimental observations of the behaviour of the saturation isothermal remanence, thermoremanence and partial thermoremanence at low temperatures are presented. The results show that there are two main contributions to this low-temperature demagnetization. The first and predominant contribution (type-1 demagnetization) is due to 'kinematic' domain state reorganization and occurs throughout cooling from room temperature to the Verwey transition, T _v , at 120–124  K. The second contribution arises from the change in anisotropy from cubic to monoclinic at T _v , which changes the overall domain structure of the grain. On warming in zero field, some domain walls will not return to their original positions but will take up a position that leads to a lower net remanence (type-2 demagnetization). In stoichiometric magnetite, demagnetization does not occur at 130  K at the isotropic point, T _k , contrary to some previous predictions. In non-stoichiometric magnetite, the influence of the Verwey transition is greatly reduced, and anomalous behaviour is observed at T _k .