脉状黝铜矿型铜矿床的稳定同位素组成及其成因意义

脉状黝铜矿型铜矿床是产于兰坪-思茅盆地中的一种新的铜矿床类型。碳酸盐及CO₂包裹体的δ¹³C值大多在-4‰～-7‰之间变化,显示碳来自地幔。矿石中铅同位素组成与盆地中喜马拉雅期碱性岩长石中铅的同位素组成一致,方解石和铁白云石的⁸⁷Sr/⁸⁶Sr比值接近或稍高于碱性岩的⁸⁷Sr/⁸⁶Sr的比值,显示矿石锶、铅与碱性岩锶、铅是同源的,均来自于上地幔。硫化物中硫的δ³⁴S值大多集中在0～-4‰之间,显示其硫主要是深部来源并有少量的地壳硫参与。包裹体水的δ¹⁸O和δD值显示成矿流体的水为大气降水、岩浆水和变质水的混合物。结合其它质、地球化学特征,笔者认为这类矿床的成因类型为壳-幔混合热液成因型。     

Stable chlorine and carbon isotope measurements of selected chlorinated organic solvents

Stable Cl and C isotope ratio results for 3 selected chlorinated solvents, perchloroethylene (PCE), trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA) provided by 4 different manufacturers are presented. The isotope ratio for all compounds range between −3.5 and +6.0‰ forδ³⁷Cl and from −37.2 to −23.3%. forδ¹³C. The greatest³⁷Cl difference between manufacturers is observed in the TCE samples which showδ³⁷CI values of −2.5%o for PPG, +2.43‰ for ICI and +4.4‰ for DOW. TCAs show a smaller range (−2.4 to +2.0‰), while the TCEs have slightly different³⁷Cl contents. The¹³C data show the most distinctδ¹³C values for PCEs (−23.3 for DOW, −24.1 for Vulcan, −33.8 for PPG and −37.2‰ for ICI) while both TCEs and TCAs show a smallerδ¹³C range, but still distinct differences. These preliminary data suggest that each manufacturer and solvent type may have distinctiveδ⁶³⁷Cl andδ¹³C values. These results show that by using a combination of³⁷Cl and¹³C, there is a potential to indicate a specific source of chlorinated solvents, as well as an ability to delineate contamination episodes caused by these compounds in groundwaters.     

Lead isotope systematics of some igneous rocks from the Egyptian shield

Lead isotope data on late Precambrian igneous rocks from the eastern desert of Egypt are presented. Previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of ${}^{37}\text{Sr}{}^{86}\text{Sr}$ (< 0.7035). The Pb data define three groups, loosely corresponding to age. An older tonalite to granodiorite (OTG) suite, with ages in the range 610–710 Ma, has Pb isotope characteristics similar to modern, mantle-derived oceanic mafic rocks. The age-corrected initial Pb isotope ratios of the OTG group lie near the “ocean regression line” in correlation diagrams of ${}^{206}\text{Pb}{}^{204}\text{Pb}$ vs. ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{203}\text{Pb}{}^{204}\text{Pb}$. The isotope data imply an origin for the OTG group analogous to calc-alkalic igneous rocks in modern intra-oceanic island arcs.A younger suite of K-rich plutonic rocks (570–595 Ma) has similar ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ initial ratios relative to the OTG suite, but have higher ${}^{207}\text{Pb}{}^{204}\text{Pb}$ initial ratios when calculated from measured whole-rock U and Pb contents. However, K-feldspar and galena, associated with this suite, indicate initial Pb isotope ratios close to the OTG suite. We interpret this to indicate that the younger granites could have originated by anatexis of OTG-type material, but they probably experienced an episode of metasomatism after emplacement.A suite of volcanic rocks (the Dokhan Volcanics) from the central eastern desert are intermediate between the younger granites and OTG in both age (～ 610 Ma) and Pb isotope characteristics.Limited data on initial ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios suggest that all the Egyptian samples originated from a source with anomalously low $\text{Th}\text{Pb}$ ratios.Although the relative abundance of granitic rocks in the Egyptian Shield decreases to the south, no evidence of north-south heterogeneity in the Pb isotope composition of the respective source regions is apparent in the data. However, the westernmost sample studied, from the Aswan area, contains distinctly more radiogenic Pb relative to the mean. This location may mark the boundary between the Late Precambrian ensimatic orogen now outcropping in the Egyptian and Saudi Arabian Shields, and an older sialic craton to the west.     

四川冕宁木落寨稀土矿床稳定同位素研究及其意义

木落寨稀土矿床在构造上位于西南新生代陆内造山带的锦屏山北缘。文章通过对木落寨2个坑道的矿石样品的C、H、O、S同位素组成的系统研究,探讨了木落寨稀土矿床成矿流体的来源及其与深部过程的关系。研究表明,石英和萤石的δ^13Cv-PDP值为-2．5‰～-9．0‰,方解石、石英、萤石和氟碳铈矿的δDv-SMOW值为-63‰～-87‰,萤石和石英的δ^18O水V-SMOW,为10．0‰～15．5‰,重晶石和方铅矿的δ^34Sv-CDT值分别为0．1‰～2．2‰和-8．6‰～-9．3‰,方解石的δ^13Cv-PDB和δ^18OV-SMOW分别为-6．6‰～-6．8‰和8．4‰～9．1‰。所有这些数据均显示木落寨稀土矿床在成矿过程中有大量地幔流体的参与,表明其成矿作用与深部过程有关。     

Mafic xenoliths from Egyptian Tertiary basalts and their petrogenetic implications

Mineralogical data, coupled with whole-rock major and trace element data of mafic xenoliths from two occurrences of the Egyptian Tertiary basalts, namely Abu Zaabal (AZ) near Cairo and Gabal Mandisha (GM) in the Bahariya Oases, are presented for the first time. Chemically, AZ basalts are sodic transitional, while those of GM are alkaline. In spite of the different petrographic and geochemical features of the host rocks, mafic xenoliths from the two occurrences are broadly similar and composed essentially of clinopyroxene, plagioclase, alkali feldspar, and Fe–Ti oxides. The analytical results of host rocks, xenoliths and their minerals suggest that the xenoliths are cognate to their host magmas rather than basement material. The mafic xenoliths are olivine-free and contain alkali feldspar contrary to the phenocryst assemblage of the host rocks, confirming that they are not cumulates from the host magma. The geochemical and mineralogical characteristics show that the precursor magmas of these xenoliths are more fractionated and possibly contaminated compared to those of the host rocks. Estimated crystallization conditions are  1–3 kbar for xenoliths from both areas, and temperature of  950–1100 °C vs. 920–1050 °C for AZ and GM, respectively. These cognate xenoliths probably crystallized from early-formed, highly-fractionated anhydrous magma batches solidified in shallow crustal levels, possibly underwent some AFC during their ascent, and later ripped-up during fresh magma pulses. The xenoliths, although rare, provide an evidence for the importance of crystal fractionation at early evolution of the Egyptian Tertiary basalts.     

Carbon and nitrogen isotope records in sediments of Lake Taihu, China, and their paleoenvironmental significance

Lake sediments can provide important historical information on records of paleoenvironments and paleoclimates and their changes.This study deals with the sedimentary history of the westem Taihu Lake based on seven geochemical indices measured in an 89-cm long sediment core. The core, corresponding to a time period from 6870 a B.P. to the present, was analyzed for δ^13Corg, δ^15N, TOC, TN, TP, C/N ratio and radiocarbon dates. Comparison of these multiple geochemical tracers helps to improve interpretations of the paleoenvironmental changes. All of the geochemical proxies used in the study change regularly, and show four major time scales that suggest different environments. During 6870-6532 a B.P., the values of all parameters slightly varied. δ^13Corg values increased in a wave pattern from -25.9‰ to -20.7‰. Similarly, δ^15N values increased from 1.6‰ to 4.5‰. TOC, TN and TP concentrations remained around 0.8%, 0.1%-3.2% and 0.5%, respectively. C/N ratios varied from 20.6 to 6.6. At 6370 a B.P., the sediment record profile showed dramatic variations in all parameters. δ^13Corg and δ^15N values dropped to -26.9‰ and 1.3‰, respectively.     

Stable isotope studies of fluid inclusions in speleothems and their paleoclimatic significance

Fluid inclusions found trapped in speleothems (cave deposited travertine) are interpreted as samples of seepage water from which enclosing calcium carbonate was deposited. The inclusions are assumed to have preserved their D/H ratios since the time of deposition. Initial ¹⁸O/¹⁶O ratios can be inferred from δD because rain- and snow-derived seepage waters fall on the meteoric water line (δD = 8δ¹⁸O + 10). Estimates of temperature of deposition of the carbonate can be calculated from inclusion D/H ratios and δ¹⁸O of enclosing calcite in Pleistocene speleothems. For most speleothems investigated (0–200,000 yr old) δ¹⁸O of calcite appears to have decreased with increasing temperature of deposition indicating that the dominant cause of climate-dependent change in δ¹⁸O of calcite was the change in K_cw, the isotope fractionation equilibrium constant, with temperature; δ¹⁸O of meteoric precipitation generally increased with increasing temperature, but not sufficiently to compensate for the decrease in K_cw.     

青海湖地区全新世风尘堆积的GDGTs化合物及其环境指示意义

青海湖晚第四纪的古环境演化和湖面变化是学术界长期关注的热点问题,争议也较多。我们对青海湖地区种羊场（ZYC）剖面（120 cm厚）全新世风尘堆积的GDGTs（甘油二烷基甘油四醚）化合物相关指标的古环境意义进行了初步研究,以期从一个新的角度对上述问题进行探讨。研究结果表明：1）ZYC剖面的TEX₈₆（由86个碳原子组成的四醚指标）与磁化率正相关,而R_i/b比值（类异戊二烯GDGTs与支链GDGTs丰度的比值）与磁化率负相关。2）ZYC剖面的TEX₈₆指标与湖泊岩芯胖真星介（Eucypris inflata）壳体δ¹⁸O指示的7月古水温及长链烯酮（U₃₇^K）指示的夏季古水温变化具有相似的变化特征,显示TEX₈₆指标较可靠地指示了青海湖地区的大气温度变化。然而,ZYC剖面的MBT（甲基化指数）和CBT（环化指数）指标在重建古气温方面具有一定的局限性,可能与青海湖地区全新世出现的干旱环境有关。3）ZYC剖面的R_i/b比值与湖泊岩芯E.inflata壳体Sr/Ca比值指示的青海湖湖水的古盐度具有较一致的变化特征,表明R_i/b比值较有效地指示了青海湖地区环境的干湿状况。4）TEX₈₆和R_i/b指标揭示青海湖区在9.5~7.5 ka B.P.为冷干时期;7.5~5.5 ka B.P.为独特的温干阶段,温度在6.2 ka B.P.左右达到最高值;5.5~1.7 ka B.P.为温湿时期;1.7~0 ka B.P.为冷干时期。5）风尘堆积的GDGTs记录表明：7.5~5.5 ka B.P.由于环境干旱,青海湖不太可能出现高湖面;5.5~1.7 ka B.P.由于环境湿润,青海湖很可能出现全新世高湖面;1.7 ka B.P.以来,青海湖地区的气候快速变干。

     

Composition and sources of lipid compounds in speleothem calcite from southwestern Oregon and their paleoenvironmental implications

We analyzed speleothem calcite from the Oregon Caves National Monument, southwestern Oregon, to determine the preservation, distribution, concentrations and sources of aliphatic lipid compounds preserved in the calcite. Maximum speleothem growth rate occurs during interglaciations and minimum during glacial intervals. Concentrations of the total lipid compounds range from 0.5 to 12.9 μg g⁻¹. They increase at times of low speleothem growth rate, suggesting dilution, whereas the apparent accumulation rate of lipid compounds tends to be highest during times of fastest speleothem growth rate. Such increased accumulation generally corresponds to times of warm (interglacial) climate, suggesting either a greater source of organic materials during interglacial times and/or greater efficiency of compound capture during more rapid calcite growth. Aliphatic lipid compounds include homologous n-alkanoic acids, n-alkanols and methyl n-alkanoates and sterols with concentrations ranging from 0.3 to 7.8 μg g⁻¹, 0.4 to 1.1 μg g⁻¹, 0.5 to 9.6 μg g⁻¹ and 0.1 to 2.7 μg g⁻¹, respectively. Minor amounts of branched methyl n-alkanoates and dimethyl n-alkanedioates are also present. The high concentrations of methyl n-alkanoates are the result of esterification reactions of free fatty acids in alkaline solutions with high pH values associated with the dripping cave waters. The distribution patterns and geochemical parameters and indices indicate that the major sources of the aliphatic lipids involved leaching from higher plants and microbial residues derived from the soil zone above the cave system. The estimated percentage of microbial inputs ranged from 42 to 90% of the total lipids and also showed an increase in accumulation during warm climates. These well-preserved lipid compounds in speleothem calcite could be used as biomarkers for paleoenvironmental study.     

鄂尔多斯盆地东胜煤田侏罗系煤中元素地球化学特征及其古环境意义

本文以鄂尔多斯盆地东胜煤田塔然高勒矿区锡尼布拉格勘查区6 5号钻孔延安组煤为研究对象,在工业分析和元素分析的基础上,采用X 射线荧光光谱仪（XRF）和电感耦合等离子体质谱仪（ICP MS）测定了煤中常量元素及微量元素含量,对煤中元素的富集程度进行了分析,并利用煤相参数和地球化学参数对延安组不同煤层的沉积古环境进行了重建。结果表明：不同煤层样品以低水分含量、（特）低灰分产率、特低硫含量和中高挥发分产率为特征,煤中硫以有机硫为主;不同煤层煤中常量元素均以SiO2和Al2O3为主,其次为CaO、Fe2O3和Na2O;煤中微量元素富集程度不同,3#煤中富集Cr、Zr、Nb、Ta和Th,轻度富集V、Cu、Zn、Ga、Y、Ba、Yb、Hf、W、Pb和U,4#煤中富集Sb,轻度富集Ba、Be、Co,5#煤中轻度富集Be和Co,6#煤中轻度富集Cr和V;稀土元素结果表明,3#煤和6#煤以重稀土富集型为特征,4#煤和5#煤以中/重稀土富集型为特征,不同煤层均存在Eu正异常现象;不同煤层煤岩显微组分均以惰质组为主,其中4#煤中惰质组含量最低;各煤层沉积水体均为氧化环境占主导,其中3#、5#及6#煤形成于酸性、富氧—次富氧、水动力较弱的干燥森林沼泽相,而4#煤形成于酸性、富氧—次富氧、水动力较弱的干燥森林沼泽相与碱性、缺氧—极贫氧环境、水体环境波动较大的潮湿森林沼泽相,这两种环境交替出现。     

Compilation of radiogenic isotope data in Mexico and their petrogenetic implications

Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos Plate were also compiled. In the MVB the isotopic ratios range as follows:⁸⁷Sr/⁸⁶Sr 0.703003-0.70841;¹⁴³Nd/¹⁴⁴Nd 0.512496-0.513098;²⁰⁶Pb/²⁰⁴Pb 18.567-19.580;²⁰⁷Pb/²⁰⁴Pb 15.466-15.647;²⁰⁸Pb/²⁰⁴Pb 38.065-38.632. The SMO shows a large variation in⁸⁷Sr/⁸⁶Sr ranging from ∼0.7033 to 0.71387.¹⁴³Nd/¹⁴⁴Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows:²⁰⁶Pb/²⁰⁴Pb 18.060-18.860;²⁰⁷Pb/²⁰⁴Pb 15.558-15.636;²⁰⁸Pb/²⁰⁴Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261 for Nd). Pb isotopes for PacOc show the following range:²⁰⁶Pb/²⁰⁴Pb 18.049-19.910;²⁰⁷Pb/²⁰⁴⁷Pb 15.425-15.734;²⁰⁸Pb/²⁰⁴Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc. Most samples with reported Sr and Nd isotopic data are spread within and around the “mantle array”. The SMO seems to have been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data in all Mexican magmatic provinces and of Nd isotopes in BC, AP, SMS, and SMOr.     

Stable isotope geochemistry of Quaternary calcretes in the Mersin area, southern Turkey – A comparison and implications for their origin

In the Mersin area, Quaternary calcretes are widespread, and occurred in a variety of forms, as namely powdery, nodular, tubular, fracture-infill, laminar crust, hard laminated crust (hardpan), pisolithic crust. They are predominantly calcite, and small amount of palygorskite associated with them as a minor component. Calcite δ¹⁸O and δ¹³C values of the calcretes vary from −4.31 to −6.82 and from −6.03 to −9.65‰ PDB, respectively. These values are consistent with values of pedogenic calcretes reported in literature from worldwide sites. The oxygen isotope values indicate formation under the influence of meteoric water at estimated temperatures from 25 to 32 °C. The carbon isotope values are typical for pedogenic calcretes, reflecting development under the C3-dominated vegetation cover and semiarid or seasonally arid climatic conditions.     

钾稳定同位素研究综述

钾稳定同位素是重要的非传统稳定同位素体系,也是近年来迅速发展的热门研究课题。对钾同位素研究历史和现状进行综述,具体包括以下内容:(1)总结了钾元素的地球化学和宇宙化学性质,包括钾在主要地质储库(地幔、地壳、海洋)中的丰度及其分配,钾在岩浆演化中的不相容大离子亲石性,常见含钾火成岩/沉积岩矿物以及钾在表生过程中的循环,也包括主要行星物质(球粒陨石、非球粒陨石、火星和灶神星陨石以及月球样品)中钾的含量、赋存状态、主要矿物,钾在太阳星云冷凝、行星积聚以及岩浆海过程中的中等挥发性质;(2)介绍了钾同位素的研究历史,从1922年Dempster利用最早的质谱仪测量钾同位素在自然界的丰度,到Taylor和Urey在1938发表的经典的钾同位素分馏实验,再到Humayun和Clayton在1995年发表的钾同位素领域的经典研究,最后到近几年的进展;(3)介绍了对钾的前处理(离子交换柱法)以及钾同位素的主要测量方法,包括早期热电离质谱仪法(TIMS),二次离子质谱仪法(SIMS)和近十几年以来高速发展的多接收电感耦合等离子体质谱仪法(MC-ICP-MS)及其不同的技术路线;(4)介绍了高精度钾同位素比值在低温地球化学和生物地球化学中的应用,包括钾同位素在地表的风化过程中、海水洋壳的反向风化作用中的分馏及其在示踪全球钾元素循环和洋壳俯冲等过程中的应用;(5)介绍了高精度钾同位素比值在高温地球化学中的应用,包括钾同位素在岩浆分异和矿物结晶过程中的分馏;(6)介绍了高精度钾同位素比值在宇宙化学中的应用,包括在太阳星云冷凝、行星凝聚、月球形成大碰撞、岩浆海、火山喷发去气过程中的分馏作用。     

新疆阿尔泰全新世双湖沉积物正构烷烃分布及其环境意义

文章对新疆阿尔泰双湖沉积物进行了古气候、古环境变化研究。双湖（48°52'N,87°02'E）位于阿尔泰山中部,是山谷岩石滑塌阻塞形成的堰塞湖,湖长约1200m,宽度约300m,最大水深9m。项目使用重力钻在双湖中心钻取了172cm无扰动岩芯,根据核素测年（¹³⁷Cs,²¹⁰Pb,AMS¹⁴C）建立了9.1cal. ka B.P.以来的时间标尺,以正构烷烃分布特征（ACL_23~33、nC₂₇/nC₃₁、P_aq）为指标,重建了该区的古植被、古气候变化。沉积物中正构烷烃以高碳数为主,中长链正构烷烃存在显著的奇碳优势。链长指标（ACL_23~33）、nC₂₇/nC₃₁指标、P_aq之间具有很强的相关性关系,表明指标可能受到相似的气候驱动因素影响,反映了区域植被和湖泊水生生物的演替历史：9.1~8.8cal. ka B.P.期间水生植被输入较少,陆生植被中草本植物相对占比较高;8.8~3.3cal. ka B.P.期间大型水生植物大量繁殖,汇水区木本植物增多;3.3cal. ka B.P.至今湖泊周围的木本植物减少,草本植物相对占比增多,水生植物输入减少。双湖沉积物ACL_23~33指标自9.1cal. ka B.P.以来呈逐渐增加的趋势,可能表明温度逐渐增加,这一结果与前人在该区利用泥炭br-GDGT重建的温度序列具有相似的变化趋势,但在百-千年尺度上,温度变化不一致,可能与指标的敏感性和定年精度有关。

     

龙门山泥盆纪腕足化石锶同位素组成特征及其古环境意义

生物成因的海相碳酸盐岩中Sr同位素组成与演化是目前研究古海平面变化、造山运动以及古气候变化的主要工具,在海相地层定年及全球等时对比研究中发挥着重要作用。对四川龙门山泥盆纪剖面腕足化石Sr同位素组成进行了研究。保存鉴定结果显示：腕足化石壳体结构保存完整;阴极射线照射下壳体不发光或部分壳体呈微弱橙黄色;大多数壳体微量元素Mn〈250μg／g、Sr〉400μg／g。表明腕足化石保存完好,后期成岩作用影响微弱。锶同位素结果显示：龙门山泥盆纪sr同位素演化同全球泥盆纪Sr同位素演化基本一致,表明龙门山泥盆纪Sr同位素演化受全球性变化影响。早泥盆世较高Sr同位素比值（甘溪组0．708437）可能是受加里东造山运动的影响,大量古陆的出露,风化作用的加强所致。其后的缓慢下降可能受海平面上升古陆面积缩小的影响。中泥盆世Sr同位素组成低值平台显示着幔源Sr和陆源Sr（风化作用较弱）输入量之间的平衡。随后的Sr同位素组成的上升可能是受中泥盆世开始温度上升的影响,致使风化作用的加强所致。     

太古宙花岗质岩石Si-O同位素对岩石成因和板块构造的约束

板块构造的起源是地球科学核心问题之一, 而表壳物质循环是现代板块构造的重要表现之一, 因此检验地球表壳物质循环起始时间是约束板块构造启动时间的重要切入点。近年来, 岩浆岩的Si-O同位素联合示踪开始被用来约束太古宙构造环境, 但由于大多太古宙样品经历了强烈的变质作用, Si同位素数据是否代表原岩信息需要进一步的评估; 此外, Si同位素在高温岩浆分异过程中变化非常小, 在分析精度不够高的情况下, 其分析结果则很可能无法揭示其潜在的变化规律。本文结合当前地球早期构造环境研究进展以及Si-O同位素的应用情况: (1)重点总结介绍了太古宙花岗质岩石(主要包括英云闪长岩-奥长花岗岩-花岗闪长岩; 简称TTG)的Si同位素应用原理, 及应用Si-O同位素约束地球早期构造环境的优势; (2)分析了目前研究中存在的问题, 并给出了具体的改进建议和方法; (3)进一步总结了太古宙TTG Si同位素和全球规模的O同位素, Ge/Si比值, 及其他地质学和地球化学指标, 确认了在大约3.8 Ga开始有表壳物质再循环特征的出现; (4)最后依据当前的研究进展, 提出了未来具体研究方向。

     

Nd-isotopes in selected mantle-derived rocks and minerals and their implications for mantle evolution

The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the _Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative _Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative _Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host.The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle — both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart.     

Hydrochemical and isotope characteristics of spring water and travertine in the Baishuitai area (SW China) and their meaning for paleoenvironmental reconstruction

A method of combining hydrochemical data logging and in situ titrating with measurement of stable carbon and oxygen isotopes was used to reveal the hydrochemical and isotopic characteristics in the Baishuitai travertine scenic area of SW China. It was found that the travertine-forming springs have a very high concentration of calcium and bicarbonate, and accordingly very high CO₂ partial pressures, which are not likely to be produced by biological activity in soil alone. Further analysis of the stable carbon isotopes of the springs shows that the high pressure of CO₂ is mainly related to an endogenic CO₂ source. That means the Baishuitai travertine is endogenic in origin. This is contrast to the commonly accepted saying that the travertine deposition in this study simply is a product of warm and humid conditions in a karst ecological environment. Rapid CO₂ degassing from the water is triggered by the much higher partial pressures in water than that of the surrounding air. Consequently, as the waters flow downstream of the spring the pH increases, the waters become supersaturated with respect to calcite, and travertine is deposited. The preferential release of ¹²CO₂ to the atmosphere results in a progressive increase of travertine ¹³C downstream. This is concluded with a preliminary discussion of variation in travertine-forming water temperatures, according to differences in stable oxygen isotopic compositions of the travertine formed in different epochs at Baishuitai. It was found that the change in water temperature is as high as 13 °C, i.e., from 23 °C at about 2500 years b.p., to 10 °C at present. This may mainly reflect that the effect of geothermal source on water temperature is decreasing. The problems involved in paleoenvironmental reconstruction with endogene travertine are also discussed. They are the impacts of "dead carbon" in radiocarbon dating and the enrichment in ¹³C of travertine by endogenic CO₂ and degassing of CO₂ from water, which has to be considered in paleovegetation reconstruction when using ¹³C data of the endogene carbonate deposits.     

太行山红层磁学性质及古环境意义初探

环境磁学研究磁性矿物特征（磁性矿物类型、含量和颗粒大小）及其转化与环境关系,在不同地质时期古气候和古环境重建中得到广泛应用。本文将磁学方法应用于太行山中元古界红层研究,并从其磁学性质角度初步探讨太行山红层的古环境。对太行山红色石英砂岩和紫红色砂质泥岩样品进行了高温磁学和常温磁学系统测试。结果显示,太行山红层主要以硬磁性的赤铁矿为主,包含少量磁铁矿,部分样品赤铁矿是唯一磁性矿物;磁性颗粒大小以单畴为主;红色石英砂岩不同样品间磁性差别显著,反映沉积环境的不稳定性。紫红色砂质泥岩中的青灰色层表现为顺磁性特征,原因可能是紫红色砂质泥岩中的赤铁矿在后期还原环境下溶解或转化为弱磁性矿物所致,说明紫红色层并非形成于长期的水下还原环境。太行山红层中普遍存在的红色波痕和泥裂说明水环境对其有短暂影响,但是红层及赤铁矿的富集说明其长时间处于氧化环境,推断太行山红层可能形成于高度氧化的陆地环境。     

鄂尔多斯盆地北岔沟砂岩沉积特征及其对天然气成藏的控制作用*

北岔沟砂岩为华北地台二叠系山西组与太原组划分对比的主要标志层,在鄂尔多斯盆地呈大面积连片分布,是上古生界重要的天然气储集层。依据地面露头调查结果与勘探资料,结合前人有关古构造和岩相古地理研究成果,笔者对鄂尔多斯盆地北岔沟砂岩的沉积特征进行了详细分析。结果表明,受益于物源区大量高石英含量的粗碎屑物质在短时间内的集中供应,北岔沟砂岩在地层中位置稳定,以含砾粗粒石英砂岩为主,石英含量高达90%左右,砂体叠置紧密,单砂体之间很少见泥岩隔夹层。这些砂体分别隶属于冲积扇、砾质辫状河和辫状河三角洲沉积体系,在鄂尔多斯盆地北部高、南部次高、中南部低的古地理背景控制下,由盆地南、北边缘向中部依次推进,沉积中心区以北发育4个规模宏大的三角洲沉积主砂带,以南发育6个规模较小的主砂带。受各主砂带砂体规模、矿物组分含量及其成岩作用差异影响,各砂带的储集物性具有较大变化,其中砂体规模最大、石英含量最高、长石等易溶矿物在酸性地层水作用下溶蚀彻底的杭锦旗—榆林—子洲—清涧—永和—吉县主砂带,储集能力最好,最有利于天然气的大规模聚集成藏,是鄂尔多斯盆地北岔沟砂岩天然气进一步勘探开发的有利场所。