宁芜地区梅山铁矿床矿物的电子探针分析和稀土元素地球化学特征

梅山铁矿床位于长江中下游成矿带宁芜盆地北段,矿体赋存于辉长闪长玢岩和下白垩统大王山组辉石安山岩的接触带。研究表明,梅山铁矿的石榴石以钙铁榴石为主,为钙铁-钙铝榴石系列,与传统意义矽卡岩矿床的石榴石组成相似;磁铁矿和赤铁矿具有斑岩铜矿和Kiruna型矿床的双重特征;赤铁矿和菱铁矿显示热液交代成因特征,但赤铁矿至少有2个成矿世代。成矿母岩辉长闪长玢岩、磁铁矿及磷灰石具有相似的稀土配分模式,暗示三者具有同源性。辉长闪长玢岩无Eu异常,代表了高氧逸度下岩浆的分离结晶作用;磁铁矿和磷灰石均具有中度负Eu异常,可能是在辉长闪长玢岩发生钠长石化的过程中,Eu以Eu2+形式在钠长石内富集,造成流体Eu亏损,后来生成的磷灰石和磁铁矿继承了流体的Eu含量特征,辉长闪长玢岩的钠长石化导致富Fe2+硅酸盐矿物淋滤铁元素进入流体,为矿床提供了铁物质。     

Estimation of Fe3+ from electron microprobe analyses: observations on calcic amphibole and chlorite

ABSTRACT Metabasites and metagreywackes from the Pelona and Rand Schists of southern California were analysed using three different electron microprobes. For all three instruments, the estimated Fe³⁺ contents of calcic amphibole, chlorite and epidote are positively correlated. For some samples, there is an additional correlation between high estimated Fe³⁺ and the presence of magnetite. These results imply that microprobe analyses can be used to discern relative differences in Fe³⁺. However, microprobe data and calculations on the sensitivity of the correction procedures to systematic analytical errors indicate that estimated values of Fe³⁺ are not significant in an absolute sense. Thus, estimates of Fe³⁺ are meaningful when comparing samples analysed with a single microprobe, but must be used with caution when comparing analyses obtained on more than one probe.     

Influence of ferric iron on the stability of mineral assemblages

Ferric iron is present in all metamorphic rocks and has the ability to significantly affect their phase relations. However, the influence of ferric iron has commonly been ignored, or at least not been considered quantitatively, mainly because its abundance in rocks and minerals is not determined by routine analytical techniques. Mineral equilibria calculations that explicitly account for ferric iron can be used to examine its effect on the phase relations in rocks and, in principle, allow the estimation of the oxidation state of rocks. This is illustrated with calculated pseudosections in NCKFMASHTO for mafic and pelitic rock compositions. In addition, it is shown that ferric iron has the capacity to significantly increase the stability of the corundum + quartz assemblage, making it possible for this assemblage to exist at crustal P–T conditions in oxidized rocks of appropriate composition.     

新疆北山小常山铁矿矿物电子探针分析及矿床成因探讨

小常山铁矿位于新疆北山裂谷带西段,矿体赋存于辉长岩、辉长岩和大理岩接触带中,部分赋存于花岗闪长岩和大理岩接触带。前人研究辉长岩年龄为276±1.2Ma,与坡北铜镍硫化物岩浆矿床形成年龄一致,同属早二叠世。小常山铁矿中可见有明显的岩浆贯入现象。矿体主要呈透镜状、块状、脉状、薄层状。金属矿物主要为磁铁矿,含极少量的褐铁矿和黄铁矿;近矿围岩蚀变较弱,主要有石榴子石化、绿帘石化、大理岩化。电子探针研究表明,磁铁矿FeOT含量范围较大,主要分布在85%范围以上,Al_2O_3含量相对较高,TiO_2-Al_2O_3-(MgO+MnO)图解、TiO_2-Al_2O_3-MgO图解均显示热液接触交代成因特点,表明小常山铁矿的形成和岩浆热液的交代作用有关。石榴子石属于钙铝榴石-钙铁榴石系列,与典型矽卡岩矿床的石榴子石端员组分组成有一定差别。结合小常山铁矿体地质特征以及电子探针分析测试等研究,认为小常山铁矿是多成因的复合型矿床,具有岩浆成因和热液成因特征,但后者是主要成因。     

Influence of ferric iron on phase equilibria in greenschist facies assemblages: the hematite‐rich metasedimentary rocks from the Monti Pisani (Northern Apennines)

We have investigated the effects of different Fe₂O₃ bulk contents on the calculated phase equilibria of low‐T/intermediate‐P metasedimentary rocks. Thermodynamic modelling within the MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (MnNKFMASHTO) chemical system of chloritoid‐bearing hematite‐rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe₂O₃ is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P–XFe₂O₃ diagrams, obtaining equilibration conditions of 475 °C and 9–10 kbar related to a post‐compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rock‐forming silicates that often yield important thermobaric information. In Fe₂O₃‐rich compositions, garnet‐ and carpholite‐in curves shift towards higher temperatures with respect to the Fe₂O₃‐free systems. The presence of a ferric‐iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P–T–XFe₂O₃ phase diagrams may also provide P–T information in common greenschist facies metasedimentary rocks.     

石榴子石Fe~(3+)含量电子探针原位分析:Flank Method方法的实例应用

电子探针分析技术主要应用于对物质进行快速无损的定性、定量分析,是材料科学和地质学领域最基础和重要的研究手段。而利用电子探针对矿物中不同价态Fe的含量进行原位定量分析,更是具有非常重要的科学意义。近来发展起来的可进行Fe~(3+) 含量定量分析的电子探针Flank Method测试方法,相比早期的实验方法具有更高的准确度和精确性。该方法主要是通过定义Fe的特征X射线L能级谱线峰位的峰侧位置(flank position),对应了不同价态Fe吸收谱线的差值线谱(difference spectrum)中强度计数最大值(FeLα*)和最小值(FeLβ*)的位置,从而避免了在早期实验中,一般通过直接测量Fe的L能级峰位(FeLα和FeLβ)可能导致的测试不确定性。同时,该测试方法通过对电子探针谱仪系统进行实时的位移误差校正,确保其在某一个稳定的实验条件(周期)下具有可重现性(精确度)。在本文中,我们根据北京大学造山带与地壳演化教育部重点实验室的JEOL JXA-8100型号电子探针的实际性能状态,在前人建立的实验步骤基础上,进行了测试流程的自适性改进和修正,建立了更符合本实验室实际情况的电子探针Flank Method测试方法,用于分析石榴子石中的Fe~(3+) 含量。通过对已知Fe~(3+) 含量的标准石榴子石样品进行检测验证,确定了该方法的可行性、可靠性以及可重复性。在对天然样品的分析中,我们选择了来自我国天山、拉萨和北祁连的洋壳俯冲类型中低温(超)高压榴辉岩中的石榴子石样品,对其进行了Fe~(3+) 含量(环带)的分析。结果显示,石榴子石中Fe~(3+) 含量可作为一个可靠的氧逸度指示剂,其(环带)变化很好地对应了榴辉岩寄主岩石的变质演化轨迹,反映了在板片俯冲过程中,榴辉岩的氧逸度具有降低的趋势。     

河北宽城蛇纹石玉的宝石矿物学特征研究

河北宽城蛇纹石玉结构细腻,颜色可呈肉红色、绿色,当地市场称之为"热河玉"。本文通过常规宝石学测试结合激光拉曼光谱、电子探针、X射线粉晶衍射、激光剥蚀等离子质谱以及可见光吸收光谱等手段对其进行了系统分析,并采用伽玛能谱仪对其放射安全性进行了检测。结果表明,肉红色样品折射率及相对密度值均较低,折射率为1.54~1.55,相对密度为2.33~2.43,主要组成矿物为斜纤蛇纹石,主要具纤维交织结构、纤维柱粒交织结构,局部可见变余生物骨架结构,次要矿物有方解石和透辉石,Fe和Mn元素含量相对较高,Fe元素含量范围在184.9×10^-6~951.7×10^-6,Mn元素含量为106.4×10^-6~287.3×10^-6;绿色样品折射率为1.55~1.56,相对密度为2.58~2.62,主要组成矿物为叶蛇纹石,具更加细腻的纤维交织结构,次要矿物为滑石及四方硫铁矿,Fe元素含量极高,可达5 979×10^-6~6 359×10^-6。结合可见光吸收光谱分析结果认为Fe³⁺、Fe²⁺以及Mn³⁺的综合作用是导致样品呈现肉红色的主要原因,而Fe²⁺和Fe³⁺则是导致绿色的主要原因。放射性检测结果显示样品的内、外照射指数及放射性比活度值均低于标准要求,对人体无放射性危害。     

西昆仑塔什库尔干地区赞坎铁矿床磁铁矿元素地球化学特征及其对成矿的制约

赞坎铁矿床是塔什库尔干地区一个典型的沉积变质型铁矿,具有多阶段成矿的特征,是塔什库尔干地区铁矿成矿作用演化的典型代表。文章将赞坎铁矿床主要矿石矿物磁铁矿的形成划分为3个世代,分别为条带状磁铁矿、浸染状磁铁矿和粗晶脉状或块状磁铁矿,分别代表3个成矿阶段的产物。电子探针和LAICP-MS原位分析表明,赞坎铁矿从条带状磁铁矿到粗晶块状磁铁矿随着磁铁矿的成矿演化主量元素中Al元素有减少的趋势,而Ti、Mn、Mg、V元素均具有增加的趋势;微量元素中Co、Nb、Hf、Ta等具有减少的趋势,Sc、Ga、Zr、Sn等元素具有增加的趋势。根据以上各成矿阶段中磁铁矿成分变化,并结合前人的研究成果发现,赞坎铁矿早期条带状磁铁矿与火山沉积作用有关,成矿后期特别是在粗晶块状和脉状磁铁矿阶段受岩浆热液影响明显,富铁矿有岩浆热液的参与。     

Comparison of elemental composition of macerals determined by electron microprobe to whole-coal ultimate analysis data

The elemental composition of the individual macerals in a suite of Australian coals has been determined in polished sections using light-element electron microprobe techniques. The analyses of the individual macerals in each coal were combined with data on maceral abundance to produce an inferred chemical composition for the organic matter of the respective whole-coal samples, and this was compared, for each sample, to the respective whole-coal ultimate analysis data, corrected to a dry, ash-free (daf) basis. Except for slightly lower values in some lower-rank coals, the inferred percentages of whole-coal C estimated from the microprobe data were found to be very close to the respective whole-coal C percentages as determined by conventional ultimate analysis. The proportion of O in the coals indicated by the microprobe study, however, appears to be as much as 2% higher than that derived from the ultimate analysis data, especially in the lower-rank coal samples. The difference it may represent errors in calculating the O percentages in ultimate analysis, errors in the microprobe analysis due to difficulties in calibration or measurement, or increased proportions of O in the coals due to factors such as take-up with storage of the polished sections. The percentages of whole-coal N calculated from the microprobe data are up to 0.5% (absolute) below the proportion of N determined directly by whole-coal ultimate analysis. This may reflect the inherent difficulty of dealing with a light element at low concentrations by the microprobe technique, or it may indicate that some of the N occurs in the coals in mineral form. The percentages of whole-coal (organic) S calculated from the microprobe study are close to the percentages of organic S determined for each sample by more conventional techniques. With the exception of (organic) O, which may be affected by other factors, and also possibly of N, the electron microprobe technique appears from the study to provide results that are consistent with ultimate analysis over a wide rank range.     

宁芜矿集区凹山铁矿床磁铁矿元素地球化学特征及其对成矿作用的制约

凹山铁矿床是一个典型的玢岩型铁矿床,成矿过程具有多阶段的特征,是宁芜矿集区凹山矿田成矿作用演化的典型代表。本次研究工作,在详细的野外地质调查研究和室内研究的基础上,将主要矿石矿物磁铁矿的形成划分为4个世代,分别为浸染状磁铁矿、角砾状磁铁矿、粗粒脉状磁铁矿和伟晶状磁铁矿,它们是四个成矿阶段的产物。电子探针和LA-ICP-MS原位分析表明,随着成矿作用的演化,磁铁矿主量元素中Ti、Mn、V含量变化微弱,Al、Mg含量增高;微量元素中Ga、Sn及高场强元素Zr、Hf、Nd、Ta含量变化较小;从角砾状矿石到伟晶状矿石Co含量逐渐增高、Sc含量逐渐降低。根据以上成矿各阶段中磁铁矿成分的变化,并结合前人研究成果得出,凹山铁矿床作为一个高温气液充填矿床,其成矿物质主要来自于岩浆演化晚期形成的高温富铁流体。在成矿过程中磁铁矿具有同源连续演化的特征,其中隐爆作用诱发了大规模铁沉淀,并为成矿提供了空间,形成了早期的浸染状和角砾状矿石;成矿过程中流体成分不断变化,后期大量挥发份的累积和外源流体的逐渐加入,形成了伟晶状矿石并使得磁铁矿具有了热液成因的特征。     

汉诺坝幔源单斜辉石巨晶氢同位素组成的离子探针分析：微尺度不均一性

运用离子探针技术测定了河北汉诺坝新生代玄武岩中3个单斜辉石巨晶的氢同位素组成，结果显示同一颗粒内部表现出微尺度的不均一性，2mm范围内δD的变化达到60‰。δD和氢含量之间不存在同步的变化，巨晶内部的化学成分均一，因此我们认为巨晶的氢同位素不均一性继续自母岩浆。母岩浆的氢同位素变化可能是去气过程中气相与熔体之间的分馏引起的。单斜辉石巨晶形成后很短的时间内即被寄主岩浆带至地表并经历了快速淬火。     

河北康保地区花岗岩独居石电子探针定年

已有年代学研究表明,康保地区西阿公单元含石榴二长花岗岩形成于中元古代.我们采用近年来新发展的独居石电子探针Th-U-总Pb定年方法对康保地区西阿公单元十硼地区变质细粒含石榴二长花岗岩样品中的独居石开展了电子探针进行测年分析,计算方法为Suzuki和Adachi等提出的方法.独居石年龄峰值分别为252Ma,265Ma,281Ma和322Ma,表明西阿公单元含石榴二长花岗岩形成时代不是中元古代,其侵位时间应为二叠纪.这一新的年代学结果与Sengor和王荃推测的华北板块与西伯利亚板块＂晚二叠世碰撞＂的模式一致.康保地区西阿公单元含石榴二长花岗岩具过铝花岗岩特征,代表古蒙古洋消失,是华北板块与西伯利亚板块碰撞造山阶段多期岩浆作用的结果,为确定古亚洲洋的闭合时间提供了重要依据.     

天然蛇纹石活性机理初探

通过对蛇纹石化学成分、晶体结构及键性的分析，将蛇纹石的活性基团划分为5类，即不饱和Si—O—Si键、O—Si—O键、含镁键类、羟基和氢键，并详细介绍了各类活性基团的特征，尤其对硅氧四面体悬挂键做了介绍。指出正是这些活性基团的存在使蛇纹石具有很高的化学活性和生物活性。纤蛇纹石是一维纳米纤维矿物，易进入生物细胞，并释放出OH^-、Mg^2 、SiO2及Ni^2 等，同时不饱和O—Si—O键发生断裂形成游离氧，由纤蛇纹石形成的这种碱性氧化溶液会严重影响生物机体的正常工作，并使其产生各种病变。     

The importance of iron speciation (Fe+2/Fe+3) in determining mineral assemblages: an example from the high‐grade aluminous metapelites of southeastern Madagascar

Metapelitic granulites from the Anosyen domain of southeastern Madagascar are exposed in three intercalated formations: the Amparihy, Bakika and Ihosy formations. Although mineralogically distinct from each other, the rocks from these formations show very similar bulk‐rock compositions when measured on a Fe_T basis. The preserved mineral assemblages thus do not reflect differences in the ratios of the main rock‐forming oxides (i.e. Al₂O₃:Fe_T:MgO), but instead reflect variations in the pre‐metamorphic oxidation state of the protolith rocks. These differences in oxidation state are manifested via differences in iron speciation – either Fe⁺² or Fe⁺³. The relatively reduced rocks of the Amparihy Formation preserve the assemblage bi–sp–sill–g–cd, which contrasts markedly with the mostly garnet and spinel‐absent bi–cd–sill–mt assemblages preserved in the strongly oxidized rocks of the Ihosy Formation. Compositionally intermediate rocks of the Bakika Formation are garnet bearing, but sillimanite‐absent, and contain the assemblage sp–g–cd–mag. Modelling of these rocks in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O system suggests that they evolved along a heating and cooling P–T path with only limited decompression accompanying cooling on the retrograde path. Peak temperatures and pressures of ～880–920 °C and 6–6.5 kbar are inferred for the majority of the Anosyen domain, with slightly lower peak temperatures (～840 °C) estimated in the extreme northwest of the area. The high‐temperature and relatively low‐pressure nature of metamorphism suggests high geothermal gradients existed during orogenesis, which in southern Madagascar is related to the amalgamation of Gondwana (580–520 Ma). Although metamorphic temperatures may have been augmented via thermal advection from the emplacement of the syn‐ to post‐tectonic Ambalavao suite, the high geothermal gradients nevertheless suggest thin and consequently hot lithosphere existed prior to orogenesis.     

Interpretation of heterogeneity effects in synchrotron X-ray fluorescence microprobe data

Heterogeneity effects often limit the accuracy of synchrotron X-ray fluorescence microprobe elemental analysis data to ± 30%. The difference in matrix mass absorption at Kα and Kβ fluorescence energies of a particular element can be exploited to yield information on the average depth-position of the element or account for heterogeneity effects. Using this technique, the heterogeneous distribution of Cu in a simple layered sample could be resolved to a 2 × 2 × 10 (x, y, z, where z is the depth coordinate) micrometer scale; a depth-resolution limit was determined for the first transition metal series and several other elements in calcite and iron oxide matrices. For complex heterogeneous systems, determination of average element depth may be computationally limited but the influence of heterogeneity on fluorescence data may still be assessed. We used this method to compare solid-state diffusion with sample heterogeneity across the Ni-serpentine/calcite boundary of a rock from Panoche Creek, California. We previously reported that Ni fluorescence data may indicate solid state diffusion; in fact, sample heterogeneity in the depth dimension can also explain the Ni fluorescence data. Depth heterogeneity in samples can lead to misinterpretation of synchrotron X-ray microprobe results unless care is taken to account for the influence of heterogeneity on fluorescence data.     

新疆雅满苏铁矿床矽卡岩和磁铁矿矿物学特征及其地质意义

雅满苏铁矿床位于东天山中段,矿体赋存于下石炭统雅满苏组安山质火山碎屑岩中,受近EW向断裂及环形断裂构造控制。矿体主要呈层状、似层状、透镜状,近矿围岩蚀变强烈,形成石榴石矽卡岩及复杂矽卡岩。电子探针分析结果表明,石榴石为钙铁榴石-钙铝榴石系列,其化学组成可表示为And45.68~100Gro0.67~57.95(A1m+Sps)11~29.03,与典型的矽卡岩型铁矿中石榴石端员组分相似。在磁铁矿Ca+Al+Mn-Ti+V图解中,大部分样品落入矽卡岩型铁矿区;TiO2-Al2O3-MgO图解中,大多数的样品落入沉积变质接触交代磁铁矿趋势区,部分早期磁铁矿落在岩浆趋势区内。结合矿床地质特征和矿物学研究,认为大多数样品经过了一个热液交代作用过程,表明雅满苏铁矿的形成与岩浆热液交代作用有关。     

汉诺坝幔源单斜辉石巨晶氢同位素组成的离子探针分析:微尺度不均一性

运用离子探针技术测定了河北汉诺坝新生代玄武岩中3个单斜辉石巨晶的氢同位素组成,结果显示同一颗粒内部表现出微尺度的不均一性,2mm范围内δD的变化达到60‰。δD和氢含量之间不存在同步的变化,巨晶内部的化学成分均一,因此我们认为巨晶的氢同位素不均一性继承自母岩浆。母岩浆的氢同位素变化可能是去气过程中气相与熔体之间的分馏引起的。单斜辉石巨晶形成后很短的时间内即被寄主岩浆带至地表并经历了快速淬火。     

辽宁营口蛇纹石玉成因分析

辽宁营口大石桥后仙峪硼矿区是蛇纹石玉的一个新产地,产出的营口玉具色深、富镁、高铁等特征。本文通过对该玉石的岩石学、岩石化学及地球化学特征分析,结合大地构造背景、定年数据、MgO-SiO2-H2O体系实验结果等,探讨了营口玉的成矿过程。认为其原岩是镁橄榄岩和镁质碳酸岩,并且受到混合岩化后期热液的交代;其主成矿期在1.95~1.85 Ga之间,成矿压力接近500×105~2 000×105 Pa,成矿温度在400~250℃之间;营口玉玉矿是中温变质热液交代矿床。     

新疆阿尔泰乌吐布拉克铁矿床矽卡岩矿物特征及其地质意义

乌吐布拉克中型铁矿床赋存于上志留统—下泥盆统康布铁堡组变质火山-沉积岩系中,矿体呈似层状、透镜状,矿体及其周围发育大量矽卡岩矿物。电子探针分析表明石榴石端员组分以钙铁榴石-钙铝榴石系列为主,辉石端员组分以透辉石为主,角闪石端员组分主要为铁镁钙闪石,这些特点表明矿区矽卡岩为交代矽卡岩中的钙矽卡岩。磁铁矿的主要组分、稀土及微量元素表明其形成与矽卡岩密切相关。结合矿床地质特征,认为矽卡岩是由岩浆热液流体交代康布铁堡组基性火山岩(熔岩和火山碎屑岩)及灰岩而形成的,磁铁矿的形成与矽卡岩的退化变质作用有关。     

Electron microprobe monazite geochronology of magmatic events: Examples from Variscan migmatites and granitoids, Massif Central, France

U–Th–Pb dating of monazite with the electron probe microanalyser (EPMA) is increasingly documented as a reliable geochronological method offering high spatial resolution. This method has been applied on monazite from the Cévennes migmatites and granitoids from the southeast of the French Massif Central. Measurements were performed on separated grains after systematic back-scattered electron (BSE) imaging. Monazites from migmatites record two main ages: (i) a protolith age of about 550–543 Ma obtained on inherited cores, and (ii) a migmatization event between 329 ± 5 and 323 ± 3 Ma recorded by monazite rims and all other monogenetic grains. Monazite from the peraluminous Rocles pluton yields a 318 ± 3 Ma age. Finally, three granite dykes are dated at 333 ± 6, 318 ± 5 and 311 ± 5 Ma; the older dyke is the most deformed of them and is interpreted as linked to the migmatization event; the two other dykes are geochronologically, petrologically and structurally coeval with the Rocles pluton. The data constrain the timing of crustal melting following Variscan thickening in the northern Cévennes area. Migmatization of Ordovician protoliths took place at 329–323 Ma and was shortly followed by intrusion of leucogranite at 318–311 Ma. The study shows that EPMA dating of monazite can be successfully used to resolve a close succession of regional melting events.