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1.
张生  陈根文 《地质科学》2015,(3):898-910
低密度的水热蒸气和超临界似气流体广泛存在于中地壳至地球表面的各种地质环境中, 是成矿金属搬运和富集的重要介质。火山喷气凝结水、火山结壳和升华物、矿床的流体包裹体气相中均存在具有地质意义的W含量或含钨矿物, 表明W同样可在含水气相中溶解和迁移。本文在350 ℃~400 ℃和压力为60~200 bar的实验条件下, 测定了WO3-H2O体系中W在水蒸气和似气流体中的溶解度, 考察了水蒸气压力对W溶解度的影响。结果显示, W在水蒸气中的逸度(或含量)远高于依据无水体系中固体WO3挥发性数据计算的蒸气压力, 证明气态溶质W与溶剂水蒸气之间存在促进W溶解的水合作用。经热力学方法分析, 认为可能形成了WO3·nH2O(g)形式的水合气体物种, 其水合数n在350 ℃、370 ℃和400 ℃时分别为1.4、1.6和2.9。因此WO3·3H2O(g)或H2WO4·2H2O(g)及H6WO6(g)在温压较高的岩浆-热液或气成-热液成矿环境中(如斑岩系统)对W的气态迁移和浓集可能具有重要作用, 而在温压较低的水热蒸气条件下, W的迁移形式可能以水合数较小的WO3·H2O(g)(或H2WO4)和WO3·2H2O(g)(或H2WO4·H2O)物种为主, 其含量或比例随水蒸气的压力而改变。某些斑岩型和脉型钨(钼)矿床常存在富气体包裹体, 伴随酸性岩浆结晶出溶的以低盐度含水蒸气占优势的岩浆流体对斑岩系统中W、Mo在高温阶段的气态迁移和矿质在花岗岩体顶部和上覆岩层的聚集具有重要意义, 之后蒸气冷凝可产生高盐度的含矿卤水或与渗流地下水混合形成低-中等盐度的成矿流体, 流体的减压沸腾(相分离)和对围岩的交代蚀变导致W、Mo等金属在不同阶段和构造-岩性部位沉淀富集。  相似文献   

2.
雄村铜金矿床是近年来西藏冈底斯成矿带内发现的一处超大型铜金矿床,该矿床位于冈底斯造山带中段南缘。对石英闪长斑岩的石英斑晶和晚期石英脉体中的流体包裹体进行了岩相学观察和同步辐射X射线荧光微探针分析。同步辐射X射线荧光微探针分析结果表明,石英闪长斑岩流体包裹体中富含成矿元素Cu和Au,在流体包裹体气相中,Mo、S、Fe、Mn、Hg、Ni、Tl、Cr等元素的含量无一例外地相对富集,Zn、Pb、Rb、As、Br则优先进入流体液相。高温热液流体在不混溶相分离过程中,成矿元素选择性迁移,在各相中进行不均匀分配,温压等物理化学条件的变化会影响成矿元素迁移分配的地球化学行为。因此,雄村矿床成矿流体是直接从岩浆熔体中出溶的富含Cu、Au、Mo、Fe等元素的岩浆流体,成矿元素在流体出溶和分离各相中的不均匀分配是成矿元素最终富集成矿的关键因素。  相似文献   

3.
桂花冲铜矿床是铜陵矿集区沙滩脚矿田内新发现的一个以斑岩型矿化为主的矽卡岩-斑岩复合型铜矿床。文章对该矿床的矿床地质和斑岩型矿化成矿流体进行了初步研究,旨在查明该矿床成矿流体的演化过程。根据脉体的穿切关系及矿物共生组合,桂花冲铜矿斑岩型矿化成矿过程可划分为钾化、硅化、石英黄铁矿、石英多金属硫化物和碳酸盐5个阶段。硅化阶段主要发育纯气体、含子矿物及富气相包裹体,石英黄铁矿阶段主要发育纯气体、富液相、富气相及含子矿物包裹体,石英多金属硫化物阶段及碳酸盐阶段主要发育富液相包裹体。从硅化阶段至碳酸盐阶段,成矿流体由高温(472.9℃)、高盐度(47.7%~74.0%)的岩浆热液逐渐向中低温(140.2~280.3℃)、低盐度(1.6%~7.7%)的岩浆热液和大气降水的混合流体演化,成矿过程中流体经历了沸腾及混合作用,混合作用是导致铜沉淀的主要机制。  相似文献   

4.
本文从火山喷气、岩浆热液矿床的成矿流体性质、金属元素在蒸汽相中的溶解及在蒸汽/卤水相的分配实验等方面概述了有关金属元素气相迁移及CO2在成矿过程中作用的研究现状。火山喷气的凝结物中高浓度的Cu、Zn、Pb、As、Ag和Au,以及斑岩型矿床中低密度流体包裹体（蒸汽相）中硫化物矿物的存在,预示着上述金属是以蒸汽相搬运的。金属元素在蒸汽相中溶解实验研究表明,金属元素在蒸汽相中以［MeXm·(H2O)n］水合物的形式存在,其溶解度随着H2O逸度和HCl逸度的增大而增加;熔体—流体体系分配实验研究揭示,NaCl—H2O体系中存在蒸汽—卤水相分离,在含S条件下Au、As等元素通常以HS-离子络合物的形式优先溶于蒸汽相,Fe、Zn、Pb、Mn、Cs等元素以Cl-离子络合物的形式优先富集于卤水相;Cu在富S热液中优先进入蒸汽相,在富Cl贫S热液中通常富集于卤水相,表明Cu在岩浆热液中是以HS-和Cl-两种络合物的形式迁移的。CO2在Au、Cu等金属元素迁移和沉淀过程中可能起重要的作用,不仅促进NaCl—H2O体系相分离,并且促使HS-络合物在蒸汽相富集以及调节成矿流体的酸碱度。斑岩型Cu—Au矿床的矿化过程可概括为3个阶段：高侵位的斑岩分异出的少量岩浆流体主要形成了青磐岩化带和部分钾硅化带,矿化通常不成规模;深部岩浆房早阶段去气作用分异出的岩浆流体主要在斑岩体早期钾化基础上叠加蚀变并形成广泛的浸染状矿化和石英—硫化物细脉,在斑岩体上部形成高级泥化带并形成低温热液型Cu—Au矿化,此阶段为主矿化期;深部岩浆房晚阶段去气作用形成的岩浆流体可能主要使斑岩体和部分围岩形成绢英岩化,并伴随晚期石英—(方解石)—硫化物脉的沉淀。  相似文献   

5.
金铜在气相中的迁移实验及矿石的成因   总被引：15，自引：1，他引：15  
文章报道了金属在高温高压气相的迁移实验研究,并阐明气相中迁移金属的成矿作用。实验是使含金属的流体在高温、高压的超临界态条件下通过相分离装置,在减低温度压力下实现气液相分离过程,收集气液相分离物,研究金属在气、液两相里的再分配。含金属(如金、铜)的NaHCO3-HCl-H2O流体在250~300℃的相分离过程实验结果表明:金属可以分布到气相里。根据金铜流体的实验结果,结合矿床调查,推测金属可以在地球深部以气体形式迁移。含金属的CO2-HCl-NaCl流体(气相)可以携带金属(金、铜),从岩石圈深部运输到地壳,甚至地表。金铜矿床矿石的形成过程中气体迁移金属起着重要作用。  相似文献   

6.
安徽铜陵胡村南铜钼矿床流体成矿过程   总被引：1，自引：0，他引：1  
胡村南铜钼矿床是在安徽铜陵铜(金)矿集区中发现的第一个矽卡岩-斑岩复合型铜钼矿床,在长江中下游成矿带具有特殊性和典型性。文章对该矿床进行了矿床地质和流体包裹体研究,旨在查明该矿床的流体成矿过程。胡村南铜钼矿床流体成矿过程可以划分为高温气成热液期、中高温热液期和低温热液期3个成矿期。高温气成热液期发育钾长石化和矽卡岩化,中高温热液期发育绿泥石化、绿帘石化和绢云母化,而低温热液期主要发育碳酸盐化。其中,中高温热液期为主要矿化期,形成辉钼矿和黄铜矿等多种硫化物网脉。高温气成热液期矿物中发育富液相和含子晶多相包裹体,中高温热液期矿物中也主要发育富液相包裹体和含子晶多相包裹体,但可见少量的富气相包裹体,低温热液期矿物中只发育富液相包裹体。从高温气成热液期经中高温热液期到低温热液期,成矿流体均一温度从435℃以上,经203~458℃,降低到156~276℃;盐度w(NaCleq)从14.0%~64.9%,经4.6%~47.5%,降低到1.0%~15.5%。成矿流体在其演化过程中发生过不混溶作用和沸腾作用。不混溶作用发生在气成热液期,使成矿流体中的成矿元素大量富集。沸腾作用发生在中高温热液期,导致成矿流体中的成矿元素卸载而沉淀出大量金属硫化物。  相似文献   

7.
云南哈播斑岩铜(-钼-金)矿床流体包裹体研究   总被引：3，自引：3，他引：0  
哈播斑岩Cu-(Mo-Au)矿床产于哀牢山富碱斑岩带的南段,形成于青藏高原后碰撞阶段构造转换环境,属于陆-陆碰撞型斑岩矿床.根据脉体的交切关系,确定哈播矿床各种脉的演化序列为早期石英脉→石英-黄铜矿脉→石英辉钼矿脉.脉中流体包裹体的岩相学、显微测温和激光拉曼光谱分析等研究结果显示,各期脉中均有富气相包裹体、富液相包裹体和含子矿物多相包裹体,各种包裹体的气相均含有CO2、SO2、H2O等气体.各期脉中多种包裹体并存并具有相似的均一温度范围,富液相包裹体均一温度149～427℃,盐度ω(NaCleq)6.0％～15.0％;富气相包裹体均一温度205～405℃,盐度ω(NaCleq) 3.4％～19.0％;含子矿物多相包裹体均一温度305～516℃,盐度w(NaCleq) 33.5％～61.0％.哈播矿床的初始成矿流体由稳定共存、不混溶的低盐度流体和高盐度流体组成,高盐度流体是哈播矿床成矿元素迁移的主要载体.成矿流体在400℃左右发生“二次沸腾”、分相,温度下降和挥发分持续逃逸可能是Cu-Au成矿的诱因.Mo元素在成矿流体多次沸腾、分相过程中,持续优先分配进入高盐度流体中而逐步富集;温度下降,使含钼硫化物在流体中溶解度降低、沉淀,形成石英-辉钼矿±黄铜矿脉.  相似文献   

8.
邹平地区与火山岩浆热液作用有关的铜矿床主要可划分为2种类型：一类为斑岩-火山角砾岩型,另一类为浅成低温热液型;代表性矿床分别为王家庄斑岩一火山角砾岩型铜（钼）矿床和南洞子浅成低温热液型铜（金）矿床。流体包裹体研究表明：王家庄铜（钼）矿床成矿流体的均一化温度和盐度偏高,出现了富气相的两相水溶液包裹体、富液相的两相水溶液包裹体和含子晶的三相水溶液包裹体共存现象,加温后,富气相包裹体均一到气相,同期富液相包裹体均一到液相的特征,这表明成矿流体在形成和演化过程中曾发生过沸腾作用。南洞子铜（金）矿床成矿流体均一温度和盐度偏低,以上3种包裹体共存的现象不明显,说明成矿流体在形成和演化过程中沸腾作用不强。上述2类矿床矿化脉石英中的δ^18OH2O-δD投影点飘离岩浆水范围,参照流体包裹体研究结果,证明邹平地区与火山岩浆热8液作用有关的铜矿成矿流体主要来源于岩浆水,后期混人大气降水。相比之下,浅成低温热液铜矿成矿流体中的大气降水混入量多。  相似文献   

9.
硼作为一种常用的地球化学指示剂和示踪剂,对研究俯冲带岩石学过程、岩浆-热液分异作用、火山活动以及稀有元素、铜、金的成矿机制具有重要意义。硼具有高水溶性和挥发性,它在气体中的分配、迁移能力和存在形式有助于理解含硼矿物的形成条件、流体化学组成的演化趋势、硼同位素的分馏效应和成矿金属的富集机理。本文在200-350℃、0.19-3.43 MPa条件下实验研究了B2O3-H2O体系中硼在共存水蒸气和富硼熔体（液体）之间的分配,平衡时气相中的B2O3含量为1.06%-32.35%。200℃、250℃、300℃和350℃时硼在气体-熔体之间的表观分配系数分别为0.035、0.042、0.20和0.33,即随温度上升,硼在含水气相中的分配和迁移能力增强。含水的富 B 熔体与硼酸稀溶液体系的气体-液体分配系数变化不大,表明B2O3-H2O±NaCl 体系中硼的气-液分配能力受液体或熔体中硼含量的影响较小,而主要与温度有关。经热力学分析,350℃、0.19-1.74 MPa条件下水蒸气中的气态硼物种可能为H3BO3和HBO2,可以预计随水蒸气压力的升高, H2O的配位数将会增大, H3BO3或其他可能的气态物种H3BO3·H2O会变得更为重要。某些火山活动区可见天然硼酸结壳（升华壳）的形成,灼热和干燥的火山岩石表面有利于硼酸从气相中凝析和沉淀。本实验结果表明,某些富硼酸的火山喷气孔气体的形成可能与地下高温火山岩浆（岩体）中存在因液态不混溶作用或晚期出溶作用产生的含水富硼的残余熔体或流体有关,熔体的去气作用或流体的减压相分离导致含水气相的产生,硼则随之大量分配至含水气体中并喷出地表。  相似文献   

10.
荣那铜(金)矿床是班公湖-怒江缝合带西段新发现的矿床,是多龙矿集区的重要组成矿床之一,已探明储量达大型规模,具有超大型矿床的成矿潜力。荣那铜(金)矿床矿石矿相学与岩相学研究显示其具有典型高硫化型浅成低温热液型矿床的矿物组合(明矾石、硫砷铜矿等)和矿化蚀变特征。通过资料收集与野外观察,本文将荣那铜(金)矿床的成矿过程划分为石英-黄铁矿阶段、石英-多金属硫化物阶段与碳酸盐阶段,其中石英-多金属硫化物阶段为主成矿阶段。为查明该矿床的成矿流体特征,进一步确定矿床成因类型,对取自深部矿石中的石英脉(均为主成矿阶段含黄铁矿、黄铜矿石英脉)开展了流体包裹体的岩相学观察、显微测温和激光拉曼光谱分析。结果表明,上述矿物中主要发育富液相、富气相和含子矿物三相包裹体;富液相包裹体的均一温度与盐度分别为:80~440℃和4.63%~11.95%NaCl eqv;富气相包裹体的均一温度和盐度分别为:320~440℃和5.55%~10.74%NaCl eqv;含子矿物三相包裹体的均一温度与盐度分别为200~400℃和29.4%~32.56%NaCl eqv;富液相与富气相包裹体的气体成分除少量N2外,气体成分均为H2O。综合分析认为,荣那矿床成矿流体发生了强烈的沸腾作用,流体沸腾作用是该矿床的重要成矿机制。可见,荣那矿床具有高硫型浅成低温热液矿床的矿物组合及蚀变特征,但主成矿阶段石英脉流体包裹体特征与典型斑岩型铜(金)矿床的流体包裹体特征相似。因此,推测荣那高硫型浅成低温热液铜金矿的深部存在斑岩型铜金矿化,该矿床应属浅成低温热液型-斑岩型铜金矿床。  相似文献   

11.
周云  黄惠兰  于玉帅  李芳  谭靖 《地球科学》2021,46(4):1248-1268
为了解锡田钨锡多金属矿床的成矿流体演化过程和成矿元素迁移机制,深入揭示成矿机制,指导该地区的下一步找矿勘探工作,对黑钨矿、锡石及透明矿物的流体包裹体进行了岩相学观察、红外显微测温及LA-ICP-MS原位分析.显示锡田钨锡多金属矿床绿柱石、黑钨矿中发育流体-熔体包裹体,均一温度最高可达760℃.早成矿阶段流体均一温度为360~500℃,盐度主要为28.4%~41.5% NaCleqv,主成矿阶段均一温度为280~450℃,盐度主要为3.0%~20.0% NaCleqv.,晚成矿阶段均一温度为120~280℃,盐度为0.4%~6.6% NaCleqv..LA-ICP-MS分析表明,超临界流体开始出溶时,W、Cu、Mo等元素优先富集于富挥发分气相中,Pb、Zn、Sn、Fe、Mn等元素则更倾向富集于高盐度卤水相中.锡田钨锡多金属矿床成矿流体来源于燕山期浅源花岗岩,钨锡成矿作用始于岩浆-热液过渡阶段,成矿流体具有高温、高盐度和富CO2等特征.成矿流体来自岩浆流体的出溶,演化过程中经历了两次不混溶作用,不混溶相分离过程中,成矿元素选择性迁移,在各相中进行不均匀分配.流体不混溶、水岩反应、流体混合和流体冷却作用是导致该矿床钨锡矿物沉淀的原因.   相似文献   

12.
台湾东北部的龟山岛浅海热液体系产生大量的热液自然硫.为了理解微量元素在自然硫中的富集规律和机制,采用激光剥蚀等离子体质谱仪（LA-ICPMS）对龟山岛自然硫进行了元素含量分析.结果显示,硫磺基底仅含有As、Se和Te等岩浆脱气产生的挥发性亲铜元素.Fe、Mn、Co、Ni等亲铁元素主要来自于安山岩基岩,富集于富铁或含硅包体中.Al、Zn、Ba、Pb、La、Ce、Au、Ag等元素显著富集于含硅包体中,表明这些元素受硅酸盐矿物控制.富铜包体具有最高的Hg、Pb、Zn等亲铜元素的单位富集程度.首次对龟山岛热液自然硫中的微量元素分布进行了原位微区分析,有助于理解微量元素在热液活动中的来源、分布和分配等地球化学行为.   相似文献   

13.
Peridotitic inclusions in alluvial diamonds from the Kankan region of Guinea in West Africa are mainly of lherzolitic paragenesis. Nevertheless, extreme Cr2O3 contents (max. 17 wt%) in some of the exclusively lherzolitic garnets document that the diamond source experienced a previous stage of melt extraction in the spinel stability field. This initial depletion was followed by at least two metasomatic stages: (1) enrichment of LREE and Sr and (2) introduction mainly of MREE–HREE and other HFSE (Ti, Y, Zr, Hf). The Ti- and HFSE-poor character of stage (1) points towards a CHO-rich fluid or carbonatitic melt, the high HFSE in stage (2) favour silicate melts as enriching agent. Eclogitic inclusions are derived from a large depth interval ranging from the lithosphere through the asthenosphere into the transition zone. The occurrence of negative Eu anomalies in garnet and clinopyroxene from both lithosphere and transition zone suggests a possible relationship to subducted oceanic crust. Lithospheric eclogitic inclusions are derived from heterogeneous sources, that may broadly be divided into a low-Ca group with LREE depleted trace element patterns and a high-Ca group representing a source with negative LREE–HREE slope that is moderately enriched in incompatible elements relative to primitive mantle. High-Ca inclusions of majoritic paragenesis are significantly more enriched in incompatible elements, such as in Sr and LREE. Calculated whole rock compositions require metasomatic enrichment even if a derivation from MORB is assumed. Received: 26 January 2000 / Accepted: 18 May 2000  相似文献   

14.
Nine fine-grained feldspathoid-, grossular-, spinel-, pyroxene-bearing inclusions from the Allende meteorite were analysed by instrumental neutron activation analysis. On the average, these inclusions are enriched in the refractory lithophile elements Ca, Sc, Ta and the rare earths by factors of 5–30 relative to Cl chondrites but are depleted in the refractory and volatile siderophiles, Ir, Co and Au. The volatile elements Fe, Cr and Zn are present at levels of 3.38–8.51%, 326–2516 ppm and 308–1376 ppm, respectively. Textural, mineralogical and chemical data suggest that the fine-grained inclusions formed in the solar nebula by the simultaneous condensation of volatiles and refractory lithophile elements which failed to condense into the coarse-grained, high-temperature condensate inclusions. The marked differences in the enrichment factors for different refractories in the fine-grained inclusions are caused by relatively small differences in their accretion efficiencies into the coarse-grained ones. The trace element data indicate that the refractories in the fine- and coarse-grained inclusions can only be the cosmic complements of one another if the fine-grained ones represent no more than ~ 20% of the most abundant refractory elements.  相似文献   

15.
Melilite and wollastonite from the Colle Fabbri stock contain silicate melt and silicate-carbonate inclusions. The homogenization temperatures of silicate inclusions are within the magmatic temperature range of mantle ultrabasic melts: about 1,320?±?15 °С. Their composition is melilititic and evolves to the composition of leucite tephrite and phonolite. The composition of silicate-carbonate inclusions are high SiO2, Ca-rich, enriched in alkalies and are similar to that of inclusions of carbonatite melts in the minerals of melilitolites of other intrusive ultramafic complexes. They are also similar to the compositions of metasomatized travertine covering the melilitolite stock. The presence of primary silicate and silicate-carbonate inclusions evidences that the melilitite magma from which melilitolites of Colle Fabbri crystallized was associated with carbonatite liquid. This liquid was highly fluidized, mobile and aggressive. Actively interacting with overlying travertine, the liquid enriched them with alkalies, aluminosilicates and incompatible elements, which resulted in the equalization of their compositions. Heterogeneous compositional dominions were formed at the contact between melilitolite and wall pelites. In the minerals of these contact facies high-Si melt inclusions of varying composition have been observed. Their occurrence is related to the local assimilation by the high-temperature melilitite magma of pelitic country rocks. The content of incompatible elements in melilitite melts and melilitolites is higher than the mantle norm and they have peculiar indicator ratios, spectra, Eu/Eu* ratio, which suggest a peculiar mantle source.  相似文献   

16.
Phlogopiie-and diopside-bearing dunite occurs as rounded inclusions in hornblende-diorite. Thepetrofabrics, mineral composition and abundances of the trace elements in the dunite indicate that the inclu-sions were derived from the upper mantle and are the residues of high-degree partial melting of mantlepeirdotite. The study of trace elements in the inclusions and their host rocks shows that the magma was origi-nated from the mantle which was enriched in incompatible elements by mantle metasomatism prior to the par-tial melting.  相似文献   

17.
Fluids or melts derived from a subducting plate are often citedas a mechanism for the oxidation of arc magmas. What remainsunclear is the link between the fluid, oxygen fugacity, andother major and trace components, as well as the spatial distributionof the impact of those fluids. To test the potential effectsof addition of a subduction-derived fluid or melt to the sub-arcmantle, olivine-hosted melt inclusions from primitive basalticlavas sampled from across the central Oregon Cascades (43°–45°N)have been analyzed for major, trace and volatile elements andfO2. Oxygen fugacity was determined in melt inclusions fromsulfur speciation determined by electron microprobe and fromolivine–chromite oxygen geobarometry. The overall rangein fO2 based on sulfur speciation measurements is from <–0·25log units to + 1·9 log units (FMQ, where FMQ is fayalite–magnetite–quartzbuffer). Oxygen fugacity is positively correlated with fluid-mobiletrace element and light rare earth element contents in basaltsgenerated by relatively low-degree partial melting. Establishinga further correlation between fO2 and fluid-mobile trace elementabundances with position along the arc requires the basaltsto be subdivided into shoshonitic, calc-alkaline, low-K tholeiiteand enriched intraplate basalt groups. Melt inclusions fromenriched intraplate and shoshonitic lavas show increasing fO2and trace element abundances closer to the trench, whereas calc-alkalinemelt inclusions exhibit no significant across-arc variations.Low-K tholeiitic melt inclusions record an increase in incompatibletrace elements closer to the trench; however, there is no correlatedincrease in fO2. The correlation observed in enriched intraplateand shoshonitic melt inclusions is interpreted to reflect aprogressively greater proportion of a fluid-rich, oxidized subductioncomponent in magmas generated nearer the subduction zone. Significantly,calc-alkaline melt inclusions with high ratios of large ionlithophile elements to high field strength elements, characteristicof ‘typical’ arc magmas, have oxidation states indistinguishablefrom low-K tholeiite and enriched intraplate basalt melt inclusions.The lack of across-arc geochemical variation in calc-alkalinemelt inclusions may suggest that these basalts are not necessarilythe most appropriate magmas for examining recent addition ofa subduction component to the sub-arc mantle. Flux and batchmelt model results produce a wide range of predicted amountsof melting and subduction component added to the mantle source;however, general trends characterized by increased melting andproportion of the subduction component from enriched intraplate,to low-K tholeiite, to calc-alkaline are robust. The model resultsdo not require enriched intraplate, low-K tholeiite and calc-alkalinemagmas to be produced from the same more fertile mantle source.However, enriched intraplate magmas, in contrast to calc-alkalineand low-K tholeiite magmas, cannot be generated from a depletedmantle source. Flux or batch melting of either the more fertileor depleted mantle sources used to generate the low-K tholeiite,calc-alkaline, and enriched intraplate magmas cannot reproduceshoshonitic compositions, which require a significantly depletedmantle source strongly metasomatized by a subduction component.The potential mantle source for shoshonitic basalts has a predictedfO2 (after oxidation) from + 0·3 to + 2·4 logunits (FMQ) whereas the mantle source for low-K tholeiite, calc-alkaline,and enriched intraplate magmas may range from –1·1to + 0·7 log units (FMQ). KEY WORDS: basalt; Cascades; melt inclusions; oxidation state; volatiles  相似文献   

18.
The first data on the composition and inner structure of zircon, one of the main ore minerals of the rare-metal metasomatites of the Gremyakha–Vyrmes alkaline-ultramafic massif, are reported. Early zircon generations are enriched in Y and REE and contain numerous inclusions of rock-forming and accessory minerals of metasomatites, as well as syngenetic fluid inclusions of calcite, thorite and thorianite. Late generations differ in the elevated Hf content and contain no inclusions. The elevated concentrations of Ca and Th in the central zones of crystals are related to the presence of numerous micron-sized inclusions of calcite and thorium phases. All zircon varieties have extremely low U and Pb contents. Concentrations and distribution patterns of incompatible and rare-earth elements in zircon from the metasomatites of the Gremyakha–Vyrmes Massif are similar to those of syenite pegmatites and magmatic carbonatites around the world. Mineral from these associations shows a positive Ce anomaly and elevated HREE contents. According to the compositions of zircon and thorite inclusion in it and experimental data on the simultaneous synthesis of these minerals, the crystallization temperature of zircon was 700–750°С. Using Ti-in-zircon temperature dependence, late zurcon was formed at temperature of 700–750°С. The rare-metal metasomatites are formed at the final stages of the massif formation, presumably after foidolites. Carbonatites could initiate metasomatic reworking of foidolites and accumulation of trace metals in them. The evolution of the primary alkaline–ultramafic melt toward the enrichment in trace elements was mainly controlled by crystallization differentiation.  相似文献   

19.
The results of study of rutile inclusions in pyrope from the Internatsionalnaya kimberlite pipe are presented. Rutile is characterized by unusually high contents of impurities (up to 25 wt %). The presence of Cr2O3 (up to 9.75 wt %) and Nb2O5 (up to 15.57 wt %) are most typical. Rutile inclusions often occur in assemblage with Ti-rich oxides: picroilmenite and crichtonite group minerals. The Cr-pyropes with inclusions of rutile, picroilmenite, and crichtonite group minerals were formed in the lithospheric mantle beneath the Mirnyi field during their joint crystallization from melts enriched in Fe, Ti, and other incompatible elements as a result of metasomatic enrichment of the depleted lithospheric mantle.  相似文献   

20.
The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K2O: 0.01 to 0.4 oxide wt%, P2O5: <0.01 to 0.2 oxide wt%, LaN: 7 to 35, YbN: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K2O: 0.32 to 0.9 oxide wt %, P2O5: 0.02 to 0.35 oxide wt%, LaN: 11 to 60, YbN: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust. Received: 2 December 1997 / Accepted: 27 August 1998  相似文献   

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