An experimental study of the grain-scale processes of peridotite melting: implications for major and trace element distribution during equilibrium and disequilibrium melting   总被引：1，自引：0，他引：1  

Mauro Lo Cascio  Yan Liang  Nobumichi Shimizu  Paul C. Hess 《Contributions to Mineralogy and Petrology》2008,156(1):87-102

The grain-scale processes of peridotite melting were examined at 1,340°C and 1.5 GPa using reaction couples formed by juxtaposing pre-synthesized clinopyroxenite against pre-synthesized orthopyroxenite or harzburgite in graphite and platinum-lined molybdenum capsules. Reaction between the clinopyroxene and orthopyroxene-rich aggregates produces a melt-enriched, orthopyroxene-free, olivine + clinopyroxene reactive boundary layer. Major and trace element abundance in clinopyroxene vary systematically across the reactive boundary layer with compositional trends similar to the published clinopyroxene core-to-rim compositional variations in the bulk lherzolite partial melting studies conducted at similar P–T conditions. The growth of the reactive boundary layer takes place at the expense of the orthopyroxenite or harzburgite and is consistent with grain-scale processes that involve dissolution, precipitation, reprecipitation, and diffusive exchange between the interstitial melt and surrounding crystals. An important consequence of dissolution–reprecipitation during crystal-melt interaction is the dramatic decrease in diffusive reequilibration time between coexisting minerals and melt. This effect is especially important for high charged, slow diffusing cations during peridotite melting and melt-rock reaction. Apparent clinopyroxene-melt partition coefficients for REE, Sr, Y, Ti, and Zr, measured from reprecipitated clinopyroxene and coexisting melt in the reactive boundary layer, approach their equilibrium values reported in the literature. Disequilibrium melting models based on volume diffusion in solid limited mechanism are likely to significantly underestimate the rates at which major and trace elements in residual minerals reequilibrate with their surrounding melt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

Sulfide-driven arsenic mobilization from arsenopyrite and black shale pyrite   总被引：1，自引：0，他引：1  

Wenyi Zhu  Nathan Yee  E. Danielle Rhine  John R. Reinfelder 《Geochimica et cosmochimica acta》2008,72(21):5243-5250

We examined the hypothesis that sulfide drives arsenic mobilization from pyritic black shale by a sulfide-arsenide exchange and oxidation reaction in which sulfide replaces arsenic in arsenopyrite forming pyrite, and arsenide (As−1) is concurrently oxidized to soluble arsenite (As+3). This hypothesis was tested in a series of sulfide-arsenide exchange experiments with arsenopyrite (FeAsS), homogenized black shale from the Newark Basin (Lockatong formation), and pyrite isolated from Newark Basin black shale incubated under oxic (21% O₂), hypoxic (2% O₂, 98% N₂), and anoxic (5% H₂, 95% N₂) conditions. The oxidation state of arsenic in Newark Basin black shale pyrite was determined using X-ray absorption-near edge structure spectroscopy (XANES). Incubation results show that sulfide (1 mM initial concentration) increases arsenic mobilization to the dissolved phase from all three solids under oxic and hypoxic, but not anoxic conditions. Indeed under oxic and hypoxic conditions, the presence of sulfide resulted in the mobilization in 48 h of 13-16 times more arsenic from arsenopyrite and 6-11 times more arsenic from isolated black shale pyrite than in sulfide-free controls. XANES results show that arsenic in Newark Basin black shale pyrite has the same oxidation state as that in FeAsS (−1) and thus extend the sulfide-arsenide exchange mechanism of arsenic mobilization to sedimentary rock, black shale pyrite. Biologically active incubations of whole black shale and its resident microorganisms under sulfate reducing conditions resulted in sevenfold higher mobilization of soluble arsenic than sterile controls. Taken together, our results indicate that sulfide-driven arsenic mobilization would be most important under conditions of redox disequilibrium, such as when sulfate-reducing bacteria release sulfide into oxic groundwater, and that microbial sulfide production is expected to enhance arsenic mobilization in sedimentary rock aquifers with major pyrite-bearing, black shale formations.  相似文献   

Simple models for dynamic melting in an upwelling heterogeneous mantle column: Analytical solutions     

Yan Liang 《Geochimica et cosmochimica acta》2008,72(15):3804-3821

Several lines of evidence suggest that the melt generation and segregation regions of the mantle are heterogeneous, consisting of chemically and lithologically distinct domains of variable size and dimension. Partial melting of such heterogeneous mantle source regions gives rise to a diverse range of basaltic magmas. In order to better assess the role of source heterogeneity during mantle melting, we have undertaken a theoretical study of trace element distribution and fractionation during concurrent melting and melt migration in an upwelling, chemically heterogeneous, two-porosity double lithology melting column. Analytical solutions for the abundance of a trace element in the matrix and channel were obtained under the assumptions that the porosity, melt and solid velocities, and solid-melt partition coefficients are constant and uniform. For simplicity, we neglected diffusion and dispersion in the melt. Chemical source heterogeneities of arbitrary size and shape were integrated into the simple melting models by allowing trace element abundance in the source region to vary as a function of time and space. Concurrent melting and melt migration in an upwelling heterogeneous mantle may be approximated as a quasi-steady state problem in which time-dependent concentration patterns produced by melting of heterogeneous source regions are superimposed on a reference steady-state concentration distribution established by melting of the ambient or background mantle. Chromatographic fractionation is especially important for the matrix melt and solid when chemical heterogeneities are involved during melting and melt migration in the mantle, giving rise to significant phase-shift between two incompatible trace elements in the matrix melt and scattered correlations among incompatible trace elements in residual peridotites. Mixing is the chief mass transfer process in the dunite channel where the chromatographic effect is negligible for most of the incompatible trace elements. The lack of chromatographic fractionation among incompatible trace elements and isotopic ratios in MORB suggests either most MORB are channel melts or mixing in magma conduit and chamber is very efficient such that the phase-shift is averaged out during magma transport and storage processes. Advection brings melt produced by smaller-degree of melting in the deeper part of the melting column to the overlying melting region, increasing the incompatible trace element abundance in the matrix and the channel. This advection-induced self-enrichment is especially important when heterogeneous sources are involved and may account for some of the enriched incompatible trace element patterns observed in residual peridotite that were previously interpreted to be a result of mantle metasomatism. Systematic studies of high-resolution spatially correlated mantle samples may help to constrain the melting history and the size and nature of chemical heterogeneities in the mantle.  相似文献   

Considerations for conducting incubations to study the mechanisms of As release in reducing groundwater aquifers     

Kathleen A. Radloff  Anya R. Manning  Brian Mailloux  Yan Zheng  M. Moshiur Rahman  M. Rezaul Huq  Kazi M. Ahmed  Alexander van Geen 《Applied Geochemistry》2008

Microbial Fe reduction is widely believed to be the primary mechanism of As release from aquifer sands in Bangladesh, but alternative explanations have been proposed. Long-term incubation studies using natural aquifer material are one way to address such divergent views. This study addresses two issues related to this approach: (1) the need for suitable abiotic controls and (2) the spatial variability of the composition of aquifer sands. Four sterilization techniques were examined using orange-colored Pleistocene sediment from Bangladesh and artificial groundwater over 8 months. Acetate (10 mM) was added to sacrificial vials before sterilization using either (1) 25 kGy of gamma irradiation, (2) three 1-h autoclave cycles, (3) a single addition of an antibiotic mixture at 1× or (4) 10× the typical dose, and (5) a 10 mM addition of azide. The effectiveness of sterilization was evaluated using two indicators of microbial Fe reduction, changes in diffuse spectral reflectance and leachable Fe(II)/Fe ratios, as well as changes in P-extractable As concentrations in the solid phase. A low dose of antibiotics was ineffective after 70 days, whereas autoclaving significantly altered groundwater composition. Gamma irradiation, a high dose of antibiotics, and azide were effective for the duration of the experiment.  相似文献   

西藏马攸木金矿床金银互化物的赋存状态   总被引：3，自引：0，他引：3  

孙燕  温春齐  多吉  霍艳 《地质与勘探》2008,44(5):42-46

马攸木金矿床是西藏近年发现的首例规模较大、矿石品位富、金成色高的岩金矿床.作者通过对马攸木金矿床矿石组构、矿石共生组合及矿物特征研究发现,金银互化物主要有自然金、含银自然金、银金矿、自然银.载金矿物为黝铜矿、针铁矿、脆硫锑铅矿及石英;金银互化物的赋存形式主要为包裹体金、裂隙金及粒间金.金银互化物的形成、富集与热液成矿作用及表生风化作用关系密切.  相似文献   

塔里木盆地西部吐木休克断裂带的主要特征和构造演化   总被引：8，自引：5，他引：3       下载免费PDF全文

朱德丰  刘和甫  吴根耀 《地质科学》2008,43(2):209-227

塔里木盆地西部的吐木休克断裂带是中央隆起(前身是晋宁期碰撞造山带)的次级单元巴楚断隆与北部坳陷的次级单元阿瓦提凹陷之间的分界。本文以大量的地质和地球物理证据,证明它是一条内部结构复杂且切穿地壳的深断裂。结合区域构造演化恢复了断裂发育史,指出它经历过三期重大的冲断活动,分别发生在加里东期、海西期和新近纪。新近纪的冲断与巴楚断隆的南界断裂带有相同的构造样式,可分为两幕:中新世的冲断受制于南天山前陆冲断带的向南扩展,更新世的冲断主要受制于西昆仑前陆冲断带的向北扩展。吐木休克断裂带的东西走向段和北西走向段的构造特征尚有若干差异,前者在加里东期活动较强,反映了构造的继承性;北西走向段则是加里东期出现的新生构造,在海西期进一步发展,更新世时因被卷入西昆仑的前陆冲断作用,表现出强烈而复杂的变形。  相似文献   

库车坳陷东部山前带古近系不同体系域内扇三角洲沉积砂体的对比研究   总被引：2，自引：0，他引：2       下载免费PDF全文

高志勇  郭宏莉  安海亭  朱如凯  张丽娟  孙玉善 《地质科学》2008,43(4):758-776

通过对塔里木盆地库车坳陷南天山山前带克孜勒努尔沟与依奇克里克野外露头沉积相的精细研究,结合山前带大量地震剖面分析与迪那201井、东秋5井等多口钻井测井相和岩心精细描述,将古近系库姆格列木群划分为2个层序。层序2低位域以冲积扇沉积为主,自湖侵体系域始,研究区广泛发育退积与进积型扇三角洲沉积。库姆格列木群层序2湖侵体系域内扇三角洲平原砂体以正韵律沉积为主,砂砾岩占地层厚度的(68.4～87)%;扇三角洲前缘砂体正、反韵律均有分布,砂砾岩占地层的百分比有所降低,为(55.2～77)%。高位体系域内主要发育以反韵律砂体为主的扇三角洲前缘沉积。古近系苏维依组只发育一个层序,其低位体系域的砂体为扇三角洲平原与前缘的分流河道砂体,湖侵体系域内扇三角洲前缘砂体正、反韵律均发育,高位体系域内主要发育扇三角洲前缘沉积,以反韵律砂体为主,正韵律砂体相对较少。对比苏维依组不同体系域内砂体储集性,湖侵域内砂体优于高位域内砂体。扇三角洲沉积体中有利储集砂体首选为湖侵体系域内的扇三角洲前缘水下分流河道和河口坝砂体,次之为低位域扇三角洲前缘(平原)分流河道砂体,最后是高位域内的河口坝及席状砂沉积砂体。特别要指出:低位体系域或者湖侵体系域底部发育的扇三角洲前缘(平原)分流河道砂体(底砂砾岩)被快速湖侵的较厚的暗色泥岩所覆盖,可形成较好储集场所。  相似文献   

喜马拉雅山脉的地质地貌特征:来自SRTM数字高程模型和降水量数据的约束   总被引：3，自引：1，他引：2       下载免费PDF全文

王猛  刘焰  何延波  魏东 《地质科学》2008,43(3):603-622

利用GTOPO30和SRTM3数字高程(DEM)数据,提取了喜马拉雅山脉(造山带)的数字高程模型并对其进行了地质地貌的初步分析。从SRTM3数字高程数据提取出坡度数据,初步分析了喜马拉雅山脉坡度和高程的特征。数字高程和坡度图清楚地展现了喜马拉雅大型断裂带(构造边界)的空间分布特征。分析了中国气象局下属的西藏、青海、四川和云南4省区气象观测台站55年来的年平均降水量观测数据、喜马拉雅山脉南坡的年平均降水量数据、喜马拉雅DEM和裂变径迹数据,发现喜马拉雅山脉从东至西,年平均降水量逐渐减少,地形起伏逐渐变小,而高程渐次升高,与此同时剥蚀速率降低;从北至南,年平均降水量逐渐增加,地形起伏增大,高程快速降低,而剥蚀速率则急剧升高。这充分说明了喜马拉雅年平均降水量大的地区,地表剥蚀作用相对较强,年平均降水量小的地区,地表剥蚀作用则较弱,即:在喜马拉雅地区,长周期的地表剥蚀过程(可长达数个百万年时间尺度)和短周期(仅仅50年)的降水量观测是耦合的。  相似文献   

柴达木盆地北缘侏罗系两类不同有机质丰度泥岩的 生物标志物特征对比研究     

杨少勇  曹剑  刘云田  胡凯  边立曾  汪立群  陈琰 《沉积学报》2008,26(4):688-696

在柴达木盆地北缘地区,分别选取有机碳含量很低和较高的侏罗系泥岩样品,对比分析了它们在生物标志物组成上的差异。结果发现,高有机质丰度泥岩的生标组成与我国西北地区侏罗纪煤系有机质的特征差异不大,相比而言,低有机质丰度泥岩的正烷烃以前主峰为特征,Pr/Ph比值在1.0左右,三环萜烷和伽马蜡烷丰度较高,并在部分样品中检出了25 降藿烷系列。结合泥岩的有机岩石学特征,认为这些差异可能反映了泥岩沉积环境和生烃母质的不同:高有机质丰度泥岩的有机显微组分以相对弱还原条件下的形态有机质为主,包括藻类体、孢子体和角质体等,而低有机质丰度泥岩的有机显微组分以相对强还原条件下的矿物沥青基质为主,其母质可能来源于低等显微菌藻类。进一步通过对比不同有机质丰度泥岩,以及区内原油生标组成之间的相互关系,讨论了研究区的油源问题。  相似文献   

延边东部五道沟岩群的单颗粒锆石SHRIMP U-Pb 年代学及其地质意义   总被引：1，自引：0，他引：1  

逄 伟孙景贵  门兰静陈 雷 陈 冬 常 艳 梁树能 张 朋 聂喜涛 《中国地质》2008,35(6):1178-1184

通过对延边地区东部五道沟岩群黑云阳起石片岩的单颗粒锆石SHRIMP U-Pb年龄测定.获得21个单点年龄,其中包括7组谐和年龄和1组不谐和年龄.谐和年龄分别为1347.8 Ma、844.8 Ma、340.2～313.7 Ma(平均值为323±23 Ma,N=4,MSWD=0.23,置信度=0.88)、292.9～288.3 Ma(平均值=291±25 Ma,N=3,MSWD=0.031.置信度=0.86)、279.2～266.2 Ma(平均值为279±28 Ma,N=4,MSWD=0.031,置信度=0.86)、127.4～124.2 Ma(206Pb/208U年龄平均值为126.5±3.7 Ma,N=5,MSWD=0.12;置信度=O.97)、116.1～106.3 Ma(平均值为115±39 Ma,N=2,MSWD=1.2置信度=0.27),不谐和年龄的下交点年龄为(451±120)Ma、上交点年龄为(1811±400)Ma(MSWD=7.2);这一结果表明:阳起石片岩的原岩主要是来自中元古代、新元古代和早古生代的碎屑物,指示五道沟群的沉积成岩作用发生在石炭世(323±23 Ma),变质作用发生在晚二叠世(291±25 Ma),之后在279.2～266.2 Ma、126.5～106.3 Ma先后受两次岩浆用和蚀变作用的改造.  相似文献