下地幔矿物研究及其进展

文中综述了20世纪90年代以来对下地幔矿物高温高压研究的进展,详细评论了下地幔温压下(Mg,Fe)SiO3钙钛矿的稳定性、(Mg, Fe)SiO3 钙钛矿和(Mg, Fe)O镁方铁矿的高压状态方程和热弹性及高压熔化、核幔边界温压下铁和硅酸盐的化学反应等几个热点问题;探讨了下地幔的矿物学组成,对下地幔的地震波速异常给出了可能的矿物学解释;介绍了国内同领域的研究工作;展望了下地幔矿物研究的发展方向。     

西藏蛇绿岩的超高压矿物：FeO、Fe、FeSi、Si和SiO2组合及其地球动力学意义

在西藏雅鲁藏布江蛇绿岩带的东段，出露罗布莎蛇绿岩块和豆荚状铬铁矿床。从豆荚状铬铁矿石中查明60－70种伴生矿物，其中包含FeO、Fe、FeSi、Si和SiO2组合。根据超高压－高温实验，该组合应形成于地球外核与下地幔之间的D“层，是地球外核的液态铁与镁硅酸盐钙钛矿（MgSiO3)相互化学反应的产物。西藏该超高压矿物组合揭示了蛇绿岩地幔活动可能深达地球外核。罗布莎蛇绿岩的该矿物组合可能是地幔－外地核之间的产物，或者是被对流作用，亦是被起源于D“层的地幔柱活动带到上地幔的。铬铁矿在地幔中结晶，并捕获了该矿物组合。     

地球下地幔矿物结构和热力学参数的研究进展与展望

下地幔从660 km到2 891 km深度,占据整个地球质量的49.2%并处于极端高温高压的状态。在下地幔相应的温度压力条件下研究主要构成矿物的物理性质,尤其是结构、密度和波速,是理解下地幔结构、物质组成以及动力学过程的关键。通过回顾过去30年高温高压矿物学实验对下地幔矿物,包括布里基曼石、铁方镁石、Ca-钙钛矿以及硅酸盐—后钙钛矿结构和热力学状态方程的重要研究进展,探讨温压条件变化、成分变化以及Fe自旋变化对这些下地幔矿物(相)密度和体波波速的影响,指出现有研究结果的不足和需要解决的问题,并对未来的研究方向提出展望。     

钙钛矿型硅酸盐的热膨胀及地幔分层

位于地下670公里深处的地震不连续带是上、下地幔的分界面,其压强为24GPa,温度为2—3千K。在这样的条件下,上地幔的主要矿物——橄榄石〔(Mg、Fe)_2SiO_4〕、辉石〔(Mg、Fe)SiO_3〕和石榴石〔(Mg、Fe、Ca)_3Al_2Si_2O_2〕将转变为斜方畸变的具钙钛矿结构的〔(Mg、Fe)SiO_3〕矿物或以钙钛矿为主的矿物组合。由于钙钛矿型的硅酸盐至少在70GPa压力下还是稳定的,因而它被认为是下地幔(或许是整个地球)中最主要的矿物。尽管钙钛矿型的硅酸盐很重要,但直到1976年才被发现,而且由于合成这种矿物所需的压力和温度条件很难达到,所以对其物理性质还不甚了解。例如,钙钛矿热膨胀系数对下地幔可能存在的成分模式起着重要的制约作用。最近我们合成了足够量的钙钛矿型的(Mg_0.9,Fe_0.1)SiO_2以便用X射线衍射法测定其在温度达840K时的无压热膨胀。在高温下测得的平均热膨胀系数为4×10~(-6)K~(-1)。这么大的热膨胀系数表明:根据土地幔组分(如地幔岩或石榴石橄榄岩,Mg值≈0.89)的标准模式求得的无压密度,要比推断的下地幔在无压条件下的密度低大纣2%。这一结果显示:上地幔与下地幔的化学性质是不同的;这与地幔热对流的分层模式是一致的。     

地幔过渡带和下地幔组成与深部热源研究进展

受时空不可及性的制约,地质学家在探究地球深部物质组成方面仍显得很被动,尤其是在探究地幔物质组成方面显得更加艰难.目前,科学家们探测地幔物质主要依靠地球物理学和实验矿物学、岩石学方法相结合的手段来进行.结果表明,地幔过渡带主要的矿物组成有瓦士利石、林伍德石、超硅石榴子石以及少量的CaSiO3.下地幔主要矿物组成有钙钛矿(Pv)、后钙钛矿(PPv)和镁方铁矿(Mw).在讨论过渡带和下地幔物质组成的基础上,归纳总结了地球内部热源的三种来源,分别是放射性元素的衰变热和初始熔融硅酸盐地球长期冷却放出的热、核幔边界在地磁场和高电导率物质的作用下产生的热以及来自地核的热.这些结论对研究地球深部动力学和热力学过程有重要意义.     

下地幔矿物及生成条件

据CatherineMcCammon等报道,在巴西Luiz河发现的金刚石中含有一些矿物包裹体,其中有（Mg,Fe,Al）（Si,Al）O3,（Mg,Fe）O,CasiO3和四面体的铁铝-镁铝榴石混合物（tetragonalalmandine－pyropephase,下简称为TAPP）,通过研究其氧化状态,就能了解一些下地幔的信息。含有包裹体的金刚石来自下地幔。通过对其内部包裹体周围的微裂隙检查,包裹体内部矿物压力测定及同位素研究,可以证明金刚石在后来的地质作用中未改变其化学成分。包裹体中的（Mg,Al）（Si,Al）O3在下地幔可能以钙钛矿的形式存在,而TAPP则是其原始相。…     

下地幔布里基曼石的铁自旋态及其地球物理意义

<正>根据地幔岩模型,下地幔主要由于布里基曼石[(Mg,Fe)(Fe,Si,Al)O3)]、铁方镁石[(Mg,Fe)O]、Ca-钙钛矿组成。Badro et al.,2003利用X光发射光谱发现,下地幔铁方镁石中的铁在高压下会经历电子的自旋转变[1]。随即发现,铁在下地幔矿物中的自旋转变会带来一系列物理性质的改变,如密度、弹性模量、波速、热传导系数以及电导率等[2]。在过去的十几年中,研究铁在下地幔矿物中的自旋态,以及自旋转变对下地     

黑金刚石:早期地壳岩石受碰撞产生的金刚石集合体

黑金刚石是产于砂矿和低级变质岩中的不规则多晶金刚石集合体。它以细小的粒度、轻碳同位素比率、范围广泛的地壳物质(硅酸盐、磷酸盐、氧化物)包体、以及不存在与来自上地幔金刚石共生的典型矿物为特征。我们设想,所有这些特征,以及在同一个黑金刚石中还存在的六方金刚石,可用含有机碳或次生石墨的地壳岩石(包括太古代和元古代的)的碰撞变质作用来解释。     

黑金刚石可能是早期陨石冲击地壳岩石形成的金刚石集合体

黑金刚石(Carbcnado)是不规则的金刚石多晶集合体,产于砂矿和低级变质岩中.其特征为细粒.碳同位素比值低,含多种地壳物质的包裹体(硅酸盐、磷酸盐、氧化物),缺乏与上地幔金刚石伴生的特征性矿物.我们认为:所有这些特征以及黑金刚石中六方金刚石的存在,均可能是含有机碳或含石墨派生物的地壳岩石(包括太古代和元古代的岩石)经冲击变质作用的结果.     

华北板块北缘中段早石炭世构造属性:察哈尔右翼后旗高分异Ⅰ型花岗岩地球化学的制约

察哈尔右翼后旗二长花岗岩岩体位于华北板块北缘中段.岩体富SiO₂、富K、富碱、低Ca和P、贫Fe和Mg,w(P₂O₅)与w(SiO₂)呈负相关,铝指数(A/CNK)为0.96~1.15,分异指数为90.36~92.96;主要造岩矿物为条纹长石、斜长石和石英,其铁镁矿物主要为黑云母,未出现碱性铁镁矿物和富铝矿物,副矿物为锆石、磁铁矿、磷灰石、榍石和钛铁矿;可见察哈尔右翼后旗二长花岗岩属高分异钙碱性I型花岗岩类.结合较低的w(∑REE)((46.8~94.4)×10^-6)、w(Th)((1.74~2.39)×10^-6)、w(U)((0.27~0.39)×10^-6)和微量元素判别图解,二长花岗岩岩浆源区可能为下地壳,源岩可能为岩石圈地幔.岩石轻重稀土分馏较强 ((La/Yb)_N= 6.52~28.39),δEu以正异常为主(0.83~3.51),富集大离子亲石元素(LILEs,Cs、Rb、Ba和K),亏损高场强元素 (HFSEs,Nb、Ta、P和Ti).地球化学特征反映了二长花岗岩具有火山弧岩石特征,且岩体侵入新元古界埃迪卡拉系什那干群,表明岩体形成于活动大陆边缘弧环境,其侵位反映了古亚洲洋的俯冲岩浆事件.     

http://www.sciencedirect.com/science/article/pii/S1674987111000909

We have performed measurements of minerals based on the synchrotron source for single crystal and powder X-ray diffraction,inelastic scattering,spectroscopy and radiography by using diamond anvil cells.We investigated the properties of iron（Fe）,iron-magnesium oxides（Fe,Mg）O, silica（SiO₂）,iron-magnesium silicates（Fe,Mg）SiO₃ under simulated high pressure-high temperature extreme conditions of the Earth’s crust,upper mantle,low mantle,core-mantle boundary,outer core, and inner core.The results provide a new window on the investigation of the mineral properties at Earth’s conditions.     

中国辽宁金刚石中高硅钙铁榴石(Majorite)等超高压矿物包裹体的发现及地质意义

在中国辽宁金刚石中获得的高硅钙铁榴石(Majorite)为一单晶碎片包裹体,与其共存的金刚石包裹体还有刚玉、碳化钛、红色金刚石碎片、钙钛矿、二氧化硅等。通过电子探针成分分析,确定该高硅钙铁榴石(Majorite)成分超硅高钙缺镁,8个分析点平均值计算的矿物分子式为(Ca2.35Fe0.49Mn0.15Mg0.02)3.01(Al1.08Fe0.48Si0.44)2.00(SiO4)3,根据Kenneth等(2000)提出的计算压力的公式得到该包裹体高硅钙铁榴石(Majorite)形成的压力为14GPa,估算形成深度达400km。用四圆单晶衍射仪测定了该石榴石的晶体结构,a=1.195 15(4)nm,求得了各原子的座标、占位度和各向异性温度因子,用I>2σ(I),计算得到R1=0.077 9,WR2=0.141 6,Goodness-of-fit(F2)=1.382。在该高硅钙铁榴石包裹体中还存在微米级二氧化硅(呈四边形断面)和氧化铁(含钠)的包裹体(析离体),它们可能是斯石英和方铁矿(或似沂蒙矿)。从高硅钙铁榴石(Majorite)的成分判断,其物质来源具壳源性质,由此推断的大陆壳俯冲深度要超过400km,这与地球物理探测郯庐断裂已切穿了上地幔、进入软流圈的看法相一致。     

Melting relations and major element partitioning in an oxidized bulk Earth model composition at 15–26 GPa

Melting experiments were performed on an FeO-rich bulk Earth model composition in the CMFAS system in order to investigate the partitioning of major elements between coexisting minerals and melts. The starting material (34.2% SiO₂, 3.86% Al₂O₃, 35.2% FeO, 25.0% MgO and 1.88% CaO), contained in Re-capsules, was a mixture of crystalline forsterite and fayalite, and a glass containing SiO₂, Al₂O₃, and CaO. Olivine is the first liquidus phase at 10 GPa but is replaced by majoritic garnet (ga) in the 15–26 GPa range. Magnesiowüstite (mw) crystallizes close to the liquidus and is joined by perovskite (pv) at 26 GPa.

The quenched melt compositions are homogeneous throughout the melt region of the charges and are only slightly enriched in Si, Ca and Fe, and depleted in Mg, relative to the starting composition. The Fe/Mg and Ca/Al ratios in all of the minerals increase rapidly below the liquidus to become compatible with the bulk composition at the solidus. At 26 GPa, a relative density sequence of mw>pv>melt>ga is observed. This indicates that majorite floating, combined with the sinking of magnesiowüstite and perovskite can be expected during the solidification of a Hadean magma ocean and in hot mantle plumes early in the Earth's history. The mineral–melt partitioning relations indicate that fractional crystallization or partial melting in the transition zone and the upper part of the lower mantle would increase the Fe/Mg and Ca/Al ratios of the melt, even if magnesiowüstite was predominant in the solid fraction. A significant contribution of accumulated mw to the segregation of the protocore is therefore unlikely. The suggested process of perovskite fractionation to the lower mantle is not capable of increasing the Mg/Si ratio in the residual melt, and the combined fractionation of perovskite and magnesiowüstite produces a melt with elevated ratios of Si/Mg, Ca/Al and Fe/Mg.     

西藏泽当地幔橄榄岩中的异常矿物及其指示意义

雅鲁藏布江蛇绿岩带内多个地幔橄榄岩体产有金刚石、碳硅石等异常矿物组合,为了进一步探讨这些异常矿物形成的物理化学条件,在前人已有的研究基础上,对泽当地幔岩体中526 kg的方辉橄榄岩样品开展人工重砂矿物学研究工作。研究表明,同雅鲁藏布江蛇绿岩带内的其他岩体相似,泽当地幔橄榄岩也选出了包含金刚石、碳硅石、锆石等30余种矿物。异常矿物组合指示泽当地幔橄榄岩中存在局部的超高压、极还原环境,可能经历了复杂的演化过程:即古老地壳物质通过深俯冲或者折沉作用,进入地幔甚至是地幔过渡带(410~660 km),随后经历了超高压、极还原环境的改造,在后续的地幔柱或地幔对流作用中从洋中脊上升至浅部环境并返回到地壳中。该过程中地幔橄榄岩中的异常矿物组合记录了岩石的演化信息,因此开展地幔橄榄岩中异常矿物组合的精细矿物学研究,对认识壳-幔物质交换以及深部地幔动力学过程都有重要的研究意义。     

An alternative interpretation of lower mantle mineral associations in diamonds

Diamonds containing ferropericlase (Mg,Fe)O and other silicate (enstatite [(Mg,Fe)SiO₃], in particular) assemblages are generally believed to be derived from the Earth's lower mantle. On the basis of the observed ratio between ferropericlase and enstatite inclusions and the FeO content of these ferropericlases, it is concluded that most of these minerals entrapped in diamonds may not represent the lithology of the lower mantle itself as has been suggested by many investigators. Instead, ferropericlases in these diamonds represent most likely the disproportionate product of ferromagnesite [(Mg,Fe)CO₃], which underwent a decarbonation reaction to form both diamond and ferropericlase simultaneously in the lower mantle. The wide variation in the Mg# of ferropericlase inclusions in diamonds is attributed to the decarbonation "loop" of the MgCO₃-FeCO₃ solid solutions. Some of the enstatite inclusions coexisting with these ferropericlases in the same diamond may represent the most abundant mineral species of (Mg,Fe)SiO₃-perovskite in the lower mantle. The latter mineral phase experienced a retrogressive transition into enstatite during the transport of diamonds to the Earth's surface.     

同步辐射激光加温DAC技术及在地球深部物质研究中的应用

实验室模拟地球深部的温度和压力环境,研究地球相关材料的物理和化学性质,是解释地震波数据、进一步了解地球内部结构和动力学过程的重要途径。用高功率的红外激光光束,加温金刚石对顶砧压腔(DAC)中的样品,可以获得深部地幔乃至地核的极端温度和压力条件,已广泛地用于地球深部矿物的相变、熔融和状态方程研究。同步辐射微束技术的发展,为激光加温DAC技术的应用开辟了新的领域,也使地幔及地核条件下的矿物研究有了重要的突破。文章介绍激光加温DAC技术的发展;阐述高温高压原位的同步辐射X射线衍射方法;例举激光加温DAC技术在地球深部物质研究中的一些应用;并对一些关键的技术问题加以分析和讨论。     

山东蒙阴金伯利岩中铬铁矿电子探针波谱分析

铬铁矿是金伯利岩型金刚石矿床中主要的指示性矿物之一.准确分析出铬铁矿化学组分中FeO、MgO、Cr203、Al203、Ti02含量,不但对铬铁矿定名起决定性作用,对金伯利岩型金刚石矿床找矿也具有重要的指示意义.利用电子探针波谱技术对铬铁矿主量化学元素分析,通过所测样品微区化学成分含量推测矿物名称.25件单矿物样品微区化...     

Petrology and geochemistry of shoshonitic plutons from the western Kunlun orogenic belt, Xinjiang, northwestern China: implications for granitoid geneses

A series of granitoids from Proterozoic to Cenozoic age occurred in the western Kunlun orogenic belt, Xinjiang, northwestern China. Several intrusions such as the West Datong (Middle Caledonian age), North Kuda (Late Caledonian age) and Kuzigan, Karibasheng, Zankan (Himalayan age) plutons have shoshonitic affinity. Their rock assemblages include (quartz) monzodiorite–(quartz) monzonite–quartz syenite (Middle Caledonian) or monzonitic granite–granite (Late Caledonian) or biotite (monzonitic) granite–diopside granite–diopside syenite (Himalayan). Generally, biotite is iron–phlogopite, with some eastonite and high Mg/(Mg+Fe_T) and Fe³⁺/Fe²⁺ ratio. Amphibole is mainly edenitic hornblende and magnesian hastingsitic hornblende, with some edenite and higher Mg/(Mg+Fe_T) and Fe³⁺/Fe²⁺ ratio. The rocks show SiO₂ contents of 52.77–71.85% and high K₂O+Na₂O (mostly >8%, average 9.14%), K₂O/Na₂O (mostly >1, average 1.50) and Fe₂O₃/FeO (0.85–1.51, average 1.01) and low TiO₂ contents (0.15–1.12%, average 0.57%). Al₂O₃ contents (13.01–19.20%) are high but variable. The granitoids are prominently enriched in LILE, LREE and volatiles such as F. However, the studied shoshonitic granitoids among the three intrusive periods also show differences in isotopic compositions and trace element concentrations, suggesting their different geneses: the origin of the West Datong pluton is probably related to the involvement of subducted oceanic crust sediments into the mantle source; the North Kuda and Himalayan plutons could have been generated by partial melting of subducted oceanic crust sediments or metasediments of thickened continental lower crust in the process of late-orogenic slab break-off or lithospheric thinning.     

TEM observations of several spinel-garnet assemblies: toward the rheology of the transition zone

In order to better identify the mineral phase which controls the rheology of the transition zone (between 410 and 660 km depth) transmission electron microscopy observations were made on several coexisting spinel-garnet assemblies: alkremite xenolith; pyrope-rich – MgO:1.1Al₂O₃ spinel assembly deformed at 1173K, 800 MPa in a Griggs apparatus; (Mg,Fe)₃(Al,Mg,Si)₂Si₃O₁₂ majorite – (Mg,Fe)₂SiO₄ spinel assembly synthesized in a laser heated diamond anvil cell. It was found that garnet crystals systematically remain undeformed while spinel crystals are plastically deformed. These results are in accord with the assumption that the rheology of majorite is stronger than the rheology of spinel, in the conditions of the transition zone.