碳酸盐化橄榄岩的电性研究

为进一步探讨上地幔的高导层成因,了解碳酸盐在上地幔电性方面的作用并估算上地幔高导层的碳酸盐含量,本文对不同碳酸盐含量的橄榄岩及玄武岩样品在2~3 GPa、300~1300℃的条件下进行了电性实验研究.研究初步发现:碳酸盐熔体显著增强橄榄岩、玄武岩样品的导电能力;单纯用含硅酸盐熔体的橄榄岩或单纯用含水橄榄岩可能难以解释上地幔某些区域的异常高导现象;同样,单纯用碳酸盐化的橄榄岩可能也难以解释上地幔某些区域的高导现象;上地幔的高导区很可能是碳酸盐熔体、硅酸盐熔体及水的共存区域.     

俯冲隧道中的部分熔融和壳幔相互作用:实验岩石学制约

俯冲隧道模型提出,俯冲板片界面相互作用是实现地球表层与内部之间物质和能量交换的基本机制.由于大陆岩石圈与大洋岩石圈在物质组成和状态上的显著差异,其深部物理和化学过程及壳幔相互作用产物必然出现一系列差异.许多实验岩石学研究已经为大洋俯冲隧道中可能发生的硅酸盐和碳酸盐岩石的部分熔融和壳幔相互作用提供了资料.无论是基性还是中酸性硅酸盐岩体系,取决于部分熔融发生的压力或深度,熔体是具有或不具有埃达克岩性质的花岗质熔体.微量CO2即可大幅降低橄榄岩的熔点,所形成的碳酸盐熔体可有效萃取岩石体系中不相容微量元素.这些硅饱和或不饱和熔体均可以在俯冲隧道或地幔深部条件下与地幔楔橄榄岩发生反应,形成复杂的反应过程和产物.但已有的实验结果主要是针对大洋岛弧环境条件而不是大陆俯冲带的环境.因此,高温高压实验需要充分考虑大陆俯冲隧道中板片-地幔界面上各种不同成分地壳及其衍生的熔/流体成分与不同橄榄岩之间的反应,并结合大陆俯冲带岩石部分熔融和壳幔相互作用的地质证据,以阐明大陆俯冲隧道过程中的变质脱水、部分熔融和地幔交代等问题.     

俯冲带中的水迁移

水从俯冲地壳迁移到地幔主要受地壳中含水矿物的稳定性支配,而俯冲带的热结构是决定俯冲地壳在哪个深度发生脱水的关键.大洋俯冲带的地温梯度变化很大,既有冷俯冲带也有热俯冲带,但是地震活动和弧火山作用在冷俯冲带相对突出.大陆俯冲带的地温梯度较低,地壳岩石总是在冷俯冲带发生变质作用,但是缺乏同俯冲弧火山作用.超冷俯冲带地温梯度很低(?5℃/km),俯冲地壳中的硬柱石可以把水带到?300km的深度.热俯冲带地温梯度很高(25℃/km),俯冲地壳在浅部就大量脱水,在80km的深度会产生长英质熔体.由于水大量溶解在这种熔体中,结果只有少量的水会运移到80~160km的弧下深度.在这两种脱水方式之外还存在大量介于两者之间的方式,使得俯冲带表现出多种水迁移现象.在暖俯冲带,低温/低压含水矿物在俯冲到60~80km的弧前深度时就发生分解,释放出大量的水.在冷俯冲带,低温/低压含水矿物随俯冲深度增加转变成低温/高压含水矿物,允许大量的水被迁移到弧下深度.无论在何种情况下,俯冲地壳的脱水不仅启动了地震活动,而且引起了地幔楔的水化.不过,总有少量水被超高压含水矿物和名义上无水矿物带至更深部地幔.俯冲板片之上的地幔楔并没有因为水的加入而立即发生部分熔融引起弧火山作用,而是首先在板片-地幔界面上发生水化.由于这里温度最低,比水化橄榄岩的湿固相线要低几百度,结果直到水化橄榄岩受到加热之后才能发生部分熔融.因此,弧火山作用一般发生在地幔楔被流体交代之后的某个时间.     

中国东部大陆岩石圈地幔及幔源岩浆岩水含量和氢同位素组成

中国东部华北陆块和华南陆块的地幔包体单矿物和全岩水含量变化很大,总体低于全球其他克拉通和非克拉通岩石圈地幔包体单矿物和全岩水含量.然而,具有新生岩石圈地幔来源的莒南橄榄岩包体单矿物和全岩水含量总体高于其他中国东部岩石圈地幔包体,指示中国东部新生岩石圈地幔的初始水含量并不低.地幔包体中单矿物水含量与Mg~#之间无明显相关关系,其中的辉石缺乏水扩散环带,因此地幔包体的低水含量与包体随玄武岩岩浆上升过程的水扩散丢失无关.大陆岩石圈地幔底部受到热软流圈地幔烘烤有可能会造成水的扩散丢失,但是熔体提取也可能是引起低水含量的原因之一.通过斑晶水含量计算得到的中国东部中生代和新生代玄武岩初始熔体水含量均高于正常洋中脊玄武岩.中生代玄武岩初始熔体水含量与岛弧玄武岩类似,而新生代玄武岩初始熔体水含量则与洋岛玄武岩和弧后盆地玄武岩类似(部分地区与岛弧玄武岩类似).这些结果表明,大陆玄武岩地幔源区相对富水,指示其地幔源区曾经受到深俯冲地壳脱水所形成的富水流体/含水熔体的交代,使其水含量升高.中国东部新生代幔源巨晶、包体矿物和玄武岩斑晶的氢同位素组成变化也很大,反映中国东部新生代岩石圈地幔氢同位素组成高度不均一.新生代岩石圈地幔具有高于亏损地幔的水含量和偏离亏损地幔值的氢同位素组成,反映其受到过俯冲太平洋板片部分熔融所产生熔体的交代.俯冲大洋板片脱水熔融产生的富水流体和含水熔体对大陆岩石圈地幔底部的交代导致其水含量增加,引起底部岩石黏滞度降低,进而导致岩石强度的降低,使其容易被构造侵蚀乃至拆沉.因此,大洋俯冲隧道中的壳幔相互作用是克拉通岩石圈减薄的重要诱因.     

俯冲带橄榄岩及其记录的壳幔相互作用

俯冲过程是板块构造运动的核心过程,而地幔楔作为俯冲系统中连接俯冲盘和仰冲盘的关键构造单元,在地球层圈之间物质循环和能量交换等方面发挥了重要作用.本研究汇总了全球代表性俯冲带橄榄岩(包括俯冲带型蛇绿岩和地幔楔型造山带橄榄岩)的研究现状,并展望未来需要解决的关键科学问题.俯冲带型蛇绿岩地幔单元和地幔楔型造山带橄榄岩分别代表着大洋和大陆俯冲带侵位的地幔岩石,是研究俯冲带壳幔相互作用的关键对象.该相互作用的本质是俯冲板片和地幔楔之间在物理过程主控下发生复杂的化学交换作用.俯冲带型蛇绿岩能够记录从大洋岩石圈产生到俯冲启动直至成熟到消亡等不同阶段复杂的熔-岩和水-岩相互作用、变形变质过程、金属成矿元素富集以及壳幔物质交换等.地幔楔型造山带橄榄岩则反映洋-陆和陆-陆俯冲/碰撞、折返等阶段强烈的变形变质历史,多种性质的熔/流体交代作用(硅酸盐熔体、碳酸盐熔体、含硅酸盐组分的C-H-O流体/超临界流体),以及复杂的壳幔物质循环过程等.利用俯冲带橄榄岩进一步探索壳幔相互作用,需要采用高空间分辨率、高精度的测试方法从微观尺度上约束复杂的化学交代过程和变质变形历史,并与宏观构造的时、空演化相联系.     

大地幔楔与克拉通破坏型金矿

环太平洋俯冲带是全球著名的金矿成矿域,其中华北克拉通和内华达是环太平洋俯冲带最重要的两个金成矿省,其成因分别与华北克拉通和怀俄明克拉通的破坏有关,成矿流体可能与俯冲板块在地幔过渡带的滞留相关.俯冲的大洋岩石圈地幔通常有数千米厚的蛇纹石化层,在板块俯冲过程中,蛇纹石在小于200km的深度内脱水转变为高压含水相——phase A,将水带到大于300km的深处.当滞留在地幔过渡带的俯冲板片中含水高压相脱水,会导致上覆大地幔楔的水化,这是克拉通破坏型金矿形成的先决条件.俯冲板片脱水释放出的富硫流体萃取周围岩石中的金等亲硫元素,形成富金流体.由于克拉通地温线低于二辉橄榄岩水饱和固相线,流体在克拉通岩石圈地幔内向上运移到100km以浅后交代岩石圈地幔,形成韭闪石等含水矿物,在克拉通岩石圈地幔中形成富水、富金的弱化层.随着克拉通岩石圈的破坏,该弱化层失稳,释放含金流体,并沿地壳浅部薄弱带迁移、聚集和沉淀,形成爆发式金矿床.由此可见,大地幔楔是形成克拉通地幔含矿弱化层,进而导致金爆发式成矿的关键.俯冲板片析出的流体富含二价铁,流体向上运移过程中,二价铁在低压下发生水解,释放出氢气;地幔楔岩浆岩可以具有较高的氧逸度,而成矿流体则是还原性的——即克拉通破坏型金矿往往属于还原性矿床.     

上地幔流变性质的研究进展

本文主要总结了近十多年来橄榄岩流变实验的研究成果,介绍了上地幔流变研究的进展和意义.经过多年的实验研究,橄榄岩流变性质在理论和应用方面都已取得重要突破.人们利用高温高压实验深入研究了熔/流体、铁含量、粒径尺度、温压条件等因素对橄榄石流变性质的影响,得到了表征橄榄石流变特性的幂率本构方程的各参数,并从理论方面探讨了橄榄石等矿物在不同深度处可能存在的变形机制.在应用方面,高温高压流变研究为许多地质现象(如软流圈的成因)的解释和地球动力学(如地幔对流)的模拟提供了实验依据和基础数据,如辉石的粒度效应可以解释上地幔板块边界变形及剪切局域化问题等.尽管对上地幔矿物的流变特性进行了深入地研究,但还有许多问题需要人们进一步厘定,如碳酸盐熔体对橄榄石流变性质的影响,橄榄石变形机制的转化等.     

中国大陆地壳上地幔电性结构及地震分布规律(一)

本文是壳慢流体演化及地震成因系列论文的第1篇．本系列由3篇论文组成，第2篇为壳内高导层成因，高温高压下卤水物态及其电导率；第3篇为壳内流体演化及地震成因．这3篇论文分别从宏观、微观和动态变化三方面，探讨了地下流体与地震的关系． 本文根据我国大地电磁测深(MT)工作者近20年的探测成果，编绘了中国大陆壳内和上地幔高导层的分布埋深图，分析了中国大陆电性结构的分区特征，探讨了上地幔高导层的成因以及壳内高导层的特征，最后总结了中国大陆不同构造区的地震孕育规律．      

大洋俯冲带变质作用、流体行为与岩浆作用

依据有效的实验岩石学和相平衡模拟结果、结合俯冲带热结构模型,讨论了大洋地壳中的基性岩、沉积岩和超基性岩在不同俯冲阶段发生的脱水和熔融作用,及其对俯冲带岩浆作用的影响.大多数洋壳在弧前(90～100km)俯冲阶段基性岩和超基性岩脱水很少,明显脱水作用发生在表层沉积物中.在弧下俯冲阶段基性岩和超基性岩都发生强烈脱水,如基性岩中90%以上的水是由绿泥石、蓝闪石、滑石和硬柱石相继在弧下100～200km深度分解释放的,这与以往基于实验模拟得到的结果大不相同;超基性岩在弧下120～220km深度发生叶蛇纹石、绿泥石和10?相脱水;但变质沉积岩在弧下深度对流体贡献不大,其主要含水矿物为多硅白云母,可以一直稳定至300km深处分解成钾锰钡矿,多硅白云母分解后直到地幔过渡带深度俯冲洋壳板片不再有明显流体释放.在少数热俯冲带中,变质沉积岩和基性岩都可以发生部分熔融(尤其是水化熔融)形成富水花岗质熔体或超临界流体,含碳酸盐矿物的沉积物可以熔融形成含钾碳酸岩熔体.在少数冷俯冲带中,超基性岩中出现A相,可把水带至地幔过渡带深处.俯冲板片特别是沉积物可以携带很多强不相容的次要元素和微量元素,通过板片流体影响俯冲带岩浆岩的地球化学成分.在弧下俯冲阶段,俯冲带的地热梯度不穿过碳酸盐化榴辉岩和橄榄岩的固相线,其中的碳酸盐矿物可被携带到深部地幔.碳酸盐化榴辉岩会在400km深度发生熔融形成富碱的碳酸岩熔体,而碳酸盐化橄榄岩则不会在俯冲带下部的地幔过渡带中发生熔融.     

青藏高原地壳高导层的成因及动力学意义探讨——大地电磁探测提供的证据

大地电磁探测结果显示,青藏高原的中下地壳普遍存在大规模的高导层,其电阻率低于10 Ωm,远低于稳定构造区地壳的平均电阻率值.通过对可能形成地壳内大规模低阻异常地质原因的分析认为,青藏高原地壳中的高导层不可能是由金属矿、石墨层或者单纯的含盐水流体引起的,而很可能是由于地壳岩石的部分熔融或者是部分熔融与含盐水流体共同导致的.这些高导层的形成是与板块运动等动力学过程密切相关的.地壳内的高导层可能是印度板块和亚洲板块俯冲的电性痕迹,其成因与板块俯冲过程中由于摩擦生热导致的岩石部分熔融和脱水作用有关,也可能与岩石圈拆沉造成幔源物质上涌有关.沿高原内主要缝合带均存在东西向连续分布的大规模高导体,有可能是青藏高原下地壳物质向东"逃逸"的证据;其中班公-怒江缝合带可能是最重要的物质运移"通道".     

The electrical conductivity of the Moon

Investigations of the lunar electrical conductivity were described in excellent reviews by Sonett (1974) and Dyalet al. (1976). In this paper we will try to consider some new aspects of this problem in comparison with the Earth's data.     

Global electrical conductivity of the earth

The Banks (1969, 1972) and Parker (1970) models of the electrical conductivity distribution are critically reviewed along with classical models by Chapman (1919), Lahiri and Price (1939), Rikitake (1950a, b, c) and others. The modern models do not seem to account for the geomagnetic variations having a continuum spectrum and S_q at the same time. A large difference in response between the 1-day and 0.5-day period components of S_q is suspected to be caused by a resonance-like induction in the superficial layer of the earth. Dufficulties in determining the conductivity of the earth's top layer are also emphasized.An overall distribution of conductivity within the earth which seems to be the most reliable at present, is drawn mostly on the basis of Banks' model.     

Integral equation of relationship on the apparent conductivity and true conductivity in induction logging

This paper investigates the relation between the induced electromotive force measured by induction logging tool and the apparent conductivity, and the relation between the apparent conductivity and the formation true conductivity. Assuming the conductivity in Green’s function to be the function of the field point coordinate, the apparent conductivity expression of electric-field intensity is derived using Green’s formula, and the integral equation has been established representing the relationship of the apparent conductivity with the true conductivity under this condition. The integral equation is analyzed and then leads to the conclusion that the equivalent conductivity is the apparent conductivity and the values of the apparent conductivity function contain the true conductivity, and the method derived the true conductivity from the apparent conductivity around the well axis is put forward. The validity of the approach given in this paper is verified through numerical calculation. On the basis of above means, the transmitter coil produces an electric-field distribution in formation at every point where the induction logging tool moves along a wellbore, and a number of the receiving coils obtain an apparent conductivity distribution; this is what induction electric-field logging is.     

混合物整体电导率的研究

研究由几种导体成分掺杂的混合物的整体电导率.对欧姆定律求平均,得到混合物电导率定义.对电流连续性方程求平均,得到混合物中电场增量方程.求电场增量方程在同种成分上的平均,并结合混合物电导率定义,得到混合物电导率公式.现有的三种混合物结构下电导率公式（电导率串联公式、并联公式和整体各向同性混合物电导率公式）都是混合物电导率公式的特例.进一步分析得出结论,混合物整体电导率是各成分电导率与整体电导率结构并联后的体积串联.     

Electrical conductivity anomalies in the earth

Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used.Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure.Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.     

Temperature-time dependence of the electrical conductivity of garnets

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Bounds on the complex conductivity of geophysical mixtures

Following previous work on bounds for complex dielectrics, bounds on the complex conductivity of a mixture of two isotropic components can be developed which are independent of any special assumption concerning the geometry of the mixture. If certain broad restrictions are assumed, such as isotropy of the mixture, then the bounds can be made more restrictive. These bounds reveal the range of the induced polarization response which can be caused by a mixture of two materials of known complex conductivity. The bounds can also be generalized for spectral responses. The bounds are conservative lithologically in the sense that many of the special models corresponding to boundary responses have lithological counterparts. The chief use for the given bounds is to gain insight into the nature of the induced polarization response. It is also possible to use the bounds to estimate the volume fractions of the components. We illustrate how this is done for the case of a general anisotropic medium.     

Models of electric conductivity in the upper mantle

Summary The accessible data of the apparent resistivity _T(T) were collected and supplemented by the spectral analysis of daily means of geomagnetic components observed at world geomagnetic observatories. The data were least-squares fitted; four models of the resistivity in the upper mantle which agree with the experimental data were obtained by solving the inverse problem using Marquardt's gradient-expansion algorithm.