地球物理学进展  2017, Vol. 32 Issue (6): 2281-2294   PDF    
地壳岩石矿物电导率实验研究进展
陈进宇, 杨晓松     
中国地震局地质研究所, 地震动力学国家重点实验室, 北京 100029
摘要:矿物和岩石电导率的实验研究一直都是国内外专家学者所关注的重点内容,目前电导率的实验测量主要采用交流阻抗谱的方法.从实验结果来看,地壳矿物和岩石电导率的影响因素主要分为外部环境和内部性质两部分,包括矿物和岩石所受到的温度和压力大小、内部赋存的流体和熔体、岩石颗粒边缘的碳膜、岩石的颗粒大小和面理方向以及矿物晶格赋存的结构水与晶格方向等.结合地球物理探测结果,前人对下地壳高导层成因提出了各自不同的看法,同时电导率的实验数据对于解释地下岩层结构、动力学特征也带来了一定的帮助.通过总结可以发现对于断层带的电导率性质目前仍然知之甚少,在今后的实验研究中则需要重点关注.
关键词地壳    岩石矿物    电导率    交流阻抗谱    影响因素    电性结构    
Review of experimental studies on electrical conductivity of crustal rocks and minerals
CHEN Jin-yu , YANG Xiao-song     
State Key Laboratory of Earthquake Dynamics, Institute of Geology, China Earthquake Administration, Beijing 100029, China
Abstract: The electrical conductivity of minerals and rocks is a research hotspot for the petrophysicists. AC impedance spectroscopy is the primary method applied in electrical conductivity measurement. For the experimental results, the influential factors of the electrical conductivity of minerals and rocks in the crust are divided into two parts:the factors of external environment and the factors of interior properties. These factors contain temperature, pressure, saline fluid, partial melting, grain boundary carbon films, grain size, the direction of foliation, water contents of minerals, orientation-related anisotropy of minerals and so on. Combining with the data of Magnetotelluric (MT), scientists have proposed many conclusions to interpret the presence of highly conductive layers, the structure of stratum and geodynamical features of the earth. Finally, we can find that the electrical conductivity of the fault zones in the upper crust is still unclear so far. In the future, we need to pay more attention to study this area.
Key words: crust     rocks and minerals     electrical conductivity     AC impedance spectroscopy     influential factor     electrical structure    
0 引言

为探测地下深部的物质结构特征,国内外的专家学者普遍采用大地电磁探测(MT)与电磁测深(GDS)等方法来约束地球内部物质组成,推断构造运动过程,并且已经取得了相当多的成果(Jones, 1992; Schilling et al., 1997; Unsworth et al., 2005; Zhao et al., 2012).解释电磁物探资料离不开对矿物和岩石导电性特征的认识,所以开展高温高压岩石电导率的实验研究就显得尤为重要(杨晓志, 2015).

对于地壳范围内的岩石来说,影响电导率的外部环境条件主要包括温度和压力等.内部因素则主要包括两方面:其中对于地下1~4 km的近地表岩石来说,孔隙度相对较大,则孔隙中含有的流体、颗粒边缘连接的碳膜和矿物颗粒的粒径大小就成为了影响电导率的重要因素;而对于中下地壳的岩石来说,其内部的结构构造则占据了主导地位,包括岩石面理方向、部分熔融程度及其空间分布、矿物晶格内部含有的结构水与矿物的组构等.与此同时,我们还可以通过这些电导率影响因素与实际测量得到的地下电性结构相结合,为推断实际的地下深部的物质组成和结构构造提供更为精确、科学的依据.

1 电导率的测量方法

电导率是表征矿物或岩石导电性质的参数,定义为

(1)

其中σ表示样品的电导率(S/m),是电阻率ρ(Ω·m)的倒数,L表示样品的长度(m),S表示电极的横截面积(m2),R表示样品的电阻(Ω).近几十年,前人曾经采用过直流法(Morin et al., 1979; Čermák and Laštovičková, 1987; Llera et al., 1990; Laštovičková, 1991)、单一频率交流法(Fuji-ta et al., 2004, 2007; Manthilake et al., 2009)测量矿物或岩石的电导率.但是这两种方法存在着表面电流和接触电阻,导致测量的电阻值会明显偏高,并且对于矿物或岩石等高阻体,上述两种测量方法会产生明显的极化作用,也无法区分岩石颗粒内部、颗粒边缘和颗粒-电极的导电机制(Karato and Dai, 2009; 黄小刚等, 2010; 李朋等, 2010).

目前测量高阻值材料的电导率主要采用交流阻抗谱法(史美伦, 2001),其原理是给系统施加一个正弦波扰动信号,测量该信号被样品扰动的响应来求出样品的电导率.相对于直流法和单频交流法,该方法具有出色的抑制直流、噪声及谐波的能力,便于建立等效电路模型,计算样品的电阻值.此方法在地球科学界应用相对较晚,直到20世纪90年代Tyburczy和Roberts (1990)才将这项技术引入到矿物和岩石的电性测量.通过阻抗谱实验得到不同频率下矿物或岩石的阻抗模值和幅角,依据该数据绘制Bode图(图 1a)和Nyquist图(图 1b)(陈进宇等, 2017),然后采用复数最小二乘法(CNLS)对实验数据进行拟合,最终获得相应矿物或岩石样品的电导率.Nyquist图中的每一个半圆弧可以等效为电阻R与电容C并联形成的电路,圆心为(R/2, 0),半径为R/2,代表了单一的导电机制.

图 1 0.2~300 MPa压力范围内、常温条件下干燥的含碳断层泥电导率测试结果 (a) Bode图; (b) Nyquist图(陈进宇等, 2017). Figure 1 The experimental results of electrical conductivity measurement at room temperature and a range of pressure between 0.2 and 300 MPa for dry carbon-bearing gouge (a)A Bode plot; (b)A Nyquist plot (Chen et al., 2017).

对于多晶体的岩石,在足够宽的频率范围之内,会出现三个完整的阻抗谱半圆弧(图 2b),自左向右分别代表岩石颗粒内部(R1)、颗粒边缘(R2C2)和电极-样品(R3C3)的导电机制.其等效电路图为一个R1电阻与两个并联的R2C2R3C3电路元件串联在一起,并且最终与系统电容Csys并联(图 2a)(Tyburczy and Roberts, 1990).

图 2 (a) 岩石电导率等效电路图, 虚线框内为岩石内部的等效电路; (b)复平面阻抗谱图(据Tyburczy and Roberts, 1990) Figure 2 (a) The equivalent circuit used to model electrical response of rocks, dashed boxes outlines elements intrinsic to material response; (b) The impedance spectra in the complex plane (Tyburczy and Roberts, 1990)

应当指出,在实际应用中,多数样品的阻抗谱弧不能完整地显示出如图 2b那样标准的半圆弧(Roberts and Tyburczy, 1993, 1994; Huebner and Dillenburg, 1995).如果岩石样品电导率的频率范围超过仪器的量程,那么复平面阻抗谱弧只能显示出其中的一部分;假如三个导电机制的弛豫时间差别不大,则会产生阻抗谱弧重叠的现象.此外,天然样品的阻抗谱弧圆心往往位于实轴以下,形成压扁的圆弧(Roberts and Tyburczy, 1991, 1993, 1994).而当颗粒边缘含有熔体、碳膜或其他高导相并形成导电通路时,R1R2C2也会从串联连接改为并联连接(Roberts and Tyburczy, 1994, 1999; Yoshino and Noritake, 2011).因此,对于单晶体或者颗粒边缘高导相连通的情况下,阻抗谱复平面上的圆弧Ⅱ会消失;同时对于含水或者电解质溶液的样品,由于电极与水的扩散作用,圆弧Ⅲ则会演变为一条直线(王多君等, 2005; Yoshino et al., 2009).

2 地壳矿物、岩石电导率的影响因素

影响矿物和岩石电导率大小的因素有很多,主要分为外部因素与内部因素两部分.外部因素指矿物和岩石所处的外部环境,如温度、压力等;内部因素则包括矿物的化学组成及晶格参数,岩石的组成、结构和构造等.

2.1 外部环境影响 2.1.1 温度

温度是影响矿物和岩石电导率最主要的外部因素(Jones et al., 2009),其电导率与温度的关系遵循Arrhenius定律,公式为

(2)

其中σ为矿物或岩石的电导率(S/m),σ0为其电导率的指前因子(S/m),Ea为活化能(J),P为压力(MPa),ΔV为活化体积,(-Ea+P×ΔV)为样品的总活化焓ΔH(J),R为气体常数(8.314 J/K),T为绝对温度(K).分别对公式(1)两边取对数值后可以看出,logσ与1/T成正比(图 3).当温度上升时,矿物晶体内部的载流子活性增加,浓度也增加,矿物和岩石的导电性则会相应地增强.由图 3所总结的实验结果可以看出,当温度由500 ℃上升至800 ℃时,矿物和岩石的电导率能够增加0.5~3个数量级.不同岩性的样品logσ增加的斜率有所不同,揭示出不同样品之间的活化焓存在着明显的差异(表 1).

图 3 几种岩石和矿物的电导率随温度变化的拟合曲线, 包括硅酸盐熔体(Gaillard, 2004)、橄榄石(ten Grotenhuis et al., 2004)、石英岩(Shimojuku et al., 2012)、玄武岩(Scarlato et al., 2004)、斜长石(Yang, 2012; Yang et al., 2012)、单斜辉石(Yang and Heidelbach, 2012)和斜方辉石(Yang et al., 2012) Figure 3 The fitting curves of temperature dependence of the electrical conductivity of several rocks and minerals, including silicic melts (Gaillard, 2004)、olivine (ten Grotenhuis et al., 2004)、quartzite (Shimojuku et al., 2012)、basalt (Scarlato et al., 2004)、plagioclase (Yang, 2012; Yang et al., 2012)、clinopyroxene (Yang and Heidelbach, 2012) and orthopyroxene (Yang et al., 2012)

表 1 几种岩石和矿物电导率的测量结果 Table 1 The experimental results of the electrical conductivity measurement of different rocks and minerals
2.1.2 压力

无论是矿物、岩石还是熔体,压力升高普遍导致电导率缓慢降低;但是对于(2)式,P×ΔV远远小于Ea,所以与温度相比,压力对电导率的影响较小(Gaillard, 2004; Scarlato et al., 2004; Yang et al., 2011, 2012).例如Yang等(2012)在6~12 kbar和300~1000 ℃范围内研究干燥的斜方辉石电导率时发现,当温度由500 ℃上升至600 ℃时,电导率上升了约0.8数量级,而压力如果上升1 GPa,电导率则仅仅下降约1.5倍.

此外,对于孔隙较多,并且赋存电解质溶液或固体高导相的岩石,压力则可以通过减小岩石孔隙度大小,改变高导相的连通性来对其电导率产生显著的影响(Glover and Vine, 1992, 1995; Mathez et al., 1995; Nover et al., 1995; Haak et al., 1997).不同的样品对压力的响应也有所不同,Glover和Vine(1992)对含有液体的麻粒岩在压力升至200~400 MPa的条件下进行了电导率的相关研究,实验结果显示对于含有电解质饱和溶液或者含有电解质不饱和溶液但是无碳的样品,电导率随压力升高而降低;而对于含有电解质不饱和溶液并且含碳的样品,电导率则随压力的升高而升高.Glover和Vine认为样品的电导率变化与麻粒岩内部形成的导电结构有关.对于前者,随着压力升高,孔隙度逐渐减小,液体的导电路径逐渐被切断,电导率逐渐降低;而对于后者,麻粒岩的电导率则主要受到碳的导电路径影响,随着压力升高,碳的连通性逐渐增强.Duba等(1994)在常温条件(25 ℃)下对含有0.3 vol%~0.8 vol% NaCl饱和溶液的角闪岩电导率与静水压力的关系进行了实验研究.在初始10 MPa压力下,电导率会略微降低.随着压力升高至250 MPa,与面理平行方向的样品,电导率会随压力升高而降低;与面理斜交的方向的样品,电导率则会随压力升高而升高.Duba等认为这是由于随着压力升高,饱水孔隙关闭,样品内部含有的石墨或硫化物等固体高导相对样品电导率产生了重要的影响.Shankland等(1997)也对含流体的角闪岩电导率进行了实验测量,结果显示电导率随着压力升高有先减小后增大的趋势,同样与含流体的裂隙逐渐关闭,高压下固体高导相发挥导电作用有关.同时初始电导率越低的样品,压力对高导相连通性影响就越显著,即dlgσ/dP越大.

2.2 岩石内部性质的影响

图 3可以看出,在同样的温度、压力等外部条件下,当样品内部性质发生改变时,电导率也会有0.5~3.5个数量级的差异,这其中包括岩石矿物组成、内部含有的流体或熔体,颗粒边缘可能赋存的石墨、硫化物等高导相,颗粒粒径大小,矿物晶格的结构水含量与晶格的组构方向等.

2.2.1 流体

对于中上地壳深度范围内的岩石来讲,岩石内部由硅酸盐矿物组成的岩石骨架导电性非常低,可以视为绝缘体(Nesbitt, 1993).但是如果岩石孔隙中含有带电解质的自由水时,其电导率有可能会有显著地提升.因此,岩石的电导率与流体电导率、岩石的孔隙度、孔隙形状以及孔隙的连通性有关(Llera et al., 1990).

岩石赋存的流体主要依靠内部溶解的部分离子(主要是Na+、K+、Ca2+与Cl-、HCO3-等,同时也含有少量的Mg2+、Fe2+、Fe3+与Mn2+等)作为导电的载流子.含有2%~10%盐度的流体可以使岩石的电导率提高1~3个数量级(Nesbitt, 1993).与流体内部离子的浓度相比,温度则对流体电导率的影响极为有限(Shimojuku et al., 2014).

对于近地表的饱水岩石来说,其电导率是由流体中的电解质导电性和岩石颗粒表面性质共同决定的(Jödicke, 1992),公式为

(3)

其中1/F是组成因子,σw是电解质的电导率,σqo是表面电导率.σqo与孔隙表面积、矿物表面电荷密度、矿物表面离子的价态与活性、温度、饱和流体的盐度等性质有关.对于黏土、胶体、泥岩、泥灰岩、弱固结页岩等低盐、低孔隙度或低渗透率的岩石,岩石中的流体连通性较低,岩石颗粒的比表面积较大,σwσ0的影响相对较弱,σqo对岩石电导率的影响则更加显著(Jödicke, 1992; Nover, 2005).Revil等(1998)通过总结前人的研究成果提出在25~200 ℃的温度范围内,表面电导率与温度具有近似的线性相关性.Jödicke(1992)提出随着深度增加,孔隙流体中的盐浓度逐渐增大,部分粘土矿物随着温度、压力的升高产生成岩作用,甚至是低级变质反应,岩石颗粒变粗,σw对岩石电导率的影响则逐渐占主导地位,最终可以将式(3)简化为Archie经验公式(Archie, 1942; Watanabe and Kurita, 1993)来计算,公式为

(4)

其中F=a/φma为常数,φ为岩石的孔隙度,m为胶结系数(在1~2之间变化).除了岩石的孔隙度以外,岩石的孔隙结构对于岩石导电性也非常得重要,在计算电导率时还应当加入描述孔隙连通性与几何形状的相关系数(Neithalath et al., 2006; 刘堂晏等, 2013; 宋延杰等, 2014).当流体含量达到1%甚至更低时,通过SEM的方法观察流体与岩石颗粒之间形成的二面角大小可以判断液体在岩石孔隙结构的连通性.固液二面角是指两个相邻的固-液界线切线间的夹角,公式为

(5)

其中,γss是两个相邻固体颗粒之间的界面能,γsl是固体颗粒与相邻液体之间的界面能.当二面角小于60°时,不论流体含量有多少都能形成连通的导电网格;而当二面角大于60°时,导电网格的连通则与流体含量有关.二面角越大,所需的液体含量也越大(Bruce Watson and Brenan, 1987; Holness, 1993; 侯渭等, 2004).

Llera等(1990)测量了含饱和KCl溶液的英安质凝灰岩、砂岩、安山岩、花岗岩和结晶灰岩在20~250 ℃条件下电阻的变化.结果显示,当温度上升至200 ℃时,样品的电阻值发生近指数性减小,其中孔隙度较高的凝灰岩电阻值在200 ℃降至稳定值.在相同温度的条件下,低孔隙度的花岗岩和结晶灰岩的电阻值相对而言要高2个数量级以上.Llera等提出饱水的岩石在温度小于200 ℃时,岩石的电阻值主要受到溶液电解质黏度的影响;而当温度大于200 ℃时,岩石的电阻值与岩石的微裂隙扩张或电解质与岩石颗粒的化学反应有关.Nesbitt (1993)通过总结前人的研究工作提出含KCl或CO2的流体在20~200 ℃的温度范围内电阻迅速降低;而当温度超过300 ℃时,电阻值逐渐回升(图 4a).Nesbitt认为在低温域,流体的黏度随着温度的增加而减小,导致离子的活性增加;而在高温域,流体HCO3-与K+和H+离子配对程度增加,载流子浓度降低,流体的密度减小.Glover和Vine(1994, 1995)将一系列中酸性原岩和变质岩填充0.5 mol/L的NaCl饱和溶液,在围压0.2 GPa、孔隙压0.18 GPa与50~900 ℃的温度范围内进行电导率的实验测量.结果显示含饱和流体的岩石随着温度增加,电导率逐渐升高;而当温度达到了350 ℃以上时,花岗岩或者花岗闪长岩等中酸性岩电导率逐渐降低,而基性麻粒岩、闪长岩、角闪片麻岩和斜方辉石等中基性岩石电导率仍然持续升高(图 4b).Glover和Vine提出在高温条件下,中酸性岩石电导率降低有可能是流体的电阻值升高所导致,也可能是岩石基质在高温下发生弱化,应变增加,从而使岩石的孔隙度与液体的连通性降低;而中基性岩石持续升高可能是内部的含水矿物发挥了作用,替代溶液成为了岩石内部导电的主要因素.Violay等(2012)利用气体压力装置在围压200 MPa、孔隙压50 MPa与温度升高至500 ℃的条件下对饱和的玄武岩进行了电导率的测量.结果显示在低温状态下,岩石的电导率随着温度上升而增加;而当温度大于300 ℃时,岩石的电导率也出现下降趋势.Violay等认为在低温条件下岩石电导率与流体的导电性质有关,在高温条件下岩石颗粒表面电导率则可能在发挥作用.Shimojuku等(2012)在1 GPa的压力与800~1100 K温度条件下测量含流体石英岩的电导率时发现含有0.43vol‰水便能够使岩石电导率上升1.5个数量级,通过SEM观察发现流体在颗粒间的二面角小于60°临界值,据此认为流体在矿物颗粒间形成了连通的导电路径.Sinmyo和Keppler (2017)发现当流体的NaCl浓度由0.01 M上升至0.1 M时,流体电导率上升了约1个数量级;但是当NaCl浓度大于0.1 M时,NaCl分解为导电的Na+和Cl-能力降低,则流体电导率增加速率变缓.

图 4 有关流体电导率的实验研究 (a)不同压力条件下, 不同KCl浓度流体的电阻率随温度的变化(Nesbitt, 1993);
(b)填充0.5M NaCl溶液的不同类型岩石的电导率随温度的变化(Glover and Vine, 1994).
Figure 4 The experimental studies about the electrical conductivity of fluid (a) Temperature dependence of electrical resistivity for different KCl concentrations and pressures (Nesbitt, 1993);
(b) Temperature dependence of electrical conducitvity of different rocks saturated with 0.5M NaCl solutions (Glover and Vine, 1994).
2.2.2 熔体

在地壳深部高温高压的条件下,尤其是在火山活动区域,岩石产生部分熔融,生成高导的熔体.当熔体在岩石颗粒边缘形成连通网格时,能够对岩石电导率的提升带来明显帮助(Schilling et al., 1997).Scarlato等(2004)发现玄武岩电导率升高与样品熔融程度增加有关,当熔体含量由8.6vol%升至79vol%时,岩石电导率提升0.3个数量级,而压力则并没有明显作用(图 3).Ferri等(2013)通过多次加热-冷却循环测量具有面理化定向结构岩石的电导率时指出熔体在岩石中呈现3D连通网格分布,电导率没有明显各向异性,当实验温度小于岩石的玻璃转换温度(Tg)时,岩石的Logσ-1/T曲线表现为非线性关系.Bagdassarov等(2004)提出硅酸盐玻璃的Tg大小与压力大小和内部的晶体结构有关,同时当温度大于Tg时,熔体的活化能也相应地升高.

此外,压力、温度和含水量对熔体电导率的影响也是近些年来的热点问题.大量的实验数据显示,无论是流纹岩(Gaillard, 2004; Guo et al., 2016b)、碱性岩(Pommier et al., 2008)、玄武岩(Ni et al., 2011a)、钠长石(Ni et al., 2011b)、英安岩(Laumonier et al., 2015)、安山岩(Laumonier et al., 2017)或者花岗岩(Chen et al., 2017),少量水都(例如约3wt%)能够提升其熔体电导率0.3~0.8个数量级(图 3)并且降低熔体的活化能,而Na+是熔体内部主要的载流子.在Na+浓度相近的条件下,钙碱性系列岩浆的电导率由流纹质至玄武质逐渐上升,与岩浆中Na+扩散率逐渐增加有关(Guo et al., 2016a).虽然压力在一定程度上能够降低熔体的电导率,但不是主要的影响因素.Gaillard (2004)认为水能够加快钠离子的流动,而压力的增加则降低了熔体的孔隙,导致钠离子活性降低,使熔体的电导率下降.Gaillard和Iacono Marziano (2005)提出熔体的孔隙度随着熔体聚合度与含水量的增加而加大,进而导致离子的流动性增强,电导率升高.Pommier等(2008)认为熔体的聚合度增加、黏度升高能够增大熔体电导率,降低活化能,增强压力对电导率的影响,温度越低含水量对熔体电导率的影响更大.Laumonier等(2015)指出随着含水量上升,活化体积上升,电导率受到压力的影响更加显著.与上述结果不一致的是Guo等(2016b)的实验结果指出熔体的含水量上升能够增加熔体的活化能,减小熔体的聚合度,增加离子孔隙度,加快离子的迁移,同时能够减弱压力对熔体电导率的影响.除了水以外,Sifré等(2014, 2015)还提出如果岩浆中含有6 wt%CO2,电导率也能够有显著提升,从而提出含有少量H2O和CO2的初始部分熔融作用是导致低速高导层的主要机制.

2.2.3 岩石颗粒边缘的碳膜

岩石颗粒边缘的高导相除了流体和熔体以外,还包括碳、金属矿物和硫化物等固体高导相(表 2),其中颗粒边缘连通的石墨膜能够在温度小于900 ℃的条件下保持稳定(图 5),对应于地壳范围内的深度(Selway, 2014).实验结果显示,对于地壳干燥的岩石,如果微量的碳(≥1 ppm)在岩石颗粒之间均匀分布形成很薄的碳膜(< 1000 Å),就能连通成导电路径,使岩石的电导率提升至0.1 S/m (Duba and Shankland, 1982; Frost et al., 1989; Mareschal et al., 1992; Glover, 1996).因此,碳也是影响地壳岩石电导率大小的主要因素之一.

表 2 几种高导相的电导率值 Table 2 Electrical conductivity of different conductive phases

图 5 岩石矿物电导率影响因素的深度适用范围(Selway, 2014) Figure 5 Expected depth ranges for important causes of enhanced conductivity of rocks and minerals (Selway, 2014)

早在20世纪80年代,Duba等(Duba and Shankland, 1982; Duba et al., 1988)就提出天然的石墨或碳具有活化能低的特点,认为碳的电导率受温度影响较小,含有很少一部分碳就可以显著地提升岩石的电导率.Frost等(1989)也认为在下地壳的高压环境下,石墨可以在岩石中稳定地存在.通过对比发现,颗粒粒径小于1 cm的含石墨层与下地壳高导层电导率值一致.Mathez等(1995)提出含碳物质能够在岩石的断裂中沉积,形成导电通路,能够有效提高断层带的电导率.Roberts等(1999)在充有CO2、CO或CH4气体,大于400 ℃的条件下发现砂岩与花岗岩在发生破裂或膨胀时,电阻值迅速地下降.Roberts等认为一定量的碳在岩石新形成的断面上沉积,生成了连续的碳膜,印证了Mathez等(1995)的观点.为了研究石墨对断层泥导电结构的影响,我们采用了石墨粉与石英粉混合的人工模拟样品研究断层泥电导率与压力的关系.实验结果显示(图 6)含有> 8wt%石墨的样品在压力上升、孔隙度减小的过程中会产生连通的导电通路,能够使样品的电导率上升10个数量级,说明样品内部的含碳结构对于断层泥电导率的提升能够带来显著的帮助.

图 6 石墨含量大于8wt%时能够在样品内部形成连通的导电路径 其中A6、A8、A10分别代表样品中含有6wt%、8wt%、10wt%的石墨. Figure 6 Fully conductive pathways are developed as the graphite content of samples larger than 8wt%
2.2.4 岩石内部结构构造

除了矿物和岩石颗粒边缘的高导相,岩石矿物晶格内部的结构水、结晶方向、粒径大小以及岩石面理方向等也是影响矿物和岩石电导率大小的重要因素.

对于含水矿物,当温压超过其稳定域时会发生脱水甚至部分熔融反应,释放导电离子,提高岩石的电导率(郭新转, 2016).朱茂旭等(2000)在测量蛇纹石的电导率时发现当温度达到500 ℃以上时,岩石的lgσ与1/T线性关系的斜率发生变化,并认为蛇纹石发生了脱水反应,形成的液相高导体在颗粒边缘构成了连通的导电网格,导电机制转变为离子导电.白利平等(2002)的辉长岩电导率测量结果显示当温度大于1000 ℃时,辉长岩中的黑云母发生脱水熔融,熔体在岩石中形成了连通的导电网格.Wang和Karato(2013)发现滑石在温度上升至840~890 K时会发生脱水反应.脱水前岩石内部的氢化学键较弱,滑石脱水后活化焓大幅度增加,变为小极化子导电.而朱茂旭等(2001a)在测量滑石电导率时发现电导率并没有随着温度的上升而发生突变,认为滑石主要依靠内部晶格导电,脱水后对于滑石电导率并没有太大的影响.

对于完全结晶的岩石来讲,如果造岩矿物具有各向异性并且定向分布的特点,则岩石在不同的方向上的导电性也会存在差异.例如Yang(2012)发现含水的斜长石存在导电的各向异性,不同晶格方向的电导率之间能相差约3~8倍(图 3).

Ten Grotenhuis等(2004)利用95%镁橄榄石和5%顽火辉石制成的模拟样品研究颗粒粒径大小对岩石电导率的影响时发现当岩石颗粒粒径减小时,单位体积边界面积增大,载流子运输更加顺畅,电导率也会明显增加(图 3, 图 7).在此基础上提出了样品电导率受粒径影响的公式为

图 7 根据立方颗粒几何模型计算得到的电导率与粒径的关系(Ten Grotenhuis et al., 2004) 其中σb为颗粒边界的电导率, σgi颗粒内部的电导率. Figure 7 Grain size dependence of electrical conductivity according to the geometrical model with cubic grains (Ten Grotenhuis et al., 2004)
(6)

其中σgi是颗粒内部的电导率,σgb是颗粒边界的电导率,δ是颗粒边界宽,d是颗粒直径.

Duba等(1994)Nover等(1995)在研究角闪岩的电导率时发现在平行或近平行面理方向,岩石的电导率随压力升高而降低;而与面理垂直或近于垂直方向,电导率随压力升高而升高.此外,平行于片麻岩面理方向的导电值也比其垂直面理方向大将近一个数量级(Fuji-ta et al., 2007; 于英杰等, 2011; 郭颖星等, 2014).Fuji-ta等(2007)认为这主要与矿物定向排列有关,分布于平行排列的矿物之间的孔隙形成了良好的导电网格.Glover和Vine(1992)在测量麻粒岩的电导率时也发现,平行面理方向的电导率要比垂直面理方向大很多,尤其是当温度和压力升高时差异就更为明显.推断可能是由于麻粒岩内部为狭长型的孔隙形状,当温度压力升高时岩芯逐渐闭合,平行面理方向可以形成更好的导电路径.朱茂旭等(2001b)提出,对于矿物颗粒边缘不存在高导相的含水榴辉岩来说,压力升高会使样品微裂隙或孔隙闭合,孔隙流体呈孤立液囊的状态,导电路径连通度下降,从而导致电导率下降.对于垂直压力方向,则裂隙更容易闭合,连通性下降更快.

2.3 其他类型的导电因素

除了以上几种影响因素之外,部分区域的导电因素还包括名义无水矿物中以缺陷形式存在的氢、岩石中的氧逸度、含铁量、颗粒边缘的硫化物等.Karato (1990)Yoshino等(2009)认为干燥的橄榄岩内部的氢非常活跃,可以成为导电的载流子,能够提高岩石内部的电导率;而不含氢的橄榄石含水量增大只能使其活化能下降,对电导率影响不大.Dai等(2015)发现在相同的温度和压力的条件下,随着氧逸度的升高(Mo+MoO2,Ni+NiO, Cu+CuO),辉长岩的电导率逐渐降低.对于名义无水矿物而言,氧逸度的增加会使氢缺陷浓度降低,电导率会相应地减小(Selway, 2014);而对于铁镁质岩石,随着氧逸度升高,三价铁离子比例升高,电导率则会相应地提升.Xu等(2000)提出当铁在铁镁比重中的含量上升0.12时,电导率能够上升1.4倍.郭颖星等(2013)通过实验提出蛇绿岩套的电导率与铁含量呈正比关系.Watson等(2010)还提出如果橄榄岩中增加少量的硫化铁,电导率也会明显增加.但是应当指出,以上几种导电因素对于地壳深度范围内矿物岩石的电导率影响有限,主要应用于地幔甚至核幔边界的深度范围.

3 电导率实验研究的意义 3.1 地下高导层的电性结构特征

地球物理探测表明,在地壳隆起区、活动造山带和克拉通底部均存在着导电异常区域(Wang et al., 2013b),该区域位于地下20~30 km的深度范围内,电导率达到约10-4~10-1 S/m (Shankland and Ander, 1983; Haak and Hutton, 1986; Jones, 1992; Korja and Hjelt, 1993; Glover and Vine, 1995; Yang, 2011),并且这些高导层(体)在水平方向上并不连续(Shankland and Ander, 1983; Evans et al., 2005),其各向异性系数能够达到约3至20以上(Yang, 2011).目前认为地壳高导层的成因机制主要包括矿物内部特殊的晶格结构和颗粒边缘存在的流体、熔体、石墨、硫化物等高导介质(杨晓志, 2014).

中、上地壳遍布含2 wt%~10wt%盐度的流体,能够使电导率升高1~3个数量级.在6 km深度范围内,随着深度增加,压力升高,有利于盐的分解,电解质大量赋存于岩石中,使电阻值逐渐降低并形成壳内10~0.01 Ω·m的高导层(Jones, 1992; Nesbitt, 1993).Guo等(2015)提出当温度达到900 K时,含有0.01流体的钠长石能够导致0.1 S/m的地下高导层.王多君等(2011)认为中下地壳高导层可能是麻粒岩脱水所导致.如果岩石颗粒边缘存在的固体高导相(包括碳或其他金属相)也能形成下地壳的高导层(Duba and Shankland, 1982; Duba et al., 1988; Frost et al., 1989; Glover and Vine, 1992; Mareschal et al., 1992; Nover et al., 1995; Glover, 1996; Jödicke et al., 2004, 2007; Yoshino and Noritake, 2011; Wang et al., 2013a).固体高导相在高温环境中能够保持相对稳定,同时在压力的作用下,其稳定性还会相应地提高(Roberts et al., 1999).

名义无水矿物对下地壳高导层同样也能够带来显著帮助(Glover and Vine, 1992; Yang et al., 2011; Shimojuku et al., 2012; Yang et al., 2012).下地壳尤其是稳定的克拉通主要成分为含有单斜辉石、斜方辉石和斜长石的麻粒岩相铁镁质岩石,并且在部分区域,单斜辉石的含量较大(>60vol%),含水量较高.Yang等(2011, 2012)通过高温高压的电导率实验研究发现,单斜辉石、斜方辉石和斜长石的矿物晶格内部都含有一定量的水,能够使岩石的活化能下降,导电机制发生变化,岩石电导率增加,而且随着含水量的提升而增大(图 3);而在相同的温度和压力条件下,单斜辉石和斜方辉石导电性较高.因此,Yang(2011)提出地壳高导层主要还是下地壳内部主要的物质组成——铁镁质麻粒岩的内部晶格导电所导致.

大量的研究成果表明每种导电成因机制在解释大地电磁测深资料时都往往存在着不确定性.换言之,地壳内部高导层(体)的形成方式可能具有多种成因.例如,对于深部流体导电机制来讲,Nover等(1995)认为岩石中的孔隙度至少要大于5%才可能使流体连通,形成壳内高导层.然而,如果要保证地壳深部能够长时间大尺度存在大量流体,并且保持稳定、互相连通状态比较困难(Mathez et al., 1995; Haak et al., 1997; Yang et al., 2011; Yang and Heidelbach, 2012).在深度达到约6 km的范围时,岩石还会因为压实作用、胶结作用、重结晶作用导致其孔隙度通常小于1%,致使流体对电导率的影响基本消失(Jödicke, 1992).同时在高温高压的条件下,流体的导电性也会大幅度降低(Nesbitt, 1993).另外,地壳的温度相对较低(通常 < 约1000 ℃),只有特定区域才会发生大规模的部分熔融反应,比如火山活动区域(Shankland and Ander, 1983; Yang, 2011),印度板块向欧亚板块俯冲形成喜马拉雅造山带下覆的低速高导层(Chen et al., 1996; Kind et al., 1996; Nelson et al., 1996; Unsworth et al., 2005)等.

对于碳来说,如果要形成良好的导电网格,岩石需要有较高的碳含量,在岩石颗粒边缘能够互相连通形成碳膜,并在相应的地质环境中稳定存在(Duba and Shankland, 1982).Mathez等(1995)在测量饱水片岩电导率时发现样品所受到的压力恢复到常压时会破坏颗粒间连通的碳膜.Yoshino(2010)通过测量高温高压条件下含有碳膜石英岩模拟样品的电导率时发现石墨膜在高温下不稳定,并且会因为动力作用而遭到破坏.Glover(1996)提出岩石颗粒边缘的碳膜经历应力释放或者受到氧化、退变质作用等影响会发生不可逆的破坏.Schilling等(1997)总结出颗粒间含碳碳膜在高氧逸度、活动造山带的环境中不能够稳定存在.

Selway(2014)提出在断层剪切带等特定区域,岩石颗粒粒径小于1 mm,粒径大小会对岩石电导率产生重要的影响.很多学者(Roberts and Tyburczy, 1991; Huang et al., 2005; Yang et al., 2011; Yang and Heidelbach, 2012)提出对于剪切带周围的围岩,岩石颗粒粒较大、电导率与岩石粒径大小关系并不显著.

总之,无论是哪种导电因素都有自己适用的温度和压力范围(图 5)(Selway, 2014).下地壳高导层也可能不仅仅是单一导电机制所导致,也存在多种导电机制共同作用的可能,需要针对特定区域具体问题具体分析.

3.2 与电导率相关的实际应用

除了对中下地壳高导层的研究工作之外,国内外的专家学者还对部分电导率异常区域进行了相关的电导率实验研究,从而为推断地震产生机制、地下岩层结构、地壳深部部分熔融程度与地球动力学模型推断等热点问题带来了一定的帮助.

Ten Grohenhuis等(2004)提出糜棱岩化作用导致岩石颗粒粒径减小,比表面积增大,提升颗粒边缘的导电性质,显著地提高岩石的导电性,并且通过分析大地电磁测深数据,可以判断剪切带的位置(图 7).Fiji-ta等(2007)将实验数据与该区域野外电磁数据进行对比后认为日本九州Higo变质带下覆片麻岩电磁探测方向与片麻岩面理方向垂直.郭颖星等(2010)通过麻粒岩的高温高压电导率实验提出华北和西南地区的下地壳主要由麻粒岩所组成.Scarlato等(2004)通过意大利埃特纳火山岩的高温高压部分熔融电导率实验提出该火山下覆的地层结构是两个高阻的辉长岩之间夹杂着一层低阻的岩浆层.Schilling等(1997)发现智利北部约20~60 km深度范围内存在1 S/m的高导层,提出该高导层是由地壳深熔作用所导致,其部分熔融程度为14 vol%~27vol%.Dai等(2014)认为干燥的花岗岩无法解释青藏高原中下地壳高导层.Hashim等(2013)利用变泥质岩脱水熔融电导率的实验数据与MT结果对比提出喜马拉雅西北部20~25 km (10 Ω·m)与西藏南部10~15 km (3 Ω·m)高导层分别是由25vol%部分熔融的变泥质岩与100%花岗质熔体所导致.Chen等(2017)则在此基础之上补充了含水花岗质岩浆电导率的实验结果,对部分熔融变泥质岩的有效电导率做了进一步的计算.认为上述两个地区高导层分别是由大喜马拉雅结晶岩系(GHC)脱水熔融(熔融程度4 vol%~16 vol%)与含水熔融(熔融程度约35 vol%)所导致,继而提出GHC中地壳深度范围内长期稳定存在部分熔融的软弱层,对印度洋板块向欧亚板块俯冲起到了润滑的作用.此外,Sinmyo和Keppler (2017)则提出含有1 vol%,浓度为5 wt% NaCl流体的中下地壳也可以解释西藏南部高导层.

3.3 浅表地震断层带电导率

对于浅部断层带而言,影响其电导率的因素非常复杂.Sibson(1977)提出地下1~4 km的地表层是松散的断层角砾或断层泥,存在大量的孔隙,可以赋存流体或者沉积石墨与金属相.Hickman等(1995)提出脆性断层与剪切带的流体主要来自于进变质作用(包括断层剪切生热)所产生的矿物脱水或者由地球深部地幔熔体上涌至地壳断层所发生的岩浆作用.通过野外采样和室内实验分析,发现很多断层剪切带内部产生的断层泥的碳含量会发生很大的变化,有时会有石墨晶体出现.比如汶川地震断层带(Togo et al., 2011; Li et al., 2013; 王焕等, 2013)、日本的Ushikubi断层(Oohashi and Kobayashi, 2008; Oohashi et al., 2012)、瑞士的Err拆离断层(Manatschal, 1999)以及德国的Bohemian地块(Haak, 1989; Haak et al., 1997; Zulauf et al., 1999)等.这些断裂带区域内的断层泥与围岩相比含碳量明显上升.而也有一些学者通过对集集地震断裂带研究(Hirono et al., 2006; Ikehara et al., 2007)发现断层带与围岩相比,含碳量明显降低.所以原岩成分和地质条件的不同,断层带含碳量就会有相应的变化.Jödicke等(2004, 2007)指出富碳的黑页岩在脆韧性转换带中会发生接近绿片岩相条件下的退变质反应,通过地壳抬升与断层剪切作用,石墨均匀涂抹在开放裂隙的剪切面上,形成连通的导电路径.与此同时,受到地震运动强烈挤压与断层摩擦的影响,断层岩的粒径变化较大,普遍具有超维数的特征(Storti et al., 2003; Ma et al., 2006; Keulen et al., 2007).

由此可见,剪切带内部流体、粒径以及碳含量的变化为地震断层带的高导性提供了可能.部分专家学者在室内研究中做了大量的工作.比如通过德国Bohemian地块所取得的超深钻岩芯,ELEKTB小组(Haak et al., 1997)发现由于受到流体的影响,中地壳脆韧性剪切带沉积了大量的石墨.经历断层摩擦剪切作用后导致剪切带的石墨化,使剪切带的导电性升高.Yamashita等(2013)在辉长岩断层泥高速摩擦实验(13 m/s)中检测出了电导率的变化.他们发现两次电导率的升高对应了熔体碎片的形成和熔体层的生长,分别导致了断层滑移的速度强化与速度弱化(图 8a);而在慢速滑移(5.3×10-3 m/s)过程中,电导率与摩擦系数呈现出很好的相关性,对应了摩擦面之间接触面积的变化(图 8b).Mathez等(2008)将石英岩填充饱和的含碳孔隙流体时,发现当石英岩发生断裂时,在断裂表面迅速形成碳膜,电导率迅速升高,而其他光滑表面的电导率则变化不明显.

图 8 模拟断层在摩擦的过程中摩擦系数、电导率、剪应力(Фr)、接触面积(Sr)和轴向位移的变化(Yamashita et al., 2013) (a) 13m/s高速摩擦的状态下; (b) 5.3×10-3 m/s慢速滑移的状态下. Figure 8 Friction coefficient, electrical conductivity, shear stress (Фr), real contact area (Sr) and relative axial displacement of the simulated fault as a function of slip distance (Yamashita et al., 2013) (a) At the seismic rate (1.3 m/s); (b) At the subseismic rate (5.3×10-3 m/s).

综上所述,对于断裂带电导率的研究,虽然国内外的专家学者已经取得了一些成果,但是对于室内系统性测量天然断层岩的电导率,尤其是研究与温度、压力、剪应力等条件的关系仍然有所欠缺,还需要在今后的研究工作当中逐步地展开.

4 结论 4.1

由以上内容可以得知,利用交流阻抗谱的测量方法,国内外专家学者对于地壳深度范围内岩石的电导率性质研究已经取得了相当多的成果.大量的研究数据表明,影响地壳矿物和岩石电导率的因素有很多.对于外部环境来讲,温度的大小最为重要,而压力则主要改变岩石中高导相的导电结构来间接影响岩石电导率的大小.除了外部环境因素外,地壳岩石的电导率主要受到岩石内部存在的高导相的影响,主要包括岩石中赋存的流体、熔体和碳.其中流体的导电性质主要与流体内部含有的电解质有关,受温度的影响比较大;含熔体的岩石电导率大小与岩石部分熔融程度有关,同时熔体内部的水也起到了重要的作用;碳则主要取决于在岩石内部是否能够形成良好的导线路径,这与碳在岩石中的含量和连通性有关.此外,矿物和岩石内部的性质也很重要,包括岩石面理方向、颗粒大小、矿物晶格方向和结构水含量等.

4.2

中下地壳高导层是目前大多数学者所关注的热点.对于中下地壳高导层成因的解释有很多种,但是不同的影响因素都有一定的局限性,个人认为不同地区的高导层很有可能是多种不同的导电因素相互作用的结果,具体的原因还需要作进一步的研究.同时,电导率的实验测量对于解释地壳结构、动力学特征也带来了一定的帮助.一系列研究结果表明,断层剪切带含有一定量的流体与石墨,同时具备细粒糜棱化的特点,相比于围岩具备高导的可能性,在未来可以予以重点关注.

致谢 感谢陈建业博士对本文的悉心帮助,感谢审稿专家对文章提出的宝贵意见,也感谢编辑部的大力支持.
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