Lithospheric Thermal-Rheological Structure of Nansha Trough Foreland Basin in South China Sea
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摘要: 为了研究南沙海槽前陆盆地深部岩石圈的热力学性质, 在前人所做的地质与地球物理研究工作的基础上, 结合各种岩石热力学流变参数, 采用有限元分析方法, 计算了现今构造逆冲之后和中中新世—全新世逆冲过程活跃时两种状态下的深部岩石圈二维温度场和流变结构.模拟计算表明, 南沙海槽前陆盆地地幔热流贡献达60%~70%, 大地热流受深部地幔控制.逆冲推覆构造作用使盆地逆冲推覆带地表热流显著升高了15%~25%, 达到70~75mW/m2.盆地沉积层温度在200℃以内, 莫霍面温度420~500℃, 地壳地温梯度25~30℃/km, 盆地热岩石圈厚度为80km左右, 横向上变化幅度不大.南沙海槽前陆盆地深部岩石圈流变性质具有明显的分层特性, 为典型的“三明治”结构.盆地岩石圈综合强度由南沙海槽向逆冲推覆带方向呈下降趋势, 盆地力学岩石圈厚度50km左右, 有效弹性厚度为30~32km.通过模拟盆地深部岩石圈热流变结构, 揭示了盆地深部岩石圈具有强地幔、弱地壳的流变学特征, 表现为高强度块体.同时南沙海域地震活动与深部岩石圈热结构、热活动和岩石圈综合强度密切相关.地壳内热构造活动弱、岩石圈强度大可能是区域地震很少发生的重要原因.Abstract: In order to study the lithospheric thermodynamic properties of Nansha trough foreland basin, we simulate the deep lithosphere temperature field and rheological structure during the thrust faults tectonic activities from Middle Miocene to Holocene and after thrusting (static) in the basin using the finite element method, based on previous studies of geological and geophysical data together with a variety of rock thermodynamic parameters.Simulation results indicate that heat flow contribution from mantle reaches 60%-70% of surface heat flow which is controlled by deep mantle in Nansha trough foreland basin.Thrusting movements increase the surface heat flow in thrust-slip fault belts by 15%-25% to 70-75 mW/m2.Our results also show that the temperature of sedimentary layer is less than 200 ℃, while the temperature at Moho varies from 420 to 500 ℃; crust geothermal gradient ranges from 25 to 30 ℃/km.And calculated thickness of thermal lithosphere is about 80 km, which varies slightly laterally.The lithosphere in Nansha waters shows obvious rheological stratification properties, as a typical "sandwich" structure.Horizontally, lithosphere strength decreases in the direction from Nansha trough to thrust belt (NW to SE).The thickness of mechanical lithosphere is about 50 km and the effective elastic thickness ranges from 30 to 32 km.The simulated thermal lithosphere rheological structure also reveals that lithosphere of Nansha basin has rheological characteristics of strong mantle and weak crust, acting as high-strength block.Earthquake activities of Nansha waters are closely related to thermal structure, thermal activity and integrated strength of lithosphere.The weak thermal activity in the crust and high lithosphere strength may be important reasons for rare earthquake occurrence in this area.
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锂离子电池具有能量密度高、功率大、使用寿命长、安全性能好等优点, 在便携式设备、电动汽车等领域展示出了良好的应用前景, 日益受到广泛关注. 尖晶石型锰酸锂(LiMn2O4)具备资源丰富、价廉、环境友好的特点, 是最有希望的下一代正极材料之一. 纯的尖晶石型锰酸锂的放电平台为4 V左右, 实验研究表明, 通过过渡金属掺杂, 锰酸锂放电平台可以升高到5 V, 因而其能量密度也将显著提高(Song et al., 2004).由于其巨大的应用前景, 这种高能量密度的掺杂锰酸锂电极材料已经成为当今研究的热点(Markovskya et al., 2004; Park and Sun, 2004).
尽管对过渡金属掺杂锰酸锂后放电平台的升高现象有众多实验研究, 但其升高机理的研究却鲜见报道.因此, 只有对锰酸锂放电平台升高机理有清楚的认识, 才能开发出性能可靠、高能量密度的锰酸锂正极材料.本文将就此进行探讨.
1. 模型与计算方法
图 1为本文采用的尖晶石型LiMn2O4的晶胞结构模型.它包含32个氧原子、16个锰原子(占据32个八面体间隙位(16d)的1/2)、8个锂原子(占据64个四面体间隙位(8a)的1/8).掺杂时, 用一个金属原子代替一个锰原子, 即金属M掺杂后的LiMn2O4可以表示成LiM0.125Mn1.875O4(M=Ti、Cr、Fe、Co、Ni、Cu、Zn、Li、Mg、Al).计算时对晶胞中的所有原子进行优化.实验研究表明, 掺杂2.5%的钴元素时晶胞的大小只减少0.03 Å(He et al., 2005), 因此进行掺杂计算时可以忽略晶胞参数的调整, 可采用实验中标准的晶胞参数, 即a=b=c= 0.824 2 nm, α=β=γ=90°(Xia and Yoshio, 1997), 理论计算也表明采用此近似不会影响体系的电子结构性质(Shi et al., 2003).
本文所有计算都采用第一原理密度泛函理论中的广义梯度近似计算方法(Tasnádi and Nagy, 2002). 计算是在Siesta软件下完成的(Artacho et al., 1999), 它对成百个原子的系统能够进行标准的密度泛函计算.用超软赝势来描述中心电子, 用双ζ的数字基组来描述价电子, 并考虑原子的自旋极化.为精确描述LiMn2O4的原子轨道, 使用广义梯度近似法(GGA), 它对很多材料都能提供非常精确的结构信息和能量信息.平面波基组的截止能量为200 Ry. 计算采用周期边界条件, 自洽迭代过程在简约布里渊区中使用了3×3×3个k点.
2. 结果与讨论
图 2为LiM0.125Mn1.875O4(M=Ti、Cr、Fe、Co、Ni、Cu、Zn)中Mn-3d、M-3d和O-2p能带的分态密度(PDOS).由图 2可知, Mn-3d、M-3d和O-2p能带之间的相互作用决定了体系的电子结构性质, O-2p能带主要分布在低能量区, 而Mn-3d和M-3d能带主要位于费米能级附近的高能量区, 掺杂M后, Mn-3d和O-2p能带的形状没有发生明显的改变, 但是, 由于Mn和O间相对位置的调整, Mn-3d和O-2p能带的位置明显地向低能量方向移动, 同时费米能级也相应地由-3 eV下移到-4.6 eV左右.然而, 仔细对比图 2中的各种掺杂情况就会发现, 当M由元素周期表中第四周期的Ti变化到Zn时, M-3d逐渐向低能量的方向移动, 即M-3d与O-2p能带间的带隙逐渐减小, 说明M-3d与O-2p之间的作用逐渐增强.
图 2 LiM0.125Mn1.875O4(M=Ti、Cr、Fe、Co、Ni、Cu、Zn)中Mn-3d、M-3d和O-2p的分态密度(PDOS)Fig. 2. Partial density of states of Mn-3d band, M-3d band and O-2p band in LiM0.125Mn1.875O4(M=Ti, Cr, Fe, Co, Ni, Cu, Zn)图 2的局部放大后能更清楚地看出M-3d与O-2p能带的相互作用, 如图 3所示.在形成M-3d能带的位置上, 出现了新的O-2p能带, 这种新的O-2p能带应是由M-3d能带诱导所致, 且随着M-3d能带逐渐向低能量的方向移动, 新的O-2p能带也逐渐向低能方向移动, 因此O的平均价态变得越来越负, 系统的静电能也越来越低.因此当Li脱出时, 需要更多的能量才能从较低的O-2p能带上获得电子, 根据嵌入电压的计算公式(Ning et al., 2006), E=-ΔGT/zF(其中, E表示嵌入电压, ΔGT表示等温条件下系统自由能的降低, z表示转移的电荷数, F表示法拉第常数), 如果新的O-2p能带出现位置的能量越低, 体系可以获得的嵌入电压就越高.如果有更多的电子转移给O, 体系的嵌入电压就会越高, 这与Aydinol et al.(1997)的观点是一致的.因此, 可以推断获得的这种高的嵌入电压, 主要是由费米能级附近新的O-2p能级决定的, 而不是由费米能级附近的M-3d能级决定的, 这与传统的认识是不同的.实际上, 锂脱出时获得的电子, 主要是由费米能级附近O-2p能带(图 3中已涂黑部分)补偿的, 而不是由低能级的O-2p能带提供319地球科学———中国地质大学学报第31卷的, 这与Ceder et al.(1997)认为的电子转移发生在Li与O之间是一致的, 与传统认为的电子由Mn转移给Li是不同的.
图 3 局部放大的LiM0.125Mn1.875O4(M=Ti、Cr、Fe、Co、Ni、Cu、Zn)中Mn-3d、M-3d和O-2p的分态密度(PDOS)Fig. 3. Enlarged partial density of states of Mn-3d band, M-3d band and O-2p band in LiM0.125Mn1.875O4(M=Ti, Cr, Fe, Co, Ni, Cu, Zn)本文还对LiM0.125Mn1.875O4(M=Ti、Cr、Fe、Co、Ni、Cu、Zn)中Li、Mn、O、M离子的净电荷进行了计算, 结果表明, 掺杂后锂离子的平均净电荷(+0.78)几乎没有变化, 与它的价键理论电荷值+1.00比较接近, 说明在尖晶石LiMn2O4晶体中, Li离子是相对独立的, 与Mn和O的相互作用不强, Li离子可以在尖晶石骨架中近似自由地嵌入/ 脱出, 这与实验结果(刘韩星等, 2001)和上述分态密度分析是一致的.虽然掺杂后Mn、O离子的净电荷都有所改变, 但是氧离子的平均净电荷(-0.58)、锰离子的平均净电荷(+1.74)与它们的价键理论电荷值-2、+3/+4相差较大, 这说明Mn-O键中离子键成分较少, 而共价键成分较多.
3. 结论
本文研究表明, 由于M-3d能带的诱导作用, 出现新的O-2p能带, 当过渡金属M由Ti变化到Zn时, M-3d能带逐渐向低能量的方向移动, 新的O-2p能带出现的位置也随之下移, 当Li脱出时, 需要更多的能量才能从新的O-2p能带上获得电子, 体系能够获得更高的嵌入电压, 可见, 这种高的嵌入电压, 主要是由费米能级附近新的O-2p能级决定的, 而不是由费米能级附近的M-3d能级决定的; 锂脱出时获得的电子, 主要是由费米能级附近O-2p能带提供的, 说明电子转移是发生在Li和O之间, 与传统认为的发生在Mn和Li之间是不同的.
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图 1 南沙海槽前陆盆地地理位置、区域构造及热流测点分布(构造和BGR01-07测线根据Franke et al., 2008; Hesse et al., 2010修改)
Fig. 1. Sketch map of tectonic and topographic map of Nansha trough foreland basin in South China Sea
图 2 南沙海槽前陆盆地94N05测线的剖面地壳结构
图中数字为岩石圈各层地震P波速度, 单位为km/s, 波速据苏达权等(1996)和Franke et al. (2008)修改
Fig. 2. Crust structure of line 94N05 profile of Nansha trough foreland basin
图 4 岩石圈现今静态(a) 和考虑中中新世—全新世逆冲推覆作用后(b) 的温度场图
图中黑色虚线为热流值; 黑色实线为温度界面, 单位为℃; 白色实线代表逆冲断层; 白色点线是岩石圈层圈界面; 白色箭头代表逆冲断层活动方向
Fig. 4. Temperature fields of deep lithosphere including static (a) and thrusting in process (b)
图 5 现今静态的(上) 和逆冲推覆过程中(下) 的岩石圈流变结构
Fig. 5. Rheological structures of deep lithosphere including static (upper) and thrusting in process (lower)
表 1 热导率相关参数K0和C值
Table 1. Parameter values K0 and C associated with the thermal conductivity
层位 K0 (W·m-1K-1) C (K-1) 沉积层 2.0 0 UTK层 3.0 1.5×10-3 下地壳 3.0 1.5×10-3 岩石圈上地幔 2.5 -4.0×10-3 注: UTK层、下地壳、岩石圈上地幔的K0按文献 Liu et al. (2005) .
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表 2 岩石圈各层流变参数(据Fernàndez and Ranalli, 1997)
Table 2. Flow law parameters of lithosphere
层位 材料 A (MPa-n·s-1) n Q (kJ·mol-1) 上地壳(沉积层) 石英岩 1×10-6 2.8 150 下地壳 石英闪长岩 3.2×10-3 3.2 270 岩石圈上地幔 橄榄岩 3.2×104 3.5 535
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