Paleozoic Magmatism in the Qinling Orogen and Its Geodynamic Significance
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摘要: 秦岭造山带记录了华南华北板块聚合的完整过程.古生代岩浆岩记录了造山过程中的壳幔相互作用和造山带演化的动力学过程.古生代的中基性岩浆岩揭示了俯冲隧道内变质脱水交代岩石圈地幔过程，其中富水基性杂岩为富钾基性岩，地球化学特征显示其地幔源区经历了洋壳沉积物的交代；看丰沟岩体为高镁闪长岩，地球化学特征显示其来自经历俯冲流体交代的地幔源区.通过对古生代岩浆岩的研究发现，其具有明显的时空分布规律，它们对应于原特提斯洋俯冲、后撤、前进和回转等过程.所以壳幔相互作用发生在原特提斯洋俯冲过程中.Abstract: The Qinling orogen records the whole amalgamation of the North China and South China blocks. The Paleozoic magmatism in the Qinling orogen registers crust-mantle interaction and geodynamics of the orogeny. In this paper, we give a brief review of the Paleozoic magmatism.Intermediate-mafic magmatic rocks derived from the metasomatic mantle reveal fluid activities in the subducted channel. The Fushui mafic complex is enriched in K, and its mantle source has been metasomatized by subducted oceanic sediment. The Kanfenggou pluton belongs to high-Mg diorite. It is inferred by the geochemical characteristics that the mantle sources of the Kanfenggou pluton have experienced subducted fluid metasomatism. The results show that they exhibit a clear spatio-temporal variation, which might be caused by the subduction, retreat, advancement, and rollback of the Proto-Tethys oceanic plate.Therefore we suggest that the Paleozoic crust-mantle interaction was induced by the subduction of the Proto-Tethys ocean.
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Key words:
- Qinling orogen /
- Paleozoic /
- magmatism /
- geodynamics
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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 秦岭-桐柏-大别造山带构造简图
Fig. 1. Schematic geological map of the Qinling⁃Tongbai⁃Dabie orogenic belt
图 2 富水基性杂岩Hf⁃Nd同位素组成
Fig. 2. Hafnium vs. Nd isotope compositions for the Fushuimafic complex
图 3 北秦岭造山带古生代构造演化
Fig. 3. Sketch model showing the Paleozoic evolution of the North Qinling orogenic belt
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