华南东部地幔过渡带顶部低速层中的熔体含量估算

周晓亚， 马麦宁*， 徐志双， 韩林， 魏东平

1 中国科学院计算地球动力学重点实验室， 北京 100049 2 中国科学院大学地球科学学院， 北京 100049



华南东部地幔过渡带顶部低速层中的熔体含量估算

周晓亚1,2， 马麦宁1,2*， 徐志双1,2， 韩林1,2， 魏东平1,2

1 中国科学院计算地球动力学重点实验室， 北京 100049 2 中国科学院大学地球科学学院， 北京 100049

地幔过渡带顶部低速层的成因及性质研究，对于认识地球内部物质运移及地幔对流过程等具有非常重要的动力学意义.最新的地震学研究显示，华南陆块东部地幔过渡带顶部的低速层存在着明显的区域性差异.一般认为，该低速层的形成与脱水引起的部分熔融有关.本文利用部分熔融体系的平衡几何模型，重点分析了熔体成分、位温、二面角和玄武质含量等因素对熔体含量的影响，并结合该低速层的分布特征，估算出研究区的南北两个子区域地幔过渡顶部熔体含量分别为～1.18 vol.%和～2.02 vol.%.这一熔体含量的显著差异可能与太平洋板片多期次俯冲作用的叠加有关.

华南； 地幔过渡带； 低速层； 部分熔融； 熔体含量

1 引言

地球内部410 km和660 km间断面之间的地幔过渡带(mantle transition zone, MTZ)是地球内部结构研究方面的热点，精细地刻画MTZ结构对认识地球深部的物质组成和地幔对流动力学过程等具有重要作用.自Revenaugh 和Sipkin(1994)首先报道了黄海、日本海下方的MTZ顶部的上地幔底部存在着低速层(low velocity layer, LVL)以来，越来越多的地震学观测在全球多个地区都发现了类似的LVL，其主要集中分布于俯冲带地区(Song et al., 2004; Obayashi et al., 2006; Courtier and Revenaugh, 2007; Jasbinsek et al., 2010; Schmandt et al., 2011)和伴有大火成岩省发育的大陆克拉通地区(Vinnik and Farra, 2002, 2007; Vinnik et al., 2003; Oreshin et al., 2011)，尤其是在俯冲带的向陆一侧更为发育(Bagley et al., 2009; Tauzin et al., 2010).

自中生代以来，华南陆块东部受到了多体系和多期次俯冲作用的影响(Hilde et al., 1977; Honza and Fujioka, 2004; Zhou et al., 2006; Sun et al., 2007; Li and van der Hilst, 2010)，其现今位置处于西太平洋俯冲带的向陆一侧(Huang and Zhao, 2006).结合前人地震层析成像结果，Huang等(2014)根据接收函数给出的MTZ间断面起伏形态，估计了其中的水含量，认为华南东部MTZ中至少部分区域是比较富水的.李国辉等(2014)基于P波三重震相给出了华南地块东部的北南两个子区域A区和B区(图1)的LVL厚度分别为57 km和40 km，相应的P波速度降分别为4.5%和2.7%.这一精细探测结果为研究本区域LVL成因提供了可靠的地震学参数.

高温高压矿物岩石物理实验研究表明，MTZ顶部LVL可能与部分熔融作用和热异常等有关(周晓亚等，2014).Bercovici和Karato(2003)认为，MTZ顶部LVL的形成与410 km间断面处瓦兹利石(wadsleyite)相变为橄榄石时，发生脱水诱发的部分熔融有关.考虑到俯冲板块通常给本区域MTZ带来的是冷却效应，这里的LVL应该与热异常无关.

虽然现有研究均倾向于华南东部MTZ顶部LVL的形成很可能与部分熔融有关，但LVL中熔体含量与低速异常之间的定量关系尚不清楚.本文根据已有研究资料，利用部分熔融体系中的平衡几何模型(equilibrium geometry model)(Takei,1998,2002; Hier-Majumder et al., 2014)，探讨了熔体成分、位温、二面角和玄武质含量等对熔体含量的影响，并根据华南东部的区域地幔环境选择出代表性参数，对LVL中的熔体含量进行了估算.

图1 华南东部LVL的分布范围虚线指示了范围，A和B所连箭头分别指示两个子区域(修改自李国辉等，2014).Fig.1 Distribution range of the LVL beneath the eastern South ChinaThe cycle of dashed line just outlines this range and the arrows adjacent to A and B point out two subdomains of the LVL.

2 平衡几何模型及参数选择

定量的估算能够导致华南东部LVL的P波速度降分别达到4.5%(A区)和2.7%(B区)的熔体含量，既需要利用一定的岩石物理模型，也要考虑熔体几何形态的影响.为此，本文选择了具有代表性的平衡几何模型(Takei, 2002; Hier-Majumder et al., 2014)来研究这一问题.

2.1 平衡几何模型

合理的几何模型对于由地震波速度变化来定量部分熔融体系中的熔体含量(体积分数)是至关重要的.相对于扁球状(Berryman, 1980)、管状(Mavko, 1980)和裂隙状(O′Connell and Budiansky, 1974)等简化的熔体形态模型，受二面角大小控制的平衡几何模型更接近于真实状况(Yoshino et al., 2005; Zhu et al., 2011).

平衡几何模型所描述的部分熔融体系中，固体颗粒之间以一定配位数(如12和14)的形式连接成格架(framework)，熔体分布在粒间孔隙中，而格架与未被填充的孔隙共同构成了骨架(skeleton)，此时体系中熔体-固体界面能与固体-固体界面能之间达到力学上的平衡.若不考虑颗粒的各向异性，则熔体形态可完全由含量和二面角确定(Yoshino et al., 2005).

用平衡几何模型计算有效体积模量和有效剪切模量主要是利用关于接触度的函数.接触度即为部分熔融岩石中颗粒与颗粒接触面积占总接触面积(固-固接触面积与固-液接触面积的总和)的比例.当熔体体积分数(φ)介于0和临界解聚分数之间时，接触度(ψ)依赖于熔体的体积分数和二面角(θ)；而当φ高于临界解聚分数时，体系结构将从颗粒支撑转变为液体支撑，颗粒孤立地悬浮于液体中(von Bargen and Waff, 1986; Wimert and Hier-Majumder, 2012).典型的临界解聚分数为20～30 vol.%(Scott and Kohlstedt, 2006; Hier-Majumder et al., 2006).

根据von Bargen和Waff (1986)给出的接触度公式计算一定的φ和θ所对应的ψ：

ψ=f(θ,φ)，

(1)

计算中采用了Takei(2000)所给出的校正.

有效弹性模量可以表达为ψ的形式：

N=μ(1-φ)g(ψ)，

(2)

Ke=

(3)

K和Km分别是固体和熔体的体积模量，μ是固体剪切模量，N和Ke分别是有效剪切模量和有效体积模量.g(ψ)和h(ψ)表示为

g(ψ)=1-(1-ψ)n1，

(4)

h(ψ)=1-(1-ψ)n2，

(5)

指数n1和n2也依赖于ψ(Takei, 2002).

根据平衡几何模型，熔体所导致的P波速度变化为

(6)

γ=μ/K，

(7)

2.2 参数选择

平衡几何模型中涉及到的参数有：熔体成分、位温(potential temperature)、二面角(dihedral angle)和玄武质含量.其中，位温指地幔绝热地温线对应的地表温度(陈凌等，2007).

要得到合理的熔体体积分数，必须选择合适的固体和熔体弹性参数范围.而固体弹性参数的主要影响因素是温度和组成成分.参考Hier-Majumder和Courtier(2011)以及Hier-Majumder等(2014)的方法，由Xu等(2008)所提供的数据库来计算固体的弹性参数.在该数据库中，成分组成简化为玄武质和方辉橄榄岩，选用玄武质体积分数来描述成分变化，平均地幔包含了18 vol.%的玄武质成分，因而本研究采用与Hier-Majumder等(2014)的选择相同的成分变化范围0～40 vol.%.由于从橄榄石到瓦兹利石的克拉伯龙相变斜率为正(Katsura et al., 2004)，过低的位温(如1300 K)可能使B区的LVL(深度为370 km)位于MTZ之内(Xu et al., 2008)，同时研究区域内并没有发育地幔柱，1800 K(Courtier et al., 2007)可以作为研究区的位温上限，因此选取的位温范围为1300～1800 K(A区)和1400～1800 K(B区).

二面角是描述熔体形态的一个重要参数，能显著地影响熔体体积分数的计算结果(Hier-Majumder and Courtier, 2011).较低压力条件下(≤3 GPa)，玄武质熔体对应的二面角范围为20°～50°(Kohlstedt, 1992)，碳酸盐熔体对应的二面角范围为25°～30°(Watson et al., 1990).但在更高的压力范围内(>3 GPa)，二面角会随着压力的增加而急剧减小，当压力大于7～8 GPa时，会小于10°，甚至接近于0°(Yoshino et al., 2007).因此，这里可以选取的范围是5°～30°.

熔体成分是弹性参数计算中的另一个重要影响因素.以等温体积模量对压力的偏导(K′)为例，橄榄岩熔体约为7，苦橄质熔体约为6，科马提岩熔体约为5，玄武质熔体约为4(Jing and Karato, 2008).如前文所述，上地幔深部的部分熔融是水参与下进行的，相对于残余固体，水更容易进入熔体(Aubaud et al., 2008)，因而生成的应该是含水熔体.不同于浅部部分熔融产生玄武质熔体(McKenzie and Bickle, 1988)，橄榄岩在上地幔底部干、湿条件下的部分熔融都生成了超基性熔体(Inoue, 1994; Herzberg and Zhang, 1996; Litasov and Ohtani, 2002).同样，碳酸盐化橄榄岩也能在上地幔底部发生部分熔融作用(Dasgupta et al., 2004; Dasgupta and Hirschmann, 2006).含水橄榄岩或碳酸盐化橄榄岩熔体更能代表上地幔底部部分熔融的熔体产物.同时为了参考和对比，也研究了干橄榄岩熔体和玄武质熔体.熔体状态方程用来计算其高压密度，所使用的参数见表1.

表1 弹性参数计算中使用的不同成分熔体数据Table 1 Data of melt used to calculate the elastic modulus

3 参数变化对熔体含量的影响

根据平衡几何模型，综合研究了一定范围内的熔体成分、位温、二面角和玄武质含量等参数对熔体含量变化的影响，结果如图2、图3和图4所示.

3.1 不同成分体系下熔体含量随位温的变化

图2给出了5种成分体系下熔体含量随位温的变化.图中可见，随着位温的增加，熔体含量明显降低.以IT8720+8wt.%H2O为例，A区位温从1300 K到1800 K，相应的熔体含量变化可达2.04 vol.%，而B区位温从1400 K到1800 K，对应的熔体含量变化为1.02 vol.%.位温相同的条件下，熔体含量大体上从IT8720、碳酸盐化橄榄岩、IT8720+2wt.%H2O、IT8720+8wt.%H2O到MORB依次增加.但这种增量较小，以1600 K位温为例，最大差值也仅为0.14 vol.%(A区)和0.08 vol.%(B区).其中碳酸盐化橄榄岩与IT8720+2wt.%H2O之间熔体含量非常接近，难以区别开.计算结果对成分变化不敏感的原因可能在于成分变化很难通过少量熔体对整体弹性性质产生较为显著的影响.

图2 不同成分体系下熔体含量随位温的变化玄武质含量为20 vol.%，二面角为10°.Fig.2 Variation of melt fraction with the increasing potential temperature within different compositions Basalt fraction is 20 vol.% and dihedral angle is 10°.

图3 不同二面角下熔体含量随位温的变化熔体成分为IT8720+8wt.%H2O，玄武质含量为20 vol.%.Fig.3 Variation of melt fraction with the increasing potential temperature for different dihedral angles Melt is IT8720+8wt.%H2O and basalt fraction is 20 vol.%.

图4 不同二面角下熔体含量随玄武质含量的变化熔体成分为IT8720+8wt.%H2O，位温为1600 K.Fig.4 Variation of melt fraction with the increasing basalt fraction for different dihedral angles Melt is IT8720+8wt.%H2O and potential temperature is 1600 K.

3.2 不同二面角下熔体含量随位温的变化

图3为不同二面角下熔体含量随位温的变化.如图所示，相同位温下，熔体含量随着二面角的增加而增加.以IT8720+8wt.%H2O，1600 K位温和20 vol.%玄武质含量为例，相应的熔体含量随二面角(5°～30°)的变化范围为1.86～2.62 vol.%(A区)和1.09～1.55 vol.%(B区).二面角越大熔体的连通性就越弱，产生相同低速异常就需要更多含量的熔体(Takei, 2002).Yoshino等(2007)的实验结果表明上地幔深部的二面角可能不大于10°，但其他相关实验中也发现熔体分布存在一定程度的各向异性(Jung and Waff, 1998)，这将部分抵消二面角随着压力增大而减小的趋势.尽管更小二面角(如5°)也是可能的，但考虑到5°与10°之间的二面角差别对熔体含量变化的影响很微弱，所以实际处理中，选择10°较为合理.

3.3 不同二面角下熔体含量随玄武质含量的变化

图4为不同二面角下熔体含量随玄武质含量的变化.二面角相同的条件下，熔体含量随着玄武质含量的增加而增加.以IT8720+8wt.%H2O，1600 K位温和10°二面角为例，随着玄武质含量的变化(0～40 vol.%)，熔体含量变化范围分别是1.71～2.42 vol.%(A区)和0.99～1.42 vol.%(B区).俯冲板片与地幔的相互作用可能将洋壳物质(玄武质)注入地幔(Xu et al., 2008; Stixrude and Lithgow-Bertelloni, 2012)，特别是MTZ(Lee and Chen, 2007).对于LVL所在的上地幔深部，注入作用可能较弱，玄武质含量可能略高于18 vol.%(全球平均组成)(Xu et al., 2008)，所以选取Hier-Majumder等(2014)采用的玄武质含量(20 vol.%).

4 讨论

上述计算部分给出了熔体含量随相关参数的变化范围，而为了获得更为确切的熔体含量值，需要结合华南东部区域地幔特征对相关参数做进一步合理约束.

华南东部内陆地区的地震活动性较弱(Zhang et al., 2003)，莫霍面起伏也很平缓(Zhang et al., 2011)，这反映了其较为平静的地幔环境，温度条件可能接近平均地幔，因而可以选择1600K作为区域的代表位温.同时，实验和热力学计算表明LVL内的熔体可能是比较富水的，水含量不小于10wt.%(Inoue et al., 2007; Hirschmann et al., 2009)，所以IT8720+8wt.%H2O可能更接近真实成分.因此，我们选择华南东部MTZ顶部LVL中可能的代表性熔体成分是IT8720+8wt.%H2O，位温1600 K，二面角10°，玄武质含量为20 vol.%.以这些参数为依据，估算出的华南东部MTZ顶部LVL中的熔体含量分别约为2.02 vol.%(A区)和1.18 vol.%(B区).需要指出的是，这里采用的平衡几何模型并没有考虑滞弹性效应(Karato and Spetzler, 1990; McCarthy and Takei, 2011)的影响，如果考虑在内，熔体含量可能会更小.

岩石学和地球化学相关资料表明，中生代晚期(110～90 Ma)华南地区的俯冲作用类型发生了剧烈的转变，从平俯冲转化为陡俯冲(Li et al., 2012).俯冲角度的增大意味着板片与地幔相互作用深度可能增加，当影响深度达到410 km间断面时，就可能扰动富水的MTZ物质上涌(Faccenna et al., 2010; Richard and Iwamori, 2010)，发生脱水部分熔融形成LVL.而在该俯冲期之前也可能存在类似的过程，其对应的LVL是否能保留至今还不清楚.此后，板块的俯冲方向又发生了多次变化(Sharp and Clague, 2006; Sun et al., 2007)，这些过程都可能不同程度地改造了LVL中的熔体含量.因此，现今LVL中熔体含量的显著差异可能是太平洋板片多期次俯冲作用叠加的结果.

5 结论

熔体很可能是华南东部下方MTZ顶部LVL速度异常的主导因素.华南东部多体系和多期次的俯冲作用既促使了MTZ的水化作用，又扰动了含水MTZ物质上涌，当穿过410 km间断面时发生脱水部分熔融作用，最终触发了LVL的形成.选择熔体成分是IT8720+8wt.%H2O，位温1600K，二面角10°，玄武质含量为20 vol.%条件下，根据平衡几何模型估算华南东部下方MTZ顶部LVL的平均熔体含量分别为2.02 vol.%(A区)和1.18 vol.%(B区).现今LVL中熔体含量的区域性显著差异可能是太平洋板片多期俯冲作用叠加的结果，想要厘清不同时期作用之间的关系还需要进一步的研究.

致谢 英国伦敦大学学院(UCL)Carolina Lithgow-Bertelloni教授提供了参考固体地幔数据，中国科学院大学周元泽副教授和青藏高原研究所李国辉博士在本文撰写中给予了很大帮助和支持，审稿人对文章提出了宝贵修改意见和建议，这里一并表示感谢！

Aubaud C, Hirschmann M M, Withers A C, et al. 2008. Hydrogen partitioning between melt, clinopyroxene, and garnet at 3 GPa in a hydrous MORB with 6 wt.%H2O.Contrib.Mineral.Petrol., 156(5): 607-625.

Bagley B, Courtier A M, Revenaugh J. 2009. Melting in the deep upper mantle oceanward of the Honshu slab.Phys.EarthPlanet.Inter., 175(3-4): 137-144.

Bercovici D, Karato S. 2003. Whole-mantle convection and the transition-zone water filter.Nature, 425(6953): 39-44.

Berryman J G. 1980. Long-wavelength propagation in composite elastic media I. Spherical inclusions.J.Acoust.Soc.Am., 68(6): 1809-1819.

Chen L, Zhu R X, Wang T. 2007. Progress in continental lithosphere studies.EarthScienceFrontiers(in Chinese), 14(2): 58-75.

Courtier A M, Jackson M G, Lawrence J F, et al. 2007. Correlation of seismic and petrologic thermometers suggests deep thermal anomalies beneath hotspots.EarthPlanet.Sci.Lett., 264(1-2): 308-316.

Courtier A M, Revenaugh J. 2007. Deep upper-mantle melting beneath the Tasman and Coral Seas detected with multiple ScS reverberations.EarthPlanet.Sci.Lett., 259(1-2): 66-76.

Dasgupta R, Hirschmann M M. 2006. Melting in the Earth′s deep upper mantle caused by carbon dioxide.Nature, 440(7084): 659-662.

Dasgupta R, Hirschmann M M, Withers A C. 2004. Deep global cycling of carbon constrained by the solidus of anhydrous, carbonated eclogite under upper mantle conditions.EarthPlanet.Sci.Lett., 227(1-2): 73-85.

Faccenna C, Becker T W, Lallemand S, et al. 2010. Subduction-triggered magmatic pulses: A new class of plumes?.EarthPlanet.Sci.Lett., 299(1-2): 54-68.

Ghosh S, Ohtani E, Litasov K, et al. 2007. Stability of carbonated magmas at the base of the Earth′s upper mantle.Geophys.Res.Lett., 34(22): L22312.

Guillot B, Sator N. 2007. A computer simulation study of natural silicate melts. Part II: High pressure properties.Geochim.Cosmochim.Acta, 71(18): 4538-4556.

Herzberg C, Zhang J Z. 1996. Melting experiments on anhydrous peridotite KLB-1: Compositions of magmas in the upper mantle and transition zone.J.Geophys.Res., 101(B4): 8271-8295.

Hier-Majumder S, Ricard Y, Bercovici D. 2006. Role of grain boundaries in magma migration and storage.EarthPlanet.Sci.Lett., 248(3-4): 735-749.

Hier-Majumder S, Courtier A. 2011. Seismic signature of small melt fraction atop the transition zone.EarthPlanet.Sci.Lett., 308(3-4): 334-342.

Hier-Majumder S, Keel E B, Courtier A M. 2014. The influence of temperature, bulk composition, and melting on the seismic signature of the low-velocity layer above the transition zone.J.Geophys.Res., 119(2): 971-983.

Hilde T W C, Uyeda S, Kroenke L. 1977. Evolution of the western Pacific and its margin.Tectonophysics, 38(1-2): 145-152, 155-165.

Hirschmann M M, Tenner T, Aubaud C, et al. 2009. Dehydration melting of nominally anhydrous mantle: The primacy of partitioning.Phys.EarthPlanet.Inter., 176(1-2): 54-68.

Honza E, Fujioka K. 2004. Formation of arcs and backarc basins inferred from the tectonic evolution of Southeast Asia since the Late Cretaceous.Tectonophysics, 384(1-4): 23-53.

Huang J L, Zhao D P. 2006. High-resolution mantle tomography of China and surrounding regions.J.Geophys.Res., 111(B9): B09305.

Huang R, Xu Y X, Luo Y H, et al. 2014. Mantle transition zone structure beneath Southeastern China and its implications for stagnant slab and water transportation in the mantle.PureAppl.Geophys., 171(9): 2129-2136.

Inoue T. 1994. Effect of water on melting phase relations and melt composition in the system Mg2SiO4-MgSiO3-H2O up to 15 GPa.Phys.EarthPlanet.Inter., 85(3-4): 237-263.

Inoue T, Kojima K, Irifune T. 2007. Water content of magma generated just above the 410 km seismic discontinuity. AGU Fall Meeting, Abstracts V24A-06.

Jasbinsek J J, Dueker K G, Hansen S M. 2010. Characterizing the 410 km discontinuity low-velocity layer beneath the LA RISTRA array in the North American Southwest.Geochem.Geophy.Geosy., 11(3): Q03008.

Jing Z C, Karato S I. 2008. Compositional effect on the pressure derivatives of bulk modulus of silicate melts.EarthPlanet.Sci.Lett., 272(1-2): 429-436.

Jung H, Waff H S. 1998. Olivine crystallographic control and anisotropic melt distribution in ultramafic partial melts.Geophys.Res.Lett., 25(15): 2901-2904.

Karato S, Spetzler H A. 1990. Defect microdynamics in minerals and solid-state mechanisms of seismic wave attenuation and velocity dispersion in the mantle.Rev.Geophys., 28(4): 399-421.

Katsura T, Yamada H, Nishikawa O, et al. 2004. Olivine-wadsleyite transition in the system (Mg, Fe)2SiO4.J.Geophys.Res., 109(B2): B02209.Kohlstedt D L. 1992. Structure, rheology and permeability of partially molten rocks at low melt fractions. ∥ Mantle Flow and Melt Generation at Mid-Ocean Ridges. 71: 103-121.

Lee C T A, Chen W P. 2007. Possible density segregation of subducted oceanic lithosphere along a weak serpentinite layer and implications for compositional stratification of the Earth′s mantle.EarthPlanet.Sci.Lett., 255(3-4): 357-366.

Li C, van der Hilst R D. 2010. Structure of the upper mantle and transition zone beneath Southeast Asia from traveltime tomography.J.Geophys.Res., 115(B7): B07308.

Li G H, Sui Y, Zhou Y Z. 2014. Low-velocity layer atop the mantle transition zone in the lower Yangtze Craton from P waveform triplication.ChineseJ.Geophys. (in Chinese), 57(7): 2362-2371, doi: 10.6038/cjg20140730.

Li Z X, Li X H, Chung S L, et al. 2012. Magmatic switch-on and switch-off along the South China continental margin since the Permian: Transition from an Andean-type to a Western Pacific-type plate boundary.Tectonophysics, 532-535: 271-290.

Litasov K, Ohtani E. 2002. Phase relations and melt compositions in CMAS-pyrolite-H2O system up to 25 GPa.Phys.EarthPlanet.Inter., 134(1-2): 105-127.

Mavko G M. 1980. Velocity and attenuation in partially molten rocks.J.Geophys.Res., 85(B10): 5173-5189.

McCarthy C, Takei Y. 2011. Anelasticity and viscosity of partially molten rock analogue: Toward seismic detection of small quantities of melt.Geophys.Res.Lett., 38(18): L18306.

McKenzie D, Bickle M J. 1988. The Volume and Composition of Melt Generated by Extension of the Lithosphere.J.Petrology, 29(3): 625-679.Obayashi M, Sugioka H, Yoshimitsu J, et al. 2006. High temperature anomalies oceanward of subducting slabs at the 410 km discontinuity.EarthPlanet.Sci.Lett., 243(1-2): 149-158.

O′Connell R J, Budiansky B. 1974. Seismic velocities in dry and saturated cracked solids.J.Geophys.Res., 79(35): 5412-5426.

Oreshin S I, Vinnik L P, Kiselev S G, et al. 2011. Deep seismic structure of the Indian shield, western Himalaya, Ladakh and Tibet.EarthPlanet.Sci.Lett., 307(3-4): 415-429.

Revenaugh J, Sipkin S A. 1994. Seismic evidence for silicate melt atop the 410 km mantle discontinuity.Nature, 369(6480): 474-476.

Richard G C, Iwamori H. 2010. Stagnant slab, wet plumes and Cenozoic volcanism in East Asia.Phys.EarthPlanet.Inter., 183(1-2): 280-287.

Sakamaki T, Suzuki A, Ohtani E. 2006. Stability of hydrous melt at the base of the Earth′s upper mantle.Nature, 439(7073): 192-194.

Schmandt B, Dueker K G, Hansen S M, et al. 2011. A sporadic low-velocity layer atop the western U. S. mantle transition zone and short-wavelength variations in transition zone discontinuities.Geochem.Geophy.Geosy., 12(8): Q08014.

Scott T, Kohlstedt D L. 2006. The effect of large melt fraction on the deformation behavior of peridotite.EarthPlanet.Sci.Lett., 246(3-4): 177-187.

Sharp W D, Clague D A. 2006. 50 Ma initiation of Hawaiian-Emperor bend records major change in Pacific plate motion.Science, 313(5791): 1281-1284.

Song T R A, Helmberger D V, Grand S P. 2004. Low-velocity zone atop the 410 km seismic discontinuity in the northwestern United States.Nature, 427(6974): 530-533.

Stixrude L, Lithgow-Bertelloni C. 2012. Geophysics of chemical heterogeneity in the mantle.Annu.Rev.EarthPlanet.Sci., 40: 569-595.

Sun W D, Ding X, Hu Y H, et al. 2007. The golden transformation of the Cretaceous plate subduction in the west Pacific.EarthPlanet.Sci.Lett., 262(3-4): 533-542.

Takei Y. 1998. Constitutive mechanical relations of solid-liquid composites in terms of grain-boundary contiguity.J.Geophys.Res., 103(B8): 18183-18203.

Takei Y. 2000. Acoustic properties of partially molten media studied on a simple binary system with a controllable dihedral angle.J.Geophys.Res., 105(B7): 16665-16682. Takei Y. 2002. Effect of pore geometry onVp/Vs: From equilibrium geometry to crack.J.Geophys.Res., 107(B2): ECV 6-1-ECV 6-12.

Tauzin B, Debayle E, Wittlinger G. 2010. Seismic evidence for a global low-velocity layer within the Earth′s upper mantle.NatureGeosci., 3(10): 718-721.

Vinnik L, Farra V. 2002. Subcratonic low-velocity layer and flood basalts.Geophys.Res.Lett., 29(4): 8-1-8-4.

Vinnik L, Farra V. 2007. Low S velocity atop the 410 km discontinuity and mantle plumes.EarthPlanet.Sci.Lett., 262(3-4): 398-412.

Vinnik L, Kumar M R, Kind R, et al. 2003. Super-deep low-velocity layer beneath the Arabian plate.Geophys.Res.Lett., 30(7): 1415.

von Bargen N, Waff H S. 1986. Permeabilities, interfacial areas and curvatures of partially molten systems: Results of numerical computations of equilibrium microstructures.J.Geophys.Res., 91(B9): 9261-9276.

Watson E B, Brenan J M, Baker D R. 1990. Distribution of fluids in the continental mantle. ∥ Continental mantle. Oxford: Oxford University Press, 111-125. Wimert J, Hier-Majumder S. 2012. A three-dimensional microgeodynamic model of melt geometry in the Earth′s deep interior.J.Geophys.Res., 117(B4): B04203.

Xu W B, Lithgow-Bertelloni C, Stixrude L, et al. 2008. The effect of bulk composition and temperature on mantle seismic structure.EarthPlanet.Sci.Lett., 275(1-2): 70-79.

Yoshino T, Takei Y, Wark D A, et al. 2005. Grain boundary wetness of texturally equilibrated rocks, with implications for seismic properties of the upper mantle.J.Geophys.Res., 110(B8): B08205.

Yoshino T, Nishihara Y, Karato S I. 2007. Complete wetting of olivine grain boundaries by a hydrous melt near the mantle transition zone.EarthPlanet.Sci.Lett., 256(3-4): 466-472.

Zhang P Z, Deng Q D, Zhang G M, et al. 2003. Active tectonic blocks and strong earthquakes in the continent of China.Sci.ChinaEarthSci., 46(2): 13-24.

Zhang Z J, Yang L Q, Teng J W, et al. 2011. An overview of the earth crust under China.Earth-Sci.Rev., 104(1-3): 143-166. Zhou X M, Sun T, Shen W Z, et al. 2006. Petrogenesis of Mesozoic granitoids and volcanic rocks in South China: a response to tectonic evolution.Episodes, 29(1): 26-33.

Zhou X Y, Ma M N, Xu Z S. 2014. Progress of the low velocity zone atop the mantle transition zone.ProgressinGeophysics(in Chinese), 29(4): 1615-1625, doi: 10.6038/pg20140417.

Zhu W L, Gaetani G A, Fusseis F, et al. 2011. Microtomography of partially molten rocks: three-dimensional melt distribution in mantle peridotite.Science, 332(6025): 88-91.

附中文参考文献

陈凌, 朱日祥, 王涛. 2007. 大陆岩石圈研究进展. 地学前缘, 14(2): 58-75.

李国辉, 眭怡, 周元泽. 2014. 基于P波三重震相的下扬子克拉通地幔转换带顶部低速层初探. 地球物理学报, 57(7): 2362-2371, doi: 10.6038/cjg20140730.

周晓亚, 马麦宁, 徐志双. 2014. 地幔过渡带顶面低速层的研究进展. 地球物理学进展, 29(4): 1615-1625, doi: 10.6038/pg20140417.

(本文编辑 胡素芳)

Estimation of the melt fraction within the low velocity layer atop the mantle transition zone beneath the eastern South China

ZHOU Xiao-Ya1,2, MA Mai-Ning1,2*, XU Zhi-Shuang1,2, HAN Lin1,2, WEI Dong-Ping1,2

1KeyLaboratoryofComputationalGeodynamics,ChineseAcademyofSciences,Beijing100049,China2CollegeofEarthSciences,UniversityofChineseAcademyofSciences,Beijing100049,China

Genesis and property of the low velocity layer (LVL) atop the mantle transition zone (MTZ) are of important implications for interior geodynamic process, such as patterns of mantle convection and material migration. Recent seismology observed an LVL beneath the eastern South China, one of typical areas associated with intensive long-term subduction. There is an obvious lateral variation in seismic behaviors of the LVL across two subdomains (A and B areas). The formation of the LVL is generally attributed to dehydration partial melting, therefore, a precise constraint of the melt fraction of the LVL is beneficial in understanding its intrinsic properties and corresponding geodynamic process.The equilibrium geometry model in a partial melting system is employed to estimate the effect of melt composition, potential temperature, dihedral angle and basalt fraction on the melt fraction. Five sets of typical melt compositions including MORB, dry peridotite (IT8720), hydrous peridotite (IT8720+2wt.%H2O; IT8720+8wt.%H2O) and carbonated peridotite are introduced. The range of potential temperature is 1300～1800K (A area) and 1400～1800K (B area). Dihedral angle varies from 5° to 30° and basalt fraction varies from 0 to 40 vol.%.Under a given condition, the melt fraction will monotonically decrease with the potential temperature while its variation is relatively insensitive to melt composition, particularly the almost undistinguished discrepancy between the IT8720+2wt.%H2O and carbonated peridotite. Additionally, our results show that the melt fraction will increase moderately with the basalt fraction and this tendency may represent relevant change in phase proportions. After evaluating the detailed mantle characteristics of the eastern South China, the creditable conditions of fore-mentioned factors may be IT8720+8wt.%H2O melt, a reference potential temperature of 1600 K, a dihedral angle of 10° and a basalt fraction of 20 vol.%. Therefore, the corresponding regional average values of melt fraction are about 2.02 vol.% (A area) and 1.18 vol.% (B area), respectively.The melt within the LVL may derive from the dehydration partial melting of ascending wet MTZ materials. The transition in subduction angle from flat to steep during about 110～90Ma provided clues for initial formation of the LVL, i.e., the interaction depth between the subduction slab and ambient mantle could extend to the MTZ and induce the upwelling of MTZ. Subsequent multiple-period subduction probably also triggered the similar process. From this point of view, current distribution of melt fraction might result from the overlap of multiple-period subduction in the northwestern Pacific.

South China; Mantle transition zone; Low velocity layer; Partial melting; Melt fraction

周晓亚， 马麦宁， 徐志双等. 2015. 华南东部地幔过渡带顶部低速层中的熔体含量估算.地球物理学报，58(9):3264-3271,

10.6038/cjg20150921.

Zhou X Y, Ma M N, Xu Z S, et al. 2015. Estimation of the melt fraction within the low velocity layer atop the mantle transition zone beneath the eastern South China.ChineseJ.Geophys. (in Chinese),58(9):3264-3271,doi:10.6038/cjg20150921.

10.6038/cjg20150921

P315

2015-04-20，2015-07-18收修定稿

国家自然科学基金(41274091，40774047)，中国科学院知识创新方向性项目(KZCX2-EW-QN602)，中国科学院与国家外国专家局创新团队国际合作伙伴计划项目(KZZD-EW-TZ-19)联合资助.

周晓亚，1986年生，博士研究生，主要从事高温高压下部分熔融体系的弹性性质研究.E-mail：zhouxiaoya10@mails.ucas.ac.cn

*通讯作者 马麦宁，1972年生，中国科学院大学副教授，主要从事岩石、矿物物理学研究.E-mail：mamn@ucas.ac.cn