華南東部地幔過渡帶頂部低速層中的熔體含量估算

周曉亞， 馬麥寧*， 徐志雙， 韓林， 魏東平

1 中國(guó)科學(xué)院計(jì)算地球動(dòng)力學(xué)重點(diǎn)實(shí)驗(yàn)室， 北京 100049 2 中國(guó)科學(xué)院大學(xué)地球科學(xué)學(xué)院， 北京 100049

?

華南東部地幔過渡帶頂部低速層中的熔體含量估算

周曉亞1,2， 馬麥寧1,2*， 徐志雙1,2， 韓林1,2， 魏東平1,2

1 中國(guó)科學(xué)院計(jì)算地球動(dòng)力學(xué)重點(diǎn)實(shí)驗(yàn)室， 北京 100049 2 中國(guó)科學(xué)院大學(xué)地球科學(xué)學(xué)院， 北京 100049

地幔過渡帶頂部低速層的成因及性質(zhì)研究，對(duì)于認(rèn)識(shí)地球內(nèi)部物質(zhì)運(yùn)移及地幔對(duì)流過程等具有非常重要的動(dòng)力學(xué)意義.最新的地震學(xué)研究顯示，華南陸塊東部地幔過渡帶頂部的低速層存在著明顯的區(qū)域性差異.一般認(rèn)為，該低速層的形成與脫水引起的部分熔融有關(guān).本文利用部分熔融體系的平衡幾何模型，重點(diǎn)分析了熔體成分、位溫、二面角和玄武質(zhì)含量等因素對(duì)熔體含量的影響，并結(jié)合該低速層的分布特征，估算出研究區(qū)的南北兩個(gè)子區(qū)域地幔過渡頂部熔體含量分別為～1.18 vol.%和～2.02 vol.%.這一熔體含量的顯著差異可能與太平洋板片多期次俯沖作用的疊加有關(guān).

華南； 地幔過渡帶； 低速層； 部分熔融； 熔體含量

1 引言

地球內(nèi)部410 km和660 km間斷面之間的地幔過渡帶(mantle transition zone, MTZ)是地球內(nèi)部結(jié)構(gòu)研究方面的熱點(diǎn)，精細(xì)地刻畫MTZ結(jié)構(gòu)對(duì)認(rèn)識(shí)地球深部的物質(zhì)組成和地幔對(duì)流動(dòng)力學(xué)過程等具有重要作用.自Revenaugh 和Sipkin(1994)首先報(bào)道了黃海、日本海下方的MTZ頂部的上地幔底部存在著低速層(low velocity layer, LVL)以來(lái)，越來(lái)越多的地震學(xué)觀測(cè)在全球多個(gè)地區(qū)都發(fā)現(xiàn)了類似的LVL，其主要集中分布于俯沖帶地區(qū)(Song et al., 2004; Obayashi et al., 2006; Courtier and Revenaugh, 2007; Jasbinsek et al., 2010; Schmandt et al., 2011)和伴有大火成巖省發(fā)育的大陸克拉通地區(qū)(Vinnik and Farra, 2002, 2007; Vinnik et al., 2003; Oreshin et al., 2011)，尤其是在俯沖帶的向陸一側(cè)更為發(fā)育(Bagley et al., 2009; Tauzin et al., 2010).

自中生代以來(lái)，華南陸塊東部受到了多體系和多期次俯沖作用的影響(Hilde et al., 1977; Honza and Fujioka, 2004; Zhou et al., 2006; Sun et al., 2007; Li and van der Hilst, 2010)，其現(xiàn)今位置處于西太平洋俯沖帶的向陸一側(cè)(Huang and Zhao, 2006).結(jié)合前人地震層析成像結(jié)果，Huang等(2014)根據(jù)接收函數(shù)給出的MTZ間斷面起伏形態(tài)，估計(jì)了其中的水含量，認(rèn)為華南東部MTZ中至少部分區(qū)域是比較富水的.李國(guó)輝等(2014)基于P波三重震相給出了華南地塊東部的北南兩個(gè)子區(qū)域A區(qū)和B區(qū)(圖1)的LVL厚度分別為57 km和40 km，相應(yīng)的P波速度降分別為4.5%和2.7%.這一精細(xì)探測(cè)結(jié)果為研究本區(qū)域LVL成因提供了可靠的地震學(xué)參數(shù).

高溫高壓礦物巖石物理實(shí)驗(yàn)研究表明，MTZ頂部LVL可能與部分熔融作用和熱異常等有關(guān)(周曉亞等，2014).Bercovici和Karato(2003)認(rèn)為，MTZ頂部LVL的形成與410 km間斷面處瓦茲利石(wadsleyite)相變?yōu)殚蠙焓瘯r(shí)，發(fā)生脫水誘發(fā)的部分熔融有關(guān).考慮到俯沖板塊通常給本區(qū)域MTZ帶來(lái)的是冷卻效應(yīng)，這里的LVL應(yīng)該與熱異常無(wú)關(guān).

雖然現(xiàn)有研究均傾向于華南東部MTZ頂部LVL的形成很可能與部分熔融有關(guān)，但LVL中熔體含量與低速異常之間的定量關(guān)系尚不清楚.本文根據(jù)已有研究資料，利用部分熔融體系中的平衡幾何模型(equilibrium geometry model)(Takei,1998,2002; Hier-Majumder et al., 2014)，探討了熔體成分、位溫、二面角和玄武質(zhì)含量等對(duì)熔體含量的影響，并根據(jù)華南東部的區(qū)域地幔環(huán)境選擇出代表性參數(shù)，對(duì)LVL中的熔體含量進(jìn)行了估算.

圖1 華南東部LVL的分布范圍虛線指示了范圍，A和B所連箭頭分別指示兩個(gè)子區(qū)域(修改自李國(guó)輝等，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 平衡幾何模型及參數(shù)選擇

定量的估算能夠?qū)е氯A南東部LVL的P波速度降分別達(dá)到4.5%(A區(qū))和2.7%(B區(qū))的熔體含量，既需要利用一定的巖石物理模型，也要考慮熔體幾何形態(tài)的影響.為此，本文選擇了具有代表性的平衡幾何模型(Takei, 2002; Hier-Majumder et al., 2014)來(lái)研究這一問題.

2.1 平衡幾何模型

合理的幾何模型對(duì)于由地震波速度變化來(lái)定量部分熔融體系中的熔體含量(體積分?jǐn)?shù))是至關(guān)重要的.相對(duì)于扁球狀(Berryman, 1980)、管狀(Mavko, 1980)和裂隙狀(O′Connell and Budiansky, 1974)等簡(jiǎn)化的熔體形態(tài)模型，受二面角大小控制的平衡幾何模型更接近于真實(shí)狀況(Yoshino et al., 2005; Zhu et al., 2011).

平衡幾何模型所描述的部分熔融體系中，固體顆粒之間以一定配位數(shù)(如12和14)的形式連接成格架(framework)，熔體分布在粒間孔隙中，而格架與未被填充的孔隙共同構(gòu)成了骨架(skeleton)，此時(shí)體系中熔體-固體界面能與固體-固體界面能之間達(dá)到力學(xué)上的平衡.若不考慮顆粒的各向異性，則熔體形態(tài)可完全由含量和二面角確定(Yoshino et al., 2005).

用平衡幾何模型計(jì)算有效體積模量和有效剪切模量主要是利用關(guān)于接觸度的函數(shù).接觸度即為部分熔融巖石中顆粒與顆粒接觸面積占總接觸面積(固-固接觸面積與固-液接觸面積的總和)的比例.當(dāng)熔體體積分?jǐn)?shù)(φ)介于0和臨界解聚分?jǐn)?shù)之間時(shí)，接觸度(ψ)依賴于熔體的體積分?jǐn)?shù)和二面角(θ)；而當(dāng)φ高于臨界解聚分?jǐn)?shù)時(shí)，體系結(jié)構(gòu)將從顆粒支撐轉(zhuǎn)變?yōu)橐后w支撐，顆粒孤立地懸浮于液體中(von Bargen and Waff, 1986; Wimert and Hier-Majumder, 2012).典型的臨界解聚分?jǐn)?shù)為20～30 vol.%(Scott and Kohlstedt, 2006; Hier-Majumder et al., 2006).

根據(jù)von Bargen和Waff (1986)給出的接觸度公式計(jì)算一定的φ和θ所對(duì)應(yīng)的ψ：

ψ=f(θ,φ)，

(1)

計(jì)算中采用了Takei(2000)所給出的校正.

有效彈性模量可以表達(dá)為ψ的形式：

N=μ(1-φ)g(ψ)，

(2)

Ke=

(3)

K和Km分別是固體和熔體的體積模量，μ是固體剪切模量，N和Ke分別是有效剪切模量和有效體積模量.g(ψ)和h(ψ)表示為

g(ψ)=1-(1-ψ)n1，

(4)

h(ψ)=1-(1-ψ)n2，

(5)

指數(shù)n1和n2也依賴于ψ(Takei, 2002).

根據(jù)平衡幾何模型，熔體所導(dǎo)致的P波速度變化為

(6)

γ=μ/K，

(7)

2.2 參數(shù)選擇

平衡幾何模型中涉及到的參數(shù)有：熔體成分、位溫(potential temperature)、二面角(dihedral angle)和玄武質(zhì)含量.其中，位溫指地幔絕熱地溫線對(duì)應(yīng)的地表溫度(陳凌等，2007).

要得到合理的熔體體積分?jǐn)?shù)，必須選擇合適的固體和熔體彈性參數(shù)范圍.而固體彈性參數(shù)的主要影響因素是溫度和組成成分.參考Hier-Majumder和Courtier(2011)以及Hier-Majumder等(2014)的方法，由Xu等(2008)所提供的數(shù)據(jù)庫(kù)來(lái)計(jì)算固體的彈性參數(shù).在該數(shù)據(jù)庫(kù)中，成分組成簡(jiǎn)化為玄武質(zhì)和方輝橄欖巖，選用玄武質(zhì)體積分?jǐn)?shù)來(lái)描述成分變化，平均地幔包含了18 vol.%的玄武質(zhì)成分，因而本研究采用與Hier-Majumder等(2014)的選擇相同的成分變化范圍0～40 vol.%.由于從橄欖石到瓦茲利石的克拉伯龍相變斜率為正(Katsura et al., 2004)，過低的位溫(如1300 K)可能使B區(qū)的LVL(深度為370 km)位于MTZ之內(nèi)(Xu et al., 2008)，同時(shí)研究區(qū)域內(nèi)并沒有發(fā)育地幔柱，1800 K(Courtier et al., 2007)可以作為研究區(qū)的位溫上限，因此選取的位溫范圍為1300～1800 K(A區(qū))和1400～1800 K(B區(qū)).

二面角是描述熔體形態(tài)的一個(gè)重要參數(shù)，能顯著地影響熔體體積分?jǐn)?shù)的計(jì)算結(jié)果(Hier-Majumder and Courtier, 2011).較低壓力條件下(≤3 GPa)，玄武質(zhì)熔體對(duì)應(yīng)的二面角范圍為20°～50°(Kohlstedt, 1992)，碳酸鹽熔體對(duì)應(yīng)的二面角范圍為25°～30°(Watson et al., 1990).但在更高的壓力范圍內(nèi)(>3 GPa)，二面角會(huì)隨著壓力的增加而急劇減小，當(dāng)壓力大于7～8 GPa時(shí)，會(huì)小于10°，甚至接近于0°(Yoshino et al., 2007).因此，這里可以選取的范圍是5°～30°.

熔體成分是彈性參數(shù)計(jì)算中的另一個(gè)重要影響因素.以等溫體積模量對(duì)壓力的偏導(dǎo)(K′)為例，橄欖巖熔體約為7，苦橄質(zhì)熔體約為6，科馬提巖熔體約為5，玄武質(zhì)熔體約為4(Jing and Karato, 2008).如前文所述，上地幔深部的部分熔融是水參與下進(jìn)行的，相對(duì)于殘余固體，水更容易進(jìn)入熔體(Aubaud et al., 2008)，因而生成的應(yīng)該是含水熔體.不同于淺部部分熔融產(chǎn)生玄武質(zhì)熔體(McKenzie and Bickle, 1988)，橄欖巖在上地幔底部干、濕條件下的部分熔融都生成了超基性熔體(Inoue, 1994; Herzberg and Zhang, 1996; Litasov and Ohtani, 2002).同樣，碳酸鹽化橄欖巖也能在上地幔底部發(fā)生部分熔融作用(Dasgupta et al., 2004; Dasgupta and Hirschmann, 2006).含水橄欖巖或碳酸鹽化橄欖巖熔體更能代表上地幔底部部分熔融的熔體產(chǎn)物.同時(shí)為了參考和對(duì)比，也研究了干橄欖巖熔體和玄武質(zhì)熔體.熔體狀態(tài)方程用來(lái)計(jì)算其高壓密度，所使用的參數(shù)見表1.

表1 彈性參數(shù)計(jì)算中使用的不同成分熔體數(shù)據(jù)Table 1 Data of melt used to calculate the elastic modulus

3 參數(shù)變化對(duì)熔體含量的影響

根據(jù)平衡幾何模型，綜合研究了一定范圍內(nèi)的熔體成分、位溫、二面角和玄武質(zhì)含量等參數(shù)對(duì)熔體含量變化的影響，結(jié)果如圖2、圖3和圖4所示.

3.1 不同成分體系下熔體含量隨位溫的變化

圖2給出了5種成分體系下熔體含量隨位溫的變化.圖中可見，隨著位溫的增加，熔體含量明顯降低.以IT8720+8wt.%H2O為例，A區(qū)位溫從1300 K到1800 K，相應(yīng)的熔體含量變化可達(dá)2.04 vol.%，而B區(qū)位溫從1400 K到1800 K，對(duì)應(yīng)的熔體含量變化為1.02 vol.%.位溫相同的條件下，熔體含量大體上從IT8720、碳酸鹽化橄欖巖、IT8720+2wt.%H2O、IT8720+8wt.%H2O到MORB依次增加.但這種增量較小，以1600 K位溫為例，最大差值也僅為0.14 vol.%(A區(qū))和0.08 vol.%(B區(qū)).其中碳酸鹽化橄欖巖與IT8720+2wt.%H2O之間熔體含量非常接近，難以區(qū)別開.計(jì)算結(jié)果對(duì)成分變化不敏感的原因可能在于成分變化很難通過少量熔體對(duì)整體彈性性質(zhì)產(chǎn)生較為顯著的影響.

圖2 不同成分體系下熔體含量隨位溫的變化玄武質(zhì)含量為20 vol.%，二面角為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°.

圖3 不同二面角下熔體含量隨位溫的變化熔體成分為IT8720+8wt.%H2O，玄武質(zhì)含量為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.%.

圖4 不同二面角下熔體含量隨玄武質(zhì)含量的變化熔體成分為IT8720+8wt.%H2O，位溫為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 不同二面角下熔體含量隨位溫的變化

圖3為不同二面角下熔體含量隨位溫的變化.如圖所示，相同位溫下，熔體含量隨著二面角的增加而增加.以IT8720+8wt.%H2O，1600 K位溫和20 vol.%玄武質(zhì)含量為例，相應(yīng)的熔體含量隨二面角(5°～30°)的變化范圍為1.86～2.62 vol.%(A區(qū))和1.09～1.55 vol.%(B區(qū)).二面角越大熔體的連通性就越弱，產(chǎn)生相同低速異常就需要更多含量的熔體(Takei, 2002).Yoshino等(2007)的實(shí)驗(yàn)結(jié)果表明上地幔深部的二面角可能不大于10°，但其他相關(guān)實(shí)驗(yàn)中也發(fā)現(xiàn)熔體分布存在一定程度的各向異性(Jung and Waff, 1998)，這將部分抵消二面角隨著壓力增大而減小的趨勢(shì).盡管更小二面角(如5°)也是可能的，但考慮到5°與10°之間的二面角差別對(duì)熔體含量變化的影響很微弱，所以實(shí)際處理中，選擇10°較為合理.

3.3 不同二面角下熔體含量隨玄武質(zhì)含量的變化

圖4為不同二面角下熔體含量隨玄武質(zhì)含量的變化.二面角相同的條件下，熔體含量隨著玄武質(zhì)含量的增加而增加.以IT8720+8wt.%H2O，1600 K位溫和10°二面角為例，隨著玄武質(zhì)含量的變化(0～40 vol.%)，熔體含量變化范圍分別是1.71～2.42 vol.%(A區(qū))和0.99～1.42 vol.%(B區(qū)).俯沖板片與地幔的相互作用可能將洋殼物質(zhì)(玄武質(zhì))注入地幔(Xu et al., 2008; Stixrude and Lithgow-Bertelloni, 2012)，特別是MTZ(Lee and Chen, 2007).對(duì)于LVL所在的上地幔深部，注入作用可能較弱，玄武質(zhì)含量可能略高于18 vol.%(全球平均組成)(Xu et al., 2008)，所以選取Hier-Majumder等(2014)采用的玄武質(zhì)含量(20 vol.%).

4 討論

上述計(jì)算部分給出了熔體含量隨相關(guān)參數(shù)的變化范圍，而為了獲得更為確切的熔體含量值，需要結(jié)合華南東部區(qū)域地幔特征對(duì)相關(guān)參數(shù)做進(jìn)一步合理約束.

華南東部?jī)?nèi)陸地區(qū)的地震活動(dòng)性較弱(Zhang et al., 2003)，莫霍面起伏也很平緩(Zhang et al., 2011)，這反映了其較為平靜的地幔環(huán)境，溫度條件可能接近平均地幔，因而可以選擇1600K作為區(qū)域的代表位溫.同時(shí)，實(shí)驗(yàn)和熱力學(xué)計(jì)算表明LVL內(nèi)的熔體可能是比較富水的，水含量不小于10wt.%(Inoue et al., 2007; Hirschmann et al., 2009)，所以IT8720+8wt.%H2O可能更接近真實(shí)成分.因此，我們選擇華南東部MTZ頂部LVL中可能的代表性熔體成分是IT8720+8wt.%H2O，位溫1600 K，二面角10°，玄武質(zhì)含量為20 vol.%.以這些參數(shù)為依據(jù)，估算出的華南東部MTZ頂部LVL中的熔體含量分別約為2.02 vol.%(A區(qū))和1.18 vol.%(B區(qū)).需要指出的是，這里采用的平衡幾何模型并沒有考慮滯彈性效應(yīng)(Karato and Spetzler, 1990; McCarthy and Takei, 2011)的影響，如果考慮在內(nèi)，熔體含量可能會(huì)更小.

巖石學(xué)和地球化學(xué)相關(guān)資料表明，中生代晚期(110～90 Ma)華南地區(qū)的俯沖作用類型發(fā)生了劇烈的轉(zhuǎn)變，從平俯沖轉(zhuǎn)化為陡俯沖(Li et al., 2012).俯沖角度的增大意味著板片與地幔相互作用深度可能增加，當(dāng)影響深度達(dá)到410 km間斷面時(shí)，就可能擾動(dòng)富水的MTZ物質(zhì)上涌(Faccenna et al., 2010; Richard and Iwamori, 2010)，發(fā)生脫水部分熔融形成LVL.而在該俯沖期之前也可能存在類似的過程，其對(duì)應(yīng)的LVL是否能保留至今還不清楚.此后，板塊的俯沖方向又發(fā)生了多次變化(Sharp and Clague, 2006; Sun et al., 2007)，這些過程都可能不同程度地改造了LVL中的熔體含量.因此，現(xiàn)今LVL中熔體含量的顯著差異可能是太平洋板片多期次俯沖作用疊加的結(jié)果.

5 結(jié)論

熔體很可能是華南東部下方MTZ頂部LVL速度異常的主導(dǎo)因素.華南東部多體系和多期次的俯沖作用既促使了MTZ的水化作用，又?jǐn)_動(dòng)了含水MTZ物質(zhì)上涌，當(dāng)穿過410 km間斷面時(shí)發(fā)生脫水部分熔融作用，最終觸發(fā)了LVL的形成.選擇熔體成分是IT8720+8wt.%H2O，位溫1600K，二面角10°，玄武質(zhì)含量為20 vol.%條件下，根據(jù)平衡幾何模型估算華南東部下方MTZ頂部LVL的平均熔體含量分別為2.02 vol.%(A區(qū))和1.18 vol.%(B區(qū)).現(xiàn)今LVL中熔體含量的區(qū)域性顯著差異可能是太平洋板片多期俯沖作用疊加的結(jié)果，想要厘清不同時(shí)期作用之間的關(guān)系還需要進(jìn)一步的研究.

致謝 英國(guó)倫敦大學(xué)學(xué)院(UCL)Carolina Lithgow-Bertelloni教授提供了參考固體地幔數(shù)據(jù)，中國(guó)科學(xué)院大學(xué)周元澤副教授和青藏高原研究所李國(guó)輝博士在本文撰寫中給予了很大幫助和支持，審稿人對(duì)文章提出了寶貴修改意見和建議，這里一并表示感謝！

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.

附中文參考文獻(xiàn)

陳凌, 朱日祥, 王濤. 2007. 大陸巖石圈研究進(jìn)展. 地學(xué)前緣, 14(2): 58-75.

李國(guó)輝, 眭怡, 周元澤. 2014. 基于P波三重震相的下?lián)P子克拉通地幔轉(zhuǎn)換帶頂部低速層初探. 地球物理學(xué)報(bào), 57(7): 2362-2371, doi: 10.6038/cjg20140730.

周曉亞, 馬麥寧, 徐志雙. 2014. 地幔過渡帶頂面低速層的研究進(jìn)展. 地球物理學(xué)進(jìn)展, 29(4): 1615-1625, doi: 10.6038/pg20140417.

(本文編輯 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

周曉亞， 馬麥寧， 徐志雙等. 2015. 華南東部地幔過渡帶頂部低速層中的熔體含量估算.地球物理學(xué)報(bào)，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

國(guó)家自然科學(xué)基金(41274091，40774047)，中國(guó)科學(xué)院知識(shí)創(chuàng)新方向性項(xiàng)目(KZCX2-EW-QN602)，中國(guó)科學(xué)院與國(guó)家外國(guó)專家局創(chuàng)新團(tuán)隊(duì)國(guó)際合作伙伴計(jì)劃項(xiàng)目(KZZD-EW-TZ-19)聯(lián)合資助.

周曉亞，1986年生，博士研究生，主要從事高溫高壓下部分熔融體系的彈性性質(zhì)研究.E-mail：zhouxiaoya10@mails.ucas.ac.cn

*通訊作者 馬麥寧，1972年生，中國(guó)科學(xué)院大學(xué)副教授，主要從事巖石、礦物物理學(xué)研究.E-mail：mamn@ucas.ac.cn