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第2大陆 The Second Continent
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第2大陆 The Second Continent
上一篇博客文章《洋壳和陆壳的深俯冲命运：来自地幔相变研究的观点》，从地幔相变研究方面，根据岩石和矿物密度关系，对大陆地壳和大洋地壳的深俯冲命运进行了介绍。其中曾提到Komabayashi等（2009）的密度关系计算表明，代表性大陆地壳岩石TTG有可能俯冲到转换带底部。本文将参考最新的研究结果作一扩展阅读，探讨TTG（大陆地壳）俯冲至地幔转换带中并稳定存在的可能性。
相关文章《第2大陆》（The Second Continent）发表在日本《地学雑誌》（Journal of Geography）上，三位作者均为著名地球科学家河合研志、 土屋卓久 、丸山茂徳（Kenji KAWAI, Taku TSUCHIYA and Shigenori MARUYAMA）。所谓“第2大陆”是指在地质历史时期由深俯冲作用带入到地幔中的大陆地壳物质的集合体，下文将详细介绍。而存在于地表的大陆则可以相对称为“第1大陆”。原文为日文，所以我将部分翻译理解的内容介绍给大家。
原文摘要如下：
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Abstract
Recent progress in our understanding of the consuming plate boundary indicates the ubiquitous occurrence of tectonic erosion of the hanging wall of the continental margin, sediment-trapped subduction, and direct subduction of immature oceanic arcs into deep mantle. Geological studies have estimated the volume of subducted tonalite-trondhjemite-granodiorite （TTG）materials to about seven times the surface total volume of continental crust. To reveal the fate of subducted crusts and how they recycle within the Earth, we studied high-pressure densities and elastic properties of TTG by means of the first principles computation method and compared them to those of peridotite. We found that TTG is gravitationally stable and its seismic velocities are remarkably faster than peridotite in the depth range from 300 to 800 km, especially from 300 to 670 km. We, therefore, propose SiO2-rich second continents in the mantle transition zone, which used to form the TTG crust on the Earth’s surface. Our proposed model may provide reasonable explanations of seismological observations such as the splitting of the 670 km discontinuity and seismic scatterers in the uppermost part of the lower mantle. The difference in seismic velocities between PREM model and experimental results in the lower part of the transition zone can be explained by 25 volumetric% of TTG, which would correspond to about six times the present volume of the continental crust. Formation and dynamics of those second continents would have controlled the Earth's thermal history over geologic time.
Key words： granite, subduction, second continent, density, first-principle calculation, identification of TTG crust in the mantle, tectonic erosion
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大陆占据了地球表面大约1/3的面积，大陆地壳（平均35km厚）是由上部大约15-20km厚的以花岗岩为主体的上地壳和基性下地壳（15-20km厚）所组成。花岗岩地壳是在板块汇聚处产生，例如现在日本东北岛弧火山作用，岩浆由太平洋板块脱水所产生的部分熔融作用所形成。酸性岩浆固化后形成的岩石密度约为2.8 g/cm3，比地幔平均值（3.5 g/cm3）小，所以大陆能够“漂浮”在地球表面。
但是在漫长的地质历史时期中，大陆并不是一直都稳定存在的，在俯冲带由于构造侵蚀（tectonic erosion）作用（参考 山本，2010），大陆物质被不断地被洋壳“刮削”到地球深部，而且被“刮削”到地球深部的大陆地壳物质总量是现今地表大陆地壳物质总量的几倍。在俯冲带大陆地壳物质进入地幔中，按照5km3/yr的速率（Clift and Hartley,2007），在过去的40亿年中俯冲下去的总量大约为地表大陆地壳总量的3倍。
Fig.1 Schematic image of mechanism by which granite is transported from the Earth's surface to the deep mantle.
花岗岩石大陆地壳中最常见的岩石类型，主要由正长石、斜长石和石英组成；化学组成非常近似的tonalite-trondhjemite-granodiorite (TTG)岩石，Komabayashi等（2009）曾按照12.5%钠长石和87.5%石英比例近似计算花岗岩物质在地幔中的密度。实验研究表明，NaAlSi3O8钠长石在2-3 GPa、1300K分解为NaAlSi2O6硬玉+SiO2石英（Birch and Cecomte,1960）。NaAlSi2O6硬玉在大约23GPa、1300-1500K条件下分解为NaAlSiO4 CF相和SiO2斯石英(Liu,1978; Yagi et al.,1994)。
NaAlSi3O8(Ab)= NaAlSi2O6(Jd)+SiO2(Qtz) (1)
NaAlSi2O6(Jd)= NaAlSiO4(CF)+SiO2(St) (2)
本研究中对硬玉、CF相和斯石英三种矿物的弹性参数进行了计算。硬玉和CF相的相关晶格常数和晶体结构如图2a所示。图2b展示了硬玉、CF相以及斯石英的晶格体积压缩曲线，相关参数利用三阶Birch-Murnaghan状态方程进行最小二乘法拟合，硬玉拟合结果与实验值比较一致，而CF相比实验值略低（见Table 1）。CF相+斯石英组合与硬玉的相对焓值（enthalpy）比较见图2c，用来界定硬玉的分解条件，图中显示硬玉在大约18 GPa会分解为CF相+斯石英。考虑温度的影响，计算结果显示该分解反应具有正当克拉伯龙斜率（Clapeyron Slope），与实验结果一直。以3.1±1.0 MPa/K (Akaogi et al.,2002) 来计算，在1500K条件下该分解反应压力为22.5±1.5 GPa (638 ± 30 km)，与660 km不连续面深度压力非常接近。
Fig.2 (a) Crystal structures of NaAlSi2O6 jadeite and NaAlSiO4 CF-type phase. Yellow, light blue, dark blue and red spheres are Na, Al, Si, and O atoms, respectively. (b)Volumes calculated within LDA （bold lines）. Triangles indicate experimental results for jadeite（red）（Zhao et al., 1997） and stishovite（green）（Ross et al., 1990; Hemley et al., 1994）. Experimental volumes of the CF-type phase are computed using a third order Birch-Murnaghan equation of state with parameters proposed by Akaogi et al.(2002) (blue dotted line). (c)The enthalpy difference of the CF-type phase and stishovite mixture relative to the jadeite calculated based on the GGA.
图3展示了硬玉、CF相和斯石英在50 GPa以内压力条件下弹性常数的计算值，其中硬玉的计算值与实验结果非常一致（见Table 2）。
Fig 3. Elastic constants as a function of pressure. （a）-（c）show longitudinal, off-diagonal, and shear elastic constants for monoclinic jadeite, respectively. Open circles and squares indicate experimental results at 0 GPa of Kandelin and Weidner （1988）. （d）-（f）show the same groups for orthorhombic CF-type phase. （g）-（i）show the same groups for stishovite（or CaCl2 at 50 GPa）.
硬玉、CF相和斯石英（以及CF相+斯石英）的体积模量、剪切模量、P波和S波速度、密度关系如图4所示。CF相和斯石英比硬玉的密度分别高13.8%和18.3%，硬玉分解为CF相和斯石英组合后密度增加15.1%，P波、S波以及bulk sound velocity 分别增加17.3%、25.0%和12.0%。
Fig. 4 (a) Aggregate bulk and shear moduli of jadeite, CF-type phase and stishovite in the pressure range from 0 to 50 GPa. Open circles indicate experimental results for jadeite at 0 GPa of Kandelin and Weidner (1988). (b) Longitudinal, bulk and shear wave velocities and densities of jadeite, CF-type phase, and stishovite. (c) Velocities and densities of jadeite and an assemblage of CF-type phase and stishovite.
根据以上这些数据就可以求得TTG的密度和速度。在大约300km深度柯石英向斯石英转变，TTG的组成为硬玉和斯石英（1：8比例）(Komabayashi et al.,2009)。在大约640km深度硬玉分解，CF相和斯石英组成比例为1：9。分解前后TTG的密度和速度见图5，在大约660km深度TTG中硬玉分解后，密度增加4.4%，P波速度增大6.1%，S波速度增大8.3%。
橄榄岩中的主要矿物目前已经比较清楚，大约15-20 GPa压力范围内为瓦兹利石（wadsleyite），20-23.5 GPa为林伍德石（ringwoodite），在23.5 GPa（约660km深度）林伍德石分解为钙钛矿（perovskite）和铁方镁石（ferropericlase）。橄榄岩中橄榄石中Fe的含量简化为大约10 mol%，其弹性随Fe变化。钙钛矿和铁方镁石之间Fe的分配系数为大约0.3（钙钛矿5 mol%，铁方镁石15 mol%）。瓦兹利石-林伍德石相变（大约520km深度）所伴随的P波和S波速度分别增大1.9%和2.4%。后尖晶石相变（大约660km深度）所伴随的P波和S波速度分别增大5.9%和13.2%。
如图5，通过比较橄榄岩和TTG的密度发现，地幔转换带中TTG比橄榄岩密度大，在大约28 GPa（750 km）密度倒转。TTG在300-750 km深度范围内重力稳定，即300 km以下TTG将有可能继续俯冲至转换带深度。波速比较：TTG在15-20 GPa范围内P波和S波速度比橄榄岩分别高8.2%和12.3%，23.5-28 GPa P波和S波速度比橄榄岩分别高5%和1.8%。TTG会滞留在28 GPa（750 km），此时P波和S波速度不连续，分别降低6.2%和4.4%。
Fig. 5 Densities (a) and velocities (b) of peridotite and TTG.
橄榄岩中橄榄石-瓦兹利石-林伍德石相变以及后尖晶石相变分别对应410、520和660 km不连续面，密度也随之而产生突变。计算结果发现，在300 km深度范围内，花岗岩比橄榄岩密度低很多，而在300-660 km之间，花岗岩比橄榄岩密度高，直到750 km两者密度相近。这一结果表明，在到达300 km深度以后，由于相变作用，花岗岩地壳将产生负浮力，密度比地幔岩石高。在1500 K条件下花岗岩在640 km深度密度再次突变上升，由4.3g/cm3突变为4.5g/cm3。300 km深度负浮力作用将使花岗岩地壳物质沉入地幔并聚集在转换带底部。
上世纪80年代变质岩中柯石英的发现，证明地表大陆地壳物质可以俯冲至100 km深度并折返回地表。地质学家随后在超高压变质岩研究中取得了许多重要的成果，最大深度约200 km（~7 GPa），这与柯石英-斯石英相变深度300 km还有一定差距，因此如果能突破300 km，那么花岗岩产生的负浮力将使其难于折返回地表（depth of no return）。
在pyrolite地幔模型中，其上地幔地震波速与地球物理模型PREM (Dziewonski and Anderson,1981)比较一致，但是在转换带下部波速与PREM等模型还存在一定的差异 (e.g. Irifune et al.,2008; Cobden et al.,2008)。根据转换带下部波速与PREM等模型的差异 (Irifune et al.,2008)，来推算转换带下部可能存在的花岗岩的含量。如图6所示，当花岗岩体积含量占25%时，P波和S波两者差异较为一致。考虑到温度影响，Irifune et al.(2008)指出pyrolite和PREM波速上的差异可能是因为滞留在转换带中的俯冲板块（stagnant slab）里含有温度相对低400K的方辉橄榄岩。但是，整个转换带温度相对低400K是难于相信的，而且在转换带中方辉橄榄岩也比地幔密度小，所以方辉橄榄岩能否在转换带下部稳定存在仍需进一步的研究考证。
现在，如果地幔转换带下部520-660 km之间140 km厚的部分含有1/4的花岗岩物质，那么这一总量大约为由花岗岩构成的上中地壳的~6倍（整个大陆体积的3倍），这是根据Rino (2007)推算的结果。实际上如下文所讨论的，花岗岩物质在下地幔顶部滞留的可能性更大。但是这种估计忽略了温度和压力的影响作用，因此还需要进一步的研究和讨论。
研究表明环太平洋地区660 km不连续面存在着分裂，如Deuss and Woodhouse (2001)的报道。根据以上的讨论，可以用地幔中的后尖晶石相变和花岗岩中的硬玉分解反应来解释。推测转换带下部温度为1800 K，这时地幔中后尖晶石相变和硬玉分解反应相变压力非常接近。但是需要注意硬玉分解反应相变和后尖晶石相变分别具有正和负的克拉伯龙斜率，而如环太平洋俯冲带温度较低，该相变应可以通过地震波观测到。因此Deuss and Woodhouse (2001)观测到的660 km不连续面的分裂或许可以用这两个相变来解释。
Kaneshima (2009)在环太平洋地区海沟下地幔顶部800-1000 km深度观测到地震波散射，并认为是洋壳物质（basaltic crust）所形成的。但是在深俯冲过程中由于脱水变质作用以及部分熔融作用会使SiO2成分流失，而洋壳中SiO2相含量不到10%，因此用洋壳物质来解释下地幔顶部的地震波散射是比较困难的，实际上可能的解释是深俯冲的花岗岩物质。
Fig. 6 Difference of P and S wave velocities from the mantle average composition in the depth range from 520 to 670 km. The green dots indicate the volumetric% of TTG. The red dot indicates the difference between the experimental results of Irifune et al. (2008) and the PREM model （Dziewonski and Anderson, 1981）.
全球花岗质地壳的可能分布见图7，详细的解释请参考原文和该图说明。
Fig. 7 Schematic illustration of the regional distribution of First and Second Continents of the Earth, which was partly modified after Fig. 7 of Maruyama et al. (2007). Second Continents are compiled from P-wave mantle tomography of Huang and Zhao (2006) and subduction history of the Earth during the past 200 Ma. The lower figure is a cross section of the Earth along the line XY in the upper figure. Second continents could occur predominantly under Asia. Under the eastern margin of Asia it is underlain by the stagnant slab. The eastern part is locally separated into two by the penetrating slab. On the contrary, under Africa, second continents occur selectively above 660 km depth, presumably due to the absence of subduction underneath since 540 Ma. Plate subduction causes tectonic erosion at the trench to transport TTG materials into the mantle transition zone as well as direct arc subduction. These transport processes developed the Second Continents over geologic time.
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