物理学家组织网（phys.org)2014年2月3日报道了一种类骨新材料，其密度比水还轻，但是其强度像钢一样硬。这种新材料由卡尔斯鲁厄理工学院( Karlsruher Institute of Technology )的 Jens Bauer和其合作者研制的，详细请浏览以下内容： New bone-like material is lighter than water but as strong as steelFebruary 3rd, 2014 in Physics / Condensed Matter Lighter, stronger, better. Credit: Jens Bauer Materials shape human progress – think stone age or bronze age. The 21st century has been referred to as the molecular age, a time when scientists are beginning to manipulate materials at the atomic level to create new substances with astounding properties. Taking a step in that direction, Jens Bauer at the Karlsruher Institute of Technology (KIT) and his colleagues have developed a bone-like material that is less dense than water, but as strong as some forms of steel. This is the first experimental proof that such materials can exist, Bauer said. Material world Since the Industrial Revolution our demand for new materials has outstripped supply. We want these materials to do many different things, from improving the speed of computers to withstanding the heat when entering Mars' atmosphere . However, a key feature of most new materials still remains in their strength and stiffness – that is, how much load can they carry without bending or buckling. All known materials can be represented quite neatly in one chart (where each line means the strength or density of the material goes up ten times): The line in the middle at 1000kg/m 3 is the density of water – all materials to its left are lighter than water and those on the right are heavier. No solid material is lighter than water unless it is porous. Porous materials like wood and bone exhibit exquisite structures when observed under a microscope, and they served as inspiration for Bauer's work. Credit: Jens Bauer/PNAS For many years, material scientists have thought that some empty areas on the compressive strength-density chart should be filled by materials that theory predicts. Computer simulations could be used to indicate an optimum microstructure that would give a material the right properties. However, nobody had tools to build materials with defined patterns at the scale of a human hair. With recent developments in lasers and 3D printing, however, a German company called Nanoscribe started offering lasers that could do just what Bauer wanted. Nanoscribe's system involves the use of a polymer that reacts when exposed to light and a laser that can be neatly focused on a tiny spot with the help of lenses. A drop of a honey-like polymer is placed on a glass slide and the laser is turned on. A computer-aided design is fed into the system and the slide carefully moves such that the laser's stationary focus touches only those points where the material is to be made solid. Once complete, the extra liquid is washed away, leaving behind materials with intricate internal structures. However, these materials on their own are not as strong as Bauer wanted. So he coats them with a thin layer of alumina (aluminium oxide) before subjecting them to stress tests. Based on the tests, he was able to improve the theoretical models he used to design the internal structure of the materials. Their results were just published in the Proceedings of the National Academy of Sciences . Even though alumina layers increase the density of these materials, all of them remain lighter than water. Bauer's strongest material has a specific honeycomb internal structure and is coated with a 50 nanometre-thick (billionth of a metre) layer of alumina. It beats all natural and man-made materials that are lighter than 1000kg/m 3 , being able to withstand a load of 280MPa (mega pascals is a unit of measuring pressure), which makes it as strong as some forms of steel. There are limitations. Nanoscribe's system can only make objects that are tens of micrometres in size. One of their newer machines can make materials in the milimetre-range, but that's about it for now, Bauer added. But that is not enough for any real-life application. However, there have been rapid improvements in all the areas this work relies on: 3D printing, new polymers and laser technology. That means we may soon have a suite of new, super lightweight materials for everything from skis to aircraft parts. If nothing else, Bauer's work shows that we are definitely in the molecular age. Source: The Conversation This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives). New bone-like material is lighter than water but as strong as steel. February 3rd, 2014. http://phys.org/news/2014-02-bone-like-material-lighter-strong-steel.html
洋壳和陆壳的深俯冲命运：来自地幔相变研究的观点(2) The Fate of Subducted Slabs：Perspectives from Studies of Phase Transitions in the Earth’s Mantle （续）接 《洋壳和陆壳的深俯冲命运：来自地幔相变研究的观点(1)》 http://bbs.sciencenet.cn/home.php?mod=spaceuid=92454do=blogid=422988 , 现为第3章和第4章。 3.大陆地壳(岩石圈)的深俯冲 大陆岩石圈（地壳）与大洋岩石圈具有较大的区别，其在年龄、物质、结构等方面均存在较大差异，地球化学组成上更加复杂。 目前对于大陆地壳的深俯冲研究资料比较有限，主要是针对代表性的大陆地壳物质成分进行高温高压实验，从矿物和岩石物性方面进行解释和分析。Irifune et al.(1994)]和Wu et al.(2009)分别对平均大陆上地壳成分（氧化物合成）和天然大陆上地壳岩石（副片麻岩）进行了高温高压相变实验。两者在物质成分上略有差异（参考Wu et al.,2009原文中的成分对比Table），区别是前者所使用的物质成分为化学合成样品，后者使用的是中国东部大别山双河地区的副片麻岩天然样品。 图14. Mineral proportion changes in the continental crust composition as a function of pressure. Point = the results of a mass-balance calculation using chemical composition data obtained in the present experiments; Cpx = clinopyroxene; Coe = coesite; Or = orthoclase; Ga = garnet; Ky = kyanite; Wd = K2Si4O9 wadeite; Hol = KAlSi3O8, hollandite; St = stishovite; CAS = unidentified Ca and Al-rich silicate; CaPv = CaSiO3 perovskite; CF = calcium ferrite-type phase. (Irifune et al.,1994) 图15. Mineral proportions of the subducted upper continental crust as a function of pressure. Ca–Pv, Ca pervoskite; Cs, coesite; Ep, epidote; C, graphite heater; Cpx, jadeite. Grt, garnet; Holl, KAlSi3O8-hollandite; Jd, jadeite; K-mica, K rich mica with unknown structure; Law, lawsonite; M, melt. Or, orthoclase; Phe, phengite; St, stishovite. (Wu et al.,2009) 随压力(深度)增加，所观测到的物相见图14和15，详细的相关系请参考原文叙述，在此不再赘述。Wu et al(2009)实验结果与 Irifune et al(1994)在氧化物体系的实验有如下两方面差异：（1）Wu et al(2009)的实验中未出现 CAS 相；（2）Wu et al(2009)研究中硬玉(Cpx)含量高于 Irifune et al(1994)的实验，且未观察到硬玉在约 24 GPa 分解为 NAL（NaAlSiO4）+斯石英。 图16. Zero-pressure density changes in the continental crust (CC) and pelagic sediment compositions (SIL = siliceous facies; ARG = argillaceous facies) as a function of pressure. Density changes in a pyrolite composition are also shown for comparison. (Irifune et al.,1994) 图17. Comparison of the calculated densities of the subducted continental crust and MORB (Aoki and Takahashi, 2004; Hirose et al., 1999) with respect to the density profile derived from PREM model (Dziewonski and Anderson, 1981). Density calculations were carried out along the three geotherms which are typical for cold and hot subduction and normal mantle. The thirdorder high-temperature Birch–Murnaghan equation of state was used in the density calculations.(from Wu et al.,2009) 结果发现，陆壳物质在8-9GPa以下其密度远远低于pyrolite的密度，但随后在斯石英和K-锰钡矿等高压矿物的形成以及连续的脱水作用下，陆壳物质的密度将超过pyrolite甚至MORB，在660km不连续面时pyrolite和俯冲陆壳物质的密度相近（图 16，17）；但随着压力继续增加进入下地幔时，林伍德石相变分解形成更高密度的钙钛矿和镁方铁矿，下地幔顶部岩石密度将再次远远大于深俯冲陆壳物质的密度。由此可见，大陆上地壳在深俯冲作用过程中或许至少可以俯冲到400km以下的地幔转换带中，这对于我们认识大陆俯冲动力学具有重要的意义。至于大陆上地壳岩石在更高压力的下地幔条件下行为如何，目前尚缺乏直接的高温高压实验数据。Irifune et al.(1994)推测，大陆上地壳物质的密度将会一直低于下地幔岩石的密度，致使俯冲陆壳板片被阻挡在地幔转换带底部处。 以上是对两个代表性的高温高压实验研究对大陆地壳深俯冲命运探索的介绍，下面继续介绍Komabayashi等（2009）对大陆代表性岩石的密度计算结果及其对大陆物质深俯冲命运的启示。 图18. Zero-pressure density profile to 27 GPa for TTG (this study), anorthosite (this study), MORB (Irifune and Ringwood, 1987; Hirose et al., 1999), pyrolite (Irifune and Ringwood, 1987), and harzburgite (Irifune and Ringwood, 1987). (from Komabayashi et al.,2009) Komabayashi等（2009）根据相关数据资料对大陆TTG岩石和斜长岩与MORB、pyrolite及方辉橄榄岩的密度对比计算表明（图18），TTG岩石在9-10 GPa左右斯石英矿物组合形成以后直至下地幔，其密度始终都是大于pyrolite；而斜长岩在9-10 GPa左右斯石英矿物组合形成以后直至转换带底部（24 GPa左右），其密度一直都大于pyrolite，但在转换带底部由于pyrolite中后尖晶石相变生成了更高密度的钙钛矿和镁方铁矿矿物组合，密度关系倒转；在25GPa左右斜长岩中的石榴石相变其密度再次短暂超过pyrolite，而在此深度以下，斜长岩的密度一直都略小于pyrolite。可见，大陆TTG岩石在深俯冲过程中是可以穿越660 km不连续面而进入下地幔甚至核幔边界；而对于斜长岩，预计中的大面积斜长岩目前在地表并未找到，可以认为大量的斜长岩在地质历史时期都发生深俯冲而进入了地幔中，由于俯冲物质具有相对较低的温度，这一温度效应可能使斜长岩在深俯冲过程中具有比周围地幔更高的密度，从而也可以穿越660 km不连续面而进入下地幔。 4. 小结 与大陆和大洋岩石圈深俯冲相关的岩石零压密度随深度的变化关系总结在图19中，在660km不连续面以上由于大陆地壳岩石和MORB中可以形成高密度的斯石英或者石榴石（榴辉岩），其密度将大于pyrolite，单从密度考虑有理由相信大陆岩石和玄武质岩洋壳可以发生深俯冲直到转换带底部。即使pyrolite中后尖晶石相变产生了更高密度的钙钛矿和镁方铁矿组合，但是俯冲带内由于具有相对较低的温度而使岩石密度可能更高，另外在下地幔顶部（700-800km）石榴石也逐渐完全转变成高密度的钙钛矿而产生拖拽力，从而使深俯冲的板块可以俯冲至下地幔。 虽然密度是非常重要的因素，然而影响板块俯冲的因素还需要考虑上下地幔的粘性以及相关的热力学因素，另外地球演化和地质历史时期中地幔内部的状态特征与目前的地幔亦有差异，这些因素对于研究古板块和现在的板块的深俯冲作用都具有重要影响作用。 图19. 几种代表性岩石零压密度随压力/深度变化的比较（周春银等，2010）.资料来源：Pyrolite（Irifune and Ringwood,1987）, MORB（Irifune and Ringwood,1987; Hirose et al.,1999）, 斜长岩（Komobayashi et al.,2009）, 副片麻岩（Wu et al.,2009）, TTG（Komobayashi et al.,2009）, 方辉橄榄岩（Irifune and Ringwood,1987）. 其中，副片麻岩在24GPa 以上压力条件下的密度变化目前尚缺乏相关的数据. (完) 参考文献： 费英伟, 2002. 地幔中的相变和地幔矿物学. In: 张有学 and 尹安 (Editors), 地球的结构、演化和动力学. 高等教育出版社, 北京, pp. 49-90. 周春银,金振民,章军锋,2010,地幔转换带:地球深部研究的重要方向,地学前缘, 17(3),90-113. Aoki, I. and Takahashi, E., 2004. Density of MORB eclogite in the upper mantle. Physics of the Earth and Planetary Interiors, 143-144: 129-143. Dziewonski, A.M. and Anderson, D.L., 1981. Preliminary reference Earth model. Physics of the Earth and Planetary Interiors, 25(4): 297-356. Kennett, B.L.N., Engdahl, E.R. and Buland, R., 1995. Constraints on seismic velocities in the Earth from traveltimes. Geophysical Journal International, 122(1): 108-124. Hirose, K., Fei, Y., Ma, Y. and Mao, H.-K., 1999. The fate of subducted basaltic crust in the Earth's lower mantle. Nature, 397(6714): 53-56. Irifune, T., Sekine, T., Ringwood, A.E. and Hibberson, W.O., 1986. The eclogite-garnetite transformation at high pressure and some geophysical implications. Earth and Planetary Science Letters, 77(2): 245-256. Irifune, T. and Ringwood, A.E., 1987. Phase transformations in a harzburgite composition to 26 GPa: implications for dynamical behaviour of the subducting slab. Earth and Planetary Science Letters, 86(2-4): 365-376. Irifune, T., 1993. Phase transformations in the earth's mantle and subducting slabs: Implications for their compositions, seismic velocity and density structures and dynamics. The Island Arc, 2(2): 55-71. Irifune, T. and Ringwood, A.E., 1993. Phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600-800 km in the mantle. Earth and Planetary Science Letters, 117(1-2): 101-110. Irifune, T., Ringwood, A.E. and Hibberson, W.O., 1994. Subduction of continental crust and terrigenous and pelagic sediments: an experimental study. Earth and Planetary Science Letters, 126(4): 351-368. Irifune T, Tsuchiya T, 2007. Mineralogy of the Earth – Phase Transitions and Mineralogy of the Lower Mantle, Treatise on Geophysics,vol2,Mineral Physics,33-62. Jamieson J.C., Fritz J.N., Manghnani M.H., Pressure measurement at high temperature in X-ray diffraction studies: gold as a primary standard, in: S. Akimoto, M.H. Manghnani (Eds.), High-Pressure Research in Geophysics, CAPJ, Tokyo, 1982, pp. 27– 48. Komabayashi, T., Maruyama, S. and Rino, S., 2009. A speculation on the structure of the D'' layer: The growth of anti-crust at the core-mantle boundary through the subduction history of the Earth. Gondwana Research, 15(3-4): 342-353. Ono, S., Ito, E. and Katsura, T., 2001. Mineralogy of subducted basaltic crust (MORB) from 25 to 37 GPa, and chemical heterogeneity of the lower mantle. Earth and Planetary Science Letters, 190(1-2): 57-63. Ono, S., Ohishi, Y., Isshiki, M. and Watanuki, T., 2005. In situ X-ray observations of phase assemblages in peridotite and basalt compositions at lower mantle conditions: Implications for density of subducted oceanic plate. J. Geophys. Res., 110: B02208,doi:10.1029/2004JB003196. Ringwood, A.E. and Irifune, T., 1988. Nature of the 650-km seismic discontinuity: implications for mantle dynamics and differentiation. Nature, 331(6152): 131-136. Tsuchiya T, First-principles prediction of the P–V–T equation of state of gold and the 660-km discontinuity in Earth’s mantle, J. Geophys. Res. 108 (2003) , doi:10.1029/2003JB002446. Wu, Y., Fei, Y., Jin, Z. and Liu, X., 2009. The fate of subducted Upper Continental Crust: An experimental study. Earth and Planetary Science Letters, 282(1-4): 275-284.
洋壳和陆壳的深俯冲命运：来自地幔相变研究的观点(1) The Fate of Subducted Slabs：Perspectives from Studies of Phase Transitions in the Earth’s Mantle 说明：由于本文插图众多，篇幅较长，不得不将原文章拆分为两部分，总共4章，第1章为前言和背景介绍，第2章讨论洋壳物质的深俯冲命运，第3章讨论陆壳物质的深俯冲命运，第4章小结。 1.前言 关于岩石圈（包括大陆岩石圈/地壳和大洋岩石圈/地壳）的深俯冲命运，是板块构造和地球动力学研究的永恒的话题。岩石圈的深俯冲命运，不可能单一地由某一地学学科（地球化学、地球物理、实验岩石学等）得到完整解释，必须结合多方面的研究成果来认识。本人学识有限，不可能面面俱到，在这里仅从地幔相变研究出发，来稍微做一点介绍。 由于本文讨论是以地幔相变为基础的，因此需要对基本的地幔矿物学知识有所了解才便于理解，大家可以参考本博《 地球内部的基本基本结构和物质组成 》 http://bbs.sciencenet.cn/home.php?mod=spaceuid=92454do=blogid=408337 及文后参考文献，以及费英伟（2002）的文章（见参考文献）；另外最重要的三类岩石的相变，即pyrolite（地幔岩）、方辉橄榄岩和玄武岩（MORB）体系的相变，本博上一篇文章《 地幔相变 Phase transitions in the Earth’s Mantle 》 http://bbs.sciencenet.cn/home.php?mod=spaceuid=92454do=blogid=419398 已经对此进行了介绍。需要强调地是，本文讨论是以《地幔转换带：地球深部研究的重要方向》一文中的3.5.2节“洋壳和陆壳的深俯冲命运”为主体而展开的，同时加入了更多的原始参考文献数据和图解，内容更加详实丰富。 本文将首先分别讨论洋壳和陆壳物质的深俯冲命运，然后结合Komabayashi等（2009）的文章对其他的代表性岩石的密度-深度剖面进行简单介绍。另外，由于实验条件有限，相关的岩石在下地幔条件下的高温高压相变实验报道很少，本文也将以作者所了解的文献为基础稍作说明。 2.大洋岩石圈的深俯冲 要认识大洋岩石圈的深俯冲命运，首先要对大洋岩石圈的基本结构有所了解。如图1所示，根据Ringwood的模型（e.g. Ringwood and Irifune,1988），大洋岩石圈顶部是一层几公里的相对较薄的玄武岩层（MORB），玄武岩层下面为20多公里厚的方辉橄榄岩层，更下部的为二辉橄榄岩以及“亏损”地幔岩成分物质。在大洋岩石圈深俯冲过程中，二辉橄榄岩以及“亏损”地幔岩层由于物质组成和性质与周围地幔相近，而将会被吸收进入周围地幔环境中，因此，玄武质洋壳（MORB）和方辉橄榄岩的性质变化决定着大洋板块深俯冲的最终命运。 图1. 大洋岩石圈结构图（from Ringwood and Irifune,1988） 2.1玄武质洋壳（MORB）深俯冲命运 The fate of subducted basaltic crust 图2. Phase relations in MORB composition up to 27GPa. Solid lines represent solidus and liquidus temperatures.（Hirose et al.,1999） 图3. Mineral proportions (wt%) in MORB as a function of depth （Perrillat et al.,2006）. The solid circles represent the phase proportions estimated in this study from Rietveld refinement of the in situ XRD spectra at 2050 K. Previous estimates by Ono et al. (2001), Hirose et al. (1999) and Irifune and Ringwood (1993) are reported as squares, triangles and open circles, respectively. Mineral abundances at shallower depth region are taken from Irifune et al.(1986). 图4. Experimental conditions and schematic phase relation of NMORB （Ono et al.,2005）. Phase assemblages are solid circles, Mg perovskite + Ca perovskite + stishovite + CaFe2O4- type aluminous phase; solid triangle, Mg perovskite + Ca perovskite + CaCl2-type silica + CaFe2O4-type aluminous phase; solid square, CaIrO3-type (Mg,Fe)SiO3 + Ca perovskite + a-PbO2-type silica + CaTi2O4-type aluminous phase. Solid diamonds present results from previous multianvil experiments . Abbreviations in the diagram of phase relation are GA, majoritic garnet; CF, CaFe2O4-type aluminous phase; MP, Mg perovskite; CP, Ca perovskite; ST, stishovite; CC, CaCl2-type silica; CT, CaTi2O4-type aluminous phase; CI, CaIrO3-type (Mg,Fe)SiO3; AP, a-PbO2-type silica. 玄武质洋壳（MORB）体系在地幔中的相变可以参考图2（上地幔），3（下地幔上部）和4（下地幔）。 随着俯冲深度的增加，其中的辉石会逐渐转变为石榴石，玄武岩相变为榴辉岩，而榴辉岩的密度超过了地幔橄榄岩（pyrolite）的密度，驱动着洋壳进一步俯冲到更深的地幔转换带底部（图5；Irifune and Ringwood,1993; Ringwood and Irifune,1988）。但是在转换带底部660处，地幔中的矿物林伍德石相变分解形成更高压高密度的矿物集合体（钙钛矿和镁方铁矿），而MORB由于Al含量比pyrolite更高，致使其中石榴石（majorite）的能够保持稳定至800km深度，石榴石密度比钙钛矿密度低~10%，那么俯冲洋壳的密度将小于周围地幔的密度，成为洋壳穿越转换带底部不连续面的阻碍（图5，6，7）。但是当MORB中的石榴石在27GPa左右完全相变为钙钛矿后，下地幔中MORB的密度将再次大于周围地幔密度（图6，7）。如果俯冲洋壳在转换带底部/上地幔顶部堆积能够突破浮力阻碍而进入下地幔，将可能继续俯冲至核幔边界（图8）。 图5. Density differences between MORB and pyrolite compositions and between harzburgite and pyrolite compositions as a function of depth. (Irifune and Ringwood,1993) 图6. Comparison of zero-pressure density changes in MORB (solid line) and pyrolite (dashed line) (Hirose et al.,1999). Solid circles represent the calculated densities at 24, 26 and 27GPa from X-ray diffraction and microprobe data. The density profle of pyrolite is from a previous study(Irifune and Ringwood,1987). Pyrolite becomes denser than MORB at 660km depth because of the transformation to perovskitite lithology, but once MORB transforms to perovskitite at 720km depth, it is no longer buoyant in the deep mantle. 图7. Comparison of calculated densities in MORB, with average mantle densities based on seismic observations (Ono et al.,2001). It was assumed that the proportions of major phases remained constant for different temperatures. Solid lines represent the isothermal density profiles. The average mantle densities are from Dziewonski and Anderson (1981): PREM and Kennett et al.(1995): AK. 图8. Net density profile of MORB composition.(Hirose et al.,2005) Pressure was calculated based on EOS of gold proposed by (a) Tsuchiya (2003) and by (b) Jamieson et al. (1982). Circles, MgPv+St+CaPv+CF; triangles, MgPv+CaCl2-type SiO2+CaPv+CF; squares, MgPP+a-PbO2-type SiO2+CaPv+CF. Closed and open symbols indicate 300 K and high temperature (1750–2290 K) data, respectively. Broken lines indicate the PREM density. The error of density is typically 0.02 g/cm3, derived from the uncertainties in volumes of coexisting phases and in mineral proportion. (a) Solid line shows a density profile at 300 K for perovskite-dominant assembly fitted to the Birch–Murnaghan equation of state. (b) Data by Ono et al. (2005) using Jamieson’s gold scale were shown for comparison (pluses). Slightly lower density reported by Ono et al. (2005) is primarily due to the lower density of CaFe2O4-type Al-phase with a different chemical composition. 2.2 大洋岩石圈方辉橄榄岩层的深俯冲命运 The fate of subducted harzburgite layer 方辉橄榄岩是俯冲大洋岩石圈中的另一种重要的岩石，它在俯冲过程中与MORB的相变行为不同。方辉橄榄岩的相变见图9. 图9. Mineral proportion changes in a harzburgite compositions as a function of pressure. (Irifune and Ringwood,1987)Opx = orthoenstatite; Cpx = clinoenstatite; St =stishovite; llm = ilrnenite. 根据前人的研究结果(Irifune and Ringwood,1987; Ringwood and Irifune,1988)，在660以上，方辉橄榄岩密度始终都小于pyrolite（图 10），这是由于方辉橄榄岩比pyrolite中Fe和Al含量均相对较低，而Al是高密度的石榴石的主要成分之一。在下地幔顶部（24-26GPa），同样由于pyrolite中Al使石榴石稳定至更深部，方辉橄榄岩的密度才略大于周围地幔以及MORB的密度。但是随着pyrolite和MORB中石榴石在27GPa左右完全转变成钙钛矿，此后一直到核幔边界，方辉橄榄岩的密度将始终略微小于相同深度的下地幔岩石（图11）。但是这并不能简单地就此而认为方辉橄榄岩完全无俯冲至下地幔的可能，且看下面的分析。 图10. Density profiles in the harzburgite, MORB and pyrolite compositions along the geotherm as a function of depth. (Irifune and Ringwood,1987) 图11. Bulks density variations of pyrolite, hartzburgite, and MORB calculated, based on the PVT-EoS of constituent mineral phases and their proportions (refer to the paper for details). Broken lines at pressures lower than 30 GPa are results in Irifune (1993).(from Irifune and Tsuchiya,2007) 根据Ringwood的大洋岩石圈模型，Irifune和Ringwood（1987，1988）将玄武岩（MORB）和方辉橄榄岩以1：4的比例（图1，玄武岩和方辉橄榄岩层厚度比大约为1：4）混合来代表深俯冲洋壳的组分，研究俯冲洋壳与周围地幔的密度关系，结果发现在650km以上俯冲洋壳始终比pyrolite密度大，但在下地幔顶部密度关系又倒转过来，而在下地幔700km左右洋壳的密度将再次大于pyrolite，在此深度以下，二者的密度非常接近（图12）。以上结果表明，影响洋壳深俯冲最终命运的关键问题在于能否突破地幔转换带底部的浮力障碍。 图12. Density differences between subducted slab and surrounding mantle. (Ringwood and Irifune,1988) The slab is assumed to consist of 20% basalt and 80% harzburgite and to be cooler than surrounding mantle by 800°C at 400 km and by 400°C at 650 km, attaining thermal equilibrium with surrounding mantle at ~900 km. The surrounding mantle follows the geotherm of Brown and Shankland (1981) and consists of pyrolite above 600 and below 700 km. Between these depths the mantle consists of a pre-existing layer of basalt and harzburgite (图13). Ringwood和Irifune则提出在转换带底部，俯冲的洋壳物质（玄武岩和方辉橄榄岩）可能在660附近堆积而形成一个“巨石”（megalith）(图13；Ringwood and Irifune,1988)，这些堆积或残留在转换带底部的洋壳物质及“巨石”可以在横向和纵向上伸展，很可能与所观测到的地震波异常有关；温度相对较低的“巨石”由于高密度而将沉入下地幔中。但是俯冲的洋壳与周围地幔的密度关系非常复杂，相边界的压力-温度斜率此时具有重要意义：转换带底部的主要矿物相是林伍德石和石榴石（majorite），后尖晶石相变和石榴石-钙钛矿相变分别具有负的和正的P-T斜率(Clapeyron Slope)，而俯冲带内（或者“巨石”）温度相对周围地幔要低300-400℃，那么意味着在下地幔顶部pyrolite中林伍德石转变成高密度的钙钛矿和镁方铁矿矿物组合后，俯冲带内相对低密度的林伍德石(橄榄石组分)仍可能保持稳定而未发生分解，而石榴石成分则可能已经转变成更高密度的钙钛矿了，由此俯冲带内林伍德石和石榴石组分此时将分别产生正的和负的浮力，二者综合作用的效果尚需进一步的研究。 图13. Model showing subduction of a cool, thick plate of differentiated oceanic lithosphere. Previous subduction episodes involving thin, thermally equilibrated plates have produced a layer of former harzburgite and basalt ('ancient oceanic lithosphere') between 600 and 700 km. The tip of a cool, thick plate experiences buoyant resistance when it penetrates this layer and encounters the discontinuity at 650 km (the 670-km discontinuity in the above figure should read 650-km discontinuity). At that depth the fonner oceanic crust and harzburgite layers may plastically thicken and buckle to form a large melange (megalith) situated mainly below the seismic discontinuity. The megalith is a transient feature and ultimately becomes entrained in the convective regime of the lower mantle. The lower layer of ductile depleted pyrolite initially at the base of the descending plate of sub-oceanic lithosphere becomes resorbed into the upper mantle by convective circulation owing to its inability to penetrate the harzburgite-basalt layer at 600-700 km because of the buoyancy relationships. (from Ringwood and Irifune,1988) (未完，下文见《洋壳和陆壳的深俯冲命运 ：来自地幔相变研究的观点(2)》 http://bbs.sciencenet.cn/home.php?mod=spaceuid=92454do=blogid=423022 ) 参考文献： 费英伟, 2002. 地幔中的相变和地幔矿物学. 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标签: 密度
