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洋壳和陆壳的深俯冲命运：来自地幔相变研究的观点(2)
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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=space&uid=92454&do=blog&id=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)
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