平衡冷魅总裁的纯纯小丫头

第57卷第4期2011年7月地质论评GEOLOGICALREVIEWVol.
57No.
4July2011注:本文为国家自然科学基金资助项目(编号10674034;10374022)的成果.
收稿日期:20110112;改回日期:20110528;责任编辑:章雨旭.
作者简介:苏文辉,男,1936年生.
教授,博导,高压高温极端条件凝聚态物理与物质合成.
通讯地址:150080,哈尔滨工业大学凝聚态科学与技术研究中心;Email:suwenhui@hit.
edu.
cn.
远离平衡相边界的柯石英形成机制及板块折返假说的物理基础苏文辉1,2,3)1)哈尔滨工业大学凝聚态科学与技术研究中心,哈尔滨,150080;2)吉林大学凝聚态物理系,长春,130023;3)中国科学院国际材料物理中心,沈阳,110015内容提要:本文从热力学、物理学、物质结构和高压高温相变的角度对板块俯冲—折返假说的物理基础进行了分析讨论.
在实际不均匀固体地球高压高温热力学体系中,地表柯石英超高压变质的非平衡态形成模式比平衡态形成模式有更大的普遍性、多样性;"静态流体地球"模型和静岩压力与深度换算公式,对其中的非平衡态体系,和地表中可能存在的局域准静水压平衡态体系,都不适用.
把压力直接换算成地球深度,断言地表矿物柯石英就是产生于上地幔的稳定相的结论,以及回避俯冲原动力,把地表柯石英和金刚石超高压变质物的存在与深俯冲之间划上恒等号,缺乏科学根据.
实际不均匀固体地球是一个天然的高压高温体系,为了保存高压高温相柯石英,岩石圈内板块折返冷却速率应与实验室中所进行的保压淬火临界速率具有可比性.
从实验室的高压高温相变规律看,如此极端缓慢的板块折返抬升冷却速率很难保持住柯石英.
有关影响折返过程保存柯石英的四个重要因素还存在很多矛盾问题.
物质中原子扩散和化学反应时间通常落在μs~ms范围,而建立平衡时间在min~102h之间.
在由超高压变质岩的同位素定年法和扩散动力学冷却速率法所得构造变质过程的时间内,己有极其充足的时间在几十千米的有限尺度中,进行扩散反应、产物变质,乃至于建立同位素平衡.
由于波速异常成因存在的多解性,板块俯冲—折返通道(轨迹),不应仅由地震层析波速异常图来确定.
板块俯冲与折返运动的变化应遵循能量和质量守恒定律,在P—T—t轨迹(通道)中应留下数量较多的俯冲—折返过程陆壳岩石、地幔物质的一次相变或二次相变等产物.
回顾板块俯冲—折返假说本身的发展过程,最关键的一步,是忽视了固体地球物质的成分、应力等局部不均匀性所造成的局部高压微区和其它非平衡热力学因素影响等事实,沿用传统的静流体模型及其压力—深度换算公式.
从物理基础看,板块深俯冲快折返的"壮观地质事件"的猜想,尚需更多更科学的证据去证实.
关键词:石英—柯石英;超高压变质;非平衡态;压力与深度换算公式;板块深俯冲快折返;同位素定年法;扩散动力学冷却速率法;动态挤压;剪切作用;局域高压微区在实验室人工合成出石英的第一个高压相柯石英(Coes,1953),并未引起人们的足够重视,直到Chopin(1984)和Smith(1984)分别在意大利和挪威地表中发现榴辉岩包裹体中含有天然柯石英,并提出和发展了地表柯石英形成机制的"地球板块俯冲—折返"假说(Chopin,1984,1987;Smith,1984,1995)时,才引起了地学界的高度重视,使得柯石英的实验室研究和地表柯石英形成机制的分析,变成了人们感兴趣的热点重大科学问题.
迄今,"板块俯冲—折返"假说一直为国内外地表柯石英形成机制的唯一主流学说,27年来,发表了包括Nature、Science在内的上千篇论文,取得了大量研究成果.
然而,国内外对实验结果的"板块俯冲—折返"假说分析,一直存在激烈的争议.
现在回过头来,利用物理学基本观点和规律剖析己经获得的重要结果的内涵,有着十分重要的学术意义.
物理学是研究宇宙不同层次物质的基本结构、相互作用,普遍的运动形式及其相互转化规律的科学,是许多其它自然科学和工程技术科学的基础.
地质构造学是研究地球构造的物质层次的科学,理应遵从物理学的普遍客观规律;也应该受构成固体地球的物质的结构、性质、行为的制约;显然,固体地球是一个天然的高压高温系统,其组成岩石矿物物质的高压高温相变与固体物质的高压高温相变规律亦应具有共性.
本文首先从热力学,然后从物理学、物质结构和高压高温相变的角度对"地球板块俯冲—折返"假说进行物理基础的分析和讨论,期望对不同的地表柯石英形成机制的争论和认识,会有重要的帮助.
笔者等以"一种无需板块俯冲折返的地表柯石英形成新机制"为题(苏文辉柯石英研究组),答复了嵇少丞教授、许志琴研究员和金振民教授等的质疑(嵇少丞等,2010),本文是其续篇,针对地球板块深俯冲快折返假说的物理基础问题进一步与同行们商榷.
1石英—柯石英高压结构转变相图国际上在实验室中广泛进行了很多石英—柯石英高压结构转变的平衡相边界研究(MacDonald,1956;KitaharaandKennedy,1964;Akella,1979;MirwaldandMassonne,1980;BohlenandBoettcher,1982;BoseandGanguly,1995;Hemingwayetal.
,1998;Gasparik,2003),1984年以前所得的石英—柯石英高压高温P—T平衡相图,成为Chopin(1984)、Smith(1984)等当时提出和发展地球板块俯冲—折返假说的重要依据之一.
众所周知,实验室开展的石英—柯石英高压转变平衡相边界研究,一般是在静水压,特别是在固态介质"准"静水压环境中,利用近平衡边界附近的压力温度条件,进行合成产物研究,由实验测定出石英—柯石英高压转变的平衡相边界.
从热力学的观点看,这种体系是除了在稳定的压力、温度热力学量的控制外没有其它的外界影响;而且体系内各局部的压力和温度近似地都与体系稳定的压力和温度相同;体系内也没有P、T以外影响石英—柯石英转变的因素;因此,这种体系是一种平衡态热力学体系.
然而,在实际的相变过程中,往往与时间变量t有关,温压亦与空间具体位置相联系,还可能会有其它相变机制或影响相变的因素的影响,因而体系处于非平衡状态,其合成产物相图是远离热力学平衡相边界的石英—柯石英相图.
当进行岩石圈中石英—柯石英高压、超高压变质研究时,持"板块俯冲—折返"假说者常常引用上述的石英—柯石英高压转变平衡相边界确认柯石英转变压力来讨论形成机制问题,并默认地表柯石英超高压变质是一平衡热力学过程;进一步,还利用把地球岩石圈看作静止均匀流体模型由帕斯卡原理获得某一点的压力P=gρh的近似公式,将压力换算成深度h(式中g为重力加速度,ρ为岩石密度).
这个模型认为地球内的压力仅由重力决定,把系统看作仅在稳定不变的压力梯度、温度梯度和密度梯度的作用下,再没有其它因素影响石英—柯石英的相变,是一种平衡态热力学系统.
显然,这种近似模型在研究地球的圈层定性模型上还是适用的;但是要应用到实际固体地球的研究,显得太粗糙,太理想化,与实际相距太远.
在实际固体地球中,特别是在岩石圈中,因组成物质、应力的不均匀性和小面积作用原理所形成的小尺度不均匀局域高压微区(苏文辉等,2009;苏文辉柯石英研究组),与静态流体平衡态热力学系统的稳定不变的压力梯度、温度梯度和密度梯度有很大的不同;还有,挤压、剪切等重要作用及其导致的形变、非晶化等效应,以及化学环境等,都直接影响石英—柯石英高压相转变(苏文辉等,2005,2009;SuWenhuietal.
,2006;苏文辉柯石英研究组),显然,它们是一种处于非平衡状态的热力学系统,其合成产物相图也是远离热力学平衡相边界的石英—柯石英相图(苏文辉等,2009;苏文辉柯石英研究组).
远离热力学平衡相边界的石英—柯石英相图,可以分成远离但大于和远离但小于热力学平衡相边界的两部分,详见图1.
以石英—柯石英平衡相边界诸直线为参照系,其上方的数据(以表示),是属于远离但大于热力学平衡相边界的非平衡状态:其中有一部分,以非晶态石英为起始原料,在高压高温作用下,经由中间亚稳相或原子的弛豫最后转变成柯石英(如非晶纳米SiO2,35GPa,800℃合成单相柯石英),与石英—柯石英平衡相边界诸直线的石英+柯石英体系不同;另一部分,晶态或非晶态石英起始原料先经过等效于挤压和剪切作用的高能机械球磨的预处理,引进了动态非平衡因素,有的还有较大不同的化学成分环境(如α石英起始原料,经高能机械球磨预处理,30GPa,650℃合成单相柯石英)(苏文辉等,2005,2009;SuWenhuietal.
,2006;苏文辉柯石英研究组),也与石英—柯石英平衡相边界诸直线的石英+柯石英体系不一样,现把这些数据标于图1中.
令人感兴趣的还有那些远离但小于石英—柯石英热力学平衡相边界诸直线、处在直线下方的非平衡系统的数据.
如号数据(非晶纳米SiO2+碳纳米管起始材料,经高能机械球磨预处理,20GPa,320℃条件下找到了尺寸为20nm的柯石英)标在图1中直线下部,验证了小尺度不均匀局域高压微区形成地表柯石英的模型(苏文辉等,2009;苏文辉柯石英研究组).
另一号数据(α—石英原粉,经高854地质论评2011年图1柯石英表观形成压力P和温度T的关系Fig.
1Therelationsofapparentpressureandtemperatureforcoesite资料来源/datafrom:点/dots:—苏文辉等,2005,2009;SuWenhuietal.
,2006;—苏文辉等,2009;苏文辉柯石英研究组;—Coes,1953;35GPa,750℃;—Nakaetal.
,1972;—Hobbs,1968;—GreenⅡ,1972;#—周永胜等,2005;—Martinezetal.
,2008);—HirthandTullis,1994线/lines:A79—Akella,1979;BB82—BohlenandBoettcher,1982;BG95—BoseandGanguly,1995;G03—Gasparik,2003;H98—Hemingwayetal.
,1998;KK64—KitaharaandKennedy,1964能机械球磨之后,加入10%的近球形微小Fe粒;在25GPa,700℃下形成柯石英,其Raman谱,XRD谱均明显可见)也列于图1中直线之下,验证了小面积作用原理形成高压微区与机械球磨作用(苏文辉柯石英研究组).
Naka等(1972)以SiO2+柯石英为起始材料,系统内有剪切力作用,测出了对应干和湿状态的两条石英—柯石英平衡相边界.
如以图1诸直线为参照系,可把剪切力看作作用于系统的外力,则该系变成了热力学非平衡系;其干状态的石英—柯石英相边界的部分数据(如23GPa,约880℃)便是处在图中直线之下方,以号表示之.
如以图1诸直线为参照系,对于有大形变造成晶格大畸变的样品情况,都可视为非平衡态系统.
Hobbs(1968)以天然和人工合成的石英单晶作起始原料,经形变428%(形变速率10-5/s),在50%,属塑性流变;在225GPa(围压10GPa+差应力1~15GPa),950℃条件下,在样品顶底端部出现柯石英,以号表示标在图中直线之下方.
周永胜等(2005)以石英岩(SiO2>990%)为起始原料,应变量81%;28GPa(围压13GPa+差应力15GPa),990℃条件下,在样品端部出现柯石英,以#表示标在图中直线之下方.
特别令人注意的是,Martinez等(2008)在非晶SiO2干凝胶基底上引入分散的叶氯素聚合体,在565℃条件下只需常压(一个大气压)环境,就可部分晶化出柯石英(以号表示,标在图中直线之下方);这意味着,不用压力而只用温度调控,也有可能获得石英的高压相柯石英,再次显示了形成柯石英途径的多样性.
为了便于比较,这里,把HirthandTullis(1994)的属于非平衡态的、大于诸直线的两个数值,以表示[①以石英岩为起始原料,454GPa(围压120GPa+差应力334GPa),500℃,应变度14%;②288GPa(围压120GPa+差应力168GPa),700℃,应变度17%.
前者属半脆性破裂,后者为半脆性流变.
柯石英均出现在顶底端部,二者还分别见于剪切破裂带内和垂直于最大主应力的颗粒边界],列在图1中.
同时把上面其它有关体系的远离但大于热力学平衡相边界的若干数据也一并列入:(36GPa,1300℃.
参见Nakaetal.
,1972);[大应变量,属半脆性破裂;425GPa(围压20GPa+差应力2~25GPa),450℃条件下,在样品顶底端部出现柯石英.
参见GreenⅡ,1972].
#[应变量75%;297GPa(围压13GPa+差应力167GPa),950℃条件下,在样品端部出现柯石英.
参见周永胜等,2005].
在图1的基础上,我们可以把非平衡态的石英—柯石英合成产物相图与前人有关石英—柯石英平衡相边界一起绘在一个图上,这样表达明晰,便于讨论,详见苏文辉柯石英研究组的图2或苏文辉等(2009)的图10.
有关石英—柯石英高压合成产物的晶态—非晶态、晶态—晶态的转变,以及平衡—非平衡态的分析,可用苏文辉柯石英研究组的图3或苏文辉等(2009)的图18的能量状态图,然后结合下面的公式进行讨论:G()PT=-32πσ33(Gc-Ga)3(Vc-Va)+ΔE()PT(1)式中G为非晶合金体系的摩尔吉布斯自由能,V为物质的摩尔体积,T为温度,P为压力,σ为常数,a、954第4期苏文辉:远离平衡相边界的柯石英形成机制及板块折返假说的物理基础c分别表示非晶相和晶化相,ΔE为扩散激活能.
因为Vc350km.
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TheFormationMechanismofCoesiteFarfromEquilibriumPhaseBoundaryandthePhysicalFoundationofHypothesisofPlateSubduction—ExhumationintheEarthSUWenhui1,2,3)1)CenterfortheCondensedMatterScienceandTechnology,HarbinInstituteofTechnology,Harbin,150080;2)DepartmentofCondensedMatterPhysics,JilinUniversity,Changchun,130023;3)InternationalCenterforMaterialsPhysics,AcademiaSinica,Shenyang,110015Abstract:Inthepresentpaper,thephysicalfoundationofthehypothesisofplatesubduction—exhumationintheEarthhasbeenanalyzedanddiscussedinthelightofthermodynamics,physics,matterstructureandhightemperaturehighpressurephasetransformation.
ForthenonequilibriumhightemperaturehighpressuresystemoftheheterogeneoussolidEarth,thenonequilibriumstateformationmodesfortheultrahighpressuremetamorphismofcoesiteintheEarth'scrustaremoregreateruniversalityanddiversitythantheequilibriumstateformationmodewhichshouldnotbeassumedtobetheonlyone.
ThemodeofstaticfluidEarthandconversionformulaoflithostaticpressureintodeptharevalidnolongerfortherealheterogeneoussolidEarthincludingnotonlythenonequilibriumstate,butalsotheequilibriumstateexistingpossiblyinsomelocalizedregionswithstablehydrostatic,quasihydrostaticpressureintheEarth'scrust.
Chopin(1984)hasfavouredtheideathatcoesitegrewundernearlystatic(hydrostatic)pressureconditionwhichbelongedanequilibriumthermodynamicsgrowthmode,hehasopenedacoesiteultrahighpressuremetamorphicstudyofitskind.
However,itislackinginscientificevidencethatthesubsequentuseofconversionformulaoflithostaticpressureintodepth,madetheformationpressure≥28GPaofnaturalcoesitetranslateinto100kmdirectly;andfurtherassertedthattheobtainedcoesiteintheEarth'scrustisastablephaseproducedfromtheEarth'smantle.
Fortheactualinhomogeneitysolidearth,thedrivingforceofplatedeepsubductionalreadywasnottheissueofovercomingfluidbuoyancywhichisdrivinglowdensitycontinentalcrustrockstoinsertintohighdensitycrust—mantlerock,butshouldresolvetheproblemoflowdensitycontinentalcrustrockstopenetrateintosolidcrust—mantlerockwithhighdensity.
Itisnecessarytohaveagiant,abnormal,andraredrivingforceforthat.
Fromthepointofviewofphysics,itisveryhardtosubductsolidcontinentalcrustrockintosolidcrust—mantlerock.
Chopinfirstlyevadedtheoriginaldrivingforce,andfurther,drewaconstantequalsignwithoutscientificbasisbetweenthetwoeventsofthecoesiteanddiamondultrahighpressuremetamorphicrockfoundintheEarth'scrustandthestablephaseproducedinthemantleorthedeepsubduction,andthenagain174第4期苏文辉:远离平衡相边界的柯石英形成机制及板块折返假说的物理基础roseuptoa"paradigm"heightthathadmisledneednooriginaldrivingforceofresearch.
Sincetheintroductionofthehypothesisofplatesubduction—exhumationsofar,theoriginaldrivingforceofplatesubduction—exhumationhasalwaysnotbeenresolved.
UnevensolidEarthisanaturalhightemperaturehighpressuresystem,inordertopreservethecoesite,ahightemperaturehighpressurephaseofquartz,therapidcoolingrateofplateisrequestedover40℃/Mathatshouldbecomparablewiththelaboratorycriticalrateofquenchingunderpressurekeeping(whichrequiresnotlessthan(05~10)*102℃/s),however,thereisahugedifferentialin1013orderofmagnitudeforbothcases.
Fromthelabphasechangeregularityofhightemperaturehighpressure,suchextremelyslowcoolingratesofplateexhumationisdifficulttokeepthelivingcoesite.
Conversely,inordertomakeplateexhumationspeedreachedlaboratorycriticalcoolingrateofkeepingcoesitequenchingunderpressure,thespeed(inair)ofplateexhumationmustachieve10~20km/s.
Thismeantthattomakeahugeamountmatterwithsuchgreatspeedfastpenetrateandexhumationinthesolidmediumthatneedsagiantabnormalraredrivingforceofexhumation,however,fromphysicsviewpointitisextremelydifficulttoachieve.
Inaddition,thefourimportantfactorsputforwardbyChopin,influencingthecoesitepreserveduringturnbackprocess,stillexistmanyproblems,andalsocouldnotovercometheaboveparadoxes.
Thetimeofatomicdiffusionandchemicalreactioninsolidmatterusuallyfallswithintherangeofμs~ms,andtocreateanequilibriumwithinmin~102h.
Bytheisotopicgeochronologyofultrahighpressuremetamorphicrocksand"Fryingicecream"modelidentification,thetimefromsubductiontoexhumationwasof10~20Ma;andthetimefromgeospeedometryincomingstructuremetamorphismprocesswasmoreshorter(<02Ma).
Comparedthetimeofatomicdiffusionandchemicalreactionofmatterwiththatofresultsobtainedfromisotopicgeochronologyandgeospeedometry,thereisthesamehugeordersofmagnitude1017~20differencebetweenthem.
Fromtheviewpointofstructureandbehaviorofmaterials,inresearchinvolvingseveralMawhichisnotlongtothegeologicaltimescaleduringplateexhumation,andseveraltotenskilomitersoflimitedsizescale,thereshouldbeextremelyplentyoftimefordiffusion,chemicalreaction,productmetamorphismandexchangewithmantlematerialstoestablishisotopeequilibrium.
Theviewof"shortstaytimeandfastturnbackspeed,withoutadequateisotopeexchangedidnotreachtheisotopicequilibrium"isunbelievable.
Thehypothesisofplatesubduction—exhumationbesidesmustbeinspectedbytheabovephysicalfoundation,stillneedstobeablesmoothlytoexplaintherelevantfeaturesofcoesiteintheEarth'scrust.
Duetotheexistenceofdiversitysolutionsofwavevelocityanomalygenesis,thesubduction—exhumationchannels(track)shouldnotdetermineonlybyseismicwavetopographicanomalymap.
Platesubductionandexhumationmovementisthebigeventwithabigenergyandlargespacechanges,andtheirenergychangesshouldfollowthelawofconservationofenergy.
InP—T—ttrack(channel),thefirstphasetransformationproductsproducedinobviousandlargequantitiesduringthesubductionprocessofcontinentalcrustrocksshouldbeleft,andthephasetransformationproductsoccurredbyquenchingunderpressurekeeping,orunderlowpressurelowtemperatureconditionsofmantlematter,andthesecondaryphasetransformationproductsofcontinentalcrustrocksduringtheexhumationprocessshouldbeleftalso.
Similarly,platesubduction—exhumationshouldcomplywiththemassconservationlaw,P—T—ttrack(channel)inuppermantleshouldappearcorrespondingcontinentalcrustrocksmatterandtheirphasetransformationproducts,andaroundcoesiteintheEarth'scrustwhereshouldexistmoremantleinteriormaterialsofupleft.
Reviewthedevelopmentprocessofsubduction—exhumationhypothesisofitself,themostcriticalstepwasneglectingthefactsthatthelocalizedhighpressuremicrozoneinducedbylocalheterogeneityofsolidEarthcomposition,stressandsoon,andtheinfluenceofothernonequilibriumthermodynamicsfactors;andadheredtothetraditionalstaticfluidEarthmodelanditspressure—depthconversionformula.
Fromtheviewpointofphysicalbasis,theconjectureof"spectaculargeologicalevent"ofplatedeepsubductionfastexhumation,stillneedsmoreandmorescientificevidencetoprove.
Keywords:Ultrahighpressuremetamorphismofquartz—coesite;nonequilibrium;conversionformulaoflithostaticpressureintodepth;platesubduction—exhumation;isotopicgeochronology;geospeedometry;dynamicextrusion;shear;localizedheterogeneityhighpressuremicroregion274地质论评2011年