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EclogiteFormationand the Rheology,Buoyancy, Seismicity,andH20 Contentof OceanicCrust Bradley R. Hacker 1 Departmentof Geologicaland EnvironmentalSciences,StanfordUniversity,Stanford,California A broad spectrumof variably altered igneous rocks with a wide range of grain sizes are compressedand heated over a wide range of pressure-temperature paths in subductionzones. Although experimentalkinetic data cannotbe extrapolatedto predictthe ratesof blueschistand eclogite formation in nature, textural data from rocks indicate that transformationbelow temperaturesof 150øCis minimal. Completetransformationof volcanicrocksoccursby •-250øC, but incompletetransformationof gabbroicrocks heatedto 800øC has been observed.There are important consequencesto the rapid transformationof volcanic rocks and the metastable persistence of gabbroicrocksinto theblueschist andeclogitestabilityfields.Fastseismicvelocities shouldbe evident first in the upper oceaniccrust and may be substantiallyretardedin the lower oceaniccrust.The upperoceaniccrustwill be denserthan asthenosphere beforethe lower oceanic crust.Early in the processof eclogiteformation,volcanicrockswill be placedin deviatorictension andthe underlyingcoarsergrainedrocksin compression; with furtherreaction,the stateof stressin gabbroicrockswill changefrom compressiveto tensile.Earthquakesat shallowdepthsshouldbe extensional in basalt and contractionalin gabbro, changing at deeper levels to extensional throughoutthe crust. INTRODUCTION Formationof eclogiteis a key factor in plate tectonics, influencingthe size and shapeof platesas well astheir age and rate of disappearance from the Earth's surface. Subductionzoneshave long beenof interestbecausethey are the buildersof continents,collectingmaterialfrom the oceanbasinsandconstructing magmaticarcs.In spiteof the dynamic nature of subduction zones, much of our understandingof their behavior stemsfrom equilibrium thermodynamics.The equilibriumviewpointhasprovena valuablefirst meansof addressing processes in subduction zones, but a more in-depth comprehension requires considerationof the ratesand mechanismsby which phase changesoccurduringsubduction. AlsoatU.S.Geological Survey, MenloPark,California This paper summarizes the rates and textures of densificationreactionsin subductionzonesby examining experimental and field studies, and then considersthe effects of these findings on the rheology, buoyancy, seismicity,andH20 contentof subducting oceaniccrust. WHAT ARE BLUESCHIST AND ECLOGITE? Blueschistfacies mafic rocks are characterizedby the coexistenceof sodic amphibole and lawsonite at low temperatureor epidote at high temperature[Ernst, 1963]. They may alsocontaingarnetor omphacite.Eclogitesensu stricto is mafic rock consistingof garnetand omphacitic clinopyroxenewith or without additional minor phases [Coleman et al., 1965]. The geophysicalimportanceof eclogiteis that it represents the highestdensitycommonly attained by crustal rocks. Omphacite and garnet have densities of3.2-3.4gm/cm 3 and3.6-4.0gm/cm 3,amarked Subduction:Top to Bottom GeophysicalMonograph96 This paperis not subjectto U.S. copyright. Publishedin 1996 by theAmericanGeophysical Union elevation beyond the density of a typical basalt at 2.9 gm/cm 3 [Carmichael, 1989].Thisshiftfrompositive to negativebuoyancyrelative to asthenosphere, which has a 337 338 ECLOGITE FORMATION IN OCEANIC CRUST densityof roughly3.23 gm/cm 3 [Cloos,1993],is as important a factor in controlling plate subductionas are conductive cooling of the lithosphereand the olivine --> spinel transformation in subductingmantle [Ahrens and 2.6 ':iiiiiiiiiiii::.:.0::::•i•i•i!ii!!111iiiiiiiiiii!!iiiiii?iiiii½•::::• .................. 2.4 2.2 '?iiiilili• 3•7 ':iiiiiii!iiiiii:. eclogite .. .......... ::::.:::::::•. .... ,,•,•,:•,•,,.. 3.24 ..:.:...: ....... Schubert, 1975]. ••' 3.10 •• • PRINCIPAL DENSIFICATION REACTIONS of basaltic rocks to blueschist 1986]. Eclogite facies conditionsexist at temperatures>500øC andpressures> 1.2 GPa (Figure 1). Eclogitefaciesrocksdo not necessarilyexist wherever such conditionsprevail in the Earth becauseof sluggishkinetics.All reactionsrequire oversteppingof equilibriumto overcomelocal free energy to attachment or detachment of atoms at interfaces, and all reactions involve diffusion or atom-atom bond rearrangement.Perceptible reaction only occurs at sufficientlyhigh diffusivitiesand reactionfree energies. HOW FAST ARE REACTIONS IN THE .......................... ........ •.... ..... ;:•:•:• ........... •-•:•:::•::,:.•'-----1! .............................. •:•'-'::ep•dota•i•i•i• ...... •:•:•i•i•i•11!'•'•:g:'•i...'. •:• ............ ......:...:.: ....:.:.....:.:.:.:.:.:....•[•?.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: ..... ......... •::• ?• .......... •?::• ...................... ...... •? .................... •........... •:• .................... 0.6 ................... ß .............. 2 93 ............ •"v ....... , 0.4 subgreenschist ..... •?' 3.5 .:•? 0.2 0 2.80 .... ii•i•}i•?" 5.2 1• ::•!•i•i:'" • 3• ..•}•?" -':3•iiii .......... Tem•ramre (øC) ;.9ff .•{•? •-' 0 :•{{•}•' •:• 7• 8• 9• Fig. 1. Metamorphic facies and reactions pertinent to eclogite formation, after [Liou, 1971], [Maresch, 1977], [Brown, 1977; Brown and Ghent, 1983; Evans, 1990], [Green and Ringwood, 1967; Ito and Kennedy,1971;Liu et al., 1993; Oh and Liou, 1990; Poli, 1993]. Three- and two-digit numberfor eachfaciesindicate density(g/cm3) and H20 content(wt%) calculated using NCMASH model of Peacock [1993] assumingequilibrium (i.e., no kinetic hindrance). and then eclogite at the low temperaturesthat prevail in most subduction zones is more complicated because of the prevalenceof disequilibriumin natureandexperiment.One reactioninferredto have transformedblueschistto eclogite at severallocalities worldwide is epidote + glaucophane --> garnet + omphacite+ paragaonite+ quartz + H20 [ElShazly et al., 1990; Ridley and Dixon, 1984; Schliestedt, increases related ....... ............. ========================== ::::::::::::::::::::::::::::::::::::: ß A broad spectrumof reactionscauserocks to become more densein subductionzones.At the high temperatures found in very hot subductionzonesthe transformationof basalticrocksto garnetgranuliteandthen eclogiteinvolves the breakdown of plagioclase, addition of Na20 to cliopyroxene and the growth of garnet (Figure 1). PT conditions for the appearance of garnet have been determinedby experimentalstudy [Green and Ringwood, 1967; Ito and Kennedy, 1971; Liu et al., 1993; Poli, 1993], and occursatlower pressures/higher temperaturesin SiO2undersaturated[Ito and Kennedy, 1971] and MgO-rich rocks [Green and Ringwood,1967]. The disappearance of plagioclase is less well defined by the same studiesand dependsmore stronglyon rock composition. The transformation ...... .....: ...... .::::::::::::.:::::::::::: EARTH? Experimental and TheoreticalInferences How long can rocks remain out of equilibrium at elevated pressuresand temperatures?Specifically, how long can rocks remain within the eclogite stability field before transformingpartially or completely to the stable phase assemblage?Information relevant to this question derivesfrom two sources:experimentand studyof natural rocks[Rubie, 1990]. Experimentalkinetic datacollectedon powderedmaterialsmay not be directlyapplicableto the majority of geologic situationswhere rocks are tightly packed crystallineaggregates--principallybecausethe interfacialfree energyof a rock is muchlessthanthatof a powder.For extrapolationto geologicconditions,dataon phase transformationsin unpowderedmaterials are required--information thatis exceedinglysparse. Recent experimentson reactionsknown to occur in subduction zones demonstrate that most kinetic data cannot be extrapolated to theEarthandthatH20 hasmajoreffects on reaction rate and texture. For instance,Holland [1980] observedjadeite growthin 24 hr from powderedalbite + quartz at 900øC and a pressureoverstepof 50 MPa. In contrast,albite -->jadeite+ quartzexperiments by Hacker et al. [1993] on albiterock showedno transformation in 24 hoursat 1100øCat a pressureoverstepof 500 MPa! Quantitative transformation-rate measurements are available for the calcite <--->aragonitereaction. As with albite, the calcite --->aragonitetransformationin Carrara marble[e.g., Hacker et al., 1992] is muchslowerthanin calcite powder. For example, at 600øC and 2.0 GPa confiningpressure,powderedcalcitetransforms50% to aragonitein <1 hour[Brar and Schloessin, 1979,p. 1409], whereas192 hourswere requiredfor similarconversionin unpowdered marble.CarlsonandRosenfeld[1981]andLtu and Yund [1993] measuredthe rate of the aragonite ---> calcite reaction;extrapolatedto geologictimescalesand grainsizes,theirdataimply that 1 mm aragonitegrainswill HACKER revert completely to calcite in less than 1 m.y. at temperatures as low as 200øC. Hacker et al. [1992] exploredthe reversetransformation in marbleand founda similar result, that the calcite --> aragonitetransformation occurswithin less than 1 m.y. at 200øC for overstepsof <100 MPa. The reactiondependsstronglyon temperature, such that even several GPa overstepat temperaturesof <100øCareunlikely to promotetransformation. The catalyticactionof H20 is well known from kinetic studiesof powders and rocks. Hacker and Kirby [1993] deformed polycrystalline, vacuum-driedalbite rocks at strain ratesof 10-4-10 -6s-1 attemperatures of 600-800øC andconfiningpressures of 1.0-2.0 GPa. In spiteof extreme pressureoverstepping andextremesamplestrain,deformed anhydrous samples contained no jadeite. Differential stressesas high as 2000 MPa producedmaximumnormal stressesup to 4.0 GPa--all at temperatureswhere the equilibriumpressureis <2.0 GPa. Someof thesesamples were strained70%, and individual grains attainedaspect 3.0 339 •-, c.....•o_.esite eclogite -: :.•feeble, c. 25%'::'""'"•:'""'"•i•:..., •:i'/!111['"'":'""'"" '"' "'"'""'":•:' ........ 2.8 2.6 ;"[]moderate, c.50% 2.4 I extensive, c.75% 2.2- I complete, 100% 2.0- '?'"'"'•jiii:. eclogite '"':":::½'{'"'"'"':' g-."i:-•i !•,•;•;*:' :' '--•cooling :.'.-':•:•:•:•:•.'.-':•:i:.'. ""•i: ......... ' lawsonite blueschist 2:iiii:: ::•::5" i•5::i•5•!'1•. .:i::::::::ii •$;";.'•: t'•;J .•i...... ..si::::::::::::::ii [g'-:$• t'•-":• • ".... ::--'•".:"-..--'i--'•::• -:•:.......... ••,.•.::lim•'-' ß :>.-r. ,'-•m: :•,-•.,-.amet granuh.t...e..... •,,.,...:.© •½.•-•...-•::• ..... :,•..'p•.•'./•.,.•-..•j • :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ................. •:•:•:•, .......... •.-.:--•:•:•:.-',•.-.::i'..:,•.-.:.,•.--:.• ................. :..... ß ........::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 1.0 • •'..•.,x -. .....!,x.•qx B .:•5{' 0.8 IIi-'• •i•'•: '...... • ; • ....'• /i:'.i:'.....'.-':•:•:5 :'••I'• •-•i .'-:::'-.:i: *!•½:.': 0.6 .-• .... : :' .-:::::: • "'""::: 5:5':" :':':':' •{•'"'&'•ø'•:e' :'":•' hi 1' subgreenschi•'t .."'"'•½':'gree '"'"' ""'"' ' nschist.':•i'-"? ':' 0.4 •.:-'•i•i.-'..: ::]-'.-'-'•:•:•-• :::::.-::-'.--'.. . 5:•5•'.':•:•: granuhte '"'•?' ':'"' '"' ' 0.2 0.0 0 1•0 2•)03•)0 Temperature (oC)6•)0 7•)08•)09&) 1000 Fig. 2. Reported extents of transformation, pressures, and temperatures,for blueschistand eclogite facies metamorphism worldwide. References and data used to compile figure are availablefrom the authoron request. ratios ofgreater than10:1.In marked contrast tothis,the addition of 1 wt% H20 produced partial reaction at temperatures as low as 600øC, even in undeformed samples. More complicatedexperimentsmeasuredthe olivine ---> spinel transformationin hot-pressedMg2GeO4 [Burnley, 1990] and Ni2SiO4 [Rubie et al., 1990] at pressuresand temperatures of 800-1900øC and 1-15 GPa. When extrapolatedto geologic conditionsthese investigations suggestthat transformationmay be suppressedto depths 150-200 km below the equilibrium boundaryin rapidly subductingslabs. In summary, existing experimentaldata indicate that H20 has a major catalyticaffect on reactionratesand that some reactionsare geologicallyfast (e.g., calcite-->aragonile) whereasothersare probablyslow (olivine --> spinel). We have essentiallyno experimentalinformationon reactionsformingblueschistandeclogite. Metamorphic Textures Observations on rocks show that transformation to blueschistand eclogitemay be suppressed long afterrocks have passedinto the respectivestability fields of these assemblages.Figure 2 was constructedfrom studies worldwide that reported not only peak metamorphic pressuresand temperaturesbut also textural information about the degree of reaction. Rocks reported as feebly recrystallizedor whosehigh-pressure phasesweresominor as to requiredetectionby electronmicroscopyare shownin Figure2 as"feeble,25%." Rockscontainingabundantrelict phases (usually clinopyroxene) or partially developed reaction coronas are denoted as "moderate, 50%", and those with rare relict phases are labeled "extensive, 75%". Metamafic rocks specificallydescribedas lacking relict phasesare shownas"complete,100%". There are some important limitations to Figure 2. 1) Reportedpeak pressuresand temperatures may not have occurred at the same time and, at best, representonly a singleway-station on thePT pathexperienced by therocks. 2) How long the rockswere at the reportedpressures and temperatures is unknown.3) Pressureestimatesfor many rocksare minimabecauseof the highvarianceof the phase assemblages; theseare denotedby U-shapedlines rather thanrectangles.4) Many high-pressure rocksexperienced substantial post-high pressure alteration that likely consumedexistingrelict phases;studiesreportingsevere alterationof thistypewereexcludedfromconsideration. 5) Fine-grainedrocksreact fasterthan coarse-grained rocks becauseof increasedinterfacialfree energy;gabbroicrocks areonly partiallytransformed at conditions wherevolcanics arecompletelyrecrystallized. Numerouspapersthatdo not specifically statewhetherrelictphases arepresent or absent had to be excludedfrom Figure 2, but this may simply mean that the absenceof relics was not significantfor the purposesof the study.ThusFigure2 shouldbe takenas an illustration of the transformation behavior of coarse- grained,i.e., gabbroicrocks. With these limitations in mind, Figure 2 indicatesthat feebly metamorphosed rocks generally persist to temperaturesof 200-250øC. Moderate transformation beginsat 200-250øC andis foundin rocksthatwereheated to just above 600øC. There are a few casesof extremely 340 ECLOGITE FORMATION IN OCEANIC CRUST limitedreactionat highte•nperature shownby dashedboxes in Figure2. Incompleteconversion of gabbroto eclogiteis known to have happenedat temperatures>700øC and pressures of 1.5-2.0 GPa in the presence of aqueous fluid [MOrk,1985]. Suppression of transformation of gabbroto eclogitewasreportedfrom rocksthatreached800-900øC andpressures of >2.8 GPain the absence of fluid [Zhanget al., 1995]. Oddly enough,thereis little differencein thePT range over which extensiveto completetransformation have been found. Both have been observed from rocks heatedto lessthan300øCandyet partialtransformation of gabbroto eclogitehasoccurredat 760-850øCand 1.6-2.0 GPa [Indares and Rivers, 1995]. In general,500-600øC marks the transitionto extensiveor completereactionfor most rocks. A few cases, however, indicate that the transformation of gabbroto eclogitemay be suppressed to muchhighertemperatures. Fe-rich rocks transform to eclogite along high P/T trajectoriesmore readily than Mg-rich rocks because reactionsinvolvingFe-richphasesgenerallyoccurat lower temperatures [Bohlenet al., 1983].Fieldevidenceof this-MgO-rich rockswith abundantigneousrelicsadjacentto Fe-rich rocks with rare relict phases--hasbeen noted at BardoneyValley in theAlps [ReynardandBalleyre,1988], at Flems0y, Norway [MOrk, 1985], and in Piedmont ophiolites[Pognante,1991;Sandroneet al., 1986]. Eclogiteformationis fasterin finer grainedrocks.For example,in the BardoneyValley of the Alps, abundant relict augites are present in gabbros, but absent in metavolcanics[Reynardand Balleyre, 1988]. In the Sesia Lanzo zone farther east, amphibolite transformed to eclogitecontainsrelics in coarserocksbut nonein finegrainedrocks[Lardeauxand Spalla, 1991]. Not only is H20 provento acceleratereactionsin the laboratory,numerouslocalitiesdemonstrate thatthisis true in natureas well. Mafic granulitein the MusgraveRanges of Australia cooled into the eclogite stability field and, thoughthe bulk of the rock is unaltered,local shearzones contain omphacite + garnet + minor zoisite [Ellis and Maboko, 1992]; the presenceof zoisite implies that the reaction was fluxed by H20. At Holsn0y in the Bergen Arcs, zoisite-bearing eclogitehaloesin undeformedrock surrounding deformedveinsalsoindicatethat fluid drove reaction[Boundyet al., 1992; Klaper, 1990]. Deformation is involved in these fluid-mediated transformations inasmuch asthefluid mustmakeits way to thereactionsite via cracks. structure of oceanic crust. Oceanic crust has been studied by dredging and drilling in ocean basins,and by the examination of ophiolites.The Samailophiolitein Omanis the best-exposed, largest,least-deformed,and perhaps most-studied ophiolitein the world.Plutonicrocksof the Samailophiolitearedivisibleintoa lowerlayeredsequence andan upperisotropicsection.The lower2.3 km (avg.)is well-foliated and lineated, layered ultramafic (chiefly wehrlite, dunite, and clinopyroxenite) through mafic cumulates [Pallister and Hopson, 1981]. Olivine predominates at thebaseof thesection,whereas plagioclase is dominantat the top [Juteauet al., 1988b].Three-quarters of the sectionis gabbro--typicallyolivine-clinopyroxene or clinopyroxenegabbrowith minorhornblendeandFeTi oxides--but at rare localitiesit is gabbronorite[Juteauet al., 1988a].The gabbroconsists of mm-sizegrainsof 55% plagioclase,35% clinopyroxene,10% olivine, andrarely Ti-pargasite [Browning, 1984]. Overlying the layered sectionare 200-900 m of isotropichigh-levelmaficrocks, chiefly hornblende-clinopyroxene gabbro [Pallister and Hopson,1981]. Grain sizesrangefrom mm to pegmatitic [Juteauet al., 1988a]. Sheeteddikes containabundantto sparseplagioclaseand augite [Hopsonet al., 1981]. The volcanic rocks are non- to moderately vesicular,almost aphyric pillows and rare brecciatedflows and massive flows [Ernewein et al., 1988]. Mid-ocean-ridge basalts(MORBs) typicallyareglassy or containplagioclasewith lessolivine [Hekinian,1982], while rockscrystallizedin intraoceanicmagmaticarcsgenerally containplagioclaseand augite [Ewart, 1982]. Midocean-ridgebasalts (MORBs) are subalkalinetholeiites that typically contain plagioclase with less olivine [Hekinian, 1982]. Oceanic plateaus, which make up a sizeableproportionof oceaniccrust, are similarin their majorelementchemistryto MORB, but are 10-40 km thick [Cloos, 1993]. Hotspot lavas and propagatingrifts are enriched in Fe and Ti relative to MORB [Flower, 1991]. Lavasproducedat fastspreading ridgessuchasthePacific have lower Mg/Fe ratios than MORB eruptedat slow spreadingridges[Batiza,1991]. ALTERATION OF IGNEOUS OCEANIC CRUST Because phasetransformations are stronglyinfluenced by theavailabilityof H20 andtheextentandtypeof preexistingalteration,understanding the alteration of oceanic crust is critical to evaluatingwhen and where phase transformations occur. Variable alteration of oceanic crust COMPOSITION OF IGNEOUS OCEANIC To understand the transformation CRUST of oceanic crust to blueschistandeclogitewe mustknowthe composition and to lower grade mineral assemblages occursby active hydrothermal circulationat spreading centersandby later weathering on theseafloor[Altet al., 1986;Humphrisand Thompson, 1978]. Somerocksescapealterationentirely, HACKER while others are wholly recrystallized to new minerals. Most typically, lavas are variably metamorphosedto subgreenschist facies,dikeschangepartially to greenschist facies, and gabbro recrystallizesweakly to amphibolitefaciesminerals.The pressuresat which thesemetamorphic assemblages form are relatively low (<300 MPa; determinedby ocean depth and the thicknessof oceanic crust) and the formation temperaturesmay range up to A Equilibriummodel 2.8 ß, base of crust 2.6 -- 2.4 best information about the alteration of oceanic lavas and dikes comes from DSDP hole 504B [Alt et al., 1989; Alt et al., 1986]. There the lavas contain -10% alterationhaloesaroundveins and cracks,leadingto about 1 wt% H20 in the bulk rock. Veins and vugs containFehydroxides, clay, celadonite, smectite, zeolites, and carbonate; groundmass minerals are altered to Fehydroxides and clay; plagioclase is partly altered to smectite;elivine is partly to totally replacedby smectite and minor calcite;glassis partly to completelyreplacedby clay; and pyroxenesare unaltered.Alteration in the dikes increases down section to a maximum of-50 • ::..::•..i•}•. 3.25 ........ !•.:...iii '"• '•" /3.02 ,]Z .:i:.:i? :'"i{{i":":'"'"•J:-5•!!: .,..-:•---'-'--}i'ii '""'"'"'"'"'"' ':'•':':•:"••••• ' 2.0- .•?;•?.":.-:-:iii:":-'?.::. O.9 OF SLOW / 1.0 .:g?siiiii!}.::..,•:i•i::iii{ii?.--•ii•¾-3': -:' 1.2:g-':.-.'-:....-'• ..... ........ xD;• ,:•557:..:'-"....'5•i ................. '"••5!5"'"•:':" / 0.8 ........ / •/'"":•:%•i;':-::•? .... ' 2.91........... '•::'"":5•ii 2.89 0.6 2.99 ......... 0.4 0.2 0.0 0 1'00 200 •00Temperature (øC) B 2.8 • Kineticmodelwith unaltered basalticandgabbroicprotolith • . [......::.-..-:3.06 3.23 3.34 .......... 2.6. /'k base Of crust / ':':•--"'"J•ii:•:0• -O•*.i;-"•'"'"'• •--::""•"'•::i5 -"•g"i•i'•'•* ............. 0.3 ':O.....-• 3.37 ß '::"?•:•. :.ii-i':"i•,•i '•:':: '••'••--.......... " --- - •:: :'• ............ -!!i. 5.:65.77.--.--:•--.• 2.4. V-',-,7 top elcrust / 2.2. i / ':':•..:.•"3.16: ..• •' •U . :!ii::'".-"-5:..-;3•i•i0.7 3.25 •.--:-:•:..... : .;...../ 0 .. 294 'g•':"i"'93 ........ •:a:..;•:.::"k. . ' i['"'" "•'•'"'"2 ':j:":=8 i .......... :.-!•.:-'•i '-"' "•' ':"'"•'"'/:• '""'"" ' ' ..... ::0.6 /2.3:. ß'?•;•i-.:' '-'.'•' '"'"'--'-'":-'-'•--'•••••iim"•-•":-----'"•....:j•i 1.0• ,' / ":•'"••'•"'•i'":•:•iii•?-'-ili• "•'"" ............ "' ' .,.:.::s::.-::.-::.-e,%-.'<-:.:e 0. 9' ............ '•.*:':.: ....... •::-:::-x u 1.2 .:..:'..-':..-':..:•:• :::s::::.'-• •.:.:...:,:s:...::.-:j;-'.:4-'.i'"'"'"" ..-•:---•:.:-%-.':i:Z:•::'-'•::':i 0.6 2.89 4'00 ., /-:...-:o.....-.•......-•-::.'•;?: 2.0 ' 2f90 :?:iiil..-"ii? i. 98' z.:,v .,,,,, ":'•--'"d•il}i 2ß89 :':";"'"":•' oh'::ii{ :::•--:".'?' 3 '::2:--•"2 :-:..-'•.:'.:'•i.-'::• 0.4 o •.oo ,,.,.o,,i.'.i-..? ß --•;•i:.-ii•:' ß7 _' • n '::":J•..:'."{.-::0 • .0.5 / 2.{I "" _ ._/ ;:..:.-'.•? : 1.3 ß-' -":-..-":..':-'-_•? 0.2 gabbro:!2S "'"0/"":'•:•. -S'O/o rea'"•h i7S0/o: 100% :::'"'"'"/'::•ction 0.0 bas91t: :50%rxn. •00%reaction i 0 C 100' 200' 300Temperature(oC)g00 000 100 •00 1000 Kineticmodelwithzeolite-facies basaltandamphibolite-facies gabbro protolith 2.6•kbase ofcrust // ':•-;"•iii•i•O.5 ...0.....:::::•,:..• •":•:""-'"•!:'i:i•5':.•"•i•11 t}..............i'"':':":'"'"::•:•'::'"• ............. '*:?'":' :•••••i!i!i::ii[.-'•i•::•--"..-':-•'• '•'':"'- ' ''.. 2.4T top ofcrust ' / ":!{3'i•:•' ? .......... ' 2.8t ' :' .' !.:.::::.:..•:.3 3.23 3.34 ............ ;::-----... , / .......! 3.31 :!"":• ' •/o:•' "::]•ii:i3.16 3.25 2.2 9 3 02 / , / / 0 35/'"'""•73•t.5 .... -'"d..-'3";!•, ....... ,.-..':•C'"'"'"'•'"'•"'•'••••• ......... . / ., •" .' i:::::::i: ß B:::::::::::: :::.-: ::::::•:-:-:-:... :-,•-:.::•:.::!:-:[.?.i:i:.::---:-:-:.... ;...•:.5:...-?. ""'"'::'•.-'-':":?: 3 O1 .:i•i:i.:i:';-i • ?'"'::'•:? ..... *'•':::;{':'"":::"'""•!','*' ..... 1'2 iii::}:{ \1/ u .......;.:-:½;•:--..! ....... ...:• """'":•':•?"':":•:•,,,.•g .i:.iii?..,.:•? :•!:i:.:..-::.......•'. -.-•. • ?.:.•:...--.•i: .......... .;.'. ................ ?•. :•:.:'•..-.•::•-.'.-..', ß :-..:i?':•. ..'•:ii:..:•:..211ii?' '•...• ':'-':. ::• ?'•" ßß '-ß-----?-':'y-':•:'":' 0.6299/254 29 0.4 . ' ß3. / 6.7. _-- "'"'•-:'•i!-':':" i ß' - '"':'--"-'-"-•57 0.2 gabbro:25'0/":'"'•0""::'•. 50%readi--':.;.•:n :75% 100%::::::::•ction 0.0 basalt: i$0% rxn.100% re?lion i [Robinsonet al., 1991]. EFFECTS •:.:,•;ii::::.:'.:'giii?..':•. :_gig ':;':::::':' 3 4 ..:::::5..`..}iiii•.........•.!{iii•..`.:iii.......•i•i•.•*•::•`.`...:..... ':.:'i!•{•.--:-.:...:......--:::-:ggiiii.::'m"'---:-:-:-:.-:•..-•..,..:.:.'-.-:.,i,e /.-2..-.-'•? 3.4 ..... 3.!0 vol%. elivine is replaced by chicrite, clay, and talc; plagioclase is partially to totally replacedby albite + zeolites,chicrite,or clay; and augitcis unalteredhigh in the sectionandrimmed by actinolitedeeperin the section.The H20 contentof the dikesaverages2 wt% [Alt et al., 1989;Alt et al., 1986]. Good information on the alteration of plutonic rocks comes from the Samail ophiolite and ODP core holes. Alterationof the Samaillayeredplutonicrocksis limitedto rare replacementof clinopyroxeneby actinolite[Lippardet al., 1986] and alterationof the isotropicplutonic rocks is limited to the growth of up to 10% actinolite and FeTi oxides,andthe infilling of cavitieswith epidote+ sphene+ prehnite + quartz [Ernewein et al., 1988]. In contrast, gabbroicrocks drilled from ODP hole 735B in the Indian Oceanshowwidespreadamphibolite-facies alterationfrom 0-100%, with an averageof perhaps20-30% [Dick et al., 1991]. elivine and pyroxeneare replacedby hornblende, thereis partial to completealterationof elivine to colorless amphiboleand talc, and 2.4 vol% of the rock consistsof hydrothermal and late magmatic veins composed of hornblende or oligoclase + diopside + epidote. H20 contentsof the 735B gabbros are mostly 0.5-1.5 wt% -----.... --..:•:!•i•:::.-::-:::-' ........... 3.31 ';iii?iii'.:•. 0 top of crust 2.2- -650øC. Our 341 0 TRANSFORMATION ON DENSITY AND H20 CONTENT When oceaniccrust is subductedit is compressedand heatedalong one of a wide variety of possiblePT paths (Fig 3). The highest-temperature path into the eclogite •60' 2•' 360Temp•a•a,e{oC)Oh0 7b0 860 900•000 Fig.3. Densities (g/cm 3) andH20 contents (wt%)forsubducted oceaniccrust calculatedusing NCMASH model of Peacockand kinetic data discussedin text. A) Equilibriummodel.B) Kinetic model with unalteredbasalt/gabbroprotolith. C) Kinetic model with subgreenschist-facies basalticprotolith and amphibolitefaciesgabbroicprotolith.Where two numbersare given,the first refersto gabbroandthe secondto basalt. 342 ECLOGITE FORMATION IN OCEANIC CRUST stability field is from the granulitefacies. In the hottestof subductionzonesinvolvinglithospherelessthana few m.y. old, it is possiblefor oceanicigneousrocksto passinto the eclogitestability field by this path [Hacker, 1991]. Rocks that recrystallized in the subgreenschistfacies during hydrothermalalteration at the mid-oceanridge will pass through the greenschistand amphibolite facies prior to enteringgranuliteconditions,and may transformvariably to greenschist,amphibolite,or granulitefaciesassemblages prior to entering the stability field of eclogite. At the opposite end of the temperature spectrum,rocks that previouslyrecrystallizedin the prehnite-pumpellyitefacies will pass through the blueschistfacies prior to entering eclogite conditions [Peacock, 1990], and may thus transform variably to blueschist prior to entering the eclogitestabilityfield. Peacock[ 1993] usedthe restrictedcompositionalsystem NCMASH (Na-Ca-Mg-AI-Si-H-O) to calculatethe mineral modes and maximum H20 contentsof subductingmafic rocks. He demonstratedthat moderately altered oceanic crustcontaining 1-2 wt% H20 beginsto dehydrateat the onsetof eclogiteor amphibolitefaciesmetamorphism,and suggestedthat the transition from blueschistto eclogite facies, associated with the breakdown of lawsonite or clinozoisite,releasesthe mostH20 duringsubduction. Peacockcalculateda rangeof pathsthattraversemostof the regionof PT spacerelevantto subductionzones.Figure 3 illustrates two paths for the uppermostand lowermost partsof subducted7-km thick oceaniccrustmcalculatedfor a subductionvelocity of 50 mm/a and shearstresses of 100 and 33 MPa. Most of the PT paths followed by the uppermostlayer of the crust (basalt) are limited by the paths identified by upright triangles, and most of paths followed by the lowermost layer of the crust (gabbro) are limited by the paths with invertedtriangles.Intermediate levelsin the crustfollow intermediatePT paths.For path 1, the uppercrustpassesprogressively throughthe greenschist to amphiboliteto eclogitefaciesconditions,while the lower crustevolvesfrom blueschistto eclogitefacies.Along path 3, all the crust remains at blueschist facies conditions to pressuresof 2.5 GPa. For pathsintermediatebetween1 and 3, most of the crust passes through the blueschist and eclogitefacies. In the spirit of Peacock's calculationsand using the information presentedin Figure 2, assumethat coarsegrainedrocks(i.e., gabbro)transform25%, 50%, 75%, and 100% at 150øC,250øC, 500øC and 550øC,respectively,in accordance with field observations described earlier. Further assume that fine-grained rocks (i.e., basalt) transform twice as fast--such that they reach 100% transformation at 250øC. Assume also that oceanic crust is made of 3 km of basalt and 4 km of gabbro.Given these assumptions, whatconclusions canbe drawnabouttherates of eclogiteformationin subductionzonesandits effecton rock physical properties? Possible variables to be consideredincludethe amountof H20 andthe composition of the crust. Figure 3A and 4A illustratethe predicteddensitiesand H20 contentsof NCMASH assemblages determinedby Peacock[ 1993] for the variousmetamorphicfacies.Figures 3B and 4B repeatthis theme,usingthe kineticinferences discussedabove.Figures3C and 4C use the samekinetic hindranceconsiderations but begin with alteredprotoliths. Because these figures are based on modeling real assemblageswith a subsetdefined by the NCMASH system,the predicteddensitiesandH20 contentscannotbe consideredexact; however, several interestingfeaturesare apparent. Figures 3B and 4B predict the behaviorof subducted basaltand gabbrousing the mineral assemblageolivine + orthopyroxene + clinopyroxene+ plagioclaseasa protolith. In the upper, volcanic crust, 50% reactionis assumedto occur at 150øC and complete reaction at 250øC; in the lower plutonic crust, 25, 50, 75, and 100% reaction is modeled as happening at 150, 250, 500, and 550øC, respectively. Forexample, thedensity of 2.97g/cm 3 for lawsonite-blueschist facies gabbro in Figure 3B was calculated using unaltered gabbro (2.89g/cm 3)transformed 50% to lawsonite blueschist (3.10 g/cm3).The H20 requiredfor hydrationin Figures3B and 4B is assumedto derivefrom sedimentsor hydratedmantle.In the absenceof sufficientH20, all faciesotherthanCE, EC, GG, and GN haveadensity of2.89g/cm 3 and0 wt%H20. The resultsfor volcanicrocksarecloseto thepredictions of Peacock'sequilibrium model (Figure 1) becausethe transformationrate is rapid. Basaltstravelingvia path 1 undergoonly slightchangesin densityprior to the appearance of garnet and the disappearanceof plagioclase.In contrast, Path 3 indicates that 5-10% volume loss occursin basaltsupon enteringboth the blueschistand eclogitefacies.As in the equilibriumcase(Figures3A and 4A), the biggeststepin dehydrationoccursat the blueschist/eclogite facies boundary;dehydrationalong path 1 is split subequally betweenthe greenschist/amphibolite and amphibolite/granulitetransitions.Basaltsmoving alongpath 3 or slightlywarmertrajectoriesloseroughlyequalamountsof H20 when entering and leaving the epidote blueschist stability field. Comparedto basalt, where volume loss occursfairly evenlyoverthe 150-450øCtemperature range, half the volume loss of gabbrooccursin the 500-550 ø interval. In contrast to basalt, which may undergo substantialhydration by 250øC, gabbro is predictedto HACKER A Equilibrtum model with unaltered basalt and gabbro protolith C Kinetic model with prehnite-pumpellyite and amphibolite facies protoliths distance (kin) 0 50 100 343 distance (kin) 150 200 Path 1 • = 2ocm/a 0 0 100 MPa.• 50 100 150 200 Path 1 • •n• v = 2ocm/a •_1%u __•..._ 0 'c= 100 MPa .•l 5.7_ ,os •• AM • • •o GG • • 50 • 311 • Pa•3 • •sGS'"••••• // •4 /// =33 MPa [ 2•9 GG •I0 •• •299 X•"•• 311 50• __ Pa•3 • -•••••• 324 / // / 1• $0 I0• $0 I0 Fig.4. Densities (g/cm 3) andH20 contents (wt%)of subducted B crust, basedon Figure 3. A) Equilibrium model with essentially Kineticmodelwith unalteredbasaltandgabbroprotolith unaltered crust. B) Kinetic model with unaltered crust. C) Kinetic distance(kin) 0 Path • • v = 10 crn/a •c -- MPa 2i89 50 2.80 100 150 200 0 modelwith crustalteredto subgreenschist-facies andamphibolitefaciesminerals.Comparewith [Peacock,1993] Figure4. Thicker lines are facies boundariesand thinner lines representchanges from 25% to 50% to 75% to complete transformation. The thickness of the crust is stretchedby a factor of 5 for clarity. Elongate rectangle at bottom of each panel shows density difference relative toasthenosphere (3.23gm/cm3). DISCUSSION: GEODYNAMIC IMPLICATIONS The retardationof reactionduring subductionillustrated in Figures 3 and 4 will affect 1) how the structure of subductedcrust is interpreted from seismic data, 2) the buoyancyforces that producesinking and bendingof the slab, 3) the distribution of stressesinduced by volume changes, 4) seismicity in the slab induced by volumechange-relatedstresses,5) the mechanicalbehaviorof the slab, 6) the distributionof H20 in the slab, and 7) the undergohydrationto temperatures up to 550øCmalthough generationof arc magmatism.All thesefactorsdiffer from at any givenconditionthe amountof H20 in gabbrois less the equilibriumstatebecausethe densificationreactionsin thanin basalt.Again, as in the equilibriummodel,the bulk descendingslabsdependon grain size, H20 content,bulk of the dehydration occurs at the blueschist/eclogite composition,and temperature.Relative to the equilibrium transition. model, transformationto blueschistand eclogiteis always Figures 3C and 4C predict the behavior of subducted delayedto greaterdepthsin the kineticmodel. altered basalt and gabbro using the prehnite-pumpellyite The first rocks to transform will be glassy basalts, faciesmineralassemblage chlorite+ albite+ pumpellyite+ followed by holocrystalline basalts, then diabases,and quartz+ calcitefor alteredbasaltand the amphibolite-facies finally gabbros.N9te that this derivesfrom thermalas well assemblagehornblende+ clinozoisite+ chlorite + quartz as kinetic causes,as the upper crust is warmer than the for alteredgabbro.The predictionsare qualitativelysimilar lower crust as well as being finer grained. The seismic to Figures3B and 4B, exceptthat the H20 contentat any velocity signatureof denserock shouldbe evidentfirst in chosenP andT is higher. the upper crust and may be substantiallyretardedin the •0• 344 ECLOGITE FORMATION IN OCEANIC CRUST lower crust.The absenceof fast velocitiesat eclogitefacies conditions cannot be taken to indicate the absence of oceaniccrust.High densitieswill appearfirst in the upper crustand may be considerablysuppressed in the lower crust.In Figure4, thelowercrustis notonlylessdensethan asthenosphere, but alsolessdensethanthe basalticupper crust.In thesesituationsthe uppercrustmay sink in the asthenosphere butthelowercrustcannot.Moreover,Kirby et al. [1996] calculated that densification reactions in Kirby and Hacker[1991] proposeda link betweenarc volcanism,intermediatedepthearthquakes, and eclogite formation.They notedthathypocenters of subduction zone earthquakes in the depthrange50-250 km frequently coincide with arc volcanoes,and proposedthat these intermediate-depth earthquakes mightbecaused by delayed eclogiteformation withinsubducting crust.Thishypothesis requiresthat eclogitedoesnot form at shallower depths becauselow temperatures and low H20 activityhinder subducting crustcan placethe crustin deviatorictension andthe mantlelithospherein deviatoriccompression. The transformation,and that the devolatilizationof amphibole differencein reactionrate betweenbasaltandgabbromeans that their calculationsare oversimplified.The initiationof densificationin the volcanicrockswill initially placethose rocksin tensionandthe underlyingcoarsergrainedrocksin and releasesvolatilesthat lead to the overlyingvolcanoes. compression. As reactionprogresses intothecoarserlower crustmustbe essentiallyanhydrous. Calculationsand conclusionspresentedabove apply crust,the stateof deviatoricstressin the gabbroicrockswill changefromcompressive to tensile.If therewereno other forcesat work in subductionzones,earthquakes at shallow depthswouldbe extensional in basaltandcontractional in in the 50-250 km depthrangetriggerseclogiteformation Figure2 suggests thattransformation to eclogiteoccursat much shallowerdepthsin most subductionzones.For KirbyandHacker'shypothesis to becorrect, thesubducting principallyto normalmid-ocean ridgecrust.Oceancrust that is anomalously thickor of unusualcomposition will transformdifferently.Oceanicplateaus, whichareup to 6 gabbro,changingasthe rocksdescenddeeperin subduction times thicker than mid-oceancrust, but compositionally similar, will have somewhat retarded transformation zonesto extensionalthroughoutthe oceaniccrust. Little is known about the mechanical behavior of becauseof the additionaltime requiredfor the subducting materialto heat;this may be partly offsethoweverby the blueschistfaciesminerals,but clinopyroxeneandgarnetare of volcanicrocks,whichwill transform two of the strongestphasesin the crust[Ji and Martignole, greaterthickness morerapidly.Field observations discussed aboveindicate 1994; Kolle and Blacic, 1983] and eclogite should be that Fe-rich rockstransformnotablyfasterthanMgO-rich strongcomparedto alteredbasalt.The replacement of finecentersoften grainedvolcanic phasesby blueschistor eclogitefacies rocks.Hydrothermalalterationat spreading markedMgO enrichment in lavas[Humphris and mineralsmay resultin strengthening, whereasthe alteration produces Thompson,1978], which would be expectedto slow of coarse-grainedminerals in gabbro to blueschistor transformation.Oceanicislandsbuilt by hotspotsandcrust eclogitephasesmay enhancedeformation[Rubie, 1983]. rifts are bothenrichedin Fe relativeto The volcanic layer, thoughhotter, may actuallybecome from propagating strongerthan the gabbroiclayer as a result of eclogite MORB, andwill transformmorerapidlythanstandardmidocean-ridgecrust.Crust eruptedat fast spreadingridges formation. It is conceivablethat suchrheologicallayering could result in a downward jump of the subduction (>60 mm/a) is alsoricher in Fe than crusteruptedat slow spreading (<50 mm/a)ridges[Niu andBatiza,1993;Sinton decollement from the volcanic into the gabbroic layer, resultingin underplatingof eclogitizeduppercrustto the and Detrick, 1992], and is expectedto transformmore rapidly. mantle hanging wall and continued subductionof the CONCLUSIONS gabbroiclayer. Crustthat undergoesslowtransformation is lesscapable The transformation of mafic rocks to blueschist and of containing H20. Figure 4 shows that a slowly zonesis complex,encompassing a transforminggabbrolayer has a maximum H20 content eclogitein subduction and grain -1/2 thatof the equilibriummodel.The flux of H20 carried wide varietyof igneousmineralassemblages intothemantle beyond adepth of70kmis 1.3x 108grams sizes,variablydevelopedandequilibratedmetamorphic permeterper yearin theequilibriummodel,butonly 0.48 x 108grams permeterperyearin thekineticmodelwith unalteredprotolith.Note thatif the only sourceof H20 in a subductionzone is the igneouscrust, situationssuch as Figure4C, wherethe H20 contentincreases with depth,are impossible.Only if there is anothersourceof H20 in the mineralassemblages, all pressurized andheatedvia a broad rangeof P T paths.Existingexperimental kineticdata cannot be extrapolated to actualpet•rotectonic settings in orderto predictwhererockstransformto blueschist and eclogite.However,texturaldatafromexhumed subduction zones worldwide indicate that little transformation occurs at temperatures below150øC.Volcanicrocksarecompletely by perhaps250øC.Coarsergabbroicrocks the H20 content of igneouscrust increasein subduction transformed zones. rarelyavoidcompleteeclogitization at temperatures above subductionzone, from subductedsedimentsor mantle, can HACKER 550øC,although examples of incomplete transformation are knownfromrocksheatedat temperatures ashighas800øC. 345 doteandassociated mineralsin low grademetamorphic rocks, Contributions toMineralogy andPetrology, 64, 123-136,1977. Brown,E.H., andE.D. Ghent,Mineralogy andphaserelations in The rapidtransformation of volcanicrocksandthe the blueschistfacies of the Black Butte and Ball Rock areas, metastable persistence of gabbroic rocksintotheblueschist NorthernCaliforniaCoastRanges, American Mineralogist, 68, 365-372, 1983. andeclogitestabilityfieldshasseveral implications. The P.,Cryptic variation withinthecumulate sequence of seismic velocitysignature of dense rocksshould beevident Browning, the Oman ophiolite: magma chamber depth and petrological firstin theuppercrustandmaybesubstantially retarded in implications, in Ophiolites andoceanic lithosphere, Geological the lower crust.The buoyancyforcescausingthe slabto Society Special Publications, edited byI.G.Gass, S.J.Lippard, sink will appearfirst in the uppercrustand may be and A.W. Shelton, pp.71-82,Geological Societyof London, London, 1984. considerablysuppressed in the lower crust.Earlier Opticaldetermination of formation of garnetin theuppercrustmaycausetheupper Carlson,W.D., andJ.L. Rosenfeld, topotactic aragonite-calcite growth kinetics: metamorphic imcrustto becomestronger thanthegabbroic layer,andmay plications, Journal ofGeology, 89,615--638, 1981. lead to underplating of eclogitizeduppercrustand Carmichael, R.S.,CRCpracticalhandbook ofphysical properties continuedsubductionof the lower crust.The initiation of of rocksandminerals, 741pp.,CRCPress, BocaRaton,FL, densificationin the volcanicrockswill initially placethose rocks in deviatoric tension and the underlying coarser 1989. Cloos, M., Lithospheric buoyancy andcollisional orogenesis: subductionof oceanicplateaus, continental margins, islandarcs, grained rocksin compression. As reactions progress into spreading ridges,and seamounts, Geological Societyof the coarserlower crust,the stateof stressin the gabbroic America Bulletin, 105, 715-737, 1993. rockswill change fromcompressive totensile. If therewere Coleman,R.G., D.E. Lee, L.B. Beatty,andW.W. Brannock, no otherforcesat work in subductionzones,earthquakes at Eclogites andeclogites--Their differences andsimilarities, shallowdepthswouldbe extensional in the basaltand contractional in the gabbro,changingat deeperlevelsto extensional throughout the crust.All theseeffectswill be minimizedin Fe-richcrustproduced at oceanicislandsand Geol. Soc.AmericaBull., 76, 483-508, 1965. Dick,H.J.B.,P.S.Meyer,S.H.Bloomer, S.H.Kirby,D.S.Stakes, andC. Mawer,Lithostratigraphic evolution of anin-situsection of oceanic layer3, Proceedings oftheOceanDrillingProgram, Scientific Results, 118,439-538,1991. A.E.-D.,R.G.Coleman, andJ.G.Liou,Eclogites and fastspreading ridgesandmostpronounced in Mg-richcrust E1-Shazly, formedat slow spreadingcenters. blueschists fromnortheastern Oman:petrologyandP-T evolution,Journalof Petrology, 31,629--666,1990. tectonics andthe Acknowledgments. Reviewed by S.H.Bloomer,S.R.Bohlen,S. - Ellis,D.J.,andM.A.H.Maboko,Precambrian DeBari,W.G. Ernst,J.G. Liou. A particularlythoroughreview wasgivenby J.H. Natland. physicochemical evolution ofthecontinental crust. I. Thegabbro-eclogite transition revisited, Precambrian Research, 55, 491-506, 1992. REFERENCES Ernewein, M., C. Pflumio,andH. Whitechurch, Thedeathof an accretionzoneas evidenced by the magmatichistoryof the Sumail ophiolite (Oman), Tectonophysics, 151,247-274, 1988. of glaucophane schists, J. Petrology, 4, itsgeophysical significance, Reviews of Geophysics andSpace Ernst,W.G.,Petrogenesis Ahrens,T.J., and G. Schubert,Gabbro-eclogite reactionrateand Physics,13,383-400, 1975. 1-30, 1963. B.W.,Phase relations of epidote-blueschists, Lithos, 25,3Alt, J.C., T.F. Anderson,L. Bonnell,and K. Muehlenbachs, Evans, 23, 1990. Mineralogy, chemistry andstableisotopic compositions of hyandpetrology of Tertiary-Recent orodrothermally alteredsheeted dikes;ODPHole504B,Leg 111, Ewart,A., Themineralogy genicvolcanic rocks: withspecial reference to theandesiteProceedings of theOceanDrillingProgram,Scientific Results, basaltic andesire compositional range,in Andesites: orogenic 111, 27-40, 1989. Alt, J.C., J. Honnorez, C. Laverne, and R. Emmermann, andesites and relatedrocks,editedby R.S.Thorpe,pp. 26-87, Wiley, Chichester,1982. Hydrothermal alteration of a 1-kmsection through theupper processes in oceanic ridgeandintraplate oceaniccrust,DeepSeaDrillingProjectHole504B;mineral- Flower,M., Magmatic in Oceanic basalts, edited byP.A.Floyd,pp.116-147, ogy,chemistry, andevolution of seawater-basalt interactions, settings, BlackieandSon,Glasgow,1991. Journalof Geophysical Research, 91, 10,309-10,335, 1986. An experimental investigation Batiza,R., PacificOceancrust,in Oceanicbasalts, editedby P.A. Green,D.H., andA.E. Ringwood, ofthegabbro-eclogite transformation anditspetrological impliFloyd,pp.264-288,BlackieandSon,Glasgow, 1991. cations, Geochimica Cosmochimica etActa,31,767-833,1967. Bohlen,S.R., V.J. Wall, andA.L. Boettcher, Geobarometry in in theformation of metagranulites, in Kineticsandequilibrium in mineralreactions, Hacker,B.R.,The roleof deformation morphic field gradients: Ridge subduction beneath the Oman editedby S.K. Saxena,pp. 141-171,Springer-Verlag, New ophiolite, Tectonics, 10,455-473,1991. York, 1983. Boundy, T.M., D.M. Fountain, andH. Austrheim, Structural de- Hacker,B.R., S.R. Bohlen,andS.H. Kirby,Albite-->jadeite+ quartztransformation in albitite, Eos,Transactions American velopment andpetrofabrics of eclogitefaciesshearzones, Geophysical Union,74,611, 1993. Bergen arcs,western Norway; implications fordeepcrustal deformational processes, Journalof Metamorphic Geology, 10, Hacker,B.R., S.H. Kirby,andS.R.Bohlen,Time andmetamorphicpetrology: thecalcite -->aragonite transformation, Science, 127-146, 1992. 258, 110-113, 1992. Brown,E.H.,Phase equilibria among pumpellyite, lawsonite, epi- 346 ECLOGITE FORMATION IN OCEANIC CRUST Hekinian, R., Petrology of the oceanfloor, 393 pp., Elsevier, Amsterdam, 1982. Holland, T.J.B., The reactionalbite = jadeitc + quartzdetermined experimentally in the range 600-1200øC, American Mineralogist, 65, 129-134, 1980. Hopson, C.A., R.G. Coleman, R.T. Gregory, J.S. Pallister, and E.H. Bailey, Geologicsectionthroughthe SamailOphioliteand associatedrocks along a Miscat-Ibra transect, southeastern Oman Mountains,Journal of GeophysicalResearch,86, 25272544, 1981. Humphris, S.E., and G. Thompson, Hydrothermal alteration of oceanic basalts by seawater, Geochimica Cosmochimicaet Acta, 42, 107-125, 1978. Indares,A., and T. Rivers, Textures, metamorphicreactionsand thermobarometry of eclo•itizedmetagabbros; a Proterozoic example,EuropeanJournalof Mineralogy,7, 43-56, 1995. Ito, K., and G.C. Kennedy,An experimentalstudyof the basaltgarnet granulite-eclogite transition, in The Structure and Physical Properties of the Earth's Crust, edited by J.G. Heacock, pp. 303-314, American Geophysical Union, WashingtonD.C., 1971. Ji, S., and J. Martignole,Ductility of garnetas an indicatorof extremely high temperaturedeformation,Journal of Structural Geology,16, 985-996, 1994. Juteau,T., M. Beurrier,R. Dahl, andP. Nehlig, Segmentation at a fossil spreadingaxis: the plutonic sequenceof the Wadi Haymiliyah area (Haylayn block, Sumail nappe, Oman), Tectonophysics, 1881, 167-197, 1988a. Juteau, T., M. Ernewein, I. Reuber, H. Whitechurch, and R. Dahl, Duality of magmatismin the plutonicsequenceof the Sumall nappe,Oman, Tectonophysics, 151, 107-136, 1988b. Kirby, S.H., E.R. Engdahl, and R. Denlinger, Intraslab earthquakesand arc volcanism:dual physicalexpressionsof crustal and uppermostmantle metamorphismin subductingslabs,in Dynamics of Subduction,GeophysicalMonograph, edited by G.E. Bebout,D. Scholl,and S. Kirby, AGU, Washington,D.C., 1996. Kirby, S.H., and B.R. Hacker, Intermediate-depthearthquakes, crustal phase changesand the roots of arc volcanoes,Eos, Transactions AmericanGeophysicalUnion,72, 481, 1991. Klaper, E.M., Reaction-enhanced formation of eclogite-facies shear zones in granulite-faciesanorthosites,in Deformation Mechanisms,Rheologyand Tectonics,GeologicalSocietyof London Special Publication, edited by R.J. Knipe, and E.H. Rutter,pp. 167-173, London,1990. Kolle, J.J., and J.D. Blacic, Deformationof single-crystalclinopyroxenes;2, Dislocation-controlledflow processesin hedenbergite, Journalof GeophysicalResearch,88, 2381-2393, 1983. Lardeaux,J.-M., and M.I. Spalla, From granulitesto eclogitesin the SesiaZone (Italian WesternAlps); a recordof the opening and closure of the Piedmont ocean, Journal of Metamorphic Geology,9, 35-59, 1991. Liou, J.G., P-T stabilities of laumontite, wairakite, lawsonitc, and relatedmineralsin the systemCaA12Si208-SiO2-H20, Journal of Petrology, 12,379-411, 1971. Lippard, S.J., A.W. Shelton, and I.G. Gass, The Ophiolites of Northern Oman, 178 pp., 1986. Maresch,W.V., Experimentalstudieson glaucophane; ananalysis of presentknowledge,Tectonophysics, 43, 109-125,1977. M0rk, M.B.E., A gabbro to eclogite transition on Flems0y, Sunnm0re,westernNorway, ChemicalGeology,50, 283-310, 1985. Niu, Y., and R. Batiza, Chemical variation trendsat fast and slow spreadingmid-oceanridges,Journalof Geophysical Research, 98, 7887-7902, 1993. Oh, C.W., andJ.G. Liou, Metamorphicevolutionof two different eclogitesin the FranciscanComplex,California,USA, Lithos, 25, 41-53, 1990. Pallister,J.S., and C.A. Hopson,Samailophioliteplutonicsuite: field relations,phasevariation,crypticvariationandlayering, and a modelof a spreadingridgemagmachamber,Journalof GeophysicalResearch,86, 2673-2645, 1981. Peacock,S.M., Numerical simulationof metamorphicpressuretemperature-timepaths and fluid productionin subducting slabs, Tectonics, 9, 1197, 1990. Peacock,S.M., The importanceof blueschist-• eclogitedehydration reactionsin subducting oceaniccrust,GeologicalSocietyof America Bulletin, 105, 684-694, 1993. Pognante,U., Petrological constraintson the eclogite- and blueschist-facies metamorphismandP-T-t pathsin the Western Alps,Journalof MetamorphicGeology,9, 5-17, 1991. Poli, S., The amphibolite-eclogite transformation; anexperimental studyon basalt,AmericanJournalof Science,293, 1061-1107, 1993. Reynard,B., andM. Balleyre,Coexistingamphiboles in aneclogitc from the WesternAlps; new constraintson the miscibility gap between sodic and calcic amphiboles, Journal of MetamorphicGeology,6, 333-350, 1988. Ridley,J., andJ.E. Dixon, Reactionpathwaysduringtheprogressive deformation of a blueschist metabasite; the role of chemi- cal disequilibrium andrestrictedrangeequilibrium,Journalof MetamorphicGeology,2, 115-128, 1984. Robinson, P.T., H.J.B. Dick, V. Herzen, and R. Pierre, Metamorphismand alterationin oceaniclayer 3; Hole 735B, Proceedingsof the OceanDrilling Program,ScientificResults, 118, 541-562, 1991. Rubie, D.C., Reaction-enhanced ductility:the role of solid-solid univariant reactions in deformation of the crust and mantle, Tectonophysics, 96, 331-352, 1983. Rubie,D.C., Role of kineticsin the formationandpreservation of eclogites,in EclogiteFaciesRocks,editedby D.A. Carswell, pp. 111-140, Blackie,Glasgow,1990. Sandrone,R., L. Leardi, P. Rossetti, and R. Compagnoni,P-T conditionsfor the eclogiticre-equilibrationof the metaophiolites from Val d'Ala di Lanzo (internal Piemontese zone, WesternAlps), Journalof MetamorphicGeology,4, 161-178, 1986. Schliestedt, M., Eclogite-blueschist relationships asevidencedby mineral equilibria in the high-pressuremetabasicrocks of Sifnos(CycladicIslands),Greece,Journal of ?errology,27, 1437-1459, 1986. Sinton,J.M., andR.S. Detrick, Mid-oceanridgemagmachambers, Journalof Geophysical Research,97, 197-216,1992. Zhang,R.Y., J.G.Liou, T.F. Yui, andD. Rumble,Transformation Liu, J., W.G. Ernst,J.G. Liou, andS.R. Bohlen,Experimental of gabbroandgranitoidto coesite-bearing eclogitefaciesrock constraintson the amphibolite-eclogitetransition,Geological from Yangkou,the SuluTerrane,easternChina,International Societyof AmericaAbstractswith Programs,25, 213, 1993. GeologicalCongress,in press,1995. Liu, M., and R.A. Yund, Transformation kinetics of polycrystalline aragoniteto calcite: new experimentaldata, modelling, and implications,Contributionsto Mineralogyand Petrology, 114,465-478, 1993. B. R. Hacker, Departmentof Geologicaland Environmental Sciences,StanfordUniversity,Stanford,California94305-2115. 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