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Home , Eclogite, Eclogitization, Laumontite, Wehrlite

Formationand the Rheology,Buoyancy, Seismicity,and H20 Contentof OceanicCrust

BradleyR. Hacker1

Departmentof Geologicaland EnvironmentalSciences, Stanford University, 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-temperaturepaths in subductionzones. Although experimentalkinetic data cannotbe extrapolatedto predict the rates of blueschistand eclogite formation in nature, textural data from rocks indicate that transformationbelow temperaturesof 150øCis minimal. Completetransformation of volcanicrocks occurs by •-250øC, but incompletetransformation of gabbroicrocks heatedto 800øC has been observed.There are important consequencesto the rapid transformation of volcanic rocks and the metastable persistenceof gabbroicrocks into the blueschistand eclogite stability fields. Fast seismic velocities shouldbe evident first in the upper oceaniccrust and may be substantiallyretarded in the lower oceaniccrust. The upper oceaniccrust will be denserthan asthenospherebefore the lower oceanic .Early in the processof eclogiteformation, volcanic rocks will be placedin deviatorictension and the underlyingcoarser grained rocks in compression;with furtherreaction, the stateof stressin gabbroicrocks will changefrom compressiveto tensile.Earthquakes at shallowdepths should be extensional in and contractionalin , changing at deeper levels to extensional throughoutthe crust.

INTRODUCTION This paper summarizes the rates and textures of densificationreactions in subductionzones by examining Formationof eclogiteis a key factor in plate , experimental and field studies, and then considersthe influencingthe size and shapeof platesas well as their age effects of these findings on the rheology, buoyancy, and rate of disappearance from the 's surface. seismicity,and H20 contentof subductingoceanic crust. Subductionzones have long been of interestbecause they are the buildersof continents,collecting material from the WHAT ARE AND ECLOGITE? oceanbasins and constructing magmatic arcs. In spiteof the dynamic nature of zones, much of our Blueschist facies rocks are characterizedby the understandingof their behavior stems from equilibrium coexistenceof sodic and at low thermodynamics.The equilibriumviewpoint has proven a temperatureor epidote at high temperature[Ernst, 1963]. valuablefirst meansof addressingprocesses in subduction zones, but a more in-depth comprehension requires They may alsocontain or .Eclogite sensu considerationof the rates and mechanismsby which phase stricto is mafic consistingof garnetand omphacitic changesoccur during subduction. clinopyroxenewith or without additional minor phases [Coleman et al., 1965]. The geophysicalimportance of Alsoat U.S. Geological Survey, Menlo Park, California eclogiteis that it representsthe highestdensity commonly attained by crustal rocks. Omphacite and garnet have Subduction:Top to Bottom densitiesof 3.2-3.4 gm/cm 3 and 3.6-4.0 gm/cm 3,a marked GeophysicalMonograph 96 elevation beyond the density of a typical basalt at 2.9 This paperis not subjectto U.S. copyright. gm/cm3 [Carmichael, 1989]. This shift from positive to Publishedin 1996 by the AmericanGeophysical Union negativebuoyancy relative to ,which has a

337 338 ECLOGITE FORMATION IN

densityof roughly3.23 gm/cm3 [Cloos, 1993],is as 2.6 ':iiiiiiiiiiii::.:.0::::•i•i•i!ii!!111iiiiiiiiiii!!iiiiii?iiiii½•::::•...... important a factor in controlling plate subductionas are 2.4 '?iiiilili• 3•7 conductive cooling of the lithosphereand the --> 2.2 ':iiiiiii!iiiiii:...... eclogite ::::.:::::::•. transformation in subductingmantle [Ahrens and ..:.:...:...... ,,•,•,:•,•,,...... : ...... 3.24 ...... Schubert, 1975]. ••' 3.10• • •:•'-'::ep•dota•i•i•i•...... ::::::::::::•...... :::::::::::: ;:•:•:•...... •:•:•i•i•i•11!'•'•:g:'•i...'.•i•iii!!i!iiiii!iiii•i•i••-•:•:::•::,:.•'-----1!======:::::::::::::::::::::::::::::::::::::...... •:•...... • ...... :...:.: ....:.:.....:.:.:.:.:.:....•[•?.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.: .....

PRINCIPAL DENSIFICATION REACTIONS ß

A broad spectrumof reactionscause rocks to become more dense in subductionzones. At the high temperatures ...... •::•?•...... •?::• ...... •?...... •...... •:•...... found in very hot subductionzones the transformationof 0.6 ...... ß...... 2 93 ...... •"v ...... , 0.4 subgreenschist.....•?' 3.5 .:•? ;.9ff .•{•? 0 basalticrocks to garnetgranulite and then eclogiteinvolves 2.80 ....ii•i•}i•?" ..•}•?" •-' :•{{•}•' 0.2 5.2 ::•!•i•i:'" -':3•iiii...... •:• the breakdown of , addition of Na20 to cliopyroxene and the growth of garnet (Figure 1). PT 0 1• • 3• Tem•ramre (øC) 7• 8• 9• conditions for the appearance of garnet have been Fig. 1. and reactions pertinent to eclogite determinedby experimentalstudy [Green and Ringwood, formation, after [Liou, 1971 ], [Maresch, 1977], [Brown, 1977; 1967; Ito and Kennedy, 1971; Liu et al., 1993; Poli, 1993], Brown and Ghent, 1983; Evans, 1990], [Green and Ringwood, and occursatlower pressures/highertemperatures in SiO2- 1967; Ito and Kennedy,1971; Liu et al., 1993; Oh and Liou, 1990; undersaturated[Ito and Kennedy, 1971] and MgO-rich Poli, 1993]. Three- and two-digit numberfor each faciesindicate rocks [Green and Ringwood, 1967]. The disappearanceof density(g/cm 3) and H20 content(wt%) calculatedusing plagioclase is less well defined by the same studiesand NCMASH model of Peacock [1993] assumingequilibrium (i.e., no kinetic hindrance). dependsmore stronglyon rock composition. The transformation of basaltic rocks to blueschist and then eclogite at the low temperaturesthat prevail in most powderedmaterials may not be directlyapplicable to the subduction zones is more complicated because of the majority of geologic situationswhere rocks are tightly prevalenceof disequilibriumin natureand experiment.One packed crystalline aggregates--principallybecause the reactioninferred to have transformedblueschist to eclogite interfacialfree energyof a rock is muchless than that of a at severallocalities worldwide is epidote + powder.For extrapolationto geologicconditions, data on --> garnet + omphacite+ paragaonite+ + H20 [El- phase transformationsin unpowdered materials are Shazly et al., 1990; Ridley and Dixon, 1984; Schliestedt, required--informationthat is exceedinglysparse. 1986]. Recent experimentson reactionsknown to occur in Eclogite facies conditionsexist at temperatures>500øC subduction zones demonstrate that most kinetic data cannot and pressures> 1.2 GPa (Figure 1). Eclogitefacies rocks do be extrapolatedto theEarth and that H20 hasmajor effects not necessarilyexist wherever such conditionsprevail in on reaction rate and texture. For instance, Holland [1980] the Earth becauseof sluggishkinetics. All reactionsrequire observedjadeite growthin 24 hr from powderedalbite + oversteppingof equilibriumto overcomelocal free energy quartz at 900øC and a pressureoverstep of 50 MPa. In increases related to attachment or detachment of atoms at contrast,albite -->jadeite + quartzexperiments by Hacker interfaces, and all reactions involve diffusion or atom-atom et al. [1993] on albite rock showedno transformationin 24 bond rearrangement.Perceptible reaction only occurs at hoursat 1100øCat a pressureoverstep of 500 MPa! sufficiently high diffusivitiesand reactionfree energies. Quantitative transformation-rate measurements are available for the calcite <--->aragonite reaction. As with HOW FAST ARE REACTIONS IN THE EARTH? albite, the calcite --->aragonite transformation in Carrara marble[e.g., Hacker et al., 1992] is muchslower than in Experimental and Theoretical Inferences calcite powder. For example, at 600øC and 2.0 GPa How long can rocks remain out of equilibrium at confiningpressure, powdered calcite transforms50% to elevated pressuresand temperatures?Specifically, how aragonitein <1 hour [Brar and Schloessin,1979, p. 1409], long can rocks remain within the eclogite stability field whereas192 hourswere requiredfor similarconversion in before transformingpartially or completely to the stable unpowderedmarble. Carlson and Rosenfeld [1981] andLtu phase assemblage?Information relevant to this question and Yund [1993] measuredthe rate of the aragonite ---> derivesfrom two sources:experiment and studyof natural calcite reaction; extrapolatedto geologictimescales and rocks [Rubie, 1990]. Experimentalkinetic data collectedon grainsizes, their data imply that 1 mm aragonitegrains will HACKER 339 revert completely to calcite in less than 1 m.y. at 3.0 2.8 •-, c.....•o_.esiteeclogite- : temperatures as low as 200øC. Hacker et al. [1992] 2.6 :.•feeble,c.25% '::'""'"•:'""'"•i•:..., •:i'/!111['"'":'""'""'"":••'"'"'"'""'":•:'...... exploredthe reversetransformation in marbleand founda ;"[]moderate, c.50% 2.4 I extensive,c.75% '?'"'"'•jiii:. eclogite similar result, that the calcite --> aragonitetransformation 2.2- I complete,100% '"':":::½'{'"'"'"':'g-."i:-•i!•,•;•;*:':' occurs within less than 1 m.y. at 200øC for overstepsof 2.0- '--• cooling :.'.-':•:•:•:•:•.'.-':•:i:.'.""•i: ...... ' <100 MPa. The reactiondepends strongly on temperature, lawsoniteblueschist such that even several GPa overstepat temperaturesof 2:iiii::::•::5"i•5::i•5•!'1•. ".... ::---'•".:"-..--'i--'•::•-:•:...... :i::::::::ii•$;";.'•: t'•;J.•i ...... • •,.•.::lim•'-' ß <100øC are unlikely to promotetransformation. ..si::::::::::::::ii[g'-:$•t'•-":• • :>.-r.,'-•m::•,-•.,-. amet granuh.t...e..... The catalyticaction of H20 is well known from kinetic 1.0 -...... !,x.•qx•,,.,...:.©B .:•5{'• •'..•.,•½.•-•...-•::•x :...... :,•..'p•.•'./•.,.•-..•jß •...... ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::...... •:•:•:•,...... •.-.:--•:•:•:.-',•.-.::i'..:,•.-.:.,•.--:.•,.-':•:•,•:•, ...... studies of powders and rocks. Hacker and Kirby [1993] 0.8 /i:'.i:'IIi-'• ...... : :' .'.-':•:•:5.-::::::.-• •:'• •I'• • ; • •-•i....'- *!•½:.':.'-:::'-.:i:•i•'•:• '...... ::]-'.-'-'•:•:•-••.:-'•i•i.-'..::::::.-::-'.--'.. . deformed polycrystalline, vacuum-driedalbite rocks at 0.6 "'""::: 5:5':" :':':':'•{•'"'&'•ø'•:e' :'":•' hi 1' 5:•5•'.':•:•:granuhte strainrates of 10-4-10-6 s-1 at temperaturesof 600-800øC 0.4 subgreenschi•'t.."'"'•½':'gree'"'"'""'"'' n s c h is t.':•i'-"? ':' '"'•?'':'"''"''"'' andconfining pressures of 1.0-2.0 GPa. In spiteof extreme 0.2 0.0 pressureoverstepping and extreme sample strain, deformed 0 anhydrous samples contained no jadeite. Differential 1•0 2•)03•)0 Temperature (oC)6•)07•)0 8•)0 9&) 1000 stressesas high as 2000 MPa producedmaximum normal Fig. 2. Reported extents of transformation, pressures, and stressesup to 4.0 GPa--all at temperatureswhere the temperatures,for blueschistand eclogite facies equilibriumpressure is <2.0 GPa. Some of thesesamples worldwide. References and data used to compile figure are were strained70%, and individual grains attainedaspect available from the authoron request. ratiosof greater than 10:1. In markedcontrast tothis, the addition of 1 wt% H20 produced partial reaction at reaction coronas are denoted as "moderate, 50%", and those temperatures as low as 600øC, even in undeformed with rare relict phases are labeled "extensive, 75%". samples. Metamafic rocks specificallydescribed as lacking relict More complicatedexperiments measured the olivine ---> phasesare shownas "complete,100%". spinel transformationin hot-pressedMg2GeO4 [Burnley, There are some important limitations to Figure 2. 1) 1990] and Ni2SiO4 [Rubie et al., 1990] at pressuresand Reportedpeak pressuresand temperaturesmay not have temperatures of 800-1900øC and 1-15 GPa. When occurred at the same time and, at best, representonly a extrapolatedto geologic conditionsthese investigations singleway-station on thePT pathexperienced by therocks. suggestthat transformationmay be suppressedto depths 2) How long the rockswere at the reportedpressures and 150-200 km below the equilibrium boundaryin rapidly temperaturesis unknown.3) Pressureestimates for many subductingslabs. rocksare minimabecause of the high varianceof the phase In summary, existing experimentaldata indicate that assemblages;these are denotedby U-shapedlines rather H20 has a major catalyticaffect on reactionrates and that thanrectangles. 4) Many high-pressurerocks experienced some reactionsare geologicallyfast (e.g., calcite-->arago- substantial post-high pressure alteration that likely nile) whereasothers are probablyslow (olivine --> spinel). consumedexisting relict phases;studies reporting severe We have essentiallyno experimentalinformation on reac- alterationof thistype were excluded from consideration.5) tionsforming blueschistand eclogite. Fine-grainedrocks react fasterthan coarse-grainedrocks becauseof increasedinterfacial free energy;gabbroic rocks Metamorphic Textures are only partiallytransformed at conditionswhere volcanics are completelyrecrystallized. Numerous papers that do not Observations on rocks show that transformation to specificallystate whether relict phases are present or absent blueschistand eclogitemay be suppressedlong after rocks had to be excludedfrom Figure 2, but this may simply have passedinto the respectivestability fields of these mean that the absenceof relics was not significantfor the assemblages.Figure 2 was constructed from studies purposesof the study.Thus Figure2 shouldbe takenas an worldwide that reported not only peak metamorphic illustration of the transformation behavior of coarse- pressuresand temperaturesbut also textural information grained,i.e., gabbroicrocks. about the degree of reaction. Rocks reported as feebly With these limitations in mind, Figure 2 indicatesthat recrystallizedor whosehigh-pressure phases were so minor feebly metamorphosed rocks generally persist to as to requiredetection by electronmicroscopy are shownin temperatures of 200-250øC. Moderate transformation Figure 2 as "feeble,25%." Rockscontaining abundant relict beginsat 200-250øC and is foundin rocksthat wereheated phases (usually clinopyroxene) or partially developed to just above 600øC. There are a few casesof extremely 340 ECLOGITE FORMATION IN OCEANIC CRUST limitedreaction at highte•nperature shown by dashedboxes structure of oceanic crust. Oceanic crust has been studied in Figure2. Incompleteconversion of gabbroto eclogiteis by dredging and drilling in ocean basins,and by the known to have happenedat temperatures>700øC and examinationof .The Samailophiolite in Omanis pressuresof 1.5-2.0 GPa in the presenceof aqueousfluid the best-exposed,largest, least-deformed,and perhaps [MOrk,1985]. Suppressionof transformationof gabbroto most-studiedophiolite in the world. Plutonicrocks of the eclogitewas reported from rocksthat reached 800-900øC Samailophiolite are divisible into a lowerlayered sequence andpressures of >2.8 GPain the absenceof fluid [Zhanget and an upperisotropic section. The lower 2.3 km (avg.)is al., 1995]. Oddly enough,there is little differencein the PT well-foliated and lineated, layered ultramafic (chiefly range over which extensiveto completetransformation wehrlite, , and clinopyroxenite) through mafic have been found. Both have been observed from rocks cumulates [Pallister and Hopson, 1981]. Olivine heatedto lessthan 300øCand yet partialtransformation of predominatesat the baseof thesection, whereas plagioclase gabbroto eclogitehas occurred at 760-850øCand 1.6-2.0 is dominantat the top [Juteauet al., 1988b].Three-quarters GPa [Indares and Rivers, 1995]. In general, 500-600øC of the sectionis gabbro--typicallyolivine-clinopyroxene marks the transitionto extensiveor completereaction for or clinopyroxenegabbro with minor hornblendeand FeTi most rocks. A few cases, however, indicate that the oxides--but at rare localitiesit is gabbronorite[Juteau et transformationof gabbroto eclogitemay be suppressedto al., 1988a].The gabbroconsists of mm-sizegrains of 55% muchhigher temperatures. plagioclase,35% clinopyroxene,10% olivine, and rarely Fe-rich rocks transform to eclogite along high P/T Ti-pargasite [Browning, 1984]. Overlying the layered trajectories more readily than Mg-rich rocks because sectionare 200-900 m of isotropichigh-level mafic rocks, reactionsinvolving Fe-rich phases generally occur at lower chiefly hornblende-clinopyroxenegabbro [Pallister and temperatures[Bohlen et al., 1983].Field evidenceof this-- Hopson,1981]. Grain sizesrange from mm to pegmatitic MgO-rich rockswith abundantigneous relics adjacentto [Juteau et al., 1988a]. Sheeteddikes containabundant to Fe-rich rocks with rare relict phases--hasbeen noted at sparseplagioclase and augite [Hopsonet al., 1981]. The BardoneyValley in the [Reynardand Balleyre,1988], volcanic rocks are non- to moderately vesicular, almost at Flems0y, [MOrk, 1985], and in Piedmont aphyric pillows and rare brecciatedflows and massive ophiolites[Pognante, 1991; Sandrone et al., 1986]. flows [Ernewein et al., 1988]. Eclogiteformation is fasterin finer grainedrocks. For Mid-ocean-ridgebasalts (MORBs) typicallyare glassy example,in the BardoneyValley of the Alps, abundant or containplagioclase with lessolivine [Hekinian,1982], relict augites are present in , but absent in while rockscrystallized in intraoceanicmagmatic arcs gen- metavolcanics[Reynard and Balleyre, 1988]. In the Sesia erally containplagioclase and augite [Ewart, 1982]. Mid- Lanzo zone farther east, transformed to ocean-ridgebasalts (MORBs) are subalkalinetholeiites eclogitecontains relics in coarserocks but none in fine- that typically contain plagioclase with less olivine grainedrocks [Lardeaux and Spalla, 1991]. [Hekinian, 1982]. Oceanic plateaus, which make up a Not only is H20 provento acceleratereactions in the sizeableproportion of oceaniccrust, are similar in their laboratory,numerous localities demonstrate that this is true majorelement chemistry to MORB, but are 10-40 km thick in natureas well. Mafic granulitein the MusgraveRanges [Cloos, 1993]. Hotspot lavas and propagatingrifts are of cooled into the eclogite stability field and, enriched in Fe and Ti relative to MORB [Flower, 1991]. thoughthe bulk of the rock is unaltered,local shearzones Lavasproduced at fast spreadingridges such as the Pacific contain omphacite + garnet + minor zoisite [Ellis and have lower Mg/Fe ratios than MORB eruptedat slow Maboko, 1992]; the presenceof zoisite implies that the spreadingridges [Batiza, 1991]. reaction was fluxed by H20. At Holsn0y in the Bergen Arcs, zoisite-bearingeclogite haloes in undeformedrock ALTERATION OF IGNEOUS OCEANIC CRUST surroundingdeformed veins also indicate that fluid drove Becausephase transformations are stronglyinfluenced reaction[Boundy et al., 1992; Klaper, 1990]. Deformation by the availabilityof H20 andthe extent and type of pre- is involved in these fluid-mediated transformations existingalteration, understanding the alterationof oceanic inasmuchas the fluid mustmake its way to thereaction site crust is critical to evaluatingwhen and where phase via cracks. transformations occur. Variable alteration of oceanic crust to lower grade assemblagesoccurs by active COMPOSITION OF IGNEOUS OCEANIC CRUST hydrothermalcirculation at spreadingcenters and by later To understand the transformation of oceanic crust to weatheringon the seafloor[Alt et al., 1986;Humphris and blueschistand eclogitewe mustknow the compositionand Thompson,1978]. Somerocks escape alteration entirely, HACKER 341

while others are wholly recrystallized to new . A Equilibriummodel 2.8

Most typically, lavas are variably metamorphosedto 2.6 ß, base of crust subgreenschistfacies, dikeschange partially to 2.4 -- top of crust -----....--..:•:!•i•:::.-::-:::-'...... 3.31 ';iii?iii'.:•. 0 facies, and gabbro recrystallizesweakly to amphibolite- 2.2- 2.0- • ::..::•..i•}•.3.25 ...... !•.:...iii'"•'•" faciesminerals. The pressuresat which thesemetamorphic .•?;•?.":.-:-:iii:":-'?.::. O.9 •:.:,•;ii::::.:'.:'giii?..':•.:_gig assemblages form are relatively low (<300 MPa; determinedby ocean depth and the thicknessof oceanic /3.02,]Z.:i:.:i? :'"i{{i":":'"'"•J:-5•!!: .,..-:•---'-'--}i'iii:f'2'•J'""'"'"'"'"'"'"'"'"':'•':':•:"•••••' crust) and the formation temperaturesmay range up to /.-2..-.-'•?':;':::::':'3.43 4 ..:::::5..`..}iiii•...... •.!{iii•..`.:iii...... •i•i•.•*•::•`.`...:...... :•...... •i• ':.:'i!•{•.--:-.:...:...... --:::-:ggiiii.::'m"'---:-:-:-:.-:•..-•..,..:.:.'-.-:.,i,e:..... 3.!0 -650øC. 1.0 Our best information about the alteration of oceanic / .:g?siiiii!}.::..,•:i•i::iii{ii?.--•ii•¾-3':.."..il-:' 1.2:g-':.-.'-:....-'• ...... xD;• ,:•557:..:'-"....'5•i...... '"••5!5"'"•:':"":: 0.8 lavas and dikes comes from DSDP hole 504B [Alt et al., 0.6 // •/'"":•:%•i;':-::•?.... ' 2.91...... '•::'"":5•ii2.89 1989; Alt et al., 1986]. There the lavas contain -10% 0.4 alterationhaloes around veins and cracks,leading to about 0.2 2.99...... 0.0 1 wt% H20 in the bulk rock. Veins and vugs containFe- 0 1'00 200 •00Temperature (øC) hydroxides, clay, celadonite, smectite, , and carbonate; groundmass minerals are altered to Fe- B Kineticmodel with unalteredbasaltic and gabbroic protolith 2.8 • • . [...... ::.-..-:3.063.23 3.34 ...... hydroxides and clay; plagioclase is partly altered to ß '::"?•:•.:.ii-i':"i•,•i'••'••---'•:'::...... " -- - - •:::'•...... smectite;elivine is partly to totally replacedby smectite 2.6.2.4./'kV-',-,7 basetopelOfcrust crust // ':':•--"'"J•ii:•:0•-!!i.5.0.3:65.77.--.--:•-- ': O.....-• -.-O•*.i;-"•'"'"'•':"'•--• ::""•"'•::i5 -"•g"i•i'•'•*•""'"...... 3.37 and minor calcite;glass is partly to completelyreplaced by 2.2. i / ':':•..:.•"3.16: 0 clay; and pyroxenesare unaltered.Alteration in the dikes increases down section to a maximum of-50 vol%. elivine ..• . :!ii::'".-"-5:..-;3•i•i0.7.3.25 •.--:-:•:...... '•i!i is replaced by chicrite, clay, and ; plagioclase is ••'U .;..... / :.. 294 'g•':"i"'93...... "•'•'"'"2•:a:..;•:.::"k.':j:":=8 i ...... :.-!•.:-'•i'-"'"•'. ' i['"'"':"'"•'"'/:•'""'"" ' ' ..... partially to totally replacedby albite + zeolites,chicrite, or ::0.6/ 2.3:. ß'?•;•i-.:' '-'.'•' '"'"'--'-'":-'-'•--'•••••iim"•-•":-----'- "•....:j•i clay; and augitcis unalteredhigh in the sectionand rimmed 1.0• / ,' ":•'"••'•"'•i'":•:•iii•?-'-ili•"•'""•::-:::-x...... "' ' .,.:.::s::.-::.-::.-e,%-.'<-:.:e 0. 9'u 1.2 .:..:'..-':..-':..:•:•:::s::::.'-• ...... •.:.:...:,:s:...::.-:j;-'.:4-'.i'"'"'"" ..-•:---•:.:-%-.':i:Z:•::'-'•::':i:.::e::'-...-.'.2:--'.Si:::::''•.*:':.: ...... by actinolitedeeper in the section.The H20 contentof the dikesaverages 2 wt% [Alt et al., 1989;Alt et al., 1986]. 0.6 ., /-:...-:o.....-.•...... -•-::.'•;?:2.0' 290 :?:iiil..-"ii?i. .,,,,, ":'•--'"d•il}i2 89 2.89 4'00:':";"'"":•' oh'::ii{:::•--:".'?' f 3 '::2:--•"2 98' z.:,v :-:..-'•.:'.:'•i.-'::•ß Good information on the alteration of plutonic rocks 0.4 o •.oo ,,.,.o,,i.'.i-..? ß --•;•i:.-ii•:'ß7 _' • n '::":J•..:'."{.-::0 • .0.5 / 2.{I "" _ ._/ ;:..:.-'.•?: 1.3 ß-' -":-..-":..':-'-_•? comes from the Samail and ODP core holes. 0.2 gabbro:!2S"'"0/"":'•:•. -S'O/orea'"•h i7S0/o:100% :::'"'"'"/'::•ction 0.0 bas91t::50% rxn. •00%reaction i Alterationof the Samaillayered plutonic rocks is limited to 0 100' 200' 300Temperature(oC)g00 000 100 •00 1000 rare replacementof clinopyroxeneby actinolite[Lippard et al., 1986] and alterationof the isotropicplutonic rocks is C Kineticmodel with -facies basalt and amphibolite-facies gabbro protolith limited to the growth of up to 10% actinolite and FeTi 2.6•k baseofcrust // ':•-;"•iii•i•O.5'*:?'":':•••••i!i!i::ii[.-'•i•::•--"..-':-•'•'•'':" '- ' ' '.....0.....:::::•,:..• •":•:""-'"•!:'i:i•5':.•"•i•11t}...... i'"':':":'"'"::•:•'::'"• ...... oxides,and the infilling of cavitieswith epidote+ sphene+ 2.4T topofcrust ' / ":!{3'i•:•'?...... ' + quartz [Ernewein et al., 1988]. In contrast, / ...... ! 3.31 "::!"":• ' 0 gabbroic rocks drilled from ODP hole 735B in the Indian 2.22.8t ' :' •/o:•'. ' !.:.::::.:..•:.3.06:]•ii:i3.163.233.343.25...... ;::-----...:-:-:•::•.-.•..:.:;s:-::-.:5.:15•:: , Oceanshow widespread amphibolite-facies alteration from 0-100%, with an averageof perhaps20-30% [Dick et al., 9 3 02 1991]. elivine and pyroxeneare replacedby hornblende, / 35. .' /'"'""•73•t.5 i:::::::i: ß ....-'"d..-'3";!•,B::::::::::::...... :::.-: :::,.-..':•C'"'"'"'•'"'•"'•'•••••:::•:-:-:-:... :-,•-:.::•:.::!:-:[.?.i:i:.::---:-:-:- ...... , ., ;...•:.5:...-?. ""'"'::'•.-'-':":?:3 O1 .:i•i:i.:i:';-i• u ...... ;.:-:½;•:--..! there is partial to completealteration of elivine to colorless / / ?'"'::'•:?.....*'•':::;{':'"":::"'""•!','*' ..... 1'2 iii::}:{\1/ ...... :•"""'":•':•?"':":•:•,,,.•g amphiboleand talc, and 2.4 vol% of the rock consistsof / •" .i:.iii?..,.:•?:•!:i:.:..-::...... •'.-.-•.• ?.:.•:...--.•i: ...... ;.'...... ?•.:•:.:'•..-.•::•-.'.-..',ß :-..:i?':•.'•...•..'•:ii:..:•:..211ii?' ':'-':.::•?'•"ß'ß-ß - - ---?-':'y-':•:'":' hydrothermal and late magmatic veins composed of hornblende or oligoclase + + epidote. H20 0.6299/254 29 0.4 . ' ß3. / 6.7. _-- "'"'•-:'•i!-':':"i ß' - '"':'--"-'-"-•57 contents of the 735B gabbros are mostly 0.5-1.5 wt% 0.2 gabbro:25'0/":'"'•0""::'•. 50%readi--':.;.•:n :75% 100%::::::::•ction [Robinson et al., 1991]. 0.0 basalt:i$0% rxn. 100% re?lion i 0 •60' 2•' 360Temp•a•a,e{oC)Oh0 7b0 860 900•000 EFFECTS OF SLOW TRANSFORMATION ON Fig.3. Densities(g/cm 3) andH20 contents(wt%) for subducted DENSITY AND H20 CONTENT oceaniccrust calculatedusing NCMASH model of Peacockand kinetic data discussedin text. A) Equilibriummodel. B) Kinetic model with unaltered basalt/gabbroprotolith. C) Kinetic model When oceaniccrust is subductedit is compressedand with subgreenschist-faciesbasaltic protolith and amphibolite- heated along one of a wide variety of possiblePT paths faciesgabbroic protolith. Where two numbersare given,the first (Fig 3). The highest-temperaturepath into the eclogite refersto gabbroand the secondto basalt. 342 ECLOGITE FORMATION IN OCEANIC CRUST

stability field is from the facies. In the hottestof made of 3 km of basalt and 4 km of gabbro.Given these subductionzones involving lithosphereless than a few m.y. assumptions,what conclusionscan be drawnabout the rates old, it is possiblefor oceanicigneous rocks to passinto the of eclogiteformation in subductionzones and its effect on eclogitestability field by this path [Hacker, 1991]. Rocks rock physical properties? Possible variables to be that recrystallized in the subgreenschistfacies during consideredinclude the amountof H20 and the composition hydrothermalalteration at the mid-oceanridge will pass of the crust. through the greenschistand amphibolite facies prior to Figure 3A and 4A illustratethe predicteddensities and enteringgranulite conditions, and may transformvariably H20 contentsof NCMASH assemblagesdetermined by to greenschist,amphibolite, or granulitefacies assemblages Peacock[ 1993] for the variousmetamorphic facies. Figures prior to entering the stability field of eclogite. At the 3B and 4B repeatthis theme,using the kinetic inferences opposite end of the temperature spectrum, rocks that discussedabove. Figures 3C and 4C use the samekinetic previouslyrecrystallized in the prehnite-pumpellyitefacies hindranceconsiderations but begin with alteredprotoliths. will pass through the blueschist facies prior to entering Because these figures are based on modeling real eclogite conditions [Peacock, 1990], and may thus assemblageswith a subset defined by the NCMASH transform variably to blueschist prior to entering the system,the predicteddensities and H20 contentscannot be eclogitestability field. consideredexact; however, several interestingfeatures are Peacock[ 1993] usedthe restrictedcompositional system apparent. NCMASH (Na-Ca-Mg-AI-Si-H-O) to calculatethe mineral Figures 3B and 4B predict the behavior of subducted modes and maximum H20 contentsof subductingmafic basaltand gabbrousing the mineral assemblageolivine + rocks. He demonstrated that moderately altered oceanic orthopyroxene+ clinopyroxene+ plagioclaseas a protolith. crust containing 1-2 wt% H20 beginsto dehydrateat the In the upper, volcanic crust, 50% reactionis assumedto onsetof eclogiteor amphibolitefacies metamorphism, and occur at 150øC and complete reaction at 250øC; in the suggestedthat the transition from blueschistto eclogite lower plutonic crust, 25, 50, 75, and 100% reaction is facies, associated with the breakdown of lawsonite or modeled as happening at 150, 250, 500, and 550øC, clinozoisite,releases the mostH20 duringsubduction. respectively.For example, the density of 2.97g/cm 3 for Peacockcalculated a rangeof pathsthat traversemost of lawsonite-blueschist facies gabbro in Figure 3B was the region of PT spacerelevant to subductionzones. Figure calculatedusing unaltered gabbro (2.89 g/cm 3) transformed 3 illustrates two paths for the uppermostand lowermost 50% to lawsoniteblueschist (3.10 g/cm3).The H20 partsof subducted7-km thick oceaniccrustmcalculated for requiredfor hydrationin Figures3B and 4B is assumedto a subductionvelocity of 50 mm/a and shearstresses of 100 derive from sedimentsor hydratedmantle. In the absenceof and 33 MPa. Most of the PT paths followed by the sufficientH20, all facies otherthan CE, EC, GG, and GN uppermostlayer of the crust (basalt) are limited by the havea densityof 2.89 g/cm 3 and0 wt%H20. paths identified by upright triangles, and most of paths The resultsfor volcanicrocks are closeto the predictions followed by the lowermost layer of the crust (gabbro) are of Peacock's equilibrium model (Figure 1) becausethe limited by the paths with inverted triangles.Intermediate transformationrate is rapid. Basaltstraveling via path 1 levels in the crustfollow intermediatePT paths.For path 1, undergoonly slightchanges in densityprior to the appear- the uppercrust passes progressively through the greenschist ance of garnet and the disappearanceof plagioclase.In to amphiboliteto eclogitefacies conditions, while the lower contrast, Path 3 indicates that 5-10% volume loss occurs in crustevolves from blueschistto eclogitefacies. Along path basaltsupon enteringboth the blueschistand eclogitefa- 3, all the crust remains at blueschist facies conditions to cies. As in the equilibriumcase (Figures 3A and 4A), the pressuresof 2.5 GPa. For pathsintermediate between 1 and biggeststep in dehydrationoccurs at the blueschist/eclogite 3, most of the crust passes through the blueschist and facies boundary;dehydration along path 1 is split sube- eclogitefacies. qually betweenthe greenschist/amphiboliteand amphibo- In the spirit of Peacock's calculations and using the lite/granulitetransitions. moving along path 3 or information presented in Figure 2, assumethat coarse- slightlywarmer trajectories lose roughly equal amounts of grainedrocks (i.e., gabbro)transform 25%, 50%, 75%, and H20 when entering and leaving the epidote blueschist 100% at 150øC,250øC, 500øC and 550øC,respectively, in stability field. Comparedto basalt, where volume loss accordance with field observations described earlier. occursfairly evenlyover the 150-450øCtemperature range, Further assume that fine-grained rocks (i.e., basalt) half the volume loss of gabbro occursin the 500-550 ø transform twice as fast--such that they reach 100% interval. In contrast to basalt, which may undergo transformation at 250øC. Assume also that oceanic crust is substantialhydration by 250øC, gabbro is predictedto HACKER 343

A Equilibrtummodelwith unaltered basaltand gabbro protolith C Kineticmodel with prehnite-pumpellyite andamphibolite faciesprotoliths distance(kin) distance(kin) 0 50 100 150 200 0 50 100 150 200

=Path 2ocm/a 1 • 0 v =Path 2ocm/a 1 •_1%u• •n• __•..._ 0 100MPa .• 'c= 100MPa .•l 5.7_,os ••

AM • GG • • GG • •o 50

• 311 • •I0•• •299 X•"• • 311 __ 50• Pa•3 • •sGS '"••••• •4 ///// Pa•3 • -•••••• 324 /// =33 MPa [ 2•9 / 1• $0I0• $0I0

Fig.4. Densities(g/cm 3) andH20 contents(wt%) of subducted B Kineticmodel with unalteredbasalt and gabbro protolith crust, basedon Figure 3. A) Equilibrium model with essentially

distance(kin) unaltered crust. B) Kinetic model with unaltered crust. C) Kinetic 0 50 100 150 200 modelwith crustaltered to subgreenschist-faciesand amphibolite- • 2.80 0 faciesminerals. Compare with [Peacock,1993] Figure4. Thicker v =Path 10 crn/a• lines are facies boundariesand thinner lines representchanges •c-- MPa 2i89 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 differencerelative to asthenosphere (3.23 gm/cm3).

DISCUSSION: GEODYNAMIC IMPLICATIONS

•0• 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 bending of the slab, 3) the distribution of stressesinduced by volume changes, 4) seismicity in the slab induced by volume- change-relatedstresses, 5) the mechanicalbehavior of the slab, 6) the distributionof H20 in the slab, and 7) the undergohydration to temperaturesup to 550øCmalthough generationof arc magmatism.All thesefactors differ from at any givencondition the amountof H20 in gabbrois less the equilibriumstate because the densificationreactions in thanin basalt.Again, as in the equilibriummodel, the bulk descendingslabs depend on 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 greaterdepths in the kinetic model. altered basalt and gabbro using the prehnite-pumpellyite The first rocks to transform will be glassy basalts, faciesmineral assemblage chlorite + albite+ pumpellyite+ followed by holocrystalline basalts, then diabases, and quartz + calcite for alteredbasalt and 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 and T is higher. the upper crust and may be substantiallyretarded in the 344 ECLOGITE FORMATION IN OCEANIC CRUST lower crust.The absenceof fast velocitiesat eclogitefacies Kirby and Hacker[1991] proposeda link betweenarc conditions cannot be taken to indicate the absence of volcanism,intermediate depth earthquakes, and eclogite oceaniccrust. High densitieswill appearfirst in the upper formation.They notedthat hypocenters of subductionzone crust and may be considerablysuppressed in the lower earthquakesin the depthrange 50-250 km frequently crust.In Figure4, thelower crust is notonly less dense than coincide with arc volcanoes,and proposedthat these asthenosphere,but alsoless dense than the basalticupper intermediate-depthearthquakes might be caused by delayed crust.In thesesituations the uppercrust may sink in the eclogiteformation within subducting crust. This hypothesis asthenospherebut the lowercrust cannot. Moreover, Kirby requiresthat eclogitedoes not form at shallowerdepths et al. [1996] calculated that densification reactions in becauselow temperaturesand low H20 activityhinder subductingcrust can placethe crustin deviatorictension transformation,and that the devolatilizationof amphibole and the mantlelithosphere in deviatoriccompression. The in the 50-250 km depthrange triggers eclogite formation differencein reactionrate betweenbasalt and gabbromeans and releasesvolatiles that lead to the overlyingvolcanoes. that their calculationsare oversimplified.The initiationof Figure2 suggeststhat transformation to eclogiteoccurs at densificationin the volcanicrocks will initially place those much shallowerdepths in most subductionzones. For rocksin tensionand the underlyingcoarser grained rocks in Kirbyand Hacker's hypothesis to becorrect, the subducting compression.As reactionprogresses into the coarser lower crustmust be essentiallyanhydrous. crust,the stateof deviatoricstress in the gabbroicrocks will Calculationsand conclusionspresented above apply changefrom compressiveto tensile.If therewere no other principallyto normalmid-ocean ridge crust. Ocean crust forcesat work in subductionzones, earthquakes at shallow that is anomalouslythick or of unusualcomposition will depthswould be extensionalin basaltand contractional in transformdifferently. Oceanic plateaus, which are up to 6 gabbro,changing as the rocksdescend deeper in subduction times thicker than mid-oceancrust, but compositionally zonesto extensionalthroughout the oceaniccrust. similar, will have somewhat retarded transformation Little is known about the mechanical behavior of becauseof the additionaltime requiredfor the subducting blueschistfacies minerals, but clinopyroxeneand garnetare materialto heat;this may be partly offsethowever by the two of the strongestphases in the crust [Ji and Martignole, greaterthickness of volcanicrocks, which will transform 1994; Kolle and Blacic, 1983] and eclogite should be morerapidly. Field observationsdiscussed above indicate strongcompared to alteredbasalt. The replacementof fine- that Fe-rich rockstransform notably faster than MgO-rich grained volcanic phasesby blueschistor eclogite facies rocks.Hydrothermal alteration at spreadingcenters often mineralsmay resultin strengthening,whereas the alteration producesmarked MgO enrichmentin lavas[Humphris and of coarse-grainedminerals in gabbro to blueschistor Thompson,1978], which would be expectedto slow eclogite phasesmay enhancedeformation [Rubie, 1983]. transformation.Oceanic islands built by hotspotsand crust The volcanic layer, thoughhotter, may actually become from propagatingrifts are bothenriched in Fe relativeto strongerthan the gabbroiclayer as a result of eclogite MORB, andwill transformmore rapidly than standard mid- formation. It is conceivablethat suchrheological layering ocean-ridgecrust. Crust eruptedat fast spreadingridges could result in a downward jump of the subduction (>60 mm/a) is also richer in Fe than crusterupted at slow decollement from the volcanic into the gabbroic layer, spreading(<50 mm/a)ridges [Niu andBatiza, 1993; Sinton resultingin underplatingof eclogitizedupper crust to the and Detrick, 1992], and is expectedto transformmore hanging wall and continued subductionof the rapidly. gabbroiclayer. CONCLUSIONS Crustthat undergoesslow transformationis lesscapable of containing H20. Figure 4 shows that a slowly The transformation of mafic rocks to blueschist and transforminggabbro layer has a maximum H20 content eclogitein subductionzones is complex,encompassing a -1/2 thatof the equilibriummodel. The flux of H20 carried wide varietyof igneousmineral assemblages and grain intothe mantle beyond a depth of 70 km is 1.3x 108grams sizes,variably developed and equilibratedmetamorphic per meterper yearin the equilibriummodel, but only 0.48 x mineralassemblages, all pressurizedand heated via a broad 108grams per meter per year in thekinetic model with rangeof P T paths.Existing experimental kinetic data unalteredprotolith. Note thatif the only sourceof H20 in a cannotbe extrapolatedto actual pet•rotectonic settings in subductionzone is the igneouscrust, situationssuch as orderto predictwhere rocks transform to blueschistand Figure4C, wherethe H20 contentincreases with depth,are eclogite.However, textural data from exhumed subduction impossible.Only if there is anothersource of H20 in the zones worldwide indicate that little transformation occurs at subductionzone, from subductedsediments or mantle, can temperaturesbelow 150øC. Volcanic rocks are completely the H20 content of igneouscrust increasein subduction transformedby perhaps250øC. Coarser gabbroic rocks zones. rarelyavoid complete at temperaturesabove HACKER 345

550øC,although examples of incompletetransformation are doteand associated minerals in low grademetamorphic rocks, knownfrom rocks heated at temperaturesas high as 800øC. ContributionstoMineralogy and , 64, 123-136,1977. Brown,E.H., andE.D. Ghent, and phase relations in The rapidtransformation of volcanicrocks and the the blueschistfacies of the Black Butte and Ball Rock areas, metastablepersistence of gabbroic rocks into the blueschist NorthernCalifornia Coast Ranges, American Mineralogist, 68, andeclogite stability fields has several implications. The 365-372, 1983. seismicvelocity signature of denserocks should be evident Browning,P., Cryptic variation within the cumulate sequence of firstin theupper crust and may be substantially retarded in theOman ophiolite: chamber depth and petrological implications,in Ophiolites and oceanic , Geological the lower crust.The buoyancyforces causing the slabto SocietySpecial Publications, edited by I.G. Gass,S.J. Lippard, sink will appearfirst in the uppercrust and may be and A.W. Shelton,pp. 71-82, Geological Society of London, considerablysuppressed in the lower crust. Earlier London, 1984. formationof garnetin theupper crust may cause the upper Carlson,W.D., and J.L. Rosenfeld,Optical determination of crustto becomestronger than the gabbroic layer, and may topotacticaragonite-calcite growth kinetics: metamorphic im- plications,Journal of Geology, 89, 615--638, 1981. lead to underplatingof eclogitizedupper crust and Carmichael,R.S., CRC practical handbook of physical properties continuedsubduction of the lower crust.The initiation of of rocksand minerals, 741 pp., CRC Press, Boca Raton, FL, densificationin the volcanicrocks will initially placethose 1989. rocks in deviatoric tension and the underlying coarser Cloos,M., Lithosphericbuoyancy and collisional orogenesis: sub- ductionof oceanicplateaus, continental margins, island arcs, grainedrocks in compression.As reactionsprogress into spreadingridges, and seamounts,Geological Society of the coarserlower crust,the stateof stressin the gabbroic America Bulletin, 105, 715-737, 1993. rockswill changefrom compressive to tensile. If therewere Coleman,R.G., D.E. Lee, L.B. Beatty,and W.W. Brannock, no otherforces at work in subductionzones, earthquakes at Eclogitesand --Their differences and similarities, shallowdepths would be extensionalin the basaltand Geol. Soc.America Bull., 76, 483-508, 1965. contractionalin the gabbro,changing at deeperlevels to Dick,H.J.B., P.S. Meyer, S.H. Bloomer, S.H. Kirby, D.S. Stakes, andC. Mawer,Lithostratigraphic evolution of anin-situ section extensionalthroughout the crust.All theseeffects will be of oceaniclayer 3, Proceedingsof the Ocean Drilling Program, minimizedin Fe-richcrust produced at oceanicislands and ScientificResults, 118, 439-538, 1991. fastspreading ridges and most pronounced in Mg-rich crust E1-Shazly,A.E.-D., R.G. Coleman, and J.G. Liou, Eclogites and formedat slow spreadingcenters. blueschistsfrom northeasternOman: petrology and P-T evolu- tion,Journal of Petrology,31,629--666, 1990. Acknowledgments.Reviewed by S.H.Bloomer, S.R. Bohlen, S. - Ellis,D.J., and M.A.H. Maboko,Precambrian tectonics and the DeBari, W.G. 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