Eclogite 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 crust.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 basalt and contractionalin gabbro, 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 tectonics, 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 Earth'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 BLUESCHIST AND ECLOGITE? oceanbasins and constructing magmatic arcs. In spiteof the dynamic nature of subduction zones, much of our Blueschist facies mafic rocks are characterizedby the understandingof their behavior stems from equilibrium coexistence of sodic amphibole and lawsonite 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 garnet or omphacite.Eclogite sensu considerationof the rates and mechanismsby which phase stricto is mafic rock 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 asthenosphere,which has a 337 338 ECLOGITE FORMATION IN OCEANIC CRUST 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 lithosphere and the olivine --> 2.2 ':iiiiiii!iiiiii:... .......... eclogite ::::.:::::::•. spinel 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 plagioclase, 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. Metamorphic facies and reactions pertinent to eclogite determinedby experimental study [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 + glaucophane powder.For extrapolationto geologicconditions, data on --> garnet + omphacite+ paragaonite+ quartz + 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