JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. Bll, PAGES 20,031-20,037, NOVEMBER 10, 1993 ElasticProperties of HydrogrossularGarnet and Implications for Water in the Upper Mantle BRII)GET O%•EII J • Departmentof Geologyand Geophysics,University of California,Berkeley JAY D. BASS Departmentof Geology,University of Illinois,Urbana GEORGE R. ROSSMAN Divisionof Geologicaland Planetary Sciences, California Institute of Technology,Pasadena The single-crystalelastic properties of a hydroussilicate gamet, hibschite (Ca3A12(SiO4)l.72(H404)l.28), were measuredusing Brillouin spectroscopic techniques. The adiabaticbulk modulusof hibschite,K s = 99.8+1.0GPa, andthe shearmodulus, •t = 64.3_--/-0.5GPa, are 40% lowerthan the bulk and shearmoduli for anhydrousgrossular gamet Ca3A12(SiO4) 3. This increasedcompressibility of hydrogarnetis attributedto increasedhydrogen bonding with pressurein the H404 tetrahedron.Density considerations indicate that hydrogametis likely to be stablerelative to an assemblagewith H20 asa separatephase throughout the upper mantleand probably the transitionzone. Assuminggamet to be the solerepository for mantlewater, the seismicwave velocitiesof a "wet" eclogiticlayer are 6-8% lower thanthose of dry eclogite. A hydrated eclogiticlayer severaltimes thicker than the oceaniccrust would probably be requiredfor a water-rich regionof themantle to be seismologicallydetectable. Lesser quantities of mantlewater than those implied by the abovescenario may be invisibleto seismictechniques. INTRODUCTION pyralspites,while in ugranditesit may rangeup to 20 wt % H20 [Rossmanand Aines,1991; Lager et al., 1989]. Some Throughoutthe evolution of the Earth it is possiblethat natural hydrogarnetsare estimatedto have equilibratedat water has been stored in the interior so that at present there pressuresof as much as •-6 GPa (•-180-km depth) and may be, inside our planet, severaltimes the amountof water as temperaturesto-•1300øC. Additionally, hydrous garnetshave is in the hydrosphere[Ahrens, 1989]. Where and in what form been synthesizedat temperaturesof 800-1200øC and pressures this water may be storedin the mantle are questionswhich may of 2-5 GPa [Ackermannet al., 19,83;Geiger et al., 1991]. be answeredby examininghydrous phases transported from the Therefore,because garnet is a substantialfraction of the upper mantle [Bell and Rossman, 1992a] and by considering the mantle, the potential of this phase as a water-bearing phaserelations and propertiesof high-pressurehydrous phases component of the Earth's interior warrants investigation. [Thompson,1992]. Among the candidate"wet" mantlephases Garnet is also relevantbecause of the possibilitythat the OH syntheticphase B has attracted attention becauseof its high containedin a variety of mantle silicatesis incorporatedvia density and the fact that it is stableto pressurescharacteristic the hydrogarnetsubstitution, H404 = SiO4. of the transitionzone [Ringwoodand Major, 1967;Akaogi and It haspreviously been arguedthat becausethe molar volume Akimoto, 1980]. However, as pointed out by Finger et al. increases with hydrogarnet substitution (-•20% at zero [1989], phase B has not yet been demonstratedto be a stable pressure),garnet would not be a favorablephase to hold water phasefor a likely bulk mantle composition. In additionto the at high pressures[Martin and Donnay, 1972]. Aines and high-pressurehydrous phases which have been synthesized, Rossman [1984] suggested,however, that if hydrogarnetis somenaturally occurring upper mantle minerals such as olivine more compressiblethan pure silicate (anhydrous) garnet, it and pyroxene are able to incorporateOH and are potential could exist as a hydrous mantle component.To addressthe storagesites for water [Bell and Rossman,1992a]. Serpentine question of hydrogarnet compressibility we have used has been shown to remain hydratedup to 25 GPa and 900 K Brillouin spectroscopyto determinethe elastic propertiesof a [Meade and Jeanloz, 1991]. Another mineral which is both single crystal of hydrogrossulargarnet at ambient conditions. petrologicallyrelevant to theupper mantle and transition Although pyrope, the magnesium end-member, maybe a more zone,and which has a hydratedform, is garnet. Aines and abundantgarnet component inthe upper mantle, we chose to Rossman[1984] and Bell andRossman [1992b] have examine a Ca-rich garnet because much more OH can be investigatedtheOH content ofmantle-derived garnetsand have incorporated intocalcic garnets: there is, in fact,complete foundhydrous components of 50-100 ppm (as H20) in solid solutionbetween the anhydrousend-member Ca3A12Si3012 (grossular) and a hydrous silica-free end- member, Ca3A12(H404)3(synthetic phase). By using samples Copyright1993 by the AmericanGeophysical Union. in this system, it should be possibleto clearly identify the Papernumber 93JB02005. effect of the H40 4 = SiO4 substitutionon the elasticproperties 0148-0227/93193JB-02005505.00 of garnets. 20,031 20,032 O'NEILL ET AL.: HYDROUSGARNET ELASTICITYAND WATER IN THE MANTLE The hydrogrossularseries is describedby the formula Seven of the natural growth faces of the hibschitewere used Ca3A12(SiO4)3_x(H404)x. Hibschite is the mineralogicalname for the Brillouin measurements.Optical goniometry was used for garnetswith 0<x<l.5, while for 1.5<x= 3.0 the garnetis to determinethe orientationof eachcrystal face relativeto the known as karoire [Passaglia and Rinaldi, 1984]. A natural crystallographic axes in order to correct the velocity hibschite,with-•57% grossularcomponent (x = 1.28+.08), measurementsfor refractioneffects [Vaughanand Bass, 1983]. was usedin our studyso that the elasticmoduli determinedfrom Details of the Brillouin scatteringtechnique are given by this sample should clearly indicate the effect, if any, of Vaughan [1979] and Sandercock [1982]. A 90ø scattering hydrogarnetsubstitution. geometrywas used with a 514.5-nm wavelengthlight source suppliedby an Ar-ion laser. Light scatteredfrom the sample EXPERIMENT was analyzed by a plane-parallel piezoelectrically driven Fabry-Perot interferometer operated in a four-pass The hibschite sample used, GRR 1457, is a euhedral, configuration. A full description of the system and the transparent,colorless crystal N200x300x300 mm in size from methodsused is given by Bass [1989]. To avoid heatingand Crestmore,California [Foshag,1920]. It wasremoved from an dehydrationof the sample during the experimentthe laser approximately 1-mm-thick vein of fine-grained calcium power was reduced to less than 25 mW, and the run time was silicates in a hand specimen that was predominantly extendedto 1-3 hoursfor eachmeasurement. Table 3 givesthe vesuvianite (Harvard Mineralogical Museum 81065). The 29 acoustic velocities which were measured in 16 different chemicalcomposition, as determinedby electronmicroprobe crystallographicdirections. All the measuredvelocities were analysis, is given in Table 1. Three random points were corrected for surface refraction effects and then inverted to analyzed,and the crystal was found to be inhomogeneousin OH' with x varying from 1.23-1.36. The valuesin Table 1 are obtainthe bestfit set of elasticmoduli, C ij [Weidnerand Carleton, 1977]. RESULTS TABLE 1. ChemicalAnalysis of Hibschite1457 Element Atomic Ratio The hibschiteelastic moduli determinedfrom the velocity data set are given in Table 4. The uncertaintiesin Table 4 are 1 N a 0.00 standarddeviation calculated from the root-mean-squareerror in Mg 0.04 velocity of the entire data set for eachmodulus. Becauseof the A1 1.87 Si 1.72 inhomogeneityof the hydrous componentin the sample, Ca 2.84 which affectsthe moduli throughan uncertaintyin density,the Mn 0.00 accuracy of the moduli is estimated to be at the 2 standard Fe 0.03 deviationlevel. The adiabaticbulk modulusKs and the shear O* 12.00 H•' 5.14 modulus•t, also given in Table 4, were calculatedusing both Voigt-Reuss-Hill averaging and Hashin-Shtrikman bounds *Oxygen was analyzeddirectly. [Watt et al., 1976], with the results from the two methods •'Calculatedassuming (Si+H/4)=3.0. being in complete agreement. The elastic constantsfor the anhydrous end-member grossular [Bass, 1989] are given for an average of the three analyses. The lattice parameter, comparisonin Table 4, as well as the isothermalbulk modulus measuredby single-crystalXray diffraction using a SyntexP21 K 0 of pure syntheticend-member katoite recentlymeasured by four-circle diffractometer,is given in Table 2, with the hydrous static compression [Olijnyk et al., 1991]. In comparing (grossular) and anhydrous (katoite) end-member values for hibschitewith grossular,a dramatic increaseis seen in all the comparison. Additionally, to interpret the Brillouin elastic moduli, such that shearingis affectedsimilarly to bulk measurementsit is necessaryto know the index of refraction. compression(see Figure 2). It shouldbe pointedout that the Values for nD and n514,measured by immersionoil techniques, elastic properties of silicate garnet have previously been are also given in Table 2. The katoite end-membervalues were observedto be remarkablyconstant with respectto changesin measuredusing a syntheticsample obtained from GeorgeLager composition[Bass, 1986]. To emphasizethe magnitudeof the of the University of Louisville. Although lattice parameter increasedcompressibility due to hydration,it can be compared, valuesfor the hydrogrossularseries (see Figure la) showsome for reference, with the difference in moduli between the low- scatter and the reported indices of refraction (Figure lb) and high-pressurepolymorphs of Mg2SiO4,