Viscous and Elastic Anisotropy in Partially Molten Rocks I: Experimental, ﬁeld, and Seismic Observations

Viscous and elastic anisotropy in partially molten rocks I: Experimental, ﬁeld, and seismic observations

Ben Holtzman (LDEO, Columbia University) Yasuko Takei (Earthquake Research Institute, U. Tokyo)

I. Consequences of viscous and elastic anisotropy Causes and a deﬁnition of “multi-scale” anisotropy

II. Experimental Observations melt alignment, melt segregation, melt migration

III. Theory New diffusion creep model

IV. Applications to a simple oceanic upper mantle structure predictions for multi-scale effective viscosity structure Geodynamic consequences of anisotropic viscosity Geochemistry (and Geophysicsreduced effective3 tackley: shearself-consis viscositytent 3-d time-d dueependent to anisotropy)plates, 1 2000G C000036 Geosystems G 1) Degree of lithosphere / asthenosphere (convectosphere) coupling ?

2) Inﬂuences convective patterns - tends to tighten streamlines at boundary layers... (Honda, 1986;a. Christensen, 1987- and renewed interestb. now)... 3) Plate boundary rheology (reduction in meso-scale effective viscosity) gives plate like behavior in self-consistent plate generation models, e.g. Tackley, 2000; Bercovici; Ogawa... uniform and moderate yield stress: c. d.

f. e. Geochemistry Geophysics 3 tackley: self-consistent 3-d time-dependent plates, 2 2000GC000043 Geosystems G strain rate weakening, melt weakening + asthenosphere:

g. h.

i. j.

k. l.

Figure 12. Cases with both strain-rate weakening and either melt viscosity reduction or depth-dependent 3 viscosity. (Figures 12a and 12b) Constant sy = 8.5 10 plus MVR with Tsol0 = 0.6 plus SRW with smax = 3 Â 4 5.7 10 . (Figures 12c and 12d) Composite sy = 1.4 10 plus depth-dependent viscosity factor 10 plus Â 4 Â SRW with smax = 1.4 10 . The color bar shows log10(viscosity), which varies between 0.1 and 10,000. The horizontal viscositÂy slice is at z = 0.97. Figure 3. Viscosity fields (left column) and temperature isosurfaces (right column) for selected cases with constant yield stress (Figures 3a and 3b) 2.8 103, (Figures 3c and 3d) 5.7 103, (Figures 3e and 3f) 7.1 103, (Figures 3g and 3h) 8.5 103, (FiguresÂ3i and 3j) 1.4 104, and (FigurÂ es 3k and 3l) 2.8 104. TheÂ Â Â Â color4.1.4.barTshowsoroidal,log10(vispoloidalcosity), which varies between 4.20.1.andIm10,000.plicationThes horizforontalEarviscosityth andsliceOthiserat z = 0.97. Isosurfaces show where the temperature is 0.1 lower than the geotherm. [80] The cases presented here and in paper 1 Planets show that regardless of which physical 4.2.1. Earth versus Venus complexities are included, cases exhibiting good platelike behavior also display toroi- [81] From these results it appears that the ex- dal:poloidal ratios in the range observed for istence of a low-viscosity layer beneath oceanic the Earth during the last 120 million years plates plays a large role in facilitating Earth- (0.3±0.5 [Lithgow-Bertelloni et al., 1993]). like plate tectonics. Neither a global astheno- Indeed, the addition of an asthenosphere or sphere nor dramatic viscosity reduction seems strain weakening does not appear to en- necessary: a factor of 10 viscosity decrease hance the toroidal:poloidal ratio and some- restricted to the regions around spreading cen- times reduces it. Toroidal energy decreases ters is sufficient, at least in the parameter range with depth, but the decrease seems to occur studied here. From this result it is tempting to in a restricted depth interval and is only a hypothesize that a reason for Venus not having factor of 2 in the absence of an astheno- plate tectonics is the commonly held belief that sphere. Anasthenosphere reduces deep-man- Venus does not have an asthenosphere, which tle toroidal motion and increases surface is generally attributed to a lack of water [Kaula, toroidal motion; however, it also enhances 1994]. However, this interpretation must be surface poloidal motion, so the ratio is not treated with caution for the following reasons: much affected. (1) Even without melting there will still be a B08313 DELOREY ET AL.: UPPER MANTLE BENEATH THE REYKJANES RIDGE B08313

letters to nature

assumption that anisotropic effects are small. Nishimura and global average, VSV is significantly slower. The anomaly associated 17,18 Forsyth studied the variations in anisotropic properties of the with the EPR is much better defined in VSH, where it is clearly Pacific upper mantle in detail—however, their analysis emphasized centred on the ridge axis; the negative VSV anomalies are instead an age dependence for both isotropic and anisotropic properties. diffuse, extending to the centre of the Pacific. The maximum The results presented here suggest that this emphasis may have been difference between the VSV and VSH perturbations of ϳ5% is too limiting. obtained for an area just southwest of Hawaii. It is notable that in Our new model S20A was derived using a wide variety of seismic the global model of Montagner and Tanimoto11, the largest aniso- observations and allowing for anisotropic heterogeneity in the tropic signal seen at this depth lies in the same area, though it has a upper mantle (see Methods for details). Figure 1 shows the per- smaller absolute amplitude. Below 150 km depth, the difference 8 turbations in VSVand VSH with respect to their values in the reference between VSH and VSV beneath the central Pacific becomes smaller model PREM19 at 50, 100 and 150 km depth in the mantle. At 50 km (Fig. 2d), and we do not believe we can resolve a difference between depth, the VSV and VSH models have very similar patterns of high the two at depths greater than 250 km with our current data set. velocities under the continents and low velocities under the mid- Two main conclusions result from our analysis. First, Fig. 1 shows ocean ridges. A pronounced difference between the models is seen that for most of the world, the anisotropy of PREM provides a good lettersbeneath the oldertoporntionsaturof theePacific plate. Here, the perturba- average. The only large region where this does not hold true is the tion in VSV is positive and large, while VSH remains close to the Pacific plate. Figure 2a, b shows average VSV and VSH values PREM value. In comparison, the difference map for the rest of the calculated for the mantle beneath the Pacific plate and for the rest assumptionworld showsthatminoranisotropicanomalieseffectsnot associatedare small. wNishimuraith identifiableand golobalf the wavoerage,rld. TheVSVaveisrasignificantlyge anisotropyslowerthat w.eTheobtaanomalyin, with VSassociatedH ϳ2–4% 17,18 Forsythtectonic elements;studied suchthe variationsdifferences,in smalleranisotrothanpic praboutopertiesϮ1%,of thewe wfasterith thethanEPRVSVisbetweenmuch betterthe Mdefinedoho andin200VSHkm,, whereis similarit is toclearlythat Pconsideracific upperto reprmantleesentinandetailunresolv—howeved backgrounder, their analysislevel. emphasized cobtainedentred oninthepreviousridge axis;detailedthe negativestudiesVSVofanomaliesthe Pacificare insteadupper 13,14,18 anAaget 100dependencekm, the differfor encesboth isotrobetweenpic theandVanisotroSH and VpicSV modelsproperties.are diffuse,mantle extending. The Ptoacifictheplatecentrise oflargetheandPacific.well sampledThe maximumby the Thesmall.resultsThe implicationpresented hereis thatsuggestthe radialthat thisanisotroemphasispy builtmayintohavPREM,e been difference between the VSV and VSH perturbations of ϳ5% is toowithlimitingVSH ϳ3%. faster than VSV, is a good average for both continents obtained for an area justAnisotropicsouthwest variationsof Hawaii. It is notable that in 11 andOuroceans.new modelHoweverS20A, atwas150derivedkm depth,usingthea wpatternide varietis dramaticallyy of seismic the global model of Montagner and Tanimoto , the largest60Naniso- obserdifferent.vationsWhereasand alloVSHwshowing fors theanisotropiccentral Pacificheterogeneityas faster thanin thethe tropic signal seen at this depth lies in the same area, though it has a upper mantle (see Methods for details). Figure 1 shows the per- smaller absolute amplitude. Below 150 km depth, the difference 8 turbations0 in VSVand VSH with respect to0 their values in the reference between VSH and VSV beneath the central Pacific becomes30Nsmaller 19 model50 PREM at 50, 100 and a150 km depth50 in the mantle. At 50bkm (Fig. 2d), and we do not believe we can resolve a difference between depth, theSVV and V models have very similar patterns of high the two at depths greater than 250 km with our current data set. 100 SVSH SH 100 velocities under the continents and low velocities under the mid- Two main conclusions result from our analysis. First, Fig. 1 shows 150 PREM SH 150 0N ocean ridges. A pronounced difference between the models is seen that for most of the world, the anisotropy of PREM provides a good B08313PREM SV 200 DELOREY ET AL.: UPPER MANTLE BENEATH THE REYKJANES RIDGE B08313 beneath200 the older portions of the Pacific plate. Here, the perturba- average. The only large region where this does not hold true is the 250 250 tion in VSV is positive and large, while VSH remains close to the Pacific plate. Figure 2a, b shows average VSV and VSH values Depth (km) Depth (km) 30S PREM300 value.PacificIn comparison, the differenc300 enon-Pacificmap for the rest of the calculated for the mantle beneath the Pacific plate and for the rest world350 showsPlateminor anomalies not350associatedPlates with identifiable of the world. The average anisotropy that we obtain, wi(seismicth VSH ϳ2–4% velocities influences by same Ϯ tectonic400 elements; suchRelateddifferences, 400smaller than seismicabout 1%, we faster observablesthan VSV between the Moho and 200 km, is similar to that 4.3 4.4 4.5 4.6 4.7 4.8 4.3 4.4 4.5 4.6 4.7 4.8 physical60S properties that cause anisotropy) consider to represent an unresolv–1 ed background level. –1 obtained in previous detailed studies of the Pacific upper S-velocity (km s ) S-velocity (km s ) 13,14,18 At 100 km, the differences between the VSH and VSV models are mantle . The Pacific plate is large and well sampled by the small.0 The implication is that the radial0anisotropy built into PREM, 150E 180E 150W 120W 90W 60W with50V ϳ3% faster than V ,cis a good50 average for both continentsd SH SV AnisotropicIsotropic variations 2) Anomalous vertical anisotropy and100oceans. However1), atT150ransversekm depth,100 the pattern Isotropyis dramatically 60N60N different.150 Whereas (Love-RayleighVSH shows the central150 Pacific asdiscrepancyfaster than the , (Love-Rayleigh discrepancy, V_sv > V_sh) 200 200 0 0 30N30N 250 Pacific V_sh250 > V_sv) (e.g. Reykjanes Ridge, Gaherty, Science, 2001,

Depth (km) 50 a Pre-Camb. Depth (km) 50 Central b 300 SV 300 100 CratonsSH e.g. Ekstrom100 Pacific and Delorey et al., JGR, 2007) 350 350 150 PREM SH 150 0N0N 400 Dziewonski,PREM SV 400 200 4.3 4.4 4.5 4.6 4.7 4.8 200 4.3 4.4 4.5 4.6 4.7 4.8 S-velocity (km s–1) S-velocity (km s–1) 250 Nature 1997;250 Ritzwoller Depth (km) Depth (km) 30S30S 300 300 Figure 2 VelocityPacificprofiles of the shear-wave velocitiesnon-PacificVSV and VSH. a, The average Plate Plates velocities350 beneath the entire Pacific plate. The350 maximum difference between VSV and400VSH occurs at around 125 km depth, in contr400ast to PREM, in which it occurs just 4.3 4.4 4.5 4.6 4.7 4.8 4.3 4.4 4.5 4.6 4.7 4.8 60S60S below the Moho.S-velocityNote a lso(kmthat s–1)the model shows that theS-velocityupper mantle (km s–1beneath) the Pacific plate is ϳ1% slow with respect to PREM down to at least 400 km depth. 0 0 150E150E 180E 150W 120W120W 90W90W 60W60W b, The average velocities calculated for all plates except the Pacific. The average 50 c 50 d structure is close to that of the starting model PREM. c, Average velocity profiles Isotropic variations calcu100lated for all Precambrian cratons withi100n the Eurasia and North America 60N –4.8% –2.4% 0.0% 2.4% 4.8% 7.2% pla150tes. These regions are sampled very well150by our data, and the deviations from the200PREM structure are large and well resolved.200 Note, however, that the difference Figure 3 Shear-wave velocity variations beneath the Pacific plate at 150 km depth. between V and V remains very close toB08314that of the starting modelRPREM.YCHERdT, ET AL.:The topSCApanelTTEREDshowsWAVEtheIMAGINGanisotropyOFdVA ϪSHARPdV onLABthe same scale as30Nthe B08314Voigt- 250 SV SH 250 SV SH Depth (km) Average velocityPre-Camb.profiles calculated for aDepth (km) cap withCentral10Њ radius centred on the averaged isotropic variation in S-wave velocity (bottom panel) calculated using 300 300 anisotropyCratonsanomaly identified in this study at 15 N,1Pacific60 W. At 130 km depth, V is the approximate expression dV Voigt ¼ 1 ðdV þ 2dV Þ. The maps clearly illustrate 3) ReceiverЊ functionsЊ SH S 3 SH SV 350 350 7% faster than VSV, almost three times the value predicted by PREM. In contrast that anisotropic velocity variations are as large as the isotropic (thermal)0N varia- with400PREM, the radial anisotr(e.g.opy in theE.shallowest N.400 Am.,mantle is small. Note that the tions. The isotropic S-wave variations correlate better with the age of the ocean 4.3 4.4 4.5 4.6 4.7 4.8 4.3 4.4 4.5 4.6 4.7 4.8 velocity discontinS-velocityuity at (km220 kms–1)is part of the starting modelS-velocityPREM, (km sand–1) not a floor than either the dVSV or dVSH maps in Fig.1. The largest deviation from this age feature that can be resolvedRychertin our inversion. et al., Nature, 2005;correlation is very clearly associated with the location of the Pacific Superswell3. 30S Figure 2 Velocity profiles of the shear-wave velocities VSV and VSH. a, The average JGR, 2007) Nature © Macmillan Publishers Ltd 1998 velocities170 beneath the entire Pacific plate. The maximum difference between VSV NATURE | VOL 394 | 9 JULY 1998 and VSH occurs at around 125 km depth, in contrast to PREM, in which it occurs just below the Moho. Note also that the model shows that the upper mantle beneath 60S the Pacific plate is ϳ1% slow with respectFigurto PREM dowe n7.to at leastT400omographickm depth. images of (left) shear wave velocity, VSV, and (right) percent anisotropy. Shear 150E 180E 150W 120W 90W 60W 4) SKS splitting b, The average velocities calculated for all plates except the Pacific. The average structure is close to that of the startingwavemodel PREM.velocityc, Average velocity andprofiles anisotropy are calculated together in(e.g.a joint Eastinversion African of the Love and Rayleigh wave calculated for all Precambrian cratons within the Eurasia and North America –4.8% –2.4% 0.0% 2.4% 4.8% 7.2% plates. These regions are sampled veryphasewell by our data,velocitiesand the deviations fr(omshown in Figure 4). The images are orientedRift, Ethiopia,in a vertical plane, normal to the ridge axis; the PREM structure are large and well rtheesolved.horizonNote, however, thatalt the differaxisence iFigurndicatese 3 Shear-wave velocitythevariationsdistancebeneath the Pfromacific plate att1he50 km depth.ridgeKendallaxis; theet al.,verti cal axis indicates depth below the between VSV and VSH remains very close to that of the starting model PREM. d, The top panel shows the anisotropy dVSV Ϫ dVSH on the same scale as the Voigt- Average velocity profiles calculated for a cap with 10Њ radius centred on the averaged isotropic variation in S-wave velocity (bottom panel) calculated using seafloor. The magnitude of anisotropyVoigt 1 is defined as 100(V2005; V )/V , where V is the shear anisotropy anomaly identified in this study at 15Њ N,160Њ W. At 130 km depth, VSH is the approximate expression dVS ¼ 3 ðdVSH þ 2dVSVÞ. The maps clearly illustrate SH SV average SH 7% faster than VSV, almost three times the value predicted by PREM. In contrast that anisotropic velocity variations are as large as the isotropic (thermal) varia- Ayele et al.,À 2004; with PREM, the radial anisotropy in thewaveshallowest mantlevelocityis small. Note thatofthea tions.horizontallyThe isotropic S-wave variationspolarizedcorrelate better withshearthe age ofwave,the ocean VSV is for a vertically polarized shear wave, and velocity discontinuity at 220 km is part of the starting model PREM, and not a floor than either the dVSV or dVSH maps in Fig.1. The largest deviation from this age Keir et al., 2005; Vaverage is the average of the two. Ratio, h, of the penalty3 constraints on VSV versus x (the anisotropy feature that can be resolved in our inversion. correlation is very clearly associated with the location of the Pacific Superswell . Bastow et al., parameter): (a)Nature ©1, Macmillan(b) Publishers1/2, Ltd 1998and (c) 2. A smoothness constraint added below (d) 135 and (e) 100 km that 170 NATURE | VOL 394 | 9 JULY 1998 squeezes lateral variations in velocity or anisotropy structure2005) to shallower depths (h = 1 used in both cases). See text for discussion.

cooling modelFigur[Te 2.urThree-dimensioncotte andal view ofSchubertthe lithosphere-asthe,nosphere2002]boundary and(calculatedsurface topography. temperatures based on a geochemical study of rare earth Red box in the inset map highlights the location of the study region within North America. Shading on the top surface indicates topography. Yellow arrow6points in the direction2 of absolute plate motion; plate using a thermalvelocitydiffusivityis 2.5 cm/yr. Red invertedoftriangles10denoteÀ stationW/mlocations.andThe loweransurfaceadiabatrepresents the elements in Reykjanes Ridge basaltic rocks (<100°C [White location of the base of the lithosphere interpolated from migrated Ps waveforms and our new migrated Sp calculated withwaveformsvariablerecorded at stationscompressibilityHRV and LMN (blue circles mark[Schmellingconversion points). The SpetHRVal.data , et al., 1995]). Note that the temperature anomaly in Figure 9a from northern back azimuths and the Ps from LBNH (grey circles mark conversion points) are not used to 2003]) predictscalculatea lithosphericthe interpolated surface becausethicknessof a discrepancy inofthe depth41to whichkmthe phaseat10migratesMa.(see exhibits a thermal inversion in the mantle, where higher section 6.2). This surface ranges from 89 km (orange) to 105 km (pink) depth. Each color band covers 2 km in depth. Black lines connect Ps piercing points to the station at which the conversion is observed. Thus the NorGreyth linesAconnectmeSpripiercingcanpointspltoathetestationiswheremtheuconversionch tihs observed.inneAllr depthsthaaren temperatures overlie slightly cooler temperatures. On the expected. calculated assuming Vp/Vs = 1.8 in subcrustal mantle. other hand, the low-velocity region could instead be caused and epicentral distances of >80°, and stacking over a wide The standard deviation error bars (grey lines in Figure 3) on [30] Beneathrange of distancesthe andlithospheredepths. At HRV we can, usewean evenpredictour deconvolveda waveforms75°Care calculatedthermalwith bootstrapby lateral variations in melt fraction. Assuming relaxed more conservative depth cutoff of 150 km by virtue of the tests in which arandom 20% of the events in the bin are anomaly overlarger volumea ofbroaddata recorded atregionthe station. In addition,centeredwe randomlybeneathreplaced by anothertrandomhe 20%,ridge.and the decon-moduli and that melt is organized in tubules at fractions do not use data from epicentral distances <55° since these volved, migrated waveforms are recalculated 100 times. This valueeventsisare simincident ilarto the discontinuityto estat anglesimabeyondtethes of anomalous mantle less than 1% and in films and tubules above 1% [Hammond critical angle for Sp transmission. After picking identifiable 2.2. Imaging S phases, our deconvolved, migrated waveforms contain 56 [12] To image the discontinuities responsible for the Sp events at HRV and 28 events at LMN (Figures 3c and 3d). and Ps converted phases, the data is first transformed into its P and SV components using a free-surface transfer matrix. Figure 7. Tomographic 3iomagesf 21 of (left) shear wave velocity, V , and (right) percent anisotropy. Shear 10 of 16 SV wave velocity and anisotropy are calculated together in a joint inversion of the Love and Rayleigh wave phase velocities (shown in Figure 4). The images are oriented in a vertical plane, normal to the ridge axis; the horizontal axis indicates the distance from the ridge axis; the vertical axis indicates depth below the seafloor. The magnitude of anisotropy is defined as 100(V V )/V , where V is the shear SH À SV average SH wave velocity of a horizontally polarized shear wave, VSV is for a vertically polarized shear wave, and Vaverage is the average of the two. Ratio, h, of the penalty constraints on VSV versus x (the anisotropy parameter): (a) 1, (b) 1/2, and (c) 2. A smoothness constraint added below (d) 135 and (e) 100 km that squeezes lateral variations in velocity or anisotropy structure to shallower depths (h = 1 used in both cases). See text for discussion.

cooling model [Turcotte and Schubert, 2002] (calculated temperatures based on a geochemical study of rare earth 6 2 using a thermal diffusivity of 10À W/m and an adiabat elements in Reykjanes Ridge basaltic rocks (<100°C [White calculated with variable compressibility [Schmelling et al., et al., 1995]). Note that the temperature anomaly in Figure 9a 2003]) predicts a lithospheric thickness of 41 km at10 Ma. exhibits a thermal inversion in the mantle, where higher Thus the North American plate is much thinner than temperatures overlie slightly cooler temperatures. On the expected. other hand, the low-velocity region could instead be caused [30] Beneath the lithosphere, we predict a 75°C thermal by lateral variations in melt fraction. Assuming relaxed anomaly over a broad region centered beneath the ridge. moduli and that melt is organized in tubules at fractions This value is similar to estimates of anomalous mantle less than 1% and in films and tubules above 1% [Hammond

10 of 16 R E P O R T S pressure and high-temperature torsion appa- scatter diffraction (EBSD). During deforma- [100] axes develop two maxima, one parallel ratus (17), in which any small element of the tion, the texture evolved through a transient to the shear direction and one oblique to the sample undergoes deformation at constant deformation texture (␥ ϳ 0.5) into a recrys- shear direction. This LPO fits with previous strain rate by simple shear (18). The shear tallization texture at high strain (Fig. 4). At low strain experimental results (13, 14) and stress and shear strain rate at the outer surface ␥ ϳ 0.5, the [010] crystallographic axes tend with numerical simulations (22, 23). At high- of the cylindrical sample were derived from to align normal to the shear plane and the er strains, the texture is much stronger and the measured torque and twist rate, respectively (19). The experiments were performed at 1200°C and 300 MPa confining pressure, Fig. 1. Shear stress versus shear strain for samples deformed at 1200°C and with the oxygen fugacity near the Fe/FeO 300 MPa at constant nominal shear buffer (10Ϫ12 bars). Fourier transform infra- strain rates of 6 ϫ 10Ϫ5 sϪ1 (gray red (FTIR) spectroscopy analyses of de- curve) and 1.2 ϫ 10Ϫ4 sϪ1 (black formed samples indicated that they contained curve). Conversion from torque and less than 30 molar parts per million (ppm) twist of torsion deformation into shear H/Si (20). Samples were deformed to differ- stress and strain was performed as in (19). The peak stresses agree well with ent amounts of bulk shear strain, under two dislocation creep flow laws for olivine constant angular displacement rates corre- aggregates determined in axial compression experiments (12). Stepping tests performed at high sponding to constant shear strain rates of strains yielded stress exponents n (n ϭ 3.2 at ␥ ϳ 1.2 and n ϭ 3.3 at ␥ ϳ 3.2) typical of either 6 ϫ 10Ϫ5 sϪ1 or 1.2 ϫ 10Ϫ4 sϪ1 at the deformation by dislocation creep. Shaded area (left) shows magnified view of the low strain interval outer surface of the sample. For both defor- at right (␥ Յ 0.2), up to equivalent strains (ϳ10%) where rheological data are typically obtained mation series, a peak stress occurred at a in compression experiments. At these low strains, deformation often appears to be steady-state because the onset of weakening is rarely reached. shear strain ␥ ϳ 0.1, followed by 15% weakening up to ␥ ϳ 0.5 (Fig. 1). At higher strains, the flow stress was nearly steady- Fig. 2. Optical micro- state, leading to a total weakening of the graphs in cross-polarized aggregate of about 20% from the peak stress light [from thin sections cut as in (18)] of olivine to the flow stress at ␥ ϳ 5. Stress exponents aggregates deformed at of n ϭ 3.3 determined after the weakening 6 ϫ 10Ϫ5 sϪ1 to different on December 2, 2007 suggest dislocation creep as the rate-limiting shear strains. Shear sense mechanism, even at high strains. is dextral. (A and B) Start- ! Microstructures were analyzed with optical ing material. Olivine ag- and electron microscopy. Thin sections were gregate hot-pressed for 12 hours, showing an cut perpendicular to the cylinder radius and equigranular fabric with within 200 to 300 ␮m of the sample outer edge. an average grain size of In such planes, deformation is nearly simple 20 ␮m. Most grains have euhedral shapes with low R E P O R T S shear (18). With increasing strain, the average S1; table S1). The samples consisted of oli- concentrate normal to the sample shear ence of chromite does not influence dislo- aspect ratios. Grain bound- vine and mid-ocean ridge basalt (MORB) and plane (the walls of the pistons) www.sciencemag.org and the a cation dynamics. grain size reduced by dynamic recrystallization olivine and MORB with either an additionallettersand ctoaxenaturs form geirdles in the shear plane A commonly observed CPO for olivine aries are straight to curved. melt (FeS) or solid (chromite) phase. The (Fig. 2A). In samples with an added third deformed in simple shear at high temperature (Fig. 2). At shear strains ␥ ϳ 0.5, a typical R E P O R T S Few deformation features Acknowledgements We thank A. Moore and P. T. Atkins for field and laboratory assistancadditionale. This phasesSeverallowerlines theof evipermeabilitydence point torel-seismicphanisotroase (olpyivindecreasinge and MORB with either chro- is characterized by a axes parallel to the shear research was supported by grants from the A. W. Mellon Foundation to L.A.D. andativeO.A.C., toandthatwofitholivinedepth iandn thMORBe uppersamplesmantle. byMost gmloibalte oone-dir FeSmensmeionlt)al, all of which formed direction, b axes normal to thewhichshearshoulddirec-occur at shear stresses higher in the melt-depleted lenses is not simple A literal extrapolation of the process dis- deformation microstructure displays evidence from the N.S.F. to L.A.D., O.A.C. and A.C.K. Causes of anisotropy such as lamellae and sub- partially pluggingmodels (thePREmeltM, Ichannels,ASP, AK13without5 and AK3n0e3t)wsohrokws hoofrimzoenltta-lrliych bands, the CPO is tion, and c axes normal to the shearthan 0.1directionMPa (14), olivine CPOs in the man- shear, but it involves substantial components cussed here may help to explain the complex propagating P waves travelling (at velocity v PH) faster than vertical tle will be affected by the presence of melt. of strain normal to the shear direction. Align- seismic anisotropy observed in many partial- Competing interests statement The authors declare that they have no competing financialsubstantially1) Latticechanging the rheological preferredprop- ch aorientationracterized by (i) signifi caofnt c oanisotropicncentra- in the shear plane crystals(Fig. 2C) (3) (SOM Second,(inText, weexperiments)argue that the transition in ment of a axes normal to the shear direction ly molten regions of the upper mantle (18). of incipient recrystallization (Fig. 2, C and D). interests. ones (at v PV), but the difference in velocity reduces with depth, New type of olivine fabric from deformation experiments grains are visible within erties (7, 8). In sheared samples of olivine tions o4f a axes normal to the shear direc- section 5). This pattern reflects the fact that in resulting in isotropic behaviour at 350 km depth . Some models at modest water contentCPOs in andgoing lofromw stresssamples of olivine and suggests that a slip on the b plane occurs Several potential examples include (i) ver- Correspondence and MORB, melt pockets align at ϳ20° to the tion; (ii) tight clusters of b axes oriented dry olivine, a slip on b planes isMORBsignificantly(Fig. 2A) to samples with melt seg- normal to the shear direction (17). Because tical fast-direction measurements beneath At ␥ ϳ 2, core-and-mantle structures suggest the grains. The micro- and requests for materials should be addressed to L.A.D. ([email protected]). (AK135 and 303) even show v PV slightly faster than v PH below shear plane1)350 Highandkm.distributeThe TS-wa olivine:veuniformlypolarization referenceacrossanisotropy15also° to decreases20° frommono-the pole to the sa2)mp lEffectse shear weaker of dissolvedthanKataayama,slip on J ungwaterc planes, andregated Karandwhichintoato networks,elevated isGeologyof bands, 2004 (Fig.stress2B) is onmuch LPOof the shear strain is accommodated in the Reykjanes Ridge south of Iceland (19), due to changes in the flow pattern rather than the bands, the components of strain normal to (ii) trench parallel fast axes measured in the the sampleton(9ica). Duringlly fromdeformationthe surface toofbesam-come isotroplanpice, batac25k-0rokmta.teFord relative to the orienta- weaker than c slip on b planes (12). In con- recrystallization by subgrain rotation (Fig. 2, E graph in (A) was taken us- a-axis parallel to shear direction a change in the deformation mechanism. The the shear direction in the lensesb have much mantle wedge above subduction zones (20), ples with reducedhorizontapermeability,lly propagatingmeltS wasegre-ves, horizotinotanllycapuoslearid zbedy wavessimple shear; an(Jungd (iii) sig -andtrast, in partially molten sampleslatterdeformedCPO is similarat to type-B CPO defined greater expression in the CPO than they or (iii) tangential patterns of anisotropy and F). The matrix wraps around porphyro- ing a gypsum plate. (C gates fromshowan initiallya higherhomogeneousvelocity (v SH)distribu-than verticallynipolarizedficant cowavnceesnt(rvaSVti)ons of c axes rotated similar temperature and stress inconditions(5), which asthe authors of that paper at- would if the bands were absent. This expres- around plume heads, e.g., Iceland (21). tion into melt-richdown to layersabout 250or “kmbandsdepth.” orientedBetween 3001km5° tando 20400° ckmoundepth,terclockwise fromKarato,the sam- 2001;described in (3), the more diffusetributeCPOto a changepat- in the behavior of dislo- sion may be enhancedcfurthera by the MPO- Each of these observations has produced a ...... e.g.v is Zhanghigher than v& , Karato,but anisotropy is five times lower than in the and D) Deformed proto- at ϳ20° toSVthe shear plane,SH separated by ple shear plane (90° from the b axes) (Fig. tern is distinguished by a and c cationsaxis girdlesat highinwater fugacity and high stress CPO effect. Thus, the “mechanism” for a-ax- range of hypotheses, a discussion of which clasts and resembles the mosaic texture de- uppermost mantle. High-resolution global tomographic models Jung, 2006; conditions. However, neither of these con- is orientation is the kinematic effect of strain is beyond the scope of this paper (22).

Downloaded from type B lith. Deformation micro- Pressure sensitivity of olivine slimelt-depletedp 1995basedregionson S-waorve“lenses.data12 ”or(Fig.on the1, inAversion2B)of. thrIn ee-componentsix samples deformed to shear the shear plane with strong concentrationsditions appliesofto our experiments. Several partitioning, not a change in the dominant However, the processes discussed here sug- and B). When melt segregates, strain concen-13 strains of 1.1, 2.1, and 3.3 Katayamato 3.5, the et scribed in natural peridotites (21). With further Bystrickysurface and body etwav al.eform 2000data support these general␥ ϭfindings, b axes normal to the shear plane.lines ofFurther,evidence disfavor a change in dis- slip system. gest detailed field-based (23) and seismo- structure at shear strain systems and seismic anisotropytrates in the weak melt-rich bands. The inter- intensity of CPOs increases with increasing when melt segregates into bands,locathetion bdyaxesnamics (e.g., a change from The influence of melt segregation and logical predictions and tests, which may with strong anisotropy characterized by v SH . v SV above 250 km al., 2004, 2006 action of straindepth.partitioningAt greater depth,and themeltse modelssegre- requirestraainstron. In gaddecreaditionse, inall samples progressive- rotate 20° antithetic to the sensedoofmishear,nant a sandlip to c slip on b planes) as an strain partitioning on CPO development will influence our interpretations of the dynam- strain, the porphyroclasts become more elon- of Earth’s upper mantle explanation for the CPO observed in our be even more effective in partially molten ics of partially molten regions of Earth. ␥ ϳ 0.5. Evidence of dis- anisotropy, with a minimum around 300 km depth. S-wave data gation leads to self-organization of the ly decrease in thickness by up to 20% with concentrations of a and c axesexpappeareriments,90di°scussed in (15). We propose regions of Earth where deformation is more 1 1 2,3 2 David Mainprice , Andreá Tommasi , He´le`ne Couvy , Patrickmelt-richCordier bands.also callTforheweakse banisotropyands form, waithnavsSVto-. v SHinatcrtheeasbaseing ofsttherainupper, accommodated by minor from their “normal” positions,a kireferrednematic extoplanation for the a-c switch, three-dimensional than in our experiments. gated, with an oblique shape fabric consistent location creep and recov- 3 mantle beneath the central Pacific and Pre-Cambrian cratons12. a References and Notes & Daniel J. Frost mosing networks (Fig. 1), with larger bands but important lengthening of the sample below as the “a-c switch” (Fig.on2C).the basis of three points, as follows: The olivine a axes will rotate if the geometry 1. D. Mainprice, Tectonophysics 279, 161 (1997). Regional surface wave studies in the Pacific and Indian ocean basins 1) The total strain in the sample partitions of the overall flow in the region permits or 2. A. Nicolas, N. I. Christensen, Formation of Anisotropy ery in the form of defor- 1 at higher angles connected by smaller normal to the shear direction. First, we must explain how the oriented with the sense and amount of shear. At ␥ ϳ 5, Laboratoire de Tectonophysique, CNRS/Universite´ de Montpellier II, F-34095 also suggest that anisotropy is present from the surface to ,250– between the melt-rich bands and melt- requires an elongation of the melt-depleted in Upper Mantle Peridotites—A Review, Geodynamic Series (American Geophysical Union, Washington, Montpellier cedex 5, France bands at lower angles1–3,14. Because the distri- Observations from detailed studies of melt pockets weaken a and c axis concentra- 300 km depth , with v SH being greater than v SV. Analysis of two- depleted lenses. Although the bands comprise lenses normal to the shear direction. An DC, 1987), vol. 16. mation lamellae and 3 to 2 ´ ´ ´ note: b c recrystallization is nearly complete and a flu- Laboratoire Structure et Proprietes de l’Etat Solide, CNRS/UniversitebdeutLilleionI,of bstationand ansurfacegles rewamveainprs ofilesindepinenthedenPt acificCPOsand Philippineand microstructuresplates by scanning and tions relative to CPOs from melt-freeonly ϳ20%sam-of the total sample volume, the anisotropic network of melt-rich layers will 3. S. Zhang, S. Karato, Nature 375, 774 (1995). F-59650 Villeneuve d’Ascq, France of strain, implythe maestilllt shalloin thwere banisotropyands muslimitedt be totransmissionthe upper 160 kmelectronof the microscopy (SEM and Bystrickyples (Fig. et 2B).al: Melt pockets providestrain rate,a andfast-thus the strain, is higher in the affect the seismic properties of regions large 4. M. Bystricky, K. Kunze, L. Burlini, J.-P. Burg, Science 4 ␮m subgrains. Grain 3Bayerisches Geoinstitut, Universita¨t Bayreuth, D-95440 Bayreuth, Germany type C 290, 1564 (2000). 15 type A+ E, weaker bands than in the stronger lenses. In in comparison to a seismic wavelength (␭) if 5. H. Jung, S. Karato, Science 293, 1460 (2001)...... moving remantlelative t.oSKSthestudiessolidcannot. The csonstrainpacing the TEM)depth ofconstrainthe anisotrothepicmicrostructural processes diffusion path for olivine components, idal mosaic microstructure (21) with a strong the bands, shear strain is oriented at ϳ20° to the separation between layers (␦ ) is much 6. G. Hirth, D. L. Kohlstedt, Earth Planet. Sci. Lett. 144, between balaynders, butis gtheovestrongrned bcorry thelatione comofpathec- directionactiveofinpolarizationthe samplesof (Fig. 1B). A map of Holyoke&enhancing Tullis:the importance of diffusion- S boundaries are curved to The mineral olivine dominates the composition of the Earth’s the sample shear plane and, therefore, in the less than a seismic wavelength (i.e., ␦ Ϸ 93 (1996). the fast shear wave with the surface geology and the observed delay type D and type A S upper mantle and hence controls its mechanical behaviourtion andlength of the sample [supporting online CPOs in the vicinity of a similar melt-rich accommodated creep and possiblylenses the grainshear plane must be back-rotated 100-3 m ϽϽ ␭ Ϸ 104 m). The configuration 7. B. K. Holtzman, N. J. Groebner, M. E. Zimmerman, foliation sub-parallel to the shear plane has times #2 s (ref. 14) suggests that SKS splitting occurs in the upper S. B. Ginsberg, D. L. Kohlstedt, Geochem. Geophys. lobate, with bulges indi- seismic anisotropy. Experiments at high temperature andmamod-terial (SOM) Text, section 2], consistent band exhibits no significant spatial varia- boundary sliding in the directionrelativeoftoshear,the sample shear plane (Fig. 3C). (comprising average thickness, spacing, an- Geosyst. 4, 8607 (2003). erate pressure, and extensive data on naturally deformedwmantleith theory200–250for pokmrouofs fthelowmantle.in a permeable tions, suggesting that deformation in the whereas movement of dislocationsThisaccommo-back-rotation is observed in the orienta- gle, and topology) of the network depends on 8. Material and Methods are available as supporting developed (Fig. 2, G and H). The ϳ5% of cating the onset of recrys- rocks, have led to the conclusion that olivine at upper-mantle Finally, a regional seismic discontinuity, called the Lehmann tion of b axes in the CPO. the physicalc properties of the solid and fluid, material on Science Online. viscous deforming medium (7, 10). bands does not modify the CPO (SOM Text, dates the remainder of the interactionsb be- 9. M. E. Zimmerman, S. Zhang, D. L. Kohlstedt, S. Karato, discontinuity, has been detected at about 220 km depth by various 2) The observed CPO predominantly re- the kinetics of the processes governing their conditions deforms essentially by dislocation creep with domi-Crystallographic preferred orientations section 3). TEM observations reveal that dis- tween grains, providing a mechanism for Geophys. Res. Lett. 26, 1505 (1999). remaining porphyroclasts consist of ribbons tallization. A weak oblique nant [100] slip. The resulting crystal preferred orientation has seismic methods (reflection, surface waves, ScS reverberations and P flects deformationain the melt-depleted lens- interactions and transport,a and the geometric 10. D. McKenzie, J. Petrol. 25, 713 (1984). (CPOs) oftooliSviconnevegrrsions),ains wemainlyre meabeneathsured focontinents.r locationsThis withdiscontinuitBurgersy vectors parallel to the modifying the von Mises criteriones, implying(13that).the deformation in the bands (kinematic) boundary conditions of regional 11. B. L. Adams, S. I. Wright, K. Kunze, Metall. Trans. 24A, been used extensively to explain the strong seismic anisotropy c b 819 (1993). with highly stretched tails (blue grains) and shape preferred orientation observed down to 250 km depth1–4. The rapid decrease of1 uaniso-ndeformhasedbeenandinterpre10 shetedaredassbeingamplduees wtoitheithera(1)axisa stroutnumberedong anisotropythose with Burgers vec- Strong anisotropy in melt-pocketis notorientationcontributing to or strongly modifying flow. Some of these properties are encom- 12. W. B. Durham, C. Goetze, J. Geophys. Res. 82, 5737 (1977). tropy below this depth has been interpreted as markingelecttheron bcausedackscabytteintenser diffdeformationraction (EBofSolivD)ine intorsa zoneparallelof mechanicalto the c axis (SOM Text, section (MPO) may randomizetypethe Aorientationsthe CPO (16of). Observationsa that the CPOtypeis Epassed in the first-order compaction length 13. M. S. Paterson, in Physics of Strength and Plasticity, A. S. is consistent with the sense of shear. (E and F) Protomylonite. Partiallycoupling recrystallizedbetween the lithosphere andmicrostructurethe asthenosphere16, or (2) barely modified in the vicinity of a band scaling argument discussed in (7) (SOM Argon, Ed. (MIT Press, Cambridge, MA, 1969), pp. 377–392. asymmetric porphyroclasts showing sub- transition from dislocation to diffusion creep in thaenuaplypseirs (8, 11). In samples of olivine and 4). The density of dislocations is not ele- and c axes, while leaving orientations(SOM Text,of slipsection 3) support this point. Text, section 2). Others remain to be studied 14. M. J. Daines, D. L. Kohlstedt, J. Geophys. Res. B102, 5 3)the Effectstransition between of pressurean anisotropic uppermost and stressmantle deforming 10257 (1997). mantle . But new high-pressure experiments suggest thatMOdislo-RB deformed in shear, the CPOs devel- vated in olivine grains adjacent to stronger planes (b axes) intact (“the MPO-CPO3) The deformationef- that produces the CPO experimentally and theoretically. at ␥ ϳ 2. Recrystallized grain size is 3 to 4 ␮m. Elongated porphyroclastsby dislocationwithcreep (whichdeformationproduces a crystal preferrlamellaeed orien- 15. The fabric in Fig. 2B would normally be interpreted as grains and deformation features (white cation creep also dominates in the lower part of the upperomantle,p an “axial” pattern in which the b axes chromite grains, suggesting that the pres- fect”). As long as melt pockets are oriented, slip on the (010)[001] system. Such a change in but with a different slip direction. Here we show that this high- tation, CPO, of olivine) and an isotropic deep mantle deforming by dominant slip system could result from locally high have aspect ratios R from 2 to 6 (R ϭ 5.8 for the finite strain ellipseCouvydiffusion atcreep et ␥(whichal.,ϭ 2005does2).not Core-and-mantleproduce CPO)5. However, recent Fig. 3. Representation of ob- stresses due to the strong chromite grains activating pressure dislocation creep produces crystal preferred orien- 4) Effects of dissolved waterserved andmelt distribution and in- the (010)[001] slip system in olivine, which is much grains). High-resolution orientation imaging high-pressure, high-temperature experiments show that even in structures and a subgrain sizetationssimilarresulting in toextremelythelow seismicrecrystallizedanisotropy, consistent Mainpricegrain size et provideal., 2005 evidenceFig. 2. Pole figuresforfor a, b, c ternal strain partitioning in ex- stronger than (010)[100] (12). However, this possi- with seismological observations below 250 km depth. These fine-grained aggregates (,20–30 mm), dislocation creep is the elevated stress on LPO perimentally deformed samples. bility is negated by the fact that the same CPO exists axes for sheared samples. The (A) Synthesis of the configura- in the olivine and MORB with FeS sample, which maps are consistent with the observed micro- recrystallization by subgrain rotation.results raise (newG questionsand Habout) Ultramylonite.the mechanical state of the Fluidaldominant deformationmosaicmechanismmicrostructureundershearconditionsplane equivalentis horizontalat and forms bands with a weak melt phase (i.e., FeS) in to those prevailing at 300 km depth9,17,18. (Holtzman et al., 2003) tion of melt bands. Bands form place of strong chromite inclusions. Furthermore, lower part of the upper mantle and its coupling with layers both shear sense is at top right. (A) anastomising networks with The pressure, or pressure interval, at which the transition from TEM images of the dislocation structures revealed no structures (Fig. 3). The spatial distribution of ␥ ϳ 5. The recrystallized matrix (abovϳe95%and belowvolume). is very homogeneous with ϳ3 ␮m equantSheared samplegrains.of olivine and larger bands at higher angles rel- evidence for a preponderance of dislocations with Despite the considerable effort to characterize olivine’s defor- 4% MORB. (B) Sheared sample ative to the shear plane (flat red [001] Burgers vectors or with high local stresses (i.e., mation mechanisms over the past 30 yr, it is only recently that arrow) connected by smaller increased dislocation density). Because no evidence small-angle misorientations within the clasts Two types of porphyroclasts with distinct crystallographic orientations remain. Highlyof olivineelongatedand chromite and 6% bands at lower angles. Smaller exists for a change in the relative strength of the slip deformation experiments could be conducted at pressure– MORB, shown in Fig. 1. The CPO arrows indicate that the samples systems, another explanation for the a-c switch must temperature conditions of the entire upper mantle6–8. New flatten and widen with shear. In be invoked. ribbon grains (blue) are in an orientation for easy slip on (010)[100] and track the bulkmeasuredstrain.in a Theirsample of olivine 16. The absence of variation in the CPO around and confirms the formation of subgrains with a simple-shear experiments on olivine aggregates at 11 GPa and and MORB with FeS melt is iden- three dimensions, the melt-rich within a band supports the conclusion that deforma- aspect ratios (R ϭ 25 to 40) are consistent1,400 8C, conditions equivalentwith tothethose finiteat depths ofstrain330 km, haveellipse at ␥ ϭ 5 (R ϭ 27).tical toAthissecondone. (C) Schematic layers connect and surround tion in the bands does not contribute to or strongly melt-depleted lenses. (B) Strain modify the CPO (SOM Text, section 3). At least two size similar to the recrystallized grains, pro- shown that deformation takes place by dislocation creep, with diagrams of three sets of three partitioning between bands reasons argue against formation of the observed CPO type of porphyroclasts (white) dominantwithactivationlowerof [001]{aspecthk0} slip systemsratios9, suggested(byRtheϽ 10) are full of subgrainspole figures for aandmelt-free oli- (anastomosing layers) and in the melt-rich bands: (i) To maintain a stable ori- concentration of [001] parallel to the shear direction and of [100] lenses. The flat arrows indicate entation during simple shear, the melt within the viding further evidence for recrystallization vine aggregate (top) [after (3)], bands must move relative to the solid (7). Thus, the deformation features, are strongly asymmetric in shape, and have an oblique lattice orientation. the total shear and the compo- and [010] normal to the shear plane (Fig. 1). Transmission olivine and oriented melt (mid- nent concentrated in the strain associated with a band at a given location will electron microscopy shows the exclusive presence of dislocations dle), and olivine and segregated probably not be high enough to modify the CPO. (ii) by subgrain rotation. bands. The narrow arrows indi- The deformation mechanisms may be different in the Both types have average grain areaswith [001]equalBurgers vetoctorsthosein a screw orientation,of thecompatiblestartingwith grains (equivalent diameter,melt (bottom), ϳall deformed20 at cate alignment of olivine a axes normal to the shear direction in the lenses. The black lines two regions because of increased melt fraction in the [001](hk0) slip. Dominant [001] slip in the deep upperFig.mantle1. Microstructures.Figure 1 Preferred(orientationA) Reflected-lightof [100], [010] andim-[001] crystallohigh Tgraphic(1473axestoin synthetic1573 K) and P (300 MPa) to shear mark the orientation of the shear plane in the lenses, “back-rotated” relative to the sample melt-rich bands. Granular flow or grain boundary LPOs were measured using electron back- ␮m), suggesting that their shapes are due entirely to strain without major coalescence. shear plane due to strain partitioning. sliding accommodated by diffusion with rigid rota- 2) Effects of dissolved waterreq uandires re -elevatedevaluation of tstresshe interpr eontatio nLPOof ani agesotroofpiacsampleolivine ofpolycrystalolivineS2954anddeformedchromiteat 1,400and8C4%and 11 GPastrainsconfininglargerpressurethanin simple␥ ϭ 1. The reference position for physical properties. For instance, the fastest P-wave velocitMORBy willshearedshear9.betweenLower hemispheretungstenequal-areapistons.projection,Thecontourstheat intervalsback-rotationof 0.5 multiplesof ofthe CPO is indicated by the no longer parallel the shear direction as in an uppermelt-richmantle bandsa uniformaredistributionvisible .as3,269themeasureddarker regionsorientations. Dextraloutlinedshear (topareas.to the right)A similaris obliquity between the www.sciencemag.org SCIENCE VOL 301 29 AUGUST 2003 1229 deforming by dominant [100](010) slip, which is the assumptionoriented 10°indicatedto 20°by half-arroto thews;sampleSD, shearwalls,direction;form-NSP, normalshearto shearplaneplane;andX, finitethestrainaxis aligned in the shear direc- traditionally used in relating flow and seismic anisotropyinginantheanastomosingextension direction.network.Shear strain(B),0.3.SEMInclinedback-black linetionmarkscanthedevelopfoliation (flatteningdue to recrystallization, discussed www.sciencemag.org SCIENCE VOL 290mantle10,1124. NOVEMBER 2000 scattered electronplane). image of one band. The further in SOM Text, section15655, but is not the origin chromite grains (white) are smaller than the of the back-rotation observed here. The relative NATURE | VOL 433 | 17 FEBRUARY 2005 | www.nature.com/nature 731 ©!!""#olivine!Naturegrains Publish(gray).ing GrouChromitep! tends to sit in seismic velocities are Va Ͼ Vc Ͼ Vb. At right is a melt (black) tubules, reducing the permeability. spatial representation of the third CPO.

1228 29 AUGUST 2003 VOL 301 SCIENCE www.sciencemag.org Geochemistry Geophysics 3 holtzman et al.: melt segregation in molten rocks 10.1029/2001GC000258 Geosystems G Causes of anisotropy a) alignment olivine + 3 vol % MORB

3) Melt distribution Multiple scales of isotropic and anisotropic organization

a) alignment (~grain size) melt pockets aligned relative to Figure 2. Reflected light optical micrograph of an olivine + MORB sample (PI-273). The melt pockets (dark) strongly align at about 20° to the shear plane (horizontal). The melt distribution is homogeneous across the sample. stress orientation (e.g. Zimmerman, (Modifiedb)from Kohlstesegregationdt and Zimmerman [1996] and Zimmerman et al. [1999].) et al., 1999) such that the melt distribution is a function of time. in 3-D (see Appendix A). Sixteen samples were We do not claim that any melt distribution reaches deformed, at varying stresses and to varying strains, a steady state, though this question will be none of which formed bands. Five of these samples addressed in much greater detail in future papers. are listed in Table 2. b) segregation (< compaction length) Therefore, when melt distributions between samples are compared, samples deformed to the same 4.2. Anorthite + Basaltic Melt strain were selected, wherever possible. In Table 2, melt segregates into bands at the experimental conditions and average properties [24] During deformation, melt migrates into melt- of the melt fraction distributions are listed for each rich bands 20 mm wide and 100 mm apart, as shown in Figure 3. The bands are oriented 10– sample. In the following sections, the results from lengscales much longer than the each group of experiments are described. 20° to the shear plane. The bands appear to form anastomosing networks. A high-resolution SEM grain size (e.g. Holtzman et al., 2003) 4.1. Olivinec)+ MOmigrationRB image of a band reveals a very high melt fraction and euhedral grains, lying ‘‘piggyback’’ on each [23] In sheared samples of olivine + 3 vol% MORB, other, providing a clear sense-of-shear indicator, as melt is uniformly, but anisotropically, distributed illustrated in Figure 3b. In addition, many of these across the sample in grain-scale pockets oriented bands root from or feed into small regions of 25° to the shear plane (20° from the maximum locally high melt fraction on the low-pressure side c) up-stress migration principal stress), as illustrated in Figure 2 (modified of the peaks in the tungsten pistons. from Kohlstedt and Zimmerman [1996] and Zim- merman et al. [1999]). The melt pockets, spaced (> compaction length) approximately a grain width apart, form parallel 4.3. Olivine + Chromite + MORB interconnected volumes often several grains long, [25] Eight shear experiments are reported here on melt migrates UP stress gradients which more closely approximate sheets than tubes this system, with varying melt fractions and finite that exist on (seismically 8 of 26 observable?) scales of plate boundaries (Takei and Holtzman, in prep. and next talk...) Christensen (1987) presented a simplified form for the case when the principle directions of the anisotropy layers are parallel to the principle directions of the stress tensor, Christensen (1987) presented a simplified form for the case when the principle directions of the anisotropy layers are parallel to the σprinciplexx directionsηN 0 of theε˙xxstress tensor, Christensen (1987) presented a simplified=form for the case when the principle directions of(15)the ! σxy " # 0 ηS $ ! ε˙xy " anisotropy layers are parallel to the principle directions of the stress tensor, σxx ηN 0 ε˙xx = 1 1 (15) where ηN = ηi ( η is the “spatial! σxyaverage" #” of0 η, η(Christensen,S $ ! ε˙xy " 1987), and ηS = (ηi)− − . The ! " ! " 1 σxx ηN 0 ε˙xx ! " anisotropy factor s = η η− . So, for our=purposes, (15) ! "! " σxy 0 ηS ε˙xy 1 1 where ηN = ηi ( η is the “spatial! average" ”#of η, (Christensen,$ ! "1987), and ηS = (ηi)− − . The ! " ! " 1 ! " anisotropy factor s = η η 1 . So, for our purpab oses,(1 ab) − − η = + 1 1 (16) where ηN = ηi ( η !is"!the “spatial" aHvBerage” of η, (Christensen,− 1987), and ηS = (ηi)− − . The ! " ! " ηb ηn ! " anisotropy factor s = η η 1 . So, for our %purposes, & 1 − ab (1 ab) − ! "! " ηHB = + − (16) ηb ηn 1 % ab (1 ab)& − The resultant curves are then incorpηHB orated= in+to t−he viscosity profiles, as perturbations to(16)the ηb ηn diffusion creep contribution to the total viscosit% y (or to the&total?)... ??? The resultant curves are then incorporated into the viscosity profiles, as perturbations to the diffusion creep contributionChristensento(1987)the presentotaltedviscosita simplifiedy (orform tofor thecasetotal?)...when the principle??? directions of the The resultant curvanisotropes arey lathenyers areincorpparallel tooratedthe principleintodirectionsthe viscositof the stressy profiles,tensor, as perturbations to the B.3 Effects of water diffusion creep contribution to the total viscositσ y (orη to0 the total?)...ε˙ ??? xx = N xx (15) This is not done yet. see discussion in paper! σ2.xy " # 0 ηS $ ! ε˙xy " B.3 Effects of water 1 1 where ηN = ηi ( η is the “spatial average” of η, (Christensen, 1987), and ηS = (ηi)− − . The ! " ! " 1 ! " mecanisotrophanismy factor s = η η− . So,A for our purpnoses, m Q V* α ThisB.3is notEffectsdone oyet.f wseeaterdiscussion !in"!pap" er 2. 1 GB diffusion(dry) 1.5e9 a1 (1 3a ) −375e3 10e-6 25 η = b + − b (16) HB η η This is not done ymecet.GBSseehanismdislodiscussioncation(dry)in papA4.7e10er 2. % n3.5b mn2 &Q600e3 V*15e-6 α35 GBDislodication(dry)ffusion(dry) 1.5e9 13.5 30 375e3 10e-6 25 mechanism A n m Q V* α GBSGB diThedisloffusion(wresultancation(dry)t curvet)es are then4.7e10incorporated3.51into t2he3 viscosit600e3y profiles,15e-6as perturbations35 to the DisloGBSGBdiffdiusioncation(dry)disloffusion(dry)creepcation(wcontributionet)to the1.5e9total viscosit3.53.51y (or 0to23the total?)...375e3??? 10e-6 25 GBDisloGBSdication(wffdislousion(wcation(dry)et)et) 4.7e10 13.53.5 302 600e3 15e-6 35 DisloB.3 cation(dry)Effects of water 3.5 0 Deformation experiments:T = 1250 C GBS dislocation(wet) 3.5 2 gas- medium P = 300 MPa 1) Melt segregationpressure and the effect of increasing strain: GBThisdiisffnotusion(wdone yet.et)see discussion in paper 2.1 3 vessel Dislocation(wet) 3.5 0 HOLTZMAN oAlivineN + chroDmite + 6K%MOROB HLSTEDT MELT SEGREGATION EVOLUTION simple shear Table 1: Flow law parameters, from Hirth & Kohlstedt (2003). experiment GBS dislocation(wmechanismet) A 3.5 n 2 m Q V* α thoriated olivine + chromite + alumina tungsten GB diffusion(dry) 1.5e9 1 3 375e3 10e-6 25 pistons Dislocation(wet) 3.5 0 pistons MORB and sample spacers GBS dislocation(dry) 4.7e10 3.5 2 600e3 15e-6 35 Ni-foil Asmeltφ =first> aligns0, at Ttheable grain1: scaleFlo w law parameters, from Hirth & Kohlstedt (2003). strain marker Dislocation(dry) 3.5 0 and then segregates into bands Nickel GB diffusion(wet) 1 3 jacket Dmelt => Dg.b. (010) (important observation for ideas in olivine GBS dislocation(wet) 3.5 2 single-crystal Ag g contact Table 1: Flow law parameters, from Hirth & Kohlstedt (2003). pistons Asϕnext=φ = talk...)>− 0, Asurface Dislocation(wet) 3.5 0 1010 µm um Dmelt => Dg.b. chromite " plugs " As φ => 0, Ag g contact Table 1: Flow law parameters, from Hirth & Kohlstedt (2003). constant load ϕγ = 1 − is applied by Asurface actuator from D≈melt => Dg.b. below Ag g contact As φ => 0, ϕ = − γγ 12 Asurface Dmelt => Dg.b. Ag g contact ≈≈ ϕ = A− P = 300 MPa surface γ 1 T = 1250 C γγ 23 γ 1 ≈≈≈ ≈ γ 2 γγ 50032 µm ≈ ≈ γ 3 ≈ ≈ γ 3 ≈500 µm 500 µm 500 µm

12 12

Fig. 4. Reflected-light optical images of samples in the ‘strain series’, deformed top-to-the-right, the x^z plane (where z is normal to the shear plane x^y, and x is parallel to the shear direction). In all images, the vertical black lines are cracks created during the quenching of the sample; the white lines are the Ni strain marker; the grey lines are the melt-rich bands. The groove spacing is 250 mm. (a) Samples with f 0 02 were ¼ Á sheared to g 1, 2 and 3. (b) Samples with f 0 06 were sheared to g 1, 2 and 3 5. ¼ ¼ Á ¼ Á

In Fig. 5c, a band at lower angle appears to connect two angles in each sample have normal distributions, as shown bands at higher angles. The observation that narrow in Fig. 6. As a function of strain, the band angles appear bands occur at lower angles than wider bands appears in remarkably constant, at 20 68 in the f 0 02 set and Æ t ¼ Á all samples. 17 68 in the f 0 06 set. The angles increase slightly Æ t ¼ Á A higher magnification view of one band in Fig. 5d with increasing strain in the f 0 02 set but not in the t ¼ Á reveals several interesting features. First, in some places, f 0 06 set, as shown in the top right box of Fig. 6. t ¼ Á the edges of the band are very well defined, marked by a Bands were not clearly visible in most of the sample with dramatic change in melt fraction; in other regions, the f 0 06 deformed to g 11 (PI-885), so no data exist for t ¼ Á ¼ Á transition from band- to non-band regions are more grada- low strain at f 0 06 in Figs. 6^8. t ¼ Á tional. Furthermore, inside the band, variations in melt In contrast to the distribution of angles, the statistical fraction are large, with islands of low melt fraction sur- distributions of the band thicknesses are clearly not rounded by melt-rich channels. In other words, a larger normal distributions; in log^log space (upper right corner band may be viewed as a cluster of smaller bands. In of Fig. 7), the distribution appears to follow a power law, Fig. 5d and e, it is clear that the melt fraction can be locally n m, where n is the frequency of occurrence of a th / tÀh th very high in a band. As annotated in the images, the oli- given thickness of a band of thickness, dth, where the vine crystals in the bands appear to be both dissolving mean value of m is 17 (values for each sample are listed in Á and growing by precipitation, as suggested by the presence Fig. 7). We cannot test the robustness of the apparent of very small grains (51 mm), incised or corrugated grain power-law nature of this distribution because the thickness boundaries of the large grains and the highly euhedral data only span up to one order of magnitude. In the images overgrowths on olivine grains, respectively. in Fig. 4, the thicknesses of the bands appears to increase between g 1 and g 2 in both the f 0 02 and ¼ ¼ t ¼ Á Melt configuration statistics f 0 06 series, but there does not appear to be a system- t ¼ Á The frequency distributions of band angles, thicknesses, atic difference between the data from samples deformed to and spacings are presented here (Table 4). The band g 2 and g 3. These visual inferences appear in the

9 HOLTZMAN AND KOHLSTEDT MELT SEGREGATION EVOLUTION

nature of the melt-rich networks. As indicated in the sche- the distribution of melt. The first-order structure is the matic drawing of 3D networks from Holtzman et al. largest bands, which are oriented at a 15^258 to the (2003b), the average length of lenses is greater in the sample shear plane (that is, the grooved piston surface) in plane normal to the shear direction than it is in the flow Fig. 13c. The population of smaller bands, the second-order plane (the usual view). Also, some sample material has structures, is oriented at a 5^158 to the sample shear been extruded laterally (in the shear plane, normal to the plane. However, if one views these structures in a local shear direction) beyond the edges of the piston. It also reference frame of the lenses, such that the shear plane in appears that the melt fraction is higher in and near this the lenses is back-rotated by 108 to the sample shear extruded material, indicating that there may be some flux plane (the piston surface), then the secondary bands are of melt from the middle of the sample towards the edges. oriented at a 15^258 with respect to this secondary shear The visual and statistical signature of the anastomosing plane, illustrated in Fig. 13c. In other words, the secondary networks is a bimodal distribution of band angles, as illus- narrow bands that cut across lenses are controlled by the trated in Fig. 13. In this image, and in all of the samples, local stress field in the lenses. This local rotation of the there are indications of some scale-invariant properties of stress field in the lenses is suggested by the back-rotation of olivine b-planes (Holtzman et al., 2003b) and by the analysis of strain partitioning (Holtzman et al., 2005). Extending this point of view one order downward in scale, the melt pockets most visible in the large bands are also oriented 208 to the wall of the band, the third-order shear plane defined by the surfaces of the melt bands. Thus, from the sample scale to the grain scale, there are three levHelOLsTZofMAscale-N AND KinOHvLarSTiEanDTce tMoEtLheT SEGorREienGATtaIOtNionEVOLofUTmIONelt alignment relative to the local and applied stress tensors. Istherealowerlimittothemelt fractionrequiredforsegre- nature of the meltg-raichtionnetwotroks.oAccurs indica?tedThein thesasche-mpltehewdiistrhibuftion of0m00elt.5Thewafirsst-dordeformer strucedture is the matic drawing of 3D networks from Holtzman et al. largest bands, which are oriented at a 15^258 to the ¼ Á (2003b), the averaaget lmodength eraof lensetes sistresgreasetersin(tthe sa1m2pl2e MshearPapl)an. eW(thaeltli-s,dthefe inedgrooved pibastonndsurfas ce) in f ¼ plane normal to tformhe sheareddi,recations shthaonwitnisinin tdheeftailow l inFig.F13igc. The. 14p.oThpulateionbofandssmallerarebandnarros, the secwond-order plane (the usual view). Also, some sample material has structures, is oriented at a 5^158 to the sample shear been extruded lateraalndly (inthtehe lenseshear plsanbe, etwnormaleento thethemplaneha. Hvoweevalmoser, if onetvinoews tvheisesibstlreucturesmelint. a local shear direction) beyond the edges of the piston. It also reference frame of the lenses, such that the shear plane in appears that the mFurtelt frhermoaction is hreigh,erthein andchronearmthiites gtherainslenses inis bathcke-roltaentedsebsy appear108 to thetosambpele shear extruded materials, tindirecatctinghedthat andthere maaly bignede some fl,ux formplaneing(the pianston surappface)aren, then tthefoliasecondtariony ba.nds are of melt from the middle of the sample towards the edges. oriented at a 15^258 with respect to this secondary shear The visual andHstaotiswticalevserigna, turewhoferethe anasbatomosndingis prplaenesen, illustt,rattedheincFhroig. 13cm. Initeothergrainswords, thearesecondary networks is a bimolargdal dierstr,ibfuewtionerof baandnd anglelesss, aels ilonlus-gatednarro,wabas ndshsothawtncuint acrFoigss l.en14se.sThare ciosnptroalledt- by the Deformationtrated in Fig. 13. In this image ,experiments:and in all of the samples, local str e ss2)field inEffectsthe lenses. This l oofcal ro taboundarytion of the conditions there are indicationsternof somsueggscale-estinsvtarhianatt cprhroopertmiesiofte gstrairessnfieldgrinowtthe hlen(seans isOssuggtestwedaldby trhiepbaenck-rotation of olivine b-planes (Holtzman-2et al., 2003b) and by the ana- on effectiveing pro ceviscosityss) is much more lefysifsicienof sttraiinn partithet1ioningpr0esen(Holcetzmofan met elal.t, 2005). Fig. 10. Reflected light images samples (a) PI-1027, (b) PI-1025, than in its absence.These varExiatendintionsg tinhis cphrooint mof ivteiewmoonerordphoer ldogownyward in and (b) PI-1020, in the stress series, sheared at constant load, top-constant load scale, the melt pockets most visible in the large bands are also oriented 208 to the wall of the band, the third-order 0 melt are not present (or nearly as clearly) in samples with more to-the-right, decreasing in applied load from (a) to (c). As with the shear plane defined by the surfaces of the melt bands. Thus, 0.04 melt previous strain series, the vertical black cracks are caused by quench- melt, suggesting that the frprometsenhe samcepleofscalemtoelthte gsraiignnscaleifican, theretlyare three ing the sample at the end of an experimental run and should be levels of scale-invari) ance to the orientation of melt align- enhances chromite grain growth. 1

ment relative to t- he local and-a3pplied stress tensors.

ignored. The groove spacing is 250 m and the melt-rich bands are s m Istherealowerlimitto1th0emelt fractionrequiredforsegre- ( the darker grey channels aligned at about 158 to the shear plane. gation to occur? The sample with f 0 005 was deformed ¼ Á at moderate stressee s (tf 122 MPa). Well-defined bands t ¼

formed, as shown in a detail in Fig. 14. The bands are narrow dislocation g.b.s. crp. and the lenses between them have almost no visible melt. Furthermore, the chroR mite grains in the lenses appear to be

stretched and alignedn , forming an apparent foliation. However, where a bai nd is pr-esen4 t, the chromite grains are a

larger, fewer and lessr elon1g0ated, as shown in Fig.14.This pattern suggests that chrot mite grain growth (an Ostwald ripen- ing process) is muchS more efficient in the presence of melt Fig. 10. Reflected light images samples (a) PI-1027, (b) PI-1025, than in its absence.These variations inchromitemorphology and (b) PconstantI-1020, in the stress series strain, sheared at cons taratent load, top- are not present (or nearly as clearly) in samples with more time to-the-right, decreasing in applied load from (a) to (c). As with the diffusion crp. previous strain series, the vertical black cracks are caused by quench- melt, suggesting that the presence of melt significantly ing the sample at the end of an experimental run and should be enhances chromite grain growth. ignored. The groove spacing is 250 mm and the melt-rich bands are -5 the darker grey channels aligned at about 158 to the shear plane. 10 2 1H0OLTZMAN AND KOHLSTEDT MELT SEGREGATION EVOLUTION Stress (MPa)

Fig. 11. Reflected light images of samples sheared under constant displacement rate conditions (that approximate constant strain rate) of (a) 3 10^4 s^1 and (b) 1 10^3 s^1. Â Â

Fig. 11. Reflected light images of samples sheared under constant displacement rate conditions (that approximate constant strain rate) of (a) 3 10^4 s^1 and (b) 1 10^3 s^1. Â higherÂ lower stress stress

Fig. 22. The pumping mechanism. (a) A schematic illustration of lenses between two underformable plates. The light grey lines in the lenses are local shear planes and the melt-rich bands are transparent. (b) Inset in (a). The open arrows indicate stress orientations and the filled grey 13 arrows indicate melt flow direction. We propose here and elsewhere (Holtzman et al., 2005) that the normal stress components on the bands switch sign above some critical angle (4308) from tensile or dilational to compressive or compactional, essentially because lenses have to 13 deform more and more the closer a band is to 458, and the system will only tolerate a certain stress difference between bands and lenses. This flattening of the bands causes the melt pressure to rise and sets up a flow into lower angle bands. (c) Inset in (a). Bands can nucleate and propagate into a lens at a low angle, and can also close off, as shown in Fig. 13. (d) Inset in (c). A grain-scale view of the tip of a propagating band.

approximately the same angle, suggesting that this angle is ubiquitous and is stress controlled (not controlled by the physical properties of the matrix). These observations suggest that bands form at the same distribution of angles at which they are observed when mature, by the collection and accumulation of individual elongated pockets. Hier-Majumder et al. (2004) adopted a micromechanical approach to this question, looking at how shear stress caused the growth of some pockets at the expense of others, and speculated that pockets at 0^458 to s1 open preferentially by stress corrosion, causing an average angle of 208. Nucleation of small bands within the well- developed lenses occurs at lower angle (5^108 relative to the shear plane), reflecting the local back-rotation of the stress field as a result of strain partitioning between lenses Fig. 23. Band spacing, dsp, as a function of compaction length, dc. and bands. This local modification of stress fields was dis- PI-1020 appears to be an outlier, as discussed in the text. The slope of cussed extensively in an earlier study (Holtzman et al., a linear fit to the rest of the data is m 0 4. ¼ Á 2005). Once elongated pockets are nucleated, the stress- driven segregation instability is initiated. occurring simultaneously in a deforming sample, as illustrated in Fig. 22. Rotation and growth As discussed by Holtzman et al. (2003a, 2005), Spiegelman Nucleation (2003) and Katz et al. (2006), bands should rotate with Why do bands nucleate and at what angle? Our observa- shear if melt is not migrating relative to the solid. The tions from the lowest shear strain experiments in the apparent rotation is caused by the simple shear, but it is strain series suggest that bands begin to form at 5^258, actually the shear parallel advection of a material line with a mean and standard deviation of 18 68. In sheared (melt-rich band) at an angle to the shear plane, causing Æ samples of olivine MORB, Zimmerman et al. (1999) the appearance of rotation. Spiegelman (2003) modified þ found that melt pockets were elongated and oriented at Stevenson’s (1989) instability analysis from a pure shear to

21 NATURE | Vol 445 | 4 January 2007 LETTERS

reached (Fig. 1c). Third, examination of post-deformation samples response to vane rotation, and that the dominant deformation mech- reveals a band of concentrated porosity at the path circumscribed by anism is Reynolds dilatancy-enabled crystal rearrangement. The the vane (Fig. 1d–f, and Supplementary Fig. 14), suggesting that the strain localization in Fig. 1d–f and Supplementary Fig. 14 can be strain softening is associated with strain localization. In this Mg alloy, explained by instabilities inherently caused by Reynolds dilatancy the shear band contains concentrated porosity at all cooling rates and fragmentation, because both decrease the local strength of the studied, and at cooling rates that result in large (>700 mm) highly region in which they occur, promoting further deformation in that branched dendrites, crystal fragmentation was additionally observed region. in the band. Shear bands also form in direct shear cell experiments. An example The shape of the M–h and sample height versus vane-rotation (h– is given for Al-7Si-0.3Mg with globular morphology in Fig. 2 and h) responses in Fig. 1b, c and the localization of deformation into Supplementary Fig. 16, where a band exists on the shear plane con- shear bands in Fig. 1d–f are typical characteristics of compacted taining a higher volume fraction of eutectic than adjacent regions cohesionless granular materials, such as dense sand or glass (positive macrosegregation). As the eutectic was liquid at fs 5 0.5, the beads7,22–26. Compacted granular materials expand when sheared shear band had a higher liquid fraction than adjacent regions at the because particles must push one another apart and increase the space end of deformation, suggesting that liquid was drawn to the band between themselves in order to rearrange (Reynolds dilatancy)1,5. The during deformation. Coh fact that partially solid alloys with fs . fs exhibit similar behaviour We find that localized bands of porosity and positive macrosegre- Coh indicates that, after growth has caused impingement (fs . fs ), the gation form only when the material is deformed within a specific crystals are sufficiently crowded that they cannot initially move past range of fs: above those at which the material flows as a dilute sus- Coh each other and that there is negligible intercrystal cohesion. The M–h pension (when fs . fs ) and below those at which the macroscopic and h–h responses suggest that crystals push one another apart in shear response is crack propagation. This range of fs is dependent on

a b 160 c 160 0.5 (mm)

0 h

0.4 ∆ 120 120

0.3 (mN m)

M 80 80 Nucleation of 0.2 Mg Al (ºC) que,

17 12 T or e, (mN m) –1 T 40 40 M 0.1 ) –1 c) Migration up stress gradients: que, or mperatur 0 0

T 0 (K s e Change in height of sample, t T 0 π/2 π 3π/2 0 π/2 π 3π/2 in two-phase metal composites, both solid state and partially molten, weak phase /d

T Vane rotation angle, θ (rad) Vane rotation angle, θ (rad) migrates UP the stress gradient d Materials Science and Engineering A 413–414 (2005) 180–185 –2 partially molten state: Shear deformation at 29% solid during solidiﬁcation of magnesium f solid state: alloy AZ91 and aluminium alloy A356

C.M. Gourlay ∗, A.K. Dahle C.M. Gourlay, A.K. Dahle / Materials Science and Engineering A 413–414 (2005) 180–185 Time, t (s) Materials ScienceCAST CRC, Scandhool ofEngineeringEngineering, The UniversityAof413–414Queensland, 4072(2005)Qld, Australia180–185 183 Received in revised form 1 July 2005d e

Abstract 5 mm Partially solid commercial Al–Si and Mg–Al alloys have been deformed in shear during solidification using vane rheometry. The dendritic mush was deformed for a short period at 29% solid and allowed to cool naturally after deformation. Both alloys exhibited yield point behaviour and deformation was highly localised at the surface of maximum shear stress. The short period of deformation was found to have a distinct+impact on the as-cast microstructure leading to fragmented dendrites in the deformation region of both alloys. In the case of the Mg–Al alloy, a concentrated Shear deformationregion of interdendriticatporosity29%was also observed solidin the deformation reduringgion. Concentrated porositysolidificationwas not observed in the Al–Si alloy. of magnesium © 2005 Elsevier B.V. All rights reserved. 0 Keyworallods: Mushy zone;y Rheology;AZ91Shear; Vane; Semi-solid;andSolidificationaluminium alloy A356

1. Introduction entangled skeleton of dendrites (the dendritic network). Metz and Flemings [3] investigated the shear yield strength of solidi- Aluminium alloy A356 and magnesiumC.M.alloyGourlayAZ91 are com- fying∗,alloA.K.ys and showedDahlethat dendritic alloys exhibit no mechan- mon casting alloys often used in automotive applications. Both ical strength before dendrite coherency and that the shear yield CASTare hypoeutecticCRC,alloScys withhoolrelatiofvelyEngineeringwide freezing ranges, The[1], Universtrength sityincreasesofeQueensland,xponentially with solid4072fractionQld,aboveAden-ustralia and they therefore contain relatively large mushy regions dur- drite coherency. Spencer et al. [4] concluded that shear yield ing solidification. In most commercial casting processes these strength is a strong function of morphology and that the yield alloys solidify with equiaxed dendrites andReceithe dendrite–liquidved in revisedstressformdecreases1dramaticallyJuly 2005at a given solid fraction as the solid mixture is subjected to a range of stresses during the casting becomes more globular. More recently, the effect of solid size process due to both the process itself and also solidification and shape on mush yield strength has been examined by Dahle related phenomena such as solidification shrinkage and ther- and Arnberg [5] who investigated a range of solidifying Al alloys 1 mm f Dilatant shear band mal contraction. Mush is deformed in processes where some 184with solid fractions in the range 0 < sC.M.< 0.5. GourlayThey were, A.K.able toDahle / Materials Science and Engineering A 413–414 (2005) 180–185 solidification occurs during filling such as in high-pressure die- explain the changes in rheological response brought about by Abstract casting or squeeze casting and also in processes where pressure grain refiner additions and compositional changes by consider- is applied to assist the feeding of solidification shrinkage after ing theirFigureeffect on the1 |solidVaneshaperheometryand size. Larger andof morepartially solidified Mg-9Al-0.(Fig. 2a and7Zn.b). Collapsea, Initially,of the dendriticreachesnetwa near-constantork destroys thevolume towards the end of deformation after global filling. irregularlyliquidshaped Mg-9Al-0.7dendrites were shoZnwn toisbegincooledto developfrom 700 C. Once nucleated,interconnectedthestructuredecreasingof the mush,volumetricallowingstrainthe collapsedof e < 0.01 (T 5 580.7 C, f 5 0.35). d–f, Post- Partially solid commercial Al–Si andOur understandingMg–Alofallothe shearys rheologyhave ofbeendendriticdeformedmush strengthinatsheara lower duringsolid fractionsolidificationand highly branched usingden- vaneurheometry. The dendritic mush Vol u s initial in both experiments,at low solid fraction has been obtained mainly from experi- drites formtemperaturemore interconnected,leadsstrongertonetwdendriticorks at a givengrowth of equiaxedre(Mg)gion tocrystals.deform Atby athe flow of discretedeformationparticlesmicrostructurin suspen- es of the Mg alloy after complete solidification in was deformed for a short period atments29%using solidAl–Si or Sn–Pband alloalloys [2–7]wed. Arnberto gcoolet al. [2]naturallysolid fraction.afterThe importancedeformation.of morphologyBothhas been furtheralloys exhibited yieldsion.pointThebehastressviourrequiredandto deform the suspension is much less stress weak phasefound thatmigratesdistinct changes in upshear response occur when the emphasisedtemperatureby Nabulsi [6] andcorrespondingSumitomo et al. [7] who shotoweda desired fs, the rotation of a four-bladed vane a sample deformed at fs 5 0.19. d, Macrograph of one-quarter of the cross- + deformation was highly localised atgrowingthedendritessurfaceimpingeofonmaximumone another at whatshearis definedstress.the roleTheplayedshortby dendriteperiodarms inofprovidingdeformationinterconnectiv- was found to havthane a distinctfor the interconnectedimpact on structure which leads to the dra- 0 Coh is initiated at 5 rotations per minute. Deformation continues for one vane section through the centre of the vane. A localized band of porosity exists at the stressthe gradientdendrite coherency solid fraction, fs . This point in the ity to the dendritic network through interlocking. Deforming matic decrease in shear stress after peak shear stress observed in the as-cast microstructure leading tosolidificationfragmentedsequence marksdendritesthe start of theindevelopmentthe deformationof an a mushrotationconstitutedregionof (12highlyof boths)branchedandallodendritesis ys.thenInrequiresstopped.thethecaseAsofthethetemperatureMg–Al alloycontinues, a concentratedto the path circumscribed by the vane. e, The equiaxed grain structure dendrites to be fragmented giving such microstructures greater Fig. 1b. The deformation rate induced by vane rotation can then region of interdendritic+ porosity was also observed in the deformation region.decrease,Concentratedsolidificationporosityprogresseswas notbyobservthe growthed in andthe Al–Sicoarseningalloyof. (Mg). At throughout the same cross-section. f, A higher magnification image of the shear strength than those containing more rounded dendrites be achieved at lower stress and the peak shear stress is never which can deform by grain reorientation alone. Experimental © 2005 Elsevier B.V. All rights reserv∗ Correspondinged. author. Tel.: +61 7 3365 3639; fax: +61 7 3365 3888. fs < 0.84, the eutectic reaction L R (Mg) 1 Mg17Al12 commences. band shown in d and e, revealing that the porosity band is ,11 grains wide. melt/weakE-mail address: [email protected] phase(C.M. Gourlay). studies on the shear behaviour of magnesium alloy AZ91 have reached in the mush away from the cylindrical surface circum- 21 b, Torque–vane-rotation responses for three typicalscribedexperimentby the vsane.conductedDeformationThethereforeshearbecomesstrain localisedrate within shear bands is c_5 1–4 s . A discussion of the flow0921-5093/$ –directionsee front matter © 2005 Elsevier B.V. All rights reserved. Coh Keywords: Mushy zone; Rheology; Shear;doi:10.1016/j.msea.2005.09.048Vane; Semi-solid; Solidification at fs . fs . Experimental parameters (temperatureat TtheinvaneuC,path.fs, andThistpeakis typicalin ofchangesgranular materialsin grain insizewhichand shape between deformation and complete kPa) as follows: top curve (580.7, 0.35, 9.1); middlefailurecurvedoes(585.5,not occur0.29,homogeneously4.9); solidification, instead beingis givenconfinedin Supplementary Information sections 1.7 and 2.1.3. initial porosity stress bottom curve (590.2, 0.22, 1.3). c, A sample expandsto bandsas it aisnumbersheared,of particlesand wide [20]. Whilst the rheology and many of the macrostructural features 0 1. Introduction entangled skeleton of dendrites (the dendriticof thesenetwalloork).ys are similarMetz, the distribution of porosity is dis- 71 radial distance tinctly dif©ferent. 2007InNAZ91,ature PanubannuluslishingofGporosityroup was observed and Flemings [3] investigated the shear yieldin thestrengthregion ofoflocalisedsolidi-deformation and fragmented dendrites Aluminium alloy A356 and magnesium alloy AZ91 are com- fying alloys and showed that dendritic alloys(Fig.exhibit4a). Itnois unlikmechan-ely that this porosity forms during deformation because, at 29% solid, the liquid is expected to be contiguous mon casting alloys often used in automotive applications. Both Fig. 4.icalAs-polishedstrengthmacrographsbeforeshowingdendritethe porosity distribcoherencution in eachyalloandy. throughoutthat the theshearsample,yieldflow channels within and between den- (a) A concentrated annulusradialof porosity distanceexists at the vane path in AZ91. (b) Large are hypoeutecticFig.allo3. Theyschangewithin meanrelatigrainvelysize withwideradialfreezingdistance from therangesvane centre.[1], roundedstrengthpores are moreincreasesevenly distributedexponentiallyin A356. with solid dritesfractionare expectedaboveto den-be broad and the mush permeability is high and they thereforeGrain containsize has beenrelatiaveragedvwithinely larradiusgecontoursmushy1 mm rinethickness.gions Thedur- drite coherency. Spencer et al. [4] concluded[21]. thatThe larshearge grainyieldsize increases the permeability further due vane edge is at 10 mm: (a) AZ91 and (b) A356. to the resulting low surface area to volume ratio of solid. It is 4. Discussion ing solidification. In most commercial casting processes these strength is a strong function of morphologymoreandlikelythatthatthetheyieldporosity formed late during solidification long after the deformation had stopped (Fig. 1a). This could be alloys solidify withannulusequiaxof porosityed dendritesis observed atandthethevanedendrite–liquidpath whilst there Instressorder todecreasesunderstand dramaticallythe mechanical behaat aviourgivenof mushsolid fraction as the solid explained if the localised deformation led to a region of higher is little porosity in the remainder of the cross-section. The at 29% solid, we must first consider the microstructure at this mixture is subjected to a range of stresses during the casting becomes more globular. More recently, theenrichedeffectliquidof solidfractionsizethan the surroundings during solidifica- width of the porosity-containing region is not constant but, is point during solidification. Fig. 2 shows that the solid is equiaxed process due to both the process itself and also solidification and shape on mush yield strength has been tion;examinedin this case,bytheDahleregion would have taken longer to solidify always somewhat broader than the width of the microstructure dendritic. The dendrite coherency solid fraction is less than 29% than the surroundings and thus would have been difficult to feed. related phenomenacontainingsuchfragmentedas solidificationdendrites. Theshrinkageporosity distribandutiontheris - for bothandofArnberthese allogys[5]cooledwhounderinvestigatedthese conditionsa range[18] andof solidifying Al alloys As AZ91 has a long non-equilibrium freezing range ( 166 C) quantified with respect to radial distance from the vane centre in the mush therefore contains a loose network of entangled and ◦ mal contraction. Mush is deformed in processes where some with solid fractions in the range 0 < fs < 0.5.andThecontainsy werelittleableeutecticto( 16%) [1], the last regions∼to solid- Fig. 5a. interlocked dendrites at 29% solid. At this solid fraction, den- ify are particularly prone to∼form porosity [22]. A356, however, solidification occursFig.during4b showsfillinga markedlysuchdifferentas inporosityhigh-pressuredistributiondie-in driteeenxplainvelopes the(the surfchangesace connectingin rheologicalboth the primaryresponseand brought about by contains a significant eutectic fraction ( 50%), and a shorter casting or squeezethe A356castingsample,andwherealsoroundedin processespores arewheredistributedpressurefairly secondarygrainarmsrefinertips ofadditionsa dendrite) areandexpectedcompositionalto contain a sig-changes by consider- non-equilibrium freezing range ( 60 C)∼[23] making interden- evenly in the cross-section. These pores are most likely hydrogen nificant liquid fraction. The remaining liquid exists between the ◦ is applied to assist the feeding of solidification shrinkage after ing their effect on the solid shape and size.dritcLarfeedingger andeasier.moreOther researchers∼ have reported material related as the sample was not degassed. Comparing Fig. 5b with dendrite envelopes. containing higher solute content than the surroundings (posi- filling. Fig. 5a shows that the A356 alloy has a higher porosity content Theirreshapegularlyof theshapedshear stress–timedendritescurveswerein Fig.sho1bwncan to begin to develop tive macrosegregation) at the deformation plane after shear at than the AZ91 alloy, and confirms that there is no increase in be explained by considering how these microstructures deform. Our understanding of the shear rheology of dendritic mush strength at a lower solid fraction and highlylow solidbranchedfraction [7]den-, supporting the idea that enriched liq- porosity at the vane path in A356. The interconnected structure of the loose dendrite network gives at low solid fraction has been obtained mainly from experi- drites form more interconnected, stronger netwuid is orksdrawn atto thea glocallyiven deforming region. Further work is ments using Al–Si or Sn–Pb alloys [2–7]. Arnberg et al. [2] solid fraction. The importance of morphologyrequiredhastobeenconfirmfurtherthe presence of positive macrosegregation Fig. 5. Quantification of the porosity distributions. Area fraction porosity is at the deformation region in the present study. found that distinct changes in shear response occur when theaveragedemphasisedwithin radius contoursby1 mNabm in thickness:ulsi [6](a) AZ91andandSumitomo(b) A356. et al. [7] who showed growing dendrites impinge on one another at what is defined the role played by dendrite arms in providing5. Conclusionsinterconnectiv- Coh the mush resistance to deformation and gives mushes with the dendrite coherency solid fraction, f . This point in the Cohity to the dendritic network through interlocking. Deforming s fs > fs their yield point behaviour [5]. At stresses τ < τpeak, Shear deformation of aluminium alloy A356 and magne- solidification sequence marks the start of the development of anthe dendritea mushnetworkconstituteddeforms whilstof larhighlygely maintainingbranchedan dendritessium alloyrequiresAZ91 at 29%thesolid showed that, in the presence of a interconnecteddendritesstructureto beandfragmentedthe mush thereforegivingbehavsuches in amicrostructuresloose dendrite netwgreaterork, both alloys exhibited distinct yield point solid-like manner where the shear stress increases near-linearly behaviour. The shear stress–time plots of the two alloys were with time.shearWhenstrengthτ = τpeak (Fig.than1), thethosestrengthcontainingof the dendriticmoreveryroundedsimilar. Adendritesshort period of shear deformation at 29% solid network is reached. In the vane experiment, the maximum shear during solidification was observed to lead to distinct changes ∗ Corresponding author. Tel.: +61 7 3365 3639; fax: +61 7 3365 3888. which can deform by grain reorientation alone. Experimental stress is at the vane tips, but a plane of maximum shear stress in the as-cast microstructure. In both A356 and AZ91, post- E-mail address: [email protected] (C.M. Gourlay). can be approximatedstudies onasthethe cshearylindricalbehasurfaceviourcircumscribedof magnesiumby deformationalloy AZ91macrostructureshave revealed that deformation is highly the vane [19]. The mush strength is therefore first exceeded at localised at the vane path, and that dendrite fragmentation occurs 0921-5093/$ – see front matter © 2005 Elsevier B.V. All rights reserved. the vane path and the dendrite network collapses at this surface. at the region of maximum shear stress. In AZ91, there was also a doi:10.1016/j.msea.2005.09.048 As the dendrites making up the network are highly branched concentrated annulus of interdendritic porosity at the vane path at 29% solid, they are entangled and interlocked. Collapsing which was proposed to result from the presence of enriched liq- an entangled network requires some dendrites to be fragmented uid at the deformation region which becomes difficult to feed late A third possibility is the “field boundary hypothesis”, which states that grain size evolves to that which allows the aggregate to deform on the boundary between diffusion and dislocation creep (e.g. deBresser et al., 2001) . This model, at present, makes a rather different prediction but I have not yet included it.

A somewhat less constrained, and very important question is the relationship between stress and depth. For now, I assume stress is a constant low value throughout the lithosphere. In the future, we can approximate a stress change at the base of the conductive layer (thermal boundary layer) and then calculate a resultant grain size distribution. What would be the physical basis for this stress change and how would we guess its magnitude? I need to consider this further, but for this application, I think we should make the simplest assumptions, since the model is most applicable to partially molten asthenospheric parts of the temperature/viscosity proﬁles anyway.

B.2 Adding the eﬀects of melt Question: The model calculations are given as values normalized by the Coble creep viscosity. Can these

normalized values be viewed as perturbations to the total viscosity (i.e. ηφ = ηφ! (η0)) or do we calculate the perturbations as perturbations to the diﬀusion creep part, such as

1 1 1 − ηtotal = (ηdisl)− + (ηφ diff)− (12) − ! " where η = η (η ). In other words, the normalized shear viscosity calculated from the φ diff φ! calc diff model becomes− a factor− by which the flow-law value is reduced. Is this a reasonable first approxi- mation? If so, another question is, does the dislocation creep value remain unaffected, because we do not know the direct effect of contiguity on dislocation creep.

The phenomenological relationship determined from experiments is: η φ = exp( αφ) (13) η0 − Hirth and Kohlstedt (2003) report different values of α for diffusion and dislocation creep. However, I am not confident that these values are so discrete. So, I will plot this equation as a perturbation to the ηtotal with several values of α.

B.3 Effects of water This is not done yet. see discussion in paper 2. A new model for grain-boundaryAs φ = diffusion> 0, creep with melt Elasticity: Takei, 1998; Viscosity: Takei and Holtzman, in prep. Dmelt => Dg.b. - formulated in terms Ag g contact of grain boundary contiguity, ϕ = A− contiguity surface A ϕ = contact (1) - relationship strainA surfacearea between traction and stress macroscopic rate velocity depends on constitutive rate of matter SS n f 1 ij j Overview relations Eij diffusion through l grain boundary, P ni 22 or 23 solved at grain scale (30)* and homogenized The The constitutive relation (elastic): upwards to give a macroscopic stresses macroscopic strain rate of solid framework macroscopic ∂up ∂ui ∂uj constitutive relation τij = λ δij + µ + or τij = λup,p11δij + µ(ui,j + uj,i) at r < R (2) 18 C C ∂xp ∂xAj A 21∂xi (29)* ! " ! " traction 3λ+2µ and λ (P+p)isl then (r R ) Lamn (r Ré ) constant and µ the shear modulus. The bulk modulus k = and Poisson’s ratio i fr i velocity 3 3k 2µ governing equations for ν = − . a grain deformation 6k+2µ B u i(r ) 8 & 13 & 14 (26 & 27 & 28)* microscopic stress microscopic deformation 2 Briiefn a graoutlin ine of theor oyf a grain Fig.1 Here I very briefly outline the paths in Figure 1. from Strain to Stress, because this is parallel to the path we use. I need to work to understand the difference between these paths, and how they differ in their solutions, later. Note that this section has some different notations, taken from YT98, describing the elastic formulation (such as the superscript f refers to ‘framework’ at the top, but later on, i think S refers to the same thing- standing for solid framework. f is ‘face number’ in all other sections). The viscous formulation will follow.

2.1 Path from Strain to Stress (A->B->C, clockwise in Fig. 1)

Path A. Macroscopic Strain to Stress First, the deﬁnition of macroscopic framework strain:

S S f 1 ∂ui ∂uj εij = + (3) 2 # ∂xj ∂xi $ and the local displacement vector at each point on grain-grain contact surface:

R f R C R ui(r ) = εijrj at X (r ) = 1 YT98 eqn(11) (4)

Path C. Grain-scale stress to macroscopic framework stress

1 σS = τ (r)dV (5) ij V ij s %r

surface. τijnj = fi at r = R. From the equilibrium condition (eqn. 6) and this deﬁnition of fi, 1 σS = (f rR + f rR)dS (6) ij 2V i j i i s %r=R

Path B. Grain-scale governing equations The equilibrium condition: ∂τij = 0 at r < R i.e. τij,j = 0 (7) ∂xj

2 A third possibility is the “field boundary hypothesis”, which states that grain size evolves to that which allows the aggregate to deform on the boundary between diffusion and dislocation creep (e.g. deBresser et al., 2001) . This model, at present, makes a rather different prediction but I have not yet included it.

A somewhat less constrained, and very important question is the relationship between stress and depth. For now, I assume stress is a constant low value throughout the lithosphere. In the future, we can approximate a (a)stress1 change at the base of the conductive layer (thermal boundary layer) and then calculate a resultant grain size distribution.semi-eWhatmpirical would be the physical basis for theoretical this stress change and how would0.8we guess its magnitude? I need to consider this further, but for this application, I think we should make the simplest assumptions, since the model is most applicable to partially molten asthenospheric parts of the temperature/viscosity proﬁles anyway. J

, 0.6 y t i u g i t

B.2 Adding the effects ofn melt o 0.4 c Qd20, 14 Question: A=2 d 30, 12 A=2.3 Q The model calculations are given0.2as values normalized by the Coble creep viscosity. Can these normalized values be viewed as perturbations to the total viscosity (i.e. η = η (η )) or do we Qd30, 14 φ φ! 0 calculate the perturbations as perturbations to the diffusion creep part, such as 0 0 0.05 0.1 0.15 0.2 Viscosity as a function of melt fraction 1 m1elt fraction, F 1 − ηtotal = (ηdisl)− + (ηφ diff)− (12) Takei and Holtzman I and− II, in prep., 100 ! " where η = η (η ).(b)In other words, thefinitenormalized melt shear viscosity calculated from the φ diff φ! calc diff model becomes− a factor− by which the flow-laR=5Mwm valuediffusivityis reduced. Is this a reasonable first approxi- c c H mation? If so, another question is, does the disloinfinitecation melt creep value remain unaffected, because we " H

-1 diffusivity , 10 do not know the direct eﬀect of y contiguity on dislocation creep. t i s

o Qd0, 14 c s i v The phenomenological relationship determined from experiments is: r

a L=18 e h s -2 η d φL=23 A=2 e 10 = exp( αφ) d 30, 12 (13) z Q i l η a 0 − A=2.3 m r o Hirth and Kohlstedt (2003) reportn different values of α for diffusion and dislocation creep. However, Qd30, 14 the presenceI am ofnot a connectedconfiden meltt that phasethese values are so discrete. So, I will plot this equation as a perturbation 10-3 dramaticallyto the reducesηtotal diffusionwith sevpatheral values of α0 . 0.05 0.1 0.15 0.2 lengths through grain boundaries. melt fraction, F B.3 Effects of water This is not done yet. see discussionThis inmodelpap helpser 2. resolve large discrepancy between melt-free olivine (Faul and Jackson,Fig.9 As φ => 0, 2007) and San Carlos olivine + MORB. See David Kohlstedt’s talk, Weds. [MR33A-01] Dmelt => Dg.b.

11 effect of homogeneous isotropic and anisotropic melt distribution for three melt L.-A. B. fractions: solidus

" /" ! ! !2 !1 0 0 0.02 0.04 10 10 10 60 60 anisotropic exponential temperature grain-scale isotropic melt 70 70

80 80

90 90 depth, km 100 100

110 110

120 120 Christensen (1987) presented a simplified form for theChristensencase when the(1987)principlepresendirtedectionassimplifiedof the form for the case when the principle directions of the anisotropy layers are parallel to the principle directionsanisotropof the stressy laytensor,ers are parallel to the principle directions of the stress tensor, Effect of segregation, calculated with Backus (1962) Christensen (1987) presenaveragingted a simplified of transverseform for theisotropiccase when mediumthe principle with layersdirection s of the anisotropy layers are parσallelxx to the principleηN 0directionsε˙ofxxthe stress tensor, L.-A. B. of differing= viscosity (Honda, 1986). (15)σxx ηN 0 ε˙xx ! σxy " # 0 ηS $ ! ε˙xy " = (15) solidus σ η 0 ε˙ xx = N xx (15)! σxy " # 0 ηS $ ! ε˙xy " ! σxy " # 0 ηS $ ! ε˙xy " where ηN = ηi ( η is the spatial average of η. Does this mean that the viscosities are weighted ! " ! " 1 1 1 by their volume whereor areaηN fraction= ηi ( η ?is),theandspatialηS a=verage(ηiof)−ηwhere. −Do.esThethiηsNmeananisotrop= thatηi they( factorviscositiesη is sthe=are spatialηweighη−ted . average of η. Does this mean that the viscosities are weighted ! " ! " ! " 1 1 ! "!1 " by their volume or area fraction ? ), and ηS = (ηi)− − . The anisotrop! " y!factor" s = η η− . 1 1 1 b! y their" volume or area fraction! "! " ? ), and ηS = (ηi)− − . The anisotropy factor s = η η− . B.3 Effects of water " /" ! " ! "! " B.3 Effects of water ! ! !2 !1 0 0 0.025 0.05 0.075 0.1 10 10 10 This is not doneThisyet.isseenot donediscussionyet.60 see indiscussionpaperin2.paper 2.B.3 60 Effects of water temperature grain-scale segregated melt melt lenses bands mechanism mechanism A A n nm mQ Q V*V* αα 70GB diffusion(dry)total 1.5e9This1 70is3not375e3done10e-6yet.25see discussion in paper 2. GB diffusion(dry)GBS dislocation(dry)1.5e9 4.7e101 3.53 2375e3600e3 10e-615e-6 3525 GBS dislocation(dry)Dislocation(dry)4.7e10 3.5 3.52 0600e3 15e-6 35 80 80 mechanism A n m Q V* α Dislocation(dry)GB diffusion(wet) 3.5 10 3 GBS dislocation(wet) 3.5 2 GB diffusion(dry) 1.5e9 1 3 375e3 10e-6 25 GB diffusion(wet) 1 3 90Dislocation(wet) 3.5900 GBS dislocation(dry) 4.7e10 3.5 2 600e3 15e-6 35 GBS dislocation(wet) 3.5 2 comprised of 0.3 Dislocation(wdepth, km et) 3.5 0 Dislocation(dry) 3.5 0 volume fraction bands, T100able 1: Flow law parameters, from100Hirth & Kohlstedt (2003). (same total melt GB diffusion(wet) 1 3 As φ => 0, fractions) 110 110 GBS dislocation(wet) 3.5 2 DTmelablet =>1:DgFlo.b. w law parameters, from Hirth & Kohlstedt (2003). Ag g contact Dislocation(wet) 3.5 0 ϕ = − Asurface As φ => 0, 120 120 Dmelt => Dg.b. Ag g contact Table 1: Flow law parameters, from Hirth & Kohlstedt (2003). ϕ = − Asurface As φ => 0, Dmelt => Dg.b. Ag g contact ϕ = − Asurface

12 Future work: Conclusions:

We can now calculate elastic and 1. In experiments, melt aligns under viscous properties from the same melt stress at the grain scale, then distribution, and are developing segregates during deformation, and predictive models for a range of then can migrate up a stress gradient. hypothetical multi-scale melt distributions... 2. In the Earth, such multi-scale viscous and elastic properties as a function processes can cause a signiﬁcant of grain boundary contiguity anisotropy in viscosity and a 0 (a) 10 reduction in effective shear viscosity, ! 1/2 elasticity easily up to two orders of magnitude. µ sk µS

-1 10 3. Such viscosity reduction on k sk kS boundary layers could have signiﬁcant consequences for plate- 10-2 viscosity 2 mantle interactions, and have a XH ! cc distinct seismic signature.

-3 HH 10 cc Normalized viscosity and elasticity 10-4 10-2 10-1 100 contiguity, J

(b) 100

elasticity 14 12 8 µ sk µS 14 12 8 8 10-1

k sk k S 8 12 12 14 14 10-2 viscosity

XHcc

-3 10 HHcc Normalized viscosity and elasticity

10-4 10-2 10-1 100 contiguity, J

Fig. 8 No More HOLTZMAN AND KOHLSTEDT MELT SEGREGATION EVOLUTION

nature of the melt-rich networks. As indicated in the sche- the distribution of melt. The first-order structure is the matic drawing of 3D networks from Holtzman et al. largest bands, which are oriented at a 15^258 to the (2003b), the average length of lenses is greater in the sample shear plane (that is, the grooved piston surface) in plane normal to the shear direction than it is in the flow Fig. 13c. The population of smaller bands, the second-order plane (the usual view). Also, some sample material has structures, is oriented at a 5^158 to the sample shear been extruded laterally (in the shear plane, normal to the plane. However, if one views these structures in a local shear direction) beyond the edges of the piston. It also reference frame of the lenses, such that the shear plane in appears that the melt fraction is higher in and near this the lenses is back-rotated by 108 to the sample shear extruded material, indicating that there may be some flux plane (the piston surface), then the secondary bands are of melt from the middle of the sample towards the edges. oriented at a 15^258 with respect to this secondary shear The visual and statistical signature of the anastomosing plane, illustrated in Fig. 13c. In other words, the secondary networks is a bimodal distribution of band angles, as illus- narrow bands that cut across lenses are controlled by the trated in Fig. 13. In this image, and in all of the samples, local stress field in the lenses. This local rotation of the there are indications of some scale-invariant properties of stress field in the lenses is suggested by the back-rotation of olivine b-planes (Holtzman et al., 2003b) and by the analysis of strain partitioning (Holtzman et al., 2005). Extending this point of view one order downward in scale, the melt pockets most visible in the large bands are also oriented 208 to the wall of the band, the third-order shear plane defined by the surfaces of the melt bands. Thus, from the sample scale to the grain scale, there are three levels of scale-invariance to the orientation of melt alignment relative to the local and applied stress tensors. Istherealowerlimittothemelt fractionrequiredforsegre- gation to occur? The sample with f 0 005 was deformed ¼ Á at moderate stresses (t 122 MPa). Well-defined bands f ¼ formed, as shown in detail in Fig. 14. The bands are narrow and the lenses between them have almost no visible melt. Furthermore, the chromite grains in the lenses appear to be stretched and aligned, forming an apparent foliation. However, where a band is present, the chromite grains are HOLTZMAN AND KOHLSTEDT MELT SEGREGATION EVOlargLUTIerON, fewer and less elongated, as shown in Fig.14.This pattern suggests that chromite grain growth (an Ostwald ripen- ing process) is much more efficient in the presence of melt Fig. 10. Reflected light images samples (a) PI-1027, (b) PI-1025, nature of the melt-rich networks. As indicated in the sche- the distribution of melt. Thethafinrstin-ordiertssatrbsenucture ceis .theThese variations inchromitemorphology and (b)mPatIi-c10dra20w, ingin ofthe3Dstnerestwsorserks fiesrom, shearHoltzmaned aett alc.onslargtaenst baloadnds,, wthop-ich are oriented at a 15^258 to the are not present (or nearly as clearly) in samples with more to-the-ri(gh200t3,b)d, ecreasthe averaingge inlengthappoflielensed los adis gfromreater in(a)theto (csam).plAe shears witplhanteh(ethat is, the grooved piston surface) in previousplastnreainnormalseriteost,hteheshearvertdirecicaltionbtlahacnkitcrais inckstheareflowcauseFig. 1d3cb. They qpuoenpulachtion- of smallermelt,bandsugs, tgeshe sectonding-ordterhat the presence of melt significantly plane (the usual view). Also, some sample material has structures, is oriented at a 5^158 to the sample shear ing the sample at the end of an experimental run and should be en hances chromite grain growth. been extruded laterally (in the shear plane, normal to the plane. However, if one views these structures in a local ignored.shTheear digrorectioovn)e bspeyoacinnd theg isedges250ofmtmhe piandston.tIhet almso eltr-eferricenhcebfandsrame ofarethe lenses, such that the shear plane in the darkaerppeargreys thacthathennelsmelt falracignedtion is haitghaerbouin tand158nearto thheis shearthe lenplsesanise.back-rotated by 108 to the sample shear extruded material, indicating that there may be some flux plane (the piston surface), then the secondary bands are of melt from the middle of the sample towards the edges. oriented at a 15^258 with respect to this secondary shear The visual and statistical signature of the anastomosing plane, illustrated in Fig. 13c. In other words, the secondary networks is a bimodal distribution of band angles, as illus- narrow bands that cut across lenses are controlled by the trated in Fig. 13. In this image, and in all of the samples, local stress field in the lenses. This local rotation of the there are indications of some scale-invariant properties of stress field in the lenses is suggested by the back-rotation of olivine b-planes (Holtzman et al., 2003b) and by the analysis of strain partitioning (Holtzman et al., 2005). Extending this point of view one order downward in constant load scale, the melt pockets most visible in the large bands are also oriented 2constant08 to the wall of the band strain, the third-ord rateer shear plane defined by the surfaces of the melt bands. Thus, from the sample scale to the grain scale, there are three levels of scale-invariance to the orientation of melt alignment relative to the local and applied stress tensors. Istherealowerlimittothemelt fractionrequiredforsegre- gation to occur? The sample with f 0 005 was deformed ¼ Á at moderate stresses (t 122 MPa). Well-defined bands f ¼ formed, as shown in detail in Fig. 14. The bands are narrow and the lenses between them have almost no visible melt. Furthermore, the chromite grains in the lenses appear to be Fig. 11. Reflected light images of samples sheared undstretcerhedconsand taalnignedt di,splformaceming anentappraarente ct ondifoliattionions. (that approximate constant strain rate) of (a) 3 10^4 s^1 and (b) 1 10^3 s^1. However, where a band is present, the chromite grains are Â Â larger, fewer and less elongated, as shown in Fig.14.This pattern suggests that chromite grain growth (an Ostwald ripen- ing process) is much more efficient in the presence of melt Fig. 10. Reflected light images samples (a) PI-1027, (b) PI-1025, than in its absence.These variations inchromitemorphology and (b) PI-1020, in the stress series, sheared at constant load, top- are not present (or nearly as clearly) in samples with more in constantto-the-right, decreasing inloadapplied lo adsamples,from (a) to (c). As w itstressh the is always increasing, previous strain series, the vertical black cracks are caused by quench- melt, suggesting that the presence of melt significantly ing the sample at the end of an experimental run and should be enhances chromite grain growth. so igsamplesnored. The groove sp acincannotg is 250 mm and treachhe melt-rich bands a aresteady state (stress and strain rate constant) the darker grey channels aligned at about 158 to the shear plane. 1012 13 102 ) s ) a a P P ( M (

y 11 t s

i 10 s s e o r c Fig. 11. Reflected lightt images of samples sheared under constant displacement rate conditions (that approximate constant strain rate) of ^4 ^1 ^3 ^1 s S (a) 3 10 s and (b) 1 10 s . i Â Â V

1 10 1010 0 1 13 2 3 4 0 1 2 3 4 Strain Strain HOLTZMAN AND KOHLSTEDT MELT SEGREGATION EVOLUTION Strain rates are limited by lenses in these spatially conﬁned samples

1012 ) s a P (

y 11 t

i 10 s o c s i V

1010 0 1 2 3 Fig.422. The pumping mechanism. (a) A schematic illustration of lenses between two underformable plates. The light grey lines in the lenses are local shear planes and the melt-rich bands are transpare-2 nt. (b) Inset in (a). The open arrows indicate stress orientations and the filled grey Strain arrows indicate melt flow direction. We propose h1ere0and elsewhere (Holtzman et al., 2005) that the normal stress components on the bands switch sign above some critical angle (4308) from tensile or dilational to compressive or compactional, essentially because lenses have to deform more and more the closer a band is to 458, and the system will only tolerate a certain stress difference between bands and lenses. This flattening of the bands causes the melt pressure to rise and sets up a flow into lower angle bands. (c) Inset in (a). Bands can nucleate speculation: and propagate into a lens at a low angle, and can also close off, as shown in Fig. 13. (d) Inset in (c). A grain-scale view of the tip of a propagating band. the constant rate samples can reach ) 1

- -3

s 10 steady state. (

e approximately the same angle, suggesting that this angle t

the organization of strain partitioning a is ubiquitous and is stress controlled (not controlled by

(between low angle and high angle bands) R the physical properties of the matrix). These observations

n suggest that bands form at the same distribution of angles i -4 is optimized to minimize dissipation (or a at which they are observed when mature, by the collection r 10 t and accumulation of individual elongated pockets. viscosity) in steady state. S Hier-Majumder et al. (2004) adopted a micromechanical approach to this question, looking at how shear stress (current experiments in torsion, see King caused the growth of some pockets at the expense of -5 others, and speculated that pockets at 0^458 to s1 open et al, [ref] 10 preferentially by stress corrosion, causing an average angle of2 208. Nucleation of small bands within the well- 10 developed lenses occurs at lower angle (5^108 relative to the shearSpltranese)s, reflec(MtPinga)the local back-rotation of the stress field as a result of strain partitioning between lenses Fig. 23. Band spacing, dsp, as a function of compaction length, dc. and bands. This local modification of stress fields was dis- PI-1020 appears to be an outlier, as discussed in the text. The slope of cussed extensively in an earlier study (Holtzman et al., a linear fit to the rest of the data is m 0 4. ¼ Á 2005). Once elongated pockets are nucleated, the stress- driven segregation instability is initiated. occurring simultaneously in a deforming sample, as illustrated in Fig. 22. Rotation and growth As discussed by Holtzman et al. (2003a, 2005), Spiegelman Nucleation (2003) and Katz et al. (2006), bands should rotate with Why do bands nucleate and at what angle? Our observa- shear if melt is not migrating relative to the solid. The tions from the lowest shear strain experiments in the apparent rotation is caused by the simple shear, but it is strain series suggest that bands begin to form at 5^258, actually the shear parallel advection of a material line with a mean and standard deviation of 18 68. In sheared (melt-rich band) at an angle to the shear plane, causing Æ samples of olivine MORB, Zimmerman et al. (1999) the appearance of rotation. Spiegelman (2003) modified þ found that melt pockets were elongated and oriented at Stevenson’s (1989) instability analysis from a pure shear to

21 40 My

L.-A. B.

solidus

T: conductive+adiabat Stress: constant grain size: fcn of stress viscosity: combined mechanisms up to peak ?

J. Warren and G Hirth: 300 MPa stress in mylonites max viscosity = 300 MPa = 3e21 Pa.s? 1e-13 /s radial section r

tangent section

Blackwell Publishing AsiaMelbourne, AustraliaIARIsland Arc1038-48712006 Blackwell Publishing Asia Pty LtdMarch 200615123Pictorial ArticleRecycled crustal materials in the mantleT. Morishita et al.

Island Arc (2006) 15, 2–3

Pictorial Article Corundum-bearing maﬁc granulites in the Horoman (Japan) and Ronda (Spain) Peridotite Massifs: Possible remnants of recycled crustal materials in the mantle

1, 2 1 3 Blackwell Publishing AsiaMelbourne, AustraliaIARIsland Arc1038-48712006 Blackwell Publishing Asia Pty LtdMarch 200615123Pictorial ArticleRecycled crustal materials in the mantleT. Morishita TOMOAKI MORISHITA, * EIICHI TAKAZAWA, SHOJI ARAI, MASAAKI OBATA, et al. TADAHIRO KODERA1,† AND FERNANDO GERVILLA4 1 Island Arc (2006) 15, 2–3 Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: [email protected]), 2Department of Geology, Faculty of Science, Niigata University, Niigata 950- 2181, Japan, 3Division of Earth & Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan and 4Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada-CSIC, 18002 Granada, Spain Pictorial Article Corundum-bearing mafic granulites in the Horoman (Japan) and Corundum-bearing mafic granulites (i.e. rocks of high-Al mafic Atsushi Toramaru for their discussions. T. Morishita thanks Ronda (Spain) Peridotite Massifs:compositions) Possible occurremnants as a minor of constituent in several orogenic Takashi Sawaguchi for his assistance in collecting samples. Con- recycled crustal materialsperidotite in the mantlemassifs of upper mantle origin, for example, Beni structive reviews by Masaki Enami and Simon Wallis improved Bousera (Morocco; Kornprobst et al. 1990), Ronda (Spain; Mori- the manuscript. shita et al. 2001) and Horoman (Japan; Morishita & Arai 2001). TOMOAKI MORISHITA,1,* EIICHI TAKAZAWA,2 SCorHOJIundum-bearing ARAI,1 MASAAKI eclogite OBATA xenoliths,3 are also rarely found in kim- TADAHIRO KODERA1,† AND FERNANDOberlite G pipesERVILLA (e.g. Sobolev4 et al. 1968). Thus, a minor but distinctive REFERENCES 1Graduate School of Natural Science and Technology, Kanazawa Universityhigh-Al geochemical, Kanazawa reser920-1192,voir Japanmay exist(email: in the upper mantle. 2 These rocks generally show geochemical signatures similar to gab- [email protected]), Department of Geology, Faculty of Science, Niigata University, Niigata 950- KORNPROBST J., PIBOULE M., RODEN M. & TABIT A. 1990. Corundum- 3 broic rocks of lower crustal origin. From these lines of evidence, 2181, Japan, Division of Earth & Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, bearing garnet clinopyroxenites at Beni Bousera (Morocco): Original Japan and 4Instituto Andaluz de Ciencias de la Tierra, Universidadcorundum-bearing de Granada-CSIC, mafic 18002 granulites/eclogites Granada, Spain are interpreted to plagioclase-rich gabbros recrystallized at depth within the mantle? be possible remnants of recycled crustal materials in the mantle. Journal of Petrology 31, 717–45. The present paper shows the occurrence of corundum-bearing MORISHITA T. & ARAI S. 2001. Petrogenesis of corundum-bearing mafic Corundum-bearing mafic granulites (i.e. rocks of high-Al mafic Atsushi (andToramar associatedu for their cor discussions.undum-free) T. Morishita mafic granulites thanks in the Horoman rock in the Horoman Peridotite Complex, Japan. Journal of Petrology compositions) occur as a minor constituent in several orogenic Takashi(F Sawaguchiigs 1,2) forand his Rondaassistance (Fin igscollecting 3,4) Massifssamples. Con-so as to provide good 42, 1279–99. peridotite massifs of upper mantle origin, for example, Beni structiveexamples reviews by of Masaki heterogeneous Enami and Simon mantle Wallis for improvedmed by mixing of recycled MORISHITA T., ARAI S. & GERVILLA F. 2001. High-pressure aluminous Bousera (Morocco; Kornprobst et al. 1990), Ronda (Spain; Mori- the manuscript.crust materials. mafic rocks from the Ronda peridotite massif, southern Spain: Signifi- shita et al. 2001) and Horoman (Japan; Morishita & Arai 2001). cance of sapphirine- and corundum-bearing mineral assemblages. Corundum-bearing eclogite xenoliths are also rarely found in kim- Lithos 57, 143–61. berlite pipes (e.g. Sobolev et al. 1968). Thus, a minor but distinctive REFERENCES MORISHITA T., ARAI S., GERVILLA F. & GREEN D. H. 2003. Closed-system high-Al geochemical reservoir may exist in the upper mantle. ACKNOWLEDGEMENTS geochemical recycling of crustal materials in the upper mantle. These rocks generally showCauses geochemical signatures of similar anisotropy to gab- broic rocks of lower crustal origin. From these lines of evidence, KORNPROBST J., PIBOULE M., RODEN M. & TABIT A. 1990. Corundum- Geochimica et Cosmochimica Acta 67, 303–10. bearing garnet clinopyroxenites at Beni Bousera (Morocco): Original We are grateful to the Board of Education of Samani Town for OBOLEV UZNETSOVA YUZIN corundum-bearing mafic granulites/eclogites are interpreted to plagioclase-rich gabbros recrystallized at depth within the mantle? S N. V., K J. I. K. & Z N. I. 1968. The petrology be possible remnants of recycled2) Layered crustal materials indistribution the mantle. Jour nalofper of mitting Psolidetrology 31us, 717–45.phases to use the of‘Apoi-dake different Shien Center isotropic’ (research or anisotropicof grospydite xenoliths properties from the Zagadochnaya kimberlite pipe in The present paper shows the occurrence of corundum-bearing MORISHITAsupport T. & A RAIcenter S. 2001. for Petrogenesis young scientists),of corundum-bearing and tomafic Akira Ishiwatari and Yakutia. Journal of Petrology 9, 253–80. (and associated corundum-free) mafic granulites in the Horoman rock in the Horoman Peridotite Complex, Japan. Journal of Petrology (Figs 1,2) and Ronda (Figs 3,4) Massifs so as to provide good 42, 1279–99. examples of heterogeneous mantle formed by mixing of recycled MORISHITA T., ARAI S. & GERVILLA F. 2001. High-pressure aluminous crust materials. mafic rocks from the Ronda peridotite massif, southern Spain: Signifi- cance of sapphirine- and corundum-bearing mineral assemblages. Lithos 57, 143–61. MORISHITA T., ARAI S., GERVILLA F. & GREEN D. H. 2003. Closed-system ACKNOWLEDGEMENTS geochemical recycling of crustal materials in the upper mantle. Geochimica et Cosmochimica Acta 67, 303–10. We are grateful to the Board of Education of Samani Town for SOBOLEV N. V., KUZNETSOVA J. I. K. & ZYUZIN N. I. 1968. The petrology permitting us to use the ‘Apoi-dake Shien Center’ (research of grospydite xenoliths from the Zagadochnaya kimberlite pipe in Fig. 1 Occurrence of aluminous mafic support center for young scientists), and to Akira Ishiwatari and Yakutia. Journal of Petrology 9, 253–80. granulites (M, corundum-free) associated with peridotites (P) in the Horoman Massif. They usually occur as thin layers (1 cm−2 m thick) alternating with the peridotite layers (P, eroded part). In this Fig. 1 Occurrence of aluminous mafic outcrop, aluminous mafic granulite is granulites (M, corundum-free) associ- more abundant than peridotite. Alumi- ated with peridotites (P) in the Horoman nous mafic granulite layers occur parallel Massif. They usually occur as thin layers to the foliation of the peridotites in the (1 cm − 2 m thick) alternating with the Blackwell Publishing AsiaMelbourne, AustraliaIARIsland Arc1038-48712006 Blackwell Publishing Asia Pty LtdMarch 200615123Pictorial ArticleRecycled crustal materials in the mantleT. Morishita et al. peridotite layers (P, eroded part). In this upper and lower part of the outcrop. outcrop, aluminous mafic granulite is Some layers show isoclinal folding Island Arc (2006) 15, 2–3 more abundant than peridotite. Alumi- (middle part of the outcrop) as well as nous mafic granulite layers occur parallel boudinage and slump-like structures to the foliation of the peridotites in the upper and Pictoriallower part Article of the outcrop. indicating strong deformation. Corundum-bearingSome mafic layers granulites show isoclinal in the folding Horoman (Japan) and *Correspondence.Ronda (Spain)(middle Peridotite part of the Massifs: outcrop) Possibleas well as remnants of boudinage and slump-like structures †Present address:recycled TKindicating service, crustal Hakusan strong materials deformation. 924-0820, Japan.in the mantle

Island Arc Recycled crustal materials in the mantleT. Morishita Received 15 November 2005;1,Blackwell accepted Publishing AsiaMelbour for publicationne, A2ustraliaIAR 02 December1038-487120061 Blackwell2005. Publishing3 Asia Pty LtdMarch 200615123Pictorial Article TOMOAKI MORISHITA, et* al. EIICHI TAKAZAWA, SHOJI ARAI, MASAAKI OBATA, *Correspondence. TADAHIRO KODERA1,† AND FERNANDO GERVILLA4 † Present address: TK service, Hakusan 924-0820, Japan. 1Graduate© School 2006 of NaturalThe Authors Science and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: doi:10.1111/j.1440-1738.2006.00517.x Received 15 November 2005; accepted for publication 02 December 2005. [email protected] compilation), 2©IslandDepartment 2006 Arc Blackwell of Geology (2006), F aculty15,Publishing 2 of– Science,3 Asia Niigata Pty University Ltd , Niigata 950- 2181, Japan, 3Division of Earth & Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, © 2006 The Authors Japan and 4Instituto Andaluz de Cienciasdoi:10.1111/j.1440-1738.2006.00517.x de la Tierra, Universidad de Granada-CSIC, 18002 Granada, Spain Journal compilation © 2006 Blackwell Publishing Asia Pty Ltd Corundum-bearing mafic granulites (i.e. rocks of high-Al mafic Atsushi Toramaru for their discussions. T. Morishita thanks compositions) occur as a minor constituent in several orogenic Takashi Sawaguchi for his assistance in collecting samples. Con- peridotite massifs of upper mantle origin, for example, Beni structive reviews by Masaki Enami and Simon Wallis improved Bousera (Morocco; Kornprobst et al. 1990), Ronda (Spain; Mori- the manuscript. shita et al. 2001) and Horoman (Japan; Morishita & Arai 2001). Pictorial Article Corundum-bearing eclogite xenoliths are also rarely found in kimberlite pipes (e.g. Sobolev et al. 1968). Thus, a minor but distinctive REFERENCES high-Al geochemical reservoir may exist in the upper mantle.Corundum-bearing mafic granulites in the Horoman (Japan) and These rocks generally show geochemical signatures similar to gab- broic rocks of lower crustal origin. From these lines of evidence, KORNPROBST J., PIBOULE M., RODEN M. & TABIT A. 1990. Corundum- bearing garnet clinopyroxenites at Beni Bousera (Morocco): Original corundum-bearing mafic granulites/eclogites are interpreted to Rondaplagioclase-rich gabbros(Spain) recrystallized at depthPeridotite within the mantle? Massifs: Possible remnants of be possible remnants of recycled crustal materials in the mantle. Journal of Petrology 31, 717–45. The present paper shows the occurrence of corundum-bearing MORISHITA T. & ARAI S. 2001. Petrogenesis of corundum-bearing mafic (and associated corundum-free) mafic granulites in the Horoman rock in the Horoman Peridotite Complex, Japan. Journal of Petrology (Figs 1,2) and Ronda (Figs 3,4) Massifs so as to provide good 42, 1279–99. recycled crustal materials in the mantle examples of heterogeneous mantle formed by mixing of recycled MORISHITA T., ARAI S. & GERVILLA F. 2001. High-pressure aluminous crust materials. mafic rocks from the Ronda peridotite massif, southern Spain: Signifi- cance of sapphirine- and corundum-bearing mineral assemblages. Lithos 57, 143–61. 1, 2 1 3 TMORISHITAOMOAKI T., ARAI S., M GERVILLAORISHITA F. & GREEN D., H. 2003.* E Closed-systemIICHI TAKAZAWA, SHOJI ARAI, MASAAKI OBATA, ACKNOWLEDGEMENTS geochemical recycling of crustal materials in the upper mantle. Geochimica et Cosmochimica Acta 67, 303–10. 1,† 4 We are grateful to the Board of Education of Samani Town for SOBOLEV N. V., KUZNETSOVA J.T I. ADAHIROK. & ZYUZIN N. I. 1968. K TheODERA petrology AND FERNANDO GERVILLA permitting us to use the ‘Apoi-dake Shien Center’ (research of grospydite xenoliths from the Zagadochnaya kimberlite pipe in support center for young scientists), and to Akira1Graduate Ishiwatari and SchoolYakutia. of Jour Naturalnal of Petrology 9Science, 253–80. and Technology, Kanazawa University, Kanazawa 920-1192, Japan (email: [email protected]), 2Department of Geology, Faculty of Science, Niigata University, Niigata 950- 2181, Japan, 3Division of Earth & Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan and 4Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada-CSIC, 18002 Granada, Spain Fig. 1 Occurrence of aluminous mafic granulites (M, corundum-free) associated with peridotites (P) in the Horoman Massif. They usually occur as thin layers Corundum-bearing mafic granulites(1 cm(i.e.−2 m rocksthick) alternating of high-Al with the mafic Atsushi Toramaru for their discussions. T. Morishita thanks peridotite layers (P, eroded part). In this compositions) occur as a minor constituentoutcrop, aluminous in maficseveral granulite orogenic is Takashi Sawaguchi for his assistance in collecting samples. Con- peridotite massifs of upper mantlemore origin,abundant thanfor peridotite. example, Alumi- Beni structive reviews by Masaki Enami and Simon Wallis improved Bousera (Morocco; Kornprobst et al.nous 1990), mafic granulite Ronda layers occur (Spain; parallel Mori- to the foliation of the peridotites in the the manuscript. shita et al. 2001) and Horoman (Japan;upper and Morishita lower part of the& outcrop.Arai 2001). Corundum-bearing eclogite xenolithsSome are layers also show rarely isoclinal found folding in kimberlite pipes (e.g. Sobolev et al. 1968).(middle Thus, part ofa theminor outcrop) but as well distinctive as boudinage and slump-like structures REFERENCES high-Al geochemical reservoir mayindicating exist strong in deformation.the upper mantle. These rocks generally show geochemical signatures similar to gab- *Correspondence. broic rocks of lower crustal origin. From these lines of evidence, KORNPROBST J., PIBOULE M., RODEN M. & TABIT A. 1990. Corundum- †Present address: TK service, Hakusan 924-0820, Japan. bearing garnet clinopyroxenites at Beni Bousera (Morocco): Original Received 15 November 2005; accepted for publication 02 Decembercor 2005.undum-bearing mafic granulites/eclogites are interpreted to plagioclase-rich gabbros recrystallized at depth within the mantle? © 2006 The Authors be possible remnants of recycled crustaldoi:10.1111/j.1440-1738.2006.00517.x materials in the mantle. Journal of Petrology 31, 717–45. Journal compilation © 2006 Blackwell Publishing AsiaThe Pty Ltdpresent paper shows the occurrence of corundum-bearing MORISHITA T. & ARAI S. 2001. Petrogenesis of corundum-bearing mafic (and associated corundum-free) mafic granulites in the Horoman rock in the Horoman Peridotite Complex, Japan. Journal of Petrology (Figs 1,2) and Ronda (Figs 3,4) Massifs so as to provide good 42, 1279–99. examples of heterogeneous mantle formed by mixing of recycled MORISHITA T., ARAI S. & GERVILLA F. 2001. High-pressure aluminous crust materials. mafic rocks from the Ronda peridotite massif, southern Spain: Signifi- cance of sapphirine- and corundum-bearing mineral assemblages. Lithos 57, 143–61. MORISHITA T., ARAI S., GERVILLA F. & GREEN D. H. 2003. Closed-system ACKNOWLEDGEMENTS geochemical recycling of crustal materials in the upper mantle. Geochimica et Cosmochimica Acta 67, 303–10. We are grateful to the Board of Education of Samani Town for SOBOLEV N. V., KUZNETSOVA J. I. K. & ZYUZIN N. I. 1968. The petrology permitting us to use the ‘Apoi-dake Shien Center’ (research of grospydite xenoliths from the Zagadochnaya kimberlite pipe in support center for young scientists), and to Akira Ishiwatari and Yakutia. Journal of Petrology 9, 253–80.

Fig. 1 Occurrence of aluminous mafic granulites (M, corundum-free) associated with peridotites (P) in the Horoman Massif. They usually occur as thin layers (1 cm−2 m thick) alternating with the peridotite layers (P, eroded part). In this outcrop, aluminous mafic granulite is more abundant than peridotite. Alumi- nous mafic granulite layers occur parallel to the foliation of the peridotites in the upper and lower part of the outcrop. Some layers show isoclinal folding (middle part of the outcrop) as well as boudinage and slump-like structures indicating strong deformation.

*Correspondence. †Present address: TK service, Hakusan 924-0820, Japan. Received 15 November 2005; accepted for publication 02 December 2005. © 2006 The Authors doi:10.1111/j.1440-1738.2006.00517.x Journal compilation © 2006 Blackwell Publishing Asia Pty Ltd L*+0.$(DW(R36$(4./%'*3&(34(H3(7$.202(5*4402*3& 5*2'/&%$('3(')$(BXIY2(-39$.(43.(H/!53-$5= G%!53-$5=(/&5(0&53-$5(2/,-6$2(34(H3(/&5 R+(36*7*&$2J((N&/6:2*2(8/2$5(3&(UVJ(I *&5*%/'$2(')/'(*&($/%)(%/2$(+./*&(5*4402*3& 3%%0..$5(*&(')$(.$+*,$(5$2%.*8$5(8:(':-$(T O*&$'*%2J((P3'$(')/'(')$(263-$(5$%.$/2$2 <+./*&(830&5/.:(5*4402*7*':(*&%.$/2$2E(*&(')$ 3.5$.(H/!53-$5=(0&53-$5=(G%!53-$5

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L*+0.$(MW(G$,*!63+(-63'(34(7*2%32*':(/2(/ 40&%'*3&(34(,$6'(4./%'*3&(43.(2/,-6$2(-.$-/.$5 4.3,(236!+$6(-395$.2(??@E /&5(2/,-6$2(4/8.*%/'$5(4.3,(-395$.$5(%.:2'/62 34(G/&(H/.632(36*7*&$(??DEJ((P3'$(')$ 6/.+$(5$%.$/2$(*&(7*2%32*':(8$'9$$&(')$()*+)! -0.*':=(,$6'!4.$$(2/,-6$(/&5(')$(*,-0.$(2/,-6$ 9*')((?J>Z(2/,-6$J((N(O$:(V0$2'*3&W(“Is this large decrease in viscosity due entirely to the addition of melt or does the associated change in grain boundary chemistry also play a role?”

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