Aug 10, 1998 - Laboratoire de Tectonophysique, Centre National de Recherche ... Montpe!!ier, France ... Kohlstedt and Goetze, 1974; Post, 1977; Chopra and Paterson,. 1981 ..... near constant flow stress was observed in only tour tests, we.
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 103, NO. B8, PAGES 18,205-18,221, AUGUST 10, 1998
A joint study of experimentaldeformationand experimentally induced microstructuresof pretextured peridotites Anke S. Wendt and David Mainprice Laboratoire de Tectonophysique, CentreNationalde Recherche Scientifique,ISTEEM, Universit6Montpe!!ierII, Montpe!!ier,France
Ernest Rutter Departmentof Earth Sciences,The Universityof Manchester,Manchester,England
Richard
Wirth
GeoForschungsZentrum, Potsdmn,Ger•nany
Abstract. A seriesof deformationtestswasperformedon samplesfi'omnaturalxenolithsand duniteswith well-defined texturesand microstructures. The rock sampleswere deformed perpendicular to the foliation,45ø to the foliation,andparallelto the foliationat temperatures
fi'om1000øCto 1200øC,at a confiningpressure of 300 MPa, anda constant strainrateof I 0-'S/s to obtaininsightinto the rheologicalbehaviorof anisotropiclithosphericmantlematerial.The experimentsshowedstrongwork hardeningfor all sampleswith a largeincreaseof the flow stress at relatively low strain.In all samples,bothbrittle andplasticmechanismswere operative.We observed nonlocalized-dilatant
and 1ocalized-dilatant
deformation
t•atures. Nonlocalized
and
localizedplasticdeformationcreatedsubgrainboundaries,deformationlamellae,and an increase in dislocationdensityin the olivine crystals.Heterogeneousdislocationactivitiesoccurredon the [ 100] (Okl)systemat 1000øC,and on the [ 100] (010) and [ 101] (010) systemsat 1200øC.Strong crystallographic fabricsdevelopedin sampleswith olivinesthat were initially orientedwith low resolvedshearstresseson their slip systems.The postdetbrmation fabric intensitywas lower in specimens with stronginitial petrofabrics.Interactionsbetweendislocations of differenttype led to highwork hardeningratesduringhigh-temperature deformation,favoringintragranular microcracking.Fracturesopenedparallelto the [100] screwdislocationsin the (010) plane, and in the { hk0} planesobliqueto the (100) plane.The strengthof the deformedsampleswas temperatureand orientationdependent.Rocksdeformedperpendicular to the foliationare strongest,andthosedeformedparallelto the foliationhaveintermediaterock strengthsimilarto thosedeformedat 45ø to the foliation.Basedon the laboratoryresults,we find important implicationsfor the rheologyof the lithosphere: (1) Peridotitesdevelopsemibrittlerheologyeven at hightemperature,andshowcharacteristic microstructures andfabricchanges.(2) The orientationof compressivestressandinitial petrofabricare importantparametersfor defininga deformationregimeif pressure,temperature,andstrainrate are fixed. (3) Interactionof slip systemsproducesstrongwork hardeningandintragranularmicrocracking. 1. Introduction
deformation
The experimental study of the mechanicalpropertiesof rocks is fundamental to the understandingof the rheological behavior of the upper mantle. Quantitative constraints for the creep strengthof the uppermostpart of the mantle have been obtained from experimentaldeformationstudiesof olivine-richnaturaland syntheticrocks [Carter and A v• Lallemant, 1970; Blacic, 1972; Kohlstedt and Goetze, 1974; Post, 1977; Chopra and Paterson, 1981, 1984;Zeuch and Green, 1984; Chopra, 1986;Mackwell et al., 1985; Cooper and Kohlstedt, 1986; Karato eta!., 1986; Hitching et al., 1989; Botch and Green, 1987; Beeman and Kohlstedt,1993]. With increasingdepth,and concurrentincreases
in pressureand temperature, rocks undergo a transition in
Copyright1998by the A•nericanGeophysicalUnion
mechanism
from
localized
brittle
fracture
to
nonlocalizedplastic flow. The transition from brittle fracture to plastic flow is important in understandingthe strength of the lithosphere,the related style of deformation and seismicsource mechanics.
Macroscopicfailure can be categorizedas brittle, fully plastic or semibrittle [Paterson, 1978; Carter and Kirby, 1978; Rutter, 1986]. During brittle fracture rock strength shows little
temperature andstrainratesensitivity.Rocksfailingby thismode dilate during deformation and show strain softening and localization.During plasticflow, strengthis pressureindependent but highly strain rate and temperaturedependent.Deformation may be homogeneous or localized. The most important deformation mechanismsduring brittle failure are cataclastic processes, includingmicrofracturing andfrictionalsliding.Plastic flow involves crystalline plasticity, twinning or diffusion. Semibrittle flow involves distributed, dilatant deformation by
Papernmnber98JB01555.
crystal plasticity and microcracking;the strengthis pressure
0148-0227/98/98JB-01555509.00
sensitive[Evans eta!., 1990].
18,205
18,206
WENDTET AL.: SEMIBRITTLEDEFORMATIONOFPERIDOTITES
Since both plasticand cataclasticdeformationmechanismsare involved during semibrittleflow, the transitionfrom brittle failure to semibrittle flow, and from semibrittle flow to fully plastic flow, may depend on rock composition,pressure,temperature, strainrate, grain size, and the presenceof a fluid phase.To obtain insighton the influence of theseparametersand on the semibrittle flow of rocks under shallow upper mantle conditions, we concentratein this paperon the semibrittledeformationof upper mantle rocks (pretextured spinel-lherzolite xenoliths and finegraineddunires).We presenthigh-temperatureaxial compression tests in different
directions
to the rocks foliation
and extended
microstructuralanalysesof the olivine crystalsin undeformedand deformed samples.
Three (VP2, BG2, NS63) of the four samplesare characterizedby a planar or isotropic distribution of their [100] and [001] axes. Only one sample (VN5) shows a linear axes distribution. The axes distribution of [100] and [001] was deduced from the low
ratiosof densitiesof the [100]/[010] and [010]/[001] axes(Figure I).
We
calculated
the absolute
values
of the individual
[100]/[010] and [010]/[001] axes density ratios (obtained fi'om calculation of pole figures on Figure 6) for each sample [comparedto the Flinn's diagram, see also Ben lsmail, 1995]. Three of our four samples,one xenolith and both dunires,have an [ 100]/[010] versus [010]/[001 ] axes density ratio close to or less than I
(0.28,
0.8,
and !.02
for VP2,
BG2,
and NS63,
respectively) and therefore have the planar or isotropic distribution of their [1001 and [001] axes as mentioned above.
2. Starting Material All (four) rocks are recrysta!lizedporphyroclasticperidotites (two spinei-lherzolitesand two dunites)with a foliation defined by tracesof chromite.Olivine is the major mineral,and elongated olivine crystals are parallel to the iineation and lie flat in the foliation plane. The grains typically are equant and have long, straight grain boundaries(see Figures 5a to5d). The chemical compositionof the olivine grainsin all samplescorrespondsto a Fo,,0.Wavy or undulousextinction appearsrarely and only in the porphyroclasts. Orthopyroxenes in the rock samples have enstatitecomposition(En,,•).Their grain sizesvary from 300 gm to 3ram in the xenoliths and from 2401Ltmto 3.2ram in the dunires.The big grains are nearly free of internaldeformationin the xenoliths,and they showcloselyspacedsubboundaries (BG2) and stronglysuturedgrain boundaries(NS63) in the dunires. 2.1. Specimen Petrology The mineralogical compositionof our samplescorrespondsto a spinel-iherzolite for both xenoliths (VN5, VP2) (Victoria, Australia) (see Figures 5a and 5b), and to a dunire for the two other rocks (BG2, Webster, North Carolina, and NS63, Norway, (see Figures5c and 5d).) Both xenoliths show a modal composition correspondingto 75% olivine, 20% orthopyroxeneand 5% chromite. The olivine grain sizes of the xenoliths vary from 1.2 to 2.8 rnm for the porphyroclastsand between0.2 and 0.4 mm tbr the recrystailized grains. The xenolith samples exhibit initial intragranular microcracks.
The dunire BG2 has a modal compositionof 93% olivine, 5% chromite, 1% orthopyroxene and 1% clinopyroxene. it is finegrainedwith a bimodal grain size distribution.Olivine grain sizes are less than 3 mm for the porphyroclastsand around 0.25 mm for the recrystailized grains. The BG2 sample is free of initial microcracks.The dunire NS63 has a modal compositionof 91% olivine, 5% chromite, 2% orthopyroxene,i% clinopyroxeneand 1% white mica. It showsflattenedgrainswith grain sizesaround !.6 mm for the olivine porphyroclasts and 0.3 rnm for the recrysta!iizedolivine grains.Only a few subgrainboundariesand dislocationsubstructuresin the olivine grainswere observed(see Figures 5a to 5d). We observed few randomly oriented intragranularcracksthroughoutthisduniresample.
Only the secondxenolith sample(VN5) showsa [1001/[010] axes ratio larger than the 10101/[0011ratio (1.6), revealing a linear distribution of its [100] and [001] axes to the foliation. The [100]
axesin the four samplesare orientedin girdle-like patternsclose to the foliation plane and define the slip direction in the (010) foliation plane. The [001] axes are concentratedin strongcentral point maxima in the foliation plane. The [010] axesare oriented about 90 ø to the foliation plane (see Figure 6). These crystallographicorientationsprobably arise from slip on [1001 (01()) which is the dominant slip system for high-temperature deformationof peridotire[e.g. Nicolas and Poirier, 1976]. The intensityof the fabrics is describedby the dimensionless texture index J that quantifies the sharpnessof the texture; J = 1 corresponds to a randomdistributionof the crystallographic axes, .I = oo corresponds to the maximum correlation of the crystallographicaxes lBunge, 1982]. We calculatedthe index .I from the spherical harmonic coefficients of the orientation distributionfunction with a truncationat order 22 resultingin a J value of 250 for a single crystal. The mathematicalprocedureis described in length by Mainprice and Silver[1993]. In our samplesthe initial fabric strengthJ is 4.8 for the spinel-iherzolite VN5 describinga weak fabric, 5.8 for the dunireBG2, 6.4 for the dunire NS63 describing an intermediate fabric strength,and 11 for the spinei-iherzoliteVP2 describinga relativelystrongfabric.
2.3. SpecimenWater Content Thermogravimetric analyses were obtained using a thcrmobalancemodel STANTON. Thermogravimetric analyses were performed on all four rock samplesusing a quantity of about 30 g of solid rock for the heat treatment.Heating in the thermobalancefrom 0øC up to 1000øCrevealeda continuousloss of volatiles that correspondsmostly to intragranularwater. The mass loss was lower than 0.5 wt% (560 ppm) for both dunite
specimensand one of the xenoliths(VN5); the secondxenolith showeda slightly higher masslossof 0.6 wt% correspondingto 671 ppm water. The valuesobtainedare in goodagreementto the lhermogravimetricanalysesof Chopra and Paterson [1984] of
their,&helm dunitesamples. Theyfounda change in weightof 0.55 wt% during heating that they attributedto a loss of initial water.
3. Experiments 2.2. Olivine
Fabrics
The samples have slightly differing initial crystallographic fabricsas shownby the preferredorientationof olivine relativeto the foliation plane.We did not seedifferencesbetweenthe fabric of the olivine porphyroclasts and the recrystallizedolivine grains.
All experiments were performed in one of three Paterson deformation gas rigs, either at the Laboratoire de Tectonophysique in Montpellier, at the Rock Mechanics Laboratoryof Manchesteror at the Rock MechanicsLaboratory at MassachusettsInstitute of Technology (MIT) in Boston.
WENDT ET AL.: SEMIBRITTLE DEFORMATION OF PERIDOTITES
2 -
18,207
Axes distribution ratios !i
//••3t
•
/•BG2, initial
• VP2,initial
initial 1
Figure 1. Axesdistribution described by thedensityratiosof the [ 100]/[010]and1010]/[001 ] crystallographic axes of olivine (modified Flinn diagram).
Experimentalconditionswere 300 MPa for the confining measurementswere carried out using a JEOL-200CX instrument pressure, 1000øCto 1200øCfor thetemperature and10-•/sl.orthe at 120 kV in Montpellier and a PHILIPS-CM-200 machinewith strainrate. We usedfor all experimentsthe sameinternalfurnace
twin lens at 200 kV at GeoforschungsZentrum in Potsdam.Initial
(number18) andassemblage setupsto ensurethe comparability grain sizes and the change of grain sizes after det.ormationwere of the experimental data. Temperatureswere calibrated to a gradientof IøC over a lengthof 5 cm aroundthe specimen. The deformationapparatuses andthe procedures usedwere similarto thosereportedby Chopra and Paterson[1981, 1984], Chopra I! 986], Karato et al. [1986] and Hitchings et al. [1989]. The
differences in stiffness of thethreemachines arelow (Montpeilier 81 kN/mm, Manchester 65 kN/mm, MIT 75 kN/mm.) The specific apparatusdistortionof each machinewas usedto reduce
the data, which ensuresthe comparabilityof the experimental datasets.We performedexperimentalaxial compression testsat temperaturesof 1000øCto 1200øC.Pressurizingand heating followed rates of 8 MPa/min and 15øC/min, respectively. Deformationstartedimmediatelyafterthe temperature stabilized to avoid annealingof the initial microstructures. The specimens were deformed without adding any water but were stored at I I 0øC and atmospherepressurebeforedet.ormation.Differential stresswas correctedfor the changein samplecrosssectionwith increasingstrain by assumingconstantvolume; data were also correctedfor the load supportedby the ironjacket andthe known
apparatus distortions. The oxygenfugacityof thesamples wasnot bufferedexternallyandwasprobablysetby the ironjacket at FeFeO.
Microstructural analyses were performed by optical
determinedby usingthe linear interceptmethodon graphsof thin sections measuring between 150 and 200 grains of any undet.ormedand det.ormedspecimen.
3.1. Sample Preparation After identifying the foliation plane of the peridotites, we impregnatedthe rocksin hot liquid surgicalpurity paraffin (ErstP.57, Merck, solidification point 60øC) at 200øC for at least 24 hours. This was usually long enoughto impregnatethe first severalcentimetersof the rock samplesto ensureeasydiamond coring. Preheated (at 100øC) rock samples were easier to
impregnatethan cold ones becausethe wax did not solidify around the cold rock surface. Wax in the drilled and flattened
sample cores was removed over 14 days in hot distilled water
bathschangedevery 12 hours.Drying of the samplecorestook 12 hoursat 200øC. Microscopicinspectionrevealedno tracesof wax after this procedure.The cylindricalspecimens,which were 1()mm in diameterand 19 to 21 mm in length,were preparedby diamondcoringof thenaturalperidotites. Their endswereground and lappedsothat they were flat and parallel. The sampleplus an aluminumoxide spacerat eachend (total
microscopy, universal stage (U-stage) measurements, and transmissionelectronmicroscopy(TEM). The water contentof
length of sample plus two spacersis 26 ram) were sealedinside long-formedjacketsof iron with a short,innertubularironjacket
the rocks was measured by thermogravimetry. U-stage measurementswere performed using an optical petrologicai microscopeequipped with a five-axes universal stage. This provided a three-dimensional representation of the fabric anisotropy of the whole rock specimen. Fabric variations throughoutthe samerock sampleare generallylow for xenoliths and dunires [e.g. Boudier and Nicolas, 1995]. TEM
aroundsampleand spacers.We useda simplestandardsample setupwith a solidspacerat the bottomanda spacerwith a central hole (diameter i mm) at the top of the specimen.Extrusionof the samplematerial or a reactionrim betweenspacerand sample were not observedfor any sample.The inner tube was addedto the standard assembly to avoid jacket puncture during pressurization.
WENDT
18,208
ET AL.' SEMIBRITTLE
DEFORMATION
OF PERIDOTITES
3.2. Experiments
3.3. Rheological Tests
Altogether, l0 samples were cored such that their axial directionwas at 0ø, 45ø, or 90ø to the tbliationplane.The two
determinedby visual inspectionof the sampleblocksand from five-axes universal stage measurements. In total, four experimentswere performedon samplescoredperpendicular to the foliation, two experimentswere conductedat 45ø to the foliation, and four experimentswere conductedparallel to the
Three types of rheologicaltestswere performed(Figure 2). Constantstrainrate testswere performedon all 10 samples.Nine good resultswere obtained(test VN5, perp.s, 1000øCfailed). During suchtests,the displacementrate of the pistonis fixed and the mechanicalresponse of the sampleis observed.Five of the 10 experimentswere characterizedby work hardeningand were stoppedat flow stressesof between400 to 700 MPa to avoid failure. The four experimentsthat attained near constantflow stresswere performedon the dunire BG2 parallel and 45ø to its fk)liationplane,on the dunireNS63 parallelto its foliationplane,
lk)liationplane.
and on the xenolith VP2, in the 45 ø direction.
We chose the foliation plane as our standard reference direction, becausethe distribution of the crystallographicaxes was planar or nearly so (see Figure 1). This choicesimplified sample preparation and allowed comparison to previous experimentalstudies.The sampleswere cored randomlywith i'espect to the azimuthal position to the foliation plane. Theoretically, there might be a small error introducedin the rheologicalevolution of the 0ø and 45ø to foliation samples duringexperimentation dueto the variationin azimuth.However, only one critical test parallel to the foliation was performedon sample(VN5), which is the specimenshowinga weak linear crystallographic anisotropy(Figure 1). We determinedthat more than 65% of its olivine [100] axes lie in an angle of +30 ø around the foliation plane by usingthe pole figures.All other olivine [100] axes are randomly oriented to the foliation plane. This
Either relaxation or creep tests followed the constant displacement rate tests (Figure 2). Relaxation tests were performed by maintaining the deformation piston in a fixed position by the servo controller [Rutter, 1972]. The stress decreasesas a functionof time owing to the nonelasticshortening of the sample. Strainsare typically small (1%), so lower strain ratesmay be reachedthan during constantstrainrate tests.Carter and A vd Lallemant [1970], Post [1977], Chopra and Paterson !1981], Karato eta!., [1986], Beeman and Kohlstedt [1993], Evans and Kohlstedt [1995], and Hirth and Kohlstedt [1995] showed for steady state dislocation creep in olivine-rich rocks, that flow datacan be describedby the empiricalpowerlaw de/dr = A if" exp (-Q/RT) (do/dt - is the strainrate,A - is a constant,ifis the differential stress,n - is the stressexponent, Q - is the activationenergyat P.....,=constantconfiningpressure,T- is the absolutetemperature,and R- is the gas constant).Even though near constant flow stress was observed in only tour tests, we
xenoliths were oriented in all three directions (0 ø, 45 ø, and 90ø), one dunire (BG2) in two directions(45 ø and 90ø), the other dunire
(Ns63)intwodirections (0øand90ø).Thefoliation planes were
suggests that the error introducedby usingthe foliationas the standard reference should be low.
Spine!-Lherzolite VNS,//sandperp.s
Iporphyroclasti• ....
soo
•
DuniteB,G2,//s, and45, ø , porp!•yroclas/lc T=•000 øC
•medium grained T=1000ø•'•'•• "2 400
fine-grained •
_
T=1200 •/' •2 ils ø
200
1
[- •,•!ed
• -2
_
_
• 100
10o
•.• •_-lOOOøc •o • Jinitial • =4.8 • • -100
I
• str•in(%) •
0
2
4
6
8
10
1- constant
strain
2-relaxation
te•t
Jinitial --5.8 • -
strain(%)
12 rate test
i
i
0
2
i
i
i
i
4
6
8
10
o 10o
3-creep te•t J-index for fabric strength
Spine!-LherzoliteVP2,//s and 45ø
porphyroc•astic •
•
-
45ø 2 400
mediumgrained
400
-"
•
15øø
1
300
300 20O
2OO 100
lOO
0
.lOOI
-5
strm (%) • strair](%> a
0
5
10
•
15
I
2O
o
2
4
6
8
lO
Figure2. Experiments performed onthedifferentsamples. Constant strain]'atetests(1) wereperformed duringall experiments tollowedeitherby relaxation (2) or creep(3) tests.Experiments weregenerally stopped at a flowstress from 400 to 500 MPa to avoid catastrophicfailure.
WENDT ET AL.' SEMIBRITTLE DEFORMATION OF PERIDOTITES
18,209
calculatedstressexponents usinga powerlaw relationship lbr all
VN5 appearedto reach a constantflow stressbelow 500 MPa
seven relaxation tests performed in the three deformation
(extrapolatedfrom the low work hardeningrate of the stressstraincurve)at a strainof about14%(Figure3c). The experiments showthatthetheologyof theperidotires was affected by temperature and deformation direction: (l) At 1100øCand 1200øCsamples workharden to highflow stresses at low strains,(2) At 1000øCthe sampleorientationhad little effect on the maximum flow stress,but a large effect on the strain
directions(0ø,45ø, and90ø) in orderto comparequalitativelyour' results with previous work. We illustrate below that brittle
deformationalsooccurs,so it is importantto emphasizethat we do not imply thatthe deformationwas"steadystate". Creeptestswere performedin threecases,two sampleswere parallelto andonewasat 45ø to the foliation.In thistypeof test, a constant stress is maintained and sample strain varies as a partitioning(alsodocumented by microstructural analyses), (3) functionof time. A short periodof transientcreep is observed Deformation perpendicularto the foliation never resultsin a befi)re a constant strain rate is obtained.
constant flow stressandshowsthestrongest workhardening, (4) Deformationat 45ø to the foliation planeshowsthe weakest behaviorof any orientation:At high temperature(1200øC) a
4. Experimental Results 4.1. Evolution
constant flow stresscan be achieved at low strain (3.8%), (5)
of Stress and Strain
The stress-strainevolutionof our 10 experimentsis shownin Figures3a to 3c. Samplesdeformedperpendicular to the foliation showed the strongest work hardening and never reached a constant flow stress during the experiment. At 1100øC and 1200øC the flow stresses of NS63 and VN5 were 700 and 420
MPa, respectively, at strains near 4%; at 1000øC flow stresses were 460 MPa at about7% strain(Figure3a). SamplesBG2 and VP2 deformed
at 45 ø to their foliation
reached a flow stress of
Deformationparallelto the foliationrevealsat low temperature (1000øC)a work hardeningbehaviorsimilarto that of samples deformedin the 45ø orientationat high temperature.Constant flow stressoccurred at strains higher than 6%. Deformation parallel to the foliation at high temperature(1200øC) revealsa strongwork hardeningand a constantflow stressat high strain (I0%).
4.2. Least Squares Regressionof StressRelaxation Data
380 MPa at 4.5% strainand380 MPa at 4.2% strain,respectively; The test resultsfor all relaxed samples(Figure 4) follow at higher strainsthe flow stresswas approximatelyconstant approximatelya straightline in a log stressversuslog strainrate (Figure3b). Samplescompressed parallelto their foliationplane plot with the slope I/n. Variationsaroundthe straightline aredue showed a strong work hardening at 1200øC with a maximum to a continuouschangeof the specimentheologywith reducing measured flow stress of 400 MPa at 3.3% strain (VP2). At flow stress.The valuesfor n were calculatedby a leastsquares 1000øC two samples(NS63 and BG2) reacheda constantflow stressof 500 MPa at 8% and 12% strain, respectively.Sample
Deformationperpendicularto foliation plane
700
I
NS63 •"
•.,.[,..d.,an %% I
I
I
I
700
-
1100øC/
-
6O0
500
-
I
-
500
-
400
300
-
300
200
-
200
100
-
100
-
0
•
NS63
•
,---
,--JYJx
.-•.'"
Deformation45øto foliationplane
400
600
-
regression of the stressrelaxation data. We observed two trends
of n values:( 1) At 1200øCthe n valuesvary between4.0 and6.0.
300
300
200
200
",
N
,
.--•oi•ooc ,
',
-'7
o
4O0
400
I
I
I
I
I
I
I
0
2
4
6
8
10
-100
-1oo .2
12
o
I
I
I
i
2
4
6
8
Strain(%)
Strain (%)
Deformationparallel to foliation plane
600
I
500
I
I
6OO
I
-
- 5O0
400 -
- 400
300 -
-300
200 -
-200
100
- 100
-
0
-loo -5
100
lO
-0
I
I
I
I
o
5
lO
15
-lOO 20
Strain(%)
Figure 3. Comparisonof the stress-strain evolutionduringexperimentsas a functionof temperatureand deformation
direction.
18,210
WENDT ET AL.' SEMIBRITTLE DEFORMATION OF PERIDOTITES VN5,//s
VN5, perp.s
J initial= 5
1200øC
1.6
spi•e!-!herz•!ite ' '
'
_spil•e!-!h•rzo!'ite ' '••
2.59 2.5
n=5aa•aa
2.58 2.57 2.56 2.35
• I I I I I 2.5.46. 8 -6.6
-6.4
-6.2
-6
-5.8
-5.6
f-i
2.3
-5.4
i
-6
-6.2
i
-5.2
l•
-
-4.8
1200øC
2.6
, ' ' ' i , , , i , , , i , , , il,
,
2,69
,_, 2.55
dunite n=13
•2,67
I
-5.4
BG2,45 ø
J initial= 6
1oooo½
2,68
I
-5.6
logstrain rate
BG2,//s 2,7
i
-5.8
log strain rate
dunite ' '
'•'2,45
2,66 2,65
"'
2,4
2,64 2.35
2,63 l,
2,62
-6 2
,
.
I
,
,
-6
,
I
,
,
,
I
,
,
-5,8 -5,6 log strain rate
ß
I
,
,
.5,4
2.3 -6.2
-5,2
i
i
i
i
i
-6
-5.8
-5.6
-5.4
-5.2
i
-4.8
log strain rate
NS63, perp.s 1000øC 2.68
i
i
i
i
dunite
J initial= 6
2.64
n=6
2.62
2.6
%'
2.58
2.56 II
2.54 -6.2
-6
-5.8
-5.6
-5.4
-5.2
log strain rate
VP2,//s 1200øC
2.65
spi•e!-!•erz'olite ....
2.6
[
• 2.4 •-2.35
..• 2.45
2.3
2.25 •
_
2.35 -6.4
2.5spin•!-!he•zolit• ' ' •2.4s
.•2.55
2.4
VP2, perp.s 1200oc
J initial= 11 2.55
-6.2
-6
-5.8
-5.6
-5.4
-5.2
-S
-4.8
log strain rate
2.2 -6.2
I -6
• -5.8
I -5.6
• -5.4
i -5.2
log strain rate
Figure4. Results of theleastsquares regression of stress relaxation data.Thesamples deformed at I!()()øCand 1200øCrevealstressexponents from4.0 to 6.0, whereas the samples deformed at !()()()øC cloveloped stress exponentsup to 25.0.
They are not systematically loweror higheras a functionof the
the relative amounts of brittle and ductile processes.The semibrittle experiments performed here do not allow us to systematicvariationwith direction,but the samplesdeformed determine the necessarymaterial parametersfor using the parallelto the foliationshoweda muchgreatervariationin stress conventionaltypesof steadystateflow laws. Nevertheless,the n exponents(6.0 to 25.0) than the others. values obtained during semibrittleflow from 4.0 to 6.0 are in The observedvariationsof n may be a functionof deformation good agreement with n values obtained during plastic clirection, samplecomposition, initialcrystallographic fabric,and deformation of peridotites previouslypublished by e.g. Chopra deformation direction. (2) At 1000øC the n values show no
WENDT ET AL.: SEMIBRITTLE DEFORMATION OF PERIDOTITES
[1980] Chopra and Paterson [1981], Karato et al. [1986], Beeman and Kohlstedt [! 993], Evans and Kohlstedt [1995], and
Hirth and Kohlstedt [1995]. This may suggestthat semibrittle flow is expressedto a certain amountin the empirical power law that describesthe steadystatecreepfor olivine.
18,211
5. !.2. Xenolith VP2 (spinel-lherzolite)(Figure 5b). Deformation at 1200øC parallel to the foliation of xenolith VP2 resultedin heterogeneousplastic and brittle deformationconcentratedin a single diffuse shearband.The olivine porphyroclastsshowedan internal strain higher than 20% and are deformed by the formation
of deformation
lamellae and microcracks.
We observed
a highintragranularcrackdensitywith no preferentialorientation. 5. Microstructural
Observations
We now describe the microstructural changes of the peridotires during experimental deformation with special attention given to the crystallographicorientation and internal grain substructuresof olivine, as it is the dominantconstituentof the samples.Microstructuralobservationson orthopyroxeneare similar for all samples.Orthopyroxenesshow generally a wavy undulosity, fractures, closely spaced subboundariesand the intergrowthof orthoenstatiteand clinoenstatitefor grainslarger than lmm throughoutthe testedsamples.Cracksdevelopedin all samplesand their interaction with dislocationsubstructuresare also described. The crystallographic fabric of olivine grains beforedeformationwas measuredin thin sectionperpendicularto the foliation (XZ or YZ). The crystallographicfabric of olivine grains alter deformation was measuredeither in the XY, XZ, or YZ planesor at 45 ø to the foliation dependenton the orientation of the sample. To obtain comparable lower hemisphere pole figures, we always rotated the foliation to the E-W direction. Ustage measurementswere performed on planes containing the long axis of the deformed core. We measuredbetween60% and 100% of the olivine grains in the thin sectionsof each of the deformedspecimens.In order to quantify the straindistributionin olivine during the compressiontests,we comparedthe elongation of all olivine porphyroclasts in at least two thin sections perpendicularto one another.This was donein all samplesbefore and alter the experiments.
Deformation at 1200øC at 45 ø to the foliation produced a single small shear band defined by the shape of the olivine crystals.The internal deformationof the olivines showeddense subboundariesor a wavy undulosity whose intensity decreased with
distance
from
the
shear
band.
The
strain
of
the
prophyroclastic olivines was 18%. !ntragranular wide-open microcracks
were
the dominant
deformation
feature
of this
sample.Their distributionwas not localized. 5.1.3. Dunite BG2 (Figure 5c). The fine-grained dunire BG2 deformedat 1000øCparallelto its foliationhad a diffusesingle shear band in the center of the cylinder. In the shear band, all olivine porphyroclastsexhibited deformation lamellae; wavy extinctionwas mostly seenin the smallerneoblasticgrains.The internal strain of all porphyroclastswas close to 15%. Grain substructures were more frequenttowardthe centerof the sample. Whereasthe undeformedsamplewas nearly free of microcracks, the deformed sample revealed many intragranular cracks. intragranular fractures opened mostly parallel or subparallelto the principal stress.A large transgranularcrack also developed throughmore than the half of the samplelength. Deformation at 1200øC at 45 ø to the foliation causeda single diffuse shear band defined by the shape of the olivine porphyroclastsand deformationlamellae in olivine perpendicular to the principal stress. The local strain in the olivine porphyroclastsapproximated 10%. Wavy extinction was only observed in small grains. The most important deformation features were the numerous intragranular microcracks without preferentialorientation. 5.1.4. Dunite NS63 (Figure 5d). After deformation at 1000øC parallelto the foliationthe medium-graineddunireshoweda very diffuse conjugateshearband definedby the shapeof the olivine 5.1 Description of Plastic and Brittle Deformation crystals. Deformation lamellae and wavy extinctiondeveloped Structures in Olivine intensely in porphyroclaststhroughoutthe whole specimenas 5.1.1. Xenolith VN5 (spinel-lherzolite)(Figure 5a). Deformation well asin the shearband.The internalstrainof the porphyroclasts at 1()()0øCparallel to the foliation of VN5 resultedin a chaotic was 20%. Toward the top and bottom of the specimen, heterogeneous patternof plasticand brittle deformation,which is experimentally induced deformation features decreased. The more intense toward the center of the sample with the cracks were distributed and occurred as intragranularcracks in development of a weak conjugate shear band defined by the three main orientations:(1) many random cracksthroughoutthe shapeof the crystals.Here the initially weak deformationfeatures whole sample. (2) small cracks oriented at about 20ø to the of the olivine porphyroclastswere replacedby densedeformation principal stress,and (3) wide-open cracks at about 30ø to the lamellae and closely spacedwavy extinction.Local strainsup to principal stress.A transgranularcrack parallel to the principal 19% developed. Very fine recrystallized grains formed along stresspropagatedthroughthe whole length of the sample(and a grain boundariesand in wedge-shapedinterstitialspaces.Cracks part of the spacer)as well as throughall previousmicrostructures. Deformationat 1000øCperpendicularto the foliation resulted were mainly intragranularwith no preferredorientation. Deformation at 1000øC perpendicularto the foliation resulted in an increasedflattening of the olivine porphyroclasts.Intense in an accentuationof the initial foliation. An increaseof subgrain plastic deformationoccurredin the olivine porphyroclasts and in boundariesin olivine was observedfor the porphyroclasticgrains. the centerof the sample.We found an averagestrainof 24% for They showeda localized strainequivalentto 15%. lntragranular all porphyroclastic grains. Additionally, we observed closely microcrackingincreasedslightly and contributedto the localized spaced subboundariesand wavy extinction whose intensity strain. decreased toward the top and the bottom of the sample. The Deformation at 1200øC perpendicularto the foliation also intragranularcracksdistributedover the whole specimenshowed no preferentialorientation. accentuated the initial foliation plane and produced a Deformation at 1100øC perpendicular to the foliation also homogeneousincrease of subgrain boundariesthrougout the whole sample. The olivine porphyroclastsrevealed an internal resulted in flattening of the inital microstructuresand led to a maximum strain of 25%. The intragranular microcrack density single shear band located in the center of the specimen.'['his shear band revealed both intenseplastic (wavy undulosity) and was a little higherthan in the undeformedsample.
18,212
WENDT ET AL.: SEMIBRITTLE DEFORMATION OF PERIDOTITES
"•"•x:
'•
....... -'
'I.:
- •:-'"':;:'•..•:.'".:' ** -':":•..'•
....'-"
5
:;•:i":-:"::'"-::-:'•":-:: ....... ' •. ' - * •..... • :;:•"'•::'.*; ..-...•**..•.•. . :::. •
'.,'.-/,/' • ..•':-:
,:-,*
...•
..-
•--.
•
.:.•. :.
,'•.."::•:'..'(-•.:..•"• ......•.,.: .ß..,...:-::.:. • '•.
"•L :'•::::::::::::::::::::::::: '•'.*:-
