Creep of dry clinopyroxene aggregates

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. B7, PAGES 13,443-13,454, JULY 10, 2001

Creep of dry clinopyroxene aggregates

Misha Bystricky • and Stephen Mackwell 2 Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania

Abstract. We have determined diffusional and dislocation creep rheologies for clinopyroxenite Ca•.0Mg0.sFe0.2Si20 6 under dry conditions by deforming natural and hot- pressed samples at confining pressures of 300-430 MPa and temperatures of 1100ø-1250øC with the oxygen fugacity buffered by either nickel-nickel oxide or iron- wtistite powders. The coarse-grained natural Sleaford Bay clinopyroxenite yielded a stress exponent of n = 4.7 + 0.2 and an activation energy for creep of Q - 760 +_ 40 kJ mol -•, consistent with deformation in the dislocation creep regime. The strength of the natural clinopyroxenite is consistent with previous high-temperature measurements of dislocation creep behavior of Sleaford Bay clinopyroxenite by Kirby and Kronenberg [1984] and Boland and Tullis [1986]. Fine-grained clinopyroxenite was prepared from ground powders of the natural clinopyroxenite. Hot-pressed samples were deformed under similar conditions to the natural samples. Mixed-mode deformation behavior was observed, with diffusional creep (n = 1) at lower differential stresses and dislocation creep (with n and Q similar to those of the natural samples) at higher differential stresses. Within the dislocation creep field the predried hot-pressed samples generally yielded creep rates that were about an order of magnitude faster than the natural samples. Thus, even at the highest differential stresses, a component of strain accommodation by grain boundary diffusion was present in the hot-pressed samples. Optical and electron microscope investigations of the deformation microstructures of the natural and hot-pressed samples show evidence for mechanical twinning and activation of dislocation slip systems. When extrapolated to geological conditions expected in the deep crust and upper mantle on Earth and other terrestrial planets, the strength of dry single-phase clinopyroxene aggregates is very high, exceeding that of dry olivine-rich rocks.

1. Introduction

Calcium-bearing clinopyroxene is a common mineral in the Earth's upper mantle comprising probably 10-15 vol % of the convecting mantle. It is also commonly found in rocks derived from the lower continental crust and may be a major component of the cratonic mantle lithosphere. Thus the mechanical behavior of calcium-bearing clinopyroxene may exert a significant influence on creep in a variety of crustal and mantle settings. In particular, the mechanical behavior of clinopyroxene under anhydrous conditions is important as such conditions may be appropriate to granulite facies metamorphism in the lower crust on Earth [e.g., Carter and Tsenn, 1987]. It may also be useful in modeling the crusts of other terrestrial planets, such as Venus, which are fundamentally dry and of gabb- roic composition [e.g., Kaula, 1990].

A number of experimental studies have investigated the high-temperature creep behavior of single crystals of clinopyroxene with compositions near that of diopside. Deformation in a Griggs apparatus yielded a dislocation creep flow law with a stress exponent n = 4.3 and an activation energy Q = 280 kJ mot-1 for diopside single crystals [Av• Lallemant, 1978] and n = 3.1 and Q - 490 kJ mol -• for chrome diopside single

•Now at Geologisches Institut, ETH-Zentrum, Zfirich, Switzerland. 2Now at Bayerisches Geoinstitut, Universitat Bayreuth, Bayreuth,

Germany.

Copyright 2001 by the American Geophysical Union.

Paper number 2001JB000333. 0148-0227/01/2001JB000333509.00

crystals [Koll• and Blacic, 1982, 1983]. Two deformation regimes for diopside single crystals were identified from experiments conducted in a 0.1 MPa dead load apparatus under controlled oxygen fugacity [Raterron and Jaoul, 1991; Ingrin et al., 1991, 1992; Raterron et al., 1994; Jaoul and Raterron, 1994]. Below a critical temperature (T c •--1130ø-1140øC), a stress exponent of n = 7 and two different activation energies Q were determined depending on which dislocation slip systems were activated, with Q = 440 kJ mol -• for an orientation

i (a + b) andQ = 740 kJ favorable for slip along { 110 } • _ , 1 (a + b) and mot -• for an orientation favorable for { 110 } 7 -

(100)[c]. Above the critical temperature, incongruent melting in the single crystals was observed, resulting in a dramatic drop in activation energy. The easiest glide systems at high temper-

l(a + b), ature (T > 800øC) were found to be {110} • _ followed by { 110 } [c] and (100) [c]. Jaoul and Rarerton [1994] report only a slight dependence of creep on oxygen fugacity for iron-bearing diopside single crystals.

Kirby and Kronenberg [1984] and Boland and Tullis [1986] investigated the high-temperature rheotogy of ctinopyroxene aggregates using the same natural material as in this work. The experiments were performed in a Griggs solid-medium apparatus (with NaC1 or NaF assemblies) and a Paterson gas- medium apparatus, respectively. In both studies, samples were deformed after only a low-temperature anneal (oven dry), which removes surface water but not hydrous minerals. Boland and Tullis [1986] did, however, obtain limited dry deformation data by venting several samples to air during the deformation. These high-temperature dry results of Boland and Tullis [1986] seem to be quite consistent with the extrapolation of the lower-

13,443

13,444 BYSTRICKY AND MACKWELL: CREEP OF DRY CLINOPYROXENE

temperature data from Kirby and Kronenberg [1984], perhaps suggesting that the NaC1 and NaF solid-medium assemblies may promote some dehydration of the samples during deformation.

In order to provide a robust flow law for dislocation creep in calcium-rich clinopyroxenite appropriate to deep crustal conditions, we have performed a study of the mechanical behavior of dry natural and hot-pressed samples of Sleaford Bay clinopyroxenite. This study is also the first where experiments were performed on both natural and hot-pressed samples from the same source material, providing important insight into the utility of fine-grained hot-pressed samples in rheological studies. In addition, as none of the previous research investigated deformation in the diffusional creep field, we have performed experiments on the hot-pressed material under conditions that favor deformation by grain boundary diffusional creep (fine grain sizes and lower stresses). Rheologies from such experiments may be important in quantifying the mechanical behavior of deep crustal shear zones.

2. Experimental Techniques and Apparatus 2.1. Sample Material

The samples used in this study were fabricated from blocks of Sleaford Bay clinopyroxenite kindly provided by J. Tullis (Brown University, Rhode Island) and S. Kirby (U.S. Geolog- ical Survey, Menlo Park, California). The material has a composition near Ca•.0Mg0.sFe0.:Si206 and a mean grain size of 330 /xm [Mauler et al., 2000]. The natural samples were prepared as 9-mm cylinders, •15-20 mm long, with the ends ground parallel. Prior to deformation, samples were annealed at 1000øC for 10 hours in a 0.1-MPa furnace with the oxygen fugacity controlled using mixed CO/CO2 gases near the Ni/ NiO phase boundary.

The fine-grained samples were prepared from powders fabricated from the same material. Pieces of the Sleaford Bay clinopyroxenite were first broken into millimeter-size frag- ments using a steel crusher, then ground in an agate mortar and pestle and sieved to yield particle-size ranges of 0-30 30-45/xm, and >45/xm. As the sieved material was found to have too broad a range of particle sizes, as measured using a Microtrac Small Particle Analyzer, the powders were subsequently gravitationally settled in distilled water. Several set- tlings were needed to obtain a suitable grain-size distribution for our experiments. Powder batches with particle-size ranges of 0-10/xm, 10-20/xm, and 20-30/xm were then annealed at 0.1 MPa and 1000øC for 10 hours under controlled oxygen fugacity conditions using CO/CO2 gas mixtures. They were subsequently stored in an oven at 150øC in order to minimize water contamination.

In order to prepare fully dense polycrystalline samples the powders were cold pressed into a nickel sleeve with a uniaxial stress of 150-175 MPa. This cold pressing produced some grain fracturing, resulting in production of a fine grain-size component for all samples. Cold-pressed samples were subsequently hot pressed at 300 MPa confining pressure and 1150 ø- 1180øC for 8 hours in an internally heated pressure vessel. Measurements of the densities of the starting material and of the hot-pressed aggregates using the Archimedean method yielded porosities of <2%. In an attempt to remove any water that may have been trapped on grain boundaries during the hot press, most samples were subsequently annealed at 0.1 MPa and 1000øC for 10 hours under controlled oxygen fugacity

conditions. A thin slice from the end of each sample was kept for microstructural and infrared analysis.

2.2. Deformation Apparatus and Experiments

Constant load "creep" experiments were performed in a gas-medium apparatus with thermal gradients in the samples of <iøC mm -•, and differential stress resolution of <5 MPa. Constant-load stepping tests were performed at temperatures from 1100 ø to 1250øC (below the solidus temperature of the material), confining pressures from 300 to 430 MPa, and differential stresses up to 430 MPa. Measured strain rates ranged from 10 -7 to 10 -3 s -•. The relatively high confining pressures available in this apparatus assist in suppressing crack nucle- ation and propagation; to minimize cracking, samples were always deformed at differential stresses less than the confining press!•re.

Samples were deformed at approximately the same oxygen fugacity as the 1000øC pretreatment. The oxygen fugacity around each sample was controlled by placing the sample in a nickel or iron sleeve (with NiO or FeO powder) and position- ing nickel or iron foil between the sample and the alumina pistons. This procedure yields oxygen fugacities near the nickel-nickel oxide (Ni/NiO) or iron-wtistite (Fe/FeO) phase boundaries, respectively. All except two deformation experiments were performed under anhydrous (predried) conditions. Flow stresses and strain rates were computed from the load displacement data and corrected for the load supported by the jacketing materials using Ni and Fe flow laws [Frost and Ashby, 1982] and for the changes in sample cross-sectional area and length.

For each deforming sample, conditions of temperature and differential stress investigated early in the experiment were revisited later in order to test for reproducibility and changes in mechanical behavior resulting from microstructural evolution. In particular, increases in grain size were expected to result in a gradual strengthening in the diffusional creep regime for the hot-pressed samples. Through measurements of strain rate at the same temperature and differential stress at different times in several experiments, we were able to define a grain growth law that, coupled with the measured starting and final grain sizes, was used to estimate grain size at all times in each experiment.

2.3. Microstructural Characterization

Both the starting material and the deformed samples were analyzed in the electron microprobe, as well as in the optical and scanning electron microscopes (SEM). Microstructural observations and grain-size measurements were performed using optical microscopy and orientation contrast in the SEM. Deformation textures were measured using electron backscat- ter diffraction techniques (EBSD) in the SEM. Both the optical and the SEM analyses were performed at ETH Ztirich and are detailed by Mauler et al. [2000].

2.4. Infrared Analysis

The presence of water within mineral aggregates is known to promote weakening during high-temperature deformation experiments (see reviews by Kirby and Kronenberg [1987] and Kohlstedt et al. [1995]). In particular, previous experiments on clinopyroxenite by Boland and Tullis [1986] suggest a reduction in creep strength by at least a factor of 2 for samples deformed in the presence of added water relative to those deformed dry at --•1250øC and 300 MPa confining pressure. We believe that

BYSTRICKY AND MACKWELL: CREEP OF DRY CLINOPYROXENE 13,445

Table 1. Experimental Conditions for Creep of Clinopyroxene Aggregates

Oxygen T Range, •:total, Experiment Preheating Buffer Pc, MPa øC % GS i, a/am GSf, a/am

PI-583 natural dried Ni/NiO 400 1175-1225 15.0 330 330 PI-592 natural dried Ni/NiO 300 1200-1250 15.9 330 330

PI-475 hot-pressed dried Ni/NiO 300 1100-1200 21.6 5.2 6.4 PI-527 hot-pressed dried Ni/NiO 300 1150-1225 27.8 5.2 6.8 P1-574 hot-pressed dried Ni/NiO 400 1150-1225 21.7 5.2 6.5 PI-576 hot-pressed dried Ni/NiO 400 1125-1250 26.8 5.2 6.9 PI-598 hot-pressed dried Ni/NiO 300 1150-1200 17.2 5.2 7.2 P I-528 hot-pressed dried Fe/FeO 300-430 1150-1225 18.8 5.2 7.0 PI-588 hot-pressed dried Fe/FeO 300 1150-1225 24.5 5.2 7.6 PI-441 hot-pressed undried Ni/NiO 300 1100-1150 25.7 5.2 5.8 PI-442 hot-pressed undried Ni/NiO 300 1150-1200 13.2 5.2 6.9

aGSi, in hot-pressed samples, initial grain size of fine-grained fraction; GSf, in hot-pressed samples, final grain size of fine-grained fraction.

our natural samples, which had been predried at 1000øC prior to deformation, were dry during deformation. However, for the hot-pressed clinopyroxenite samples, some water may have been incorporated in cold-pressed samples as adsorbed water on grain surfaces. Subsequent hot pressing in sealed assemblies probably failed to completely dehydrate the samples. For this reason, we normally annealed these samples at 0.1 MPa and 1000øC under controlled oxygen fugacity to remove any resid- ual water. While this technique will probably remove any water dissolved on grain boundaries or sample surfaces, it is unlikely to completely dehydrate water trapped in intracrystalline fluid inclusions formed due to grain boundary migration during hot pressing. Such trapped water may slowly escape the inclusions during protracted deformation experiments at high temperature a, nd result in some water weakening of the sample. Thus it is important to test for the presence of hydrous species in the samples after hot pressing and deformation.

It is generally not possible to quantify water contents in aggregates where the grain size is small relative to the thick- ness of the infrared sample, as is the case for the hot-pressed clinopyroxene samples. Significant uncertainties in the true water content result from the introduction of fine hydrated microcracks along grain boundaries during sample preparation that are difficult to dehydrate without altering the initial water content. In an attempt to quantify the water fugacity in experiments on hot-pressed samples we placed small olivine single crystals at the end of several samples during cold pressing. These crystals were oriented with the [010] axes parallel to compression so that they would neither deform under the creep conditions nor interfere with the deformation of the clinopyroxenite. As the relationship between water fugacity and hydroxyl solubility in olivine has been determined [Kohl- stedt et al., 1996], infrared measurements of the water content of the olivine allow an estimation of the water fugacity in the sample during the deformation experiment.

Infrared measurements of the water contents of the hot-

pressed and deformed clinopyroxene and the included olivine crystals were performed using the Bruker infrared microscope attachment on the IFS 120 HR high-resolution Fourier trans- form infrared spectrometer (FTIR) at the Bayerisches Geoin- stitut in Bayreuth, Germany. The microscope was purged during the measurements with a stream of water and carbon dioxide-free purified air. Two hundred scans were accumulated for each spectrum at a resolution of 1 cm-•.

3. Results

Two suites of experiments were performed on samples of clinopyroxenite. Experiments on dried natural clinopyroxenite specimens from Sleaford Bay only investigated deformation in the dislocation creep regime. The second set of experiments was conducted on samples hot-pressed from powders of the same material. As the grain sizes were considerably smaller, experiments investigated deformation in both the diffusional and the dislocation creep fields. Sample information and experimental conditions for all deformed clinopyroxene aggregates are listed in Table 1. The creep data derived from the deformation experiments are given in Table 2.

3.1. Natural Samples

Two samples of predried natural Sleaford Bay clinopyroxenite were deformed at an oxygen fugacity defined by the Ni/NiO buffer (Tables 1 and 2). The rheological data can be described by a dislocation flow law of the form

t} = Aais o-n exp (-Qdis/Rr), (1)

where k is the strain rate, rr -- (rr• - rr3) is the differential stress, T is the temperature, R is the gas constant, and A di,•, n, and Q dis are empirical parameters for the preexponential term, the stress exponent, and the activation energy for dislocation creep, respectively [Frost and Ashby, 1982].

A nonlinear least squares regression procedure was used to fit (1) to the experimental data for all temperatures, strain rates, and differential stresses. Using this analysis, we obtained the following flow law for dry coarse-grained clinopyroxenite:

(-760-+40kJmo1-1) /} = 109'8+0'50 '4'7+-0'2 exp RT , (2) where k is in s -•, rr is in MPa, and T is in kelvins. The uncertainties in the fitted parameters given in (2) were determined in the regression analysis and represent errors of one standard deviation. Perhaps a more useful indication of the quality of the fit is given by A, the root mean variance of the measured values of log strain rate from those predicted by the fitted flow law. For the natural data, the value for this param- eter is A = 0.09. The values of n and Q are suggestive of control of deformation by dislocation creep. Figure 1 shows all the experimental data for natural samples with constant temperature lines determined using this flow law.


Table 2. Creep Data for Natural and Hot-Pressed Clinopyroxene-Aggregates

Table 2. (continued)

Experiment T, øC Stress, MPa Strain Rate, s -• Experiment T, øC Stress, MPa Strain Rate, s -•

PI-583 a 1200 217 5.1 _+ 0.6 x 10 -7 PI-574 1150 106 2.3 __+ 0.1 X 10 -6 325 3.8 + 0.2 x 10 -6 165 3.4 _+ 0.2 x 10 -6 383 9.1 ___ 0.2 x 10 -6 273 8.9 __+ 0.6 x 10 -6 286 2.0 __+ 0.1 X 10 -6 312 1.3 _+ 0.1 x 10 -s 377 9.8 ___ 0.2 x 10 -6 1175 88 3.2 __+ 0.3 x 10 -6

1225 322 9.1 ___ 0.1 x 10 -6 168 6.8 __+ 0.6 x 10 -6 379 2.1 _+ 0.1 x 10 -s 123 4.3 __+ 0.2 X 10 -6 223 1.9 __+ 0.l X 10 -6 392 3.4 + 0.2 x 10 -s 267 4.3 __+ 0.1 x 10 -6 225 9.3 __+ 0.7 x 10 -6 334 1.1 _+ 0.1 x 10 -s 1200 226 1.9 _+ 0.1 x 10 -s

1175 334 2.4 __+ 0.1 X 10 -6 332 4.8 _+ 0.3 x 10 -s 372 4.0 + 0.1 x 10 -6 109 6.0 __+ 0.2 x 10 -6

1200 371 9.2 __+ 0.4 X 10 -6 175 1.0 _+ 0.1 x 10 -s 1225 323 1.2 _+ 0.2 x 10 -s 275 2.9 _+ 0.1 x 10 -5

PI-592 a 1250 169 1.8 __+ 0.1 x 10 -6 64 3.2 ___ 0.2 x 10 -6 231 7.8 + 0.2 X 10 -6 220 1.7 _+ 0.1 x 10 -5 276 2.2 _+ 0.1 x 10 -s 1225 224 3.6 _+ 0.1 x 10 -s 200 3.4 __+ 0.1 x 10 -6 86 9.1 __+ 0.5 x 10 -6 181 1.9 + 0.1 X 10 -6 160 1.8 _+ 0.1 x 10 -5 223 5.8 ___ 0.1 X 10 -6 332 9.3 _+ 0.8 x 10 -s 264 1.7 + 0.1 x 10 -s 219 3.4 _+ 0.1 x 10 -s

1200 277 2.7 __+ 0.1 x 10 -6 PI-576 1126 182 2.7 __+ 0.2 x 10 -6 261 1.7 + 0.1 x 10 -6 277 5.9 ___ 0.2 x 10 -6 307 4.1 + 0.3 x 10 -6 1150 174 3.6 __+ 0.2 x 10 -6 291 3.1 __+ 0.1 X 10 -6 219 5.2 _+ 0.2 x 10 -6 238 1.1 __+ 0.1 X 10 -6 356 1.7 _+ 0.1 x 10 -s

1250 238 9.9 __+ 0.1 x 10 -6 135 2.3 + 0.1 x 10 -6 278 2.4 + 0.1 x 10 -s 227 4.7 __+ 0.8 x 10 -6

PI-475 1100 225 1.1 __+ 0.1 x 10 -6 278 7.2 _.+ 0.4 x 10 -6 259 1.7 __+ 0.1 X 10 -6 1201 278 3.2 + 0.4 x 10 -s

1150 256 1.1 _+ 0.1 x 10 -s 129 6.9 ___ 0.7 X 10 -6 165 4.8 __+ 0.1 X 10 -6 219 1.8 _+ 0.1 x 10 -5 100 1.6 __+ 0. l X 10 -6 330 5.0 + 0.1 x 10 -s 215 5.3 _--_ 0.4 X 10 -6 176 1.1 _+ 0.1 x 10 -s 259 7.5 ___ 0.3 X 10 -6 295 3.7 __+ 0.2 x 10 -s

1200 260 4.4 _+ 0.2 x 10 -s 73 3.2 + 0.2 X 10 -6 167 1.9 _+ 0.1 x 10 -s 274 2.9 _+ 0.2 x 10 -s 85 8.8 + 0.8 x 10 -6 1250 274 2.2 ___ 0.1 x 10 -4

136 1.4 _+ 0.1 x 10 -s 214 1.0 __+ 0.l X 10 -4 52 5.7 __+ 0.3 x 10 -6 127 3.4 + 0.2 x 10 -5

109 1.0 _+ 0.1 x 10 -s 176 7.7 _+ 0.2 x 10 -s 26 2.7 ___ 0.2 x 10 -6 1200 131 7.9 + 0.6 x 10 -6

224 3.1 + 0.1 x 10 -s 312 3.8 +_ 0.5 x 10 -s 36 3.9 + 0.4 X 10 -6 217 1.7 _+ 0.1 x 10 -s

267 4.4 _+ 0.3 x 10 -s 1150 221 3.4 __+ 0.6 X 10 -6 1150 269 7.1 __+ 0.3 x 10 -6 311 7.3 ___ 0.6 x 10 -6 1175 269 1.7 _+ 0.1 x 10 -s 219 2.7 __+ 0.3 X 10 -6

171 8.3 ___ 0.7 x 10 -6 PI-598 1150 162 1.8 + 0.1 x 10 -6 221 9.2 __+ 0.2 x 10 -6 218 2.7 __+ 0.3 x 10 -6

PI-527 1200 106 1.0 + 0.1 x 10 -s 261 3.8 ----- 0.2 X 10 -6 72 5.0 + 0.3 x 10 -6 189 1.9 __+ 0.1 x 10 -6

171 2.4 _+ 0.1 x 10 -s 1200 187 1.4 + 0.1 x 10 -s 212 4.2 _+ 0.1 x 10 -s 141 8.6 ___ 0.4 X 10 -6 266 9.0 _+ 0.4 x 10 -s 107 6.7 ___ 0.2 X 10 -6 140 1.4 _+ 0.1 x 10 -s 76 4.7 __+ 0.1 X 10 -6 209 3.0 _+ 0.2 x 10 -s 43 2.3 ___ 0. l X 10 -6

1150 212 4.3 __+ 0.1 x 10 -6 32 1.8 ___ 0.1 x 10 -6 293 1.2 + 0.2 x 10 -s 76 3.5 + 0.1 X 10 -6 143 2.2 __+ 0.1 x 10 -6 107 3.9 + 0.1 x 10 -6 185 3.2 ___ 0.1 x 10 -6 186 9.1 ___ 0.2 x 10 -6 251 6.3 __+ 0.2 x 10 -6 1150 184 1.1 + 0.1 x 10 -6 222 4.4 __+ 0.4 x 10 -6 136 5.4 _+ 0.2 x 10 -7

1175 225 1.7 _+ 0.2 x 10 -s 220 1.3 + 0.1 X 10 -6 116 2.9 __+ 0.2 x 10 -6 PI-528 b 1150 228 2.3 ___ 0.1 x 10 -6 188 6.7 ___ 0.5 x 10 -6 259 2.5 -+- 0.1 x 10 -6 290 2.6 __+ 0.3 x 10 -s 300 3.4 + 0.2 X 10 -6 165 6.4 __+ 0.3 x 10 -6 178 1.1 + 0.1 x 10 -6 229 1.3 + 0.1 x 10 -s 1200 178 4.3 __+ 0.4 x 10 -6

1225 225 1.3 ___ 0.1 x 10 -4 222 6.9 + 0.3 x 10 -6 102 1.9 _+ 0.1 x 10 -s 256 9.8 ___ 0.5 X 10 -6 152 3.4 + 0.2 x 10 -s 134 2.3 __+ 0.2 X 10 -6 195 6.3 _+ 0.2 x 10 -s 157 3.2 + 0.2 X 10 -6


Table 2. (continued)

Experiment T, øC Stress, MPa Strain Rate, s -•

226 7.5 _ 0.6 x 10 -6 235 5.9 _+ 0.4 x 10 -6 271 6.8 _+ 0.6 x 10 -6 339 1.6 _+ 0.1 x 10 -s 310 1.2 __+ 0.1 x 10 -s 383 3.0 _+ 0.1 x 10 -s 231 6.0 __+ 0.3 x 10 -6

1225 180 1.6 _+ 0.1 x 10 -s 224 2.3 _+ 0.2 x 10-s 270 3.8 _+ 0.1 x 10 -s 115 5.8 ___ 0.4 x 10 -6 159 1.1 _+ 0.1 x 10 -s

PI-588 b 1225 217 6.7 _+ 0.5 x 10 -s 84 1.4 + 0. I x !0 -s

161 3.4 _+ 0.2 x 10 -s 256 9.2 _ 0.3 x 10-s

50 8.1 +__ 0.3 x 10 -6 120 1.8 _+ 0.1 x 10 -s 222 5.7 _+ 0.2 x 10 -s

1200 227 2.4 _+ 0.1 x 10 -s 174 1.0 __+ 0.1 x 10 -s 253 2.5 _+ 0.1 x 10 -s 101 4.8 __+ 0.1 x 10 -6 64 2.6 + 0.1 x 10 -6

141 5.7 ___ 0.4 x 10 -6 210 1.1 _+ 0.1 x 10 -s 252 1.7 + 0.1 x 10 -s

1150 251 3.4 + 0.1 x 10 -6 215 1.9 __+ 0.1 x 10 -6 276 3.5 __+ 0.2 x 10 -6

1175 277 7.4 __+ 0.9 x 10 -6 190 3.1 ___ 0.1 x 10 -6 234 4.6 + 0.5 x 10 -6

1200 190 7.3 __+ 0.2 x 10 -6 232 9.8 __+ 0.3 x 10 -6

PI-441 c 1100 162 2.8 __+ 0.l X 10 -6 257 9.4 ___ 0.6 x 10 -6 187 3.9 +_. 0.2 x 10 -6 305 1.5 __+ 0.1 x 10 -5 229 6.3 __+ 0.3 x 10 -6 165 3.1 __+ 0.1 x 10 -6 205 4.4 __+ 0.2 x 10 -6 135 1.8 ___ 0.1 x 10 -6

1153 104 6.2 __+ 0.2 x 10 -6 191 1.7 _+ 0.1 x 10 -s 134 7.1 __+ 0.4 x 10 -6 217 2.2 +_ 0.1 x 10 -s 169 1.2 _+ 0.1 x 10 -s 243 2.9 _+ 0.1 x 10 -s 119 6.4 __+ 0.2 x 10 -6 203 2.0 +__ 0.1 x 10 -s 307 5.5 _+ 0.4 x 10 -s

66 2.9 + 0.2 x 10 -6 200 1.6 _+ 0.1 x 10 -s

84 4.0 _+ 0.1 x 10 -6 PI-442 c 1150 217 2.2 _+ 0.1 x 10 -s

235 2.5 +_ 0.1 x 10 -s 142 7.3 __+ 0.3 x 10 -6

1200 158 5.8 +_ 0.1 x 10 -s 62 1.4 _+ 0.1 x 10 -s

aNatural samples. bDeformed at Fe/FeO buffer. CNot dried at high temperature after hot pressing.

Microstructures of the starting material and of the deformed samples were examined using optical and electron microscopy. Deformed samples show features characteristic of intracrystalline plasticity, such as deformation lamellae, undulose extinction, some limited formation of subgrains, and deformation

-3.5

-4.0

-4.5

-5.0

o -5.5

-6.0

-6.5

o / MPa

200 300 400 I I

Dry Sleaford Bay Clinopyroxenite Ni/NiO

1250 øC 1225 øC 1200 øC 1175øC

1250 øC

5OO

I

1225 øC

1175 øC

•= 109.8 (j4.7 e-760/RT

-7.0 I I I 2.2 2.4 2.6

log / MPa

Figure 1. Strain rate versus stress plot of creep data from experiments on dry natural Sleaford Bay clinopyroxenite. The experiments were performed using the Ni/NiO oxygen buffer at confining pressures of 300 or 400 MPa and temperatures between 1175 ø and 1250øC. The solid lines represent the dislocation creep flow law for dry natural clinopyroxenite (equation (2)) at 1175 ø, 1200 ø, 1225 ø, and 1250øC.

twins (Figure 2). Grain boundaries show evidence of bulging, suggestive of incipient recrystallization. Detailed analysis of the microstructures and deformation textures is given by Mauler et al. [2000].

Infrared measurements of the water content were performed on the predried natural samples prior to and after deformation. Infrared spectra of individual grains within the deformed samples show only a small absorption band near 3650 cm -•, corresponding to <20 molar ppm H/Si, and no broad absorption band. These measurements indicate that the experiments were conducted under essentially dry conditions.

3.2. Hot-Pressed Samples

The experimental conditions and creep data for the clinopyroxenite samples that were hot-pressed from powders of Slea- ford Bay clinopyroxenite are also given in Tables 1 and 2. All of the samples discussed below were predried at 1000øC for 10 hours under controlled oxygen fugacity conditions prior to deformation. In contrast to the experiments on the natural coarse-grained material the hot-pressed samples showed deformation behavior characterized by a low value of the stress exponent at lower stresses and finer grain sizes, and behavior characterized by a higher stress exponent at higher stresses and larger grain sizes. Figure 3 illustrates such characteristics for


<,.,.•,e.•, :;:,•,½- '::.-'•:• ....... •.,•:j- '-?;• . •. ':- -,'--,.½.• '•4:c--:' •,.•.-•"•'• "- •:,• .... .----•-7 <'2 ...... $ ..... •.:./:'•::5 .... 7.• :- • ...-:: ......... "::'7.%.":"•. ':' ' -,." ..:':'5--;;:" :•,.:•::,2•5:<' :'""•'•4 -,:•?•:::." ..,..;..7 • •.,•:,•-

--<--•&.•,,: .: .. ......... ?-,•:,:5:..::5..;: --•-.:,,:..•:., • .• . ,•: ...... :• .......... '"•--w• '"•;•D•;.•,•;.::-•,• ...... --•:':•,•,, "::•-•-•: ....... ,.• '•

Figure 2. Polarized light micrographs showing natural clinopyroxene aggregates (from Figure 3 of Maukr et al. [2000], reprinted with pemission from Elsevier Science). (a) Natural aggregate dried at 1000øC, unde- formed, and (b) natural aggregate dried at 1000øC, deformed to 16% shortening.

sample PI-574, which was deformed at various stresses and temperatures. All samples showed some evidence of grain growth as indicated by a gradual strengthening within the creep regime characterized by the low stress exponent. On the basis of these observations and following data analysis similar to that of Hirth and Kohlstedt [1995a], we assume that the region with a low stress exponent is representative of creep dominated by grain boundary diffusional (CoNe) creep (stress exponent of 1, grain-size exponent of 3) and that deformation within the region characterized by the higher stress exponent is dominated by dislocation creep.

On the assumption that the diffusional and the dislocation creep components are independent, the creep rate of the aggregate can be described by

0-1

• = Adi f • exp (-Qdif/RT) + Aais O-n exp (-Qdis/RT),

(3)

where k is the total strain rate, 0- = (0-1 - 0'3) is the differential stress, d is the grain size, T is the temperature, R is the gas

constant, and A dif, Q dif, A dis,/'/, and Q dis are empirical parameters characterizing the diffusional and dislocation creep components. For any set of experimental conditions the component that yields the faster strain rate will dominate the behavior. However, as can be seen from Figure 3, much of the range of conditions accessed experimentally falls within the transitional range between the two creep components. For this reason, it was necessary to fit (3) to the entire data set for all experiments on predried hot-pressed samples deformed with the Ni/NiO buffer.

As noted above, the grain size of the hot-pressed clinopyroxenite samples increased throughout the experimental dura- tion. In order to fit (3) to the data we had to determine the grain size for each creep interval during each experiment. As we only measured the starting and final grain sizes and each sample was deformed at multiple temperatures, we had to infer the grain size at each intermediate set of conditions by assuming a normal grain growth law of the form

D] - D/2 = K0 t exp ( - Qgg/R T) (4)


o / MPa

100 200 300400

Dry Hot-pressed Clinopyroxenite PI-574

-4.0- Ni/NiO ß - '• 1225 øC

.' ß 1200 øC • 1175 øC ,o

ß 1150 øC •"

-5.0 - ß .."i, .." _

ß ß ,

ß ß

-6.0 - 1200 øC ß ß

ß ß

ß

,,•' ß

ß ß

ß ß

ß

1150 øC

-7.0 - -

..... i;= 1015'l ill d -3 e-56ø/Rr + 101ø'8 (j4.7 e-760/Rr

1.2 1.6 2.0 2.4

log / MPa

Figure 3. Strain rate versus stress plot of creep data from experiment PI-574 on a dry hot-pressed clinopyroxene aggregate. The experiment was performed using the Ni/NiO oxygen buffer at a confining pressure of 400 MPa and temperatures between 1150 ø and 1225øC. The dotted lines represent the flow law for dry hot-pressed clinopyroxene (equation (5)) at 1150 ø and 1250øC.

similar to that determined by Karato [1989] for single-phase olivine aggregates hot pressed in the same manner. The values of Ko and Q gg were calculated using the creep data from two experiments on very fine grained clinopyroxene aggregates that were deformed predominantly in the diffusional creep field. In these experiments the same stress and temperature conditions were revisited at different times [see, e.g., Hirth and Kohlstedt, 1995a, 1995b]; the K o and Qgg parameters were adjusted until all data for the same conditions yielded the same strain rates. Thus approximate values for the fitted parameters are Ko = 6 x 10 -3 m 2 s -• and Qgg -- 370 kJ mol -•. While the grain growth law determined in this manner can only be regarded as zeroth order, it is sufficiently accurate for the small interpolations in grain size required for this study (Figure 4 and Table 1).

In this way, we were able to determine the creep parameters in (3) for the hot-pressed samples deformed dry with a Ni/NiO buffer using a nonlinear least squares regression technique to fit the deformation data. Unfortunately, as there were too few data collected at stresses high enough to access dislocation creep alone, our regression techniques converged slowly on values for n and Q that were unrealistic relative to our measurements on dislocation creep in the natural material. Thus we refit the data after fixing the values of n = 4.7 and Q dis = 760 kJ mol -•, yielding the following flow law for the hot- pressed samples:

-560 +_ 30 kJ mo1-1) 1015.1+_0.70-1d-3 exp R T

(-760kJmo1-1) -Jr- 1010'8+_0'90 '4'7 exp RT '

where k is in s-•, 0- is in MPa and T is in kelvins. A value of A = 0.16 was determined for the root mean variance of the

measured values of the log of strain rate from those predicted by the fitted flow law. When we performed least squares regression fits to data from individual experiments on hot- pressed samples that were collected only at high stress conditions, we obtained similar values of n and Qdis to those given in (5). All data for dry hot-pressed samples deformed under Ni/NiO conditions and corrected to a grain size of 8 txm are plotted in Figure 5. The solid lines represent (5) for various temperatures. We also deformed samples under Fe/FeO oxygen buffered conditions (Tables 1 and 2). Within the scatter of the data these results cannot be distinguished from samples deformed under Ni/NiO conditions (Figure 6).

Microstructures of the hot-pressed samples were examined using optical and scanning electron microscopy (Figure 4) [Mauler et al., 2000]. The samples exhibit deformation features such as deformation lamellae, undulose extinction, and deformation twins. In the coarser samples we observe ---15-25% recrystallization to a grain size of---8 txm. Grain-size measurements indicate that grain growth has occurred in the fine- grained samples from a starting grain size of 5.2 to ---7 txm. Detailed microstructural and textural analyses on some of these samples are reported by Mauler et al. [2000].

The flow law for hot-pressed clinopyroxene (equation (5)) was derived only from creep data on samples that were predried at 1000øC for 10 hours under controlled oxygen fugacity conditions prior to deformation. Infrared spectra of the olivine crystals imbedded in these samples showed no evidence of hydroxyl bands within a resolution of ---10 molar ppm H/Si. Because of the fine grain size of the hot-pressed clinopyroxene samples we could not obtain infrared spectra of individual clinopyroxene grains. Although we always observed a broad absorption band in the infrared spectra around 3200-3600 cm-•, it is likely that our samples were contaminated by water during preparation of the samples for infrared analysis. While the magnitude of this absorption feature varied from sample to sample, it represented no more than ---100 molar ppm H/Si. However, on the basis of the lack of hydroxyl in the olivine spectra we believe that these samples were deformed under dry conditions.

By contrast, infrared spectra of samples deformed after hot pressing without high-temperature heat treatment showed an absorption band near 3650 cm -• and a broader absorption band near 3200-3600 cm -• than observed in the predried samples, corresponding to more than 1000 molar ppm H/Si. This latter feature most likely results from hydroxyl associated with grain boundaries and/or fluid inclusions. Unfortunately, neither of these samples were deformed with an imbedded olivine crystal for more accurate determination of water content. Both of these undried samples deformed at significantly higher strain rates in both the dislocation and diffusional creep regimes than the predried samples under otherwise similar conditions (Tables 1 and 2 and Figure 7).


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Fibre 4. Polarized li•t micrographs showing fine-grained fraction in hot-pressed dinop•oxene aggregates (from Fibre 5 of Mauler et al. [2000], reprinted with pe•ission from Elsevier Science). (a) Sample hot pressed from 10 to 20 •m dinopyroxene powder and dried at 1000øC, and (b) sample hot pressed from the s•e powder, dried at 1000øC, and defo•ed to 25% sho•ening.

4. Discussion

4.1. Effect of Oxygen Fugacity

The high-temperature deformation behavior of clinopyroxenite has been characterized under a range of conditions with variation in stress, temperature, and oxygen fugacity and in the presence or absence of water. While most samples were deformed under relatively oxidizing conditions, at the Ni/NiO solid buffer, two samples were deformed at •4 orders of magnitude more reducing conditions, at the Fe/FeO buffer. Within the scatter of the data, no difference in creep rate could be detected between samples deformed under these two oxygen buffer conditions (Figure 6). Consequently, further experiments at the Fe/FeO buffer were not considered necessary. Previous deformation experiments on single crystals of diopside [Jaoul and Raterron, 1994] also .showed little or no dependence on oxygen fugacity.

4.2. Effect of Water

Creep data on predried and undried samples of hot-pressed clinopyroxenite demonstrate that the presence of small quan-

tities of water has a major weakening effect on deformation within both the diffusional and dislocation creep regime (Fig- ure 7). While predried samples were deformed under essentially dry conditions, infrared spectra clearly indicate the presence of hydroxyl in the undried samples. The absorption band near 3650 cm -• in these spectra probably results from intracrystalline hydroxyl. In contrast, the broad absorption band near 3200-3600 cm -• is more ambiguous. Although most of this absorption may be associated with hydrated grain boundaries or fluid inclusions, some is likely to result from hydration of microfractures during preparation of the samples for infrared analysis. The enhanced dislocation creep rates in the undried samples probably result from the presence of water within the grains, which may enhance recovery by dislocation climb [Boland and Tullis, 1986], while the diffusional creep enhancement likely results from hydration of the grain boundaries. This hydrolytic weakening appears to result in a decrease in the activation energies for both deformation mechanisms (Figure 7 and Table 2), to ---420 kJ mol -• for diffusional creep and ---450 kJ mol -• for dislocation creep. Further experiments


under carefully controlled water fugacities and careful analyt- ical work are necessary to understand the interplay between hydrogen defects, the movement of dislocations within grains, and the diffusion of chemical species along grain boundaries.

4.3. Comparison With Previous Studies

It is not possible to compare directly the stress-strain rate results of single-crystal deformation experiments with aggregate behavior, as multiple slip systems are required for homogeneous deformation of a polycrystal. However, it is notewor- thy that the more recent single-crystal experiments on diopside by Raterron and colleagues [Rateffort and Jaoul, 1991; Ingrin et al., 1991, 1992; Rateffort et al., 1994; Jaoul and Rateffort, 1994] also reported a high stress exponent (n = 7, relative to our value of 4.7) and an activation energy for dislocation creep of Q = 740 kJ mo1-1 (relative to our value of 760 kJ mo1-1) when multiple slip systems were activated. In addition, the activation energy for our undried samples (Q -450 kJ mo1-1) is similar to that obtained by Raterron and colleagues (Q = 440 kJ mo1-1) for single crystals when there was no contribu- tion from the slip system (100)[c], suggesting that the presence of hydroxyls may cause de-emphasis of this hardest slip system. They also report only a weak dependence of creep behavior on oxygen fugacity, consistent with our results.

-4.0

-5.0

-6.0

-7.0

o/MPa

100 200 300400

Dry Hot-pressed Clinopyroxenite / - Ni/NiO • ß -

ß 1250 øC ,/ ß / ß 1225 øC ß 1200 øC . 117s øc '

- • 1125øC .... o• • •• * '/ -

, 1100Oc •z,u c

1200 øC 1150 øC

1100 øC

{; = 1015'l {J1 d-3 e-560mr + 10 •ø'8 04'7 e -76ømr

1.2 1.6 2.0 2.4

log • / MPa

Figure 5. Strain rate versus stress plot of creep data from experiments on dry hot-pressed clinopyroxene aggregates deformed using the Ni/NiO oxygen buffer. The experiments were performed at confining pressures of 300-430 MPa and temperatures between 1100 ø and 1250øC. The data are corrected to a grain size of 8/xm. The solid lines represent the flow law for dry hot-pressed clinopyroxene (equation (5)) at 1100 ø, 1150 ø, 1200 ø, and 1250øC.

-4.0

-5.0

-6.0

-7.0

o / MPa

100 200 300400

Dry Hot-pressed Clinopyroxenite PI-588

- Fe/FeO

,• 1225 øC ß 1200 øC * 1175 øC ß 1150 øC

..'

ß

ß ..

ß ..'" ..':• &

ß .." ...*'& ..' ,•, .. ,,- ß

1225 øC ..... ..4 "

1200 øC" .'""'

1175 øC 1150 øC

..... {;= 10 TM o 1 d -3 e-560mr + 101ø'8 04.7 e-760/Rr

1.2 1.6 2.0 2.4

log o / MPa

Figure 6. Strain rate versus stress plot of creep data from experiment PI-588 on a dried hot-pressed clinopyroxene aggregate deformed using the Fe/FeO oxygen buffer. The experiment was performed at a confining pressure of 300 MPa and temperatures between 1150 ø and 1225øC. The data are corrected to a grain size of 8/xm. The dashed lines represent the flow law for dry hot-pressed clinopyroxene (equation (5)) at 1150 ø, 1175 ø, 1200 ø, and 1225øC.

Experimental deformation of predried natural Sleaford Bay clinopyroxenite resulted in a grain-size-independent dislocation creep rheology. Previous research on the same material by Boland and Tullis [1986] in a gas-medium apparatus under drained (dry) conditions yielded essentially the same results as in our study at similar conditions (Figure 8). By contrast, the results of their experiments under water-added conditions are more than a factor of 2 weaker than under dry deformation conditions. Our results on clinopyroxenite samples with and without predrying also suggest a significant weakening in the presence of water (Figure 7). While we were not able to quantify the water fugacity in our experiments, the magnitude of the weakening is similar in both cases.

While their experiments were performed at lower temperatures, the results of Kirby and Kronenberg [1984] are largely consistent with our deformation data (Figure 8). As noted earlier, their experiments were performed at higher pressures in a solid-medium Griggs deformation apparatus with NaC1 or NaF assemblies. They determined a stress exponent of n -- 5.3 for their high-temperature deformation regime. While their activation energy of 380 kJ mo1-1 is rather lower than ours, they report an increase in activation energy with increas- ing temperature, and their high-temperature data (>1000øC) are in agreement with the extrapolation of our dry deformation


-4.0

-5.0

-6.0

-7.0

o / MPa

100 200 300400

Undried Hot-pressed Clinopyroxenite

- Ni/NiO

ß 1200 øC ß 1150-1153 øC ß 1100 øC

1100 øC (dry).. .....

..... t= 10 •5'2 152 d -3 e -56ømr + 1020'8 04'7 e -76ømr

1.2 1.6 2.0 2.4

log o / MPa

Figure 7. Strain rate versus stress plot of creep data from experiments on undried hot-pressed clinopyroxene aggregates deformed using the Ni/NiO oxygen buffer. The experiments were performed at confining pressures of 300 MPa and temperatures between 1100 ø and 1225øC. The data are corrected to a grain size of 8 •m. The dotted line represent the flow law for dry hot-pressed clinopyroxene (equation (5)) at 1100øC.

data to lower temperatures. It must also be noted that while they did not effectively dehydrate their samples prior to deformation, some loss of water to the solid-medium assembly may have occurred during deformation. It is therefore unclear whether their results represent dry or wet rheologies.

4.4. Natural Versus Hot-Pressed Aggregates: Effect of Grain Boundary Processes

Our experimental data indicate that dry hot-pressed clinopyroxenite deforms approximately an order of magnitude faster in the dislocation creep regime than the natural samples under identical deformation conditions (Figure 8). Similar observations of strength differences for coarse natural and fine- grained hot-pressed materials have previously been discussed for olivine deformation by Hirth and Kohlstedt [1995a, 1995b]. On the basis of the von Mises criterion that homogeneous deformation in monomineralic aggregates requires activation of five independent slip systems, they suggest that the difference in strength results from de-emphasis of the stronger dislocation slip systems due to grain boundary accommodation processes. Certainly, grain boundary diffusion may contribute to the total strain even in the dislocation creep regime. Such accommodation may be more evident in hot-pressed samples where the grain size is small and the grain boundaries may be imperfectly formed, with higher defect densities and hence

faster grain boundary diffusion rates. Finer grain sizes require shorter diffusion distances for accommodating a smaller number of independent slip systems.

Results from our study and that of Hirth and Kohlstedt [1995a, 1995b] suggest that longer durations and higher temperatures of hot pressing may reduce this difference in strength between natural and hot-pressed samples through grain growth. However, which of the two types of dislocation creep flow laws is more appropriate to the Earth remains controver- sial. On the one hand, one may argue that deformation experiments on hot-pressed or fully synthetic aggregates underestimate the aggregate strength in the dislocation creep field. On the other hand, experiments to higher strains on natural dunite [Chopra and Paterson, 1981, 1984] and natural clinopyroxenite [this study; Mauler et al., 2000] show some dynamic recrystallization to a grain size similar to that of our hot-pressed aggregates. Therefore it could be argued that a fine-grained rheology is more appropriate to model steady state creep in the Earth, since deformation of coarse-grained natural rocks in the laboratory is not performed at microstructural "steady state" (D. L. Kohlstedt, personal communication, 2000).

4.5. Transitional Rheologies

As noted previously, over much of the range of conditions investigated for the hot-pressed clinopyroxenite samples, con- tributions from both diffusional and dislocation creep are observed. The breadth of this region in log rr space predominantly reflects the distribution of grain sizes within the samples. For instance, in samples hot-pressed from 20 to 30 •m powder, the area fraction of grains with sizes below 10 •m

T /øC

1200 1100 1000 900 -2 • • • t

Clinopyroxenite Flow Laws

BT o = 250 MPa

.•...Hip ...... d = 8 gm ".. Dis

Nat Dis • ".....'•.

-10 t I , f I t 6.5 7.0 7.5 8.0 8.5

104/T /K -1

Figure 8. Strain rate versus inverse temperature plot of flow laws determined in this study for dislocation creep of dry natural Sleaford Bay clinopyroxenite (Nat Dis) and dislocation and diffusional creep of dry hot-pressed clinopyroxenite (Hip Dis and Hip Dif, respectively). The flow laws are adjusted to a differential stress of 250 MPa and the diffusional creep flow law is plotted for a grain size of 8 •m. Also plotted are the dislocation creep flow laws (KK) from Kirby and Kronenberg [1984] for oven-dried samples and (BT) from Boland and Tullis [1986] for water-added samples. The star indicates the average of the experimental results of Boland and Tullis [1986] for dry samples deformed at temperatures near 1250øC.


is up to 20% [Mauler et al., 2000]. The presence of a significant proportion of fine grains will extend a component of diffusional creep well into higher stress conditions where one might expect only dislocation creep. In part, the broad distribution in grain sizes results from comminution of grains during cold pressing and the inability of grain growth during hot pressing to tighten the grain-size distributions. Despite experimenting with different stresses during cold pressing and durations of hot pressing, we were unable to obtain significant improve- ments in grain-size distributions.

Although we classified powders into several distinct grain- size ranges, the comminution of grains during cold pressing always produces a significant population of grains in the 0-10 •m range (Figure 4) [Mauler et al., 2000]. As deformation in the grain boundary diffusional creep regime displays a dependence of strain rate on grain size with an exponent of -3, this fine-grained component may dominate the mechanical behavior. Our creep data for samples prepared using all powder-size distributions were best fit on the assumption that the initial grain size was the same as for the fine-grained samples (5.2 •m). This observation suggests that working with coarser powders in an attempt to generate coarser grained samples may not work, as grain refinement during cold pressing will intro- duce a fine-grained fraction, resulting in promotion of grain- size-dependent creep processes.

4.6. Diffusional Creep

For reasons discussed earlier, when fitting the data for the low stress deformation of the hot-pressed samples, we assumed a stress exponent of n - 1. Two fine-grained samples, which were only deformed at low stresses, were fitted to a single- component diffusional creep rheology. This fit yielded values of n = 1.0 _+ 0.2. In this deformation regime we always observed some strengthening of the aggregates as a function of time, reflecting progressive grain growth. The fitting of the diffusional creep law to the data required that we correct for changes in grain size. In order to do this, we used starting and final grain sizes and a grain growth law described in section 3. There is potential error associated with this assumption of a grain growth law, given the complexity of the stress- temperature history of these samples. In addition, another complication results from the high grain-size dependence of diffusional creep that emphasizes the small grain-size fraction. Thus, although grain-size measurements for some samples may yield relatively high means [Mauler et al., 2000], the fine grain- size fraction (Figure 4) may dominate the rheology. This effect increases the uncertainty of the diffusional creep law. Thus the quoted uncertainties in the constitutive parameters for diffusional creep of clinopyroxenite likely underestimate the true errors.

4.7. Geophysical Applications

While clinopyroxenite exists only rarely in the Earth as a monomineralic aggregate, clinopyroxenes with compositions of diopside-augite form a major mineral component of many lower crustal and upper mantle lithologies. Thus the mechanical behavior of this important mineral is critical in understand- ing in particular the rti•-6iøgy of the lower continental crust. We can gain a significant insight into such rheologies by ex- trapolating laboratory data to the conditions appropriate in such settings. As much of the lower continental crust exists under conditions of granulite-facies metamorphism, which is

generally considered to be anhydrous, experimental rheologies determined under dry conditions are most applicable.

Our experimentally determined dislocation creep rheologies for dry clinopyroxenite (for either natural or hot-pressed material) can be extrapolated to conditions of the lower continental crust assuming a continental geotherm from Chapman [1986]. We obtain very high strengths for this material that are in excess of those predicted from an extrapolation of dry olivine aggregate deformation to the same conditions [Chopra and Paterson, 1984]. These high predicted strengths for calcium-bearing clinopyroxenite, despite modest strengths for clinopyroxenite in the laboratory, result from the high values of the stress exponent and activation energy for creep for this rock. Thus one might expect that clinopyroxene grains would not, in general, deform under dry lower crustal conditions. On the other hand, our results on undried samples suggest that water can have a major weakening effect on these aggregates.

Localization of deformation and channeling of fluids along plastic flow zones within the deep crust will also tend to decrease the overall strength of this region. Grain refinement of clinopyroxene due to dynamic recrystallization, which was observed in the microstructures of a number of our samples [Mauler et al., 2000], will tend to promote grain-size-dependent diffusional creep, resulting in a significant reduction in strength. Fluids channeled along such zones of high deformation will also tend to reduce the strength, as suggested by our observations of water weakening in both the dislocation and diffusional creep regimes.

Of course, the presence of other phases may also result in a significant reduction in aggregate strength. Experimental studies by Mackwell et al. [1998] on deformation of dry diabase samples (Maryland diabase), composed predominantly of calcium-rich pyroxene (mostly augite) and plagioclase feldspar, yield much lower predictions for deep crustal strength than those from the present study. Interestingly, in the diabase study the mechanical behavior was dominated by the (weaker) plagioclase grains, suggesting a much higher strength for the clinopyroxenite consistent with the present work. It must also be noted that while deformation studies have yielded rather low stress exponents for natural (n < = 4) and synthetic (n = 3 for dislocation creep and n = 1 for diffusion creep) plagioclase aggregates [Rybacki and Dresen, 2000], the stress exponent reported for dry diabase by Mackwell et al. [1998] is identical to that reported here for dislocation creep of dry clinopyroxenite.

Acknowledgments. This research was made possible by grants from the Planetary Geology and Geophysics Program at NASA. We wish to thank Jan Tullis and Steve Kirby for providing slabs of the Sleaford Bay clinopyroxenite. The assistance of Paul Raterron, Jean-Claude Doukhan, Jeff Lawlis, Sean Finan, Iona Stretton, Florian Heidelbach, Karsten Kunze, Alexandra Mauler, David Bruhn, Mark Zimmerman, and David Kohlstedt in various aspects of this research is gratefully acknowledged. We are also grateful for helpful comments on the manuscript by David Kohlstedt, Greg Hirth, and an anonymous re- viewer.

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M. Bystricky, Geologisches Institut, ETH-Zentrum, CH-8092 Zti- rich, Switzerland. ([email protected])

S. Mackwell, Bayerisches Geoinstitut, Universitfit Bayreuth, D-95440 Bayreuth, Germany.

(Received July 19, 2000; revised March 20, 2001; accepted March 25, 2001.)

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Creep of dry clinopyroxene aggregates