JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B6, PAGES 4257-4269, JUNE 10, 1984

EXPERIMENTAL STUDIES OF THE ROLE OF WATER IN THE PLASTICITY OF QUARTZITES

D. H. Mainprice 1 and M. S. Paterson

Research School of Earth Sciences, Australian National University

Abstract. Experiments on several quartzites at 300 MPa confining pressure show them to behave as if near the brittle-ductile transition at 900øC

when oven-dry and in a more ductile fashion at 1000oc when extra water is added. In both cases

the stress-strain curves are substantially higher than would be expected on the basis of the strength of hydrolyrically weakened synthetic crystals containing less "water" than the rocks, as measured by the broad infrared absorption at low temperature. Thus it would appear that, relative to the single crystals, the "effective" concentration of the water-related species that is active in mechanical weakening within the grains is only a small fraction of the "water" present. The presence of glass in the grain boundaries of "wet" deformed specimens suggests that grain boundary weakening may also be significant in these experiments. Questions of equilibrium and of geological extrapolation are discussed.

Introduction

Studies on single crystals of quartz have sho•n that their plastic deformation is strongly dependent on the presence of trace amounts of "water" in the structure. The "water" can be incorporated during growth or by suitable heat treatment in an aqueous environment at high temperature and pressure and gives rise to a

Parrish et al., 1976; Tullis and Yund, 1980; Jaoul et al., this issue]. This paper will examine some of the relevant factors in making the connection. It will both present new results and draw on those in the literature

in attempting to analyse the role of water.

Experimental Observations

Specimens and Procedures

Experiments were carried out on a variety of quartzites, varying in grain size from 0.2 mm to nearly 2 am. Some showed evidence of previous deformation (for example, undulatory extinction), while others did not (Table 1).

Specimens were cored with a diamond drill, normal to the bedding plane if visible, and their ends were ground to within 0.01 mm of parallelism. Their dimensions were usually 7 mm in diameter and 15 mm in length, and they were dried in an oven at 110øC after drilling in order to remove any superficial water, especially that introduced in the drilling operation. During testing the specimens were enclosed in a copper jacket of 0.25 mm wall thickness, sealed with press-fitted molybdenum alloy (TZM) rings to a hollow sintered alumina piston and an end piece. An alumina spacer, interposed between specimen and piston, had a small axial venting hole when dried specimens were tested (to ensure that the effective confining

remarkable plasticizing effect known as hydrolyric pressure was equal to the applied gas pressure weakening œGriggs and Blacic, 1964, 1965; Griggs, 1967; Paterson and Kekulawala, 1979, and references therein]. The nature of the water-

related species continues to be under discussion and in this paper will be referred to simply as "water" or as hydroxyl, since the measure of its concentration that is used here is the strength of infrared absorption in the 3-•m region deriving from O-H stretching vibrations.

The discovery of hydrolytic weakening in single crystals of quartz naturally suggests that the effect will be important in facilitating the deformation of quartz-rich rocks in nature where water is ubiquitous and evidence of plastic deformation of quartzites is widespread. However, the connection between hydrolytic weakening of quartz single crystals and the plasticity of polycrystalline quartz rock remains to be fully clarified in laboratory experiments, although it is usually accepted that water also plays a role in the latter [Heard and Carter, 1968;

1Now at Laboratoire de Tectonophysique, Universit• de Nantes.

Copyright 1984 by the American Geophysical Union.

Paper number 3B1581. O148-0227/84 / 003B- 1581 $05. O0

even if slight jacket leaks occurred) and no hole in cases where water was added. In spite of close attention to polishing the sealing faces of the alumina pistons and to other aspects of the assembly, sealing of the jackets with respect to the gas confining medium was not always successful, and in the case of obviously leaking jackets the results have been rejected (a new jacketing procedure which overcomes these problems has been more recently developed [Paterson et al., 1982]). It should be noted that even when blind spacers are used between specimen and piston there is normally some loss of water in case of wet specimens but a substantial part is retained, as will be discussed later.

The deformation experiments were carried out in compression in a gas medium, high-pressure deformation apparatus previously described [Paterson, 1970], with internal furnace and internal load cell. All stress-strain tests

were carried out in the B-quartz stability field at 300-MPa confining pressure in argon gas. After raising the confining pressure to near the working level, the temperature was raised in the course of « to 1 hour and the specimen held at temperature for a period of up to about « hour before commencing the straining. The duration of straining to 10% at 10-5s-! strain rate is about 3 hours (actually somewhat longer since the machine has usually been driven at a constant motor speed that corresponds

4257

4258 Mainprice and Paterson: Water in the Plasticity of, Duartzites

TABLE 1. Specimen Materials

Name Locality Grain $izel mm Notes and Donors

Black Hills quartzite South Dakota

Chewings Range quartzite Central Australia

Heavitree quartzite Central Australia

Mt. Barker quartzite South Australia

0.2

1.7

0.4

0.6

Ouadrant quartzite U. S .A. 0.4

Simpson quartzite U.S.A. 0.4

Quartzitite Bommel Quarry, 1.9 Massif Central, France

Well-annealed (J. Tullis)

Natural deformation

features and some

fine recrystallized grains. (J.C. Wilkie)

Overgrowths; traces of mica (S.M. Schmid)

(A. Spry)

(J. Tullis)

Overgrowths (H.C. Heard)

Lack of natural

deformation features

(J. Paquet)

1The grain sizes were determined by the linear intercept method and corrected for the sectioning effect [Exner, 1972; Pickering, 1976]; the standard deviation was about 7% in each case except for the two coarse- grained rocks where it was twice as large.

to the required strain rate at a steady state, which gives a strain rate two to three times lower in the elastic and early yielding stages because of the additional elastic distortion of

the apparatus during these stages). At the conclusion of straining, the load was removed and the deformed length was verified by retouching the specimen while temperature and pressure were maintained, taking about 30 s. The temperature was then reduced to 700øC in approximately 2 min, with an accompanying drop in confining pressure to about 230 MPa. The specimen was then slowly cooled through the beta to alpha transition region over a period of thirty minutes to avoid excessive cracking. Finally, the temperature and pressure were reduced together to ambient. In calculating the stress-strain curves, correction has been made for apparatus distortion and the stresses are calculated on current cross-sectional area, assuming the specimen to remain a cylinder of constant volume during the deformation. The load supported by the copper jacket is negligible in these experiments.

Infrared Absorption Measurements

Infrared absorption spectra were recorded with unpolarized radiation in the 3-Bm region by using a Perkin-Elmer 180 spectrophotometer with the specimen at 77K in a cryostat with calcium fluoride windows, appropriate precautions being taken to avoid water vapor contamination. The

is a sloping background that was subtracted -1 approximately by taking the absorbance at 4000 cm as a reference level and assuming that the back- ground varied as (wavelength)-4 as in Rayleigh scattering. The absorption coefficient for the intrinsic absorption of the specimen material was then calculated as a function of wave-number

and typical results are given in Figure la to lg. High accuracy cannot be claimed for these

results because of the large background losses and the consequent poor signal quality and also because the grain size is in many cases comparable with the thickness of the section, tending to lead to an underestimate of the amount of hydroxyl concentrated near grain boundaries. However, it is believed that the results are at least of

semiquantitative significance, permitting useful conclusions to be drawn, as follows:

1. Comparison with the shape of the mica and ice hydroxyl bands shown in Figure lh indicates the presence of both mica and free water (f•ozen at 77K and presumably occupying the bubbles seen in thin section). However, allowing for these absorption bands and for minor Si02 absorption from the quartz itself, the greater part of the absorption is of a type that is generally similar to the broad hydroxyl absorption shown by wet synthetic quartz and clear amethyst single crystals œKekulawala et al., 1978, 1981; Aines and Rossman, this issue].

2. Using the calibration suggested by Paterson [1982], the hydroxyl content of the as-received rocks, excluding that contributing

specimens were mostly 'between 0.1 and 0.4 •n thick, to ice and mica bands, varies from at least polished on both sides, and with epoxy setting 1000to more than 3000 H/106Si (0.015 to 0.045 resins finally extracted with ethanol. Owing to wt percent H20 ) . These numbers represent lower scattering at cracks and grain boundaries, there limits if there is a heterogeneous distribution

Mainprice and Paterson: Water in the Plasticity of Ouartzites 4259

of hydroxyl on the scale of the section thickness.

3. Spectra with sufficiently low background to resolve the hydroxyl absorption were not obtained from specimens that were deformed without the addition of water. The deformability of the specimens presumably indicates that at least some of the hydroxyl mentioned above was retained during an experiment, although the appearance of small amounts of glass in the vent hole adjacent to the specimen suggests that some was also lost.

4. In the case of specimens deformed with the addition of water, the final hydroxyl content was substantially higher than for the original rock and was of the order of 5000 to 10000 H/106Si or 0.08 to 0.15 wt%. (Figures li and lj).

Stress-Strain Results

Approximately fifty experiments, thought to be successful from the point of view of experimental problems, were carried out on the various quartz rocks, mainly on Black Hills, Heavitree, and

Quartz Polycrystals 900øC 1(f5• 1 Oven Dry 1400

1200

1000

800

600

400

200

0 0

• Blackhills (4170)

- •-Q•Jadrant (4180) •• .•Quartzitite (4407)

•••...••,.•.• -- ( 1000øC ;,"• •••Heavitree

J •_ Barrel _(• 1, 79) Chewings Range (3480)

Simpson (4183)

I I I I 2 4 6 8 10

Strain (%)

Fig. 2. Stress-strain curves (full lines) measured at 10-5s -1 strain rate, 900øC under 300-MPa confining pressure for various quartz rocks in oven-dried condition, vented (exception: the quartzitite was deformed at 1000oc).

2ø•a 151 Heavitree

5

0

15 Simpson

'E 10 .?,o e

• Chewings Range ._e

._.o 5 o

õ o ._

• lO •o g • Quartzitite

5

o 20

b

Quadrant

Mt Barker

Muscovite

•..Water

J Heavitree (4240) 3800 3400 3000 2600 3800 3400 3000 2600

Wavenumber (cr• l)

Fig. 1. Infrared absorption measured at 77K (except for water, at room temperature) and corrected for background scattering as described in text: (a)-(g) The various rocks in as- received condition; (h) spectra for mica, water and ice (arbitrary scale for absorption coefficient); (i) and (j) spectra after deformation under "water-added" conditions at

300 MPa confining pressure.

Simpson quartzites. Some were tested in the oven- dry condition, some with added water and some after prior heat treatment at 1500-MPa pressure.

Typical stress-strain curves are shown in Figure 2 for vented, oven-dried specimens at 900•C, 10-5s-1 strain rate and 300 MPa confining pressure(designated as "dry" conditions; the infrared spectra are the same as for the as- received rock). They exhibit broad similarity in behavior under comparable conditions with the exception of Simpson quartzite. The latter, the only rock with substantial porosity (~ 7%), gives a continually rising stress-strain curve while the remainder show a peak, reached at around 3% strain, followed by a gradual decrease in strength during which a shear zone develops, as described later. Thus, under these conditions, all the rocks except Simpson quartzite behave as if they are in the brittle-ductile transitional zone in the confining pressure range, as might be expected for a flow stress of around 800-1200 MPa, which is around three to four times the confining pressure (Mogi [1965, 1966] and Paterson [1978, p. 166]; a recent study by Belfield and Twiss [1981] has explored the transition in Heavitree quartzite in more detail). The effects of varying the temperature and the strain rate are shown in Figure 3 for Heavitree quartzite.

Other specimens of Heavitree, Black Hills, and Simpson quartzites were tested under "wet" conditions, using a nominally unvented assembly, as already described, and either water-saturating the specimen or adding 5 B1 of water (0.3% by weight or 20,000 H/106 Si). No difference in results was detected for the two modes of adding water. Considerable practical difficulties were encountered with sealing all the water in throughout the tests, but infrared absorption measurements (Figures li and lj) indicate that at least some of the water is retained (no free water is seen visually on opening the jacket) and that the total water content of the specimen in these tests is substantially higher than in the case of vented, oven-dry specimens. Figure 4 shows stress-strain curves obtained under both

dry and wet conditions at 1000øC and 10-5s -1

4260 Mainprice and Paterson: Water in the Plasticity of Ouartzites

Oven Dry Heavitree Quartzite

12001

t ldSE 1 900øC (4184) 1000 ,• 165• 1 1000øC /,•'-• (4196) • 800 •-

• 600

= oooøc • 400

Q 200• 0 i I I I 0 2 4 6 8 10

Strain (%)

Fig. 3. Stress-strain curves at 300-MPa confining pressure for oven-dry Heavitree quartzite at the different rates marked, for oven-dry, vented conditions at 900øC (full lines) and 1000øC (dashed lines).

strain rate. The flow stresses for the wet

conditions are lower by 20 to 30% than for the dry conditions, and the behavior is more ductile, as defined by the absence of any tendency for the stress-strain curves to turn down after reaching a peak or for the specimens to develop shear failures; this trend might be expected from the more favorable ratio of flow stress to confining pressure. However, in spite of the nearly constant flow stress, changes were observed in relaxation behavior measured at successively larger strains, indicating that the specimens were not deforming in a true steady state but that the stress exponent n is increasing, possibly due to gradual loss of water or change in a melt phase. An additional result for wet Simpson quartzite shows that the flow stress is further decreased, and in a more substantial degree for these conditions than for the dry conditions, when the strain rate is reduced to 10-6s-1 (dashed line in Figure 4). Relaxation tests carried out after 6-8% strain also

indicated that the strain-rate dependence was much greater for the wet than the dry conditions; interpreted in terms of a power law fit, the stress exponent n for Black Hills and Simpson quartzites was found to be around 3 to 6 for 1000øC wet conditions compared with 20 to 30 for 900øC dry conditions (similar results have been reported in the abstract by Belfield and Twiss [1981]). Some attempts to influence the stress- strain behavior by further modification of the specimen environment, either by using seawater or 0.1-m lithium hydroxide solution in place of water or by introducing a molybdenum wire wound spirally on the specimen under wet conditions in order to lower the oxygen fugacity, did not reveal any changes that were clearly outside normal specimen variability, although the sealing problems encountered may have obscured the issue in some cases.

Further tests were done after prior heat treatment at 1500 MPa quasihydrostatic pressure. It has been reported that the strength of quartz- rich rocks can be substantially reduced by

raising the confining pressure to around 1500 MPa •Tullis et al., 1979]. Experiments were therefore carried out with a view to confirming that weakening observed at 1500 MPa confining pressure could be retained after reducing the pressure and temperature to ambient to demonstrating the weakening in a subsequent test at 300 MPa confining pressure, 900øC (cf. the procedure suggested by Paterson and Kekulawala •1979•). About a dozen oven-dried specimens were given a preliminary heat treatment for periods of « hour to 24 hours at 900øC and 1500 MPa in a Boyd- England type piston-cylinder apparatus, using an assembly in which the specimen was enclosed in a copper container, separated by a thin boron nitride sleeve from the graphite heating tube which was surrounded by a sodium chloride pressure medium (Figure 5). The temperature-pressure paths in both approaching and returning from the heat treatment conditions were chosen to be in the s-quartz field. The stress-strain curves shown in Figure 6a, measured subsequently in the gas apparatus at 300 MPa, indicated that the 1500-MPa treatment for a short period without special precautions to seal the copper container around the specimen produced some residual weakening, although not nearly as much as the tests at 1500 MPa had shown, but that the degree of weakening diminished with increased duration of heating at 1500-MPa; the type of stress-strain curve is generally similar to that obtained without the 1500-MPa heat treatment.

Since the trend with time suggested loss of water from the assembly during the 1500-MPa heat treatment, further experiments were done with grooves in the ends of the copper container where contact is made with the lid, as shown in Figure 5, so as to introduce an unsupported area for better sealing. Paradoxically, no significant weakening was now observed in a subsequent 300-MPa test, and when water was added during the heat treatment the strength and ductility even appeared to be slightly increased (Figure 6b). Two further, similar experiments were done with specimens of the dry synthetic quartz single crystal A6-13 (used as reference dry crystal by Kekulawala et al. [1981]), with addition of water to the copper container. Heat treatment at 1500 MPa, 900øC for 6 hours left the crystal with a yield

•oooOc 1 ooo

'• Heavitree (4196) } a. 'Dry" :• 800 Simpson (4226) ß • Blackhills (4233) • Heavitree (4240) 'Wet •e 600 Simpson (4229)

• 400 son 1(•6• 1 (3229)

;5 200

o o 2 4 6 8 lO 12 14 16

Strain (%)

Fig. 4. Stress-strain curves (full lines) measured at 10-5s -1, 1000øC under 300-MPa confining pressure for various quartz rocks with water added and also for Simpson quartzite at 10-gs -1 under the same conditions otherwise (dashed line).

Mainprice and Paterson: Water in the Plasticity of Ouartzites 4261

I C/T/

I_

':.'-":...":• p YROPHYLLITE COPPER

JACKET • TALC

:."..'• ALUMINA mnl

I'"i :t s,,,,,, I I '--f-

BN

15ram • • 7.37mm

Fig. 5. Sample assembly used in the 1500-MPa heating experiments in the solid-medium apparatus.

stress at 300-MPa confining pressure still in excess of 1400 MPa, the limit to which it was felt safe to load the piston in the gas apparatus. A 24-hour treatment of a specimen that had been additionally put with the water into a platinum jacket, welded up so as to ensure retention of the water, led to the stress-strain curve shown in Figure 6b; this curve is still much higher than the one found by Blacic [1975] for similar specimens tested at 1500 MPa. In conclusion, there may be uncontrolled variability in these experiments owing to the solid medium assembly being open to the escape of water in some degree, but it is felt that there remains some element of difference between tests carried out under the 1500-MPa conditions and those done at 300 MPa subsequent to 1500-MPa heat treatment. This difference reflects either a change in the condition of the specimens associated with the cycling through atmospheric pressure and temperature to reach the 300-MPa conditions or a basic difference in mechanical behavior of given specimens at 1500-MPa and 3.00-MPa confining pressure (see Mackwell and Paterson [1982] for further experiments of this type).

Microstructural Observat ions

At the optical microscope scale the micro- structures of the deformed specimens were heterogeneous, although less so with decreasing strain rate and increasing water content. The oven-dried specimens showed well-developed narrow shear fractures at about 30 ø to the compression axis, as already mentioned, within which was sometimes discernible a fault gouge, shown by electron microscopy to be a crush zone [cf. Tullis and Yund, 1977]; adjacent to the shear zones there was some development of axially oriented microcracking and of deformation bands of rotated extinction within

the grains (Figures 7a and 7b). Thus there was optical evidence of intragranular plastic deformation, but it was accompanied by brittle processes as well. In specimens to which water had been added, the microfracturing was less marked, the tendency to form a shear failure zone largely suppressed, and the undulatory extinction evidence of intragranular deformation more accentuated (Figure 7c). The deformed specimens with 1500-MPa pre-treatment generally

4262 Mainprice and Paterson: Water in the Plasticity of Quartzites

1200

Heavitree Quartzite

1500 MPa Precook

('open' system)

•1000

soo

600

400

200

•.- Untreated (4184)

urs (3214) I hour (3209)

(3204)

i i i i i øo 2 4 6 8 10 12

Strain (%)

1400j ,• 1200J-

Heavitree Quartzite

1500 MPa Precook

('closed' system)

.,/24 hours (3238)J .... (5 j•l water)J

,•• •ours (3220) 000

/•// •Untreated (4184) 8oo

••///1' • hour (3219) 600

r/I 400

iii 200

O0 2 4 6 8 10 12 14 Strain (%)

Fig 6. (a) Stress-strain curves measured at 10 -5 1 ß s- , 900 øC under 300-MPa confining pressure on specimens of heavitree quartzite that had previously been heated, for the times marked, at 1500 Pa. 900øC in the solid-medium apparatus. (b) Stress-strain curves measured at 10-•s -1 900øC under 300-MPa confining pressure on specimens of Heavitree quartzite (full lines) heat treated as under (a) except that additional steps to seal the specimen assembly had been taken apJ 5 B1 of water were added .in the case

of run 3220 (see text), and on a specimen of "dry" synthetic quartz crystal A•-13 (dashed lines) heated in a similar assembly with added water at 1500 MPa, 900-C in the solid-medium apparatus.

showed similar features to the oven-dried

specimens (specimens deformed at 1500 MPa, kindly supplied by J. Tullis, showed strong development of deformation features, including lamellae and deformation bands, the latter being broader than in the specimens deformed at 300 MPa and accompanied by some subgrain development).

Electron microscope observations showed that the oven-dried specimens had developed a very heterogeneous dislocation structure on the micron

and were much more numerous in specimens with added water, where many also appeared to have amorphous rims within them. All specimens pretreated at 1500 MPa and subsequently deformed at 300 MPa were similar to each other with

respect to bubble density and dislocation structure, all, for example, showing crystallographic control of the dislocations.

To investigate further the effect of hydrostatic pressure on microstructure during pre-treatment, two oven-dried specimens of

scale, with a strong tendency for the dislocations Heavitree quartzite were heated at 900øC for to be crystallographically aligned parallel to 2 hours in the solid medium apparatus without <a> or <½.> directions; the dislocation special measures to seal the copper container,

--

density tended to be higher and more heterogeneous one at 500-MPa and one at 1000-MPa pressure. near the shear fractures. In cases where water

had been added, the dislocation structure was less heterogeneous and the dislocations were typically curved and tangled where associated with bubbles (quartzite deformed at 1500 MPa, mentioned above, showed a fairly homogeneous distribution of curved dislocations). Electron microscope observations also revealed that in all specimens deformed with added water small amounts of a new, amorphous phase were seen on grain boundaries (typically in 0.1-•m thickness) and at triple junctions (Figure 7d). This phase could be distinguished from epoxy resin by differences in diffraction pattern and tendency to electron damage, and analytical electron microscopy confirmed the presence of silicon in it. It is therefore concluded that the

amorphous phase was a glass resulting from partial melting in the presence of water and probably incorporating other phases as well as quartz, such as mica and ferrous orthoclase, both identified in the as-received rock. Such

glass was also seen in the specimens deformed at 1500 MPa, both with and without added water, as well as in deformed Chewings Range quartzite in the oven-dried condition. Bubbles were seen

in all specimens, not always on dislocations,

The 500-MPa specimen showed no significant change in dislocation structure, while the 1000-MPa specimen showed substantial rearrangement, dipoles, and loops now being common and the loops often being associated with 0.1-•m bubbles; also bubbles were present in large numbers on grain boundaries and in grains in the 1000-MPa case, probably to a greater extent than in untreated oven-dried specimens. The pre-treated single crystal that did not yield macroscopically up to 1400-MPa differential stress in the test at

300-MPa confining pressure showed a similar curved or looped dislocation structure in the few places where dislocations were present in significant density, reminiscent of those seen in "wet" synthetic crystals deformed above the critical weakening temperature [Morrison-Smith et al., 1976].

Discussion

Comparison With Other Studies

A comparison is made with other experimental results in Figure 8. In this conncection the following points should be noted:

Mainprice and Paterson: Water in the Plasticity of Ouartzites 4263

•.....•,...• ½,..•., ,,... ',½•:..•j'?"- .....'...-'-""'.•;'.-:-...,,,,' -c.'......,•,,..'., "' .... "'•'"! .•.:.:•'.% ..... .-•' :;!;""' '"" :•..,." .... •' ' '"*'"":"'

' ' ";'"' 4'• • 4,.-.'.'..,,;;7 .... ::'•; • ...... '-.;7 ' . ':-'?"" ½-. .. - ....

"'" "< ...... '• .... ". -"t ::' ...,..-.,.;/; .... * .- . • ..... i -'• ,•.... --::. -..• .•,-. ...;•, ...... <:..... . ...... . .....::;?* ;•.- ..... ?:;. . ".'-..:.,. "•.I' .

?• ?::.•; / -•., .... •-": •--:"..:"':c '• ::":':...*." .;•. :?,, ....... .... -:-..:-.- . • .:........:¾..:.---. : . ,. ..... ..

.:,:; :/:?:t.::.:.-:,;...--.:,". ...... ':;-.. .... ......• . ). •: • .. ':•' ....... ......

B' ':'•'* ':' '-'? "; ':*' •; ':'*'"' •:':: *' ß ..-,:',-.---.'...:,:*:.--....:'•....-:.-:• ' : .............. ".. "'".:..." -?;'•?.,74 -' -. :.....';'.-.:::::.....:,

t ............ I ..............................

Fig. 7. (a) Blackhills quartzite, as received. (b) Blackhills quartzite (run 4170)

after 9% strain at 900øC, 10-øs -•, 300-MPa confining pressure 'zn ov•n-•ried condition. (c) Blackhills quartzite (run 3227) after 13% strain at 1000øC, 10-'s- , 300-MPa con- fining pressure with 5 •! water added. (d) Heavitree quartzite (run 4240) after 8% strain at 1000øC, 10-øs -• 300-MPa confining pressure with water added showing amor- phous phase on grain boundary. Figures 7a through 7c are optical micrographs with cross polarizers and the scale bar is 300 •m. Figure 7d is an electron micrograph of an ion-thinned specimen taken at 200 kV and the scale bar is 1 pro.

4264 Mainprice and Paterson: Water in the Plasticity of Quartzites

t400

1200

(• 1000

• 800

.-• 600

• 400

Present

Work

[]

0

[] Heavitree 1 0 Simpson "Dry" (as-is) 900øC ß Heavitree

{ Canyon Ck "Wet" (water added) 1000øC

ß Simpson [] Jaoul et al. (as-is in CaCO 3)

o Christie et al.

0 Heard/Carter x(Jaoul et al.

as-is in NaCl) []

- '1 ß Christie et al. Jaoul et al.

200- { Parrish et al. ß1 ("wet") O0 200 400 600 800 1000 1200 1400 1600 1800 2000

Confining Pressure (MPa)

Fig 8. Relative flow stresses of quartzites at 10 -5 -1 ß s and 900øC, measured at various confining pressures in the present work and by Heard and Carter [1968], Parrish et al. [1976], J. M. Christie et al. (unpublished manuscript, 1982), and $aoul et al. [this issue], interpolated or extrapolated as discussed in text.

1. The present results and the result of Heard and Carter [1968] (interpolated from their stress-strain curves) refer to the maximum of the stress-strain curve (which is in many cases followed by a shear failure), while the other results purport to be "steady state" flow stresses.

2. The results of Parrish et al. [1976], J.M. Christie et al. (unpublished manuscript, 1982) and Jaoul et al. [this issue], all from solid-medium apparatus, are derived from the flow laws that they have fitted to their results and, in the last two cases, represent some extrapolation.

used, which is thought by Jaoul et al. to retain water more effectively. Thus it would appear that the flow stress is very sensitive to the actual water content during the run and experiments with better control on the water content are

needed before conclusions can be drawn about a

pressure effect in the "dry" cases; the present spread of results may in fact in some significant degree reflect variations in water content rather than pressure effects. (Another flow law, reported in abstract by Hansen and Carter [1982] for quartzite designated as "dry", gives a flow stress of 370 MPa at 900øC, 10-5s -1, 1000 MPa confining pressure, slightly lower than the result of Jaoul

3. The designation "dry" means the as-received et al., for Heavitree and again suggesting some or ~ 100oC oven-dried condition; "wet" means with retention of initial hydroxyl). added water or with water available from

decomposition of talc. J.M. Christie et al. (unpublished manuscript, 1982) showed by infrared

measurements, however, that their "dry" deformed specimens contained a substantial amount of hydroxyl, more than half that in the "wet" specimens, and it can be assumed that all "dry" specimens contain some hydroxyl. Jaoul et al. also made measurements on specimens furnace-dried in vacuum at 1000•C which ga.ve an extrapolated flow stress at 900•C, 10-5s -ñ of over 200 MPa a• 1500 MPa confining pressure, while for similar material Kronenberg and Tullis [this issue] found a flow stress at 10% strain of nearly 1400 MPa under conditions of approximately 1000 MPa confining pressure, 1000•C and 10-6s -1 strain rate.

In the "dry" cases it is seen, first, that for Simpson quartzite the measurements at 800 and

In the "wet" cases, the results of Jaoul et al. [this issue] show increased weakening with increased amount of added water, again indicating strong sensitivity to water content. The actual hydroxyl contents are not known in these cases but an infrared measurement by us on Simpson quartzite heat treated in talc at 1500 MPa confining pressure revealed a lower OH content than for the "wet" specimens in our mechanical tests at 300 MPa, indicating that the "wet" specimens of Jaoul et al. at 1500 MPa were possibly lower in OH content than ours at 300 MPa, while our specimens were stronger; if genuine, such an effect would point to an inverse pressure dependence of flow stress in "wet" specimens, contrary to the usual situation with increasing confining pressure and indicating that any pressure effect associated with a cataclastic component of strain must be absent or subordinate.

1000 MPa confining pressure give rather lower flow This conclusion is consistent with the observations stresses than at 300 MPa, suggesting a pressure of Tullis et al. [1979], who noted a change-over effect. However, for Heavitree quartzite, the from a positive to a negative pressure effect as 1500-MPa strength is comparable to that at 300 MPa the temperature was raised above 700-800•C for when a CaC03 pressure medium has been used in the several silicate and quartz rocks (see almo former case, whereas it is one-half when NaCl is Kronenberg and Tullis [this issue])-. No conclusion

Mainprice and Paterson: Water in the Plasticity of Quartzites 4265

can be drawn from the present work about melt to be approximately of granitic composition, difference of strengths in the • and • stability the pressure increase from 300 MPa to 1000 MPa fields. or higher would not significantly decrease the

solidus temperature (~600øC) under water-excess The Role of Grain Boundary Melt conditions and would even raise it somewhat in the

absence of the excess water [Best, 1982, p. 373]. Weakening from addition of water and/or increase Hence, since for a given chemical composition the

in confining pressure may be associated, at least amount of melt formed depends on the amount by in part, with the presence of melt at grain which the test temperature exceeds the solidus boundaries. This suggestion is consistent with temperature, we conclude that similar amounts of the following observations: melt would be expected at 300 M•a and 1500 MPa.

1. All of our specimens deformed "wet" showed This conclusion is consistent with preliminary glass in the grain boundaries whereas, among "dry" electron microscope observations and further specimens, only Chewings Range Quartzite showed an suggests that the mechanical role of the small appreciable amount of glass in the grain boundaries fraction of partial melt is not the main factor after deformation and this rock gave the lowest determining difference in strengths at 300 M•a stress-strain curve, except for Simpson quartzite and 1500 MPa but that the water itself plays an the result for which is thoughtto be affected by important role, either in the melt or by its larger porosity. penetrating the grains.

2. The higher flow stress in our tests on "dry" At this point, a distinction should be drawn Simpson quartzite compared with Heard and Carter's between the effect just discussed and the partial can probably be correlated with greater loss of melting effect studied by van der Molen and initial water in our case, as evidenced by the Paterson [1979]. In the latter case, much larger presence of small amounts of glass in the vent hole amounts of melt were involved, the mechanism of adjacent to the specimen (there was no vent hole in deformation was largely cataclastic, and the role the spacer immediately adjacent to Heard and of the melt appeared to be to permit freer Carter's specimens although their piston was vented). relative movement of grains or fracture fragments

3. The lesser degree of weakening in our "wet" that were not undergoing internal deformation. experiments relative to the "dry" than for the In the present situation the main part of the other cases in Figure $ may again be related to deformation continues to be intracrystalline, some loss of water from our "wet" assemblies since although modified more or less by the conditions observations on weight loss indicated partial loss at the grain boundaries and by some cataclasis. of water, presumably by migration of the fluid phase along the spacer-jacket interface since a solid spacer was used.

In spite of an approximately constant flow stress being developed in the "wet" experiments, the presence of the melt films and changes in behavior on reloading indicate that chemical- mechanical equilibrium has not been reached. Also, no obvious correlation exists between the approximate initial hydroxyl contents (Figure 1) and the flow stresses. However, the relative strengths of the various quartzites are preserved in the "dry" and "wet" environments (for example, Blackhills quartzite is always stronger than Heavitree), suggesting that there are differences in properties that persist and continue to influence the behavior.

The presence of a melt film would destroy the capacity to transmit shear stress across grain boundaries and lead to increased grain boundary sliding with attendant reduction in strength in the power law creep regime œChen and Argon, 1979]. According to the theory of Chen and Argon (their equation (6a)) the maximum reduction in flow

Therefore, possible intracrystalline changes must now be considered.

The Strength of the Quartz Grains

Since there is presumably some degree of hydrolytic weakening in the grains in the quartzites, it is of interest to compare their flow stresses with those of synthetic hydroxyl-rich single crystals. This comparison needs to be done on the basis of resolved shear stresses on the active slip systems. The problem of relating the macroscopically

øobserved flow stress to the resolved shear stress

on active slip systems within the constituent grains in polycrystalline quartz can be expected to be similar to that in the case of the hexagonal metals since the point symmetry in B-quartz is the same with respect to basal, prismatic, and pyramidal slip systems. Therefore, as a reasonable first approximation we shall apply the self-consistent theory of Hutchinson [1977], which assumes a power creep law and allows for variation of strain from grain to grain. In this case, four independent slip systems are sufficient for the polycrystalline

stress at given strain rate and temperature due deformation, compared with the five required by the to this effect would be by a factor (3/5)1/n where von Mises criterion in the case of the homogeneous n is the stress exponent in the power creep law, straining assumed in the Taylor-Bishop-Hill theory, that is, by at most 10 to 20 percent for n = 6 to 3. and so the calculations of stresses should be more However, the observed strength reduction is approximately 25% at 300-MPa confining pressure and even greater at higher pressures (Figure 8), indicating that other phenomena are important in the strength reduction.

The solidus temperature in the grain boundary regions, which would be about 1000øC for the pure

reliable than in the latter theory when pyramidal slip is much harder than prismatic or basal slip. Hutchinson writes the creep law for slip on individual slip systems as • = •(T/TK)n , where • and T are the resolved shear strain rate and resolved shear stress on the K'th slip system, • is a constant (a reference strain rate), n is

Si02-H20 system [Kennedy et al., 1962], is locally another constant (the stress exponent), and •K is reduced by the presence of muscovite and ferrous a reference resolved shear stress for the K'th orthoclase in the Heavitree quartzite. Taking the system. He then establishes the relationship

4266 Mainprice and Paterson: Water in the Plasticity of 0uartzites

& = •(O/oo)n between the macroscopic uniaxial --

stress o and strain rate & in the polycrystalline aggregate, where • and n are the same constants as for the slip systems in the grains and o o is the uniaxial reference stress for the aggregate.

--

The T K and o o can be taken as measures of the relative "strengths" of the slip systems and the aggregate, respectively. Hutchinson considers basal, prismatic and pyramidal slip systems for which the •K are represented by •A, •B, and TC , respectively.

For application to quartz we note first that single crystal studies such as those of Balderman [1974], Kirby and McCormick [1979], and Linker and Kirby [1981] suggest n -- 3 as a first approx- imation applicable to both basal and prismatic

by intracrystalline slip, with basal and prismatic systems being of approximately equal strength and pyramidal systems being about five times stronger, the application of Hutchinson's model suggests that the individual grains would have a compressive strength in lm_ or 0 • orientation of roughly 400 MPa at 10-5s-1 strain rate for the 900øC "dry" conditions and 300 MPa for the 1000øC "wet" conditions, at 3% strain. In this approximation, these figures will not be greatly changed by moderate variations in the assumptions about relative strengths of slip systems or values of

n. These values of flow stress may be compared with a flow stress of about 140 MPa at 3% strain

at 800•C, 300-MPa confining pressure, for a synthetic single crystal of ñm_ orientation of

slip in the 8 field in the hydrolytically weakened about 400 H/106Si [Kekulawala et al., 1981]; condition and we shall assume this also to apply such a strength also appears to be in approximate to any pyramidal slip that may occur. If the agreement with the observations at atmospheric deformation of the polycrystalline quartz occurred pressure on i TM_ and 0 + orientations made by entirely by intracrystalline slip, one would then Linker and Kirby [1981] (for example, their expect the stress exponent in its creep relation- specimens LC53 and LC55 at 750•C with initial ship to be also equal to 3. In fact, relaxation broadband hydroxyl content of 370 H/106Si), measurements point to much higher values of allowing for some precipitation of water and the n(20 to 30) for the 900•C "dry" conditions, which slightly lower temperature. Adjusting the 800•C suggests either that the rate of slip in the strength to 900øC and 1000•C using an apparent grains is much more sensitive to stress level than activation energy in the 8 field of approximately corresponds to n = 3, perhaps due to being near 100 kJ mo1-1 [Linker and Kirby, 1981] gives the critical weakening temperature, or that there respectively about 100 MPa and 80 MPa. Thus the is some other factor affecting the strain rate in single crystal strength for the quartz grains in the aggregate, such as a component of cataclastic the aggregate deduced above, neglecting any grain- deformation; n = 3 is more nearly consistent with boundary weakening effects, is about four times the observations (3 to 6) for the 1000•C "wet" that of a synthetic crystal with 400 H/106Si of conditions but this correlation may be coincidental broadband hydroxyl. If we now assume, as suggested if the melt at grain boundaries is significantly affecting the flow stress.

Overlooking these difficulties, we consider further the relationship between •o and the ZK' Single crystal studies such as those of Ba•ta and Ashbee [1969, 1970], Hobbs et al. [1972], and Linker and Kirby [1981] indicate that ZA and •B can be taken as being roughly equal in a first approximation (neglecting, for example, the difference in strength of the two prismatic slip systems deduced by Linker and Kirby, a factor of 3 or so, and comparing a mean of these with the basal slip strength), whereas T C seems to be around five times 9rearer. Hutchinson 9ives the

by Kirby and McCormick [1979], that the strain rate is proportional to the hydroxyl content and we take n = 3, the hydroxyl content that would give four

times the strength of the 400 H/106Si content, at given strain rate and temperature in power law creep, would be 400/43; that is, about 6 H/106Si, or even less if n > 3.

A figure of much below 10 H/106Si for the structural or mechanically-effective hydroxyl content of the grains in the quartzites would meet a difficulty in connection with the critical weakening temperature. In order to have a critical weakening temperature below 900•C (say, not more than 850•C in order to avoid a very high

result of a calculation for the case TA/T B -- 1/10 temperature dependence around 900-1000øC) an OH and n = 3 , which gives •o/TB = 2.6. Increasing content of at least 30 H/106Si would be needed if ß A/•B to around 1 would appear likely to

increase this ratio by about a factor of 2 judging by the parallel effect for the case of n = 1 given by his formula (10); we could

then expect •o/wA = •o/TB TM 5 approximately. Hutchinson's Figure 4 also suggests that this ratio will increase as n increases but by a maximum factor of substantially less than two. Thus we may expect TA • TB • •o/5 as a very approximate relationship between the strengths of the most active slip systems and the strength

one can validly extrapolate the data of Blacic and Griggs [see Blacic and Christie, this issue] and Hobbs et al. [1972] (for an extrapolation formula, see Paterson and Kekulawala, [1979]). It may be noted, bowever, that the strength observations of Heard and Carter [1968] do not show any dramatic changes in temperature dependence down to 500•C, putting the concept of a critical weakening temperature in some question in this context, although a component of cataclastic deformation may be obscuring the

of the aggregate. Or, if we compare the aggregate issue. case to a uniaxial test on a single crystal in a "soft" orientation such as im_ prism or 0 + (45 • to a and c axes) where the Schmid factor

--

is nearly 0.5, the compressive strength of the single crystal would be expected to be around 2/5th's of the compressive strength of the aggregate.

Thus, if the deformation in the quartzites studied here were assumed to take place entirely

While admitting the uncertainties surrounding both the critical weakening temperature concept and the Hutchinson analysis, one may conclude that, in terms of synthetic crystal behavior, the best estimate at present for the mechanically effective hydroxyl content of the gra•ins in the quartzites is of the order of 10 H/10•Si, with an uncertainty of possibly less than a factor of 10. In comparing this figure with the total hydroxyl

Mainprice and Paterson: Water in the Plasticity of Ouartzites 4267

content of the rocks under "dry" conditions, which varied from around 1000 to 3000 H/106Si (excluding contributions from mica bands and bulk water but not allowing for some possible loss during the run), it follows that only of the order of 1% of the broadband hydroxyl present in the rock is effective in hydrolyric weakening if we take a synthetic crystal with about 400 H/106Si at 900øC, 300 MPa as a reference. Such a reference raises a number of serious

questions.

infrared absorption (Figures li and lj). Thus in the wet specimens a substantial part at least of the total OH content detected by infrared absorption probably resides in the grain boundary films but in view of the uncertainties in estimation of mean

film thickness and in interpretation of the infrared observations no firm conclusion can be drawn about

the OH content of the grains. In the case of the as-received rock, where such films are absent, the infrared absorption would evidently have to be attributed to an appreciable extent to OH present within the grains themselves. However,

Questions Concerning Hydrolytic Weakenin8 Concepts unfortunately, owing to the lack of suitable spectra from deformed "dry" specimens and to

The question immediately arises as to why the broadband hydroxyl in the quartzites should be relatively ineffective in mechanical weakening compared with that in the single crystals. One way of resolving this paradox would be to suppose that most of the broadband hydroxyl is also initially segregated in the rock in some way, as the molecular water is localized in bubbles, and that diffusion is too slow at 900øC, 300 MPa for this situation to be changed appreciably within the time available in the experiments. Regarding the question of where, and in what state, the main part of the broadband hydroxyl actually resides in the rock, we consider two possible types of localized site:

1. Films in bubbles. We first consider the

case of a gellike water-rich coating on the walls

evidence of the loss of some water from the

specimens during the runs, we again cannot draw any firm conclusions about the OH content of the grains during the deformation. However, the possibility remains that the relatively high strength of the grains reflected in the poly- crystal strengths at 300-MPa confining pressure is evidenced in spite of there being substantial amounts of OH within the grains themselves, as has been observed recently in single crystals by Mackwell and Paterson [1983].

In the experiments under wet conditions it is likely that the melt film acts as a water reservoir, similarly as in dunire under wet conditions [Chopra and Paterson, 1981, unpublished manuscript, 1983]. Furthermore, amorphous films on silicate surfaces are known to be. zones of high

of bubbles that is giving rise to the broad infra- ionic mobility, typically six orders of magnitude red absorption band (we assume that free water higher than for the host silicate œWikby, 1974; would freeze and give an ice band). A lower limit Olbert and Doremus, 1983], so we may expect the to the required volume fraction of bubbles is water activity to be readily equilibrated and so given if we assume that all their volume is buffered throughout the grain boundary regions occupied by the surface coating. At this limit, by this water reservoir, thus maximizing the the ratio of spacing • to diameter d for possible influence of water in grain boundary equally sized and spaced bubbles is given by sliding. The low-energy configuration, including g/d = (•/6x)1/3, where x is the volume fraction the absence of broken b•onds, at the interface of

of the specimen occupied by the coating. If the coating is assumed to contain about 10% water by weight, the maximum amount normally found in silicate glass [Wu, 1980], then its H/Si ratio will be 0.7 (700,000 H/166Si). Therefore its volume fraction x would need to be 1/700 in order to contribute 1000 H/106Si averaged over the whole rock, or a little more if the coating is of lower density than quartz. With x • 1/700 , g/d has to be not greater than 7.

No such closely spaced population of clearly resolved bubbles is seen in the quartzite specimens and could only exist as "bubbles" of less than the order of 10-nm diameter tha• are

either undetectable or obscured by beam damage. Therefore we rule out the idea that the broadband

hydroxyl might reside in coatings in the bubbles that are visible in the electron microscope, as postulated by Kekulawala et al. [1981] for single crystals.

2. A grain boundary phase. In specimens of

amorphous and crystalline tetrahedrally coordinated solids[Spaepen, 1978] may also facilitate the entry of hydroxyl or other species into the quartz grains, further movement being assisted by pipe diffusion in the large number of dislocations intersecting the quartz-melt surface (Figure 7d). However, the extent of penetration of extra hydroxyl into grains in the "wet" experiments remains conjectural. It is quite possible that it is not very significant at 300 MPa but increases appreciably toward 1500 MPa as the hydroxyl diffusion rates become very high [Blacic, 1981; Mackwell and Paterson, 1983], thus explaining lower strengths at higher confining pressure. This explanation seems more likely than one involving the known increased solubility of water in silicate melt at higher pressure [for example, Tomlinson, 1956; Kurjian and Russell, 1958] since the water content of the melt is probably already relatively high at 300 MPa; but differences in the degree of retention

Heavitree quartzite deformed under wet conditions, of added water may be of some importance. the glassy film observed in grain boundaries, of Under "dry" conditions, on the other hand, approximate thickness t = O.1Bm, corresponds to there may be relatively little melt to act as a volume fraction x of about 0.15% (calculated a buffer, leading to the strength being more on a cubic grain approximation for which x = 3t/d sensitive to small differences in total water where d is the grain size, taken to be 200 Bm). content and contributing to the substantial Thus, as in the last paragraph, if this film differences between our results and those at contained 10% water by weight, it would contribute 1500 MPa. In addition, there may be effects a bulk water content of approximately 1000 H/106Si, from continued water loss from specimens during compared with several thousand H/lOVSi detected by the runs, reflected in hardening observed during

4268 Mainprice and Paterson: Water in the Plasticity of Ouartzites

long-term relaxation tests. However, further studies are needed to establish the hydroxyl content of the grains themselves and its influence on their strength, and to relate this influence to the hydrolytic weakening effects observed in synthetic single crystals.

Questions Concerning Geological Relevance

Department of Solid State Physics for access to their infrared spectrophotometer,to J.C.Christie for permission to quote results of stress-strain tests at 1500 MPa by him and his colleagues, and to the University for a Scholarship (D.H.M.). They also thank the persons mentioned in Table 1 for supplying specimens and many colleagues for useful discussions, especially J.N. Boland, P.N. Chopra, I. Jackson, B.E. Hobbs, B.G. Hyde,

The first question connected with extrapolating K.R.S.S. Kekulawala, A.C. McLaren, S.J. Mackwell, laboratory measurements on quartzite to geological I. van der Molen, and J.C. Wilkie. Finally, they conditions is whether the specimens in the tests are in equilibrium with the experimental environment in respect of their "water" content and of any other variables that may be relevant. If we take the high-temperature strength of quartz as a measure of its kinetically effective "water" content (in terms of synthetic crystal behavior), then the previous considerations point to the content in the case of the quartzites being an order of magnitude less than in the case of a single crystal of synthetic quartz that is thought to be in equilibrium with water at 900øC, 300 MPa• Therefore, it would appear that either (1) the quartzites are not in equilibrium in the experiments in respect of the distribution of "water" due to diffusive sluggishness or (2) if the "water" is uniformly distributed in the grains, then there must be other factors that result in the quartz grains in the "wet" quartzites behaving differently from "wet" synthetic crystals.

A second question is bow to relate the behavior of a quartzite in equilibrium under experimental conditions to one in equilibrium under geological conditions in respect of "water" or other relevant defect content,

geological conditions being around 350-650øC and 300-600 MPa confining pressure for typical situations in which extensive flow in quartzites

thank the U.S. Geological Survey for the opportunity to present this work at the Carmel Workshop and to be able to study the other papers presented when revising the manuscript for publication, thereby promoting valuable cross- fertilization of ideas.

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(Received October 19, 1982; revised September 13, 1983; accepted September 13, 1983).