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The OH content of pyrope at high pressure
Chemical Geology 147 Ž1998. 161–171
The OH content of pyrope at high pressure Anthony C. Withers ) , Bernard J. Wood, Michael R. Carroll CETSEI, Department of Geology, UniÕersity of Bristol, Bristol, BS8 1RJ, UK
Abstract The OH contents of pyrope garnets synthesised at 10008C and pressures between 2 and 13 GPa have been measured by infrared spectroscopy. We find that under the same conditions of pressure, temperature, aH 2 O and aSiO 2 the OH content of pyrope is similar to that of grossular. This shows that observed differences in nature between water contents of pyrope and grossular are related to paragenesis rather than reflecting higher intrinsic solubility in grossular. In the presence of excess SiO 2 the H 2 O content increases with pressure to 5 GPa Ž1000 ppm. then decreases to below the detection limit at pressures above 7 GPa. Thus garnet becomes more hydrous with pressure to some critical value, beyond which dehydration occurs even under H 2 O-saturated conditions. This observation is consistent with the measured partial molar volume of water in hydrogarnet which becomes greater than that in fluid water within the pressure range of this study. When corrected to upper mantle conditions we find that pyrope should dehydrate with increasing depth below about 250 km. Pyrope is unlikely therefore to be a major site of water storage in the transition zone and upper mantle. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Mantle; Pyrope; Pressure; Hydroxyl ion; Infrared spectroscopy
1. Introduction The abundance of H 2 O in the earth’s mantle has important effects on its viscosity, the nature of convection and on related processes such as partial melting and volcanic degassing. From typical H 2 O contents of mid-ocean ridge basalts of 0.1–0.3 wt.% ŽMichael, 1988., one would estimate, assuming that water is completely incompatible, that the upper mantle source region contains around 200 ppm water by weight. Recent measurements of the water contents of upper mantle minerals from peridotite xenoliths, summarised by Rossman Ž1996., suggest that )
Corresponding author. Fax: q44 Ž117. 925-3385; e-mail: [email protected]
concentrations of this order would be present not as a free fluid phase, but rather dissolved in nominally anhydrous minerals such as olivine, pyroxene and garnet. Apart from olivine, which can dissolve up to 0.12 wt.% H 2 O at 10 GPa ŽKohlstedt et al., 1996., it is not known whether these measured concentrations are close to upper mantle saturation levels, or whether the xenolith minerals have extensively re-equilibrated on the way to the surface. In the context of water storage at high pressures, garnet is a particularly important phase because grossular ŽCa 3 Al 2 Si 3 O 12 . is known to be capable of dissolving up to 20.80 wt.% H 2 O ŽPassaglia and Rinaldi, 1984. and significant water contents have been found in pyropic garnet from xenoliths ŽBell and Rossman, 1992.. Since garnet increases in abun-
0009-2541r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 7 . 0 0 1 7 9 - 4
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A.C. Withers et al.r Chemical Geology 147 (1998) 161–171
dance from about 5% by volume at 150 km depth to close to 30% at 650 km depth ŽRingwood, 1991., it is important to know whether this phase can dissolve large amounts of H 2 O throughout its compositional and P–T stability range. Water may be incorporated in garnets as OH defects associated with charge balancing or oxidation-reduction reactions, or may substitute for Si in the hydrogarnet substitution: x Si SiŽ gt. q 2H 2 OŽfluid . s w 4H x Si Ž gt . q SiO 2 Žqz r cs.
Ž 1.
The latter mechanism, first verified in grossular by Cohen-Addad et al. Ž1967., has been shown to occur also in natural and synthetic garnets of other compositions, notably pyrope Že.g. Ackermann et al., 1983; Geiger et al., 1991., and in other nominally anhydrous silicate phases, e.g. quartz ŽMcLaren et al., 1983; Weil, 1984; Purton et al., 1992; Lin et al., 1994; McConnell et al., 1995.. In calcic garnets there is complete solid solution between grossular and a hydrogarnet end-member, where Si is completely replaced by groups of four protons in the hydrogarnet substitution. In this paper, ‘hydrogarnet’ will be used to refer to the completely substituted end-member, X 3 Al 2 ŽH 4 O4 . 3 , of a solid solution series with a silicate garnet. ‘Hydrogrossular’ and ‘hydropyrope’ will refer, respectively, to the X s Ca and X s Mg hydrogarnets, a fictive species in the case of hydropyrope where there is limited hydrogarnet substitution. The highest water content thus far recorded in a natural, pyropic garnet of mantle origin is 226 ppm H 2 O ŽRossman, 1996., while naturally occurring grossular garnets may contain over 50 times this amount. On the basis of these observations it has been argued that the pyrope-rich garnets of the mantle can contain only limited amounts of the hydrogarnet component ŽMartin and Donnay, 1972; Lager et al., 1989. and that they are therefore poorer in H 2 O than grossular garnets under similar conditions. Since, however, the paragenesis of hydrogrossular and of mantle pyrope are very different in terms of pressure, temperature and f H 2 O , this inference is difficult to quantify. Our aim here is to characterise the H 2 O contents of synthetic pyrope garnets as a function of pressure from 2 to 13 GPa. This will enable us to constrain
the role of garnet in water storage in the upper mantle through into the transition zone. 2. Experimental methods 2.1. Synthesis experiments End-member pyrope garnets were synthesised over a pressure interval of 2.0–13.0 GPa using piston cylinder and multi-anvil apparatuses. The starting material for all syntheses was an intimate mix of reagent grade oxides, ground under ethanol in an agate pestle and mortar to a grain size of 20–30 mm, fired at 13008C for 24 h, reground, refired for a similar time and reground again. Composition of the starting mix was pyrope q10 wt.% SiO 2 , to which was added 10 wt.% deionised water prior to sealing in Pt capsules. Following recovery, all capsules were checked for the presence of excess water. The products of all synthesis experiments were examined optically as grain mounts in oils of known refractive index. Garnets were checked for optical isotropy and coesite Žor quartz, in experiments - 2.5 GPa, or stishovite ) 9 GPa. was invariably found to be present. The ubiquitous presence of a silica phase fixes the activity of SiO 2 in equilibrium Ž1. at a known value. Garnet crystals varied from 10 to 1000 mm in size and contained fluid, and sometimes solid, inclusions. An isothermal suite of experiments, at a temperature of 10008C, was conducted to investigate the effect of pressure on water incorporation in pyrope. The temperature of all experiments was measured using W97 Re 3 –W75 Re 25 Žtype D. thermocouples. No correction for pressure was applied to the thermocouple emf. Run duration varied from 5 to 96 h ŽTable 1.. Experiments at 2.0–4.0 GPa were performed in an end-loaded piston cylinder apparatus, using NaCl as a pressure medium. The assembly has been calibrated for pressure using the albite–jadeite–quartz reaction determined by Holland Ž1980.. Capsules were pressed into NaCl pedestals and protected from thermocouple intrusion by a 0.5 mm thick Pt disc. On disassembly, the thermocouple junction was checked to have been within 0.5 mm of the capsule and the NaCl pressure medium was dissolved from around the capsule to enable comparison of pre- and
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171
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Table 1 Synthesis conditions and results Label
Run duration Žh.
T Ž8C.
P ŽGPa.
Crystals analysed
na
D b Žcmy1 .
CH 2O c Žwt.% H 2 O.
Uncertainty d Žwt.% H 2 O.
Py77 Py34 Py21 Py95 Py33 Py37 Py98 Py70 Py54 Py88 Gr86 Gr83 Gr71
25.0 5.0 5.0 95.2 5.0 5.0 8.0 6.0 5.0 5.0 41.2 45.8 7.0
1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
2.0 2.5 3.0 3.0 3.5 4.0 6.0 7.0 8.0 13.0 2.0 3.0 8.0
1 3 2 2 4 3 1 1 4 1 1 1 1
2 5 9 2 6 8 1 4 15 3 1 1 1
187 720 735 574 856 1346 1275 0 0 0 539 328 966
0.014 0.054 0.055 0.043 0.064 0.101 0.096 0 0 0 0.036 0.022 0.065
"0.002 "0.015 "0.009 "0.008 "0.007 "0.019 "0.038 q0.016 e q0.003 e q0.018 e –f –f –f
a
Total number of spectra collected and averaged. Integral absorption coefficient, D s HK Ž n . d n , where K s logŽ I0 rIt .rd, I0 is the incident intensity of the infrared beam, It is the transmitted intensity and d is the path length Žsample thickness. in cm. c Concentration of H 2 O in garnet, expressed as wt.% H 2 O, calculated using Eq. Ž3.. The densities of pyrope and grossular were taken to be 3.58 and 3.59 g cmy3 , respectively. The values of I given by Bell et al. Ž1995. and Rossman and Aines Ž1991. were used for pyrope and grossular, respectively. d Errors are "1 sd, except for experiment Py98, where error is estimated. Other exceptions are noted. e Detection limit Žsee Appendix A.. f No errors are given for single-point analyses. b
post-experiment capsule weights. In this way, water loss from the capsule was shown to be insignificant; weighing precision was "0.1 mg. A Walker-style multi-anvil apparatus ŽWalker et al., 1990. was used for experiments at pressures greater than 4.0 GPa. 1 inch cubic anvils with 12 mm truncated edge length were used in experiments at 5.0–8.0 GPa, and a 6 mm truncated edge length assembly was used for the 13.0 GPa experiment. Octahedra with integral gasketing fins were cast from MgO-based ceramic. Furnaces were constructed by coring sintered LaCrO 3 and the furnace ends capped with 1 mm thick Mo discs. The cell was assembled with the thermocouple in an axial position and the thermocouple junction in contact with the Pt capsule. The sample occupied up to 1.5 mm of the length of the capsule, which was positioned so that this part of the capsule was in the centre Žhot spot. of the furnace. Further details of experimental procedure and pressure calibration are given in Pawley and Wood Ž1995.. Quench rates in the multi-anvil apparatus were typically observed to exceed 3008C sy1 .
2.2. Infrared spectroscopy Several crystals were selected from each experiment and mounted individually in epoxy resin so that doubly-polished crystal slabs could be prepared. After polishing, the crystal slabs were extracted from the resin and cleaned using CCl 4 . An electronic micrometer with precision of 1 mm was used to measure sample thickness, which ranged between 25 and 300 mm. Crystals were typically euhedral and contained both fluid and solid inclusions which were invariably concentrated in the crystal centres. Concentrations of inclusions in crystals synthesised at 2.0 GPa were sufficiently high as to prevent IR analysis in most cases; only the very thin Ž40 mm. rim of these crystals would allow a clear path for analysis. In contrast, pyrope synthesised at G 3.0 GPa typically formed larger Ž) 100 mm. crystals with fewer inclusions, thus allowing several analyses per crystal. Similar differences in inclusion density were described by Geiger et al. Ž1991.. When polishing crystals ) 500 mm in diameter, the thickness of the polished slab was a compromise between being
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thin enough to provide an inclusion-free window while attempting to maximise thickness to enable a sufficiently high signal-to-noise ratio for the analysis of low OH concentrations. A Nicolet 800 FTIR spectrometer was used to analyse for OH using an unpolarised IR beam, KBr ŽGe on KBr substrate. beamsplitter, globar source and liquid N2 cooled HgCdTe detector attached to a microscope. Where samples were large enough, a focused 100 mm spot size on the crystal surface and redundant aperturing were employed. Smaller apertures were used to analyse areas of down to 40 = 40 mm. The free-standing crystal slabs were placed over suitably sized apertures Ž50–500 mm in diameter. in Mo plates, and backgrounds were collected through similar apertures. Wherever possible clear portions of crystals were chosen for analysis. In order to increase spectral quality the microscope was enclosed in a chamber which could be purged with N2 gas, thereby ensuring low, constant atmospheric water levels. Spectra were collected at resolutions of 4 and 0.5 wavenumbers. Spectral baselines were fitted by computer and correction was made for broad band absorption, assigned to molecular water and, if necessary, interference fringes were subtracted. Further details are given in the Appendix A.
3. Results All pyropes synthesised at pressures - 7.0 GPa have an asymmetric peak in the absorption spectrum centred at 3630 cmy1 ŽFig. 1., which corresponds exactly to those in the OH spectra of synthetic pyropes of a number of previous studies ŽAckermann et al., 1983; Geiger et al., 1991; Hosch ¨ and Langer, 1996.. Ackermann et al. Ž1983. suggested that this peak is caused by OH-vibrations of tetrahedral ŽOH.44clusters, as observed in hydrogrossular Že.g. CohenAddad et al., 1967.. The assignment of the 3630 cmy1 band in pyrope to a hydrogarnet component was supported by the low-temperature peak splitting behaviour of the band observed by Geiger et al. Ž1991.. The hydrogarnet peak in grossular garnets synthesised under similar conditions is centred at 3622 cmy1 , with a shoulder at 3612 cmy1 ŽFig. 1.. In grossular, the principal hydrogarnet peak is much sharper than that in pyrope, with a full width at
Fig. 1. Unpolarised IR spectra in the OH stretching region between 3300 and 3800 cmy1 : Ža. grossular Žexperiment Gr83 of Table 1. synthesised at 3.0 GPa and 10008C, 0.25 cmy1 resolution, and Žb. pyrope Žexperiment Py21., also synthesised at 3.0 GPa and 10008C. Resolutions 4 cmy1 . Spectra are normalised to 1 mm thickness.
half-height ŽFWHH. of 7 cmy1 , as opposed to 55 cmy1 in the case of pyrope. The grossular spectra also sometimes display weak absorbance bands at positions of 3561, 3602, 3656, and 3665 cmy1 . In general, the crystals produced in our study are too small to allow traversal analysis, but where this has been attempted no significant zoning was observed. A rim to rim traverse of one 1000 mm pyrope crystal, analysing discrete 100 mm spots, showed no indication of systematic zoning in OH, although "17% variation in absorbance was observed. This is in contrast to Geiger et al. Ž1991. who found that in some cases absorbance was higher in the central regions of synthetic pyrope crystals than at the rims. Wherever possible relatively inclusion-free areas were chosen for analysis, avoiding cracks within the crystal and poorly polished surfaces. Spectra were consistent in shape and peak height. In the case of all grossular syntheses and pyrope experiments at 6.0, 7.0 and 13.0 GPa, the small crystal size ŽF 100 mm. made FTIR analysis difficult. In each of these experiments only one crystal was analysed and the analysed spot size was similar to the size of the crystal, so we cannot tell
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171
from these analyses whether there is any zoning in OH concentration within the crystals. Proportional error in thickness measurement is also greater in these cases. Pyrope garnets synthesised at 7–8 GPa and 10008C contain inclusions of euhedral, acicular crystals of up to 100 mm in length and ; 1 mm in diameter. These inclusions were identified as topazOH ŽAl 2 SiO 3 ŽOH. 2 . on the basis of optical properties and IR spectroscopy. In grain mounts the crystals are cyclically twinned, as reported by Wunder et al. Ž1993. for topaz-OH. IR analyses of pyrope crystals containing the inclusions produce absorption spectra with two peaks in the OH stretching region, centred at 3600 and 3530 cmy1 , which is consistent
165
with the OH absorption pattern for topaz-OH ŽWunder et al., 1993.. The peak at 3630 cmy1 , attributed to the hydrogarnet substitution in pyrope synthesised at - 7 GPa, is not present in crystals synthesised at 7–8 GPa. The intensities of the absorption bands at 3600 and 3530 cmy1 are proportional to the inclusion density, confirming that these peaks are caused solely by inclusions of the hydrous topaz phase. IR absorption spectra of several pyrope garnets synthesised in this study are shown in Fig. 2. The OH concentration in pyrope increases over the pressure range 2.5–4.0 GPa, as indicated by the increasing intensity of the peak at 3630 cmy1 in the spectrum. At intermediate pressures of 3.5–6.0 GPa the OH concentration remains constant within analytical error, while at pressures G 7.0 GPa no IR absorption due to hydrogarnet is detectable, even though the experiments were H 2 O-saturated. In contrast, the hydrogarnet band in grossular persists at least to 8.0 GPa at 10008C; under the same conditions pyrope was found to be truly anhydrous. 3.1. Absolute OH concentrations Infrared spectroscopy is not an intrinsically quantitative technique. In order to determine the concentration of a particular species a constant of proportionality Žthe molar absorptivity. must first be determined through some independent analytical technique. Molar absorptivity varies with frequency and, because the vibrational energy of the OH bond is sensitive to its atomic environment, the frequency of OH absorbance bands differs between crystal structures and between crystallographic sites within a lattice. The concentration of a species may be calculated using the Beer–Lambert law: cs
1
D
˜ K Ž n . dÕ˜ s H I Õ˜ I Õ2
Ž 2.
1
Fig. 2. Unpolarised IR spectra of pyrope in the OH stretching region between 3000 and 4000 cmy1 . All spectra are normalised to 1 mm thickness and labelled with the synthesis pressure; temperature of all experiments was 10008C.
where c is the concentration of the species in mol ly1 , K Ž n . is the absorption coefficient as a function of wavenumber Ž n . and I is the integral molar absorption coefficient. K s logŽ I0rIt .rd, where d is the path length Žsample thickness. in cm, I0 is the incident intensity of the infrared beam and It is the
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171
166
transmitted intensity. The mass fraction of H 2 O may be calculated as wt.% using the relation: CH 2 O s
1.8 = D I=r
Ž 3.
where C H 2 O is the concentration of H 2 O in wt.% and r is the density of the host in g cmy3 . Bell et al. Ž1995. have determined the value of I for H 2 O in pyropic garnet based on H 2 manometry. The sample used in the calibration was a garnet m eg acry st o f ap p ro x im ate co m p o sitio n Py67 Alm 21Gr11 , which had broad absorbance peaks in the OH stretching region of the IR spectrum centred at 3571 and 3512 cmy1 . Bell et al. Ž1995. derived a value of 6700 " 670 l moly1 cmy2 for I H 2 O , which is around 20 times the value obtained by Aines and Rossman Ž1984. for garnets of composition Py 75 Alm 25 . Bell et al. Ž1995. suggest that the discrepancy is caused by impurities in the samples and by analytical errors in the earlier study. Aines and Rossman Ž1984. derived a value of 8000 l moly1 cmy2 for a near end-member grossular, and a value of 7492 l moly1 cmy2 may be calculated from the calibration of two hydrogrossular garnets by Rossman and Aines Ž1991.. The concentrations of OH Žexpressed as wt.% H 2 O. in Table 1 and Fig. 3 were calculated using the
integral molar absorption coefficients of Bell et al. Ž1995. and Rossman and Aines Ž1991. for pyrope and grossular, respectively. Using these calibrations, OH concentrations in pyrope synthesised in this study vary from 0.014 to 0.101 wt.% H 2 O at pressures - 7 GPa, while the maximum OH content measured in grossular is 0.065 wt.% H 2 O at 8.0 GPa and 10008C. At pressures - 7 GPa, therefore, it appears that under fixed pressure, temperature and aSiO 2 conditions the solubility of OH in pyrope is similar to that in grossular. The OH vibration energies and FWHH of the synthetic end-member garnets of this study are different from those observed in the natural garnets on which calibrations are based. The absolute concentrations of H 2 O in pyrope reported in this paper are, therefore, subject to revision in the event of an independent calibration for H 2 O in end-member pyrope. However, the relative changes in concentration are independent of an absolute calibration, so the conclusions of this paper concerning the effect of pressure on the relatiÕe solubility of H 2 O in pyrope will be unaffected.
4. Discussion 4.1. Attainment of equilibrium
Fig. 3. Variation in concentration of H 2 O in pyrope Ž C H 2 O . with pressure at 10008C. C H 2 O expressed as wt.% H 2 O, calculated from FTIR analysis using integral molar absorption coefficient, I s6700 l moly1 cmy2 ŽBell et al., 1995.. Error bars are "1 standard deviation, except for point at pressures6.0 GPa, where error is estimated. Where no hydrogarnet band is observed in the IR spectrum a detection limit is given, based on sample thickness and spectral noise levels Žsee Appendix A..
Based on dehydrogenation experiments using natural garnet megacrysts, Wang et al. Ž1996. calculate that the H 2 O concentration in the centre of a 1 mm pyrope crystal can be reduced by ; 20% in 2 hours. Under hydrothermal annealing at 10008C and 300 MPa, hydrogen diffusion in olivine crystals can occur over distances of millimetres in time scales of hours or minutes ŽMackwell and Kohlstedt, 1990.. Re-equilibration of hydrogen defects in olivine requires transport of Si, O and metal defects on the scale of the sample ŽMackwell et al., 1988.. If similar diffusion rates were applicable to pyrope one might anticipate complete re-equilibration in less than 5 h at 10008C ŽS.J. Mackwell, pers. commun... However, in a previous study significant zoning in H 2 O concentration has been measured in synthetic pyrope ŽGeiger et al., 1991., which indicates that there are problems with equilibration of the hydrous
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171
component in pyrope. The difficulty of re-equilibration has been confirmed by performing synthesis experiments of varying duration at 10008C and 3.0 GPa. No detectable difference in OH concentration was found between experiments of 5 h and 96 h duration. This suggests that re-equilibration of the hydrogarnet defect is actually rather slow under the conditions of the synthesis experiments. It was therefore impossible to prove unequivocally that equilibrium has been attained by means of reversal experiments. 4.2. FTIR spectroscopy— band assignment The asymmetry and broadening of the hydrogarnet peak in pyrope relative to grossular may indicate that the OH atomic positions are less tightly constrained in pyrope than in grossular, or that there is increased anharmonicity in the OH vibration. Part of the apparent asymmetry is due to convolution of the two vibrations predicted for a site of D 2d or S 4 symmetry ŽHarmon et al., 1982.. Resolution of these two bands has been previously observed in low-temperature Ž78 K. IR measurements of pyrope crystals ŽGeiger et al., 1991. and the emergence of a shoulder on the high-frequency side of the hydropyrope peak was evident in the low-temperature IR spectrum measured by Hosch ¨ and Langer Ž1996.. In grossular, the hydrogarnet peak can be seen to be composite from the presence of the shoulder at 3612 cmy1 ŽFig. 1.. Both natural ŽRossman and Aines, 1991. and synthetic ŽHosch and Langer, 1996. grossular ¨ garnets often display numerous peaks in the OH stretching region, in addition to those attributed to hydrogrossular, suggesting a role for incorporation mechanisms other than the hydrogarnet substitution. Natural grossular garnets of this type were analysed in the single-crystal NMR study of Cho and Rossman Ž1993.. Their results provide evidence of an alternative incorporation mechanism involving pairs of protons, rather than the four proton clusters of the hydrogarnet substitution. In contrast, neither the pyrope nor the grossular spectra which form the basis of our study show absorption bands in the OH stretching region other than those which we assign to hydrogarnet, so that the latter appears to be the major solution mechanism under the conditions of synthesis.
167
4.3. Crystal chemical and thermodynamic considerations It has been observed that garnets in which the shared edge of the octahedral site is longer than the ˚ unshared edge, i.e. those in which r X4 ) 1.01 A ŽNovak and Gibbs, 1971., are more likely to have a higher hydrous component than those, like pyrope, where the shared edge is shorter than the unshared ŽSacerdoti and Passaglia, 1985.. Distance-leastsquares calculations indicate that incorporation of hydrogarnet decreases the length of the shared edge and increases that of the unshared, leading to the suggestion that a limit to the amount of hydration of the garnet structure will be reached at lower OH concentrations in pyrope than in grossular ŽLager et al., 1989.. We note, however, that under the same physicochemical conditions there is little difference in OH contents of end-member pyrope and grossular ŽTable 1.. Measurements of the lattice parameters of endmembers of the grossular–hydrogrossular series ŽSkinner, 1956; O’Neill et al., 1993. enable us to calculate the partial molar volume of water in the hydrogarnet substitution in grossular, V H 2 OŽ gr . s 14.302 cm3 moly1 at 298 Kr0.1 MPa. When we compare this with the volume–pressure curve for water at elevated temperatures Žcalculated using a parameterisation of the results of the molecular dynamics study of Brodholt and Wood, 1993., we find that the molar volume of free fluid water Ž VH 2 O . is greater than in garnet at low pressure and crosses over to become lower at high pressure ŽFig. 4.. This means, in principle, that garnet should become more hydrous with increasing pressure until the crossover point is reached and then progressively dehydrate at higher pressures. The predicted behaviour is broadly what we observe in pyrope ŽFig. 3.. In order to extend the model to our experimental observations we need however to consider the equilibrium between pyrope, H 2 O fluid, the hydropyrope component and coesite: Mg 3 Al 2 Si 3 O 12 q 6H 2 O s Mg 3 Al 2 Ž H 4 O4 . 3 pyrope
fluid
hydropyrope
q 3SiO 2 coesite
Ž 4.
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Fig. 4. Molar volume of H 2 O Ž V H 2 O ., compared to partial molar volume of H 2 O in garnet, which was calculated on the basis of equilibrium with a free SiO 2 phase ŽEq. Ž1... VH 2 O in the fluid was calculated using a parameterisation of the molecular dynamics simulation of Brodholt and Wood Ž1993.. Dashed line: partial molar volume of H 2 O in grossular Ž V H 2 OŽ gr . ., calculated using unit cell and compressibility data for grossular and hydrogrossular ŽOlijnyk et al., 1991., with thermal expansivity and compressibility for grossular and coesite from Fei Ž1995. and Bass Ž1995.. The same thermal expansivity was used for grossular and hydrogrossular. Line a – a: V H 2 OŽ py . calculated assuming the same increase in compressibility and a larger increase in cell volume than in the grossular–hydrogrossular system. Line b – b: VH 2 OŽ py . calculated assuming equal compressibility for pyrope and hydropyrope.
To a good approximation, the point in the pressure domain where OH concentrations in pyrope reach their maximum corresponds to the point at which DV for Eq. Ž4. becomes zero in passing from negative to positive values. On the basis of our experimental results, this is shown to occur at a pressure of ; 5 GPa. The point at which DV becomes zero is illustrated graphically in Fig. 4 as the point where VH 2 OŽ py . intersects the VH 2 O curve Ž DVreaction s 6Ž VH OŽ py . y V H O ... If we assume that 2 2 the increase in volume that occurs when Si is replaced by 4H in pyrope is the same as that which occurs in grossular, we can use the measured volume data for the end-members of the grossular–hydrogrossular series to calculate V H 2 OŽ gt. . When, however, account is taken of the expansivities and compressibilities of the phases involved in Eq. Ž4. ŽOlijnyk et al., 1991; Fei, 1995; Bass, 1995., and we assume similar compressibility contrasts in the
grossular–hydrogrossular and pyrope–hydropyrope systems, the calculated V H 2 OŽ gt. curve does not intersect the V H 2 O curve at pressures of - 9 GPa. The estimated partial molar volume of water in garnet is apparently too low. The discrepancy would be resolved if the compressibility of hydropyrope were less than that of hydrogrossular Žone possibility is that the hydrogarnet defect concentration is sufficiently small for the surrounding lattice structure to maintain its incompressibility., or if VH 2 OŽ py . is greater than V H 2 OŽ gr . Ži.e. the increase in volume which occurs with the 4H for Si substitution is greater in pyrope than in grossular.. These are taken as end-member cases. Constraining the V H 2 OŽ py . curve to intersect the V H 2 O curve at 5 GPa, as inferred from the experimental data, and assuming a similar contrast in elastic properties to those calculated using the grossular–hydrogrossular data, we find that V H 2 OŽ py . of 15.3 cm3 moly1 Žat 298 K, 0.1 MPa. is required to explain the experimental observations. Alternatively, if the elastic properties of hydropyrope and pyrope are assumed to be similar to one another then V H 2 OŽ py . of 14.3 cm3 moly1 will yield the crossover at 5 GPa. 4.4. Implications for water storage and transport in mantle garnets Most natural garnets of mantle provenance show simple IR absorption in the OH stretching region, which has led to the suggestion that the hydrogarnet substitution is the dominant mechanism for water incorporation ŽAines and Rossman, 1984.. However, both natural and synthetic garnets with low water contents sometimes show a more complicated spectrum with up to 18 bands ŽGeiger et al., 1991; Rossman and Aines, 1991; Rossman, 1996; Hosch ¨ and Langer, 1996., indicating that other incorporation mechanisms can occur. Complicated spectra of the latter type were not observed, however, in the end-member pyrope garnets that are the basis of our study ŽFig. 1.. Our goal now is to apply our experimental observations to the mantle, assuming that pyrope is a good analogue for mantle garnet. To do this we need to take account of two differences between the experimental conditions and those of the mantle: the mantle is not an isothermal system, and SiO 2 is not
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171
likely to exist as a free phase Žcoesite. in the mantle. If we assume as a first approximation that SiO 2 activity in the mantle is controlled by equilibrium between olivine and orthopyroxene: Mg 2 SiO4 q SiO 2 s 2MgSiO 3 forsterite
Ž 5.
enstatite
then Eq. Ž1. can be rewritten as: x Mg 2 SiO4 q X 3 Al 2 Si 3 O 12 q 2 x H 2 O s 2 x MgSiO 3 q X 3 Al 2 Ž SiO4 . 3yx Ž H 4 O4 . x
Ž 6.
or in defect notation: x x x Si SiŽ gt. q 4H ŽH 2 O. s w 4H x Si Ž gt . q Si Žen .
169
enstatite is about 19.0 cm3 moly1 at 298 Kr0.1 MPa, while the molar volume of coesite is 20.6 cm3 moly1 . Taking this into account, and using a mantle geotherm ŽWyllie, 1988., we calculate that, for either of the end-member cases considered above, the crossover point Žonset of dehydration. will occur at depths - 300 km ŽFig. 5.. This remains true for any reasonable mantle geotherm. We therefore conclude that the hydrogarnet substitution in pyrope does not provide a viable mechanism for incorporation of water within the transition zone.
Ž 7.
The effective partial molar volume of water in garnet is lower in this case than when coesite is present because the partial molar volume of SiO 2 in
Fig. 5. Molar volume of H 2 O Ž V H 2 O ., compared to partial molar volume of H 2 O in pyrope Ž VH 2 OŽ py . .. VH 2 OŽ py . calculated on the basis of equilibrium with olivineqpyroxene ŽEq. Ž6... Line a – a: V H 2 O in pyrope assumed to be larger than in grossular as derived from Fig. 4 Žsee text.. Line b – b: V H 2 OŽ py . assumed equal to V H 2 OŽ gr . but less compressible than in grossular Žsee Fig. 4 and text.. Solid lines correspond to a geotherm for upwelling mantle beneath thin lithosphere and chain lines are calculated for a cratonic geotherm ŽWyllie, 1988..
5. Conclusions Ž1. When pyrope and grossular are synthesised under similar conditions with respect to pressure, temperature, f H 2 O , aSiO 2 and at pressures - 7 GPa, a hydrogarnet component is present in similar concentrations in each. At higher pressure a hydrogarnet component persists in grossular, while pyrope becomes anhydrous. Ž2. With increasing pressure, the hydrogarnet component in pyrope synthesised at 10008C, P H 2 O s P TOT increases to a maximum of about 1000 ppm at 5–6 GPa, then decreases to near zero at pressures G 7 GPa. This is in accord with the estimated partial molar volume of H 2 O in garnet, which becomes greater than the molar volume of free water at the pressure where maximum solubility is reached. Ž3. Using the constraint of a solubility maximum for OH in pyrope at 5–6 GPa imposed by the experimental data, we calculate that structural water may be present in the form of hydrogarnet in mantle garnets at up to 9 GPa Ž270 km depth., the hydrogarnet component persisting to pressures greater than the limiting pressures we observe in experiments with excess SiO 2 . Within and below the transition zone, however, the hydrogarnet substitution appears not to be a viable incorporation mechanism for water in pyrope.
Acknowledgements A.C.W. acknowledges the support of a NERC research studentship. We thank K. Langer and S.J. Mackwell for constructive reviews.
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Appendix A. Spectral analysis The baseline in the 3000–3800 cmy1 region of the spectrum was fitted with a cubic function. The second-order differentials at the end points of this range were prescribed; they were chosen by arbitrarily pre-averaging the spectral data outside of the area covered by the fitted function and the quality of fit judged by eye. Consistency of fits was judged by refitting spectra on separate occasions and comparing the baseline-subtracted spectra. A broad absorption band centred at 3415 cmy1 , attributed to molecular water, was present when fluid inclusions were present in the IR beam path. The intensity of this band was proportional to the fluid inclusion density, as observed in the synthetic pyrope of Ackermann et al. Ž1983.. When crystal size was small, it was sometimes only possible to analyse a single spot within the crystal; in these cases the broad band absorbance often formed a significant proportion of the absorption spectrum in the OH stretching region. All spectra were corrected for this molecular absorption by subtracting a scaled broad band spectrum for pure water from the composite sample spectrum. The small size of the crystals produced in many experiments necessitated the use of small Ž40 = 40 mm. apertured analysis spots of thinly ground crystal slabs. This introduced several analytical problems, not least of which was the increased difficulty in preparing the thinly sectioned crystals, mounting them for analysis and accurately measuring their thickness. A strong interference fringe was often present in spectra from thin crystals andror small analysed spot sizes. When these interferences made a significant contribution to the spectrum Ži.e. when the amplitude of the sinusoidal interference was significant relative to the intensity of the OH absorption peaks., they were fitted with an appropriate function and subtracted from the spectrum in order to generate spectra which could be directly compared ŽFig. 6.. Where no hydrogarnet absorption band was evident in the IR spectrum, the detection limit for this species was calculated on a per crystal basis. An ideal hydrogarnet absorption peak was scaled such that its apparent intensity was just significant relative to the spectral ‘noise’ in this region. The apparent noise was primarily due to variations in background,
Fig. 6. Removal of interference fringes: a sin c function Ždotted line. is subtracted from the raw spectrum Žtop. to enable comparison of spectra and integration of peaks.
atmospheric water concentrations. The intensity of the reduced hydrogarnet peak represented the minimum detectable concentration of OH in the sample. The detection limit was calculated from the integral absorption coefficient for the scaled hydrogarnet peak.
References Ackermann, L., Cemic, ´ L., Langer, K., 1983. Hydrogarnet substitution in pyrope: a possible location for ‘water’ in the mantle. Earth Planet. Sci. Lett. 62, 208–214. Aines, R.D., Rossman, G.R., 1984. The hydrous component in garnets: pyralspites. Am. Mineral. 69, 1116–1126. Bass, J.D., 1995. Elasticity of minerals, glasses, and melts. In: Ahrens, T.J. ŽEd.., AGU Reference Shelf 2—Mineral Physics and Crystallography, a Handbook of Physical Constants. American Geophysical Union, Washington, DC, pp. 45–63. Bell, D.R., Rossman, G.R., 1992. The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa. Contrib. Mineral. Petrol. 111, 161–178. Bell, D.R., Ihinger, P.D., Rossman, G.R., 1995. Quantitative analysis of trace OH in garnet and pyroxenes. Am. Mineral. 80, 465–474. Brodholt, J.P., Wood, B.J., 1993. Simulations of the structure and thermodynamic properties of water at high pressures and temperatures. J. Geophys. Res. 98, 519–536. Cho, H., Rossman, G.R., 1993. Single-crystal NMR studies of
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171 low-concentration hydrous species in minerals: grossular garnet. Am. Mineral. 78, 1149–1164. ´ Cohen-Addad, C., Ducros, P., Bertaut, E.F., 1967. Etude de la substitution du groupement SiO4 par ŽOH.4 dans les composes ´ Al 2 Ca 3 ŽOH.12 et Al 2 Ca 3 ŽSiO4. 2.16 ŽOH. 3.36 de type grenat. Acta Crystallogr. 23, 220–230. Fei, Y., 1995. Thermal expansion. In: Ahrens, T.J. ŽEd.., AGU Reference Shelf 2—Mineral Physics and Crystallography, a Handbook of Physical Constants. American Geophysical Union, Washington, DC, pp. 29–44. Geiger, C.A., Langer, K., Bell, D.R., Rossman, G.R., Winkler, B., 1991. The hydroxide component in synthetic pyrope. Am. Mineral. 76, 49–59. Harmon, K.M., Gabriele, J.M., Nuttal, A.S., 1982. Hydrogen bonding in the tetrahedral O4 H 44y cluster in hydrogrossular. J. Molec. Struct. 82, 213–219. Holland, T.J.B., 1980. The reaction albites jadeiteqquartz determined experimentally in the range 600–12008C. Am. Mineral. 65, 129–134. Hosch, A., Langer, K., 1996. Single-crystal MFTIR spectra at ¨ various temperatures of synthetic end member garnets in the OH valence vibrational region. Phys. Chem. Miner. 23, 305– 306. Kohlstedt, D.L., Keppler, H., Rubie, D.C., 1996. Solubility of water in the a-phase, b-phase and g-phase of ŽMg,Fe. 2 SiO4 . Contrib. Mineral. Petrol. 123, 345–357. Lager, G.A., Armbruster, T., Rotella, F.J., Rossman, G.R., 1989. OH substitution in garnets: X-ray and neutron diffraction, infrared, and geometric-modeling studies. Am. Mineral. 74, 840–851. Lin, J.S., Payne, M.C., Heine, V., McConnell, J.D.C., 1994. Ab-initio calculations on ŽOH.4 defects in a-quartz. Phys. Chem. Miner. 21, 150–155. Mackwell, S.J., Kohlstedt, D.L., 1990. Diffusion of hydrogen in olivine: implications for water in the mantle. J. Geophys. Res. 95, 5079–5088. Mackwell, S.J., Dimos, D., Kohlstedt, D.L., 1988. Transient creep of olivine: point defect relaxation times. Philos. Mag. A— Phys. Condens. Matter Defects Mech. Properties 57, 779–789. Martin, R.F., Donnay, G., 1972. Hydroxyl in the mantle. Am. Mineral. 57, 554–570. McConnell, J.D.C., Lin, J.S., Heine, V., 1995. The solubility of w4HxSi defects in a-quartz and their role in the formation of molecular water and related weakening on heating. Phys. Chem. Miner. 22, 357–366. McLaren, A.C., Cook, R.F., Hyde, S.T., Tobin, R.C., 1983. The mechanisms of the formation and growth of water bubbles and associated dislocation loops in synthetic quartz. Phys. Chem. Miner. 9, 465–482.
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Michael, P.J., 1988. The concentration, behavior and storage of H 2 O in the suboceanic upper mantle: implications for mantle metasomatism. Geochim. Cosmochim. Acta 52, 555–566. Novak, G.A., Gibbs, G.V., 1971. The crystal chemistry of silicate garnets. Am. Mineral. 56, 791–825. Olijnyk, H., Paris, E., Geiger, C.A., Lager, G.A., 1991. Compressional study of katoite wCa 3 Al 2 ŽO4 H 4 . 3 x and grossular garnet. J. Geophys. Res. 96, 14313–14318. O’Neill, B., Bass, J.D., Rossman, G.R., 1993. Elastic properties of hydrogrossular garnet and implications for water in the upper mantle. J. Geophys. Res. 98, 20031–20037. Passaglia, E., Rinaldi, R., 1984. Katoite, a new member of the Ca 3 Al 2 ŽSiO4 . 3 –Ca 3 Al 2 ŽOH.12 series and a new nomenclature for the hydrogrossular group of minerals. Bull. Mineral. ´ 107, 605–618. Pawley, A.R., Wood, B.J., 1995. The high pressure stability of ˚ phase: potential storage sites for H 2 O in talc and 10 A subduction zones. Am. Mineral. 80, 998–1003. Purton, J., Jones, R., Heggie, M., Oberg, S., Catlow, C.R.A., 1992. LDF pseudopotential calculations of the a-quartz structure and hydrogarnet defect. Phys. Chem. Miner. 18, 389–392. Ringwood, A.E., 1991. Phase-transformations and their bearing on the constitution and dynamics of the mantle. Geochim. Cosmochim. Acta 55, 2083–2110. Rossman, G.R., 1996. Studies of OH in nominally anhydrous minerals. Phys. Chem. Miner. 23, 299–304. Rossman, G.R., Aines, R.D., 1991. The hydrous components in garnets: grossular–hydrogrossular. Am. Mineral. 76, 1153– 1164. Sacerdoti, M., Passaglia, E., 1985. The crystal structure of katoite and implications within the hydrogrossular group. Bull. Mineral. ´ 108, 1–8. Skinner, B.J., 1956. Physical properties of end-members of the garnet group. Am. Mineral. 51, 428–436. Walker, D., Carpenter, M.A., Hitch, C.M., 1990. Some simplifications to multianvil devices for high-pressure experiments. Am. Mineral. 75, 1020–1028. Wang, L., Zhang, Y., Essene, E.J., 1996. Diffusion of the hydrous component in pyrope. Am. Mineral. 81, 706–718. Weil, J.A., 1984. A review of electron spin spectroscopy and its application to the study of paramagnetic defects in crystalline quartz. Phys. Chem. Miner. 10, 149–165. Wunder, B., Rubie, D.C., Ross, C.R., Medenbach, O., Seifert, F., Schreyer, W., 1993. Synthesis, stability, and properties of Al 2 SiO4 ŽOH. 2 — a fully hydrated analog of topaz. Am. Mineral. 78, 285–297. Wyllie, P.J., 1988. Magma genesis, plate tectonics, and chemical differentiation of the Earth. Rev. Geophys. 26, 370–404.
The OH content of pyrope at high pressure Anthony C. Withers ) , Bernard J. Wood, Michael R. Carroll CETSEI, Department of Geology, UniÕersity of Bristol, Bristol, BS8 1RJ, UK
Abstract The OH contents of pyrope garnets synthesised at 10008C and pressures between 2 and 13 GPa have been measured by infrared spectroscopy. We find that under the same conditions of pressure, temperature, aH 2 O and aSiO 2 the OH content of pyrope is similar to that of grossular. This shows that observed differences in nature between water contents of pyrope and grossular are related to paragenesis rather than reflecting higher intrinsic solubility in grossular. In the presence of excess SiO 2 the H 2 O content increases with pressure to 5 GPa Ž1000 ppm. then decreases to below the detection limit at pressures above 7 GPa. Thus garnet becomes more hydrous with pressure to some critical value, beyond which dehydration occurs even under H 2 O-saturated conditions. This observation is consistent with the measured partial molar volume of water in hydrogarnet which becomes greater than that in fluid water within the pressure range of this study. When corrected to upper mantle conditions we find that pyrope should dehydrate with increasing depth below about 250 km. Pyrope is unlikely therefore to be a major site of water storage in the transition zone and upper mantle. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Mantle; Pyrope; Pressure; Hydroxyl ion; Infrared spectroscopy
1. Introduction The abundance of H 2 O in the earth’s mantle has important effects on its viscosity, the nature of convection and on related processes such as partial melting and volcanic degassing. From typical H 2 O contents of mid-ocean ridge basalts of 0.1–0.3 wt.% ŽMichael, 1988., one would estimate, assuming that water is completely incompatible, that the upper mantle source region contains around 200 ppm water by weight. Recent measurements of the water contents of upper mantle minerals from peridotite xenoliths, summarised by Rossman Ž1996., suggest that )
Corresponding author. Fax: q44 Ž117. 925-3385; e-mail: [email protected]
concentrations of this order would be present not as a free fluid phase, but rather dissolved in nominally anhydrous minerals such as olivine, pyroxene and garnet. Apart from olivine, which can dissolve up to 0.12 wt.% H 2 O at 10 GPa ŽKohlstedt et al., 1996., it is not known whether these measured concentrations are close to upper mantle saturation levels, or whether the xenolith minerals have extensively re-equilibrated on the way to the surface. In the context of water storage at high pressures, garnet is a particularly important phase because grossular ŽCa 3 Al 2 Si 3 O 12 . is known to be capable of dissolving up to 20.80 wt.% H 2 O ŽPassaglia and Rinaldi, 1984. and significant water contents have been found in pyropic garnet from xenoliths ŽBell and Rossman, 1992.. Since garnet increases in abun-
0009-2541r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 7 . 0 0 1 7 9 - 4
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dance from about 5% by volume at 150 km depth to close to 30% at 650 km depth ŽRingwood, 1991., it is important to know whether this phase can dissolve large amounts of H 2 O throughout its compositional and P–T stability range. Water may be incorporated in garnets as OH defects associated with charge balancing or oxidation-reduction reactions, or may substitute for Si in the hydrogarnet substitution: x Si SiŽ gt. q 2H 2 OŽfluid . s w 4H x Si Ž gt . q SiO 2 Žqz r cs.
Ž 1.
The latter mechanism, first verified in grossular by Cohen-Addad et al. Ž1967., has been shown to occur also in natural and synthetic garnets of other compositions, notably pyrope Že.g. Ackermann et al., 1983; Geiger et al., 1991., and in other nominally anhydrous silicate phases, e.g. quartz ŽMcLaren et al., 1983; Weil, 1984; Purton et al., 1992; Lin et al., 1994; McConnell et al., 1995.. In calcic garnets there is complete solid solution between grossular and a hydrogarnet end-member, where Si is completely replaced by groups of four protons in the hydrogarnet substitution. In this paper, ‘hydrogarnet’ will be used to refer to the completely substituted end-member, X 3 Al 2 ŽH 4 O4 . 3 , of a solid solution series with a silicate garnet. ‘Hydrogrossular’ and ‘hydropyrope’ will refer, respectively, to the X s Ca and X s Mg hydrogarnets, a fictive species in the case of hydropyrope where there is limited hydrogarnet substitution. The highest water content thus far recorded in a natural, pyropic garnet of mantle origin is 226 ppm H 2 O ŽRossman, 1996., while naturally occurring grossular garnets may contain over 50 times this amount. On the basis of these observations it has been argued that the pyrope-rich garnets of the mantle can contain only limited amounts of the hydrogarnet component ŽMartin and Donnay, 1972; Lager et al., 1989. and that they are therefore poorer in H 2 O than grossular garnets under similar conditions. Since, however, the paragenesis of hydrogrossular and of mantle pyrope are very different in terms of pressure, temperature and f H 2 O , this inference is difficult to quantify. Our aim here is to characterise the H 2 O contents of synthetic pyrope garnets as a function of pressure from 2 to 13 GPa. This will enable us to constrain
the role of garnet in water storage in the upper mantle through into the transition zone. 2. Experimental methods 2.1. Synthesis experiments End-member pyrope garnets were synthesised over a pressure interval of 2.0–13.0 GPa using piston cylinder and multi-anvil apparatuses. The starting material for all syntheses was an intimate mix of reagent grade oxides, ground under ethanol in an agate pestle and mortar to a grain size of 20–30 mm, fired at 13008C for 24 h, reground, refired for a similar time and reground again. Composition of the starting mix was pyrope q10 wt.% SiO 2 , to which was added 10 wt.% deionised water prior to sealing in Pt capsules. Following recovery, all capsules were checked for the presence of excess water. The products of all synthesis experiments were examined optically as grain mounts in oils of known refractive index. Garnets were checked for optical isotropy and coesite Žor quartz, in experiments - 2.5 GPa, or stishovite ) 9 GPa. was invariably found to be present. The ubiquitous presence of a silica phase fixes the activity of SiO 2 in equilibrium Ž1. at a known value. Garnet crystals varied from 10 to 1000 mm in size and contained fluid, and sometimes solid, inclusions. An isothermal suite of experiments, at a temperature of 10008C, was conducted to investigate the effect of pressure on water incorporation in pyrope. The temperature of all experiments was measured using W97 Re 3 –W75 Re 25 Žtype D. thermocouples. No correction for pressure was applied to the thermocouple emf. Run duration varied from 5 to 96 h ŽTable 1.. Experiments at 2.0–4.0 GPa were performed in an end-loaded piston cylinder apparatus, using NaCl as a pressure medium. The assembly has been calibrated for pressure using the albite–jadeite–quartz reaction determined by Holland Ž1980.. Capsules were pressed into NaCl pedestals and protected from thermocouple intrusion by a 0.5 mm thick Pt disc. On disassembly, the thermocouple junction was checked to have been within 0.5 mm of the capsule and the NaCl pressure medium was dissolved from around the capsule to enable comparison of pre- and
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Table 1 Synthesis conditions and results Label
Run duration Žh.
T Ž8C.
P ŽGPa.
Crystals analysed
na
D b Žcmy1 .
CH 2O c Žwt.% H 2 O.
Uncertainty d Žwt.% H 2 O.
Py77 Py34 Py21 Py95 Py33 Py37 Py98 Py70 Py54 Py88 Gr86 Gr83 Gr71
25.0 5.0 5.0 95.2 5.0 5.0 8.0 6.0 5.0 5.0 41.2 45.8 7.0
1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
2.0 2.5 3.0 3.0 3.5 4.0 6.0 7.0 8.0 13.0 2.0 3.0 8.0
1 3 2 2 4 3 1 1 4 1 1 1 1
2 5 9 2 6 8 1 4 15 3 1 1 1
187 720 735 574 856 1346 1275 0 0 0 539 328 966
0.014 0.054 0.055 0.043 0.064 0.101 0.096 0 0 0 0.036 0.022 0.065
"0.002 "0.015 "0.009 "0.008 "0.007 "0.019 "0.038 q0.016 e q0.003 e q0.018 e –f –f –f
a
Total number of spectra collected and averaged. Integral absorption coefficient, D s HK Ž n . d n , where K s logŽ I0 rIt .rd, I0 is the incident intensity of the infrared beam, It is the transmitted intensity and d is the path length Žsample thickness. in cm. c Concentration of H 2 O in garnet, expressed as wt.% H 2 O, calculated using Eq. Ž3.. The densities of pyrope and grossular were taken to be 3.58 and 3.59 g cmy3 , respectively. The values of I given by Bell et al. Ž1995. and Rossman and Aines Ž1991. were used for pyrope and grossular, respectively. d Errors are "1 sd, except for experiment Py98, where error is estimated. Other exceptions are noted. e Detection limit Žsee Appendix A.. f No errors are given for single-point analyses. b
post-experiment capsule weights. In this way, water loss from the capsule was shown to be insignificant; weighing precision was "0.1 mg. A Walker-style multi-anvil apparatus ŽWalker et al., 1990. was used for experiments at pressures greater than 4.0 GPa. 1 inch cubic anvils with 12 mm truncated edge length were used in experiments at 5.0–8.0 GPa, and a 6 mm truncated edge length assembly was used for the 13.0 GPa experiment. Octahedra with integral gasketing fins were cast from MgO-based ceramic. Furnaces were constructed by coring sintered LaCrO 3 and the furnace ends capped with 1 mm thick Mo discs. The cell was assembled with the thermocouple in an axial position and the thermocouple junction in contact with the Pt capsule. The sample occupied up to 1.5 mm of the length of the capsule, which was positioned so that this part of the capsule was in the centre Žhot spot. of the furnace. Further details of experimental procedure and pressure calibration are given in Pawley and Wood Ž1995.. Quench rates in the multi-anvil apparatus were typically observed to exceed 3008C sy1 .
2.2. Infrared spectroscopy Several crystals were selected from each experiment and mounted individually in epoxy resin so that doubly-polished crystal slabs could be prepared. After polishing, the crystal slabs were extracted from the resin and cleaned using CCl 4 . An electronic micrometer with precision of 1 mm was used to measure sample thickness, which ranged between 25 and 300 mm. Crystals were typically euhedral and contained both fluid and solid inclusions which were invariably concentrated in the crystal centres. Concentrations of inclusions in crystals synthesised at 2.0 GPa were sufficiently high as to prevent IR analysis in most cases; only the very thin Ž40 mm. rim of these crystals would allow a clear path for analysis. In contrast, pyrope synthesised at G 3.0 GPa typically formed larger Ž) 100 mm. crystals with fewer inclusions, thus allowing several analyses per crystal. Similar differences in inclusion density were described by Geiger et al. Ž1991.. When polishing crystals ) 500 mm in diameter, the thickness of the polished slab was a compromise between being
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thin enough to provide an inclusion-free window while attempting to maximise thickness to enable a sufficiently high signal-to-noise ratio for the analysis of low OH concentrations. A Nicolet 800 FTIR spectrometer was used to analyse for OH using an unpolarised IR beam, KBr ŽGe on KBr substrate. beamsplitter, globar source and liquid N2 cooled HgCdTe detector attached to a microscope. Where samples were large enough, a focused 100 mm spot size on the crystal surface and redundant aperturing were employed. Smaller apertures were used to analyse areas of down to 40 = 40 mm. The free-standing crystal slabs were placed over suitably sized apertures Ž50–500 mm in diameter. in Mo plates, and backgrounds were collected through similar apertures. Wherever possible clear portions of crystals were chosen for analysis. In order to increase spectral quality the microscope was enclosed in a chamber which could be purged with N2 gas, thereby ensuring low, constant atmospheric water levels. Spectra were collected at resolutions of 4 and 0.5 wavenumbers. Spectral baselines were fitted by computer and correction was made for broad band absorption, assigned to molecular water and, if necessary, interference fringes were subtracted. Further details are given in the Appendix A.
3. Results All pyropes synthesised at pressures - 7.0 GPa have an asymmetric peak in the absorption spectrum centred at 3630 cmy1 ŽFig. 1., which corresponds exactly to those in the OH spectra of synthetic pyropes of a number of previous studies ŽAckermann et al., 1983; Geiger et al., 1991; Hosch ¨ and Langer, 1996.. Ackermann et al. Ž1983. suggested that this peak is caused by OH-vibrations of tetrahedral ŽOH.44clusters, as observed in hydrogrossular Že.g. CohenAddad et al., 1967.. The assignment of the 3630 cmy1 band in pyrope to a hydrogarnet component was supported by the low-temperature peak splitting behaviour of the band observed by Geiger et al. Ž1991.. The hydrogarnet peak in grossular garnets synthesised under similar conditions is centred at 3622 cmy1 , with a shoulder at 3612 cmy1 ŽFig. 1.. In grossular, the principal hydrogarnet peak is much sharper than that in pyrope, with a full width at
Fig. 1. Unpolarised IR spectra in the OH stretching region between 3300 and 3800 cmy1 : Ža. grossular Žexperiment Gr83 of Table 1. synthesised at 3.0 GPa and 10008C, 0.25 cmy1 resolution, and Žb. pyrope Žexperiment Py21., also synthesised at 3.0 GPa and 10008C. Resolutions 4 cmy1 . Spectra are normalised to 1 mm thickness.
half-height ŽFWHH. of 7 cmy1 , as opposed to 55 cmy1 in the case of pyrope. The grossular spectra also sometimes display weak absorbance bands at positions of 3561, 3602, 3656, and 3665 cmy1 . In general, the crystals produced in our study are too small to allow traversal analysis, but where this has been attempted no significant zoning was observed. A rim to rim traverse of one 1000 mm pyrope crystal, analysing discrete 100 mm spots, showed no indication of systematic zoning in OH, although "17% variation in absorbance was observed. This is in contrast to Geiger et al. Ž1991. who found that in some cases absorbance was higher in the central regions of synthetic pyrope crystals than at the rims. Wherever possible relatively inclusion-free areas were chosen for analysis, avoiding cracks within the crystal and poorly polished surfaces. Spectra were consistent in shape and peak height. In the case of all grossular syntheses and pyrope experiments at 6.0, 7.0 and 13.0 GPa, the small crystal size ŽF 100 mm. made FTIR analysis difficult. In each of these experiments only one crystal was analysed and the analysed spot size was similar to the size of the crystal, so we cannot tell
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from these analyses whether there is any zoning in OH concentration within the crystals. Proportional error in thickness measurement is also greater in these cases. Pyrope garnets synthesised at 7–8 GPa and 10008C contain inclusions of euhedral, acicular crystals of up to 100 mm in length and ; 1 mm in diameter. These inclusions were identified as topazOH ŽAl 2 SiO 3 ŽOH. 2 . on the basis of optical properties and IR spectroscopy. In grain mounts the crystals are cyclically twinned, as reported by Wunder et al. Ž1993. for topaz-OH. IR analyses of pyrope crystals containing the inclusions produce absorption spectra with two peaks in the OH stretching region, centred at 3600 and 3530 cmy1 , which is consistent
165
with the OH absorption pattern for topaz-OH ŽWunder et al., 1993.. The peak at 3630 cmy1 , attributed to the hydrogarnet substitution in pyrope synthesised at - 7 GPa, is not present in crystals synthesised at 7–8 GPa. The intensities of the absorption bands at 3600 and 3530 cmy1 are proportional to the inclusion density, confirming that these peaks are caused solely by inclusions of the hydrous topaz phase. IR absorption spectra of several pyrope garnets synthesised in this study are shown in Fig. 2. The OH concentration in pyrope increases over the pressure range 2.5–4.0 GPa, as indicated by the increasing intensity of the peak at 3630 cmy1 in the spectrum. At intermediate pressures of 3.5–6.0 GPa the OH concentration remains constant within analytical error, while at pressures G 7.0 GPa no IR absorption due to hydrogarnet is detectable, even though the experiments were H 2 O-saturated. In contrast, the hydrogarnet band in grossular persists at least to 8.0 GPa at 10008C; under the same conditions pyrope was found to be truly anhydrous. 3.1. Absolute OH concentrations Infrared spectroscopy is not an intrinsically quantitative technique. In order to determine the concentration of a particular species a constant of proportionality Žthe molar absorptivity. must first be determined through some independent analytical technique. Molar absorptivity varies with frequency and, because the vibrational energy of the OH bond is sensitive to its atomic environment, the frequency of OH absorbance bands differs between crystal structures and between crystallographic sites within a lattice. The concentration of a species may be calculated using the Beer–Lambert law: cs
1
D
˜ K Ž n . dÕ˜ s H I Õ˜ I Õ2
Ž 2.
1
Fig. 2. Unpolarised IR spectra of pyrope in the OH stretching region between 3000 and 4000 cmy1 . All spectra are normalised to 1 mm thickness and labelled with the synthesis pressure; temperature of all experiments was 10008C.
where c is the concentration of the species in mol ly1 , K Ž n . is the absorption coefficient as a function of wavenumber Ž n . and I is the integral molar absorption coefficient. K s logŽ I0rIt .rd, where d is the path length Žsample thickness. in cm, I0 is the incident intensity of the infrared beam and It is the
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166
transmitted intensity. The mass fraction of H 2 O may be calculated as wt.% using the relation: CH 2 O s
1.8 = D I=r
Ž 3.
where C H 2 O is the concentration of H 2 O in wt.% and r is the density of the host in g cmy3 . Bell et al. Ž1995. have determined the value of I for H 2 O in pyropic garnet based on H 2 manometry. The sample used in the calibration was a garnet m eg acry st o f ap p ro x im ate co m p o sitio n Py67 Alm 21Gr11 , which had broad absorbance peaks in the OH stretching region of the IR spectrum centred at 3571 and 3512 cmy1 . Bell et al. Ž1995. derived a value of 6700 " 670 l moly1 cmy2 for I H 2 O , which is around 20 times the value obtained by Aines and Rossman Ž1984. for garnets of composition Py 75 Alm 25 . Bell et al. Ž1995. suggest that the discrepancy is caused by impurities in the samples and by analytical errors in the earlier study. Aines and Rossman Ž1984. derived a value of 8000 l moly1 cmy2 for a near end-member grossular, and a value of 7492 l moly1 cmy2 may be calculated from the calibration of two hydrogrossular garnets by Rossman and Aines Ž1991.. The concentrations of OH Žexpressed as wt.% H 2 O. in Table 1 and Fig. 3 were calculated using the
integral molar absorption coefficients of Bell et al. Ž1995. and Rossman and Aines Ž1991. for pyrope and grossular, respectively. Using these calibrations, OH concentrations in pyrope synthesised in this study vary from 0.014 to 0.101 wt.% H 2 O at pressures - 7 GPa, while the maximum OH content measured in grossular is 0.065 wt.% H 2 O at 8.0 GPa and 10008C. At pressures - 7 GPa, therefore, it appears that under fixed pressure, temperature and aSiO 2 conditions the solubility of OH in pyrope is similar to that in grossular. The OH vibration energies and FWHH of the synthetic end-member garnets of this study are different from those observed in the natural garnets on which calibrations are based. The absolute concentrations of H 2 O in pyrope reported in this paper are, therefore, subject to revision in the event of an independent calibration for H 2 O in end-member pyrope. However, the relative changes in concentration are independent of an absolute calibration, so the conclusions of this paper concerning the effect of pressure on the relatiÕe solubility of H 2 O in pyrope will be unaffected.
4. Discussion 4.1. Attainment of equilibrium
Fig. 3. Variation in concentration of H 2 O in pyrope Ž C H 2 O . with pressure at 10008C. C H 2 O expressed as wt.% H 2 O, calculated from FTIR analysis using integral molar absorption coefficient, I s6700 l moly1 cmy2 ŽBell et al., 1995.. Error bars are "1 standard deviation, except for point at pressures6.0 GPa, where error is estimated. Where no hydrogarnet band is observed in the IR spectrum a detection limit is given, based on sample thickness and spectral noise levels Žsee Appendix A..
Based on dehydrogenation experiments using natural garnet megacrysts, Wang et al. Ž1996. calculate that the H 2 O concentration in the centre of a 1 mm pyrope crystal can be reduced by ; 20% in 2 hours. Under hydrothermal annealing at 10008C and 300 MPa, hydrogen diffusion in olivine crystals can occur over distances of millimetres in time scales of hours or minutes ŽMackwell and Kohlstedt, 1990.. Re-equilibration of hydrogen defects in olivine requires transport of Si, O and metal defects on the scale of the sample ŽMackwell et al., 1988.. If similar diffusion rates were applicable to pyrope one might anticipate complete re-equilibration in less than 5 h at 10008C ŽS.J. Mackwell, pers. commun... However, in a previous study significant zoning in H 2 O concentration has been measured in synthetic pyrope ŽGeiger et al., 1991., which indicates that there are problems with equilibration of the hydrous
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171
component in pyrope. The difficulty of re-equilibration has been confirmed by performing synthesis experiments of varying duration at 10008C and 3.0 GPa. No detectable difference in OH concentration was found between experiments of 5 h and 96 h duration. This suggests that re-equilibration of the hydrogarnet defect is actually rather slow under the conditions of the synthesis experiments. It was therefore impossible to prove unequivocally that equilibrium has been attained by means of reversal experiments. 4.2. FTIR spectroscopy— band assignment The asymmetry and broadening of the hydrogarnet peak in pyrope relative to grossular may indicate that the OH atomic positions are less tightly constrained in pyrope than in grossular, or that there is increased anharmonicity in the OH vibration. Part of the apparent asymmetry is due to convolution of the two vibrations predicted for a site of D 2d or S 4 symmetry ŽHarmon et al., 1982.. Resolution of these two bands has been previously observed in low-temperature Ž78 K. IR measurements of pyrope crystals ŽGeiger et al., 1991. and the emergence of a shoulder on the high-frequency side of the hydropyrope peak was evident in the low-temperature IR spectrum measured by Hosch ¨ and Langer Ž1996.. In grossular, the hydrogarnet peak can be seen to be composite from the presence of the shoulder at 3612 cmy1 ŽFig. 1.. Both natural ŽRossman and Aines, 1991. and synthetic ŽHosch and Langer, 1996. grossular ¨ garnets often display numerous peaks in the OH stretching region, in addition to those attributed to hydrogrossular, suggesting a role for incorporation mechanisms other than the hydrogarnet substitution. Natural grossular garnets of this type were analysed in the single-crystal NMR study of Cho and Rossman Ž1993.. Their results provide evidence of an alternative incorporation mechanism involving pairs of protons, rather than the four proton clusters of the hydrogarnet substitution. In contrast, neither the pyrope nor the grossular spectra which form the basis of our study show absorption bands in the OH stretching region other than those which we assign to hydrogarnet, so that the latter appears to be the major solution mechanism under the conditions of synthesis.
167
4.3. Crystal chemical and thermodynamic considerations It has been observed that garnets in which the shared edge of the octahedral site is longer than the ˚ unshared edge, i.e. those in which r X4 ) 1.01 A ŽNovak and Gibbs, 1971., are more likely to have a higher hydrous component than those, like pyrope, where the shared edge is shorter than the unshared ŽSacerdoti and Passaglia, 1985.. Distance-leastsquares calculations indicate that incorporation of hydrogarnet decreases the length of the shared edge and increases that of the unshared, leading to the suggestion that a limit to the amount of hydration of the garnet structure will be reached at lower OH concentrations in pyrope than in grossular ŽLager et al., 1989.. We note, however, that under the same physicochemical conditions there is little difference in OH contents of end-member pyrope and grossular ŽTable 1.. Measurements of the lattice parameters of endmembers of the grossular–hydrogrossular series ŽSkinner, 1956; O’Neill et al., 1993. enable us to calculate the partial molar volume of water in the hydrogarnet substitution in grossular, V H 2 OŽ gr . s 14.302 cm3 moly1 at 298 Kr0.1 MPa. When we compare this with the volume–pressure curve for water at elevated temperatures Žcalculated using a parameterisation of the results of the molecular dynamics study of Brodholt and Wood, 1993., we find that the molar volume of free fluid water Ž VH 2 O . is greater than in garnet at low pressure and crosses over to become lower at high pressure ŽFig. 4.. This means, in principle, that garnet should become more hydrous with increasing pressure until the crossover point is reached and then progressively dehydrate at higher pressures. The predicted behaviour is broadly what we observe in pyrope ŽFig. 3.. In order to extend the model to our experimental observations we need however to consider the equilibrium between pyrope, H 2 O fluid, the hydropyrope component and coesite: Mg 3 Al 2 Si 3 O 12 q 6H 2 O s Mg 3 Al 2 Ž H 4 O4 . 3 pyrope
fluid
hydropyrope
q 3SiO 2 coesite
Ž 4.
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Fig. 4. Molar volume of H 2 O Ž V H 2 O ., compared to partial molar volume of H 2 O in garnet, which was calculated on the basis of equilibrium with a free SiO 2 phase ŽEq. Ž1... VH 2 O in the fluid was calculated using a parameterisation of the molecular dynamics simulation of Brodholt and Wood Ž1993.. Dashed line: partial molar volume of H 2 O in grossular Ž V H 2 OŽ gr . ., calculated using unit cell and compressibility data for grossular and hydrogrossular ŽOlijnyk et al., 1991., with thermal expansivity and compressibility for grossular and coesite from Fei Ž1995. and Bass Ž1995.. The same thermal expansivity was used for grossular and hydrogrossular. Line a – a: V H 2 OŽ py . calculated assuming the same increase in compressibility and a larger increase in cell volume than in the grossular–hydrogrossular system. Line b – b: VH 2 OŽ py . calculated assuming equal compressibility for pyrope and hydropyrope.
To a good approximation, the point in the pressure domain where OH concentrations in pyrope reach their maximum corresponds to the point at which DV for Eq. Ž4. becomes zero in passing from negative to positive values. On the basis of our experimental results, this is shown to occur at a pressure of ; 5 GPa. The point at which DV becomes zero is illustrated graphically in Fig. 4 as the point where VH 2 OŽ py . intersects the VH 2 O curve Ž DVreaction s 6Ž VH OŽ py . y V H O ... If we assume that 2 2 the increase in volume that occurs when Si is replaced by 4H in pyrope is the same as that which occurs in grossular, we can use the measured volume data for the end-members of the grossular–hydrogrossular series to calculate V H 2 OŽ gt. . When, however, account is taken of the expansivities and compressibilities of the phases involved in Eq. Ž4. ŽOlijnyk et al., 1991; Fei, 1995; Bass, 1995., and we assume similar compressibility contrasts in the
grossular–hydrogrossular and pyrope–hydropyrope systems, the calculated V H 2 OŽ gt. curve does not intersect the V H 2 O curve at pressures of - 9 GPa. The estimated partial molar volume of water in garnet is apparently too low. The discrepancy would be resolved if the compressibility of hydropyrope were less than that of hydrogrossular Žone possibility is that the hydrogarnet defect concentration is sufficiently small for the surrounding lattice structure to maintain its incompressibility., or if VH 2 OŽ py . is greater than V H 2 OŽ gr . Ži.e. the increase in volume which occurs with the 4H for Si substitution is greater in pyrope than in grossular.. These are taken as end-member cases. Constraining the V H 2 OŽ py . curve to intersect the V H 2 O curve at 5 GPa, as inferred from the experimental data, and assuming a similar contrast in elastic properties to those calculated using the grossular–hydrogrossular data, we find that V H 2 OŽ py . of 15.3 cm3 moly1 Žat 298 K, 0.1 MPa. is required to explain the experimental observations. Alternatively, if the elastic properties of hydropyrope and pyrope are assumed to be similar to one another then V H 2 OŽ py . of 14.3 cm3 moly1 will yield the crossover at 5 GPa. 4.4. Implications for water storage and transport in mantle garnets Most natural garnets of mantle provenance show simple IR absorption in the OH stretching region, which has led to the suggestion that the hydrogarnet substitution is the dominant mechanism for water incorporation ŽAines and Rossman, 1984.. However, both natural and synthetic garnets with low water contents sometimes show a more complicated spectrum with up to 18 bands ŽGeiger et al., 1991; Rossman and Aines, 1991; Rossman, 1996; Hosch ¨ and Langer, 1996., indicating that other incorporation mechanisms can occur. Complicated spectra of the latter type were not observed, however, in the end-member pyrope garnets that are the basis of our study ŽFig. 1.. Our goal now is to apply our experimental observations to the mantle, assuming that pyrope is a good analogue for mantle garnet. To do this we need to take account of two differences between the experimental conditions and those of the mantle: the mantle is not an isothermal system, and SiO 2 is not
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171
likely to exist as a free phase Žcoesite. in the mantle. If we assume as a first approximation that SiO 2 activity in the mantle is controlled by equilibrium between olivine and orthopyroxene: Mg 2 SiO4 q SiO 2 s 2MgSiO 3 forsterite
Ž 5.
enstatite
then Eq. Ž1. can be rewritten as: x Mg 2 SiO4 q X 3 Al 2 Si 3 O 12 q 2 x H 2 O s 2 x MgSiO 3 q X 3 Al 2 Ž SiO4 . 3yx Ž H 4 O4 . x
Ž 6.
or in defect notation: x x x Si SiŽ gt. q 4H ŽH 2 O. s w 4H x Si Ž gt . q Si Žen .
169
enstatite is about 19.0 cm3 moly1 at 298 Kr0.1 MPa, while the molar volume of coesite is 20.6 cm3 moly1 . Taking this into account, and using a mantle geotherm ŽWyllie, 1988., we calculate that, for either of the end-member cases considered above, the crossover point Žonset of dehydration. will occur at depths - 300 km ŽFig. 5.. This remains true for any reasonable mantle geotherm. We therefore conclude that the hydrogarnet substitution in pyrope does not provide a viable mechanism for incorporation of water within the transition zone.
Ž 7.
The effective partial molar volume of water in garnet is lower in this case than when coesite is present because the partial molar volume of SiO 2 in
Fig. 5. Molar volume of H 2 O Ž V H 2 O ., compared to partial molar volume of H 2 O in pyrope Ž VH 2 OŽ py . .. VH 2 OŽ py . calculated on the basis of equilibrium with olivineqpyroxene ŽEq. Ž6... Line a – a: V H 2 O in pyrope assumed to be larger than in grossular as derived from Fig. 4 Žsee text.. Line b – b: V H 2 OŽ py . assumed equal to V H 2 OŽ gr . but less compressible than in grossular Žsee Fig. 4 and text.. Solid lines correspond to a geotherm for upwelling mantle beneath thin lithosphere and chain lines are calculated for a cratonic geotherm ŽWyllie, 1988..
5. Conclusions Ž1. When pyrope and grossular are synthesised under similar conditions with respect to pressure, temperature, f H 2 O , aSiO 2 and at pressures - 7 GPa, a hydrogarnet component is present in similar concentrations in each. At higher pressure a hydrogarnet component persists in grossular, while pyrope becomes anhydrous. Ž2. With increasing pressure, the hydrogarnet component in pyrope synthesised at 10008C, P H 2 O s P TOT increases to a maximum of about 1000 ppm at 5–6 GPa, then decreases to near zero at pressures G 7 GPa. This is in accord with the estimated partial molar volume of H 2 O in garnet, which becomes greater than the molar volume of free water at the pressure where maximum solubility is reached. Ž3. Using the constraint of a solubility maximum for OH in pyrope at 5–6 GPa imposed by the experimental data, we calculate that structural water may be present in the form of hydrogarnet in mantle garnets at up to 9 GPa Ž270 km depth., the hydrogarnet component persisting to pressures greater than the limiting pressures we observe in experiments with excess SiO 2 . Within and below the transition zone, however, the hydrogarnet substitution appears not to be a viable incorporation mechanism for water in pyrope.
Acknowledgements A.C.W. acknowledges the support of a NERC research studentship. We thank K. Langer and S.J. Mackwell for constructive reviews.
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Appendix A. Spectral analysis The baseline in the 3000–3800 cmy1 region of the spectrum was fitted with a cubic function. The second-order differentials at the end points of this range were prescribed; they were chosen by arbitrarily pre-averaging the spectral data outside of the area covered by the fitted function and the quality of fit judged by eye. Consistency of fits was judged by refitting spectra on separate occasions and comparing the baseline-subtracted spectra. A broad absorption band centred at 3415 cmy1 , attributed to molecular water, was present when fluid inclusions were present in the IR beam path. The intensity of this band was proportional to the fluid inclusion density, as observed in the synthetic pyrope of Ackermann et al. Ž1983.. When crystal size was small, it was sometimes only possible to analyse a single spot within the crystal; in these cases the broad band absorbance often formed a significant proportion of the absorption spectrum in the OH stretching region. All spectra were corrected for this molecular absorption by subtracting a scaled broad band spectrum for pure water from the composite sample spectrum. The small size of the crystals produced in many experiments necessitated the use of small Ž40 = 40 mm. apertured analysis spots of thinly ground crystal slabs. This introduced several analytical problems, not least of which was the increased difficulty in preparing the thinly sectioned crystals, mounting them for analysis and accurately measuring their thickness. A strong interference fringe was often present in spectra from thin crystals andror small analysed spot sizes. When these interferences made a significant contribution to the spectrum Ži.e. when the amplitude of the sinusoidal interference was significant relative to the intensity of the OH absorption peaks., they were fitted with an appropriate function and subtracted from the spectrum in order to generate spectra which could be directly compared ŽFig. 6.. Where no hydrogarnet absorption band was evident in the IR spectrum, the detection limit for this species was calculated on a per crystal basis. An ideal hydrogarnet absorption peak was scaled such that its apparent intensity was just significant relative to the spectral ‘noise’ in this region. The apparent noise was primarily due to variations in background,
Fig. 6. Removal of interference fringes: a sin c function Ždotted line. is subtracted from the raw spectrum Žtop. to enable comparison of spectra and integration of peaks.
atmospheric water concentrations. The intensity of the reduced hydrogarnet peak represented the minimum detectable concentration of OH in the sample. The detection limit was calculated from the integral absorption coefficient for the scaled hydrogarnet peak.
References Ackermann, L., Cemic, ´ L., Langer, K., 1983. Hydrogarnet substitution in pyrope: a possible location for ‘water’ in the mantle. Earth Planet. Sci. Lett. 62, 208–214. Aines, R.D., Rossman, G.R., 1984. The hydrous component in garnets: pyralspites. Am. Mineral. 69, 1116–1126. Bass, J.D., 1995. Elasticity of minerals, glasses, and melts. In: Ahrens, T.J. ŽEd.., AGU Reference Shelf 2—Mineral Physics and Crystallography, a Handbook of Physical Constants. American Geophysical Union, Washington, DC, pp. 45–63. Bell, D.R., Rossman, G.R., 1992. The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa. Contrib. Mineral. Petrol. 111, 161–178. Bell, D.R., Ihinger, P.D., Rossman, G.R., 1995. Quantitative analysis of trace OH in garnet and pyroxenes. Am. Mineral. 80, 465–474. Brodholt, J.P., Wood, B.J., 1993. Simulations of the structure and thermodynamic properties of water at high pressures and temperatures. J. Geophys. Res. 98, 519–536. Cho, H., Rossman, G.R., 1993. Single-crystal NMR studies of
A.C. Withers et al.r Chemical Geology 147 (1998) 161–171 low-concentration hydrous species in minerals: grossular garnet. Am. Mineral. 78, 1149–1164. ´ Cohen-Addad, C., Ducros, P., Bertaut, E.F., 1967. Etude de la substitution du groupement SiO4 par ŽOH.4 dans les composes ´ Al 2 Ca 3 ŽOH.12 et Al 2 Ca 3 ŽSiO4. 2.16 ŽOH. 3.36 de type grenat. Acta Crystallogr. 23, 220–230. Fei, Y., 1995. Thermal expansion. In: Ahrens, T.J. ŽEd.., AGU Reference Shelf 2—Mineral Physics and Crystallography, a Handbook of Physical Constants. American Geophysical Union, Washington, DC, pp. 29–44. Geiger, C.A., Langer, K., Bell, D.R., Rossman, G.R., Winkler, B., 1991. The hydroxide component in synthetic pyrope. Am. Mineral. 76, 49–59. Harmon, K.M., Gabriele, J.M., Nuttal, A.S., 1982. Hydrogen bonding in the tetrahedral O4 H 44y cluster in hydrogrossular. J. Molec. Struct. 82, 213–219. Holland, T.J.B., 1980. The reaction albites jadeiteqquartz determined experimentally in the range 600–12008C. Am. Mineral. 65, 129–134. Hosch, A., Langer, K., 1996. Single-crystal MFTIR spectra at ¨ various temperatures of synthetic end member garnets in the OH valence vibrational region. Phys. Chem. Miner. 23, 305– 306. Kohlstedt, D.L., Keppler, H., Rubie, D.C., 1996. Solubility of water in the a-phase, b-phase and g-phase of ŽMg,Fe. 2 SiO4 . Contrib. Mineral. Petrol. 123, 345–357. Lager, G.A., Armbruster, T., Rotella, F.J., Rossman, G.R., 1989. OH substitution in garnets: X-ray and neutron diffraction, infrared, and geometric-modeling studies. Am. Mineral. 74, 840–851. Lin, J.S., Payne, M.C., Heine, V., McConnell, J.D.C., 1994. Ab-initio calculations on ŽOH.4 defects in a-quartz. Phys. Chem. Miner. 21, 150–155. Mackwell, S.J., Kohlstedt, D.L., 1990. Diffusion of hydrogen in olivine: implications for water in the mantle. J. Geophys. Res. 95, 5079–5088. Mackwell, S.J., Dimos, D., Kohlstedt, D.L., 1988. Transient creep of olivine: point defect relaxation times. Philos. Mag. A— Phys. Condens. Matter Defects Mech. Properties 57, 779–789. Martin, R.F., Donnay, G., 1972. Hydroxyl in the mantle. Am. Mineral. 57, 554–570. McConnell, J.D.C., Lin, J.S., Heine, V., 1995. The solubility of w4HxSi defects in a-quartz and their role in the formation of molecular water and related weakening on heating. Phys. Chem. Miner. 22, 357–366. McLaren, A.C., Cook, R.F., Hyde, S.T., Tobin, R.C., 1983. The mechanisms of the formation and growth of water bubbles and associated dislocation loops in synthetic quartz. Phys. Chem. Miner. 9, 465–482.
171
Michael, P.J., 1988. The concentration, behavior and storage of H 2 O in the suboceanic upper mantle: implications for mantle metasomatism. Geochim. Cosmochim. Acta 52, 555–566. Novak, G.A., Gibbs, G.V., 1971. The crystal chemistry of silicate garnets. Am. Mineral. 56, 791–825. Olijnyk, H., Paris, E., Geiger, C.A., Lager, G.A., 1991. Compressional study of katoite wCa 3 Al 2 ŽO4 H 4 . 3 x and grossular garnet. J. Geophys. Res. 96, 14313–14318. O’Neill, B., Bass, J.D., Rossman, G.R., 1993. Elastic properties of hydrogrossular garnet and implications for water in the upper mantle. J. Geophys. Res. 98, 20031–20037. Passaglia, E., Rinaldi, R., 1984. Katoite, a new member of the Ca 3 Al 2 ŽSiO4 . 3 –Ca 3 Al 2 ŽOH.12 series and a new nomenclature for the hydrogrossular group of minerals. Bull. Mineral. ´ 107, 605–618. Pawley, A.R., Wood, B.J., 1995. The high pressure stability of ˚ phase: potential storage sites for H 2 O in talc and 10 A subduction zones. Am. Mineral. 80, 998–1003. Purton, J., Jones, R., Heggie, M., Oberg, S., Catlow, C.R.A., 1992. LDF pseudopotential calculations of the a-quartz structure and hydrogarnet defect. Phys. Chem. Miner. 18, 389–392. Ringwood, A.E., 1991. Phase-transformations and their bearing on the constitution and dynamics of the mantle. Geochim. Cosmochim. Acta 55, 2083–2110. Rossman, G.R., 1996. Studies of OH in nominally anhydrous minerals. Phys. Chem. Miner. 23, 299–304. Rossman, G.R., Aines, R.D., 1991. The hydrous components in garnets: grossular–hydrogrossular. Am. Mineral. 76, 1153– 1164. Sacerdoti, M., Passaglia, E., 1985. The crystal structure of katoite and implications within the hydrogrossular group. Bull. Mineral. ´ 108, 1–8. Skinner, B.J., 1956. Physical properties of end-members of the garnet group. Am. Mineral. 51, 428–436. Walker, D., Carpenter, M.A., Hitch, C.M., 1990. Some simplifications to multianvil devices for high-pressure experiments. Am. Mineral. 75, 1020–1028. Wang, L., Zhang, Y., Essene, E.J., 1996. Diffusion of the hydrous component in pyrope. Am. Mineral. 81, 706–718. Weil, J.A., 1984. A review of electron spin spectroscopy and its application to the study of paramagnetic defects in crystalline quartz. Phys. Chem. Miner. 10, 149–165. Wunder, B., Rubie, D.C., Ross, C.R., Medenbach, O., Seifert, F., Schreyer, W., 1993. Synthesis, stability, and properties of Al 2 SiO4 ŽOH. 2 — a fully hydrated analog of topaz. Am. Mineral. 78, 285–297. Wyllie, P.J., 1988. Magma genesis, plate tectonics, and chemical differentiation of the Earth. Rev. Geophys. 26, 370–404.