Mineralogical Magazine, December 2000, Vol. 64(6), pp. 1057±1065
Evidence for ice VI as an inclusion in cuboid diamonds from high P-T near infrared spectroscopy
1,2, 3 3 3 3 3 H. KAGI *, R. LU ,P.DAVIDSON ,A.F.GONCHAROV , H.-K. MAO AND R. J. HEMLEY 1 Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan 2 Center for High Pressure Research (NSF Science and Technology Center) and Department of Geosciences, ESS Building, State University of New York at Stony Brook, Stony Brook, NY 11794-2100, USA 3 Geophysical Laboratory and Center for High Pressure Research*, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, D.C. 20015-1305, USA
ABSTRACT Near infrared absorption (NIR) spectra of natural morphologically cubic polycrystalline diamonds (cuboid) were obtained at room temperature, and the stretching plus bending combination band of molecular water was observed. The spectrum consisted of the main band at 5180 cm 1 due to liquid water and a shoulder at 5000 cm 1. The 5000 cm 1 band suggests the presence of a phase with stronger hydrogen bonding in inclusions in the diamond. This shoulder absorption decreased on heating to 1208C. The combination band of H2O at high pressure and temperature was measured using a resistively heated diamond cell and the pressure dependence of the peak position was obtained. Comparison with the present experimental results indicates that the spectral changes induced by heating of the cuboid corresponded to melting of a high-pressure form of ice, and the shoulder absorption at 5000 cm 1 arises from ice VI at 1.9 GPa. On the other hand, the liquid water, a main component of the fluid inclusions in the cuboid, was not under high pressure judging from the frequency of the combination band. This contrast might relate to the texture of the cuboid diamond. The spectral observation enables us to estimate the residual pressure of mantle fluid encapsulated in these diamonds. The diamond-cell data also provide high-P-T NIR fingerprint spectra that could be useful for identifying H2O phases and confining pressures in other samples.
KEYWORDS: ice VI, cuboid diamond, near-IR spectroscopy.
Introduction al., 1999). Furthermore, solid carbon dioxide was observed by IR spectroscopy in a natural NATURAL minerals of high-pressure origin with diamond, and the blue shift seen in the CO2 high strength have the potential to retain high absorption bands suggested that the solid CO2 was internal pressures on material trapped within these present at pressures as high as 5 GPa (Schrauder crystals. Previously reported infrared and Raman and Navon, 1993). More recently, the combina- spectra have revealed that high internal pressures tion of micro-Raman spectroscopy and single- can be retained in some diamond inclusions. crystal X-ray diffraction (XRD) has been used to Frequency shifts in IR spectra disclosed that the obtain pressures of 5.5 GPa for coesite inclusions residual internal pressures for quartz inclusions in in Venezeulan diamonds (Sobolev et al., 2000). cuboid diamonds were 1.5 2.1 GPa (Navon, These experimental results showed that the 1991). Using micro-Raman spectroscopy, pres- diamond is strong enough to encapsulate high sures of 0.13 to 0.65 GPa were measured inside pressure on the order of gigapascals. olivine inclusions in Siberian diamonds (Izraeli et Some natural diamonds are rich in mantle- derived ¯uid inclusions (Chrenko et al., 1967; Guthrie et al., 1991; Navon, 1991; Navon et al., 1988; Schrauder and Navon, 1993, 1994). The * E-mail: [email protected] phase diagram of water (Bridgman, 1935;
# 2000 The Mineralogical Society H. KAGI ETAL.
Pistorius et al., 1963; Fei et al., 1993) illustrated Experimental methods in Fig. 1 shows that the high internal pressure retained in diamonds is high enough to crystallize A morphologically cubic diamond (cuboid liquid water into ice VI or VII at room diamond) with a size of ~2 mm, from the temperature. Thus one may expect that water Democratic Republic of Congo, containing can exist as ice inclusions in diamonds at ambient water inclusions was investigated. The NIR temperature, but ice inclusions have not been absorption spectra of the cuboid diamond were reported although there are numerous reports of recorded using an FTIR spectrometer (Perkin liquid water. Navon (1991) demonstrated that Elmer, Spectrum 2000) with a halogen lamp, water contained in cuboid diamonds did not quartz beam splitter and InSb detector. Spectral freeze to ice even at liquid nitrogen temperature, measurements were carried out at 4 cm 1 and he suggested that the high surface-to-volume resolution, and interferograms from 1000 scans ratio of the micro-inclusions and/or the high were averaged to obtain one spectrum. Mid-IR solute content of the residual ¯uid resulted in the measurements were performed with the combina- supercooled ¯uid. It was presumed that the size tion of a Globar light source, TGS detector and and chemical composition of the inclusions KBr beam splitter operating at a spectral caused supercooling even at high pressure and resolution of 4 cm 1. The cuboid diamond was room temperature in which ices are the stable set in a holder made from copper plate of 2 mm form of water. Most of the water in cuboid thickness with a square hole of ~2 mm to ®t the diamonds is contained in liquid form as reported cuboid diamond. A small space between diamond previously, but the possibility of the presence of and the copper holder was sealed with aluminium ice as an inclusion in diamonds cannot be ruled foil to get high heat conduction. The beam size of out given the P-T condition of present inclusions the IR incident light (10 mmf) was larger than in diamonds. In this study, we searched for ice as the diamond specimens. The sample holder was a solid inclusion in natural diamonds using NIR heated with a belt heater up to 1208C and the spectroscopy. temperature of the diamond sample surface was
FIG. 1. Schematic phase diagram of water at high pressure (after Bridgman, 1935; Pistorius et al., 1963; Fei et al., 1993). The arrows show two possible heating sequences for natural cuboid diamond samples at the pressures estimated exclusively from the frequency of the combination band of water. Case I corresponds to the ice VI at 1.9 GPa and Case II corresponds to ice VII at 5 GPa.
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monitored by a thermocouple. For comparison, the phase of water, attention must be paid to the NIR spectra of liquid water sandwiched between shape of fundamental OH-stretching absorption 1 two CaF2 disks were also measured. band around 3400 cm which is sensitive to the The NIR spectra of water at high pressure were extent of hydrogen bonding of water. However, measured in a diamond anvil cell consisting of a the intrinsic absorption of diamond, the ®rst pair of type II diamonds with 500 mm ¯at culets. overtone of the H2O bending mode, and the Spectra were recorded using a Bruker IFS66v/s fundamental stretching mode due to the OH ion FTIR spectrometer equipped with a Bruker II IR from possible hydrous minerals can overlap the microscope (Irscope II) with KBr beam splitter, OH-stretching band derived from molecular globar source and MCT detector. Pressure was water. calibrated by the ruby ¯uorescence method. In order to overcome these dif®culties, espe- Measurements were carried out at room tempera- cially acute for natural samples, it is necessary to ture and as a function of temperature to 1208Cat observe a vibrational mode of water that is free high pressure, with heating coils placed around from interference of other modes related to the entire diamond cell. hydrous species and is sensitive to the extent of hydrogen bonding of water molecule. In this study, we focused on the combination stretching Results and discussion plus bending mode of the H2O molecule in the Figure 2 shows the mid-IR absorption spectrum of condensed phases. In Figure 3, the combination a natural diamond sample. As previously band is shown for the cuboid diamond (Fig. 3a) reported, our sample contained water, silicates, and pure liquid water (Fig. 3b), respectively. The etc. as inclusions. Water concentration in this large rise at the right end on the spectrum of sample was estimated to be 65 ppm using a molar cuboid diamond in Fig. 3a is a tail from a sharp absorptivity value for the OH-stretching mode of absorption at 4500 cm 1 derived from a combina- liquid water (81 liter mol. 1 cm 1) reported by tion of CH vibrations typical of natural diamonds Thompson (1965). Generally in order to identify (Davies, 1977; Davies et al., 1984) at 1405 cm 1
FIG. 2. Mid-IR absorption spectra of the cuboid diamond.
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and 3107 cm 1. These two spectra in Fig. 3a,b peak positions, the band shapes differ sharply reveal the same peak position at ~5180 cm 1.It from each other between the diamond and liquid suggests most of the water in the diamond is water. Figure 3 also shows these two spectra liquid water. In contrast to the similarity in these superimposed in order to highlight the difference
FIG. 3. NIR absorption spectra of water involving the stretching plus bending combination band at room temperature. The upper spectrum (a) is from the cuboid diamond, and lower spectrum (b) was from liquid water. Both spectra show a peak at ~5200 cm 1, but the upper spectrum shows a convex upward feature on the low wavenumber side of the band in contrast to the concave upward feature for the lower spectrum. The two spectra are superimposed in the lower part of the ®gure in order to clarify the difference in band shapes between them.
10 60 ICE VI INCLUDED IN CUBOID DIAMONDS in the band shapes. A spectrum of the cuboid temperature. Furthermore, by cooling to room diamond shows convex upward features in the temperature, the combination mode band reverted lower wavenumber side of the combination band, to the convex upward feature in the lower while liquid water shows concave upward feature wavenumber side, the same as the spectrum in the lower wavenumber side. This convex obtained before heating. This demonstrates that upward band shape observed for the diamond the spectral changes were reversible. (Fig. 3a) suggests that the water in the latter For the purpose of assigning the component at contains some additional components giving rise 5000 cm 1 in the spectra, the stretching plus to the absorption at lower wavenumbers other bending combination band of ices were measured than liquid water. at high pressure using a diamond anvil cell. The frequency of the OH-stretching vibration Representative spectra measured at room decreases with increasing strength of hydrogen temperature are shown in Fig. 5. The pressure bond (e.g. Nakamoto et al., 1955; Lutz, 1995; dependences of the peak positions of the Goncharov et al., 1996). The presence of a combination band are illustrated in Fig. 6. The component in the lower wavenumber side frequency of the band decreases with increasing (Fig. 3a) suggests that water in the diamond pressure, and the phase transitions from liquid contained some additional phase of water with water through ice VII are re¯ected in the clear stronger hydrogen bonding. This might be ice discontinuity of frequency. Pressure derivatives of inclusions under pressure. Water molecules in ice frequency estimated from the present results were are strongly hydrogen bonded and typically show negative for liquid water, ice VI and ice VII lower OH-stretching frequencies than liquid water ( 34.0 cm 1 /GPa, 106 cm 1 /GPa and (e.g. Bertie and Whalley, 1964; Marckmann and 36.3 cm 1/GPa, respectively). The pressure Whalley, 1964; Bertie et al., 1968; Engelhardt derivative for ice VII at 1008C gave a value and Whalley, 1979; Wong and Moffatt, 1987; ( 35.1 cm 1/GPa) similar to that taken at 208C. Aoki et al., 1996). Therefore, the combination Larsen and Williams (1998) reported that the band of ice should be observed at a lower pressure derivative of the frequency for ice VII frequency compared with that of liquid water. was 13+4cm 1/GPa in the wide pressure If the material contained in the diamond range extended to 30 GPa. The difference associated with stronger hydrogen bond is ice, a implies the decrease of the pressure derivative high pressure polymorph of water, it should melt in the higher pressure region. This indicates that on increasing temperature, as indicated by the the frequency of the combination band can serve phase diagram of water shown in Fig. 1. In this as a pressure marker of ¯uid inclusions in study, the diamond sample was heated to 1208C diamonds especially within the range of internal and a change in shape of the combination band pressure inside of natural diamonds (<10 GPa). was observed. In Fig. 4 combination bands The decreasing component at 5000 cm 1 for the measured for the diamond at room temperature diamond sample on heating corresponds to ice VI (208C) and high temperature (1208C) are shown. at 1.9 GPa (Case I in Fig. 1) or ice VII at 5 GPa We can clearly see that the convex upward (Case II in Fig. 1) at room temperature for the feature, denoted by the arrow in the room NIR frequencies, respectively. However, ice VII temperature spectrum, (Fig. #129;4a) disappeared can be excluded by consideration of the phase in the spectrum obtained at 1208C (Fig. 4b). diagram (see Fig. 1), assuming that the phase Figure 4c shows the difference spectrum. The relations in the sample correspond to those of bulk concave upward shape of the spectrum in Fig. 4b) H2O. Since ice VII near 5 GPa melts at is similar to that of liquid water taken at room temperatures far above 1208C, we can conclude temperature (Fig. 3b). The decreased component that the cuboid diamond studied here contained suggests that a high-pressure phase of water (an inclusions of ice VI at 1.9 GPa. Residual internal ice phase) melts to liquid water and had almost pressure of such diamonds previously estimated disappeared at 1208C. The peak position of the from quartz inclusions was 1.5 2.1 GPa decreasing component was at ~5000 cm 1 as (Schrauder and Navon, 1994). The consistency shown in Fig. 4c, which was lower by 200 cm 1 of the present results with those obtained for when compared with the combination band of quartz inclusions further indicates that the liquid water at room temperature. This observa- inclusions in cuboid diamonds are under hydro- tion suggests that a portion of the water in the static conditions and the effects of solutes on diamond exists as ice and it melts with increasing melting relations are minimal. Furthermore, the
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FIG. 4. NIR absorption spectra of the cuboid diamonds obtained (a) at 1208C and (b) at room temperature. The combination band of molecular water was observed in both spectra. While the band shape of the combination mode appeared to be convex upward before heating, it changed to concave upward at 1208C as well as liquid water (Fig. 2a). The spectral change is denoted by the arrow. The difference spectra between (a) and (b) are displayed in part (c).
inferred con®ning pressure in ice VI will result in the physical conditions of mantle ¯uid fall within the same ¯uid pressure value of 4 7 GPa as the thermodynamic stability ®eld of diamond. estimated from the residual internal pressure of The relative intensity of the 5000 cm 1 quartz. Navon (1991) obtained this pressure value shoulder on the main absorption around by an extrapolation to the mantle temperature 5200 cm 1 can give us information on the relative with the assumption of constant volume due to the abundance of ice in the cuboid diamond. A compensating thermal expansion and compressi- contribution from the shoulder absorption on the bility of diamond. The pressure range showed that low wavenumber side of the combination band
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FIG. 5. Representative NIR spectra of water at high pressure and room temperature observed using a diamond anvil cell. Pressure-induced phase transitions (liquid water ice VI ice VII) were observed.
was 10 20% of the main peak as shown in the because cuboid diamonds have a polycrystalline lower illustrations in Figs. 3 and 4. The present texture, especially for the inclusion-rich outer result demonstrates that most of the water in the portion of diamond grain. We can speculate that diamond sample exists as liquid, but a consider- the grain boundaries were completely closed from able amount (10 20%) of liquid water coexists as outside during the crystal growth and the inside of ice VI. The extent to which the ¯uid inclusions the crystal and grain boundary would be observed here are trapped in the inside of indistinguishable. The majority of the water in diamond crystals or in between the grain thecuboiddiamondscouldbeliquidina boundary of diamond crystals is not clear, supercooled condition resulting from the small
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FIG. 6. Pressure dependence of the OH-stretching plus HOH bending combination band frequency for liquid water, ice VI and ice VII at room temperature (208C; open squares) and 1008C (®lled squares).
inclusion droplet size or high solute content. Here, Acknowledgements we have to mention the physicochemical condi- tions of liquid water observed in the present study. One of the authors (HK) was funded a by As shown in Fig. 3, the combination band for the Postdoctoral Fellowship for Research Abroad cuboid diamond measured at room temperature from the Japan Society for the Promotion of exhibited a peak at 5180 cm 1. This peak position Science (JSPS). CHiPR is jointly supported by the is identical to that of pure water at ambient National Science Foundation under the grant EAR pressure within the experimental error (spectral 89-20239 and the State University of New York at resolution of 4 cm 1). The pressure dependence Stony Brook. Research at the Geophysical of the combination band of liquid water (Fig. 6) Laboratory was also supported by NSF EAR- demonstrates that the liquid water in the diamond 9814819. This is MPI publication No. 280. is not under the same high pressure as the ice VI (1.9 GPa). This suggests two water phases being References at two distinct residual pressures in the same diamond. One might speculate that the liquid phase is embedded in boundaries of diamond Aoki, K., Yamawaki, H. and Sakashita, M. (1996) crystals and the ice VI is inside single crystals. Observation of fano interference in high pressure ice This possibility will require intensive study by VII. Phys. Rev. Lett., 76, 784 6. taking the salinity of the ¯uids into account Bertie, J.E. and Whalley, E. (1964) Infrared spectra of (Navon, 1999). The speciation of water in natural ices II, III, and V in the range 4000 to 350 cm 1. J. diamonds is important for considering the Chem. Phys., 40, 1646 59. geochemical stages of incorporation of mantle Bertie, J.E., Labbe, H.J. and Whalley, E. (1968) Infrared ¯uid during the formation of cuboid diamonds. spectrum of ice VI in the range 4000-50 cm 1. J. Further study by IR microscopy may provide Chem. Phys., 49, 2141 4. additional insight into these issues. Bridgman, P.W. (1935) The phase diagram of water to
10 6 4 ICE VI INCLUDED IN CUBOID DIAMONDS
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