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Evidence for ice VI as an inclusion in cuboid diamonds from high P-T near infrared spectroscopy

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

^* E-mail: kagi{at}eqchem.s.u-tokyo.ac.jp

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 120°C. 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

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