HR: 0830h
AN: V31D-0959    [PDF]
TI: The Anvils as Pressure Calibrants in the Hydrothermal Diamond Anvil Cell
AU: * Davis, M K
EM: mkdavis@umich.edu
AF: University of Michigan Department of Geological Sciences, 2534 C.C. Little Building, Ann Arbor, MI 48109 United States
AU: Panero, W R
EM: wpanero@umich.edu
AF: University of Michigan Department of Geological Sciences, 2534 C.C. Little Building, Ann Arbor, MI 48109 United States
AU: Stixrude, L P
EM: stixrude@umich.edu
AF: University of Michigan Department of Geological Sciences, 2534 C.C. Little Building, Ann Arbor, MI 48109 United States
AB: Throughout the crust and the upper part of the mantle, water is an important agent of heat and mass transport in processes ranging from metasomatism to magma generation in arc environments. One of the important properties of water in this regime: its ability to dissolve significant amounts of solids, presents a substantial challenge to the experimental study of water-rich systems. Many commonly used pressure standards, such as quartz and ruby, dissolve in water under the conditions accessible to the hydrothermal diamond anvil cell (up to 1200 K and 5 GPa). For this reason, it is important to develop alternative pressure calibrants. Two methods have been developed by other groups for pressure calibration in the HDAC in the presence of water. One method relies on the equation of state of the ambient fluid and the observation that the sample chamber remains approximately isochoric on heating. Disadvantages of this method include our imperfect knowledge of the equation of state of water over the relevant pressure-temperature interval, possible changes in fluid composition, and sample chamber assembly relaxation at temperatures above 800 K. The second method is based on the Raman signal from diamond chips loaded with the sample. Synthetic $^{13}$C diamond is used to avoid overlap with the much stronger signal from the anvils. Diamond is an ideal pressure sensor since it is chemically inert and unaffected by water. Therefore, we use the tips of the diamond anvils as "internal" sensors. The primary disadvantage of this method is that the stress distribution inside the anvils is non-hydrostatic and inhomogeneous, although the normal stress across the diamond-sample interface must be continuous. Using confocal micro-Raman spectroscopy we are able to characterize both the inhomogeneity and the non-hydrostaticity of the diamond stress field by combining axial and radial transects with peak shapes. We find that on room temperature loading there is substantial inhomogeneity in the diamond stress field: variations of up to 2.3 cm$^{-1}$ or about 0.8 GPa over a pressure range of 0 to 3.5 GPa. However, heating substantially reduces inhomogeneity in the vicinity of the diamond-sample interface allowing the derivation of a useful pressure calibration. Preliminary results show that the primary Raman line of diamond shifts with respect to temperature according to the equation 1332.15 - 0.0016x - 3.5e-5x$^{2}$ + 7.1e-11x$^{3}$ where x is temperature. The same Raman line of diamond shifts with pressure according to the equation 1332.15 + 3.4*P where the pressure, P, is in GPa. We find that the effects of temperature and pressure are independent of one another so that an independent measurement of temperature (with thermocouples) together with the measured Raman shift determines the pressure with an accuracy of 0.27 GPa at 800K and 2 GPa. We compare our calibration to the quartz and ruby calibration scales over the range where they are stable. We also compare our calibration to previous experiments using independent pressure calibrants.
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting