This study aims at providing an accurate method to determine in situ the pressure in the sample chamber during a diamond-anvil-cell experiment. We report the calibration of the frequency shift with pressure of the Raman band of a 98.3 at.% 13C diamond against ruby and SrB4O7:Sm2+. This method is suitable for the study of geological fluids during hydrothermal experiments performed at pressures up to 16 GPa and temperature up to 800°C, particularly for subduction-zone processes involving fluids or/and silicate melts. Experiments were carried out in a new hydrothermal diamond-anvil cell (HDAC) designed for the study of geological fluids at crustal and mantle pressures.

We propose the following equation to determine the pressure up to 16 GPa at temperatures up to 800°C:

P(GPa) = 373.97(26) − 16.55 ⋅ [510.41(35) + vRC − v(P, T) + 6.9(3) ⋅ 10−3 ⋅ T(K) − 2.32(5) ⋅ 10−5⋅T2(K)]0.5

where P is the pressure in GPa, v is the Raman frequency under pressure and temperature, vRC is the Raman frequency at room conditions and T is the temperature in kelvins.

As an application, we have determined the pressure of the total miscibility between water and a silicate melt of haplogranite composition using the Raman shift of 13C-rich diamond, which is compared with the pressure calculated using the equation of state of water.

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