The Use of 13c Diamond As Pressure Sensor for Hydrothermal Diamond Anvil Cell Experiments

Abstract:

The knowledge of deep hydrous geological fluids has been strongly improved thanks to characterizations performed in situ in hydrothermal diamond anvil cells (HDAC) experiments. However, such investigations are suffering from pressure and temperature limitations (in most of the cases: up to 2 GPa, 1100°C), and from the need of pressure sensors at low pressures (from room pressure to a few GPa) and at high temperatures.

This study aims at providing an accurate method to determine in situ the sample pressure in a diamond anvil cell experiment. Therefore we report the calibration of the pressure shift of the Raman line of a 98.3 atom. % ¹³C diamond and we propose a new equation to determine the pressure up to 16 GPa and 800°C.This method is suitable for the study of geological fluids during hydrothermal experiments performed at low pressures (up to a few GPa) and high temperature (up to 800°C), particularly for subduction zone processes involving aqueous fluids or/and silicate melts. We also present a new designed hydrothermal diamond anvil cell allowing the study of geological fluids at crustal and mantle conditions. This HDAC is externally heated, it can be combined with laser heating. It is equipped with a pressure driven membrane, allowing isothermal decompressions. It is versatile enough to be used with any kind of diamond anvil (standard design, Boelher-Almax design or plates).

As an application, we have determined the total miscibility between water and a silicate melt of haplogranite composition and compared the pressure determined with the equation of state of water with the pressure measured through the ¹³C diamond sensor. We show that the use of the equation of state of water to calculate the pressure for such experiments is less accurate but valid.