The New Generation Griggs-type Apparatus

Objective

This technique is a solid-medium tri-axial apparatus dedicated to explore deformation processes of rocks over a large range of pressures (0.3-5 GPa) and temperatures (20-1300 °C), such as encountered in the lithosphere of the Earth.

Principle

Based on the piston-cylinder technology, the Griggs-type apparatus has been formerly designed by David T. Griggs in the 60’s^[1], and then modified by Harry W. Green in the 80’s^[2]. In both cases, the Griggs apparatus is characterized by a metal frame that includes: 1) three horizontal platens mounted on vertical columns, 2) a main hydraulic cylinder (confining pressure ram) suspended to the middle platen and 3) a deformation gear box and piston/actuator fixed on top of the upper platen. The “confining” ram and deformation actuator are each connected to independent pistons that transmit forces to the sample assembly within a pressure vessel. With such a vessel, deformation can be achieved at confining pressures of up to 2 or 5 GPa, depending on the apparatus and diameter of the sample assembly. Thanks to a resistance furnace (graphite), the sample temperature is increased by Joule effect (up to ≈1300 °C), while the pressure vessel is water cooled on top and bottom. In Green’s design, the Griggs apparatus also includes an end-load system that homogenizes the pre-stress in the pressure vessel. This permits to achieve deformation experiments at higher pressures (max. 5 GPa), particularly using a small bore in the pressure vessel. For further details about the Griggs press, the readers are referred to the excellent description of the modified Griggs apparatus design by Rybacky et al.^[3].

A new generation apparatus^[⁴^]

Arising from a close collaboration with the École Normale Supérieure de Paris (ENS Paris, France) and Sanchez Technologies company (Core Lab France), the new generation Griggs-type apparatus is directly based on the design from H. W. Green, but some improvements have been made to comply with European standards for safety of high-pressure experiments^[4]. In this new press, the confining and deformation actuators are driven by servo-controlled hydraulic syringe pumps, giving the possibility to perform either constant load or constant displacement experiments at high pressures (up to 5 GPa). The confining (isostatic) pressure, force, and displacement are respectively monitored using oil pressure sensors, a load cell (max. 200 kN) and displacement transducers^[4]. The pressure vessel is made of an inner tungsten-carbide (WC) core inserted into a 1° conical steel ring and pre-stressed using the strip winding technique.

For transmitting forces, the pressure vessel and sample assembly lie between WC-removable pistons that include a deformation piston (σ₁), confining piston (σ₃), end-load piston and base plate. Together with regular cooling on top and bottom of the pressure vessel, water flows through the steel vessel around the tungsten-carbide core within 6 mm diameter holes for better cooling. The hydraulic cylinder for the confining pressure is also cooled by silicon oil flow. In addition, the deformation apparatus in Orléans employs larger sample size up to 8 mm diameter, so that 1) microstructures can be better developed, 2) the Griggs press and Paterson press share a common sample dimension for future comparisons, and 3) higher shear strain can be applied during non-coaxial experiment. This requires an increased diameter of the WC bore in the pressure vessel (27 mm instead of 1 inch, i.e., 25.4 mm), reducing the maximum attainable pressure to 3 GPa.

The sample assembly (see video JoVE)

The sample assembly refers to all “consumable” pieces around the sample and required to perform a Griggs-type experiment. This includes NaCl as the confining medium, a graphite furnace and two copper discs to heat by joule effect, and other pieces (lead, pyrophyllite, alumina pistons, etc.) to increase pressure and transmit forces. Such a sample assembly is fully appropriate to perform either co-axial (pure shear) or non-coaxial (general shear) deformation experiments over the whole range of pressures and temperatures of the Griggs-type apparatus^[4]. While a pure shear experiment typically requires a cored drill sample of a certain length (commonly ≈2 times the sample diameter), a general shear deformation is commonly applied to a zone cut at 45° to the piston axis. The sample material can either be a slice of a core sample or fine-grained powder of a chosen grain size. The shear pistons and sample are wrapped into a metal foil and jacketed within a platinum tube welded (or folded flat) at both sides. The temperature is commonly monitored using either S-type (Pt_90%Rd_10%alloy) or K-type (Ni alloy) thermocouple.

References

[1] Griggs, D. J. Hydrolytic weakening of quartz and other silicates. Geophys. J. Int. 14(1-4), 19 – 31, doi:10.1111/j.1365-246X.1967.tb06218.x (1967).

[2] Green, H. W., and Borch, R. S. A New Molten Salt Cell for Precision Stress Measurements at High Pressure. Eur. J. Mineral. 1(2), 213 – 219, doi:10.1127/ejm/1/2/0213 (1989).

[3] Rybacky, E., Renner, J., Konrad, K., Harbott, W., Rummel, F., Stöckhert, B. A Servohydraulically-controlled Deformation Apparatus for Rock Deformation under Conditions of Ultra-high Pressure Metamorphism. PAGEOPH 152, 579 – 606, doi:10.1007/s000240050168 (1998).

[4] Précigout, J., Stünitz, H., Pinquier, Y., Champallier R., and Schubnel, A. High-pressure, High-temperature Deformation Experiment Using the New Generation Griggs-type Apparatus. Journal of Visualized Experiments 134, e56841, doi:103791/56841 (2018).