A. To reach Higher pressures
The maximum pressure range so far explored is limited. For a DDIA, deformation experiments have been performed only to ~10 GPa (to ~1600 K), and for a RDA to ~18 GPa (at ~1800 K). In order to investigate the whole mantle rheology including the lower mantle by laboratory experiments, we need to extend the pressure range to ~25-30 GPa (and to temperature of ~2000 K) (conditions at ~700-900 km in Earth’s mantle).

One strategy to increase pressure is to modify the system from the cubic geometry inherent to the DIA, to a 6 - 8 geometry associated with a Kawai type design. Under similar sample volumes, the 6 – 8 system has traditionally out performed the DIA. Weidner along with others has designed such a system to work with the T-cup concept (VAUGHAN et al., 1998). In this system, 1 cm cubes, which can be sintered diamond or cubic boron nitride, form the second stage that acts on the standard octahedral cell assembly. The hydrostatic T-cup has delivered 29 GPa at room temperature with cubic boron nitride anvils. A D-Tcup has been designed and ordered from the French company that builds the Paris – Edinburgh cell. The system will use the 500 ton press with two additional pistons that push the top and bottom cubic anvils, in much the same manner as the DDIA. We are expecting delivery in 2006. This system will be tested, cells will be developed for it, and it will provide the sample all of the x-ray access that is currently available for the DDIA. We will initially install this press on the new monochromatic side station that has recently been tested at the NSLS. As we are still in the commissioning stage for the side station, we will not have competing proposals for beam time for this station during the first year. By that time, we expect to have the fundamental aspects of the system working. Continued improvements of cell assemblies will add capability to the system. During this research program, Weidner will pursue the implementation of the D-Tcup system for high pressure rheological measurements.

At GSECARS, Wang and others have successfully pushed the pressure range to about 18 GPa using 2 mm truncations in the DDIA. They designed independent anvil gaskets to provide lateral support for the x-ray transparent anvils, apparently with some success. In contrast, essentially all of the more conventional 4-mm truncation (6 mm cube) configurations currently employ self gasketing, wherein material from the edges and corners of the cube itself (i.e., the pressure medium) flows into the gaps between the anvils, thus forming the gasket. Persistent premature failure of 4-mm truncated x-ray transparent anvils suggests that lateral support for those anvils is poor, so to push the 6-mm cell with its larger sample to higher pressures, perhaps reaching the Transition Zone, we may explore the possibility of independent gasketing.

Additionally, we will modify the DDIA in order to gain in pressure by building a large deformation machine, DDIA-30 (30 mm anvils for a 1000 ton press). We have received funds from the COMPRES Infrastructure Development Committee and GSECARS to design and build a prototype DDIA-30. The apparatus is expected to be completed and tested in 2007, and will enhance the current Rheology Grand Challenge project in at least two aspects. (1) As a single-stage machine, DDIA-30 will be capable of deforming large samples (up to 10 mm in diameter), up to about 10 GPa. This capability will permit faulting experiments with acoustic emission detection. Up to six acoustic detectors will be mounted on the back end of each DDIA anvils to accurately locate sources of events to a spatial resolution of below 50 microns. Coupled with in-situ X-ray diffraction and imaging, DDIA-30 will allow us to study earthquake mechanisms at pressure conditions down to 350 km depths. (2) With the introduction of a pair of Drickamer anvils in a large cubic cell assembly, the DDIA-30 can be used as a double-stage deformation machine under ultrahigh pressures. Preliminary tests, conducted in the smaller DDIA using 4 mm truncation on the first stage anvils, have generated up to about 50 GPa pressure on 0.25 mm diameter samples. We expect to be able to reach higher pressures in DDIA-30 with sample diameters about 0.5 mm. This will allow us to conduct deformation experiments under lower mantle conditions with controlled strain and strains rates. During this research program, Wang will pursue the implementation of the DDIA-30 and related Drickamer adaptation for rheological studies at high pressure.

Pressure generation is more efficient with a RDA. Already with the initial design (4 mm truncation), we have generated ~17-18 GPa (at ~1800 K). The pressure range of deformation experiments will be extended by two ways. First, the reduction of truncation to 3 mm will further increase the maximum pressure of operation. Second, we plan to use polycrystalline diamond at the tip of the anvils. We have already explored various polycrystalline diamonds, and one of the promising product is HIME-DIA developed by Irifune’s group at Ehime (IRIFUNE et al., 2004). HIME-DIA is made through the transformation of graphite at very high pressures and temperatures. Because the P-T conditions of synthesis far exceed the phase boundary between graphite and diamond, the kinetics of nucleation is so fast that one can produce submicron grain-size polycrystalline diamond without any binder (SUMIYA et al., 2004) and it has a very high hardness (SUMIYA and IRIFUNE, 2004). Although a large block of HIME-DIA has not been manufactured, a thin disk-shaped one is already produced at several mm diameter and 1-2 mm thickness. One option is to use a polycrystalline diamond disk at the tip of the anvil for RDA. Karato has already contacted Professor Irifune to collaborate on this project (see attached letter). In this proposed program, we will aggressively pursue different hard anvil strategies. We will investigate a wide range of commercial products for this purpose. We will work to develop ‘hard anvil friendly’ cell assemblies.