Ivan C. Getting

The deformation-DIA: A new apparatus for high temperature triaxial deformation to pressures up to 15 GPa

A new deformation apparatus has been developed, based on the widely used cubic-anvil apparatus known as the DIA. Two differential rams, introduced in the upper and lower guide blocks, allow independent control of the differential strain and stress field under high confining pressure. Testing experiments with synchrotron x rays have demonstrated that this deformation DIA (D-DIA) is capable of generating up to 30% axial strain on a 1–2 mm long sample under confining pressures up to 15 GPa at simultaneous high temperatures. Various compressional strain rates from 10−3 to about 5×10−6 s−1 have been achieved. Extensional experiments have also been carried out successfully. Strains are measured by x-ray imaging of the sample which has a length measurement precision of ∼0.1 μm; pressures are monitored using standard materials with well established equations of state. X-ray transparent anvils made of sintered polycrystalline cubic boron nitride have been successfully tested, with a two-dimensional x-ray charge co...

A New Ultrasonic Interferometer for the Determination of Equation of State Parameters of Sub-millimeter Single Crystals

A new giga-Hertz ultrasonic interferometer has been developed, based on ultrasonic microscopy technology. The interferometer operates from 0.3 GHz to 1.5 GHz. The high frequency and associated small wavelengths together with the large bandwidth make it possible to measure travel times in samples with thicknesses of several microns and allow for unprecedented accuracy in bond corrections. An absolute accuracy of 1 part in 105 in travel time measurements is achievable in single crystals (thickness of ∼200 microns) or glasses of interest to the earth sciences. The high precision travel time data, combining with sample length measurements using a laser interferometer built in our laboratory, yield very high precision ultrasonic velocities.The interferometer is intended for use in conjunction with a newly developed 4 GPa gas piston cylinder apparatus (Getting andSpetzler, 1993) for equation of state measurements under simultaneous pressure and temperature. A separate correction for the bond will be made for each datum at every point in temperature pressure space.

Shear attenuation and dispersion in MgO

Abstract An understanding of anelastic effects on seismic wave propagation is crucial for developing accurate seismic models of the mantle. The lower mantle is thought to contain a significant volume fraction, approximately 20%, of magnesiowustite, (Mg,Fe)O, in addition to the dominant silicate perovskite. Weaker magnesiowustite may dominate the rheological response of the lower mantle. It may also account for the shear attenuation observed in the lower mantle. We have measured the complex torsional modulus of a single crystal of the magnesiowustite end member periclase, MgO, at small strains, seismic frequencies, and high temperature. The attenuation is approximately 0.030 ( Q approximately 33) at 0.003 Hz and 1500 K with a dislocation density of approximately 5 × 109 m−2. The corresponding dispersion reduces the shear velocity of MgO at 0.001 Hz and 1500 K by approximately 4% relative to ultrasonic values. To the extent that magnesiowustite behaves similarly to MgO, it appears to be a good candidate to account for a major portion of lower-mantle attenuation.

The strength and rheology of commercial tungsten carbide cermets used in high-pressure apparatus

Uniaxial compressive stress-strain curves have been measured on a suite of 26 commercial grades of tungsten carbide cermets and three maraging steels of interest for use in high-pressure apparatus. Tests were conducted on cylindrical specimens with a length to diameter ratio of two. Load was applied to the specimens by tungsten carbide anvils padded by extrudable lead disks. Interference fit binding rings of maraging steel were pressed on to the ends of the specimens to inhibit premature corner fractures. Bonded resistance strain gages were used to measure both axial and tangential strains. Deformation was exremely uniform in the central, gauged portion of the specimens. Tests were conducted at a constant engineering strain rate of 1 × 10−5 S−l. The composition of the specimens was principally WC/Co with minor amounts of other carbides in some cases. The Co weight fraction ranged from 2 to 15%. Observed compressive strengths ranged from about 4 to just above 8 GPa. Axial strain amplitude at failure varied from ∼ 1.5% to ∼9%. Representative stress-strain curves and a ranking of the grades in terms of yield strength and strain at failure are presented. A power law strain hardening relation and the Ramberg-Osgood stress-strain equation were fit to the data. Fits were very good for both functions to axial strain amplitudes of about 2%. The failure of these established functions is accompanied by an abrupt change in the trend of volumetric strain consistent with the onset of substantial microcrack volume.

Seismic attenuation in artificial glass cracks: Physical and physicochemical effects of fluids

Attenuation and stiffness of artificial, fluid containing cracks are measured from 3 mHz to 10 Hz. The cracks are wedge-shaped; made from glass microscope slides. To explain the frequency dependence of both the attenuation and the stiffness (akin to a modulus), we need to appeal to well known fluid flow mechanisms and to the physicochemical interaction between the fluid and crack surface. By altering the wettability of the crack surfaces, surfactants change the mobility of water and thereby change the frequency dependence of the fluid flow effects by several orders of magnitude.

Cross pressure and temperature derivatives of selected elastic moduli for olivine from gigahertz ultrasonic interferometry

Techniques have been developed to measure the cross pressure and temperature dependence of elastic moduli of olivine using the gigahertz ultrasonic interferometric method in a gas pressure vessel (to 500 MPa) equipped with an internal heater (to 500 K). Special attention has been drawn to the effect of the bond which couples the sample to the buffer rod. We obtained ∂ 2 C 22 /∂P∂T = (2.8 ± 0.9) x 10 -3 K- for a San Carlos olivine sample with a lapped contact. Selective values of ∂ 2 K T /∂P∂T (0, 0.5 x 10 -3 , 1.0 x 10 -3 K -1 ) are used to calculate their effect on cell volume, bulk modulus, and bulk sound velocity for olivine at pressures to 14 GPa and temperatures to 1800 K (410-km depth), using the high-temperature form of the Birch-Murnaghan equation of state. The largest difference between the cell volumes for ∂ 2 K T /∂P∂T = 1 x 10 -3 K -1 and for ∂ 2 K T /∂P∂T = 0 is about 0.6% at the highest pressure and the highest temperature (14 GPa, 1800 K). Therefore PVT data with precision at least better than 0.6% are required to distinguish between the extreme values of ∂ 2 K T /∂P∂T for olivine if the pressure and temperature data are free of errors. If the cross pressure and temperature derivative term is neglected (∂ 2 K T /∂P∂T = 0) in extrapolating laboratory elasticity data to mantle pressure and temperature conditions, the bulk modulus of olivine can be underestimated by as much as 10% and the bulk sound velocity by 3% at the 410-km seismic discontinuity. Thus the present study highlights the need to measure elastic properties of mantle candidate minerals at the P-T conditions of the mantle transition zone in order to establish a more precise model of the mantle mineralogy.

Selected elastic moduli and their temperature derivatives for olivine and garnet with different Mg/(Mg+Fe) contents: Results from GHz ultrasonic interferometry

Selected P-wave acoustic travel times through single crystal samples of olivine and garnet with different Mg/(Mg+Fe) ratios were measured at elevated temperatures (from 293 K to 363 K) using a newly developed GHz ultrasonic interferometer. These high precision data allow us to examine aspects of the elasticity-composition systematics of olivine (forsterite-fayalite) and garnet (pyrope-almandine) solid solution series. For every 10% decrease in the Mg/(Mg+Fe) content, the P-velocity of olivine along the b-axis decreases about 3% and ∂lnC22/∂T decreases about 7%; for garnet ∂ln⌊C44 + (C11 + C12) / 2⌋/∂T decreases about 6%. If such velocity-composition systematics are typical and hold at the P-T regime of the mantle, only a 3–4% variation in the Mg/(Mg+Fe) content of olivine is required to explain the lateral seismic velocity variation in the upper portion of the mantle within one tectonic region (dlnVp∼l%), and 7–8% variation in the Mg/(Mg+Fe) content to explain the 2% lateral seismic velocity variation of the upper mantle globally.

Gas‐charged piston‐cylinder apparatus for pressures to 4 GPa

An end‐loaded piston‐cylinder high pressure apparatus which utilizes an argon pressure medium has been developed. This device has a pressurized volume in excess of 3 cm3 and offers the absolute pressure calibration of the piston‐cylinder apparatus with the truly hydrostatic and chemically inert enviornment of argon. The vessel is precharged to 0.5 GPa through a port and valve system built into the end‐load hardware. No port is required in the wall of the vessel. After precharge, the piston is advanced into the chamber to generate pressure to 4 GPa.Solid argon, at pressures greater than ∼1.3 GPa, is so weak that it requires gas sealing techniques to contain it. The seals consist entirely of unsupported area mitre rings in chamfers. No O‐rings or other soft seal components are used. This configuration permits extensive cycling through the melting‐freezing pressure of argon without failure. Leak rates are exceedingly low over the entire pressure range. A safety barrier have been constructed which comfortably...

Implication of the complete travel time equation of state for a new pressure scale

A complete equation of state results from ultrasonic travel time measurements at simulataneous temperature and pressure, and thermal expansion and heat capacity measurements as a function of temperature at zero pressure. When unit cell dimensions are measured simultaneously (by X‐ray methods) a second determination of the equation of state is obtained. It is possible to eliminate the pressure as a dependent variable between the two EOSs and thus determine it from the other equation of state parameters. Present accuracy obtainable by X‐ray and ultrasonic methods suggests a new pressure scale which is more accurate than present absolute scales based on the dead‐weight tester.

Characterizing Sub-millimeter Crystals with a Wide-band GHz Interferometer

The demand from geophysics and material sciences to characterize small crystal samples has propelled the development of a wide-band GHz ultrasonic interferometer. With this interferometer, acoustic travel times through single crystal samples as small as 100 μm in thickness and through a thin film of cyanoacrylate (super glue, ∼5 μm) have been determined with unprecedented accuracy. The bond effect on the sample travel time is much better understood than it was in the past. The interferometer is intended to operate at pressures up to 4 GPa (40 kbar) and at temperatures to 1500 K.