Simultaneous SHG (second harmonic generation) and XRD (x-ray diffraction) diagnostics have been a... more Simultaneous SHG (second harmonic generation) and XRD (x-ray diffraction) diagnostics have been applied to examine the phase behavior of energetic materials, HMX (octahydro-1,3,5,7tetranitro-l,3,5,7-tetrazocine) and TATB (l,3,5-triamino-2,4,6 trinitrobenzene). This unique capability provides information about both volume and surface effects that occur during the solid-solid transformation process. This paper reports XRD results for HMX and TATB at elevated temperatures and on simultaneous SHG and XRD experiments on HMX at fixed temperature. Our results do not indicate that a solid-solid phase transformation occurs for TATB even at temperatures up to 340°C. XRD results on HMX held at 165°C and 1 bar, indicate that the (3 to 5 transformation is incomplete after a period of 4.5 hours which do not temporally correlate with SHG. Overall information indicates that the observed SHG intensities from surface effects can, in some cases, dominate over volume generated SHG contributions. Finally, we have run in situ AFM scans of HMX at 180°C and 184°C that show HMX surface area increases by many orders of magnitude after the 5-phase transformation is completed.
This is a preprint of a paper intended for publication in a journal or proceedings. Since changes... more This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.
In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, ... more In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C 44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V. Although body-centered-cubic (bcc) metals have one of the simplest crystal structures in the periodic table, they display a rich variety of physical properties and thus provide an important benchmark for the validation of modern first-principle theory 1. In particular, the lattice dynamics of bcc transition metals have attracted great scientific attention. The Kohn anomaly in the phonon dispersion of bcc transition metals, and its dependence upon pressure and temperature, has been a challenge for first principle calculations to capture 2,3. The strong differences displayed by the phonon dispersion of the various elements of group V (vanadium, niobium and tantalum) suggest that there is a profound dependence of the phonon energies on the electronic structure and the topology of the Fermi surface 4,5. The high superconducting temperature (T c = 9.25 K for Nb and T c = 5.3 K for V) and its notable increase with pressure have also been suggested to be due to electron-phonon coupling and Fermi-surface properties 6-8. The stability at high pressure of the bcc structure is speculated to critically hinge on the topology of the Fermi surface as well, and an intra-band nesting is theoretically predicted to give rise to shear phonon instabilities 9. Focusing on vanadium, calculations of shear instabilities arising from phonon softening 9 have prompted the reinvestigation of the structural stability of V under high pressure. X-ray powder diffraction showed a transition from the bcc to a rhombohedral phase at 69 GPa 10 and subsequent calculations have confirmed the nature of the rhombohedral distortion-even though different transition pressures were proposed 5,11-13. Interestingly, under hydrostatic conditions the transition is hindered, and non-hydrostaticity helps in overcoming the energy barrier associated with the structural phase change 14. Irrespective of the exact pressure at which the transition occurs, the bulk of theoretical work points towards a common mechanism: the progressive intra-band nesting at the Fermi surface that eventually leads to an electronic topological transition (ETT) with a concomitant transverse acoustic phonon mode softening. Specifically, at a critical pressure, parts of the 3rd electronic, partially occupied, conduction band of d symmetry move into the close vicinity of the Fermi level. The nesting vector, already responsible for the Kohn anomaly in the transverse acoustic phonon mode along the (ξ, 0, 0) direction at ξ = 0.25 at ambient pressure 8 , reduces to zero and the ETT takes place, with instability in the shear elastic constant C 44 9. This anomalous softening of the elastic response causes an energy gain that counterbalances the standard elastic strain energy
Using 36 Cl cosmic ray exposure dating we obtained continuous exposure histories for 7-12 m-high ... more Using 36 Cl cosmic ray exposure dating we obtained continuous exposure histories for 7-12 m-high limestone surfaces at two sites (10 km apart) on the Sparta normal fault scarp. As each major earthquake adds new surface to the cumulative scarp exposing new material to cosmic-ray bombardment these exposure histories allow the slip history to be constrained. The results show that an earthquake occurred on this fault 2800 ± 300 yr ago. We infer that this is the seismic event that destroyed ancient Sparta in 464 B.C. Four earlier earthquakes ruptured the Sparta fault in the last 13 ka with similar slip amplitudes of about 2 m and with time intervals ranging from 500 yr to 4500 yr. The observations also confirm that the Sparta scarp is post-glacial, supporting the hypothesis that similar scarps elsewhere in the Mediterranean region have a comparable age. The absence of any event since 464 B.C. could suggest a future event is imminent. However, the irregularity of earthquake time intervals could also be due to changes of loading with important consequences for the mechanics of continental deformation.
ABSTRACT The velocity of an acoustic wave propagating at the interface formed by a single crystal... more ABSTRACT The velocity of an acoustic wave propagating at the interface formed by a single crystal of cobalt in contact with liquid helium has been measured using impulsive stimulated light scattering. Dispersion curves of velocity versus in-plane propagation direction to 10 GPa have been obtained. The data is used to determine the pressure dependence of the elastic tensor element c44 assuming only the density of cobalt. The additional elements c12 and c13 are also obtained by assuming the values for c11 and c33 that were determined in a separate study using inelastic x-ray scattering [1]. Absolute values of elastic constants and pressure derivatives are compared to the results of first principles theoretical calculations in the generalized gradient and local density approximations. The validity of such calculations for predicting the high pressure elasticity of metals of hexagonal symmetry is discussed. [1] D. Antonangeli, M. Krisch, G. Fiquet, D. Farber, C. Aracne, J. Badro, F. Occelli, and H. Requardt, Phys. Rev. Lett. 93, 215505 (2004).
ABSTRACT The chemical activation energy barrier of metastable anisotropic solid phase decompositi... more ABSTRACT The chemical activation energy barrier of metastable anisotropic solid phase decomposition reactions depends, in part, on crystal symmetry. Further, a detailed understanding of solid-solid phase transitions is critical for accurate modeling of energetic material decomposition. Indeed, delta phase HMX is more sensitive to shock initiated chemical reactions than the beta phase polymorph. Thus, to learn more about pressure-induced beta to delta transition dynamics we have employed a second harmonic generation optical diagnostic. In addition we have monitored the corresponding pressure and temperature throughout the transition. In some instances we include real-time digital micro-photographs that temporally correspond to each optical pressure measurement. Our goal is to quantify the isothermal kinetic phase diagram of HMX and the corresponding stability fields. X-ray analysis before and after the pressure-induced transition provides an added degree of certainty to inferences made from our optical measurements. These experiments provide a unique data set for the understanding of ignition conditions that lead to the stable or metastable decomposition reactions of delta phase HMX.
The phase transformation of P-HMX (> 0.5% RDX) to 5 phase has been studied for over twenty years ... more The phase transformation of P-HMX (> 0.5% RDX) to 5 phase has been studied for over twenty years and more recently with an high-contrast optical second harmonic generation technique. Shock studies of the plastic binder composites of HMX have indicated that the transition is perhaps irreversible, a result that concurs with the static pressure results published by F. Goetz et al. [l] in 1978. However, the stability field favors the p polymorph over 6 as pressure is increased (up to 5.4 GPa) along any thermodynamically reasonable isotherm. In this experiment, strict control of pressure and temperature is maintained while x-ray and optical diagnostics are applied to monitor the conformational dynamics of HMX. Unlike the temperature induced p->6 transition, the pressure induced is heterogeneous in nature. The 1 bar 25°C 6->p transition is not immediate, occuring over tens of hours. Transition points and kinetics are path dependent and consequently this paper describes our work in progress.
Using 36 Cl cosmic ray exposure dating we obtained continuous exposure histories for 7-12 m-high ... more Using 36 Cl cosmic ray exposure dating we obtained continuous exposure histories for 7-12 m-high limestone surfaces at two sites (10 km apart) on the Sparta normal fault scarp. As each major earthquake adds new surface to the cumulative scarp exposing new material to cosmic-ray bombardment these exposure histories allow the slip history to be constrained. The results show that an earthquake occurred on this fault 2800 ± 300 yr ago. We infer that this is the seismic event that destroyed ancient Sparta in 464 B.C. Four earlier earthquakes ruptured the Sparta fault in the last 13 ka with similar slip amplitudes of about 2 m and with time intervals ranging from 500 yr to 4500 yr. The observations also confirm that the Sparta scarp is post-glacial, supporting the hypothesis that similar scarps elsewhere in the Mediterranean region have a comparable age. The absence of any event since 464 B.C. could suggest a future event is imminent. However, the irregularity of earthquake time intervals could also be due to changes of loading with important consequences for the mechanics of continental deformation.
ABSTRACT Direct cosmogenic dating of exhumed earthquake scarps provide a new way to determine the... more ABSTRACT Direct cosmogenic dating of exhumed earthquake scarps provide a new way to determine the slip-rate and the earthquake history of a fault. Here, we have applied in situ 36Cl cosmic ray exposure dating to a limestone fault scarp produced by the Kaparelli fault. The Kaparelli Fault is located at the eastern extremity of the Gulf of Corinth. In 1981, a sequence of three earthquakes with magnitude greater than 6 struck this region (Jackson et al. E.P.S.L. 57, 1982, King et al. G.J.Int, 80, 1985). The Kaparelli fault was responsible for the third event (March, 4, 1981, Ms ~6.4) with an associated rupture of 12 km and a maximum vertical displacement of 70 cm. For about 5 km, the surface break followed a 3-4 m high limestone fault scarp along which the 1981 event exhumed 40 cm of new scarp. On the best preserved section of this scarp we sampled a continuous 3.5 m vertical profile, with a 10 cm resolution. 36Cl is produced at shallow depths in calcite predominantly by spallation of calcium. This production decreases exponentially with depth beneath the surface but varies little with height once a section of the scarp has been tectonically exhumed. The concentration of 36Cl and of stable chlorine in the 35 samples has been measured by accelerator mass spectrometry (AMS) at the LLNL Center for Accelerator Mass Spectrometry. The 36Cl concentration increases at an approximately constant rate with the height of the scarp, suggesting that the slip-rate of the fault is constant with time. The 36Cl concentrations indicate that the scarp is probably more than ten thousand years old. Moreover, the 36Cl pattern suggests that this scarp might have formed as a result of 6 events. Further analysis of our data and refinement of 36Cl production rates will allow us to further test these hypotheses. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
ABSTRACT We have measured the sound velocity of Ice VII by Brillouin spectroscopy using membrane-... more ABSTRACT We have measured the sound velocity of Ice VII by Brillouin spectroscopy using membrane-style diamond anvil cell with an external Mo-wire resistance heater at elevated temperatures to 873 K at pressures of 2 GPa to 19 GPa. The unit cells of Ice VII and Au were determined by synchrotron X-ray diffraction, using Au as an in situ pressure gauge. All our samples contained were contained within and chemically insulated from the Re-gasket hole by a gold liner. We determined the melting of Ice VII by monitoring the sound velocity drop and the disappearance of diffraction pattern of Ice VII upon melting. Below ~8 GPa, our results are consistent with the previously determined melting curves (Datchi et al. 2000; Frank et al. 2004; Lin et al. 2004; Goncharov et al. 2005). However, our determination of the melting temperature at 12.5 GPa results in a discrepancy of at least 130 K. Given the care taken in the present experiments to avoid potential contamination of the water sample due to reactions at high temperatures between the sample and the gaskets and/or pressure gauges, our new measurements likely provide the first measurements on pure water which displaies an extended stability field of the solid phase. Thus, our new measurements suggest that the melting curve of H2O at high pressure needs to be reevaluated.
ABSTRACT The laser-heated diamond anvil cell (LHDAC) is a valuable quantitative tool for the eart... more ABSTRACT The laser-heated diamond anvil cell (LHDAC) is a valuable quantitative tool for the earth and planetary sciences because it can reproduce the thermodynamic conditions in planetary interiors, making possible measurements of material properties for those portions of planets that are not directly observable. The utility of the LHDAC is diminished, however, when systematic imaging-system-related errors lead to difficulty in comparing the results of different experiments and different labs. We present calculations detailing systematic errors in spectroradiometric temperature measurements due to the diamond anvil itself. Wavelength-dependent index of refraction and absorption can lead to errors of several hundred K. In addition, we assess methods for interpreting data already in the literature with respect to these sources of error.
Cerium is a rare-earth metal with many of its physical and chemical properties governed by the co... more Cerium is a rare-earth metal with many of its physical and chemical properties governed by the complex behavior of its 4f electron in contributing to the bonding of the crystal structure. Specifically, its gamma-alpha volume-collapse transition under pressure is the only known solid-solid transition in an element with the phase boundary ending at a critical point, and the detailed mechanism
ABSTRACT Recent activity of normal faults in Greece has produced steep limestone fault scarps at ... more ABSTRACT Recent activity of normal faults in Greece has produced steep limestone fault scarps at the base of the mountain fronts. For example, on the Sparta fault located in the Peloponnese, and responsible for the 464 B.C. M˜7 earthquake, a continuous fresh scarp cuts limestone bedrock and indurated conglomerates. The scarp is nearly continuous dipping at 65-68o with well-preserved slickensides. The maximum height of the scarp is 10-12 metres, progressively decreasing towards the ends. The few local variations are associated with active streams where hangingwall erosion causes the scarp to be locally higher. The regularity of the scarp is powerful evidence that the footwall and hangingwall surfaces were originally continuous and the scarp surface represents fault slip alone. It also suggests that there was no significant erosion or deposition on the hanging-wall (except near active gullies) since the scarp began to form. Such observations have led to the suggestion that numerous well-preserved limestone escarpments around the eastern Mediterranean, similar to those in Sparta, are post-glacial in age. Using 36Cl cosmogenic dating we tested the foregoing ideas. Limestones (largely calcite) contain an abundance of calcium, which is a major target element for cosmogenic 36Cl production. Samples were collected from the limestone scarp surface to recover the continuous exposure history of the scarp and also from the footwall and hanging wall surfaces as well as a depth profile in the hanging wall wedge. The concentration of 36Cl and of stable chlorine has been measured by accelerator mass spectrometry (AMS) at the LLNL-CAMS for each of the samples (about 150). The result confirm: 1- that the fault scarp was formed between about 2000 B.P. and 13 ka B.P. as the result of 5 earthquakes (including the known event in 464 B.C.) with similar slip amplitudes of about 2m and with time intervals ranging from 500yr to 4500yr (Benedetti et al., GRL, 2002), 2- that both hanging wall and footwall surfaces have exposure ages of 40-50 ka, and 3- that the colluvial wedge was formed during three episodes of deposition that occurred between 50 to 70 ka B.P. It is not clear whether those episodes, that are also stratigraphically recognizable, are seismically or climatically related. The climatic hypothesis is that during the cold and dry climate of the last glaciation the slopes were unstable so that the free-faces formed during repeated earthquakes were immediately buried. At the end of the glaciation, as a consequence of the wet and warm climate, slopes stabilized and earthquake slip began to accumulate and to form the present day cumulative escarpements.
The amount and timing of river incision along the western margin of the Altiplano can provide ins... more The amount and timing of river incision along the western margin of the Altiplano can provide insight into the climatic, volcanic, and tectonic history of this region. While many studies have focused on the Tertiary geologic history of the western margin of the Altiplano, the Quaternary geologic history of this margin remains largely unstudied. The Pacific draining Rio Tambo river
ABSTRACT Measurements of the chemistry and physics of the pure endmembers of the C-O-H-N-S system... more ABSTRACT Measurements of the chemistry and physics of the pure endmembers of the C-O-H-N-S system are important building blocks in the study of planetary ices. To that end, we performed x-ray diffraction measurements on pure oxygen using the laser-heated diamond anvil cell. Using a novel experimental setup and a combined interpretation of the diffraction patterns in conjunction with spectroradiometric observations, the temperature and pressure stability field of the ε -phase has been studied. We also present measurements of the melting temperature of oxygen at 47 and 55 GPa, pressures more than twice as high as previously achieved. The ensemble of our experimental evidence highlights the remarkable thermodynamic stability of the O8 cluster structure with respect to lower pressure molecular arrangements and points to non-magnetic ordering as one of the main reasons for that.
The phase transformation of P-HMX (> 0.5% RDX) to 5 phase has been studied for over twenty years ... more The phase transformation of P-HMX (> 0.5% RDX) to 5 phase has been studied for over twenty years and more recently with an high-contrast optical second harmonic generation technique. Shock studies of the plastic binder composites of HMX have indicated that the transition is perhaps irreversible, a result that concurs with the static pressure results published by F. Goetz et al. [l] in 1978. However, the stability field favors the p polymorph over 6 as pressure is increased (up to 5.4 GPa) along any thermodynamically reasonable isotherm. In this experiment, strict control of pressure and temperature is maintained while x-ray and optical diagnostics are applied to monitor the conformational dynamics of HMX. Unlike the temperature induced p->6 transition, the pressure induced is heterogeneous in nature. The 1 bar 25°C 6->p transition is not immediate, occuring over tens of hours. Transition points and kinetics are path dependent and consequently this paper describes our work in progress.
While the canonical view is that active deformation in the Andes is mostly confined to the easter... more While the canonical view is that active deformation in the Andes is mostly confined to the eastern slopes in the subandean fold and thrust belt, recent studies have highlighted the existence of active deformation in the central Andean forearc (Audin et al, 2003, Hartley et al., 2000, Husson and Sempere, 2003, Farias et al., 2005). The forearc of southern Peru,
Simultaneous SHG (second harmonic generation) and XRD (x-ray diffraction) diagnostics have been a... more Simultaneous SHG (second harmonic generation) and XRD (x-ray diffraction) diagnostics have been applied to examine the phase behavior of energetic materials, HMX (octahydro-1,3,5,7tetranitro-l,3,5,7-tetrazocine) and TATB (l,3,5-triamino-2,4,6 trinitrobenzene). This unique capability provides information about both volume and surface effects that occur during the solid-solid transformation process. This paper reports XRD results for HMX and TATB at elevated temperatures and on simultaneous SHG and XRD experiments on HMX at fixed temperature. Our results do not indicate that a solid-solid phase transformation occurs for TATB even at temperatures up to 340°C. XRD results on HMX held at 165°C and 1 bar, indicate that the (3 to 5 transformation is incomplete after a period of 4.5 hours which do not temporally correlate with SHG. Overall information indicates that the observed SHG intensities from surface effects can, in some cases, dominate over volume generated SHG contributions. Finally, we have run in situ AFM scans of HMX at 180°C and 184°C that show HMX surface area increases by many orders of magnitude after the 5-phase transformation is completed.
This is a preprint of a paper intended for publication in a journal or proceedings. Since changes... more This is a preprint of a paper intended for publication in a journal or proceedings. Since changes may be made before publication, this preprint is made available with the understanding that it will not be cited or reproduced without the permission of the author.
In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, ... more In spite of the simple body-centered-cubic crystal structure, the elements of group V, vanadium, niobium and tantalum, show strong interactions between the electronic properties and lattice dynamics. Further, these interactions can be tuned by external parameters, such as pressure and temperature. We used inelastic x-ray scattering to probe the phonon dispersion of single-crystalline vanadium as a function of pressure to 45 GPa. Our measurements show an anomalous high-pressure behavior of the transverse acoustic mode along the (100) direction and a softening of the elastic modulus C 44 that triggers a rhombohedral lattice distortion occurring between 34 and 39 GPa. Our results provide the missing experimental confirmation of the theoretically predicted shear instability arising from the progressive intra-band nesting of the Fermi surface with increasing pressure, a scenario common to all transition metals of group V. Although body-centered-cubic (bcc) metals have one of the simplest crystal structures in the periodic table, they display a rich variety of physical properties and thus provide an important benchmark for the validation of modern first-principle theory 1. In particular, the lattice dynamics of bcc transition metals have attracted great scientific attention. The Kohn anomaly in the phonon dispersion of bcc transition metals, and its dependence upon pressure and temperature, has been a challenge for first principle calculations to capture 2,3. The strong differences displayed by the phonon dispersion of the various elements of group V (vanadium, niobium and tantalum) suggest that there is a profound dependence of the phonon energies on the electronic structure and the topology of the Fermi surface 4,5. The high superconducting temperature (T c = 9.25 K for Nb and T c = 5.3 K for V) and its notable increase with pressure have also been suggested to be due to electron-phonon coupling and Fermi-surface properties 6-8. The stability at high pressure of the bcc structure is speculated to critically hinge on the topology of the Fermi surface as well, and an intra-band nesting is theoretically predicted to give rise to shear phonon instabilities 9. Focusing on vanadium, calculations of shear instabilities arising from phonon softening 9 have prompted the reinvestigation of the structural stability of V under high pressure. X-ray powder diffraction showed a transition from the bcc to a rhombohedral phase at 69 GPa 10 and subsequent calculations have confirmed the nature of the rhombohedral distortion-even though different transition pressures were proposed 5,11-13. Interestingly, under hydrostatic conditions the transition is hindered, and non-hydrostaticity helps in overcoming the energy barrier associated with the structural phase change 14. Irrespective of the exact pressure at which the transition occurs, the bulk of theoretical work points towards a common mechanism: the progressive intra-band nesting at the Fermi surface that eventually leads to an electronic topological transition (ETT) with a concomitant transverse acoustic phonon mode softening. Specifically, at a critical pressure, parts of the 3rd electronic, partially occupied, conduction band of d symmetry move into the close vicinity of the Fermi level. The nesting vector, already responsible for the Kohn anomaly in the transverse acoustic phonon mode along the (ξ, 0, 0) direction at ξ = 0.25 at ambient pressure 8 , reduces to zero and the ETT takes place, with instability in the shear elastic constant C 44 9. This anomalous softening of the elastic response causes an energy gain that counterbalances the standard elastic strain energy
Using 36 Cl cosmic ray exposure dating we obtained continuous exposure histories for 7-12 m-high ... more Using 36 Cl cosmic ray exposure dating we obtained continuous exposure histories for 7-12 m-high limestone surfaces at two sites (10 km apart) on the Sparta normal fault scarp. As each major earthquake adds new surface to the cumulative scarp exposing new material to cosmic-ray bombardment these exposure histories allow the slip history to be constrained. The results show that an earthquake occurred on this fault 2800 ± 300 yr ago. We infer that this is the seismic event that destroyed ancient Sparta in 464 B.C. Four earlier earthquakes ruptured the Sparta fault in the last 13 ka with similar slip amplitudes of about 2 m and with time intervals ranging from 500 yr to 4500 yr. The observations also confirm that the Sparta scarp is post-glacial, supporting the hypothesis that similar scarps elsewhere in the Mediterranean region have a comparable age. The absence of any event since 464 B.C. could suggest a future event is imminent. However, the irregularity of earthquake time intervals could also be due to changes of loading with important consequences for the mechanics of continental deformation.
ABSTRACT The velocity of an acoustic wave propagating at the interface formed by a single crystal... more ABSTRACT The velocity of an acoustic wave propagating at the interface formed by a single crystal of cobalt in contact with liquid helium has been measured using impulsive stimulated light scattering. Dispersion curves of velocity versus in-plane propagation direction to 10 GPa have been obtained. The data is used to determine the pressure dependence of the elastic tensor element c44 assuming only the density of cobalt. The additional elements c12 and c13 are also obtained by assuming the values for c11 and c33 that were determined in a separate study using inelastic x-ray scattering [1]. Absolute values of elastic constants and pressure derivatives are compared to the results of first principles theoretical calculations in the generalized gradient and local density approximations. The validity of such calculations for predicting the high pressure elasticity of metals of hexagonal symmetry is discussed. [1] D. Antonangeli, M. Krisch, G. Fiquet, D. Farber, C. Aracne, J. Badro, F. Occelli, and H. Requardt, Phys. Rev. Lett. 93, 215505 (2004).
ABSTRACT The chemical activation energy barrier of metastable anisotropic solid phase decompositi... more ABSTRACT The chemical activation energy barrier of metastable anisotropic solid phase decomposition reactions depends, in part, on crystal symmetry. Further, a detailed understanding of solid-solid phase transitions is critical for accurate modeling of energetic material decomposition. Indeed, delta phase HMX is more sensitive to shock initiated chemical reactions than the beta phase polymorph. Thus, to learn more about pressure-induced beta to delta transition dynamics we have employed a second harmonic generation optical diagnostic. In addition we have monitored the corresponding pressure and temperature throughout the transition. In some instances we include real-time digital micro-photographs that temporally correspond to each optical pressure measurement. Our goal is to quantify the isothermal kinetic phase diagram of HMX and the corresponding stability fields. X-ray analysis before and after the pressure-induced transition provides an added degree of certainty to inferences made from our optical measurements. These experiments provide a unique data set for the understanding of ignition conditions that lead to the stable or metastable decomposition reactions of delta phase HMX.
The phase transformation of P-HMX (> 0.5% RDX) to 5 phase has been studied for over twenty years ... more The phase transformation of P-HMX (> 0.5% RDX) to 5 phase has been studied for over twenty years and more recently with an high-contrast optical second harmonic generation technique. Shock studies of the plastic binder composites of HMX have indicated that the transition is perhaps irreversible, a result that concurs with the static pressure results published by F. Goetz et al. [l] in 1978. However, the stability field favors the p polymorph over 6 as pressure is increased (up to 5.4 GPa) along any thermodynamically reasonable isotherm. In this experiment, strict control of pressure and temperature is maintained while x-ray and optical diagnostics are applied to monitor the conformational dynamics of HMX. Unlike the temperature induced p->6 transition, the pressure induced is heterogeneous in nature. The 1 bar 25°C 6->p transition is not immediate, occuring over tens of hours. Transition points and kinetics are path dependent and consequently this paper describes our work in progress.
Using 36 Cl cosmic ray exposure dating we obtained continuous exposure histories for 7-12 m-high ... more Using 36 Cl cosmic ray exposure dating we obtained continuous exposure histories for 7-12 m-high limestone surfaces at two sites (10 km apart) on the Sparta normal fault scarp. As each major earthquake adds new surface to the cumulative scarp exposing new material to cosmic-ray bombardment these exposure histories allow the slip history to be constrained. The results show that an earthquake occurred on this fault 2800 ± 300 yr ago. We infer that this is the seismic event that destroyed ancient Sparta in 464 B.C. Four earlier earthquakes ruptured the Sparta fault in the last 13 ka with similar slip amplitudes of about 2 m and with time intervals ranging from 500 yr to 4500 yr. The observations also confirm that the Sparta scarp is post-glacial, supporting the hypothesis that similar scarps elsewhere in the Mediterranean region have a comparable age. The absence of any event since 464 B.C. could suggest a future event is imminent. However, the irregularity of earthquake time intervals could also be due to changes of loading with important consequences for the mechanics of continental deformation.
ABSTRACT Direct cosmogenic dating of exhumed earthquake scarps provide a new way to determine the... more ABSTRACT Direct cosmogenic dating of exhumed earthquake scarps provide a new way to determine the slip-rate and the earthquake history of a fault. Here, we have applied in situ 36Cl cosmic ray exposure dating to a limestone fault scarp produced by the Kaparelli fault. The Kaparelli Fault is located at the eastern extremity of the Gulf of Corinth. In 1981, a sequence of three earthquakes with magnitude greater than 6 struck this region (Jackson et al. E.P.S.L. 57, 1982, King et al. G.J.Int, 80, 1985). The Kaparelli fault was responsible for the third event (March, 4, 1981, Ms ~6.4) with an associated rupture of 12 km and a maximum vertical displacement of 70 cm. For about 5 km, the surface break followed a 3-4 m high limestone fault scarp along which the 1981 event exhumed 40 cm of new scarp. On the best preserved section of this scarp we sampled a continuous 3.5 m vertical profile, with a 10 cm resolution. 36Cl is produced at shallow depths in calcite predominantly by spallation of calcium. This production decreases exponentially with depth beneath the surface but varies little with height once a section of the scarp has been tectonically exhumed. The concentration of 36Cl and of stable chlorine in the 35 samples has been measured by accelerator mass spectrometry (AMS) at the LLNL Center for Accelerator Mass Spectrometry. The 36Cl concentration increases at an approximately constant rate with the height of the scarp, suggesting that the slip-rate of the fault is constant with time. The 36Cl concentrations indicate that the scarp is probably more than ten thousand years old. Moreover, the 36Cl pattern suggests that this scarp might have formed as a result of 6 events. Further analysis of our data and refinement of 36Cl production rates will allow us to further test these hypotheses. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48.
ABSTRACT We have measured the sound velocity of Ice VII by Brillouin spectroscopy using membrane-... more ABSTRACT We have measured the sound velocity of Ice VII by Brillouin spectroscopy using membrane-style diamond anvil cell with an external Mo-wire resistance heater at elevated temperatures to 873 K at pressures of 2 GPa to 19 GPa. The unit cells of Ice VII and Au were determined by synchrotron X-ray diffraction, using Au as an in situ pressure gauge. All our samples contained were contained within and chemically insulated from the Re-gasket hole by a gold liner. We determined the melting of Ice VII by monitoring the sound velocity drop and the disappearance of diffraction pattern of Ice VII upon melting. Below ~8 GPa, our results are consistent with the previously determined melting curves (Datchi et al. 2000; Frank et al. 2004; Lin et al. 2004; Goncharov et al. 2005). However, our determination of the melting temperature at 12.5 GPa results in a discrepancy of at least 130 K. Given the care taken in the present experiments to avoid potential contamination of the water sample due to reactions at high temperatures between the sample and the gaskets and/or pressure gauges, our new measurements likely provide the first measurements on pure water which displaies an extended stability field of the solid phase. Thus, our new measurements suggest that the melting curve of H2O at high pressure needs to be reevaluated.
ABSTRACT The laser-heated diamond anvil cell (LHDAC) is a valuable quantitative tool for the eart... more ABSTRACT The laser-heated diamond anvil cell (LHDAC) is a valuable quantitative tool for the earth and planetary sciences because it can reproduce the thermodynamic conditions in planetary interiors, making possible measurements of material properties for those portions of planets that are not directly observable. The utility of the LHDAC is diminished, however, when systematic imaging-system-related errors lead to difficulty in comparing the results of different experiments and different labs. We present calculations detailing systematic errors in spectroradiometric temperature measurements due to the diamond anvil itself. Wavelength-dependent index of refraction and absorption can lead to errors of several hundred K. In addition, we assess methods for interpreting data already in the literature with respect to these sources of error.
Cerium is a rare-earth metal with many of its physical and chemical properties governed by the co... more Cerium is a rare-earth metal with many of its physical and chemical properties governed by the complex behavior of its 4f electron in contributing to the bonding of the crystal structure. Specifically, its gamma-alpha volume-collapse transition under pressure is the only known solid-solid transition in an element with the phase boundary ending at a critical point, and the detailed mechanism
ABSTRACT Recent activity of normal faults in Greece has produced steep limestone fault scarps at ... more ABSTRACT Recent activity of normal faults in Greece has produced steep limestone fault scarps at the base of the mountain fronts. For example, on the Sparta fault located in the Peloponnese, and responsible for the 464 B.C. M˜7 earthquake, a continuous fresh scarp cuts limestone bedrock and indurated conglomerates. The scarp is nearly continuous dipping at 65-68o with well-preserved slickensides. The maximum height of the scarp is 10-12 metres, progressively decreasing towards the ends. The few local variations are associated with active streams where hangingwall erosion causes the scarp to be locally higher. The regularity of the scarp is powerful evidence that the footwall and hangingwall surfaces were originally continuous and the scarp surface represents fault slip alone. It also suggests that there was no significant erosion or deposition on the hanging-wall (except near active gullies) since the scarp began to form. Such observations have led to the suggestion that numerous well-preserved limestone escarpments around the eastern Mediterranean, similar to those in Sparta, are post-glacial in age. Using 36Cl cosmogenic dating we tested the foregoing ideas. Limestones (largely calcite) contain an abundance of calcium, which is a major target element for cosmogenic 36Cl production. Samples were collected from the limestone scarp surface to recover the continuous exposure history of the scarp and also from the footwall and hanging wall surfaces as well as a depth profile in the hanging wall wedge. The concentration of 36Cl and of stable chlorine has been measured by accelerator mass spectrometry (AMS) at the LLNL-CAMS for each of the samples (about 150). The result confirm: 1- that the fault scarp was formed between about 2000 B.P. and 13 ka B.P. as the result of 5 earthquakes (including the known event in 464 B.C.) with similar slip amplitudes of about 2m and with time intervals ranging from 500yr to 4500yr (Benedetti et al., GRL, 2002), 2- that both hanging wall and footwall surfaces have exposure ages of 40-50 ka, and 3- that the colluvial wedge was formed during three episodes of deposition that occurred between 50 to 70 ka B.P. It is not clear whether those episodes, that are also stratigraphically recognizable, are seismically or climatically related. The climatic hypothesis is that during the cold and dry climate of the last glaciation the slopes were unstable so that the free-faces formed during repeated earthquakes were immediately buried. At the end of the glaciation, as a consequence of the wet and warm climate, slopes stabilized and earthquake slip began to accumulate and to form the present day cumulative escarpements.
The amount and timing of river incision along the western margin of the Altiplano can provide ins... more The amount and timing of river incision along the western margin of the Altiplano can provide insight into the climatic, volcanic, and tectonic history of this region. While many studies have focused on the Tertiary geologic history of the western margin of the Altiplano, the Quaternary geologic history of this margin remains largely unstudied. The Pacific draining Rio Tambo river
ABSTRACT Measurements of the chemistry and physics of the pure endmembers of the C-O-H-N-S system... more ABSTRACT Measurements of the chemistry and physics of the pure endmembers of the C-O-H-N-S system are important building blocks in the study of planetary ices. To that end, we performed x-ray diffraction measurements on pure oxygen using the laser-heated diamond anvil cell. Using a novel experimental setup and a combined interpretation of the diffraction patterns in conjunction with spectroradiometric observations, the temperature and pressure stability field of the ε -phase has been studied. We also present measurements of the melting temperature of oxygen at 47 and 55 GPa, pressures more than twice as high as previously achieved. The ensemble of our experimental evidence highlights the remarkable thermodynamic stability of the O8 cluster structure with respect to lower pressure molecular arrangements and points to non-magnetic ordering as one of the main reasons for that.
The phase transformation of P-HMX (> 0.5% RDX) to 5 phase has been studied for over twenty years ... more The phase transformation of P-HMX (> 0.5% RDX) to 5 phase has been studied for over twenty years and more recently with an high-contrast optical second harmonic generation technique. Shock studies of the plastic binder composites of HMX have indicated that the transition is perhaps irreversible, a result that concurs with the static pressure results published by F. Goetz et al. [l] in 1978. However, the stability field favors the p polymorph over 6 as pressure is increased (up to 5.4 GPa) along any thermodynamically reasonable isotherm. In this experiment, strict control of pressure and temperature is maintained while x-ray and optical diagnostics are applied to monitor the conformational dynamics of HMX. Unlike the temperature induced p->6 transition, the pressure induced is heterogeneous in nature. The 1 bar 25°C 6->p transition is not immediate, occuring over tens of hours. Transition points and kinetics are path dependent and consequently this paper describes our work in progress.
While the canonical view is that active deformation in the Andes is mostly confined to the easter... more While the canonical view is that active deformation in the Andes is mostly confined to the eastern slopes in the subandean fold and thrust belt, recent studies have highlighted the existence of active deformation in the central Andean forearc (Audin et al, 2003, Hartley et al., 2000, Husson and Sempere, 2003, Farias et al., 2005). The forearc of southern Peru,
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Papers by Daniel Farber