Quentin Williams - Academia.edu (original) (raw)

Papers by Quentin Williams

Earth and Planetary Science Letters, 1998

Uranium series disequilibria have been used to study the physical melting regime along a section ... more Uranium series disequilibria have been used to study the physical melting regime along a section of the southern Mid-Atlantic Ridge where the largest mantle Bouguer anomaly yet observed on ridges exists. Zero age excesses for ( 230 Th)=( 238 U) and ( 231 Pa)=( 235 U) are age constrained by the presence of measured ( 226 Ra)=( 230 Th) disequilibria. By this criterion, 7 out of 15 samples are younger than 8000 years. Zero age ( 230 Th)=( 238 U) range between 1.06 and 1.25 while zero age ( 231 Pa)=( 235 U) range between 1.77 and 3.57. Concentrations of Th range from 100 to 866 ppb and strongly correlate with K=Ti. The U-series data differentiate the segment with the large gravity anomaly from the surrounding segments. Specifically, this segment shows combined 231 Pa and 230 Th systematics unlike any observed in MORB thus far, having exceptionally low zero-age 231 Pa excesses. The U-series data can be modeled using upwelling rates similar to the half-spreading rate of this area of ridge. The disequilibria systematics for the anomalous segment 4 possibly suggest a solid upwelling rate that is a factor of two higher than the adjacent segments. The similarity between melting at segment 4 and at ocean islands is noted. magmatic phenomenon operates. However, a unified understanding derived from geophysics, geochemistry and seismology has been uncommon. One of the major unanswered geophysical questions about ridges is what causes the 'bull's eye' pattern of the mantle Bouguer anomaly (MBA) observed at several ridge segments on slow spreading ridges . Specifically, does this pattern simply relate to variation in crustal structure or does it identify an area of active mantle upwelling ? The answer will influence the basic model for upwelling and melting, at least beneath slow spreading ridges. However, probes

The American Mineralogist, 1992

... Cation field effects on orthosilicate glass vibrations. Quentin Williams, and Thomas F. Coone... more ... Cation field effects on orthosilicate glass vibrations. Quentin Williams, and Thomas F. Cooney Univ. Calif. at Santa Cruz, Inst. Tectonics, Santa Cruz, CA, United States Univ. Hawaii at Manoa, United States. This record provided courtesy of AGI/GeoRef. ...

Phys Rev B, 1996

The indirect energy gaps of NaCl-structured CdS and CdSe are measured using absorption spectrosco... more The indirect energy gaps of NaCl-structured CdS and CdSe are measured using absorption spectroscopy to approximately 55 and 42 GPa, respectively. The gaps shift linearly with pressure and their pressure dependence generally is in good agreement with previous measurements to 16 GPa. Also, band calculations using pseudopotentials and density-functional theory are conducted on CdS as a function of pressure. The shift in the energy gap of CdS with pressure (dEgap/dP) is experimentally determined to be -5.7(6)×10-3 eV/GPa, in good agreement with the theoretical calculation of -5.6×10-3 eV/GPa. The pressure dependence of the gap of CdSe is measured to be -4.0(2)×10-3 eV/GPa, while previous theoretical work yielded -6.0×10-3 eV/GPa. Relative to a previous experimental result on CdSe, our measured shift is in considerably closer agreement with theory. The energy gap in both materials is indirect throughout the pressure range of these measurements. There is also no compression-induced change in the location in k space of the valence-band maxima for either the experimental or theoretical results. In contrast to previous inferences for CdS, the gap appears to remain from the L point to the X point up to our maximum pressure. Based on a simple linear extrapolation of our data, we estimate the metallization pressures of NaCl-structured CdS and CdSe to be above 250 and 150 GPa, respectively. Because phase transitions occur in both materials below these pressures, it is unlikely that either of the NaCl-structured phases will undergo metallization.

Geophysical Monograph, 2005

Journal of Geophysical Research, 1993

American Mineralogist, Dec 1, 1992

... Vibrational spectra of magnesite (MgCO 3 ) and calcite-III at high pressures. Quentin William... more ... Vibrational spectra of magnesite (MgCO 3 ) and calcite-III at high pressures. Quentin Williams, Benjamin Collerson, and Elise Knittle University of California at Santa Cruz, Earth Sciences Board, Santa Cruz, CA, United States. This record provided courtesy of AGI/GeoRef. ...

The vibrational properties of a natural sample of chondrodite, a humite-structured phase with an ... more The vibrational properties of a natural sample of chondrodite, a humite-structured phase with an ideal composition of 2Mg2SiO4Mg(OH)2, are examined to pressures in excess of 40 GPa. Chondrodite samples are characterized both under static compression at 300 K, and following laser heating to peak temperatures in excess of 1500 K. Additionally, experiments on reactions of water with pyroxene compositions at pressures between 30 and 40 GPa also document the presence and vibrational properties of hydroxyl units in the hydrous phase D of Liu (1987). In the case of both chondrodite and the pyroxene composition, all high pressure phases examined have hydroxyl stretching frequencies of less than 3400 cm-1, implying that these phases have a larger degree of hydrogen bonding than lower pressure hydroxylated phases in the MgO-SiO2-H2O system. Two of the three hydroxyl stretching vibrations of chondrodite observed at high pressure shift to higher frequency with compression. Such anomalous positive shifts of these hydroxyl stretching vibrations are attributable to the role of inter-hydrogen repulsion within the chondrodite structure: this repulsion is likely to play a primary role in the destabilization of this structure at high pressures. In the chondrodite samples, no evidence is seen of dehydration at high pressures and temperatures; new phases observed appear to be hydroxyl-bearing crystals. This observation, when coupled with the synthesis of the hydroxyl-bearing phase D in the pyroxene system indicates that hydroxyl-bearing crystalline phases exist at lower mantle pressures in the MgO-SiO2-H2O system, and that the lower mantle could plausibly represent a major geochemical reservoir of hydrogen.

Geochimica Et Cosmochimica Acta, Oct 1, 2010

A means for estimating pressures in natural samples based on both the coupled substitution (Na+) ... more A means for estimating pressures in natural samples based on both the coupled substitution (Na+) [1+] (Ti + [VI]Si) [4+] = (M) [2+] (Al + Cr) [3+], and the classic pyroxene-stoichiometry majorite-substitution into garnet at high-pressure, is derived for garnets with majoritic chemistry. The technique is based on a compilation of experimental data for different bulk compositions. It is compositionally and thermally robust and can be used to estimate pressures experienced by natural materials during formation of majoritic garnet. In addition, it can be used either retrospectively, or in new experimental studies to establish the pressures of crystallization of reaction products, and determine if disequilibrium is recorded by the chemistries of majoritic garnets. Pressures are calculated based on majoritic chemistries in chondritic meteorites and diamond inclusions. Majoritic garnets associated with Mg perovskite in shocked L chondrites ( n = 4) yield uniform pressures of 23.8 ± 0.2 GPa that are slightly higher than pressures recorded by majoritic garnet in shock-derived melt veins in L chondrites (22.4 ± 0.6 GPa; n = 5). Similar pressures are also exhibited by shock-derived majoritic garnets in H chondrites (22.2 ± 1.1 GPa; n = 3). Diamond inclusions with eclogitic and peridotitic majoritic garnet chemistries exhibit mean pressures of 10.7 ± 2.7 GPa ( n = 30) and 8.3 ± 1.6 GPa ( n = 15) respectively, consistent with a sub-lithospheric origin. However, pressures defined by majoritic diamond inclusions from Jagersfontein (22.3 ± 0.8 GPa and 16.9 ± 1 GPa), Monastery (15.7 ± 7 GPa) and Kankan (15.5 ± 0.2 GPa) show that these inclusions originated from the mantle transition zone. Thus, this new single-phase method for pressure estimation has unmatched potential to map the depth of formation of garnets with majoritic chemistries that occur as diamond inclusions in all parageneses except those that include Ca silicate perovskite. The derived pressures confirm the sub-lithospheric origin of eclogitic majoritic diamond inclusions, and thus provide a more comprehensive picture of the important role of storage of oceanic lithosphere in the transition zone.

In recent years a broad range of seismic discoveries have painted a picture of Earth's core-mantl... more In recent years a broad range of seismic discoveries have painted a picture of Earth's core-mantle boundary (CMB) that is far more complex than a simple one-dimensional boundary between the molten outer core and the solid silicate mantle. Over the last ten years several anomalous regions of the lowermost mantle have been detected using a multitude of seismic phases and approaches. These regions are characterized by reductions of \textit{P}-wave and \textit{S}-wave velocities by at least 10%. These ultra-low velocity zones (ULVZ) are characterized as having strong variability at short scale length (e.g. ,≤ 100 km) and do not appear to be a global layer. Using 300 deep focus Tonga-Fiji subduction zone earthquakes recorded at the Warramunga array in central Australia, we detect an extremely localized low-velocity feature at the CMB that lies beneath low shear wave velocities in D'' south of New-Caledonia. An array analysis of \textit{ScP} reveals anomalous precursors that are most robustly explained by a dense ULVZ with the following characteristics: 8.5~km thick, ˜50~km wide, \textit{P}- and \textit{S}-wave reductions of 10 and 25%, respectively, and a density increase of 10% (±5%). These parameters are best explained by the presence of dense partial melt in the ULVZ. A model to keep the dense lens of partially molten material from spreading out along the CMB includes the entrapment of melt by intercumulus crystal growth after drainage from the anomalously hot overlying mantle. Therefore, this region may be closely related to thermal instabilities at the thermal boundary layer of the CMB that will influence the stability, genesis and persistence of mantle plumes.

CaCO 3 -aragonite has been examined to pressures of 50 GPa using angle-dispersive synchrotron X-r... more CaCO 3 -aragonite has been examined to pressures of 50 GPa using angle-dispersive synchrotron X-ray diffraction at 300 K. A progressive pressure-induced change takes place in the diffraction patterns, with the (021) and (111) diffraction lines of aragonite converging toward one ...

This chapter contains sections titled: Introduction Experimental Results Conclusions Introduction... more This chapter contains sections titled: Introduction Experimental Results Conclusions IntroductionExperimentalResultsConclusions

Journal of physics. Condensed matter : an Institute of Physics journal, Jan 3, 2016

Single-crystal B4.3C boron carbide is investigated through the pressure-dependence and inter-rela... more Single-crystal B4.3C boron carbide is investigated through the pressure-dependence and inter-relation of atomic distances, optical properties and Raman-active phonons up to ~70 GPa. The anomalous pressure evolution of the gap width to higher energies is striking. This is obtained from observations of transparency, which most rapidly increases around 55 GPa. Full visible optical transparency is approached at pressures of >60 GPa indicating that the band gap reaches ~3.5 eV; at high pressure, boron carbide is a wide-gap semiconductor. The reason is that the high concentration of structural defects controlling the electronic properties of boron carbide at ambient conditions initially decreases and finally vanishes at high pressures. The structural parameters and Raman-active phonons indicate a pressure-dependent phase transition in single-crystal (nat)B4.3C boron carbide near 40 GPa, likely related to structural changes in the C-B-C chains, while the basic icosahedral structure app...

Science, Jul 24, 1998

The statistical correlation between the locations of hot spots at the surface of Earth and the di... more The statistical correlation between the locations of hot spots at the surface of Earth and the distribution of ultra-low-velocity zones at the base of the mantle has about a 1 percent chance of arising randomly. This correlation is more significant than that between hot spots and negative velocity anomalies in tomographic models of deep mantle compressional and shear velocity. This correlation is consistent with the notion that many hot spots originate in a low-velocity, probably partially molten layer at the core-mantle boundary and undergo little lateral deflection on ascent.

Journal of Geophysical Research, 2000

Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in single-crystal olivine (α-(Mg,N... more Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in single-crystal olivine (α-(Mg,Ni,Fe)2SiO4), polycrystalline β phase (β-(Mg,Ni,Fe)2SiO4), and silicate spinel (γ-(Mg,Ni,Fe)2SiO4) at 1473 K between 1 and 14 GPa under controlled thermodynamic conditions. In olivine, DMg-Niα ranges from 10-17 m2 s-1 at 1 GPa to 4×10-18 m2 s-1 at 9 GPa, and DMg-Feα ranges from 3×10-15 m2 s-1 at 1 GPa to 10-15 m2 s-1 at 4 GPa. At 9 GPa the cation diffusion rates in γ spinel are ˜3 orders of magnitude higher than those in olivine; between 9 and 14 GPa, DMg-Niγ decreases from 8×10-15 to 9×10-16 m2 s-1, and DMg-Feγ decreases from 2×10-13 to 5×10-14 m2 s-1. Between 10 and 14 GPa the diffusion coefficients for both Fe and Ni are similar in magnitude and pressure dependence between β phase and γ spinel. The activation volumes for Mg-Ni diffusion are 3.2 and 6.7 cm3 mol-1 in the α and γ(β) phases, respectively, and for Mg-Fe diffusion are 5.4 and 6.1 cm3 mol-1 in the α and γ(β) phases, respectively. Our results demonstrate that extreme differences in transport properties are expected to occur across the 400 km discontinuity. A model for electrical conductivity in the upper mantle involving two mechanisms for electrical conduction, with one mechanism being associated with a hydrogen or defect-related conduction process and the other involving a straightforward application of the Nernst-Einstein equation to our cationic diffusion data, reproduces geomagnetic sounding results for the upper mantle and shallow transition zone. This modeling yields an order of magnitude increase in electrical conductivity across the 400 km discontinuity, with little or no change in conductivity occurring near 520 km depth. Our modeling suggests that divalent cation diffusion is the dominant mechanism of charge transport between 400 and 670 km depth and that the high-pressure olivine polymorphs dominate the electrical behavior of the deep upper mantle. Comparison between field-based deep-mantle conductivity profiles derived from magnetotelluric studies and extrapolation of our model suggests that a discontinuity in electrical transport properties exists at the base of the transition zone as well. Similarly, given the dramatic differences in chemical diffusion between different polymorphs of olivine, it is possible that other transport property-mediated processes, such as viscous flow, could dramatically change across the 400 and 670 km discontinuities; such changes are less likely to occur at 520 km depth.

Nature, 1998

... Nature 392, 461-468 (2 April 1998) | doi :10.1038/33083. The core–mantle boundary layer and d... more ... Nature 392, 461-468 (2 April 1998) | doi :10.1038/33083. The core–mantle boundary layer and deep Earth dynamics. ... Top of page Core–mantle chemical reactions. ...

American Mineralogist, 1996

Raman spectra of Na2Si20s glass and Na2Si20s liquid have been collected in situ to pressures of 1... more Raman spectra of Na2Si20s glass and Na2Si20s liquid have been collected in situ to pressures of 16 GPa at 300 K and 10 GPa at 885 K. In glass compressed at 300 K, bands associated with intertetrahedral bridging 0 atoms decrease in intensity and ultimately become unresolvable above 8 GPa. Over this same pressure range, new peaks appear in the glass between 600 and 800 cm-I; these new features are consistent with the formation of SiOs or Si06 polyhedra through the destruction of nonbridging 0 atoms. Between -10 and 16 GPa, band intensities shift, the total integrated Raman scattering intensity of the glass decreases by more than a factor of ten, and the pressure dependence of mode shifts changes. These changes are consistent with a new densification mechanism initiated in the glass above -8 GPa, probably from the formation of linkages between two or more highly coordinated Si atoms. No vibrations attributed to bridged silica tetrahedra are observed in the liquid above 8 GPa, indicating that at least 50% of the Si in the liquid at 8 GPa is highly coordinated. Upon decompression of thermally quenched samples at 300 K, bands associated with bridged tetrahedra reappear at pressures below 1 GPa. Similarly, glasses that have been compressed only at 300 K and then decompressed have bands associated with bridged tetrahedra. The formation of bands associated with bridged tetrahedra on decompression both in glasses quenched from liquids formed at high-pressure and in glasses compressed at 300 K demonstrates that these species arise from structural reorganizations during decompression. Such reorganizations plausibly involve the breakdown of highly coordinated Si polyhedra and the resultant formation of bridged tetrahedra. The nearly complete decomposition ofthe high-pressure structure in glasses on decompression documents that in situ high-pressure measurements are crucial in deriving accurate constraints on the structure of silicate liquids at high pressure.

Journal of Geophysical Research, 2000

Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in singlecrystal olivine (a-(Mg,Ni... more Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in singlecrystal olivine (a-(Mg,Ni,Fe)2SiO4), polycrystalline/3 phase (/3-(Mg,Ni,Fe)2SiO4), and silicate spinel (•/-(Mg,Ni,Fe)2SiO4) at 1473 K between 1 and 14 GPa under controlled thermodynamic conditions. In olivine, D•g Ni ranges from 10 -17 m 2 s -1 at 1 GPa to 4 x 10 -•8 m 2 s -1 at 9 GPa, and D•g_Fe ra'•ges from 3 x 10 -is m 2 s -• at 1 GPa to 10 -•s m 2 s -• at 4 GPa. At 9 GPa the cation diffusion rates in 3/spinel are -3 orders of magnitude higher than those in olivine; between 9 and 14 GPa, DMg_Ni decreases from 8 x 10 -•s to 9 x 10 -16 m 2 s -1, and D•4g_Fe decreases from 2 x 10 -•3 to 5 X 10 -•4 m 2 s -•. Between 10 and 14 GPa the diffusion coefficients for both Fe and Ni are similar in magnitude and pressure dependence between/3 phase and 3/spinel. The activation volumes for Mg-Ni diffusion are 3.2

Earth and Planetary Science Letters, 1998

Uranium series disequilibria have been used to study the physical melting regime along a section ... more Uranium series disequilibria have been used to study the physical melting regime along a section of the southern Mid-Atlantic Ridge where the largest mantle Bouguer anomaly yet observed on ridges exists. Zero age excesses for ( 230 Th)=( 238 U) and ( 231 Pa)=( 235 U) are age constrained by the presence of measured ( 226 Ra)=( 230 Th) disequilibria. By this criterion, 7 out of 15 samples are younger than 8000 years. Zero age ( 230 Th)=( 238 U) range between 1.06 and 1.25 while zero age ( 231 Pa)=( 235 U) range between 1.77 and 3.57. Concentrations of Th range from 100 to 866 ppb and strongly correlate with K=Ti. The U-series data differentiate the segment with the large gravity anomaly from the surrounding segments. Specifically, this segment shows combined 231 Pa and 230 Th systematics unlike any observed in MORB thus far, having exceptionally low zero-age 231 Pa excesses. The U-series data can be modeled using upwelling rates similar to the half-spreading rate of this area of ridge. The disequilibria systematics for the anomalous segment 4 possibly suggest a solid upwelling rate that is a factor of two higher than the adjacent segments. The similarity between melting at segment 4 and at ocean islands is noted. magmatic phenomenon operates. However, a unified understanding derived from geophysics, geochemistry and seismology has been uncommon. One of the major unanswered geophysical questions about ridges is what causes the 'bull's eye' pattern of the mantle Bouguer anomaly (MBA) observed at several ridge segments on slow spreading ridges . Specifically, does this pattern simply relate to variation in crustal structure or does it identify an area of active mantle upwelling ? The answer will influence the basic model for upwelling and melting, at least beneath slow spreading ridges. However, probes

The American Mineralogist, 1992

... Cation field effects on orthosilicate glass vibrations. Quentin Williams, and Thomas F. Coone... more ... Cation field effects on orthosilicate glass vibrations. Quentin Williams, and Thomas F. Cooney Univ. Calif. at Santa Cruz, Inst. Tectonics, Santa Cruz, CA, United States Univ. Hawaii at Manoa, United States. This record provided courtesy of AGI/GeoRef. ...

Phys Rev B, 1996

The indirect energy gaps of NaCl-structured CdS and CdSe are measured using absorption spectrosco... more The indirect energy gaps of NaCl-structured CdS and CdSe are measured using absorption spectroscopy to approximately 55 and 42 GPa, respectively. The gaps shift linearly with pressure and their pressure dependence generally is in good agreement with previous measurements to 16 GPa. Also, band calculations using pseudopotentials and density-functional theory are conducted on CdS as a function of pressure. The shift in the energy gap of CdS with pressure (dEgap/dP) is experimentally determined to be -5.7(6)×10-3 eV/GPa, in good agreement with the theoretical calculation of -5.6×10-3 eV/GPa. The pressure dependence of the gap of CdSe is measured to be -4.0(2)×10-3 eV/GPa, while previous theoretical work yielded -6.0×10-3 eV/GPa. Relative to a previous experimental result on CdSe, our measured shift is in considerably closer agreement with theory. The energy gap in both materials is indirect throughout the pressure range of these measurements. There is also no compression-induced change in the location in k space of the valence-band maxima for either the experimental or theoretical results. In contrast to previous inferences for CdS, the gap appears to remain from the L point to the X point up to our maximum pressure. Based on a simple linear extrapolation of our data, we estimate the metallization pressures of NaCl-structured CdS and CdSe to be above 250 and 150 GPa, respectively. Because phase transitions occur in both materials below these pressures, it is unlikely that either of the NaCl-structured phases will undergo metallization.

Geophysical Monograph, 2005

Journal of Geophysical Research, 1993

American Mineralogist, Dec 1, 1992

... Vibrational spectra of magnesite (MgCO 3 ) and calcite-III at high pressures. Quentin William... more ... Vibrational spectra of magnesite (MgCO 3 ) and calcite-III at high pressures. Quentin Williams, Benjamin Collerson, and Elise Knittle University of California at Santa Cruz, Earth Sciences Board, Santa Cruz, CA, United States. This record provided courtesy of AGI/GeoRef. ...

The vibrational properties of a natural sample of chondrodite, a humite-structured phase with an ... more The vibrational properties of a natural sample of chondrodite, a humite-structured phase with an ideal composition of 2Mg2SiO4Mg(OH)2, are examined to pressures in excess of 40 GPa. Chondrodite samples are characterized both under static compression at 300 K, and following laser heating to peak temperatures in excess of 1500 K. Additionally, experiments on reactions of water with pyroxene compositions at pressures between 30 and 40 GPa also document the presence and vibrational properties of hydroxyl units in the hydrous phase D of Liu (1987). In the case of both chondrodite and the pyroxene composition, all high pressure phases examined have hydroxyl stretching frequencies of less than 3400 cm-1, implying that these phases have a larger degree of hydrogen bonding than lower pressure hydroxylated phases in the MgO-SiO2-H2O system. Two of the three hydroxyl stretching vibrations of chondrodite observed at high pressure shift to higher frequency with compression. Such anomalous positive shifts of these hydroxyl stretching vibrations are attributable to the role of inter-hydrogen repulsion within the chondrodite structure: this repulsion is likely to play a primary role in the destabilization of this structure at high pressures. In the chondrodite samples, no evidence is seen of dehydration at high pressures and temperatures; new phases observed appear to be hydroxyl-bearing crystals. This observation, when coupled with the synthesis of the hydroxyl-bearing phase D in the pyroxene system indicates that hydroxyl-bearing crystalline phases exist at lower mantle pressures in the MgO-SiO2-H2O system, and that the lower mantle could plausibly represent a major geochemical reservoir of hydrogen.

Geochimica Et Cosmochimica Acta, Oct 1, 2010

A means for estimating pressures in natural samples based on both the coupled substitution (Na+) ... more A means for estimating pressures in natural samples based on both the coupled substitution (Na+) [1+] (Ti + [VI]Si) [4+] = (M) [2+] (Al + Cr) [3+], and the classic pyroxene-stoichiometry majorite-substitution into garnet at high-pressure, is derived for garnets with majoritic chemistry. The technique is based on a compilation of experimental data for different bulk compositions. It is compositionally and thermally robust and can be used to estimate pressures experienced by natural materials during formation of majoritic garnet. In addition, it can be used either retrospectively, or in new experimental studies to establish the pressures of crystallization of reaction products, and determine if disequilibrium is recorded by the chemistries of majoritic garnets. Pressures are calculated based on majoritic chemistries in chondritic meteorites and diamond inclusions. Majoritic garnets associated with Mg perovskite in shocked L chondrites ( n = 4) yield uniform pressures of 23.8 ± 0.2 GPa that are slightly higher than pressures recorded by majoritic garnet in shock-derived melt veins in L chondrites (22.4 ± 0.6 GPa; n = 5). Similar pressures are also exhibited by shock-derived majoritic garnets in H chondrites (22.2 ± 1.1 GPa; n = 3). Diamond inclusions with eclogitic and peridotitic majoritic garnet chemistries exhibit mean pressures of 10.7 ± 2.7 GPa ( n = 30) and 8.3 ± 1.6 GPa ( n = 15) respectively, consistent with a sub-lithospheric origin. However, pressures defined by majoritic diamond inclusions from Jagersfontein (22.3 ± 0.8 GPa and 16.9 ± 1 GPa), Monastery (15.7 ± 7 GPa) and Kankan (15.5 ± 0.2 GPa) show that these inclusions originated from the mantle transition zone. Thus, this new single-phase method for pressure estimation has unmatched potential to map the depth of formation of garnets with majoritic chemistries that occur as diamond inclusions in all parageneses except those that include Ca silicate perovskite. The derived pressures confirm the sub-lithospheric origin of eclogitic majoritic diamond inclusions, and thus provide a more comprehensive picture of the important role of storage of oceanic lithosphere in the transition zone.

In recent years a broad range of seismic discoveries have painted a picture of Earth's core-mantl... more In recent years a broad range of seismic discoveries have painted a picture of Earth's core-mantle boundary (CMB) that is far more complex than a simple one-dimensional boundary between the molten outer core and the solid silicate mantle. Over the last ten years several anomalous regions of the lowermost mantle have been detected using a multitude of seismic phases and approaches. These regions are characterized by reductions of \textit{P}-wave and \textit{S}-wave velocities by at least 10%. These ultra-low velocity zones (ULVZ) are characterized as having strong variability at short scale length (e.g. ,≤ 100 km) and do not appear to be a global layer. Using 300 deep focus Tonga-Fiji subduction zone earthquakes recorded at the Warramunga array in central Australia, we detect an extremely localized low-velocity feature at the CMB that lies beneath low shear wave velocities in D'' south of New-Caledonia. An array analysis of \textit{ScP} reveals anomalous precursors that are most robustly explained by a dense ULVZ with the following characteristics: 8.5~km thick, ˜50~km wide, \textit{P}- and \textit{S}-wave reductions of 10 and 25%, respectively, and a density increase of 10% (±5%). These parameters are best explained by the presence of dense partial melt in the ULVZ. A model to keep the dense lens of partially molten material from spreading out along the CMB includes the entrapment of melt by intercumulus crystal growth after drainage from the anomalously hot overlying mantle. Therefore, this region may be closely related to thermal instabilities at the thermal boundary layer of the CMB that will influence the stability, genesis and persistence of mantle plumes.

CaCO 3 -aragonite has been examined to pressures of 50 GPa using angle-dispersive synchrotron X-r... more CaCO 3 -aragonite has been examined to pressures of 50 GPa using angle-dispersive synchrotron X-ray diffraction at 300 K. A progressive pressure-induced change takes place in the diffraction patterns, with the (021) and (111) diffraction lines of aragonite converging toward one ...

This chapter contains sections titled: Introduction Experimental Results Conclusions Introduction... more This chapter contains sections titled: Introduction Experimental Results Conclusions IntroductionExperimentalResultsConclusions

Journal of physics. Condensed matter : an Institute of Physics journal, Jan 3, 2016

Single-crystal B4.3C boron carbide is investigated through the pressure-dependence and inter-rela... more Single-crystal B4.3C boron carbide is investigated through the pressure-dependence and inter-relation of atomic distances, optical properties and Raman-active phonons up to ~70 GPa. The anomalous pressure evolution of the gap width to higher energies is striking. This is obtained from observations of transparency, which most rapidly increases around 55 GPa. Full visible optical transparency is approached at pressures of >60 GPa indicating that the band gap reaches ~3.5 eV; at high pressure, boron carbide is a wide-gap semiconductor. The reason is that the high concentration of structural defects controlling the electronic properties of boron carbide at ambient conditions initially decreases and finally vanishes at high pressures. The structural parameters and Raman-active phonons indicate a pressure-dependent phase transition in single-crystal (nat)B4.3C boron carbide near 40 GPa, likely related to structural changes in the C-B-C chains, while the basic icosahedral structure app...

Science, Jul 24, 1998

The statistical correlation between the locations of hot spots at the surface of Earth and the di... more The statistical correlation between the locations of hot spots at the surface of Earth and the distribution of ultra-low-velocity zones at the base of the mantle has about a 1 percent chance of arising randomly. This correlation is more significant than that between hot spots and negative velocity anomalies in tomographic models of deep mantle compressional and shear velocity. This correlation is consistent with the notion that many hot spots originate in a low-velocity, probably partially molten layer at the core-mantle boundary and undergo little lateral deflection on ascent.

Journal of Geophysical Research, 2000

Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in single-crystal olivine (α-(Mg,N... more Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in single-crystal olivine (α-(Mg,Ni,Fe)2SiO4), polycrystalline β phase (β-(Mg,Ni,Fe)2SiO4), and silicate spinel (γ-(Mg,Ni,Fe)2SiO4) at 1473 K between 1 and 14 GPa under controlled thermodynamic conditions. In olivine, DMg-Niα ranges from 10-17 m2 s-1 at 1 GPa to 4×10-18 m2 s-1 at 9 GPa, and DMg-Feα ranges from 3×10-15 m2 s-1 at 1 GPa to 10-15 m2 s-1 at 4 GPa. At 9 GPa the cation diffusion rates in γ spinel are ˜3 orders of magnitude higher than those in olivine; between 9 and 14 GPa, DMg-Niγ decreases from 8×10-15 to 9×10-16 m2 s-1, and DMg-Feγ decreases from 2×10-13 to 5×10-14 m2 s-1. Between 10 and 14 GPa the diffusion coefficients for both Fe and Ni are similar in magnitude and pressure dependence between β phase and γ spinel. The activation volumes for Mg-Ni diffusion are 3.2 and 6.7 cm3 mol-1 in the α and γ(β) phases, respectively, and for Mg-Fe diffusion are 5.4 and 6.1 cm3 mol-1 in the α and γ(β) phases, respectively. Our results demonstrate that extreme differences in transport properties are expected to occur across the 400 km discontinuity. A model for electrical conductivity in the upper mantle involving two mechanisms for electrical conduction, with one mechanism being associated with a hydrogen or defect-related conduction process and the other involving a straightforward application of the Nernst-Einstein equation to our cationic diffusion data, reproduces geomagnetic sounding results for the upper mantle and shallow transition zone. This modeling yields an order of magnitude increase in electrical conductivity across the 400 km discontinuity, with little or no change in conductivity occurring near 520 km depth. Our modeling suggests that divalent cation diffusion is the dominant mechanism of charge transport between 400 and 670 km depth and that the high-pressure olivine polymorphs dominate the electrical behavior of the deep upper mantle. Comparison between field-based deep-mantle conductivity profiles derived from magnetotelluric studies and extrapolation of our model suggests that a discontinuity in electrical transport properties exists at the base of the transition zone as well. Similarly, given the dramatic differences in chemical diffusion between different polymorphs of olivine, it is possible that other transport property-mediated processes, such as viscous flow, could dramatically change across the 400 and 670 km discontinuities; such changes are less likely to occur at 520 km depth.

Nature, 1998

... Nature 392, 461-468 (2 April 1998) | doi :10.1038/33083. The core–mantle boundary layer and d... more ... Nature 392, 461-468 (2 April 1998) | doi :10.1038/33083. The core–mantle boundary layer and deep Earth dynamics. ... Top of page Core–mantle chemical reactions. ...

American Mineralogist, 1996

Raman spectra of Na2Si20s glass and Na2Si20s liquid have been collected in situ to pressures of 1... more Raman spectra of Na2Si20s glass and Na2Si20s liquid have been collected in situ to pressures of 16 GPa at 300 K and 10 GPa at 885 K. In glass compressed at 300 K, bands associated with intertetrahedral bridging 0 atoms decrease in intensity and ultimately become unresolvable above 8 GPa. Over this same pressure range, new peaks appear in the glass between 600 and 800 cm-I; these new features are consistent with the formation of SiOs or Si06 polyhedra through the destruction of nonbridging 0 atoms. Between -10 and 16 GPa, band intensities shift, the total integrated Raman scattering intensity of the glass decreases by more than a factor of ten, and the pressure dependence of mode shifts changes. These changes are consistent with a new densification mechanism initiated in the glass above -8 GPa, probably from the formation of linkages between two or more highly coordinated Si atoms. No vibrations attributed to bridged silica tetrahedra are observed in the liquid above 8 GPa, indicating that at least 50% of the Si in the liquid at 8 GPa is highly coordinated. Upon decompression of thermally quenched samples at 300 K, bands associated with bridged tetrahedra reappear at pressures below 1 GPa. Similarly, glasses that have been compressed only at 300 K and then decompressed have bands associated with bridged tetrahedra. The formation of bands associated with bridged tetrahedra on decompression both in glasses quenched from liquids formed at high-pressure and in glasses compressed at 300 K demonstrates that these species arise from structural reorganizations during decompression. Such reorganizations plausibly involve the breakdown of highly coordinated Si polyhedra and the resultant formation of bridged tetrahedra. The nearly complete decomposition ofthe high-pressure structure in glasses on decompression documents that in situ high-pressure measurements are crucial in deriving accurate constraints on the structure of silicate liquids at high pressure.

Journal of Geophysical Research, 2000

Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in singlecrystal olivine (a-(Mg,Ni... more Mg-Fe and Mg-Ni interdiffusion coefficients have been measured in singlecrystal olivine (a-(Mg,Ni,Fe)2SiO4), polycrystalline/3 phase (/3-(Mg,Ni,Fe)2SiO4), and silicate spinel (•/-(Mg,Ni,Fe)2SiO4) at 1473 K between 1 and 14 GPa under controlled thermodynamic conditions. In olivine, D•g Ni ranges from 10 -17 m 2 s -1 at 1 GPa to 4 x 10 -•8 m 2 s -1 at 9 GPa, and D•g_Fe ra'•ges from 3 x 10 -is m 2 s -• at 1 GPa to 10 -•s m 2 s -• at 4 GPa. At 9 GPa the cation diffusion rates in 3/spinel are -3 orders of magnitude higher than those in olivine; between 9 and 14 GPa, DMg_Ni decreases from 8 x 10 -•s to 9 x 10 -16 m 2 s -1, and D•4g_Fe decreases from 2 x 10 -•3 to 5 X 10 -•4 m 2 s -•. Between 10 and 14 GPa the diffusion coefficients for both Fe and Ni are similar in magnitude and pressure dependence between/3 phase and 3/spinel. The activation volumes for Mg-Ni diffusion are 3.2