This study supplements GyPSM-S modeling (Chapter 2) with a focus on the influences of slab fluid ... more This study supplements GyPSM-S modeling (Chapter 2) with a focus on the influences of slab fluid source lithology and fluid transport mechanisms on melt geochemistry, the implications of mantle source depletion related to fluid fluxing, and potential melt migration processes. Slab age and convergence velocity, which contribute to the slab thermal structure, are significant for the locations of dehydration reactions within the different lithological layers of the slab. The fluid source lithology determines the fluid flux and the fluid-mobile trace element input to the wedge. Through an investigation of changing fluid inputs based on changing subduction model parameters, there is a progression of geochemical characteristics described in studies of cross-arc and along-arc lavas that can be duplicated assuming (i) limited fluid-rock interaction within the mantle wedge and (ii) melt migration preserves the spatial distinction among melts initiated in different areas of the wedge. Specifi...
Introduction: Shock pressure conditions recorded in meteorites and terrestrial impact rocks provi... more Introduction: Shock pressure conditions recorded in meteorites and terrestrial impact rocks provide essential information for constraining their impact histories. The pressure thresholds for forming shockinduced transformational and deformational features in meteoritical minerals are used to extract these shock conditions [1-2]. Nevertheless, common deformation features, e.g. fractures, undulatory extinction and mosaicism occur in a wide range of pressures and may not be indicative [1]. High-pressure transformations, in contrast, have well-defined pressure field but occur only in a small portion of shocked meteorites, depending strongly on the shock temperature and duration [2]. It is desirable to build a shock barometer that can accurately indicate progressive pressure increase and is applicable to most naturally shocked rocks. Previous studies proposed to use e.g. spectroscopic properties of feldspars, the orientations preference of fractures and width of shock lamellae in olivine...
At the 26 th AIRAPT conference in 2017, a task group was formed to work on an International Pract... more At the 26 th AIRAPT conference in 2017, a task group was formed to work on an International Practical Pressure Scale (IPPS). This report summarizes the activities of the task group toward an IPPS ruby gauge. We have selected three different approaches to establishing the relation between pressure (P) and ruby R1-line shift (Δλ) with three groups of optimal reference materials for applying these approaches. Using a polynomial form of the second order, the recommended ruby gauge (referred as Ruby2020) is expressed by: P[GPa] = 1.87(+0.01) × 10 3 Dl l 0 1 + 5.63(+0.03) Dl l 0 , where λ 0 is the wavelength of the R1-line near 694.25 nm at ambient condition. In June of 2020, the Executive Committee of AIRAPT endorsed the proposed Ruby2020. We encourage highpressure practitioners to utilize Ruby2020 within its applicable pressure range (up to 150 GPa), so that pressure data can be directly compared across laboratories and amended consistently as better scales emerge in the future.
We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Châ... more We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Château-Renard, based on co-located Raman spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and electron backscatter diffraction, electron microprobe analysis, and transmission electron microscopy (TEM) with selected-area electron diffraction. A single polished section contains a network of melt veins from ~40 to ~200 μm wide, with no cross-cutting features requiring multiple vein generations. We find high-pressure minerals in veins greater than ~50 μm wide, including assemblages of ringwoodite + wadsleyite, ringwoodite + wadsleyite + majorite-pyrope, and ahrensite + wadsleyite. In association with ahrensite + wadsleyite at both SEM and TEM scale, we find a sodic pyroxene whose Raman spectrum is indistinguishable from that of jadeite but whose composition and structure are those of omphacite. We discuss constraints on the impact record of this meteor...
Five-component icosahedral quasicrystals with compositions in the range Al68-73Fe11-16Cu10-12Cr1-... more Five-component icosahedral quasicrystals with compositions in the range Al68-73Fe11-16Cu10-12Cr1-4Ni1-2 were recently recovered after shocking metallic CuAl5 and (Mg0.75Fe0.25)2SiO4 olivine in a stainless steel 304 chamber, intended to replicate a natural shock that affected the Khatyrka meteorite. The iron in those quasicrystals might have originated either from reduction of Fe2+ in olivine or from the stainless steel chamber. In this study, we clarify the shock synthesis mechanism of icosahedral quasicrystals through two new shock recovery experiments. When CuAl5 and Fe2+-bearing olivine were isolated in a Ta capsule, no quasicrystals were found. However, with only metallic starting materials, numerous micron-sized five-component icosahedral quasicrystals, average composition Al72Cu12Fe12Cr3Ni1, were found at the interface between CuAl5 and stainless steel, demonstrating nucleation of quasicrystals under shock without any redox reaction. We present detailed characterization of rec...
We evaluate the petrography and geochemistry of an unusual suite of subduction-related Phanerozoi... more We evaluate the petrography and geochemistry of an unusual suite of subduction-related Phanerozoic high-MgO rocks from the Othris ophiolite complex in Greece, some of which have previously been described as komatiitic lavas. In particular, we study ultramafic, olivine-phyric lavas from the Agrilia area and high-Mg basaltic dykes from the Pournari area. We seek to define primary
The carbon budget of the continental lithospheric mantle is poorly known, although it is believed... more The carbon budget of the continental lithospheric mantle is poorly known, although it is believed to be substantially fed by subduction zones and a source of the widespread non volcanic CO 2 degassing occurring in active tectonic intracontinental settings. We propose new constraints on the CO 2 budget of the subcontinental lithospheric mantle below the Pannonian
Proceedings of the National Academy of Sciences, 2016
Significance The singular occurrence, to date, of natural quasicrystals requires an explanation b... more Significance The singular occurrence, to date, of natural quasicrystals requires an explanation both of the possibility and of the rarity of their formation outside of the laboratory. Successful synthesis by an experimental shock, with starting materials similar to the exotic intermetallic alloys in the Khatyrka meteorite, demonstrates a mechanism that is feasible in space but not in any natural setting on Earth. The previously unrecognized composition of the synthesized quasicrystal, the first, to our knowledge, to be created by shocking discrete bulk starting materials, demonstrates a method for discovery of previously unknown quasicrystal compositions.
Constraints on water storage capacity and actual content in the mantle must be derived not only f... more Constraints on water storage capacity and actual content in the mantle must be derived not only from experimental studies, but also from investigation of natural samples. Olivine is one of the best-studied, OH-bearing "nominally anhydrous" minerals, yet there remain multiple hypotheses for the incorporation mechanism of hydrogen in this phase. Moreover, there is still debate as to whether the mechanism is the same in natural samples vs. experimental studies, where concentrations can reach very high values (up to ~o.6 wt% H 2 0) at high pressures and temperatures. We present new observations and review IR and TEM data from the literature that bear on this question. Hydrogen incorporation in natural olivine clearly occurs by multiple mechanisms, but in contrast to some previous assertions we find that there are strong similarities between the IR signatures of experimentally annealed olivines and most natural samples. At low pressures (lower than ~2 GPa) in both experiments and natural olivines, hydrogen incorporation might be dominated by a humite-type defect, but the nature of the defect may vary even within a single sample; possibilities include point defects, planar defects and optically detectable inclusions. IR bands between 3300 and 3400 cm-1 , ascribed previously to the influence of silica activity, are apparently related instead to increased oxygen fugacity. At higher pressures in experiments, the IR band structure changes and hydrogen is probably associated with disordered point defects. Similar IR spectra are seen in olivines from xenoliths derived from deeper parts of the mantle (below South Africa and the Colorado Plateau) as well as in olivines from the ultra-high pressure metamorphic province of the Western Gneiss Region in Norway.
Shock temperature measurements via optical pyrometry are being conducted on <100> single-crystal ... more Shock temperature measurements via optical pyrometry are being conducted on <100> single-crystal MgO preheated beftire compression to 1905-1924 K. Planar shocks were generated by impacting hot Mo(driver plate)-MgO targets with Mo or Ta flyers launched by the Caltech two-stage light-gas gun up to 6.6 km/s. Quasi-brightness temperature was measured with 2-3% uncertainty by a 6-channel optical pyrometer with 3 ns time resolution, over 500-900 nm spectral range. A high-power, coiled irradiance standard lamp was adopted ftir spectral radiance calibration accurate to 5%. In our experiments, shock pressure in MgO ranged from 102 to 203 GPa and the corresponding temperature varied from 3.78 to 6.53 kK. For the same particle velocity, preheated MgO Hugoniot has about 3% lower shock velocity than the room temperature Hugoniot. Although model shock temperatures calculated ftir the solid phase exceeded our measurements by ~5 times the uncertainty, there was no clear evidence of MgO melting, up to the highest compression achieved.
Subduction initiation may unfold via different pathways in response to plate strength, plate age,... more Subduction initiation may unfold via different pathways in response to plate strength, plate age, and driving mechanism. Such pathways infl uence volcanism on the overriding plate and may be preserved in the sequence of erupted volcanic products. Here, we parameterize melting in a mechanical model to determine the volcanic products that form in response to different subduction initiation modes. We fi nd that with a mode of continuous initiation with infant-arc spreading, the foundering of the subducting slab and water release from the slab govern a succession from basalts with compositions similar to mid-ocean-ridge basalts (MORB) to boninites. The modeled transition from MORB-like to boninite composition typically occurs within a few million years. When plate strength is reduced, the subducting slab tends to segment, with extensive melting occurring when the slab breaks; most melting occurs close to the trench. When plate strength increases, subduction initiation becomes continuous without infant-arc spreading; such a mode leads to a limited, very low degree of melting occurring during a long interval of plate convergence before subduction initiation starts, although extensive melting near the trench is still possible when subduction initiation starts after a protracted period of plate convergence (~10 m.y.). If the subduction initiation is driven by constant stresses, such as through ridge push, the slab subducts rapidly in response to continuous acceleration of the plate under action of the far-fi eld push; signifi cant melting, including boninite eruption, can be generated within a few million years with no trench migration. Based on the tectonic and volcanic evolution, these different modes may be applicable to the initiation of the Izu-Bonin-Mariana arc (infant-arc spreading and a sequence from MORB-like to boninites), the New Hebrides arc (slab segments in the upper mantle), the Puysegur Trench in New Zealand (scarce distribution of volcanism and no infant-arc spreading), and the Aleutian Trench (strong volcanism and no infant-arc spreading).
Widespread bodies of garnet^spinel metaperidotites with pyroxenitic layers occur in the ultrahigh... more Widespread bodies of garnet^spinel metaperidotites with pyroxenitic layers occur in the ultrahigh-pressure metamorphic Kimi Complex. In this study we address the origin of such peridotite^pyroxenite associations in the context of polybaric melting regimes. We conduct a detailed geochemical investigation of major and trace element relations and compare them with a range of major element modelling scenarios. With increasing bulk-rock MgO content, the garnet^spinel metaperidotites exhibit decreasing CaO, Al 2 O 3 , TiO 2 , and Na 2 O along with increasing Ni and a gradually increasing Zr/Zr à anomaly, consistent with an origin as residues after variable degrees of melt extraction.The major element modelling further suggests a polybaric adiabatic decompression melting regime beginning at high to ultrahigh pressure, with an intermediate character between pure batch and fractional melting and a mean extent of melting of 9^11%. The pyroxenites exhibit major element compositions that cannot be reproduced by experimental or calculated melts of peridotite. Moreover, the Kimi pyroxenites have highly variable Ni and Sc contents and a wide range of Mg-number (0Á 76^0Á 89), inconsistent with an origin as frozen melts or the products of melt^peridotite interaction. However, both the major element systematics and the observed rare earth element patterns, with both convex and concave shapes, can be explained by an origin as clinopyroxene-rich, highpressure cumulates involving garnet and/or Cr-spinel.
Journal of Geophysical Research: Solid Earth, 2012
We have conducted new equation of state measurements on liquid Fe 2 SiO 4 in a collaborative, mul... more We have conducted new equation of state measurements on liquid Fe 2 SiO 4 in a collaborative, multi-technique study. The liquid density (r), the bulk modulus (K), and its pressure derivative (K′) were measured from 1 atm to 161 GPa using 1-atm double-bob Archimedean, multi-anvil sink/float, and shock wave techniques. Shock compression results on initially molten Fe 2 SiO 4 (1573 K) fitted with previous work and the ultrasonically measured bulk sound speed (C o) in shock velocity (U S)-particle velocity (u p) space yields the Hugoniot: U S = 1.58(0.03) u p + 2.438(0.005) km/s. Sink/float results are in agreement with shock wave and ultrasonic data, consistent with an isothermal K T = 19.4 GPa and K′ = 5.33 at 1500 C. Shock melting of initially solid Fe 2 SiO 4 (300 K) confirms that the Grüneisen parameter (g) of this liquid increases upon compression where g = g o (r o /r) q yields a q value of-1.45. Constraints on the liquid fayalite EOS permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system CaO-MgO-Al 2 O 3-SiO 2-FeO at elevated temperatures and pressures. In our model a whole mantle magma ocean would first crystallize in the mid-lower mantle or at the base of the mantle were it composed of either peridotite or simplified "chondrite" liquid, respectively. In regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra-low velocity zones, neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales, but our model defines a compositional range of liquids that would be gravitationally stable.
We report major, trace, and volatile element data on basaltic glasses from the northernmost segme... more We report major, trace, and volatile element data on basaltic glasses from the northernmost segment of the Eastern Lau Spreading Center (ELSC1) in the Lau back-arc basin to further test and constrain models of back-arc volcanism. The zero-age samples come from 47 precisely collected stations from an 85 km length spreading center. The chemical data covary similarly to other back-arc systems but with tighter correlations and welldeveloped spatial systematics. We confirm a correlation between volatile content and apparent extent of melting of the mantle source but also show that the data cannot be reproduced by the model of isobaric addition of water that has been broadly applied to back-arc basins. The new data also confirm that there is no relationship between mantle temperature and the wet melting productivity. Two distinct magmatic provinces can be identified along the ELSC1 axis, a southern province influenced by a ''wet component'' with strong affinities to arc volcanism and a northern province influenced by a ''damp component'' intermediate between enriched mid-ocean ridge basalts (E-MORB) and arc basalts. High-field strength elements and rare earth elements are all mobilized to some extent by the wet component, and the detailed composition of this component is determined. It differs in significant ways from the Mariana component reported by E. Stolper and S. Newman (1994), particularly by having lower abundances of most elements relative to H 2 O. The differences can be explained if the slab temperature is higher for the Mariana and the source from which the fluid is derived is more enriched. The ELSC1 damp component is best explained by mixing between the wet component and an E-MORB-like component. We propose that mixing between water-rich fluids and low-degree silicate melts occurs at depth in the subduction zone to generate the chemical diversity of the ELSC1 subduction components. These modified sources then rise independently to the surface and melt, and these melts mix with melts of the background mantle from the ridge melting regime to generate the linear data arrays characteristic of back-arc basalts. The major and trace element framework for ELSC1, combined with different slab temperatures and compositions for difference convergent margins, may be able to be applied to other back-arc basins around the globe.
Journal of Geophysical Research: Solid Earth, 2009
We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa ... more We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low-density samples (MgSiO 3 glass) and high-density, polycrystalline aggregates (MgSiO 3 perovskite + majorite). The latter samples were synthesized using a large-volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existing P-V-T data from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third-order Birch-Murnaghan isentropes, offset with a Mie-Grüneisen-Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value of K 0 significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth-order isentrope. We also provide a new EoS for Mg 2 SiO 4 liquid, calculated in a similar manner. The Grüneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra-low-velocity zones associated with melting at the core-mantle boundary, and crystallization of a terrestrial magma ocean.
Journal of Geophysical Research: Solid Earth, 2003
We present a detailed and quantitative examination of the thermodynamics and phase change mechani... more We present a detailed and quantitative examination of the thermodynamics and phase change mechanisms (including amorphization) that occur upon shock wave loading and unloading of silica. We apply Debye-Grüneisen theory to calculate both the Hugoniot of quartz and isentropic release paths. Quartz converts to stishovite (or a stishovite-like phase) between 15 and 46 GPa, and persistence of the solid phase above its liquidus (i.e., superheating) is confirmed between 77 and 110 GPa. Calculations compare favorably to measurements of shock and post-shock temperatures. For silica, the method of measuring post-shock temperature is insensitive to predicting whether phase transitions actually occur during release. Measurements of release states in pressure-particle velocity space are compared to computed frozen-phase release paths. This comparison suggests transformation of a stishovite-like phase to lower density phases including quartz, liquid, or dense amorphous glass. Transformations to liquid or glass occur upon release from peak pressure of 26 GPa and above. The isentropic release assumption appears to be approximately valid. A shock pressure-temperature scale relating metamorphism of silica in shock-loaded quartz is proposed. Neither recovery of coesite nor substantial quantities of crystalline stishovite-like phases upon shock loading of quartz is predicted. Trace amounts of crystalline stishovite-like phases from shock loading between 15 and 26 GPa are expected.
Manganese contents and the iron/manganese ratio of igneous rocks have been used as a method of pr... more Manganese contents and the iron/manganese ratio of igneous rocks have been used as a method of probing the heterogeneity in the Earth's mantle during melting of peridotite and pyroxenite lithologies. Most previous work has assumed that changes in these parameters require changes in either source lithology or composition based on experiments that suggest manganese is slightly incompatible during melting and that the iron/manganese ratio is fixed by the presence of olivine. However, the presence of volatiles in the mantle drives melting at lower temperatures and with different compositions than is seen in volatile-free systems, and thus the partitioning behavior of Fe and Mn may in fact vary. We have produced silicate liquids in equilibrium with a peridotite assemblage under hydrous conditions at 3 GPa pressure which show that manganese can also be unexpectedly compatible in garnet at 1375 °C and Mn partitioning between solids and liquids can be strongly affected by temperature and liquid composition. The compatibility of Mn in garnet provides a mechanism for large variations of Mn contents and the Fe/Mn ratio in silicate melts solely involving melting of mantle peridotite with fairly small compositional changes. Correlations between Mn variations and other indices indicative of melting in the presence of garnet may provide a means of more completely understanding the role of garnet at high pressures in peridotite melting.
This study supplements GyPSM-S modeling (Chapter 2) with a focus on the influences of slab fluid ... more This study supplements GyPSM-S modeling (Chapter 2) with a focus on the influences of slab fluid source lithology and fluid transport mechanisms on melt geochemistry, the implications of mantle source depletion related to fluid fluxing, and potential melt migration processes. Slab age and convergence velocity, which contribute to the slab thermal structure, are significant for the locations of dehydration reactions within the different lithological layers of the slab. The fluid source lithology determines the fluid flux and the fluid-mobile trace element input to the wedge. Through an investigation of changing fluid inputs based on changing subduction model parameters, there is a progression of geochemical characteristics described in studies of cross-arc and along-arc lavas that can be duplicated assuming (i) limited fluid-rock interaction within the mantle wedge and (ii) melt migration preserves the spatial distinction among melts initiated in different areas of the wedge. Specifi...
Introduction: Shock pressure conditions recorded in meteorites and terrestrial impact rocks provi... more Introduction: Shock pressure conditions recorded in meteorites and terrestrial impact rocks provide essential information for constraining their impact histories. The pressure thresholds for forming shockinduced transformational and deformational features in meteoritical minerals are used to extract these shock conditions [1-2]. Nevertheless, common deformation features, e.g. fractures, undulatory extinction and mosaicism occur in a wide range of pressures and may not be indicative [1]. High-pressure transformations, in contrast, have well-defined pressure field but occur only in a small portion of shocked meteorites, depending strongly on the shock temperature and duration [2]. It is desirable to build a shock barometer that can accurately indicate progressive pressure increase and is applicable to most naturally shocked rocks. Previous studies proposed to use e.g. spectroscopic properties of feldspars, the orientations preference of fractures and width of shock lamellae in olivine...
At the 26 th AIRAPT conference in 2017, a task group was formed to work on an International Pract... more At the 26 th AIRAPT conference in 2017, a task group was formed to work on an International Practical Pressure Scale (IPPS). This report summarizes the activities of the task group toward an IPPS ruby gauge. We have selected three different approaches to establishing the relation between pressure (P) and ruby R1-line shift (Δλ) with three groups of optimal reference materials for applying these approaches. Using a polynomial form of the second order, the recommended ruby gauge (referred as Ruby2020) is expressed by: P[GPa] = 1.87(+0.01) × 10 3 Dl l 0 1 + 5.63(+0.03) Dl l 0 , where λ 0 is the wavelength of the R1-line near 694.25 nm at ambient condition. In June of 2020, the Executive Committee of AIRAPT endorsed the proposed Ruby2020. We encourage highpressure practitioners to utilize Ruby2020 within its applicable pressure range (up to 150 GPa), so that pressure data can be directly compared across laboratories and amended consistently as better scales emerge in the future.
We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Châ... more We report the first discoveries of high-pressure minerals in the historical L6 chondrite fall Château-Renard, based on co-located Raman spectroscopy, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy and electron backscatter diffraction, electron microprobe analysis, and transmission electron microscopy (TEM) with selected-area electron diffraction. A single polished section contains a network of melt veins from ~40 to ~200 μm wide, with no cross-cutting features requiring multiple vein generations. We find high-pressure minerals in veins greater than ~50 μm wide, including assemblages of ringwoodite + wadsleyite, ringwoodite + wadsleyite + majorite-pyrope, and ahrensite + wadsleyite. In association with ahrensite + wadsleyite at both SEM and TEM scale, we find a sodic pyroxene whose Raman spectrum is indistinguishable from that of jadeite but whose composition and structure are those of omphacite. We discuss constraints on the impact record of this meteor...
Five-component icosahedral quasicrystals with compositions in the range Al68-73Fe11-16Cu10-12Cr1-... more Five-component icosahedral quasicrystals with compositions in the range Al68-73Fe11-16Cu10-12Cr1-4Ni1-2 were recently recovered after shocking metallic CuAl5 and (Mg0.75Fe0.25)2SiO4 olivine in a stainless steel 304 chamber, intended to replicate a natural shock that affected the Khatyrka meteorite. The iron in those quasicrystals might have originated either from reduction of Fe2+ in olivine or from the stainless steel chamber. In this study, we clarify the shock synthesis mechanism of icosahedral quasicrystals through two new shock recovery experiments. When CuAl5 and Fe2+-bearing olivine were isolated in a Ta capsule, no quasicrystals were found. However, with only metallic starting materials, numerous micron-sized five-component icosahedral quasicrystals, average composition Al72Cu12Fe12Cr3Ni1, were found at the interface between CuAl5 and stainless steel, demonstrating nucleation of quasicrystals under shock without any redox reaction. We present detailed characterization of rec...
We evaluate the petrography and geochemistry of an unusual suite of subduction-related Phanerozoi... more We evaluate the petrography and geochemistry of an unusual suite of subduction-related Phanerozoic high-MgO rocks from the Othris ophiolite complex in Greece, some of which have previously been described as komatiitic lavas. In particular, we study ultramafic, olivine-phyric lavas from the Agrilia area and high-Mg basaltic dykes from the Pournari area. We seek to define primary
The carbon budget of the continental lithospheric mantle is poorly known, although it is believed... more The carbon budget of the continental lithospheric mantle is poorly known, although it is believed to be substantially fed by subduction zones and a source of the widespread non volcanic CO 2 degassing occurring in active tectonic intracontinental settings. We propose new constraints on the CO 2 budget of the subcontinental lithospheric mantle below the Pannonian
Proceedings of the National Academy of Sciences, 2016
Significance The singular occurrence, to date, of natural quasicrystals requires an explanation b... more Significance The singular occurrence, to date, of natural quasicrystals requires an explanation both of the possibility and of the rarity of their formation outside of the laboratory. Successful synthesis by an experimental shock, with starting materials similar to the exotic intermetallic alloys in the Khatyrka meteorite, demonstrates a mechanism that is feasible in space but not in any natural setting on Earth. The previously unrecognized composition of the synthesized quasicrystal, the first, to our knowledge, to be created by shocking discrete bulk starting materials, demonstrates a method for discovery of previously unknown quasicrystal compositions.
Constraints on water storage capacity and actual content in the mantle must be derived not only f... more Constraints on water storage capacity and actual content in the mantle must be derived not only from experimental studies, but also from investigation of natural samples. Olivine is one of the best-studied, OH-bearing "nominally anhydrous" minerals, yet there remain multiple hypotheses for the incorporation mechanism of hydrogen in this phase. Moreover, there is still debate as to whether the mechanism is the same in natural samples vs. experimental studies, where concentrations can reach very high values (up to ~o.6 wt% H 2 0) at high pressures and temperatures. We present new observations and review IR and TEM data from the literature that bear on this question. Hydrogen incorporation in natural olivine clearly occurs by multiple mechanisms, but in contrast to some previous assertions we find that there are strong similarities between the IR signatures of experimentally annealed olivines and most natural samples. At low pressures (lower than ~2 GPa) in both experiments and natural olivines, hydrogen incorporation might be dominated by a humite-type defect, but the nature of the defect may vary even within a single sample; possibilities include point defects, planar defects and optically detectable inclusions. IR bands between 3300 and 3400 cm-1 , ascribed previously to the influence of silica activity, are apparently related instead to increased oxygen fugacity. At higher pressures in experiments, the IR band structure changes and hydrogen is probably associated with disordered point defects. Similar IR spectra are seen in olivines from xenoliths derived from deeper parts of the mantle (below South Africa and the Colorado Plateau) as well as in olivines from the ultra-high pressure metamorphic province of the Western Gneiss Region in Norway.
Shock temperature measurements via optical pyrometry are being conducted on <100> single-crystal ... more Shock temperature measurements via optical pyrometry are being conducted on <100> single-crystal MgO preheated beftire compression to 1905-1924 K. Planar shocks were generated by impacting hot Mo(driver plate)-MgO targets with Mo or Ta flyers launched by the Caltech two-stage light-gas gun up to 6.6 km/s. Quasi-brightness temperature was measured with 2-3% uncertainty by a 6-channel optical pyrometer with 3 ns time resolution, over 500-900 nm spectral range. A high-power, coiled irradiance standard lamp was adopted ftir spectral radiance calibration accurate to 5%. In our experiments, shock pressure in MgO ranged from 102 to 203 GPa and the corresponding temperature varied from 3.78 to 6.53 kK. For the same particle velocity, preheated MgO Hugoniot has about 3% lower shock velocity than the room temperature Hugoniot. Although model shock temperatures calculated ftir the solid phase exceeded our measurements by ~5 times the uncertainty, there was no clear evidence of MgO melting, up to the highest compression achieved.
Subduction initiation may unfold via different pathways in response to plate strength, plate age,... more Subduction initiation may unfold via different pathways in response to plate strength, plate age, and driving mechanism. Such pathways infl uence volcanism on the overriding plate and may be preserved in the sequence of erupted volcanic products. Here, we parameterize melting in a mechanical model to determine the volcanic products that form in response to different subduction initiation modes. We fi nd that with a mode of continuous initiation with infant-arc spreading, the foundering of the subducting slab and water release from the slab govern a succession from basalts with compositions similar to mid-ocean-ridge basalts (MORB) to boninites. The modeled transition from MORB-like to boninite composition typically occurs within a few million years. When plate strength is reduced, the subducting slab tends to segment, with extensive melting occurring when the slab breaks; most melting occurs close to the trench. When plate strength increases, subduction initiation becomes continuous without infant-arc spreading; such a mode leads to a limited, very low degree of melting occurring during a long interval of plate convergence before subduction initiation starts, although extensive melting near the trench is still possible when subduction initiation starts after a protracted period of plate convergence (~10 m.y.). If the subduction initiation is driven by constant stresses, such as through ridge push, the slab subducts rapidly in response to continuous acceleration of the plate under action of the far-fi eld push; signifi cant melting, including boninite eruption, can be generated within a few million years with no trench migration. Based on the tectonic and volcanic evolution, these different modes may be applicable to the initiation of the Izu-Bonin-Mariana arc (infant-arc spreading and a sequence from MORB-like to boninites), the New Hebrides arc (slab segments in the upper mantle), the Puysegur Trench in New Zealand (scarce distribution of volcanism and no infant-arc spreading), and the Aleutian Trench (strong volcanism and no infant-arc spreading).
Widespread bodies of garnet^spinel metaperidotites with pyroxenitic layers occur in the ultrahigh... more Widespread bodies of garnet^spinel metaperidotites with pyroxenitic layers occur in the ultrahigh-pressure metamorphic Kimi Complex. In this study we address the origin of such peridotite^pyroxenite associations in the context of polybaric melting regimes. We conduct a detailed geochemical investigation of major and trace element relations and compare them with a range of major element modelling scenarios. With increasing bulk-rock MgO content, the garnet^spinel metaperidotites exhibit decreasing CaO, Al 2 O 3 , TiO 2 , and Na 2 O along with increasing Ni and a gradually increasing Zr/Zr à anomaly, consistent with an origin as residues after variable degrees of melt extraction.The major element modelling further suggests a polybaric adiabatic decompression melting regime beginning at high to ultrahigh pressure, with an intermediate character between pure batch and fractional melting and a mean extent of melting of 9^11%. The pyroxenites exhibit major element compositions that cannot be reproduced by experimental or calculated melts of peridotite. Moreover, the Kimi pyroxenites have highly variable Ni and Sc contents and a wide range of Mg-number (0Á 76^0Á 89), inconsistent with an origin as frozen melts or the products of melt^peridotite interaction. However, both the major element systematics and the observed rare earth element patterns, with both convex and concave shapes, can be explained by an origin as clinopyroxene-rich, highpressure cumulates involving garnet and/or Cr-spinel.
Journal of Geophysical Research: Solid Earth, 2012
We have conducted new equation of state measurements on liquid Fe 2 SiO 4 in a collaborative, mul... more We have conducted new equation of state measurements on liquid Fe 2 SiO 4 in a collaborative, multi-technique study. The liquid density (r), the bulk modulus (K), and its pressure derivative (K′) were measured from 1 atm to 161 GPa using 1-atm double-bob Archimedean, multi-anvil sink/float, and shock wave techniques. Shock compression results on initially molten Fe 2 SiO 4 (1573 K) fitted with previous work and the ultrasonically measured bulk sound speed (C o) in shock velocity (U S)-particle velocity (u p) space yields the Hugoniot: U S = 1.58(0.03) u p + 2.438(0.005) km/s. Sink/float results are in agreement with shock wave and ultrasonic data, consistent with an isothermal K T = 19.4 GPa and K′ = 5.33 at 1500 C. Shock melting of initially solid Fe 2 SiO 4 (300 K) confirms that the Grüneisen parameter (g) of this liquid increases upon compression where g = g o (r o /r) q yields a q value of-1.45. Constraints on the liquid fayalite EOS permit the calculation of isentropes for silicate liquids of general composition in the multicomponent system CaO-MgO-Al 2 O 3-SiO 2-FeO at elevated temperatures and pressures. In our model a whole mantle magma ocean would first crystallize in the mid-lower mantle or at the base of the mantle were it composed of either peridotite or simplified "chondrite" liquid, respectively. In regards to the partial melt hypothesis to explain the occurrence and characteristics of ultra-low velocity zones, neither of these candidate liquids would be dense enough to remain at the core mantle boundary on geologic timescales, but our model defines a compositional range of liquids that would be gravitationally stable.
We report major, trace, and volatile element data on basaltic glasses from the northernmost segme... more We report major, trace, and volatile element data on basaltic glasses from the northernmost segment of the Eastern Lau Spreading Center (ELSC1) in the Lau back-arc basin to further test and constrain models of back-arc volcanism. The zero-age samples come from 47 precisely collected stations from an 85 km length spreading center. The chemical data covary similarly to other back-arc systems but with tighter correlations and welldeveloped spatial systematics. We confirm a correlation between volatile content and apparent extent of melting of the mantle source but also show that the data cannot be reproduced by the model of isobaric addition of water that has been broadly applied to back-arc basins. The new data also confirm that there is no relationship between mantle temperature and the wet melting productivity. Two distinct magmatic provinces can be identified along the ELSC1 axis, a southern province influenced by a ''wet component'' with strong affinities to arc volcanism and a northern province influenced by a ''damp component'' intermediate between enriched mid-ocean ridge basalts (E-MORB) and arc basalts. High-field strength elements and rare earth elements are all mobilized to some extent by the wet component, and the detailed composition of this component is determined. It differs in significant ways from the Mariana component reported by E. Stolper and S. Newman (1994), particularly by having lower abundances of most elements relative to H 2 O. The differences can be explained if the slab temperature is higher for the Mariana and the source from which the fluid is derived is more enriched. The ELSC1 damp component is best explained by mixing between the wet component and an E-MORB-like component. We propose that mixing between water-rich fluids and low-degree silicate melts occurs at depth in the subduction zone to generate the chemical diversity of the ELSC1 subduction components. These modified sources then rise independently to the surface and melt, and these melts mix with melts of the background mantle from the ridge melting regime to generate the linear data arrays characteristic of back-arc basalts. The major and trace element framework for ELSC1, combined with different slab temperatures and compositions for difference convergent margins, may be able to be applied to other back-arc basins around the globe.
Journal of Geophysical Research: Solid Earth, 2009
We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa ... more We present new equation-of-state (EoS) data acquired by shock loading to pressures up to 245 GPa on both low-density samples (MgSiO 3 glass) and high-density, polycrystalline aggregates (MgSiO 3 perovskite + majorite). The latter samples were synthesized using a large-volume press. Modeling indicates that these materials transform to perovskite, postperovskite, and/or melt with increasing pressure on their Hugoniots. We fit our results together with existing P-V-T data from dynamic and static compression experiments to constrain the thermal EoS for the three phases, all of which are of fundamental importance to the dynamics of the lower mantle. The EoS for perovskite and postperovskite are well described with third-order Birch-Murnaghan isentropes, offset with a Mie-Grüneisen-Debye formulation for thermal pressure. The addition of shock data helps to distinguish among discrepant static studies of perovskite, and for postperovskite, constrain a value of K 0 significantly larger than 4. For the melt, we define for the first time a single EoS that fits experimental data from ambient pressure to 230 GPa; the best fit requires a fourth-order isentrope. We also provide a new EoS for Mg 2 SiO 4 liquid, calculated in a similar manner. The Grüneisen parameters of the solid phases decrease with pressure, whereas those of the melts increase, consistent with previous shock wave experiments as well as molecular dynamics simulations. We discuss implications of our modeling for thermal expansion in the lower mantle, stabilization of ultra-low-velocity zones associated with melting at the core-mantle boundary, and crystallization of a terrestrial magma ocean.
Journal of Geophysical Research: Solid Earth, 2003
We present a detailed and quantitative examination of the thermodynamics and phase change mechani... more We present a detailed and quantitative examination of the thermodynamics and phase change mechanisms (including amorphization) that occur upon shock wave loading and unloading of silica. We apply Debye-Grüneisen theory to calculate both the Hugoniot of quartz and isentropic release paths. Quartz converts to stishovite (or a stishovite-like phase) between 15 and 46 GPa, and persistence of the solid phase above its liquidus (i.e., superheating) is confirmed between 77 and 110 GPa. Calculations compare favorably to measurements of shock and post-shock temperatures. For silica, the method of measuring post-shock temperature is insensitive to predicting whether phase transitions actually occur during release. Measurements of release states in pressure-particle velocity space are compared to computed frozen-phase release paths. This comparison suggests transformation of a stishovite-like phase to lower density phases including quartz, liquid, or dense amorphous glass. Transformations to liquid or glass occur upon release from peak pressure of 26 GPa and above. The isentropic release assumption appears to be approximately valid. A shock pressure-temperature scale relating metamorphism of silica in shock-loaded quartz is proposed. Neither recovery of coesite nor substantial quantities of crystalline stishovite-like phases upon shock loading of quartz is predicted. Trace amounts of crystalline stishovite-like phases from shock loading between 15 and 26 GPa are expected.
Manganese contents and the iron/manganese ratio of igneous rocks have been used as a method of pr... more Manganese contents and the iron/manganese ratio of igneous rocks have been used as a method of probing the heterogeneity in the Earth's mantle during melting of peridotite and pyroxenite lithologies. Most previous work has assumed that changes in these parameters require changes in either source lithology or composition based on experiments that suggest manganese is slightly incompatible during melting and that the iron/manganese ratio is fixed by the presence of olivine. However, the presence of volatiles in the mantle drives melting at lower temperatures and with different compositions than is seen in volatile-free systems, and thus the partitioning behavior of Fe and Mn may in fact vary. We have produced silicate liquids in equilibrium with a peridotite assemblage under hydrous conditions at 3 GPa pressure which show that manganese can also be unexpectedly compatible in garnet at 1375 °C and Mn partitioning between solids and liquids can be strongly affected by temperature and liquid composition. The compatibility of Mn in garnet provides a mechanism for large variations of Mn contents and the Fe/Mn ratio in silicate melts solely involving melting of mantle peridotite with fairly small compositional changes. Correlations between Mn variations and other indices indicative of melting in the presence of garnet may provide a means of more completely understanding the role of garnet at high pressures in peridotite melting.
The Pannonian Basin in East-Central Europe has been affected by lithospheric fluxing by mantle CO... more The Pannonian Basin in East-Central Europe has been affected by lithospheric fluxing by mantle CO 2 -rich fluids as evidenced by alkaline/calc-alkaline volcanism and basin gas geochemical data [1]. The source, timing and amplitude of this CO 2 flux remain enigmatic and are of prime importance for petroleum exploration.
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Papers by Paul Asimow