bluuebunny/crossref_metadata_2025 huggingface dataset:
Role of drifts and global merged interaction regions in the long‐term modulation of cosmic rays
Authors: McDonald Frank B. , Lal Nand , McGuire Robert E.
Published: February 2008

It is shown that the long‐term modulation of 0.2‐ to 3‐GV galactic and anomalous cosmic rays over the 22‐year heliomagnetic cycle is principally a combination of two solar related processes, the cumulative effect of long‐lived global merged interaction regions (GMIRs) and large‐scale particle gradient and curvature drifts in the interplanetary magnetic field. This paradigm for cosmic ray modulation is based on the observed changes in cosmic ray intensity from solar minimum to solar maximum over successive solar cycles (21 and 22) using data from 1 AU and from the outer heliosphere. For cycle 21 (when positive ions flow in over the solar pole and out along the heliospheric neutral current sheet; qA>0), the 1977–1980 modulation is dominated by GMIRs. While drifts may play a role in particle transport in the heliosphere at this time, the changing inclination of the heliospheric neutral sheet has a negligible effect on the intensity of cosmic ray nuclei. In cycle 22 when this flow pattern is reversed, it is shown that drifts are an important but not dominant factor for galactic cosmic ray modulation with the current sheet related drift effects decreasing with increasing rigidity R and heliocentric distance r. Anomalous cosmic rays are much more sensitive to changes in the current sheet inclination. The observed changes in the galactic cosmic ray intensity over the 1987–1988 period due to the increasing inclination of the current sheet, α, are a factor of 3–5 smaller than predicted by the time‐dependent model of Potgieter and Le Roux (1992). The latitude variation at Voyager 1 (heliolatitude 32°) of the dependence of galactic cosmic ray intensity on α is consistent with theoretical expectations. The strong role of GMIRs and their episodic nature requires a long lifetime (1.5–1.8 years) and a magnetic structure that effectively extends over the solar poles. Such a long GMIR lifetime implies a modulation boundary of the order of ∼175 AU.
Anomalous aspects of magnetosheath flow and of the shape and oscillations of the magnetopause during an interval of strongly northward interplanetary magnetic field
Authors: Chen Sheng‐Hsien , Kivelson Margaret G. , Gosling Jack T. , Walker Raymond J. , Lazarus Allan J.
Published: February 2008

On February 15, 1978, the orientation of the interplanetary magnetic field (IMF) remained steadily northward for more than 12 hours. The ISEE 1 and 2 spacecraft were located near apogee on the dawnside flank of the magnetotail. IMP 8 was almost symmetrically located in the magnetosheath on the dusk flank and IMP 7 was upstream in the solar wind. Using plasma and magnetic field data, we show that (1) the magnetosheath flow speed on the flanks of the magnetotail steadily exceeded the solar wind speed by 20%, (2) surface waves of ∼5‐min period and very nonsinusoidal waveform were persistently present on the dawn magnetopause and waves of similar period were present in the dusk magnetosheath, and (3) the magnetotail ceased to flare at an antisunward distance of 15 RE. We propose that the acceleration of the magnetosheath flow is achieved by magnetic tension in the draped field configuration for northward IMF and that the reduction of tail flaring is consistent with a decreased amount of open magnetic flux and a larger standoff distance of the subsolar magnetopause. Results of a three‐dimensional magnetohydrodynamic simulation support this phenomenological model.
Spectrometric and photometric observations of low‐latitude aurorae
Authors: Rassoul H. K. , Rohrbaugh R. P. , Tinsley B. A. , Slater D. W.
Published: February 2008

The properties of a low‐latitude aurora of September 22, 1982, observed with photometers and a spectrograph from McDonald Observatory, Texas, and an all‐sky mapping photometer from a site near Boulder, Colorado, are discussed. The dominant emission was [OI] 630 nm with a 630/558 nm emission ratio of 3.8. Other weaker emissions were present that are characteristic of heavy particle aurorae and can be attributed to energetic (keV) ions and neutral atoms from the ring current. The properties of this and other low‐latitude aurorae are discussed in the context of stable auroral red (SAR) arcs and type d aurorae, and of energetic ion and neutral aurorae. The emissions associated with heavy particle precipitation are characteristic of main phase conditions. The observed equatorial movement of main phase low‐latitude red aurorae is consistent with the inward convection of ring current particles and the inward movement of the plasmapause. Previous work has hypothesized that coulomb collisions of plasmaspheric electrons with ring current O+ cause the energization of the electrons exciting SAR arcs. Calculations of [OI] 630 and 558 nm emissions show that for red aurorae with 630/558 nm ratios ∼4, additional energization processes producing electrons in the energy range 10–10,000 eV are required. We propose some clarification of nomenclature for several types of aurorae occurring at low latitudes.
Measurements of the time constant for steady ionization in shaped‐charge barium releases
Authors: Hoch Edward L. , Hallinan Thomas J.
Published: February 2008

Quantitative measurements of three solar illuminated shaped‐charge barium releases injected at small angles to the magnetic field were made using a calibrated color television camera. Two of the releases were from 1989. The third release, a reanalysis of an event included in Hallinan's 1988 study of three 1986 releases, was included to provide continuity between the two studies. Time constants for ionization, measured during the first 25 s of each release, were found to vary considerably. The two 1989 time constants differed substantially, and both were significantly less than any of the 1986 time constants. On the basis of this variability, we conclude that the two 1989 releases showed evidence of continuous nonsolar ionization. One release showed nonsolar ionization which could not be attributed to Alfvén's critical ionization velocity process, which requires a component of velocity perpendicular to the magnetic field providing a perpendicular energy greater than the ionization potential.
Diffusion coefficients of energetic water group ions near comet Giacobini‐Zinner
Authors: Tan L. C. , Mason G. M. , Richardson I. G. , Ipavich F. M.
Published: February 2008

When the ICE spacecraft flew past the comet Giacobini‐Zinner on September 11, 1985, the ultra low‐energy charge analyzer and energetic particle anisotropy spectrometer sensors observed an intensity gradient in the flux of fast (800‐1400 km s−1) water group ions in the so‐called transition region downstream of the cometary bow wave. We have combined the data from the two sensors and applied a single, self‐consistent analysis technique to derive a single‐particle spectrum from ∼200‐1600 km s−1. Using this information, along with the deduced bulk flow speed of the ions, we calculate a parallel diffusion coefficient in the transition region downstream of the cometary bow wave of (2.3 ± 0.5) × 1017 cm² s−1; the corresponding scattering mean free path λ is (6 ± 1) × 104 km. We also find that the parallel diffusion coefficient depends on the collision frequency ν+ (ν−) of water group ions with Alfvén waves, which we assume are propagating parallel (antiparallel) to the magnetic field. Our observations are consistent with ν+ = (1.6 ± 0.4) × 10−2 s−1 and ν−/ν+ = 0.3 ± 0.2; i.e., most of the waves are propagating in the antisunward direction.
Evidence of preferential directions for gravity wave propagation due to wind filtering in the middle atmosphere
Authors: Taylor M. J. , Ryan E. H. , Tuan T. F. , Edwards R.
Published: February 2008

All‐sky TV images of wave structure in the near‐infrared hydroxyl (OH) nightglow emission were recorded over a 3‐month period during May, June, and July 1988 from a high‐altitude site at the Mountain Research Station (40.0° N, 105.6° W, 3050 m), near Nederland, Colorado. Well‐defined, coherent wave patterns associated with the passage of short period (<1 hour) gravity waves were observed on a total of 22 occasions. The wave motions exhibited similar spatial and temporal properties during each month but a distinct tendency for northward propagation (68% of the wave azimuths within ± 40.0° N), with some eastward motion in May and June, was observed throughout the campaign. Although it is theoretically well known that upward propagating gravity waves can be blocked at a critical layer produced by the interaction of the waves with the horizontal background wind, observational evidence of this phenomenon is rare. To investigate the possibility that the asymmetry in the wave propagation directions was caused by the critical layer, a model based on mean climatological background winds and numerical tidal wave modes valid for any mid‐latitude site and time of the year was constructed to show the regions forbidden to upward gravity wave propagation from critical layer theory. These “blocking diagrams” which vary with height and time were constructed for the OH altitude (∼87 km) for the present paper. Comparison of the predicted (i.e., least restricted) and the observed directions of the wave motion show almost complete agreement. This suggests that middle atmospheric winds can play an important role in determining the flux and the azimuthal distribution of short‐period waves reaching the upper atmosphere.
Common origin of positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes
Authors: Prölss G. W.
Published: February 2008

Anomalous increases of the ionization density at middle latitudes (positive ionospheric storms) and anomalous increases of the neutral gas density at low latitudes (the geomagnetic activity effect) are prominent features of upper atmospheric storms. The present study investigates the idea that both phenomena are caused by traveling atmospheric disturbances (TADs). According to theory, such TADs are generated during magnetic substorm activity and propagate with high velocity from polar to equatorial latitudes. To examine the above hypothesis, magnetic, ionospheric, and neutral atmospheric data are compared for five different disturbance events. These case studies demonstrate that (1) there is a good temporal correlation between magnetic substorm activity at high latitudes, daytime positive ionospheric storms at middle latitudes, and the geomagnetic activity effect at low latitudes; (2) the initial phase of positive ionospheric storms propagates with high velocity toward lower latitudes; (3) this velocity is roughly consistent with the time lag of the geomagnetic activity effect at low latitudes; (4) the ionospheric disturbance is a conjugate phenomenon of global extent; and (5) it cannot be explained as an electric field effect. In summary, our data are fully consistent with the idea that TADs are responsible for both positive ionospheric storms at middle latitudes and the geomagnetic activity effect at low latitudes.
Modeling the Arecibo nighttime F2 layer: 1. Overhead properties
Authors: Meléndez‐Alvira D. J. , Burnside R. G. , Walker J. C. G.
Published: February 2008

The servo model of Rishbeth et al. (1978) is extended and applied to fit the height and plasma density of the nighttime F2 layer, as measured from the Arecibo Observatory in Puerto Rico during solar maximum by Burnside (1984). The model equations are integrated numerically to fit the observed peak height and density. Specifically, the fitting is made by applying a theoretical zonal current (or drift), as needed so as to match the observed F2 peak. Moreover, the applied current/drift employs the measured ion velocity, whereas the component due to the neutral wind is freely adjusted. An improvement over previous servo‐model fits is made by considering the time variations in all the model neutral and plasma densities and temperatures. All the background neutral densities are obtained from the MSIS model (Hedin, 1987). The following results are obtained: (1) the model adequately reproduces the observed behavior of the F2 layer; (2) the additional terms in the extended servo‐height equation affect the peak height computation by no more than 10% on average; (3) the applied current is generally eastward though it becomes westward during the postmidnight collapse or descent of the layer; (4) differences between model and optically measured meridional wind speeds appear to be related to the presence of large vertical shears in the wind; (5) the observed peak density can be reproduced to within 20% to 40% accuracy; and (6) variable plasmaspheric fluxes of the order of 1013 (m−2 s−1) contribute to the maintenance and variability of the nighttime peak density. Also, horizontally divergent plasma fluxes remove plasma away from the local F2 peak; however, this is relatively minor. The overall model fit to the data is improved with the ion‐neutral collision frequency increased by 70% (Burnside et al., 1987).
Measurement of direct current electric fields and plasma flow speeds in Jupiter's magnetosphere
Authors: Kellogg Paul J. , Goetz K. , Howard R. L. , Monson S. J. , Balogh A. , Forsyth R. J.
Published: February 2008

During the encounter of Ulysses with Jupiter, we have measured two components of the dc electric field and deduced from them the flow speed in the Io torus, as well as the presence of a polar cap region and what we interpret as a cleft region. Within the torus the flow speed is approximately equal to the speed of a plasma corotating with Jupiter but has significant deviations. The dominant deviations have an apparent period of the order of Jupiter's rotation period, but this might be a latitudinal effect. Other important periods are about 40 min and less than 25 min.
Simultaneous high‐P, high‐T X ray diffraction study of β‐(Mg,Fe)2SiO4 to 26 GPa and 900 K
Authors: Fei Yingwei , Mao Ho‐kwang , Shu Jinfu , Parthasarathy G. , Bassett W. A. , Ko Jaidong
Published: February 2008

The lattice parameters of β phase [(Mg0.84Fe0.16)2SiO4] have been determined by X ray diffraction using synchrotron radiation under simultaneous high‐pressure and high‐temperature conditions. The experiments were conducted up to a pressure of 26 GPa and a temperature of 900 K. High pressures were generated in a Mao‐Bell type diamond anvil cell using neon gas as pressure medium. The sample was heated with an external Ni80Cr20 wire heater. Gold was used as an internal high‐pressure calibrant at high temperatures. Using 4.0 for (∂KT/∂P)T, we found that the 300‐K isothermal bulk modulus, KT0, and its temperature derivative at constant pressure, (∂KT/∂T)P, are 174(±3) GPa and −2.7(±0.3)×10−2 GPa/K, respectively. The value of the Anderson‐Grüneisen parameter δT of the β phase is 5.1(±0.8) above the Debye temperature. The experimental results provide direct measurements of the pressure effect on the thermal expansivity and the temperature dependence of bulk modulus for the β phase.
Cenozoic volcanism in the California Coast Ranges: Numerical solutions
Authors: Liu Mian , Furlong Kevin P.
Published: February 2008

We present the results of numerical simulations of Cenozoic volcanism in the Coast Ranges of California, associated with the northward migration of the Mendocino Triple Junction (MTJ). Three aspects of the thermal evolution and magmatism in the wake of the MTJ are investigated: (1) mantle flow and thermal perturbation in the “slabless window”; (2) pressure‐release partial melting in the upwelling asthenosphere; and (3) thermally induced crustal anatexis. Numerical results show that upwelling of asthenosphere in the “slabless window” causes a drastic change in the thermal structure of the lithosphere and results in significant pressure‐release partial melting in the upwelling asthenosphere. Up to 4–5 km of basaltic magma may be produced. Most of this mafic magma is likely stored near the base of the crust. Thermally induced crustal anatexis mainly occurs in the deep crust (below 20 km), where a high degree (>50%) of partial melting may be predicted. Most of the crustal melt is probably generated within 1 m.y. after the passage of the MTJ. This work also suggests that the spatial and temporal distribution of Cenozoic volcanism in the California Coastal Ranges is directly related to the variation of the relative velocity between the North America and Pacific plates in the last 30 m.y. The voluminous volcanic rocks in the northern California Coast Ranges correlate to the relatively high velocity between these two plates in the last 10 m.y. The reduced volcanism in the central Coast Ranges may be related to the slow migration of the MTJ between 10 and 20 Ma.
A mathematical model of push‐up block formation: An example from the Big Bend Region, Texas
Authors: Erdlac Richard J. , Erdlac Richard J.
Published: February 2008

Laramide age development of the Terlingua‐Solitario push‐up structure was investigated by field mapping, experimental study, and theoretical analysis. Field mapping suggested a four‐stage development involving Laramide age compression followed by Basin and Range age tension. The push‐up structure is composed of Cretaceous limestones, shales, and marls. Thick competent units of limestone (rigid plates) are imbedded within thick formations of thinner bedded incompetent shale and marl. Deformation occurred when the tectonic load rose to sufficient magnitude to overcome the strength of the competent units. The Santa Elena Limestone was investigated experimentally to determine its burial depth and mechanical properties. The critical buckling stress and the state of stress were then determined within the Santa Elena, using field mapping to establish “boundary conditions” for the theoretical work. Plate bending theory, linear elasticity, and fracture mechanics were used to describe incipient deformation of the push‐up structure. The critical stress required to initiate cylindrical bending was calculated at 7.9 MPa (79 bars) and reflects the regional Laramide tectonic stress acting on the outer edge of the Santa Elena plate. The state of stress within the plate was determined by linear elasticity theory combined with Laramide tectonic strain measurements. Compressive principal stresses of 209.2 MPa (2092 bars), 40.8 MPa (408 bars), and 18.6 MPa (186 bars) were calculated and are much higher in magnitude than the critical stress. However, near preexisting flaws (i.e., pore space, microcracks), tensile stresses of several hundred bars are required by fracture mechanics. This local tensile loading caused small cracks to propagate and eventually to coalesce to form joints and fractures, many of which later developed into strike‐slip and dip‐slip faults parallel tp Laramide σ1.
Detection of changes in ridge‐crest morphology using repeated multibeam sonar surveys
Authors: Fox Christopher G. , Chadwick William W. , Embley Robert W.
Published: February 2008

A quantitative analysis method is developed to objectively identify significant bathymetric changes between multibeam sonar surveys of the same seafloor area. The technique involves mathematical gridding of bathymetric surveys from different years, co‐registration of grids based on prominent features covered in both surveys, calculation of slope‐weighted depth difference grids, and determination of the boundaries of statistically significant differences. The resulting difference grids are used in interpreting geological processes and calculating the volume of new material added to the seafloor. The method is applied to data from the southern Juan de Fuca Ridge, where a seafloor volcanic eruption of 0.05 km3 is inferred to have occurred between 1983 and 1987. Camera‐tow data and submersible observations provide ground truth which confirms the validity of the quantitative analysis method, and indicates empirically that the detection limit of the method allows new seafloor landforms measuring 1 × 105 m2 in area (or about 200–300 m in diameter) and 5–15 m in thickness to be detected through repeated Sea Beam bathymetric surveys.
A lithium isotope study of hot springs and metabasalts from Mid‐Ocean Ridge Hydrothermal Systems
Authors: Chan Lui‐Heung , Edmond John M. , Thompson Geoffrey
Published: February 2008

The Li isotopic compositions of seawater and of fresh and altered basalts are distinct and therefore applicable to the study of the hydrothermal processes in the oceanic crust and the Li balance in the ocean. High‐temperature fluids from seven vents on the East Pacific Rise (EPR) at 21°N and 11°–13°N have δ6Li values ranging between −6 and −11‰, i.e., 3–7‰ heavier than fresh basalt values. The intervent variations in Li concentration and isotopic composition correlate well with water/rock ratios. No temporal change in the isotopic value was observed between 1981 and 1985. Metabasalts show both Li depletion and enrichment relative to fresh basalt. They display light Li isotopic values which reflect incorporation of Li mobilized from fresh basalt. From these observations we conclude that the Li isotopic composition of submarine hot springs is controlled by a path‐dependent process involving dissolution of primary minerals and precipitation of Li with alteration phases. δ6Li values of fluids from the Mid‐Atlantic Ridge (−6 to −8‰) fall in the range of the EPR, indicating similar reaction controls at the two ridge systems. The lack of 7Li enrichment in the fluids from slower spreading ridges indicates that Li is not recycled from older weathered crust. Thus the difference between the 3He and 87Sr/ 86Sr based hydrothermal flux and the crustal transfer rate of Li cannot be reconciled by the inclusion of secondary Li from older crust.
On the partitioning of heat flux between diffuse and point source seafloor venting
Authors: Schultz A. , Delaney J. R. , McDuff R. E.
Published: February 2008

The first direct time series measurements of diffuse hydrothermal percolation velocities and temperatures have been obtained from a warm effluent source within a large vent field at the Endeavour Segment of the Juan de Fuca Ridge, 47°57′N, 129°06′W. A newly developed instrument using principles of electromagnetic induction to measure variations in small scale flow rates, and an integral array of thermocouples and thermistors, was emplaced directly atop a sulfide edifice from which vigorous diffuse hydrothermal percolation was observed. Samples of effluent temperatures and velocities were collected continuously for the period July 19 through September 2,1988. Temperatures were found to be modulated by variations at semidiurnal tidal periods. The coherence between temperature and velocity signals was found to be strong at periods longer than 12 hours with a strong negative correlation between the two signals and no significant coherence at shorter periods. Effluent temperatures were found to vary from about 7°C to 13°C, and vertical effluent velocities were found to vary between 0.07 and 0.15 m s−1. Examination of transient events in the temperature and velocity records indicates that short time scale variations in effluent discharge rates appear to lead corresponding pulses in fluid temperature. A general synchronicity between effluent temperature variations and an energetic episode of local seismicity is indicated. Finally, the heat flux density from the top of the sulfide body due to this diffuse mode of effluent discharge was found to vary between about 2 and 4 MW m−2, with an arithmetic mean value of 2.91 (±0.23) MW m−2. Studies of point source “smoker” venting in this vent field, as well as extensive geological mapping of diffuse and point hydrothermal sources leads us to infer that heat flow out of the system due to diffuse mode flow may exceed that due to high‐temperature venting by a factor of 5.
Partitioning of strike‐slip and convergent motion between Eurasia and Arabia in eastern Turkey and the Caucasus
Authors: Jackson James
Published: February 2008

The plate motion model NUVEL‐1 (De Mets et al., 1990) predicts a strongly oblique component to the convergence between Arabia and Eurasia at the longitude of the Caucasus. Earthquake focal mechanisms suggest that this oblique convergence is partitioned into pure right‐lateral strike‐slip motion in eastern Turkey and pure shortening by thrusting in the Greater Caucasus. Such partitioning offers an explanation for the apparent eastward continuation of the right‐lateral faulting of the North Anatolian fault from Turkey into Iran. From a summation of seismic moment tensors between 1911 and 1991 it appears that most of the right‐lateral strike‐slip motion in eastern Turkey is accommodated by faulting in large earthquakes, while most of the shortening in the Caucasus appears to occur aseismically.
On the stability of isostatically compensated mountain belts
Authors: Zhou Shaohua , Sandiford Michael
Published: February 2008

The formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence. The evolution of potential energy stored in an orogen is dependent on (1) the density structure, (2) the thermal evolution, and (3) the way convergent deformation is partitioned between crust and mantle lithosphere. It is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension, or collapse, may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened (e.g., England, 1987). In this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the “Brace‐Goetze” model for the rheology of the lithosphere. The reference lithosphere, defined to be in potential energy and isostatic balance with the mid‐ocean ridges, changes with different thermal parameters of the lithosphere. The instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts (measured relative to the reference state) with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range. It is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates (greater than a few times 10−16 s−1) and leading to significant finite extension providing the Moho temperatures exceed about 650–700°C, a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust. Consequently, processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse. The calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening (i.e., fl ≤ fc/2, where fl and fc are the lithospheric and crustal thickening factors).
Differential shear wave attenuation and its lateral variation in the North Atlantic Region
Authors: Sheehan Anne F. , Solomon Sean C.
Published: February 2008

Because the attenuation of seismic waves is sensitive to variations in temperature and to partial melting, mapping seismic Q in the upper mantle is an important tool for assessing the processes responsible for lateral heterogeneity. Utilizing a digital data base of over 150 seismograms and a spectral ratio technique, we have measured SS‐S differential attenuation in the North Atlantic region. Differential attenuation is positively correlated with SS‐S travel time residual, and both differential attenuation and travel time residual decrease with increasing seafloor age. In an earlier paper we showed that the age dependence of SS‐S travel time residual can be explaned entirely by the cooling of the oceanic lithosphère, i.e., contributions from the asthenosphere or from a mantle melt fraction are not required. The assumption that plate cooling also dominates the variation of differential attenuation with age permits the derivation of an empirical relation between Q−1 and temperature for the oceanic thermal lithosphere, i.e., the depth interval 0–125 km. However, both the absolute values and the depth distribution of Q we obtain under this assumption are at variance with the results of surface wave attenuation studies, and the variation of Q−1 with temperature that we derive is not as strongly dependent on temperature as indicated by some laboratory measurements. We therefore develop models for Q in which lateral variations include contributions from the asthenospheric low‐Q zone as well as from lithospheric cooling. The Q models obtained under this alternative assumption are in good agreement with those obtained from surface wave studies and are therefore preferred over those models with lateral variations confined to the upper 125 km. Systematic long‐wavelength (1000–7000 km) variations in differential attenuation, corrected for seafloor age, are evident along the axis of the Mid‐Atlantic Ridge. These variations can be qualitatively correlated with long‐wavelength variations in SS‐S differential travel time residuals and are attributed to along‐axis differences in upper mantle temperature.
Oxygen diffusion in a fine‐grained quartz aggregate with wetted and nonwetted microstructures
Authors: Farver John R. , Yund Richard A.
Published: February 2008

Oxygen grain boundary and bulk diffusion rates have been measured in a 1.2‐μm grain diameter quartz aggregate (Arkansas novaculite) preannealed in wetting and nonwetting fluids to determine the effect of microstructure. Samples were preannealed at 800°C and 150 MPa confining pressure for 6 to 12 days with pure water, pure CO2, or water + NaCl (∼6M) to produce equilibrium microstructures. The samples were then annealed at 450° to 800°C and 100 MPa confining pressure with 98%18O‐enriched water to determine the diffusion rates. Profiles of18O/(18O+16O) with depth from the surface were measured using an ion microprobe, and the product of the grain boundary diffusion coefficient (D′) and effective boundary width (δ) or the bulk diffusion coefficient (Dbulk) determined. Compared to the starting material, the D′δ values for samples texturally equilibrated with pure CO2(nonwetted microstructure) and pure water are about the same and about 4 times greater, respectively. Bulk diffusivities in samples preannealed with 6MNaCl (wetted microstructure) are 4 to 5 orders of magnitude greater than the bulk diffusivities in samples preannealed with pure CO2(nonwetted). In addition, the D'δ value for a sample texturally equilibrated first with 6MNaCl and then with pure CO2is about a factor of 3 greater than the value for the starting material, demonstrating that the textural equilibration process is essentially reversible. The results of this study indicate that transport rates are strongly influenced by the wetting characteristics of coexisting fluids in a polycrystalline aggregate. In nonwetted samples, changes in the effective grain boundary width or the formation of small isolated pores at triple junctions can change D′δ values. However, these effects are minor compared to the change in transport rate for samples with nonwetted versus wetted microstructures. The much faster rates for samples with a wetted microstructure are believed to be due to oxygen transport along the interconnected channels by ionic diffusion in a static fluid. The bulk diffusivities obtained for samples with a wetted microstructure are consistent with previously determined values for oxygen exchange at an igneous intrusive contact.
Deformations in a layered crust due to a system of cracks: Modeling the effect of dike injections or dilatancy
Authors: Roth Frank
Published: February 2008

The elasticity theory of dislocations is used for sources with Burgers' vector normal to the dislocation surface. It leads to a source model for an opening crack consisting of a center of dilatation and a double force without moment. The work on deformations around a tension crack in a homogeneous half‐space is combined with the results for subsurface deformations in a layered half‐space. Thus, a method is obtained to calculate the displacement and stress throughout a medium accounting for vertical changes in its elastic parameters. By superposition of the elastic fields of point sources, an extended crack or a system of cracks can be simulated. The results agree very well with those for inclined crack point sources in a homogeneous half‐space. Surface displacements, subsurface displacement and tilt fields obtained for realistic situations are measurable by existing methods. The influence of layering is demonstrated in comparison with homogeneous models and the results of a model consisting of several point cracks are given. The method can be applied to the forward modeling of crustal deformation caused by dike injections in volcanic areas, by mid‐ocean ridges, and by continental rifts. Thus, the so‐called magma chamber of a volcano can be modeled as a system of cracks instead of a cavity. Moreover, the deformational effects of dilatancy in rock and the opening of cracks during hydrofracturing can be simulated.
Melting experiments on the enstatite‐pyrope join at 80–152 kbar
Authors: Gasparik Tibor
Published: February 2008

Melting temperatures of the assemblage pyroxene + garnet on the enstatite‐pyrope join were experimentally determined at 11 different pressures between 80 and 152 kbar with a split‐sphere anvil apparatus (USSA‐2000). The compositions of pyroxene, garnet, and melt coexisting along the solidus were also determined. Comparison with previous data at lower temperatures revealed a large temperature dependence of the compositions of garnet coexisting with pyroxene. It is proposed that the observed variation in the garnet composition is caused by disorder in garnet. The obtained alumina contents of orthopyroxene coexisting with garnet on the solidus at 80–110 kbar contributed to an improvement of the garnet peridotite thermobarometry. An internally consistent set of thermodynamic parameters was derived, which allows calculation of the temperature‐pressure phase diagram for the enstatite‐pyrope join at 20–260 kbar.
The effects of temperature‐ and pressure‐dependent viscosity on three‐dimensional passive flow of the mantle beneath a ridge‐transform System
Authors: Shen Yang , Forsyth Donald W.
Published: February 2008

To estimate the effects of temperature‐ and pressure‐dependent viscosity on passive flow beneath midocean spreading centers, we have developed a numerical approach combining a finite element method to solve for the three‐dimensional flow and a finite difference solution for temperatures. This numerical method is applied to study an idealized spreading center consisting of a 100‐km transform fault offsetting two ridge segments. Viscosities are given according to the temperature and pressure dependence of thermally activated creep for Newtonian and non‐Newtonian rheology. Melt production rates are calculated by using an adiabatic melt relation and our flow and temperature fields. The density variation of mantle due to thermal expansion is converted into gravity anomaly. The results are compared with those of a model with uniform viscosity. The thickening of the high‐viscosity layer causes mantle upwelling in the variable‐viscosity model to be stronger but narrower. Faster upwelling reduces heat loss to the surface during ascent, leading to greater melt production. Under the same boundary conditions, the variable‐viscosity model creates thicker crust and the predicted thickness is more nearly independent of spreading rate. The zone of melt production is narrower and the rate of melt production decreases more abruptly near the transform fault. The gravity anomaly caused by the mantle thermal structure for the variable‐viscosity model is similar to that of the constant‐viscosity model with a maximum difference of a few milligal.
The steady part of the secular variation of the Earth's magnetic field
Authors: Bloxham Jeremy
Published: February 2008

The secular variation of the Earth's magnetic field results from the effects of magnetic induction in the fluid outer core and from the effects of magnetic diffusion in the core and the mantle. Adequate observations to map the magnetic field at the core‐mantle boundary extend back over three centuries, providing a model of the secular variation at the core‐mantle boundary. Here we consider how best to analyze this time‐dependent part of the field. We propose that the first step should be to seek the steady core flow that best fits the field maps, isolating what we call the steady secular variation, the part of the secular variation explained by steady core flow. To calculate steady core flow over long time periods, we introduce an adaptation of our earlier method of calculating the flow in order to achieve greater numerical stability. We perform this procedure for the periods 1840–1990 and 1690–1840 and find that well over 90% of the variance of the time‐depedent field can be explained by simple steady core flow. The core flows obtained for the two intervals are broadly similar to each other and to flows determined over much shorter recent intervals. Although we can explain a large part of the signal with these flows, none of them provides an adequate explanation of the secular variation. In particular, the fit to the original observations from which the field models were derived is inadequate. We examine the residual secular variation, the part of the signal that remains. We argue that it is dominated by the effects of nonsteady flow and magnetic diffusion. We do not find any compelling evidence for torsional oscillations as the dominant ingredient of this nonsteady part of the secular variation.
Chemosynthetic microbial activity at Mid‐Atlantic Ridge hydrothermal vent sites
Authors: Wirsen Carl O. , Jannasch Holger W. , Molyneaux Stephen J.
Published: February 2008

Chemosynthetic production of microbial biomass, determined by 14CO2 fixation and enzymatic (RuBisCo) activity, at the Mid‐Atlantic Ridge (MAR) 23° and 26°N vent sites was found in various niches: warm water emissions, loosely rock‐attached flocculent material, dense morphologically diverse bacterial mats covering the surfaces of polymetal sulfide deposits, and filamentous microbes on the carapaces of shrimp (Rimicaris exoculata). The bacterial mats on polymetal sulfide surfaces contained unicellular and filamentous bacteria which appeared to use as their chemolithotrophic electron or energy source either dissolved reduced minerals from vent emissions, mainly sulfur compounds, or solid metal sulfide deposits, mainly pyrite. Moderately thermophilic Chemosynthetic activity was observed in carbon dioxide fixation experiments and in enrichments, but no thermophilic aerobic sulfur oxidizers could be isolated. Both obligate and facultative chemoautotrophs growing at mesophilic temperatures were isolated from all chemosynthetically active surface scrapings. The obligate autotrophs could oxidize sterilized MAR natural sulfide deposits as well as technical pyrite at near neutral pH, in addition to dissolved reduced sulfur compounds. While the grazing by shrimp on the surface mats of MAR metal sulfide deposits was observed and deemed important, the animals’ primary occurrence in dense swarms near vent emissions suggests that they were feeding at these sites, where conditions for Chemosynthetic growth of their filamentous microbial epiflora were optimal. The data show that the transformation of geothermal energy at the massive polymetal sulfide deposits of the MAR is based on the lithoautotrophic oxidation of soluble sulfides and pyrites into microbial biomass.
Mechanical erosion and reheating of the lithosphere: A numerical model for hotspot swells
Authors: Monnereau Marc , Rabinowicz Michel , Arquis Eric
Published: February 2008

It is currently debated if either thermal erosion of the lithosphere or dynamical support is the source of topography and geoid anomalies. The origin of tins controversy lies probably in the difficulty to model simultaneously these two effects. For this purpose we have studied the time dependent behavior of two‐dimensional convection with a temperature and pressure dependent viscosity. The use of a control volume method allows us to define a rigid zone simulating the mechanical lithosphere. The interface between the lithosphere and the convective mantle is determined by a viscosity cutoff. First, some experiments model the rise of a plume below the lithosphere in order to observe the evolution of the uplift and thus to appreciate the various processes involved in the swell formation. Before the plume reaches the base of the thermal lithosphere, an uplift a few hundred meters in amplitude develops which can only be ascribed to a pure dynamical support. The major uplift occurs when the ductile part of the lithosphere, the convective boundary layer, is squeezed by the plume. The reheating of the mechanical lithosphere takes place after this transient stage of dynamical erosion. However, this late process is very slow but can magnify the amplitude of the swell if the lithosphere stays long enough above the plume. These results shed some light on the different mechanisms occurring during the swell formation, but the configuration modeled does not correspond to the one expected for actual hotspot swells. They feature plume rising up to the lithosphere while natural situations correspond to lithosphere drifting above preexisting plumes. An experiment with a moving lithosphere was run and shows that thermal erosion does not affect significantly a moving lithosphere even for relatively slow drifting velocities (few centimeters/year). Indeed, the thermal structure of the lithosphere is not modified above the 800°C isotherm except for a motionless plate. In this case the resulting swell should be greater: this could explain why Azores, Crozet or Cap Verde swells are so high. On the other hand, the shape of a swell over a moving lithosphere is strikingly reminiscent of the Hawaiian swell.
Atmospheric dust dispersal analyzed by granulometry of the Misers Gold Event
Authors: Wohletz K. H. , Raymond R.
Published: February 2008

Granulometric analysis of ejecta from Misers Gold high explosive cratering experiment demonstrates that atmospheric dust dispersal can be evaluated by particle‐size distribution data. From size analyses of the Misers Gold preshot test bed alluvium, ejecta, and sweep‐up materials collected out to 35 crater radii (1227 m), we find that approximately 5.9 × 106 kg (∼11% of the total crater ejecta mass) was depleted from the crater ejecta deposits and likely represents the portion of the cratered mass initially lofted into atmospheric suspension. The dominant size range of this lofted dust was 88 to 2000 μm with a mean diameter by mass of 384 μm,. In addition to the dust lofted in the explosion column, dust in the size range of 100–800 μm was swept up from the ground by the explosive air blast and base surge dominantly between ranges of 10 and 18 crater radii (360 and 650 m from ground zero). This sweep‐up dust was convectively drawn into the column and contributed up to 2% of the total mass of lofted. Based on the measured abundance of coarse (>250 μm diameter) dust particles in the estimated lofted dust, it is likely that about 70% of this lofted mass fell out within about 1 hour, such that the remaining dust cloud mass was ∼1.8 × 106 kg, which is equivalent to a dust lofted per unit blast yield of 0.5 kT/kT or about 4% of the crater ejecta mass. This study supports the hypothesis that if initial distributions can be constrained, the volume of dust lofted into atmospheric suspension from large surface explosions can be estimated from analysis of particle‐size distributions of ejecta deposited near the explosion. This result may have particular applications to study of the atmospheric effects of historic and prehistoric volcanic eruptions.
A similarity model of incremental fracture growth in submarine hydrothermal systems
Authors: Rovetta Mark R.
Published: February 2008

Knowledge of the fluid permeability within all parts of submarine hydrothermal systems is needed to understand hydrothermal circulation in ocean crust and the heat budget of mid‐ocean ridges. If hydrothermal circulation extends into the gabbro layer of young ocean crust, and possibly to within meters of a crustal magma chamber, then the fluid permeability close to the interface between the magmatic and hydrothermal parts of the ridge system may originate by the growth of fractures induced by hydrothermal cooling. Similar incremental growth mechanisms for thermal contraction fractures located at the edges of lava flows, dikes, and crustal plutons are suggested by the occurrence of discrete microearthquakes during the solidification of basalt lava lakes and the presence of “joint striations” on the surfaces of some columnar joints in basalt. This similarity of fracture mechanisms approaching magma‐solid interfaces may provide a means of estimating the spacing of thermal contraction fractures in submarine hydrothermal systems. Microearthquakes originating beneath the active hydrothermal vents at 21°N East Pacific Rise have seismic moments of 1017 dyn cm and frequencies of 0.1–1.0 d−1. This scale of microearthquake activity is inconsistent with model seismic sources consisting of incrementally growing fractures spaced less than several meters apart and suggests that fluid permeability near the magma chamber boundary is 10 −18 m2 or larger. The lower bound on fracture spacing and fluid permeability computed from the incremental fracture model is independent of any fluid transport model but is consistent with laboratory measurements of permeability and with the assumed permeabilities of recent numerical models of hydrothermal circulation.
The natural thermoluminescence of meteorites: 5. Ordinary chondrites at the Allan Hills Ice Fields
Authors: Benoit Paul H. , Sears Hazel , Sears Derek W. G.
Published: February 2008

Natural thermoluminescence (TL) data have been obtained for 167 ordinary chondrites from the ice fields in the vicinity of the Allan Hills in Victoria Land, Antarctica, in order to investigate their thermal and radiation history, pairing, terrestrial age, and concentration mechanisms. Using fairly conservative criteria (including natural and induced TL, find location, and petrographic data), the 167 meteorite fragments are thought to represent a maximum of 129 separate meteorites. Natural TL values for meteorites from the Main ice field are fairly low (typically 5–30 krad, indicative of terrestrial ages of ∼400 ka), while the Farwestern field shows a spread with many values 30–80 krad, suggestive of < 150‐ka terrestrial ages. There appear to be trends in TL levels within individual ice fields which are suggestive of directions of ice movement at these sites during the period of meteorite concentration. These directions seem to be confirmed by the orientations of elongation preserved in meteorite pairing groups. The proportion of meteorites with very low natural TL levels (<5 krad) at each field is comparable to that observed at the Lewis Cliff site and for modern non‐Antarctic falls and is also similar to the fraction of small perihelia (<0.85 AU) orbits calculated from fireball and fall observations. Induced TL data for meteorites from the Allan Hills confirm trends observed for meteorites collected during the 1977/1978 and 1978/1979 field seasons which show that a select group of H chondrites from the Antarctic experienced a different extraterrestrial thermal history to that of non‐Antarctic H chondrites,