bluuebunny/crossref_metadata_2025 huggingface dataset:
Cusp/cleft region as observed by the Viking UV imager
Authors: Garbe G. P. , Murphree J. S. , Cogger L. L. , Woch J.
Published: February 2008

The cusp/cleft region is composed of direct entry magnetosheath and magnetospheric boundary layer plasma. Much effort has recently been put forth in distinguishing the individual particle signatures of these regions. This report uses the Viking UV imager to show variable optical signatures of the cusp/cleft region. The region has been classified into four categories via their particle signatures: the cusp proper, cusp, cleft, and plasma mantle. Time series plots of intensity profiles along the Viking footprint are used to show the evolution of the cusp/cleft region during different auroral morphologies. These profiles establish that the observed emissions are located at the footprint of the cleft region. In addition, the region defined as the cusp was found poleward of this continuous emission. These data are also compared to recent particle studies of the cusp/cleft region and to appropriate topological models.
FUV‐visible photometric imaging of aurorae
Authors: Tobiska W. K. , Gladstone G. R. , Chakrabarti S. , Shepherd M. G. , McConnell J. C. , Link R. , Schmidtke G. , Stasek G.
Published: February 2008

Photometric images of 130.4‐, 337.1‐, 391.4‐, and 557.7‐nm auroral airglow emissions are presented for November 16, 1980, and December 9, 1981. Unique observations and new data reduction techniques allow 360° imaging of the local auroral emissions on both days and a view of spatial and temporal variability. The 1980 aurorae, one nonpulsating and one pulsating, are located in the morning sector. We find that the 1980 diffuse, nonpulsating aurora was located < 75 km from the rocket in a northern location with respect to the rocket and was produced by electrons with a characteristic energy of 200‐300 eV. The pulsating aurora was located > 75 km in a western location and was produced by electrons with a characteristic energy of 2‐3 keV. Both the near (northern) and far (western) aurorae had motion and size characteristics consistent with northeasterly drift rates of 200‐500 m s−1 and 30‐50 km in size, respectively. The 1981 flight observed a stable arc in the evening sector. The arc was produced by electrons with a characteristic energy of 1‐3 keV and had a probable 1/e width of approximately 75 km. For both flights, the 557.7/391.4 and 337.1/391.4 ratios were approximately constant.
Derivation of the ion temperature partition coefficient β∥ from the study of ion frictional heating events
Authors: McCrea I. W. , Lester M. , Robinson T. R. , St.‐Maurice J. ‐P. , Wade N. M. , Jones T. B.
Published: February 2008

This paper reports the results of a study evaluating the ion temperature partition coefficient β∥ over a range of F region altitudes. The data have been selected from three EISCAT CP‐0 experiments, each of which displayed clear evidence of ion frictional heating. The data were averaged at a time resolution of 30 s. These observations, made in an observing direction parallel to the Earth's magnetic field, have the advantage that the line‐of‐sight ion thermal velocity distribution can be closely approximated to a Maxwellian, while still giving a line‐of‐sight ion temperature which can be interpreted on the basis of a well‐known energy balance equation. The technique for determining β∥ depends upon fitting the variation of the field‐parallel ion temperature to simplified forms of this balance equation. The method is an extension of the curve‐fitting approach previously used by Glatthor and Hernández (1990) to deduce both ion temperature partition coefficients at a single F region altitude. The results of this procedure are compared to the theoretical predictions for β∥ obtained from an equation originally due to St.‐Maurice and Hanson (1982). By introducing measured and modeled parameters, it is found that values of β∥ close to those expected for resonant charge exchange collisions are predicted around the F region peak. At greater heights, however, the increased influence of Coulomb collisions is predicted to give rise to an increase in β∥ with altitude, corresponding to ion thermal velocity distributions which tend toward isotropy. This height dependence, which has been theoretically predicted in some other recent studies, would be an important factor in calculations of the energy balance in the upper F region. While the experimentally derived values of β∥ are close to the RCE predictions near the F region peak, the expected increase in β∥ with altitude is seen in only one of the three selected events. In the remaining cases, the increases are notably smaller. Possible reasons for this discrepancy are discussed. Changes in ion and neutral composition, together with effects such as heat conduction and thermal diffusion can act to bias the curve‐fitting technique. The degree of coupling between the ionized and neutral atmospheres is also an important factor, and there is considerable uncertainty in some of the terms used in the theoretical part of the study. Nonetheless, the results suggest that Coulomb collisions play a potentially important role in determining the energy balance of the upper F region. A clear need exists for further studies in this area to establish more fully the contribution of Coulomb processes.
Mirror instability: 1. Physical mechanism of linear instability
Authors: Southwood David J. , Kivelson Margaret G.
Published: February 2008

The mirror instability is prevalent in planetary and cometary magnetosheaths and other high beta environments. We review the physics of the linear instability. Although the instability was originally derived from magnetohydrodynamic fluid theory, later work showed that there were significant differences between the fluid theory and a more rigorous kinetic approach. Here we point out that the instability mechanism hinges on the special behavior of particles with small velocity along the field. We call such particles resonant particles by analogy with other uses of the term, but there are significant differences between the behavior of the resonant particles in this instability and in other instabilities driven by resonant particles. We comment on the implications of these results for our understanding of the observations of mirror instability‐generated signals in space.
Mid‐Pleistocene basalt from the Seguam Volcanic Center, central Aleutian Arc, Alaska: Local lithospheric structures and source variability in the Aleutian Arc
Authors: Singer Bradley S. , Myers James D. , Frost Carol D.
Published: February 2008

Recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern Aleutian Arc and the sources and processes that produce the broad spectrum of Aleutian basaltic lavas. Mid‐Pleistocene basalt from the Seguam Island volcanic center in the central Aleutian Arc erupted through strongly extended arc crust, contains 4–8% MgO, is low in K2O (<0.5%), high in Al2O3 (>18%), and is distinguished from Aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements (REE) and low Sr and Pb content. The Ba/La (47–72) and Pb/La (0.7–1.5) ratios are high, incompatible element ratios are variable, and REE patterns cross one another. The 87Sr/86Sr ratio (0.70365 ± 0.00003) is the highest value obtained from the Aleutian islands, also 206Pb/204Pb (18.91 ±0.02), 207Pb/204Pb (15.60 ±0.02), and 208Pb/204Pb (38.51 ±0.04) ratios are among the most radiogenic values known, whereas the 143Nd/144Nd ratio (0.51297 ±0.00003) is among the lowest measured. Low abundances of incompatible elements, high Ba/La and Pb/La ratios, uniform Sr, Nd, and Pb isotope ratios, and high eruptive temperatures (∼1160° C) suggest that basalt erupted at Seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust. Partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics. Trace element abundances rule out an eclogite source and suggest instead a peridotite‐dominated source with ∼5–10% of a subducted sediment component. This peridotite may reside in the mantle wedge or in the Amlia Fracture Zone (AFZ) of the Pacific plate that was subducted beneath Seguam ∼1 Ma. In addition, the AFZ possibly enhanced the subduction of terrigenous sediment into the source region beneath Seguam and this extra terrigenous component may explain the Ba/La, Pb/La and Sr, Nd, and Pb isotope ratios. The distinctive composition of Seguam basalt reflects a complex set of physical conditions, including fracture zone subduction and intraarc extension, that affected both the source region and thermo‐chemical history of magma ascent. Local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes. Models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the Aleutian Arc.
Petrology, elasticity, and composition of the mantle transition zone
Authors: Ita Joel , Stixrude Lars
Published: February 2008

We compare the predictions of compositional models of the mantle transition zone to observed seismic properties by constructing phase diagrams in the MgO‐FeO‐CaO‐Al2O3‐SiO2system and estimating the elasticity of the relevant minerals. Mie‐Grüneisen and Birch‐Murnaghan finite strain theory are combined with ideal solution theory to extrapolate experimental measurements of thermal and elastic properties to high pressures and temperatures. The resulting thermodynamic potentials are combined with the estimated phase diagrams to predict the density, seismic parameter, and mantle adiabats for a given compositional model. We find that the properties of pyrolite agree well with the observed density and bulk sound velocity of the upper mantle and transition zone. Piclogite significantly underestimates the magnitude of the 400‐km velocity discontinuity and overestimates the velocity gradient in the transition zone. Substantially enriching piclogite in Al provides an acceptable fit to the observations. Invoking a chemical boundary layer between the uppermost mantle and transition zone leads to poor agreement with observed seismic properties for the compositions considered. Within the transition zone, the dissolution of garnet to Ca‐perovskite near 18 GPa may explain the proposed 520‐km seismic discontinuity. Below 700 km depth, all compositions disagree with observed bulk sound velocities, implying that the lower mantle is chemically distinct from the upper mantle.
An application of synthetic seismicity in earthquake statistics: The Middle America Trench
Authors: Ward Steven N.
Published: February 2008

This paper demonstrates how synthetic seismicity calculations which are based on the concept of fault segmentation and incorporate the physics of faulting through static dislocation theory can improve earthquake recurrence statistics and hone the probabilities of hazard. Compared to forecasts constructed from a handful of earthquake recurrence intervals, forecasts constructed from synthetic seismicity are more robust in that they embody regional seismicity information over several units of magnitude, they can extrapolate seismicity to higher magnitudes than have actually been observed, and they are formulated from a catalog which can be extended as long as needed to be statistically significant. Synthetic seismicity models can also be used to judge the stability of common rate estimates and the appropriateness of idealizations to the earthquake cycle. I find that estimates of fault slip rate are unbiased regardless of sampling duration, while estimates of earthquake recurrence time are strongly biased. Recurrence intervals estimated from seismicity samples less than about 10 times the actual recurrence interval will almost certainly be too short. For the Middle America Trench (MAT), it would take 200 and 400 years of monitoring to constrain slip and recurrence rates to ±10%. Events M ≥ 6 have as much as a 60% probability of recurrence within 5 years due to the clustering of small earthquakes in foreshocks and aftershocks. This probability drops to less than 15% for M ≥ 7 events. Increasing gap time generally increases conditional probability of earthquake occurrence, but the effect is weak. For the MAT, the spread parameters of the best fitting lognormal or Weibull distributions (≈0.75) are much larger than the 0.21 intrinsic spread proposed in the Nishenko‐Buland hypothesis. Stress interaction between fault segments disrupts time or slip predictability and causes earthquake recurrence to be far more aperiodic than has been suggested.
Physical properties of ash flow tuff from Yucca Mountain, Nevada
Authors: Nelson Philip H. , Anderson Lennart A.
Published: February 2008

We have measured the density and determined the porosity of 198 samples of ash flow tuffs from three boreholes at Yucca Mountain, Nevada. The electrical properties, velocity, and permeability of many of these samples have also been determined. We use mineralogical and physical data from other sources to determine the dependence of measured physical properties upon petrology. Porosity in the samples varies over a wide range, from as low as 1% in the densely welded tuffs to 53% in the zeolitized nonwelded tuffs. Porosity is the primary control on the other measured physical properties that consequently vary over broad ranges. Alteration (zeolites and clays) is a primary control on grain density and a significant secondary control on bulk density, resistivity, velocity, and permeability. Sorting the samples into rock classes defined in terms of the degree of welding and gross mineralogy enables us to separate the influence of zeolites and clays from that of porosity. Empirical rock property relationships established for sandstones can be applied with good success to tuffs. Archie's law relating resistivity to porosity is found to fit the unaltered samples with an exponent m of 2.0. An empirical expression relating compressional velocity to porosity and clay content forms an excellent upper bound to the velocity‐porosity data. Permeability‐porosity plots are similar in form to those obtained for clastic rocks, although the permeability is considerably lower than in clastic rocks of similar porosity. Zeolites and clays reduce the grain density, increase the electrical conductivity, reduce the compressional velocity, and reduce the permeability.
The role of lithospheric mantle in continental flood volcanism: Thermal and geochemical constraints
Authors: Arndt Nicholas T. , Christensen Ulrich
Published: February 2008

Continental flood basalts (CFB) are commonly said to form by direct melting of metasomatized lithospheric mantle, either during major lithospheric extension or when a mantle plume impinges on the base of the lithosphere. We tested these ideas in a thermomechanical model that combines lithospheric dynamics and mantle convection. Dry melting was assumed, and the proportions of melt from different source regions were monitored. In all cases, >96% of melt was found to come from asthenosphere or plume, with minimal amounts from continental lithosphere. During passive lithosphere extension the total amount of melt is small, and the proportion from the lithosphere is <3%. During plume interaction and concurrent extension, magmas formed during the first half of a magmatic episode contain virtually no lithospheric melt, and during the second half the proportion reaches only ∼2%. The proportion of lithosphere heated above its solidus is greater but never exceeds 3–4% of the total source. These results were used to test the concept that the distinctive chemical signature of CFB is a result of melting of lithospheric mantle. Our calculations show that if CFB are hybrid magmas and the proportion of lithospheric melt is less than 10–30%, the composition required for the lithospheric source is unrealistic (e.g., εNd < −40; Nb/U negative). Crustal contamination of asthenospherederived magmas can explain many of the characteristics of CFB if the primary magmas were produced by melting in plumes at sublithosphere depths (>100 km) and have high concentrations of both MgO (>20%) and incompatible trace elements (K2O ∼ 1%). The very low Nb and Ta concentrations in certain CFB cannot, however, be explained by this process. Ratios of Nb to elements such as La or U are lower in many flood basalts and picrites than in all likely source materials: they are almost as low as in most rocks from the continental crust, and they are far lower than in peridotites from the lithospheric or asthenospheric mantle. Another process must therefore fractionate Nb and Ta. We suggest that this takes place during the passage of magma through the lithospheric mantle, perhaps because of differences in the reaction rates of minerals in metasomatized peridotite. The probability that CFB are hybrid magmas containing material from aesthenospheric and lithospheric mantle and in many cases from continental crust, as well as the possibility that some elemental ratios change during magma‐lithosphere interaction, casts serious doubt on the reliability of such rocks as probes of the lithospheric mantle.
Quantitative characterization of abyssal Hill morphology along flow lines in the Atlantic Ocean
Authors: Goff John A.
Published: February 2008

The morphology of the seafloor, when sampled along flow lines of relative plate motion, represents an indirect geologic time series record of the processes which form it. Eventually we hope to be able to interpret this information so that we may improve our understanding of the workings of ridge‐crest and offridge processes through time. This study takes an important step toward that goal by quantitatively characterizing, through stochastic modeling, the abyssal hill morphology along the available Sea Beam flow line data sets in the Atlantic. The six Sea Beam flow line data sets used in this study include two near the Kane Fracture Zone and four near the Rio Grande and Moore Fracture Zones. Stochastic parameters estimated from these flow lines, such as the mis height, lineament azimuth, characteristic widths, and planview aspect ratio, are plotted as a function of crustal age. In addition to testing available hypotheses, the purpose of this work is to investigate how the abyssal hill morphology changes with age and to correlate this behavior to known changes in ridge characteristics with age. These results place important constraints on our understanding of the process of seafloor creation, and begin to establish a baseline against which morphology may be interpreted in the absence of other information. Along the Kane Fracture Zone, variations in the estimated abyssal hill lineament azimuth, rms height, and plan‐view aspect ratio with crustal age appear to be correlated in various ways with the segmentation history and also possibly with large changes in plate motion direction that have occurred there within the past 80 m.y. Along the Rio Grande and Moore Fracture Zones, variations in the lineament azimuth, rms height and other parameters as a function of crustal age are often significant on a ∼5–10 m.y. time scale. These variations are difficult to interpret as yet. To emphasize longer time scale variations, the estimated rms heights from the South Atlantic flow lines are averaged over common age bins. This demonstrates a strong negative correlation of rms height with the spreading rate. However, when characteristic widths are averaged in this manner, no strong correlation with spreading rate is found. Large variations in this parameter occur on a ∼25 m.y. time scale.
Further paleomagnetic results from the Builth Wells Ordovician Inlier, Wales
Authors: McCabe Chad , Channell James E. T. , Woodcock Nigel H.
Published: February 2008

A characteristic magnetization has been isolated from 18 sites in Middle Ordovician igneous and sedimentary rocks from the Builth inlier, Wales. Two conglomerate tests demonstrate that the characteristic magnetization in two of the volcanic units was acquired early, perhaps during initial emplacement. Six sites from the northern and central parts of the inlier give a mean direction of D=153.8°, I=69.8° (k=45.8; α95=10°) that passes a regional fold test. However, 12 sites from the southern end of the inlier, in and near the Llanelwedd quarry, yield a significantly different direction after correction for local tilt: D=176.4°, I=44.5° (k=42.4; α95=6.7°). The Llanelwedd quarry data are anomalous with respect to both results from the northern parts of the inlier, and those obtained from approximately coeval rocks from the Shelve inlier. We have searched for an explanation for the discrepancy between the magnetization directions of the two areas and have now ruled out some possibilities, including superimposed magnetization components at one of the two areas. The Llanelwedd quarry lies within a wedge of dominantly andesitic volcanic rocks that appear only locally, and which are overlain by two unconformities that are also not present elsewhere in the inlier. One possible explanation for the anomalous nature of the Llanelwedd quarry magnetization is that the strata were not horizontal at the time of magnetization, either due to early volcano‐tectonic tilting or primary dips on the flanks of a volcano. Another possibility is that the Llanelwedd quarry rocks were extruded rapidly enough that the geocentric coaxial dipole field hypothesis cannot be applied. Results from Cenozoic volcanic rocks from the western United States suggest that even highly anomalous fields can be preserved virtually unchanged over a number of separate flow events. We currently regard this to be the most viable explanation for the anomalous nature of the Llanelwedd quarry magnetization. We conclude that the data from the 6 northern sites represent the best estimate of the Middle Ordovician axial dipole field for the inlier.
Paleomagnetic constraints on the geodynamic history of the major blocks of China from the Permian to the present
Authors: Enkin Randolph J. , Yang Zhenyu , Chen Yan , Courtillot Vincent
Published: February 2008

Paleomagnetic study of China and its environs has been the center of a major international effort for the last 10 years. In this paper, we critically review all available paleomagnetic poles of Upper Permian to Tertiary age from the main blocks of China with the goal of placing constraints on models of the formation and the subsequent deformation of the region. After selecting “reliable” poles by applying objective criteria, we divide our analysis into first‐order (motions of blocks) and second order (deformation within blocks). For first order analysis, apparent polar wander paths are constructed for the major blocks. We discuss the compatibilities and contradictions between the geological and paleomagnetic records. A sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented. In general, the major blocks were probably in contact throughout the Permian and Triassic, but the Jurassic was the key age during which most of the movement toward China's present configuration took place. Our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events. During the Cretaceous, Chinese poles agree with poles from other continents transferred onto Eurasia. At the second order, we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region, indicating that much of China has been affected by small (< 20°) differential rotations. This we interpret as deformation caused in part by the extrusion of the Chinese blocks away from the Indian collision. The complete annotated list of poles is given as an appendix.
Geometric improvement of the spot satellite orbit using Doris Doppler residual data
Authors: Fat B. Nhun , Biancale R. , Valette J.‐J. , Minster J.‐F.
Published: February 2008

After adjustment of a satellite orbit by classical dynamical methods, the residuals of tracking data from a Doppler satellite tracking network still contain information. They are used to further adjust geometrically the orbit, the station coordinates, and Doppler biases and drifts for each station. After extensive testing in simulation, the method is applied to 21 days of the DORIS data from the SPOT2 satellite. As this tracking network has an excellent coverage and a very low measurement noise, the orbit reproducibility in the tangential direction is reduced from 40–100 cm root‐mean‐squares (rms) with a classical dynamical orbit calculation to 7–32 cm rms for the geometrical orbit, based on the difference between 2‐day orbit arcs overlapping over 1 day. The reproducibility in the radial direction is slightly improved to 6–30 cm rms on the same arcs, with the most significant effect on the 1/revolution frequency. The formal error is also mostly reduced on this frequency. The Doppler residuals are reduced from 2–13 to 0.7–1.4 mm/s. This is only 3 times the measurement noise level and allows identification of ionospheric (magnetic storms), tropospheric (large atmospheric water vapor), and geometric effects (low elevation angles) in the data. Five‐day orbit arcs of this improved orbit are used to recalculate station positions: A reproducibility of 4–13 cm is achieved on short distances (50 km), which is better than what is obtained with 15‐day arcs of the initial dynamical orbit. Furthermore, for nine stations collocated with laser satellite tracking stations, the rms difference between their position calculated with 15 days of orbit and their independently estimated geodetic values is reduced from 56 cm rms with the dynamical orbit to 33 cm rms with the geometric orbit: This demonstrates that the improved reproducibility corresponds to an improved accuracy.
Electron microscopy of (Mg, Fe)SiO3 Perovskite: Evidence for structural phase transitions and implications for the lower mantle
Authors: Wang Yanbin , Guyot Francois , Liebermann Robert C.
Published: February 2008

A transmission electron microscopy study has been carried out on twin‐domain structure in (Mg, Fe)SiO3 perovskite and in five analogue perovskites: CaTiO3, CaGeO3, MnGeO3, LaGaO3, and SmAlO3. Three crystallographically distinct twin laws are found in all of these GdFeO3‐type (space group Pbnm) perovskites: reflection twins across the {112} and {110} planes, and 90° rotation twins about the [001] axis. Twin‐domain morphology of MgSiO3 perovskite is examined as a function of the temperatures from which the specimens were quenched in high‐pressure synthesis experiments. Crystals quenched from temperatures exceeding 1600°C contain significantly higher twin densities than those quenched from below 1300°C, suggesting that structural phase transitions may have taken place in high‐temperature MgSiO3 perovskite during quenching. On the basis of theoretically predicted twin laws and of the twin‐domain morphology observations on analog perovskites that undergo a variety of phase transitions, it appears that the most likely phase transitions in the silicate perovskite are (with decreasing temperature) cubic‐tetragonal‐orthorhombic. These results support our earlier suggestion that under the experimental synthesis conditions, and perhaps in certain regions of the Earth's lower mantle, the stable phase of MgSiO3 may have the cubic perovskite structure and that structural phase transitions may occur in the lower mantle.
P‐SV conversions at a shallow boundary beneath Campi Flegrei Caldera (Italy): Evidence for the magma chamber
Authors: Ferrucci F. , Hirn A. , De Natale G. , Virieux J. , Mirabile L.
Published: February 2008

Seismograms from an active seismic experiment carried out at Campi Flegrei caldera (near Naples, Italy) show a large‐amplitude 5V‐polarized shear wave, following by less than 1.5‐s P waves reflected at wide angle from a deep crustal interface. Early arriving SV‐polarized waves, with the same delay to direct P waves, are also observed in seismograms from a regional, 280‐km‐deep, magnitude 5.1 earthquake. Such short delays of S to P waves are consistent with a P‐SV conversion on transmission occurring at a shallow boundary beneath the receivers. The large amplitude of the converted‐SV phase, along with that the P waves are near vertical, requires a boundary separating a very low rigidity layer from the upper caldera fill. The converted phases are interpreted as a seismic marker of a magma chamber. The top of this magma chamber is located slightly deeper than the deepest earthquakes observed during the 1982–1984 unrest of Campi Flegrei.
Evidence of regional structural controls on vent distribution: Springerville Volcanic Field, Arizona
Authors: Connor Charles B. , Condit Christopher D. , Crumpler Larry S. , Aubele Jayne C.
Published: February 2008

Quantitative analysis of the geographic distribution of vents and comparison with regional structural, petrologic, and vent age data provide insight into the processes governing the emplacement of vents in the Springerville volcanic field, Arizona. A total of 409 vents in the Springerville volcanic field (SVF) have a mean distance to nearest neighbor vents of 955 m, a much closer spacing than is common in some platform‐type volcanic fields. Based upon a cluster analysis search radius parameter of 4500 m, these vents comprise seven geographic clusters, with only five outlying vents occurring in the entire field. Cinder cone clusters in the western portion of the field are significantly older than clusters in the eastern portion of the field (pvalue of <0.001), and there is a tendency for cluster age to decrease to the east. This is particularly evident when mean cluster ages are calculated for tholeiite, alkaline olivine basalt, and evolved alkaline rock types independently. Application of the two‐point azimuth and Hough transform methods demonstrates that regional cinder cone alignments transect these clusters. The most prominent of these alignments trend ENE in the eastern portion of the field and WNW in the western portion of the field, creating an overall arcuate pattern that is subparallel to the trend of the Mogollon Rim and the Colorado Plateau/Transition Zone boundary. These observations suggest that vents (and clusters) migrated from west to east in response to plate motion, but the general pattern of vent migration was complicated by regional structures, which enhanced the volume and duration of magmatism in some areas. The fractures or faults implied by vent alignments indicate thatShminis oriented radial to the Colorado Plateau in the SVF. Preferred vent alignment orientations may be related to extension resulting from plateau uplift, and to a much smaller degree from a minor Basin and Range imprint. While regional in extent, the implied structures appear to differ significantly from some of those in several other plateau‐marginal fields in that they cannot be related to major reactivated Precambrian structures. Our vent alignment data differ from those seen by other workers in the Zuni‐Bandera and Mount Taylor fields, suggesting the stress field for the SVF is different from other fields in the proposed Jemez lineament. The stress field implied by vent alignment data, combined with structural data, suggests that the southwestern tectonic boundary of the Colorado Plateau of Brumbaugh (1987) should be extended southeastward to include the SVF at the plateau's southern boundary.
Alteration of hydrovolcanic basaltic ash: Observations with visible and near‐infrared spectrometry
Authors: Farrand William H. , Singer Robert B.
Published: February 2008

Ash of basaltic composition that is produced by hydrovolcanic activity occurs frequently on Earth and may be much more widespread on Mars. Altered basaltic tephras from tuff rings and tuff cones across the Basin and Range were examined using visible and near infrared (Vis/IR) reflectance spectrometry and a variety of other techniques. It was found that Vis/IR Spectrometry is sensitive to subtle changes in the hydration and oxidation state of these tephras. Hydration produces an increase in overall reflectance with gradual development of water and hydroxyl absorptions at 1.4 μm and 1.9 μm. The oxidation of basaltic tephras can be observed spectrally through changes in the position and slope of the Fe3+‐ O2−charge transfer edge, inflections on that edge due to ferric oxide minerals and changes in the position and depth of the 1 μm iron crystal‐field band. Significantly in the tuffs examined, ferrous and ferric iron minerals produce one composite absorption feature rather than two resolvable bands. Different styles of alteration were noted between thinly bedded tuff ring deposits emplaced by high energy, largely dry, pyroclastic surge and massively bedded tuff cone deposits emplaced by wetter, lower energy pyroclastic flow. The former class of tephras become hydrated and only moderately oxidized with some limited development of dioctahedral smectite clay minerals and minimal palagonitization. The latter class of tephras are highly palagonitized which implies a high degree of hydration and oxidation. In fact, in the most highly altered tephras, all the Fe2+that is initially in the volcanic glass is converted to Fe3+within nanophase and bulk ferric oxide phases in the palagonite. There is also more extensive development in the tuff cone beds of zeolite and phyllosilicate minerals. The differences in reflectance spectra of altered basaltic tephras can be traced back to initial differences in the water/magma ratio extant at the volcanic vent; thus, Vis/IR spectrometry holds the potential to be used as a tool for investigations into the volcanic history and paleoclimate of volcanic terrains.
Compositional trends of minerals in oceanic cumulates
Authors: Elthon Don , Stewart Matthew , Ross D. Kent
Published: February 2008

Five differences are noted between the compositions of minerals in oceanic cumulates and minerals that crystallize from mid‐ocean ridge basalts (MORBs) at 1 atm. These differences are (1) Mg numbers of clinopyroxene (CPX) in oceanic cumulates are often higher (>88) than found in experimental studies of MORBs at 1 atm; (2) Mg numbers of orthopyroxene (OPX) in oceanic cumulates are often higher (>81) than found in experimental studies of MORBs at 1 atm; (3) the low‐Ca pyroxene in oceanic cumulates (with some notable exceptions) is OPX rather than (inverted) pigeonite; (4) Na2O contents of CPX in oceanic cumulates are often higher (0.3 to 0.8% Na2O) than in 1‐atm experiments or phenocrysts in MORBs (typically 0.15 to 0.35% Na2O); and (5) TiO2 contents of CPX in oceanic cumulates are often higher than in 1‐atm experiments. These characteristics are consistent with crystallization of many oceanic gabbros at moderate pressures (3–10 kbar). The Na2O and TiO2 contents of liquids calculated to be in equilibrium with CPXs from oceanic cumulates generally are comparable to the abundances of these oxides in MORBs. The notable exception is the suite of cumulates from Deep Sea Drilling Project site 334, which appears to have crystallized at low pressures from strongly depleted Na2O‐poor, TiO2‐poor, SiO2‐rich basalts. We suggest that such strongly depleted basaltic liquids are a widespread, but minor, component in mid‐ocean ridge petrogenesis even though such liquids have never been directly sampled.
Magnetic domain state and coercivity predictions for biogenic greigite (Fe3S4): A comparison of theory with magnetosome observations
Authors: Ricci Juan C. Diaz , Kirschvink Joseph L.
Published: February 2008

The discovery of bacteria that precipitate greigite within intracellular organelles (magnetosomes) offers new evidence about the origin of greigite in natural environments. Unlike magnetite, only scarce information is available about the magnetic characteristics of greigite. For this reason, and the present inability to grow these microorganisms in pure culture, it is not known whether or not the magnetosomes in the newly discovered greigite‐precipitating bacteria are of single‐domain (SD) size, as are the magnetosomes from magnetite‐precipitating bacteria. Hie hypothesis of natural selection for magnetotactic behavior predicts that the greigite‐bearing magnetosomes should also be single magnetic domains. Using previously reported magnetic properties and crystallographic features for greigite, we have calculated the size and shape boundaries expected for SD and superparamagnetic (SPM) behavior in this mineral. For further characterization of the greigite crystals, we analyzed the domain state at various length/width ratios assuming crystal shapes of parallelepipeds and prolate spheroids. Magnetite was used as control for the current theories supporting these calculations. We also present a simple algorithm to calculate the upper size limit of single‐domain grains. Our results show that the crystals of bacterial greigite characterized so far are located in the region close to the single‐domain superparamagnetic boundary and should have relatively low coercivity. If these crystals contribute to the magnetization of sediments, remanence produced by bacterial greigite could be mistaken for large, multidomain magnetite in alternating field demagnetization studies.
Rock magnetism of remagnetized Paleozoic carbonates: Low‐temperature behavior and susceptibility characteristics
Authors: Jackson Mike , Rochette Pierre , Fillion Gérard , Banerjee Subir , Marvin Jim
Published: February 2008

We have conducted a new set of rock magnetic experiments on samples of remagnetized Paleozoic carbonates of eastern North America. These experiments were designed to investigate the origin of the unusual hysteresis behavior of these rocks, by evaluating (1) the importance of ferrimagnetic pyrrhotite as a remanence carrier, and (2) the sources of low‐field susceptibility. Low‐temperature measurements of saturation isothermal remanent magnetization (SIRM) indicate that the pyrrhotite magnetic transition at 32 K is absent in the Trenton and Onondaga limestones of New York. This transition is observed but poorly expressed in the Knox Group of east Tennessee: room‐temperature SIRMs, cooled to 10 K in zero field, lose a small fraction of their intensity between 30 and 35 K. Samples from all three formations show a broad peak in IRM intensity at about 200 K, which is typical of pyrrhotite. In the Trenton and Onondaga samples, the ferrimagnetic component of low‐field susceptibility is significantly larger than the ratio Mrs/Hcr, and is thus probably due dominantly to magnetite; in some of the Knox samples the reverse is true, suggesting an important pyrrhotite contribution. For all of the samples, the ferrimagnetic susceptibility, normalized by the saturation magnetization, is anomalously high, about a factor of 5 or 10 higher than the typical value for magnetite. We believe that this indicates a very substantial contribution from superparamagnetic particles. Strong frequency dependence of susceptibility and very high ratios of anhysteretic to saturation remanence confirm the importance of ultrafine particles, spanning the superparamagnetic—single‐domain boundary. All three of these chemically remagnetized carbonates units exhibit the following properties, which have not previously been found together for any rock or synthetic analog, and which therefore appear to constitute a diagnostic set of rockmagnetic criteria for recognizing chemically‐remagnetized rocks: Mrs/Ms ≈ 0.89 (Hcr/Hc)−0.6; kƒ/Ms ≈ 50 μm/A; ARM/SIRM ≈ 20%; ka/(Mrs/Hcr) ≈ 50.
Volatile and trace element composition of melt inclusions from the Lower Bandelier Tuff: Implications for magma chamber processes and eruptive style
Authors: Dunbar Nelia W. , Hervig Richard L.
Published: February 2008

The Lower Bandelier Tuff, erupted from the Valles Caldera at 1.51 Ma, is composed of a Plinian tephra and associated ignimbrite. Based on ion and electron microprobe analyses of melt inclusions (MI) in quartz, sanidine, and pyroxene phenocrysts, a strong volatile gradient was present in the upper portion of the magma chamber. In the 20 km3 of magma that produced the Plinian tephra, most H2O and F contents of MI are between 4 to 5 wt % and 0.18 to 0.20 wt%, respectively, and between 3 to 4 wt% and 0.12 and 0.14 wt% in the first‐deposited basal ignimbrite. Measurements of the H2O and F contents of the 380 km3 of magma which produced the bulk of the ignimbrite are concentrated between 2 to 3 wt% and 0.05 to 0.14 wt% respectively. The invariance of H2O (and possibly Cl and B) relative to F in the magma which formed the Plinian tephra suggests that this portion of the magma chamber was saturated with respect to an H2O‐rich vapor phase. The trace element composition of MI are varied, and overlap with composition of pumice lumps. A group of MI are rich in Ti, Sr, Ba and low in B, Cl, Rb, Y, Nb, and Th, and are compositionally similar to analyses of matrix glass from some ignimbrite pumice lumps. These represent the mixing of a second rhyolite into the base of the Lower Bandelier magma chamber. MI influenced by the second magma have not been found in Plinian samples, but occur in ignimbrite samples from base to top of the deposit. When these MI are removed from the data set, the remainder show strong linear correlations between Nb and Rb, Y, Zr, and Th. These correlations can be most easily explained by fractionation of approximately ∼40% quartz and alkali feldspar (with trace amounts of chevkinite). MI from the Plinian tephra are similar to bulk Plinian pumice composition, suggesting that magmatic evolution was well‐progressed at the time that the MI were trapped. However, fractional crystallization is difficult to reconcile with the observed distribution of some other elements, including H, B, Li, F and Cl. The enrichment of H2O and F in the upper portion of the magma chamber, along with their lack of correlation with Nb, Rb, Zr, Y, and Th, suggest that these sets of elements were decoupled during magmatic evolution. The upward enrichment of H2O and F must have occurred at a faster rate than other trace elements. The observed volatile gradient may have influenced the eruption dynamics of the magma, initially causing discrete layers of magma to be removed when the gradient was large, followed by chaotic eruption of the bulk of the ignimbrite when the gradient was small. Comparison with the Bishop Tuff eruption, however, suggests that the chaotic eruption style of the LBT may be related to vent geometry and conduit evolution, promoting high discharge rates, not differences in the absolute concentration of volatiles.
Southwestern limits of Indian Ocean Ridge Mantle and the origin of low 206Pb/204Pb mid‐ocean ridge basalt: Isotope systematics of the central Southwest Indian Ridge (17°–50°E)
Authors: Mahoney J. , le Roex A. P. , Peng Z. , Fisher R. L. , Natland J. H.
Published: February 2008

Basalts from the Southwest Indian Ridge reflect a gradual, irregular isotopic transition in the MORB (mid‐ocean ridge basalt) source mantle between typical Indian Ocean‐type compositions on the east and Atlantic‐like ones on the west. A probable southwestern limit to the huge Indian Ocean isotopic domain is indicated by incompatible‐element‐depleted MORBs from 17° to 26°E, which possess essentially North Atlantic‐ or Pacific‐type signatures. Superimposed on the regional along‐axis gradient are at least three localized types of isotopically distinct, incompatible‐element‐enriched basalts. One characterizes the ridge between 36° and 39°E, directly north of the proposed Marion hotspot, and appears to be caused by mixing between hotspot and high ∈Nd, normal MORB mantle; oceanic island products of the hotspot itself exhibit a very restricted range of isotopic values (e.g., 206Pb/204Pb = 18.5–18.6) which are more MORB‐like than those of other Indian Ocean islands. Between 39° and 41°E, high Ba/Nb lavas with unusually low 206Pb/204Pb (16.87–17.44) and ∈Nd (−4 to +3) are dominant; these compositions are not only unlike those of the Marion (or any other) hotspot but also are unique among MORBs globally. Incompatible‐elementenriched lavas in the vicinity of the Indomed Fracture Zone (∼46°E) differ isotopically from those at 39°–41°E, 36°–39°E, and both the Marion and Crozet hotspots. Thus, no simple model of ridgeward flow of plume mantle can explain the presence or distribution of all the incompatible‐element‐enriched MORBs on the central Southwest Indian Ridge. The upper mantle at 39°–41°E, in particular, may contain stranded continental lithosphere, thermally eroded from Indo‐Madagascar in the middle Cretaceous. Alternatively, the composition of the; Marion hotspot must be grossly heterogeneous in space and/or time, and one of its intrinsic components must have substantially lower 206Pb/204Pb than yet measured for any hotspot. The origin of the broadly similar but much less extreme isotopic signatures of MORBs throughout most of the Indian Ocean could be related to the initiation of the Marion, Kerguelen, and Crozet hotspots, which together may have formed a more than 4400‐km‐long band of juxtaposed plume heads beneath the nearly stationary lithosphere of prebreakup Gondwana.
Thermodynamic and anharmonic properties of forsterite, α‐Mg2SiO4: Computer modelling versus high‐pressure and high‐temperature measurements
Authors: Reynard Bruno , Price Geoffrey D. , Gillet Philippe
Published: February 2008

Self‐consistent minimum free‐energy calculations of the structure and lattice dynamics of forsterite at high pressure (up to 30 GPa) and high temperature (up to 1850 K) have been performed using an approach based on the Born model of solids. In the free‐energy minimization procedure, lattice dynamics and thermodynamic properties are calculated self‐consistently within the quasi‐harmonic approximation (QHA). The results of the free‐energy minimizations are compared with recent spectroscopic studies of forsterite at high pressure and high temperature. The predicted variations of mode frequencies with compression are consistent with those obtained from high‐pressure spectroscopy. On the other hand, the variations with temperature are underestimated. Because the variations of lattice dynamics with pressure are related to anharmonic properties such as the Gruneisen parameter and thermal expansion, it is concluded that most of the extrinsic (i.e., volume dependent) anharmonicity can be accounted for within the QHA. On the other hand, the variations of the lattice dynamics with temperature also include intrinsic (i.e., volume free) anharmonic effects, which are not accounted for in the QHA. In forsterite, these effects become significant for thermodynamic properties (heat capacities and thermal expansion) above 1200 K.
Time‐dependent mapping of the magnetic field at the core‐mantle boundary
Authors: Bloxham Jeremy , Jackson Andrew
Published: February 2008

We consider the problem of constructing a time‐dependent map of the magnetic field at the core‐mantle boundary. We use almost all the available data from the last 300 years to produce two maps, one for the period 1690–1840 and the other for 1840–1990. We represent the spatial dependency of the field using spherical harmonics, the time dependency using a cubic B‐spline basis, and seek the smoothest solutions compatible with the observations. Particular attention must be paid to the effects of the crustal field in the data. We argue that for observations from permanent magnetic observatories the most efficient strategy is to use first‐differences of annual means; for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field. The resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models, demonstrating that very simple models fit the data. The resulting time‐dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies.