bluuebunny/crossref_metadata_2025 huggingface dataset:
Matrix effects for reflectivity spectra of dispersed nanophase (superparamagnetic) hematite with application to Martian spectral data
Authors: Morris Richard V. , Lauer Howard V.
Published: February 2008

The effect of the matrix on reflectivity spectra of nanophase (superparamagnetic) hematite (np‐Hm) dispersed within it was studied over the 350–2200‐nm wavelength range. Np‐Hm is characterized by particle diameters less than ∼10 nm and has properties distinctly different from larger diameter hematite particles (bulk‐Hm). Data were obtained for four series of powder samples which have different matrix properties and variable concentrations of np‐Hm dispersed within discrete powder particles. The matrix materials are two size ranges of each of two compositions which have different internal scattering characteristics. Reflectivity data show that matrix properties have a large influence on the reflectivity spectra of pigmentary np‐Hm. Samples with the same Fe2O3, content can have np‐Hm absorption edges characterized by very different positions (color) and consequently, very different slope (R600/R450) and curvature ((R600)×(R450)/(R550)2) indices, where, for example, R600 is the reflectivity at 600 nm. Conversely, samples with equivalent absorption edges can have very different Fe2O3 concentrations. For all samples, ranges of slope and curvature indices are 0.99–8.55 and 0.50–2.38, respectively. Although it may be possible to place limits, quantitative relationships between positions of ferric absorption edges and Fe2O3 concentrations are unreliable without knowledge of matrix properties of the system under consideration. As an example, the curvature index is a measure of relative crystallinity (relative proportion of bulk‐Hm) for a series of samples containing relatively increasing proportions of bulk‐Hm over np‐Hm only at constant Fe2O3 concentration and constant matrix effects. In the absence of the stated conditions, variations in the curvature index can result from any combination of crystallinity, matrix effects, and pigment (hematite) concentration. It is possible to match the Fe2O3 concentration, magnetic properties, and spectral data for Martian surface material with a laboratory mixture whose only ferric‐bearing phase is hematite. What made this possible is a matrix material with suitable optical scattering properties and differences in optical and magnetic properties of np‐Hm and bulk‐Hm. Most of the hematite is present as strongly magnetic np‐Hm, but some bulk‐Hm is required to give a shallow ∼860‐nm band. Hematite is thus very likely present on Mars and may even be the dominant ferric mineralogy.
Mechanical stability of shallow magma chambers
Authors: Sartoris G. , Pozzi J. P. , Philippe C. , Le Moüel J. L.
Published: February 2008

The mechanical stability of the wall of a magma chamber depends on the value of the stresses likely to be developed in the immediate vicinity of this boundary in relation to the conditions for failure to occur. Various structural and physical factors contributing to these stresses are systematically analyzed. Models of volcanic systems having an axial symmetry around a vertical axis with an isolated reservoir, filled with magma under pressure, in a homogeneous elastic medium are investigated. Spheroidal magma chambers of different aspect ratios (0.7–2) and approximately the same volume (13.5–14 km3), with tops at 3.5–4.5 km depth, are considered. Stress distributions including the effect of gravity, free surface of the Earth, and remote stresses increasing with depth are calculated by using a numerical finite element method. The development of tensile tangential stresses greater than 10 MPa in the elements adjacent to the wall of the chamber is assumed to be a sufficient condition for its instability. The standard state of stress assumed for the crust (i.e., the boundary conditions imposed at large horizontal distance from the chamber) is varied in a continuous range having as lower limit a state of uniaxial strain in the vertical direction and as upper limit a state of hydrostatic stress. Calculations are first performed by making the assumption that the magma pressure P acting at the top of a chamber equals the lithostatic pressure of the overlying rocks. Alternatively, the upper limit of the values of P (critical pressure Pc) for which the initial shape of a chamber may be stable, according to the assumed necessary conditions for stability, is determined. For boundary conditions corresponding to uniaxial strain and Poisson's ratio v = 0.25, Pc turns out to be approximately half of the lithostatic pressure of the overlying rocks. A simple criterion is proposed to estimate if a chamber may evolve toward a new stable shape when P > Pc. This is highly improbable if P equals the lithostatic pressure of the overburden. Larger values of Poisson's ratio (0.30–0.35) favor stability, yielding critical pressures exceeding the lithostatic pressure. Stability is extremely sensitive to the standard state of stress assumed for the crust. Boundary conditions progressively approaching a hydrostatic state of remote stresses yield increasingly higher critical pressures. It turns out that the density contrast between magma and host rocks is not crucial for stability (for Δρ/ρ < 11%). For the rather large spheroidal chambers investigated, the shape is not a critical factor either. The effect of topographical details (e.g., a volcanic edifice, 1.5 km high and 6 km in radius) is practically irrelevant.
Obtaining earth surface and spatial deflections of the vertical from free‐air gravity anomaly and elevation data without density assumptions
Authors: Gleason David M.
Published: February 2008

Moritz (1980) presents a density‐free scheme allowing for the analytical or regular continuation of a given set of free‐air gravity anomalies, referenced to the ground, to any desired level surface if a corresponding set of elevations (e.g., above mean sea level) is available. An efficient spectral implementation of this scheme is discussed by Sideris (1987). A subsequent spectral execution of the planar Vening‐Meinez equation on the continued anomalies yields deflections of the vertical on the chosen level surface. The deflections are brought back to the Earth's surface via a spectrally implemented Taylor series. Deflections at a constant altitude above the level surface are obtained through a routine spectral execution of the planar upward continuation integral. Two sites, having diverse topographies, were surveyed for 1 arc min by 1 arc min mean free‐air anomaly and elevation values and for smaller sets of astronomically determined deflections to serve as control or “truth” values. In a topographically tranquil but gravimetrically turbulent Oklahoma site the overall RMS of the differences between true and predicted deflections was 0.3 arc sees and in a rugged New Mexico site it was 0.6 arc sec. Accurate first derivative terms (in both continuation steps) require a 1 arc min data set as interpolation‐free as possible. A 1 arc min data grid is shown to be insufficient for meaningful computations of the higher order series terms. Potential pitfalls of the two‐dimensional fast Fourier transform pair are discussed with an emphasis on unwanted circular convolution effects which, if unaccounted for, can increase the error in individual predicted deflections by as much as 100%.
Shock‐induced temperatures of CaMgSi2O6
Authors: Svendsen Bob , Ahrens Thomas J.
Published: February 2008

Optical radiation from CaMgSi2O6 crystal (diopside) shock‐compressed to 145–170 GPa yields shock‐induced temperatures of 3500–4800 K, while that from CaMgSi2O6 glass, with a density 86% that of CaMgSi2O6 crystal, shock‐compressed to 96–98 GPa, yields shock‐induced temperatures of 3700–3900 K. The observed radiation histories of of the targets containing CaMgSi2O6 crystal and glass imply that the shock‐compressed states of both are highly absorptive, with effective absorption coefficients of ≥ 500–1000 m−1. Calculated shock‐compressed states for both CaMgSi2O6 crystal and glass, when compared to experimental results, imply the presence of a high‐pressure phase (HPP) along both Hugoniots over the respective pressure ranges. The CaMgSi2O6 crystal experimental results are consistent with a standard temperature and pressure (STP) HPP mass density of 4100±100 kg/m3, a STP HPP bulk modulus of 250±50 GPa, and a difference in specific internal energy (SIE) between (metastable) HPP and the CaMgSi2O6 crystal states at STP (“energy of transition”) of 2.2±0.5 MJ/kg. The CaMgSi2O6 glass results are “best‐fit” by the same (median) values of all three parameters; except for the STP SIE difference between the CaMgSi2O6 glass and HPP states, however, they are less sensitive to parameter variations than the crystal results because they are at lower pressure. All these model constraints are insensitive to the range of values (1–2) assumed for the STP HPP Gruneisen's parameter. The relatively high value of the STP SIE difference between HPP and CaMgSi2O6 crystal or glass most likely implies that CaMgSi2O6 glass and crystal experience both solid‐solid and solid‐liquid phase transformations along their respective Hugoniots below 96 and 144 GPa, respectively. The HPP CaMgSi2O6 Hugoniot constrained by the crystal experimental results lies between 2500–3000 K in the pressure range (110–135 GPa) of the lowermost mantle (D′′)] our results imply that CaMgSi2O6 is at least partly molten at these pressures and temperatures. Seismically constrained compositional models for this region of Earth's lower mantle suggest that it could contain a significant amount of Ca (25–30 wt % CaO). If so, our results imply that the temperature of the D′′ region must be below ≈ 3000 K, since the finite S‐wave velocity of the D′′ region implies that it must be (at least at seismic frequencies) predominantly solid.
He and Ne isotopic studies on the extraterrestrial material in deep‐sea sediments
Authors: Matsuda Jun‐ichi , Murota Miho , Nagao Keisuke
Published: February 2008

Elemental abundances and isotopic compositions of He and Ne were determined by stepwise heating for a magnetic separate of deep‐sea sediments and its 3 M HCl resistant fractions. High 3He/4He ratios (2–3×10−4) and solar 20Ne/22Ne ratios (12–14) were observed in all temperature fractions for these samples. The major release temperatures of 4He were at 1000°C, whereas 20Ne at 1200°C. These results suggest that solar He and Ne are tightly trapped inside the grains with nearly uniform isotopic ratios. The magnetic separate lost about 75% of its 3He against 37% weight loss during the 3 M HCl treatment, indicating that solar He and Ne are relatively enriched in the fine fraction and/or on the surface of magnetite grains. It is likely that the solar wind with its high energy tail is responsible for the extraterrestrial noble gases in deep‐sea sediments. Alternatively, solar wind He and Ne were redistributed in the magnetite during atmospheric entry and tightly trapped inside the grains.
Partitioned waveform inversion and two‐dimensional structure under the network of autonomously recording seismographs
Authors: Nolet Guust
Published: February 2008

Nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms, provided the number of model parameters is small. To reduce the size of the large‐scale two‐or three‐dimensional seismic modelling problem, path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves. This partitions the large‐scale optimization problem into a number of independent small‐scale problems and reduces the computational effort by several orders of magnitude. The path integral parameters are transformed such that their correlation matrix becomes diagonal. A linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes Backus‐Gilbert resolution analysis possible. The method has been applied to body wave and surface wave data from the broadband portable Network of Autonomously Recording Seismographs (NARS) in western Europe. Using a surface wave ray approximation to generate synthetics by mode summation, waveforms were fitted for frequencies as high as 50 mHz. Variations in S velocity as large as 6±1% have been established with a vertical resolution of 100–150 km and a horizontal resolution as small as 400 km in parts of the model. Although a low velocity layer is present under the southern part of the NARS array, such layer is very weak or absent north of the Paris Basin. Horizontal velocity gradients are well‐resolved down to 400 km depth.
Robust description of statistically heterogeneous seafloor topography through its slope distribution
Authors: Shaw Peter R. , Smith Deborah K.
Published: February 2008

Statistical heterogeneity of abyssal hill properties is often evident in seafloor topography, even under periods of relatively constant spreading direction and rate. In this paper we relate the statistics of topographic slopes computed on finite spatial scales to the autocovariance function and investigate the practicality of using these functions in describing such heterogeneous abyssal hill terrains. For a two‐dimensional homogeneous surface, a direct relation exists between the sample autocovariance and the slope distributions at different spatial scales. However, for a heterogeneous field characterized by large transient signals, the computed autocovariance estimate no longer has a clear statistical interpretation and becomes dominated by the transients. In contrast, the family of slope distributions can still be used to derive stable descriptors of the field. Slope statistics are thus useful in deriving a more robust estimate of the autocovariance than the usual sample autocovariance. Moreover, slope statistics may also be used to derive stable estimates of quantities not measurable with the autocovariance function or power spectra, such as the statistical asymmetry of features. Examples of the use of slope statistics and a comparison with autocovariance methods are presented. We document and quantify evidence of statistical asymmetry in a region of abyssal hills in the northeast Pacific and, in a second example, the presence of multiple lineations in a region where a fracture zone cuts through abyssal hill terrain.
Relationship between the location of chemosynthetic benthic communities and geologic structure on the Cascadia Subduction Zone
Authors: Lewis Brian T. R. , Cochrane Guy C.
Published: February 2008

Chemosynthetic benthic communities, which live symbiotically with microbes capable of metabolizing nutrients dissolved in water seeping out of the seafloor, are widespread along the Cascadia subduction zone. These seeps and vents are therefore indicative of one mode of fluid migration out of the subduction zone sediments. We have used deep‐towed seismic methods, including hydrophones mounted on Alvin, to examine the detailed geologic structure under two of these vent sites. At one of the sites, located on a seaward dipping thrust zone, the benthic communities are associated with a disruption of the subsurface acoustic layering in the thrust zone. It appears that at this site, dewatering is occurring along fractures in the disrupted sediments which connect to permeable layers in the undeformed sediments and not along the main thrust fault. The other site is located near the top of a ridge which has been uplifted by thrusting along a landward dipping thrust. Most of the benthic communities are found to exist at the outcrop of a steeply dipping unconformity between recent slope‐basin sediments and the older uplifted sediments, with the unconformity serving as the fluid pathway. Underlying this unconformity at the most active dewatering location is a complexly deformed structure which appears to enhance the flow of fluid to the unconformity. The source of the fluids could be the older uplifted sediments or the recent slope‐basin turbidites. In either case the source is shallow, less than about 1 km.
A tomographic glimpse of the upper mantle source of magmas of the Jemez Lineament, New Mexico
Authors: Spence William , Gross Richard S.
Published: February 2008

The 800‐km‐long Jemez lineament is the most active volcanic feature in the southwestern United States. It is the southeastern tectonic boundary of the Colorado Plateau and crosses the Rio Grande rift at the Jemez Mountains. The primary volcanism of the lineament is basaltic and has occurred in the last 4.5 m.y. To infer spatial distributions of partial melt in the upper mantle source zones for the Rio Grande rift and the Jemez lineament, we investigated the lateral variations of P wave velocity in the upper mantle beneath these features. We used teleseismic P wave delays recorded at a 22‐station network to perform a damped least squares, three‐dimensional inversion for these lateral variations. Our technique employed velocity interpolation within a three‐dimensional grid of points, rather than using blocks of constant P wave velocity. This method allows highly realistic computation of seismic ray paths as well as accurate computation of the matrix elements in our system of equations. Determinations of resolution of results were done in two independent ways, both of which gave consistent estimates of resolution. In our best resolved volume the inversion showed no significant concentration of relative low velocity for P waves beneath the Rio Grande rift. However, directly beneath the Jemez lineament there is a ∼100‐km‐wide, 1–2% low‐velocity feature in the depth range of 50–160 km. Because of the association of the low P wave velocity with the Jemez volcanic lineament but not with the Rio Grande rift, because lowered P wave velocity can be associated with increased partial melt, and because the volume of recent volcanism at the lineament greatly exceeds that at the rift, we infer that a large magmatic source zone exists beneath the Jemez lineament but not beneath the Rio Grande rift. This implies that the volcanic potential of the Jemez lineaments continues to greatly exceed that of the Rio Grande rift. The mantle source zones for volcanics of the Jemez lineament are not overridden by, but rather track, the motion of the North American plate; this implies that these sources are within the lithospheric plate, as is clarified in the discussion. The magmatic source zones of the Jemez lineament are modeled as due to clockwise rotation of the Colorado Plateau about a pole in northeastern Colorado. This rotation caused extension of the lithosphere beneath the Jemez lineament, permitting concentration there of partially melted rock in the upper mantle.
Oriented microfractures in Cajon Pass Drill Cores: Stress field near the San Andreas Fault
Authors: Wang Chi‐Yuen , Sun Yan
Published: February 2008

Oriented cores recovered from the Cajon Pass deep drill hole near the San Andreas fault in southern California were examined under both optical and electronic microscopes to study the distributrion and orientation of microfractures in the rocks. The objective was to determine whether there may be any systematic relationship between the microfractures in the rocks and the stress field in this area and to test the hypothesis that the observed shear wave birefringence can be caused by in situ microfractures. Three sets of microfractures, all dipping subvertically, appear in the rock cores studied. The oldest set (set 2) consists of mineralized microfractures with an orientation of N40°–50°E. A younger set (set 3), less prominent in appearance, consists of mineralized microfractures with an orientation of N68°–76°E. The most recent set (set 1) consists of open microfractures with fresh walls, with orientation of N14°–28°W. The two mineralized sets of microfractures were most likely formed in situ. The last set of microfractures with fresh walls was probably formed during core extraction and stress release. Based on elementary consideration in fracture mechanics, the second and the last sets (sets 3 and 1, respectively) of microfractures are consistent with a mean orientation of the present state of stress at the Cajon Pass drilling site, with a mean direction of the maximum horizontal stress at about N70°E, as determined from recent analyses of stress‐induced well bore breakouts, hydraulic fractures, and shear wave birefringence. The oldest set of microfractures (set 2) could have formed during an earlier strain event when the crystalline basement beneath the Cajon Pass area experienced an earlier strain episode when the basement block occupied a more northerly position during which the maximum horizontal stress was oriented at about N45°E. On the other hand, it could also have formed under the same state of stress as the present one, but the basement block could have rotated counterclockwise.
Fabric‐related velocity anisotropy and shear wave splitting in rocks from the Santa Rosa Mylonite Zone, California
Authors: Kern H. , Wenk H.‐R.
Published: February 2008

The directional dependence of P and S wave velocities have been measured at pressures (up to 600 MPa) and temperatures (up to 700°C) in rocks from the Santa Rosa Mylonite Zone (southern California). During tectonism, these were progressively deformed from granodiorite protolith to mylonite and ultimately phyllonite. The mineralogical and chemical composition of protolith and mylonite is nearly identical. Thus these rocks provide excellent material for documenting the effect of microstructural and textural changes on rock anisotropy. Velocity anisotropy increases significantly with the degree of deformation, whereas average velocities and densities do not change. At low pressure (50 MPa) the velocity anisotropy ranges from 1.7% in granodiorite up to 19% in phyllonite and is due to both oriented microfractures and crystallographic preferred orientation. At high pressure (600 MPa), the residual anisotropy up to 12% is mainly due to preferred mineral orientation, in particular of biotite. Significant shear wave splitting is measured parallel to the foliation plane and shows a good correlation with the biotite texture. These observations confirm that oriented microcracks and preferred orientation of minerals should be taken into account in the interpretation of seismic reflection and refraction data in terranes with deformed rocks.
Scanning and transmission electron microscope observations of magnetite and other iron phases in Ordovician carbonates from east Tennessee
Authors: Suk Dongwoo , Van Der Voo Rob , Peacor Donald R.
Published: February 2008

Previous paleomagnetic observations for the carbonates of the Lower Ordovician Knox Group have indicated that ancient magnetizations in these rocks are of the same age as the late Paleozoic Alleghenian Orogeny. Rock magnetic properties strongly suggest magnetite as the carrier of the magnetization, but the textural and crystalline characteristics, sizes, morphologies, and mineral associations of these magnetites are poorly known. We have examined magnetic extracts and iron oxides in thin sections with scanning (SEM) and scanning/transmission (STEM) electron microscope techniques to determine whether the observed iron‐oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers. Several different forms of magnetite in limestones and dolomites, which in places are host to Mississippi‐Valley type deposits, are documented by imaging and energy‐dispersive analysis using SEM and STEM, by X ray diffraction and electron diffraction patterns using STEM. The magnetite is either spherical with a dimpled surface or nonspherical in the form of void‐filling single grains or grain aggregates. Most of the iron oxides have the composition of pure end‐member magnetite, but occasional titanomagnetite and hematite, including rare zincian hematite, have been observed (only in limestone). Wherever found in thin section, nonspherical magnetites occur in association with secondary dolomite, potassium‐feldspar, illite, and quartz. Some iron oxides have, in fact, inclusions of K‐feldspar and quartz. Some of the magnetite (spherical and nonspherical) is polycrystalline; this implies that the larger observed grains may consist of single domains or pseudo‐single domains. This provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single‐domain (but interacting?) subgrains, on the basis of remanent coercivities and blocking temperatures. We interpret the pure end‐member magnetite to be authigenic, having formed at approximately the same time as the K‐feldspars, which in nearby areas have yielded late Paleozoic radioisotopic ages (278–322 Ma). The Knox carbonates therefore are inferred to carry a chemical rémanent magnetization. Iron‐rich clays or original iron‐titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution‐precipitation processes. These processes require rock‐fluid interactions which are thought to be related to migrating connate brines during the Alleghenian Orogeny.
Geodesy by radio interferometry: The application of Kalman Filtering to the analysis of very long baseline interferometry data
Authors: Herring Thomas A. , Davis James L. , Shapiro Irwin I.
Published: February 2008

We discuss the application of Kalman filtering techniques to the analysis of very long baseline interferometry (VLBI) data. The VLBI observables are geometrically related to the geodetic and astrometric parameters which can be determined from them. However, contributions to the observables from the clocks at, and the atmospheres above, the VLBI sites must be accounted for if reliable estimates of geodetic and astrometric parameters are to be obtained. Here an implementation of a Kaiman filter to account for stochastic behavior on those parameters which vary during the course of a VLBI experiment is discussed. Both the nature of the stochastic processes which should be used in the model for the VLBI data and the implementation of the Kaiman filter estimator are considered. From the results obtained, we conclude that the Kaiman filter is appropriate for analyzing VLBI data. The choice of stochastic model does not unduly affect the estimates of the geodetic parameters and the quality of these estimates is higher than that for conventional weighted least squares estimators.
Volcanic and structural morphology of the south flank of Axial Volcano, Juan de Fuca Ridge: Results from a Sea MARC I side scan sonar survey
Authors: Appelgate T. Bruce
Published: February 2008

A 1500 km2 Sea MARC I side scan sonar survey south of Axial volcano investigated the geological structure and constitution of the South Axial Rift Zone (SARZ) and the northern 30 km of the Vance spreading segment. Relative age assignments based on structural relationships indicate that recent volcanism on the southern flank of Axial volcano has been restricted to the SARZ. The surficial volcanic morphology of the SARZ changes downrift, from linear volcanic ridges (north) to small (1‐km diameter) cratered cones (south), perhaps indicating a variation in eruptive vent geometry from fissures to point sources downrift. Recent lavas on the northern and central SARZ erupted along preexisting faults in the underlying crust. By acting as pathways for SARZ magmas, these faults may have controlled the orientation of SARZ volcanic ridges and perhaps the orientation of the entire SARZ edifice. The Vance spreading segment terminates at 45°40′N and is not linked with a transform fault. The axis of spreading between 45°40′N and 46°00′N is offset west of the Vance segment and is constrained to lie between 129°54′W and 130°06′W. A discrete, structurally defined spreading axis, however, is not evident over the southern flank of Axial volcano. The divergent plate boundary may now underlie the SARZ massif, although South Helium Basin (an embayment in the southeast flank of Axial volcano) has undergone, and may continue to accommodate, crustal extension. A 48 km2 lava field discovered east of the SARZ is composed of lavas that are inferred to have erupted from the SARZ. These lavas can be traced eastward from the SARZ into the axial valley of the northern Vance segment and partially fill the valley with lavas up to 60 m thick. The eruption of these lavas, which occupy an estimated volume of 1.8 km3, may have contributed to the formation of Axial volcano's summit caldera.
Distribution and composition of hydrothermal plume particles from the ASHES Vent Field at Axial Volcano, Juan de Fuca Ridge
Authors: Feely R. A. , Geiselman T. L. , Baker E. T. , Massoth G. J. , Hammond S. R.
Published: February 2008

In 1986 and 1987, buoyant and neutrally buoyant hydrothermal plume particles from the ASHES vent field within Axial Volcano were sampled to study their variations in composition with height above the seafloor. Individual mineral phases were identified using standard X ray diffraction procedures. Elemental composition and particle morphologies were determined by X ray fluorescence spectrometry and scanning electron microscopy/X ray energy spectrometry techniques. The vent particles were primarily composed of sphalerite, anhydrite, pyrite, pyrrhotite, chalcopyrite, barite, hydrous iron oxides, and amorphous silica. Grain size analyses of buoyant plume particles showed rapid particle growth in the first few centimeters above the vent orifice, followed by differential sedimentation of the larger sulfide and sulfate minerals out of the buoyant plume. The neutrally buoyant plume consisted of a lower plume, which was highly enriched in Fe, S, Zn, and Cu, and an upper plume, which was highly enriched in Fe and Mn. The upper plume was enriched in Fe and Mn oxyhydroxide particles, and the lower plume was enriched in suspended sulfide particles in addition to the Fe and Mn oxyhydroxide particles. The chemical data for the water column particles indicate that chemical scavenging and differential sedimentation processes are major factors controlling the composition of the dispersing hydrothermal particles. Short‐term sediment trap experiments indicate that the fallout from the ASHES vent field is not as large as some of the other vent fields on the Juan de Fuca Ridge.
Constraints on early events in Martian history as derived from the cratering record
Authors: Barlow Nadine G.
Published: February 2008

The shapes and densities of crater size‐frequency distribution curves are used to constrain two major events early in Martian history: termination of high obliteration rates and viability of the multiple impact origin of the crustal dichotomy. Distribution curves of fresh craters superposed on uplands, intercrater plains, and ridged plains display shapes and densities indicative of formation prior to the end of heavy bombardment. This observation correlates with other geologic evidence, suggesting a major change in the erosional regime following the last major basin size impact (i.e., Argrye). In addition, the multisloped nature of the curves supports the idea that the downturn in the crater size‐frequency distribution curves reflects the size‐frequency distribution of the impactors rather than being the result of erosion. The crustal dichotomy formed prior to the heavy bombardment intermediate epoch based on distribution curves of knobby terrain; if the dichotomy resulted from a single gigantic impact, this observation places constraints on when this event happened. An alternate theory for dichotomy formation, the multiple‐impact basin idea, is questioned: since distribution curves of large basins as well as heavy bombardment era units are not represented by a −3 differential power law function, this study finds fewer basins missing on Mars compare to the Moon and Mercury than previously reported. The area covered by these missing basins is less than that covered the northern plains.
Resurfacing of the Martian Highlands in the Amenthes and Tyrrhena region
Authors: Craddock Robert A. , Maxwell Ted A.
Published: February 2008

The southern cratered highlands of Mars contain a large population of flat‐floored, rimless craters which have previously been interpreted to have formed by aeolian mantling or flood volcanism. Neither of these geologic processes accurately explains the observed morphology or the crater statistics. Geologic mapping in the Amenthes and Tyrrhena region indicates that craters with this morphology occur on undulating intercrater materials near the dichotomy boundary and on more rugged materials farther into the highlands containing numerous ancient valley networks. Cumulative size‐frequency distribution curves indicate ages of N(5) = 790 (early to middle Noachian) and N(5) = 540 (middle Noachian) for the cratered plateau and cratered highland materials, respectively, opposite the observed stratigraphic relations. For crater diameters >16 km the population of impact craters is consistent with stratigraphy, but the population of smaller craters in the region indicates the importance of resurfacing. Superposed, fresh craters indicate a resurfacing event that ceased at N(5) = 200–250 (late Noachian to early Hesperian). Crater counts divided into 5° latitudinal bins show an increase in the number of craters between the 8‐ and 50‐km‐diameter range with increasing northerly latitudes, suggesting that the resurfacing was not a single event. Statistical modelling of an erosive event capable of removing the continuous ejecta deposits from the craters, eroding them to reduce the apparent diameter, and simultaneously burying smaller eroded craters explains both the morphology of the flat‐floored, rimless craters, and their population distribution. Matching the slope of the modelled curves with that of the cratered highland materials suggests that up to 1400 m of erosion may have taken place in the highlands. Some of this eroded material probably remained locally, burying the smaller craters. Later stripping of this material indicated by exposed layering and inverted topography has exhumed the smaller craters in the cratered plateau material resulting in the apparent age discrepancy determined from crater counts. Up to 1.0 km of material may have been removed by this later erosive event to explain the low density of ancient valley networks in the cratered plateau materials.
Nature of the mantling deposit in the heavily cratered terrain of northeastern Arabia, Mars
Authors: Moore Jeffrey M.
Published: February 2008

A portion of northeastern Arabia (∼20°–35°N, ∼285°–315°W) appears to be overlain by an extensive, several‐hundred‐meter‐thick deposit of horizontally layered material subsequently subjected to erosion. Preexisting topography is visible but subdued, suggesting that it has been mantled rather than enveloped (infilled). Where uneroded, the surface of the deposit is smooth at ∼50 m/pixel resolution. Large craters on this landscape become less distinct with decreasing size until most craters ∼10 km in diameter are shallow, rimless circular depressions. The relative age of the mantle, as indicated by the crater density, appears to be consistent with deposition during the period immediately following the decline of heavy bombardment. The mantling deposit was episodically emplaced over a period of time, sufficient to permit a succession of channel formation, infilling, erosion, and subsequent new channeling of the Auqakuh Vallis, and variations over time in the amount of mantling material on large craters within the study area. In places, the deposit has either been extensively eroded to produce remanent mesas or entirely removed. Erosion of this extent indicates that the material is easily disintegrated to loose and freely transportable particles, possibly dust‐sized. The observation that the mantle mutes but does not mask preexisting large‐scale topography yet buries small‐scale features implies that its constituents were deposited from atmospheric suspension (or saltation). Speculative origins for the deposit include formation as a differentially welded pyroclastic tuff or a differentially compacted, zonally indurated aeolian dust deposit.
Flank tectonics of Martian volcanoes
Authors: Thomas Paul J. , Squyres Steven W. , Carr Michael H.
Published: February 2008

On the flanks of Olympus Mons is a series of terraces, concentrically distributed around the caldera. Their morphology and location suggest that they could be thrust faults caused by compressional failure of the cone. In an attempt to understand the mechanism of faulting and the possible influences of the interior structure of Olympus Mons, we have constructed a numerical model for elastic stresses within a Martian volcano. In the absence of internal pressurization, the middle slopes of the cone are subjected to compressional stress, appropriate to the formation of thrust faults. These stresses for Olympus Mons are ∼250 MPa. If a vacant magma chamber is contained within the cone, the region of maximum compressional stress is extended toward the base of the cone. If the magma chamber is pressurized, extensional stresses occur at the summit and on the upper slopes of the cone. For a filled but unpressurized magma chamber, the observed positions of the faults agree well with the calculated region of high compressional stress. Three other volcanoes on Mars, Ascraeus Mons, Arsia Mons, and Pavonis Mons, possess similar terraces. Extending our analysis to other Martian volcanoes, we find that only these three and Olympus Mons have flank stresses that exceed the compressional failure strength of basalt, lending support to the view that the terraces on all four are thrust faults.
Fault propagation folds induced by gravitational failure and slumping of the central Costa Rica Volcanic Range: Implications for large terrestrial and Martian volcanic edifices
Authors: Borgia Andrea , Burr Jeremiah , Montero Walter , Morales Luis Diego , Alvarado Guillermo E.
Published: February 2008

Long sublinear ridges and related scarps located at the base of large volcanic structures are frequently interpreted as normal faults associated with extensional regional stress. In contrast, the ridges bordering the Central Costa Rica volcanic range (CCRVR) are the topographic expression of hanging wall asymmetric angular anticlines overlying low‐angle thrust faults at the base of the range. These faults formed by gravitational failure and slumping of the flanks of the range due to the weight of the volcanic edifices and were perhaps triggered by the intrusion of magma over the past 20,000 years. The thrust fault underlying Alajuela ridge rises from a depth of ∼700 m at a dip of ∼15° over a distance of a few kilometers. It ramps up at ∼600 m depth on a ∼40° slope and terminates in the axis of the forward syncline, ∼400 m below the surface. The hanging wall of this fault has been thrusted about 200 m away from the volcanic range axis. Thrusting on the lower flanks of the CCRVR is compensated by extension on the range axis with formation of summit grabens. Shallow seismicity in the volcanic range suggests that thrusting is still active. We postulate that at Kilauea volcano, Hawaii, the gravity‐ and magma‐induced slumping of the southern flank on low‐angle thrust faults generates hanging wall anticlines similar to those observed in Costa Rica. These anticlines are hypothesized to occur along the base of the volcano, where the thrust faults ramp up toward the sea bottom. Ridges and scarps between 2000 and 5000 m below sea level are interpreted as the topographic expression of these folds. We further suggest that the scarps of the CCRVR and Kilauea are valid scaled terrestrial analogs of the perimeter scarp of the Martian volcano Olympus Mons. We suggest that the crust below Olympus Mons has failed under the load of the volcano, triggering the radial slumping of the flanks of the volcano on basal thrusts. The thrusting would have, in turn, formed the anticlinal ridges and scarps that surround the edifice. The thrusting seems to be preferential toward the northwest and southeast, and a northeast‐southwest rift zone seems to have developed, perpendicular to the regional topographic gradient. The thrust faults may extend all the way to the base of the Martian crust (about 40 km), and they may have been active until almost the end of the volcanic activity. We suggest that gravitational failure and slumping of the flanks of volcanoes is a process common to most large volcanic edifices. In the CCRVR and at Kilauea this slumping of the flanks is a slow intermittent process, but it could evolve to rapid massive avalanching leading to catastrophic eruptions. Thus monitoring of uplift and displacement of the folds related to the slump tectonics could become an additional effective method for mitigating volcanic hazards.
Depth and geoid anomalies over oceanic hotspot swells: A global survey
Authors: Monnereau Marc , Cazenave Anny
Published: February 2008

The broad depth and geoid anomalies associated with 23 hotspot swells in oceanic areas have been analyzed. Maximum height and geographical extent of the topographic swell, and of the geoid anomaly as well, have been measured for each hotspot. The results indicate a clear increase of the topographic swell height with age of the underlying lithosphere, from values in the range 300–500 m at young ages to values in the range 1500–2000 m at ages larger than 100 Ma. The geoid anomaly amplitude also increases with plate age from nearly zero close to mid‐ocean ridges, to 6–8 m over old plates. On the other hand, the geographical extent of the swell does not show any clear relationship with plate age. The mean lateral extent of swells range from 1000 to 1500 km. Swells located close to spreading ridges show a significant non zero depth anomaly but are associated with negligeable geoid signal. These results complete those of a previous study where the apparent compensation depth of oceanic hots pot swells was showed to increase linearly with the square root of plate age and coincide roughly with the base of the thermal lithosphere. This trend may either be interpreted in terms of lithospheric thinning or dynamical support. Besides both seem necessary to explain the observed bathymetry, in proportion evolving with aging of the lithosphere.
The structure of 0‐ to 0.2‐m.y.‐old oceanic crust at 9°N on the East Pacific Rise from expanded spread profiles
Authors: Vera E. E. , Mutter J. C. , Buhl P. , Orcutt J. A. , Harding A. J. , Kappus M. E. , Detrick R. S. , Brocher T. M.
Published: February 2008

We analyze four expanded spread profiles acquired at distances of 0, 2.1, 3.1, and 10 km (0–0.2 m.y.) from the axis of the East Pacific Rise between 9° and 10°N. Velocity‐depth models for these profiles have been obtained by travel time inversion in the τ‐p domain, and by x‐t forward modeling using the WKBJ and the reflectivity methods. We observe refracted arrivals that allow us to determine directly the uppermost crustal velocity structure (layer 2A). At the seafloor we find very low Vp and VS/Vp values around 2.2 km/s and ≤ 0.43. In the topmost 100–200 m of the crust, Vp remains low (≤ 2.5 km/s) then rapidly increases to 5 km/s at ∼500 m below the seafloor. High attenuation values (Qp < 100) are suggested in the topmost ∼500 m of the crust. The layer 2–3 transition probably occurs within the dike unit, a few hundred meters above the dike‐gabbro transition. This transition may mark the maximum depth of penetration by a cracking front and associated hydrothermal circulation in the axial region above the axial magma chamber (AMC). The on‐axis profile shows arrivals that correspond to the bright AMC event seen in reflection lines within 2 km of the rise axis. The top of the AMC lies 1.6 km below the seafloor and consists of molten material where Vp ≈ 3 km/s and VS = 0. Immediately above the AMC, there is a zone of large negative velocity gradients where, on the average, Vp decreases from ∼6.3 to 3 km/s over a depth of approximately 250 m. Associated with the AMC there is a low velocity zone (LVZ) that extends to a distance no greater than 10 km away from the rise axis. At the top of the LVZ, sharp velocity contrasts are confined to within 2 km of the rise axis and are associated with molten material or material with a high percentage of melt which would be concentrated only in a thin zone at the apex of the LVZ, in the axial region where the AMC event is seen in reflection lines. Away from the axis, the transition to the LVZ is smoother, the top of the LVZ is deeper, and the LVZ is less pronounced. The bottom of the LVZ is probably located near the bottom of the crust and above the Moho. Moho arrivals are observed in the profiles at zero and at 10 km from the rise axis. Rather than a single discontinuity, these arrivals indicate an approximately 1‐km‐thick Moho transition zone.
Lanthanides in silicate glasses: A vibrational spectroscopic study
Authors: Ellison Adam J. G. , Hess Paul C.
Published: February 2008

Parallel‐ and perpendicular‐polarized Raman and KBr pellet transmission IR spectra of quenched 10K2O‐50SiO2‐nR2O3 (R = La, Gd, Yb; n = 0, 1, 5, 10 mol) glasses are presented. Increasing lanthanide oxide concentration produces partially‐polarized high‐frequency bands at 1030, 940, and 860 cm−1, assigned to the symmetric stretching modes of SiO4 tetrahedra containing 1, 2, and 4 nonbridging oxygen, respectively, in which the nonbridging oxygen coordinate primarily with lanthanides. Lanthanides therefore form silicate anions that are depolymerized relative to the bulk liquid and which have no counterparts in R2O3‐SiO2 binary systems. The spectra indicate that there is little sharing of nonbridging oxygen by K and lanthanides. The spectra of glasses containing different lanthanides at the same concentration are qualitatively and quantitatively very similar. Lanthanides have energetically unfavorable interactions with the network structure of polymerized liquids compared to cations of lower valence. If lanthanides coordinate nonbridging oxygen without the aid of K, then lanthanide saturation concentrations will show modest increases with increasing (Na,K)/Al in peralkaline liquids, except in liquids in which P2O5 concentration is comparable to the total lanthanide concentration. Since differences in lanthanide ionic radii have small effects upon the spectra, lanthanide solution mechanisms in silicate glasses (and by inference silicate liquids) are probably very similar.
Geochemical and isotopic characteristics of lithospheric mantle beneath West Kettle River, British Columbia: Evidence from ultramafic xenoliths
Authors: Xue Xianyu , Baadsgaard Halfdan , Irving Anthony J. , Scarfe Christopher M.
Published: February 2008

A group of spinel peridotite xenoliths from West Kettle River, British Columbia, represents essentially undepleted to moderately depleted lithospheric mantle rocks in terms of major and compatible trace elements. Whole rock Sr isotopic composition for most of these xenoliths, and whole rock Sm‐Nd isotopic composition and LREE contents for some of them, seem to have been perturbed by near‐surface processes. Sr and Nd isotopic results for acid‐cleaned clinopyroxenes separated from these spinel peridotites reveal an isotopically mid‐ocean ridge basalt (MORB)‐like mantle. Seven spinel Iherzolites gave Nd model ages of 1.5–3.6 Ga, similar to MORB, and on a Sm‐Nd isotope diagram plot close to a reference Nd isochron with an age of 0.7 Ga and an initial εNd of +7. These features likely resulted from multiple mantle depletion. The isotopic similarities of these xenoliths with MORB suggest that this area is underlain by oceanic lithospheric mantle, possibly accreted to North America during the mid‐Jurassic. The Nd isochron age could record the time when the oceanic lithosphere was isolated from the asthenosphere. Recent enrichment event may have acted on such a depleted mantle, as indicated by the low Sm/Nd ratios of two spinel harzburgites.