bluuebunny/crossref_metadata_2025 huggingface dataset:
Three‐dimensional structure of the upper mantle beneath the Atlantic Ocean inferred from long‐period Rayleigh waves: 2. Inversion
Authors: Mocquet A. , Romanowicz B.
Published: February 2008

The three‐dimensional shear wave velocity structure of the upper mantle beneath the Atlantic Ocean is investigated by inversion of fundamental mode of Rayleigh wave group and phase velocities. The results are reliable down to 300 km depth and improve upon the image of upper mantle structure given by global tomographic models. The inversion of two‐dimensional lateral distributions of group and phase velocities, regionalized without a priori constraints, shows that shear wave velocities are correlated with lithospheric age beneath the North Atlantic. Our data set fails to quantify the correlation between shear velocities and lithospheric age, because of the small lateral extent of young regions (<22 Ma) and of strong transverse gradients. In contrast with similar studies in the Indian and Pacific oceans, slow velocities associated with the location of the Mid‐Atlantic Ridge are restricted to the lithosphere. Therefore the Atlantic mid‐oceanic ridge behaves as a shallow structure, which is weakly related to deep upwelling asthenospheric flows. At depths greater than 200 km, the central part of the Atlantic is characterized by the highest velocity anomaly. This anomaly can be interpreted in different ways. Part of this anomaly might correspond to a negative (VSH–VSV)/VSV ratio implicitely included in the isotropic inversion. We also tentatively interpret this anomaly as an important contributor to a shallow degree two pattern of shear wave velocity distribution for the Atlantic area.
Modeling of residual spheres for subduction zone earthquakes: 1. Apparent slab penetration signatures in the NW Pacific caused by deep diffuse mantle anomalies
Authors: Zhou Hua‐Wei , Anderson Don L. , Clayton Robert W.
Published: February 2008

We have computed focal residual spheres for 145 subduction zone earthquakes along the northwest edge of the Pacific using regional and global mantle velocity models from tomographic inversions. The mantle models explain much of the observed residual sphere data and, to a certain extent, suggest the location of mantle velocity heterogeneities which are responsible for various residual sphere patterns. For most deep events considered, the fast slablike residual sphere anomalies are caused by diffuse heterogeneities, mainly of deep lower mantle and receiver mantle origin rather than by an extension of the slab. The region immediately below the deepest earthquakes, depths of 650–1500 km, has an effect usually smaller than or comparable to the effect of other regions of the mantle. Without a proper account of the teleseismic effect, attributing the long‐wavelength anomalies of the residual sphere to near‐source slab effects alone, or even primarily, is not valid. The fast bands in many observed residual spheres agree with seismicity trends. Once the deep mantle and receiver mantle effects are removed, these may give the approximate orientation, but not the depth extent, of near‐source fast velocities. For most deep earthquakes under Japan the predominant fast band is subhorizontal rather than near vertical. This type feature would be overlooked in conventional residual sphere studies using only steeply diving rays and cosine weighting of the data.
Reflected and mode‐converted seismic waves within the shallow Aleutian Subduction Zone, Southern Kenai Peninsula, Alaska
Authors: Stephens C. D. , Page R. A. , Lahr J. C.
Published: February 2008

Pronounced secondary phases observed in local recordings of quarry shots and earthquakes on the southern Kenai Peninsula, Alaska, are identified as reflected P and S and converted S‐to‐P phases originating within four depth ranges: in the upper few kilometers of the Cook Inlet Tertiary basin, at midcrustal depths within the overthrust North American plate, at about 35 km depth near the top of the Wadati‐Benioff seismic zone in proximity to the inferred interplate megathrust, and at about 5–10 km below the megathrust in the subducted Pacific plate. Beneath the landward margin of the accreted Chugach terrane, the mid‐upper plate (MUP) discontinuity dips as steeply as 20°–30° to the west‐northwest over the depth range 12–18 km. At shallower depths it flattens and possibly arches with the crest at about 10 km depth. Similar midcrustal reflectors have been imaged about 125 km to the southwest and about 350 km to the northeast along the structural trend of the Chugach terrane. The extensive reflectors may have a common origin, possible caused by the presence of underplated rocks. Relative amplitudes and polarities of the secondary phases originating at MUP depths provide few constraints on the nature of the discontinuity. It appears that the MUP discontinuity is seismically inactive and does not represent a brittle‐ductile transition zone within the upper plate. The two converted S‐to‐P phases generated near the top of the subducted plate could indicate a low‐velocity zone associated with subducted oceanic crust.
Electrical conductivity of olivine, a dunite, and the mantle
Authors: Constable Steven , Duba Al
Published: February 2008

Laboratory studies of the electrical conductivity of rocks and minerals are vital to the interpretation of electromagnetic soundings of the Earth's mantle. To date, the most reliable data have been collected from single crystals. We have extended these studies with electrical conductivity measurements on a dunite from North Carolina, in the temperature range of 600°–1200°C and under controlled oxygen fugacity. Observations of conductivity as a function of oxygen fugacity and temperature demonstrate that conduction in the dunite is indistinguishable from conduction in single olivine crystals. Thus the common practice of exaggerating the single‐crystal conductivities to account for conduction by grain boundary phases in the mantle is unnecessary. Because the dunite conductivity is consistent with that published for single crystals under similar conditions, we have made a combined analysis of these data. Conductivity as a function of temperature between 600° and 1450°C displays three conduction mechanisms whose activation energies may be recovered by nonlinear least squares fitting, yielding activation energies of 0.21±2.56×10−19 J (0.13±1.60 eV) below 720°C, 2.56±0.02×10−19 J (1.60±0.01 eV) between 720°C and 1500°C and 11.46±0.90×10−19 J (7.16±0.56 eV) above 1500°C. The behavior of conductivity as a function of oxygen fugacity is well explained by a model in which an ƒo2‐independent population of charge carriers is supplemented at high oxygen fugacities with a population that is proportional to ƒo20.3. This parametrization produces a clear correlation of the ƒo2 dependent term with iron content, which is otherwise obscured by variations in conductivity among olivines.
Subduction‐induced fragmentation of the Nazca Plate off Peru: Mendana Fracture Zone and Trujillo Trough revisited
Authors: Huchon Philippe , Bourgois Jacques
Published: February 2008

The Mendana Fracture Zone and the Trujillo Trough are two major intra‐oceanic features on the Nazca Plate off Peru. During the SEAPERC cruise of the R/V Jean Charcot in 1986, Sea Beam bathymetric data, single‐channel seismic records, geomagnetic and gravity measurements, and heat flow data have been obtained over these two areas. In this paper, we confirm that the Mendana Fracture Zone is actively opening perpendicular to its trend, resulting in the formation of new oceanic crust since about 3.5 Ma. We calculate that this new rift is propagating westward along the fracture zone at a velocity of about 10 cm yr−1 (or 2 cm yr−1 with respect to the trench). This extensional feature appears to be genetically linked with the Trujillo Trough, located 200 km to the north. This trough is bounded to the east by a left: lateral transpressive fault which became active probably at the time when the Mendana Rift started. We then discuss a possible mechanism to explain the origin of these two areas of intraoceanic deformation.
Late Quaternary rate of slip along the San Jacinto Fault Zone near Anza, southern California
Authors: Rockwell Thomas , Loughman Christopher , Merifield Paul
Published: February 2008

The Anza section of the San Jacinto fault, southern California, has not experienced a groundbreaking earthquake since at least 1918 and probably since before 1899, leading previous workers to designate this 20‐km‐long fault segment as a seismic gap. Study of displaced Pleistocene and Holocene alluvial fan and fluvial deposits across the San Jacinto fault near Anza, California, dated using 14C and soil development age control, indicates a minimum right‐lateral slip rate of 9.2±2 mm/yr since 9.5 ka, and a slip rate of 11+9−5 mm/yr since 14 ka, 12+9−5 mm/yr since 17 ka, and 13+10−6 mm/yr since 50 ka. These estimates agree with a previously determined 700 ka to present minimum slip rate of 10±2 mm/yr. Based on an estimated average slip rate of 12 mm/yr from the longer‐term slip rate estimates, about 0.8 m of potential slip has accumulated if the 1918 earthquake broke the Anza segment; over 1.1 m of potential slip has accumulated if the last earthquake occurred prior to 1899. These data attest to the potential for an earthquake ≥ M6.5 for this section of the fault.
Rates of fluid expulsion across the Northern Cascadia Accretionary Prism: Constraints from new heat row and multichannel seismic reflection data
Authors: Davis E. E. , Hyndman R. D. , Villinger H.
Published: February 2008

One hundred and ten closely spaced probe heat flow measurements provide new constraints on the thermal regime of the northern Cascadia accretionary sedimentary prism off Vancouver Island. Complementary heat flow values have been obtained from the depth of a bottom‐simulating seismic reflector (BSR) that is interpreted to mark the thermally controlled base of a methane hydrate layer. The only local heat flow variations observed are associated with a sediment slump that is seen in SeaMARC II acoustic images and with the outcrop of several major thrust faults. These anomalies appear to be the result of material displacement only; strong localized fluid flow along faults or elswhere is not evident in the probe or BSR results. Fluid expulsion resulting from the dewatering of the prism sediments appears to occur regionally in the 10–20‐km‐wide zone landward of the deformation front. In this area there is a significant disagreement between the probe and BSR heat flow estimates (roughly 30%) that can be explained by a regionally uniform vertical fluid flow at a rate of about 8×10−10 m s−1. This is in good agreement with the estimated fluid expulsion rate required by the decrease in porosity landward of the deformation front, as estimated from the increase in seismic velocities derived from multichannel reflection data. The heat flow in Cascadia Basin seaward of the deformation front, corrected for the effect of sedimentation, is in excellent agreement with that predicted by cooling plate models. Landward, there is a regional trend of decreasing heat flow across the accretionary prism, which is consistent with a model of simple tectonic thickening. Temperatures at the top of the oceanic crust are estimated to be over 200°C beneath the 2–3 km of sediment in Cascadia Basin. Temperatures at the interface between the prism and the oceanic crust continue to increase landward, and reach 400°–450°C beneath the middle to inner continental shelf. Initiation of megathrust earthquake failure along the main subduction thrust may be thus restricted by the high temperatures to the zone beneath the continental slope and outer shelf. Early and shallow metamorphism of the prism material probably is another important consequence of the early deep burial of the young oceanic crust in this setting.
Thrust mechanics in highly overpressured accretionary wedges
Authors: Platt J. P.
Published: February 2008

Rapid tectonic loading of wet sediment in accretionary wedges is likely to cause the fluid pressure to rise until it is sufficient to cause dilatant fracturing. Dewatering of sediment that has been underthrust and accreted beneath the wedge can produce a large steady supply of such highly overpressured fluid. Dilatant fracturing will create escape routes, so the fluid pressure is likely to be buffered at the value required for the transition between shear and oblique tensile (dilatant) fracture, which is slightly in excess of the load pressure if the maximum compression is nearly horizontal. This in turn buffers the strength of the wedge at the cohesive strength, which is not pressure‐dependent, and will not vary greatly throughout the wedge. Near the wedge front the strength is likely to be that of the cohesion on existing thrust faults in the wedge. The shear resistance on the base of the wedge will also be fairly constant and related to the cohesive strength of the weak sediment layer that acts as the basal detachment. These assumptions allow the application of a simple plastic continuum model, which successfully predicts the observed gently convex taper of accretionary wedges. They also allow the use of a mechanical model for frontal imbrication, which predicts the spacing of imbricate thrusts at the front of the wedge from the strength and thickness of the imbricated layer, the ramp angle, and the factional resistances on the bounding faults and décollement surfaces. Reconstruction of thrust geometries from seismic data across the Makran and other accretionary wedges allows the determination of the geometrical parameters, and the thrust imbrication model allows the mechanical parameters to be calculated in terms of an assumed cohesive strength for the incoming sediment layer. The calculated values are reasonable, and successful forward modelling of the profile of the Makran shows that the postulate of a buffered value, of fluid pressure in excess of the load pressure is consistent with observation.
The effect of carbonate cementation of underthrust sediments on deformation styles during underplating
Authors: Sample James C.
Published: February 2008

Fluids expelled during deformation at convergent margins lead to a variety of diagenetic reactions, including carbonate cementation, which alter the rheology of the deforming sediment. The volume of carbonate cement in shallow sediments at modern convergent margins is small, but sandstones in an ancient accretionary complex in Alaska contain significant amounts of carbonate cement that formed before underplating occurred. Other than vein calcite, carbonates are early calcite replacing framework grains or filling pore spaces, followed by ankerite that occurs mainly as pore‐filling cement. Amounts range from 2–16 weight percent in typical samples, averaging about 9 weight percent. Mean porosity filled by carbonate in Kodiak sandstones corresponds to porosities expected at depths of 4–5 km in normal compacting sands underthrust at modem convergent margins. The temperatures of formation are difficult to determine precisely, but were probably between 100°C and 200°C. The bulk of cementation occurred before the formation of thrust faults, folds, and slaty cleavage related to underplating. Lithification by cementation changed the rheology of sandstones by (1) filling pore space and increasing peak strength, and (2) increasing cohesion so that paniculate flow was no longer a viable deformation mechanism. Lithification before significant deformation contributed to underplating of the Kodiak Formation as relatively coherent duplexes.
State of lithospheric stress and borehole stability at Deep Sea Drilling Project Site 504B, eastern equatorial Pacific
Authors: Morin Roger H. , Newmark Robin L. , Barton Colleen A. , Anderson Roger N.
Published: February 2008

Hole 504B in the eastern equatorial Pacific is the deepest hole to penetrate oceanic basement, extending more than 1500 m beneath the seafloor. Two borehole televiewer (BHTV) logs were obtained in this hole, one through the upper 1000 m of basement during Deep Sea Drilling Project leg 83 and the other extending an additional 260 m deeper into the crust during Ocean Drilling Program leg 111. The two BHTV records have been combined and processed in terms of both acoustic amplitude and travel time in order to evaluate the extent and distribution of rock failure along the borehole wall. A histogram of borehole enlargements versus azimuth depicts a dominant breakout azimuth of N122.5°E which corresponds to the direction of minimum principal stress Sh and correlates well with stress orientations inferred from solutions to regional earthquake focal mechanisms. Furthermore, the bimodality of this histogram, with a secondary mode orthogonal to Sh, indicates that a significant number of enlargements are coalesced tensile fractures occurring along the orientation of SH, the maximum principal stress. The appearance of this orthogonal, bimodal distribution suggests that the regional horizontal stress field is highly anisotropic, a condition supported by seismic data which show all of the earthquakes in the area to be strike‐slip. The frequency of borehole enlargements increases with increasing depth and depicts a systematic structural deterioration of the well bore. The tensile fractures along the SH azimuth contribute to this degradation and appear to be induced by thermal stresses due to the injection of cold water into hot rock. The frequency of these extensional features does not increase with depth. Rather, their appearances can be directly correlated with shipboard efforts at deliberately cooling the well and/or with the sudden resumption of drilling after the hole had been allowed to reequilibrate thermally for several days. These latter borehole enlargements are more pronounced then those commonly associated with hydraulic fracturing. The mechanisms for fracture initiation and growth, based upon temperature contrasts between the well bore fluid and the adjacent rock may enhance rock failure.
Transversely isotropic phase velocity analysis from slowness estimates
Authors: Gaiser J. E.
Published: February 2008

Near‐offset vertical seismic profiling (VSP) is an established technique to obtain interval velocities for lithology characterization, time‐to‐depth calibrations, and seismic data processing. Unfortunately, these velocities are often insufficient to distinguish lithology types and can disagree with surface seismic stacking velocities. Multiple‐source offset VSPs provide additional travel time information for calculating interval anisotropic velocities that can help discriminate lithology. In this paper, a technique is developed for computing transversely isotropic phase velocities over intervals less than the dominant seismic wavelength. Independent estimates of vertical and horizontal slowness are made for each receiver depth and source offset pair. These provide the magnitude and direction of phase velocity for calculating the in situ elastic properties of a transversely isotropic medium. This technique requires the assumption of lateral homogeneity because horizontal slowness components are related to surface properties between sources and not to rock properties between receivers at depth. Factors that violate this assumption such as source elevation differences and dipping layers are evaluated with synthetic data. Six impulsive source P wave offset VSPs in a 663‐m well in east Texas are analyzed and a near offset SH wave vibrator VSP provides vertical shear wave phase velocities. Strong transversely isotropic velocity variations observed in the Tertiary and Cretaceous sediments penetrated by this well correlate with lithology types, suggesting that this technique is sensitive to local properties at the receiver. These results may not indicate exact intrinsic elastic properties and could be due in part to dipping layers, vertical heterogeneities, and unknown local surface variations. Percent “phase velocity” anisotropy appears to increase from 6 to 20% for P wave and 10 to 50% for SV wave with increasing amounts of calcareous material and shale in the rock. (Percentage anisotropy is defined as the ratio of the horizontal to vertical velocity for P waves and the ratio of velocity at 45° to vertical velocity for SV waves.) Sandstones and chalk exhibit the least amount of anisotropy, while the shales and calcareous shales exhibit the most.
Effects of the internal shear wave window: Comparison with anisotropy induced splitting
Authors: Liu Enru , Crampin Stuart
Published: February 2008

In the absence of anisotropy, the observed polarizations of shear waves should be those radiated from the source, modified only by interaction with internal interfaces and free surface topography. It has been suggested that shear wave polarizations may be systematically distorted by internal interfaces, which could lead to difficulties in interpreting anisotropy‐induced shear wave splitting. Since the literature now reports many investigations of shear wave splitting, possible complications caused by internal interfaces would be important and need to be assessed. In this paper, we investigate the polarization of shear waves in full‐wave synthetic seismograms incident from a point source at an isotropic‐to‐isotropic interface, over a range of angles of incidence. Particle motion due to plane and curved wave fronts has different characteristics. Internal shear wave windows can be defined, analogous to the shear wave window at the free surface. Within the innermost window, the polarizations at subsurface geophones (as in vertical seismic profiles) are recorded with little distortion, but outside this window the waveforms become increasingly distorted for larger angles of incidence. This is similar to the behavior of shear waves with respect to the shear wave window at the free surface, except that the behavior at internal interfaces is more complicated because of additional critical angles, and additional windows. We conclude that the interaction of shear waves with internal interfaces is only a serious difficulty when interpretating wide offset vertical seismic profiles, wide angle reflections, and cross‐hole data.
"Variations in trace metal and halogen ratios in magmatic gases through an eruptive. Cycle of the Pu'u O'o Vent, Kilauea, Hawaii: July–August 1985""
Authors: Miller Theresa L. , Zoller William H. , Crowe Bruce M. , Finnegan David L.
Published: February 2008

Particle and gas samples were obtained before and after eruptive episode 35 in July and August 1985 at the fuming Pu'u O'o vent, Kilauea volcano, Hawaii. The sampling system employed consisted of a particle filter followed by four 7LiOH treated filters to collect acidic gases. The filters were analyzed using instrumental neutron activation analysis (INAA). The results indicate that Br/Cl and Re/Cl ratios do not fluctuate through an eruption cycle but the F/Cl, F/Br and metal/Cl ratios (In and Cd) do change through the cycle. An inverse relationship between F/Cl and metal/Cl was observed. The changes are probably due to influxes of relatively undegassed magma during the repose period releasing fume with lower F/Cl, F/Br and higher metal/Cl ratios. As the magma in the Pu'u O'o conduit gradually degasses either before or several days after an eruptive episode, F/Cl and F/Br ratios increase and the metal/Cl ratios decrease. One sample collected on July 24, two days before eruptive episode 35, did not follow this general trend. This can be explained by a gas pulse from a deeper, less degassed portion of magma making its way to the top of the conduit.
Evolution of sulfide mineralization on Mars
Authors: Burns Roger G. , Fisher Duncan S.
Published: February 2008

The presence of komatiitic igneous rocks on Mars, based on geochemical evidence from SNC meteorites and Viking X ray fluorescence analyses of the regolith, suggests that massive and disseminated iron sulfide mineralization occurs near the Martian surface. Analogies are drawn between possible ultramafic Fe‐Ni sulfides on Mars and terrestrial pyrrhotite‐pentlandite ore deposits associated with Archean komatiites formed during early crustal development on Earth. Partial melting of the mantle as a result of high radiogenic heat production then, extrusion of turbulent high‐temperature ultramafic lavas, segregation of immiscible FeS melts during cooling, gravitational settling and fractional crystallization of sulfide minerals in magma chambers or lava flows produced massive and disseminated sulfide mineralization associated with terrestrial komatiites. Comparable processes probably occurred on Mars where, on account of the inferred higher Fe/(Fe + Mg) ratio of the X ray mantle (estimated to contain ∼4.5 wt % S), iron‐rich basaltic magmas were produced by partial melting at depths and temperatures exceeding 165 km and 1400°C, respectively. Adiabatic diapiric emplacement of these iron‐rich, very low viscosity basaltic melts transported significant concentrations of dissolved sulfur as S2− and HS− from the mantle. Ensuing sulfide mineralization may have been either thinly disseminated within ultramafic lavas erupting over large areas of Mars or concentrated locally at the base of structural depressions. Cumulate ore deposits several meters thick may occur at the base of intrusions or in near‐surface magma chambers. The evidence for insignificant plate tectonic activity on Mars and minimal interactions of Martian mantle with crust, hydrosphere and atmosphere has restricted the evolution of sulfide ore deposits there. Thus terrestrial porphyry copper and molybdenum deposits, granite‐hosted mineralization, and related continental crust‐derived ores, including PbS‐ZnS deposits in sedimentary rocks, are unlikely to have formed on Mars. Ultramafic Fe‐Ni sulfides and perhaps iron‐rich sediments (gossans and abiotic banded iron formations) derived from chemical weathering of the basaltic crust, as well as cumulate chromites, are likely to be the only ore deposits present on Mars.
The relation of catastrophic flooding of Mangala Valles, Mars, to faulting of Memnonia Fossae and Tharsis Volcanism
Authors: Tanaka Kenneth L. , Chapman Mary G.
Published: February 2008

Detailed stratigraphic relations indicate two coeval periods of catastrophic flooding and Tharsis centered faulting (producing Memnonia Fossae) in the Mangala Valles region of Mars. Major sequences of lava flows of the Tharsis Montes Formation and local, lobate plains flows were erupted during and between these channeling and faulting episodes. First, Late Hesperian channel development overlapped in time the Tharsis‐centered faulting that trends north 75° to 90°E. Next, Late Hesperian/Early Amazonian flooding was coeval with faulting that trends north 55° to 70°E. In some reaches, resistant lava flows filled the early channels, resulting in inverted channel topography after the later flooding swept through. Both floods likely originated from the same graben, which probably was activated during each episode of faulting. Faulting broke through groundwater barriers and tapped confined aquifers in higher regions west and east of the point of discharge. The minimum volume of water required to erode Mangala Valles (about 5×1012 m3) may have been released through two floods that drained a few percent pore volume from a relatively permeable aquifer. The peak discharges of the floods may have lasted from days to weeks. The perched water discharged from the aquifer may have been produced by hydrothermal groundwater circulation induced by Tharsis magmatism, tectonic uplift centered at Tharsis Montes, and compaction of saturated crater ejecta due to loading by lava flows.
Origins of Marslike spectral and magnetic properties of a Hawaiian palagonitic soil
Authors: Morris Richard V. , Gooding James L. , Lauer Howard V. , Singer Robert B.
Published: February 2008

A palagonitic soil (HWMK1) from Mauna Kea, Hawaii, has Marslike spectral and magnetic properties. Reflectivity spectra of <20 μm size separates resemble spectra for Martian bright regions, and reflectivity spectra of coarser size separates have some spectral characteristics in common with spectra for Martian dark regions. The bulk soil is magnetic with a saturation magnetization of 0.98 A m2/kg of sample (∼10 A m2/kg of Fe) at 296 K. Petrographic observations show that the soil contains separate populations of black and orange tephra particles which are relatively unaltered fragments of tachylite (merocrystalline basalt) and extensively weathered (palagonitized) fragments of sideromelane (clear, brown basalt glass), respectively. Although both types of particles contain plagioclase and olivine microphenocrysts, their iron‐oxide mineralogies are substantially different, as documented by Mossbauer and static‐magnetic measurements. A strongly magnetic, opaque iron‐oxide (titanomagnetite) is associated with black particles and is responsible for the magnetic nature of the soil. A relatively weakly magnetic, superparamagnetic ferric‐oxide and minor hematite are associated with orange particles and are responsible for the color. The more highly weathered state (higher state of oxidation) of orange particles is a consequence of the faster weathering (palagonitization) rate for their sideromelane precursors than for tachylite. Increasing state of oxidation, decreasing saturation magnetization, and increasing near‐IR reflectivity with decreasing particle diameter of bulk soil are manifestations of the increasing proportions of palagonitic material as particle diameter decreases.
Infrared spectra and crystal chemistry of scapolites: Implications for Martian mineralogy
Authors: Swayze Gregg A. , Clark Roger N.
Published: February 2008

Near‐infrared and midinfrared spectra of a wide range of scapolite compositions were studied to determine the cause of the 2.36‐μm features that have been correlated with similar features in the near‐IR spectrum of Mars. We attribute the 2.36−μm features to vibrations caused by HCO3− and HSO4− in the anion sites of scapolite. The 2.36‐μm absorption complex consists of four overlapping bands with the following assignments: absorptions at 2.33 and 2.35 μm are attributed to combinations of hydroxyl stretches and a O‐H … O bend from HCO3−, absorptions at 2.39 and 2.41 μm are attributed to combinations of hydroxyl stretches and a O‐H ··· O bend from HSO4−. The relative intensities of all four bands vary according to the HCO3−/HSO4− ratio and disordered anion site occupancy. Oriented single‐crystal polarized spectra of a bicarbonate‐bisulfate bearing scapolite show that the 3.9‐μm hydrogen bond stretch, from HCO3−, is polarization controlled. This verifies that HCO3− is an intimate part of the scapolite crystal structure and that hydrogen in HCO3− is forming hydrogen bonds with T1 framework oxygens. The association of yellow fluorescence, attributed to S2− in the anion site, with vibrational features of HCO3− and HSO4− in the rims and fractures of scapolite grains suggests that some common geochemical process is incorporating these ions into the crystal structure during later stage alteration of scapolite. The positional disorder of HCO3− and HSO4− in the low‐symmetry anion site of scapolite gives the 2.36‐μm band complex a unique spectral signature not likely to be duplicated in any other mineral.
Variations of Mars gravitational field and rotation due to seasonal CO2 exchange
Authors: Chao B. Fong , Rubincam David Parry
Published: February 2008

About a quarter of the Martian atmospheric mass is exchanged between the atmosphere and the polar caps in the course of a Martian year: CO2 condenses to form (or add to) the polar caps in winter and sublimes into the atmosphere in summer. This paper studies the effect of this CO2 mass redistribution on Martian rotation and gravitational field. Two mechanisms are examined: (1) the waxing and waning of solid CO2 in the polar caps and (2) the geographical distribution of gaseous CO2 in the atmosphere. In particular, the net peak‐to‐peak changes in J2 and J3 over a Martian year are both found to be as much as ∼6×10−9. A simulation suggests that these changes may be detected by the upcoming Mars Observer under favorable but realistic conditions.
The evolution of atmospheric CO2 on Mars: The perspective from carbon isotope measurements
Authors: Wright I. P. , Grady M. M. , Pillinger C. T.
Published: February 2008

In situ measurements of 12C/13C of atmospheric CO2 on Mars made by Viking are, within error, similar to values normally encountered on Earth. However, high precision isotopic measurements made on SNC meteorites show there to be a 40‰ fractionation between carbon associated with silicate minerals (δ13C ≈ −25‰, 12C/13C ≈ 91.3) and trapped CO2 gas or carbonate minerals (δ13C ≈ +15‰, 12C/13C ≈ 87.7). To a first approximation, it can be considered that the silicate‐sited carbon in SNC meteorites carries the isotopic signature of the element present at depth in the Martian crust; herein it is considered that this magmatic carbon is representative of bulk Mars. On the other hand, carbon in the form of carbonate most probably results from secondary processes in operation at the surface of the planet. Trapped CO2 is thought to be a sample of Martian atmospheric gas. The significant difference in isotopic composition between the different components in SNC meteorites is difficult to interpret in terms of closed‐system equilibrium or kinetic isotopic fractionation effects. Rather, it seems that the relative enrichment of 13C in surficial carbon is due to preferential removal of the lighter isotope during an atmospheric loss process. Between 30 and 70 mbar of CO2 may have been lost from the atmosphere to outer space.
The origin and implications of undercooled andesitic inclusions in rhyolites, clear lake volcanics, California
Authors: Stimac James A. , Pearce Thomas H. , Donnelly‐Nolan Julie M. , Hearn B. Carter
Published: February 2008

At Clear Lake, California, undercooled andesitic inclusions are common in rhyolitic to dacitic lava flows and pyroclastic deposits. One group of rhyolite lava flows contains about 1 vol % undercooled andesitic inclusions <2 cm in size. The inclusions are evenly distributed in several flows and do not vary significantly in size or abundance. They consist of plagioclase, orthopyroxene, ilmenite, apatite needles, and vesicular rhyolitic glass ±olivine±clinopyroxene. Microphenocryst size in these small inclusions correlates positively with inclusion size, suggesting that inclusions have not undergone extensive disaggregation since formation. Texturally similar inclusions up to 50 cm in size occur in associated pyroclastic deposits. None of the inclusions represent the mafic end‐member of the system. Rather, they represent intermediate magmas formed by liquid‐crystal fractionation and hybridization of distinct andesitic magmas at or near the base of a silicic magma chamber. The large inclusions are transitional in composition and mineralogy between Clear Lake basaltic andesites and smaller inclusions, which show more evidence of hybridization. These differences suggest similar but distinct sources and mechanisms of formation for the large and small inclusions. The small inclusions probably formed by episodic mingling across an interface or hybrid zone separating rhyolitic and andesitic magmas. We speculate that earthquakes generated along the San Andreas fault system may have triggered mingling events. This occurrence demonstrates that dispersion of undercooled intermediate to mafic inclusions in silicic magma chambers is the dominant mode of mafic‐felsic magma interaction in some systems.
Dynamical effects from equation of state on topographies and geoid anomalies due to internal loading
Authors: Hong Hanjing , Yuen David A.
Published: February 2008

The effects of mantle compressibility on geoid and topographic signatures caused by internal loading in the mantle are studied. For a Cartesian compressible model the Green's functions of these geophysical signatures are determined by solving a two‐point boundary value problem with depth for each horizontal wave number associated with the density anomaly. Various types of equations of state have been considered, which include both constant and depth‐dependent thermodynamic parameters. Surface topographies are not changed too much by the effects of mantle compressibility, even for long wavelengths. The deformation of bottom boundary is not seriously influenced by mantle compressibility, except for long wavelengths and large viscosity contrasts between the upper and lower mantles. Geoid signals with horizontal wavelengths, exceeding 10,000 km, can be greatly influenced by mantle compressibility. For large viscosity contrasts, differences of up to 100% between incompressible and compressible geoid responses can be found. The admittance function also shows great sensitivity to mantle compressibility at long wavelengths. There are significant differences in the geophysical signatures from the ways in parameterizing the viscosity stratification in the mantle. An exponentially dependent viscosity model produces smaller geoid and surface topographical signals than for a single step function viscosity model.
The electronic band structures of iron, sulfur, and oxygen at high pressures and the Earth's core
Authors: Boness D. A. , Brown J. M.
Published: February 2008

In order to better understand properties of iron and iron mixtures under earth core conditions, Linear Muffin‐Tin Orbital (LMTO) band structure calculations for iron, sulfur, oxygen, and iron‐sulfide (in CsCl, WC, and AuCu 3 structures) were undertaken. The computationally efficient LMTO method has proven accurate in determining equation‐of‐state behavior; even for iron (where predicted properties represent near worse case examples) the deviations between theoretical and measured densities in the 100‐GPa pressure regime are found to be less than 2%. Sulfur and oxygen have differing high‐pressure behavior. Sulfur experiences a marked s‐p state to d state electronic transition with increasing compression that leads to an atomic volume compatible with ideal mixing in close‐packed iron‐dominated structures. In the iron‐sulfide system, sulfur and iron have overlapping bands that further enhance solid solution behavior. In contrast, oxygen, with no closed‐shell p electrons does not become a d band metal at high pressure. Its smaller atomic volume excludes alloy solid solution behavior with iron. Electronic contributions to specific heat are accurately estimated under ambient conditions by the LMTO calculations. Under Earth core and high‐pressure Hugoniot conditions, the electronic contribution to specific heat is 2/3 as large as the lattice contribution. Hugoniot temperatures for å iron at 200 GPa determined through integration of thermodynamic quantities are found to be within experimental errors of results based on radiative measurements of shocked iron interfaces. Based on Hugoniot measurements, the melting temperature for iron at 243 GPa is 5600±1500 K. The difference in density between the solid inner core and liquid outer core may be larger than that expected for chemistry dominated by an iron‐sulfur solid solution series. In contrast, the expected eutectic behavior between iron and FeO may be consistent with the seismic model, if core chemistry lies to the iron‐rich side of the eutectic composition. Under this assumption, a coremantle boundary temperature of 3200 K is estimated.
Monteregian hotspot track: A long‐lived mantle plume
Authors: Sleep Norman H.
Published: February 2008

The Monteregian Hills, the younger White Mountains, and the New England Seamounts constitute a well‐defined hotspot track. The evolution of the plume through time is obtained from the marine track. The buoyancy flux of the plume when it made the New England Seamounts was about 2 Mg s−1 which would make it the seventh or eighth most active plume today. The hotspot declined in activity after it made Nashville Seamount. The buoyancy flux when it made Corner Seamounts and today is less than 0.5 Mg s−1. The changes in the flux of the marine hotspot limit how far the marine flux can be extrapolated to the west onto the continental track and what can be learned in general about vulnerability of the continental lithosphere to hotspots. The lack of an obvious track west of Montréal may be due either to failure of the plume to penetrate the Canadian Shield, to the lack of recognizable intrusions, or to strengthening of the plume when it approached Montreal.
The role of fluctuations of vertical and horizontal wind and particle concentration in the deposition of dust suspended by wind
Authors: Gillette Dale A. , Porch William M.
Published: February 2008

Fluctuating quantities of vertical wind speed, horizontal wind speed, and concentration of particles with radii between 0.5 and 3 μm were observed for dusty air generated by wind erosion of soil a short distance upwind of the sensors. Probability distributions for the fluctuating quantities showed a form that was explained by a strong correlation of particle concentration with wind speed, momentum flux downward, and particle flux downward. A distribution showing percentage of the particle flux indicated that the most effective fluctuation scales were within 1–3 standard deviations of the means. The distribution showed that much of the flux was counter to the gradient in a rather inefficient turbulent mixing process. Observations that a large contribution to total particle flux occurs at horizontal wind speeds as high as 3 standard deviations above the mean would indicate that wind tunnel tests with a distribution of horizontal wind speeds narrower than distributions of natural winds may give significantly different suspension results for wind erosion studies. The authors interpret the strong correlation of horizontal wind speed and particle concentration to show a very fast response of small particle input by ‘sandblasting’ (disaggregation of small particles from larger particles and splashing of loose small particles from the surface). This sandblasting is driven by saltation of larger sand grains, a process which is highly responsive to gusts.
Simulation of the differences and similarities of warm and cold surface frontogenesis
Authors: Gidel Louis T.
Published: February 2008

Various differences and similarities between warm and cold frontogenesis are numerically modeled. The hydrostatic adiabatic Boussinesq primitive equations are integrated on a two‐dimensional grid. The frontogenesis is forced by an along‐front gradient of potential temperature and by a vertically sheared cross‐front wind field. The model develops fronts with the proper vertical circulations, strengths, and slopes; more positive relative vorticity than negative relative vorticity is produced, and the frontal zone at the surface develops in a zone of convergence. Model results indicate that cold fronts will propagate faster than warm fronts and that the fronts will develop on the time scale of 1–3 days. Nonlinear advections brake the frontogenesis for cold fronts in the model and are almost entirely responsible for realistic frontogenesis of warm fronts in the model. Conceptual models of both warm and cold frontogenesis are developed which clarify the origin of the vertical circulation and some of the frontogenesis processes.
Ozone profiles and chemical loss rates in the tropical stratosphere deduced from backscatter ultraviolet measurements
Authors: Frederick J. E. , Guenther B. W. , Hays P. B. , Heath D. F.
Published: February 2008

Analysis of data obtained by the backscatter ultraviolet (BUV) experiment on the Atmosphere Explorer E satellite has provided equatorial ozone mixing ratio profiles for equinox and solstice conditions. The combination of these results with a pure oxygen chemical model yields the rate of odd oxygen loss due to the sum of the odd hydrogen, nitrogen, and chlorine cycles. Use of recent mid‐latitude stratospheric measurements of HOx, NOx, and ClOx with the BUV data provides an independent calculation of the catalytic loss. Below 45 km the agreement between the two sets of loss rates is satisfactory. At higher altitudes the odd hydrogen cycle provides far more Ox loss than can be tolerated by the BUV measurements if the photodissociation of O2 is the only source and has the currently accepted magnitude. The results suggest either a tropical HOx concentration smaller than is now believed or the presence of a very large source of odd oxygen in the upper stratosphere and lower mesosphere.
Stratospheric positive ion chemistry of formaldehyde and methanol
Authors: Fehsenfeld F. C. , Dotan I. , Albritton D. L. , Howard C. J. , Ferguson E. E.
Published: February 2008

The reactions of H3O+·nH2O ions with CH2O and CH3OH have been studied in flowing afterglow and flow drift reaction systems. Hydrated protonated formaldehyde, CH2OH+·nH2O, reacts with H2O to form H3O+ hydrates and CH2O for small n. Therefore protonated formaldehyde ions are not expected to have significant atmospheric concentrations. Methanol reacts rapidly with H3O+ ·nH2O ions for n = 0, 1, 2, and 3. The absence of protonated methanol ions in the atmosphere above 40 km, where H3O+ ·nH2O (n = 0, 1, 2, and 3) are observed to be the dominant ions, yields a useful upper limit on the atmospheric concentration of methanol.
Mixed layer velocities induced by internal waves
Authors: D'Asaro Eric A.
Published: February 2008

The velocity spectrum in an idealized mixed layer due to the underlying internal wave field is computed by using the linearized internal wave equation and the model internal wave spectrum proposed by Desaubies (1976). For typical mixed layers the induced velocities will be as large as the expected turbulent velocities and will exist throughout the mixed layer. With a sufficiently large density step at the mixed layer base, internal wave energy can be concentrated in the mixed layer.
A VHF technique for space‐time mapping of lightning discharge processes
Authors: Taylor William L.
Published: February 2008

Instrumentation using fast logic circuits and a wide band receiving system (20–80 MHz) to determine direction of arrival of individual VHF electromagnetic impulses from lightning discharge processes was tested successfully at the Kennedy Space Center (KSC) during the 1976 Thunderstorm Research International Program (Trip 76). Time difference of arrival of impulses was used to measure the azimuth from a pair of horizontally spaced antennas and the elevation from a pair of vertically spaced antennas. Although the base line of each pair was only 13.74 m, the time difference measurements were made with 0.4‐ns resolution so that the direction to the source could be determined to an accuracy of ±0.5°. Directions of the individual impulses were represented in real time on a 60° azimuth and 30° elevation format and photographed with a 16‐mm movie camera. Two stations separated by about 17.8 km simultaneously recorded direction data for discharges from over three dozen thunderstorms in the KSC area. A description of this technique and the results of the Florida tests are presented. Analysis of these data revealed that (1) fine structure of lightning discharges could be accurately located and tracked with this technique by using triangulation methods, (2) branching and structure in the discharge processes could be observed, (3) most lightning activity occurred about 5–6 km high, near the −10°C temperature level, and (4) considerable movement of discharge centers occurred as the lightning processes permeated the thunderstorm volume at progression speeds of about 50–150 km/s.
Atmospheric trace gases and land and sea breezes at the Sepik River coast of Papua New Guinea
Authors: Wilkniss P. E. , Lamontagne R. A. , Larson R. E. , Swinnerton J. W.
Published: February 2008

Trace gas measurements (222Rn, CO, CH4), weather satellite images, and conventional meteorological observations established the existence of a pronounced land and sea breeze regime at the Sepik River coast. Its interaction with the synoptic wind field was evaluated. Considerable natural variability of atmospheric CO and CH4 was observed. CCl4 behaved similarly to man‐made CCl3F.