bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of Apollo 17 orange and black glass beads
Authors: Arndt J. , von Engelhardt W.
Published: February 2008

Size frequency distributions of the cross sections of orange and black glass beads in the 125–250 µm fraction of soil 74220 have been determined. Cross section medians of black vitrophyric and orange glassy beads in the 125–250 µm fraction are 0.16 mm and 0.14 mm, respectively. The vitrophyric beads contain skeletal olivine crystals mantled by ilmenite. Some beads contain fragments and euhedral crystals of endogeneous olivine. “Free‐flight” cooling rates and the critical cooling rate for crystallization have been determined with a synthetic melt of orange glass composition. The critical cooling rate is approximately 100° C/s. The “free‐flight” cooling rate of droplets 0.16 mm in diameter is approximately 1080° C/s. It is inferred that Apollo 17 orange melt droplets were erupted together with a large mass of hot gas and/or formed a dense cloud of heat‐radiating droplets.
Ubiquitous high‐FeO silicates in enstatite chondrites
Authors: Lusby David , Scott Edward R. D. , Keil Klaus
Published: February 2008

Our electron‐microprobe studies of four type 3 enstatite chondrites show that olivines and low‐Ca pyroxenes with more than 3 wt % FeO are more abundant than previously recognized: 4–8% of clear pyroxene grains and a higher proportion of dusty pyroxenes have Fs 5–20. From scanning electron microscope and electron probe surveys of these four EH3 chondrites, we discovered 542 objects that contain silicates (very largely pyroxene) with >3 wt % FeO: 18 chondrules, 381 rimmed or unrimmed grains, and 143 mineral aggregates with Fs 5–27 pyroxene or Fa 3–14 olivine. Aggregates and grains may have been derived from chondrules by comminution. Rims of enstatite with Fs 0–3 on many grains formed by reduction or growth in the nebula or parent body. In two EH4‐5 chondrites, we identified 89 FeO‐rich objects that are all enstatite grains with Fs 1–10 having FeO‐poor enstatite rims, which grew during planetary metamorphism. Two of the 18 FeO‐rich chondrules in EH3 chondrites have textures and mineral compositions that are consistent with crystallization of FeO‐rich melts, in others these textures are modified by nebula or planetary reduction along cracks and grain edges that formed low‐FeO silicates and metallic Fe, Ni. However, several FeO‐rich chondrules contain crystals of both high‐ and low‐FeO silicates that appear to have igneous origins but could not have formed by closed system crystallization of chondrule melts. These probably reflect either reduction of chondrule melts by the nebula during crystallization, or collisions of partly molten chondrules having very different FeO concentrations. An origin involving partial melting of chondrule precursor grains seems less plausible.
Gas chromatographic instrumentation for the analysis of aerosols and gases in Titan's atmosphere
Authors: Scattergood T. W. , Valentin J. R. , O'Hara B. J. , Kojiro D. R. , Carle G. C.
Published: February 2008

During the next decade or so, NASA, in conjunction with the European Space Agency, plans to send a spacecraft to the Saturnian system so that local studies of Saturn and its satellite, Titan, can be made. In order to study the atmosphere of Titan, analyses of both aerosols and gases will have to be made. To accomplish this, gas chromatographic instrumentation for the collection and analysis of organic gases and aerosols in Titan's atmosphere is being developed. The aerosols will be collected and then subjected to pyrolysis‐gas chromatography. Results using a simple pyrolysis‐GC system and tholin, made by subjecting a nominal Titan mixture (96.8% N2, 3% CH4, 0.2% H2) to laser‐supported shocks, show that many compounds, including hydrocarbons and simple nitriles, can be identified by this technique. Atmospheric gases will be collected using large volume (>10 cm3) sample loops and then analyzed by gas chromatography. Large volume samples are required because the ambient pressures, where the probe instruments are first deployed, will be low (<10 mbar). Preliminary studies using a 20 cm3 sampling system and a very sensitive meta‐stable ionization detector show that hydrocarbon components at the 10 ppb level can be detected. Work will continue to improve GC sensitivity, minimize analysis time, and develop interfaces with suitable sample collectors for analysis of atmospheres by future spacecraft.
Long‐Term Seismic Potential of the San Andreas Fault Southeast of San Francisco, California
Authors: Thatcher Wayne , Lisowski Michael
Published: February 2008

The long‐term (decade or longer) seismic potential of the Peninsular San Andreas fault, the 125‐km‐long segment southeast of San Francisco, has been assessed by comparing the slip accompanying the 1906 Mw = 7.7 earthquake with the amount of this slip deficit made up since 1906. Of particular importance is the longstanding factor‐of‐2 inconsistency between 1906 surface offsets (≤1.5 m) and geodetically determined coseismic slip (2.7 m) on the southern 90 km of the Peninsular San Andreas. In reexamining this issue we find that the discrepancy cannot be attributed to either large observational uncertainties in the geodetic data or shortcomings in fault modeling. Rather, the problem lies in the complexity of this 90‐km‐long fault reach, where the San Andreas trend changes by 6°, the zone of recent deformation is 0.5–2 km wide, and much of the fault is heavily vegetated and inaccessible. Farther northwest on the 1906 rupture, where the fault geometry is much simpler, evidence from deformed fences shows that right‐lateral coseismic slip was commonly distributed across a zone ∼ 20–600 m wide, with only about 70% of the total slip occurring at the main fault trace. The general importance of distributed coseismic deformation and the evidence for fault zone complexity indicate that the best estimate of 1906 slip on this 90‐km fault segment is the geodetically constrained value of 2.6 ± 0.2 m. Farther northwest on the Peninsular San Andreas the coseismic slip ranges from 3.4 to 4.4 m, and geodetic slip and surface offsets show considerably better agreement. Following previous work, we have assessed the risk posed to the southern Peninsular San Andreas by two broad classes of earthquakes. These appraisals use the geodetic slip distribution, apply a long‐term slip rate of 15 mm/yr suggested by geologic and geodetic studies, and assume the approximate correctness of the time‐predictable recurrence model. The possibility of an M ≃ 6 1/2 earthquake on the 30‐km‐long segment northwest of San Juan Bautista cannot be completely precluded, but the evidence bearing on its likelihood is contradictory. In contrast with several previous assessments we find that the risk of an earthquake of M ≃ 7 rupturing the southern. ∼ 90 km of the Peninsular San Andreas during the next several decades is quite low.
Compression of oceanic lithosphere: An analysis of intraplate deformation in the Central Indian Basin
Authors: Zuber Maria T.
Published: February 2008

Intraplate compressional deformation in the Central Indian Basin is expressed as linear E‐W trending topographic undulations and geoid anomlies with characteristic spacings of approximately 200 km. To elucidate the nature of this deformation, we develop models of an oceanic lithosphere in a state of horizontal compression. The lithosphere is treated as a viscous or plastic layer of uniform strength which overlies a viscous half space in which strength decreases exponentially with depth. We compare models in which deformation with a dominant wavelength develops in response to flexural buckling and the hydrodynamic growth of instabilities. For the viscous flexural buckling model, deformation is constrained to occur in the antisymmetric mode by flexural folding, which is characterized by uniform bending of the layer. In the hydrodynamic flow model a range of deformational styles are possible. In a strong viscous lithosphere, deformation of the layer occurs by flexural folding at a wavelength which agrees with that predicted from flexural buckling theory. In a lithosphere of intermediate strength, the layer deforms by folding characterized by thickening which localizes beneath topographic highs. In a relatively weak lithosphere, the layer incurs an even greater amount of localized thickening and deforms in the symmetric or pinch‐and‐swell mode by inverse boudinage. If the layer has a plastic rheology, deformation occurs by inverse boudinage independent of the yield stress. Results from models in which the layer folds either flexurally or with periodic thickening are consistent with the observed depth distribution of seismicity in the Central Indian Basin and with experimental rock rheological data. Because of the trade‐off between buoyancy forces and layer strength in determining the dominant wavelength, a lithosphere which was overlain by weak sediments when intraplate deformation initiated can be characterized by a compressive layer strength almost a factor of 3 less than a lithosphere which was initially sediment‐free. For a range of plausible models which consider both a sediment‐covered and sediment‐free lithosphere, the observed wavelength requires deviatoric stresses in the Indian plate of several hundreds of MPa.
Probing the evolving Andean Lithosphere: Mid‐Late Tertiary magmatism in Chile (29°–30°30′S) over the modern zone of subhorizontal subduction
Authors: Kay S. Mahlburg , Maksaev V. , Moscoso R. , Mpodozis C. , Nasi C.
Published: February 2008

In the mid‐late Tertiary (27–6 m.y. ago), volcanism diminished, then ceased in the main Andean Cordillera in Chile between 29° and 30°30′S. Over the same time span the subduction angle shallowed under this region, which today lies in the modern shallowly dipping, nonvolcanic segment of the south central Andes (28°–33°S). The trace and major element chemistry and mineralogy of the volcanic rocks in this region record the detailed pattern of lithospheric cooling and crustal thickening that accompany this major tectonic transition. Volcanic changes include, in addition to a decreased rate of volcanism, a decrease in the range of compositions from basaltic andesites to rhyolites in the oldest rocks to only rhyolite in the youngest rocks, a change in source and fractionating mineralogy with an increased role for amphibole and garnet in the younger rocks, and an increase in crustal assimilation. These volcanic changes also correlate in time with deformational events that help to account for crustal thickening and the space‐time pattern of deformation and volcanism in the foreland region. Although the model stands alone, support for the tectonic setting and magmatic source region of these Tertiary volcanic rocks can be found in comparisons with recent volcanic rocks erupting over more steeply dipping modern Andean segments. The older Doña Ana Formation (27–18 m.y.) is similar to volcanic rocks in the Southern Volcanic Zone, while the younger Cerro de las Tortolas (14–10 m.y.) and Vallecito (6 m.y.) formations are similar to volcanic rocks over regions of thicker crust in the Central Volcanic Zone and the northern part of the Southern Volcanic Zone.
Point source solutions and coupling parameters in cratering mechanics
Authors: Holsapple K. A. , Schmidt R. M.
Published: February 2008

Many phenomena of interest in impact cratering occur on a time and distance scale large compared to those of the impactor. As a consequence, the details of the impactor may be of little consequence, and the effects of the impactor can be replaced by an equivalent point source of energy and momentum. This point source assumption is developed and proved to define a single scalar “coupling parameter” measure, C ∝ αUμδν of the impactor radius α, velocity U, and density δ. Idealized examples are given where this measure can range from the cube root of the impactor kinetic energy, with μ = 2/3, to cases where it is the cube root of its momentum with μ = 1/3. For real materials the correct choice of μ depends on the material of the target. Materials with little dissipation have the exponent μ ≃ 0.6; while materials with large dissipation have μ ≃ 0.4. The assumption of such a measure is shown to lead to a unified theory of crater scaling in terms of the exponent μ. Results are given for crater size, ejecta distributions, growth histories, time of formation, melt volume, and shock decay. The results are compared to experiments and code calculations. (Craters, impacts, scaling, point source, self‐similar, coupling parameter).
Crustal refraction surveys across the Trans‐Hudson Orogen/Williston Basin of south central Canada
Authors: Morel‐à‐l'Huissier P. , Green A. G. , Pike C. J.
Published: February 2008

Eight reversed seismic refraction profiles have been recorded across the Phanerozoic Williston sedimentary basin that overlies the margins of the Superior and Wyoming Archean cratons and the Proterozoic Trans‐Hudson orogen. These data have been modeled using two‐dimensional ray‐tracing techniques to match the times and amplitudes of primary and coherent secondary arrivals. Within the southern extension of the Superior craton margin (Thompson belt) a high‐velocity (∼ 6.4 km/s) lid overlies a narrow region of high electrical conductivity, and a zone of westerly dipping reflections is nearly coincident with a weak velocity discontinuity that seems to delineate the western edge (Thompson fault) of the craton margin. Complex zones of low‐velocity material and the North American Central Plains electrical conductivity anomaly are concluded to lie within the southern extension of the Reindeer‐South Indian Lakes terrain, a component of the Trans‐Hudson orogen. A high‐velocity (≳ 7.0 km/s) lower crustal layer, which represents a zone of complexity separating “normal” crust from the upper mantle, extends throughout the region. The crust is relatively thick, with depths to Moho varying smoothly between 41 and 48 km. There is no evidence in the relatively low‐resolution seismic refraction data for abrupt changes in the level of the Moho, but there may be a minor (3–4 km) thinning of the crust near the center of the Williston Basin.
Introduction to Special Section on Geophysical Investigations of Proposed Radioactive Waste Disposal Sites
Authors: Oliver H. W.
Published: February 2008

A symposium on “Geophysical Investigations of Proposed Radioactive Waste Disposal Sites” was held at the Fall Meeting of the American Geophysical Union, December 13, 1982. Since then, five of the papers presented at the symposium have been published in the Journal of Geophysical Research and an additional six papers are included in this issue. Three of the current papers involve geophysical research at Yucca Mountain, Nevada; two papers are on subsurface structure and fracturing of the Strath‐Halladale granite in northern Scotland, a prime candidate for rad waste storage in the United Kingdom; and a general paper is included on the application of various geophysical methods for characterizing all the potential storage sites in the United States under consideration by the U.S. Department of Energy.

In 1982, the following nine sites in the United States (Figure 1) were under consideration by the U.S. Department of Energy for the first U.S. repository of high‐level radioactive waste (HLW). The host rock at each site is noted in parentheses (from NW to SE): Hanford, Washington (Miocene basalt flows); Yucca Mountain, Nevada (Tertiary tuff); Davis Canyon, Utah, (bedded salt); Lavender Canyon, Utah (bedded salt); Deaf Smith, Texas (Permian bedded salt); Swisher County, Texas (Permian bedded salt); Vacherie dome, Louisiana (domal salt); Richton dome, Mississippi (domal salt); and Cypress Creek dome, Mississippi (domal salt)
Seismic wave propagation in a very permeable water‐saturated surface layer
Authors: Géli Louis , Bard Pierre‐Yves , Schmitts Denis P.
Published: February 2008

According to Biot's (1956a, b) model, the presence of water plays an important role in the propagation of seismic waves in at least three different ways: (1) in an infinite medium, water saturation induces an attenuation that can be accounted for by a complex formulation of wave velocities, as in viscoelastic media; (2) at the boundaries of the saturated medium, pore pressure and water flux determine specific continuity conditions; and (3) there is a second compressional wave, called the P2 wave. In this paper, we discuss the latter two effects. Biot's model is presented first, with homogenization theory used to provide the numerical values of the different coefficients used in the model. In an infinite medium, the model is of practical interest when the frequency ƒ is about the same order of magnitude as a characteristic frequency noted ƒc, which depends on the properties of the constituents. This limits the application of Biot's model to a few particular fields in geophysics. In a layered medium, Biot's model has a wider scope in that it provides a tool for modeling fluid‐solid interaction at the boundaries of the saturated medium. This is illustrated in our paper for the case of a very permeable water‐saturated surface layer over an elastic half‐space. Two examples are given; in the first example (rigid sands) we discuss the physics of the strongly attenuated P2 wave predicted by Biot, the amplitude of which becomes significant when the ƒ/ƒc ratio is about equal to or greater than 0.1. In the second example (soft unconsolidated sediments) the P2 wave is negligible, but the calculation of the complete wave field is required when the ƒ/ƒc ratio is about 0.01. There is no adequate equivalent single phase model that gives a correct estimation of the amplitude of the ground motion. In this case, we argue that the P2 wave is not important in itself, but Biot's model allows the description of the fluid‐solid interaction at the water table; continuity of effective stress and pore pressure can be explicitly formulated.
Tensile fracture resistance mechanisms in brittle polycrystals: An ultrasonics and in situ microscopy investigation
Authors: Swanson Peter L.
Published: February 2008

A zone of distributed microcracking is often suggested to accompany tensile macrocrack propagation in rocks and ceramics. The microcracking is said to be largely responsible for (1) high values of fracture energy, (2) increasing resistance to fracture with crack extension and (3) the dependence of fracture mechanics data on the experimental setup. In the present paper, the material breakdown processes in imperfectly elastic Westerly granite are investigated using ultrasonic wave probing and in situ microscopy during mode I fracture experiments. These observations are compared with an in situ reflection/transmission microscopy investigation of mode I fracture in a near‐ideal elastic polycrystalline alumina (Al2O3). As defined by the spatial distribution of longitudinal and surface wave attenuation in wedge‐loaded double‐cantilever beam specimens of Westerly granite, the fracture process zone is elongate in the direction of fracture propagation (15–40 mm long by 1–2 grain dimensions wide; grain size 0.75 mm). As revealed by in situ reflection microscopy, the ultrasonic wave energy is partially transmitted through the developing fracture surfaces via two sources of crack interface traction: (1) remnant islands of unfractured material left behind the advancing fracture front and (2) geometrical interlocking of the microstructurally rough fracture surfaces. A similar zone of crack flank tractions is found in the alumina (greater than 2000 μm long; grain size 20–100 μm). No evidence of a diffuse kidney‐shaped cloud of microcracking distributed ahead of the main fracture tip (predicted by many fracture models) was found in either material. Instead, interface‐localized microcracking was observed to operate at positions where the tractions, or restraining forces, are transmitted across the nascent fracture surfaces. Crack flank tractions shield the main crack tip from high levels of stress and are relieved by friction‐induced microcracking and microcrack rupture of intact‐material bridges. As a consequence of the crack history dependence of the crack tip shielding, it is proposed that, even under small‐scale inelastic deformation conditions appropriate to linear elastic fracture mechanics, neither R‐curve behavior nor applied‐KI/subcritical crack velocity relationships are intrinsic properties of these and similar materials.
Computation of the gravity vector from torsion balance data in southern Ohio
Authors: Arabelos D. , Tscherning C. C.
Published: February 2008

The method of least squarescollocation was used for gravity vector estimation from torsion balance measurements in southern Ohio. The results were dependent on the covariance function used and on the selection of a proper signal to noise ratio. Here a standard deviation of the noise of 10 EU (10−9 s−2) gave the best results. Expressed in terras of standard deviations of observed minus predicted differences, the best results were 0.4 arc sec for deflections of the vertical and 2 mGal for gravity anomalies. This compares to signal standard deviations of 4 arc sec and 22 mGal for deflections and gravity anomalies, respectively.
Till beneath ice stream B: 3. Till deformation: Evidence and implications
Authors: Alley R. B. , Blankenship D. D. , Bentley C. R. , Rooney S. T.
Published: February 2008

Most of the velocity of ice stream B near the Upstream B camp (UpB), West Antarctica, appears to arise from deformation of a seismically detected, subglacial till layer that averages 6 m thick. Available evidence indicates that the entire thickness of this till layer is deforming and is eroding subjacent bedrock into flutes parallel to ice flow and hundreds of meters across. The resulting till flux beneath UpB is equivalent to an average erosion rate of about 0.4 mm yr−1 in the catchment area and suggests that till deltas tens of kilometers long have been deposited at the grounding line during the Holocene. Such deltas should be characterized by partial ice‐till decoupling across a water film and by a small ice‐air surface slope; they may have been discovered by recent geophysical work.
Slow waves trapped in a fluid‐filled infinite crack: Implication for volcanic tremor
Authors: Ferrazzini Valerie , Aki Keiiti
Published: February 2008

The dynamics and seismic radiation of fluid‐filled cracks have been studied by numerous authors, as models for tremor and for long‐period events observed at volcanoes. One of the most intriguing results of the recent models is the existence of a very slow wave propagating along the crack boundary. In order to better understand this slow wave, which has so far only been studied numerically, we studied analytically normal modes trapped in a liquid layer sandwiched between two solid half‐space. A slow wave, similar to the tube wave found by Biot, exists for all wavelengths. In the short wavelength limit, this wave approaches the Stoneley wave for the liquid‐solid interface. Unlike the tube wave, however, as the wavelength increases to infinity, both the phase and group velocities approach zero, in inverse proportion to the square root of wavelength. The phase velocity and amplitude of this slow wave are in good agreement with those obtained by the numerical studies on the dynamics of fluid‐filled cracks by two dimensional and three‐dimensional finite difference methods. In the past the size of a magma body has been estimated from volcanic tremor periods and the acoustic velocity in the fluid. These estimates should be drastically reduced if the slow wave dominates the tremor. For example, the extremely long‐period volcanic tremor, with periods up to 7s, observed at Mount Aso may be generated by a fluid‐filled crack of modest size, a magma body 0.5 m thick and 0.5 km long.
An electrochemical series of redox couples in silicate melts: A review and applications to geochemistry
Authors: Schreiber Henry D.
Published: February 2008

An electrochemical series of redox couples has been developed for use in silicate melt systems that model basaltic magmas. The redox couples are ordered in terms of their reference reduction potentials, which are measures of their ease of reduction in silicate melts. Although the numerical value of the reference reduction potential for a particular redox couple is quite sensitive to the exact melt composition and temperature, the order of the couples in the series appears to be relatively independent of these variables. Thus, upon calibration to a desired composition, oxygen fugacity, and temperature, this electrochemical series can provide estimates of redox state proportions in basaltic magmas on different planetary bodies. The geochemical electrochemical series can also be used to understand the interrelationship of the redox state of the magma and the presence of volatile species such as oxygen, water, sulfur gases, and carbon gases.
Laboratory study of the strain field and acoustic emissions during the failure of a barrier
Authors: Sobolev Guennadi , Getting Ivan C. , Spetzler Hartmut
Published: February 2008

A barrier on a fault was constructed in a concrete model. Two sets of lubricated glass plates were cast into the specimen to represent the previously broken, low friction part of a fault. An intact concrete barrier in line with and between the sets of glass plates was caused to fail under axial loading. The fault plane crossed the sample, a rectangular parallelopiped, at an angle of 35° with respect to the long (also the loading) axis. As the sample was loaded, axial and transverse strains in the vicinity of the barrier were monitored with conventional strain gages. Differential surface uplift was measured by double‐exposure holography. Acoustic emissions were recorded digitally from the signals of piezoelectric compressional wave transducers. The axial loading was controlled such that the shortening rate of the sample was constant. Initial elastic deformation was followed by nonuniform strain concentrated in the barrier region. Acoustic emissions in the early stages of loading occurred diffusely and were of predominantly tensional character, i.e., compressional arrivals on all transducers. Intensive deformation was concentrated near the barrier ends. After the maximum applied force had been reached, maxima in axial deformation were observed, first far from the barrier and slowly migrating toward the eventual rupture. Small force drops, i.e., small local failures, were associated with axial extension far from the fault and axial shortening near the fault. Shear‐type acoustic emissions were observed to dominate in the later phases of deformation leading to the catastrophic failure of the barrier.
May 18, 1980, Mount St. Helens deposits in South Coldwater Creek, Washington
Authors: Fisher Richard V. , Glicken Harry X. , Hoblitt Richard P.
Published: February 2008

Beginning at 0832 Pacific Daylight Time (PDT), Mount St. Helens, Washington, produced a rockslide debris avalanche and a lateral explosion (blast) that generated a laterally directed pyroclastic surge, called here a blast surge. The resulting deposits are thickest in the east‐west trending valley of North Fork Toutle River immediately north of the volcano but are almost as thick in South Coldwater Creek, a small tributary of the Toutle River, in the lee of an east‐west trending ridge (Johnston Ridge) just north of the North Fork Toutle River. The avalanche surmounted Johnston Ridge (300 m relief) and flowed down the valley of South Coldwater Creek. Closely following the avalanche and in part contemporaneously with it, the blast surge left thin (up to ∼0.5 m) deposits on Johnston Ridge and thick (up to ∼ 15 m) deposits in South Coldwater Creek. These deposits have a lower layer (layer A1) which is friable, massive, poorer in fines (particles < 1/16 mm), and more poorly sorted than the upper layer (layer A2). Layer A2 is less friable because it contains more silt‐ and clay‐sized material. Beneath layer A1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface. This zone, designated as layer A0, is a poorly mixed layer containing schlieren of soil, shattered volcanic fragments, and considerable quantities of organic material. Layer A1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge; layer A2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head; layer A0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge. It is concluded that a strongly fines‐depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high‐velocity particulate density current.
The geochemistry of the Inyo Volcanic Chain: Multiple magma systems in the Long Valley Region, eastern California
Authors: Sampson Daniel E. , Cameron Kenneth L.
Published: February 2008

Chemical data from the Inyo volcanic chain of eastern California illustrate that the most recent eruption (about 600 years B.P.) sampled at least two separate magmatic systems. Two rhyolitic lava types were produced in the eruption, one coarsely porphyritic (CP) and one finely porphyritic (FP). These two lavas are chemically, mineralogically, and texturally distinct from one another, are physically intermixed, and are distributed unequally among three lava flows. The FP lava comprises a range of compositions that form coherent but discontinuous trends on variation diagrams. Quantitative major and trace element modeling indicates that the trends were not produced by fractionation of the observed phenocryst assemblage, but rather that the high‐silica end‐member could have been produced from the low‐silica end‐member by fractionation of an equivalent hydrous assemblage. This and compositional discontinuity in the trends could indicate that two slightly zoned FP end‐members were generated at higher water pressure, and then back‐mixed to form the present array of FP lava compositions. In contrast, chemical distinctions, physical nature, and chemical modeling suggest that the CP and FP magmas evolved separately along distinct, and possibly parallel, liquid lines of descent. Since these are the most recent lavas to erupt in the Long Valley caldera, this suggests that at least two chemically isolated magmatic systems are present beneath the region. The three lava flows produced in the 600‐year‐B.P. eruption lie along a north‐trending line that crosses the northwestern boundary of the caldera. The CP lava is not found outside the caldera, increases in abundance toward the caldera center, and shows marked physical and chemical similarity to 100,000‐year‐old lava erupted in the caldera's northern topographic moat. The more crystalline nature of the CP lava, coupled with its similarity to older Long Valley lava, may indicate that it came from a source within the caldera, possibly the same magma body as the one detected seismically beneath the northwest moat of the caldera.
Long‐term nongeocentric axial dipole directions and a geomagnetic excursion from the Middle Pleistocene sediments of the Humboldt River Canyon, Pershing County, Nevada
Authors: Negrini Robert M. , Verosub Kenneth L. , Davis Jonathan O.
Published: February 2008

A paleomagnetic record has been obtained from the middle Pleistocene sediments of the Humboldt River Canyon, Pershing County, Nevada. The age of the sediments is constrained by two tephra layers, the 610,000‐year‐old Lava Creek tephra and the approximately 495,000‐year‐old Dibekulewe tephra. The magnetization of the sediments is strong and stable, and the similarity of data from replicate sections strongly suggests that the data set represents an accurate record of the magnetic field except for the presence of postmagnetization rotations of less than 10°. The influence of postmagnetization rotations was eliminated in one of the sections (HR1A) by rotating the paleomagnetic directions in accordance with the direction of the Lava Creek welded tuff previously determined from five sampling sites near Yellowstone National Park. After correcting the mean directions we observed nongeocentric axial dipole (non‐GAD) mean directions of moderate amplitude (19° ± 13° in declination and 22° ± 8° in inclination) in the record from the HRlA section which may represent as much as 115,000 years of magnetic field behavior. In the youngest part of the Humboldt River Canyon sediments a zone of anomalous directions was recorded which we interpret to be a detailed record of an unconfirmed geomagnetic excursion. This record exhibits behavior consistent with that of previously reported polarity transitions. Because of the widespread nature of the tephras, especially the Lava Creek tephra, the prospects are very good for improving estimates of the magnitude and duration of the non‐GAD paleomagnetic directions and, also, for confirming the excursion.
Structure of the southern Juan de Fuca Ridge from seismic reflection records
Authors: Morton Janet L. , Sleep Norman H. , Normark William R. , Tompkins Donald H.
Published: February 2008

Twenty‐four‐channel seismic reflection records were obtained from the axial region of the southern Juan de Fuca Ridge. Two profiles are normal to the strike of the spreading center and intersect the ridge at latitude 44°40′N and 45°05′N; a third profile extends south along the ridge axis from latitude 45°20′N and crosses the Blanco Fracture Zone. Processing of the axial portions of the cross‐strike lines resolved a weak reflection centered beneath the axis. The reflector is at a depth similar to seismically detected magma chambers on the East Pacific Rise and a Lau Basin spreading center; we suggest that the reflector represents the top of an axial magma chamber. In the migrated sections the top of the probable magma chamber is relatively flat and 1–2 km wide, and the subbottom depth of the chamber is greater where the depth to the ridge axis is greater.
Beryllium 10 in hydrothermal vent deposits from the East Pacific Ridges: Role of sediments in the hydrothermal processes
Authors: Vallet‐Silver J. N. , Tera F. , Klein J. , Middleton R.
Published: February 2008

Beryllium 10 concentrations were determined for 31 handpicked hydrothermal sulfides, six oxyhydroxides, seven basalts, and five sediments collected from the hydrothermally active areas of the East Pacific ridges. The samples include specimens from the East Pacific Rise (EPR) at 21°N and 13°N, the Galapagos Rift, the Guaymas Basin, and the Gorda and the Juan de Fuca ridges. Additional samples from massive sulfides associated with the Oman ophiolites were studied. In all samples, we obtained values ranging from 0.04 × 106 atoms/g to 125 × 106 atoms/g, with the lowest values being very close to our blank (0.015 × 106 atoms/g). The data show systematic variations with sample location and type. The 10Be concentrations measured for the mid‐ocean basalts are of the order of 0.3 × 10 atoms/g and reach 3800 × 106 atoms/g for the pelagic deep‐sea sediments collected near the EPR 21°N. Based on their 10Be concentrations, we can clearly distinguish two categories of sulfides: sulfides containing low 10Be concentration (<106 atoms/g) sitting directly on the mid‐ocean basalt (EPR or Juan de Fuca), and sulfides with high 10Be concentration (>106 atoms/g) located atop of a thick pile of young sediments (Guaymas Basin or Gorda Ridge). The oxy‐hydroxides, collected on some of the Juan de Fuca chimneys, are always very rich in 10Be with concentrations reaching 125 × 106 atoms/g. We propose three mechanisms to explain these data: (1) mechanical entrapment of sediments in the hydrothermal chimneys as observed in the Gorda Ridge sulfides, (2) leaching of 10Be from the ocean crust sediments, transport by hydrothermal fluids, and coprecipitation with heavy metals as hypothesized to explain the Be concentrations observed in the Guaymas Basin, and (3) passive adsorption of 10Be from seawater, preferred hypothesis oxy‐hydroxides of the areas with no sediment cover.
A constitutive model for quasi‐static behavior of rock salt
Authors: Desai C. S. , Varadarajan A.
Published: February 2008

A general constitutive model is proposed to characterize short‐term quasi‐static stress‐strain behavior of rock salt. It is a simplified model based on rate independent theory of elastoplasticity and includes such factors as hardening, stress path, cohesive and tensile strengths, and variations of yield behavior with mean pressure. The constants for the model are determined from laboratory tests under different initial pressures and stress paths using a multiaxial testing device and standard triaxial test results available in literature for rock salts. The model is verified with respect to tests used for finding the constants and a test not used for finding the constants. The proposed model requires less or equal number of constants as compared with other models with similar capabilities.
Behavior and origin of helium, neon, argon, and nitrogen from active faults
Authors: Sugisaki Ryuichi
Published: February 2008

Compositional variations of gas species occluded in fracture zones (type I) within active faults are essentially the same as those in bubble gases (type II) separating from mineral springs along faults. The variation in type II is hydrogeochemically interpreted as a result of dissolving and degassing of atmospheric air in groundwater at various temperatures. The comparison of these variations in both types of gases suggests that Ne, Ar, and N2 in the type I gases are of atmospheric origin and their fluctuation is strongly controlled by the circulation of groundwater, which recharges and discharges atmospheric air at various temperatures. Further hydrogeochemical evaluation shows that a part of He observed in fault zones is of radiogenic origin but its proportion is very small in active faults. In these zones, juvenile He emission related to the tectonic stresses causing earthquake may be substantially modified by the compositional fluctuation of gases controlled by the water circulation within the fault system. Accordingly, monitoring of rare gases except for H2 from historically active faults may be less effective for earthquake prediction. In contrast, gas bubbles of high He content issuing from mineral springs which usually occur along prehistorically active faults are less contaminated by air and may provide some general precursory information concerning regional stresses at depth prior to an earthquake.
Temporal variation of 3He/4He ratio recorded in deep‐sea sediment cores
Authors: Takayanagi M. , Ozima M.
Published: February 2008

He isotopic composition was measured for 72 deep‐sea sediments from 16 sites in the Pacific and Atlantic oceans and the Japan Sea. 3He content ranges from less than 10−12 cm3 STP/g to about 6 × 10−11 cm3 STP/g and 3He/4He from about 10−8 to 1.67 × 10−4. The He isotopic data lie fairly well on a mixing curve in a 3He/4He‐3He diagram, where the end‐members of the mixing curve are extraterrestrial He in interplanetary dust particles (IDP's) and terrestrial He in ocean sediments. This confirms the view that there are significant amounts of extraterrestrial He in ocean sediments from global fallout of IDP's into deep‐sea sediments. 3He flux estimated for two long cores gives a value of about (1.5 ± 1.0) × 10−15 cm3 STP/cm2 yr over a time span of 40 m.y., which corresponds to an annual fallout rate of IDP's (size < 10 μm) of about 400 t. In one of the cores, we observe a “3He‐spike” which is associated with anomalously high Ir content (1.86 ± 0.56 ppb). This may be due to meteorite debris.
Paleomagnetism of Cretaceous Pacific Seamounts revisited
Authors: Hildebrand John A. , Parker Robert L.
Published: February 2008

The paleomagnetism of Cretaceous Pacific seamounts is reexamined. Herein techniques for nonuniform magnetic modeling are applied to determine paleomagnetic pole positions and their associated confidence limits. Modeling techniques are presented for reconstruction of both uniform and nonuniform components of the seamount magnetization. The uniform component yields an estimate of the paleomagnetic pole position, and the nonuniform component accounts for irregularities in the seamount magnetization. A seminorm minimization approach constructs maximally uniform magnetizations and is used to represent seamount interiors. A statistical modeling approach constructs random nonuniform magnetizations and is used to determine the confidence limits associated with each pole position. Mean paleopoles are calculated for groups of seamounts, including their associated error bounds. The mean paleopole for seven reliably dated Upper Cretaceous seamounts is located close to the position predicted by Pacific‐hotspot relative motion. The paleopole for five seamounts with Cretaceous minimum dates is located west of the hotspotpredicted apparent polar wander path and may represent a Lower Cretaceous or Upper Jurassic pole.
Crustal underplating and evolution in the Red Sea Rift: Uplifted gabbro/gneiss crustal complexes on Zabargad and Brothers Islands
Authors: Bonatti Enrico , Seyler Monique
Published: February 2008

The nature of the crust of the Red Sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of Zabargad and The Brothers in the central and northern Red Sea. Zabargad exposes, in addition to mantle‐derived peridotites, a crustal section consisting of metagabbros and metapyroxenites, associated with silicic gneisses and cut by doleritic dikes. Seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern Red Sea. The Zabargad peridotitesilicic gneiss‐gabbroic association provides a sample of upper mantle‐lower crust in a continent before the inception of a major rifting event. The petrology and mineral chemistry of the Zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure (>900–1000 MPa or >9–10 kbar), i.e., over 30 km deep. The gneisses have tonalitic‐trondhjemitic affinity and were originally equilibrated in the garnet granulite facies, implying also high pressure and derivation from the lower continental crust. This suggests that after the subcrustal continental lithosphere along the proto‐Red Sea rift had been consumed possibly by convective thinning, but before any significant thinning of the crust, basaltic melts intruded the base of the continental crust, underplating it with gabbroic complexes. Thus crustal rifting was probably preceded by a thermal anomaly in the mantle, which supports an “active rifting” hypothesis. The gabbro/pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing PT conditions, from granulite to amphibolite to greenschist facies implying decompression and uplift, probably related to the gradual thinning of the continental crust during rifting. During this stage, additional gabbroic intrusions and basaltic dikes, exemplified by the gabbros and dolerites from The Brothers Islands, were emplaced at decreasing depths beneath the thinned continental crust. The northern Red Sea crust is presently in this stage. It is neither oceanic nor continental: it lacks a continuous, thick, basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust. Subsequently, basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete, equidistant “hot points” (as observed presently in the central Red Sea), which by axial propagation evolved into nearly continuous zones of oceanic crust accretion (as observed presently in the southern Red Sea). Processes similar to those proposed here for the evolution of the Red Sea rift can be documented in passive margins of the Atlantic and in some Alpine ophiolite complexes.
Multichannel seismic evidence for a subcrustal intrusive complex under Oahu and a model for Hawaiian volcanism
Authors: Ten Brink Uri S. , Brocher Thomas M.
Published: February 2008

Coincident multichannel seismic reflection and refraction data acquired during a wide‐aperture two‐ship experiment provide evidence for a complex crust‐mantle (C‐M) transition under Oahu, Hawaii. Several large‐aperture common depth point lines and three expanding spread profiles suggest the existence of an anomalously thick (3–6 km) C‐M transition zone underneath the volcanic ridge which extends for distances of 100 km to the north and south from the center of Oahu. The anomalous C‐M transition may represent a plutonic complex which intruded into the upper mantle and the lower crust in a 200‐km‐wide area centered at Oahu. The existence of such a large volume of intrusions near the base of the crust implies that the surficial expression of volcanism constitutes only a small fraction of the amount of melt generated at depth under the Hawaiian Islands. This interpretation is in accord with previous petrological models which predict trapping and accumulation of upwelling magma at and below the Moho. We have constructed a model which suggests that the interaction between the upwelling magma and the lithospheric flexural stress field may modulate the characteristic eruption history of Hawaiian volcanoes. In particular, the model for the plane stress field which accompanies the flexure of the oceanic crust around island chains indicates that the stress field under individual volcanoes varies considerably with its position relative to the tip of the chain. As a Hawaiian‐sized volcano develops, the magnitude of deviatoric compressive stresses under it is probably sufficient to block the conduits of the upwelling magma within the oceanic crust and to terminate eruptions. Further upwelling magma is predicted by the models to be ponded at the base of the crust. Resumption of posterosional volcanism seems to occur at a constant distance behind the center of active shield volcanism, as the horizontal compressive stresses along the axis of the chain are released. Observed orientations of dikes of this volcanic phase agree with the directions of the maximum calculated stresses. Our model implies that magma upwells over a 300‐km‐wide zone and that the oceanic plate may not be fractured under the islands.
Lava fountain heights at Pu'u 'O'o, Kilauea, Hawaii: Indicators of amount and variations of exsolved magma volatiles
Authors: Head James W. , Wilson Lionel
Published: February 2008

We review the factors determining lava fountain heights in Hawaiian‐style basaltic eruptions such as those occurring during the recent series of eruptive episodes at the Pu'u ‘O’ o vent, east rift zone of Kilauea Volcano, Hawaii. Numerical solutions to the equations describing the fluid dynamics of such eruptions predict that lava fountain heights, which are indicators of the velocity of magmas in vents, should be controlled much more strongly by amounts of exsolved volatiles than by any other factors. The next most important factor is the width of the conduit system, which determines frictional losses and can be characterized by the volume flux of magma. The diameter of the surface vent required to accommodate a given discharge (i.e., instantaneous volume rate of eruption) is also a function of exsolved magma volatile content but is less dependent on this factor than is the fountain height. We simulate fountain heights for typical discharges at Pu'u ‘O’ o and find implied exsolved gas contents very close to those determined by other methods. The corresponding predicted vent diameters are comparable with (and somewhat smaller than) the observed diameter of the Pu'u ‘O’ o vent. The discrepancy is probably a consequence of processes occurring in the lava pond which is present over the vent during eruptions. The variation of fountain height with time during eruptive episodes can be used as a measure of changing exsolved magma volatile content, which can be related to processes occurring in the subsurface dike system.