bluuebunny/crossref_metadata_2025 huggingface dataset:
Petrology of gabbroic rocks from the Mid‐Cayman Rise Spreading Center
Authors: Elthon Don
Published: February 2008

Mineral compositions of a diverse suite of gabbroic rocks (anorthosites, leuco‐troctolites, troctolites, leuco‐olivine gabbros, olivine gabbros, leuco‐gabbros, and gabbros) from the Mid‐Cayman Rise spreading center indicate that these cumulate rocks have been produced by the crystal fractionation of basaltic liquids. Olivine compositions range from Fo87.8 to Fo73.0; plagioclase compositions range from An74 to An33; clinopyroxene Mg numbers range from 89.1 to 63.0. The covariations of these petrogenetic indicators are very similar to those produced during the experimental crystallization of mid‐ocean ridge basaltic (MORB) magmas. Certain features of these gabbroic rocks, however, are inconsistent with the occurrence of this fractionation at low pressures (1 atm to 2 kbar). Many of the rocks have pyroxenes with Mg numbers higher than those formed during the low‐pressure crystallization of MORBs. The low‐Ca pyroxene is orthopyroxene rather than pigeonite (the low‐Ca pyroxene that crystallizes from MORBs at low pressures). Ilmenite occurs as a minor phase even in some gabbroic rocks with very forsteritic olivines (up to Fo87.3), which contrasts with results of phenocryst and experimental studies of MORBs that demonstrate the low‐pressure crystallization of ilmenite only in highly fractionated magmas. Additionally, the olivine‐out reaction that typically produces a low‐Ca pyroxene at low pressures generally is not recorded in these rocks; olivine disappears by a reaction relationship, but a low‐Ca pyroxene is not generally found in more fractionated gabbroic rocks. Although the experimental data are not available to conclusively demonstrate that these effects were produced at moderate pressures (5–10 kbar), the effects are similar to those predicted thermodynamically and to those observed in limited experimental studies within this pressure range. It is suggested, for this reason, that the most likely scenario for the production of these gabbroic rocks is the moderate‐pressure crystallization of basaltic magmas within deep‐seated magma chambers underneath this slow‐spreading center.
LAGEOS orbit decay due to infrared radiation from Earth
Authors: Rubincam David Parry
Published: February 2008

Infrared radiation from the earth may be the principal reason for the decay of LAGEOS' orbit. The radiation heats up the laser retroreflectors embedded in LAGEOS' aluminum surface. This creates a north‐south temperature gradient on the satellite. The gradient in turn causes a force to be exerted on LAGEOS because of recoil from photons leaving its surface. The delayed heating of the retroreflectors due to their thermal inertia gives the force a net along‐track component which always acts like drag. A simple thermal model for the retroreflectors indicates that this thermal drag accounts for at least half the observed average along‐track acceleration of −3.33 × 10−12 m s−2 and perhaps much more. The contribution from the aluminum surface to this effect is negligible. Thermal drag, which is a variant of the Yarkovsky effect, can explain some, but not all, of the observed fluctuations in drag. In view of these results, charged particle drag may not be as important as previously believed for LAGEOS.
Serpentinized ultramafics and hydrothermal activity at the Mid‐Atlantic Ridge crest near 15°N
Authors: Rona P. A. , Widenfalk L. , Boström K.
Published: February 2008

The initial occurrence of serpentinized ultramafic rocks at the nontransform intersection of a wall of a rift valley with the wall of a fracture zone is described from a site at the Fifteen Twenty Fracture Zone. The ultramafics crop out in block‐faulted terrain on the upper portion of the eastern intersection between the rift valley and fracture zone walls in water depths between 2910 and 3300 m. They comprise cumulate harzburgites, pyroxenites, Iherzolites, and wehrlites, as well as gabbronorites, olivine gabbronorites, gabbropegmatites, and alteration products including serpentinites, bastite serpentinites, and asbestos. The Ti‐Zr‐Y relations and relatively constant Zr/Ti ratio in basalts recovered with the ultramafic rocks indicate a cogenetic relation from a common magmatic source that has undergone a late stage differentiation in the lower crust. Ongoing hydrothermal activity is indicated by chemical anomalies (δ3He, Mn) in the near‐bottom water at the ultramafic outcrop. The upwelling hydrothermal circulation apparently follows crust‐penetrating faults that may have controlled the diapiric ascent of the serpentinites and that continue to tap degassing magma and/or mantle. The observations presented indicate that ultramafic cumulates form beneath the rift valley adjacent to long‐offset (>100 km) ridge‐ridge transform faults, where they are serpentinized by hydrothermal processes within the initial 1 × 106 years of generation of lithosphère at a slow spreading axis. The corners formed by the intersections of the walls of a rift valley with both the transform (RT corner) and the nontransform (RN corner) portions of these fracture zones are principal loci of diapiric emplacement of serpentinized ultramafics.
Geophysical constraints on lunar bulk composition and structure: A reassessment
Authors: Hood L. L. , Jones J. H.
Published: February 2008

Theoretical lunar mantle seismic velocity and density profiles are calculated for a series of proposed bulk compositions, present‐day selenotherms, and models for early differentiation and evolution. The seismic velocity profiles are compared to the most recent observational model derived from the complete Apollo data set to evaluate compositional implications. The mantle density profiles are compared to mean density, moment‐of‐inertia, and crustal density/thickness constraints to evaluate the need for a metallic core. The calculated seismic velocity profiles are in qualitative agreement with the observational model in two general respects: (1) a quasicontinuous velocity decrease occurs in the upper mantle due to the large thermal gradients that characterize all three of the considered thermal models; (2) a discontinuous velocity increase occurs at the spinel‐to‐garnet transition (approximately 500 km depth) due mainly to the higher garnet velocities as compared to olivine and orthopyroxene. In the upper mantle, all four of the considered bulk compositions (spanning a range of plausible FeO and Al2O3 contents) are allowed when uncertainties in the temperature profile and depth of differentiation are taken into account. However, in the middle mantle (depths > 500 km), the most aluminous models (those with larger garnet abundances) consistently produce larger velocity increases that are most nearly in accord with the observational model. In general, models that assume differentiation of the upper mantle only, that are more aluminous in bulk composition, and that have strong thermal gradients in the upper mantle but smaller gradients in the middle mantle result in calculated velocity profiles that most nearly agree with the observational velocity model. An alternate differentiation model in which the middle mantle is dominated by MgO‐rich mafic silicates is also in accord with the observational seismic velocity model but results in density decreases that appear to violate stability arguments. In every case, calculated mantle density increases are insufficient to match the one‐standard‐deviation upper limit on the moment‐of‐inertia even when the most favorable combinations of allowed crustal thickness and density are assumed. Agreement with the moment‐of‐inertia upper limit is obtained if metallic cores representing 1–4% of the lunar mass are added to the bulk composition. Such cores are larger than needed to explain the observed depletions of lunar siderophile elements if the moon formed entirely from terrestrial mantle material.
Mineralogy of a basaltic clast in lunar highland regolith breccia 60019
Authors: Takeda Hirosh , Miyamoto M. , Galindo C. , Ishii T.
Published: February 2008

The mare basalt component in two lunar meteorites ALHA81005 and Y791197 has been useful in constraining their ejection sites. Although Apollo 16 regolith breccias with glassy matrices have been proposed to be the best analogs for the lunar meteorites, no mare basalt component has been found in such breccias studied so far (e.g., 60019). However, a basaltic clast, Ba‐2, was found on the cut surface of a new slab of 60019, so we have studied Ba‐2 using mineralogical techniques for comparison with the lunar meteorites. Ba‐2 is a coarse‐grained, granular basalt clast, 2.5 × 5 mm. It consists of plagioclase, high‐Ca pyroxene, ilmenite, silica, and mesostasis. The pyroxene and plagioclase crystals show moderate chemical zoning. Most of the pyroxene crystals are augite, with minor iron‐rich rims adjacent to the mesostases. The An content of plagioclase ranges from 95 to 75. Compared to Apollo and Luna mare basalts, the Ba‐2 mineral chemical trends are most like high‐alumina, moderate‐TiO2 basalts found in the Apollo 16 rake samples and to Luna 16 basalt fragments; however, olivine is not present in Ba‐2. The bulk chemical composition of Ba‐2 estimated by modal combination supports the similarity. The highland pyroxene types in the lunar meteorites bear a strong resemblance to one another and to those in 60019, but 60019 appears to have significantly fewer pyroxenes with the extremely low Mg contents of the VLT basalt component in the lunar meteorites. This difference can be explained by the fact that the amount of mare basalt in 60019 is extremely small (less than 0.1%) and the fact that Fe enrichment of pyroxene in the basalt is minor.
The meteorite component of Apollo 16 noritic impact melt breccias
Authors: Korotev Randy L.
Published: February 2008

Noritic melt rocks (alias LKFM and VHA) from Apollo 16 have concentrations of Ni approximately three to five times those of impact melt rocks of similar composition from other lunar landing sites. The melt rock associated with Apollo 16 dimict breccias has the highest mean concentration of Ni of any lunar rock type (1150 µg/g in 11 samples). The Ni is contained in Fe‐Ni metal that is dissimilar in composition to metal from ordinary chondrites in having a lower Ni concentration (6%) and lower Ni/Co ratio (17). It also has Ir/Ni and Ir/Au ratios of about one‐half and one‐third those of chondrites. The metal in the noritic impact melts is the carrier of the siderophile element signature of ancient meteorite group 1H. The composition of the Fe‐Ni metal is dissimilar to that of chondrites because it derives from metal‐rich meteorites, probably irons, that created the impact melts 3.9 Ga ago. The metal from these iron meteorites dominates the siderophile element concentrations of polymict samples at Apollo 16. Models that attempt to estimate indigenous concentrations of Ni and Co in Apollo 16 materials overestimate their abundance by assuming that the meteoritic component has chondritic ratios of Ir to Ni and Co. The type‐2 melt (VHA) associated with the dimict breccias and the type‐1 melt (Apollo 16 LKFM) were most likely produced by impact of two related iron meteoroids that impacted near the Apollo 16 site. The type‐2 melt found in samples from station 7 is probably from the same impact as that producing the dimict breccias, although it could represent the impact of a third, related iron.
Constraints on the lithospheric structure of Venus from mechanical models and tectonic surface features
Authors: Zuber Maria T.
Published: February 2008

Radar images of the surface of Venus show numerous structures that are interpreted as having formed due to horizontal compression and extension of the lithosphere. Many of these features exhibit characteristic scales (widths or spacings) of deformation, one of 10–20 km and another of 100–300 km. For a range of simple models, we test the hypothesis that these length scales are controlled by dominant wavelengths arising from unstable compression or extension of the Venus lithosphere. Results show that the existence of tectonic features that exhibit both length scales can be explained if, at the time of deformation, the lithosphere consisted of a crust that was relatively strong near the surface and weak at its base and an upper mantle that was stronger than or of nearly comparable strength to the upper crust. The spacings of these features imply crustal thicknesses in the approximate range 5–30 km and a thermal gradient not greater than 25 K km−1. Features that exhibit only the smaller scale of deformation can be explained by either a lithosphere with a thick crust that overlies a weak mantle or a lithosphere with a strong mantle but with small internal strength contrasts. For a broad range of parameters, the models require not only that smaller scale compressional and extensional features have similar spacings, but also that the widths and spacings of larger scale compressional features be greater than those formed in extension. This is consistent with observed differences in the characteristic length scales of larger scale surface features, as well as the similarities in spacings of observed smaller scale features.
Rare earth patterns in shergottite phosphates and residues
Authors: Laul J. C.
Published: February 2008

Leaching experiments with 1M HCl on ALHA 77005 powder show that rare earth elements (REE) are concentrated in accessory phosphate phases (whitlockite, apatite) that govern the REE patterns of bulk shergottites. The REE patterns of whitlockite are typically light REE depleted with a negative Eu anomaly and show a hump at the heavy REE side, while the REE pattern of apatite (in Shergotty) is light REE enriched. Parent magmas are calculated from the modal compositions of residues of ALHA 77005, Shergotty, and EETA 79001. The parent magmas lack a Eu anomaly, indicating that plagioclase was a late‐stage crystallizing phase and it probably crystallized before the phosphates. The parent magmas of ALHA 77005 and Shergotty have similar REE patterns with a subchondritic Nd/Sm ratio. However, the Sm/Nd isotopics require a light REE depleted source for ALHA 77005 (if the crystallization age is <600 m.y.) and a light REE enriched source for Shergotty. Distinct Nd and Sr isotopic signatures may suggest different source regions for shergottites.
Olivine‐spinel transitions: Experimental and thermodynamic constraints and implications for the nature of the 400‐km seismic discontinuity
Authors: Bina Craig R. , Wood Bernard J.
Published: February 2008

The sequence of high‐pressure phase transitions α→β→γ in olivine is traditionally used as a model for seismic velocity variations in the 200‐ to 650‐km‐depth interval in a mantle of peridotitic bulk composition. It has been proposed that the observed seismic velocity increase at 400‐km depth is too sharp and of too small a magnitude to be attributable to the α→β phase change and that the upper mantle must thus be chemically stratified, with the 400‐km discontinuity being due either to a combination of phase changes in a layer of pyroxene/garnet‐rich “piclogite” composition or to a chemical boundary between such a piclogite layer and an overlying peridotitic layer. Using available calorimetric, thermoelastic, and synthesis data (and their associated experimental uncertainties), we have derived internally consistent high‐pressure phase relations for the Mg2SiO4‐Fe2SiO4 join. We find that the divariant transition α→α+β→β, which is generally regarded as occurring over a broad depth interval for mantle olivine compositions, is, in fact, extremely sharp. The seismic discontinuity corresponding to the α→α+β→β transition in (Mg0.9Fe0.1)2SiO4 should occur over a depth interval (isothermal) of about 6 km at a depth of approximately 400 km; the sharpness of this transition is quite insensitive to uncertainties in the constraining calorimetric, thermoelastic, and synthesis data. In addition, we have computed seismic velocity profiles for a model mantle consisting of pure olivine of (Mg0.9Fe0.1)2SiO4 composition. Comparison of these computed profiles to those derived from recent seismic studies indicates that the magnitude of the observed velocity increase at 400‐km depth is consistent with a mantle transition zone composed of about 60–70% olivine. We conclude that there is no need to infer the existence of pyroxene/garnet‐rich compositions, such as eclogite or “piclogite,” in the transition zone, since an upper mantle of homogeneous olivine‐rich peridotitic composition is consistent with the available seismic velocity data.
The effects of free water on snow gliding
Authors: McClung David M. , Clarke Garry K. C.
Published: February 2008

Conventional equations that relate snowpack drag to glide velocity take no account of the possible influences of free water at or near the snow‐earth interface. For this reason two of the most important applications of snow mechanics, predicting maximum forces on structures and predicting the release of full‐depth avalanches, lie beyond the scope of present theory. Here we discuss two effects that can decrease snowpack drag and increase glide velocity: (1) reduction of snow viscosity by the presence of water in the lowest layers, and (2) partial separation of the snowpack from the glide interface. For partial separation we show how the presence of free water covering portions of the interface reduces the effective roughness and increases glide speed; for significant enhancement of the glide rate a substantial fraction of the bed must be water covered.
Crustal structure of the Adirondacks determined from broadband teleseismic waveform modeling
Authors: Owens Thomas J.
Published: February 2008

Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of the Department of Energy's Regional Seismic Test Network station RSNY are analyzed to examine the crustal structure beneath the Adirondack Highlands of upstate New York. Radial receiver functions are inverted in the time domain to determine the vertical shear velocity structure at four distinct back azimuths. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. Southeast of RSNY, our model consists of a thick crust, including a broad crust‐mantle transition. The most prominent structure is a high‐velocity zone (shear velocity >4.0 km/s) between 18 and 26 km depth which overlies a lower crust of low average shear velocity (< 3.7 km/s). The extent of the high‐velocity zone correlates in depth with a highly reflective zone in Consortium for Continental Reflection Profiling profiles SE of RSNY, while the deep low‐velocity zone may be correlated with a broad electrical conductivity anomaly in the Adirondacks. This general structure is also seen SW of RSNY, but is more difficult to document at NW and NE azimuths.
Ice‐covered water volcanism on Ganymede
Authors: Lee Allison M. , Clifford Stephen M.
Published: February 2008

Eruption of liquid H2O magmas along extensional fractures and graben‐bounding normal faults may have played a critical role in the development of Ganymede's grooved terrain. The resurfacing potential of a water magma is dependent on a variety of factors, including the areal extent of the source region, the rate of discharge, the thickness of the flow, and the time that it takes the flow to completely freeze to its base. In this paper the thermal evolution of such a flow is considered in detail. The minimum unfrozen lifetime of a 5‐m flow is approximately 12.5 days while a 10‐m flow would survive for at least 50 days. Heating resulting from factional head loss could reasonably extend these lifetimes by 50% or more. With a discharge rate of the order of 1–10 km3 d−1, an individual volcanic water flow could flood an area of ∼104 km2 before freezing. As the flow solidifies, its volume will increase, thus lifting and arching its protective ice cover. Extensional fractures may then develop in the ice subparallel to the graben walls. These fractures could result in grooves directly, given a sufficiently thick (∼1 km) flow, or they may simply act to concentrate various tectonic forces that could initiate groove‐producing faults.
Borehole tensor strain measurements in California
Authors: Gladwin Michael T. , Gwyther Ross L. , Hart Rhodes , Francis Max , Johnston M. J. S.
Published: February 2008

Two continuous borehole plane strain monitoring sites have been operational in California since late 1983, using borehole tensor strain monitors implanted at a depth of 150 m. Shear strain data at subtidal sensitivies were available immediately after installation without contamination by bond curing or thermal recovery signals. At Pinon Flat Observatory, data indicate a constant shear strain accumulation of 0.6 microstrain per annum with the axis of maximum compression oriented 50° ± 5° west of north. This result differs significantly from regional geodetic estimates, the amplitude being dominated by continued viscoelastic response of the hole. Variations of measured strain rate exceeding 100 nstrain/yr would be significant at this instrument site. At San Juan Bautista, measurements of the angle of maximum compression (10° ± 3° west of north) agree closely with previous geodetic and hydrofracture estimates. After 2 years of operation the residual shear strain rate is 1 μstrain/yr. Preliminary analysis of strain steps observed at San Juan Bautista during the Morgan Hill earthquake of April 24, 1984, show good agreement with calculations from seismically determined source parameters for this event.
The Steens Mountain (Oregon) Geomagnetic Polarity Transition: 3. Its regional significance
Authors: Mankinen Edward A. , Larson Edwin E. , Gromme C. Sherman , Prevot Michel , Coe Robert S.
Published: February 2008

Study of the variations of direction and intensity of the geomagnetic field as recorded by the Miocene lava flows on Steens Mountain, southeastern Oregon, has resulted in a detailed description of total field behavior during a reversal in polarity. In addition to information about the polarity reversal itself, the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15.5 Ma. In order to test the feasibility of using this record as a means of correlation in this part of the western United States, comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the Steens Basalt or rocks of equivalent age. Despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes, convincing similarities among some of the records are evident. The Steens Basalt paleomagnetic record does, indeed, have potential as a correlation tool during this time of widespread basaltic volcanism. Additionally, paleomagnetic data from flows that were sampled in detail yield a middle Miocene paleomagnetic pole at 88.3°N, 209.0° (α95 = 6.3°) for the High Lava Plains of Oregon. This pole position is statistically indistinguishable from the earth's rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted. Data from selected sites within the coeval part of the Columbia River Basalt Group yield a paleomagnetic pole at 88.7°N, 171.6°E (α95 = 4.0°). The Columbia River Basalt Group pole is statistically indistinguishable from either the rotational axis or from the High Lava Plains pole. These findings indicate no post‐20 Ma differential rotation between south‐eastern Washington and south‐central Oregon, in contrast to previous interpretations.
The morphology of ice streams A, B, and C, west Antarctica, and their environs
Authors: Shabtaie Sion , Whillans Ian M. , Bentley Charles R.
Published: February 2008

Airborne radar soundings of the ice sheet surface made in 1984–1985, together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams A, B, and C and much of the region around them. The flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking. Ice thicknesses were also measured along the flight lines but have not yet been mapped. The surfaces of active ice streams A and B exhibit a longitudinal ridgetrough topography of uncertain origin. Prominent surface valleys are associated with most, but not all, of their marginal shear zones. There is a deep subglacial trough beneath the grid northeastern side of ice stream A that connects to the subglacial trough beneath Reedy Glacier. The boundary between ice streams Bl and B2, the two tributaries of ice stream B, persists as a suture zone for 250 km downstream from their merger. Ice stream Bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream B until it disappears close to the grounding line. Between ice streams Bl and B2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice, which suggest to us that “rafts” of ice are being incorporated into the ice streams. The previously reported negative mass balance for the drainage system of ice stream B may arise from the expansion of the ice stream into the interior of the West Antarctic ice sheet. Crevasse bands at some places on ridge AB, together with high glacial velocities reported elsewhere, also suggest this expansion. Close to the Ross Ice Shelf, ice stream B widens into a nearly flat area where the mean surface slope is only 3.5 × 10−4, and the surface elevations are only a few tens of meters above hydrostatic equilibrium. Inactive ice stream C differs from active ice streams A and B in surficial and basal characteristics. No elongated ridges and troughs are observed; instead, the ice stream surface exhibits several terraces, including some maxima and minima in elevation. Radar sounding reveals areas where basal echoes are strong and steady, indicating subglacial water, alternating with areas of weaker echoes with short fading lengths. Calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick, depending on the electrical conductivity of the water. Due to reversals in the surface slope, the hydrologic potential gradient changes sign several times along ice stream C, which implies that subglacial water should collect in some locations.
Quartz‐cemented breccias from the Mid‐Atlantic Ridge: Samples of a high‐salinity hydrothermal upflow zone
Authors: Delaney John R. , Mogk David W. , Mottl Michael
Published: February 2008

Textural, mineralogical, chemical, and fluid inclusion studies on a suite of quartz‐rich, sulfide‐bearing greenstone breccias indicate that high‐salinity, high fluid flow systems were recently active on the Mid‐Atlantic Ridge south of the Kane Fracture Zone. These systems involved fluids with temperatures close to 300°C and salinities almost 3 times that of seawater. Rounded, pebblelike fragments enclosed within a matrix of euhedral quartz crystals appear to have been cemented by quartz deposition from a moving fluid which attained velocities in excess of 1 m/s. Homogenization temperatures and freezing point depression effects in fluid inclusions from texturally early and late quartz crystals indicate temporal evolution of fluid temperatures and NaCl equivalent salinities from 290°C and 10 wt % to 200°C and 4.7 wt % during alteration and cementation of the breccias. Highly variable cobalt concentrations in the evolving hydrothermal solutions are indicated by fine scale oscillatory zoning in euhedral pyrite within the breccias. A continuous supply of the high‐salinity fluids may be generated either by supercritical two‐phase separation at depths well below the level of inclusion entrapment or by retrograde dissolution of hydroxy chloride phases in the deeper portions of the hydrothermal system. Either model requires the initial temperature of the deeply circulating fluids to have been distinctly higher than 415°C and the fluid to have been cooled by conduction or by mixing with lower‐temperature seawater prior to entrapment within the inclusions.
Venus gravity: A harmonic analysis
Authors: Bills Bruce G. , Kiefer Walter S. , Jones Robert L.
Published: February 2008

An improved model of Venusian global gravity has been obtained by fitting an eighteenth‐degree and eighteenth‐order spherical harmonic series to 78 orbital arcs of high altitude (950–1350 km at periapsis) tracking data and 351 orbital arcs of lower‐altitude (150–200 km at periapsis) data from the Pioneer Venus Orbiter (PVO). Compared to a recently published tenth degree model [Mottinger et al, 1985], which is based on the 78 high‐altitude arcs only, the current model provides a significant improvement in resolution and fidelity. As a measure of this improvement, we note that for the low‐altitude arcs alone, the variance of the residuals for the present model is reduced to 19% of the data variance, compared to 51% for the tenth‐degree model. Venus differs significantly from the Earth in that it exhibits a significant correlation between long‐wavelength topography and gravity. The gravity/topography spectral admittances are inconsistent with either Airy or Pratt isostasy, but they are consistent with dynamic support by mantle convection.
Analytical method for evaluating superficial fault displacements in volcanic air fall deposits: Case of the Hirayama Fault, south of Tanzawa Mountains, central Japan, since 21,500 years B.P.
Authors: Ito Tanio , Uesugi Yo , Yonezawa Hiroshi , Kano Ken'ichi , Someno Makoto , Chiba Tatsuro , Kimura Toshio
Published: February 2008

The establishment of an analytical method for evaluating superficial fault displacements must contribute greatly to the improvement of methods for earthquake prediction in the Quaternary volcanic areas where many volcanic air fall deposits of known age are widely traceable. However, because the air fall deposits of uniform thickness cover many surfaces of blocks separated by faults, cumulative displacements along faults cannot always be detected in the deposits. Then, the study of geomorphological developments along a fault becomes important in analysis of superficial faults in air fall deposits. The Hirayama fault, south of the Tanzawa mountains, central Japan, is analyzed as an optimal example for studying the methodology in air fall deposits. The most important result of the detailed analysis is that the displacement rate of the Hirayama fault has been decreasing since 21,500 years B.P.; that is, the Hirayama fault has nonuniform history of movement. The result consequently indicates that the assumption of uniform history in fault movement is not always safe in earthquake prediction.
Faunal composition and organic surface encrustations at hydrothermal vents on the southern Juan De Fuca Ridge
Authors: Tunnicliffe Verena , Fontaine A. R.
Published: February 2008

Examination of a small collection of macroinvertebrates from three vents of the southern Juan de Fuca vent field reveals differences between the vents with respect to species composition, species habits, and microbial and metallic deposits on their surfaces. TWo apparently new vestimentiferan species were found, and for the first time the Juan de Fuca palm worm was observed to dwell on smokers. High acidity values recorded in this system may interfere with the process of shell calcification in an archaeogastropod snail. The surfaces of vestimentifer an tubes at two vents are heavily encrusted with microbial and metallic accumulations. Scanning electron microscope, transmission electron microscope, and energy dispersive X ray microanalysis observations show that iron‐based crusts on orange tubes are built from accumulations of an Fe‐rich particle of distinctive size and shape. Morphological evidence is presented for the microbial origin of Fe‐rich particles. Zn‐rich particles found on black tubes are not of microbial origin. We suggest that iron deposition on surfaces in the vent environment is initially biocatalytic but subsequent deposits may build by simple inorganic reactions.
Pb, Sr, and Nd isotopes in basalts and sulfides from the Juan de Fuca Ridge
Authors: Hegner E. , Tatsumoto M.
Published: February 2008

Pb, Sr, and Nd isotopes of seven basalt glasses collected by the submersible Alvin from the southern Juan de Fuca Ridge (SJFR) are almost identical (206Pb/204Pb ∼ 18.45, 207Pb/204Pb ∼ 15.47, 208pb/204pb ∼ 37.81 87Sr/86Sr ∼ 0.70249, 143Nd/144Nd ∼ 0.51315). Whereas all basalts appear cogenetic, four of the samples have uniform abundances of U, Th, Rb, Nd, Sm, Pb, and Sr, indicating that they are also comagmatic. Two basalt glasses dredged previously at the SJFR have similiar isotopic compositions but higher concentrations of U, Th, and Pb. The 206Pb/204Pb ratios are intermediate between generally less radiogenic ridge basalts from south of the Juan de Fuca Ridge (JFR) and often more radiogenic basalts from the northern JFR and NE Pacific seamounts. Sr and Nd isotopic compositions closely resemble data of other ridge basalts from the northernmost East Pacific Rise and are intermediate between isotopically more diverse seamount basalts produced nearby. Eight sulfides collected from three hydrothermal vent sites at SJFR have very similar Pb isotopic compositions to that of the associated basalts. Sr and Nd isotopic ratios of a single sulfide sample lie on a mixing curve between seawater and basalt data, indicating that the sulfide precipitated after mixing of hydrothermal solutions and ambient seawater.
U, Th, and Pb isotopes in hot springs on the Juan de Fuca Ridge
Authors: Chen J. H.
Published: February 2008

The concentrations and isotopic compositions of U, Th, and Pb in three hydrothermal fluids from the Juan de Fuca Ridge were determined. The samples consisted of 10.2–57.6% of the pure hydrothermal end‐members based on Mg contents. The Pb contents of the samples ranged from 34 to 87 ng/g, U from 1.3 to 3.0 ng/g, and Th from 0.2 to 7.7 pg/g. These samples showed large enrichments of Pb and Th relative to deep‐sea water and some depletion of U. They did not show coherent relationships with Mg, however, indicating nonideal mixings between the hot hydrothermal fluids and cold ambient seawater. Particles filtered from these hydrothermal fluids contained significant amounts of Th and Pb which may effectively increase the concentration of these elements in the fluids when acidified. The 234U/238U values in all samples show a 234U enrichment relative to the equilibrium value and have a seawater signature. The Pb isotopic composition of the Juan de Fuca hydrothermal fluids resembles that of 21°N East Pacific Rise and has a uniform mid‐ocean ridge basalt signature. The hydrothermal systems at oceanic spreading ridges have circulated through a large volume of basalts. Therefore Pb in these fluids may represent the best average value of the local oceanic crust. From the effects of U deposition from seawater to the crust and Pb extraction from rock to the ocean, the U/Pb ratio in the hydrothermally altered oceanic crust may be increased significantly.
Noble gases in diamonds: Occurrences of solarlike helium and neon
Authors: Honda M. , Reynolds J. H. , Roedder E. , Epstein S.
Published: February 2008

We have measured noble gases in 17 diamond samples, mostly inclusion free, from diverse, known locations. The 3He/4He ratios are characterized by a large spread (104), ranging from values below atmospheric to close to the solar ratio. Highest ratios were seen for an Australian colorless diamond composite and an Arkansas diamond. These samples also have imprecise but intriguing neon isotopic ratios, which are close to the solar value. An origin for the solarlike He and Ne in the diamond samples is unlikely to be accounted for by the presence of nucleogenic or spallogenic components. For single diamond stones a positive correlation is found between 3He/4He and 13C/12C, possibly indicating that heavy carbon is accompanied by primordial helium. However, the He result for the Australian colorless diamond composite with low δ13C value requires another explanation, possibly sedimentary carbon contaminated with cosmic dust. The wide variation in 4He/40*Ar ratios observed from diamond samples suggests a complex history for the source regions and the diamond crystallization processes. Results for two Australian diamond composites (colorless and colored), which came from the same kimberlite pipe, are especially notable: the colorless stones contain no radiogenic components but solarlike He and Ne isotopic ratios, whereas the colored stones are enriched in radiogenic and fissiogenic components. Seemingly the Australian diamonds crystallized in a heterogeneous environment in the mantle source region. A pair of Arkansas diamonds, believed to be from a single pipe, exhibits similar anomalies.
The central anomaly magnetic high: Implications for ocean crust construction and evolution
Authors: Tivey Maurice A. , Johnson H. Paul
Published: February 2008

The central anomaly magnetic high (CAMH) is a long, lineated, short‐wavelength (<10 km wide at the sea surface) magnetic anomaly overlying many of the world's midocean spreading centers. This anomaly is particularly well developed over the northern Juan de Fuca Ridge, including the Northern Symmetrical and Endeavour segments. The ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near‐axis crustal plate evolution. Recently obtained deep‐tow magnetometer data from segments of the Juan de Fuca spreading center have caused us to revise the present models of the source of the CAMH, and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates. In this model, we attribute the near‐axis magnetization contrast to tectonically controlled low‐temperature alteration of the upper vertical section of extrusive basement. Extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis. The correlation between fissured crustal blocks and near‐bottom magnetic lows, and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off‐axis tow over an outer flanking ridge.
Elastic stress and deformation near a finite spherical magma body: Resolution of the point source paradox
Authors: McTigue D. F.
Published: February 2008

Approximate solutions are obtained for the stress and displacement fields due to a pressurized spherical cavity in an elastic half‐space. The solutions take the form of series expansions in powers of ε = a/d, where a is the cavity radius and d is the depth. The leading‐order term in the expression for the surface uplift, which arises at O(ε3), recovers the well‐known result of Mogi for the response to a point dilatation. The first higher‐order correction accounts for a cavity of finite size and thus offers the possibility of fitting leveling data for not only the depth but also the radius and pressure increment. However, this correction is of O(ε6) and, consequently, is weak. The result provides a formal explanation for the success of the point dilatation model in representing uplift data even when it is known independently that ε is not small. The higher‐order correction causes the surface uplift to fall off more rapidly in the radial direction, implying that a fit of the point source solution tends to underestimate the depth d. In contrast to the surface displacement, the stress field near the cavity is affected profoundly by the proximity of the free surface. Three higher‐order corrections to the stress field are obtained, which result in a uniformly valid approximation to O(ε5). The hoop stress at the cavity exhibits a tensile maximum at the circle of tangency with a cone with its apex at the free surface. This result appears to be consistent with the locus of fractures radiating outward from the magma body inferred by seismic methods in Long Valley, California.
Fluctuations in surface height of active lava lakes during 1972–1974 Mauna Ulu Eruption, Kilauea Volcano, Hawaii
Authors: Tilling Robert I.
Published: February 2008

An “active” lava lake is one that forms directly on top of a magma column, as opposed to an “inactive” one formed by passive ponding of lava within a preexisting crater. Two active lava lakes operating within summit craters of newly constructed volcanic shields at Mauna Ulu and Alae were the longest‐lived of any observed in a Hawaiian rift zone to date. Small‐scale fluctuations (a few to 20 m) in the height of lake surfaces are typically associated with degassing (“gas piston”) activity. Larger fluctuations, as great as 200 m, reflect major changes in eruptive activity. Measurements of surface height fluctuations of Mauna Ulu lake show gross positive correlation with variation in summit tilt, which in turn reflects the inflation‐deflation state of Kilauea's summit magma reservoir. A more tenuous conclusion is that a sympathetic variation possibly exists between surface heights of the Mauna Ulu and Alae lakes. Collectively, these observations (1) require the existence of a fully engorged, connected, open, and hydraulically efficient magma conduit system linking the summit reservoir with the lava lakes, (2) provide additional evidence for the postulated regime of “quasi‐steady‐state” magma transfer that characterized much of the eruption, (3) imply that fluctuations in lake surface reflect variation in magma supply rate to the reservoir and/or outflow rate from the Mauna Ulu‐Alae vents, and (4) suggest undetected episodes of magma intrusion or storage in Kilauea's east rift zone during 1973.
Shear instability in a viscoelastic material as the cause of deep focus earthquakes
Authors: Ogawa Masaki
Published: February 2008

The mechanism of deep focus earthquakes has been examined by numerical and linear analysis of shear instability in subducting slabs. We assume subducting slabs deform such that the spatially averaged strain rate
becomes time independent. Furthermore, we assume that rheology depends on temperature in the manner of an activation process. Using this model, we investigate quantitatively the conditions under which shear instability takes place, i.e., the deformation in the material concentrates in a thin layer whose temperature increases explosively until melting occurs. First, we study instability in a material whose rheological properties are spatially homogeneous. Shear instability takes place in a homogeneous material when
exceeds a certain critical level
, which depends strongly on the stress in the slabs but does not depend on the detail of the creep law (the value of the activation energy). Next, we study instability in a material that contains shear zones similar to those often observed by field geologists, i.e., a thin layer made of material with lower vicosity than the surrounding region. We find that shear zones trigger shear instability even for the case of
provided that G, the ratio of characteristic time of thermal diffusion to that of shear heating in shear zones, exceeds 1, and that f, the ratio of stiffness of elastic deformation in the surrounding region to that of ductile deformation in shear zones, becomes lower than 1. Applying this condition to subducting slabs with shear zones, we find that shear zones trigger shear instability even when the average strain rate
is lower than
by an order of magnitude. From these results, and from an estimate of
in subducting slabs, we conclude that shear instability is a mechanism likely to induce deep focus earthquakes.
The relationship between Wadati‐Benioff Zone geometry and P, T and B axes of intermediate and deep focus earthquakes
Authors: Apperson K. Denise , Frohlich Cliff
Published: February 2008

For 865 earthquakes with focal depths exceeding 90 km this paper presents a concise global summary of the distributions of P, T, and B axes of their focal mechanisms with respect to the downdip, along‐strike, and normal axes of Wadati‐Benioff zones. The focal mechanisms are best double‐couple solutions for the centroid moment tensors reported by the Harvard group, with the P, T, and B axes corresponding to the principal axes of the individual moment tensors. We find, as did previous investigators, that events below 300 km have predominantly downdip P axes, with 50% of all P axes of deep focus events lying within 28° of downdip. For events between 90‐ and 300‐km depth there are regional differences, but most regions possess events with nearly downdip P axes or events with nearly downdip T axes. In almost all regions having downdip P or T axes, Bingham statistics show that the greatest concentration of P or T axes lies somewhat below the plane of the Wadati‐Benioff zone. For events of all focal depths in nearly all regions, B axes cluster along the strike direction, with 50% occurring within 38° of the strike. From these regional results we define a “typical” focal mechanism as one having the B axis within 30° of the along strike and having either the T or P axis within 30° of the downdip. Such a focal mechanism might occur in response to a stress tensor having its greatest or least principal stresses channeled approximately along the downdip direction of the subducting slab, with the intermediate principal stress lying horizontally within the slab, controlled possibly by slab bending. However, by this criterion, only 29% of the focal mechanisms for deep and intermediate events are typical. Thus the majority of the individual events cannot be explained simply as occurring in response to a downdip and slab‐normal greatest and least principal stress system, with a horizontal along‐strike intermediate principal stress. A few regions, such as Java and the Marianas, are clearly anomalous in that they possess significantly fewer typical focal mechanisms than do most other regions.
A unified lunar control network: The near side
Authors: Davies Merton E. , Colvin Tim R. , Meyer Donald L.
Published: February 2008

The Lunakhod 2, Apollo 11, 14, and 15 retroreflectors, and the Apollo 15, 16, and 17 ALSEP antenna locations are the most accurately located positions on the Moon. The Apollo 15 retroreflector and the Apollo 15, 16, and 17 ALSEP antennas have been identified on high‐resolution Apollo panoramic pictures and their locations transferred to Apollo mapping camera frames. Three Apollo control networks have been computed, two by the Defense Mapping Agency Aerospace Center and one by the National Ocean Survey/U.S. Geological Survey, based on measurements of points on the Apollo mapping camera frames. Transformations involving translation, rotation, and scale were determined that adjusted the coordinates of the points of the three control networks to the center of mass origin and the mean Earth/polar axis coordinate system defined by the lunar ranging retroreflector experiment. The transformed DMA/603 control network was the best fit to the ALSEP coordinates and was selected as a base to transform all points in D. L. Meyer/s (DMAAC) telescopic network to the new origin and coordinate system. New coordinates of 1156 of these points are given as are 10 points from a Mariner 10 solution in the north polar region. These coordinates are presented in the appendix and are available only in the microfiche version of this journal.