bluuebunny/crossref_metadata_2025 huggingface dataset:
Galapagos Hot Spot‐Spreading Center System: 1. Spatial petrological and geochemical variations (83°W–101°W)
Authors: Schilling J.‐G. , Kingsley R. H. , Devine J. D.
Published: February 2008

We report on the petrology and geochemistry of basalts dredged at 40–50 km intervals along the Galapagos Spreading Center, between 83°W and 101°W (40 stations). Emphasis is on spatial variations of ‘whole rock’ major elements, rare earths, trace metals of the first transition series, and the nature of phenocryst assemblages and their abundances. These results provide new constraints on the nature and scale of mantle source heterogeneities, melting conditions, thermal field, and dynamics of crustal formation of the region. We suggest that ridge segments outside the high magnetic amplitude zone are at a steady state as a result of passive seafloor spreading. Basalts from these segments are apparently derived from an asthenosphere relatively uniformally depleted in incompatible elements, which appears of worldwide extent. We reject Vogt and DeBoer's [1976] model invoking damming at fracture zones of subaxial astenosphere flow of crystal slushes and increasing fractional crystallization down the flow line, because this model would not explain the gradients in REE observed about the Galapagos Platform. Our preferred model combines the mantle‐plume binary mixing model of Schilling [1973] with the concept of recurring rift propagation proposed by Hey [1977a]. We further propose that pulsating mantle plume flux, perhaps in the form of a chain of blobs, may initiate the development of new rifts and their propagation. The present position of the tips of such new propagating rifts locate the wave fronts of such pulsating mantle plume flow. A two million year period is suggested for the last 4 m.y. from Wilson and Hey's [1979] information. Rigorous testing of our preferred model is possible.
Systematics of rare gas isotopes in basic lavas and ultramafic xenoliths
Authors: Kyser T. Kurtis , Rison W.
Published: February 2008

Mid‐ocean ridge (MOR) tholeiites and several ultramafic xenoliths from a variety of localities have identical 3He/4He ratios of 1.3×10−5 which indicate that a large portion of the mantle has a constant helium isotopic composition. In contrast, megacrysts from alkali basalts, phenocrysts from andesites, and some nodules have 3He/4He ratios of 0.3 to 1.3×10−5, and submarine tholeiites from Kilauea have ratios greater than 1.3×10−5. The low ratios of some nodules may be due either to loss of 3He by prior outgassing events that lowered the 3He/(U + Th) ratio or to interactions with metasomatic fluids having low 3He/4He or 3He/(U + Th) ratios. Andesites and nodules associated with subduction zones have 3He/4He ratios indicative of mixing between the helium in MOR tholeiites and subducted helium with a low 3He/4He ratio. High 3He/4He ratios of Hawaiian tholeiites, like the ratios measured in materials from other hot spots, reflect a deeper, less depleted source than do MOR tholeiites. Some xenoliths and lavas have greater 21Ne/22Ne and 20Ne/22Ne ratios than does the atmosphere. Correlation of excess 21Ne with 4He in most samples suggests that the 21Ne is nucleogenic. The excesses of 20Ne are probably the result of mass fractionation processes. Hawaiian submarine lavas, which may sample deeper, less depleted portions of the mantle than do MOR tholeiites, not only have high 3He/4He ratios but also low40Ar/36Ar ratios. Many nodules and megacrysts related to alkali basalts have lower 3He/4He ratios but higher 40Ar/36Ar ratios than do MOR tholeiites because of prior outgassing events that have lowered the 3He/(U + Th) and 36Ar/K ratios. Lavas and nodules associated with subduction zones can have variable 3He/4He and 40Ar/36Ar ratios because of the addition of subducted gases and the disturbance of the relative position of depleted and undepleted portions of the mantle.
The role of iron partitioning in mantle composition, evolution, and scale of convection
Authors: Watt J. Peter , Ahrens Thomas J.
Published: February 2008

The effect on composition and evolution of the mantle of the recently‐observed strong concentration of iron in (Mg, Fe) O‐magnesiowüstite (mw) at the expense of (Mg, Fe) SiO3‐perovskite (pv) structure is studied by calculating a temperature‐ and pressure‐dependent iron partitioning coefficient for the lower mantle. The value of the standard entropy for MgSiO3‐perovskite is found to be 69.4±10.3 J/mole deg from the recently determined phase diagram of forsterite. Iron remains concentrated in (Mg, Fe) O throughout the entire lower mantle if account is taken of an FeO phase change, with the partitioning coefficient (xpvFe/xpvMg)/(xmwFe/xmwMg) increasing from 0.04 to 0.8 between 670 Km a the core‐mantle boundary. Partitioning has negligible effect on gross density and elastic properties Of the lower mantle. By using recent shock wave and static compression results for FeO and MgSiO3‐perovskite, we find that the lower mantle is more pyroxene‐rich than the upper mantle and as iron‐rich, or somewhat less so, than the upper mantle. Mg/(Mg + Fe) = 0.93–0.95 for the lower mantle compared with 0.85–0.90 for the uppermost mantle. The lower mantle Mg/Si ratio is closer to chondritic values (0.99± 0.03) (≈1.5 for a peridotite with px/ol = 0.4(molar)), thus supporting the idea of a chemically layered mantle with implications for the style of mantle convection. While partitioning of iron has no significant effect on gross lower mantle density, we find that the (Mg, Fe) O and perovskite components of the lower mantle have essentially the same densities. Mantles with higher bulk iron contents have (Mg, Fe) O denser than the perovskite component; for a bulk magnesium mole fraction of, 0.80, the density difference is 0.7–0.8 g/cm3. We investigate the feasibility of the Mao, Bell, and Yagi gravitational separation hypothesis of mantle evolution in which a mantle more iron‐rich than present loses iron through gravitational sinking of the denser (Mg, Fe) O, and we conclude that the process cannot successfully compete with solid state convection unless implausibly large grain sizes or unacceptably low viscosities are invoked. A likely explanation for removal of iron from an initially iron rich lower mantle is upward extraction of FeO‐enriched basalts or picrites and concentration of iron in upper mantle garnets during accretion of the earth or subsequent convection with the entire mantle passing through the partial melt zone. Thus the lower mantle was depleted of iron relative to both the upper mantle and the mantles of the small terrestrial planets and satellites, which do not have mantle pressures sufficient to form perovskite‐structure silicates, or which had lower accretional temperatures and less extensive melting. On this basis, Venus would be expected to have a mantle similarly depleted in iron.
Crustal structure near the Iceland Research Drilling Project borehole from a seismic refraction survey
Authors: Thomson William H. , Garmany Jan D. , Lewis Brian T. R.
Published: February 2008

A seismic refraction survey was conducted at Reydarfjördur, Iceland, as part of the Iceland Research Drilling Project in July 1978. An array of 15 seismometers was used to measure apparent velocities from 25 explosions along a 23 km refraction line. The experiment yielded data used to determine the P‐ and S‐wave velocity structures beneath the drill site and to obtain bounds on the depth to seismic ‘layer 3.’ The observed apparent velocities vary continuously as a function of range with a marked increase in velocity at a range of 16 km. This corresponds to an observed cross‐over in the travel time data. The measurement of apparent velocities has yielded data of the form X(p), the distance to a measured ray parameter, and τ(p), the delay time. Joint inversion of the X(p) and τ(p) data has been performed by linear programing. Layers of constant slowness gradient are used as the basis for the inversion process. Estimates of error in the data are used to produce extremal bounds on the family of earth models that satisfy the data. When low velocity zones are assumed to be absent, the resulting envelope of P‐wave velocity models indicates a continuous variation of velocity with depth. Two regions of high velocity gradient are observed, one near the surface and the other at depths between 2.6 and 3.9 km associated with layer 3 in Iceland. These results suggest that modeling of Iceland's upper crust with a few thick homogeneous constant velocity layers is an inadequate representation of the velocity structure. Some features observed in the seismic data suggest the presence of a low velocity zone with a lid between 1.6 and 2.5 km depth. Rough synthetic modeling places this lid at the shallower end of this range.
Primary and secondary alkali and halogen element distribution in Iceland Research Drilling Project basalts from eastern Iceland
Authors: Floyd P. A. , Fuge R.
Published: February 2008

The degree of oxidation and hydration, and the variation of Fe, Mg, Li, Na, K, Rb, F, Cl, and I are reported for hydrothermally altered basaltic lavas and dikes intersected in the Iceland Research Drilling Project (IRDP) borehole sequence from eastern Iceland. The concentrations of water‐soluble Na, Cl, and I have also been determined and represent approximately 1%, 40%, and 53%, respectively, of the total content of each element. Hydration and oxidation in both lavas and dikes generally increase downward in the total lava pile to maxima toward the base of the drilled section. Three lava groups and two dike groups are recognized at different depths on the basis of the relative distribution of Zr and Y. Each group exhibits a trend toward more primitive compositions upward in the pile. Within group and vertical chemical variations reflect both (1) primary chemical features such as parental magma composition and degree of crystal fractionation and (2) secondary mobilization during hydrothermal alteration. The latter caused variable alkali and halogen element dispersion, but initial differences between the chemical groups appear still to be discernible. Mobility sequences were Rb > K > Li > Na and Cl > F > I. Although all the analyzed elements were mobilized at the relatively low levels of hydration, few systematic trends with progressive hydration are observed: e.g., K and Rb were leached and Li was sometimes enriched. The main effect of alteration was a localized redistribution of mobile elements into stable secondary phases. Apparent depletions of mobile elements at the base of the sequence reflect primary variations rather than extensive leaching.
The trace element composition of the lavas and dikes from a 3‐km vertical section through the lava pile of eastern Iceland
Authors: Gibson Ian L. , Kirkpatrick R. James , Emmerman Rolf , Schmincke Hans‐Ulrich , Pritchard Gary , Oakley Peter J. , Thorpe Richard S. , Marriner Gizelle F.
Published: February 2008

The Tertiary lava succession of eastern Iceland dips at shallow angles towards and beneath the axial Quaternary volcanic zone. A 3‐km vertical section through part of this Tertiary pile, studied during the Iceland Research Drilling Project, contains flows ranging in composition from basalt to icelandite. This succession can be divided into three distinct stratigraphic groups: upper and lower groups, 1.7 and 0.6 km thick with Zr/Y ratios of about 4.5, and a 0.7‐km‐thick middle group with a Zr/Y ratio of about 6.5. The middle group also has a significantly higher Ce/Yb ratio and higher elemental Sr abundances. The data on within‐group variation are equally compatible with high‐level fractionation of a plagioclase‐olivine‐clinopyroxene assemblage with accessory apatite and opaque minerals or with open system fractionation models. Dikes occur throughout the 3‐km section and are comparable in trace element composition to the lavas of the upper group. The dikes are part of a N‐S trending swarm originating from the Breiddalur volcanic center some 15 km to the south. These dikes were injected laterally, northward into the Reydarfjordur lava pile at the time of the eruption of the upper group lavas. The lower two major lava groups may be related to similar, but unidentified, discrete volcanic centers perhaps buried downdip. The new data on this deeper section corroborate earlier studies which indicated that on a gross scale, the eastern Iceland lava pile becomes less mafic with depth. This vertical change occurs because each major volcanic center erupted more silicic lavas close to the center and more magnesium‐rich compositions toward the periphery. The formation of the lava pile occurred by tilting toward the spreading center and overlapping repetition of such major units which resulted in the more mafic edges of the lava groups forming the topographically higher parts of the crustal section.
Density and porosity logging in the IRDP hole, Iceland
Authors: Jónsson Geirfinnur , Stefánsson Valgardur
Published: February 2008

The results from the gamma‐gamma density log and the neutron‐neutron porosity log in the continuously cored IRDP hole are presented. As the core recovery was 99.7%, there is an unique opportunity to compare the response to logging tools in basaltic environment to laboratory measurements on the core samples. Special calibration pits for density calibration were constructed, and the logging data for densities were found to be in an agreement with laboratory measurements on core samples. For porosity, the laboratory data were found to be lower than those obtained from the logs. The reason for this was found to be that effective porosity, as measured in the laboratory, is generally significantly lower in basalt than the total porosity measured by the logs. The mean deviation in the repeatability of the density is ±160 kg/m3 and for the apparent limestone porosity index ±3%. A correlation coefficient of −0.71 is found between the matrix density and the concentration of SiO2 in the rocks, and a correlation coefficient of −0.76 is found between the matrix density and the gamma ray intensity of the rocks. It is proposed that the presence of zeolites and other secondary minerals cause the observed systematic scatter in the density‐porosity relation within individual units in the hole. Owing to a lower porosity the mean bulk density of the intrusions is higher than the mean bulk density of the flows. In contrast, the matrix densities of flows and dikes are shown to be equal within limits of error. The average density of the 1900‐m‐thick section logged is (2.85±0.27)×103 kg/m3 for the bulk density and (3.03±0.20)×103 kg/m3 for the matrix density. The average porosity is 9%. Significant variations in the matrix density with depth are shown to be associated with three phases in the volcanic activity building up to basaltic pile in Reydarfjördur. The matrix density obtained from the logs in the IRDP hole is consistent with other published determinations of the matrix density of the oceanic crust.
Northern East Pacific Rise: Magnetic anomaly and bathymetric framework
Authors: Klitgord Kim D. , Mammerickx Jacqueline
Published: February 2008

The oceanic crust in the eastern Pacific between 7°N and 30°N and east of 127°W contains a fairly complete history of the spreading centers associated with the East Pacific Rise since 25 m.y. B.P. (late Oligocene). In this paper, we have summarized the seafloor spreading magnetic‐anomaly data and the bathymetric data that reflect the record of this tectonic history. The well‐defined magnetic lineations north of the Clarion fracture zone, in the mouth of the Gulf of California, and on the east flank of the East Pacific Rise (EPR) are carefully examined and used to provide a guide for interpreting the spreading pattern between the Clarion and Clipperton fracture zones, southward of the Rivera fracture zone over the Mathematician Ridge, and over the entire EPR east of the Mathematician Ridge between the Rivera and Siqueiros fracture zones. The bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas. The fracture zone bathymetry and the seafloor spreading magnetic lineations on the EPR south of the Rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations. All these data provide a good record of the plate reorganizations in the middle Miocene at magnetic anomaly 5 A time (12.5 to 11 m.y. B.P.), in the late Miocene at magnetic anomaly 3′−4 time (6.5 m.y. B.P.), and in the Pliocene at magnetic anomaly 2′−3 time (3.5 m.y. B.P.). Several abandoned spreading centers, including the Mathematician Ridge, were left behind as a result of these reorganizations. The Mathematician Ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5A.
Detection and location of earthquakes in the Central Aleutian Subduction Zone using island and ocean bottom seismograph stations
Authors: Frohlich Cliff , Billington Selena , Engdahl E. R. , Malahoff Alexander
Published: February 2008

A network of eight University of Texas ocean bottom seismographs (OBS) operated for 6 weeks in 1978 about 50 km offshore of Adak Island, Alaska, and nearby islands. In 1979 a similar network of nine instruments was deployed for 7 weeks farther offshore within and up to 100 km seaward of the Aleutian trench. For shallow earthquakes on the outer trench slope, for shallow earthquakes in the thrust zone, and for intermediate‐depth events we have analyzed the OBS and island‐based network data and evaluated the island network's capabilities for earthquake detection and location and for focal mechanism determination. Our three major conclusions are presented. The first concerns shallow earthquakes on the outer trench slope. In 1979 about 30 earthquakes occurred within the Aleutian trench and up to 60 km seaward of the trench axis. The island network located none of these events and detected P phases for only three of them. Ray tracing shows that the islands lie in a geometric shadow zone for events on the outer trench slope. The best located events are shallower than 20 km and exhibit first motions consistent with normal faulting. Several authors have suggested that these events are caused by bending of the oceanic lithosphere at the outer rise prior to subduction. If so, then the event locations reported here show that the bending stresses exceed the strength of lithosphere only in a narrow zone extending about 10 km landward and 60 km seaward of the trench axis. The second conclusion concerns shallow earthquakes in the thrust zone. Epicenters determined by island stations alone are virtually identical to epicenters determined using data from both island and OBS stations. The locations are similar whether they are determined using flat‐layered velocity models or a more realistic model and a ray‐tracing scheme. Nearly all the earthquakes detected by the OBS network are also recorded by island stations. Three composite focal mechanism solutions that use data from both island and OBS stations have P axes parallel to the direction of plate convergence. The thrust zone events are separated from events on the outer trench slope by a gap in seismicity about 50 km wide situated on the inner trench slope. The third conclusion concerns earthquakes deeper than 70 km. Epicenters determined using island network stations alone lie 10 to 80 km south of those determined using OBS and island stations, with the differences between epicenters depending both on event depth and on the velocity model used. Ray tracing with a realistic velocity model shows that for events at 200‐km depth locations are about 80 km north of locations determined from island data alone. These results do not support the observed increase in the dip of the Benioff zone beneath 100‐km depth that is suggested by locations determined from island network data alone using a flat‐layered velocity model.
In situ study of the physical mechanisms controlling induced seismicity at Monticello Reservoir, South Carolina
Authors: Zoback Mark D. , Hickman Stephen
Published: February 2008

In two ∼1.1‐km‐deep wells, the magnitudes of the principal in situ stresses, pore pressure, permeability, and the distribution of faults, fractures, and joints were measured directly in the hypocentral zones of earthquakes induced by impoundment of Monticello Reservoir, South Carolina. Analysis of these data suggests that the earthquakes were caused by an increase in subsurface pore pressure sufficiently large to trigger reverse‐type fault motion on preexisting fault planes in a zone of relatively large shear stresses near the surface. The measurements indicated (1) near‐critical stress differences for reverse‐type fault motion at depths less than 200–300 m, (2) possibly increased pore pressure at depth relative to preimpoundment conditions, (3) the existence of fault planes in situ with orientations similar to those determined from composite focal plane mechanisms, and (4) in situ hydraulic diffusivities that agree well with the size of the seismically active area and time over which fluid flow would be expected to migrate into the zone of seismicity. Our physical model of the seismicity suggests that infrequent future earthquakes will occur at Monticello Reservoir as a result of eventual pore fluid diffusion into isolated zones of low permeability. Future seismic activity at Monticello Reservoir is expected to be limited in magnitude by the small dimensions of the seismogenic zones.
Magnetotelluric experiment over the ROSE area
Authors: Filloux J. H.
Published: February 2008

Seafloor observations of natural electromagnetic fluctuations diffusing into the earth have been made in the vicinity of the Pacific Rise at 12°N and 21°N to gather information on the electrical conductivity structure of the oceanic basement near a spreading ridge. At 21°N the close proximity of land (Baja California) introduces a noticeable distortion of the EM fields, and consequently the magnetotelluric sounding for this area must be interpreted with caution. At both locations the conductance of the upper 200 km appears exceptionally high and the conductivity rises rapidly below, reaching 1 S m−1 or more at 350 km. The statistical significance of a prominent conductivity increase near a depth of 22.5 km detected in the inverted data from 12°N and of a less developed one at 30 km in the case of 21°N is discussed. Records of the magnetic variations at 21°N taken above the spreading center, compared to records from 40 and 120 km to the west, reveal a conspicuous enhancement of the high‐frequency horizontal variations in a direction slightly west of magnetic north. We interpret this distinctive feature as the signature of an electric current sheet concentrated in an extremely high conductance, relatively shallow layer electrically bridging the two seafloor areas adjacent to the spreading center. An approximate calculation suggests a conductance in excess of 3000 S per meter of ridge crest in the close vicinity of the accretion center, equivalent to 1 to 8 km of molten basalt, depending on temperature (1000°–1200°C?) and depth (2–10 km?). This structure is consistent with seismic refraction information for the same area and with stratigraphic reconstruction of a fossil accretion center in Samail ophiolite. An interpretation in terms of a well‐developed magma chamber is irresistible, although confirmation of the existence of this feature as well as improvement of its resolution by means of additional EM data would be of major interest.
Location of instruments on the seafloor by joint adjustment of instrument and ship positions
Authors: Creager Kenneth C. , Dorman LeRoy M.
Published: February 2008

The problem of determining accurate locations for arrays of underwater instruments is similar to the problem of locating local earthquakes by utilizing a limited number of receivers but is complicated by the fact that estimated errors in longitude and latitude of a given receiver (ship) position are nonzero and are typically correlated. Estimates of ship positions at sea are commonly determined by fixes from the Navy Transit navigation satellite system. This system calculates position error ellipses which vary in size and orientation from fix to fix. Deviations in the orientations of the principal axes of the error ellipses from north‐south and east‐west result in the introduction of off‐diagonal elements in the ship position variance matrix. Tying each ship fix to a stationary array of ocean bottom seismometers (OBS's) using acoustic ranging allows averaging of fixes, thus solving simultaneously for ship and OBS positions, puts tight constraints on the system as a whole and especially on the relative positions of the OBS's. In addition to ship positions, the data required to locate the instruments are two‐way travel times between the ship and transponders on the instruments, and initial depth estimates of OBS's read from the bathymetry. The parameters which we solve for are the locations of the instruments (including depth) and of the ship at each fix. The forward problem, calculating acoustic travel times from distances between ship fixes and OBS's, is solved by a ray shooting algorithm. This system of equations is linearized and transformed to diagonalize the covariances; it is then solved iteratively by inverting the data jointly using singular value decomposition and step‐length damping. The solution includes the covariances of the parameters. We show calculations of positions of four OBS's for deployments during three experiments including the Rivera oceanic seismic experiment (ROSE). Typical fix error ellipses (1 σ) are 300 m by 200 m and resulting instrument position uncertainties are approximately 100 m. Relative instrument positions are accurate to about 50 m. Relative errors can be decreased to 10 to 20 m by using simultaneous rather than sequential ranging.
Geophysical evidence for the absence of a crustal magma chamber under the northern Juan de Fuca Ridge: A contrast with ROSE results
Authors: McClain Kevin J. , Lewis Brian T. R.
Published: February 2008

Deep‐tow reflection data, seismic refraction data to ocean bottom seismometers, and gravity interpretations of the northern Juan de Fuca Ridge all suggest a very narrow or absent crustal magma chamber under this portion of the rise axis. The seismic features that have been suggested as indicating crustal magma chambers on other parts of the East Pacific Rise, such as travel time delays and reflections from the top of a shallow magma reservoir, are not found on the northern part of this ridge. These results indicate that at spreading rates of 3 cm/yr crustal cooling (probably by water penetration) is sufficiently rapid to prevent the existence of a steady state crustal magma chamber and therefore crustal accretion may be episodic.
Analysis of the Petatlan aftershocks: Numbers, energy release, and asperities
Authors: Valdés Carlos , Meyer Robert P. , Zuñiga Ramón , Havskov Jens , Singh Shri K.
Published: February 2008

The Petatlan earthquake of March 14, 1979 (Ms = 7.6), occurred between the Middle America trench and the Mexican coast, 15 km southwest of Petatlan, Guerrero, Mexico. From seismograms recorded on smoked paper, FM, and digital tapes, we have identified 255 aftershocks with coda lengths greater than 60 s that occurred 11 hours to 36 days after the main shock. Based on these events, the aftershock epicentral area defined during the period between 11 and 60 hours was about 2000 km2; between 11 hours and 6 days it was about 2400 km2. Although the area grew to 6060 km2 in 36 days, most of the activity was still confined within the area defined after 6 days. This suggests that the smaller aftershock area might represent an asperity. The distribution of events and energy release per unit area confirm the existence of heterogeneity in the aftershock area. Thus our data support the concept of an inhomogeneous rupture area that includes an asperity, as suggested by Chael and Stewart (1982) to account for the differences they computed for the body and surface wave moments from WWSSN data. However, the combination of the moments Reichle et al. (1982) report for body and surface waves from IDA data and the rupture areas reported in this paper results in a solution that is most physically realizable in terms of stress drop and slip. We calculate stress drops of 5 and 15 bars, the former for the average over the entire area, the latter for the asperity, and an average slip of 60 cm for the entire area and 120 cm for the asperity. These values for slip are 30% and 60%, respectively, of the convergence of the Cocos plate relative to the North America plate during the 36‐year period between the last two major earthquakes in the Petatlan area. Hypocenters of the aftershocks define a zone about 25 km thick, dipping 15° with an azimuth of N20°E, which is perpendicular to the Middle America trench. Most aftershocks are below the main shock. The b value estimated for aftershocks in the range 3.0 ≤ MCL ≤ 4.5 is 60% higher than one determined similarly by Hsu (1981) for the foreshocks.
Heat flow, arc volcanism, and subduction in northern Oregon
Authors: Blackwell David D. , Bowen Richard G. , Hull Donald A. , Riccio Joseph , Steele John L.
Published: February 2008

A total of 75 heat flow measurements are presented for the part of the Oregon Cascade Range and surrounding provinces, including the Willamette Valley, the High Lava Plains, the Deschutes‐Umatilla Plateau, and the Blue Mountains, between 43°30′ and 45°05′N. The data document a major change in regional heat flow from values of 40±2 mW m−2. in the Western Cascade Range and Willamette Valley provinces to 100±5 mW m−2 along the boundary between the High Cascade Range and Western Cascade Range provinces. This variation has a half width of approximately 10 km and appears to be exactly the same along the whole study area (a strike length of over 150 km). This heat flow change coincides with a line of hot springs and occurs approximately 20–30 km west of the axis of the High Cascade Range where the major andesite stratovolcanoes of Mount Jefferson and the Three Sisters occur. The heat flow boundary corresponds to a steep gravity gradient with about the same half width and an amplitude of approximately −25 mGal. Both sets of data can be explained by a hot, low‐density region about 60 km wide in the crust below 7–10 km. This heat source region is probably a zone of temporary residence of magmas melting off the subduction zone. Such crustal staging chambers may be commonly involved in the complex petrologic evolution of andesites and granitic rocks in volcanic arcs. The ratio of apparent intrusive to extrusive rocks ranges from 1:1 counting the total volcanism to 10:1 or 50:1 if only andesitic or rhyolitic volcanics are included. The overall heat flow pattern is consistent with active subduction of the Juan de Fuca plate under the Pacific Northwest. The absorption of heat from the continental outer arc by the sinking plate and the occurrence of the high heat flow associated with the volcanic arc are documented. The lack of earthquakes in the Pacific Northwest is probably related to the proximity to the ridge and the fact that several kilometers of sediment overlie the basaltic basement where it begins to be subducted, so that it is already at a minimum temperature of 100°–200°C. During the Oligocene and Miocene the edge of the volcanic arc was in the Western Cascade Range at least 40–60 km west of its present position. During the last 6–10 m.y. the heat flow has decreased from high volcanic arc values to subnormal outer arc values. The effectiveness of subduction as a heat sink is thus clearly demonstrated. The eastward migration of the volcanic arc axis might be associated with the proposed rotation of the Western Cascade Range due to Basin and Range spreading.
Martian dust mantling and surface composition: Interpretation of thermophysical properties
Authors: Christensen Philip R.
Published: February 2008

The surface block abundance, fine component thermal inertia, thermal emissivity, and relative atmospheric dust content have been determined for selected regions on Mars, using broadband infrared spectral properties measured at 7, 9, 11, and 20 m by the Viking infrared thermal mapper (IRTM). Each of the surface and atmospheric properties produce characteristic spectral signatures which have been successfully modeled. The surface was modeled by a mixture of a high‐inertia ‘block’ component and a fine component, with wavelength dependent emissivity. The abundance of blocks or patches of exposed high‐inertia material varies from 5 to 20% between 0 and 30°N. Low‐inertia regions have the fewest blocks: these may be exposed patches of bonded material. The ubiquity of nonaeolian materials exposed on the surface suggests that there has not been substantial net deposition within this region. However, some dust deposition may be occurring in low‐inertia regions. Observed global variations in the fine component inertia may be due to changes in the degree of bonding of the surficial material. The surface thermal emissivity is strongly correlated with albedo. The nearly linear decrease from near unity at all wavelengths for materials with an albedo greater than 0.28, to 0.98, 0.94, 0.92, and 0.92 at 7, 9, 11, and 20 m, respectively, for materials with an albedo of 0.10 suggests that material on the surface of Mars is composed of mixtures of the light, high‐emissivity and dark, low‐emissivity components. During the relatively clear period studied, the atmospheric opacity was found to vary spatially, being uniformly ‘low’ between 30 and 60°N and uniformly ‘high’ between 0 and −30° S, Between 0 and 30°N the opacity was higher over dark regions than over bright, low‐inertia regions. This variability is probably due to local, surface property‐dependent differences in the rates of dust generation and removal. Dark regions are sources of dust through removal of material deposited by global storms. Dust accumulates in bright, low‐inertia regions.
Seasonal carbon dioxide exchange between the regolith and atmosphere of Mars: Experimental and theoretical studies
Authors: Fanale Fraser P. , Banerdt W. Bruce , Saunders R. Steven , Johansen L. A. , Salvail James R.
Published: February 2008

Measurements of the rate of penetration of a CO2 ‘pressure wave’ through basalt‐clay soils have been conducted under conditions of PCO2 (∼6 mbar), ΔPCO2 (∼2 mbar), soil density (ρ ∼ 1.3), and T (−40° to −70°C) which simulate the penetration of the cap‐induced seasonal CO2 pressure wave through the topmost regolith on Mars. Results suggest that existing theoretical models for diffusion of a gas through a porous, highly adsorbing medium may be reliably used to assess the importance of Martian seasonal regolith‐atmosphere CO2 exchange. Results also suggest that although Mars topsoils with a substantial component of fine‐grained weathering products could harbor, within a few meters of the surface, a substantial fraction of the atmospheric CO2 inventory, seasonal cap‐atmosphere CO2 exchange is still much more important than seasonal regolith‐atmosphere exchange. This is so because the high adsorptive capacity of such soils greatly slows CO2 diffusion through them owing to the typically high ratio of mean residence time for CO2 in the (static) adsorbed phase to that in the pore gas at T
−40°C. At such temperatures, typical of the Martian regolith, the CO2 diffusivity of such soils may be ∼1 × 10−2 cm2 s−1—only somewhat higher than the thermal diffusivity and not high enough to allow seasonal isothermal regolith buffering of the cap‐induced CO2 pressure wave to be effective. On the other hand, even the adsorption‐slowed diffusivities are still high enough to allow thermally driven regolith atmosphere exchange to be effective on a time scale of the obliquity cycle (∼105 yr). We also examined the maximum effect of thermally driven exchange between the topmost seasonally thermally affected regolith and the atmosphere. While of somewhat greater potential importance than the isothermal exchange, the thermally driven exchange would be recognizable only if the pressure wave from CO2 thus exchanged at high latitudes did not propagate though the atmosphere at a faster rate than that at which the exchange itself occurred—an unreasonable assumption. Our results are insensitive to soil geometry and temperature and suggest that both isothermal and thermally driven regolith‐atmosphere seasonal CO2 exchange effects are likely to be greatly dominated by cap‐atmosphere exchange. In principle, it is conceivable that a more profound understanding of the cap‐atmosphere‐regolith interaction could eventually allow us to use the externally imposed pressure waves as a natural experiment to set limits on global or regional soil properties. However, the regolith effects, while quite possibly dominant for H2O, are seemingly too weak and too complex for CO2 to allow this to be accomplished.
Global mean sea surface computation using GEOS 3 altimeter data
Authors: Marsh James G. , Martin Thomas V. , McCarthy John J.
Published: February 2008

A mean sea surface map has been determined for the global ocean areas between +62° and −62° latitude using GEOS 3 altimeter data. A grid of laser reference orbits computed using the GEM 10B gravity model has been used to orient the altimeter data in a center of mass coordinate system. The density of the altimeter tracks has enabled the computation of the sea surface heights above the reference ellipsoid on a 1°×1° grid in most of the oceanic areas. In the northwest Atlantic the dense coverage has enabled computations on a 0.25°×0.25° grid. Comparisons of the global surface with an independently computed mean sea surface based upon SEASAT altimeter data indicate an rms agreement of a little over a meter. Comparisons of the regional solution in the northwest Atlantic with SEASAT profiles indicate a precision of a few decimeters in this surface. An analysis of the global crossover differences has indicated the possibility of a timing error in the altimeter data. Timing bias values of 9.2±0.5 ms for 1975 and 18.7±0.6 ms for 1976 have been recovered from an analysis of the altimeter data.
Laboratory simulation of planetesimal collision
Authors: Matsui Takafumi , Waza Toshihiko , Kani Kouki , Suzuki Shoji
Published: February 2008

Low‐velocity impact experiment on rocks was performed to reveal a nature of collision process of planetesimals. Projectiles of mild steel (S15CK) and rocks (tuff and basalt) with velocities of 17 to 270 m/s were impacted against four kinds of rocks (tuff, basalt, granite, and dunite) with various shapes and sizes. Imparted energy divided by target mass Ei ranges 106 to 108 erg/g. The phenomena associated with the impact of mild steel against spherical targets were classified into five categories. The categories vary with increase in impact velocity as follows: (1) rebound of projectile with no contact damage on target, (2) rebound of projectile with fragmentary chips, and creations of a contact damage (crater?) and radiating fissures on target, (3) rebound of projectile with longitudinal splitting of target, (4) destruction of target with producing shatter conelike fragments, and (5) complete destruction of target. We cannot see any difference in the above categories between various kinds of rocks. The shape of the target also does not influence the categories significantly. The size distribution of the fragments produced by the complete destruction was well expressed by an inverse power law relation, n(l) dl ∝ l−α−1dl, where n(l) is the incremental number of the fragments within a linear increment dl, l is the size, and α is constant; α seems to increase with increase in Ei,. An empirical relationship between α and Ei was approximately given by α∝ log Ei. Empirical formulae for the maximum fragment mass normalized by the original target mass (Ml/Mt) and Ei were also presented for each rock type. It is found from the shape distribution of the fragments produced by the shatter cone type destruction that the length ratios between the shortest and the longest axes, c/a, and between the intermediate and the longest axes, b/a, were greater than 0.2 and 0.3, respectively. Average length ratio of a:b:c is roughly given by 2:√2:1. Collisional phenomena associated with low‐velocity impact experiment were shown to be different from those observed in the high‐velocity impact experiment. The difference in mechanical properties between projectile and target significantly influenced the collisional behavior. For collision between rocks with similar mechanical properties, relative size between projectile and target played an important role in determining the collisional mode: Only the smaller body was destroyed completely, and the larger body suffered no damage even if released Ei was enough to destroy both colliding bodies simultaneously. This observation suggests an important implication in the early stage of planetary formation, that runaway growth of the largest planetesimal takes place even when the collision condition prefers catastrophic destruction to other collision types.
Apollo Lunar Seismic Experiment—Final summary
Authors: Nakamura Yosio , Latham Gary V. , Dorman H. James
Published: February 2008

Processing and initial analysis of the entire set of Apollo lunar seismic data collected continuously from 1969 through 1977 have now been completed. Recent results include: 1) better defined deep moonquake locations, which appear to be bounded rather sharply between about 800 km and 1000 km depths with concentrations near both boundaries; and 2) middle mantle (∼500 to 1000 km depth) seismic velocities of Vp, = 8.3 ± 0.4 km/sec and Vs = 4.6 ± 0.2 km/sec, which are significantly higher than previous estimates and represent an increase of velocities from the upper mantle as opposed to a decrease in previous estimates.
The Apollo 14 Regolith: Petrology of cores 14210/14211 and 14220 and soils 14141, 14148, and 14149
Authors: Simon Steven B. , Papike James J. , Laul J. C.
Published: February 2008

New modal data are presented for continuous polished thin sections from double drive tube 14210/14211, and single drive tube 14220, and for polished grain mounts of four soils from the double drive tube, one from the single drive tube, and the soils 14148 (trench top), 14149 (trench bottom), and 14141 (Cone Crater). Modal data show that the Cone Crater soil is immature, whereas the ‘smooth plains’ soils are mature and rich in agglutinates and breccias. Neither core exhibits any major variations with depth. Microprobe analyses of mineral and glass fragments are consistent with derivation of the soils predominantly from the local rocks, with ∼5–11% exotic mare component indicated by the modal data. About 4% of the glasses are SiO2‐ and K2O‐rich granitic glasses which are comminuted mesostasis from the local melt rocks. The soils are depleted in feldspar relative to the source rocks. The preferred explanation for this depletion is that feldspar is concentrated in the <10 μm fines and is consumed in the formation of agglutinates, regolith breccias, and feldspathic glass.
Relationship between an iridium anomaly and the North American microtektite layer in core RC9‐58 from the Caribbean Sea
Authors: Glass B. P. , DuBois David L. , Ganapathy R.
Published: February 2008

In a previous publication, an iridium anomaly was reported in core RC9‐58 from the Caribbean Sea, about 30 cm below the peak abundance of North American microtektites. In order to determine more precisely the relationship between the iridium anomaly and the North American microtektite layer, we searched for microtektites in the samples that were used for the iridium studies. We found that the North American microtektite layer is actually two layers, with the peak abundances separated by 25 cm. The upper layer consists of ‘normal’ North American microtektites and the lower layer consists of previously described clinopyroxene‐bearing spherules. The iridium anomaly was found to correlate with the lower layer. Although the two layers appear to be the result of two separate events, several lines of evidence suggest that they were produced by a single event. The separation into two layers may have been produced by differential settling in the sediment due to density variations. The correlation between the iridium anomaly and the North American microtektite layer supports the terrestrial impact origin for tektites.
Regionalization of crustal coda Q in the continental United States
Authors: Singh Sudarshan , Herrmann Robert B.
Published: February 2008

A contour map of crustal Q0 values for the entire United States is presented based on a scattering model to explain the coda waves of local and near regional earthquakes. These coda Q0 values are in good agreement with Q of Lg waves. Data are obtained from over 250 local earthquakes, with most magnitudes between 3.0 and 5.0, recorded by 25 WWSSN and 27 LRSM stations throughout the continental United States. These two sets of data provide a range of frequencies from 0.5 to 3.5 Hz. A frequency dependence of Q is observed in the range of frequencies considered. A power law dependence of the form Q = Q0(f/f0)η is assumed. The value of frequency dependence η, is found to be maximum in the tectonically active western United States and least in the stable regions of the central and south central United States. The lowest Q0 values are obtained in the western United States, with values ranging from 140 to 200 in the Coastal Plains. Average crustal Q0 values for the Basin and Range province vary from 200 to 300, increasing gradually to 400 in the Colorado Plateau region. Q0 values in the Columbia Plateau vary from 200 to 400. Q0 values increase very rapidly along the central and southern Rocky Mountains from 400 to 800. East of the Rocky Mountains Q0 values increase gradually in the Interior Plains to a maximum value of around 1300 in the Mississippi Embayment region. Farther east Q0 values decrease gradually to an almost constant value of 1000 along the Appalachian Mountains. Coastal regions of eastern and northeastern United States have a crustal Q0 value between 900 to 700. Regions of north central and Gulf Coastal Plains show low Q0 values, ranging from 600 to 400 and less. The source factor B(fp), a measure of the intensity of scattering, was obtained for several different regions of the continental United States. Regions of high Q0 values exhibit low intensity of scattering.
Seismic velocity structure of the lunar mantle
Authors: Nakamura Yosio
Published: February 2008

I have inverted the recently completed set of seismic arrival times from the Apollo lunar seismic network to estimate the average seismic velocities in three sections of the lunar mantle: two for the upper mantle and one for the middle mantle. The method used is a variation of the linearized least squares inversion where the inversion is accomplished in steps. The estimated average velocities in the upper mantle decrease from Vp = 7.74 km/s and Vs = 4.49 km/s in the section above 270‐km depth to Vp = 7.46 km/s and Vs = 4.25 km/s in the section between 270‐ and 500‐km depth, confirming the earlier finding of negative gradients based on seismic amplitude variations. The average velocities in the middle mantle between the depths of 500 km and 1000 km of Vp = 8.26 km/s and Vs = 4.65 km/s are significantly higher than those in the upper mantle, contradicting earlier estimates based on more limited data. The higher velocities may suggest initial melting of the moon down to at least 1000‐km depth.