bluuebunny/crossref_metadata_2025 huggingface dataset:
Seamount statistics in the Pacific Ocean
Authors: Smith Deborah K. , Jordan Thomas H.
Published: February 2008

We apply the wide‐beam sampling technique of Jordan et al. (1983) to approximately 157,000 km of wide‐beam profiles to obtain seamount population statistics for eight regions in the eastern and southern Pacific Ocean. Population statistics derived from wide‐beam echograms are compared with seamount counts from Sea Beam swaths and with counts from bathymetric maps. We find that the average number of seamounts with summit heights h ≥ H is well‐approximated by the exponential frequency‐size distribution: ν(H)=νoe−βH. The exponential model for seamount sizes, characterized by the single scale parameter β−1, is found to be superior to a power‐law (self‐similar) model, which has no intrinsic scale, in describing the average distribution of Pacific seamounts, and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance. Large‐scale regional variations in seamount populations are documented. We observe significant differences in seamount densities across the Murray fracture zone in the North Pacific and the Eltanin fracture zone system in the South Pacific. The Eltanin discontinuity is equally evident on both sides of the Pacific‐Antarctic ridge. In the South Pacific, regions symmetrically disposed about the ridge axis have very similar seamount densities, despite the large difference between Pacific plate and Antarctic plate absolute velocities; evidently, any differences in the shear flows at the base of the Pacific and Antarctic plates do not affect seamount emplacement. Systematic variations in νo and β are observed as a function of lithospheric age, with the number of large seamounts increasing more rapidly than small seamounts. These observations have been used to develop a simple model for seamount production under the assumptions that (1) an exponential size‐frequency distribution is maintained, (2) production is steady state, and (3) most small seamounts are formed on or near the ridge axis. The limited data available from this study appear to be consistent with the model, but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters. However, the data from the South Pacific indicate that the off‐axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m.
Depth and age in the north Pacific
Authors: Renkin Miriam L. , Sclater John G.
Published: February 2008

The concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor. Analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer. In past studies, problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor. In this study we evaluate different tectonic histories for the “quiet zones” and construct a revised isochron chart for the North Pacific. We check and modify the current sediment thickness charts using new deep‐sea drilling data and revised ideas concerning reflectors in the southwestern North Pacific. Averaging the SYNBAPS digital data base into half‐by‐half‐degree elements and correcting for sediment load, we plot basement depth versus age for the entire North Pacific. Because of the skewness in the data associated with seamounts, we present the data as contours about the mode and find that two‐thirds of the observations lie within 300 m of this value. On crust between 0 and 80 Ma, the ocean floor subsides with the square root of age. On ocean floor older than 100 Ma the depths in general flatten with age with areally correlateable swells and valleys. The Hawaiian and Line Island swells correlate positively, and the Mariana Basin valley negatively, with geoid height. The flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor. Further, the correlation of the swells and valleys with geoid height presents evidence for organized convection in the upper mantle beneath older lithosphere in the North Pacific.
Origin of the Louisville Ridge and its relationship to the Eltanin Fracture Zone System
Authors: Watts A. B. , Weissel J. K. , Duncan R. A. , Larson R. L.
Published: February 2008

We have combined shipboard and Seasat altimeter derived data in an intergrated geological and geophysical study of the Louisville Ridge; a 3500‐km‐long seamount chain extending from the Tonga trench to the Eltanin Fracture Zone. A break in the smooth trend of the ridge at latitude 37.5°S has been recognized in both bathymetric and altimetric data. The 40Ar‐39Ar dating of rocks dredged either side of the break suggest that it is analogous to the bend in the Hawaiian‐Emperor seamount chain. Although the general trend of the ridge can be fit by small circles about Pacific absolute motion poles determined from other seamount chains, the new bathymetric and age data allow us to refine Pacific absolute motion poles. The continuity in smooth trend of the ridge and the Eltanin Fracture Zone suggests some relationship between them. However, a major offset developed on this transform between 60 and 80 Ma, prior to the oldest dated rocks from the ridge. Although magmatism was more or less continuous on the ridge during 28–60 Ma, it probably occurred on crust with little or no offset. Thus magmatism appears to have been little influenced by the developing fracture zone. By 28 Ma, the distance between the magmatic source and the fracture zone had decreased sufficiently for a portion of the ridge to have been emplaced on crust with an offset. After about 12 Ma, however, volcanic activity on the Louisville Ridge apparently waned, despite a possible influence on the magmatism of the fracture zone.
Backthrusting in accretionary wedges
Authors: Silver Eli A. , Reed Donald L.
Published: February 2008

Backthrusting of the rear of the accretionary wedge, arcward over the rocks of the forearc basin, is a common aspect of accretionary tectonics. An older assumption that backstops of accretionary wedges dip back toward the arc, and that accreted material is emplaced below such backstops, is contradicted by observations from many active forearcs that indicate (1) backthrusting is common, (2) forearc basins are nearly ubiquitous associates of accretionary wedges, and (3) forearc basement, where imaged, appears to diverge from the sedimentary package, dipping under the wedge while the overlying sediments are often lifted up against it. Backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust. We find ratios of Lt/Lb averaging 2.64±0.8, where Lt is the horizontal distance from the crest of the wedge to the front of the toe, and Lb is the distance from the crest of the wedge to the surface projection of the backthrust. This ratio is predicted to increase with total convergence and with sedimentation, because the toe undergoes higher deformation rates than the backthrust, and sedimentation in the trench favors outbuilding of the toe, whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust.
Structure and development of the Midocean Ridge Plate Boundary in the Gulf of Aden: Evidence from GLORIA Side Scan Sonar
Authors: Tamsett Duncan , Searle Roger C.
Published: February 2008

A GLORIA side scan sonar survey along the crest of the midocean ridge in the Gulf of Aden has provided information on the small‐scale morphology of its spreading centers and fracture zones. Spreading center offsets increase in length to the east. This is thought to result from changes in the behavior of the mantle with increasing distance from the Afar hot spot. A 300‐km‐long spreading center in the western Gulf of Aden is oriented obliquely to the spreading direction. Nested within the obliquely oriented median valley is an en echelon arrangement of a number of smaller, elongated basins, each oriented approximately normal to the spreading direction. Close to the axis of spreading, volcanic and closely spaced tectonic structures run approximately normal to the spreading direction. Higher on the walls of the median valley, more‐widely‐spaced normal faults have formed subparallel to the valley. In the western Gulf of Aden there are no indications of sonar targets that might represent transform faults parallel to the NNE spreading direction. In the east, spreading center offsets of 15–25 km are made at fracture zones which are best described as narrow zones of highly oblique spreading. Offsets of greater than 25 km are achieved on single transform faults, and the associated tectonic fabric has a spreading direction trend. The boundaries between crusts of different ages along inactive transform fault extensions lie along the bases of fracture zone scarps. These scarps are situated in the older crust and face the younger. This is the opposite of the situation predicted on the basis of simple plate cooling models. At some places in the Gulf of Aden the fracture zone topography is very subdued, and this is attributed to the failure of the two slabs of lithosphere at the end of the transform fault to couple completely before the younger lithosphere on the nontransform side of the spreading center completes its ascent of the median valley wall. There is evidence of a change of spreading direction 3.5 m.y. ago, from about 040° to 022°. It is argued that the small‐scale morphology of the transform fault/spreading center intersections can be understood in terms of the transfer of shear fracture on a transform fault to tensile fracture on a spreading center propagating away from the intersection. It is concluded that the transform fault must be weaker than the spreading center.
Growth and persistence of Hawaiian volcanic rift zones
Authors: Dieterich James H.
Published: February 2008

Hawaiian volcanic rift zones are modeled by representing the rifts and adjacent volcano flanks as long ridges with the geometry of flattened triangular prisms. The intrusion of dikes along the axis of a rift requires a mechanism to generate the appropriate dike‐trapping stress field within the prism. Possible factors that affect the state of stress in the prism include multiple dike intrusion along the ridge axis, faulting, and gravitational sagging of the topography. In extreme models with very steep slopes and high Poisson's ratio, corresponding to the gelatin models of rift zones by Fiske and Jackson (1972), results of finite element calculations indicate that gravity‐induced stresses are sufficient to trap a dike into propagating within the prism and parallel to the rift zone as proposed by Fiske and Jackson. However, the mechanism does not work for gently sloping flanks or a more acceptable Poisson's ratio of about 0.25. Additionally, trapping stresses in the gravity‐loading and density stratification models will not persist after a few dike injection episodes. Therefore in mature Hawaiian rift zones with possibly thousands of dikes, additional processes must act to control the stresses that permit continued dike intrusion and rift persistence. It is proposed that accommodation to dike emplacement occurs by slip on deep faults, possibly of the type proposed for the 1975 Kalapana, Hawaii, earthquake. As suggested by others for this earthquake, the faults could coincide with the contact of the volcano with the seafloor within the weak seafloor sediments. Such faulting not only provides a means for the flanks to adjust continuously to intrusions but also generates the stress patterns needed to constrain future dikes to propagate along the rift axis. Other possible faulting mechanisms, such as shallow gravity slides and normal faulting of the flanks, do not appear to favor rift zone persistence. In this model the horizontal stress generated by a standing column of magma at the time of dike emplacement, the stresses in the ridges, and the fault strength are coupled. This results in a feedback between the maximum height that magma can rise along the rift, fault friction, and fault width. Through this feedback the slope of the volcano flank is controlled by the fault friction. When applied to Kilauea Volcano, the model yields an estimate for the coefficient of fault friction as high as 0.39 assuming normal hydrostatic pore fluid pressure. An implication of this model, supported by other studies, is that rift intrusion and lateral spreading could be major contributors to volcano growth.
The cogenetic organ cauldron and batholith, south central New Mexico: Evolution of a large‐volume ash flow cauldron and its source magma chamber
Authors: Seager William R. , McCurry Michael
Published: February 2008

The Organ cauldron and its underlying plutonic roots are exposed through a vertical section 5–6 km thick in the Organ Mountains of south central New Mexico. Emplaced within a 1 m.y. time span at 33.7–32.8 Ma, both cauldron fill tuffs and lavas, as well as the underlying Organ batholith, document emplacement of a chemically and mineralogically zoned magma chamber at shallow depths. The tuffs, as much as 3.0 km thick near the center of the cauldron, show rather simple normal chemical and mineralogical gradients which indicate zoning in the magma chamber from aphyric rhyolite (77% SiO2) at the top to crystal‐rich rhyolite (68% SiO2) at deeper levels. Lava flows, more than 0.3 km thick, overlie the tuffs and apparently represent still deeper, more mafic (68–61% SiO2), crystal‐rich parts of the erupted magma volume. The oldest phase of the batholith may also be compositionally zoned from quartz syenite (68% SiO2) at the top to monzodiorite (55% SiO2) at the lowest exposed level. This phase of the batholith, which forms both a concordant floor beneath the volcanic roof and a thick dikelike body along part of the cauldron margin, may be the undrained residue of the magma chamber whose silicic cap was erupted as tuffs and lavas. Small bodies of fine‐grained, porphyritic alkali feldspar granite at the top of the batholith may represent undrained remnants of the silicic cap. Diapirlike intrusions of the deeper, more mafic parts of the batholith into higher more silicic parts may be related to eruption mechanisms. Three younger plutonic phases of the batholith seemingly represent three stages of silicic core magma generation and intrusion during the progressive crystallization and differentiation of the batholith. Major normal faults which offset cauldron fill and are invaded by plutonic rocks suggest the cauldron evolved in a regional extensional stress field.
Mineralization, alteration, and hydrothermal metamorphism of the ophiolite‐hosted Turner‐Albright sulfide deposit, southwestern Oregon
Authors: Zierenberg Robert A. , Shanks Wayne C. , Seyfried William E. , Koski Randolph A. , Strickler Michael D.
Published: February 2008

The Turner‐Albright sulfide deposit, part of the Josephine ophiolite, formed on and below the seafloor during Late Jurassic volcanism at a back arc spreading center. Ore fluids were probably localized by faults which were active on the seafloor at the time of sulfide deposition. The uppermost massive sulfide formed on the seafloor at hydrothermal vents, similar to sulfide‐depositing hot springs on modern spreading centers. The bulk of the sulfide mineralization formed below the seafloor within olivine basalt hyaloclastite erupted near the time of mineralization. Infiltration of hydrothermal fluid into the hyalo‐clastite altered the rock to quartz + Fe‐chlorite and quartz + sericite + Fe‐chlorite. Intense alteration resulted in replacement of the protolith by quartz + sulfide. The fluid responsible for the hydrothermal alteration was evolved seawater with low pH and Mg and high Fe. Sulfide δ534S values average 4.7‰ and indicate contribution of sulfur from both basalt and seawater sulfate. The average value of sulfide δ34S and the difference between sulfide and contemporaneous seawater sulfate δ34S values are similar to ophiolite‐hosted sulfide deposits in Cyprus. Chlorite‐rich hydrothermally altered basalt has δ18O values of 3.0–6.8‰ due to high‐temperature (250°–350°C) reaction with a seawater‐derived ore fluid. Quartz in silicified basalt has higher δ18O values of 12.3–15.5‰, probably reflecting lower‐temperature recrystallization of amorphous silica formed during initial alteration of basalt. Mudstone and clinopyroxene basalt above the sulfide horizons were not altered by the ore‐transporting hydrothermal fluid, but these rocks were hydrothermally metamorphosed by altered seawater heated by deep circulation into hot oceanic crust. This subseafloor metamorphism produced a mineral assemblage typical of prehnite‐pumpellyite facies metamorphism. Exchange with altered seawater increased the whole‐rock δ18O of the basalts to values of 9.4–11.2‰.
Temperature and pressure derivatives of elastic constants with application to the mantle
Authors: Anderson Don L.
Published: February 2008

The temperature and pressure derivatives of the elastic moduli M of solids can be cast into the form of dimensionless logarithmic anharmonic {DLA} parameters, ∂ ln M/∂ In p = {M} at constant temperature, pressure, or entropy (T, P, S), where p is the density. These parameters show little variation from material to material and are expected to show little variation with temperature at high temperature. Most of the available derivative data for ionic solids has been renormalized and analyzed for dependency on ion type, crystal structure, and other parameters. The {DLA} parameters exhibit little variation and little correlation with crystal structure for most close‐packed halides and oxides. There are small systematic variations with ionic radius, Gruneisen's γ, and the bulk modulus‐rigidity ratio (K/G). Temperature and pressure derivatives are correlated because of the importance of the volume‐dependent, or extrinsic, terms. The intrinsic terms {K}v and {G}v are also highly correlated, even for open‐packed structures, where ∂ ln M/α∂T = {M}v. These correlations make it possible to estimate the derivatives of high‐pressure phases. The spinel forms of olivine are predicted to have “normal” derivatives, and therefore the magnitude of the modulus or velocity jump associated with the olivine‐spinel transition near 400 km should be similar to that measured in the laboratory. The actual size of the 400‐km discontinuity is much less, indicating the presence of substantial quantities of minerals other than olivine in the upper mantle or transition region. Recent calculations in apparent support of a homogeneous olivine‐rich (>60%) mantle are based on choices for the derivatives of β‐ and γ‐Mg2SiO4, which are unlike other ionic crystals. There is no evidence that these phases should be anomalous in their physical properties. The temperature and pressure derivatives of ionic crystals depend on the nature of the ions and their coordination.
Upper mantle cross section from Tonga to Newfoundland
Authors: Graves Robert W. , Helmberger Donald V.
Published: February 2008

A record section of long‐period U.S. and Canadian observations of multibounce SH waves from a Tonga event is used to study lateral variation in the upper mantle. Source to receiver distances range from 81° to 131° with triplication patterns apparent in S4 for the western North America stations and S5 for the more distant stations. A corresponding synthetic section explaining the timing separation and waveform distortions for the various multiple S phases was derived by forward modeling. Initial trial models consisted of connecting existing pure path models, assuming ATL (old Atlantic) as an analog for the old Pacific, TNA for the western portion of the United States, and SNA for the shield portion of North America. Isovelocity depth contours were constructed to connect these models and corresponding synthetics were computed following a modified WKBJ method. Lateral heterogeneity is found to exist in the upper 350 km of the Pacific Ocean region and can be modeled reasonably well with a high‐velocity lid (Vs = 4.7 km/s) which thickens with increasing oceanic crustal age at the expense of the underlying low‐velocity zone. Velocities in the upper 200 km of the older southwest Pacific are found to be up to 6% faster than those at comparable depths in the younger tectonic region near western North America. The best fitting model indicates that the upper mantle structure of the old Pacific is similar to that of the old Atlantic, although perhaps slightly slower.
Stabilization of rapid frictional slip on a weakening fault by dilatant hardening
Authors: Rudnicki John W. , Chen Chao‐Hsun
Published: February 2008

Frictional slip is often accompanied by dilatancy due to uplift in sliding over asperities and micro‐cracking in the adjacent material. If dilatancy occurs more rapidly than pore fluid can flow into the newly created void space, the local pore pressure is reduced and the effective normal stress is increased in compression, tending to inhibit further slip. This dilatant hardening is analyzed for a simple model. One surface of a slab is loaded by compressive stress and shear displacement and connected to a reservoir of pore fluid held at constant pressure. The other boundary is a frictional surface, assumed to have formed at peak stress, on which the shear stress decreases from a peak value τp to a residual value τr as slip increases from zero to δ0. In the absence of pore fluid effects an instability corresponding to an unbounded slip rate occurs when the slope of the shear stress versus slip relation is more negative than the unloading stiffness of the surrounding material. Dilatant hardening prevents this instability provided that the pore pressure in the reservoir is high enough. If the pressure in the reservoir is too low, the pressure at the fault surface can be reduced to the point at which the pore fluid bulk modulus decreases rapidly, eliminating the stabilizing effect. When the analysis is modified to include normal stress changes simulating those in the axisymmetric compression test, the prediction of the critical pressure in the reservoir agrees to within a factor of 2 or 3 with that observed by Martin in tests on Westerly granite. The predictions are also consistent with the trends observed by Martin of decreasing critical reservoir pore pressure with increasing effective confining stress and decreasing nominal strain rate.
Numerical simulation of the crystallization of multicomponent melts in thin dikes or sills: 1. The liquidus phase
Authors: Spohn Tilman , Hort Matthias , Fischer Heide
Published: February 2008

Crystal size frequency distributions, crystallization histories, and thermal histories were calculated for the liquidus phase of a two‐component melt to model the crystallization of the liquidus phase of a multicomponent magma injected in a dike or sill. The crystal sizes, the variances of the crystal size frequency distributions, and the crystallization times depend on the competitive rates of crystallization and heat transfer by heat conduction, on the nucleation and growth rate functions, and on the boundary conditions. It is shown that the nucleation and growth rate functions can be parameterized by their maximum amplitudes and by the temperatures of the maxima. The texture of the crystallized intrusion depends on the difference between the initial undercooling and the undercooling of the maximum nucleation rate. The initial undercooling decreases with increasing distance from the margin of the intrusion and with increasing crystallization rate and wall rock temperature. The crystals are moderately sorted if the initial undercooling is larger than the undercooling of the maximum nucleation rate with bimodal and possibly trimodal size distributions. The crystals become increasingly well sorted with decreasing initial undercooling. If the undercooling is smaller than the undercooling of the maximum nucleation rate, the size distributions are unimodal. The unimodal distributions at the central regions of the intrusion typically have positive skewness about the mode and are platykurtic. If crystal embryos and impurities contribute significantly to heterogenous nucleation, the size distributions have a strong negative skewness. The crystal size frequency distributions and the thermal and crystallization histories are systematically different from those of one‐component models, and the solidification times are different from those calculated for purely thermal Stefan models. The analysis can be used to estimate kinetic parameters from the textures of natural intrusions. Peak nucleation and growth rates of plagioclase calculated from the crystal sizes of three dikes are found to be internally consistent and to compare favorably with published laboratory data.
The hydrothermal outflow plume of Valles Caldera, New Mexico, and a comparison with other outflow plumes
Authors: Goff Fraser , Shevenell Lisa , Gardner Jamie N. , Vuataz Francois‐D. , Grigsby Charles O.
Published: February 2008

Stratigraphic, temperature gradient, hydrogeochemical, and hydrologic data have been integrated with geologic data from previous studies to show the structural configuration of the Valles caldera hydrothermal ouflow plume. Hydrologic data suggest that 25–50% of the discharge of the Valles outflow is confined to the Jemez fault zone, which predates caldera formation. Thermal gradient data from bores penetrating the plume show that shallow gradients are highest in the vicinity of the Jemez fault zone (up to 190°C/km). Shallow heat flow above the hydrothermal plume is as high as 500 mW m−2 near core hole VC‐1 (Jemez fault zone) to 200 mW m−2 at Fenton Hill (Jemez Plateau). Chemical and isotopic data indicate that two source reservoirs within the caldera (Redondo Creek and Sulphur Springs reservoirs) are parents to mixed fluids flowing in the hydrothermal plume. However, isotopic data, borehole data, basic geology, and inverse relations between temperature and chloride content at major hot springs indicate that no single reservoir fluid and no single diluting fluid are involved in mixing. The Valles caldera hydrothermal plume is structurally dominated by lateral flow through a belt of vertical conduits (Jemez fault zone) that strike away from the source reservoir. Stratigraphically confined flow is present but dispersed over a wide area in relatively impermeable rocks. The Valles configuration is contrasted with the configuration of the hydrothermal plume at Roosevelt Hot Springs, which is dominated by lateral flow through a near‐surface, widespread, permeable aquifer. Data from 12 other representative geothermal systems show that outflow plumes occur in a variety of magmatic and tectonic settings, have varying reservoir compositions, and have different flow characteristics. Although temperature reversals are commonly observed in wells penetrating outflow plumes, reversals are not observed in all plumes. Less information is available on the absolute age of hydrothermal outflow plumes, although the data show that they can be as old as 106 years and display episodic behavior.
Tests on the recovery of precise geoid height differences from gravimetry
Authors: Kearsley A. H. W.
Published: February 2008

To transform Global Positioning System (GPS) derived ellipsoidal height differences, observable to 2–3 ppm of the station separation, into orthometric heights without loss of precision, geoid‐ellipsoid spatial height differences (ΔN) need to be evaluated to this same precision. One of the gravimetric techniques which has been shown, in theory, to be capable of achieving this precision uses ring integration to sense the short wavelength contribution of the local gravity field to ΔN, in combination with a geopotential model to provide the medium to long wavelength contribution. In this paper, comparisons of ΔN, computed by this method, with ΔN values derived from GPS and conventional levelling, have answered many questions relating to computational procedure and have demonstrated the viability of the method. Agreements in ΔN have been accomplished at about 2 to 3 ppm for shorter (<60 km) lines, and at <2 ppm for longer lines. This has been achieved by integrating reduced gravity anomalies over a spherical cap of radius about 1° to find the short wavelength features in ΔN. The gravity anomalies are reduced to a high‐order geopotential model (OSU81, nmax= 180), and this same model is used to generate the longer‐wavelength features of ΔN. It appears that, for a model of this high order, the radius of the cap size is not critical. However, using a lower‐order (e.g., nmax= 90) model as a reference, it appears that the optimum radius, while being in the region of π/nmax, is more critical, although comparisons in ΔN at 2 to 3 ppm are still achievable. The ability of geopotential model OSU81 alone to recover ΔN varies with location and degree. In two of the areas the comparison was at about 10 ppm or worse for nmax= 180, with the fit improving with decrease in nmax. By contrast, the other two tests showed control ΔN at about 3 to 4 ppm for nmax= 180, with the fit deteriorating with decreasing nmax, the behavior one would expect. Apparently the quality of the geopotential model varies with location. It is striking, however, that no matter how poorly the model recovered ΔN, the addition of an inner zone contribution, integrated over a surprisingly small cap size (<0.5°), improved the comparisons and in each case brought them down to the level of 3 to 4 ppm. These tests serve to confirm the viability of the ring integration method for evaluating ΔN, while giving valuable guidance as to the optimum configuration for the technique.
Permeability, conductivity, and pore geometry of sandstone
Authors: Doyen Philippe M.
Published: February 2008

Using Bruggeman's effective medium theory, the hydraulic permeability and dc electrical conductivity of sandstone are both expressed in terms of the ratio of two microscopic lengths, which are the dimensions of a characteristic pore throat and pore chamber, respectively. The theory is consistent with Kozeny‐Carman formulas in the limit of a perfectly microscopically homogeneous pore structure. Based on the effective medium approximation, the transport properties of Fontainebleau sandstone are predicted from a quantitative study of the pore space morphology. A series of epoxy‐impregnated thin sections of Fontainebleau sandstone was prepared from cores with porosity ranging between 5 and 22%. Using an image analyzer, throat and pore size distributions were constructed from the digitized and segmented microsections. For each sample the transport coefficients are calculated from the characteristic lengths, which are estimated directly from the experimental size histograms. The changes of both permeability and conductivity are predicted within a factor of 3 over the whole range of porosity. The variations of transport coefficients with porosity are interpreted from the contrasting evolution of pores and throats during diagenesis. Large pore chambers alternate with narrow passages in Fontainebleau sandstone. With decreasing porosity, some of the large pores remain stable, while the throats gradually shrink and are finally eliminated. The microscopic inhomogeneity of the pore geometry of Fontainebleau sandstone implies that its flow properties deviate from Kozeny‐Carman predictions.
Phase relationships in Fe‐Ni alloys at high pressures and temperatures
Authors: Huang Eugene , Bassett William A. , Weathers Maura S.
Published: February 2008

Diffusionless phase transformations in Fe‐Ni alloys with up to 35 wt % Ni were studied using resistance‐heated diamond anvil cells and synchrotron radiation. At temperatures up to 600°C and pressures up to 25 GPa, Fe35%Ni showed only the face‐centered cubic (fee) phase while Fe25%Ni and Fe10%Ni showed the body‐centered cubic (bcc) and hexagonal close‐packed (hep) phases in addition to the fee phase. The phase relationships of these solid phases are topologically similar to those of pure iron. The triple point becomes lower in temperature, and the slope (dT/dP) of the fcc/hcp boundary decreased as Ni‐content increases in the alloys. From our results we infer that Fe‐Ni alloys in the Earth's inner core might exist as two phases, fcc and hcp. The existence of these two phases could have played an important role in the separation of the inner and outer core.
Mechanisms of sediment accretion in the Middle America Trench off Mexico
Authors: Moore Gregory F. , Shipley Thomas H.
Published: February 2008

High‐resolution seismic reflection and Sea Beam bathymetrie data provide insights into the processes of sediment offscraping and accretion in the Middle America Trench off southern Mexico. Thick terrigenous sediments that are transported down Ometepec Canyon and accumulate along the trench floor are scraped off the oceanic plate and accreted in thrust packets to the lower trench slope. The packets offscraped represent most of the trench strata. Underlying hemipelagic deposits that accumulate on the seafloor seaward of the trench are subducted landward of the toe of the slope. Horizontal displacement on the thrusts is less than 1 km. Leading edge folds are the surface expressions of the thrusts and strike subparallel to the base of the trench slope. The folds are continuous for as much as 10 km and have amplitudes as high as 200 m and wavelengths of 0.5 to 2 km. Folds are best developed along sections of the trench with interbedded silly turbidite and mud deposits. Folds are absent where thick coarse‐grained fan deposits occur. Thickening of the thrust packets occurs by large‐scale thrust duplication, by layer‐parallel shortening, and by deposition of material that slumps off the leading edge of older upslope thrust blocks.
Weathering, the carbon cycle, and the differentiation of the continental crust and mantle
Authors: Van Groos A. F. Koster
Published: February 2008

Weathering produced a total of 2–4 × 1025 g of carbonates, clay minerals and SiO2 (chert and quartz) in approximately equal proportions during the history of the Earth. This amounts to approximately 1% of the mass of the mantle. A third was produced during the Hadean, before 3900 Ma. The amount of carbon consumed is greatly in excess of the crustal carbon reservoir and is equivalent to the total amount of carbon in the upper mantle. It is concluded that most of the material produced was subducted and that only a small fraction is preserved in sedimentary rocks. The chemistry of the subducted sediments was highly diverse in response to the variability of the climatological and depositional environment. This resulted in significant mantle heterogeneities on different scales. The absence of Hadean sedimentary rocks is explained by postulating that all the sedimentary material produced in this period was subducted after being mixed with a large amount of basaltic and komatiitic impact debris derived from the intense meteorite flux. The meteorite flux diminished to 0.2–0.4 km3/yr at the Hadean‐Archean boundary, which resulted in a large reduction of the amount of impact debris, and sediments began to accumulate at the earth's surface. At the same time copious amounts of felsic intrusive rocks, derived from silica‐enriched subducted material, were produced and a continental crust began to grow rapidly. The model indicates that the oldest sedimentary rocks contain a substantial proportion of impact‐derived material and that careful investigation of these rocks may provide support for the model.
The nature of magnetic single domains in volcanic glasses of the KBS Tuff
Authors: Schlinger Charles M. , Griscom David , Papaefthymiou Georgia C. , Veblen David R.
Published: February 2008

Magnetic and optical variations in volcanic glasses of the East African Rift have been investigated by means of experimental and analytical study. The results add to the formal understanding of magnetism in volcanic glasses and give us new perspectives on the nature, origin, and significance of fine‐grained magnetic minerals and on the magnetic petrology of glassy rocks. The samples that we have studied in detail, glass shards from the KBS Tuff of northern Kenya, contain a superparamagnetic precipitate of microcrystals, ∼2 to ∼10 nm in size. The precipitate is likely to have formed at elevated temperature, during or subsequent to eruption, about 1.9 m.y. ago. We have studied these microcrystals by means of transmission electron microscopy (TEM), ac and dc magnetic methods, electron spin resonance (ESR), Mössbauer spectroscopy, and optical spectroscopy. Our efforts have concentrated on determining the magnetic mineralogy and understanding the mechanism(s) responsible for the optical and magnetic behavior of these glasses. The particle size determination for the larger of these microcrystals has been achieved using TEM imaging and Mössbauer spectroscopy. The mineralogy is nonstoichiometric magnetite, based on Mössbauer, X ray diffraction, and electron diffraction results. The ratio of saturation rémanent magnetization to saturation magnetization for the larger microcrystals is very nearly that predicted by single‐domain theory for a uniaxial anisotropy, the origin of which may lie with stress or surface effects. We interpret magnetic and ESR results to infer the possible existence of microcrystals or amorphous clusters of Fe atoms, ∼1 nm or smaller, in KBS glasses that appear homogeneous in high‐resolution TEM images. Results from optical absorption experiments show that the color variations in KBS shards arise due to optical absorption by the precipitate, a result consistent with classical electromagnetic theory.
Cumulate maturation and melt migration in a temperature gradient
Authors: Lesher C. E. , Walker D.
Published: February 2008

A potential driving force for cumulate compaction and melt segregation in magmas arises from the change in crystal solubility with temperature. Spatial thermal variation in a cumulus crystal pile sets up a gradient in interstitial melt composition, which in turn provides a diffusion potential for silicate liquid species. Mass transport in response to this potential, referred to as thermal migration, drives adcumulus mineral growth in the cooler regions of a cumulus crystal pile and segregation of interstitial melt toward warmer regions. Heat loss through the cumulate mush zone to the country rock promotes expulsion of intercumulus liquid back toward the magma reservoir. Heat loss from the mush to cooler convective draughts within the chamber promotes trapping of intercumulus liquid. Liquid state Soret diffusion alters this mechanism only in minor chemical detail except when Soret fractionation produces changes in liquid chemistry that exceed those produced by crystal fractionation. This can cause solubility curves to be intersected at high temperature rather than low T, burying the intercumulus liquid in the boundary layer and preventing its escape back into the magma. Quartz saturation in silicic systems may be affected in this way. We demonstrate this process with laboratory experiments in the systems Mg2SiO4‐SiO2 and mid‐ocean ridge basalt. Cumulate compaction rates by thermal migration are limited by diffusion rate, but length scales for compaction increase linearly in time as long as the saturation conditions and temperature are unchanged. Adcumulus crystal growth remote from the accumulation front by thermal migration, although locally at equilibrium with the intercumulus melt, is not in chemical equilibrium with the main magma reservoir. The operation of thermal migration can reconcile two commonly observed, but seemingly contradictory, features of adcumulate rocks: unzoned cumulus crystals (reflecting isothermal crystallization at one place) within cryptically varying mesoscopic layers (reflecting spatial variations of temperature). The chemical details of this process are complicated by selective interdiffusion of components in the intercumulus liquid along compositional/thermal gradients. Compatible and incompatible elements show anomalous enrichments, thus complicating estimates of residual intercumulus liquid porosity and magma composition from cumulus rock chemistry. Thermal migration does not operate independently of magma body cooling or to the exclusion of the other processes of cumulate crystal compaction. Physically, the most promising opportunities for thermal migration to produce observable results occur within individual layers of large layered sequences. Large mafic intrusions cool slowly enough that mass diffusion in the intercumulus melt can observably rearrange constituents of individual layers of cumulus crystal pile in chemistry, modal mineralogy, and texture. Some of the expected consequences of the process are found in layered sequences of mafic intrusions.
San Jacinto Intrusive Complex: 1. Geology and mineral chemistry, and a model for intermittent recharge of tonalitic magma chambers
Authors: Hill R. I.
Published: February 2008

Geological mapping within the San Jacinto Mountains of southern California has delineated three major and numerous minor plutons of the Cretaceous Peninsular Ranges batholith. Early minor intrusives emplaced into quartz‐rich metasedimentary sequences span the compositional range olivine gabbro to granite. The three large (to 250 km2) plutons span a limited compositional range between mafic tonalite (color index (CI) > 15) and K‐feldspar‐poor granodiorite (CI ≃ 10). All units are composed of plagioclase (An30–40) (50–55%), quartz (20–30%), K‐feldspar (1–8%), biotite (10–15%), hornblende (0–5%), titanite (0–2%), and accessory zircon, apatite, allanite, and ilmenite. Variations in mineral abundances are geographically systematic only within the youngest major mass (unit III), which grades from marginal mafic tonalite to central K‐feldsparpoor granodiorite. Mineral foliations and banding, schlieren, and inclusion orientation within each unit usually parallel the nearest contact. Alignment of foliations and apparent flow‐sorting and scour features are interpreted as reflecting flow patterns within each chamber. Mafic synplutonic dykes (of quartz diorite and tonalite) intruded the tonalites and were broken up to form extensive inclusion trains. Dyke‐tonalite relations are interpreted as showing that (1) magma adjacent to the pluton wall had considerable yield strength, (2) magmatic flow adjacent to pluton walls was capable of moving material some distance (up to kilometers) to form the inclusion trains, and (3) the dykes represent conduits through which a considerable amount of liquid was added to the inflating magma chambers. Mineral compositions throughout the major plutons are relatively uniform. Mean plagioclase composition ranges from An40 in the most mafic tonalites to An30 in the most felsic granodiorites; the total microprobe‐observed range is An44 to An25 (and to An47 in a mafic inclusion). Mg/(Mg + Fe + Mn) of biotite and hornblende drop similarly from 0.44 to 0.36 reflecting changes in rock Mg/(Mg+Fe). The sole oxide mineral is almost pure ilmenite. The limited ranges of mineral compositions imply considerable stability of physico‐chemical conditions during the crystallization of each pluton. The field, petrographic, and mineral chemical data are interpreted in terms of an intermittently recharged continuously solidifying magma chamber.
Statistics of the geomagnetic secular variation for the past 5 m.y.
Authors: Constable C. G. , Parker R. L.
Published: February 2008

A new statistical model is proposed for the geomagnetic secular variation over the past 5 m.y. Unlike previous models, which have concentrated upon particular kinds of paleomagnetic observables, such as VGP or field direction, the new model provides a general probability density function from which the statistical distribution of any set of paleomagnetic measurements can be deduced. The spatial power spectrum of the present‐day nondipole field is consistent with a white source near the core‐mantle boundary with Gaussian distribution. After a suitable scaling, the spherical harmonic coefficients may be regarded as statistical samples from a single giant Gaussian process; this is our model of the nondipole field. Assuming that this characterization holds for the fields of the past, we can combine it with an arbitrary statistical description of the dipole. We compute the corresponding probability density functions and cumulative distribution functions for declination and inclination that would be observed at any site on the surface of the Earth. Global paleomagnetic data spanning the past 5 m.y. are used to constrain the free parameters of the model, i.e., those giving the dipole part of the field. The final model has these properties: (1) with two exceptions, each Gauss coefficient is independently normally distributed with zero mean and standard deviation for the nondipole terms commensurate with a white source at the core surface; (2) the exceptions are the axial dipole g10 and axial quadrupole g20 terms; the axial dipole distribution is bimodal and symmetric, resembling a combination of two normal distributions with centers close to the present‐day value and its sign‐reversed counterpart; (3) the standard deviations of the nonaxial dipole terms g11 and h11 and of the magnitude of the axial dipole are all about 10% of the present‐day g10 component; and (4) the axial quadrupole reverses sign with the axial dipole and has a mean magnitude of 6% of its mean magnitude. The advantage of a model specified in terms of the spherical harmonic coefficients is that it is a complete statistical description of the geomagnetic field, capable of simultaneously satisfying many known properties of the field. Predictions about any measured field elements may be made to see if they satisfy the available data.
Mid‐Cretaceous paleomagnetic reference field for North America: Restudy of 100 Ma intrusive rocks from Arkansas
Authors: Globerman Brian R. , Irving E.
Published: February 2008

We have obtained new paleomagnetic data from three middle Cretaceous alkalic intrusive complexes in central Arkansas: Magnet Cove and Potash Sulfur Springs intrusions (about 100 Ma, 20 sites), and Granite Mountain stock (about 88 Ma, 6 sites). Out of 254 specimens measured, 147 from 20 sites were accepted. The remainders have scattered magnetization directions which experiments indicate were caused by lightning occurring during frequent electrical storms in this region. The geologic evidence indicates that the intrusions have not been tilted since emplacement. The directions of magnetization calculated with respect to present‐day horizontal yield a mean paleopole at 192.5°E, 74.1°N (N = 20, K = 34, A95 = 5.7°). This does not differ at 95% confidence from that determined previously by Scharon and Hsu, but the error is diminished by half. The angular dispersion (Sd) of site virtual geomagnetic poles (VGPs) is 13.6°, similar to the value of Sd (about 15°) predicted by current paleosecular variation models. The revised Arkansas paleopole is combined with four other cratonic North American paleopoles of mid‐Cretaceous age to yield a revised middle Cretaceous reference pole at 196°E, 71°N, A95 = 4.9°. The mean of nine paleopoles derived from overprints in rock units from the craton and craton margin, which range in age from Proterozoic to Jura‐Cretaceous, is 202°E, 68°N, A95 = 8.2°. This is not different from the reference paleopole, indicating that overprinting was very widespread across North America in mid‐Cretaceous time.