bluuebunny/crossref_metadata_2025 huggingface dataset:
A direct measurement of the distance between a hypocenter in a Benioff‐Wadati Zone and the Slab‐Asthenosphere contact
Authors: Stefani Joseph P. , Geller Robert J. , Kroeger Glenn C.
Published: February 2008

We made a direct measurement of the distance from a hypocenter in the Benioff‐Wadati zone to the boundary between the downgoing slab and overlying asthenosphere. This was accomplished by identifying a low amplitude P wave reflection off the slab‐asthenosphere contact which arrives at teleseismic stations several seconds after the initial P arrival. The measured delay time (after P) of the reflected phase shows a consistent azimuthal variation, thus eliminating possible source or receiver effects. The relative amplitude, polarity, and delay time of this observed phase are consistent with a model in which the distance between hypocenter and slab‐asthenosphere contact at this depth is about 38±5 km. The event studied was an intermediate depth (181 km) earthquake in the southern Kuriles which occurred in the lower layer of a double seismic zone. Our measured distance, when compared to the distance between the two seismic zones, indicates that the upper layer of seismicity is within 15 km of the slab‐asthenosphere contact.

Appendix is available with entire article, on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, U.C. 20009. Document JB1‐010; $1.00. Payment must accompany order.
The origin of hotspot traces: Evidence from eastern Australia
Authors: Pilger Rex H.
Published: February 2008

Interpretation of available isotopic ages and of published geologic maps of igneous rocks in eastern Australia indicates a north‐south time transgressive pattern of cessation of igneous activity along a curvilinear trace, beginning about 35 Ma at 20°S and extending to the present, near 37°S. However, volcanic activity began 70–60 Ma over the length of the Highlands and persisted through the early Cenozoic until progressive cessation began. The latitudinal rate of termination is compatible with that predicted by plate reconstructions relative to the Hawaiian ‘hotspot’, but the trend of the trace is incompatible with that predicted unless the Hawaiian‐Emperor bend is somewhat greater than 50 m.y. in age relative to the LaBrecque et al. [1977] time scale. The volcanic pattern appears to be incompatible with a hotspot or plume hypothesis. Paleostress and contemporary stress indicators as well as the volcanic pattern support a model in which the trace forms, due to intraplate extension normal to the trend of the trace, resulting in pressure release and melting at the base of the lithosphere, beginning in the Late Cretaceous. This stress field persisted until 35 Ma when progressive reorientation of the stress field occurred. Cessation of igneous activity reflected onset of compression normal to the trend of the trace. As a consequence, a migrating stress node is recorded in the progressive pattern of extinction of volcanic activity along the trace. Evidence from other ‘hotspot’ traces suggests that they too represent intraplate stress controls. It is not necessary to postulate deep sources (mantle plumes) for the origin of hotspot traces; intraplate stress due to drag, plate margin forces, thermal contraction, and nonsphericity of the earth can also reflect ‘absolute’ motion.
Global Q estimates from antipodal Rayleigh waves
Authors: Chael Eric P. , Anderson Don L.
Published: February 2008

Global average estimates of the group velocity and attenuation of long‐period (120–300 s) Rayleigh waves were made using seismograms from the epicenter's antipode (Δ≃180°). Focusing at the antipode produced amplified arrivals with favorable signal‐to‐noise ratios. The high‐quality data yielded very stable attenuation values, with excellent agreement between the results from successive Rayleigh arrivals for a single event and between the results for two different events. Lateral heterogeneities in earth structure can cause systematic biasing of attenuation measurements based on antipodal records. The initial, uncorrected results therefore provide a lower bound estimate of global Q. An ellipsoidal perturbation in shape was used to simulate the effects of lateral velocity heterogeneities on Rayleigh wave propagation. Using the agreement of repeated attenuation measurements as a constraint, we estimated both the bias in those measurements and the splitting widths of the Rayleigh modes. At a period of 200 s, the estimated splitting width is 0.30% this agrees closely with calculations by Luh (1974) for an earth model with different continental and oceanic velocity profiles. The estimated bias varied from 30% to zero over the 120‐ to 260‐s band. After correcting for bias, the antipodal Q values range from 108 at 120 s to 188 at 260 s. These Q are within the range of previous measurements but are lower than the mean values from typical great circle studies, implying that the globally averaged upper mantle is slightly more attenuative than has been generally recognized.
Paleomagnetism of small basalt exposures in the West Puget Sound Area, Washington, and Speculations on the accretionary origin of the Olympic Mountains
Authors: Beck Myrl E. , Engebretson David C.
Published: February 2008

Twenty‐two sites from two small exposures of basalt of probable Eocene age immediately east of the Olympic Mountains have the following mean paleomagnetic direction: declination, 176.5°; inclination, −66.5°; circle of 95% confidence, 8.6°. This is the only paleomagnetic study of pre‐Miocene rocks anywhere in western Washington or Oregon that matches the polar wandering curve for North America. However, a marked fanning of paleomagnetic declinations around the Olympic core suggests that concordance in this case is fortuitous and results from oroclinal bending during or immediately after Olympic orogenesis. Creation of the Olympic Mountains involved accretion of thickened basaltic crust followed by underthrusting of large quantities of oceanic sedimentary debris, as the Farallon plate pressed northeastward into a recess formed by the thick crystalline blocks of Vancouver Island and the North Cascades.
Comments on ‘Tectonic setting of ophiolite obduction in Oman’ by Robert Coleman
Authors: Rod Emile
Published: February 2008

The Oman ophiolite project [Coleman, 1981a] contributed a considerable amount of valuable detailed geologic, petrologic, and geophysical data which added to our understanding of the origin and emplacement of the ophiólites. However, as in any study involving a large number of researchers, some inconsistencies appear. Hop son et al. [1981] are aware of this problem. In the structural and tectonic interpretations particularly, there is a certain tendency to present in text and illustrations one particular line of arguments in support of one preconceived idea.
Two‐dimensional compressional wave velocity structure under San Francisco Volcanic Field, Arizona, from teleseismic P residual measurements
Authors: Stauber Douglas A.
Published: February 2008

A low compressional‐wave velocity region in the midcrust below the San Francisco Mountain stratovolcano, Arizona, has been detected by the teleseismic P residual technique. This region is approximately 6 km wide, lies between elevations of 9 km and 34 km below sea level, and has a compressional velocity reduction of more than 6% with respect to the surrounding rocks. Several mechanisms are found to be quantitatively sufficient to produce such a feature. These include (1) a cool silicic pluton enclosed in a more mafic crust, (2) high temperature (near but below the solidus) in a quartz‐bearing rock in the low‐velocity region, (3) high density of water‐filled cracks having pore pressures nearly equal to lithostatic pressure, and (4) the presence of melt, either in intergranular pores or in crystal‐poor dikes.
Geotomography in oil shale
Authors: Daily W. D. , Lytle R. J. , Laine E. F. , Okada J. T. , Deadrick F. J.
Published: February 2008

Geophysical tomographs were made of oil shale at the Occidental Oil Shale, Inc., mine near DeBeque, Colorado. These tomographs mapped the in situ radio frequency electromagnetic attenuation of oil shale between 1 and 40 MHz in a plane 27 by 12 m. Measured attenuation rates (inverse skin depth) ranged from 0.1 to 0.5 m−1 with the attenuation rate approximately proportional to the square root of frequency. At 20 MHz, the measured relative permittivity of oil shale is about 9. Geotomographs were made in another plane 16.8 by 12 m, before and again after the shale in this region was explosively rubbled. Measured electromagnetic attenuation rates at 20 MHz were generally larger in the rubble than in the undisturbed shale, suggesting that geotomography may be useful in mapping a characteristic of the rubbled zone such as void content or void size distribution. Model calculations of the rubble using a theory of dielectric mixtures predict a smaller attenuation rate in the rubble, implying that the measurements result from phenomena such as scattering or oil shale anisotropy, which are not accounted for in the model.
Observation of 18.6‐year modulation tide at the South Pole
Authors: Rydelek P. A. , Knopoff L. , Zürn W.
Published: February 2008

We have observed long‐period variations in the amplitudes of the diurnal and semidiurnal gravity tides at the South Pole which we attribute to the tidal effects of the 18.6‐year regression of the nodes of the lunar orbit. The observations are in agreement with theoretical values of the 18.6‐year amplitude modulation of these tides.
Information theory lateral density distribution for Earth inferred from global gravity field
Authors: Rubincam David Parry
Published: February 2008

Information theory inference, better known as the maximum entropy method, is used to infer the lateral density distribution inside the earth. The approach assumes that the earth consists of indistinguishable Maxwell‐Boltzmann particles populating infinitesimal volume elements and follows the standard methods of statistical mechanics (maximizing the entropy function). The GEM 10B spherical harmonic gravity field coefficients, complete to degree and order 36, are used as constraints on the lateral density distribution. The spherically symmetric part of the density distribution is assumed to be known. The lateral density variation is assumed to be small in comparison with the spherically symmetric part. The resulting information theory density distribution for the cases of no crust removed, 30 km of compensated crust removed, and 30 km of uncompensated crust removed all give broad density anomalies extending deep into the mantle, but with the density contrasts being the greatest toward the surface (typically ±0.004 g cm−3 in the first two cases and ±0.04 g cm−3 in the third). None of the density distributions resemble classical organized convection cells. The information theory approach may have use in choosing standard earth models, but the inclusion of seismic data into the approach appears difficult.
The evidence of ‘CHO1’ in sediment core samples of Pacific Ocean collected among FGGE
Authors: Peng Han‐chang , Zhao Kui‐huan , Chen Sui‐tian
Published: February 2008

Three new microtektite sites have been identified in addition to the known world distributions. This is of significance for confirming and extending the known range of tektite distributions and for a thorough study of their origin.
A brief K‐Ar age study of the IRDP borehole, Reydarfjördur, eastern Iceland
Authors: Albertsson K. J. , Hooker P. J. , Miller J. A.
Published: February 2008

Five core samples from a depth of 182 to 1899 m were studied in order to ascertain if any geologically sensible K‐Ar ages could be determined on such highly altered material. Ages were obtained ranging from 8.9 to 10.9 Ma, seemingly consistent with previous K‐Ar work on correlated updip surface exposures. Serious radiogenic argon depletions or total overprinting by the hydrothermal metamorphism were not observed.
Changes in gravity resulting from changes in topography
Authors: Walsh Joseph B.
Published: February 2008

The horizontal and vertical components of gravity change when material is transported from one place to another. The change at a station fixed in a position is the sum of two effects. Consider the change in gravity when material is moved toward the station from a remote point. One part, ΔgM, of the total change in gravity is due to the increased attraction between the mass of the transported material and the gravimeter. The other part, ΔgF, is due to deformation of the earth caused by forces exerted on the surface by the transported material. I assume that deformation of the earth caused by topographical changes can be simulated by applying appropriate shear and normal tractions to a homogeneous, elastic half space. Using the reciprocal theorem, I derive a simple integral giving the changes in the horizontal and vertical components of gravity due to arbitrary tractions in the half space. Evaluating the integral for features having various shapes leads to the conclusion that both the horizontal and vertical components of the gravity change can be expressed in the form ΔgF = −cΔgM(ρgd/μ), where c is a constant, ρ is density, g is gravity, μ is rigidity, and d is a linear dimension, either the distance between the station and the feature or the lateral size of the feature. ΔgF becomes appreciably large in comparison with ΔgM when d is 100 km or larger.
The ROSE Seismic Data Storage and Exchange Facility
Authors: LaTraille Sharon L. , Gettrust Joseph F. , Simpson Michael E.
Published: February 2008

The Rivera Ocean Seismic Experiment was a multi‐institutional marine seismic experiment held off western Mexico during early 1979. Dense spatial sampling was provided by ocean bottom seismometers and ocean bottom hydrophones recording both natural events and explosions. The ROSE experiment and a companion land experiment generated a data base of approximately 76,000 ‘events’ (source‐instrument pairs). Procedures for efficient data storage, retrieval, and exchange were designed and implemented at the Hawaii Institute of Geophysics. Data capabilities of the exchange are demonstrated by using data from several participants to obtain qualitative estimates of the attenuation of seismic energy in the oceanic mantle near the East Pacific Rise and through the ocean/continent margin near Petatlan, Guerrero, Mexico. The data base is now available for general use.
The correction for the travel time effects of seafloor topography in the interpretation of marine seismic data
Authors: Purdy G. M.
Published: February 2008

Problems associated with the application of topographic corrections to deep‐ocean marine seismic data are described. Synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path. Water path corrections place the shots at the ray entry points on the seafloor, require no assumptions about the structure under investigation, and maintain the correct distribution of data with range. The water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time‐slowness space. There is, then, no requirement to make determinations of slowness, and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity. Water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity.
Variation in young oceanic crust and upper mantle structure
Authors: Gettrust Joseph F. , Furukawa Kazuo , Kempner William B.
Published: February 2008

Seismic refraction and single‐channel reflection data taken along 0.5‐, 2.5‐, and 4.5‐m.y.‐age isochrons near the East Pacific Rise during Project ROSE are used to determine if a systematic change in the P velocity‐depth function with age can be resolved. Inversion of these data suggests that any change in crustal P velocity structure related to age is smaller than variability in the seismic velocity‐depth function along an isochron. The emergence of a ‘normal’ crust‐mantle transition by 4.5‐m.y.‐age is seen in these data. Crust and crust‐mantle transition zone heterogeneity along these isochrons may be related to the along strike variability in processes at the ridge crest. The velocity‐depth functions for the three split profile refraction lines are compared with velocity‐depth functions for the Samail ophiolite, which is thought to represent oceanic crust of similar age. The velocity‐depth functions for the ROSE data are bounded by different velocity‐depth models for the Samail ophiolite; this suggests that those models are not in disagreement but represent the lateral heterogeneity that can be expected in young oceanic crust.
Heat and mass transfer in a fault‐controlled geothermal reservoir charged at constant pressure
Authors: Goyal K. P. , Narasimhan T. N.
Published: February 2008

A two‐dimensional mathematical model of a fault‐controlled geothermal reservoir has been developed. Heated lighter water, rising in the fault, is assumed to charge a reservoir which, in turn, is overlain by a thin, impermeable, thermally conducting cap rock. The mass flow rate or the pressure associated with the charging process at the fault inlet is unknown and can only be estimated. Thus in this paper the pressure in the fault at the bottom of the reservoir is assumed to be prescribed. Quasianalytic solutions for the velocity, pressure, and temperature are obtained in the fault‐reservoir system for a high Rayleigh number flow. In this approximation, the upwelling fluid does not cool off appreciably until it reaches the cold upper boundary of the reservoir and encounters conductive heat loss. This thermal boundary layer, which is thin at the top of the fault, grows outward laterally and occupies the full thickness of the aquifer far away from the fault. The mathematical model is based on the flow of liquid water in a saturated porous medium. The solution techniques involve the combination of perturbation methods, boundary layer theory, and numerical methods. The analysis of this generic model can be applied to liquid‐dominated geothermal systems where the thickness of the impermeable cap rock is very small compared to the depth of the reservoir.
A comparison of synthetic P‐Tau sections for proposed models of two ophiolites
Authors: Brocher Thomas M. , Kempner William C. , Gettrust Joseph F.
Published: February 2008

Analyses of slant stacks of synthetic record sections generated from velocity models for ophiolites by the reflectivity method show that even 90‐km‐long refraction lines having a 10‐Hz source and a 1‐km intertrace spacing barely distinguish between fine‐scale crustal structures which are of current interest. Such data are sufficient, however, to distinguish between young and old oceanic crust. Tripling the spatial sampling period degrades the quality of the synthetic slant stacks for this temporal frequency content. A major implication of the study is that most currently available field data, being sparser and poorer in quality (due to both noise and instrument responses), offer little hope of resolving thin low‐velocity layers or small changes in velocity gradients by using newly proposed inversion techniques based on slant stacking. Last, a simple, first‐order correction to a downward continuation algorithm, namely, the inclusion of the phase shift caused by postcritical reflection at each ray parameter, is proposed.
Proceedings of the Third International Colloquium on Mars: Dedicated to Thomas A. Mutch, 1931–1980
Authors: Arvidson Raymond E.
Published: February 2008

Thomas (Tim) A. Mutch, professor of Geological Sciences, Brown University, was killed in a tragic mountain climbing accident in the Himalayas in October 1980. At the time, he was on a leave of absence from Brown University, serving since September 1979 as the Associate Administrator for Space Sciences of the National Aeronautics and Space Administration. Tim's death leaves a significant gap in the cadre of scientists involved in planetary research. He is missed as a researcher capable of synthesizing a wide variety of data, as an active and eloquent proponent of solar system exploration, and perhaps most importantly, as a dear friend to many of us.
Foreword [to Proceedings of the Third International Colloquium on Mars]
Authors: Edelson B. I.
Published: February 2008

Our understanding of the solar system has advanced rapidly during the past two decades of spacecraft exploration. Mars has been of particular interest throughout this period. Beginning 18 years ago with Mariner 4 and continuing today with the operation of the Mutch Memorial Station, the quality and quantity of Mars data have increased continuously. The data from each of these missions have dramatically changed our ideas about this planet; we now know that Mars is a diverse, scientifically fascinating world.
Classification of surface units in the equatorial region of Mars based on Viking Orbiter color, albedo, and thermal data
Authors: Arvidson R. E. , Guinness E. A. , Zent A. P.
Published: February 2008

Viking Orbiter color, albedo, and thermal inertia data for the equatorial region of Mars were utilized as a five‐dimensional data set to search for clusters that correspond to mappable surface units. A principal components analysis shows that 84% of the variance within the data for the equatorial region (30° of equator) can be carried along two vector directions. These vector directions typify the dominant trend of Martian surface materials. The first direction indicates that as the surface materials become brighter at red wavelengths they also brighten at green and violet wavelengths. The second direction implies that as the albedo increases, the thermal inertia decreases. These dominant trends were deemphasized by stretching the data from a five‐dimensional elliptical swarm into a hypersphere through the use of principal component techniques. The decorrelated data were then plotted in a triangle diagram with red/violet, albedo, and thermal inertia apices to facilitate discrimination of inherent clusters. Unit maps for the region within 15° latitude of the equator and bounded by 0° to 50° and 280° to 360°W longitude were generated on the basis of the clusters. Up to eight clusters can be identified and mixing between clusters appears to be important. The three major clusters consist of bright, red materials with low thermal inertia; dark, relatively gray materials with high inertia; and materials with intermediate values of color, albedo, and thermal inertia. Mantling by aeolian deposits or volcanic materials is prevalent in the brightest, reddest unit with the lowest thermal inertia, while the darker, grayer units with higher thermal inertia are dominated by marelike plains. The mantling is apparent in Viking Orbiter images with resolutions from 8 to 30 m per pixel.
A global system of spiraling geosutures
Authors: Neev David , Hall John K.
Published: February 2008

A global system of geosutures was identified. These form a major counterclockwise converging spiral pattern whose ‘eye’ stretches along the Alpine‐Himalayan orogenic belt. The system of lithospheric slices, which are bounded by these geosutures, approaches the Alpine‐Himalayan ‘eye’in different ways: the slices converging from the south collide frontally, whereas the northern ones tangentially and sequentially slide along one another. Another smaller clockwise converging spiral pattern was identified close to the earth's north pole. In addition to the Alpine‐Himalayan belt, many other important tectonic features appear to be associated with the geosutures of the proposed system, thereby offering explanations for their genesis. The correlation between some of the geosutures and hydrocarbon accumulations as well as metallic ore deposits may stimulate new concepts and leads for economic explorations elsewhere. Another system of geosutures of planetary dimensions was also noticed on Mars. The largest spiral pattern converges counterclockwise on an ‘eye’ located along the Martian equator. Two smaller spiral patterns were also noticed on both poles of Mars. The northern spiral is clockwise, while the southern one is counterclockwise. If the Martian spirals are indeed analogous to those on earth, then a third spiral showing counterclockwise convergence may exist close to the earth's south pole. The coherence and smoothness of the newly identified patterns on earth build confidence in their reliability. Once the kinematics involved in this system are understood, we expect that their origin will also be deciphered. However, this last is beyond the scope of the present paper.
Effects of fractional crystallization and cumulus processes on mineral composition trends of some lunar and terrestrial rock series
Authors: Longhi John
Published: February 2008

A plot of Mg of mafic minerals versus An of plagioclase in cumulate rocks from various lunar and terrestrial rock series shows each series to have a distinct curvilinear trend. The slopes of these trends vary from nearly vertical in the case of lunar anorthosites and Mg‐norites to nearly horizontal in the case of gabbros from the mid‐Atlantic ridge. Calculations based upon known major element partitioning between mafic minerals, plagioclase and subalkaline basaltic liquids indicate that fractional crystallization coupled with cotectic accumulation of mafic minerals and plagioclase will produce mineral composition trends on the Mg versus An diagram with slopes >1 for cases where An ≳ Mg. Furthermore, fractional crystallization of basaltic magmas with alkali concentrations approaching zero will produce near vertical Mg versus An trends. Therefore, the steep slopes of the lunar rock series are consistent with relatively simple fractionation processes. The relatively flat slope of mineral compositions from gabbros collected from the mid‐Atlantic ridge at 26°N is inconsistent with simple fractionation processes, and calculations show that periodic refilling of a fractionating magma chamber with picritic magma cannot simply explain this flat slope either.
Comparison of thermal history of orthopyroxenes between Lunar Norites 78236, 72255, and diogenites
Authors: Takeda Hiroshi , Mori Hiroshi , Miyamoto Masamichi
Published: February 2008

Orthopyroxene crystals from lunar norite 78236 and Civet Cat clast 72255 were compared with those of two diogenites, Johnstown and Ibbenbüren. Their exsolution and deformation textures were studied by analytical transmission electron microscope (ATEM) in addition to single crystal X ray diffraction and electron microprobe techniques. 72255 pyroxenes contain clinobronzite lamellae similar to those in Johnstown, which were interpreted to have formed by stress‐induced transformation by shock events. The X ray and TEM observation of the orthopyroxene (En76Fs21Wo3) in 78236 shows no augite exsolution with (100) in common. Abundant GuinierPreston zones, with several unit‐cells wide, are present. The textures in conjunction with the thermal history proposed by chronologic data suggest that 78236 experienced slow cooling deep in the crust above 1000°C, cooled rapidly due to excavation by an impact event, and then cooled slowly again at moderate temperature. This history is similar to that proposed for Johnstown. Computer simulation on the development of the (100) augite lamellae in 72255 and Ibbenbüren suggests a deep origin in the crust.
The Apennine Front Core 15007/8: Irradiational and depositional history
Authors: Bogard D. D. , Morris R. V. , Johnson P. , Lauer H. V.
Published: February 2008

Ferromagnetic resonance (FMR), magnetic, and noble gas measurements were made on soils from the Apennine Front core 15007/8 as part of a multidisciplinary study. Values of the Is/FeO surface exposure (maturity) index and FeO concentrations are reported for 0.5 cm depth intervals; noble gas and FMR data are reported for 13 soils and 2 clasts selected from various depths in the ∼57‐cm‐long core. These data, along with mineralogical and stratigraphic data reported by others, indicate four major core units whose contacts are at ∼18, 49, and 55 cm. An in situ reworking zone at the top of the core is not indicated by the Is/FeO data, and the cosmogenic gas data show that the core soils have experienced multistage exposure to cosmic rays. The 0–18 cm unit has values of cosmogenic 131Xe/ 126Xe that are higher than underlying soil and are indicative of irradiation at average depths >50 cm; thus this unit cannot have a simple irradiational relationship to soil beneath it. This unit is probably the ejecta from the crater on whose rim the core was collected. The 18–49 cm unit is characterized by relatively constant values of Is/FeO and abundances of cosmogenic gases and most likely represents a soil zone well mixed, possibly by downslope movement during the precrater era. Is/FeO drops by about 30 units at the 49‐cm contact. The two units below it are correspondingly more immature than overlying soil and, in addition, contain soils with either high concentrations of green glass or relatively low FeO contents. The significant heterogenetics in these two units indicate they have not been involved in the mixing layer which is presumed to occur due to downslope soil movement. Comparison of the 15007/8 core to other cores suggests that soils on slopes tend to be mixed and not contain units with long in situ irradiation profiles. Such cores are less likely to contain old or exotic soil layers.
Low temperature volatilization on the Moon
Authors: von Gunten Hans R. , Wegmüller Fritz , Krähenbühl Urs
Published: February 2008

The concentrations of volatile elements in grain‐size separated agglutinate and mineral fractions of samples 76240 (permanently shadowed) and 76260 were compared in heating experiments. The surface concentrations of Hg were found to be considerably higher in the shadowed sample. For the rest of the investigated elements only small or insignificant differences in the concentrations were observed between the two samples which differ in the storage temperature during the last 104 y. The difference in the concentrations of Hg must be the result of the shadowing. Based on the present investigation and on our earlier heating experiments, it can be concluded that Hg and Cd are mobilized by solar heating of the uppermost lunar soil and are trapped in cold places on the moon. This low‐temperature volatilization leads to a redistribution of volatiles (e.g., Hg and Cd) on the lunar surface.
Carbon components and their isotopic compositions in the Allende meteorite
Authors: Swart P. K. , Grady M. M. , Wright I. P. , Pillinger C. T.
Published: February 2008

High resolution stepwise oxidations have been undertaken on two bulk fractions and on an acid residue of the Allende meteorite. The results show the existence of at least three carbon phases and suggest the possible presence of two more. The bulk of the carbon has a temperature of peak release closely corresponding to amorphous (free) carbon and an isotopic composition of approximately −20‰. At a slightly lower temperature of release we believe there to be carbon depleted in 13C. As far as variations in experimental technique allow a decision to be made, combustion of this component appears to take place under conditions which previous workers have shown to liberate exotic compositions of nitrogen and xenon.
The Polymict Eucrites Elephant Moraine A79004 and A79011 and the regolith history of a basaltic achondrite parent body
Authors: Delaney Jeremy S. , Prinz M. , Nehru C. E. , O'Neill C.
Published: February 2008

The achondrites Elephant Moraine A79004 and A79011 are closely related polymicteucrites of a type previously unrecognized in Antarctica. They contain an early, metamorphically homogenized breccia containing clasts of great textural diversity very similar to those in howardites, and a late injected mafic clast component that contains a suite of basalts that may, in part, be the result of impact melting rather than internal igneous processes. The basaltic achondrite parent body (BAP), of which they are regolith samples, had a complex igneous history of (1) early igneous activity, (2) brecciation to form polymict breccias, (3) localized metamorphism resulting in partial homogenization of areas of the regolith, (4) late igneous activity resulting from internal igneous or impact melting processes, and (5) further brecciation and mixing of late formed mafic rocks with early breccias.
The Tucson iron and its relationship to enstatite meteorites
Authors: Nehru C. E. , Prinz M. , Delaney J. S.
Published: February 2008

Tucson is a unique iron meteorite with Si‐Cr bearing metal and about 8 vol.% of silicates that are almost free of Fe, Cr, Mn, and alkalis. The silicates are mainly forsterite (66.4%) and enstatite (30.2%) with minor diopside (2.7%), anorthite plus feldspathic glass (0.7%),and traces of Mg‐Al spinel and brezinaite. ‘Enstatite’ contains 0.5 to 21% Al2O3 and ‘diopside’ from 5 to 18%. Al2O3 generally increases away from early formed forsterite. The pyroxene is the most aluminous known in nature and contains close to the theoretical limit of Mg‐Tschermaks solution. These compositions crystallized metastably because of the difficulty of nucleating feldspar in the rapidly cooled assemblage. The preferred hypothesis is that the present forsterite‐enstatite silicate assemblage is similar to the precursor silicate asssemblage, with little or no high temperature metal silicate interaction. The metal and silicates were turbulently impact mixed at temperatures high enough to produce melt and forsterite (about 1500°C), with volatilization of Ga, Ge, and As from the metal and Na, K, and Mn from the silicates, and then rapidly cooled so that silicates formed metastable aluminous pyroxenes and anorthitic glass. The precursor forsterite enstatite assemblage is a previously unrecognized new type of chondritic or achondritic highly reduced assemblage related to the enstatite meteorites.
Study of 18O diffusion in magnesium orthosilicate by nuclear microanalysis
Authors: Jaoul Olivier , Houlier Bernard , Abel François
Published: February 2008

The self‐diffusion of 18O‐enriched oxygen in forsterite was determined by measuring the bulk concentration using the nuclear reaction 18O(p,α)15N. Important technical improvements led to a more precise determination of the 18O diffusion profiles. These results indicate that the self‐diffusion coefficient D* is independent of the annealing duration and the oxygen partial pressure. In addition, the D* values are found to be lower by nearly 1 order of magnitude than those previously published (preexponential factor D0 ∼ 2.3×10−6cm2/s, activation energy, ∼70 kcal/mole).
On the stability of shear cracks and the calculation of compressive strength
Authors: Dienes J. K.
Published: February 2008

Griffith's 1921 theory of crack stability is extended to account for the frictional energy dissipation associated with interfacial sliding when the crack faces are in contact. This global energy balance approach is essentially different from Griffith's 1924 local theory of compression cracks, which assumes that instability occurs when the stress at any point exceeds the intrinsic strength of the material. An explicit expression for critical crack size in compression is obtained. On the basis of this expression we find the most critical crack orientation and define theoretical strength as the critical stress for that orientation. This uniaxial compression strength significantly exceeds the tensile strength. For example, when the coefficient of friction is unity, the ratio of compressive to tensile strength is 8.7, and the crack plane lies at 13° to the loading direction. Following this, the heating due to interfacial sliding is found by a one‐dimensional thermoelastic analysis. It is concluded that under normal conditions the stress field acting on the crack faces is only slightly affected by frictional heating, though the temperature rise may be very high, and melting may occur within microseconds.
Core and crustal geomagnetic fields
Authors: Alldredge Leroy R.
Published: February 2008

The spherical harmonic content of fields from core sources and that from extended crustal sources overlap so much that very long wavelength anomalies obtained by subtracting a core field model up to only degree 10 from the observations will be distorted so badly that interpretation in terms of geological sources will be difficult.
Rayleigh waves excited by the discontinuous advance of a rupture front
Authors: Harris John G. , Achenbach Jan D. , Norris Andrew N.
Published: February 2008

Analytical results are presented for Rayleigh waves excited by a sudden change in the rate of growth of a subsurface zone of rupture. The curved rupture front advances across an inclined plane. The rupture can be brittle or cohesive tractions can act at its front. The analysis consists of two parts: First, ray theory is used to calculate wavefront approximations to the waves emitted when the rupture front speed suddenly changes. Secondly, a representation integral for the Rayleigh wave, where the integration is performed over a surface enclosing the rupture front, is constructed by using the emitted waves in combination with art appropriate Green's tensor. This integral is evaluated asymptotically. Synthetic accelerographs are constructed which illustrate how the rupture process, and the geometry of the rupture front and the fault plane affect the excitation of Rayleigh waves.
Heat and mass transfer in a fault zone‐controlled geothermal reservoir: Numerical results
Authors: Pratt R. , Kassoy D. R. , Gary J.
Published: February 2008

A model is described for planar geothermal reservoir charging by upflow in a permeable fault zone in the earth's crust. Liquid water, heated at depth in basement material, rises in the fracture system associated with the faulted region. When the fault is intersected by a horizontal layer of permeable material, charging of hot water into an adjacent aquifer can occur. The heat and mass transfer process for this system is defined in terms of natural and forced convection in a thermally active porous medium. Solutions to the basic mathematical model are obtained by finite‐difference methods. The fluid mechanics and energy transport are resolved in both the narrow fault zone and the adjacent aquifer. The results show that previously developed asymptotic solutions, derived for large Rayleigh numbers and narrow fault zones, provide quite good predictions of the system heat and mass transport. The pressure field induced by fluid motion, typically about 5% of the cold hydrostatic head, is found to be quite sensitive to the input mass flux when the system Rayleigh number R is between 100 and 500. When R = 100, the convective upflow in the fault zone is cooled by conduction to the surface down to 60% of the system depth. In contrast, when R = 500, only about 20% of the fault is affected by cooling. The temperature variation with depth in the adjacent aquifer is nearly the background conductive form at about 5 fault depths away from the fault zone when R = 500. In comparison, when R = 100, the analogous distance is reduced to about 1 fault depth. The surface heat flux distribution is strongly dependent on the R value. Above the fault the heat flux value increases linearly with R in the range 100 to 500.
Effect of a Hadean terrestrial magma ocean on crust and mantle evolution
Authors: Hofmeister Anne M.
Published: February 2008

This paper presents a model for the thermal and chemical evolution of a global magma ocean created by impact heating during the earth's accretion. Detailed calculations are given for a melt depth of 120 km for end member picritic and peridotitic compositions. Application of fluid mechanics and heat transfer principles shows as a first approximation that the chemical evolution can be portrayed by idealized fractional crystallization occurring only on the the bottom of the magma ocean. The fractional crystallization sequence for the picritic model is 90 km of mafics overlain by 15 km of ol‐sp‐gabbronorite and topped by 15 km of iron‐rich leucodiorite; whereas the peridotite sequence consists of 99 km of dunite, harzburgite, and websterite overlain by 5 km of ol‐sp‐gabbronorite and capped by 16 km of ferroleucogabbronorite. Although subsolidus convection would rehomoginize all but the felsic layers of the picritic model, the layers of the peridotite would remain intact owing to their stable density profile. The mineralogy of the felsic layers is largly independent of the basic model assumptions. However, the composition of the mafic layers is model dependent. The most likely results for the mafic mafic layers (lherzolite or primitive mantle peridotite) are similar to mantle xenoliths, suggesting that a terrestrial magma ocean may have existed. However, comparison of the chemistry of the calculated upper felsic layer with that of the Canadian shield shows that crystallization of a global magma ocean would not directly produce the Archean continental crust. Multiple melting and differentiation of the Fe‐leucodiorite to Fe‐leucogabbronorite layer and of the underlying primitive mantle would be required to form an Archean granitoid crust.
A study of the importance of nonlinearity in the inversion of earthquake arrival time data for velocity structure
Authors: Pavlis Gary L. , Booker John R.
Published: February 2008

All techniques that currently exist to invert arrival time data from earthquake sources for velocity structure rest on a local linearization of the nonlinear equations relating these data to the velocity model. We have studied the limitations imposed by this approximation using synthetic data in which a one dimensional velocity model is the only variable. We have found that this problem is sufficiently nonlinear that some iterative scheme is generally required to obtain results that are quantitatively correct. On the other hand, convergence studies using the algorithm recently developed by Pavlis (1982) suggest that this problem is somewhat better behaved than one might think. Convergence to a solution linearly close to the truth is fairly readily achieved whenever the resolution of the data is sufficient to resolve the structure of the true velocity model. Convergence to solutions that are not linearly close where found to occur, however, when the true velocity model contained significant unresolvable structure.
Petrographic and physical factors controlling thermal conductivity of granitic rocks in Illinois deep holes UPH 1, 2, AND 3
Authors: Eckstein Y. , Dahl P. S. , Vitaliano C. J.
Published: February 2008

Twenty‐four granitic core samples from the Illinois deep drill holes yield thermal conductivity values in the range 29.3–37.6 mW/cm °C, wet bulk densities in the range 2.608–2.960 g/cm3, and water accessible porosity in the range 0.12–0.78%. Thermal conductivity values vary with modal quartz content of the rocks at all depths, and both parameters decrease linearly with depth between 673 and 1016 m in drill hole UPH 3. Below 1070 m there is no systematic variation in modal quartz content with depth and hence no systematic variation in thermal conductivity. Similarly, no systematic variations in modal quartz or physical properties with depth are observed in drill hole UPH 2.
The shear‐wave velocity gradient at the base of the mantle
Authors: Lay Thorne , Helmberger Donald V.
Published: February 2008

The relative amplitudes and travel times of ScS and S phases are utilized to place constraints on the shear‐wave velocity gradient above the core‐mantle boundary. A previously reported long‐period ScSH/SH amplitude ratio minimum in the distance range 65° to 70° is shown to be a localized feature, apparently produced by an amplitude anomaly in the direct S phase, and therefore need not reflect the velocity gradient at the base of the mantle. The amplitude ratios that are free of this anomaly are consistent with calculations for the JB model or models with mild positive or negative velocity gradients in the lowermost 200 km of the mantle. ScSV arrivals are particularly sensitive to the shear velocity structure just above the core‐mantle boundary. The apparent arrival time of the peak of ScSV is as much as 4 s greater than that of ScSH in the distance range 75° to 80° for Sea of Okhotsk events recorded in North America. This can be explained by interference effects produced by a localized high velocity layer or strong positive S wave velocity gradient in the lowermost 20 km of the mantle. A velocity increase of about 5% is required to explain the observed shift between ScSV and ScSH. This thin, high velocity layer varies laterally, as it is not observed in similar data from Argentine events. Refined estimates of the outermost core P velocity structure are obtained by modeling SKS signals in the distance range 75° to 85°.
Critical evaluation of segregation vesicles in mid‐ocean ridge basalts as a rock orientation tool
Authors: Van Wagoner Nancy A.
Published: February 2008

Segregation vesicles in seafloor basalts generally are sperical in shape, 0.1–1.3 mm in diameter, and partly filled with a dark, cup‐shaped meniscus of chilled residual melt. This residue presumably enters the vesicles sometime after crystallization is sufficient to maintain the shape of the vesicle but before final crystallization and flows to the bottom of the vesicle before solidifying. The orientation of the meniscus can therefore be used as an indicator of the cooling orientation of the rock. Results of statistical tests on 17 seafloor basalt samples previously oriented by megascopic geopetal features indicate that the meniscuses in segregation vesicles of a given rock sample are preferentially oriented about the earth's gravity field, as indicated by the megascopic features. In all cases there is a consistency of segregation vesicle and megascopic orientations. As well, the distributions of magnetic inclinations of rocks oriented by megascopic features and segregation vesicles are similar. The segregation vesicle technique can be used to orient dredge and submersible samples for paleomagnetic polarity studies and to determine the dip and predeformational magnetic inclination of rocks recovered in drill cores, which have a known, in‐place vertical orientation, if care is taken to select samples properly. Analyses of the scatter of segregation vesicle inclinations of 34 rock samples indicate that the error associated with dip and magnetic inclination determinations generally are less than ±36° and ±44°, respectively. Of 158 submersible and dredge samples examined, 22% have segregation vesicles which can be used for rock orientation purposes and 30% of the rocks examined from DSDP cores have such vesicles. Routine application of the technique to such samples has the potential of significantly increasing knowledge of the structure and paleomagnetism of oceanic crust. A simple method for application of this technique is presented.