bluuebunny/crossref_metadata_2025 huggingface dataset:
Frequency dependent ultrasonic properties of high‐porosity sandstones
Authors: Winkler Kenneth W.
Published: February 2008

Compressional wave phase velocity and attenuation have been measured in several high‐porosity (20–26%) sandstones as a continuous function of frequency from 400 to ∼2000 kHz. Samples include Massilon, Berea, and Boise sandstones and a synthetic fused glass bead sample. Both dry and brine saturated samples have been studied at effective pressures up to 40 MPa. Dry samples all show negative velocity dispersion (velocity decreasing with increasing frequency) and attenuation (dB/m) increasing as the third to fourth power of frequency. These data are interpreted as evidence of scattering within the samples. Brine saturated rocks show positive velocity dispersion and attenuation increasing with a first‐ to second‐power frequency dependence. These data are interpreted as evidence for a local fluid‐flow loss mechanism. Fused glass beads show no indication of flow related losses.
Bulk thermoelastic attenuation of composite materials
Authors: Budiansky Bernard , Sumner Eric E. , O'Connell Richard J.
Published: February 2008

A theoretical derivation is made for the effective, frequency‐dependent thermoelastic bulk modulus
of an isotropic composite subjected to hydrostatic stress
producing volumetric strain
. No heat transfer is permitted to occur at the outer boundary of the composite, but as a consequence of differential heating, heat transfer among the constituents, and hence dissipation, occurs in the interior. Thus between the limits ω = 0 and ω = ∞K*(ω) is complex and the thermoelastic damping may be measured by QK−1 = Im (K*)/Re(K*). Parametric studies show the influence of various elastic and thermal properties on thermoelastic damping. Numerical calculations are made for a hypothetical lower mantle assemblage of stishovite and magnesiowüstite. The results show that thermoelastic dissipation in the lower mantle can account for the observed attenuation of the fundamental radial normal mode and can also provide interesting constraints on grain sizes.
The production curve for agglutinates in planetary regoliths
Authors: McKay David S. , Basu Abhijit
Published: February 2008

Models for the production of agglutinates are developed that can be applied to the lunar surface or to any planetary or asteroidal body lacking an atmosphere. Models are developed using rate equations for progressively more complex situations and range from Model 1, which is a simple linear increase of agglutinate content with time, to Model 4, which includes provision for recycling of existing agglutinates and replenishment and burial of exposed soil. Model 4 has some aspects of a steady state because, depending on the rate constants, agglutinate content may be limited to an intermediate value, even for long exposure times. In an extreme case, agglutinate content may be limited to a value near zero. These models predict that agglutinates should be low in abundance in areas of thin regolith, such as the Lunokhod‐2 site on the moon, and on asteroids. The models may also help explain the apparent low agglutinate abundances of lunar regolith breccias and meteorite regolith breccias.
Mineral chemistry of the shergottites, nakhlites, chassigny, brachina, pallasites and urelites
Authors: Smith Joseph V. , Steele Ian M. , Leitch Catherine A.
Published: February 2008

The mineral chemistry is compared for selected achondrites. Olivine in the ALHA 77005 and EETA79001 shergottites, olivine‐rich Chassigny and Brachina, and the nakhlites contains Ni indicative of oxidizing conditions, whereas pallasitic and ureilitic olivines contain much lower Ni due to reducing conditions. The Brachina olivine and pyroxene have distinctively higher Fe/Mn than the shergottites and Chassigny further indicating that Brachina is unique. The Chassigny and 77005 olivines contain lower Cr2O3 (0.03 wt%) than the Brachina and 79001 olivines. Values of Fe/Mn for cumulus augites in nakhlites are higher than for the shergottites, whereas those for ferropigeonites are not. The 77005 shergottite contains troilite FeS in contrast to 79001, Shergotty, Zagami, and Chassigny, which contain pyrrhotite. Further analyses are needed, but the present survey indicates that at least Brachina is not chemically cogenetic with the other ‘oxidized achondrites’ and that the Fe/Mn ratio of the cumulus augites in nakhlites is a problem for the assignment of the nakhlites, shergottites, and Chassigny to a single genetic group.
Impact melt‐rock clasts in the Hvittis Enstatite chondrite breccia: Implications for a genetic relationship between El chondrites and aubrites
Authors: Rubin Alan E.
Published: February 2008

The Hvittis enstatite chondrite breccia contains 5±2 vol% dark clasts which appear to be impact melt‐rocks. The largest clast (Clast A) is 2.5 cm in diameter; it has an igneous texture, no chondrules, and is depleted in metallic Fe, Ni and sulfide. Its fine grain size, low‐Ni kamacite and Schreibersite, and rare martensite resulted from rapid cooling. The abundance of melt‐rock clasts in Hvittis suggests that the breccia resided at its parent body surface. However, the apparent absence of EL chondrites with solar wind‐implanted rare gases suggests that the EL parent body was too small to develop a substantial regolith. The bulk composition of Clast A is very similar to that of average aubrites, except for a sevenfold enrichment in S, Cr, and Mn. These elements are concentrated in the sulfides. It thus is possible that the clast was derived from one of the sulfide‐rich regions of the Hvittis matrix. If so, then aubrites may be genetically related to EL chondrites.
26Al‐production rates and 53Mn/26Al production rate ratios in nonantarctic chondrites and their application to bombardment histories
Authors: Herpers U. , Englert P.
Published: February 2008

The long‐lived spallogenic radionuclides 26Al and 53Mn were systematically studied in a large number of nonantarctic meteorites by nondestructive γ‐γ‐coincidence technique and neutron activation, respectively. From the 26Al‐activities normalized to the main target element, silicon, an average production rate of 298±55 (dpm/kg Siequ) was derived. Based on 15 chondrites with exposure ages ≥12×106a and depth profiles of Dhurmsala and Keyes, an average production rate ratio [53Mn/26Al] prod × 1.48±0.15 [dpm/kg Fe / dpm/kg Siequ] was calculated, which seems to be depth‐independent for meteorites with preatmospheric radii R<35 cm. 53Mn/26Al‐radiation ages for 29 stones with short exposure ages were determined. A comparison of our results with the respective 53Mn‐ and 21Ne‐exposure ages generally shows a good agreement. The cosmic ray bombardment age scale covered by this method is 1×106<Trad<10×106a.
Rock 67015: A feldspathic fragmental breccia with KREEP‐rich melt clasts
Authors: Marvin Ursula B. , Lindstrom Marilyn M.
Published: February 2008

Breccia 67015, one of the feldspathic fragmental breccias (FFBs) from the rim of North Ray Crater, contains a wider range of clast lithologies than has been described previously in FFB's. Our petrologic and INAA studies have revealed two components of major petrologic and geologic significance. One of these is a dark, vesicular, KREEP‐rich melt rock of VHA (very high alumina) basalt composition, which occurs in clasts up to 6 cm long. Melt rocks of this type are common at the Apollo 16 site, but in previous years had not been discovered as prominent constituents of the feldspathic fragmental breccias. Their apparent absence led to hypotheses, no longer valid, that the KREEPy melt rocks were younger than the anorthositic breccia matrices and that the two lithologies might represent Imbrian and Nectaris ejecta, respectively. The second lithology we discovered is a large clast of medium‐grained ferroan anorthositic norite with a composition that falls among the most magnesian members of the ferroan anorthosite suite of pristine cumulate rocks. This material constitutes a previously ‘missing’ member of certain mixing models designed to account for the North Ray Crater melt rocks and bulk breccias.
The thermoluminescence carrier in the Dhajala chondrite
Authors: Sparks M. H. , McKimmey P. M. , Sears D. W. G.
Published: February 2008

The thermoluminescence (TL) sensitivities of 58 chondrules separated from the Dhajala (H3.8) chondrite have been measured in order to investigate the cause of the 105‐fold variation in the TL sensitivity of ordinary chondrites. The TL sensitivities of the individual chondrites range over nearly two orders of magnitude, from 0.002 to 0.12 (where 4 mg of bulk Dhajala powder = 1), with no correlation with chondrule mass, cross‐sectional area or diameter. Some chondrules have TL per unit mass ten times that of bulk Dhajala powder and mass‐TL balance arguments suggest that these chondrules are a major TL carrier. The composition of 15 chondrules was also determined and it was found that high TL chondrules tend to have Ca contents at the upper end of the range observed. These observations are consistent with the TL sensitivity variation in ordinary chondrites, which is related to metamorphism, being caused by the devitrification of glass to produce feldspar and with feldspar being the dominant TL phosphor in Dhajala. That chondrules are an important TL carrier would also be consistent with primary feldspar as the TL phosphor, but petrologic observations probably make this appear unlikely.
The characterization of viscous remanent magnetization in large and small magnetite particles
Authors: Tivey Maurice , Johnson H. Paul
Published: February 2008

Previous work on the properties of viscous remanent magnetization (VRM) have shown that it can play an important role in some paleomagnetic investigations. The results of this study on the properties of VRM of very fine, single‐domain (SD) and coarse, multidomain (MD) magnetite particles ranging in size from 0.02 to 60 μm indicate that the MD samples are very sensitive to the previous magnetic history of the samples. The af demagnetization causes a dramatic decrease in the ability of MD samples to acquire VRM compared to the same sample with an undemagnetized thermal remanent magnetization (TRM). Thermal demagnetization of the same MD samples causes an increase in the ability to acquire VRM components. Submicron SD samples show VRM acquisition and decay curves that are independent of previous magnetic treatment. Both MD and SD samples are extremely sensitive to the length of time of zero and weak field storage that occurs prior to the application of the external VRM field. A model using the total energy of the system is used to describe the above results.
A method of determining demagnetizing factor from multidomain hysteresis
Authors: Dunlop David J.
Published: February 2008

The average internal magnetic field Hi inside a multidomain (MD) grain is the sum of the applied field Ho and a demagnetizing field involving the demagnetizing factor N of the grain. It is shown that NMD governing MD wall displacements is not the same as NSD for single‐domain (SD) rotations of the same grain when saturated, as determined by ferromagnetic resonance, for example. NMD can be determined from the measured hysteresis parameters XO (initial susceptibility), Hc (coercive force), Hcr (remanent coercive force), Jrs (saturation remanence), and JD (magnetization when HO = Hcr) by one of three formulas: (1) N = XO−1 − Xi−1, (2) N = (Hc/Jrs) − Xi−1 and (3) N = JD−1(Hcr − HC) − Xi−1. These formulas, which resemble equations derived recently by Smith and Merrill, reduce to conventional approximations for N as the intrinsic susceptibility Xi = dJ/dHi → ∞. In order to use the exact formulas, Xi must be estimated independently by assuming XiHc ≈ constant. Using hysteresis measurements for 2200‐Å cubes of magnetite, which should theoretically have a two‐domain or three‐domain structure, N is calculated to be about 2, in satisfactory agreement with predicted values of NMD and about one half NSD = 4π/3 for a cube.
Modeling near‐field and teleseismic observations from the Amchitka Test Site
Authors: Burdick L. J. , Wallace Terry , Lay Thorne
Published: February 2008

During the Amchitka testing program, recordings were made of the near‐field ground velocity for events MILROW and CANNIKIN. These records exhibited a P arrival, a feature associated with near‐field pP and a developing Rayleigh wave. In this investigation, simple models were found which match the complete seismograms. They simultaneously predict the amplitude, wave shape, timing, and periodicity of both the body waves and the surface waves. Models for both the Amchitka crustal structure and the source time functions were developed. The crustal model is similar to previous models in its average properties, so travel time constraints are still satisfied. Small modifications have been made to match the waveform data. The events on Amchitka can apparently be represented as isotropic point sources with the types of simple source time histories predicted by classical yield scaling laws. A source model for event LONGSHOT was developed on the basis of the yield scaling laws since no near‐field records are available for it. The source models were tested by comparing the teleseismic body waveforms they predict with observations. Short‐period P waves, long‐period P waves, and long‐period pS waves were considered. The short‐period P waves provided the best constraint. To match them, it was necessary both to develop a model for teleseismic pP since it arrives later than the near‐field model would predict and to estimate the average value of t*. The MILROW and CANNIKIN teleseismic short‐period P waves indicate that the average effective tα* for teleseismic ray paths from Amchitka is 0.9 s. The scatter about this value, however, is very substantial. The accuracy of the scaling laws was tested by comparing synthetics for LONGSHOT to teleseismic observations. The predicted amplitudes and wave shapes fall within the scatter in the observations, though again this scatter is substantial.
Coseismic and postseismic vertical movements associated with the 1940 M7.1 Imperial Valley, California, earthquake
Authors: Reilinger Robert
Published: February 2008

Elevation changes derived from precise leveling surveys conducted by the National Geodetic Survey in 1931, 1941, 1972, and 1974 along two routes crossing the Imperial fault in southern California represent the first documented examples of spatially coherent vertical movements associtated with strike‐slip faulting in the United States. The leveling routes cross the northern section of the fault that broke at the time of the 1940, M7.1 Imperial Valley earthquake. The observed vertical movements, which extend approximately 25 km east and west of the fault, are among the largest observed anywhere in the United States. These movements are consistent with simple elastic models of coseismic and postseismic “secondary” vertical deformation due to right‐lateral, strike‐slip movement along a finite length fault. The spatial and temporal patterns of deformation suggest a simple scenario consisting of large coseismic slip on the southern part of the Imperial fault which transferred stresses to the northern part of the Imperial fault and the Brawley fault (en echelon fault northeast of Imperial fault). These stresses were subsequently released primarily by aseismic creep. The estimates of coseismic faulting determined from the leveling observations are roughly consistent with field observations of horizontal surface offset made after the earthquake as well as with some estimates derived from triangulation measurements. The postseismic fault offsets reported here suggest slip to shallower levels than previous estimates of postseismic slip determined from repeated triangulation measurements. The overall shallow slip (coseismic plus postseismic) required by the leveling observations is larger on the southern part of the Imperial fault than on the northern part of this fault or on the Brawley fault. The 1979, M6.6 Imperial Valley earthquake apparently eliminated some of this difference in overall shallow slip by causing surface offsets on the northern part of the Imperial fault and the southern Brawley fault, even though the epicenter for the 1979 event was located on the southern Imperial fault. Other geophysical observations in this part of the Imperial Valley (fault creep, seismicity, seismic faulting) suggest that stick‐slip on the southern part of the Imperial fault and creep on the northern part of this fault and on the Brawley fault may characterize the long‐term fault behavior in the Imperial Valley.
Compressional and shear wave structure of the East Pacific Rise at 11°20′N: Constraints from three‐component ocean bottom seismometer data
Authors: Bratt Steven R. , Solomon Sean C.
Published: February 2008

We report the analysis of seismograms from two orthogonal refraction lines in the ROSE area of the East Pacific Rise at 11°20′N recorded by a three‐component ocean bottom seismometer (OBS). Converted shear waves are clearly observed on the horizontal seismometers for both lines, including paths crossing the rise axis. Neither a travel time delay nor measurable attenuation is observed for shear or compressional waves which have passed beneath the rise crest at depths down to 4 km. On the basis of these observations we conclude that no large crustal magma chamber is present beneath the rise at this latitude. We invert the travel times of P and converted S waves to infer the crustal structure in the vicinity of the rise, and we use P wave amplitudes to constrain further the range of solutions. The P wave velocity distribution does not differ markedly from that of “normal” young oceanic crust, but the shear velocity distribution results in a Poisson's ratio (0.32) that may be more typical of highly fractured crust in rise‐axis regions. A thin (∼80 m) layer of sediment is inferred to be present beneath the OBS on the basis of the measured particle motions, the observation of an Sp phase, and the reverberatory character of the seismograms. A combined consideration of the results of this experiment and other observations of rise‐crossing crustal phases elsewhere in the ROSE area leads us to conclude that any magma within the crust beneath the rise axis of this region must be confined to narrow dikes or pipes or to small isolated bodies with a vertical thickness of less than about 1 km.
Strain accumulation across the Asal‐Ghoubbet Rift, Djibouti, East Africa
Authors: Ruegg J. C. , Kasser M. , Lépine J. C.
Published: February 2008

The Asal Ghoubbet rift in the Republic of Djibouti is a subaerial continuation of the Gulf of Aden ridge system. Many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system. Geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm/yr perpendicular to the normal faults that bound the rift. Periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm/yr. Although the observed deformations could be contamined by postseismic relaxation after a rifting episode in November 1978, the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley. The deformation is uniformly distributed in time and in space inside the inner floor of the rift. The necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process.
Radiative heat transfer in molten and glassy obsidian
Authors: Gable C. W. , Shankland T. J.
Published: February 2008

We have measured optical transmittance spectra in rhyolitic obsidian samples in the wavelength range λ = 380–5500 nm and at temperatures T from 19°–1145°C, above and below the softening point. From the transmittance, we calculated the absorption coefficient α(λ, T) and the radiative thermal conductivity KR(T). KR ranged from 3×10−3 cal cm−1 s−1 K−1 (1.2 W m−1 K−1) at 700°C to 12×10−3cal cm−1 s−1 K−1 (5 W m−1K−1) at 1145°C. The 700°C value is comparable with lattice thermal conductivity KLof about 4×10−3 cal cm−1 S−1K−1 (1.7 W m −1K −1). Removing scattering effects due to bubbles from the transmittance spectra by lowering the absorption baseline increased KR to 20×10−3 cal cm−1 s−1 K−1(8.4 W m−1 K−1) at 1145°C. Because scattering by bubbles is likely to be small in confined magmas, these numbers are probably minimum values for KR and indicate that in active plutons radiative heat transport could be greater than lattice conductivity by more than a factor of 2 at 1000°C. Thus melting markedly strengthens KR, and radiative heat transport is probably the dominant component of the total conductivity K = KL + KR in silicic magmas. These relatively large values of K can be applied to models of the thermal evolution of magma bodies and to cooling of intrusives.
Shock temperatures in CaO
Authors: Boslough Mark B. , Ahrens Thomas J. , Mitchell Arthur C.
Published: February 2008

Blackbody temperatures of CaO shocked to pressures from 140 to 182 GPa have been measured in the 3750 to 6000 K range using the Lawrence Livermore National Laboratory light gas gun. These shock temperatures, along with Hugoniot data for single crystal and porous CaO and isothermal data, are used to construct equations of state for the high pressure (B2) phase of CaO. The zero‐pressure density of the B2 phase is between 3.8 and 4.0 Mg/m3 and the B1‐B2 transition energy is 2.1 to 2.3 MJ/kg. The density and bulk modulus at pressures from 70 to 135 GPa are similar to seismically determined values for the lower mantle of the earth. Thus the lower mantle could have a substantial inventory of Ca‐bearing minerals, and mixed oxide models, for the composition of the lower mantle will be insensitive to the quantity of CaO assumed.
Introduction to Calderas Special Issue
Authors: Lipman Peter W. , Self Stephen , Heiken Grant
Published: February 2008

Large pyroclastic eruptions and associated caldera collapse structures represent the most catastrophic geologic events that have occurred on the earth's surface in Phanerozoic time, other than large impacts of asteroids or meteorites. Calderas are large collapse depressions, more or less circular in form, the diameters of which are many times greater than the diameter of any included vents [Williams and McBirney, 1979]. The largest known calderas associated with silicic pyroclastic eruptions that produce ash flow tuffs or ignimbrites are as much as 60 km in diameter; related pyroclastic deposits have magmatic volumes exceeding 103 km3. In addition to potential problems of volcanic hazards, calderas and associated volcanic activity are important controls on many of the earth's ore deposits and geothermal energy resources, as well as providing key insights into the generation and evolution of large‐volume silicic magma bodies.
Some numerical experiments on the withdrawal of magma from crustal reservoirs
Authors: Spera Frank J.
Published: February 2008

Compositional heterogeneity in the form of continuous or discontinuous chemical and thermal gradients in lava and/or pyroclastic flows is very common. An understanding of the dynamics of magma withdrawal is essential to palinspastic reconstruction of intensive variable gradients in magma reservoirs. Important parameters governing the extent of subterranean magma mixing triggered by an eruption include the vertical structure of density and viscosity within the chamber, the discharge, the size and shape of the chamber, and whether eruption takes place along a sublinear fissure, a ring fracture, or a central vent. A numerical model has been set up to study isoviscous magma withdrawal from a central vent conduit as a function of the Reynolds number, the reservoir to conduit width ratio, reservoir aspect ratio (width/depth), and differing kinematic boundary conditions. Both open (magma recharge) and closed (caldera collapse) system behavior are considered. Finite difference solutions to the vorticity transport and Poisson equations enable determination of vorticity, stream function, and velocity fields as a function of time. The most petrologically significant output is the stream function (particle trajectories) and evacuation isochron diagrams. An evacuation isochron represents the locus of points within the chamber such that magma parcels along a given isochron arrive at the bottom of the volcanic conduit concurrently. Open systems evolve toward a time invariant state (fully developed flow). Spin‐up times depend on chamber aspect ratio (Br/Dr), reservoir/conduit width ratio (Br/Bc), and Reynolds number (Re). Br, Dr, and Bc represent chamber half‐width, depth, and conduit half‐width, respectively. Spin‐up times are relatively small (1/10 to 1/5) fractions of typical eruption durations. The shape and orientation of evacuation isochrons (EI) depend on Re (increasing Re decreases withdrawal depth) and geometric factors (increasing Br/Bc at constant Re and Br/Dr or decreasing Br/Br at constant Br/Bc and Re increases withdrawal depth). A significant amount of roofward magma can remain untapped in a chamber even for long duration eruptions. Systems driven by caldera collapse also involve juxtaposition of roofward and deep‐seated magma during the course of an eruption. Relative to the magma recharge (open system) situation, EI's are laterally elongated. The extent of vertical mixing is thus smaller although still significant in this case. Maximum withdrawal depths vary monotonically in both cases. There is excellent qualitative agreement between predictions based on the numerical experiments and Fe‐Ti oxide temperatures for a thermally zoned ash flow deposit south of Mono Lake in eastern California (Bishop Tuff).
Hydrodynamic aspects of caldera‐forming eruptions: Numerical models
Authors: Wohletz K. H. , McGetchin T. R. , Sandford M. T. , Jones E. M.
Published: February 2008

Comparison of results from a two‐dimensional numerical eruption simulation (KACHINA) to calculations based upon a shock tube analog supports the conclusion that the hydrodynamics during the initial minutes of large caldera‐forming ash flow eruptions may be dominated by blast wave phenomena. Field evidence for this phenomenology is pyroclastic surge deposits commonly occurring both directly below caldera‐related ash flow sheets, on top of a preceding Plinian fall deposit (ground surge), and separating individual ash flow units. We model the eruption of the Tshirege member of the Bandelier Tuff (1.1 Ma B.P.) from the Valles caldera, New Mexico. In the model a magma chamber at 100 MPa (1 kbar) and 800°C is volatile rich, with an average H2O abundance above saturation greater than 8.7 wt % increasing to nearly 100 wt % near the very top of the chamber. Using a shock tube analogy, decompression of the chamber through a wide‐open dikelike vent 0.1 km wide and 1 to 5 km long forms a shock wave of 3 MPa (≃3O atm) with a velocity greater than 1.0 km s−1. Steady flow of material erupted from the vent begins after 20 to 100 s based upon a 7‐km depth from the ground surface to a reflective (density) boundary in the chamber and a rarefaction wave velocity of 100 to 600 m s−1. The velocity of the ash front behind the shock wave is 300 to 500 m s−1. The shock tube model serves as a basis to evaluate the consistency of the KACHINA code results which are similar to a one‐dimensional problem along the symmetry axis. The results of the KACHINA simulation show in some detail the effect of multiple reservoir rarefaction reflections and possibly Prandtl‐Meyer expansion in generating compressive wave fronts following the initial shock. The rarefaction resonance not only prolongs unsteady flow in the vent but tends to promote surging flow of ash behind the leading shock. Furthermore, these results are consistent with a blast wave characterized as a shock front followed by one or more pulses of entrained ash. The blast wave shocks ambient air to higher pressures and temperatures, the magnitudes of which depend strongly on the initial chamber overpressure, distance, and direction from the vent. In consideration of volcanic hazards our numerical model shows that a shock wave compressed the atmosphere to pressures of ≃0.2 to 0.7 MPa (2–7 atm) and temperatures of ≃200° to 300°C for distances to 10 km from the Bandelier vent(s).
The Long Valley/Mono Basin Volcanic Complex: A preliminary magnetotelluric and magnetic variation interpretation
Authors: Hermance John F. , Slocum Warren M. , Neumann Gregory A.
Published: February 2008

The Long Valley/Mono Basin volcanic complex in eastern California is one of the few major silicic caldera systems in western North America which have exhibited volcanic activity so recently that they may still be potentially active. Whereas, in principle, magnetotelluric measurements offer a significant opportunity to study such systems through mapping subsurface electrical features associated with hydrothermal and magmatic processes in the crust, our studies in this area indicate that such techniques are profoundly affected by the highly three‐dimensional structures associated with these complicated volcanic terranes. We argue that while simple plane‐layered (one‐dimensional) interpretations are likely to be misleading in such a complicated three‐dimensional environment, magnetotelluric observations can delineate important physical features associated with caldera structures; in particular, the method can closely determine the location and magnitude of major boundary faults. Because of the high resistivity contrast between basin fill and crystalline basement, these techniques are very useful for characterizing the subsurface hydrologie regime as well. In addition, sites have been occupied in the southwest moat of Long Valley caldera, an area that is currently exhibiting a variety of tectonic activity. Both the telluric field and magnetic induction arrows imply the presence of a structurally controlled east‐west electric current system at relatively shallow depth in the crust. This elongated east‐west zone is aligned along the belt of recent seismic activity in the southwest moat, a zone of seismic shear wave attenuation, and a zone of known hydrothermal alteration at the surface. Finally, we summarize observations from the vicinity of Pumice Valley (adjacent to Mono Craters) which has been thought to be an active incipient caldera. Since our present data do not indicate an observable decrease in resistivity at shallow depths beneath the valley, we are led to conclude that the parent magma body feeding Mono Craters (and perhaps Inyo domes) is either too thin or too deep to be resolved or is significantly displaced from a position beneath the center of the inferred ring fractures.
Caldera development during the Minoan eruption, Thira, Cyclades, Greece
Authors: Heiken Grant , McCoy Floyd
Published: February 2008

The well‐known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption of 1400 B.C., Thira consisted of three Java shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half. Andesitic lavas formed the overlapping shields of the north and were contemporaneous with and, in many places, interbedded with the southern tuff deposits. Although there appears to be little difference between the composition of magmas erupted, differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial, producing lava flows, whereas in the south, eruptions within a flooded depression produced a sequence of mostly phreatomagmatic tuffs. Many of these tuffs are plastered onto the walls of what appears to have been an older caldera, most probably associated with an eruption of rhyodacitic tephra 100,000 years ago. The Minoan eruption of about 1400 B.C. had four distinct phases, each reflecting a different vent geometry and eruption mechanism. The Minoan activity was preceded by minor eruptions of fine ash. (1) The eruption began with a Plinian phase, from subaerial vent(s) located on the easternmost of the lava shields. (2) Vent(s) grew toward the SW into the flooded depression. Subsequent activity deposited large‐scale base surge deposits during vent widening by phreatomagmatic activity. (3) The third eruptive phase was also phreatomagmatic and produced 60% of the volume of the Minoan Tuff. This activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding. This deposit contains 5–40% lithic fragments that are typical of the westernmost lava shield and appears to have been erupted when caldera collapse began. (4) The last phase consisted of eruption of ignimbrites from vent(s) on the eastern shield, not yet involved in collapse. Collapse continued after eruption of the ignimbrites with foundering of the eastern half of the caldera. Total volume of the collapse was about 19 km3, overlapping the older caldera to form the caldera complex visible today. Intracaldera eruptions have formed the Kameni Islands along linear vents concomitant with vents that may have been sources for the Minoan Tuff.
A dislocation model of the stress history dependence of frictional slip
Authors: Olsson William A.
Published: February 2008

Frictional slippage on material interfaces is pervasive in mechanical problems of all types, but it can be particularly important in geomechanics. Extant models of cracked or jointed rock usually take frictional resistance along rock interfaces to be described by uniform Coulomb friction. A simple theory is presented which incorporates nonuniform friction. The description is by means of continuous distributions of infinitesimal dislocations. The resulting stress‐plastic strain behavior is nonlinear and stress history dependent and is a result of the generation, interaction, and annihilation of slip zones of unlike sign. The theory provides an explanation for the discrete memory effect observed during cyclic stressing of rock and also indicates potential difficulties in the design and interpretation of experiments on jointed rock.
A model for the tectonic development of the Southeastern Colorado Plateau Boundary
Authors: Aldrich M. J. , Laughlin A. W.
Published: February 2008

Stress data show that the contemporary tectonic boundary of the southeastern Colorado Plateau is coincident with the segment of the Jemez lineament between the White Mountains in east‐central Arizona and the Jemez Mountains in north‐central New Mexico. The lineament is actually a broad ( ∼50 km wide) technically active zone that trends N52° and approximately coincides with a Precambrian province boundary. It is characterized by extensive late Tertiary‐Quaternary volcanic fields, NNE trending (∼N25°E) en echelon faults, and both normal and strike‐slip faulting. Pliocene‐Quaternary normal oblique‐slip displacements have been observed on faults within and south of the lineament but not north of it. Late Laramide NE‐SW compression, which caused widespread tectonic shortening throughout the western Cordillera, apparently was transformed into left slip along the Jemez lineament. As the Precambrian basement underwent strike‐slip faulting, a belt of NNE trending en echelon faults developed in the overlying Phanerozoic sedimentary strata. By the end of the Laramide orogeny 40 Ma ago the principal structural elements of the lineament, seen today, had been formed. Volcanism in the Datil‐Mogollon volcanic field began after the cessation of Laramide tectonism and heated the crust around the southeastern edge of the Colorado Plateau. These elevated temperatures further weakened the part of the physiographic plateau immediately south of the Jemez lineament so that with the onset of SW‐NE extension in the southern Basin and Range Province, about 30 Ma ago, this area deformed more in response to the extension than to the stess field of the plateau interior. The emplacement of large NW trending dikes into the area between 27 and 29 Ma ago was directly related to this extensional event. During the late Miocene the direction of spreading changed to a W and WNW orientation allowing the Colorado Plateau to begin a small clockwise rotation. Coeval left slip and extension occurred across the Jemez lineament. Major volcanism on the lineament was initiated by the change in spreading direction. Volcanic activity started at the weakest points first, where the lineament is intersected by the Rio Grande Rift (Jemez Mountains) and Capitan lineament (White Mountains). With increasing extension across the lineament between 7 and 4 Ma ago the NNE trending faults opened, and by the earliest Pliocene ( ∼4.5 Ma ago), volcanism was occurring along the entire southeastern tectonic boundary (Jemez lineament) of the Colorado Plateau.
Dispersed fluid flow in fractured reservoirs: An analysis of tracer‐determined residence time distributions
Authors: Robinson Bruce A. , Tester Jefferson W.
Published: February 2008

A methodology for analyzing the internal flow characteristics of a fractured geothermal reservoir using tracer‐determined residence time distribution curves is outlined. Emphasis is placed on comparison of the statistical quantities obtained from the tracer curves of different reservoirs or of the same reservoir under different conditions. In this way, model‐independent information may be used unambiguously to construct empirical reservoir performance correlations. Downhole measurements of the tracer response exiting from discrete fracture zones permit further characterization of reservoir fluid flow behavior. Tracer experiments conducted in prototype hot dry rock geothermal reservoirs in fractured rock are examined using these statistically based data analysis methods.
The Geisha Guyots: Multibeam bathymetry and morphometric interpretation
Authors: Vogt Peter R. , Smoot N. Christian
Published: February 2008

Multibeam bathymetric charts (100 fm = 183 m contour interval) are presented and analyzed for the ∼1100‐km‐long, 120° trending Geisha chain of eight guyots and at least 13 other seamounts (>1 km height) in the northwest Pacific; these guyots are compared with 23 others in the North Pacific. Edifice distribution is nonrandom and nonuniform and is best described as clusters (200 km apart), some of which are composed of subclusters (30–50 km apart) and edifices 10–20 km from their neighbors. Published radiometric dates from the two ends of the Geisha chain (94 Ma for Makarov in the southeast and 102 Ma for Seiko in the northwest) are consistent with rapid (14 cm/yr) plate motion over a fixed hotspot, but other mechanisms cannot be excluded. Crustal age (from magnetic lineations) increases south westward from 141 to 156 Ma along the chain, i.e., from 39 to 62 Ma at times of seamount volcanism. Total edifice volume is small (2×104 km3), with Makarov guyot at 6000 km3 and the other guyots 700–1500 km3 each. Summit plateau depths (averaging 1420±60 m minimum depth and 1600±120 m break depth) are remarkably uniform along the chain; plateau areas (87±68 km2, ranging from 14 to 246 km2) are the smallest for any guyot chain. Original island heights (0.5–1.3 km) and minimum volumes eroded (2–100 km3) are estimated from the height/area relation of modern volcanic islands. Summit plateau relief (∼100–200 m) is explained by simultaneous shoreline erosion (∼1 km/Ma) and subsidence of thermally rejuvenated lithosphere. There is no bathymetric evidence for guyot volcanism or faulting postdating subsidence below wave base. The Geisha guyots rise 4.5–4.9 km above the regional basement, implying a hotspot‐generated swell height of 0.5–1 km and lithosphere thermally reset to about 45% of its age at time of volcanism. Average upper guyot slopes (21°±3°) exceed those of other guyots (13°+3°) but resemble small seamount slopes, suggesting that flank slopes decrease, above about 5000 km 3 edifice volume, due to higher effusive rates and larger flows. The Geisha guyots exhibit prominent but short radial protuberances interpreted as flank rift zones (FRZ). A relation between base‐edifice differential magma pressure and FRZ length suggests three regimes: FRZ's are absent on seamounts <2 km in height, increase slowly (0.2 km/MPa) for large seamounts and small guyots like the Geishas, and above 100 MPa increase rapidly (3 km/MPa) with large scatter. FRZ's exceeding 100–150 km length on large edifices suggest hot, low‐viscosity rift conduits along which magma can flow under low‐pressure gradients.
On the brittle‐ductile behavior of iron meteorites: New experimental constraints
Authors: Matsui Takafumi , Schultz Peter H.
Published: February 2008

High‐velocity (∼2 to ∼5 km/sec) impacts into octahedrite iron meteorites (Gibeon, El Sampal, and Arispe), steel, and iron targets were made at the NASA‐Ames Vertical Gun Facility. The experiments clearly revealed the brittle behavior of meteorites at very low temperatures (<200° K) as indicated by spallation of the crater rim and the development of deeply penetrating fractures in the meteorite sample. Although failure also occurred at higher temperatures (290°K), it was restricted to near‐surface spalls. High‐velocity (∼5 km/sec) impacts resulted in a net mass loss regardless of the brittle‐ductile behavior of the target or projectile, whereas low‐velocity (∼2 km/sec) impacts resulted in a net mass gain for ductile projectiles into brittle and ductile targets. Consequently, even if iron meteorites (octahedrites) were originally the core of a layered parent body, they might be subsequently destroyed in a brittle manner. Such results are consistent with the hypothesis by Matsui and Mizutani (1977) that the brittle behavior of iron‐like planetesimals in the low‐temperature asteroid zone prevents growth into full‐size planets.
Lu‐Hf and Sm‐Nd evolution in lunar mare basalts
Authors: Unruh D. M. , Stille P. , Patchett P. J. , Tatsumoto M.
Published: February 2008

Lu‐Hf and Sm‐Nd data for mare basalts combined with Rb‐Sr and total REE data taken from the literature suggest that the mare basalts were derived by small (≤10%) degrees of partial melting of cumulate sources, but that the magma ocean from which these sources formed was light REE and Hf‐enriched. Calculated source compositions range from lherzolite to olivine websterite. Nonmodal melting of small amounts of ilmenite (≤3%) in the sources seems to be required by the Lu/Hf data. A comparison of the Hf and Nd isotopic characteristics between the mare basalts and terrestrial oceanic basalts reveals that the εHf/εNd ratios in low‐Ti mare basalts are much higher than in terrestrial oceanic basalts. The results are qualitatively consistent with the hypothesis that terrestrial basalt sources are partial melt residues whereas mare basalt sources are cumulates. Alternatively, the results may imply that the terrestrial mantle has evolved in two (or more) stages of evolution, and that the net effect was depletion of the mantle during the first ∼1–3 b.y. followed by enrichment during the last 1–2 b.y.; or simply that there is a difference in Lu‐Hf crystal‐liquid partitioning (relative to Sm‐Nd between the lunar and terrestrial mantles.
Relict and other anomalous grains in chondrules: Implications for chondrule formation
Authors: Kracher Alfred , Scott Edward R. D. , Keil Klaus
Published: February 2008

We have identified relict olivine and pyroxene grains in chondrules from ordinary and carbonaceous chondrites that probably did not crystallize in situ. Some of these olivines are clear, but others contain fine‐grained Fe,Ni (‘dusty olivines’) and resemble previously described occurrences in ordinary chondrites. There are also chondrules in which all olivine is dusty. We conclude: (1) not all relict olivines are dusty, (2) not all dusty olivines crystallized outside the chondrule in which they occur,and (3) some dusty olivines were produced during chondrule formation by a reduction process that affected the whole chondrule. The occurrence of dusty olivines and relict pyroxenes and olivines in chondrules from carbonaceous as well as ordinary chondrites supports the argument that chondrules from all chondrites had similar origins and histories. We propose that chondrules and mineral fragments were transported across f(O2) gradients in the solar nebula while they were hot, or were reheated in an environment different from the one in which they formed. Partially molten chondrules sometimes incorporated mineral grains or chondrules with different redox states, producing compound chondrules and chondrules containing anomalous grains. Dusty olivines may also have formed when hot chondrules were transported to regions of lower oxygen fugacity.
Chemical and structural changes induced by thermal annealing of shocked feldspar inclusions in impact melt rocks from Lappajärvi Crater, Finland
Authors: Bischoff A. , Stöffler D.
Published: February 2008

Plagioclase and microcline of clastic mineral and rock inclusions in impact melt rocks (kärnäite) were sampled from the coherent melt sheet located in the central crater area of the Lappajärvi meteorite crater and analyzed by optical, electron optical, X ray, and microprobe methods. Both types of feldspars display strong chemical, textural, and structural alterations compared to the feldspars of their parental crystalline basement rocks as a result of the thermal energy supplied by the surrounding superheated impact melt. Increase of shock experienced by the clasts prior to their incorporation into the melt results in increasing thermal alteration by the melt. In a lower degree of shock both primary plagioclase and microcline transform to ‘checkerboard’‐textured grains that consist of 5–10–μm‐sized subgrains of parallel optical orientation embedded in a mesostasis of eutectically grown sanidine and quartz. The subgrains are strongly zoned, disordered labradorite to andesine and rimmed by sanidine. In a high degree of shock the subgrains are larger and randomly oriented. The subgrains are the result of a nonequilibrium fractional crystallization from a liquid state during which a strong chemical exchange between the melt and the feldspar inclusions took place. On the basis of model calculations it is concluded that the temperature of the kärnäite melt at the time of the incorporation of the clastic debris was in the range of 1800° to 2100°C. The equilibration temperature of the clast‐melt mixture is estimated accordingly to be at least in the range of 1100° to 1140°C or higher, depending on the magnitude of the clast's postshock temperature. This temperature is above the solidus and in part above the liquidus temperature of a dry granodioritic melt and is also above the solidus temperatures of the feldspars. The clast‐laden melt was at rest when the equilibrium temperature was reached, suggesting that the readjustment of the transient crater cavity by central uplifting of the crater took place within a time scale of hours.