bluuebunny/crossref_metadata_2025 huggingface dataset:
The Apollo 15 yellow impact glasses: Chemistry, petrology, and exotic origin
Authors: Delano J. W. , Lindsley D. H. , Ma M.‐S. , Schmitt R. A.
Published: February 2008

The Apollo 15 yellow impact glasses are characterized by moderate TiO2 (∼ 4.8%) and high abundances of the large ion lithophile elements (e.g., K, P, Hf, Th, REE). Since the chemistry of these glasses cannot be duplicated by any combination of local components presently known to occur at the Apollo 15 landing site, these yellow glasses seem to be exotic to that area. Chemical and petrologic constraints suggest that these samples were produced by impact melting of an immature mare regolith developed upon an unusual variety of mare basalt. We speculate that the target basalts were the youngest lava flows known to exist on the moon (i.e., Eratosthenian‐age lavas in Oceanus Procellarum and Mare Imbrium). Specific tests are proposed for evaluating this provocative hypothesis.
Chemistry and phase relations of VLT Volcanic glasses from Apollo 14 and Apollo 17
Authors: Chen H.‐K. , Delano J. W. , Lindsley D. H.
Published: February 2008

The VLT (very low titanium) volcanic glasses from Apollo 14 and Apollo 17 have been analysed for the major elements and trace Ni. These glasses display compositional trends that may not result entirely from olivine fractionation. The phase relations have been determined experimentally. Olivine is the liquidus phase in the pressure interval from 0 to ∼18 kbar. Pigeonite is the liquidus phase from ∼18 kbar to ≥ 22 kbar. If these VLT volcanic glasses are samples of primary melts that had been in chemical equilibrium with olivine and low‐Ca pyroxene in their source regions, then these liquids were derived from depths of 360–380 km within the moon.
Deuterium enrichments in type 3 ordinary chondrites
Authors: McNaughton N. J. , Fallick A. E. , Pillinger C. T.
Published: February 2008

The majority of sixteen type 3 ordinary chondrites studied contain D/H ratios above the typical terrestrial range. We observe the most extreme deuterium enrichments in the least metamorphosed samples, and a trend of decreasing D/H with increasing metamorphism as measured by thermoluminescence sensitivity. Stepwise pyrolysis of bulk meteorite samples demonstrates that the amount of water released decreases rapidly with increasing temperature, and that the 450°C–750°C heating step releases the most deuterium enriched water. More detailed stepwise pyrolysis of one of the least metamorphosed ordinary chondrites, Semarkona, produces water from the 550°C–650°C heating step which has a δD of +5740°/‰ (SMOW). Although this is the most deuterium‐rich composition yet encountered in a solar system material, it may not represent a limiting value. It is suggested that a deuterium enrichment of this magnitude may only be produced by ion‐molecule reactions in interstellar clouds.
Refractory residues, condensates and chondrules from solar furnace experiments
Authors: King Elbert A.
Published: February 2008

Vertical access solar furnace experiments have produced refractory residues, condensates and chondrules that are similar to components of chondritic meteorites. In particular, Ca‐Al‐rich refractory residues similar in chemistry to inclusions in carbonaceous chondrites have been produced by partial evaporation of basaltic bulk rock samples. Fe‐Mg‐Si‐rich condensates with distinctive microbotryoidal morphology have been collected from the same sample runs. Particle coatings and aggregates with virtually identical microbotryoidal morphology and major element chemistry have been identified in both the Allende and Murchison meteorites. Spattered drops from melt beads undergoing heating and partial evaporation resemble some meteoritic chondrules in their mineralogies, textures, grain size, and sorting. The spatter mechanism is highly efficient in the production of chondrules. If any of the refractory inclusions in chondrites are, in fact, partial evaporation residues, many meteoritic fluid drop chondrules must have been formed by this process. The hot central portion of the solar nebula, acting on a cloud of dust and gas, is the probable source of heat required to produce the fractionated chemistry and physical state of many of the components of chondritic meteorites.
Microcrack connectivity in rocks: A renormalization group approach to the critical phenomena of conduction and failure in crystalline rocks
Authors: Madden Theodore R.
Published: February 2008

A study is made of the critical phenomena associated with the onset of conductivity and the onset of failure in a rock with a random distribution of microcracks using a renormalization group theory approach. The interaction between different length scales is an important element in the theory, which is a departure from the results of equivalent media theory. The crack parameter that determines if the rock can conduct or is to fail is the crack count per unit area times the square of the crack length. The parameter space is divided up into regions or phases. These phases are those of no conduction and no failure, conduction and no failure, and conduction and failure. The predictions of the model agree well with SEM data on the microcrack populations of stressed and unstressed rock samples. The crack densities seen in unstressed Westerly granite fall in the range that predicts conduction but no failure, but only a 50% increase in density is needed to reach the failure criteria. A sample stressed to 65% of the failure stress has greatly increased conduction connectivity but seems no closer to failure. The sample stressed to failure shows an appropriate crack density increase.
Anomalous heat flow in the northwest Atlantic: A case for continued hydrothermal circulation in 80‐M.Y. crust
Authors: Embley Robert W. , Hobart Michael A. , Anderson Roger N. , Abbott Dallas
Published: February 2008

A detailed study of a 60×150 km area at 60°W, 24°N at the eastern end of the Nares Abyssal Plain indicates that hydrothermal circulation is still active in the 80 m.y. B.P. oceanic crust. The 58 heat flow measurements made at five stations in the area have revealed (1) constant heat flow over the abyssal plain (56 mW m−2), (2) a cyclic heat flow over the abyssal hills (mean of 77 mW m−2), and (3) a large anomaly of 710 m W m−2 over one of several small domes which protrude from the abyssal plain. The domes are 0.5–1.0 km in diameter near the top and rise 50 m above the level of the abyssal plain. They are recognized from surface echo sounders by an abrupt disappearance in the abyssal plain subbottom reflectors, but on near‐bottom pinger records they appear as steep‐walled structures which are covered by ∼10 m of sediment (compared to ∼75 m on the surrounding abyssal hills). From analogy with active ridge crests, these features are probably small volcanoes. The heat flow anomaly over one of the domes is matched well by a finite element convection model with the following characteristics: (1) recharge at one basement outcrop and discharge at another, (2) 300 m of sediment fill between outcrops, and (3) permeabilities of 10−10 cm2 for basalt and 10−13 cm2 for sediment. In other words, we believe that there is very effective convective heat transfer within the crust and out of the relatively permeable, thinly sedimented basement dome, resulting in the local high heat flow. Overall, the results from the Nares survey vividly show the age independent muting effect of sediment on the surface manifestation of crustal convection. In our survey area the mode of heat transfer varies from purely conductive in the more thickly sedimented abyssal plain areas (∼300 m sediment cover) to moderate amplitude convection pattern beneath the abyssal hills (∼75 m sediment cover) to a very large thermal anomaly over the small dome or ‘chimneylike’ structure (∼10 m sediment cover). The domes are possibly active analogues to the presently inactive basement chimney drilled at DSDP site 417A.
On the terrestrial heat flow and physical limnology of Crater Lake, Oregon
Authors: Williams David L. , Von Herzen Richard P.
Published: February 2008

The heat flow into 7000‐year‐old Crater Lake caldera is dominated by hydrothermal processes. The average of the conducted heat flow value is 138±121 mW/m2 (3.3±2.9 HFU). The total heat flow is estimated to be between 669 and 1381 mW/m2 (16 and 33 HFU). Hydrothermal fluids penetrate into the lake floor to a depth of at least 1.5 to 2.0 km and possibly much deeper. Two thermal spring areas were discovered on the deep lake floor. These springs, and probably other as yet undiscovered ones, have a dramatic effect on the water temperature, circulation, and stratification of the lake. Dense thermal waters pond in the southwest basin. The deep waters of the east and northwest basin are warmed to the point that they convect, causing the lake to be vertically mixed. A hyperadiabatic temperature gradient results from this convection and possibly the stagnating effects of small quantities of suspended solids. Thermal fluids carrying dissolved solids cause anomalously high concentrations of chloride and sulfate in the relatively pure lake water.
Source times and scaling relations of large earthquakes
Authors: Furumoto Muneyoshi , Nakanishi Ichiro
Published: February 2008

Source time is a kinematic fault parameter corresponding to the duration of seismic source time functions and is accurately determined from phase spectra of long‐period surface waves. Source times are determined for 36 large earthquakes during the last three decades. Scaling relations among source times, seismic moments, and fault dimensions are derived. Seismic moment is proportional to the cube of source time, and fault dimension is proportional to source time. Source times for low‐angle thrust earthquakes along deep‐sea trenches are found to be longer than those of other types, such as intraplate shocks and deep shocks. They are also significantly longer than the rupture times expected from a Haskell model, suggesting that, generally, there exists an introductory stage of faulting that precedes the main stage of the rupture propagation.
Viscoelastic stress relaxation on deep fault sections as a possible source of very long period elastic waves
Authors: Bonafede M. , Boschi E. , Dragoni M.
Published: February 2008

Observations of very long period elastic waves are attributed to ‘silent’ earthquakes, events which are presumed to occur at depth in a nonbrittle regime. The fault at depth is modeled as a thin viscoelastic gouge layer embedded in an elastic medium. The source of silent earthquakes is envisaged as a prolonged, slow slippage of the fault faces, which yields a radiation spectrum strongly depleted in the high‐frequency components and dominated by the near‐field term. The results of the model are compared with the dominant frequencies, mean duration, and amplitude of a long‐period perturbation recorded at Trieste in the years preceding the 1976 Friuli (Italy) earthquake. The role that silent earthquakes may play in the overall earthquake mechanism (in particular, as precursory phenomena) is discussed.
Deformation of single‐crystal clinopyroxenes: 2. Dislocation‐controlled flow processes in Hedenbergite
Authors: Kollé J. J. , Blacic J. D.
Published: February 2008

Laboratory deformation experiments were carried out on two single‐crystal clinopyroxenes: chrome diopside and hedenbergite. The tests were made in a Griggs solid‐medium, triaxial, not creep tester. A confining pressure of 1000 MPa was applied in all the experiments. The crystals were deformed at strain rates from 10−4 s−1 to 10−8 s−1 and at temperatures from 400°C to 1200°C. Two orientations of the crystals with respect to the maximum principal compressive stress were tested. The first orientation subjects the (100), [001] and (001), [100] mechanical twinning systems in clinopyroxene to a high, resolved shear stress appropriate for mechanical twinning to occur. The second orientation subjects these systems to an equal resolved shear stress, but in a way that mechanical twinning is not possible. The effects of temperature and strain rate on the flow stress were observed for the two clinopyroxene compositions. Mechanical twinning on the system (100), [001] was observed to be the primary deformation mechanism in crystals oriented favorably for twinning. Above 850°C, deformation occurs by dislocation glide and is strongly temperature and strain‐rate dependent. In the orientation for which mechanical twinning is not possible, crystals deformed by two different dislocation glide mechanisms, depending on temperature. From 400°C to 900°C, deformation in hedenbergite occurs by dislocation glide controlled by lattice resistance. This thermally activated mechanism has an activation energy of 285 kJ/mol (68 kcal/mol) and an activation area that expresses the effect of the applied stress on the total activation energy. This mechanism only operates at stresses above 520 MPa. At temperatures above 900°C and applied stresses less than 520 MPa, flow in hedenbergite occurs by climb‐controlled dislocation glide. This leads to a thermally activated power law flow equation. The activation energy is 523kJ/mol (125 kcal/mol). Strain rate depends on the stress raised to the power 3.6. The flow law derived for hedenbergite at high temperatures and low stresses may be extrapolated to a strain rate of 10−14 s−1, thought to be appropriate for flow in the mantle. This flow law predicts an equivalent viscosity of hedenbergite of 1021 poise at 800°C and 1019 poise at 1200°C.
Direct inversion of one‐dimensional magnetotelluric data
Authors: Coen Shimon , Quercia Franchesca , Mackiewicz Maria
Published: February 2008

It is shown that the conductivity profile of a layered earth can be obtained directly (noniteratively) from one‐dimensional magnetotelluric observations. The approach we use is based on the Born approximation to the electric field integral equation. We apply the inversion algorithm to two data sets. The first one is analytic, and we are able to show analytically all the steps in the inversion algorithm. The resulting conductivity profile is an adequate approximation to the actual profile. The second data set contains the apparent resistivities from a particular field site at 21 frequencies, and the resulting conductivity profile compares adequately with well log measurements. The results that we obtained are promising enough to consider the direct inversion of two‐dimensional data.
Interpretation of whole‐disk photometry of Phobos and Deimos
Authors: Pang Kevin D. , Rhoads Jack W. , Hanover Gene A. , Lumme Kari , Bowell Edward
Published: February 2008

The phase curves (whole‐disk brightness vs. solar phase angle) of the Martian satellites were compiled out to 125° phase angle from ground‐based and spacecraft photometric observations. The data were modeled with the Lumme‐Bowell theory. Accurate phase integrals, zero‐phase geometric albedos and Bond albedos were obtained from the best fit theoretical curves. The microstructural and particulate surface properties of Phobos differ little from those of Deimos. The satellites' very similar whole‐body densities and geochemical and microphysical makeup suggest a common origin. However, having spent a considerable portion of their lifetimes in proximity to Mars probably did modify their surface characteristics. The value of the surface roughness (less than that on an asteroid of comparable size), the debris mantle on Deimos and the grooves on Phobos are probably evidence of such modifications. Consequently, Phobos and Deimos should be used as models of asteroid‐class objects only with reservation.
Total‐moment spectra of fourteen large earthquakes
Authors: Silver Paul G. , Jordan Thomas H.
Published: February 2008

Total‐moment spectra MT(ω) = ‖M(ω)‖/
, where M is the moment rate tensor, are computed for 14 large earthquakes recorded by the International Deployment of Accelerometers (IDA) network using the scalar‐moment retrieval method proposed by Silver and Jordan [1982]. For each event we obtain estimates of MT averaged over the 10 disjunct, 1‐mHz intervals in the low frequency band 1–11 mHz; typical IDA record sets from events with MT ≳ 0.2 A (1 A = 1027 dyne · cm) yield standard errors on the 1‐mHz averages that are generally less than 20%. Our multiple‐band estimates of MT are usually consistent with comparable single‐band values found by other investigators. From the total‐moment spectra we derive the zero‐frequency (static) moment
≡ MT(0) and the characteristic source duration τc ≡ 2 (Var [f(t)])½, where f(t) is the time function of the moment rate tensor, assumed to be the same for all components. The parameter τC formally depends on the second‐degree temporal, spatial, and mixed spatial‐temporal moments of an extended source, but calculations with realistic source geometries indicate that the interpretation of TC strictly in terms of the second temporal moment leads to very little error. A plot of τc against
shows considerable scatter; some events lie significantly above the empirical scaling curve of Kanamori and Given [1981] (‘slow earthquakes’) and some below (‘fast earthquakes’). Examples of the former include all three deep‐focus events analyzed here as well as the 1970 Colombia and 1963 Peru‐Bolivia deep‐focus earthquakes, whose total‐moment spectra are calculated from the moment tensor solutions of Gilbert and Dziewonski [1975]. An example of the latter is the great Sumbawa earthquake of August 9, 1977 (
= 24±3 A), which is inferred to have a relatively shallow spatial centroid (< 20 km) and a nearly flat moment spectrum, suggesting that the faulting was confined to the upper, more brittle portion of the oceanic lithosphere. Three other large earthquakes—Tonga (June 22, 1977;
= 23±2 A), Kuril Islands (December 6, 1978;
= 3.6±0.2 A), and Tumaco (December 12, 1979;
= 25±2 A)—exhibit moment spectra which significantly decrease toward higher frequencies, indicating larger values of τc. The aftershocks of both the Tonga and Kuril Islands events are distributed over a considerable range of depths, consistent with rupture into the lower, more ductile portions of the oceanic lithosphere. We speculate that the characteristic source duration may be causally related to the depth of coseismic rupturing within the lithosphere.
Deep crustal geothermal measurements, hole 504B, Costa Rica Rift
Authors: Becker Keir , Langseth M. G. , Von Herzen R. P. , Anderson R. N.
Published: February 2008

We report an extensive suite of geothermal measurements in the deepest borehole yet drilled into the oceanic crust, hole 504B of the Deep Sea Drilling Project. Located in 6.2‐m.y.‐old crust of the Costa Rica Rift, hole 504B was cored during legs 69 and 70 in late 1979 and leg 83 in late 1981, to a total depth of 1350 m beneath the seafloor, through 274.5 m of sediment and 1075.5 m of basalt. During the three drilling legs, downhole temperatures were logged 11 times, and the thermal conductivities of 239 sediment and basalt samples were measured. The results indicate a dominantly conductive mode of heat transfer through the complete section, at 190±10 mW/m2. This is consistent with the predicted plate heat transfer and the hypothesis that the thick sediment cover acts as a seal against hydrothermal circulation of seawater to basement. For over 2 years after this sediment seal was penetrated, borehole temperatures were nearly isothermal to about 350–370 m, indicating that ocean bottom water was flowing down the hole into the upper ∼100 m of basement. This downhole flow was driven by the underpressure of the basement pore fluids, which is of indefinite, but possibly hydrothermal, origin (Anderson and Zoback, 1982). The flow rate decreased from 6000–7000 1/h in late 1979 to about 1500 1/h 2 years later; altogether over 50×106 kg of seawater has been drawn into the basement. We estimate a permeability of ≳6×10−14 m2 for the reservoir in the upper ∼100 m of basement. This zone seems to correspond to a layer of high apparent porosity (Becker et al., 1982), which has been tentatively identified as a thin layer 2A (Anderson et al., 1982a).
Present seismic evidence for a boundary layer at the base of the mantle
Authors: Doornbos D. J.
Published: February 2008

To improve the global coverage needed in investigating the observational consequences of boundary layer models, the existing data set has been extended with long‐period diffracted P and SH from the SRO network, and short‐period diffracted P from the NORSAR array. It was found that on average, the amplitude decay of diffracted SH is relatively small, in accordance with the majority of previously published WWSSN data, and consistent with the observed weakness in dispersion of this phase. These results, in combination with the apparent absence of anomalously high dT/dΔ, can be explained by a relatively thin (≤100 km) low‐velocity zone at the base of the mantle. Long‐period diffracted P wave data probably lack the required resolution for detecting a relatively thin transition zone. Such a zone, with relatively low velocity, is allowed by an average of travel time and amplitude data across the core shadow boundary and appears to be required by the NORSAR observations of short‐period diffraction far into the shadow. However, recent results of individual short‐period P wave studies pertaining to geographically limited regions only (including also the NORSAR observations) are partly conflicting unless lateral variations exist. On balance, current thermal models appear to be consistent with (but not required by) the present seismic evidence.
Preseismic rupture progression and great earthquake instabilities at plate boundaries
Authors: Li Victor C. , Rice James R.
Published: February 2008

We present a procedure for modeling the initially quasi‐static upward progression of a zone of slip from some depth in the lithosphere toward the earth's surface, along a transform plate margin, culminating in a great crustal earthquake. Stress transmission in the lithosphere is analyzed with a generalized Elsasser model, in which elastic lithospheric plates undergo plane stress deformation and are coupled by an elementary foundation model to a Maxwellian viscoelastic asthenosphere. Upward progression of rupture over a finite length of plate boundary, corresponding to a seismic gap along strike, is analyzed by a method based on the ‘line‐spring’ concept, whereby a two‐dimensional antiplane analysis of the upward progression provides the relation between lithospheric thickness‐averaged stress and slip used as a boundary condition in the generalized Elsasser plate model. The formulation results in a nonlinear integral equation for the rupture progression as a function of time and distance along strike. A simpler approximate single degree of freedom analysis procedure is described and shown to lead to instability results that can be formulated in terms of the slip‐softening slope at the boundary falling below the elastic unloading stiffness of the surroundings. The results also indicate a delay of ultimate (seismic) instability due to the stiffer short versus long time asthenospheric response and predict a final period of self‐driven creep toward instability. The procedures for prediction of rupture progression and instability are illustrated in detail for an elastic‐brittle crack model of slip zone advance, and parameters of the model are chosen consistently with great earthquake slips and stress drops. For example, an effective crack fracture energy of the order 4×106 J/m2 at the peak, 7 to 10 km below surface, of a Gaussian bell‐shaped distribution of fracture energy with depth, with variance of the order 5 km, simulating strength build‐up in a seismogenic layer, leads to prediction of nominal seismic stress drops of 30 to 60 bars and slips of 2 to 5 m in great strike slip earthquake ruptures breaking 100 to 400 km along strike. Precursory surface straining in the self‐driven stage is predicted to proceed at a distinctly higher rate over time intervals beginning 3 to 10 months before such an earthquake, this interval being greater for longer distances along strike over which the preseismic upward rupture progression takes place.
Breaking of a single asperity: Rupture process and seismic radiation
Authors: Das S. , Kostrov B. V.
Published: February 2008

The problem of spontaneous shear rupture of a single circular asperity on an infinite fault plane is studied. Initially, the fault plane is broken everywhere except at a circular asperity. An applied displacement at infinity results in a stress concentration along the bounding edge of the asperity. The frictional stress on the broken part of the fault plane is taken to be a constant. Once a point on the asperity breaks, the stress there drops to the same value as on the ‘main’ fault surface. The rupture is started by relaxing the shear stress at a point on the asperity edge and is then allowed to propagate spontaneously, using a critical stress level fracture criterion. The rupture process is calculated numerically. It is found that for asperities of constant strength, the rupture first propagates around the edge of the asperity and then inward, a phenomenon best described by the well‐known term of classic military maneuver: ‘the double encircling pincer movement.’ In the appendix, the expressions for the far‐field seismic radiation due to the rupture of such an asperity are derived. It is shown that the nth Cartesian component of the far‐field displacement at (x, t) for P, SV, and SH waves, using the notation of Aki and Richards (1980), is given by un(x, t) = (Dni/4πρc2R)∫∫s0τi3{ξ, t − [(R − ξ · γ)/c]} dS(ξ). Thus the far‐field pulses can be directly found from the stress drops on the fault plane. This formula is also true for ‘crack’ or ‘dislocation’ problems. The directivity function Dni for displacement for the asperity problem is found to be that for the double couple, modified by some factor. In particular, the fault plane is a nodal plane for SV waves. For the rupturing of asperities on a finite fault, these directivity functions are applicable only to the initial part of observed pulses at a receiver, provided the receiver is not located on the fault plane outside the broken part of the main crack edge, in which case it is inapplicable for all time.
State of stress, permeability, and fractures in the Precambrian granite of northern Illinois
Authors: Haimson Bezalel C. , Doe Thomas W.
Published: February 2008

In situ fracture logging, permeability tests, and stress measurements have been conducted in UPH 3, a 1600‐m‐deep hole drilled into the Precambrian granitic basement of northern Illinois. Two major fracture zones are revealed, which cannot be discerned in UPH 2, a similarly deep hole about 1 km away. The segments of the UPH 3 core that were oriented indicate the existence of three sets of subvertical joints striking at N55°E, N40°W and E–W. These sets correspond to surface and shallow borehole joint directions in the Precambrian and Paleozoic rock of southern Wisconsin as well as other areas of the Midwest. The permeability values in UPH 3 display an overall reduction with depth from about 10−4 darcy at 700 m to 10−8–10−9 darcy at 1600 m. Permeability is highest in the zones of greatest fracturing, one of which occurs near the top of the granite and is probably related to fractures which were formed when the granite was at the surface in late Paleozoic times. Permeability reduction with depth is consistent with previous laboratory and field results in crystalline rocks. Hydrofracturing measurements in UPH 3 reveal a compressional stress field with the largest stress horizontal and oriented at N48°E (±30°). Based on linear regression of 13 test results in the depth range of 686–1449 m, the greatest horizontal stress has a magnitude of [20.5+(0.023×depth(m))] MPa. The least horizontal compression has a value of [8.7+(0.019×depth(m))] MPa. The vertical stress, based on density measurements, is given by [−1.3+(0.026xdepth(m))] MPa. Both magnitudes and directions support previous results in the technically stable Great Lakes region of the midcontinent. However, a mb = 4.4 earthquake did occur in 1972 some 90 km south of UPH 3, at a depth of 13 km. The focal mechanism solution revealed strike slip motion with the pressure axis horizontal and trending northeast, in accord with our measured stress directions and relative magnitudes but not predicted from a frictional sliding criterion based on Byerlee's law.
Thermal conductivity of minerals at high pressure: The effect of phase transitions
Authors: Roufosse Micheline C. , Jeanloz Raymond
Published: February 2008

Theory predicts and measurements confirm that anharmonic properties such as the lattice thermal resistivity, the Grüneisen parameter, and the coefficient of thermal expansion generally decrease as a function of increasing density upon compression of solids. The Debye‐Grüneisen continuum model further predicts a discontinuous decrease of anharmonic properties during a phase transition with a density increase, in contradiction with available experimental data on alkali halides. We present results calculated from a model based on interatomic potentials. This model correctly predicts an increase in anharmonic properties for simple compounds transforming from a six‐ to an eight‐coordinated structure. These results stress the importance of interatomic spacing as well as density in determining changes in thermodynamic properties due to polymorphism. The change in crystal structure across a phase transition also affects the thermal conductivity via Brillouin zone summations over the interacting phonon modes. We have used in the calculations either the bulk sound speed, or the Debye average velocity calculated from the longitudinal and transverse velocities (in both cases, theoretically derived from the interatomic potentials). We have found that the pressure dependence of thermal conductivity determined from the bulk sound speed agrees much better with experimental data for each phase than that determined from the average velocity. Apparently, shear modes contribute less to the change of thermal conductivity with compression than has been thought.
Convection in the mantle wedge above the slab and tectonic processes in subduction zones
Authors: Ida Yoshiaki
Published: February 2008

Some nonconductive heat transport in the mantle wedge above the subducting lithosphere is necessary to explain the volcanic and thermal activities in subduction zones. The physical mechanisms that have been proposed to explain the observed thermal processes are classified into the following three categories: (1) secondary convection mechanically driven by the motion of the slab, (2) thermal convection due to a large horizontal temperature gradient associated with the presence of the cool slab, and (3) flow induced by ascending mantle diapirs containing low‐density melt. The relative importance of these three mechanisms is evaluated, using a simple model in which the conduit of the convective flow is prescribed. The first mechanism of mechanically driven flow dominates the other two when the effective viscosity of the mantle wedge exceeds 1020 Pa s (1021 P). Known estimates of wedge viscosity suggest that either the second or third mechanism is more likely to be the one that actually governs the tectonic process. It is proposed that both the second and third mechanisms are realized in different stages of a single tectonic sequence and that arc volcanism and back arc spreading occur, corresponding to these different modes of convection.
Reflection properties of phase transition and compositional change models of the 670‐km discontinuity
Authors: Lees Alison C. , Bukowinski M. S. T. , Jeanloz Raymond
Published: February 2008

Seismic observations of the amplitude ratio amplitude P′670P′/amplitude P′P′ suggest a reflection coefficient R ∼ 7–13% for reflections from the underside of the 670‐km discontinuity. This can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region. Velocity and density profiles through the transition region were calculated for olivine, pyroxene, garnet, and olivine + garnet model mineral assemblages. For models with identical upper and lower mantle compositions, the computed reflection coefficients from the phase transitions pertinent to the 670‐km discontinuity are less than 0.5%. These phase transitions produce their maximum R if they are effectively discontinuous, which gives R ∼ 2–3%, except for the garnet to perovskite transformation which gives R ∼ 8%. In general, given realistic impedance contrasts across a phase transition, the change in material properties must occur over a region less than 3 km thick for R to be of the order of 5%. A discontinuous change in chemical composition at 670‐km depth, coupled with a change in phase, can also produce a reflection coefficient of the observed magnitude: R ∼ 2–3% for a first‐order change in phase at 670 km with different iron concentrations on each side of the discontinuity, and R ∼ 4–6% for cases with a discontinuous increase in silica content. Based on our analysis, the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient. Thus although limited by considerable uncertainties, the mineralogical and seismic data suggest a two‐layered model of the mantle with a discontinuous change in both phase and composition at 670 km.
Analysis of surface deformation data, Kilauea Volcano, Hawaii: October 1966 to September 1970
Authors: Dvorak John , Okamura Arnold , Dieterich James H.
Published: February 2008

A least squares inversion technique has been applied to surface displacement measurements made at Kilauea Volcano, Hawaii, in an attempt to locate centers of intrusive activity. Utilizing analytic expressions for the displacement field arising from dilatational sources in an elastic half space, this technique allows for the simultaneous inversion of level, tilt, and horizontal distance measurements in terms of the location and volume of the intrusive body. A variety of simple elastic model geometries, representing possible shapes for intrusive bodies in the summit region of Kilauea, were examined. None of the model geometries considered significantly improved the quality of the model fit to the available data over that obtained by a point source model. Calculations based on elastic models to consecutive level surveys were compared to available tilt data and horizontal distance measurements. These comparisons indicate occasional departures from ideal elastic behavior. The departure of measured tilt patterns from calculated tilt values may be the result of earthquake activity or episodic block movements superimposed on larger elastic displacements. The departure of measured and calculated horizontal measurements reveals a gradual north‐south extensional strain across the summit caldera of Kilauea in addition to the elastic uplift and subsidence patterns determined from level surveys. A minimum estimate for the magma supply rate to Kilauea is provided by the elastic models to the level data. The average rate for the period from October 1966 to September 1970 was 0.07 km3 per year.
Regional tectonic and earthquake hazard implications of a crustal fault zone in southwestern Washington
Authors: Weaver Craig S. , Smith Stewart W.
Published: February 2008

The St. Helens seismic zone (SHZ) is a crustal earthquake zone in southwestern Washington that is defined by small‐ to moderate‐magnitude (2.5 to 5.5) earthquakes and focal mechanisms. This zone, seismically active along a distance of 90 km, is interpreted as a fault zone capable of generating a moderate‐ to large‐magnitude shallow earthquake. At the northern end of this seismic zone (about 15 km north of the Cowlitz River) a large area of seismic quiescence (for earthquakes above magnitude 3.0) has predominated since 1977. This quiescence leaves open the question whether the SHZ continues northward into southern Puget Sound. Earthquake focal mechanisms along the SHZ indicating nearly vertical strike‐slip faulting with the preferred fault planes striking north–south. From these focal mechanisms we infer that the direction of maximum compression is northeast, approximately parallel with the direction of plate convergence between the North American and Juan de Fuca plates. We interpret the focal mechanism data as evidence for locked subduction. This tectonic model raises the possibility of a large‐magnitude subduction earthquake. Neither the possibility of a moderate to large shallow crustal earthquake nor a major subduction event is part of the current earthquake hazard assessment for Washington.
Density and size distribution of seamounts in the eastern Pacific inferred from wide‐beam sounding data
Authors: Jordan Thomas H. , Menard H. William , Smith Deborah K.
Published: February 2008

Theoretical expressions relating the distribution of apparent seamount heights per unit length of ship track to the distribution of actual seamount heights per unit area of seafloor are derived for both narrow‐beam and wide‐beam echo sounders. The shapes of seamounts are approximated as truncated right‐circular cones of constant slope ε, height‐to‐radius ratio ξ, and flatness f; i.e., a seamount of basal radius r has a height h = ξr, a flat top of radius fr, and a slope angle of ϕ = arctan ε. The set of all seamounts in a given physiographic province is assumed to be a Poissonian ensemble specified by a function v(R), the average number of seamounts per unit area for which r ≥ R. A simple choice for the size distribution of seamounts, and one shown to be consistent with data in the eastern Pacific, is to specify that it decays exponentially with R: v(R) = vo exp (−αR). In the case of a profile of length L taken in water of depth D with a wide‐beam echo sounder (half beam width > ϕ), the shallow‐slope, deep‐water approximation (ε << 1, ε/α << D) yields that the expected number of seamounts with apparent heights
is
. We apply this sampling theory to apparent‐height data measured from 44,020 km of wide‐beam profiles on Pacific crust of post‐Mesozoic age. Although the spatial distribution of seamounts in the eastern Pacific is certainly not Poissonian, the abundance statistics do not involve spatial correlations, and we show they are robust with respect to deviations from the Poisson model, provided the statistics are interpreted as averages over the area actually sampled by the track lines. The observed falloff of apparent heights in the eastern Pacific is very nearly exponential, justifying our use of an exponential model of seamount size distribution. The shape parameters are estimated from multibeam surveys of Pacific seamounts; an analysis of 30 features in the height range 200–4000 m yields ξ = 0.21±0.011 and f = 0.21±0.028. Using these parameters, we obtain vo = (4.0±1.1) × 10−9 m−2 and α = (6.3±0.8) × 10−4 m−1. These values imply that per 106 km2 in the eastern Pacific there are 1600±400 seamounts with summit heights greater than 300 m and 200±60 greater than 1000 m; integrated over the entire size distribution, seamounts occupy about 6% of the seafloor area and constitute about 0.4% of the oceanic crustal volume. Our estimates of seamount abundance are appreciably greater than those of most other authors, a discrepancy we attribute primarily to the sampling bias of previous studies, which generally used uncorrected map counts. This hypothesis is corroborated by a comparison of our statistics with map count data for seamounts of heights exceeding 2000 m compiled for the eastern central Pacific from the Scripps charts; above 3000 m the map counts are consistent with the predictions of the exponential model, but below this height they are biased to low values.
Use of the magnetotelluric method for a better understanding of the West African Shield
Authors: Ritz M.
Published: February 2008

Magnetotelluric soundings have been carried out in the Republic of Upper Volta and in the Niger Republic to gain a better understanding of the structure of the West African shield. The sounding stations are situated from east to west on a sedimentary basin on the Central African mobile belt (Pan‐African orogeny at 550±100 m.y.) and on the West African craton (Eburnean orogeny at 1850±250 m.y.). The results show that the craton is characterized by a zone of high resistivity in the crust and uppermost mantle. The absence of a conductive zone at the interface between the crust and the uppermost mantle is consistent with the hypothesis of Hyndman and Hyndman (1968) on the dehydration of the crust of the stable shields. The first conductive layer of the craton is situated at a depth of 130 km with a temperature of about 860°C. In the mobile belt and the basin the presence of a low resistivity layer at a depth of 30–40 km has been established. If water is present in the lower crust, it can explain the origin of this conductive zone. A plausible explanation is that this layer may have been formed by the slow infusion of water from the mantle during the last thermal reactivation. Regional differences in electrical conductivity structure between the Central African mobile belt and West African craton appear to extend deeper than 200 km. Modeled conductivity variations with depth in the upper mantle obtained from magnetotelluric results are compared to other continental models (thermal model‐seismic structure) in order to define a lower lithospheric boundary for West Africa. At the present stage the problem is not yet resolved (mobile crustal plate or anchorage in the African plate).
Mare glasses from Apollo 17: Constraints on the Moon's bulk composition
Authors: Delano J. W. , Lindsley D. H.
Published: February 2008

Two previously unreported varieties of mare volcanic glass have been discovered in Apollo 17 samples. Twenty‐three chemical types of volcanic glass have now been analyzed from the six Apollo landing sites. These volcanic glasses, which may be samples of primary magmas derived from the differentiated lunar mantle, define two linear arrays that seem to reflect regional, if not global, regularities among the source regions of these melts. Additional systematics among these glasses have been used to estimate the bulk composition of the moon. The results suggest that the refractory lithophile elements (e.g., Ca, Al, Ti, REE, U) are present at abundances of 1.7×chondrites. The silicate portion of the moon appears to have a major‐element composition similar to a volatile (Si, Na, K)‐depleted, earth's upper mantle. The theory involving an earth‐fission origin of the moon can be tested further through trace element analyses (e.g., Ni) on the volcanic glasses, and through determination of the (N/36Ar) ratio and noble gas isotopes from primordial lunar gas trapped within vesicles associated with mare volcanic glass.
Comparison of Yamato and Victoria Land polymict eucrites: A view from mineralogical and isotopic studies
Authors: Takeda Hiroshi , Wooden J. L. , Mori Hiroshi , Delaney J. S. , Prinz M. , Nyquist L. E.
Published: February 2008

New interior samples of four Yamato polymict eucrites (Y74159, Y74450, Y75011, and Y75015) have been studied by electron microprobe, transmission electron microscope, and X ray diffraction techniques, and compared with several samples of the Victoria Land polymict eucrites. These same samples have been analysed using Rb‐Sr and Nd‐Sm isotopic systematics. Several grains of inverted pigeonite, with blebby augite similar to those in Binda and Moama, have been identified in all four Yamato eucrites. Coarse‐grained mesostasis‐rich subophitic basalts, which contain Mg‐rich pigeonite (with Fe‐rich olivine veinlets) zoned outward to a subcalcic ferroaugite rims, have also been found. These unique clasts were not found in ALH76005, 77302, 78040, 78158, and 78165 and EET eucrites. The tight grouping of Rb/Sr and Sm/Nd ratios, and similar modal compositions of the Yamato group indicate that they are most likely to be pieces from a single fall, and distinct from the ALH and EET groups. However, the Yamato, Allan Hills, and Elephant Moraine groups may sample a few distinct magmas or similar but different source regions on the same parent body.
Sedimentary processes and crustal cycling on Venus
Authors: Warner Jeffrey L.
Published: February 2008

Sediment exists on the Venus surface. It is observed in Venera images between outcrops and boulders of sedimentary rocks. Sediment is produced by pyroclastic volcanism and chemical weathering. Chemical weathering is driven by an enhanced activity of water and an elevated surface temperature. Sediment is transported by wind action and lithified by cementation and diagenesis. Cementation may be by carbonate or silica cement; diagenesis may be by products of chemical weathering acting as cement, or by compaction and recrystallization of sediment into a texture with interlocking grains. Sediment may be transported from the top of sialic continents (such as Ishtar) to the modal plains where it is deposited, lithified, and integrated into the local crust. As new layers are added, the bottom of the crust melts and is, in part, returned to the mantle. A steady‐state chemical exchange might exist by this mechanism of crustal cycling that links atmosphere, continents, modal plains, and mantle.
Sixth foray for pristine nonmare rocks and an assessment of the diversity of lunar anorthosites
Authors: Warren Paul H. , Taylor G. Jeffrey , Keil Klaus , Kallemeyn Gregory W. , Rosener Paula S. , Wasson John T.
Published: February 2008

Eleven nonmare samples are characterized, generally both compositionally and petrographically. Five are almost certainly pristine (ferroan anorthosite 10056c, alkali anorthosite 14047c, ferroan anorthosite 14312c, anorthositic troctolite 14321c3, and granite 14321c4), and four are probably pristine (norite 14318c, noritic anorthosite 67215, ‘quasi‐pristine’ ferroan anorthosite 73217c, and norite 78527). Five are unique anorthosites, extending the diversity of lunar anorthosites considerably. Alkali anorthosites, particularly 14047c, are extremely rich in incompatible elements. The first anorthosite from Apollo 11 with manifestations of pristinity (10056c) appears to be a normal member of the ferroan group. However, the first pristine ferroan anorthosites from Apollo 14 and 17 to be studied in detail (14312c and 73217c) both have unusually sodic plagioclase by ferroan standards. With only 75% plagioclase, 67215 is the first large (273 g) probably pristine ferroan sample with less than ∼83% plagioclase. The nature of the correlations between longitude and such parameters as Eu and Sm concentrations, and Sc/Sm, Ti/Sm, and Ca/Na ratios, among pristine ANT samples has been elucidated, because five of the new pristine samples are from a western site (Apollo 14). Interestingly, ferroan anorthosite 14312c has Ca/Na, Sc/Sm, and Ti/Sm ratios close to the normal ferroan values, but significantly displaced in the direction of the nonferroan western pristine rocks. This suggests that the composition‐longitude relationships are as old as the late stages of the crystallization of the putative magma ocean.
Magnetite in CI chondrites
Authors: Hyman M. , Rowe M. W.
Published: February 2008

On the basis of 939 measurements of the CI chondrites, their magnetite contents have been determined to be: Alais, 8.9±0.9%, Ivuna, 11.1±0.3%, Orgueil, 11.0±0.4%, Revelstoke, 9.4% and Tonk, 8.5%. Standard statistical analysis of the data indicates the existence of two subgroups, one comprised of Ivuna and Orgueil, the other of Alais, Revelstoke, and Tonk. While we are not proposing a change in classification, it is clear that distinctive differences exist between the two subgroups. The precision of the method is ≈±3% so that the standard deviations listed reflect the extent of actual variation in magnetite distribution. The wide range, from 6 to nearly 50% Fe3O4, in previous estimates of the magnetite content of Orgueil is due to experimental error, not to a real heterogeneity.