bluuebunny/crossref_metadata_2025 huggingface dataset:
Petrologic variations in Apollo 16 surface soils
Authors: Houck Karen J.
Published: February 2008

Detailed modal analyses of fifteen surface soils were compared with rake and rock sample data to provide information about source rocks, maturation history, and intrasite variations in Apollo 16 regolith. Triangular plots of source rock components [defined as anorthosites, crystalline (melt) matrix breccias, and metamorphosed breccias] show that, with the possible exception of soils 64501 and 68501, Apollo 16 soils have similar source rocks that are well homogenized throughout the site. Modal abundance variations in submillimeter size fractions tend to reflect those of rake and rock samples. Maturity plots in agglutinate‐monomineralic fragment‐lithic and crystalline breccia space show Apollo 16 soils to be generally mature except for those collected from trenches (61221 and 61241) and from locations near North Ray Crater (stations 11 and 13). Data produced by other workers is combined with data from this study and used to divide the Apollo 16 site into three soil petrographic provinces. Central site soils are mature, well homogenized, and enriched in glass, but patches of immature and/or inhomogenous material are also present. They are probably the most typical Cayley Plains materials present at the site. North Ray soils are immature to submature, containing North Ray ejecta in the form of feldspathic fragmental breccia and plagioclase. South Ray soils are mature, but contain small amounts of fresh impact melts and plagioclase, possibly from breakdown of blocky South Ray ejecta. South Ray and North Ray ejecta differ in composition and physical properties, supporting the hypothesis that the South Ray event excavated Cayley material while the North Ray event excavated Descartes material.
Shock disturbance of the I‐XE system
Authors: Caffee M. W. , Hohenberg C. M. , Horz F. , Hudson B. , Kennedy B. M. , Podosek F. A. , Swindle T. D.
Published: February 2008

Three separate samples of the meteorite Bjurbole were artificially shocked at pressures of 70 kb, 200 kb, and 400 kb. Analysis of xenon released in stepwise heating shows that the I‐Xe system of the 400 kb sample is substantially altered by the shock loading and it is no longer possible to infer an age or trapped xenon composition for that sample. The 200 kb and 70 kb samples display isotopic structures progressively less altered demonstrating the gradations in shock disturbance likely to be found in natural systems. Interpretations of the I‐Xe and 40Ar‐39Ar systems for several naturally shocked meteorites are also presented. New data for Arapahoe do not confirm the previously reported age and trapped xenon composition, demonstrating instead that its I‐Xe structure has been strongly disturbed by shock.
A note against a small‐body origin for shergottites, nakhlites, and chassignites
Authors: Ryder Graham
Published: February 2008

With the possible exception of Brachina, shergottites, nakhlites, and chassignites all crystallized about 1.3 b.y. ago on the same planet, and they were probably ejected from it together about 180 m.y. ago. No breccias from the parent planet have yet been identified, thus a dominantly impact‐processed surface or a small body internal melt scenario is virtually untenable. These meteorites must have formed in a magmatic complex, and on a planet from which it was difficult to remove material: as far as we know, it only happened once. Everything we know about shergottites, nakhlites, and Chassigny is consistent with an igneous origin on Mars.
An inventory of particles from stratospheric collectors: Extraterrestrial and otherwise
Authors: Fraundorf P. , McKeegan K. D. , Sandford S. A. , Swan P. , Walker R. M.
Published: February 2008

Examination of 526 particles in the 2–50 μm size range obtained from stratospheric impaction collectors has shown that most of them fit into one of three groups on a simple plot of Mg/Si versus Al/Si energy dispersive X ray peak ratios. The particles within a given group exhibit strong correlations in composition and mineralogy to one another, and the groups appear to largely consist either of earth crustal, man‐made, or extraterrestrial materials. The temporal variability and relatively flat size distribution of material in the earth crustal group, in particular, suggests an aircraft‐related, as opposed to stratospheric, source for these particles.
Nucleation and growth of strike slip faults in granite
Authors: Segall Paul , Pollard David D.
Published: February 2008

Fractures within granodiorite of the central Sierra Nevada, California, were studied to elucidate the mechanics of faulting in crystalline rocks, with emphasis on the nucleation of new fault surfaces and their subsequent propagation and growth. Within the study area the fractures form a single, subparallel array which strikes N50°–70°E and dips steeply to the south. Some of these fractures are identified as joints because displacements across the fracture surfaces exhibit dilation but no slip. The joints are filled with undeformed minerals, including epidote and chlorite. Other fractures are identified as small faults because they display left‐lateral strike slip separations of up to 2 m. Slickensides, developed on fault surfaces, plunge 0°–20° to the east. The faults occur parallel to, and in the same outcrop with, the joints. The faults are filled with epidote, chlorite, and quartz, which exhibit textural evidence of shear deformation. These observations indicate that the strike slip faults nucleated on earlier formed, mineral‐filled joints. Secondary, dilational fractures propagated from near the ends of some small faults contemporaneously with the left‐lateral slip on the faults. These fractures trend 25°±10° from the fault planes, parallel to the direction of inferred local maximum compressive stress. The faults did not propagate into intact rock in their own planes as shear fractures. Rather, adjacent faults were linked together by secondary, dilational fractures. Extensive secondary fracturing between faults produced larger fault zones that accommodate 10–100 m of left‐lateral slip. As deformation progressed, faulting evolved from relatively short, closely spaced faults to longer, more widely spaced fault zones.
The relationship between surface topography, gravity anomalies, and temperature structure of convection
Authors: Parsons Barry , Daly Stephen
Published: February 2008

Normal stresses associated with convection in a fluid layer, whose boundaries can deform, produce topography on those boundaries. When the equations of motion are linear, integral relations between topography on the boundaries and the temperature structure can be found as a function of wavelength. Expressions of this kind have been derived for the case of convection in a constant viscosity fluid when inertial effects are negligible. The total gravity anomaly is the sum of contributions due to the topography on the two boundaries and to temperature variations within the fluid, and similar integral relations between gravity and the temperature structure can also be derived. At wavelengths large compared to the depth the shape of the kernels in the integrals, particularly that for gravity, is sensitive to the boundary conditions used. The transfer function between gravity and topography is also characteristic of the boundary conditions at long wavelengths. In all the cases considered, there is a transition between short‐and long‐wavelength behavior which occurs at wavelengths proportional to the layer depth. If the bottom boundary can deform as well as the upper boundary, the gravity anomaly tends to zero at long wave‐lengths as the gravity kernel tends to zero everywhere. The surface topography kernel is always zero at the bottom boundary. The lack of any significant surface expression produced by temperature variations near the bottom of the layer provides an explanation of the similar relationships between gravity and topography associated with the dissimilar temperature structures produced by different modes of heating. At least for cellular convection, surface topography and gravity anomalies largely reflect temperature variations in the vicinity of the upper thermal boundary layer. This is consistent with an explanation of geoid anomalies over mid‐ocean swells in terms of convection beneath the lithosphere, where the lower part of the thermally defined plate acts as the upper thermal boundary layer of the convection.
Magnetic polarity stratigraphy: Stochastic properties of data, sampling problems, and the evaluation of interpretations
Authors: Johnson Noye M. , McGee Victor E.
Published: February 2008

A statistical model has been derived which can be used either to test the interpretation of a given magnetic stratigraphy or to make time estimates from the paleomagnetic data. The model is based on the presently accepted statistical properties of the earth's magnetic field and the stochastic nature of the paleomagnetic sampling process. Given knowledge of the number of paleomagnetic sites, the distribution of the sites, an estimate of the geologic time span sampled, and the mean length of the magnetic polarity zones, the expected success of the sampling program can be calculated and compared with its actual performance. In a number of test cases the theoretical results have agreed with realized results. The mean time span for polarity intervals in the late Neogene has been established from existing data at 120,000 years. Once the mean time span for polarity intervals has been established, estimates for stratigraphic time span can be made by means of the statistical model. Of three models studied, a time uniform sampling program is the most efficient for establishing a local magnetic polarity stratigraphy; the least efficient strategy is an exponential distribution of samples.
Global postseismic deformation in a stratified viscoelastic Earth: Effects on Chandler Wobble Excitation
Authors: Dragoni Michele , Yuen David A. , Boschi Enzo
Published: February 2008

The principal purpose of this paper is to examine whether movement of matter in a stratified viscoelastic earth can enhance the excitation of the Chandler wobble. We have constructed analytically a two‐layer model consisting of an elastic lithosphere overlying a Maxwell viscoelastic mantle in order to calculate explicitly the temporal evolution of the inertia tensor from earthquake faulting. It is found that the ability of the viscoelastic flows to produce an additional increment of the inertia tensor depends critically on R, the ratio of the inner radius to the outer radius. Only for R ≳ 0.7 is there a substantial increase. For the earth (R ≃ 0.98) an amplification of between four and six is obtained from the models considered. However, the time scales over which this enhancement of the moment of inertia takes place is about 10 Maxwell times, thus making this model unlikely to excite the Chandler wobble for values of the mantle viscosity derived from postglacial uplifts. This phenomenon of strain amplification occurs by virtue of ‘transient membrane mechanics,’ as the ratio between the horizontal wavelength of the deformation field is large compared with the elastic shell. By means of a similarity argument we have found that for angular orders less than 40 to 45 ‘transient membrane mechanics’ can be important in global postseismic deformation in the earth. This mode of transient deformation arises from the three‐dimensional nature of the problem and hence cannot be modeled with conventional flat earth models.
Growth and dynamics of the Nordre Strømfjord Shear Zone
Authors: Sørensen Kai
Published: February 2008

The Nordre Strømfjord shear zone is exposed for a length of 170 km within the Precambrian Nagssugtoqidian Mobile Belt of West Greenland. Its western 100 km consists of a 15–20 km wide zone of syndeformational granulite facies rocks. The movement was sinistral with a displacement of 120 km across the zone. The strain across the shear zone margin increases exponentially but levels off to a near constant value over major sections of the inner zone. This profile suggests a growth model, in which the marginal zones are focii of instantaneous deformation that moves outward as hardening occurs along the inner boundaries and weakening occurs along the outer boundaries. This growth model allows the minimum possible viscosity change associated with the zone to be calculated from the logarithmic strain gradient and from the width across which weakening occurs. When applied to an area within the granulite grade part of the shear zone, this model indicates a decrease in viscosity by a factor 10 to 50, and the functional form of this weakening shows that the parameter undergoing change enters the exponential part of the appropriate constitutive equation. The available evidence seems to exclude shear heating as a viable weakening mechanism but supports chemical weakening. It is hypothesized that ‘hydrolytic weakening' was responsible. At higher crustal levels a fundamental change in geometry of the Nordre Strømfjord zone occurs. The applicability of the present model is thus restricted to deep crustal levels. In conclusion it is illustrated that the model is of relevance to small scale shear zones as well and may also be representative of mantle deformation.
Phase transitions and equations of state for the sodium halides: NaF NaCl, NaBr, and NaI
Authors: Sato‐Sorensen Yosiko
Published: February 2008

Phase changes and equations of state of four sodium halides, NaF, NaCl, NaBr, and NaI, have been studied up to ∼60 GPa at room temperature with the use of a diamond anvil cell. NaF and NaCl transform from the B1 phase to the B2 phase at ∼23 GPa and ∼29 GPa, respectively. At ∼33 GPa, NaBr transforms from the B1 phase to an unidentified phase with a lower symmetry than the B2 phase. NaI also transforms at ∼32 GPa to a new phase which seems to have the same structure as that of the high‐pressure phase of NaBr. The B2 phase of NaF has bulk modulus K0 = 103 (±19) GPa and volume ratio at 1 atm Vo2/Vo1 = 0.811 (±0.025), and the B2 phase of NaCl has K0 = 121 (±23) GPa and V02/V01 = 0.734 (±0.022). The volume change across the transition was estimated to be ΔV/V1 ≃ −10.4% for NaF and −5.8% for NaCl.
Anharmonicity of three minerals at high temperature: Forsterite, fayalite, and periclase
Authors: Anderson O. L. , Suzuki I.
Published: February 2008

Recent data on Ks (the adiabatic bulk modulus) and α (the volume coefficient of thermal expansion) versus T (temperature) at high temperatures (500°C < T < 1000°C) have been published or are in press. These data, taken at ambient pressure, extend the measurement of single‐crystal elastic constants for forsterite, fayalite and periclase to record temperatures. The high temperature anharmonic properties of forsterite and fayalite are presented for the first time in this paper, and they are compared with similar previously published data for MgO. The anharmonic properties referred to above concern the dependence of γ (the Grüneisen ratio), PTH (the thermal pressure), and Cv (the specific heat) with T. If γ (at constant V) is independent of T at high T, the anharmonicity in γ is said to be nil; similarly, for Cv. If PTH at constant V is proportional to T at high T, then the anharmonicity in PTH is said to be nil. The anharmonicity determined by these experiments indicates that the minerals are not alike with regard to their properties γ, PTH, and Cv. The γ versus T at constant V indicates that there is anharmonicity for all three minerals, but the effects are opposite in fayalite and forsterite in such a way that anharmonicity should be absent in olivine. For PTH at 1 bar, anharmonicity is detectable and positive in forsterite, absent in fayalite, and detectable and negative in periclase. It would be slight in olivine. In all three solids, anharmonicity in Cv is pronounced and positive.
Grabens, basin tectonics, and the maximum total expansion of the Moon
Authors: Golombek Matthew P. , McGill George E.
Published: February 2008

The presence of grabens and wrinkle ridges indicates that stresses sufficient to deform or fault crustal rocks have existed for at least part of lunar history. These structures have been explained as due to global expansion and contraction, basin subsidence, or both. Even if the extension implied by all lunar grabens were due to global expansion, the total radius increase permissible after terminal bombardment would be much less than that predicted by the most conservative models of lunar thermal evolution. The obvious areal association of most grabens and wrinkle ridges with impact basins suggests that a genetic relationship exists also. But it is necessary to develop either kinematic or dynamic models to determine if basin subsidence alone can account for these structures. Kinematic models are the most useful because fewer assumptions regarding material properties are needed than for dynamic models. For both types, the grabens and wrinkle ridges provide the data that must be predicted by the models. Considerations of geometry, kinematics, and mechanics suggest that the simple grabens on the moon are reliable indicators of crustal extension. Two end‐member kinematic models of postimpact basin modification are derived: one assuming that subsidence is accomplished entirely by an increase in radius of curvature of the basin floor, the other assuming that subsidence is accomplished without any change in radius of curvature of the basin floor. Any combination of the two also would be possible. Both models predict enough extension to account completely for basin‐related grabens, and thus they indicate that virtually all of the extension implied by lunar grabens can be provided by modest amounts of basin subsidence. In addition, both models also predict total radial and hoop shortening of the basin floor of about the correct magnitude to explain wrinkle ridges if these features are structures resulting from compression. Consequently, almost all extension and shortening of the lunar crust implied by grabens and wrinkle ridges can be explained by basin subsidence alone; significant global expansion after ∼3.9 b.y. is unlikely, and significant global contraction is unnecessary.
Focal depths of intracontinental and intraplate earthquakes and their implications for the thermal and mechanical properties of the lithosphere
Authors: Chen Wang‐Ping , Molnar Peter
Published: February 2008

We investigate the distribution of focal depths for earthquakes that do not appear to be associated with zones of recent subduction, using both new results from analyses of individual events recorded at teleseismic distances and published data for both microearthquakes and larger events. The deepest events in oceanic regions occur in old lithosphere (≥100 Ma), and excluding earthquakes in active mountain belts, the deepest crustal events occur in old cratons (tectonic age ≥800 Ma). Therefore, the temperature at the source region is likely to be an important factor determining whether deformation occurs seismically or not. From estimates of the temperatures at depths of the deepest events, we conclude that those limiting temperatures are about 250°–450°C and 600°–800°C for crustal and mantle materials, respectively. In several regions of recent continental convergence, in addition to shallow crustal seismicity, there is seismic activity in the uppermost mantle. The lower crust, however, is essentially aseismic. We infer that both the upper crustal and the mantle seismic regions correspond to zones of relatively high strength and that they are separated by a zone of lower strength in the lower crust where aseismic, ductile deformation predominates. This simple interpretation is qualitatively in agreement with extrapolated values of brittle and ductile strengths of geologic materials studied under appropriate pressure and temperature conditions in the laboratory. A low‐strength zone in the lower crust might allow detachment of crystalline nappes from the underlying mantle (and lower crustal) lithosphere. The apparently greater strength of mantle materials than crustal materials at the same temperature implies that oceanic lithosphere is much stronger than continental lithosphere, and this difference may account for why plate tectonics works well in oceanic regions but not in continents.
Measurements of frictional heating in granite
Authors: Lockner David A. , Okubo Paul G.
Published: February 2008

A large (150×150×40 cm) granite sample, sawn diagonally in half to simulate a fault, was deformed in a biaxial rock press at normal stress up to 6.41 MPa. Displacements and local shear stress were monitored along the fault (200×40 cm). Temperature transients as large as 10 m°C were recorded following stick slip events at distances of 0.2 to 1.0 cm from the fault and were related to stress, displacement, and total work. The temperature measurements were used to calculate the heat generated on the fault during slip. Frictional heating was found to be 94±2% of the total work expended in each event, implying a seismic efficiency of 4–8%. When water was injected onto the fault, both fractional stress drop and static frictional stress increased. The efficiency of frictional heating was not lowered by the presence of water. Heat generated during deformation of a 0.15‐cm‐thick layer of simulated gouge was also measured for sliding rates from 0.09 to 9.1 μm/s. In these gouge experiments, temperature rises were less than 0.1°C and were proportional to sliding rate.
Error analysis for the determination of tectonic rotation from paleomagnetic data
Authors: Demarest Harold H.
Published: February 2008

Appendix B is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document B83‐001; $2.50. Payment must accompany order.

Recent determinations of tectonic rotation and flattening from paleomagnetic data have exaggerated the size of the confidence limits or error bars by about 25%. This has been caused by the misuse of conventional error propagation formulas to derive one degree of freedom error bars from a two dimensional cone of confidence (α95). An analytical calculation, which is valid when error bars are small, shows that the correct error bars are 75% to 80% of the values derived in previous studies. I have used numerical integration to check on the applicability of the analytical solution and to calculate error bars when the analytical approximation cannot be used. The analytical approximation may be used whenever α95 is less than 10° and the mean inclination is not too close to vertical. Confidence limits given for tectonic rotation and flattening in recent literature are too large and should be reduced by 20–25%. Narrowing the error bars in this way permits a more accurate interpretation of paleomagnetic data.
Stress corrosion and crack propagation in Sioux quartzite
Authors: Peck Lindamae
Published: February 2008

Two aspects of the brittle deformation of Sioux quartzite have been studied: water‐induced changes in the amount of energy consumed during fracture of the quartzite and the occurrence of microcracking during crack propagation. Wedge‐loaded double cantilever beam test specimens were cracked in several environments (air, water, aqueous solutions of NaOH, HCl, NaCl, methanol) at crack velocities of 4.2×10−2 mm/s and 4.2 mm/s and temperatures of 17°–60°C. Scanning electron microscopy and the determination of enhanced permeability were used to detect fracture induced microcracks. The fracture experiments measured the fracture energy of the quartzite, the specific energy required to produce a unit increment of crack extension, using the analysis of linear elastic fracture mechanics. The fracture energy of Sioux quartzite in water is 15% lower than that of quartzite cracked in air (ambient humidity). The dependence of the quartzite fracture energy on crack velocity (time) and on the concentration of water in the test environment is characteristic of the occurrence of stress corrosion as a mechanism of enhanced crack growth in the presence of water. Other theories of the chemomechanical weakening of rock, reduction in surface energy and changes in dislocation mobility, do not explain the observed environmentally induced changes in the fracture energy of the quartzite. Although increased crack velocity in quartzite cracked in hot water is evidence of thermally enhanced water weakening, the fracture energy of quartzite cracked in hot air or hot water (40°–60°C) is 12–13% higher than room temperature values. The increase in fracture energy is attributed to changes (reduction in residual stress, existence of thermal stresses) in the net‐stress field responsible for microcracking and crack propagation in the quartzite.
Impact experiments on ice
Authors: Kawakami Shin‐Ichi , Mizutani Hitoshi , Takagi Yasuhiko , Kato Manabu , Kumazawa Mineo
Published: February 2008

The results of cratering and fragmentation experiments on pure ice are reported. The projectiles used are cylindrical aluminum, poly‐carbonate, teflon, and pyrophyllite fired at velocities between 110 m/sec and 680 m/sec, with kinetic energies at impact between 2 and 500 joules. Crater diameters (pit diameters) in the ice were about two times larger than craters in the same energy range in basalts. The ratios of (pit diameter)/(spall diameter) are about three and the ratios of (depth of crater)/(spall diameter) are between 0.1 and 0.3 which are close to the depth/diameter ratios observed in basalts. The crater diameter in ice is also well expressed as a single function of the ‘late‐stage effective energy’ defined recently by Mizutani et al. [1983]. The specific energy for complete destruction of ice target is about 50 J/kg which is two orders of magnitude smaller than that of basalt. The present experimental data on cratering and fragmentation of ice show that the impacts associated with the largest craters on Callisto and Mimas must have severely fractured the whole satellites, and that those giant impacts with the kinetic energy of 1023 to 1027 joules probably affected significantly the evolution of the satellites.
Crustal and upper mantle structure of the northern and central Sierra Nevada
Authors: Mavko Barbara B. , Thompson George A.
Published: February 2008

Supplement available with entire article on microfiche. Order from the American Geophysical Union, 2000 Florida Avenue, N. W., Washington, D.C., 2009. Document B83‐002; $2.50. Payment must accompany order.

Teleseismic data were recorded within the Sierra Nevada to look for lateral variations in the upper mantle. The data were collected at both temporary and permanent stations, and P wave residuals were computed. After correcting the P residual data for crustal and topographic effects, there is still a variation of as much as 0.5‐0.6 s from the north end of the Sierra Nevada to Mono Lake, located east of the central part of the range. In addition, there are significant variations in travel time patterns, depending on the azimuth of wave arrivals. Two simple modeling approaches have been used to infer the upper mantle velocity structure from the observed variations in travel time. In the first, it is assumed that the velocity variations are distributed throughout a depth range of about 60–160 km. One model that fits the data indicates a north‐to‐south trend in upper mantle P velocities from about 7.9 km/s at the north end of the Sierra to 7.6 km/s near Mono Lake. Superimposed is a west‐to‐east trend from 7.85 km/s under the Sierra Nevada crest to 7.7 km/s east of Lake Tahoe. In the second modeling approach, a fixed velocity contrast is assumed between the lithosphere and asthenosphere with variations in travel time resulting from variations in lithospheric thickness. One model, consistent with the observations, indicates a thinning of the lithosphere from 110 km at the north end of the Sierra to 60 km near Mono Lake.
Hot spot heat transfer: Its application to Venus and implications to Venus and Earth
Authors: Morgan Paul , Phillips Roger J.
Published: February 2008

Gross similarities between earth and Venus suggest that both planets might be expected to lose heat by the same mechanism. Available data do not resolve plate tectonics on Venus, however, and other heat loss mechanisms, such as hot spot heat loss, have been suggested. Using a model which gives a relationship among surface elevation, lithospheric thickness, and heat flux, we test the hot spot heat loss mechanism for Venus and determine that it can easily explain the predicted heat loss of the planet with a modest number of hot spots (of the order of 35). Approximately 93%of the mapped topography of Venus can be explained solely on the basis of lithospheric thickness variations. Additional compensation is required for topography above a radius of 6053 km, and this can be effected by incorporating a variable thickness crust into the model. If crust is assumed to be generated on the crests of the hot spots, probably by processes associated with volcanism, the model is consistent with almost 99% of the mapped Venus topography. The model is also basically consistent with available gravity data and interpretations which indicate compensated topography and great depths of compensation (100–1000 km) for the mid‐latitudes of the planet. The remaining approximately 1% of the topography not explained by hot spot crustal generation is thought to be compensated at a shallower depth primarily by variations in crustal thickness which are not directly related to hot spot volcanism. The models, relating surface elevation to lithospheric thickness and heat flow, are grossly consistent with either a plate tectonic or hot spot heat loss mechanism for Venus. The range of lithospheric thicknesses implied by the range of elevations on Venus implies that if plate tectonics is the dominant heat loss mechanism on Venus, the mean plate velocities are slower than on earth. Slow spreading ridges should be resolvable by the available Venus topographic data but are not apparent. Comparison and analysis of the hypsometric curves for earth and Venus suggest basic differences between the curves, the skewed terrestrial ocean floor curve being consistent with plate tectonics, but the relatively symmetrical venusian curve indicates that either plate tectonics differs on Venus from the terrestrial analogue or that plate tectonics does not dominate the topographic expression of Venus tectonism. The venusian curve is consistent with a hot spot heat loss mechanism. A hot spot crustal genesis mechanism is proposed which would probably result in crust very similar to the granite‐greenstone terranes of many terrestrial Archaean cratons. We speculate that hot spot heat loss may have preceded plate tectonics as the dominant terrestrial heat loss mechanism.
Varying geomagnetic anomalies and secular variation
Authors: Alldredge Leroy R.
Published: February 2008

The time variations of the geomagnetic field at 10 European observatories were examined in detail. Fourier analyses were made for three field components at each of the observatories. Curves for each component were synthesized for three passbands: A low passband for periods greater than 30 years, a mid passband for periods equal to or less than 30 years but greater than 13 years, and a high passband for periods equal to or less than 13 years. The low passband results show only very smooth variations from which the linear prediction of secular variation, as is now in use, might give a good average result for 10–15 years if there were no other changes. The high passband results show the solar cycle effect that has been adequately treated before. The mid passband has an amplitude of tens of nT which changes differently at each observatory in such a way as to infer core sources. These time‐varying anomalies are important to the understanding of secular variation.
Strength of mineral absorption features in the transmitted component of near‐infrared reflected light: First results from RELAB
Authors: Pieters Carlé M
Published: February 2008

Bidirectional reflectance measurements are the only type of reflectance data available to the remote observer. For compositional interpretations, data are desired not only for identification of possible mineral components but also for modal abundance. The latter requires detailed information about the strength of absorption features. Using a new laboratory facility, the RELAB, laboratory data in the near infrared are presented that document effects of particle size, mineral mixtures, and viewing geometry for selected materials with well‐developed absorption bands. The commonly observed increase in reflectance with decrease in particle size is also observed for absorption bands as well as a related decrease in absorption strength. For small particles in parts of the spectrum of maximum reflectance, however, a minor decrease in reflectance with a decrease in particle size is sometimes observed. Small particles dominate the observed characteristics of particulate surfaces, which contain a range of particle sizes. The mean optical path length (transmission through particles) of reflected radiation measured for a variety of particle sizes has an apparent upper limit of about 2 mm for particles <250 μm. The typical number of particles involved in the optical path is less than 50.
Seismicity, fault plane solutions, depth of faulting, and active tectonics of the Andes of Peru, Ecuador, and southern Colombia
Authors: Suárez Gerardo , Molnar Peter , Burchfiel B. Clark
Published: February 2008

The intracontinental seismicity of the Andes of Peru, Ecuador, and southern Colombia is concentrated along the easternmost flank of the Cordillera beneath the western margin of the sub‐Andes. The focal depths and fault plane solutions of the largest events were constrained by comparing the observed long‐period P waves with synthetic waveforms. In general, the fault plane solutions show reverse faulting on steeply dipping nodal planes striking northwest–southeast and reflect crustal shortening perpendicular to the range, probably in response to the subduction of the Nazca plate to the west. Earthquakes in the sub‐Andes occur at depths of between 8 and 38 km, indicating that much of the crust deforms in a brittle manner. The seismicity seems to reflect antithetic underthrusting of the Brazilian shield beneath the eastern margin of the Andes. However, the earthquakes are too deep and their nodal planes are too steep to be associated with the thin‐skinned decollement of the sedimentary cover. Instead, they appear to reflect the tectonic deformation of the underthrust basement west of the zone of active deformation at the surface in the sub‐Andes. Hence the earthquakes on the east side of the Andes may reflect the style of deformation in the hinterland of fold and thrust belts, such as in the Canadian Rockies, while much of the decollement farther east occurs aseismically. In contrast, normal faulting occurs in the high Andes on planes parallel to the strike of the mountain belt. Thus, there is a delicate balance between the compressive stress applied to the Andes in the direction of subduction, which causes thrust faulting in the sub‐Andes, and the gravitational body forces acting on the topographically higher parts and the crustal root of the Andes, which may cause normal faulting at high elevation.
An anomalous low velocity region above the deep earthquakes in the Japan Subduction Zone
Authors: Suyehiro Kiyoshi , Sacks I. Selwyn
Published: February 2008

P wave travel time residuals for local intermediate and deep focus earthquakes are studied to infer a velocity model of the asthenosphere beneath the southern Okhotsk Sea and the western Japan Sea. Tectonically, both regions belong to plate subduction zones. Large positive travel time anomalies are observed at stations above the deepest earthquakes. Consideration of source, path, and receiver effects leads to a conclusion that within the asthenosphere there is a lower velocity region below about 300 km depth with velocities about 5–10% below the Jeffreys‐Bullen velocity model.
The petrology of high‐MG dikes from the Beartooth Mountains, Montana: A search for the parent magma of the Stillwater Complex
Authors: Longhi John , Wooden Joseph L. , Coppinger Karyn D.
Published: February 2008

Six geochemically distinct groups of Precambrian high‐Mg dikes have been examined to determine if any could have been a sample of or closely related to the parent magma of the Stillwater Complex. Only two groups have sufficiently magnesian olivine and orthopyroxene, but neither of these has sufficiently calcic plagioclase. In terms of major elements there appear to be only two distinct magma types: one has a high orthopyroxene component and is petrographically similar to noritic dikes and sills near the Great Dyke of Zimbabwe and the Bushveld Complex of South Africa; the other is similar to primitive tholeiites with higher plagioclase and diopside components than the first. All of the groups are enriched in light‐REE and have unusually high K2O concentrations. Model calculations indicate that crustal contamination (30–50%) of fractionating komatiitic magmas is consistent with the major and trace element characteristics of the first magma type. Less extensive contamination of komatiitic magmas may produce the parental magmas of the large layered intrusions.
Chemistry of the Apollo 11 Highland Component
Authors: Laul J. C. , Papike J. J. , Simon S. B. , Shearer C. K.
Published: February 2008

Chemical data (28 major, minor, and trace elements) were obtained by instrumental neutron activation analysis (INAA) for 38 hand‐picked highland fragments (1–20 mg) from coarse fines 10085. The Apollo 11 highland chemical groups are represented by a range of petrographic types. Six major chemical highland groups are identified in the Apollo 11 fragments. These are (1) high‐K KREEP; (2) anorthosite (e.g., 15415, 60015) with 10× chondrite rare‐earth‐elements (REE) abundances, a positive Eu anomaly, anorthosite with 30× chondrite abundances, and a positive Eu anomaly; (3) anorthosite‐norite‐troctolite (ANT) (67075); (4) Low‐K Fra Mauro (LKFM) (77135); (5) anorthositic gabbro (68415) with no Eu anomaly, anorthositic gabbro (65055, 67955) with a positive Eu anomaly, and anorthositic gabbro (68516, 60015) with a negative Eu anomaly; and (6) Apollo 11 ‘dominant highland component,’ 2×–10× chondrite REE abundances with a positive 10×–14× chondrite Eu anomaly. There are three newly‐recognized groups in the Apollo 11 highland suite, based on the REE patterns. These are: (a) ANT with 5× chondrite La, 22× chondrite abundances, and a positive Eu anomaly; (b) 10× chondrite flat pattern with 14× chondrite positive Eu anomaly; and (c) 2–3× chondrite flat pattern with 10× chondrite positive Eu anomaly. The Apollo 11 highland suite is very low in potassium (non‐KREEPy). Overall, the Apollo 11 highland suite is quite similar to the Apollo 16 suite. Most previously recognized highland chemical groups are represented in the Apollo 11 highland component.
Spectral chemistry of green glass‐bearing 15426 regolith
Authors: Burns Roger G. , Dyar M. Darby
Published: February 2008

The detection of appreciable concentrations of ferric iron in a synthetic green glass equilibrated at an oxygen fugacity of 10−11 atm prompted a Mossbauer spectral study of pristine emerald‐green glass spherules carefully handpicked from regolith sample 15426. No Fe3+ ions were detected in this lunar sample or in a synthetic green glass simulant equilibrated at f0 2 = 10−14 atm, suggesting that the green glass clods in rock 15426 formed under conditions of correspondingly low oxygen fugacities. The Mossbauer spectra indicated the presence of olivine crystallites in the lunar emerald green glass spherules. Measurements of homogeneous and partially devitrified synthetic silicate glasses revealed that significant changes of coordination environment about Fe2+ ions in the glass structure occur during crystallization of olivine crystals from the melt.
Impact versus internal origins for mesosiderites
Authors: Hewins Roger H.
Published: February 2008

Mesosiderites are very intimately mixed stony‐iron breccias, including little recrystallized and highly recrystallized varieties (I‐III) and a subgroup (IV) with melted matrix. All mesosiderite breccias contained some material that was hot, as indicated by pigeonite that inverted at 1150°C. Subgroup IV contained silicate and metal liquids at temperatures of at least 1300°C and 1350°C, respectively. Cooling rates are not rigorously defined, but are approximately 1°/year in the high temperature range and 1°/m.y. below 500°C. The latter cooling requires a depth of 40 km in an asteroid with a radius of 400 km, or less if porous, low‐conductivity surface material is present. Cratering with metal projectiles appears incapable of generating mesosiderites: a normal impact produces rapidly cooled, metal‐poor breccias. Impacts on a crust in which metal‐silicate segregation had taken place could generate stony‐iron breccias, but burial under an insulating blanket would be necessary to obtain slow cooling. Disruption and reassembly of two colliding bodies and accretion of differentiated planetesimals could form stony‐iron breccias at depth but, as with the previous two models, the easiest way to explain the low olivine of mesosiderites is to postulate derivation from olivine‐poor chondritic material. Two internal processes, turbulent metal‐crust mixing during core formation and crustal blocks sinking into the core, provide a deep location and withhold olivine from the breccia. However, these mechanisms do not explain how the silicate and metal liquids in subgroup IV (otherwise interpreted as impact melt‐rocks) could have failed to segregate in a deep, slowly cooled environment. No single model for mesosiderite origins is wholly satisfactory.
Microirghizites recovered from a sediment sample from the Zhamanshin Impact Structure
Authors: Glass B. P. , Fredriksson K. , Florensky P. V.
Published: February 2008

Microscopic glass spherules, called microirghizites, have been recovered from a stream deposit in the Zhamanshin crater. The microirghizites partly overlap the irghizites (tektites) and SiO2‐rich zhamanshinites (impactites) in composition, but exhibit greater variations in composition and have lower average SiO2 contents. A similar relationship has been observed between microtektites and tektites in the Australasian, Ivory Coast, and North American strewn fields. The compositions and oxide trends observed for the microirghizites, SiO2‐rich zhamanshinites and irghizites are rather similar to those observed for the Australasian microtektites and tektites. The major differences are the Al2O3 and FeO versus SiO2 trends, which could be explained by differences in amount of meteoritic contamination. The irghizites and microirghizites contain higher nickel contents (up to 0.41%) than the SiO2‐rich zhamanshinites and Australasian tektites and microtektites. The major oxides versus silica trends for the microirghizites support a previous conclusion that the impacting body was a chondrite meteorite and the bulk parent material was terrestrial. Although the irghizites may have been more contaminated by the impacting body than were the SiO2‐rich zhamanshinites, we find that the major differences in composition between the irghizites and SiO2‐rich zhamanshinites may be due to different parent materials rather than meteoritic contamination.
The role of phosphorus in lunar samples—A chemical study
Authors: Jovanovic S. , Reed G. W.
Published: February 2008

Up to 60% of the REE, P, halogens, and U in basalt 75055 and basalt mineral separates, KREEP‐rich breccia 14312, and components from highland breccia 66095 are in phases that are rapidly dissolved by pHl HNO3 acid; these could be phosphates. The REE patterns of the pHl acid soluble fractions are distinctly different from those of the postleach residues. The REE patterns of the dilute acid soluble fractions in the breccia samples are similar. The basalt samples also have similar acid soluble REE patterns but different from the breccia samples. Significant fractions of P (25%), LREE (40%), and HREE (16%) in whole rock basalt are pHl acid soluble. Large fractions of P (47% and 61%) and small fractions of LREE (10%) associated with plagioclase and pyroxene mineral separates are acid soluble. Thus for this igneous rock, the acid soluble phases associated with the whole rock and with the major minerals may be different.
Decrease of the solar flare/solar wind flux ratio in the past several aeons deduced from solar neon and tracks in lunar soil plagioclases
Authors: Wieler R. , Etique Ph. , Signer P. , Poupeau G.
Published: February 2008

We determined the He, Ne, and Ar concentrations and isotopic compositions of mineral separates of six lunar subsurface samples and of two regolith breccias which were exposed to the sun as early as 2–3 b. y. ago. The results are compared with our noble gas data obtained previously on mineral separates of lunar surface soil samples most of which contain recently implanted solar gases. The mean solar flare track densities were determined on aliquots of several of the plagioclase separates analyzed for noble gases. Solar wind retentive mafic minerals and ilmenites show that a possible secular increase of the 20Ne/22Ne ratio in the solar wind during the last 2–3 Ga is <2%. In plagioclases, which are less retentive for solar wind Ne, a considerable fraction of the retained Ne is of solar flare origin. The data are compatible with values for 20Ne/22Ne of ∼11.3–11.8, reported for solar flare Ne retained in plagioclase separates from lunar soils. The solar flare track data and the Ne data independently show that plagioclases exposed to the sun over the last 108 years recorded a lower mean ratio of solar flare to solar wind intensities than samples exposed about 1–3 b.y. ago. On the basis of track data we estimate these ratios to differ by a factor ∼2.
Do oblique impacts produce Martian meteorites?
Authors: Nyquist L. E.
Published: February 2008

Geochronological and geochemical characteristics of several achondritic meteorites match those expected of Martian rocks. Several authors have suggested that these meteorites might have originated on Mars, but no satisfactory explanation has been given of how they may have been ejected from the Martian surface. It is suggested here that the oblique impact of large meteoroids may produce ejecta which is entrained with the ricocheting projectile and accelerated to velocities in excess of Martian escape velocity. This suggestion is based on earlier experimental studies of oblique impacts and on the observation of several large Martian craters with the characteristic ‘butterfly’ ejecta pattern produced by low angle impacts. Several acceleration mechanisms may act on the Martian ejecta. At high impact velocities, the ricocheting projectile should be vaporized and fluid dynamic drag should act on the entrained ejecta. The drag equation can be integrated for an idealized representation of ricochet and explosion of the projectile. It is shown that large ejecta fragments, on the order of 1–10% of the initial projectile radius, could be drag accelerated to velocities in excess of the Martian escape velocity. Fragments greater than or equal to about a meter in size will escape the Martian atmosphere if they are launched at velocities significantly in excess of the Martian escape velocity. Impacts capable of launching sizable ejecta are expected to occur at a maximum rate of about 4×10−8 yr−1 on Mars and at a comparable or lower rate on the moon. The long transit time between Mars and earth would lead to establishment of a steady state population of potential Martian meteorites. The much shorter transit time from the moon to earth would cause any lunar ejecta to arrive in spurts which are short in comparison to their repetition rate. Thus the absence of lunar meteorites from our collection could be a simple observational effect due to the short terrestrial lifetime of meteorites. These considerations are preliminary in the sense that several simplifications and assumptions are made. However, they suggest that a Martian origin of the shergottite meteorites is dynamically possible.