bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrothermal activity at the Trans‐Atlantic Geotraverse Hydrothermal Field, Mid‐Atlantic Ridge crest at 26°N
Authors: Rona P. A. , Thompson G. , Mottl M. J. , Karson J. A. , Jenkins W. J. , Graham D. , Mallette M. , Von Damm K. , Edmond J. M.
Published: February 2008

The first submersible observations of the only known active submarine hydrothermal field on a slow‐spreading oceanic ridge are incorporated with results of 10 prior years of investigation to derive an understanding of periodicity, duration, and varying intensity of hydrothermal activity at the Trans‐Atlantic Geotraverse (TAG) Hydrothermal Field on the Mid‐Atlantic Ridge crest near latitude 26°N. Hydrothermal activity has persisted at this location for at least 1×106 years based on the distribution of hydrothermal and hydrogenous mineralization with respect to crustal age. The hydrothermal activity has been cyclic, multistage, and episodic. Prior high‐temperature hydrothermal venting stages with a periodicity of the order of 1×104 years and duration of the order of 101 years are deduced from the estimated ages of discrete sedimentary layers anomalously enriched in Cu, Fe, and Zn and correspond with the independently determined periodicity of volcanic eruptive cycles on the Mid‐Atlantic Ridge. The most recent episode of high‐temperature venting is inferred to have ceased in the recent past based on metal enrichment (Cu, Fe, Zn) in the surficial sediment layer. Low‐temperature hydrothermal venting stages with a duration of the order of 1×104 years intervene between the short high‐temperature stages and produce stratiform deposits of layered and earthy manganese oxide, iron oxide, hydroxide, and silicate. Bivalve‐like forms with the characteristics of vent clams in various stages of dissolution are identified on bottom photographs. The fresh appearance of intact tubules composed of iron hydroxide that acted as vents on relict deposits, conductive heat flow anomalies in the sediment column, and the record of temperature anomalies and excess 3He in the near‐bottom water column, suggest that the low‐temperature hydrothermal discharge is intermittent at individual vents on a time scale of years.
Pristine highland clasts in Consortium Breccia 14305: Petrology and geochemistry
Authors: Shervais John W. , Taylor Lawrence A. , Laul J. C. , Smith M. R.
Published: February 2008

The Apollo 14 site continues to provide pristine non mare rocks that contrast sharply with those from Apollo 16, the archetypical highland site. We characterize six small (<1 cm) samples of pristine highland crust that occur as clasts in polymict lunar breccia 14305. One is an alkali anorthosite; the others include troctolite, norite, and anorthosite of the Mg‐suite. The alkali anorthosite (,400) is a plagioclase adcumulate with minor intercumulus augite. Cerian fluorapatite with LREE‐contents ≅30,000 x chondrite occurs as an inclusion in plagioclase. Enstatite troctolite, 358 and magnesian anorthosite, 361 have trace element characteristics similar to “eastern” Mg‐suite rocks, highlighting the importance of local variations in geochemical systematics that are superimposed on moon‐wide, longitudinal variations. The highly fractionated trace element abundances and highly magnesian mineral compositions of olivine orthopyroxenite, 389 suggest a new, previously unknown magma‐type related to the Mg‐suite. This magma may have formed in equilibrium with garnet. Alkali anorthosites and magnesian anorthosites‐troctolites cannot be related to a single parent magma by fractional crystallization or by variable KREEP assimilation. These suites represent separate parent magmas, each of which may have been modified by mixing with KREEP.
Carbon‐14 ages of Antarctic meteorites with accelerator and small‐volume counting techniques
Authors: Jull A. J. T. , Donahue D. J. , Zabel T. H. , Fireman E. L.
Published: February 2008

Carbon‐14 terrestrial ages of six Antarctic meteorites have been measured by accelerator mass spectrometry. These six and three additional Antarctic meteorites were also measured by small counters. The results are in general agreement, with some differences that are related to the calibration samples used. Yamato 75102 was found to be the youngest meteorite yet measured from the Antarctic collections, 3×103 yrs. The ages for the Yamato meteorites are younger than those from sites in the Allan Hills region. Accelerator data on Allan Hills (ALHA) samples, 77232 and 80101, show limit ages >48×103 yrs. Counter measurements on ALHA 77208, 77269, and 79025 give limit ages >38×103, >34×103, and >3 ×103 yrs, respectively. Accelerator measurements on MBRA 76001, a sample previously measured by counting, gives a finite age of 39±2×103 yrs. Tne younger ages of meteorites of the Yamato region are suggested to be the result of the larger recovery area and the more “porous” barrier at the Yamato Mountains.
Lu‐Hf constraints on the evolution of lunar basalts
Authors: Fujimaki Hirokazu , Tatsumoto Mitsunobu
Published: February 2008

Very low Ti basalts and green glass samples from the moon show high Lu/Hf ratios and low Hf concentrations. Low‐Ti lunar basalts show high and variable Lu/Hf ratios and higher Hf concentrations, whereas high‐Ti lunar basalts show low Lu/Hf ratios and high Hf concentrations. KREEP basalts have constant Lu/Hf ratios and high but variable Hf concentrations. Using the Lu‐Hf behavior as a constraint, we propose a model for the mare basalts evolution. This constraint requires extensive crystallization of the primary lunar magma ocean prior to formation of the lunar mare basalt sources and the KREEP basalts. Mare basalts are produced by the melting of the cumulate rocks, and KREEP basalts represent the residual liquid of the magma ocean.

Lu and Hf concentrations and the Hf isotopic data of lunar rocks suggest that assimilation cannot be accepted as a major process to explain the diversity of the lunar mare basalts. The urKREEP hypothesis is also unnecessary. Both high‐ and low‐Ti basalts show enough iron enrichment to be regarded as melting products of the last stage cumulate rocks from the lunar magma ocean. The KREEP basalts are also rich in iron and may be regarded as the final, residual liquid left after the crystallization of the major portion of the primary lunar magma ocean.
The oblique impact hypothesis and relative probabilities of lunar and Martian meteorites
Authors: Nyquist L. E.
Published: February 2008

The author has previously suggested that shergottites and possibly other SNC meteorites were launched from the Martian surface by the oblique impact of large meteoroids. This hypothesis is further evaluated in the context of the subsequent discovery of a lunar meteorite. The oblique impact hypothesis is consistent with observations concerning the lunar and putative Martian meteorites. Oblique impacts are comparatively rare, consistent with the observation that ejecta from only one lunar event and one or two putative Martian events have been recognized. The hypothesis is also consistent with the observation of ejecta from a volatile‐poor object, the moon, as well as the probable existence of ejecta from a more volatile‐enriched object, Mars. Furthermore, in spite of the comparative rarity of oblique impacts, there exists at least one candidate source crater for the lunar meteorite in the lunar highlands adjacent to a source of the VLT basalt fragment discovered in it. There also exist at least two candidate source craters on Martian terrain of appropriate age to be the source terrain of the SNC meteorites. The candidate source craters are of appropriate size to be representative of events that have contributed to the current inventory of a continuous background of space stored lunar and Martian ejecta. Finally, it is shown that within calculational uncertainties and the current poorly known fall statistics of lunar and probable Martian meteorites, the oblique impact hypothesis gives a satisfactory explanation of the relative abundances of lunar, Martian, and other meteorites. These results support the oblique‐impact‐on‐Mars origin of SNC meteorites and give a satisfactory explanation for the occurrence of a lunar meteorite.
Support of long‐wavelength loads on Venus and implications for internal structure
Authors: Banerdt W. B.
Published: February 2008

A thin‐shell formulation for computing stress distributions on the surfaces of one‐plate planets is developed. The lithospheric model includes laterally varying density anomalies at two depths, corresponding to undulations on a crust‐mantle boundary and variations in upper mantle density. For a given set of model parameters, values for the vertical displacement, crustal thickness anomaly, and mantle density anomaly required to satisfy the topographic and gravitational boundary conditions are computed, along with the resulting stress field. The computed stresses can then be compared to stress directions inferred from observed tectonic features. This formulation is used to study loads on Venus's lithosphere, and it is found that most highland areas appear to have been formed in a state of isostasy or by uplift, with a significant portion of their support probably derived from dynamic processes in the mantle. A possible exception is Ishtar Terra, whose circumferential mountain belts are most consistent with a radially compressive stress regime generated by flexural loading. Reasonable lithospheric models predict low‐density mantle beneath Atla, Beta, Eisila, Thetis, and Tethus Regiones and thick crust accompanied by higher‐density mantle under Ishtar Terra and Ovda Regio. Thus western Aphrodite Terra appears to be more closely related to Ishtar than to the rest of the equatorial highlands. These results favor a hot spot model of Venus tectonics in which Beta, eastern Aphrodite, Eisila, and Tethus are expected to be relatively young and thermally active, while Ovda and Ishtar should be older and less active.
Velocity determination from offset VSP data: Tests of the method
Authors: Pujol Jose , Burridge Robert , Smithson Scott B.
Published: February 2008

A previously reported iterative generalized inversion to determine interval velocities from vertical seismic profiling (VSP) data is tested with respect to the uniqueness of the solution and the selection of the optimal value for a damping parameter. The model used here consists of flat, horizontal layers. For error‐free data the solution for the velocities of any two adjacent layers is definitely nonunique if there is no travel time information for those layers. For these layers, the solution depends strongly on the initial estimates. If the same initial values are used for several adjacent layers without travel times, the final velocities differ from the initial ones by a constant amount and give an approximate rms velocity. The effect of errors is analyzed in connection with the optimal value for the damping parameter λ. During the initial stages of the inversion the effect of λ is to assure the convergence of an iterative solution to a nonlinear problem. As the iterations proceed, the value of λ is decreased, and at a certain stage the problem may be considered linear. At this point the selection of the optimal value of λ becomes critical because it determines the effect of errors in the final velocity structure. When errors are added to synthetic travel times, the effect of λ is seen clearly: a value of λ that is too small introduces large errors in the velocities; a value of λ that is too large reduces the resolution of the velocity structure. In general, there is no objective way to determine the optimal λ that will minimize the effect of errors on velocities. However, the differences between synthetic times and times computed for the inversion velocities remain almost the same for λ below a certain threshold. This gives an indication of the best value of λ consistent with the observations when they are contaminated by errors. When dealing with actual data there are two additional pieces of evidence that can be used to determine the optimal λ: the velocities obtained from the sonic logs, and the comparison between the actual VSP and the synthetic VSPs computed for different values of λ. The optimal λ should be the one that gives the best agreement between actual and synthetic VSPs. Both the location of the reflections and their relative amplitudes should be taken into account in the comparison.
Sediment accretion, subduction, and dewatering at the base of the trench slope off Costa Rica: A seismic reflection view of the décollement
Authors: Shipley Thomas H. , Moore Gregory F.
Published: February 2008

Sea Beam bathymetric and water gun seismic reflection data off the west coast of Costa Rica image the base of the trench slope, its underlying structure, and the behavior of a décollement surface in the presence of relief on the subducting plate. A low‐angle, landward‐dipping reflection marks the master décollement, above which only about 80 m of a 350 m oceanic sedimentary section is offscraped and accreted to the lower trench slope. A small horst block at least 9 km long, 1.5 km wide, and 300 m high is being obliquely subducted beneath the landward trench slope. On five seismic lines the décollement rises over the horst and then descends into the next incoming graben as the fault zone maintains a specific stratigraphic level. The undulating geometry of the décollement implies that the subducting grabens are not sites for sediment entrapment and subduction and that the horsts are not causing tectonic erosion of the landward trench wall. Substantial dewatering within the subducting sediments is documented within about 4 km of the deformation front. The dewatering may favor development of bedding‐parallel zones of low shear strength within the subducting sediments and the master décollement may seek one of these zones.
Crystallization and compositional convection in a porous medium with application to layered igneous intrusions
Authors: Kerr Ross C. , Tait Stephen R.
Published: February 2008

Growing crystals can provide a local source of buoyancy by depleting melt of certain chemical components, and vigorous convective motions in the fluid can result. We report a quantitative experimental study, in which saturated aqueous solutions of CuSO4 and Na2SO4 in porous media of either glass balls or CuSO4•5H2O crystals were crystallized by cooling from below. The crystals growing in the pore space released light, chemically depleted solution which had the potential to convect out of the porous medium and be replaced by additional saturated solution from above. We explore the physical and chemical effects of compositional convection during the complete solidification of the interstitial solution by varying the permeability of the porous medium, the cooling rate, and the initial composition of the solution. This type of convection can form adcumulate rocks by enabling continuous circulation of melt from the main magma reservoir through the pores of a cumulus pile. Adcumulus growth can occur by this mechanism as long as the convective velocity in the porous medium is greater than the velocity of the solidification front. We scale the experimental results to magmatic conditions and infer that this process should occur commonly in layered igneous intrusions. When our porous media had a small permeability, compositional convection enabled the adcumulate growth of a completely solid contact between melt and cumulate. Trapping of dynamically unstable melt beneath this impermeable layer of crystals also occurred in these experiments and may be important geologically. Small, shallowly emplaced intrusions and the cumulate zones at the base of komatiites are probably cooled too rapidly for significant intercumulus convection to occur.
Gabbroic xenoliths and host ferrobasalt from the southern Juan de Fuca Ridge
Authors: Dixon Jacqueline Eaby , Clague David A. , Eissen Jean‐Philippe
Published: February 2008

Rare isotropic gabbroic xenoliths occur in sheet and lobate flow fragments of nearly aphyric ferrobasalt collected along a 12‐km section of the southern Juan de Fuca Ridge. Xenoliths comprise ≪ 1% of the dredge contents and range in size from 1 cm3 (glomerocryst) to 240 cm3. The xenoliths have ophitic to intersertal texture with 5–50% interstitial glass of ferrobasaltic composition more evolved than the host lava. On the basis of texture and mineralogy, the xenoliths have been subdivided into three types: type I, plagioclase + olivine + glass; type II, plagioclase + augite + glass ± olivine; and type III, plagioclase + augite + olivine + glass ± pigeonite (partially inverted) + Fe‐Ti oxides. Mineral and glass inclusion compositions suggest a sequence of evolution for the three xenolith types in which type I is the least evolved and type III is the most evolved. Application of a graphical pyroxene geothermometer to augite in xenolith types II and III yields crystallization temperatures of 1100°–1200°C and to host‐lamellae pairs in inverted pigeonite yields subsolidus equilibrium temperatures of 1100°–1150°C. Coexisting titanomagnetite‐ilmenite pairs in type III xenoliths yield temperature estimates of 1000°–1070°C and log f02 = −9.7 to −10.8. We infer that the xenoliths represent the partially crystalline “mush” boundary zone of a magma chamber based on the abundance of interstitial glass, zonation of mineral grains in the most crystalline samples, and coherence of chemical trends between interstitial glass, glass inclusions, and mineral phases. The evolved composition of the xenoliths provides evidence for the presence of melts more fractionated than the host ferrobasalt in the magma chamber. The erupted ferrobasalt is a hybrid lava formed by mixing these highly evolved melts with more primitive melts.
Plastic anisotropy and texture development in calcite polycrystals
Authors: Wenk H. R. , Takeshita Toru , Van Houtte P. , Wagner F.
Published: February 2008

In the past, most information on the plasticity of rocks has been derived from axisymmetric compression experiments. These data are the basis for Arrhenius‐type flow laws which relate stress and strain, treating them essentially as scalars and not accounting for the deformation history. This assumption is valid for a very limited range of mechanisms, for example, superplastic flow, dislocation climb, and some mechanisms of recrystallization. Wherever dislocation glide is involved in the deformation process, preferred orientation develops which depends on the strain mode. The development of texture has a profound influence on the stress‐strain curve: (1) a geometric factor due to crystal orientation and expressed by the Taylor factor, (2) strain hardening due to interaction of dislocations in active slip systems, and (3) latent hardening because of microstructural difficulties in activating inactive slip systems at expected critical shear stresses. We have analyzed the influence of geometric factors on the plasticity of calcite polycrystals using the full constraint Taylor theory and observed that effects of strain mode are even stronger than in metals where the anisotropy of plastic flow was first documented. For example, at low temperature in axial deformation the specimen hardens rabidly, whereas in plane strain it softens during straining. The differences can be attributed to activation of different slip systems (number and relative importance) in each case, particularly the role of mechanical twinning. Wherever dislocation glide is significant, anisotropic flow laws should be considered, also in geological materials, to explain deformation. Particularly emphasized is the importance of deformation experiments in geometries different from axisymmetric compression.
Zirconium, hafnium, and rare earth element partition coefficients for ilmenite and other minerals in high‐Ti lunar mare basalts: An experimental study
Authors: McKay G. , Wagstaff J. , Yang S‐R.
Published: February 2008

Partition coefficients were determined for Gd, Lu, Hf, and Zr among ilmenite, armalcolite, and synthetic high‐Ti mare basaltic melts at temperatures from 1122° to 1150°C, and at oxygen fugacities of IW × 100.5, by in situ analysis with the electron microprobe, using samples doped to percent concentration levels. Coefficients for Zr were also measured for samples containing 600–1600 ppm Zr, using a special high‐sensitivity electron microprobe technique. In addition, coefficients were determined for Hf and Zr between chromian ulvospinel and melt, for Hf between pigeonite and melt, and for Lu between olivine and melt by microprobe analysis of samples doped to percent levels. Recommended values for ilmenite are D(Gd) = 0.006, D(Lu) = 0.09, D(Hf) = 0.42, and D(Zr) = 0.33. Recommended values for armalcolite are D(Gd) = 0.006, D(Lu) = 0.046, D(Hf) = 1.5, and D(Zr) = 0.9. Values measured using the microprobe are in general agreement with values measured by analyzing mineral separates from the sanle run products by isotope dilution at the USGS Isotope Lab in Denver. Agreement between microprobe and ID is within 3% for Hf, and within 10% for Zr. For Lu, the ID results were 15–35% lower than the microprobe values. The source of this disagreement has not been identified. Parition coefficients measured in this study and the collaborative ID study confirm the general results of earlier measurements. D values for ilmenite are less than 0.01 for the LREE (light rare earth element), are around 0.1 for the HREE (heavy rare earth elements), and are several times greater than this for Zr and Hf. These results constitute the first simultaneous experimental measurement of ilmenite distribution coefficients for the three important elements Lu, Hr, arid Zr in mare basaltic systems.
A potpourri of pristine Moon rocks, including a VHK mare basalt and a unique, augite‐rich Apollo 17 anorthosite
Authors: Warren Paul H. , Shirley David N. , Kallemeyn Gregory W.
Published: February 2008

Analysis of previously unstudied Apollo lithic fragments continues to yield surprising results. Among this year's samples is a small anorthosite fragment, 76504,18, the first pristine anorthosit e found from Apollo 17. This unique lithology strongly resembles the main type of Apollo anorthosites (ferroan anorthosites), but 76504, 18 has a far higher ratio (about 9) of high‐Ca pyroxene to low‐Ca pyroxene, higher Na in its plagioclase, higher contents of incompatible elements such as REE, and a higher Eu/Al ratio. Assuming that 76504, 18 is a cumulate with less than 45% trapped liquid, its parent melt probably had a negative Eu anomaly. In all these respects, 76504,18 seems more likely than (other) ferroan anorthosites to be closely related to typical mare basalts. Apparently this anorthosite was among the latest to form by plagioclase flotation above a primordial magmasphere; typical mare basalt source regions probably accumulated at about the same time or even earlier. Another previously unstudied fragment, 14181c, is a VHK (very high potassium) basalt that is similar in most respects to typical (“aluminous”) Apollo 14 mare basalt but has a K/La ratio of 1050. This lithology probably formed after a normal Apollo 14 mare basaltic melt partially assimilated granite. New data for siderophile elements in Apollo 12 mare basalts indicate that only the lowest of earlier data are trustworthy as being free of laboratory contamination.
Hydrostatic compression of Fe1‐xO Wüstite
Authors: Jeanloz Raymond , Sato‐Sorensen Yosiko
Published: February 2008

Hydrostatic compression measurements on Fe0.95 wüstite up to 12 GPa yield a room temperature value for the isothermal bulk modulus of K0T = 157 (±10) GPa at zero pressure. This result is in accord with previous hydrostatic and nonhydrostatic measurements of K0T for wüstites of composition: 0.89 ≤ Fe/O ≤ 0.95. For comparison, dynamic measurements of the bulk modulus by ultrasonic, shock wave and neutron‐scattering experiments have yielded values of K0T between 140 and 185 GPa. Both high‐precision compression data and simple models of the effects of defects on elastic constants indicate that the bulk modulus of wüstite depends little on composition. Systematics among the bulk moduli of transition metal monoxides can be understood in terms of a crystal field model, although crystal field contributions to the individual elastic moduli of wüstite are relatively small.
Static compression of iron to 78 GPa with rare gas solids as pressure‐transmitting media
Authors: Jephcoat Andrew P. , Mao H. K. , Bell Peter M.
Published: February 2008

New data are presented for the room temperature, static compression of iron to 78 GPa with solid neon and argon as pressure‐transmitting media. X ray diffraction studies have been performed on a geophysically relevant material, for the first time to such pressures under quasihydrostatic conditions, in a diamond anvil cell. The hydrostatic technique leads to increased precision in the measurement of high pressures and has placed closer constraints on the equation of state of ε iron. From a linear least squares fit of a finite strain equation of state to the present data combined with earlier, low‐pressure data for metastable ε iron, the preferred values for the zero‐pressure isothermal bulk modulus, K0, and first pressure derivative, K0′, are 192.7 (±9.0) GPa and 4.29 (±0.36), respectively. The zero‐pressure volume for the ε phase is 6.687 (±0.018) cm3/mol. On the basis of the pressure‐volume curve calculated from fits of the finite strain equation of state, ε iron appears to be less compressible under nonhydrostatic conditions, but the differences are within the error of the nonhydrostatic experiment. The results also confirm that the absence of a soft medium in static compression experiments with the diamond anvil cell results in an overestimate of the unit cell volume (measured with the incident X ray beam parallel to the load axis) for pressures calculated with the nonhydrostatic ruby calibration scale. It is found that for ε iron, substantial compensation for this nonhydrostatic effect is implicit in the nonhydrostatic ruby pressure scale up to intermediate strains. The hydrostatic data and the ε iron isotherm derived from shock wave experiments on iron samples are in very close agreement.
Isotopic and trace element constraints on the petrogenesis of subduction‐related lavas from Martinique, Lesser Antilles
Authors: Davidson J. P.
Published: February 2008

The Sr, Nd, and Pb isotopic compositions of orogenic lavas from Martinique span considerable ranges, from values close to those of mid‐ocean ridge basalts to compositions more characteristic of continental crust. The isotopic compositions, together with rare earth element (REE) and other trace element distributions, are used to characterize a number of distinct volcano‐stratigraphic “centers” on Martinique within which magma evolution is largely controlled by fractional crystallization processes. The range in REE distributions, incompatible trace element ratios, and isotopic compositions between centers on Martinique is, however, not compatible with an origin purely through fractional crystallization. In general, La/Yb ratios increase with decreasing 143Nd/144Nd. Similarly, K/Rb ratios, while decreasing only slightly within individual centers, vary by an order of magnitude for all Martinique volcanics and correlate well with isotopic ratios. These geochemical and isotopic features are thought to reflect contamination by a continentally derived sediment or melt thereof. The behavior of oxygen isotopes relative to radiogenic isotopes cannot be reconciled with a model in which subducted sediments alone are involved, and significant contamination of ascending magmas by terrigenous sediments intercalated within the arc crust is thought to occur. A model is proposed whereby parental magmas are derived from a subduction‐modified mantle wedge. During ascent, derivative magmas assimilate continentally derived sediments intercalated in the arc crust. This process increases δ18O and 87Sr/86Sr, and decreases 143Nd/144Nd. The proportion of assimilated intracrustal sediment involved in producing the most contaminated compositions may be more than 50%.
A “no‐lid” zone in the central Chang‐Thang platform of Tibet: Evidence from pure path phase velocity measurements of long period Rayleigh waves
Authors: Brandon Catherine , Romanowicz Barbara
Published: February 2008

Pure path phase velocities of fundamental mode Rayleigh waves, in the period range 30–80s, are obtained for different paths across the Tibet Plateau, using a “two‐event” method, with events located in Tibet and aligned with World‐Wide Standard Seismograph Network stations at teleseismic distance. Evidence is found for significant lateral variations of structure of the crust and uppermost mantle in Tibet. In particular, phase velocities obtained on paths contained in the Chang Thang platform are higher than average in Tibet, at periods shorter than 60s, while remaining very low at longer periods. When interpreting these phase velocities in terms of simple layered S velocity models, it is found that, contrary to other areas in Tibet, they are incompatible with the existence of a high velocity lid. S velocities of 4.4–4.5 km/s are found immediately beneath the Moho, and the crust is somewhat thinner (50–60 km) than average in Tibet. This indicates a possible upwelling of hot asthenospheric material, compatible with Landsat evidence for subrecent basaltic volcanism, and independent seismic evidence such as inefficient transmission of Snwaves. On the other hand, a 70 km thick crust and very high upper mantle S velocities are found on the western border of Tibet (Karakorum, Hindu Kush) compatible with previous finer‐scale results concerning the associated zone of intermediate depth seismicity.
Attenuation and scattering of broadband P and S waves across North America
Authors: Taylor Steven R. , Bonner Brian P. , Zandt George
Published: February 2008

Differential attenuation (δt*) measurements of teleseismic P and S waves are made in the frequency range 0.05–5.0 Hz across North America using broadband seismic data from the Department of Energy's Regional Seismic Test Network and Lawrence Livermore National Laboratory's Basin and Range network. Data from four events have been analyzed by first assuming frequency independent Q. The observed regional variations for (δt*) are consistent with previous studies and show greater attenuation for stations in the Basin and Range relative to those in shield regions of North America. A significant component of apparent P attenuation appears at frequencies above about 0.5 Hz in both the Basin and Range and shield which reduces the ts*/tp* ratio to values possibly as low as 2. The second part of the analysis involved an extension to an absorption band attenuation model for both P and S waves. A linearized inversion for δt*(f) is then performed on the averaged spectral ratios for the Basin and Range and shield. Both S and P attenuation decrease with increasing frequency in both provinces, but the frequency dependence of Qs−1 is more pronounced. Attenuation is greater for the Basin and Range, and the data suggest enhanced S attenuation between about 0.2 and 1.4 Hz relative to the shield. The ratio of Qs−1/Qp−1(Qp/Qs) decreases from ∼2.0 at 0.05 Hz to less than 1.0 at frequencies greater than 1 Hz with similar effects in both provinces. Comparisons of our observations with different types of geophysical data lead us to conclude that dissipative mechanisms involving partial melt may be acting beneath the Basin and Range. The decrease in Qs−1/Qp−1 at high frequencies is difficult to explain on both the Basin and Range and shield using intrinsic absorption mechanisms. The broadband data set used in this study appears to span the frequency band where intrinsic attenuation mechanisms evident at low frequencies are dominated by effects due to scattering at higher frequencies. If P and S waves interact with the same heterogeneities, the frequency where the attenuation due to scattering will be strongest is proportional to the inverse of the wavelength (i.e., where ka ∼ 1, where k is the wave number and a is the scale length of heterogeneity). Thus, in the presence of scattering, a decrease in the apparent S to P attenuation should be observed at higher frequencies, which is consistent with our observations.
Geodetic measurement of horizontal deformation in the northern San Francisco Bay region, California
Authors: Prescott William H. , Yu Shui‐Beih
Published: February 2008

Analysis of geodolite observations in the coast ranges of northern California indicates that relative plate motion is spread over a wide zone. Little or no deformation occurs to the southwest of the San Andreas Fault itself. To the northeast of the San Andreas fault, a region transected by the parallel Rodgers Creek and Maacama faults, 25 ± 6 mm/yr of right‐lateral slip is distributed over about 60 km. There is a peak shear strain of 0.6 ± 0.1 μrad/yr near the San Andreas fault. Elsewhere in the deforming zone, shear strain rates are 0.3 ± 0.04 μrad/yr. At the northeastern boundary of the study area the strain rate is less than 0.2 μrad/yr. Except for the high strain rate near the San Andreas fault, there is no evidence of any localization of deformation to the surface traces of the faults, nor is there any evidence of creep or aseismic slip on any of the faults in the area. The data indicate that the relative motion between the Pacific and North American plate occurs primarily to the northeast of the San Andreas fault. The observations are inconsistent with models that include slip at depth on only the San Andreas fault. The data can be fit equally well by models with slip at depth on all three of the major faults or with motion distributed across the entire zone from the San Andreas fault to the Napa Valley fault. Preliminary analysis of observations to the Farallon Islands, about 40 km southwest of the San Andreas fault, suggests that little or no additional relative motion is accommodated on that side of the San Andreas fault. In particular, it is unlikely that the 20‐mm/yr difference between the plate motion rate (about 55 mm/yr) and the San Andreas fault system rate (about 35 mm/yr) can be explained by deformation between the San Andreas fault and the Farallon Islands.
Empirical reflection travel time versus depth and velocity versus depth functions for the deep‐sea sediment column
Authors: Carlson R. L. , Gangi A. F. , Snow K. R.
Published: February 2008

Numerous correlations of one‐way reflection travel times and depths to reflecting horizons have been proposed in the Initial Reports of the Deep Sea Drilling Project (DSDP). Having carefully reviewed proposed correlations, we selected 233 correlated depths (ranging from 27 m to 1.4 km) and one‐way travel times (ranging from 0.02 to 0.7 s) from 154 DSDP sites worldwide. Assuming that the velocity/depth function for deep‐sea sediments can be approximated by an exponential form, we have used these data to establish statistically well‐constrained global models relating depth to travel time and velocity to depth. There is remarkably little scatter in the data. Based on nonlinear least squares fits, the best depth/time function is Z (km) = −3.03 1n[1 − 0.52T(s)]; the rms error in depth is 26 m. The corresponding average velocity/depth function is V(km/s) = 1.59exp(0.33Z). We estimate that the real site‐to‐site variability of velocities at depth is less than 0.20 km/s. Any dependence of velocity on composition is below the resolution of the data, and available evidence suggests that age has no significant effect. These results imply that to a depth of 1.4 km the physical state of sediments depends on in situ overburden pressure and temperature, which are related to depth of burial, and that chemical and mechanical equilibrium is achieved in a period of time that is geologically short. Our results are consistent with laboratory measurements of P wave velocities in deep‐sea sediments, but velocity/depth functions for terrigenous and calcareous sections derived from sonobuoy wide‐angle reflection surveys are incompatible with the time and depth data, and overestimate velocities by as much as 10% and 30%, respectively. These systematic errors may result from the fact that the marked anisotropy of deep‐sea sediments is neglected in the interpretation of reflection data.
Data requirements for determining precise relative geoid heights from gravimetry
Authors: Kearsley A. H. W.
Published: February 2008

Global Positioning System (GPS)‐based receivers appear capable of obtaining Δh, the ellipsoidal height differences, to about 2–4 ppm of the distance separating the end points. The historical record of vertical crustal motion has been obtained from spirit leveling and therefore gives changes in elevation in terms of orthometric height. For GPS‐based height differences to be useful in extending the record of height change, the geoid‐ellipsoid separation ΔNneeds to be evaluated. But is it possible to find ΔNgravimetrically to the precision to which Δhcan be measured? This analysis shows that the inner zone contribution to ΔNcould be more precise than is Δhfrom GPS. This contribution to ΔNshould be better than ±5 cm over 100 km if the random errors in the 10 km by 10 km mean gravity anomalies do not exceed ±3 mGal and provided the gravity and elevation data used to obtain the gravity means are connected onto the national gravity and (at least) third‐order leveling networks, respectively. For lines of mean elevation greater than 2 km the error in ΔNwill exceed specifications unless the density of the subsurface material can be estimated to better than ±200 kg m−3. Results of other tests suggest that the remote zone contribution to ΔN(ψ > 2.0°) is adequately represented by high degree (nmax≅ 180) geopotential models. A small test with the suggested method, performed in the White Sands Test Area, appears to confirm the above data specifications and supports the conclusions of two other investigations that ΔNcan be evaluated from gravimetry to precisions which match that of Δhfrom GPS, at least over shorter lines.
Buoyancy‐driven instabilities of low‐viscosity zones as models of magma‐rich zones
Authors: Whitehead J. A.
Published: February 2008

The low Reynolds number dynamics of a thin layer of fluid bounded below by a flat horizontal boundary and above by a fluid of another viscosity and greater density is reported. Three distinct stages of growth were observed. The first stage is a Rayleigh‐Taylor instability, in which disturbances of one specific wave number grow most rapidly. If ε is the ratio of the viscosity of the thick layer to the viscosity of the thin layer, fastest growth is for wave number ε−⅓. In the second stage, distortion of the interface is large, and it is found experimentally that the fluid moves out of the thick layer as circular columns surrounded by relatively broad regions of descending material. In the third stage, fully matured structures are formed. If the upwelling material has less viscosity than the surrounding material, the structure develops a rim syncline and a pronounced overhang and eventually ascends as a spherical pocket of fluid fed by a conduit. Two applications to geophysics are given: The first application follows from the fact that a melt source must exist in the mantle below mid‐ocean ridges. This source can be approximated as a cylindrical body with lower viscosity and density compared to the overlying mantle. If the cylinder develops a gravitational instability, it will develop regularly spaced vertical protrusions. Estimates of the spacing are compared to high‐resolution segmentation data, and some constraints on viscosity of the mantle below spreading centers are made. These are that viscosity of the mantle is 1018±1 P, and the ratio of this viscosity to the viscosity of the cylinder is less than 100. In the second application, the upwelling conduits are measured experimentally and solitary waves are observed. A recently found analog with magma rising up through the pores of a viscous crystalline matrix is discussed.
Rheology and structure of olivine‐basalt partial melts
Authors: Cooper R. F. , Kohlstedt D. L.
Published: February 2008

Recent experiments demonstrate that solution‐precipitation (pressure‐solution) processes in the liquid basalt‐olivine system are exceedingly rapid. The effect of liquid basalt on the diffusional (Newtonian) creep rate of polycrystalline olivine is significant; fine‐grained, chemically and texturally equilibrated, partially molten assemblages of olivine plus basalt deform at rates which are a factor of 2–5 faster than polycrystalline olivine specimens without the liquid basalt second phase. Measured values of the activation energy for creep suggest that while the kinetics of deformation in this partially molten system are affected by short‐circuit diffusion through melt‐filled triple junctions, the deformation process is rate‐limited by matter transport through melt‐free grain boundaries. A simple geometrical model, based on a dynamic equilibrium partial‐melt morphology which consists of melt‐free olivine grain boundaries coupled with an interconnected melt network along triple junction channels, adequately describes our deformation results. The extent to which the liquid phase penetrates from the triple junctions into the grain boundaries of texturally equilibrated partial melts determines the contribution of solution‐precipitation mechanisms to the creep rate; the critical physical parameters, therefore, are those which affect the solid‐liquid and solid‐solid interfacial energies, such as various composition‐related chemical activities in the liquid phase and the composition and spatial distribution of mineral phases in the crystal residuum.
Melting of pyrope Mg3Al2Si3O12 up to 10 GPa: Possibility of a pressure‐induced structural change in pyrope melt
Authors: Irifune Tetsuo , Ohtani Eiji
Published: February 2008

The melting of pyrope Mg3Al2Si3O12 has been determined experimentally at pressures up to 10 GPa. Pyrope melts congruently in the pressure range 3.5–10 GPa. At pressures below 7 GPa, aluminous enstatite of pyrope composition crystallizes during quenching, whereas quench pyrope appears at higher pressures. Moreover, the slope of the melting curve of pyrope decreases significantly at pressures above 7 GPa. These changes in both the nature of the quench phase and the slope of the melting curve suggest a pressure‐induced coordination change for Al3+ in pyrope melt at around 7 GPa. This pressure‐induced coordination change of Al3+ probably occurs even in magmatic liquids generated in the deep upper mantle and should affect some of their physical properties.
Discrimination among tectonic settings using trace element abundances of basalts
Authors: Butler John C. , Woronow Alex
Published: February 2008

A wealth of chemical information may provide a false sense of security to the petrologist interested in extracting petrogenetic information from a set of major and/or trace element abundances. Such data are said to be closed, as each analysis sums to a constant (100%). Failure to sum to a constant results from a combination of errors of commission and omission. A correlation coefficient between a pair of closed components has a contribution from the linear association between the components and a contribution from the effects of closure itself. Standard statistical procedures cannot separate these two sources; therefore, the investigator cannot tell if a strong correlation (as revealed by a nearly linear trend on a binary scatter diagram, for example, is due to a strong linear association between the components or due to closure. Techniques developed by Aitchison (1984a, b) appear to be capable of providing a framework within which the user can begin to assess the relationship among closed components. These techniques are applied to a set of 35 TiO2, Zr, Y, and Sr analyses of basalts. A statistical analysis permits rejection of Aitchison's (1984a, b) hypothesis of complete subcompositional independence, indicating that there is a degree of dependency within the data set. The first two principal components extracted from the covariance matrix of the log‐centered form of these data account for more than 90% of the total variation, and three major tectonic‐related fields can be clearly recognized in the space defined by the first two principal components: ocean floor basalts, within‐plate basalts and arc‐related basalts. Analyses from six additional data sets taken from the literature were plotted on this diagram, and all reclaim the tectonic settings stated in the literature. Simple modeling reveals that the addition or subtraction of Sr from an analysis results in a linear locus of points which is parallel to the boundary between the ocean floor and the within‐plate basalts, suggesting that such a diagram may prove useful in the analysis of basalts that have been subjected to low‐ to moderate‐grade metamorphism/metasomatism.
Tectonic processes at the collision of the D'Entrecasteaux Zone and the New Hebrides Island Arc
Authors: Fisher Michael A.
Published: February 2008

The d'Entrecasteaux zone (DEZ), a large submarine mountain chain composed of two east trending, subparallel ridges, collides with the north trending New Hebrides island arc. The direction of convergence of the oceanic plate and arc suggests that the DEZ is being subducted at a slightly oblique angle (10°–15°), so that the DEZ creeps slowly north, parallel to the trench. One possible consequence of this northward creep is that slope strata south of the DEZ should bear a structural overprint caused by ridge collision, whereas strata north of the DEZ may show no such overprint. Migrated multichannel seismic data presented in this report reveal the probable form of this overprint: Within the collision zone, strata forming the arc's accretionary wedge dip toward the trench, possibly recording progradation of slope rocks across the ocean floor, and neither outer arc ridge nor slope basin is present. Seismic data from south of the DEZ, however, show no evidence for this overprint because both an outer arc ridge and slope basin are present and the structure of slope strata on opposite sides of the DEZ appears broadly similar, involving a nearly invariant stratal dip toward the arc. Nevertheless, the slope's morphology shows that collision has modified the structure of rocks below the arc's slope and that the arc responded differently to collision of the DEZ's two ridges. The north ridge is nearly flat‐topped, and mid‐ocean ridge basalt has been dredged from its flanks. Isobaths of the slope bulge westward where the ridge is being subducted, suggesting that the slope has been uplifted by the ridge. In contrast, the south ridge shows sharp relief and is made up of stratified, possibly sedimentary, rocks, and the Bougainville Spur, a sharp peak, projects above the arc's lowermost slope along the trend of this ridge. The spur may be a peak, similar to one of those surmounting the south ridge, that rammed the arc's slope.